Paleomagnetic correlation of surface and subsurface basalt flows in the central and southwestern part of the Idaho National Laboratory, Idaho

Mary Hodges; Allison R. Trcka; Duane E. Champion

doi:10.3133/sir20255020

Paleomagnetic Correlation of Surface and Subsurface Basalt Flows in the Central and Southwestern Part of the Idaho National Laboratory, Idaho

Scientific Investigations Report 2025-5020

DOE/ID-22263

Prepared in cooperation with the U.S. Department of Energy

By: Mary Hodges, Allison R. Trcka, and Duane E. Champion

https://doi.org/10.3133/sir20255020

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Plate: Plate 1 (476 KB pdf) - Subsurface stratigraphic fence diagrams interpreted from paleomagnetic inclination data from coreholes in the southern part of the Idaho National Laboratory, Idaho, pl. 1
Data Release: USGS data release - Paleomagnetic inclination data collected from Coreholes EREF-GW-1, STF-PIE-AQ-02, TAN 2336, USGS 138, USGS 139, USGS 142, USGS 143, USGS 144, USGS 145, USGS 147, and USGS 148A, located at and near the Idaho National Laboratory, Idaho
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Abstract

The U.S. Geological Survey, in cooperation with the U.S. Department of Energy, used paleomagnetic data from 22 coreholes to construct 3 fence diagrams of subsurface basalt flows in the southern part of the Idaho National Laboratory. These diagrams provide comprehensive descriptions of the horizontal and vertical distribution of basalt flows and sediment layers beneath the surface, aiding geological studies and contributing valuable data to numerical models of groundwater flow and contaminant transport. The correlations established though these diagrams include spatial correlations between basalt flows found in multiple coreholes. Correlations were identified by matching average paleomagnetic inclinations and confirming or denying these correlations using petrology, geochemistry and radiometric ages.

The fence diagrams aid in identifying potential locations of subsurface vents, volcanic vents that have been buried by more recent volcanic activity, associated to subsurface basalt flows. By tracing the subsurface flows and analyzing where the greatest thickness occurs, the locations of buried vents can be inferred. Some subsurface flows exhibit correlations across several coreholes and may indicate yet unidentified surface or buried vents, thereby enhancing our understanding of the volcanic history and subsurface geology of the region.

Introduction

The Idaho National Laboratory (INL), operated by the U.S. Department of Energy (DOE), was established in 1949 to develop peacetime atomic energy, nuclear safety research, defense programs, and advanced energy concepts. Currently, the INL is developing advanced energy concepts and novel energy solutions to ensure the future security of the Nation’s energy resources and technologies. It encompasses approximately 890 square miles (mi²) of the eastern Snake River Plain (ESRP) in southeastern Idaho (fig. 1).

Locations of the coreholes, fence diagrams cross sections, the Arco-Big Southern Butte
and the Axial Volcanic Zones, selected vents, and selected facilities, at and near
the Idaho National Laboratory, Idaho. — Figure 1.
Locations of the coreholes, fence diagrams cross sections, the Arco-Big Southern Butte and the Axial Volcanic Zones, selected vents, and selected facilities, at and near the Idaho National Laboratory, Idaho.

Wastewater disposal sites at the INL have been principal sources of radioactive- and chemical-waste contaminants in water from the ESRP aquifer (fig. 1). Growing concern about the subsurface movement of contaminants from these wastes have led to an increase in the number and types of studies of subsurface geology and hydrology. These studies aim to provide information for the development of conceptual and numerical groundwater-flow and contaminant-transport models (Anderson and Lewis, 1989; Anderson, 1991; Anderson and Bartholomay, 1995; Anderson and Bowers, 1995; Anderson and others, 1996a; Anderson and Liszewski, 1997; Ackerman and others, 2006, 2010 2).

Basalt flows comprise more than 85 percent of the volume of the subsurface of the ESRP, with sedimentary interbeds comprising the remaining 15 percent of the volume (Ackerman and others, 2006, table 6). The basalt and sediment exhibit significant variations in hydraulic conductivity and this three-dimensional distribution of materials plays a crucial role in governing groundwater movement in the ESRP aquifer (Welhan and others, 2002). Despite the small volume of sediment relative to basalt, subsurface sediments have great influence on groundwater movement (Fisher and Twining, 2011). The basalt flows are primarily composed of olivine tholeiite basalts, which are rich in iron and magnesium, characterized by a significant olivine content and have low silica levels (Kuntz and others, 1992). Although these basalts are chemically very similar to each other (Kuntz and others, 1992), their paleomagnetic inclination and polarity vary for each basalt flow from different vents. Paleomagnetic data record the angle (inclination) and orientation (polarity) of the Earth’s magnetic field at the time of eruption. Paleomagnetic inclination data were used to correlate subsurface basalt flows based on similar paleomagnetic inclination measurements and polarity.

Results of this study will be used to extend a three-dimensional subsurface stratigraphic framework of the INL (Champion and others, 2011; Champion and others, 2013; Hodges and Champion, 2016) for geologic and hydrologic studies, including the evaluation of potential volcanic hazards to the INL. This study will also help to refine the geologic framework of the conceptual and numerical models of groundwater flow and contaminant transport through the ESRP aquifer at and near the INL.

Purpose and Scope

Knowledge about the surface and subsurface geologic framework of the ESRP is needed to aid in refining conceptual and numerical models of groundwater flow and contaminant transport at and near the INL. This report describes subsurface stratigraphy of the central part of the INL derived from paleomagnetic inclination data, used to build 3 fence diagrams from 22 coreholes (fig. 1; pl. 1). Supporting paleomagnetic inclination data for coreholes STF-AQ-PIE-02, USGS 138, USGS 139, USGS 142, USGS 143, USGS 144, and USGS 145 are included in multiple data publications and reports (Champion and others, 2011; Hodges and Champion, 2016; Trcka and others, 2024).

Subsurface stratigraphic fence diagrams interpreted from paleomagnetic inclination
data from coreholes in the southern part of the Idaho National Laboratory, Idaho. — Plate 1.
Subsurface stratigraphic fence diagrams interpreted from paleomagnetic inclination data from coreholes in the southern part of the Idaho National Laboratory, Idaho. [Available for downloading at https://doi.org/10.3133/sir20255020]

Previous Investigations

Basalt flows of different ages may have identical paleomagnetic inclinations. Therefore, it is essential to utilize additional data from previous studies, such as lithology, petrology, age determinations, geophysical logs, and geochemical analyses, in conjunction with paleomagnetic inclination to effectively confirm or reject correlations (table 1). For example, whole-rock compositions were used to assess the initial correlations between select coreholes and select surface vents, and olivine phenocryst characterization was evaluated as a correlation tool (Sutherland, 2022).

Kuntz and others (1980) described the petrology, subsurface stratigraphic framework, and potential volcanic hazards of the Radioactive Waste Management Complex (RWMC) by using drill core data, which include K-Ar ages, paleomagnetic inclination and polarity data, and petrography. Anderson and Lewis (1989) expanded the stratigraphic framework of the unsaturated zone at the RWMC to numerous other boreholes by using natural gamma geophysical logs as a primary correlation tool in conjunction with core described by Kuntz and others (1980). The stratigraphic framework for the conceptual model for groundwater flow (Ackerman and others, 2006) was based on a limited number of cores and natural gamma geophysical logs from more than 200 uncored wells (Anderson and Lewis, 1989; Anderson, 1991; Anderson and Bartholomay, 1995; Anderson and Bowers, 1995; Anderson and others, 1996b; Anderson and Liszewski, 1997; Anderson and others, 1999). However, more recent paleomagnetic stratigraphy studies (Champion and others, 2011, 2013 21; Hodges and Champion, 2016) have been used to evaluate and, in many cases, confirm this stratigraphic framework. This report is a more detailed comparison of the stratigraphy through the central parts of the INL where groundwater contamination occurred (Treinen and others, 2024). A list of all the reports published by the U.S. Geological Survey (USGS) on project work completed at the INL can be found in Fisher (2022).

Table 1.

Summary of selected previous investigations of the geology, paleomagnetism, and stratigraphy of the eastern Snake River Plain and Idaho National Laboratory, Idaho.

^{[Abbreviations: CFA, Central Facilities Area; ESRP, eastern Snake River Plain; INL, Idaho National
Laboratory; INTEC, Idaho Nuclear Technology and Engineering Center (also known as
Idaho Chemical Processing Plant [ICPP]); MFC; Materials and Fuels Complex; NRF, Naval
Reactors Facility; NPR, New Production Reactor; RWMC, Radioactive Waste Management
Complex; SRP, Snake River Plain; TAN, Test Area North; USGS, U.S. Geological Survey]}

Table 1. Summary of selected previous investigations of the geology, paleomagnetism, and stratigraphy of the eastern Snake River Plain and Idaho National Laboratory, Idaho.
Reference	Area of investigation	Reference summary
Anders and Sleep (1992)	ESRP	Thermal and mechanical effects of the Yellowstone hotspot
Anderson and Liszewski (1997)	INL	INL unsaturated and ESRP aquifer stratigraphy, based on core and natural gamma logs
Anderson and others (1996a)	INL	INL stratigraphy based on natural gamma logs
Anderson and others (1996b)	INL	INL surficial sediment thickness
Anderson and others (1999)	ESRP, INL, and vicinity	Geologic controls on hydraulic conductivity
Bestland and others (2002)	INL, Big Lost Trough	Sedimentary interbeds in the Big Lost Trough, corehole 2-2A
Braile and others (1982)	ESRP	Seismic profiling of ESRP
Champion and Shoemaker (1977)	ESRP	Paleomagnetism of ESRP volcanic rocks
Champion and Greeley (1978)	Wapi Lava Field, Idaho	Geology of the Wapi Lava Field, south of INL
Champion and others (1981)	INL	Radiometric ages and paleomagnetism at corehole Site E (NPR Test)
Champion and others (1988)	INL	Radiometric ages and paleomagnetism at corehole Site E (NPR Test), description of Big Lost Cryptochron
Champion and Lanphere (1997)	INL, MFC	Age and paleomagnetism of two drill cores at MFC
Champion and Herman (2003)	INL, INTEC area	Paleomagnetism of basalt from drill cores near INTEC
Champion and others (2002)	INL and vicinity, ESRP	Accumulation and subsidence based on paleomagnetism and geochronology
Champion and others (2011)	South INL	Paleomagnetic correlation of surface and subsurface basalts, southern INL
Champion and others (2013)	INL, NRF	Paleomagnetic correlation of surface and subsurface basalts at NRF and INL
Geslin and others (2002)	Big Lost Trough, INL	Pliocene and Quaternary river drainage and sediment
Grimm-Chadwick (2004)	INL, CFA	Stratigraphy, geochemistry, and descriptions of high K₂O flow in cores
Hackett and Smith (1992)	ESRP	Description of ESRP volcanism including development of Axial Volcanic Zone
Helmuth and others (2020)	Butte City, ID, 7.5” map	Geologic map of the Butte City 7.5” quadrangle
Hodges and others (2015)	INL age dates	Radiometric ages of selected basalt flows
Hodges and Champion (2016)	INL	Paleomagnetic correlation of surface and subsurface basalts, central INL
Kuntz and Kork (1978)	INL, RWMC	Geology of RWMC area
Kuntz and others (1980)	INL, RWMC	Radiometric dating, paleomagnetism on cores from RWMC
Kuntz and others (1986)	ESRP	Radiocarbon dates on Pleistocene and Holocene basalt flows
Kuntz and others (1992)	ESRP	ESRP basaltic volcanism, including eruption styles, landforms, petrology, and geochemistry
Kuntz and others (1994)	INL	Geologic map of INL, including radiometric ages and paleomagnetism
Kuntz and others (2003)	North INL	Geologic map of north and central INL, radiometric ages and paleomagnetism
Lanphere and others (1994)	INL, TAN cores	Petrography, age, and paleomagnetism of basalt flows at and near TAN
Lanphere and others (1993)	INL, at and near NRF	Petrography, age, and paleomagnetism of basalt flows at and near NRF
Mazurek (2004)	Central INL	Genetic alteration of basalt in SRP aquifer
Miller (2007)	INL, RWMC	Geochemistry and descriptions of the B flow and stratigraphy of corehole USGS 132
Morse and McCurry (2002)	INL	Base of the aquifer, alteration in basalts
Pierce and Morgan (1992)	ESRP	Age progression of Yellowstone Hot Spot
Pierce and others (2002)	ESRP	Age progression of Yellowstone Hot Spot
Reed and others (1997)	INL, ICPP	Geochemistry of lava flows in cores at ICPP
Rightmire and Lewis (1987)	INL, RWMC	Unsaturated zone geology, geochemistry of sediment and alteration products
Russell (1902)	Snake River Plain and aquifer	Geology and water resources of the Snake River Plain
Scarberry (2003)	INL, RWMC cores	Geochemistry of the F flow (now referred to as the Big Lost Reversed Polarity Cryptochron flows) and distribution in several coreholes at the INL
Shervais and others (2006)	INL, TAN cores	Cyclic geochemical variations in basalt in TAN drill cores
Stroup and others (2008)	INL	Statistical stationarity of sediment interbed thickness
Tauxe and others (2004)	SRP	Paleomagnetism of the SRP
Trcka and others (2024)	INL	Paleomagnetic inclination measurements on subsurface basalt flows
Twining and others (2008)	INL	Construction diagrams, lithological, and geophysical logs for coreholes
Walker (2000)	ESRP	Volcanology of the SRP
Welhan and others (2002)	INL, ESRP	Morphology of inflated pahoehoe flows
Welhan and others (2007)	INL	Geostatistical modeling of sediment abundance
Wetmore and Hughes (1997)	INL	Model morphologies of subsurface lava flows
Wetmore and others (1999)	INL	Axial Volcanic Zone construction

Geologic Setting and Framework

The eastern Snake River Plain (ESRP) developed after the North American tectonic plate moved southwestward over a fixed upper mantle-melting anomaly beginning about 17 million years ago (Ma; Pierce and Morgan, 1992; Pierce and others, 2002; Morgan and McIntosh, 2005). Thermal disruption from the Yellowstone Hot Spot resulted in a time-transgressive series of silicic volcanic fields, characterized by positive geoid anomalies, rhyolitic resurgent caldera eruptions, emplacement of a mid-crustal mafic sill, and subsidence with later basaltic plains magmatism (Braile and others, 1982; Anders and Sleep, 1992; Rodgers and others, 2002; Shervais and others, 2006). Resurgent calderas of the Picabo volcanic field (10.2±0.06 Ma to 7.9±0.4 Ma; Kellogg and others, 1994; McCurry and Hughes, 2006), and the Heise volcanic field (7.05±0.04 Ma to 4.43±0.08 Ma; Pierce and Morgan, 1992; Pierce and others, 2002; Morgan and McIntosh, 2005; McCurry and Hughes, 2006) were active in the area now occupied by the Idaho National Lab (INL).

The ESRP is subsiding in the wake of the Yellowstone Hot Spot calderas (Braile and others, 1982; Anders and Sleep, 1992; McQuarrie and Rodgers, 1998; Rodgers and others, 2002). Silicic material from the caldera eruptions was buried by ESRP olivine tholeiite basalt, and loess and fluvial sediments (Bestland and others, 2002; Blair, 2002; Geslin and others, 2002). Nearly all subsurface sediments are loess, a fine-grained material deposited by wind; fluvial sediments are a minor component of subsurface sediment. Fluvial sediments can be identified and roughly correlated from corehole to corehole using petrography and detrital zircon analysis (Geslin and others, 2002). Loess sediment layers cannot be distinguished from one another except by identifying the basalt flows that overlie and underlie them, so sediment layers are illustrated in the fence diagrams, but not described.

The ESRP is considered the main example of basaltic plains volcanism (Greeley, 1982). This form of basaltic volcanism is intermediate in style between flood basalts, such as the Columbia River Basalt Group, and shield volcano eruptions, such as those in Hawaii and Iceland. Basaltic eruptions on the ESRP generated a land surface formed from coalesced shield volcanoes that produced voluminous tube-fed pahoehoe basalt flows and fissure eruptions (Greeley, 1982). Basaltic plains volcanism is characterized by relatively low effusion rates, long recurrence intervals, low total volumes of lava erupted, and the prevalence of monogenetic volcanoes (Kuntz, 1992). ESRP olivine tholeiite basalt flows may be as much as 22 miles (mi) long, and the accumulated volume of a shield volcano may be as large as 1.7 cubic miles (mi³; Kuntz and others, 1992). The flank areas of typical ESRP low shield volcanoes have slopes of less than 1 degree, and summit and vent areas have slopes of approximately 5 degrees (Greeley, 1982). Large, old vents are sometimes preserved as low buttes (kipukas) surrounded by basalt flows from younger vents; a prominent example is AEC Butte (fig. 2; Kuntz and others, 1994, 2003, 2007 44 46; Skipp and others, 2009). The total volume of basalt filling the ESRP is about 9,600 mi³ (Kuntz, 1992).

