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Research Paper THEMED ISSUE: CRevolution 2: Origin and Evolution of the Colorado River System II

GEOSPHERE Paleogeographic implications of late Miocene lacustrine and nonmarine deposits in the region: GEOSPHERE; v. 12, no. 3 Immediate precursors to the Colorado River doi:10.1130/GES01143.1 James E. Faulds1, B. Charlotte Schreiber2, Victoria E. Langenheim3, Nicholas H. Hinz1, Thomas H. Shaw4, Matthew T. Heizler5, Michael E. Perkins6, 19 figures; 7 tables Mohamed El Tabakh7, and Michael J. Kunk8 1Nevada Bureau of Mines and , University of , Reno, Nevada 89557, USA CORRESPONDENCE: jfaulds@​unr​.edu 2Department of Earth and Space Sciences, University of Washington, Seattle, Washington 98195, USA 3U.S. Geological Survey, Menlo Park, California 94025, USA 4LK Energy, 1729 Harold Street, , 77098, USA CITATION: Faulds, J.E., Schreiber, B.C., Langen- 5New Mexico Bureau of Geology and Resources, Tech, Socorro, New Mexico 87801, USA heim, V.E., Hinz, N.H., Shaw, T.H., Heizler, M.T., 62025 E. White Circle, Lake City, 84109, USA Perkins, M.E., El Tabakh, M., and Kunk, M.J., 2016, 7154-78 71st Avenue, Queens, 11367, USA Paleogeographic implications of late Miocene lacus- 8U.S. Geological Survey, Reston, Virginia 20192, USA trine and nonmarine evaporite deposits in the Lake Mead region: Immediate precursors to the Colorado River: Geosphere, v. 12, no. 3, p. 721–767, doi:10​ ​ .1130​/GES01143.1. ABSTRACT the northern Grand Wash, Mesquite, southern Detrital, and northeastern Las Vegas basins. New tephrochronologic data indicate that the upper part of the Received 17 October 2014 Thick late Miocene nonmarine evaporite (mainly halite and ) and halite in the Hualapai basin is ca. 5.6 Ma, with rates of deposition of ~190–450 Revision received 12 July 2015 related lacustrine limestone deposits compose the upper basin fill in half gra- m/m.y., assuming that deposition ceased approximately coincidental with the Accepted 8 February 2016 Published online 24 March 2016 bens within the Lake Mead region of the Basin and Range Province directly arrival of the Colorado River. A 2.5-km-thick halite sequence in the Hualapai west of the Colorado Plateau in southern Nevada and northwestern Arizona. basin may have accumulated in ~5–7 m.y. or ca. 12–5 Ma, which coincides with Regional relations and geochronologic data indicate that these deposits are lacustrine limestone deposition near the present course of the Colorado River late synextensional to postextensional (ca. 12–5 Ma), with major extension in the region. bracketed between ca. 16 and 9 Ma and the abrupt western margin of the The distribution and similar age of the limestone and evaporite depos- Colorado Plateau established by ca. 9 Ma. Significant accommodation space its in the region suggest a system of late Miocene axial lakes and extensive in the half grabens allowed for deposition of late Miocene lacustrine and evap- continental playas and salt pans. The playas and salt pans were probably fed orite sediments. Concurrently, waning extension promoted integration of ini- by both groundwater discharge and evaporation from shallow lakes, as evi- tially isolated basins, progressive enlargement of drainage nets, and develop- denced by sedimentary textures. The elevated terrain of the Colorado Plateau ment of broad, low gradient plains and shallow water bodies with extensive was likely a major source of water that fed the lakes and playas. The physical clastic, carbonate, and/or evaporite sedimentation. The continued subsidence relationships in the Lake Mead region suggest that thick nonmarine evapo- of basins under restricted conditions also allowed for the preservation of rites are more likely to be late synextensional and accumulate in basins with particularly thick, localized evaporite sequences prior to development of the relatively large catchments proximal to developing river systems or broad through-going Colorado River. elevated terranes. Other basins adjacent to the lower Colorado River down- The spatial and temporal patterns of deposition indicate increasing stream of Lake Mead, such as the Dutch Flat, Blythe-McCoy, and Yuma basins, amounts of freshwater input during the late Miocene (ca. 12–6 Ma) immedi- may also contain thick halite deposits. ately preceding arrival of the Colorado River between ca. 5.6 and 4.9 Ma. In axial basins along and proximal to the present course of the Colorado River, evaporite deposition (mainly gypsum) transitioned to lacustrine limestone INTRODUCTION progressively from east to west, beginning ca. 12–11 Ma in the Grand Wash Trough in the east and shortly after ca. 5.6 Ma in the western Lake Mead re- Thick and widespread evaporite deposits can develop in internally drained gion. In several satellite basins to both the north and south of the axial basins, basins within continental rift settings in arid environments, as extension com- evaporite deposition was more extensive, with thick halite (>200 m to 2.5 km monly fragments continental crust into numerous fault blocks and attendant For permission to copy, contact Copyright thick) accumulating in the Hualapai, Overton Arm, and northern Detrital ba- half grabens. During the early phases of rifting, drainage networks may be Permissions, GSA, or editing@​geosociety​.org. sins. Gravity and magnetic lows suggest that thick halite may also lie within rela­tively small and limit the extent and thickness of evaporite deposition.

© 2016 Geological Society of America

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A However, broader drainage networks integrating multiple basins typically Alluvial Fan Saline Mudflat evolve over time and may be coupled with regional subsidence, which col- Sand Flat/Carbonate Gypsum lectively promote accumulation of more extensive evaporite sequences (e.g., Limestone ­McKenzie, 1978; Royden et al., 1980; Sclater and Christie, 1980; Bally and Deposition Salt Pan Dry Mudflat Oldow, 1984). Marine incursions into restricted embayments may further ac- celerate this process. Extensive syn- to postextensional evaporite deposits are therefore common in continental rifts, including many passive continental margins (Holwerda and Hutchinson, 1968; Lowell and Genik, 1972; Jackson and Seni, 1983; Tankard and Balkwill, 1989). For example, large provinces of extensive evaporite deposition in extensional settings are well documented especially in relatively arid regions, including the Gulf of Suez (Evans, 1988; Steckler et al., 1988), East Africa (Catuneanu et al., 2005; Chorowicz, 2005; ­Ebinger, 2005), Angola (Hudec and Jackson, 2002, 2004; Dickson et al., 2003), Ground (Dickinson, 2009; Pindell and Kennan, 2009; Stern et al., 2010), Water and Brazil (Meisling et al., 2001; Japsen et al., 2012). Table Most of these evaporite provinces were largely derived from marine-fed water bodies, but some are related to extensive arid-region lake and playa B Map View deposits (Fig. 1), as exemplified in East Africa (Chorowicz, 2005; Ebinger, 2005) and the (Niemi et al., 1997). Depending on water composition and evaporation rates, may fill developing accommodation space very Springs Saline Mudflat Salt Pan Inflow rapidly and can become particularly thick if the basins continue to subside. In particular, halite can collect at rates in excess of 10 cm/yr (Schreiber and Hsü, Inflow 1980). Evaporites deposited in lakes are commonly quite similar in appearance Outflow to many marine-fed deposits except for differences in trace-element content Gypsum and the lack of tides affecting deposition. Generally, lake sedimentation is the Limestone product of water with entrained sediment inflow, but in arid regions evapo- Deposition Open (through-flowing) Lake Closed (terminal) Lake ration rates become more important and water influx may become restricted to groundwater and springs, bringing little or no clastic load. In this case, the C Cross Section ionic content, pH, and rate of evaporation of the water become paramount controls (Benison et al., 2007), as described in models for basin filling pre- Evaporation Salt Pan sented in Lowenstein and Hardie (1985) and further developed in Renaut and Inflow Outflow Inflow Evaporation Saline Mudflat Gierlowski-Kordesch (2010). Although not as well documented, the thickness and extent of Miocene Spill to Quaternary evaporite deposits in the Basin and Range Province within the point southwestern (Fig. 2) may rival those of some world-renowned Groundwater Groundwater regions of evaporite deposition (e.g., Peirce, 1976, 1981; Johnson and Gon- Flow Gypsum Limestone Flow Deposition zales, 1978; Smoot and Lowenstein, 1991; Faulds et al., 1997; Rauzi, 2002a, 2002b). Many basins in this region contain substantial volumes of carbonate Fresh to Brackish Saline and Ca- deposits ( and gypsum), but some basins contain sub- Stable Lake Level Unstable Lake Level stantial amounts of halite. Particularly thick or widespread halite deposits oc- cupy the Overton Arm, Detrital, and Hualapai basins in the Lake Mead region Figure 1. Generalized model for evaporite deposition in terrestrial basins (modified from Renaut and Gierlowski-Kordesch, 2010). of southern Nevada and northwestern Arizona (Fig. 3; Mannion, 1963; Faulds (A) Playa lake model for closed-basin sedimentation. Gypsum-anhydrite accumulates in the saline mudflats, with gypsum and/or limestone deposits forming at the margins of these mudflats. Halite is deposited in the salt pans. If synextensional, wedge-shaped et al., 1997), Luke and Picacho basins of central Arizona (Peirce, 1976), and geometries are likely to develop in the basin-fill deposits, with thickening toward the major bounding fault. (B) Plan or map view of Bristol Lake basin in southeastern California (Fig. 2; Rosen, 2000). Some of interconnected basins containing open (through-flowing) lake and closed (terminal) lake or playa. Carbonates would more likely ac- the halite deposits have little or no carbonate and limited amounts of sulfate, cumulate in the open lakes, whereas gypsum-anhydrite and halite would be more probable in the terminal lakes. (C) Cross-sectional hydrologic view of interconnected basins, which may be linked largely through groundwater flow with or without periodic spill overs environments much like those found today in evaporative lakes in southern and/or interconnecting drainages. Western (Benison et al., 2007; Jagniecki and Benison, 2010).

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Pliocene, but definitive age constraints are sparse. Detailed sedimentological studies of the Quaternary (Smith, 1979, 2000) and Bristol GSL evaporites (Rosen, 1991, 2000) provide important contemporary analogues for interpreting older and buried evaporites in the region. Lenticular wedge- shaped geometries of some of the evaporite deposits, such as the Quaternary Glauber salt bed of the Great Salt Lake (Eardley, 1962; Mikulich, 1971) and Red Utah ee Lake halite in the Hualapai basin of northwestern Arizona (Faulds et al., 1997), n Nevada o suggest a synextensional origin for at least some of these evaporite deposits. zZ B The halite deposits and related evaporites have important paleogeographic

A n o CR implications for the southwestern United States. They record a period of in­ S i t i I U ternal drainage that predates integration of most of the region by the Colorado N s A n E River and its tributaries. As such, they provide a paleogeographic marker that a A T r A T can provide important temporal and spatial constraints on overall drainage N L

D P evolution. In the Lake Mead region, for example, thick and widespread, late

Miocene evaporite and lacustrine carbonate deposits appear to immediately R O

A Grand D predate arrival of the Colorado River. However, the temporal and spatial re- A N CR CanyonNevada lationships between the evaporite and lacustrine deposits and the relevance G R O of widespread late Miocene evaporite deposits in the Lake Mead region (e.g., E L halite, gypsum, and anhydrite) to development of the Colorado River have not O P C Arizona been studied systematically. Further, the overall size and extent of the deposits R SB O in individual basins or groups of interconnected basins may reflect the relative V length of time of the ponding episode and/or overall size of the catchment BLB IN DFB C 100 km area. Such relations can be greatly impacted, however, by paleoclimate varia- T E r tions, which significantly affect the freshwater influx from adjacent ranges and an si potential for postburial halite dissolution. CB tio BlB n Neogene halite deposits of the southwestern United States also have im- LB Zzoo nnee portant economic implications, having been exploited for mineral salt produc- California CR GR tion, evaluated for low-temperature geothermal resources, and assessed for YB storage of natural gas, natural gas liquids, compressed air energy, and nuclear MkB PB wastes (Netherland and Sewell, 1977; Johnson and Gonzales, 1978; FERC, 1982; Rauzi, 2002b; Kostick, 2013). In the Lake Mead region, halite deposits in Figure 2. Digital elevation model showing the Colorado Plateau and Basin and Range Province the Overton Arm area (Fig. 3) were exploited in several mines prior to the filling in the southwestern United States. In contrast to broad transition zones throughout much of Utah and Arizona, the Colorado Plateau gives way abruptly westward to the Basin and Range of Lake Mead and were analyzed for their potential effects on the water quality Province in the Lake Mead region of northwestern Arizona. White box encompasses the study of the reservoir (U.S. Bureau of Reclamation, 1950). To the south of Lake Mead area in the Lake Mead region and corresponds to the area covered in Figures 3 and 4. Red boxes in northwestern Arizona, studies of the Red Lake halite in the Hualapai basin enclose other areas with documented thick halite deposits (south-central Arizona) or possi- ble but as yet undocumented, thick evaporite sequences (lowermost Colorado River Valley in for gas storage date back to efforts by Southwest Gas Corporation in the mid- westernmost Arizona and southeastern Calfornia). Lighting is from the northwest. Abbrevia- 1950s (Neal and Rauzi, 1996; Rauzi, 2002b). Additionally, various developers tions: BlB—Blythe-McCoy basin; BLB—Bristol Lake basin; CB—Chemehuevi basin; CR—Colo- have proposed and, in some cases, initiated permitting of natural gas storage rado River (blue line); DFB—Dutch Flat basin; GR—Gila River; GSL—Great Salt Lake; LB—Luke basin; MkB—Mohawk basin; PB—Picacho basin; SB—Sacramento basin; YB—Yuma basin. projects at Red Lake. The purpose of this paper is therefore to (1) provide an overview of the distribution and origin of late Miocene lacustrine and evaporative deposits The regional setting and abundant textural and geochemical evidence indi­ (mainly halite) in major basins in the Lake Mead region; (2) review age con- cate that most of the Miocene to Quaternary evaporite deposits within the straints on these deposits, including existing data from previous efforts and Basin and Range Province were deposited in continental playas and shallow several new 40Ar/39Ar dates and tephrochronologic correlations presented lakes in enclosed basins as opposed to a marine origin (Peirce, 1976; Rosen, herein; (3) discuss the paleogeographic implications of these deposits, particu- 1991; Faulds et al., 1997). Within Arizona and neighboring parts of Nevada larly in terms of elucidating the evolution of the Colorado River and the depo­ and California, these evaporites are inferred to be middle Miocene to early sitional­ environment characterizing the Lake Mead region immediately prior

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Utah A B Arizona Mormon Pf Figure 3. (A) Digital elevation model of the Mts Mesquite Basin Lake Mead region, showing distribution of r n e m late Miocene limestone and gypsum, as iv R MesquiBasite well as major faults (compiled from Faulds n gi er Da Vir ver Mts Ri et al., 2001c; Page et al., 2005; Beard et al., in Beav irg 2007; Felger and Beard, 2010). Basement V Mormon depth contours are shown for major ba- ge rgin River Depression n Vi sins and based on inversion of gravity data a Basin MormonBasin

