Scientific Investigations Report 2006–5316
U.S. GEOLOGICAL SURVEY
Scientific Investigations Report 2006–5316
DOE/ID-22201
The ESRP is an elongate, downwarped basin containing approximately 3,500 ft of Quaternary and late Tertiary basalt lava flows and sedimentary interbeds overlying late Tertiary rhyolitic volcanic rocks. The basin is characterized by plains-style volcanism (Greeley, 1982) comprising thin (3–100 ft), basaltic, pahoehoe lava flows and coalesced shield volcanoes. Estimates of basalt accumulation rates and eruptive recurrence intervals in 23 boreholes across the INL were published by Champion and others (2002). An analysis of their data reveals that accumulation rates are normally distributed, with pre-800 Ka (thousand years) basalts having a higher mean accumulation rate than post-800 Ka basalts (125 ft/Ka versus 79 ft/Ka).4 Recurrence intervals are lognormally distributed with a ±1 σ interval about the median of 66 [33–167] Ka, and pre-800 Ka basalts have a shorter median recurrence interval than post-800 Ka basalts (54 Ka versus 100 Ka).5
Basalt lava flows are highly fractured at their margins, less so in their interiors, and the rubbly contacts between individual flows (interflow zones) can have greatly enhanced porosity and permeability that focuses ground-water movement (Lindholm and Vaccaro, 1988). The morphology and stacking of lava flows imparts a complex, anastomosed hydrostratigraphy of preferred flow paths at the local scale and a subhorizontal stratigraphic character at larger scales (Welhan and others, 2002b).
Lengthy periods of quiescence during volcanic construction were marked by accumulation of fine-grained eolian, playa, and lacustrine sediment, and lesser amounts of coarser, channelized fluvial deposits within the basalt pile. Beneath the Idaho National Laboratory, sedimentary interbeds constitute about 15 percent by volume of the Snake River Plain aquifer and its unsaturated zone (Anderson and Liszewski, 1997). Because most interbeds are fine-grained, the presence of sediment in the subsurface tends to reduce the aquifer’s bulk hydraulic conductivity. The amount of sediment penetrated by individual boreholes ranges from less than 5 percent of the stratigraphic column to more than 50 percent (up to 100 percent in shallow wells) near the terminus of the Big Lost and Little Lost Rivers in an inferred depositional center known as the Big Lost Trough (Anderson and others, 1996; Geslin and others, 1999).
The Big Lost Trough is situated between the Axial Volcanic Zone to the east, the mountains and associated valleys of the Basin and Range to the north, and the northwest-trending volcanic rift zones in the northern and southern parts of the INL (fig. 2). The Axial Volcanic Zone is a topographic high of linearly clustered volcanic vents that have shed numerous basalt flows towards the northwest and southeast. Over time, these lava flows interfingered with, and trapped, sediments carried onto the Snake River Plain by the Big Lost and Little Lost Rivers. Similar clusters of volcanic vents associated with the northwest-trending rift zones may have further confined the geographic extent of this depositional center.
Although volcanic construction likely exerts the primary control on local geomorphology and the development of depositional centers, it is unclear whether and to what extent regional tectonic subsidence influences larger-scale patterns of sedimentary and volcanic accumulation. Paleomagnetic and radiometric evidence suggests that the overall rate of basalt accumulation in the vicinity of the INL decreased about 200 Ka before present (Anderson and others, 1997; Champion and others, 2002). This conjecture explains the increased proportion of sediment in the stratigraphic section over the latter half of the Quaternary, but leaves unanswered the question of whether sediment supply rates and (or) sources also changed.
Of particular significance to this study is the stratigraphic discontinuity between older and younger stratigraphic units in the northernmost part of the INL (figs. 2 and 3). South of the discontinuity, rocks and sediments younger than about 800 Ka overlie older units, dipping northeastward into the discontinuity; north of the discontinuity, these younger units are almost completely absent. Because different patterns and frequency of sediment accumulation may characterize the older and younger composite units, the statistical nature of their sediment distribution was investigated to determine if units had to be segregated and modeled separately.
4 Significant at a 95 percent confidence level; a Student’s-t test of similarity of the mean accumulation rates returned a p-value of 0.017 (for pre- and post-800 thousand year basalt sample sizes of 5 and 18, respectively.
5 Significant at a 99.9 percent confidence level; a Mann-Whitney rank-sum test of similarity of the median recurrence intervals returned a p-value of 0.0006 (for pre- and post-800 thousand year basalt sample sizes of 13 and 80, respectively).
U.S.
Department of the Interior | U.S. Geological
Survey
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