Select volcanic vents and prominent topographic features at and near the Idaho National
Laboratory, Idaho — Figure 2.
Select volcanic vents and prominent topographic features at and near the Idaho National Laboratory, Idaho.

More than 95 percent of the total volume of basalt in the ESRP is composed of tube-fed pahoehoe basalt flows erupted from monogenetic shield volcanoes and lava cones (Kuntz and others, 1992). The greatest numbers of eruptive centers are in the Axial Volcanic Zone (fig. 1). The Axial Volcanic Zone is the informal name of a constructional volcanic highland (an elevated landform created by the accumulation of volcanic materials) that parallels the long axis of the ESRP (Hackett and Smith, 1992; Kuntz and others, 1992, 1994 45; Anderson and others, 1996b; Anderson and Liszewski, 1997; Anderson and others, 1999; Hughes and others, 1999; Wetmore and others, 1999). Near Middle Butte (fig. 1) the Axial Volcanic Zone merges with the Arco-Big Southern Butte Volcanic Zone trapping the Big Lost River, the Little Lost River and Birch Creek into a closed basin. Basalt flows from the Axial Volcanic Zone and the Arco-Big Southern Butte Volcanic Zone flowed into the closed basin, and interfingered in the subsurface. This interaction is shown in fence diagram B-B′ (pl. 1), which shows the abundant sediment layers between the basalt flows in coreholes USGS 145, USGS 136, Middle 1823, Middle 2050A, NPR TEST/W-02, and USGS 139 (Champion and others, 2011; Hodges and Champion, 2016; Trcka and others, 2024).

Basaltic eruptions at and near the INL have occurred about every 32–140 thousand years (ka; Champion and others, 2002). Eruptions in what is now the northern part of the INL occur at longer intervals, but the eruptions on or near the Axial Volcanic Zone have the shortest recurrence intervals and have higher accumulation rates (Champion and others, 2002). Each eruption leads to the formation of a shield near the vent and lava tubes that followed streams and then inflated. Shields and inflated lava tubes are elevated terrain, and subsequent eruptions tend to occur around and adjacent to existing basaltic formations.

Most ESRP basalts are olivine tholeiites, which are the result of small quantities of magma that rise to the surface from subcrustal sources over short periods of time without significant fractionation or crustal contamination (Kuntz and others, 1992). Most basalt flows at the INL are petrographically similar and contain olivine, plagioclase, clinopyroxene, ilmenite, magnetite, glass, and accessory apatite (Kuntz and Kork, 1978). Extrusive rocks of more evolved compositions also are found on the ESRP, but the volume of these rocks is small in comparison to the volume of olivine tholeiite basalts. Evolved composition lavas (like andesite) are exposed on the surface at Cedar Butte, East Butte and Big Southern Butte in the southern part of the INL and are found at depth in coreholes near the southern INL boundary (Anderson and others, 1996a; Hodges and Champion, 2016).

Geomagnetic Framework

The Earth’s magnetic field varies through time in polarity and intensity. Approximately 0.78 Ma ago, the polarity of the Earth’s magnetic field changed from “reversed” to “normal,” during which the north pole became the south pole, with “normal” referring to the polarity state that now exists. The event is named the Brunhes-Matuyama boundary (Ogg and Smith, 2004). Geomagnetic polarity reversals have occurred many times through Earth history; figure 3 is a graphic portrayal of reversals since 3.85 Ma ago (Gradstein and others, 2004; Gradstein and others, 2020). In paleomagnetic terminology, a chron is a time period greater than 0.2 Ma, during which the magnetic field is dominantly normal or reversed polarity. During normal or reversed polarity chrons, geomagnetic excursions may occur, resulting in shorter time periods of opposite polarity, known as “subchrons,” 0.03 to 0.2 Ma long. Cryptochrons are time periods of opposite polarity of less than 0.03 Ma duration (McElhinny and McFadden, 2000). Most of the basalts studied for this report were erupted during the normal polarity Brunhes Chron (0.78 Ma to present; Ogg and Smith, 2004) which includes the reversed polarity Big Lost Cryptochron, and the reversed polarity Matuyama Chron (2.581–0.78 Ma; Ogg and Smith, 2004), the normal polarity Cobb Mountain Cryptochron (1.208–1.187 Ma; Ogg and Smith, 2004) and the normal polarity Jaramillo Subchron (1.07–0.99 Ma; Ogg and Smith, 2004; fig. 3).

Mid-Pliocene to latest Holocene geomagnetic time scale, modified from Champion and
others and Gradstein and others — Figure 3.
Mid-Pliocene to latest Holocene geomagnetic time scale, modified from Champion and others (1988) and Gradstein and others (2004).

The remanent magnetization recorded by ferromagnetic minerals in basalt lava flows aligns with the geomagnetic field vector as the basalt crystallizes and cools. The local geomagnetic field vector usually varies with an average angular motion of 4–5 degrees per 100 years in latest Pleistocene and Holocene basalt flows, with extreme variance from 0 to 10 degrees per 100 years (Champion and Shoemaker, 1977).

The Snake River Plain monogenetic volcanic fields, an area with multiple volcanic features created from a single magma source, have volumes of 4 cubic kilometers (km³; 0.96 mi³) or more and typically record a single direction of remanent magnetization. This indicates that individual basalt flow units belonging to a basalt flow erupted in a brief enough period that any change in the local geomagnetic field vector was too small to detect (less than 100 years). Large eruptions that lasted for long periods (more than 100 years) may show small changes in directions. During, before and after, magnetic field reversals, the Earth’s magnetic field changes rapidly, and the change in magnetic directions may be captured in the erupting lavas (Champion and others, 1988).

Volcanoes at and near the INL most likely erupted for only a few days to a few decades of time. Paleomagnetic inclination and declination can be measured from samples collected from surface basalt flows because the original orientation of the field core sample can be measured at the surface. The subcore plugs, cylindrical samples extracted from drill cores, used in this study only yield inclination data because the original azimuth, which is necessary for determining declination, was not preserved during drilling process. The locations of the inclination data are found in table 2.

Table 2.

Corehole data for fence diagram correlations, Idaho National Laboratory, Idaho.

^{[Local name: Local well identifier used in this study. Latitude and longitude: In degrees, minutes, seconds and based on the North American Datum of 1983 (NAD83).
Land-surface altitude: In feet above North American Vertical Datum of 1988 (NAVD88). Fence diagram: Noted A-A′, B-B′, and C-C′ as shown on plate 1 and figure 1. Hole depth: Drilled depth in feet below land surface. Site number: Unique numerical identifier used to access well data which can be found in U.S. Geological Survey (2025).]}

Table 2. Corehole data for fence diagram correlations, Idaho National Laboratory, Idaho.
Local name	Latitude	Longitude	Land-surface altitude	Fence diagram	Hole depth	Citation for paleomagnetic data table	Site number
ARA-COR-005	43°30'44.67′′	112°49'02.99′′	5,056	C-C′	860	Champion and others (2011)	433045112490301
ANL-DH-50	43°35'44.68′′	112°38'02.97′′	5,164	B-B′	252	Champion and others (2011)	433545112380301
ANL-OBS-A-001	43°35'44.75′′	112°39'44.32′′	5,124.61	B-B′	1,910	Champion and others (2011)	433545112394101
C1A	43°30'23.30′′	113°02'06.73′′	5,028.70	A-A′	1,805	Champion and others (2011)	433023113020301
MIDDLE 1823	43°34'18.64′′	112°58'20.59′′	4,942.86	B-B′	1,653	Champion and others (2011)	433418112581701
MIDDLE 2050A	43°34'09.14′′	112°57'08.37′′	4,931.71	B-B′	1,427	Champion and others (2011)	433409112570500
MIDDLE 2051	43°32'16.59′′	113°00'52.37′′	5,000.80	A-A′	1,179	Champion and others (2011)	433217113004900
NRF-15	43°39'41.84′′	112°54'53.59′′	4,845.37	A-A′	759	Champion and others (2013)	433942112545001
STF-AQ-01	43°31'11′′	112°53'51′′	4,951	C-C′	713	Champion and others (2011)	No data
STF-AQ-02	43°31'26.67′′	112°53'34.99′′	4,940.85	C-C′	549	Trcka and others (2024)	433127112533201
USGS 131-131A	43°30'36.69′′	112°58'18.75′′	4,979.61	C-C′	1,198	Champion and others (2011) and Hodges and Champion (2016)	433036112581800
USGS 132	43°29'06.34′′	113°02'53.93′′	5,032.08	C-C′	1,238	Champion and others (2011)	432906113025000
USGS 134	43°36'10.81′′	113°00'01.28′′	4,972.38	A-A′	949	Champion and others (2011)	433611112595800
USGS 135	43°27'53.13′′	113°09'38.63′′	5,139.38	C-C′	1,198	Hodges and Champion (2016)	432753113093600
USGS 136	43°34'47.38′′	112°58'14.99′′	4,938.51	B-B′	1,048	Hodges and Champion (2016)	433447112581501
USGS 138	43°46'14.61′′	112°55'38.05′′	4,802.33	A-A′	334	Trcka and others (2024)	434615112553501
USGS 139	43°38'22.90′′	112°46'06.55′′	4,954.19	B-B′	788	Trcka and others (2024)	433823112460401
USGS 142	43°38'36.98′′	113°01'12.42′′	4,995.32	A-A′	1,880	Trcka and others (2024)	433837113010901
USGS 143	43°37'35.39′′	112°34'15.73′′	5,187.95	B-B′	830	Trcka and others (2024)	433736112341301
USGS 144	43°30'20.57′′	112°55'27.98′′	4,954.22	C-C′	620	Trcka and others (2024)	433021112552501
USGS 145	43°33'58.24′′	113°04'26.98′′	5,134.87	A-A′; B-B′	1,368	Trcka and others (2024)	433358113042701
NPR-TEST/W-02	43°34'50.67′′	112°52'34.99′′	4,933.77	B-B′	5,000	Champion and others (2011)	433451112523201

Most basalt flows and vents on the surface in the southern part of INL have normal remanent magnetic polarity (Kuntz and others, 1994). These basalt flows and vents erupted during the normal polarity Brunhes Chron, indicating that they are less than 0.78 Ma in age (fig. 3). Some surface basalt flows and vents in the northern part of the INL have reversed magnetic polarity and erupted during the reversed polarity Matuyama Chron, fig. 3 (0.78–2.581 Ma; Kuntz and others, 1994; Gradstein and others, 2004).

Reversed polarity is readily identified in paleomagnetic analysis but cannot be positively identified without paleomagnetic analysis. A reversed polarity basalt flow was detected at depths between 324 and 765 ft in the following coreholes (fig. 1; table 2; pl. 1):

• C1A,
• Middle 2051,
• USGS 145,
• NPR-TEST/W-02,
• USGS 131-131A,
• USGS 132,
• USGS 135,
• USGS 144,
• STF-PIE-AQ-01,
• STF-PIE-AQ-02, and
• ARA-COR-005.

This basalt flow named the Big Lost flow, erupted during the reversed polarity Big Lost Cryptochron during the normal polarity Brunhes Chron (Champion and others, 1988). The age of the Big Lost flow was revised from 565±14 ka to 560±4 ka in Hodges and others (2015). Earlier reports called the Big Lost flow: the “Big Lost flow group” (Champion and others, 2011), the “G” flow (Champion and others, 1988), and the “F” flow (Anderson and others, 1996a; Scarberry, 2003). The Big Lost flow shows stratigraphically shallowing mean magnetic inclinations from coreholes that are roughly 5 mi apart, from −47 degrees in USGS 131-131A to −26 degrees in USGS 137A (pl. 1). This may indicate that the ambient magnetic field present at the time of the reversed polarity Big Lost Cryptochron was changing rapidly (Champion and others, 1988).

Sampling and Analytical Methods

The drill cores collected for this study were logged and sampled using protocols described in Johnson and others (2005). Prior to sampling, the core materials were described, and the tops and bottoms of basalt flows, and flow units were identified. In this study, the term “flow” refers to all the eruptive products from the same eruptive event, and a basalt flow unit is defined herein as the minimum subdivision of a basalt flow, possessing quenched bottom and top surfaces, and typically part of a nearly contemporaneous group of other basalt flow units. Depths were measured by tape in feet (ft) and tenths of a foot from footage marked on the cores in the core boxes at the drill site. Cores are marked for depth and up direction in the field, and the recorded measured depths are logged at the end of each core run.

Paleomagnetic samples were collected and processed using protocols described in Trcka and others (2024). Seven samples were collected from each identified basalt flow or flow unit, when enough core material was available. The samples were than analyzed using a cryogenic magnetometer to measure the inclination, unoriented declination, and intensity of magnetizations. A secondary weak remanent magnetization is commonly found on many samples which can be easily removed. However, in rare instances, a much stronger magnetization can be imparted to the basalt flow when it’s on the surface and struck by lightning. This effected basalt flow is then buried by subsequent basalt flows (Champion and others, 2011; Hodges and Champion, 2016; Trcka and others, 2024).

The top of an underlying basalt flow in a drill core may have been deformed, rotated, or both during emplacement, rendering the uppermost part of the basalt flow unsuitable for paleomagnetic analysis. Samples with inconsistent inclination measurements typically are discarded and not included in the average inclination value of the basalt flow(s) (Champion and others, 2011; Hodges and Champion, 2016; Trcka and others, 2024).

The remanent magnetization of the top of any subsurface basalt flow may be thermally remagnetized by subsequent eruption and emplacement of the overlying basalt flow. Thermal remagnetization is most easily detected on the contacts between basalt flows of opposing polarity. Since 2000, the demagnetization protocol changed such that several samples below polarity boundaries were routinely thermally demagnetized (Champion and others, 2011; Hodges and Champion, 2016; Trcka and others, 2024).

Every sample collected from any basalt flow recording the reversed polarity Big Lost Cryptochron was thermally demagnetized to remove the weak remanent magnetization and to assess the relative declination orientations of overlying and underlying normal polarity basalt flows, because they overprint and are overprinted, respectively, by subsequent basalt flows. Assuming the bracketing normal polarity basalt flows have declinations near 0 degrees, an assessment of the declination of the reversed polarity Big Lost Cryptochron may be made. Champion and others (1988) provide further details of this analysis.

Paleomagnetic Correlation of Basalt Flow Units

Basalt flows in each drill core can be characterized for polarity and average paleomagnetic inclinations over the entire length of the core (Champion and others, 2011; Hodges and Champion, 2016; Trcka and others, 2024). Basalt flows in nearby drill cores (hundreds of feet to a few miles apart) may be correlated to one to another at similar depth intervals having the same polarity and inclination measurements. Hole-to-hole fence diagrams were then constructed using correlations derived from paleomagnetic inclination and other available data, which may include depth, petrology, age, geophysical logs, and geochemistry.

When coreholes are farther apart, it becomes more difficult to identify basalt flow similarities, and the correlation between them becomes less certain. In this study, the furthest distance between coreholes is 7.5 mi, from NRF-15 to USGS 138. However, the correlation between these two cores is reliable because USGS 138 has abundant sediment and only three basalt flows.

It is important to note that basalt flows may terminate before reaching the next drill core, or additional or alternative basalt flow(s) may be present in that drill core. Basalt flows may be correlated at greater distances if they have the same petrography, polarity, and average inclination measurements, but the depths at which they correlate may be significantly different (about ±50–100 ft) because of topographic or structural controls, or both, on basalt flows during and after emplacement. When available, supporting data such as geophysical logs, geochemical analyses, and isotopic dating help make correlations when coreholes are far apart.

Paleomagnetic Polarity Transitions

The Earth’s magnetic field changed from reverse polarity to normal polarity at the time of the Brunhes-Matuyama polarity transition (0.780 Ma; Gradstein and others 2004), commonly referred to as the “Brunhes-Matuyama boundary.” The change from reverse polarity (Matuyama Chron) to normal polarity (Brunhes Chron) is seen in all the fence diagrams signified by the dashed red line (pl. 1). The boundary is nearest the surface in USGS 138 (fence diagram A-A′), where it occurs in a thick sediment zone. USGS 138 is the corehole nearest to the edge of the ESRP basin of all the coreholes in this study, and the stratigraphic section there is thinnest.