R M (modified slightly from Langenheim et al., n

u M d a o d u CWf d d y d y d na 2001a, 2010). Abbreviations for physio­ n y ad

nge Virgin R zo a

i R i

v ev C i u graphic features and towns (black text): Ra e v

Mts rough N

r e Ar rough w r o BC—Black Canyon; BGL—Big Gypsum gas hern Gran r t sh T r ke Basin n thern Gran Sheep A Ledges; CR—Colorado River; DS—Dolan n Wa Nor Wash T y La Nor e Las Ve Springs; FM—Frenchman Mountain; igure 14 HRf NGf Dr F ash Lak eau on Basi GM—Grapevine Mesa; ML—Mohave Lake; rt LV ton Arm Basi NM Colorado Platea e f NM—Nevershine Mesa; SnP—Snap Point; sz er Ov o Plat s h

BH SP—Sandy Point; WR—Wheeler Ridge. Ov i Nellis Basin g

l s u C

a ke Grand W o Abbreviations for faults (red text): BGf—

Las Wf h Las Vegas a r

s L T Nellis Basin Colorad Ve h h Muddy Mts WR a s Blind Goddess Mine fault; BHf—Bitter­ gas g Basin Basin Virgin a W i u SnP Las Veg o South r d W Ridge-Hamblin Bay fault; CMf—Cerbat FM Mts n d LV T a sz T h r alapa Lake n SP s e G n a Mountains fault; CWf—California Wash a Hu alapai r m Lake e T Boulder Basin p Boulder Basi G l SWf W e e Las Veg ak m Hu fault; Df—Detrital fault; Ff—Frenchman BGL B d L p n a Figure 11 r e n e le River Mt r e Ff n n B i B a Lake Mountain fault; HRf—Hen Springs–Rogers i n a SIf MSf r as n h Lak s s Lak i t a Gra r i Co a G n s lo a B s u s d r Mead B a a Springs fault; LBf—Lost Basin Range fault; B Mead a i n a o C Black l n GM r B B d s S g a a e e l i t g o s g n g n a LVsz—Las Vegas Valley shear zone; MMf— i h Figure 8 t r e t g n i t y R r e u o r a n iv e Co t r M G o Mockingbird Mine fault; MSf—Meadview R e

D G r CR S ts n l C BC orado Platea De i o BGf Colorado Slope fault; NGf—northern Grand Wash s l o a

B r Spring Mts a fault; Pf—Piedmont fault; SGf—southern

t d

o L

Eldorado s

LBf

ak o

L Grand Wash fault; SIf—Saddle Island de-

Basin Df e Platea

Gr SWf R s Fi tachment fault; SWf—South Virgin–White u i a gure 15 White Hill v e nd

r Hills detachment fault; Wf—Wheeler Ridge SG Gran Wa u f d fault. The Lake Mead fault system includes

s W sh Eldorado Basin Detrit s a the Bitter Ridge–Hamblin Bay and Hen s h H H C Springs–Rogers Springs faults, as well as u McCullough Mt al l ua i Detrit a B l asin l east-northeast–striking sinistral faults to a s a DS p Eldorado Mt p a al Basin a the southwest and northeast in southern MMf i B i B Cerba a a Black Mts s Nevada. (B) Digital elevation model show- sin in A Ne Iv rizona ing outlines of late Miocene lakes (Lakes anpa t Mts Bas va CMf d Grand Wash, Hualapai, and Las Vegas), as in h ML Figure 13 a adapted and in some cases modified (see discussion in text) from House et al. (2008) Normal fault, solid where certain; dashed if concealed; and Spencer et al. (2013). Each subsequent red where Quaternary offset; balls on downthrown side lake probably filled at least the lower parts Low-angle normal fault; solid where certain; dashed if of the older lake basins. Known thick (>200 m), subsurface halite (red dashed concealed; tics on downthrown side lines) and potentially thick subsurface Strike-slip fault, with arrows showing relative motion; ­halite deposits (green dashed lines) are solid where certain; dashed where concealed; red where also shown. Quaternary offset

to integration by the river; and (4) consider whether other basins in the region gional geologic setting, geologic maps, well data in some areas, detailed grav- may contain previously unrecognized thick halite deposits through a review of ity data (Fig. 4) with derivative depth-to-basement maps (Fig. 3; Langenheim gravity and magnetic data. Our study area includes the broader Lake Mead re- et al., 2001a, 2001b, 2010), and aeromagnetic data were incorporated into our gion, which for the purposes of this study includes the area extending from Las analysis of late Miocene basin-fill sedimentary sequences in the Lake Mead Vegas Valley on the west to the Grand Wash Cliffs along the western margin region. For the purposes of this paper, a basin is defined as a discrete structural of the Colorado Plateau on the east, as well as basins extending ~75–100 km entity bounded on at least one side by a major fault and containing relatively north and south of the present course of the Colorado River (Fig. 3). The re- thick (>300 m) sequences of basin-fill sediments.

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115°30′ 115° 114°30′ 114° 115°30′ 115° 114°30′ 114° A mGal B nT 37° 37° 20 550 MB 16 MB 500 12 450 8 400 MMB 4 MMB 350 DLB DLB 0 300 NGB ′ NGB 36°30′ 36°30 –4 250 –8 200 OB OB –12 LVB LVB NB 150 –16 NB 100 BB –20 50 TB SGB BB TB –24 SGB 0 –28 –50 36° 36° –32 –100 DB DB –36 EB –150 EB –40 –200 –44 –250 HB –48 HB –300 –52 –350 DB 35°30′ DB 35°30′ IB –56 IB –400 01020 km 01020 km

Figure 4. (A) Isostatic gravity map. Contour interval is 4 mGal. Anomalies reflect density variations in upper to middle crust. Yellow line is the outline of Lake Mead. (B) Total-field aeromagnetic map. Contour interval is 50 nT. Light-blue line is the outline of Lake Mead. Thick dark-blue lines are state boundaries. See Langenheim et al. (2001a, 2010) for details on data sources. Abbreviations: BB—Boulder basin; DB—Detrital basin; DLB—Dry Lake basin; EB—Eldorado basin; HB—Hualapai basin; IB—Ivanpah basin; LVB—Las Vegas basin; MB—Mesquite basin; MMB—Mormon Mesa basin; NB—Nellis basin; NGB—northern Grand Wash Trough; OB—Overton Arm basin; SGB—southern Grand Wash Trough; TB—Temple Bar basin.

REGIONAL GEOLOGIC SETTING: CENOZOIC PALEOGEOGRAPHY Mead region, extension generally began in early to middle Miocene time, as evidenced by tilt fanning (i.e., progressive upward decrease in tilt) in half gra- The Cenozoic paleogeographic evolution of the region is marked by a 180° bens and exhumation of the footwalls of major normal faults (e.g., Anderson drainage reversal that reflects an evolving tectonic setting. In the early Tertiary et al., 1972; Faulds et al., 1992, 2001a, 2001b; Beard, 1996; Duebendorfer and (Paleocene to Oligocene), streams flowed northeastward from broad high- Sharp, 1998; Fitzgerald et al., 2009). As large-magnitude east-west extension lands to the west of the Colorado Plateau into vast lowlands in the Colorado swept through the region in the early to middle Miocene, major detachment Plateau region (e.g., Potochnik and Faulds, 1998; Wernicke, 2011), as evidenced faults and arrays of upper-plate normal faults dissected the former highlands, by widespread, southwesterly derived Paleocene–Eocene gravels along the fragmenting them into panels of northerly trending fault blocks. In addition, western margin of the Colorado Plateau (Young, 1982). These highlands were major strike-slip faulting (Anderson, 1973; Longwell, 1974; Bohannon, 1984; built by crustal shortening and arc magmatism associated with the Sevier and Duebendorfer and Simpson, 1994; Duebendorfer et al., 1998; Fryxell and Laramide orogenies. Extensive early Tertiary erosion stripped much of the Duebendorfer,­ 2005; Umhoefer et al., 2010b) and a component of north-south sedimentary­ and volcanic cover from these highlands. shortening (Anderson et al., 1994; Duebendorfer and Simpson, 1994) further The highlands foundered in middle Tertiary time (Oligocene to Miocene) deformed the Lake Mead region north of the Colorado River in late Miocene as the convergent plate margin transitioned into a transform boundary (e.g., time. As the region foundered and subsided during the early to late Miocene, ­Atwater and Stock, 1998), and extensional to transtensional tectonism took the early Tertiary northeast-flowing drainage system was disrupted, and a long hold within the Basin and Range Province (e.g., Wernicke, 1992). In the Lake period of internal drainage ensued. Enclosed basins (generally half grabens)

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began filling with locally derived sediment, as evidenced by abundant early to addressed whether the evolving regional drainage system and ultimately the middle Miocene fanglomerate, fluvial, and lacustrine deposits (e.g., Bohannon, Colorado River also left behind a series of markers in the form of evaporite 1984; Beard, 1996; Hickson et al., 2010; Lamb et al., 2010). Throughout much of deposits in both axial and satellite basins (e.g., Faulds et al., 1997). We there- the region (particularly areas south of Lake Mead), voluminous intermediate fore evaluate the overall distribution and apparent age of known evaporite volcanism accompanied the onset of extension, transitioned to widespread bi- (e.g., halite) and related lacustrine deposits in the late Miocene basin fill, as modal magmatism during peak extension, and was followed by more isolated well as evidence for possible undiscovered thick sequences of halite in the alkalic volcanism during the latter stages of rifting (Faulds et al., 1995, 2001a; Lake Mead region. Although early to middle Miocene sedimentary deposits Gans and Bohrson, 1998). Thus, some half grabens filled with thick sequences are widespread and also record key elements in the Cenozoic structural and (>1 km) of volcanic rock. paleogeographic evolution of the region (e.g., Bohannon, 1984; Beard, 1996; By late Miocene to Pliocene time, the southern reaches of the transform Umhoefer et al., 2010a; Hickson et al., 2010; Lamb et al., 2010, 2015), this paper boundary jumped inland to the eastern side of the Peninsula Ranges, the focuses on the late Miocene sections due to their potential relevance to devel- Gulf of California developed (Stock and Hodges, 1989; Oskin and Stock, 2003; opment of the Colorado River. Fletcher et al., 2007), and extension and strike-slip faulting waned in the Lake Mead region. The reduction in strain rates promoted widespread aggrada- tion within composite basins, and a rugged middle Miocene topography of Lake Mead Region tilt blocks was buried by sediments in multiple basins, as evidenced by grav- ity studies (Langenheim et al., 2010). Although most of the region has been In northwestern Arizona and southern Nevada, the Colorado River crosses integrated­ into the Colorado River, facies patterns in late Tertiary basin fill are an abrupt boundary between the Colorado Plateau and Basin and Range Prov- generally congruent with modern topography, as alluvial fans derived from ince (Figs. 2, 3, and 5). Essentially flat, relatively unextended strata on the flanking ranges interfinger with and give way to floodplain, lacustrine, and/or high-standing Colorado Plateau give way to moderately to steeply tilted fault continental playa environments toward the basin centers. blocks in the Basin and Range Province across a system of west-dipping nor- Reduced strain rates and regional aggradation in late Tertiary time facili- mal faults, including the Grand Wash fault zone (Lucchitta, 1966, 1979) and tated evolution of regional drainage systems that ultimately integrated large South Virgin–White Hills detachment fault (Duebendorfer and Sharp, 1998; networks of basins. Because much of the region, especially large composite Brady et al., 2000; Fig. 3). The conspicuous, west-facing fault-line escarpment basins, had subsided beneath the level of the Colorado Plateau and base level of the Grand Wash Cliffs, consisting of subhorizontal Paleozoic strata, marks lowered to sea level as the Gulf of California opened to the south, the de- the western margin of the Colorado Plateau within the footwall of the Grand veloping late Miocene to Pliocene drainage network ultimately had an outlet Wash fault zone and rises ~1.3 km above several east-tilted half grabens in the to the south and southwest, opposite to the early Tertiary northeast-flowing Basin and Range Province, including the Grand Wash Trough and Hualapai drainage. basin. Major extension progressed northward across this region in the Mio- The Colorado River ultimately became the preeminent drainage system in cene, with the main episode occurring ca. 16–13 Ma to the south of Lake Mead the southwestern United States, as it captured much of the Rocky Mountains, and in the eastern Lake Mead region (Anderson et al., 1972; Faulds et al., 1992, most of the Colorado Plateau, and large portions of the Basin and Range Prov- 2008, 2010; Beard, 1996; Duebendorfer and Sharp, 1998) and ca. 13–9 Ma in ince (Fig. 2). Although controversy still surrounds its long-term evolution and the western Lake Mead area (Duebendorfer and Simpson, 1994; Harlan et al., the origin of the Grand Canyon (e.g., Flowers and Farley, 2012, versus Karl- 1998; Castor et al., 2000). Extension had generally ceased by ca. 6 Ma, espe- strom et al., 2014), a consensus is developing that the lower Colorado River cially to the south of Lake Mead. However, widely spaced Quaternary faults cut (i.e., from Lake Mead southward) developed through sequential south-directed the northern part of the Lake Mead region (U.S. Geological Survey, 2010) and filling and spilling of a chain of lakes through formerly closed basins (Spencer include segments of the Grand Wash, Wheeler Ridge, and Piedmont faults, and Patchett, 1997; House et al., 2008) between ca. 6 and 5 Ma (Faulds et al., which bound the Grand Wash Trough, Gregg basin, and Mesquite basin, re- 2001c; Dorsey et al., 2007; House et al., 2008). As each lake spilled over to spectively (Fig. 3). the south, the Colorado River lengthened in its wake. Limestone and siltstone The Colorado River emanates from the Grand Canyon within the western of the Bouse Formation record the trail of lakes in multiple basins extending part of the Colorado Plateau and traverses the Lake Mead region from east to south from the Lake Mead region to the Blythe area (Spencer and Patchett, west orthogonal to the northerly trending structural grain (Figs. 2, 3, and 5). 1997; House et al., 2008; Spencer et al., 2008, 2013). The origin of the Grand Canyon and Colorado River in this region has been the Substantial previous work has focused on the deposits left by the precur- subject of significant study and debate (e.g., Powell, 1875, 1895; Blackwelder, sor lakes and Colorado River along the axial part of the drainage, including 1934; Longwell, 1946; Hunt, 1956; Lucchitta. 1966, 1972, 1979, 1989; Lovejoy, the Bouse Formation (e.g., Spencer et al., 2008, 2013) and fluvial deposits of 1980; Young and Spamer, 2001; Hill et al., 2008; Wernicke, 2011; Flowers and the Colorado River (House et al., 2005, 2008), but relatively little research has Farley, 2012; Karlstrom et al., 2014). Recent work has refined the timing of in-

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Grand Canyon Colorado Plateau

Grand Wash Cliffs

Grapevine Canyon

Figure 5. View looking east at the Grand Airport Point Wash Trough and western margin of the Grand Wash Colorado Plateau. Grapevine Mesa is Grapevine Mesa capped by the Hualapai Limestone and es- Trough sentially marks the floor of a late Miocene lake (Lake Grand Wash) that immediately predates arrival of the Colorado River.

