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Scientific Investigations Report 2009–5014

Scientific Investigations Report 2009–5014

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Geologic Setting

The upper Humboldt River basin consists of several deep structural basins in which basin-fill deposits of Tertiary and Quaternary1 age and volcanic rocks of Tertiary age have accumulated. The bedrock of each basin and adjacent mountains are composed of carbonate and clastic sedimentary rocks of Paleozoic age, and crystalline rocks of Cambrian, Jurassic, and Tertiary age. Numerous geologic studies have been conducted in the area since about the 1930s in efforts to identify and characterize the different rocks and deposits that underlie the study area and to map their distribution.

1This term, and others such as Tertiary or Paleozoic, denotes ranges of geologic age. The geologic time scale on the inside front cover of this report gives ages in millions of years for these terms.

Hydrogeologic Units

The numerous rock units and sedimentary deposits identified in previous studies were grouped into hydrogeologic units by Maurer and others (2004). These hydrogeologic units were regrouped into six hydrogeologic units in this report (pl. 1, table 3). The units, in order of decreasing age, are: (1) carbonate rocks and interbedded clastic sedimentary rocks of Cambrian to Permian age; (2) clastic sedimentary rocks of Ordovician to Devonian age; (3) crystalline rocks consisting of granitic intrusive and metamorphic rocks of Cambrian, Jurassic, and Tertiary age; (4) volcanic rocks of Tertiary age; (5) older basin-fill deposits of Tertiary age that comprise most of the alluvial fill in each basin; and (6) younger basin-fill deposits of Quaternary age that consist mostly of deposits along stream flood plains. Basin-fill deposits and carbonate rocks can have high permeability and transmit ground water, whereas, the other rocks generally have low permeability and impede the flow of ground water (Maurer and others, 2004). The lithology and water-bearing characteristics of each unit are discussed below and summarized in table 3.

Carbonate and Clastic Sedimentary Rocks

Carbonate and clastic sedimentary rocks consist of: (1) carbonate rocks (limestones and dolomites) with interbedded shales and sandstones of Cambrian through Devonian age, (2) mostly shales and sandstones of Mississippian and Pennsylvanian age, and (3) interbedded carbonate rocks, sandstones, and shales of Pennsylvanian and Permian age (pl. 1, table 3). The thickness of this sequence of rocks is at least 20,000 ft in the southern Ruby Mountains, 10,000 ft in the Pinon Range and Snake Mountains, and 4,000 ft in the Independence Mountains (Coats, 1987, p. 13–47). Parts of the unit that consist of carbonate rocks of Cambrian to Devonian age are exposed extensively in the southern Ruby Mountains, southern Pinon Range, and to a limited extent in western parts of the Snake Mountains and northeastern parts of the Independence Mountains (Coats, 1987, pl. 1). Clastic sedimentary rocks, such as sandstone and shale of the Diamond Peak Formation and Chainman Shale, are exposed extensively in the Pinon Range, Adobe Range, and Peko Hills where they overlie the older carbonate rocks (Coats, 1987, pl. 1).

The permeability of the combined unit of carbonate and clastic sedimentary rocks undoubtedly varies over a wide range because of the differing lithologies present. The permeability of clastic parts of the unit probably is relatively low. In contrast, carbonate rocks can be very permeable where circulating ground water has widened fractures through geologic time. Hydraulic conductivity ranges from 0.0005 to 900 ft/d based on estimates from 23 carbonate rock aquifer tests conducted throughout the Great Basin (Plume, 1996, p. 13). Additionally, the hydraulic conductivity of the carbonate rocks ranged from 0.1 to greater than 150 ft/d at two large gold mines (west of the study area) in the vicinity of Carlin (Maurer and others, 1996, p. 9–11). Lowest values reflect hydraulic properties of dense, unfractured rock and highest values reflect hydraulic properties of fracture zones that have been widened by dissolution. This range of values illustrates the importance of faulting and fracturing in the development of secondary porosity and permeability in carbonate rocks. A qualitative indication of the high permeability of carbonate rocks in the study area is the absence of perennial streams in watersheds of the southern Ruby Mountains (fig. 1), which are underlain almost entirely by carbonate rocks (pl. 1; Coats, 1987, pl. 1). In other parts of the study area, perennial mountain streams are common.