Other paleomagnetic events recorded in basalt flows in the study area include:

• The reversed polarity Big Lost Cryptochron of the normal polarity Brunhes Chron, recorded in the Big Lost flow, present in A-A′ coreholes C1A, Middle 2051 and USGS 145; B-B′ coreholes USGS 145 and NPR-TEST/W-02; and all the C-C′ coreholes;
• Basalt flows erupted during the normal polarity Jaramillo Subchron of the reversed polarity Matuyama Chron, are found in coreholes C1A, Middle 2051, USGS 145, USGS 134, USGS 142, NRF 15, USGS 136, Middle 1823, Middle 2050A, USGS 132 and USGS 131-131A; and
• Basalt flows erupted during the normal polarity Cobb Mountain Cryptochron of the reversed polarity Matuyama Chron are found in USGS 139 and in ANL-OBS-A-001.

Fence Diagram Correlations of Basalt Flows

Flow and Vent Naming Conventions

Surface Vents and Flows

The surface vents and associated basalt flows in this study almost all erupted from monogenetic eastern Snake River Plain shield volcanoes. Names used for surface vents and their associated surface and subsurface basalt flows at and near the INL derive from (1) formal geographic names, (2) informal names, such as AEC Butte, that follow the naming convention used by researchers at the Idaho National Laboratory (Champion and others, 2011; Champion and others, 2013; and Hodges and Champion, 2016; table 3), and (3) spot elevations on topographic maps, such as Vent 5206 (Kuntz and others, 1980). When there is more than one vent at the same elevation but located in different locations, a modifier is attached, such as “East Vent 5350.” The only formal names used in this report are Quaking Aspen Butte (Geologic Names Information system [GNIS] feature ID 398018), Sixmile Butte (GNIS feature ID 375048), and Tin Cup Butte (GNIS feature ID 375532). For a list of new and previously used surface vent and flow informal names, along with citations to their previous use where applicable, please refer to table 3, which is cited at the first instance of an informal flow name herein.

Table 3.

List of new and previously used surface vent and flow informal names, along with citations to their previous use, Idaho National Laboratory, Idaho.

^{[Source: “Renamed” indicates informal name has been changed from previous reports; “New”
indicates informal name newly used in this report]}

Table 3. List of new and previously used surface vent and flow informal names, along with citations to their previous use, Idaho National Laboratory, Idaho.
Informal name	Source
Vent 5206	Champion and others (2011)
Lavatoo Butte	Champion and others (2011)
Crater Butte	Champion and others (2011)
Pond Butte	Hodges and Champion (2016)
Vent 5244	Renamed
State Butte	Champion and others (2013)
AEC Butte	Champion and others (2013)
Topper Butte	New
Microwave Butte	New
Deuce Butte	New
Mid Butte	Hodges and Champion (2016)
Radio Facility Butte	New
East Vent 5350	Hodges and Champion (2016)
Vent 5252	Champion and others (2011)
East Vent 5305	New
Vent 5298	New
Vent 5398	New
Vent 5148	New
Vent 5119	Hodges and Champion (2016)
West of Atomic City Vent	New
High K₂O	Champion and others (2011)
Low K₂O	Champion and others (2011)
D3	Hodges and Champion (2016)
East of Middle Butte Vent	Champion and others (2011)
G	Champion and others (2011)
East 65 and 67	New
South CFA Buried Vent	Champion and others (2011)
North INTEC	Champion and others (2011)
Big Lost 3	Renamed
Big Lost 2	Renamed
Big Lost 1	Renamed
CFA Buried Vent	Champion and others (2011)
Late Basal Brunhes	Champion and others (2011)
Middle Basal Brunhes	Champion and others (2011)
Early Basal Brunhes	Champion and others (2011)
North Late Matuyama	Champion and others (2011)
South Late Matuyama	Champion and others (2011)
Late Matuyama	Renamed
Post Jaramillo	Champion and others (2011)
Jaramillo 3	Renamed
Jaramillo 2	Renamed
Jaramillo 1	Renamed
Cobb Mountain	Renamed
East Matuyama Upper	Champion and others (2011)
East Matuyama Middle	Champion and others (2011)
East Matuyama Lower	Champion and others (2011)
Middle Matuama 4	Renamed
Middle Matuama 3	Renamed
Middle Matuama 2	Renamed
Middle Matuama 1	Renamed
Post Olduvai	Champion and others (2011)
Olduvai	Champion and others (2011)
Walcott Tuff designated B	New

A number of subsurface basalt flows in this study have been traced to surface vents, most of which have informal names (pl. 1; fig. 2; tables 3 and 4). Tracing a subsurface basalt flow to its surface vent is crucial as it aids in reconstructing the volcanic history of the region and assessing potential volcanic hazards. For simplicity, only the subsurface basalt flows with tracible surface vents are shown on figure 2. However, it is almost certain that more subsurface basalt flows have surface vents, but tracing the basalt flows from the subsurface to their surface vents has not yet been done. Examples of basalt flows for which surface vents likely exist but have not yet been identified include (table 3; Champion and others, 2011; Hodges and Champion, 2016):

• the High K₂O flow,
• the D3 flow,
• the East of Middle Butte flow, and
• the G flow.

There may be others; for example, basalt flows from State Butte (table 3; Champion and others, 2002; Champion and others, 2013; Hodges and others, 2015) are exposed on the surface, and basalt flows in core that are stratigraphically above the State Butte flows may also have surface vents.

Table 4.

Summary of data for surface vents, which have subsurface flows that can be traced to them at and near the Idaho National Laboratory (INL), Idaho (pl. 1).

^{[Vent names taken from informal names shown in table 3. Radiometric age dates published in Hodges and others (2015) unless otherwise noted. Latitude and longitude: Shown in decimal degrees, North American Datum of 1983 (NAD83). Vent altitude: In feet above National Geodetic Vertical Datum of 1988 (NGVD88). Vent paleomagnetic inclination and declination: From published reports with a 1-sigma uncertainty. Fence diagrams A-A′, B-B′, and C-C′ as shown on plate 1. Symbols:±, plus or minus; —, no data]}

Table 4. Summary of data for surface vents, which have subsurface flows that can be traced to them at and near the Idaho National Laboratory (INL), Idaho (pl. 1).
Vent and flow name	Age (ka)	Vent		Vent altitude	Vent paleomagnetic inclination (degrees)	Vent paleomagnetic declination (degrees)	Flow appears in fence diagram
Vent and flow name	Age (ka)	Latitude	Longitude	Vent altitude	Vent paleomagnetic inclination (degrees)	Vent paleomagnetic declination (degrees)	Flow appears in fence diagram
Arco-Big Southern Butte Volcanic Zone (West INL vents)
Quaking Aspen Butte	60±16	43.393	-113.207	5,864	63.6	344.8	C-C′
Vent 5206	63±9	43.455	-113.024	5,206	63.9	12.6	C-C′, B-B′
Lavatoo Butte	211±16¹	43.531	-113.094	5,216	55.5	356.1	A-A′, C-C′
Crater Butte	292±58²	43.592	-113.150	5,570	61.8	344.0	A-A′, B-B′
Pond Butte	293±16⁷	43.584	-113.187	5,371	74.0	48.0	A-A′, B-B′
Sixmile Butte	395±25³	43.513	-113.254	5,695	51.0	347.6	A-A′, B-B′
Tin Cup Butte	352±5⁴	43.468	-113.205	5,590	74.6	66.2	C-C′
Vent 5244	550±33	43.413	-113.115	5,244	66.0	7.6	A-A′, C-C′
Central Vents
State Butte	621±9	43.673	-112.878	4,871	75.0	22.3	A-A′
AEC Butte	721±21³	43.595	-112.950	4,953	53.4	359.2	B-B′
Axial Volcanic Zone (East INL vents)
Topper Butte	—	43.638	-112.552	5,313	42.6	5.5	B-B′
Microwave Butte	—	43.599	-112.590	5,493	64.1	342.8	B-B′
Deuce Butte	208±8⁷	43.588	-112.560	5,454	64.5	13.0	B-B′
Mid Butte	195±39	43.520	-112.774	5,295	52.3	340.9	B-B′, C-C′
Radio Facility Butte	535±10⁷	43.538	-112.585	5,568	69.6	14.3	B-B′
East Vent 5350	313±8⁷	43.518	-112.701	5,350	62.7	352.8	B-B′, C-C′
Vent 5252	350±40⁶	43.515	-112.748	5,252	71.2	358.9	B-B′, C-C′
East Vent 5305	—	43.635	-112.586	5,305	64.5	12.7	B-B′
Vent 5298	—	43.633	-112.527	5,298	65.3	5.0	B-B′
Vent 5398	355±12⁷	43.483	-112.744	5,398	52.9	350.1	C-C′
Vent 5148	—	43.662	-112. 511	5,148	53.4	316.8	B-B′
Vent 5119	572±43⁴	43.459	-112.772	5,119	57.6	12.7	C-C′
West of Atomic City Vent	551±8⁷	43.449	-112.844	5,011	48.0	13.9	C-C′

¹

Champion and others (2002)

²

Skipp and others (2009)

³

Champion and others (2013)

⁴

Hodges and Champion (2016)

⁵

Kuntz and others (1994)

⁶

Champion and others (1988)

⁷

Turrin and others (2023)

Buried Flows

Buried flows are paleomagnetically identified basalt flows that appear in enough cores to make a reasonable inference of a vent location (table 5). The flows and their associated vents are under- and over-lain by basalt flows from vents that have been traced to surface vents. The locations of the buried vents are inferred from cores that have enough thickness data to infer a direction of thickening and are of ages and depths that make a surface vent unlikely. However, due to the concentration of cores near INL facilities, errors in identification and location of buried vents may occur, due to the uneven distribution of cores. Names used for buried flows and their associated buried vents at and near the INL are derived from informal names which are based off (1) paleomagnetic and geochemical characteristics of the basalt flow, (2) proximity to INL facilities, and (3) geography. The informal names follow the naming convention used by researchers at the Idaho National Laboratory (Champion and others, 2011; Champion and others, 2013; and Hodges and Champion, 2016; table 3). For a list of new and previously used buried vent and buried flow informal names, along with citations to their previous use where applicable, please refer to table 3, which is cited at the first instance of an informal flow name herein.

Table 5.

Summary of data for basalt flows with enough data to infer buried vent locations and untraced flows which have no identified surface vent at and near the Idaho National Laboratory, Idaho (pl. 1).

^{[Buried and untraced flow informal names shown in table 3. Buried flows are listed in approximate order from youngest to oldest, with negative
inclination values indicating reversed polarity. Paleomagnetic inclination and declination
from published reports in table 1 with a 1-sigma uncertainty. General vent area: In and around the INL. Fence diagram A-A′, B-B′, and C-C′ as shown on plate 1. Radiometric ages published in Hodges and others (2015) unless otherwise noted. Abbreviations: INL, Idaho National Lab; RWMC, Radioactive Waste Management Complex; ka, thousand
years; Ma, million years;±, plus or minus; —, no data; U, upper; L, lower]}

Table 5. Summary of data for basalt flows with enough data to infer buried vent locations and untraced flows which have no identified surface vent at and near the Idaho National Laboratory, Idaho (pl. 1).
Flow name	Age (ka or Ma)	General vent area	Paleomagnetic inclination (degrees)	Fence diagram
Buried flows
South CFA buried Vent	U: 440±7⁴ ka L: 452±88 ka	Central INL	61 to 63	All
North INTEC	—	Central INL	54	B-B′, C-C′
Big Lost 3	—	RWMC	−34 to −32	All
Big Lost 2	—	RWMC	−40 to −35	A-A′, C-C′
Big Lost 1	560±4 ka	RWMC	−47 to −39	All
CFA buried	536±63 ka	Central INL	60 to 70	All
Late Basal Brunhes	—	Arco-Big Southern Butte	60 to 63	A-A′, B-B′
Middle Basal Brunhes	717±7⁴ ka	Southeast INL	64	All
Early Basal Brunhes	—	Central INL	52 to 55	All
North Late Matuyama	1.462±0.067 Ma⁴	Axial Volcanic Zone	−46	A-A′
South Late Matuyama	792±18⁴ ka	Southeast INL	−59	A-A′, C-C′
Late Matuyama	914±5⁴ ka	South INL	−67 to −75	All
Post Jaramillo	—	South INL	−66 to −64	All
Jaramillo 3	—	South INL	51 to 55	All
Jaramillo 2	—	South INL	57 to 63	A-A′, B-B′
Jaramillo 1	—	South INL	44 to 53	A-A′, B-B′
Cobb Mountain	1.194±0.019 Ma⁴	Axial Volcanic Zone	59, 60	B-B′
East Matuyama upper	1.246±0.011 Ma⁴	Axial Volcanic Zone	−38, −41	B-B′
East Matuyama middle	—	Axial Volcanic Zone	−54	B-B′
East Matuyama lower	—	Axial Volcanic Zone	−56, −59, −57	B-B′
Middle Matuyama 4	1.21±0.102 Ma¹	Axial Volcanic Zone	−59 to −56	B-B′
Middle Matuyama 3	1.256 Ma²	Axial Volcanic Zone	−23,5	B-B′
Middle Matuyama 2	1.37 Ma³	Undetermined	−70, −68, −69, −66, −69	A-A′, B-B′
Middle Matuyama 1	1.44±0.042 Ma¹	Axial Volcanic Zone	−64 to −61	B-B′
Post Olduvai	—	Undetermined	−67 to −64	A-A′, B-B′
Olduvai	—	Axial Volcanic Zone	57, 70	B-B′
Untraced flows
High K₂O	289±8 ka	Axial Volcanic Zone	52	B-B′, C-C′
D3	515±57⁴ ka	Arco-Big Southern Butte	59 to 61	all
East of Middle Butte	—	Arco-Big Southern Butte	60	C-C′
Unknown 47 and 49 degree	—	Axial Volcanic Zone	47, 49	B-B′
G	591±32⁴	Arco-Big Southern Butte	59	C-C′
East 65 and 67	—	Axial Volcanic Zone	65, 67	B-B′

¹

Champion and Lanphere (1997)

²

Duane Champion, U.S. Geological Survey, written commun. (2012)

³

Brent Turrin, Rutgers University, written commun. (2010)

⁴

Turrin and others (2023)

Untraced Vents and Flows

Untraced flows are basalt flows that are believed to have surface vents based on their stratigraphic position but have not yet been identified (table 5). They are located high in the stratigraphic section and are under- and over-lain by basalt flows from vents that have been traced to surface vents. Several basalt flows in this report are young enough to have surface vents but such surface vents have not yet been traced. Names used for untraced flows and their associated vents at and near the INL derive from informal names which are based off (1) paleomagnetic and geochemical characteristics, (2) proximity to INL facilities, and (3) geography. The informal names follow the naming convention used by researchers at the Idaho National Laboratory (Champion and others, 2011; Champion and others, 2013; and Hodges and Champion, 2016). For a list of new and previously used untraced vent and untraced flow informal names, along with citations to their previous use where applicable, please refer to table 3, which is cited at the first instance of an informal flow name herein. More subsurface basalt flows may have surface vents than those listed in table 5.

Fence Diagrams

Three fence diagrams were created using paleomagnetic inclination data for basalt flows in select coreholes (table 2; pl. 1). Fence diagram A-A′ was created because coreholes USGS 145 and USGS 142 were recently drilled in the western part of the INL, and new mapping (Helmuth and others, 2020) revealed that the Pond Butte flow is more extensive than previously thought (table 3; Hodges and Champion, 2016). Fence diagram B-B′ was created to examine the interfingering of flows from the Arco-Big Southern Butte Volcanic Zone and the Axial Volcanic Zone. Fence diagram C-C′ was created to examine the southern limit of the Big Lost Trough.

Fence diagrams are valuable tools for visualizing paleomagnetic inclination data from basalt flows; however, they can oversimplify complex geological relationships. The non-linear paths depicted in diagrams A-A′, B-B′, and C-C′ may obscure the intricate three-dimensional nature of subsurface formations (pl. 1). Factors such as the length of time an eruption continued, coring through different parts of a lava tube, slight displacement of core blocks in the drilling process, and rapid change in the magnetic field, can lead to variations in inclination values that are inadequately represented in a two-dimensional format. This distortion risks misunderstandings about the spatial relationships among different basalt flows and sediments. Therefore, it is essential to approach fence diagrams with a critical mindset. Although they provide a structured visual representation of inclination values across various basalt flows, the inherent distortions may oversimplify the geological context. Variations in thickness may be due to the place where the drill bit encounters a lava surface. Coring through the edge of a lava tube will likely yield a thinner core section than coring through the middle of a lava tube, and basalt from the sides of a lava tube may have anomalous directions from displacement due to fractures.