Wheeler Ridge

Sandy Point Lake Mead

ception of the Grand Canyon and Colorado River (Spencer et al., 2001, 2013; The pre–Colorado River, late Miocene lacustrine deposits commonly Faulds et al., 2001c; House et al., 2005; Dorsey et al., 2007) and models for overlie and interfinger with playa deposits, with sections composed of inter- drainage development (Spencer et al., 2001, 2013; House et al., 2005, 2008; calated limestone, gypsum, anhydrite, halite, siltstone, and claystone. The Crossey et al., 2015). finer-grained deposits grade laterally into generally locally derived fluvial Several basins in southern Nevada and northwestern Arizona record a sandstones and coarser-grained alluvial fan facies toward basin margins. This transition from dominantly lacustrine to through-flowing fluvial deposition entire package of sediments, ranging from alluvial fans proximal to mountain between ca. 6 and 4.5 Ma that marks the arrival of the Colorado River (Faulds fronts to siltstones and evaporites in the central parts of basins, has commonly et al., 2001c; House et al., 2005, 2008). Late Miocene lacustrine carbonate de- been referred to as the Muddy Creek Formation (Bohannon, 1984). Numerous posits, in particular, chronicle a westward progression of lakes from ca. 12 to 40Ar/39Ar dates and tephra correlations constrain the Muddy Creek Formation 5.5 Ma across the Lake Mead region. These lakes appear to predate full de- and its equivalents throughout the region to the late Miocene–earliest Pliocene velopment of the Colorado River. From east to west, these lakes have been (Feuerbach et al., 1991; Spencer et al., 2001; Beard et al., 2007; Muntean, 2012). referred to as Lake Grand Wash, Lake Hualapai, and Lake Las Vegas (e.g., Sedimentologic and detrital zircon data indicate that the Muddy Creek Forma- Spencer et al., 2013; Pearthree and House, 2014; Fig. 3B). The maximum levels tion in the area accumulated in interior drained basins and was not of these lakes have been based on the uppermost elevations of late Miocene associated with a proto–Colorado River (e.g., Pederson, 2008; Forrester, 2009; limestone deposits in the various basins, as discussed in subsequent sections. Dickinson et al., 2014).

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Halite Distribution and Origin Possible sources of in the region include Permian redbeds (Supai Formation) in the Colorado Plateau, which are rich in chloride and A group of poorly exposed, but thick (>1 km) late Tertiary evaporite depos- interbeds of halite (Lucchitta, 1966; Eaton et al., 1972), thick Miocene calc-alka- its, consisting primarily of halite, stretches from southern Arizona to the Lake line volcanic piles in the Basin and Range, and hydrothermal waters (Faulds Mead region of southern Nevada (e.g., Holser 1970; Peirce 1976; Faulds et al., et al., 1997). The Permian redbeds are a likely source of both Na+ and Cl–, be- 1997; Rauzi, 2002a, 2002b, 2002c) (Figs. 2 and 3). Unusually thick evaporites cause the nearby Colorado Plateau provides a large, topographically elevated have been documented within this group, including 1800 m of anhydrite in the region from which to derive groundwater and surface runoff (Faulds et al., Picacho basin (Peirce, 1976), >1200 m of halite in the Luke basin (Eaton et al. 1997). However, similar to the central Andes, where unusually thick nonmarine 1972), >500 m of halite in the Overton Arm basin (Mannion, 1963), and ~2500 m halite deposits were derived from volcanic source areas (Ericksen and Salas, of halite in the Red Lake area of the Hualapai basin (Faulds et al., 1997) (Figs. 2 1989; Alonso et al., 1991), 1–4-km-thick Oligocene–Miocene volcanic piles in and 3). Most of the thicker known evaporite deposits occur near the margin of the Basin and Range and associated hydrothermal deposits may have also the Basin and Range Province, proximal to the topographically elevated Colo- provided a significant source of Na+ and Cl–. Modern hypersaline and soda rado Plateau, as best exemplified by the thick halite in the Hualapai basin along lakes are commonly associated with active volcanic regions in rift settings the western margin of the Colorado Plateau (Fig. 3). (Kempe and Degens, 1985). On the basis of the terrestrial Cenozoic setting for the bulk of the region It is important to note that multiple processes modify the original textures and geochemical evidence, most of these halite and related evaporite deposits of halite and other evaporites in continental playa settings and hypersaline have been interpreted as nonmarine in origin (Mannion 1963; Holser, 1970; lakes. These processes include desiccation, dissolution, recrystallization, and Eaton et al., 1972; Peirce 1976; Bohannon 1984; Faulds et al., 1997). For ex- growth of displacive salt in muds. Such processes can obliterate or signifi- ample, low bromine contents (2–6 ppm) in halite from the Luke (Eaton et al., cantly modify original bedding, crystalline form, and fluid inclusions (e.g., 1972), Overton Arm (Holser, 1970), and Hualapai basins (Faulds et al., 1997), Shearman, 1970). This, in turn, can create difficulties in accurately dating such as well as S and O isotopic values from capping anhydrite in the Hualapai deposits utilizing such techniques as tephrochronology. Tephras are com- ­basin (Faulds et al., 1997), all indicate a nonmarine origin. Lovejoy (1980) re- monly well preserved in lacustrine environments and form widespread uni- lated the unusually thick halite in the Hualapai basin to evaporation of Colo- form beds, which can be geochemically correlated over broad regions (e.g., rado River water, hypothesizing that a proto–Colorado River flowed through Perkins and Nash, 2002). In contrast, dissolution, desiccation, and recrystalliza- the Grand Canyon, debauched into the Grand Wash Trough, and terminated tion can significantly disrupt layers of tephra in evaporite sequences, making in the Hualapai basin. Interpretations of a late Miocene–early Pliocene marine it much less likely to recognize such layers in drill core. The thick sequences incursion of a proto–Gulf of California into much of the lower Colorado River of halite in the region and dominance of halite in some stratigraphic columns region (Lucchitta, 1979; Buising, 1990) have been largely refuted, at least in (e.g., Hualapai basin; Faulds et al., 1997) compounds the dating dilemma. northern areas, on the basis of Sr isotopic values (0.7102–0.7114) and lack of Nonetheless, without interbedded lavas, tephras offer the most viable means marine fossils or marine geochemical signatures in late Miocene lacustrine by which to date the halite deposits, and thus we dedicated significant effort to limestone (e.g., Spencer and Patchett, 1997; Spencer et al., 2008, 2013). How- identifying tephras in available core from the region. ever, paleontologic evidence suggests that late Miocene–early Pliocene marine It should also be noted that that evaporites can accumulate rapidly. High incursions may have extended as far north as the Blythe basin (McDougall, rates of evaporation in arid regions permit optimum accumulation of 2008; McDougall and Martinez, 2014) (Fig. 2), well south of the Lake Mead area, at rates of 1–40 m/1000 yr and halite at 10–100 m/1000 yr, as observed in mod- although marine organisms may have been transported by birds into saline ern marine-sourced basins (Schreiber and Hsü, 1980). For example, deposition lakes in the Blythe area (Spencer et al., 2013). of the 2-km-thick upper Miocene evaporite section on the floor of the Medi- Abundant textural evidence (Fig. 6) suggests that the evaporites accumu- terranean occurred within ~0.63 m.y. (CIESM, 2008; Roveri et al., 2008; Manzi lated in continental playas. For example, poorly developed bedding, coarse et al., 2009). In continental basins with sufficient accommodation space, evap- interlocking halite , the common occurrence of intercrystalline shale orite deposition may be as rapid as in marine-fed basins. However, assuming within the halite, and isolated displacive halite crystals and desiccation cracks in the ionic input into a continental-sourced basin is only one-tenth that of marine the associated shale (Faulds et al., 1997) indicate that the halite in the Hualapai sources, the time interval available for deposition of 2.5 km of halite, as in the basin originated primarily through intrasedimentary displacive growth in desic­ Hualapai basin, may have been as long as several million years. cated pans (playa mudflats), together with intervals of deposition in flooded Halite is preserved when it is buried beneath the level of the stagnant point saline pans (e.g., Gornitz and Schreiber, 1981; Rosen, 1994; ­Curial and Moretto, in the phreatic zone. The upper phreatic zone is typically fresh water recharged 1996; Schubel and Lowenstein, 1997). Relatively pure beds of finer-grained by topographically driven flow from the adjacent, basin-bounding ranges. ­halite (cumulates) probably originated from subaqueous growth in a stratified Below the stagnant point, lateral groundwater movement is slow, and the hyper­saline water body (e.g., Smoot and Lowenstein, 1991). groundwater becomes saturated with respect to halite by dissolution of halite

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A B

Figure 6. Photos showing sedimentary structures and textures of the evaporite sequence in the Hualapai basin. (A) Bed- ding plane view of typical massive halite 5 cm 5 cm consisting of interlocking, subhedral to ­euhedral halite crystals with little ­matrix; from 1247 m (4092 ft) depth in EP-1 well C E (Table­ 3). (B) Typical texture in halite se- quence, with displacive halite crystals and lesser amounts of interstitial, pale red- dish-brown claystone; from 612 m (2008 ft) depth in EP-1 well. This texture indicates halite deposition induced by groundwater discharge in a continental playa. (C) Bed- ding plane view of halite filling en echelon veins (black arrows) cutting reddish-brown siltstone from 603 m (1979 ft) depth in the 5 cm EP-1 well. This texture indicates that halite grew in desiccation cracks due to ground- water discharge. (D) Mudcracks (black arrows) cutting anhydrite-rich siltstone at F 625 m (2052 ft) depth in EP-1 well. (E) Large displacive halite­ in reddish-brown, Tephra fine-grained sandstone from 620 m (2035 ft) depth in EP-1 well. (F) Tephra inter­calated 5 cm in halite sequence at 626 m (2053 ft) depth in EP-1. Black dashed line shows approxi- mate contact of tephra with under­lying Halite halite. This tephra geochemically correlates D with the 5.62 ± 0.11 Ma Conant Creek Tuff. (G) Displacive nodular anhydrite in a sandy ­matrix near the top of the evaporite section 5 cm from 427 m depth (1401 ft; drill hole KM-1; Table 3). G

5 cm 5 cm

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near the basin margins. Buried halite is therefore preserved in the basin depo­ illustrated in Figure 7. This anomaly is certainly resolvable and can be used center. Preservation of halite occurs when subsidence results in burial of the to delineate known halite accumulations or identify potential halite deposits. halite below the stagnant point and/or where the groundwater becomes satu- However, thick, porous, sedimentary fill will also yield a gravity low relative to rated with respect to halite. Preservation conditions are met during periods of crystalline and metamorphic basement. In our analysis of individual basins, a prolonged arid climate, rapid subsidence, and/or where the bounding ranges we compared the maximum extent of subsurface halite by assuming a con- lack sufficient elevation to provide the hydraulic head to flush the central basin stant density-depth relationship of the same density of halite with those previ- by topographically driven flow. ously published estimates of basin fill assuming no halite shown in Figure 3. Additional methods, however, are needed to test whether a gravity low is a halite deposit or a thick accumulation of clastic sediments and to estimate the METHODS proportion of halite to clastic sediments within the basin fill. Halite also has a low magnetic susceptibility (–10.1 k × 10–6 SI) in contrast to Geophysical iron-bearing quartzose sandstones (440 k × 10–6 SI) and shales (641 k × 10–6 SI) (Prieto, 1993). This weak contrast in magnetic susceptibility has been used to Although no new geophysical data were obtained in this study, available map halite bodies using aeromagnetic data in the Gulf of Mexico (Vixo et al., detailed gravity and aeromagnetic surveys (Fig. 4) and depth-to-basement esti- 1987; Prieto, 1993), because such bodies have a characteristically low magnetic mates derived from the gravity data (Fig. 3A; Langenheim et al., 2001a, 2001b, response on the order of a few nT. In the Lake Mead region, this susceptibil- 2010) were utilized to interpret the extent of halite in several basins in the Lake ity contrast is enhanced by clasts of magnetic volcanic and basement rocks Mead region. Halite has specific density and magnetic properties that make and by iron and manganese coating of clasts (as observed in buried gravity and aeromagnetic data well suited to identify basins likely to contain deposits) resulting from the prolonged arid climate. The magnetic response salt deposits and to delineate the extent of halite within basins with known of halite produces anomalies of 4–20 nT, depending on the depth of burial, accumulations. Both methods offer significant advantages, versus drilling, re- with a shallow halite body producing the largest amplitude anomaly and the sistivity, or seismic methods, in that there are substantial historic databases, characteristic lower and to higher values over the edges of a tabular body. This both in the public domain and commercially available. signal, although measurable with the existing data, may be difficult to isolate, Data acquisition and inversion methods are described in Langenheim et al. particularly if the basement rocks are also strongly magnetic. (2001a, 2001b, 2010). Gravity measurements are precise to ~0.1 mGal, and the Seismic-reflection data are available in some areas and have been applied resulting anomalies are estimated to be ±0.5 mGal or better. Aeromagnetic to interpreting basin geometries and evolution, most notably in the Hualapai, anomalies are estimated to be ±1–2 nT or better. To estimate thickness of basin Detrital, and Las Vegas basins, as well as the Virgin River depression (Bohan- fill, the gravity field is separated into a component caused by density varia- non et al., 1993; Faulds et al., 1997; Langenheim et al., 2001a, 2001b). These tions within the basement and a component caused by low-density basin fill. data provide important constraints for the various depth-to-basement esti- The basin-fill anomaly is then converted to thickness using density-depth rela- mates using gravity. The sonic velocity of halite (4500–5000 km/s) is consider- tionships, in the case of this region, based on sonic velocity information (logs ably higher than predicted by its low density using standard velocity-density and interval velocities used to migrate seismic-reflection data) as a proxy for relationships (4500 km/s predicts a density of 2540 kg/m3 using the Gardner density. An important assumption is that the density of the basin-fill deposits et al. (1974) relationship), so comparison of basin thickness from seismic-­ can be related to velocity using standard relationships (e.g., Gardner et al., reflection data with that derived from gravity can help constrain whether halite 1974). For the depth-to-basement map shown in Figure 3, the density of the is a significant component of the basin fill. upper 500 m is assumed to be ~2100 kg/m3, but it increases with depth to Electrical methods can also be useful for identifying subsurface halite bod- ~2400 kg/m3 by 1.2–1.3 km. The effect of the assumed density-depth relation- ies. When groundwater wells are geophysically analyzed, they are typically ship means that the shape of the basin is fairly well constrained, but that the surveyed with conductivity, or resistivity wire line logs. Freshwater has rela- absolute depth of the basin may be overestimated if the real density contrast tively poor conductivity (e.g., high resistivity). As chloride content increases, is higher than that assumed and underestimated if the actual density contrast however, conductivity increases and resistivity decreases due to dissolution is lower than that used in the inversion. of halite by drilling fluids or increasingly brackish groundwater proximal to Halite deposits have an approximate mean density of 2100 kg/m3. In con- evaporite deposits (Dewan, 1983). Therefore, although evaporites may not trast, quartzose and feldspathic sandstones and conglomerates have a den- be penetrated by a groundwater well, trends in conductivity/resistivity can sity of 2300–2600 kg/m3, increasing in density with burial compaction. This indicate proximity to buried evaporites. Electrical surveys, both airborne and contrast in density produces a negative gravity response, or gravity “low.” ground based, can also be used to map conductivity anomalies associated An example of a modeled gravity response (3 mGal) for a half graben con- with brackish or saline water. These data are available in a few areas and taining a 300-m-thick halite deposit and a half graben without any halite is have been applied to groundwater issues, in particular interpreting grain size,