Clastic Sedimentary Rocks

Shale, siliceous shale, chert, quartzite, siltstone, and minor amounts of limestone and andesitic volcanic rocks of Ordovician through Devonian age were deposited in a deep-water marine environment adjacent to the continental shelf of Western North America, offshore from where carbonate rocks were being synchronously deposited. During Late Devonian to Early Mississippian time, the clastic sedimentary rocks were thrust eastward as much as 90 mi over the carbonate rocks along a low-angle fault named the Roberts Mountains thrust (Stewart, 1980, p. 36). This tectonic event is known as the Antler orogeny (Stewart, 1980, p. 36). Along the Roberts Mountains thrust in the study area, these clastic sedimentary rocks overlie carbonate and clastic rocks of equivalent age (Coats, 1987, p. 80–81). This hydrogeologic unit is exposed extensively in the Snake and Independence Mountains and to a lesser extent in the Adobe and Pinon Ranges and Diamond and Ruby Mountains (pl. 1).

The permeability of clastic sedimentary rocks of Ordovician to Devonian age varies widely depending on the degree to which the unit has been affected by faulting. At two large gold mines in the area of Carlin just west of the study area, the hydraulic conductivity of this unit was found to range from 0.001 to 0.5 ft/d in unfractured rock to as much as 100 ft/d along faults (Maurer and others, 1996, p. 9–11).

Crystalline Rocks

Two types of crystalline rocks are found in the study area—metamorphic rocks and granitic rocks (pl. 1). Metamorphic rocks occur in the central and northern Ruby Mountains and East Humboldt Range. They formed as a result of the metamorphism (re-crystallization due to heat and pressure) of carbonate and clastic sedimentary rocks of Cambrian to Devonian age during part of the Paleozoic and again in the Mesozoic (Coats, 1987, p. 77–79). Textures and compositions include metaquartzite, calcite marble, gneiss, and schist. The thickness of metamorphic rocks may be as much as 20,000 ft, which is similar to that of unmetamorphosed carbonate rocks in southern parts of the Ruby Mountains.

Granitic rocks occur in the central Ruby Mountains, Elko Hills, southern Independence Mountains, and Pinon Range (pl. 1). Compositions include granite of Jurassic age and granodiorite of Tertiary age in the Ruby Mountains and alaskite of Tertiary age in the southern Independence Mountains and Pinon Range (Coats, 1987, pl. 1, p. 73–77). These rocks extend to great depth, and their distribution at depth can be much greater than that indicated by outcrop area.

The low permeability of crystalline rocks can be inferred from the presence of numerous perennial streams in the central and northern Ruby Mountains and East Humboldt Range. Every watershed in these parts of the mountain ranges has a stream that is perennial at least to the mountain front.

Volcanic Rocks and Sedimentary Basin-Fill Deposits

A thick sequence of alternating sedimentary deposits and volcanic rocks accumulated in structural basins of the study area from Eocene time to Holocene time (Coats, 1987, p. 50–71). The sequence consists of three hydrogeologic units listed in table 3 and shown on plate 1—volcanic rocks, older basin-fill deposits, and younger basin-fill deposits. Herein, the three units are discussed together because they are complexly interbedded. The composite thickness2 of the three units ranges from 1,000 ft to more than 5,000 ft in a deep narrow structural basin that extends from southern Huntington Valley to the southern Marys River Area paralleling the Ruby Mountains and East Humboldt Range (fig. 3). Thicknesses also range from 1,000 ft to more than 5,000 ft in northern parts of the Marys River and North Fork Areas and in part of the Elko Segment. Sixteen oil exploration wells drilled since 1951 penetrated differing thicknesses of basin-fill deposits and volcanic rocks overlying older bedrock (fig. 3; table 4) as follows:

2Combined basin fill and volcanic rock thicknesses discussed above and shown in figure 3 are from a depth to pre-Tertiary basement grid developed for northern Nevada. The depths shown should be considered estimates that do not always agree with depths recorded for oil wells in table 4. (D.A. Ponce, U.S. Geological Survey, written and oral commun., 2007). The process of developing the grid and its uncertainties are described by Ponce (2004, p. 71–74 and figs. 6–3 and 6–9).

Well 13 in Huntington Valley penetrated 11,926 ft of basin-fill deposits and never encountered pre-Tertiary bedrock. Logs for several of the wells also illustrate the complex interbedding of older and younger basin-fill deposits with volcanic rocks. Well 2 penetrated 1,690 ft of older basin fill, 1,110 ft of volcanic rocks, and another 1,430 ft of older basin fill. Well 10 penetrated 3,420 ft of older basin fill, 900 ft of volcanic rocks, and another 1,170 ft of older basin fill. Well 11 penetrated 909 ft of younger basin fill, 243 ft of older basin fill, 1,243 ft of volcanic rocks, and another 684 ft of older basin fill.