The stratigraphic units in the fence diagrams (pl. 1) are represented as rectilinear blocks that indicate similar paleomagnetic inclinations for units in that flow. The values in the blocks represent the average paleomagnetic inclination value of a flow to the nearest whole degree (for example, 0.4 degree rounded down, 0.5 degree rounded up).

The top few basalt flows in each fence diagram correlate to basalt flows from vents that are exposed at the surface and have been mapped geologically by Kuntz and others (1994) and Helmuth and others (2020). The surface sample sites have completely oriented directions of remanent magnetization. For the basalt flows that can be traced from surface to subsurface, the subsurface corehole inclination means agree with the surface site inclination data.

Basalt flows identified in the subsurface by their paleomagnetic inclinations that do not correlate to flows in nearby cores are labeled “unknown.” If an unknown sample can be correlated to one other core, the blocks are given colors, if no correlation can be made, blocks are left white.

Fence Diagram A-A′

A north-northeast to southwest cross section line, A-A′ begins at C1A, in the Arco-Big Southern Butte Volcanic Zone, and bends north towards Middle 2051 in the Big Lost Through near the Big Lost River, then proceeds northwest to USGS 145, northeast to USGS 134, northwest to USGS 142, eastward to NRF 15, and finally northward to USGS 138, near the mouth of the Little Lost River (fig 1; pl. 1).

The profile is around 26-mi long with the shortest distance between coreholes being 2.4 mi (between C1A and Middle 2051) and the longest distance of 7.5 mi (between NRF 15 and USGS 138). The depths of the coreholes range from 334 ft below land surface (BLS; USGS 138) to 1,800 ft BLS (USGS 142), with an average depth of 1,182 ft BLS. The surface elevation averages 4,968 ft with the highest elevation at 5,135 (USGS 145) and the lowest at 4,802 (USGS 138).

A-A′ intersects as many as 24 different basalt flows and numerous interbedded sediments, and all the coreholes have sediment as the top layer. Middle 2051 is near the Big Lost River, and had the greatest amount of sediment, with approximately 128 ft at the top with a surface elevation of 5,001 ft.

Fence Diagram B-B′

A west to east cross-section line, B-B′ begins at USGS 145, which is on the shoulder of the Arco-Big Southern Butte Volcanic Rift Zone, proceeds eastward through the Big Lost Trough to Middle 1823, then takes a norward bend to USGS 136, a southeast bend to Middle 2050A, then proceeds to NPR-TEST/W-02, and takes a northeast bend to USGS 139. It goes to ANL-OBS-A-001 which is within the Axial Volcanic Zone and proceeds to ANL-DH-50 and takes a northeast bend to USGS 143 (fig. 1; pl. 1).

USGS 136 was included in the fence diagram because the topmost 500 ft of Middle 1823 is uncored, and there is no deep core between NPR-TEST/W-02 and ANL-OBS-A-001. Additionally, USGS 143 is the deepest core available east of the Materials Fuel Complex (MFC), and both USGS 139 and USGS 143 are new cores in the central part of the INL.

The profile is around 29-mi long, with the shortest distance between coreholes being 0.5 mi (between Middle 1823 and USGS 136) and the longest distance of 6.7 mi (between NPR TEST/W-02 and USGS 139). The depths of the coreholes range from 252 ft BLS (ANL-DH-50) to 5,000 ft BLS (NPR TEST/W-02) with an average depth of 1,586 ft BLS. The surface elevation averages 5,018 ft with the highest elevation at 5,188 (USGS 143) and the lowest at 4,931 (Middle 2050A).

B-B′ intersects as many as 48 different basalt flows and numerous interbedded sediments, and all the coreholes have sediment as the top layer. Middle 2050A is near the Big Lost River, and had the greatest amount of sediment, with approximately 78 ft at the top with a surface elevation of 4,931 ft. ANL-OBS-A-001 and USGS 145 had the least surface sediment, 4 and 3 ft as their first layers, respectively. USGS 145 and USGS 143 are at the greatest elevations, 5,134 ft and 5,184 ft respectively due to their locations within the volcanic zones (table 2).

Fence Diagram C-C′

A west to east cross-section line, C-C′ intersects as many as 28 different basalt flows and numerous interbedded sediments (fig. 1; pl. 1). It begins at USGS 135, located in the Arco-Big Southern Butte Volcanic Zone, and passes through USGS 132 and then USGS 131-131A on a line with azimuth approximately 10 degrees north of east (pl. 1; fig. 1). At USGS 131-131A, the section line enters the Big Lost Trough and bends about 20 degrees east-southeast towards USGS 144. The line then bends northward, and then passes through STF-AQ-01 and STF-AQ-02. From STF-AQ-02, the section turns east-southeast and ends at ARA-COR-005, situated on the west shoulder of the Axial Volcanic Zone.

The profile is around 18-mi long, with the shortest distance between coreholes being 0.38 mi (between STF-AQ-01 and STF-AQ-02) and the longest distance of 5.8 mi (between USGS 135 and USGS 132). The depths of the coreholes range from 620 ft BLS (USGS 144) to 1,198 ft BLS (USGS 131-131A/USGS 135) with an average depth of 910 ft BLS. The surface elevation averages 5,008 ft with the highest elevation at 5,139 (USGS 135) and the lowest at 4,941 (STF-AQ-02).

All the fence diagram C-C′ coreholes have sediment as the top layer. The sediment is thin, as C-C′ runs semi-parallel to the Arco-Big Southern Butte Volcanic Zone and is at a higher elevation than the majority of the Big Lost Trough. The thickest surface sediment is 20-ft thick, found in STF-OBS-AQ-01 within the Big Lost Trough, which has an elevation of 4,951 ft. USGS 135 and ARA-COR-005 are at the greatest elevations, 5,135 ft and 5,043 ft, respectively, due to their locations within the volcanic zones.

Volcanic Vents and Associated Basalt Flows

Olivine tholeiite basalt volcanoes on the ESRP emplace as low shields, fissure flows, and lava tubes (Greeley, 1982). As the ESRP subsides (McQuarrie and Rodgers, 1998), ongoing sediment deposition and basalt eruptions partially fill the basin. Younger lavas are emplaced alongside older basalts until enough lava is erupted to overcome the older topography. Subsurface basalt flows have been traced to 24 surface vents (fig. 2; table 4). Basalt flows and their corresponding vents are discussed from surface to depth, that is, youngest to oldest.

Surface Vents

Arco-Big Southern Butte Volcanic Zone Vents

Quaking Aspen Butte

The basalt of Quaking Aspen Butte (GNIS feature ID 398018; Champion and others, 2011) is the top basalt flow of corehole USGS 132 in fence diagram C-C′. The preferred age of the basalt flow is 60±16 ka (table 4; Hodges and others, 2015), and it correlates to the Quaking Aspen Butte surface site which has a 63.6 degree inclination (table 4; Champion and others, 2011). The Quaking Aspen Butte flow only has one basalt flow unit with an inclination of 60 degrees and is 22-ft thick.

Vent 5206

The basalt flow of Vent 5206 (table 3; Champion and others, 2011) is found in corehole C1A in fence diagram A-A′, and in coreholes USGS 132, USGS 131-131A, USGS 144, STF-PIE-AQ-01, and SFT-PIE-AQ-02 in fence diagram C-C′ (pl. 1). It is the topmost basalt flow in coreholes C1A, USGS 131-131A, USGS 144, STF-PIE-AQ-01, and SFT-PIE-AQ-02, and is overlain by sediment and the Quaking Aspen Butte flow in USGS 132. The preferred age of the basalt flow is 63±9 ka (table 4; Hodges and others, 2015), and it correlates to the Vent 5206 surface site which has a 63.9-degree inclination (table 4; Champion and others, 2011).

In each corehole, the Vent 5206 flow is comprised of up to two basalt flow units. The basalt flow units within a given corehole have the following inclinations and total thicknesses:

• C1A, 65 and 66 degrees and 102-ft thick;
• USGS 132, 64 and 62 degrees and 92-ft thick;
• USGS 131-131A, 64 degrees and 83-ft thick;
• USGS 144, 62 degrees and 84-ft thick;
• STF-PIE-AQ-01, 65 degrees and 51-ft thick; and
• SFT-PIE-AQ-02, 66 degrees and 28-ft thick.

The average inclination across the basalt flow is 64.4 degrees, with a standard deviation of 1.6 degrees.

Lavatoo Butte

The basalt flow of Lavatoo Butte (table 3; Champion and others, 2011) is found in coreholes C1A and Middle 2051 in fence diagram A-A′, and in coreholes USGS 132, USGS 131-131A, and USGS 144 in fence diagram C-C′. The Lavatoo Butte basalt flow is overlain by the Vent 5206 flow in C1A, by sediment in Middle 2051, by sediment and the Vent 5206 flow in USGS 132 and USGS 144, and by the East of Middle Butte flow in USGS 131-131A. The top part of Middle 2051 has considerable sediment, so the top of the Lavatoo Butte flow may not have been recovered in drilling. The preferred age of this basalt flow is 211±16 ka (table 4; Champion and others, 2002), and it correlates to the Lavatoo Butte surface site which has a 55.5-degree inclination (table 4; Champion and others, 2011).

The Lavatoo Butte flow only has one basalt flow unit within a given corehole with the following inclinations and total thicknesses:

• C1A, 55 degrees and 118-ft thick;
• Middle 2051, 57 degrees and 6-ft thick;
• USGS 132, 54 degrees and 71-ft thick;
• USGS 131-131A, 55 degrees and 62-ft thick; and
• USGS 144, 51 degrees and 78-ft thick.

The average inclination across the basalt flow is 54.6 degrees, with a standard deviation of 1.8 degrees.

Crater Butte

The basalt flow of Crater Butte (table 3; Champion and others, 2011) is found in coreholes Middle 2051, USGS 145, and USGS 134 in fence diagram A-A′, and in coreholes USGS 145 and USGS 136 in B-B′. It is overlain by the Lavatoo Butte flow in Middle 2051, and is the topmost flow in USGS 145, USGS 134, and USGS 136, where it is overlain by sediment. The preferred age of this basalt flow is 292±58 ka (table 4; Skipp and others, 2009), and it correlates to the Crater Butte surface site which has a 61.8-degree inclination (table 4; Champion and others, 2011).

In each corehole, the Crater Butte flow is comprised of up to two basalt flow units. The basalt flow units within a given corehole have the following inclinations and total thicknesses:

• Middle 2051, 64 degrees and 16-ft thick;
• USGS 145, 60 and 61 degrees and 130-ft thick;
• USGS 134, 61 degrees and 4-ft thick; and
• USGS 136, 60 degrees and 46-ft thick.

The average inclination across the basalt flow is 61.2 degrees, with a standard deviation of 1.9 degrees.

Pond Butte

The basalt flow of Pond Butte (table 3; Hodges and Champion, 2016) is found in coreholes C1A, Middle 2051, USGS 145, USGS 134, USGS 142, NRF 15, and USGS 138 in fence diagram A-A′, and in coreholes USGS 145, USGS 136, and Middle 2050A in fence diagram B-B’. It is the topmost flow in USGS 142, NRF 15, and USGS 138, where they are overlain by sediment. It is overlain by sediment and the Lavatoo Butte flow in corehole C1A, by sediment and the Crater Butte flow in Middle 2051 and USGS 136, by an unknown uncorrelated 54 degree flow in USGS 145, by the Crater Butte flow in USGS 134, by sediment and the Mid Butte flow in Middle 2050A. The preferred age of this basalt flow is 293±16 ka (table 4; Turrin and others, 2023), and the basalt flow correlates to the Pond Butte surface site which has a 74 degree inclination (table 4; Champion and others, 2011).

In each corehole, the Pond Butte flow is comprised of up to two basalt flow units. The basalt flow units within a given corehole have the following inclinations and total thicknesses:

• C1A, 75 degrees and 50-ft thick;
• Middle 2051, 74 degrees and 71 ft-thick;
• USGS 145, 74 and 75 degrees and 76-ft thick;
• USGS 134, 76 degrees and 113-ft thick;
• USGS 142, 78 degrees and 54-ft thick;
• NRF 15, 77 degrees and 54-ft thick;
• USGS 138, 76 degrees and 13-ft thick;
• USGS 136, 77 degrees and 57-ft thick; and
• Middle 2050A, 72 degrees and 9-ft thick.

The average inclination across the basalt flow is 75.4 degrees, with a standard deviation of 1.8 degrees.

Sixmile Butte

The basalt flow of the Sixmile Butte (GNIS ID 375048; Hodges and Champion, 2016) is found in coreholes USGS 134, USGS 142 and NRF 15 in fence diagram A-A′, and in coreholes USGS 136 and NPR-TEST/W-02 in B-B′. It is overlain by sediment and the Pond Butte basalt flow. The preferred age of this basalt flow is 376±81 ka (table 4; Champion and others, 2002), and it correlates to the Sixmile Butte on the land surface which has a 51 degree inclination (table 4; Champion and others, 2011).

The Sixmile Butte flow only has one basalt flow unit within a given corehole with the following inclinations and total thicknesses:

• USGS 134, 54 degrees and 125-ft thick;
• USGS 142, 56 degrees and 80-ft thick;
• NPR-TEST/W-02, 56 degrees and 96-ft thick;
• NRF 15, 58 degrees and 79-ft thick; and
• USGS 136, 56 degrees and 32-ft thick.

The average inclination across the basalt flow is 56 degrees, with a standard deviation of 1.4 degrees.

Tin Cup Butte

The basalt flow of Tin Cup Butte (GNIS ID 375532; Champion and others, 2011) is the top basalt flow in USGS 135 in fence diagram C-C′. The preferred age of this basalt flow is 550±22 ka (table 4; Hodges and Champion, 2016), and the basalt flow correlates to the Tin Cup Butte on the surface site which has a 74.6 degree inclination (table 4; Champion and others, 2011).

The Tin Cup Butte flow is comprised of up to three basalt flow units with the following inclinations and total thicknesses:

• USGS 135, 71, 72, and 72 degrees and 120-ft thick.

The first basalt flow unit and the second basalt flow unit are overlain by sediment. The average inclination across the basalt flow is 72 degrees, with a standard deviation of 0.6 degrees.

Vent 5244

The basalt flow of Vent 5244 (table 3) is found in corehole C1A in fence diagram A-A′ and in coreholes USGS 135 and USGS 132 in fence diagram C-C′. It is overlain by the D3 flow in USGS 135 and in C1A, and by the Vent 5334 flow in USGS 132. The preferred age of this basalt flow is 550±22 ka (table 4; Hodges and others, 2015), and the basalt flow correlates to the Vent 5244 surface site which has a 66-degree inclination (table 4; Champion and others, 2011).

The Vent 5244 flow only has one basalt flow unit within a given corehole with the following inclinations and total thicknesses:

• C1A, 67 degrees and 93-ft thick;
• USGS 135, 67 degrees and 60-ft thick; and
• USGS 132, 68 degrees and 130-ft thick.

The average inclination across the basalt flow is 67.3 degrees, with a standard deviation of 0.6 degrees.

Central Vents

State Butte

The basalt flow of State Butte (table 3; Champion and others, 2013) is found in corehole NRF 15 in fence diagram A-A′, and it is overlain by the CFA Buried Vent flow. The preferred age of this basalt flow is 621±9 ka (table 4; Hodges and others, 2015), and the basalt flow correlates to the State Butte surface site which has a 75-degree inclination (table 4; Champion and others, 2011).

The State Butte flow is comprised of up to two basalt flow units with the following inclinations and total thicknesses:

• NRF 15, 73 and 72 degrees and 156-ft thick.

The average inclination across the basalt flow is 72.5 degrees, with a standard deviation of 0.7 degrees.

AEC Butte

The basalt flow of AEC Butte (table 3; Champion and others, 2013) flow is found in coreholes USGS 136, Middle 1823, Middle 2050A, and NPR-TEST/W-02 in fence diagram B-B′. It is overlain by the CFA Buried Vent flow in USGS 136 and Middle 1823 and by sediment and CFA Buried Vent flow in Middle 2050A and NPR-TEST/W-02. The preferred age of this basalt flow is 721±21 ka (table 4; Champion and others, 1988), and the basalt flow correlates to the AEC Butte surface site which has a 53.4-degree inclination (table 4; Champion and others, 2011).

In each corehole, the AEC Butte flow is comprised of up to two basalt flow units. The basalt flow units within a given corehole have the following inclinations and total thicknesses:

• USGS 136, 56 degrees and 102-ft thick;
• Middle 1823, 55 and 55 degrees and 111-ft thick;
• Middle 2050A, 52 and 53 degrees and 105-ft thick; and
• NPR-TEST/W-02, 53 and 52 degrees and 102-ft thick.