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–20 –20

–21 –21

–22 –22

–23 –23

miliGa l –24 –24

–25 –25

–26 –26

–27 –27 0 2 4 6 8 10 12 14 16 18 0 2 4 6 8 10 12 14 16 18

0 0 P=2.40 P=2.30 P=2.40 P=2.30 P=2.40 P=2.40 P=2.40 P=2.40 P=2.40 P=2.40 P=2.40 1 P=2.40 1 P=2.10 P=2.40 P=2.40 P=2.50 P=2.50 P=2.40 P=2.40

P=2.50 P=2.50 P=2.50 P=2.50 2 2 P=2.60 P=2.50 P=2.60 P=2.50

Depth (kilometer) P=2.60 P=2.60 P=2.60 P=2.60 P=2.67 P=2.60 P=2.67 P=2.60 3 3 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 Horizontal Distance (kilometer) Horizontal Distance (kilometer)

–99.0 1.9 2.0 2.1 2.2 2.3 2.4 2.5 2.6 99.0 DENSITY

Figure 7. Modeled gravity profile results of a half graben with (left) and without halite (right) showing a –3 mGal anomaly generated by a 305 m (1000 ft) thick halite body at a depth of 915 m (3000 ft) below the surface. Density (P) is in g/cm3.

groundwater quality, and possible presence of evaporites in the upper 400– A3). A more rigorous data discussion can be obtained at www​.ees.nmt​ .edu​ ​ 500 m in parts of the Hualapai, Detrital, Sacramento, and Virgin River basins /Geol/labs​ /Argon​ _Lab​ /NMGRL​ _homepage​ .html​ (click on data) for the NMGRL (Zohdy et al., 1994; Truini et al., 2012). data. Basalt samples were subjected to step-wise heating and yielded simple plateau age spectra that correspond to eruption ages. The sanidine ages are 40Ar/39Ar Geochronology determined from laser fusion of many splits (15–30) of fine-grained separates and are therefore susceptible to xenocrystic contamination. Eruption ages are Seven 40Ar/39Ar dates were obtained to constrain the ages of basin de- based on either combining the youngest population of apparent ages (sam- posits and timing of major extension, particularly within the Grand Wash ples JF-97-144, JF-98-308, and MW-98-36) or calculating maximum eruption Trough (Table 1). Six samples were dated at the New Mexico Geochronology ages from the youngest population of results that yield indistinguishable Research Laboratory (NMGRL), and one sample was dated at the U.S. Geo- 40Ar/39Ar ratios (JF-98-155). A maximum eruption age is assigned to these, logical Survey in Denver. Interpretations of the ages are summarized briefly because the total 40Ar measured in these samples is assumed to be equal to herein, and analytical data are presented in Appendix A (Tables A1, A2, and the radiogenic 40Ar.

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TABLE1. 40Ar/39Ar AGE DETERMINATIONS AND TEPHROCHRONOLOGY, LAKE MEAD REGION Location Material Area Latitude Longitude Tilt dated or Apparent age Sample Number (°N) (°W) magnitude Rock type correlated (Ma) Source A JF-97-76 SGWT 36°06′53″ 114°06′41″ 0° Basalt lava, Sandy Point Groundmass4.49± 0.23 USGS BMW-98-36 SGWT 36°00′39″ 114°02′55″ ~5° East Ash-fall tuff Sanidine 7.52 ± 0.11*NM CMW-97-18 SGWT 36°06′00″ 114°02′24″ ~4° East Tuff of Grapevine Wash, Airport Point Glass 11.01± 0.03 UU D JF-97-144 SGWT 36°03′19″ 114°00′53″ ~4° East Tuff of Grapevine Wash, Grapevine Wash Glass 11.01± 0.03 UU E JF-97-144 SGWT 36°03′19″ 114°00′53″ ~4° East Tuff of Grapevine Wash Sanidine 11.20± 0.13 NM F JF-98-155 SGWT 36°06′54″ 114°00′01″ ~5°–10° East Tuff of Pearce Ferry Sanidine 13.26± 0.06*NM G JF-98-308 SGWT 36°03′03″ 114°05′32″ 30°–35° East Nonwelded tuff Sanidine 15.51± 0.04 NM H JF-99-459 GWC 36°10′17″ 113°47′53″ 0° Basalt lava, Snap Point Groundmass8.93± 0.03 NM I JF-99-460 GWC 36°14′27″ 113°51′51″ 0° Basalt lava, Nevershine Mesa Groundmass8.94± 0.05 NM J JF99-455 NB 36°15′12″ 114°56′37″ 0° Tephra Glass 5.62 ± 0.11 UU K RL2053 HB 35.619°113.987° 0° Tephra Glass 5.62 ± 0.11 UU Note: For area: GWC—Grand Wash Cliffs; HB—Hualapai basin; NB—Nellis basin; SGWT—southern Grand Wash Trough, south of Lake Mead. For source: NM—laser-fusion (sanidine) or weighted mean plateau (groundmass) ages from Matt Heizler at New Mexico Geochronology Research Laboratory; USGS (U.S. Geological Survey)—weighted mean plateau ages from M. Kunk, USGS (Denver); UU—glass geochemistry correlation (tephrochronology) from M. Perkins, University of Utah. See Figure 7 for locations of samples from the Grand Wash Trough. *Maximum eruptive age.

Tephrochronology ses of major elements in glass shards, the dominant material in the tephra ­layers. Isotopic ages provide a chronologic calibration of select tephra in Tephrochronology is the characterization, correlation, and age calibration these basins, while remaining ages are estimated by interpolation between of tephra layers. It has contributed substantially to knowledge of the stra- dated horizons. tigraphy and chronology of late Neogene sedimentary strata in the western Glass shards from three tephra beds from surface exposures in the Grand United States (e.g., Sarna-Wojcicki and Davis, 1991; Christiansen and Yeats, Wash Trough and Nellis basin, as well as one sample from core in the Hualapai 1992; Perkins et al., 1998; Perkins and Nash, 2002). Of particular importance basin, were analyzed at the University of Utah following methods described in in this area are large-magnitude explosive eruptions generated along the Perkins et al. (1998). Glass shard compositional data from electron microprobe Yellow­stone hotspot track and the more focused eruptions from the south- analyses of individual shards are presented for the four samples in Table 2. west Nevada volcanic province. Extensive tephra layers from these erup- The core sample (RL1-2053) correlates with the 5.62 ± 0.11 Ma Conant Creek tions, ranging in age from ca. 16 to 5 Ma, provide both relative and quanti- Tuff. One sample from the Nellis basin correlates with the 5.62 ± 0.11 Ma tuff of tative ages for many extensional basins in the Basin and Range (Perkins and Wolverine Creek. Two samples from the Grand Wash Trough correlated with Nash, 2002; Perkins et al., 2014). Relative dating is provided by the chemical the 11.01 ± 0.03 Ma Cougar Point Tuff. The context of these correlations is dis- correlation of tephra from basin to basin using electron microprobe analy- cussed in the individual descriptions of each basin.

TABLE 2. ELECTRON MICROPROBE ANALYSES OF TEPHRAS Sample Location (°N) (°W) NSiTiAlFeMnMgCaBaKNa Cl FOTotalCorrelation JF99-455 NB 36.253 114.944 20 34.39 0.066 6.15 0.96 0.026 0.039 0.367 0.028 4.55 2.14 0.131 0.167 50.84 99.84 5.62 Ma Wolverine Creek Tuff RL1-2053 HB 35.619 113.987 20 34.65 0.0646.110.950.023 0.039 0.350 0.036 2.85 3.05 0.130 0.141 50.02 98.405.62 Ma Conant Creek Tuff MW97-18 SGWT 36.100 114.040 12 34.09 0.129 6.12 1.53 0.033 0.023 0.487 0.086 4.08 1.47 0.042 0.237 51.42 99.75 11.01 Ma CPT XIII tuff JF97-144 SGWT 36.055 114.015 22 33.97 0.1316.091.540.025 0.024 0.484 0.088 4.67 1.26 0.041 0.256 51.7 100.28 11.01 Ma CPT XIII tuff Note: N—Number of analyses, with values for each element representing average for all analyses. Locations: SGWT—southern Grand Wash Trough; HB—Hualapai basin, El Paso Natural Gas #1 Red Lake–Federal Well; NB—Nellis basin. The tuff of Wolverine Creek and Conant Creek Tuff are from the same ash flow but sampled at two separate localities. CPT—Cougar Point Tuff.

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RESULTS Grand Wash Trough

In the sections below, we describe the late Miocene evaporite and lacus- The Grand Wash Trough is a 75-km-long basin bounded on the east by the trine deposits in several individual basins in the Lake Mead region (Fig. 3) Grand Wash Cliffs along the western margin of the Colorado Plateau and on in order to glean the spatial and temporal relations of such deposits to one the west by the White Hills and South (Fig. 3A). The southern another and the Colorado River. For each basin, we present (1) geological part of the trough includes two east-tilted half grabens, which are separated by overviews, (2) descriptions of the known distribution of late Miocene lacus- Wheeler Ridge in the north and the Lost Basin Range in the south. The south- trine and evaporite deposits, (3) geochronological constraints on the tim- eastern half graben, here termed the southern Grand Wash Trough, developed ing of basin development and late Miocene sedimentary facies, (4) poten- in the hanging wall of the west-dipping northern Grand Wash fault, is centered tial for undiscovered thick sections of halite in the basin-fill sediments, and in the Grapevine Wash area, and contains less than ~1 km of Tertiary basin-fill (5) a summary of late Miocene depositional environments. More detailed sedimentary rocks (Langenheim et al., 2010). In the western part of the Grand descriptions of the Grand Wash Trough and Hualapai basin are provided Wash Trough, the Gregg basin is a relatively narrow east-tilted half graben due to superb exposures and availability of core for analysis, respectively. that lies in the hanging wall of the west-dipping Wheeler Ridge and Lost Basin These attributes, in turn, furnished a sedimentologic, geochronologic, and Range faults. As the Wheeler Ridge fault dies out northward to the north of geophysical framework with which to evaluate and catalogue lacustrine and Lake Mead, Gregg basin and southern Grand Wash Trough coalesce to form evaporite deposits elsewhere in the Lake Mead region. For all analyzed ba- a large composite basin more than 4 km deep (Langenheim et al., 2010), here sins in the Lake Mead region in this study, Table 3 summarizes the age, referred to as the northern Grand Wash Trough (Fig. 3). thickness, and elevation of late Miocene lacustrine limestone, gypsum-­ Dissection by the Colorado River and its tributaries has produced excel- anhydrite, and halite deposits, overall basin-fill thicknesses, and potential lent exposures of the upper part of the Tertiary section in both the southern for thick ­halite deposits. Grand Wash Trough and Gregg basin (Figs. 3 and 5). The middle to late Mio-

TABLE 3. LATE MIOCENE BASIN-FILL SEDIMENTS, LAKE MEAD REGION Limestone Gypsum-anhydrite Halite Basin-fill thickness Age Thickness Elevation Age Thickness Elevation Age Thickness Elevation Basin (km) (Ma) (m) (m asl) 87Sr/86Sr (Ma) (m) (m asl) (Ma) (m) (m asl) Halite potential Grand Wash S: <1 12–<7.5 210 360–912 0.7114–0.7195 13–9 500 360–720 12–<5.6(?) NR —Likely (northern) Trough N: >4 <1500 north Hualapai 4—NR ——<5.6 ~395 Top: 12–<5.6>1230 Top: 209–418 Confirmed 286–440 ~2500 likely to bottom at least 972 bsl and >2 km bsl Temple Bar <1.5 10–<6 25–100 360–720 0.7137–0.7145 10–6 >200 360–439 —NR —Unlikely <1000 Overton 3–4 —NR— —11–6 >100 400 asl to Ca. 11–6 305–>530 Top: 400–260 bsl Confirmed ~400 bsl <1500 to bottom: >706 bsl Detrital N: <2 10–<6 25–100 360–720 0.7137–0.7145 10–<6>200 360–488 10–<6 229 338–414 Confirmed in S: 2–3 <800 north north; likely in <1500 south south Boulder <1 —NR— —8.5–5.5>40 360–660 —NR— Unlikely

Nellis <2 <5.6 ≤50 595–720 0.7107–0.7109 8.5–5.5>40 450–660 —NR —Unlikely <800 Las Vegas 2–4 —NR— —8.5–5.5(?) ??8.5–5.5(?) NR —Possible in NE <300 NE Virgin River E: 8–10 —NR— — 10–4 >400 303 bsl–90 10–5.5(?) NR —Likely depression W: 5 <1000 Note: asl—above sea level; bsl—below sea level; NR—not recognized. 87Sr/86Sr ratios for the Bouse Limestone and contemporary Colorado River are 0.7102–0.7114 and 0.7103– 0.7108, respectively. The dashes indicate that the lithologic unit (e.g., limestone) has not been observed. Italicized text denotes potential thicknesses inferred from geophysical data. E— eastern; N—northern; S—southern; W—western.