The oldest basin-fill deposits and volcanic rocks in the study area, consisting of basal conglomerate overlain by a sequence of welded tuffs, deposits of the Elko Formation (claystone, siltstone, shale, limestone, and tuff), and rhyolitic lava flows and domes, are of Eocene and earliest Oligicene age and are almost entirely north of the Humboldt River (Coats, 1987, p. 51–58). All of the basin-fill deposits are tuffaceous to differing extents. The total thickness exceeds 3,000 ft; however, these rocks and deposits apparently do not constitute a continuous blanket over northern parts of the study area. According to Henry (2008), these deposits accumulated in and along at least three deep and wide eastward draining valleys during Eocene time. The valleys were separated by uplands from which any air-fall tuffs were eroded and re-deposited in the valleys.

From late Eocene to middle Miocene, the upper Humboldt River basin probably was an area undergoing erosion since deposits and volcanic rocks of this age span are absent. About 15–14 Ma (millions of years before present), during the middle Miocene, the Elko basin began to form as low-angle and high-angle faulting began along the west sides of the Ruby Mountains, East Humboldt Range, and Snake Mountains (Wallace and others, 2008, p. 58–61). The Elko basin was large, extending from southern Huntington Valley to northern Marys River and from the structurally active Ruby Mountains–East Humboldt Range–Snake Mountains on the east to the structurally inactive Adobe and Pinon Ranges on the west (Wallace and others, 2008, p. 58). Materials eroded from these mountain ranges spread across the basin accumulating as fine-grained lake deposits in lowlands and as alluvial fan and stream flood-plain deposits toward basin margins. This pattern of deposition continued into late Miocene (10–9 Ma) when the Elko basin began to drain externally resulting in non-deposition of sediments and erosion of existing ones (Wallace and others, 2008, p. 63). Non-deposition, erosion, and transport of sediments out of the basin continued through late Miocene and most of the Pliocene except for a brief period in middle Pliocene when ash-rich sediments similar to those of middle Miocene age accumulated (Wallace and others, 2008, p. 61).

Younger basin-fill deposits in the upper Humboldt River basin consist mostly of unconsolidated sand and gravel along active stream channels (pl. 1; Coats, 1987, p. 70–71). The deposits also form a thin cover overlying pediments of older basin-fill deposits in the northern Marys River and North Fork Areas (pl. 1 and A.R. Wallace, U.S. Geological Survey, oral and written commun., 2008). The younger basin-fill deposits in Huntington Valley, especially those on the east side, also could be a thin veneer of glacial outwash of Pleistocene age from the Ruby Mountains overlying older basin fill of middle Miocene age (A.R. Wallace, U.S. Geological Survey, oral commun., 2008). In Huntington Valley, thicknesses of younger basin-fill deposits penetrated by oil exploration wells range from 1,710 to 3,400 ft (wells 14, 15, and 16; table 4). However, distinguishing younger basin fill from older at such depths is problematic and could be open to different interpretations.

The hydraulic properties of basin-fill deposits and volcanic rocks have not been evaluated in the upper Humboldt River basin. Farther west, however, the hydraulic properties of basin-fill deposits have been evaluated at large gold mines along the Carlin Trend. The basin-fill deposits in this area are of Miocene age and accumulated under conditions similar to those of the middle Miocene Elko basin (Wallace and others, 2008, p. 52–58). Near Carlin at the Gold Quarry mine, the transmissivity of older basin-fill deposits ranges from 780 to 3,600 ft2/d and hydraulic conductivity ranges from 2 to 7 ft/d (Plume, 1995, p. 17). Using well drillers’ logs to determine the ratio of coarse- to fine-grained sediments in the upper 100 ft of flood-plain deposits along the Humboldt River (about 60 mi west of the study area), Bredehoeft and Farvolden (1963, p. 201) estimated the sand-shale ratio to vary from 20 to 70 percent. The hydraulic conductivity determined from specific capacity of selected wells varied from 25 to 40 ft/d (Bredehoeft and Farvolden, 1963, p. 201). These ratios and values of hydraulic conductivity also may apply to the upper Humboldt River basin, and a similar analysis of well logs would be very useful for making estimates of basin-fill aquifer properties in the study area.

Structural Features

Faults and related fractures can function as enhanced conduits for ground water flow, or impede flow where hydrogeologic units of differing permeability are juxtaposed or filled by fault gouge (pulverized rock along the fault zone produced by friction when a fault moves). Near large gold mines along the Carlin Trend, faults impede the movement of ground water where carbonate rocks are juxtaposed against volcanic and clastic sedimentary rocks (Plume, 2005, p. 6–7). The evidence that faults are barriers to flow in this area is the substantial water level difference, greater than 1,000 ft, across the faults after more than 15 years of pumping for mine dewatering (Plume, 2005, p. 6). In other cases, however, the effects of faults may not be known until large-scale pumping stresses are applied to an aquifer.

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