The average inclination across the basalt flow is 53.7 degrees, with a standard deviation of 1.6 degrees.

Axial Volcanic Zone Vents

Topper Butte

The basalt flow of Topper Butte (table 3) is the topmost basalt flow in USGS 143 in fence diagram B-B′. The flow correlates to the Topper Butte surface site which has a 42.6-degree inclination (table 4; Champion and others, 2011), and is overlain by sediment. The Topper Butte flow only has one basalt flow unit with an inclination of 42 degrees and is 7-ft thick.

Microwave Butte

The basalt flow of Microwave Butte (table 3) is the topmost flow in coreholes USGS 139, ANL-OBS-A-001, and ANL-DH-50 in fence diagram B-B′. The basalt flow correlates to the Microwave Butte surface site which has a 64.1-degree inclination (table 4; Champion and others, 2011), and is overlain by sediment.

The Microwave Butte flow only has one basalt flow unit within a given corehole with the following inclinations and total thicknesses:

• USGS 139, 67 degrees and 41-ft thick;
• ANL-OBS-A-001, 66 degrees and 46-ft thick; and
• ANL-DH-50, 66 degrees and 59-ft thick.

The average inclination across the basalt flow is 66.3 degrees, with a standard deviation of 0.6 degrees.

Deuce Butte

The basalt flow of Deuce Butte (table 3) is found in coreholes USGS 139, ANL-DH-50, and USGS 143 in fence diagram B-B′. The basalt flow is overlain by the Microwave Butte flow in USGS 139, by sediment and the Microwave Bute flow in ANL-DH-50, and by the Topper Butte flow in USGS 143. The preferred age of this basalt flow is 208±8 ka (table 4; Turrin and others, 2023), and the basalt flow correlates to the Deuce Butte surface site which has a 64.5 degree inclination (table 4; Champion and others, 2011).

The Deuce Butte flow only has one basalt flow unit within a given corehole with the following inclinations and total thicknesses:

• USGS 139, 65 degrees and 51-ft thick;
• ANL-DH-50, 66 degrees and 15-ft thick; and
• USGS 143, 64 degrees and 72-ft thick.

The average inclination across the basalt flow is 65 degrees, with a standard deviation of 1 degree.

Mid Butte

The basalt flow of Mid Butte (table 3; Hodges and Champion, 2016) is the top basalt flow in coreholes Middle 2050A and NPR-TEST/W-02 in fence diagram B-B′, and in coreholes STF-PIE-AQ-01, STF-PIE-AQ-02, and ARA-COR-005 in fence diagram C-C′. It is overlain by sediment in Middle 2050A, NPR-TEST/W-02, and ARA-COR-005, by the Vent 5206 flow in STF-PIE-AQ-01, and by the Vent 5206 flow in STF-PIE-AQ-02. The preferred age of this basalt flow is 195±39 ka (table 4; Hodges and others, 2015), and the basalt flow correlates to the Mid Butte surface site which has a 52.3-degree inclination (table 4; Champion and others, 2011).

In each corehole, the Mid Butte flow is comprised of up to two basalt flow units. The basalt flow units within a given corehole have the following inclinations and total thicknesses:

• Middle 2050A, 53 degrees and 41-ft thick;
• NPR-TEST/W-02, 55 degrees and 94-ft thick;
• STF-PIE-AQ-01, 55 degrees and 53-ft thick;
• STF-PIE-AQ-02, 52 degrees and 66-ft thick; and
• ARA-COR-005, 54 and 58 degrees and 88-ft thick.

The average inclination across the basalt flow is 53.8 degrees, with a standard deviation of 1.3 degrees.

Radio Facility Butte

The basalt flow of Radio Facility Butte (table 3) is found in coreholes USGS 139, ANL-OBS-A-01, and ANL-DH-50 in fence diagram B-B′. It is overlain by sediment and the Deuce Butte flow in USGS 139 and in ANL-DH-50, and an unknown uncorrelated 74-degree flow in ANL-OBS-A-001. The preferred age of this basalt flow is 535±10 ka (table 4; Turrin and others, 2023), and the basalt flow correlates to the Radio Facility Butte surface site which has a 69.6-degree inclination (table 4; Champion and others, 2011).

In each corehole, the Radio Facility Butte flow is comprised of up to two basalt flow units. The basalt flow units within a given corehole have the following inclinations and total thicknesses:

• USGS 139, 71 and 69 degrees and 140-ft thick;
• ANL-OBS-A-01, 69 and 70 degrees and 198-ft thick; and
• ANL-DH-50, 71 and 69 degrees and 145-ft thick.

The average inclination across the basalt flow is 69.8 degrees, with a standard deviation of 0.9 degrees.

East Vent 5350

The basalt flow of East Vent 5350 (table 3; Hodges and Champion, 2016) is found in corehole NPR-TEST/W-02 in fence diagram B-B′, and in coreholes STF-PIE-AQ-01, STF-PIE-AQ-02, and ARA-COR-005 in C-C′. It is overlain by sediment and the Mid Butte flow in NPR-TEST/W-02, by the High K₂O flow in STF-PIE-AQ-01, and by sediment and the High K₂O flow in STF-PIE-AQ-02 and ARA-COR-005. The preferred age of this basalt flow is 313±8 ka (table 4; Turrin and others, 2023), and it correlates to the East Vent 5350 surface site which has a 62.7-degree inclination (table 4; Champion and others, 2011).

The East Vent 5350 flow only has one basalt flow unit within a given corehole with the following inclinations and total thicknesses:

• NPR-TEST/W-02, 64 degrees and 21-ft thick;
• STF-PIE-AQ-01, 61 degrees and 21-ft thick;
• STF-PIE-AQ-02, 60 degrees and 19-ft thick; and
• ARA-COR-005, 64 degrees and 6-ft thick.

The average inclination across the basalt flow is 62.3 degrees, with a standard deviation of 2.1 degrees.

Vent 5252

The basalt flow of Vent 5252 (table 3; Champion and others, 2011) is found in coreholes USGS 136, Middle 2050A, NPR-TEST/W-02, and USGS 139 in fence diagram B-B′ and in coreholes USGS 131-131A, USGS 144, STF-PIE-AQ-01, STF-PIE-AQ-02, and ARA-COR-005 in fence diagram C-C′. It is overlaid by sediment and the Pond Butte flow in USGS 136, by sediment and the High K₂O flow in Middle 2050A and USGS 131-131A, by sediment and the East Vent 5350 flow in NPR-TEST/W-02, STF-PIE-AQ-01 and ARA-COR-005, by the Radio Facility Butte flow in USGS 139, and by the East Vent 5350 flow in STF-PIE-AQ-01. The preferred age of this basalt flow is 350±40 ka (table 4; Champion and others, 1988), and the basalt flow correlates to the Vent 5252 surface site which has a 71.2-degree inclination (table 4; Champion and others, 2011).

The Vent 5252 flow only has one basalt flow unit within a given corehole with the following inclinations and total thicknesses:

• USGS 136, 73 degrees and 46 ft-thick;
• Middle 2050A, 70 degrees and 46 ft-thick;
• NPR-TEST/W-02, 73 degrees and 107-ft thick;
• USGS 139, 72 degrees and 25-ft thick;
• USGS 131-131A, 73 degrees and 11-ft thick;
• USGS 144, 72 degrees and 70-ft thick;
• STF-PIE-AQ-01, 70 degrees and 86-ft thick;
• STF-PIE-AQ-02, 73 degrees and 79-ft thick; and
• ARA-COR-005, 69 degrees and 186-ft thick.

The average inclination across the basalt flow is 71.7 degrees, with a standard deviation of 1.6 degrees.

East Vent 5305

The basalt flow of East Vent 5305 (table 3) is found in corehole USGS 143 in fence diagram B-B′. It is overlain by sediment and the Deuce Butte flow. The basalt flow correlates to the East Vent 5305 surface site which has a 64.5-degree inclination (table 4; Champion and others, 2011). The East Vent 5305 flow only has one basalt flow unit with an inclination of 63 degrees and is 63-ft thick.

Vent 5298

The basalt flow of Vent 5298 (table 3) is found in corehole ANL-OBS-A-001 and USGS 143 in fence diagram B-B′. It is overlain by the Radio Facility Butte flow in ANL-OBS-A-001 and by sediment and the Vent 5305 flow in USGS 143. The basalt flow correlates to the Vent 5298 surface site which has a 65.3-degree inclination (table 4; Champion and others, 2011).

In each corehole, the Vent 5298 flow is comprised of up to two basalt flow units. The basalt flow units within a given corehole have the following inclinations and total thicknesses:

• ANL-OBS-A-001, 65 and 66 degrees and 139-ft thick; and
• USGS 143, 65 and 65 degrees and 201-ft thick.

The average inclination across the basalt flow is 65.3 degrees, with a standard deviation of 0.5 degrees.

Vent 5398

The basalt flow of Vent 5398 (table 3) is found in coreholes STF-PIE-AQ-02 and ARA-COR-005 in fence diagram C-C′. It is overlain by sediment and an unknown uncorrelated 55 degree flow in STF-PIE-AQ-02 and by the Vent 5252 flow in ARA-COR-005. The basalt flow correlates to the Vent 5398 surface site which has a 52.9-degree inclination (table 4; Champion and others, 2011).

The Vent 5398 flow only has one basalt flow unit within a given corehole with the following inclinations and total thicknesses:

• STF-PIE-AQ-02, 52 degrees and 51-ft thick; and
• ARA-COR-005, 53 degrees and 62-ft thick.

The average inclination across the basalt flow is 52.5 degrees, with a standard deviation of 0.7 degrees.

Vent 5148

The basalt flow of Vent 5148 (table 3) is found in coreholes USGS 139, and ANL-OBS-A-001 in fence diagram B-B′. It is overlain by sediment and the Vent 5252 flow in USGS 139, and by the Vent 5298 flow in ANL-OBS-A-001. The preferred age of this basalt flow is 355±12 ka (table 4; Turrin and others, 2023), and the basalt flow correlates to the Vent 5148 surface site which has a 53.4-degree inclination (table 4; Champion and others, 2011).

In each corehole, the Vent 5148 flow is comprised of up to three basalt flow units. The basalt flow units within a given corehole have the following inclinations and total thicknesses:

• USGS 139, 55, 53, and 56 degrees and 160-ft thick; and
• ANL-OBS-A-001, 55 degrees and is 40-ft thick.

There is sediment between the second basalt flow unit and third basalt flow unit in corehole USGS 139. The average inclination across the basalt flow is 55 degrees, with a standard deviation of 1.3 degrees.

Vent 5119

The basalt flow of Vent 5119 (table 3; Hodges and Champion, 2016) is found in coreholes STF-PIE-AQ-01 and ARA-COR-005 in fence diagram C-C′. It is overlain by sediment and the Big Lost flow. The preferred age of this basalt flow is 572±43 ka (table 4; Hodges and others, 2015), and the basalt flow correlates to the Vent 5119 surface site which has a 57.6-degree inclination (table 4; Champion and others, 2011).

The Vent 5119 flow only has one basalt flow unit within a given corehole with the following inclinations and total thicknesses:

• STF-PIE-AQ-02, 54 degrees and 41-ft thick; and
• ARA-COR-005, 56 degrees and 85-ft thick.

The average inclination across the basalt flow is 55 degrees, with a standard deviation of 1.4 degrees.

West of Atomic City Vent

The basalt flow of West of Atomic City Vent (table 3) is found in corehole ARA-COR-005 in fence diagram C-C′. It is overlain by an unknown uncorrelated, 60-degree flow. The preferred age of this basalt flow is 551±8 ka (table 4; Turrin and others, 2023), and the basalt flow correlates to the West of Atomic City Vent surface site which has a 48-degree inclination (table 4; Champion and others, 2011).The West of Atomic City Vent flow only has one basalt flow unit with an inclination of 45 degrees and is 65-ft thick.

Untraced and Buried Flows

Untraced Flows

High K₂O

The basalt flow named High K₂O (table 3; Champion and others, 2011) is found in corehole Middle 2050A, in fence diagram B-B′ and in coreholes USGS 131-131A, USGS 144, STF-AQ-01, STF-AQ-02 and ARA-COR-005 in fence diagram C-C′. It is overlain by the Pond Butte flow in Middle 2050A, by sediment and the Lavatoo Butte flow in USGS 131-131A, by the Lavatoo Butte flow in USGS 144, and by sediment and the Mid Butte flow in STF-AQ-01, STF-AQ-02 and ARA-COR-005. The preferred age of this basalt flow is 289±8 ka (table 5; Hodges and others, 2015), and its surface vent is inferred to be in the Axial Volcanic Zone.

The High K₂O flow only has one basalt flow unit within a given corehole with the following inclinations and total thicknesses:

• Middle 2050A, 51 degrees and 29-ft thick;
• USGS 131-131A, 53 degrees and 16-ft thick;
• USGS 144, 50 degrees and 26-ft thick;
• STF-AQ-01, 52 degrees and 43-ft thick;
• STF-AQ-02, 55 degrees and 21-ft thick; and
• ARA-COR-005, 55 degrees and 88-ft thick.

The average inclination across the basalt flow is 55.6 degrees, with a standard deviation of 2.1 degrees.

D3

The basalt flow named D3 (table 3; Hodges and Champion, 2016) is found in coreholes C1A, Middle 2051, USGS 145, USGS 134, USGS 142 in fence diagram A-A′, coreholes USGS 145 and Middle 2050A in fence diagram B-B′, and in coreholes USGS 135, USGS 131-131A, USGS 144, STF-AQ-01, STF-AQ-02 and ARA-COR-005 in fence diagram C-C′. It is overlain by sediment and the Lavatoo Butte flow in C1A, Middle 2051, and USGS 145, by sediment and the Sixmile Butte flow in USGS 134, USGS 142, by sediment and the Pond Butte flow in USGS 145, by sediment and the Vent 5252 flow in Middle 2050A, by an unknown uncorrelated 51-degree flow in USGS 135, by the South CFA Buried Vent flow in USGS 131-131A, USGS 144, STF-AQ-01, and STF-AQ-02, and by the West of Atomic City Vent flow in ARA-COR-005. The preferred age of this basalt flow is 515±57 ka (table 5; Turrin and others, 2023), and its surface vent is inferred to be in the Arco-Big Southern Butte Volcanic Zone.

In each corehole, the D3 flow is comprised of up to three basalt flow units. The basalt flow units within a given corehole have the following inclinations and total thicknesses:

• C1A, 61 degrees and 38-ft thick;
• Middle 2051, 59 and 59 degrees and 112-ft thick;
• USGS 145, 60 degrees and 78-ft thick;
• USGS 134, 59 degrees and 16-ft thick;
• USGS 142, 62 degrees and 43-ft thick;
• Middle 2050A, 59, 59, and 61 degrees and 91-ft thick;
• USGS 135, 60 degrees and 79-ft thick;
• USGS 131-131A, 60 degrees and 131-ft thick;
• USGS 144, 61 degrees and 46-ft thick;
• STF-AQ-01, 59 degrees and 33-ft thick;
• STF-AQ-02, 60 degrees and 34-ft thick; and
• ARA-COR-005, 60 degrees and 46-ft thick.

The average inclination across the basalt flow is 59.9 degrees, with a standard deviation of 0.9 degrees.

East of Middle Butte

The basalt flow named East of Middle Butte (table 3; Champion and others, 2011) is found in corehole USGS 131-131A in fence diagram C-C′. It is overlain by sediment and the Vent 5206 flow, and its surface vent is inferred to be to be east of Middle Butte in the Arco-Big Southern Butte Volcanic Zone. The East of Middle Butte flow only has one basalt flow unit within a given corehole with an inclination of 61 degrees and is 26-ft thick.

Unknown 47-and 49-Degree

An unknown correlated basalt flow is found in coreholes USGS 139 and ANL-OBS-A-001 in fence diagram B-B′. It is overlain by sediment and the Vent 5148 flow in USGS 139 and ANL-OBS-A-001. The flow thickens eastward, so the surface vent is inferred to be in the Axial Volcanic Zone.

In each corehole, the unknown correlated flow is comprised of up to two basalt flow units. The basalt flow units within a given corehole have the following inclinations and total thicknesses:

• USGS 139, 49 degrees and 48-ft thick; and
• ANL-OBS-A-001, 47 degrees and 122-ft thick.

The average inclination across the basalt flow is 48 degrees, with a standard deviation of 1.4 degrees.

