Review and Reconnaissance of the Hydrogeology of Tertiary Sedimentary Rocks in the Vicinity of Frenchman Flat, Nevada Test Site

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REVIEW AND RECONNAISSANCE OF THE HYDROGEOLOGY OF TERTIARY SEDIMENTARY ROCKS IN THE VICINITY OF FRENCHMAN FLAT, NEVADA TEST SITE U.S. Department of Energy Nevada Operations Office under contract No. DE-AC08-96NV11718

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This report was prepared as an account of work sponsored by the United States Government Neither the United States nor the United States Department of Energy, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process or service by trade name, mark, manufacturer, or otherwise, does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof. This report has been reproduced from the best available copy. Available to DOE and DOE contractors from: Office of Scientific and Technical Information For prices call (423) 576-8401 Available to the public from: National Technical Information Service U.S. Department of Commerce 5285 Port Royal Road

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REVIEW AND RECONNAISSANCE OF THE HYDROGEOLOGY OF TERTIARY SEDIMENTARY ROCKS IN THE VICINITY OF FRENCHMAN FLAT, NEVADA TEST SITE

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Table of Contents List of Figures ................................................................................................................................... ii List of Acronyms and Abbreviations ...........................................................................................iii 1.2 Methods of Study ............................................................................................. 1-2 2.0 HYDROGEOLOGY OF THE TERTIARY SEDIMENTARY ROCKS ........... 2-1 2.1 Tertiary Sedimentary Rocks South and Southwest of Frenchman 2.1.1 Rocks of Winapi Wash...................................................................... 2-2 2.1.1.1 Extent and Thickness ...................................................... 2-2 2.1.1.2 Age and Correlation........................................................ 2-2 2.1.1.3 Lithology and Hydraulic Character............................... 2-4 2.1.2 Rocks of Pavits Springs..................................................................... 2-5 2.1.2.2 Age and Correlation........................................................ 2-6 2.1.2.3 Lithology and Hydraulic Character............................. 2-7 2.2 Pre-Crater Flat Volcanic Rocks North of Frenchman Flat ................... 2-13 2.2.1 Extent and Thickness........................................................................ 2-14 2.2.2 Age and Correlation......................................................................... 2-14 2.2.3 Lithology and Hydraulic Character................................................ 2-19 4.0 RECOMMENDATIONS FOR FURTHER STUDY ............................................. 4-1 Appendix A Field Data .............................................................................................................A-l

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List of Figures Number . Title Page Figure 1-1 Generalized Geologic Map of the Frenchman Flat Vicinity.......................1-3 Figure 2-1 Evolution of Stratigraphic Nomenclature for Tertiary Sedimentary Rocks in the Frenchman Flat Vicinity................................................. 2-3 Figure 2-2 Schematic Cross Section from Hampel Wash to Nye Canyon................. 2-8 Figure 2-3 Exposure of Rocks of Pavits Spring showing the bedded nature of the Figure 2-4 Exposure of Rocks of Pavits Spring showing the undulating, hummocky topography characteristics of the formation ...........................2-11 Figure 2-5 Conglomerate lenses intercalated with tuff in Nye County, north of Figure 2-6 Conglomerate lens truncating green peralkaline ash bed in Nye

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List of Acronyms and Abbreviations Corrective Action Unit Federal Facility Agreement and Consent Order Lower Carbonate Aquifer Lower Clastic Confining Unit Nevada Test Site southwestern Nevada volcanic field Tuff Confining Unit Timber Mountain Aquifer Underground Test Area (subproject) Volcaniclastic Confining Unit Wahmonie Volcanic Confining Unit

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Work is currently underway within the Underground Test Area (UGTA) subproject of the U.S. Department of Energy/Nevada Operations Office Environmental Restoration Program to develop corrective action plans in support of the overall corrective action strategy for the Nevada Test Site (NTS) as established in the Federal Facility Agreement and Consent Order (FFACO, 1996). A closure plan is currently being developed for Frenchman Flat, which has been identified in the FFACO as a Corrective Action Unit (CAU). Part of this effort requires that hydrogeologic data be compiled for inclusion in a CAU-specific hydrologic flow and transport model that will be used to predict contaminant boundaries. Hydrogeologic maps and cross sections are currently being prepared for use in the model to define the nature and extent of aquifers and confining units that might influence the flow of contaminated groundwater from underground nuclear tests conducted in During this effort, it has been found that older Tertiary-age sediments might be hydrogeologically important in the Frenchman Flat model area. Although the character and extent of these units are poorly known, there is reason to believe that in some parts of Frenchman Flat they may lie between the regional Lower Carbonate Aquifer (LCA) and the younger Tertiary saturated alluvium and volcanic units in which several underground nuclear tests were conducted. It was not possible to quickly determine their extent, or ascertain whether or not these units might act as confining units or aquifers. The work described in this report was done to gain a better understanding of the hydrogeology of The main objective of this study was to develop a simplified conceptual hydrogeologic model of the Tertiary sedimentary rocks in the vicinity of Frenchman Flat so that these rocks could be accurately depicted on Frenchman Flat CAU cross sections and unit-extent maps. To develop a realistic conceptual model, the following two critical questions regarding the hydrogeology of these rocks had to be answered: (1) What are the hydraulic characteristics of these rocks? (i.e., are they aquifers or confining units?), and (2) What is the thickness and extent of these rocks beneath Frenchman Flat? A secondary objective of the study was to document the assumptions, generalizations, and constraints on the hydrogeology of the Tertiary sedimentary rocks as applied to the construction of the cross sections and unit-extent maps. Finally, this report is intended to