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cene section in these areas has been referred to as the rocks of the Grand 13 Ma. Additionally, our dates and those from Crossey et al. (2015) bracket Wash Trough after Bohannon (1984). The upper part of this section tempo- the age of the Hualapai Limestone in the Grand Wash Trough between ca. rally correlates with the Muddy Creek Formation. In ascending order, the Ter- 12 and 7.5 Ma. It is important to note that the middle to upper parts of the tiary section in the southern Grand Wash Trough consists of at least 250 m of Hualapai Limestone onlap the Grand Wash fault in the southern Grand Wash middle to late Miocene fanglomerate, more than 120 m of a sandstone-silt- Trough (Fig. 8), indicating that this part of the fault zone became inactive prior stone facies with locally interbedded gypsum, and as much as 210 m of late to ca. 8–9 Ma. Miocene limestone (Figs. 8 and 9; Longwell, 1936; Lucchitta, 1966; Bohan- 40Ar/39Ar dates from basalt flows along the Grand Wash Cliffs to the north non, 1984; Wallace, 1999; Blythe, 2005; Wallace et al., 2005; Crossey et al., of Lake Mead further constrain the structural and paleogeographic evolu- 2015). The Miocene units interfinger­ and thicken eastward toward the deeper tion of the Grand Wash Trough. Billingsley and Wellmeyer (2003) suggested parts of the half graben. The limestone in the Grand Wash Trough is known that a basalt flow capping Snap Point on the elevated western margin of the as the Hualapai Limestone and has been correlated with similar limestone Colorado Plateau directly east of the Grand Wash Trough was erupted from elsewhere in the central to eastern Lake Mead region (Longwell, 1928, 1936; a dike at Snap Point and flowed down the steep erosional escarpment of Lucchitta, 1966). In the Grand Wash Trough, exposures of the Hualapai Lime- the Grand Wash Cliffs through Snap Canyon to the Nevershine Mesa area stone range from ~500 to 912 m above sea level (asl). Gypsum crops out as in the Grand Wash Trough, where a presumed correlative basalt is intercalated low as ~360 m asl (Fig. 8 and ­Table 3; Wallace et al., 2005). Detrital zircon data in alluvial fan sediments (Fig. 11). The basalt flow at Snap Point had yielded a indicate a local provenance for the sandstone-siltstone facies in the southern K-Ar age of 9.07 ± 0.80 Ma (Reynolds et al., 1986), but the Nevershine Mesa Grand Wash Trough (Crossey et al., 2015). basalt had not been previously dated. Our 40Ar/39Ar dates and geochemical The rocks of the Grand Wash Trough are bracketed between ca. 15.5 and analyses of the basalt flows at Snap Point and Nevershine Mesa support the 7.5 Ma, as constrained by 40Ar/39Ar dates originally reported by Faulds et al. Billingsley and Wellmeyer (2003) hypothesis. The two basalts yielded essen- (2001b) but with the analytical data and further discussion presented herein tially identical geochemistry and 40Ar/39Ar dates of 8.93 ± 0.03 Ma and 8.94 ± (Fig. 9 and Table 1). The older age is based on a 15.51 ± 0.04 Ma 40Ar/39Ar date 0.05 Ma, respectively (Fig. 12). The basalt at Nevershine Mesa overlies proba- on sanidine from a rhyolite tuff tilted ~30°–35° near the base of the Tertiary sec- ble correlatives of the Hualapai Limestone, and both the limestone and basalt tion on the west flank of Grapevine Mesa (Figs. 5 and 8; Table 1; Wallace et al., flow onlap the Grand Wash fault (Figs. 11A and 11D). These relations suggest 2005). A gently tilted (~5°–10°) nonwelded tuff in the lower exposed part of the that the Hualapai Limestone in this area is older than 8.9 Ma and that the pres- sandstone-siltstone facies in the Pearce Ferry area (Figs. 8 and 10D) has yielded ent physiography of the Grand Wash Trough and abrupt western margin of the a maximum 40Ar/39Ar age on sanidine of 13.26 ± 0.06 Ma, whereas tephras Colorado Plateau were largely established by 8.9 Ma. in the lowermost part of the Hualapai Limestone in Grapevine Wash (Figs. Another basalt flow at Sandy Point in the Grand Wash Trough provides an 8 and 10C) and directly beneath the Hualapai Limestone near Airport Point additional, critical paleogeographic marker, as it overlies Colorado River grav- both geochemically correlate with an 11.01 ± 0.03 Ma tuff derived from the els ~100 m above the present grade of the river (Figs. 5, 8, and 10A). This basalt Bruneau-Jarbidge volcanic field in southernmost Idaho (Table 2; Wallace et al., flow yielded an40 Ar/39Ar age of 4.49 ± 0.23 Ma (Fig. 9; Tables 1 and A2). It was 2005). Fine-grained sanidine from this tephra also yielded an 40Ar/39Ar age of originally dated at 3.79 ± 0.46 Ma via the K/Ar method (Damon et al., 1978). The 11.20 ± 0.13 Ma (Fig. 9; Table 1), which is compatible with the geochemical underlying river gravels accumulated within a canyon cut significantly below correlation. In the same part of the section, Crossey et al. (2015) geochemically the level of the Hualapai Limestone. Thus, the first appearance of the Colorado correlated a tephra with 12.07–11.31 Ma ash-flow tuffs derived from calderas River in the Grand Wash Trough is bracketed between ca. 7.5 Ma (age of the in the Snake River Plain region. Furthermore, as reported herein, K-feldspar upper Hualapai Limestone) and 4.5 Ma (age of Sandy Point basalt). from a tephra intercalated in the upper part of the nearly flat-lying Hualapai The Wheeler Ridge and Lost Basin Range faults accommodated 275–500 m Limestone at Grapevine Mesa yielded an 40Ar/39Ar maximum eruptive age of of offset of the Hualapai Limestone (Lucchitta, 1966; Wallace et al., 2005; Seixas 7.52 ± 0.11 Ma (Figs. 8 and 9; Table 1). et al., 2015) and continued to accommodate displacement as the Colorado These data bracket both the timing of tilting and age of the Hualapai Lime- River incised from ca. 5 Ma to present (Seixas et al., 2015), as evidenced by stone in the Grand Wash Trough. Significant tilting and presumably exten- gentle local tilting (~5°) of both the early Pliocene basalts and Colorado River sion occurred between ca. 15.5 and 7.5 Ma, as indicated by the progressive sediments (Howard and Bohannon, 2001; Howard et al., 2010), as well as offset upward decrease in tilt (from 35° to subhorizontal) within the middle to late early Pleistocene alluvial fan deposits (Wallace et al., 2005). Thus, mild exten- Miocene section. However, the base of the Tertiary section is tilted much less sion continued to affect the area in Pliocene–Pleistocene time. than underlying Paleozoic strata (35° versus ~60°+), suggesting an earlier on- Tertiary strata are generally not well exposed in the deeper (>4 km) northern set to extension. On the basis of regional relations (e.g., Beard, 1996; Faulds part of the Grand Wash Trough (Fig. 3) nor has this area been penetrated by deep et al., 2010), we infer that major extension began ca. 16 Ma in the Grand Wash wells. The Grand Wash fault in this area has accommodated Quaternary displace- Trough and that the main episode of extension occurred between ca. 16 and ment in contrast to areas to the south, where late Miocene strata onlap the fault.

GEOSPHERE | Volume 12 | Number 3 Faulds et al. | Late Miocene lacustrine and nonmarine evaporite deposits, Lake Mead region Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/12/3/721/3335088/721.pdf 734 by guest on 24 September 2021 on 24 September 2021 by guest Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/12/3/721/3335088/721.pdf Research Paper SCF—Sheep Canyon fault; SCVF—South Cove fault; SP—Sandy Point; WRF—Wheeler Ridge fault. Red italic letters (A, B, etc.) are associated with GWC—Grand Wash Cliffs; ICF—Iceberg Canyon fault; LBF—Lost Basin Range fault; LP—Lookout Point; MF—Meadview fault; NGWF—northern Grand Wash fault; PF—Pearce Ferry; Figure 8. Generalized geologic map and cross section of the Grand Wash Trough (modified from Wallace et al., 2005). AF—Airport fault; AP—Airport Point; GC—Grapevine Canyon; whereas the Paleozoic-clast conglomerate (Tcp) was largely sourced from the Grand Wash Cliffs to the east. of the central to western Lake Mead region (e.g., Bohannon, 1984; Beard, 1996). Proterozoic-clast conglomerate (Tcg) was derived from the South Virgin Mountains to the west, do not correspond to location of dated samples. Ths unit may temporally correlate with the Horse Spring Formation, which accumulated in early to middle Miocene basins in much and correspond to specific samples and data provided in Tables 1 and 2. Dates shown on cross section represent approximate stratigraphic positions of dated horizons but generally meters 1000 –600 –400 –200 200 400 600 800 WNW 0 A

? Lake Mead

? 36° 7′ 30″ N

Line of cross sectio n on downthrown side. accurate, dotted where concealed. Bar and ball Normal fault Solid where certain and location A 36° 00′ N — GREGG BASIN YX u QT Tby Tgy Thl Qf Tt 11 cs

4° 7′ 30″ A′ Mead 8 Yx u Tbr Basalt of Sandy Point

Late Miocene gypsum Late Miocene Hualapai Limestone Middle to late Miocene tephras Colorado River sediment Quaternary-pliocene fanglomerate units Wa Lake

A te r Qf Tby Ts A W1

Tc LBF — Thl g

W 4 QT H Thl SP

EE Yx u

LE R MF cs RID GE

FA 4 ULT G ? WR Yx u s

Tbr F YX u Pz

50 SCVF Pz

85 ICF Tbr 25 65

Strike and dip of bedding Intermittent stream City limit of Meadview SCF ? ? Meadvie LP 60 5 Pz Tc Thl g Inclined w

4 AF 5 B 15.5 Ma

? 13.3 Ma 3 AP WR

, F Ths? Pz YXu Arizona Tc Tc Tb r Pz Ts C p g Ts ? Th l 3 Middle to late Miocene sandstone-siltstone facies Middle to late Miocene rock avalanche deposits Middle to late Miocene Paleozoic clast conglomerate Middle to late Miocene Proterozoic clast conglomerat Proterozoic gneiss and diabas Paleozoic sedimentary strata Middle Miocene sedimentary and tu Tc Ts g GC Tc — GRAND W g 4 Sample point

G 4 r a p e v D i n e ? PF W 14° 00′ W a s h A′ 40 Geochemical correlation (tephrochronology) E Ar / Tg y F 39 s Ar date 5 kilometers ASH 5 Th l Mead Lake 4 Ths? Tc e p 5 TROUGH 7.5 Ma ff aceous rocks GWC NGWF GWC 4 Tc Pz

? N 4 p — 40 Ts Ar/ Tg y 39 11 e Thl? Ar dates or tephrochronology Ma Ts

? Tc p ESE feet A′ 3000 –200 –100 0 1000 2000 0 0

GEOSPHERE | Volume 12 | Number 3 Faulds et al. | Late Miocene lacustrine and nonmarine evaporite deposits, Lake Mead region 735 Research Paper

Grand Wash Trough Hualapai Basin 4.49+/-0.23 Ma Tby A QTcs 100 80 100 100

Ar* QTa Thl 60 40 40 B % Ts 20 60 %40Ar* 60 0 10 100 5.62+/-0.11 Ma

E 1 10 K/Ca

0.1 K/Ca

Ma 11 7.52+/-0. Tgy 1 K 10 0.01 20 N 20 9 A JF-97-76 Ts JF-97-76-2 0 8 0 MW-98-36 JF-97-144 7 B E Tcg 7.52± 0.11 Ma 11.20 ± 0.13 Ma Tha 6 .20+/-0.13 11 Ma A n = 1 MSWD = 0.54 A B 4.49 ± 0.23 Ma (MSWD = 0.29) D 5 B 4.48 ± 0.23 Ma (MSWD = 1.72) n = 23 C C D E E Apparent age (Ma) H Ts F 4 F FGG H Tcg 13.26+/-0.06 Ma 3

2 Relative probability 0 10 20 30 40 50 60 70 80 90 100 Tbr 5110 52025 5 Cumulative %39Ar Released 10 15 20 25 Tvsy Tcg Age (Ma) Age (Ma)

15.51+/-0.04 Ma 100 F1 F2 G Tvso

Tbr %40Ar* 60 %40Ar* 100 100

10 10 K/Ca Ths K/Ca G 1 1 20 N N Mr 0 1 y y JF-98-155 JF-98-155 JF-98-308 b m 13.72 ± 0.61 15.51 ± 0.04 Ma 13.26 ± 0.06 Ma Xg t MSWD = 0.19 MSWD = 1.57 n = 15 MSWD = 2.52 n = 23 n = 7 Ds Relative probabilit Relative probabilit

Xg

Figure 9. Generalized stratigraphic columns of the Grand Wash Trough and Hualapai basin, showing approximate tilts, relative unit thicknesses, and 40Ar/39Ar results. Dashed purple lines are tephras or tuffs that have been dated using the 40Ar/39Ar technique or geochemically correlated using tephrochronology (e.g., 5.62 Ma tephra intercalated with halite in the Hualapai basin; see Table 2). Sample locations for dated samples are shown on Figure 8 (for Grand Wash Trough) and listed in Table 1, with letters as corresponding labels in each. For Grand Wash Trough: QTcs—Colorado River sediments; Tby—early Pliocene basalts (e.g., 4.5 Ma Sandy Point basalt); Thl—Hualapai Limestone; Ts—sandstone-siltstone facies; Tgy—gypsum; Tcg—conglomerate derived from South Virgin Mountains to the west; Ths—tuffaceous sedimentary rocks probably correlative with Horse Spring Formation (e.g., Beard, 1996); Mr—Redwall Limestone; Ds—Sultan Limestone; Єm—Muav Limestone; Єb—Bright Angel Formation; Єt—Tapeats Sandstone; Xg—Paleoproterozoic gneiss. For Hualapai basin: QTa—Holocene–late Miocene shale, conglomerate, gypsum, and anhydrite; Tha—late Miocene (ca. 12–5.6 Ma) halite and lesser shale and anhydrite; Tcg—locally derived late Miocene fanglomerate; Tvsy—middle Miocene (ca. 13–16 Ma) volcanic and ; Tvso—early to middle Miocene (ca. 20–16 Ma) volcanic and sedimentary rock, possibly resting on a thin section of Cambrian strata; Xg—Proterozoic gneiss, granite, and diabase. For 40Ar/39Ar results, age spectrum, K/Ca, and radiogenic yield diagrams are shown for basalt groundmass concentrates (sample A—two samples of Sandy Point basalt), as provided by the U.S. Geological Survey, and relative probability and auxiliary diagrams are shown for sanidine (samples B, E, F, and G). Samples F, G, and E yield normal distributions for the majority of the analyses and yield robust tephra deposition ages of variable precision. Sample F1 is the same as F2 with the assumption that all analyses are 100% radiogenic. The youngest group yields a precise measure of a maximum deposition age. Sample B (MW-98-36) is highly contaminated with inherited grains as old as ca. 300 Ma, and thus youngest data are a maximum tephra depositional age. A single younger grain from this tephra yielded an age of 7.52 ± 0.11 Ma and was chosen as the maximum depositional age. Open symbols in the ideograms were not used for weighted mean age calculations. See Appendix A for analytical data.

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A B

Tby

4.5 Ma Sandy Point Basalt

Figure 10. Key stratigraphic features in Tertiary section, Grand Wash Trough. (A) Looking northeast at 4.5 Ma basalt QTcr at Sandy Point along the shores of Lake Mead. The basalt overlies Colorado River QTcr gravels (QTcr). (B) Looking south at con- tact between Sandy Point basalt (Tby) and Colorado River gravels (QTcr), encompass- ing ~4 m in height. Note the imbrication of the clasts in the gravels (white arrow). (C) Circa 11 Ma tephra near the base of the Hualapai Limestone (Thl) in Grape- vine Canyon in the Grand Wash Trough. Black arrows point to ~1–2-m-thick light- C D gray tephra beneath a ledge of limestone. Thl Grand Wash Cliffs (D) Looking north at 13.3 Ma tephra along Lake Mead in the Pearce Ferry area. The 13.3 Ma Tephra 13.3 Ma date is a maximum eruptive age (see Fig. 9). The Grand Wash Cliffs, com- posed primarily of Paleozoic carbonates, loom on the horizon.