G

The basalt flow named G (table 3; Champion and others, 2011) is found in coreholes USGS 135 and USGS 132 in fence diagram C-C′. It is overlain by sediment and the Big Lost flow in USGS 135 and USGS 132. The preferred age of this basalt flow is 591±32 ka (table 5; Turrin and others, 2023), and the surface vent is inferred to be in the Arco-Big Southern Butte Volcanic Zone due to the basalt flows being thickest in cores by the southeast INL boundary.

In each corehole, the G flow is comprised of up to four basalt flow units. The basalt flow units within a given corehole have the following inclinations and total thicknesses:

• USGS 135, 59, 58, 59, and 60 degrees and 233-ft thick; and
• USGS 132, 58 degrees and 50-ft thick.

The average inclination across the basalt flow is 58.8 degrees, with a standard deviation of 0.8 degrees.

East 65 and 67

The basalt flow named East 65 and 67 (table 3) is found in coreholes USGS 143 and USGS 139 in fence diagram B-B′. It is overlain by an unknown uncorrelated 63-degree flow in USGS 143, and by sediment and an unknown uncorrelated 57-degree flow in USGS 139. The basalt flow thickens eastward, so the vent is inferred to be in the Axial Volcanic Zone.

In each corehole, the East 65 and 67 flow is comprised of up to two basalt flow units. The basalt flow units within a given corehole have the following inclinations and total thicknesses:

• USGS 143, 67 and 67 degrees and 254-ft thick; and
• USGS 139, 65 degrees and 80-ft thick.

The average inclination across the basalt flow is 66.3 degrees, with a standard deviation of 1.2 degrees.

Buried Flows

South CFA Buried Vent

The basalt flow named South CFA Buried Vent (table 3; Champion and others, 2011) is found in corehole NPR-TEST/W-02 in fence diagram B-B′, and in coreholes USGS 131-131A, USGS 144, STF-PIE-AQ-01, and STF-PIE-AQ-02 in fence diagram C-C′. It is overlain by sediment and the Vent 5252 flow in NPR-TEST/W-02, USGS 131-131A, and USGS 144, by an unknown uncorrelated 59-degree flow in STF-PIE-AQ-01, and by sediment and an unknown uncorrelated 56-degree flow in STF-PIE-AQ-02. In USGS 144 the basalt flow is separated by 2 ft of sediment. The preferred age of this basalt flow of the upper flow is 440±7 ka (table 5; Turrin and others, 2023), and the preferred age of the lower flow is 452±88 ka (table 5; Hodges and others, 2015). The basalt flow is thickest to the south of the Central Facilities Area where the surface vent is inferred to be located.

In each corehole, the South CFA Buried Vent flow is comprised of up to two basalt flow units. The basalt flow units within a given corehole have the following inclinations and total thicknesses:

• NPR-TEST/W-02, 63 degrees and 30-ft thick;
• USGS 131-131A, 62 degrees and 21-ft thick;
• USGS 144, 62 and 63 degrees and 100-ft thick;
• STF-PIE-AQ-01, 61 degrees and 118-ft thick; and
• STF-PIE-AQ-02, 61 degrees and 66-ft thick.

The average inclination across the basalt flow is 62 degrees, with a standard deviation of 0.9 degrees.

North INTEC

The basalt flow named North INTEC (table 3; Champion and others, 2011) is found in coreholes USGS 136, Middle 2050A in fence diagram B-B′ and in corehole USGS 144 in C-C′. It is overlain by sediment and the Sixmile Butte flow in USGS 136, and by the D3 flow in Middle 2050A and in USGS 144. The basalt flow is thickest to the north of the INTEC facility where the surface vent is inferred to be located at (Champion and others, 2011; Hodges and Champion, 2016).

In each corehole, the North INTEC flow is comprised of up to two basalt flow units. The basalt flow units within a given corehole have the following inclinations and total thicknesses:

• USGS 136, 53 degrees and 75-ft thick;
• Middle 2050A, 47 and 53 degrees and 49-ft thick; and
• USGS 144, 54 degrees and 73-ft thick.

The average inclination across the basalt flow was 51.8 degrees, with a standard deviation of 3.2 degrees.

Big Lost

The basalt flow named Big Lost (table 3; Champion and others, 2011) is found in coreholes C1A, Middle 2051, and USGS 145 in fence diagram A-A′, in coreholes USGS 145 and NPR Test/W-02 in fence diagram B-B′, and in coreholes USGS 135, USGS 132, USGS 131-131A, USGS 144, STF-PIE-AQ-01, STF-PIE-AQ-02, and ARA-COR-005 in fence diagram C-C′. It is overlain by Vent 5244 flow in C1A, by the D3 flow in Middle 2051 and USGS 131-131A, STF-AQ-01, and ARA-COR-005, by sediment and the D3 flow in USGS 145, by the Low K₂O flow (table 3; Champion and others, 2011) in NPR-TEST/W-02, by the Vent 5244 flow in USGS 135 and USGS 132, and by the North INTEC flow in USGS 144. The Big Lost flow is prevalent in the southern part of the INL site, and the Big Lost vent is almost certainly inferred to be in the subsurface below RWMC, where it is thickest. Unlike many other basalt flows, the Big Lost flow is in enough cores that it is possible to accurately infer a vent location. The preferred age of this basalt flow is 560±4 ka (table 5; Hodges and others, 2015), and the basalt flow erupted during the reversed polarity Big Lost Cryptochron.

The Big Lost flow is divided into three subunits:

• Big Lost 3,
• Big Lost 2, and
• Big Lost 1.

These subunits are distinguished by their mean inclinations. Big Lost 1 has the steepest inclinations, while Big Lost 3 has the shallowest.

In each corehole, the Big Lost 3 (table 3) flow is comprised of up to two basalt flow units. The basalt flow units within a given corehole have the following inclinations and total thicknesses:

• USGS 145, −34 degrees and 42-ft thick; and
• USGS 144, −32, and −33 degrees and 107-ft thick.

The average inclination across the basalt flow was −33 degrees, with a standard deviation of 1 degree.

In each corehole, the Big Lost 2 (table 3) flow is comprised of up to three basalt flow units. The basalt flow units within a given corehole have the following inclinations and total thicknesses:

• C1A, −35 and −37 degrees and 80-ft thick;
• USGS 135, 40 degrees and 20-ft thick;
• USGS 131-131A, −35 degrees and 55-ft thick;
• USGS 132, −38 degrees and 35-ft thick;
• USGS 144, −37 degrees and 4-ft thick;
• STF-AQ-01, −35, −37, and −40 degrees and 105-ft thick; and
• ARA-COR-005, −35 degrees and 53-ft thick.

The average inclination across the basalt flow is −35.8 degrees, with a standard deviation of 2.3 degrees.

In each corehole, the Big Lost 1 (table 3) basalt flow is comprised of up to two basalt flow units. The basalt flow units within a given corehole have the following inclinations and total thicknesses:

• C1A, −39 degrees and 31-ft thick;
• Middle 2051, −41 degrees and 43-ft thick;
• NPR-TEST/W-02, −41 degrees and 36-ft thick;
• USGS 132, −42 degrees and 35-ft thick; and
• USGS 131-131A, −44 and −47 degrees and 75 ft-thick.

The average inclination across the basalt flow is −42.3 degrees, with a standard deviation of 2.8 degrees.

CFA Buried Vent

The basalt flow named CFA Buried Vent (table 3; Champion and others, 2011) is found in coreholes C1A, Middle 2015, USGS 145, USGS 134, USGS 142, NRF 15, and USGS 138 in fence diagram A-A′, in coreholes USGS 145, USGS 136, Middle 1823, Middle 2050A, NPR-Test/W-02, and USGS 139 in fence diagram B-B′, and in coreholes USGS 132, USGS 131-131A and STF-PIE-AQ-01 in fence diagram C-C′. It is overlain by sediment and the Big Lost flow in C1A, Middle 2051, USGS 145, by the D3 flow in USGS 134, by an unknown uncorrelated 53 degree flow in USGS 142, by the Sixmile Butte flow in NRF 15, by sediment and the Pond Butte flow in USGS 138, by sediment and the North of INTEC flow in USGS 136 and Middle 2050A, by the Big Lost flow in NPR-TEST/W-02 and USGS 131-131A, by sediment and the Low K₂O flow in USGS 139, by sediment and the G flow in USGS 132, and by sediment and the Vent 5119 flow in STF-PIE-AQ-01. The preferred age of this basalt flow is 536±63 ka (table 5; Hodges and others, 2015). The basalt flow is thickest near the Central Facilities Area where the surface vent is inferred to be at (Champion and others, 2011).

In each corehole, the CFA Buried Vent flow is comprised of up to four basalt flow units. The basalt flow units within a given corehole have the following inclinations and total thicknesses:

• C1A, 66 degrees and 50-ft thick;
• Middle 2051, 68, 68, 69 and 68 degrees and 179-ft thick;
• USGS 145, 70, 64, and 65 degrees and 154-ft thick;
• USGS 134, 64 and 65 degrees and 136-ft thick;
• USGS 142, 66 and 67 degrees and 57-ft thick;
• NRF 15, 64 degrees and 21-ft thick;
• USGS 138, 64 degrees and 79-ft thick;
• USGS 136, 65 and 69 degrees, and 170-ft thick;
• Middle 1823, 67 degrees and 27-ft thick;
• Middle 2050A, 66, 66, 60, and 65 degrees and 177-ft thick;
• NPR-TEST/W-02, 66 degrees and 102-ft thick;
• USGS 139, 65 degrees and 29-ft thick;
• USGS 132, 65 degrees and 28-ft thick;
• USGS 131-131A, 66 and 67 degrees and 128-ft thick; and
• STF-PIE-AQ-01, 67 degrees and 16-ft thick.

The average inclination across the basal flow is 63.9 degrees, with a standard deviation of 11.9 degrees.

Late Basal Brunhes

The basalt flow named Late Basal Brunhes (table 3; Champion and others, 2011) is found in coreholes C1A, Middle 2051, USGS 145, USGS 134, and USGS 142 in fence diagram A-A′, and in coreholes USGS 145, USGS 136, Middle 1823, and Middle 2050A in fence diagram B-B′. It is overlain by sediment and the CFA Buried Vent flow in C1A, USGS 145 and USGS 142, by the CFA Buried Vent flow in Middle 2051, by an unknown uncorrelated 57-degree flow in USGS 134, and by the AEC Butte flow in USGS 136, Middle 1823, and Middle 2050A. The basalt flow is thickest in the west-central part of INL, so the vent is inferred to be in the Arco Big Southern Butte Volcanic Zone (Champion and others, 2011). The basalt flow is one of three basalt flows that marks the boundary that separates the normal polarity Brunhes Chron and the reversed polarity Matuyama Chron.

In each corehole, the Late Basal Brunhes flow is comprised of up to three basalt flow units. The basalt flow units within a given corehole have the following inclinations and total thicknesses:

• C1A, 60 degrees and 25-ft thick;
• Middle 2051, 62 degrees and 50-ft thick;
• USGS 145, 61 and 63 degrees and 148-ft thick;
• USGS 134, 62 degrees and 103-ft thick;
• USGS 142, 60, 60, and 60 degrees and 152-ft thick;
• USGS 145, 61 and 63 degrees and 148-ft thick;
• USGS 136, 61 degrees and 54-ft thick;
• Middle 1823, 63 degrees and 44-ft thick; and
• Middle 2050A, 62 degrees and 33-ft thick.

The average inclination across the basalt flow is 61.4 degrees, with a standard deviation of 1.2 degrees.

Middle Basal Brunhes

The basalt flow named Middle Basal Brunhes (table 3; Champion and others, 2011) is found in coreholes C1A, Middle 2051, and USGS 142 in fence diagram A-A′, in coreholes USGS 136 and Middle 1823 in fence diagram B-B′, and in coreholes USGS 135, USGS 132, and USGS 131-131A in fence diagram C-C′. It is overlain by the Late Basal Brunhes flow in C1A and USGS 142; by sediment and the Late Basal Brunhes flow in Middle 2051, USGS 136 and Middle 1823, by sediment and the G flow in USGS 135, and by sediment and the CFA Buried Vent flow in USGS 132 and USGS 131-131A. The preferred age of this basalt flow is 717±5 ka (table 5; Turrin and others, 2023). It is thickest in the southern part of the INL, so the vent is inferred to be in the southeast part of the INL (Champion and others, 2011). The basalt flow is one of three basalt flows that marks the boundary that separates the normal polarity Brunhes Chron and the reversed polarity Matuyama Chron.

In each corehole, the Middle Basal Brunhes flow is comprised of up to three basalt flow units. The basalt flow units within a given corehole have the following inclinations and total thicknesses:

• C1A, 68 degrees and 53-ft thick;
• Middle 2051, 66 degrees 37-ft thick;
• USGS 142, 66 and 65 degrees and 83-ft thick;
• USGS 136, 67 degrees and 43-ft thick;
• Middle 1823, 64 degrees and 29-ft thick;
• USGS 135, 66 degrees and 36-ft thick;
• USGS 132, 66 degrees and 50-ft thick; and
• USGS 131-131A, 66, 65, and 68 degrees and 97-ft thick.

The average inclination across the basalt flow is 65.2 degrees, with a standard deviation of 2.4 degrees.

Early Basal Brunhes

The basalt flow named Early Basal Brunhes (table 3; Champion and others, 2011) is found in corehole USGS 134 in fence diagram A-A′, in coreholes USGS 136, Middle 1823, Middle 2050A, and NPR-TEST/W-02 in fence diagram B-B′, and in corehole USGS 131-131A in fence diagram C-C′. It is overlain by sediment and the Late Basal Brunhes flow in USGS 134, by the Middle Basal Brunhes flow in USGS 136, Middle 1823, and USGS 131-131A, by the Late Basal Brunhes flow in Middle 2050A, and by sediment and the AEC Butte flow in NPR-TEST/W-02. It is thickest in the Big Lost Trough so the vent is inferred to be in the central part of the INL (Champion and others, 2011). The basalt flow is one of three basalt flows that marks the boundary that separates the normal polarity Brunhes Chron and the reversed polarity Matuyama Chron.

In each corehole, the Early Basal Brunhes flow is comprised of up to three basalt flow units. The basalt flow units within a given corehole have the following inclinations and total thicknesses:

• USGS 134, 55 degrees and 52-ft thick;
• USGS 136, 57 degrees and 21-ft thick;
• Middle 1823, 54 degrees and 11-ft thick;
• USGS 131-131A, 54 degrees and 14-ft thick;
• Middle 2050A, 52 and 53 degrees and 8- ft thick; and
• NPR-TEST/W-02, 55 degrees and 9-ft thick.

The average inclination across the basalt flow is 54.3 degrees, with a standard deviation of 1.8 degrees.

North Late Matuyama

The basalt flow named North Late Matuyama (table 3; Champion and others, 2011) is found in coreholes NRF 15 and USGS 138 in fence diagram A-A′. It is overlain by sediment and an unknown uncorrelated −42-degree flow in NRF 15, and by sediment and the CFA Buried Vent flow in USGS 138. The preferred age of this basalt flow is 1.462±0.067 Ma (table 5; Turrin and others, 2023), and the vent is inferred to be in the Axial Volcanic Zone (Champion and others, 2011).

In each corehole, the North Late Matuyama flow is comprised of up to two basalt flow units. The basalt flow units within a given corehole have the following inclinations and total thicknesses:

• NRF 15, −46 and −48 degrees and 31-ft thick; and
• USGS 138, −46 degrees and 21-ft thick.

The average inclination across the basalt flow is −46.6 degrees, with a standard deviation of 1.2 degrees.

South Late Matuyama

The basalt flow named South Late Matuyama (table 3; Champion and others, 2011) is found in coreholes C1A, USGS 142, and NRF 15 in fence diagram A-A′, and in coreholes USGS 135, USGS 132, and USGS 131-131A in fence diagram C-C′. It is overlain by sediment and the Middle Basal Brunhes flow in C1A, USGS 135, USGS 132, by an unknown uncorrelated −54-degree flow in USGS 142, by sediment and the North Late Matuyama flow in NRF 15, and by sediment and the Early Basal Brunhes flow in USGS 131-131A. The preferred age of this basalt flow is 792±18 ka (table 5; Turrin and others, 2023), and the vent is inferred to be in the southeast part of the INL (Champion and others, 2011).

In each corehole, the South Late Matuyama flow is comprised of up to three basalt flow units. The basalt flow units within a given corehole have the following inclinations and total thicknesses:

• C1A, −61 degrees and 28-ft thick;
• USGS 142, −59 and −61 degrees and 47-ft thick;
• NRF 15, −58 degrees and 14-ft thick;
• USGS 135, −61, −59, and −59 degrees and 225-ft thick;
• USGS 132, −62 and −59 degrees and 111-ft thick; and
• USGS 131-131A, −65 and −61 degrees and 88-ft thick.