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serve as a resource base on the current state of knowledge on the geology and hydrogeology of the Tertiary sedimentary rocks in the southern portion of the NTS. 1.2 Methods of Study The study began with the gathering and review of published geologic and hydrogeologic information on the Tertiary sedimentary rocks. This information included geologic maps, drill hole reports, and various geologic and hydrogeologic reports (see Section 6.0, References). Because much of the information and work on these rocks has not yet been published, personal communication with geologists of the United States Geological Survey and the Los Alamos National Laboratory was a critical part of the study. The authors conducted a field reconnaissance of the Tertiary sedimentary rocks to gain an understanding of the hydraulic character of the rocks and to determine the stratigraphic and hydrogeologic relationships between the older Tertiary rocks north of Frenchman Flat and those exposed south and southwest of Frenchman Flat. The reconnaissance involved visits to outcrop exposures where photographs and field sketches were made and samples collected (Appendix A). Later the samples were examined and described using a microscope (Appendix B). During both outcrop and sample examination, particular attention was given to those properties and characteristics that could influence the ability of rocks to transmit groundwater. Samples collected during the reconnaissance are available from the United States Geological Survey Core Library in Mercury, Nevada. Photographs and field sketches are on file at the Bechtel Nevada Geology/Hydrology office in Mercury, Nevada. In addition, drill cuttings, core, geophysical logs, and hydrologic data from existing drill holes that encountered the rocks of interest were also A brief synopsis of the geology and hydrogeology of Frenchman Flat is presented in this section. More detail can be found in the references listed in Section 6.0 of this report. Frenchman Flat is a closed intennontane basin located in the southeastern portion of the NTS. It is bounded on the north by Massachusetts Mountain and the Halfpint Range, on the east by the Buried Hills, on the south by the Spotted Range, and on the west by the Wahmonie volcanic center (Figure 1-1). The sparsely vegetated valley floor slopes gently toward a central playa lakebed. Ground level elevations range from 938 meters (m) (3,078 feet [ft]) above sea level at the playa, to over 1,219 m (4,000 ft) in the nearby

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E3 Quaternary playa deposits Tertiary volcanic rocks Rocks of Pavits Spring ^ Rocks of Winapi Wash Source: Modified from Barnes and others (1982). Figure 1 -1. Generalized geologic map of the Frenchman Flat vicinity.

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The generalized stratigraphic section for the Frenchman Flat area consists of (from oldest to youngest) Proterozoic and Paleozoic clastic and carbonate rocks, Tertiary sedimentary rocks, Tertiary volcanic rocks, and Quaternary and Tertiary alluvium. In the northern portion of Frenchman Flat volcanic rocks, erupted mostly from calderas located to the northwest of Frenchman Flat, unconformably overlie Paleozoic carbonate and clastic rocks. To the south, most of these volcanic units, including the Ammonia Tanks Tuff, Rainier Mesa Tuff, Topopah Spring Formation, and Crater Flat Group, either thin considerably or pinch out altogether. Tuffs, lavas, and debris flows from the Wahmonie volcanic center dominate the volcanic section in the western portion of the valley. To the south and southeast, most of the volcanic units are absent and older Tertiary (Miocene and Oligocene) sedimentary rocks, which unconformably overlie the Paleozoic rocks in the southern portion of Frenchman Flat, dominate the Tertiary section. In most of the Frenchman Flat area, the older rocks are covered by alluvium. Alluvium thicknesses range from a thin veneer along the valley edges to perhaps as much as 1,000 m (3,300 ft) The structural geology of the Frenchman Flat region is complex. During the late Mesozoic, the region was subjected to compressional deformation resulting in folding, thrusting, uplift, and erosion of the pre-Tertiary rocks. In more recent times (Miocene to Quaternary), the region has undergone extensional deformation during which the present basin-and-range topography was developed and the Frenchman Flat basin was formed. Extensional deformation produced east-northeast-striking strike-slip faults such as the Rock Valley fault, as well as numerous generally north-striking normal faults that displace the Tertiary and pre-Tertiary rocks. The rocks beneath Frenchman Flat can be divided into seven hydro strati graphic units based on their hydrologic properties and stratigraphic position (modified from Winograd and Thordarson, 1975). In ascending order, these units are: the Lower Clastic Confining Unit (LCCU), LCA (the regional aquifer), Volcaniclastic Confining Unit (VCU [comprising the Tertiary sedimentary rocks discussed in this report]), Wahmonie Volcanic Confining Unit (WVCU), Tuff Confining Unit (TCU), Timber Mountain Aquifer (TMA), and the Alluvial Aquifer (AA). The LCCU and LCA are regional units and therefore inferred to be present everywhere beneath Frenchman Flat The distribution of the younger hydrostratigraphic units is governed by local depositional patterns, structure, and