In the transitional region between the northern and southern parts of the evidenced by the intercalated gypsum, thin bedding, and mudcracked bedding Grand Wash Trough, the Hualapai Limestone overlies extensive late Miocene planes (Wallace et al., 2005). The lack of fluvial textures, such as crossbeds or gypsum, which may approach 500 m in thickness (Figs. 3 and 11; Billingsley ripple marks, local abundance of gypsum rinds, interfingering and bordering and Wellmeyer, 2003). Similar strata may lie in the subsurface farther north, fanglomerate facies, and interbedded gypsum appear to rule out a through-go- where gravity data indicate the thickest (>4 km) sedimentary fill within the ing drainage. However, the relatively thick deposits of gypsum (~500 m) and Grand Wash Trough. Here, a 30–40 mGal gravity low (Fig. 4A; Langen­heim possible thick halite (as much as 1.5 km) in the central to northern reaches of et al., 2010) may indicate that halite makes up some of the basin fill. Based on the Grand Wash Trough suggest significant input of groundwater and/or sur- inversion of the anomaly and assuming a constant density contrast consistent face water to the basin in late Miocene time. with halite, the maximum thickness of halite in this area is ~1.5 km. Such a The basin transitioned to a wetter setting ca. 12–11 Ma as reflected by the thickness would produce reasonable basement gravity values and is also con- Hualapai Limestone. Although some early interpretations suggested a marine sistent with low magnetic values in the area (Fig. 4B). origin­ for the Hualapai Limestone (Blair, 1978; Blair and Armstrong, 1979), 87Sr/86Sr The late Miocene depositional environment of the Grand Wash Trough isotopic ratios (0.7114–0.7195; Spencer and Patchett, 1997; Roskowski et al., 2010; ranged from alluvial fans proximal to the margins to continental playa and Lopez Pearce et al., 2011), fossil assemblages, petrography, and d13C-d18O isotopic lacustrine generally in the interior portions. Deposition of the sandstone-silt- geochemistry (Faulds et al., 1997, 2001c; Wallace, 1999; Wallace et al., 2005; Crossey stone facies probably occurred in an evaporative interior continental playa, as et al., 2015) indicate deposition in a lacustrine to nonmarine wetland setting.

GEOSPHERE | Volume 12 | Number 3 Faulds et al. | Late Miocene lacustrine and nonmarine evaporite deposits, Lake Mead region Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/12/3/721/3335088/721.pdf 737 by guest on 24 September 2021 on 24 September 2021 by guest Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/12/3/721/3335088/721.pdf Research Paper Wellmeyer, 2003). Thus, it appears that the basalt lava cascaded down the Grand Wash Cliffs ca. 8.9 Ma. between these two locations. Basalt dikes that have yielded 9.10 ± 0.13 Ma K/Ar dates atop the Colorado Plateau were probable feeders to these flows (Billingsley and composition ( ­ and the Grand Wash Trough modified from Billingsley and Wellmeyer (2003). The basalt flows at Snap Point and Nevershine Mesa are essentially identical in age and strata at an elevation of ~2000 probably correlates with the Hualapai Limestone. (C) View north toward Snap Point and upper Grand Wash Cliffs. Here, an 8.93 ± 0.03 Ma basalt flow overlies Permian elevation of ~950 m and is intercalated in fanglomerates that cover and are not displaced by the northern Grand Wash fault. The fanglomerates overlie limestone that of the cliffs above the concealed trace of the northern Grand Wash fault. (B) View southwest of Nevershine Mesa, where an 8.94 ± 0.05 Ma basalt flow crops out at an River adapted and simplified from Billingsley and Wellmeyer (2003). Snap Point resides at the crest of the Grand Wash Cliffs, whereas Nevershine Mesa lies at the base Figure 11. Grand Wash Cliffs and 8.9 D A B Table 1; Fig.12). No significant post–ca. 9 Ma faults lie between Snap Point and Nevershine Mesa. Scattered small exposures of correlative basalt crop out SE A LEVEL Nevershine Mesa METER S 1000 150 0 2000 2500 500 Nevershine Mes A Ts Qf m. Photos courtesy of George Billingsley. (D) Cross section A–A

WHEELER RIDGE FAULT Pz Ma basalt flow. (A) Generalized geologic map of the Grand Wash Cliffs and Grand Wash Trough ~10–15 Th s a 11 BASIN GYP 3 o 50 ′W Tc Qf HILL S AND RANG p Tg y 8.9 Ma Basal t VER E Qf TICA L EXAGGERA Snap Point A′ Qf Tc Tb y Th l p NEVERSHINE MESA TION x2

Qf NORTHERN GRAND WASH FAULT Tc p

Xg r C

′N 10 36 Pz o A ′ (location shown in A) extending from Snap Point to Nevershine Mesa Qf Line of cross section

downthrown side. Major concealed normal fault A Tc on downthrown side. accurate, dotted where concealed. Bar and ball Normal fault Solid where certain and location SNAP CANYON Strike and dip of bedding QT p GRAND W Tg y Tb y Th s Xg r Tc Tc Th l Qf Pz Ts 5 COLORADO PL AT a p g Inclined Quaternary-Pliocene alluviu Late Miocene gypsum Late Miocene Hualapai Limestone Basalt of Snap Point Quaternary fanglomerate units Early Proterozoic gneiss Paleozoic sedimentary strata Middle to late Miocene sandstone-siltstone facies Middle to late Miocene Paleozoic clast conglomerate Middle to late Miocene Proterozoic clast conglomerate Middle Miocene sedimentary and tuf ASH CLIFFS

Tb y A′ Snap Point (8.9 Ma Basalt) EAU

Xg r Pz Tb y

SNAP POINT Bar and ball on METER S A′ SE A LEVE L 1000 1500 500 2000 2500 m km north of the Colorado faceous rocks

GEOSPHERE | Volume 12 | Number 3 Faulds et al. | Late Miocene lacustrine and nonmarine evaporite deposits, Lake Mead region 738 Research Paper

Location Snap PointNevershine Mesa C Sample Number JF99-459 JF99-460 80 A B Latitude 36o10′17″ N36o14′27″ N Longitude113o47′53″ W113o51′51″ W 40

%40Ar Normalized Results (Weight %) 0 1 SiO2 49.36 49.94 TiO2 1.688 1.555 Al2O3 16.12 16.23 0.1 FeO 10.98 10.09 Ca MnO0.178 0.179 12 0.01 MgO6.49 6.97 CaO 10.60 10.48 JF99-459 JF99-460 Na2O 3.17 3.24 K2O1.060.97 11 P2O5 0.366 0.340 Total99.28 99.12 8.93 ± 0.03 Ma (MSWD = 1.11) Trace Elements (ppm) 10 Sc 28 30 8.94 ± 0.05 Ma (MSWD = 2.19) I V220 214 Cr 266263 Ni 12589 9 Cu 49 53 Zn 92 94

Aarent age (Ma) F E E G H F G Ga 17 20 D D Rb 14 14 8 Sr 443 430 I H Y2925 Zr 156 139 Integrated Age = 8.93 ± 0.07 Ma Integrated Age = 9.16 ± 0.13 Ma 7 Nb 21.9 19.3 Ba 607 476 0 20 40 60 80 100 0 20 40 60 80 100 La 15 36 39 39 Ce 46 55 Cumulative % Ar eleaed Cumulative % Ar eleaed Pb 16 Th 45

Figure 12. 40Ar/39Ar (A and B) and geochemical (C) data for Snap Point and Nevershine Mesa basalt flows. (A) and (B) Age, K/Ca, and radiogenic yield diagrams for basalt groundmass concentrates provided by New Mexico Geochronology Research Laboratory. Age spectra are either completely or nearly flat and yield well-defined and identical plateau ages. Plateau ages provide robust erup- tion ages. See Appendix A for analytical data. As described in the appendix, errors for plateau and integrated ages are calculated differently. (C) Major- and trace-element geochemical data provided by Washington State GeoAnalytical Laboratory. The geochemical data are also nearly identical and support the correlation of the basalt flows at Snap Point and Nevershine Mesa. MSWD—mean square of weighted deviates.

This lake and/or wetlands has been referred to as Lake Grand Wash (e.g., Hualapai Basin Spencer et al., 2013), with a maximum level of ~912 m (Fig. 3B) corresponding to the current elevations of the uppermost exposures of the Hualapai Lime- The Hualapai basin is an ~60-km-long, north-northwest–trending depres- stone. Because the lake was fresh, it probably had an outlet either through sur- sion situated ~40 km south of the Colorado River and sandwiched between face runoff or groundwater sapping into nearby basins. 87Sr/86Sr and d13C-d18O the western margin of the Colorado Plateau on the east and Cerbat Mountains trends through the section of limestone suggest that Lake Grand Wash was on the west (Fig. 3). The basin is a gently to moderately east-tilted half graben initially fed by springs sourced from the Colorado Plateau ca. 12–8 Ma and that developed in the hanging wall of the southern Grand Wash fault (Fig. 13B). meteoric groundwater was progressively introduced after ca. 8 Ma just prior The half graben probably formed primarily during the main episode of middle to development and integration by the Colorado River (Crossey et al., 2015). Miocene extension (ca. 16–13 Ma), but minor amounts of normal faulting have It is important to note that the absence of exotic rounded river gravels and deformed the area since ca. 8.7 Ma (Faulds et al., 2010). Quaternary scarps fluvial textures in the sandstone-siltstone facies combined with the presence have not been observed along the southern Grand Wash fault or elsewhere in of the capping lacustrine Hualapai Limestone suggest that the Colorado River the basin. Despite its proximity to the Colorado River and Grand Canyon, the was not responsible for deposition of the late Miocene sediments in the Grand Hualapai basin remains an internally drained, closed depression and contains Wash Trough (e.g., Longwell, 1946; Lucchitta, 1966, 1972). the Red Lake playa. Due to a lack of dissection by tributaries of the Colorado

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A Colorado Plateau

RLGS-1D A′ Figure 13. Hualapai basin halite deposit. The Hualapai basin contains at least a 867 TA-1 KM-2 1.2-km-thick sequence of late Miocene KM-1 halite, as observed in drill holes. The Lake Grand Wash 849 EP-1 ­halite sequence may reach 2.5 km in thick- ness, as suggested by seismic-reflection 850 855 data (Faulds et al., 1997). (A) Oblique 3D view (from Google Earth) of Hualapai 396 basin showing location of five drill holes –97 418 209 308 2.5 km (marked by green and gray circles), depth- 96 204 to-basement (i.e., thickness of basin-fill 1.5 km sediments) contours from Figure 3 (black 55 1.0 km numbers in km), maximum elevation (~912 m) of Lake Grand Wash in blue, Hualapai Basin and cross section shown in (B). Halite is shown as green in the drill holes, with A gypsum-anhydrite shown in dark pink. The wells that penetrate halite lie in the central to northern reaches of the deepest part of –972 N the basin and are marked by green circles. Cerbat Mts Purple unit in the upper left part of the image marks the southernmost exposures of the Hualapai Limestone, and pink poly- 2 km gons denote gypsum exposures. Brown transparent polygon shows the extent of Lake Hualapai (elevation at ~720 m). Num- B bers adjacent to drill holes correspond to A A Hualapai Basin Colorado Plateau ′ collar elevation (black text), top elevation SGW of halite (dark green), and bottom eleva- Cerbat Mts Tvso Pz QTa KM2 EP1 2 tion of well (red). All elevations are in Tvso ­meters. See Table 4 for descriptions of drill 0 holes. (B) 1:1 cross section constrained by Tha a migrated seismic-reflection profile (from Tcg –2 Faulds et al., 1997). SGW—southern Grand Xg Wash fault. Unit labels same as in Figure 9. Xg Tvsy –4 Pz—Paleozoic strata, undivided. –6 –8 km SW NE

River, synextensional middle to late Miocene strata within the basin are ob- contains a thick (~3.9 km) sequence of Miocene to Quaternary basin-fill strata, scured by more recent flat-lying sediments in contrast to the highly dissected with tilts decreasing up-section from ~25° to 0°. basins along the course of the Colorado River to the north, such as the Grand As inferred from exposures in surrounding mountain ranges and along the Wash Trough. Nonetheless, gravity (Fig. 4A; ~50–60 mGal low in the northern western margin of the Colorado Plateau, analysis of drill core, and seismic-­ part of the basin; Langenheim et al., 2010), drill-hole (Fig. 13A; Table 4), and reflection profiles, the stratigraphy of the Hualapai basin consists of a thick seismic-reflection data (Faulds et al., 1997) indicate that the Hualapai basin section of primarily Miocene sedimentary and volcanic rocks. In ascending

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TABLE 4. DRILL HOLES IN KNOWN HALITE BASINS, LAKE MEAD REGION Halite top Halite top Halite bottom Well number Basin °N °W Collar elevationWell depth depth elevation elevationHalite thickness KM-1 Hualapai 35.605 114.025850 795432 418<55 >362 KM-2 Hualapai 35.619 113.991855 651459 396<204>192 EP-1 Hualapai 35.620 113.987855 1827 547308 <–972 >1280 TA-1 Hualapai 35.646 113.991849 753640 209<96 >113 RLGS-1D Hualapai 35.726 114.045867 964– –– – SSC-1 Overton Arm 36.40598 114.40550408 932645 –237 <–524 >287 SSC-2 Overton Arm 36.40738 114.38990402 1108 573–171<–706 >535 SSC-3 Overton Arm 36.42283 114.39226429 682266 163–132~295 SSC-4 Overton Arm 36.41432 114.40391420 408313 107<12 >95 SCB-1 Overton Arm 36.34809 114.44289430 829– –– – SCB-2 Overton Arm 36.32490 114.45454428 859688 –260 –339 79 GCM-1A Detrital 35.937 114.497513 254– –– – GCM-10 Detrital 35.918 114.533637 133– –– – GCM-8 Detrital 35.917 114.515562 325183 379<237>142 GCM-5 Detrital 35.915 114.496545 371142 403<174>229 GCM-9 Detrital 35.911114.506 543291 142401 <252 >149 GCM-2 Detrital 35.922 114.488543 319129 414<224>190 GCM-3 Detrital 35.907 114.493569 183174 395<386>9 GCM-4 Detrital 35.902 114.496575 424187 388231 157 GCM-7A Detrital 35.907 114.514543 375205 338170 168 GCM-11 Detrital 35.899 114.534619 274– –– – GCM-6 Detrital 35.893 114.505574 346236 338<228>110 PDC-3 Detrital 36.006 114.482462 56 –– –– PDC-4 Detrital 36.005 114.482463 233– –– – PDC-1 Detrital 35.995 114.493502 212– –– – PDC-2 Detrital 35.992 114.493500 229– –– – Notes: For Hualapai and Detrital basins, data are from the Arizona Geological Survey. For the Overton Arm basin, data are from Netherland and Sewell (1977). Well locations are shown on Figures 2, 13B, 14C, and 15. Many of the drill holes bottomed in halite; thus, halite thicknesses are minimums. All elevations, depths, and thicknesses are expressed in meters above or below sea level. Dashes indicate that halite was not present.

order, this section includes: (1) ~750 m of lower to middle Miocene volcanic Tables 3 and 4) penetrated a thick succession of halite from depths of 597 m and sedimentary rock possibly resting on Cambrian strata and/or Proterozoic (1958 ft depth) to 1827 m (5995 ft) near the center of the Hualapai basin (Fig. gneiss and granite; (2) ~335 m of middle Miocene volcanic and sedimentary 13B), yielding a minimum thickness of halite of ~1230 m. Documented halite rock; (3) fanglomerates along the margins that interfinger with evaporites in in the EP-1 well ranges from 258 m above sea level to ~972 m below sea level the central part of the basin; 4) up to 2500 m of halite referred to as the Red (bsl). On the basis of the reflective character of the halite in a seismic-reflection Lake halite and intercalated with minor shale (5%–10%) and anhydrite; and profile, the total thickness of halite has been inferred at ~2500 m (Faulds et al., (5) ~600 m of late Miocene–Quaternary shale and lesser amounts of gypsum, 1997), suggesting that the halite may extend downward to elevations in excess anhydrite, and conglomerate (Fig. 9; Faulds et al., 1997). It is important to note of 2 km below present sea level. These elevations are consistently lower than that the thick halite does not crop out anywhere, either within or along the the exposures of the Hualapai Limestone in the Grand Wash Trough, although margins of the basin. minor tectonism between the two basins since the late Miocene cannot be Drill-hole and geophysical data constrain the overall thickness and extent ruled out. Gravity data are compatible with the interpretation that the Hualapai of the Red Lake halite deposit. In four drill holes within the basin, the top of the basin is dominated by a 2.5-km-thick sequence of halite (Figs. 3, 4, and 13B). In- halite varies from 209 to 418 m in elevation (Fig. 13A; Tables 3 and 4), whereas version of the gravity low using a relationship of increasing density with depth the top of the gypsum-anhydrite sequence that caps the section of evaporites produces a basin thickness of more than 10 km, which does not agree with the ranges from 286 to 440 m asl. The EP-1 well (drilled by El Paso Natural Gas; seismic-reflection data. Modifying the density-depth relationship to include a