The average inclination across the basalt flow is −58.6 degrees, with a standard deviation of 5.1 degrees.

Late Matuyama

The basalt flow named the Late Matuyama (table 3) is found in coreholes C1A, Middle 2051, USGS 145, USGS 134, and NRF 15 in fence diagram A-A′, in coreholes USGS 145, USGS 136, Middle 1823, Middle 2050A, and NPR-TEST/W-02 in fence diagram B-B′, and in coreholes USGS 135, USGS 132, and USGS 131-131A in fence diagram C-C′. It is overlain by sediment and the South Late Matuyama flow in C1A, USGS 135, and USGS 132, by sediment and an unknown uncorrelated 60-degree flow in Middle 2051, by sediment and the Late Basal Brunhes flow in USGS 134, USGS 145, and by sediment and the Early Basal Brunhes flow in USGS 134, USGS 136, Middle 1823, Middle 2050A, NPR-TEST/W-02, and USGS 131-131A. The preferred age of this basalt flow is 919±9 ka (table 5; Turrin and others, 2023), and it is thickest in the south, so the vent is inferred to be south of the southern INL boundary (Champion and others, 2011).

In each corehole, the Late Matuyama flow is comprised of up to seven basalt flow units. The basalt flow units within a given corehole have the following inclinations and total thicknesses:

• C1A, −71, −72, −70, −71, and −71 degrees and 318-ft thick;
• Middle 2051, −66, −69, and −70 degrees and 359-ft thick;
• USGS 145, −75, −71, −67, −69, −69, and −70 degrees and 292-ft thick;
• USGS 134, −70 degrees and 56-ft thick;
• NRF 15, −69 degrees and 8-ft thick;
• USGS 136, −70, −70, and −69 degrees and 126-ft thick;
• Middle 1823, −72, −69, −69, −68, −68, −69, and −68 degrees and 193-ft thick;
• Middle 2050A, −72, −67, −68, and −69 degrees and 147-ft thick;
• NPR-TEST/W-02, −68 degrees and 94-ft thick;
• USGS 135, −72, −73, −70, −70, and −73 degrees and 268-ft thick;
• USGS 132, −72 and −73 degrees 267-ft thick; and
• USGS 131-131A, −72 degrees and 254-ft thick.

The average inclination across the basalt flow is −70 degrees, with a standard deviation of 1.9 degrees.

Post Jaramillo

The basalt named Post Jaramillo (table 3; Champion and others, 2011) is found in coreholes Middle 2051, USGS 134 and USGS 142 in fence diagram A-A′, in corehole USGS 136 in fence diagram B-B′, and in coreholes USGS 135 and USGS 132 in fence diagram C-C′. It is overlain by the Late Matuyama flow in Middle 2051, USGS 136, USGS 135 and USGS 132, by sediment and the Matuyama flow in USGS 134, and by sediment and the South Late Matuyama flow in USGS 142. The basalt flow erupted during the reversed polarity Matuyama Chron after the normal polarity Jaramillo Subchron, and its inferred vent location is south of the INL.

In each corehole, the Post Jaramillo flow is comprised of up to two basalt flow units. The basalt flow units within a given corehole have the following inclinations and total thicknesses:

• Middle 2051, −63 degrees and 88-ft thick;
• USGS 134, −62 and −64 degrees and 103-ft thick;
• USGS 142, −64 degrees and 72-ft thick;
• USGS 136, −66 degrees and 53-ft thick;
• USGS 135, −66 degrees and 31-ft thick; and
• USGS 132, −64 degrees and 46-ft thick.

The average inclination across the basalt flow is 64.1 degrees, with a standard deviation of 1.5 degrees.

Jaramillo

The basalt flow named Jaramillo (table 3) is found in coreholes C1A, Middle 2051, USGS 145, USGS 134, USGS 142, and NRF 15 in fence diagram A-A′, in coreholes USGS 145, USGS 136, and Middle 2050A in fence diagram B-B′, and in coreholes USGS 132, and USGS 131-131A in fence diagram C-C′. It is overlain by sediment and the Late Matuyama flow in C1A, NRF 15, USGS 145, Middle 1823, Middle 2050A, by sediment and the Post Jaramillo flow in Middle 2051, USGS 134, USGS 142, and USGS 136, by the Post Jaramillo flow in USGS 132, and by the Late Matuyama flow in USGS 131-131A. The basalt flow erupted during the normal polarity Jaramillo Subchron of the reversed polarity Matuyama Chron, and its inferred vent location is south of the INL.

The Jaramillo flow is divided into three subunits: Jaramillo 3, Jaramillo 2, and Jaramillo 1. These subunits are distinguished by their mean inclinations. Jaramillo 3 is the youngest and Jaramillo 1 is oldest.

In each corehole, Jaramillo 3 flow (table 3) is comprised of up to five basalt flow units. The basalt flow units within a given corehole have the following inclinations and total thicknesses:

• C1A, 52, 54, 53, 55, and 54 degrees and 244-ft thick;
• Middle 2051, 51 degrees and 85-ft thick;
• USGS 134, 54 degrees and 134-ft thick;
• USGS 142, 56 degrees and 17-ft thick;
• NRF 15, 54 degrees and 68-ft thick;
• USGS 136, 55 degrees and 100-ft thick;
• Middle 1823, 55, 55, and 53 degrees and 72-ft thick;
• Middle 2050A, 53, 52, and 51 degrees and 81-ft thick;
• USGS 132, 52 degrees and 67-ft thick; and
• USGS 131-131A, 51, 51, and 54 degrees and 303-ft thick.

The average inclination across the basalt flow is 53.3 degrees, with a standard deviation of 1.6 degrees.

In each corehole, Jaramillo 2 flow (table 3) is comprised of up to three basalt flow units. The basalt flow units within a given corehole have the following inclinations and total thicknesses:

• C1A, 58 degrees and 22-ft thick;
• USGS 145, 59, 63, and 63 degrees and 131-ft thick;
• USGS 142, 61 degrees and 19-ft thick;
• USGS 136, 60 degrees and 32-ft thick;
• Middle 1823, 57 degrees and 69-ft thick; and
• Middle 2050A, 58 degrees and 51-ft thick.

The average inclination across the basalt flow is 57.4 degrees, with a standard deviation of 2.9 degrees.

In each corehole, Jaramillo 1 flow (table 3) is comprised of up to two basalt flow units. The basalt flow units within a given corehole have the following inclinations and total thicknesses:

• C1A, 52 degrees and 103-ft thick;
• Middle 2051, 44 and 45 degrees and 82-ft thick;
• Middle 1823, 49 degrees and 27-ft thick; and
• Middle 2050A, 49 and 53 degrees and 53-ft thick.

The average inclination across the basalt flow is 48.6 degrees, with a standard deviation of 3.6 degrees.

Cobb Mountain

The basalt flow named Cobb Mountain (table 3) is found in coreholes ANL-OBS-AQ-001 and USGS 139 in fence diagram B-B′. The basalt flow erupted during the normal polarity Cobb Mountain Cryptochron of the reversed polarity Matuyama Chron, and the preferred age is 1.194±19 Ma (table 5; Turrin and others, 2023). The vent is inferred to be located in the Axial Volcanic Zone near the Materials and Fuels Complex.

In each corehole, the Cobb Mountain flow only has one basalt flow unit within a given corehole with following inclinations and total thicknesses:

• USGS 139, 60 degrees and 31-ft thick; and
• ANL-OBS-AQ-001, 59 degrees and 316-ft thick.

The average inclination across the basalt flow is 59.5 degrees, with a standard deviation of 0.7 degrees.

East Matuyama Upper

The basalt flow named East Matuyama Upper (table 3; Champion and others, 2011) is found in corehole ANL-OBS-A-001 in fence diagram B-B′. The basalt flow erupted during the reversed polarity Matuyama Chron. It is overlain by the Cobb Mountain flow, and the preferred age is 1.246±0.011 Ma (table 5; Turrin and others, 2023). The vent is inferred to be in the Axial Volcanic Zone. The East Matuyama Upper basalt flow only has one basalt flow unit with an inclination of −48 degrees and is 124-ft thick.

East Matuyama Middle

The basalt flow named East Matuyama Middle (table 3; Champion and others, 2011) is found in coreholes NPR-TEST/W-02 and ANL-OBS-A-001 in fence diagram B-B′. It is overlain by sediment and the Late Matuyama flow in NPR-TEST/W-02 and by the Eat Matuyama Upper flow in ANL-OBS-A-001. The basalt flow erupted during the reversed polarity Matuyama Chron and its vent is inferred to be in the Axial Volcanic Zone.

The East of Middle Butte flow only has one basalt flow unit within a given corehole with following inclinations and total thicknesses:

• ANL-OBS-A-001, −41 degrees, and 48-ft thick; and
• NPR-Test/W-02, −38 degrees and 17-ft thick.

The average inclination across the basalt flow is −39.5 degrees, with a standard deviation of 2.1 degrees.

East Matuyama Lower

The basalt flow named East Matuyama Lower (table 3; Champion and others, 2011) is found in coreholes NPR-Test/W-02 and ANL-OBS-A-001 in fence diagram B-B′. It is overlain by the East Mutuyama Middle flow in NPR-Test/W-02 and ANL-OBS-A-001. The basalt flow erupted during the reversed polarity Matuyama Chron, and its vent is inferred to be in the Axial Volcanic Zone.

In each corehole, East Matuyama Lower is comprised of up to two basalt flow units. The basalt flow units within a given corehole have the following inclinations and total thicknesses:

• NPR-Test/W-02, −54 degrees and 68-ft thick;
• ANL-OBS-A-001, −54, and −54 degrees, and 99-ft thick.

The average inclination across the basalt flow is −54 degrees, with a standard deviation of 0 degrees.

Middle Matuyama

The basalt flow named Middle Matuyama (table 3) is found in coreholes C1A, USGS 142, and NRF 15 in fence diagram A-A′, and in coreholes Middle 1823, Middle 2050A, NPR-TEST/W-02 and ANL-OBS-A-001 in fence diagram B-B′. It is overlain by sediment and the Jaramillo flow in C1A and NRF 15, by sediment and an unknown uncorrelated −77 degree flow in USGS 142, by a correlated unknown −63 degree flow in Middle 1823, by a correlated unknown −63 degree flow in Middle 2050A, by an unknown uncorrelated −67 degree flow in NPR-Test/W-02, and by sediment and the East Matuyama Lower flow in ANL-OBS-A-001. The basalt flow erupted during the reversed polarity Matuyama Chron between the normal polarity Jaramillo Subchron and the normal polarity Olduvai Subchron.

The Middle Matuyama flow is divided into four subunits:

• Middle Matuyama 4,
• Middle Matuyama 3,
• Middle Matuyama 2, and
• Middle Matuyama 1.

These subunits are distinguished by their ages.

In each corehole, Middle Matuyama 4 flow (table 3) is comprised of one basalt flow unit with the following inclinations and total thicknesses:

• USGS 142, −57-degrees and 20-ft thick;
• NRF 15, −58-degrees and 73-ft thick;
• Middle 1823, −57-degrees and 16 ft thick;
• Middle 2050A, −56-degrees and 53-ft thick;
• NPR-Test/W-02, −59-degrees and 55-ft thick; and
• ANL-OBS-A-001, −57-degrees and 148-ft thick.

The average inclination across the basalt flow is −57.3 degrees, with a standard deviation of 1 degree. The preferred age of this basalt flow is 1.21±0.015 Ma (table 5; Champion and Lanphere, 1997), and the vent is inferred to be in the Axial Volcanic Zone.

In each corehole, Middle Matuyama 3 flow (table 3) is comprised of two basalt flow unit with the following inclinations and total thicknesses:

• NPR-TEST/W-02, −23 and 5 degrees and 111 ft.

The average inclination across the basalt flow is −10 degrees, with a standard deviation of 21.2 degrees. The preferred age of the basalt flow is 1.256±0.010 Ma (table 5; Duane Champion, USGS, written communication, 2012), and the vent location is undetermined.

In each corehole, Middle Matuyama 2 flow (table 3) is comprised of one basalt flow unit with the following inclinations and total thicknesses:

• C1A, −70 degrees and 61-ft thick;
• USGS 142, −68 degrees and 30-ft thick;
• Middle 1823, −69 degrees and 38-ft thick;
• Middle 2050A, −66 degrees and 87-ft thick; and
• NPR-TEST/W-02, −69 degrees and 10-ft thick.

The average inclination across the basalt flow is −68.4 degrees, with a standard deviation of 1.5 degrees. The preferred age of this basalt flow is 1.37±0.18 Ma (table 5; Brent Turrin, Rutgers University, written commun., 2010), and the vent is inferred to be in the Axial Volcanic Zone.

In each corehole, Middle Matuyama 1 flow (table 3) is comprised of up to two basalt flow units. The basalt flow units within a given corehole have the following inclinations and total thicknesses:

• NPR-TEST/W-02, −61 degrees and 73-ft thick; and
• ANL-OBS-A-001, −61 and −64 degrees and 159-ft thick.

The average inclination across the basalt flow is −62 degrees, with a standard deviation of 1.7 degrees. The preferred age of this basalt flow is 1.44±0.04 Ma (table 5; Champion and Lanphere, 1997), and the vent is inferred to be in the Axial Volcanic Zone.

Uncorrelated 11-Degree

An unknown uncorrelated 11-degree basalt flow is found in corehole Middle 2050A in fence diagram B-B′. It is overlain by 139 ft of sediment and the Middle Matuyama flow. Middle 2050A was not drilled through the base of this flow. Because the 11-degree flow underlies a thick sediment, it may be much older than the Middle Matuyama flow. The unknown uncorrelated 11-degree flow is comprised of one basalt flow unit with an inclination of 11 degrees and is a minimum of 9-ft thick.

Four Uncorrelated Flows

There are four unknown uncorrelated basalt flows found in coreholes ANL-OBS-A-001 in fence diagram B-B′. It is overlain by the Middle Matuyama flow, and have the following inclinations and thicknesses, from youngest to oldest:

• −51 degrees and 43-ft thick;
• −41 degrees, and 43-ft thick;
• −42 degrees and 35-ft thick; and
• −51 degrees and 120-ft thick.

Post Olduvai

The basalt flow named Post Olduvai (table 3; Champion and others, 2011) is found in corehole C1A in fence diagram A-A′ and in coreholes Middle 1823, NPR-TEST/W-02 and ANL-OBS-A-001 in fence diagram B-B′. It is overlain by sediment and the Middle Matuyama flow in C1A, by sediment and an unknown uncorrelated −42-degree flow in Middle 1823, by sediment and an unknown uncorrelated −55-degree flow in NPR-TEST/W-02, and by an unknown uncorrelated −58-degree flow in ANL-OBS-A-001. In C1A, the flow overlies an unknown uncorrelated reversed polarity −63 and −48-degree flows which are separated by sediment. The −48-degree flow is the deepest unit penetrated by C1A. The basalt flow erupted after the normal polarity Olduvai Subchron of the reversed polarity Matuyama Chron.

In each corehole, the Post Olduvai flow is comprised of up to two basalt flow units. The basalt flow units within a given corehole have the following inclinations and total thicknesses:

• C1A, −67 and −66 degrees and 140-ft thick;
• Middle 1823, −67 and −66 degrees and 97-ft thick;
• NPR-TEST/W-02, −65 degrees and 55-ft thick; and
• ANL-OBS-A-001, −64 degrees and 21-ft thick.

The average inclination across the basalt flow is −65.8 degrees, with a standard deviation of 1.2 degrees.

Olduvai

The basalt flow named Olduvai (table 3, Champion and others, 2011) is found in coreholes NPR-TEST/W-02 and ANL-OBS-A-001 in fence diagram B-B′. It is overlain by sediment and an unknown uncorrelated −70-degree flow in NPR-TEST/W-02, and by sediment and an unknown uncorrelated −53-degree flow in ANL-OBS-A-001. NPR-TEST/W-02 was drilled to 5,000 ft total depth, but the rest of the NPR-TEST/W-02 core is not discussed in this report because there are no more basalt correlations. The basalt flow erupted during the normal polarity Olduvai Subchron of the reversed polarity Matuyama Chron, which ended 1.728 Ma and began 1.925 Ma (Gradstein and others, 2004). The Olduvai flow is older than 1.44 Ma.

In each corehole, Olduvai flow is comprised of one basalt flow unit with the following inclinations and total thicknesses:

• NPR-TEST/W-02, 70-degrees and 37-ft thick; and
• ANL-OBS-A-001, 56-degrees and a minimum of 8-ft thick, because the corehole ends in this flow without going through it.