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The static water level in Frenchman Flat is relatively flat, with an elevation of about 731 m (2,400 ft) above sea level (Winograd and Thordarson, 1975). However, depth to water below ground level varies with topography, ranging from 209 m (687 ft) in the central portion of Frenchman Flat to about 393 m (1,290 ft) at Well UE-1 lb in the north. The static water level is generally located within the VCU, TCU, or the WVCU but, in the deeper portions of Frenchman Flat, the TMA and lower portion of the AA are also saturated. Perched and semiperched water is recognized within altered units (primarily the WVCU) in several drill holes along the western margin of the valley. Regional groundwater flow is generally to the south-southwest (Laczniak et al., 1996).

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2.0 HYDROGEOLOGY OF THE TERTIARY SEDIMENTARY ROCKS 2.1 Tertiary Sedimentary Rocks South and Southwest of Tertiary sedimentary rocks exposed south and southwest of Frenchman Flat have been mapped as two formations. An older Oligocene-age unit was designated the Horse Spring Formation by Hinrichs (1968) and Barnes et al. (1982). However, these rocks are older than the Horse Spring Formation mapped in the Lake Mead region by Bohannon (1984). Wahl et al. (1996) included Oligocene-age sedimentary rocks in the Frenchman Flat area within an Oligocene- to Early Miocene-age compilation unit they called “prevolcanic sedimentary rocks.” This compilation unit included Tertiary sedimentary rocks of the NTS area that appear to predate southwestern Nevada volcanic field (SWNVF) activity (Wahl et al., 1996). Yount (personal communication, 1996) designated the Oligocene-age sedimentary rocks exposed in the southern portion of the NTS, particularly in the vicinity of Winapi Wash, as the Rocks of Winapi Wash. Throughout the remainder of this report, Oligocene-age sedimentary rocks that occur south and southwest of Frenchman Flat will be referred to as Rocks of Winapi Wash after Yount (personal communication, 1996). This is based on the following reasons: • The rocks do not correlate with the Horse Spring Formation of Bohannon (1984). • They form a distinct stratigraphic unit within the study area. • Correlation to rocks outside of the study area is uncertain, and beyond the scope of this Overlying the Oligocene-age sedimentary rocks south and southwest of Frenchman Flat is a diverse assemblage of interbedded Miocene-age volcanic and sedimentary rocks that were designated Rocks of Pavits Spring by Hinrichs (1968) and Barnes et al. (1982). Wahl et al. (1996) included the Rocks of Pavits Spring within a Miocene-age compilation unit called “older synvolcanic sedimentary rocks.” This compilation unit encompassed Miocene-age sedimentary deposits of the NTS area that are older than the Timber Mountain Group and which were deposited at least partly contemporaneous with SWNVF activity (Wahl et al., 1996). Yount (personal communication, 1996) used the same designation for these rocks as Hinrichs (1968) and Barnes et al. (1982), referring to them as the Rocks of Pavits Spring. This report will also refer to the Miocene-age sedimentary rocks that occur south and southwest of Frenchman Flat as Rocks of Pavits Spring after Hinrichs (1968) and Barnes et al. (1982). This is because these rocks form a distinct

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stratigraphic unit within the study area, and because correlation to rocks outside the study area is uncertain and beyond the scope of this study. Figure 2-1 depicts the evolution of stratigraphic nomenclature for the Tertiary sedimentary rocks in the vicinity of Frenchman 2.1.1 Rocks of Winapi Wash 2.1.1.1 Extent and Thickness The Rocks of Winapi Wash are exposed along the flanks of highlands that bound Frenchman Flat to the south and southwest (Figure 1-1). These Tertiary sedimentary rocks unconformably overlie Cambrian to Ordovician-age sedimentary rocks (Hinrichs, 1968; and Barnes et al., 1982). It is likely that the Rocks of Winapi Wash are present beneath the southern portion of Frenchman Flat, and may extend as far north as the central portion of the basin. The formation is probably not present beneath the northern portion of Frenchman Flat The upper contact of the formation is probably an unconformity, which may account for the observed variability in the thickness of the formation. The maximum thickness appears to be less than 305 m (1,000 ft) in the area (Barnes et al., 1982; Hinrichs, 1968). However, numerous faults in the area and the lateral discontinuous nature of individual units within the formation make estimates of formation thickness very uncertain. Beneath the central portion of Frenchman Flat, gravity data (Wahl, 1995) and numerous cross sections constructed through the area for the Frenchman Flat CAU generally constrain the thickness to less than 100 m (328 ft) (Drellack and Prothro, 1997). 2.1.1.2 Age and Correlation The Rocks of Winapi Wash have been determined to be Oligocene in age (Yount, personal communication, 1996), based partly on an age date of 30.2 million years obtained from an interbedded tuff (Marvin et al., 1970). The rocks were formerly mapped as the Horse Spring Formation (Hinrichs, 1968; and Barnes et al., 1982), but are older than the Miocene-age Horse Spring Formation in the …