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significant thickness of low-density fill, such as halite, produces basin depths mentary processes probably contributed to halite deposition in the Hualapai more consistent with the seismic-reflection data (Langenheim et al., 2010). basin: (1) evaporation from a standing body of , and (2) intra-sedimentary On the basis of regional relations, the age of the Red Lake halite was pre- displacive growth of halite in a desiccated salt pan. The common displacive viously bracketed between ca. 13 and 8 Ma (Faulds et al., 1997), but our new growth textures (Fig. 6B) and relative lack of interlayered fine clastics indi- tephrochronologic data (Table 2) and recent work indicate a somewhat younger cate that much of the halite was derived from groundwater under arid age of ca. 12–5.6 Ma. Because the halite occupies the gently tilted upper part of conditions, which accounts for lack of clastic influx into the basin center area. the basin fill and extension in the region is constrained in nearby areas to ca. The basin playa most likely served as a prolonged, regional discharge zone 16–8 Ma (Faulds et al., 2010), with the main pulse occurring from 16 to 13 Ma, for large volumes of generally saline groundwater (Faulds et al., 1997). The the gently tilted halite is probably younger than ca. 13 Ma. Our analysis of displacive growth of halite probably took place by both seeping, hypersaline core from the EP-1 well in the Hualapai basin (Fig. 13) revealed an intercalated groundwater and from incoming fresh water at the surface of the salt pan and tephra at a depth of 626 m (2053 ft) or 229 m in elevation. The tephra lies ~79 m subsequent evaporation (Fig. 1). Poorly defined bedding throughout much of beneath the top of the halite and 136 m beneath the top of the capping an- the sequence resulted from a sporadic influx of undersaturated waters that dis- hydrite. This tephra geochemically correlates (Table 2) with the Conant Creek solved and then reprecipitated the underlying halite both as irregular crystal Tuff, which is a Yellowstone hotspot tuff sourced from the eastern Snake River overgrowths and as interstitial precipitates upon re-concentration due to rapid Plain. The Conant Creek Tuff has yielded an 40Ar/39Ar age of 5.62 ± 0.11 Ma from evaporation. During the early stages of halite deposition, the groundwater in the near source ash-flow tuff (relative to an age of 28.201 ± 0.46 Ma for sanidine the basin may have been relatively fresh but would become saline through from the Fish Canyon Tuff; Kuiper et al., 2008). Thus, the halite in the Hualapai discharge and subsequent evaporation. However, as the halite sequence grew basin is now bracketed between ca. 12 to slightly younger than 5.6 Ma and is in thickness, any groundwater within the central part of the basin would have largely contemporaneous with the Hualapai Limestone to the north. likely been saline due to dissolution of some of the previously deposited, sub- The textures (Fig. 6) and bromine content of the halite and S and O isotopic surface halite. values of intercalated and capping anhydrite indicate that halite deposition re- sulted from groundwater discharge and ponding in an intracontinental playa (Faulds et al., 1997). Our recent, more thorough analysis of core from the EP-1 Virgin-Detrital Trough well revealed a compositionally, texturally, and sedimentologically monoto- nous sequence of halite. This sequence consists of halite (>90%), interbedded The Virgin-Detrital trough is a continuous ~140-km-long, 15–40-km-wide, with lesser fine clastics and anhydrite (Figs. 6B, 6C, and 6E). Generally, the northerly trending topographic depression that straddles Lake Mead and the halite layers are massive, pure, and poorly bedded. In addition, salt crystals Colorado River in southern Nevada and northwestern Arizona (Fig. 3). It con- are clear, lack primary sedimentary features, generally exceed 2 cm in length sists of three distinct parts, including a central portion referred to as the Temple (Fig. 6A), and range up to 10 cm long. Some of the halite fills dissolution cavi- Bar basin, a northern segment called the Overton Arm basin, and a southern ties within the halite masses. Fine clastics or anhydrite stringers outline some leg referred to as the Detrital basin. of the halite crystals, with relatively common displacive halite crystals in silt- stone (Fig. 6B). The observed fabrics are typical of postdepositional textures, which resulted from rock and groundwater interaction. Primary sedimentary Temple Bar Basin halite is typically small in crystal size and rich in milky fluid inclusions (Benison and Goldstein, 1999; Goldstein, 2001). Entrapment of fluid inclusions generally The Temple Bar basin occupies the central Lake Mead region proximal to takes place at high rates of crystal growth during primary deposition of the the Colorado River and central part of the Virgin-Detrital trough (Fig. 3). It con- halite. The observed lack of fluid inclusions and large blocky crystals indicate tains the confluence of the Virgin and Colorado Rivers. It is as much as ~25 km that much of the Red Lake halite formed from slow growth and/or recrystalliza- wide and is sandwiched between the northern White Hills and South Virgin tion processes probably due to periods of exposure, localized dissolution, and Mountains to the east and Black Mountains to the west. The Temple Bar basin reprecipitation in shallow playas. is essentially an east-tilted half graben in the hanging wall of the South Virgin– The extreme thickness of the halite, apparent lack of limestone, common White Hills detachment fault and largely consists of east-tilted middle to late displacive growth textures, and relative lack of interlayered clastics suggest Miocene fanglomerate, megabreccia, and volcanic rocks bracketed between that the Hualapai basin had no outlet and served as a regional sink for saline ca. 15.2 and 6 Ma. Upward-decreasing tilts suggest synextensional deposition groundwater and surface water that evaporated in extensive salt pans within a of the Miocene section, with the main episode of extension occurring from ca. continental playa (e.g., Fig. 1; Neev and Emery, 1967; Smoot and Lowenstein, 16.5 to 11 Ma (Duebendorfer and Sharp, 1998; Blythe et al., 2010). Quaternary 1991). A similar depositional environment presently characterizes the Dead fault scarps have only been observed along the northeastern margin of the Sea area (Neev and Emery, 1967; Abed and Yaghan, 2000). Two coeval sedi­ Temple Bar basin to the north of Lake Mead (Fig. 3; U.S. Geological Survey,

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2010) and possibly along the far western edge along the northern part of the levels of the lake have been inferred at ~720 m. Sr isotopes from the limestone Detrital fault. However, late Miocene strata in the area are locally tilted gently (0.7137–0.7145; Roskowski et al., 2010) suggest that the lake had a similar ori­ (<15°) and cut by minor faults (Beard et al., 2007). gin to Lake Grand Wash to the east, with springs and possibly some runoff The upper part of the basin fill consists of little-deformed late Miocene serving as the primary source of water and a probable outlet that prevented fanglomerate that interfingers with and gives way to the Hualapai Limestone significant evaporation. and gypsum toward the basin center (Howard et al., 2010). The upper Miocene section onlaps the bounding detachment fault and fills paleovalleys cut into Proterozoic rocks in the footwall of the detachment. Some of these paleo- Overton Arm Basin channels connect eastward to the Gregg basin in the Grand Wash Trough (Howard et al., 2010). Scattered exposures of Hualapai Limestone rim the cen- The Overton Arm basin is an ~50-km-long, ~10–15-km-wide, northerly tral part of the basin (Beard et al., 2007), crop out at elevations ranging from trending topographic depression that extends northward along the Virgin ~360 m to as high as 720 m (Spencer et al., 2013), and range in thickness River from Lake Mead between the South Virgin Mountains to the east and from ~25–100 m (Table 3; Howard et al., 2010). Interbedded tephras and an Muddy Mountains to the west (Fig. 3). Gravity data indicate that basin-fill sed- 8.4 Ma basalt flow bracket the age of the limestone and related sediments iments in the southern part of the basin are relatively thin (generally <1 km (including gypsum) between ca. 10 and 6 Ma (Howard et al., 2010). A tephra thick). However, a 25 mGal gravity low occupies the northern part of the basin interbedded within the middle part of the Hualapai Limestone directly south (Fig. 4A) and lies between two left-lateral faults, the Bitter Ridge–Hamblin Bay of Temple Bar yielded an 40Ar/39Ar age of 5.97 ± 0.04 Ma (Spencer et al., 2001), and Hen Spring–Rogers Spring faults. This part of the basin contains as much which suggests that the limestone in the Temple Bar area is largely younger as 3–4 km of basin-fill sediments (Langenheim et al., 2010) and has been inter- than that in the Grand Wash Trough (Spencer et al., 2013). This date has been preted as a pull-apart within a left step in the sinistral fault system (Campagna widely used as a younger age constraint for the Hualapai Limestone in the and Aydin, 1994). Significant deformation has affected the Overton Arm basin Lake Mead area. since the late Miocene, as evidenced by faulting and folding of the Muddy Gypsum and mudstone indicate a depocenter in the central part of the Creek Formation (Figs. 14B and 14D). Quaternary fault scarps are limited but ­basin directly west of Temple Bar (Fig. 3; Longwell, 1936; Beard et al., 2007). include segments of the northeast-striking sinistral Rogers Spring fault (Fig. Here, extensive gypsum crops out along the shores of Lake Mead in an area 14B) on the west side of the basin and northerly trending normal faults on the known as the “Big Gypsum Ledges.” The gypsum is exposed at elevations eastern margin (Fig. 3). ranging from 360 to 439 m asl (Table 3; Peirce, 1974; Beard et al., 2007) and con- Significant halite occupies the Overton Arm basin 20–30 km north of the tains tephras correlated between 7.5 and 6.3 Ma (Howard et al., 2010). Gypsum Colorado River, as evidenced by drill holes directly west of Lake Mead and in this area is at least ~200 m thick. sparse outcrops (Fig. 14; Longwell, 1928, 1936; Mannion, 1963). Exposures Gravity data and surface exposures of gently tilted strata indicate that the of halite in this area are the only observed outcrops of appreciable halite in sediments in the Temple Bar basin are as much as 1.5 km thick (Langenheim the Lake Mead region. The exposed halite crops out as discrete sedimentary et al., 2010). If composed solely of halite, the basin fill is no more than 1 km layers as thick as ~4 m (Fig. 14C); these layers are not diapirs. Halite in the thick. Smooth, generally low magnetic values are consistent with the presence Overton Arm area was used as a source of salt by Native Americans and of some halite, although the survey flight lines are widely spaced in most of early settlers in the region and was mined prior to the filling of Lake Mead this area. Halite observed in drill holes in the nearby northern Detrital basin (Longwell, 1936; Netherland and Sewell, 1977). However, nearly all halite ex- (Fig. 3) is also suggestive of some halite in the Temple Bar area. posures are now submerged beneath the reservoir. The halite is generally The late Miocene depositional environment in the Temple Bar basin was pure and coarsely crystalline (Netherland and Sewell, 1977), with textures probably similar to that of the Grand Wash Trough, as it ranged from allu- resembling those in the halite of the Hualapai basin. Siltstone interbeds are vial fans proximal to the margins (e.g., Blythe et al., 2010) to continental playa common within the halite, and anhydrite is typically interbedded in the upper and lacustrine in interior portions. Deposition of the gypsum and mudstone and lower parts of the halite sequence. is also relatively common occurred in an evaporative playa through groundwater and/or surface water and makes up more than 3% of the evaporite sequence. The halite interfingers input. The lack of widespread fluvial sediments suggests that a through-going with and is overlain by gypsum, siltstone, sandstone, and clay of the Muddy drainage did not exist. The basin transitioned to a wetter setting ca. 10–6 Ma Creek Formation (Mannion, 1963; Muntean, 2013). The Muddy Creek Forma- as reflected by the Hualapai Limestone, which accumulated in a lacustrine tion in the area is roughly bracketed between ca. 11 and 6 Ma (Howard et al., to nonmarine wetland environment. This lake has been referred to as Lake 2010). A 6 Ma basalt flow (Feuerbach et al., 1991) is intercalated in the Muddy Hualapai (Fig. 3B; Spencer et al., 2013). On the basis of current elevations of Creek Formation ~40–400 m above the top of the halite (Mannion, 1963) and the uppermost exposures of the Hualapai Limestone between the Detrital and places a younger age constraint on the halite in the Overton Arm basin (Figs. Temple Bar basins (e.g., Beard et al., 2007; Felger and Beard, 2010), maximum 14B–14D).

GEOSPHERE | Volume 12 | Number 3 Faulds et al. | Late Miocene lacustrine and nonmarine evaporite deposits, Lake Mead region Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/12/3/721/3335088/721.pdf 743 by guest on 24 September 2021 on 24 September 2021 by guest Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/12/3/721/3335088/721.pdf Research Paper A D C B –370 –270 –170 Lake Las Ve –700 1300 30 0 Feet Tha 0 0 0 We A st Echo Ba y Js Muddy Creek Formation (Tmcl Basalt (Tby ) Tr ga s Impure halite with silt & glauberit Halite (Tha Gypsum (Tgy ) iassic-Jurassic sedimentary rocks (Js Muddy Mountains SCB 2

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A –23 7 408 Tmc l –52 4 Area in B –70 42 0 Js 6 –1 12 71 –253 SSC4

42 9 Overton Overton 6 Ma Basalt – Tb y 16 3

SSC3

1.0 km 1.0 2.5 km 2.5

Tmcl Arm Basin Arm

Th a 5 km 5

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2 km

Tg y N Salt Cov e

C 1 km N East A′ Meter s –800 –400 0 40 0 movement of the halite. regional deformation rather than diapiric is shown in B. Folding is probably due to from Mannion (1963). Cross-section line section of the Overton Arm area modified and below the halite. (D) Schematic cross gypsiferous siltstone and sandstone above gently west conformable with bedding in stratigraphic layer ~4 m thick and dips labeled “C.” The halite forms a distinct tion of photo is shown in B by black arrow exposure directly north of Salt Cove. Loca - mentary rocks. (C) Looking west at halite Miocene); Js—Triassic and Jurassic sedi - Trs—Red Sandstone Formation (middle Miocene); Tha—halite; Tgy—gypsum; Tmcl—lower Muddy Creek Formation (late Tby—late Miocene (ca. 6 Ma) basalt lavas; older alluvium and alluvial fan sediments; alluvium in recently active washes; Qf— the area penetrate halite. Unit labels: Qa— covered by Lake Mead. Four drill holes in cur in this area, with the largest outcrops Cove area. Small exposures of halite oc - mapping of halite exposures in the Salt fied from Muntean (2013), with our new western part of Overton Arm basin modi ­ shown in B. (B) Geologic map of the of ~720 m. Black box encompasses area Vegas based on a maximum elevation parent polygon shows limits of Lake Las for descriptions of drill holes. Green trans - elevations shown in meters. See Table 4 bottom elevations of hole (red), with all text), top elevations of halite (green), and correspond to collar elevations (black drill holes. Numbers adjacent to drill holes thickest halite lies to the northeast of the est part of the basin, suggesting that the halite lies on the periphery of the deep - stone-siltstone dark yellow. The known is dark pink; basalt is black; and clay green in the drill holes; gypsum- ­ numbers) from Figure 3. Halite is shown as ness of basin fill shown in km with black depth-to-basement contours (i.e., thick - which penetrate halite (green circles), and showing location of six drill holes, five of Google Earth) of the Overton Arm basin Arm basin. (A) Oblique 3D view (from Figure 14. Halite deposits in the Overton anhydrite -