The average inclination across the basalt flow is 63 degrees, with a standard deviation of 9.9 degrees.

Walcott Tuff Designated B (Rhyolite)

The rhyolite flow named Walcott Tuff designated B (table 3) is found in corehole USGS 142 in fence diagram A-A′. It is overlain by the Middle Matuyama flow. The preferred age of this rhyolite flow is 6.42±0.07 Ma (Schusler and others, 2020). The tuff is a minimum of 438-ft thick. USGS 142 did not penetrate the base of the rhyolite, so the total thickness of the rhyolite tuff below the base of USGS 142 is unknown. Schusler and others (2020) correlated the tuff in USGS 142 with Walcott Tuff, designated B (Anders and others, 2014) at 4,160 to 4,240 ft BLS in the NPR-TEST/W-02 (Anders and others, 2014), using geochronology, mineralogy, geochemistry and isotopic data.

Sediment

Fence diagram A-A′ runs west and north of the Big Lost River. Sediment in A-A′ is thickest at the surface in corehole Middle 2051, which is near the Big Lost River; however, there is abundant sediment below the Jaramillo flow, in C1A, Middle 2051, USGS 145, and USGS 142. NRF 15 and USGS 138 were not drilled deeply enough to show how much sediment is below the Jaramillo flow. A thick sediment zone is overlain by the Jaramillo flow in coreholes USGS 145 and USGS 142, and is the deepest unit penetrated by USGS 134. The thick sediment is underlain by an unknown correlated 74-degree normal polarity flow in both USGS 145, where it is 63-ft thick, and in USGS 142, where it is 30-ft thick.

Fence diagram B-B′ crosses perpendicular to the Big Lost River. Coreholes USGS 136, Middle 1823, and Middle 2050A are closest to the river and which have prevalent surface sediment. USGS 145 is on the shoulder of the Arco-Big Southern Butte Volcanic Zone and has a 115-ft thick sediment zone that underlies the Jaramillo flow and overlies the unknown correlated 74 degree flow. Corehole ANL-OBS-A-001 and the coreholes to the east have little sediment because they are in the Axial Volcanic Zone. The Arco-Big Southern Butte Volcanic Zone and the Axial Volcanic Zone are volcanic highlands, with more vents, more frequent, and more voluminous eruptions (Champion and others, 2002).

Fence diagram C-C′ is on the shoulder of the Arco-Big Southern Butte Volcanic Zone and the Axial Volcanic Zone. The thickest sediment in the C-C′ section underlies the Vent 5206 flow in USGS 132, USGS 131-131A, USGS 144, and STF-AQ-01, and the next thickest underlies the Lavatoo Butte flow in USGS 131-131A. The older parts of the C-C′ section have little sediment.

Updated Interpretations of Volcanic Flows

Several flow and vent names have been updated from earlier subsurface stratigraphic studies (Champion and others, 2011, 2013 21; Hodges and Champion, 2016) based on new paleomagnetic data, petrological findings, age determinations, geophysical logs, and geochemical analyses.

Vent 5244

The Vent 5244 flow in fence diagrams A-A′ and C-C′ were labeled “E Flow” in Champion and others (2011) and Hodges and Champion (2016). Since those reports were published, the E flow has been traced to Vent 5244 which is south of the INL boundary.

Sixmile Butte

The Sixmile Butte flow identified in fence diagrams A-A′ and B-B′ was previously labeled ATR-Complex unknown vent (Champion and others, 2011) and Vent 4959 (Champion and others, 2013; Hodges and Champion, 2016). Since those reports were published, the basalt flow has been traced to Sixmile Butte (Dietz, 2024). The Sixmile Butte flow flowed northeast, past the older AEC Butte to the NRF area, and to the east, passing south of AEC Butte.

Cobb Mountain

The Cobb Mountain flow was previously labeled as an unknown, uncorrelated 59-degree flow in ANL-OBS-A-001 (Champion and others, 2011). Since then the basalt flow has been correlated and labeled Cobb Mountain, due to its preferred age (Champion and Lanphere, 1997).

Big Lost

The Big Lost flow, which was previously described by Champion and others (2011) and Hodges and Champion (2016), has been divided into three subunits. It was separated based on progressive shallowing of mean paleomagnetic inclination values with shallowing depth. Big Lost 3, the oldest subunit, has flow units characterized by inclination values of around 30-degrees. The next oldest, Big Lost 2, has flow units with steep inclination values of high 30-degrees. Big Lost 1 is the youngest of these subunits, and has flow units with low inclination values of around 40-degrees. The Big Lost flow erupted during the reversed polarity Big Lost Cryptochron, a time when the magnetic field changed very quickly, which is reflected in the inclinations recorded in the Big Lost flows, which erupted from a single volcano over a relatively short time. The time separation of the three subunits is thought to be on the order of months to years of time.

Late Matuyama

In this report, the name of the Matuyama flow, which was previously described by Champion and others (2011) and Hodges and Champion (2016), has been changed to “Late Matuyama.” This change is due to the renaming of the Middle Matuyama flow. Although both flows are chronologically correlated, they remain distinct entities.

Jaramillo

The Jaramillo flow, which was previously described by Champion and others (2011) and Hodges and Champion (2016), has also been divided into three subunits. The normal polarity Jaramillo Subchron of the reversed polarity Matuyama Chron lasted from 1.07 to 0.99 Ma (Gradstein and others, 2004; fig. 3). Unlike the Big Lost flow, which shows consistent variation through time, the variation in the Jaramillo flow inclinations may mean that the Jaramillo flow likely came from more than one vent, erupting at different times. Their time separations may be from thousands to tens of thousands of years. Jaramillo 1 is the oldest and has approximately 50-degree mean inclination flow units. Jaramillo 2 is the next oldest and has steep 50-degree flow units. Jaramillo 3 is the youngest of these units and has low 50-degree flow units. The Jaramillo flow likely came from different volcanoes, the length of time spanned by the eruptions is much longer than typical recurrence in the INL part of the ESRP.

Middle Matuyama

The Middle Matuyama flow was previously separated into four separate basalt flows: Matuyama, 1.21 Ma; Matuyama, 1.256 Ma; Matuyama, 1.37 Ma; and Matuyama, 1.44 Ma, respectively (Champion and others, 2011; Hodges and Champion, 2016). In this report, the flows were combined into a single basalt flow named Middle Matuyama. This basalt flow has been divided into four subunits representing the previously separated basalt flows, based on their respective ages. The subunits of Middle Matuyama are arranged chronologically, with Middle Matuyama 1 being the oldest, dating back to 1.44 Ma. It is followed by Middle Matuyama 2, which is slightly younger at 1.37 Ma. Middle Matuyama 3 is next in the sequence, dating back to 1.256 Ma. The youngest of the subunits is Middle Matuyama 4, which dates back to 1.21 Ma.

Other Flows

In USGS 134, an uncorrelated flow is comprised of three basalt flow units with the following inclinations and total thickness:

• 58, 57, and 57 degrees and 99-ft thick.

It is overlain by the CFA Buried Vent flow. The basalt flow was identified as the AEC Butte flow in an earlier publication (Hodges and Champion, 2016), but its paleomagnetic inclinations are slightly steeper than those usually found in the AEC Butte flow, so they have been labeled uncorrelated in this report.

Summary

Subsurface and surface correlations were identified by average paleomagnetic inclinations of flows, and depth from land surface in coreholes. Subsurface flows that correlate to surface vents include, from youngest to oldest:

• Quaking Aspen Butte,
• Vent 5206,
• Lavatoo Butte,
• Crater Butte,
• Pond Butte,
• Sixmile Butte,
• Tin Cup Butte,
• Vent 5244,
• State Butte,
• AEC Butte,
• Topper Butte,
• Microwave Butte,
• Deuce Butte,
• Mid Butte,
• Radio Relay Butte,
• East Vent 5350,
• Vent 5252,
• East Vent 5305,
• Vent 5298,
• Vent 5398,
• Vent 5148,
• Vent 5119, and
• West of Atomic City Vent.

There are likely more vents that can be correlated to subsurface flows, but the vents and flows are too far apart, with too few coreholes between them to trace correlations.

Vents buried in the subsurface are tentatively identified by finding the greatest thickness of the basalt flow, but due to the concentration of cores near facilities, the greatest thickness may not reflect the true location of buried vents. The following are basalt flows which vent locations can be inferred:

• the South CFA Buried Vent flow;
• the North of INTEC flow;
• the Big Lost flow;
• the CFA Buried flow;
• the Late Basal Brunhes flow;
• the Middle Basal Brunhes flow;
• the Early Basal Brunhes flow;
• the North Late Matuyama flow;
• the South Late Matuyama flow;
• the Late Matuyama flow;
• the Post Jaramillo flow;
• the Jaramillo flow;
• the Cobb Mountain flow;
• the East Matuyama Upper flow;
• the East Matuyama Middle flow;
• the East Matuyama Lower flow;
• the Middle Matuyama flow;
• the Post Olduvai flow; and
• the Olduvai flow.

Some subsurface basalt flows that do not correlate to surface vents, do correlate over several coreholes, and may correlate to buried vents. Subsurface flows which correlate across several coreholes, but not to a surface vent include the High K₂O flow, the D3 flow, the East of Middle Butte Vent flow, the Unknown 47 degree flow, the Unknown 49 degree flow, the G flow, and the East 64 and 67 degree flow. The location of vents buried in the subsurface by younger basalt flows can be inferred if their flows are penetrated by several coreholes, by tracing the flows in the subsurface, and determining where the greatest thickness occurs.

New data presented in this report changed interpretations of some basalt flows discussed in earlier reports. Changes include:

• The former E flow has been traced to the Vent 5244 west of the southwest corner of the INL.
• The former ATR-Complex unknown vent flow (Champion and others, 2011) and Vent 4959 flow (Champion and others, 2013; Hodges and Champion, 2016) has been named Sixmile Butte flow and is correlated with the Sixmile Butte surface vent.
• The former unknown uncorrelated 59 degree flow has been named Cobb Mountain, and is inferred to have erupted during the normal polarity Cobb Mountain Subchron of the reversed polarity Matuyama Chron.
• The Big Lost flow has been subdivided into Big Lost 1, Big Lost 2, and Big Lost 3, based on the average inclination values. All the Big Lost flow units are products of a single monogenetic volcano.
• The former Matuyama flow has been renamed the Late Matuyama flow.
• The Jaramillo flow has been divided, from youngest to oldest, into Jaramillo 1, 2, and 3. Jaramillo 1, 2, and 3 flows are separated from one another by layers of sediment, and have dissimilar distribution, so it is unlikely that they are from a single monogenetic center.
• The former flows named Matuyama, 1.21 Ma; Matuyama, 1.256 Ma; Matuyama, 1.37 Ma; and Matuyama, 1.44 Ma have been combined into one flow name Middle Matuyama and were subdivided into Middle Matuyama 4 (1.21 million years ago [Ma]), Middle Matuyama 3 (1.256 Ma), Middle Matuyama 2 (1.37 Ma), and Middle Matuyama 1 (1.44 Ma), based on the ages of the previously separate flows.
• The basalt flow formerly identified as the AEC Butte flow in USGS 134 (Hodges and Champion, 2016) is now an uncorrelated flow.

Acknowledgments

The authors would like to acknowledge Linda C. Davis and Matthew Gilbert of the U.S. Geological Survey for outstanding work that made this report possible.

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Conversion Factors

U.S. customary units to International System of Units


Multiply	By	To obtain
Length
inch (in.)	2.54	centimeter (cm)
inch (in.)	25.4	millimeter (mm)
foot (ft)	0.3048	meter (m)
mile (mi)	1.609	kilometer (km)
yard (yd)	0.9144	meter (m)
Area
square mile (mi²)	2.590	square kilometer (km²)

International System of Units to U.S. customary units


Multiply	By	To obtain
Length
centimeter (cm)	0.3937	inch (in.)
millimeter (mm)	0.03937	inch (in.)
meter (m)	3.281	foot (ft)
kilometer (km)	0.6214	mile (mi)
meter (m)	1.094	yard (yd)
Area
square kilometer (km²)	0.3861	square mile (mi²)

Datums

Vertical coordinate information is referenced to the North American Vertical Datum of 1988 (NAVD 88).

Horizontal coordinate information is referenced to the North American Datum of 1983 (NAD 83).

Altitude, as used in this report, refers to distance above the vertical datum.

Abbreviations

DOE: U.S. Department of Energy
INL: Idaho National Laboratory
Ma: million years ago
Ka: thousand years ago
ESRP: eastern Snake River Plain
NRF: Naval Reactors Facility
CFA: Central Facilities Area
MFC: Materials and Fuels Complex
ATRC: Advanced Test Reactor Complex
INTEC: Idaho Nuclear Technology and Engineering Center
RWMC: Radioactive Waste Management Complex
USGS: U.S. Geological Survey

For information about the research in this report, contact the

Director, Idaho Water Science Center

U.S. Geological Survey

230 Collins Rd

Boise, Idaho 83702-4520

https://www.usgs.gov/centers/id-water

Manuscript approved on March 04, 2025

Publishing support provided by the U.S. Geological Survey

Science Publishing Network, Tacoma Publishing Service Center

Edited by Jeff Suwak and Vanessa Ball

Layout and design by Yanis X. Castillo

Illustration support by JoJo Mangano

Disclaimers

Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the U.S. Government.

Although this information product, for the most part, is in the public domain, it also may contain copyrighted materials as noted in the text. Permission to reproduce copyrighted items must be secured from the copyright owner.

Suggested Citation

Hodges, M.K.V., Trcka, A.R., and Champion, D.E., 2025, Paleomagnetic correlation of surface and subsurface basalt flows in the central and southwestern part of the Idaho National Laboratory, Idaho: U.S. Geological Survey Scientific Investigations Report 2025–5020, 38 p., 1 pl., https://doi.org/10.3133/sir20255020.

ISSN: 2328-0328 (online)

Study Area

Additional publication details
Publication type	Report
Publication Subtype	USGS Numbered Series
Title	Paleomagnetic correlation of surface and subsurface basalt flows in the central and southwestern part of the Idaho National Laboratory, Idaho
Series title	Scientific Investigations Report
Series number	2025-5020
DOI	10.3133/sir20255020
Publication Date	June 05, 2025
Year Published	2025
Language	English
Publisher	U.S. Geological Survey
Publisher location	Reston, VA
Contributing office(s)	Idaho Water Science Center
Description	Report: vi, 38 p.; 1 Plate: 50.00 x 32.00 inches; Data Release
Country	United States
State	Idaho
Other Geospatial	Idaho National Laboratory
Online Only (Y/N)	Y
Additional Online Files (Y/N)	Y

Paleomagnetic Correlation of Surface and Subsurface Basalt Flows in the Central and Southwestern Part of the Idaho National Laboratory, Idaho

Table of Contents

Links

Abstract

Introduction

Purpose and Scope

Previous Investigations

Table 1.

Geologic Setting and Framework

Geomagnetic Framework

Table 2.

Sampling and Analytical Methods

Paleomagnetic Correlation of Basalt Flow Units

Paleomagnetic Polarity Transitions

Fence Diagram Correlations of Basalt Flows

Flow and Vent Naming Conventions

Surface Vents and Flows

Table 3.

Table 4.

Buried Flows

Table 5.

Untraced Vents and Flows

Fence Diagrams

Fence Diagram A-A′

Fence Diagram B-B′

Fence Diagram C-C′

Volcanic Vents and Associated Basalt Flows

Surface Vents

Arco-Big Southern Butte Volcanic Zone Vents

Quaking Aspen Butte

Vent 5206

Lavatoo Butte

Crater Butte

Pond Butte

Sixmile Butte

Tin Cup Butte

Vent 5244

Central Vents

State Butte

AEC Butte

Axial Volcanic Zone Vents

Topper Butte

Microwave Butte

Deuce Butte

Mid Butte

Radio Facility Butte

East Vent 5350

Vent 5252

East Vent 5305

Vent 5298

Vent 5398

Vent 5148

Vent 5119

West of Atomic City Vent

Untraced and Buried Flows

Untraced Flows

High K2O

D3

East of Middle Butte

Unknown 47-and 49-Degree

G

East 65 and 67

Buried Flows

South CFA Buried Vent

North INTEC

Big Lost

CFA Buried Vent

Late Basal Brunhes

Middle Basal Brunhes

Early Basal Brunhes

North Late Matuyama

South Late Matuyama

Late Matuyama

Post Jaramillo

Jaramillo

Cobb Mountain

East Matuyama Upper

East Matuyama Middle

East Matuyama Lower

Middle Matuyama

High K₂O