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Five holes drilled by Stauffer Chemical Company in the early 1960s pene- heim et al., 2010). It is bound by the steeply east-dipping Detrital fault on the trated halite in the Overton Arm basin with thicknesses ranging from ~305 m west and west-dipping Blind Goddess fault on the east. Quaternary scarps and to more than 530 m (Fig. 14A; Table 4). Three of the holes bottomed in ­halite a major north-trending gravity and magnetic gradient mark the Detrital fault. (Netherland and Sewell, 1977). The top of the halite ranges from ~400 m (sur- The Detrital fault appears to have accommodated modest west-tilting of the face exposures above sea level) down to 260 m bsl (Table 3). The halite extends ­Detrital basin, as evidenced by gently west-dipping reflectors in a seismic-­ to at least 706 m bsl, as recorded in well SSC2. Documented gypsum-anhydrite­ reflection profile imaging the basin (Faulds, 1999). in the area ranges up to ~100 m in thickness, as observed in outcrops and Several holes drilled by Goldfield Consolidated Mining Company and drill holes. The late Miocene section is, however, deformed into folds and cut Phelps Dodge Corporation from 1957 to 1959 in the northern lobe penetrated by faults in the area. The halite thins and becomes more impure to the south evaporites, including gypsum ~16 km south of Lake Mead and halite farther toward Echo Bay. Overall, the known halite extends over an area exceed- south (Fig. 15). Evaporite tops in these wells range in elevation from 360 to ing ~90 km2. 488 m asl (Peirce, 1974) with the uppermost halite at 338–414 m asl (Tables 3 The thickest known part of the halite (>530 m in the SSC2 drill hole) cor- and 4), similar to elevations of the top of the halite in the Hualapai basin to the responds to the margin of the deepest part of the basin, as marked by the east. Maximum observed thickness of halite in wells within the Detrital basin negative gravity anomaly (Figs. 3, 4, and 14C) and would account for about half is 229 m, but this is a minimum as several wells bottom in halite. Transient of the observed gravity low. However, the thickest halite probably lies ~5 km electromagnetic­ profiles flown across this basin indicate low resistivity val- to the northeast of the drill holes near the center of the gravity low and pre- ues to depths of 400 m, suggesting that evaporite deposits (including halite) sumably in the deepest part of the basin. Based on a gravity inversion with a extend to those depths (Truini et al., 2012). A gravity inversion with a con- constant density contrast of –600 kg/m3, the maximum thickness of halite in stant density contrast of –600 kg/m3 suggests that the maximum thickness of this basin is probably ~1.5 km. Such a thickness is consistent with basement ­halite in this basin is ~800 m. The presence of substantial halite in the northern gravity values and low magnetic values in the area (Fig. 4). Gypsum-anhydrite ­Detrital basin is particularly notable, as this area is not marked by a large nega- is also probably significantly thicker in the deeper part of the basin as com- tive gravity anomaly in contrast to the thick halite deposits in the Hualapai and pared with that observed in outcrops and drill holes to the southwest. Overton Arm basins. Although core samples were not available for analysis, we envision a late The halite in the northern Detrital basin appears to be similar in composition Miocene depositional environment in the Overton Arm basin similar to that of and age to other halite deposits in the region. Although no samples were avail- the Hualapai basin. Proximal alluvial-fan facies graded basinward to a broad able, well logs indicate that the Detrital basin halite largely consists of massive continental playa, where groundwater and/or surface water evaporated in interlocking crystals with minor amounts of intercalated siltstone (logs on file salt pans yielding relatively thick sections of halite and gypsum. In contrast with Arizona Geological Survey), similar to that within the Hualapai basin. The to the Temple Bar basin and southern Grand Wash Trough, however, lacus- Detrital basin halite appears to directly underlie and partially interfinger with trine limestone did not accumulate in the Overton Arm basin, suggesting that gypsum deposits that correlate with the ca. 7.5–6.3 Ma gypsum in the western a long-standing fresh water lake with a relatively stable level did not exist in part of the Temple Bar basin (Virgin basin of Howard et al., 2010). We therefore the area in the late Miocene. The broad extent and appreciable thickness of infer that this halite is similar in age to the >6 Ma halite in the Overton Arm the sandstone-siltstone facies in the Muddy Creek Formation in the Overton basin and also temporally correlative with at least the upper part of the halite Arm basin (Fig. 14B) suggest that clastic input into this basin was significantly in the Hualapai basin. It is also probable that the evaporite sequence in the greater than that in the Hualapai basin, which in turn implies a greater fluvial Detrital basin is similar in age to the ca. 10–6 Ma evaporite-lacustrine section contribution. in the Temple Bar basin, because the two basins essentially merge in the Lake Mead area. The southern lobe of Detrital Valley is underlain by a narrow (~15-km-wide), Detrital Basin north- to northwest-trending composite basin. The southern and eastern parts of this composite basin (Fig. 3) merge southward with the northern depo­ The Detrital basin is a 10–15-km-wide, northerly trending elongated basin center of Sacramento Valley and probably comprise an east-tilted half graben that extends southward from Lake Mead for more than 90 km, with the Black developed in the hanging wall of the west-dipping Cerbat Mountains fault Mountains on the west and White Hills and Cerbat Mountains on the east (Langenheim et al., 2010). The northern part of this southern lobe is probably (Fig. 3). It connects northward with the Temple Bar and Overton Arm basins. a west-tilted half graben developed in the hanging wall of the Mockingbird Two major lobes comprise the Detrital basin and are separated by an inter- Mine fault and/or southern continuation of the Detrital fault zone. Although basinal high ~35–50 km south of Lake Mead, where basin deposits are less basin-fill sediments are generally less than 2 km thick, the southern lobe of than 500 m thick. The northern lobe is a broad relatively shallow graben with Detrital basin may locally contain as much as ~3 km of sediment (Fig. 3), with sediments as much as 1.2 km thick, as evidenced by gravity data (Langen­ a major depocenter (2–3-km-thick basin fill) to the west of Dolan Springs, as

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1.0 km Lake Hualapai 1.0 km PDC3 PDC2 PDC4 PDC1 462/463 502 406/230 White Hills 500 290 271 GCM1A GCM2 GCM5 513 GCM8 543 259 GCM9 414 GCM3 GCM7A 545 224 403 569 GCM4 GCM10 562 543 DB08-06 174 395 575 637 379 401 386 543 252 388 237 338 151 504 574 168 1.5 km GCM6 GCM11 338 2.5 km 228 619 1.0 km 345

Black Mountains N Detrital Basin

1 km

Figure 15. Oblique 3D view (from Google Earth) of the northern Detrital basin showing location of 15 drill holes, eight of which penetrate halite, and depth-to-basement contours from Figure 3. The basin in this area is ~5–10 km wide. Halite is shown as bright green in the drill holes. The known halite lies in a relatively shallow part of the basin. The basin is much deeper ~30 km to the south, where thicker halite may be present (Fig. 3). Numbers adjacent to drill holes correspond to collar elevation (black text), top elevation of halite (green), and bottom elevation of drill hole (red). See Table 4 for descriptions of drill holes. Pink and purple areas to the east of the basin mark outcrops of gypsum and the Hualapai Limestone, respectively.

evidenced by Bouguer and isostatic gravity data (Oppenheimer and Sumner, and eastern parts of the gravity low at depths of ~200–400 m, which were 1981; Langenheim et al., 2010). interpreted as saturated fine-grained basin fill (Truini et al., 2012). An alternate Because drill-hole data indicate appreciable halite to the north in the shal- interpretation for these deposits is evaporites, such as halite. However, thick lower part of the basin, we speculate that the deeper southern lobe may also halite has yet to be documented by drill holes in this area. contain a relatively thick sequence of halite, and thus the depths to basement The late Miocene depositional environment in Detrital basin was similar to shown in Figure 3 may be overestimated. A gravity inversion, assuming that that in the Overton Arm and Hualapai basins. It appears to have been charac- halite comprises all basin fill, suggests that the maximum depth of the basin is terized by proximal alluvial-fan facies grading basinward to a continental playa, ~1.5 km and thus the maximum thickness of the halite is also ~1.5 km. Airborne where groundwater and/or surface water evaporated in salt pans yielding transient electromagnetic profiles indicate low-resistivity values in the central ­halite and gypsum. Available well data suggest relatively minor clastic ­input,

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which implies a minimal fluvial contribution in the central part of the basin. tiary basin lies directly north of the Frenchman Mountain block and contains The apparent lack of carbonate deposition suggests that most of the ­Detrital as much as 2 km of basin-fill sediment. This basin has been referred to as basin did not contain a long-standing fresh-water lake during the late Miocene the Nellis basin, because it contains the eastern part of Nellis Air Force Base but was instead dominated by ephemeral saline lakes and playas. (Castor et al., 2000). The west-northwest elongated basin may have developed in a right step or pull-apart near the eastern end of the Las Vegas Valley shear zone (Fig. 3; Langenheim et al., 2001b). Isostatic gravity data define a 7-km- Boulder, Nellis, and Las Vegas Basins long basin that trends ~N70°W, parallel to the eastern part of the shear zone. If the basin is filled with sediments of the same density of halite, the basin would The Boulder basin in the western Lake Mead region essentially merges be no more than 800 m deep. A pronounced magnetic low is also present here, northwestward with the narrow (5–10-km-wide), west-northwest–trending although most, if not all, of the amplitude of this low results from the juxtaposi- ­Nellis basin, which in turn wraps around the north end of Frenchman Moun- tion of magnetic basement beneath Frenchman Mountain to the south against tain and joins the Las Vegas basin to the west (Fig. 3). The northern part of the nonmagnetic basement to the north across the Las Vegas Valley shear zone. north-northeast–striking Frenchman Mountain fault zone truncates the Nellis The Las Vegas Valley is a broad basin bound by Frenchman Mountain and basin on the west and forms an abrupt eastern margin to a 2–4-km-deep sub- the River Mountains on the east, Spring Mountains on the west, McCullough basin of Las Vegas Valley. The right-lateral Las Vegas Valley shear zone, which Mountains to the south, and a series of northerly trending ranges along the Las has accommodated ~65 km of slip since the middle Miocene (Longwell, 1974; Vegas Valley shear zone to the north (Fig. 3A). The basin contains a complex Wernicke et al., 1988), essentially bounds all three basins on the north. The system of subbasins associated with major normal faults and strands of the main episode of extension in this area occurred ca. 13–9 Ma, as evidenced by right-lateral Las Vegas Valley shear zone, as evidenced by isostatic residual and decreasing tilts up-section within major half grabens (e.g., Duebendorfer and aeromagnetic data (Langenheim et al., 2001b). A system of west-dipping nor- Wallin, 1991; Castor et al., 2000). mal faults and right-lateral faults (e.g., the Frenchman Mountain fault) bounds On the basis of gravity data, the Boulder basin appears to be a broad shal- Las Vegas basin on the east (Matti et al., 1993; Castor et al., 2000). Quaternary low (generally <1-km-thick section of basin-fill sediments) basin that lies directly scarps mark several of these faults. Some of the deeper subbasins (2–4 km) east of Frenchman Mountain and consists primarily of middle to late Miocene occupy the northern and northeastern parts of Las Vegas Valley and are prob- sedimentary strata. Miocene strata in the basin are generally tilted gently to ably associated with right steps in the Las Vegas Valley shear zone (Langen- moderately eastward, with the magnitude of tilting progressively decreasing heim et al., 2001b). Quaternary sediments blanket most of Las Vegas basin and up-section from ~50° in middle Miocene strata to subhorizontal in latest Mio- obscure older basin fill. Seismic-reflection data suggest >2-km-thick sections cene (ca. 6 Ma) units (Castor et al., 2000). Thus, the basin may correspond, of basin-fill sediments in subbasins along the northern margin of the valley. at least in part, to an east-tilted half graben developed in the hanging wall of Taylor et al. (2008) noted that well logs show gypsum units in the Miocene the Saddle Island detachment (Duebendorfer and Wallin, 1991). However, the section within Las Vegas basin, but little information is available on the overall northern extent of the Saddle Island detachment is poorly defined. The north- composition of sediments and facies distribution within the basin. If the basin ern part of Boulder basin is complicated by an east- to northeast-trending fold fill is composed entirely of halite, the inversion using the seismic constraints belt. If once a half graben or series of half grabens, the Boulder basin has since produces basement gravity values that are 10–20 mGal higher than gravity been significantly modified by north-south shortening, possibly associated measurements on nearby basement outcrops. Given the uncertainties of the with the intersection of the left-lateral Lake Mead fault system and right-lat- geophysical data, halite as thick as 300 m could be present in the basin. eral Las Vegas Valley shear zone (see Anderson et al., 1994; Duebendorfer and Widespread late Miocene gypsum and a capping limestone unit mark the Simpson, 1994). Although Boulder basin contains as much as 3 km of middle uppermost part of the Miocene section in the northern part of the Boulder to late Miocene sedimentary rocks, it has a relatively subdued gravity signa- basin and adjacent Nellis basin. There appears to be little break in deposi- ture indicative of a relatively shallow (generally <1 km) basin. This may result tion between gypsum and limestone, as evidenced by concordant dips and from the gentle dips of some of the bounding faults, such as the Saddle Island an apparent lack of major unconformities. Surface exposures and exploratory detachment. Another possible contributing factor arises from higher densities drilling indicate that the late Miocene gypsum covers at least 13 km2 and lo- of these older, generally more consolidated sedimentary rocks as compared cally exceeds 40 m in thickness (Castor and Faulds, 2001). The limestone unit to those assumed in the gravity inversion. Quaternary faults are sparse within is 2–50 m thick and both overlies and interfingers with the upper part of the the Boulder basin, with one Holocene scarp (Meadview Slope fault) along the gypsum. The limestone crops out at elevations ranging from ~595 m to 720 m southeastern margin (Fig. 3; U.S. Geological Survey, 2010). However, late Mio- asl, whereas the gypsum is exposed at elevations ranging from ~450 m to cene strata are locally deformed by minor faults and folds. 660 m asl. Fossil assemblages, textures, and 87Sr/86Sr isotopic values indicate a Relatively thick sections of middle to late Miocene sedimentary rocks and nonmarine origin for the limestone (Castor and Faulds, 2001; Roskowski et al., isostatic gravity data (Langenheim et al., 2001b) indicate that a small late Ter- 2010). On the west, limestone exposures are truncated by northern strands of

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the Frenchman Mountain fault (Fig. 3A). A downthrown correlative of the lime- Gravity and seismic-reflection data indicate that the depression consists stone and gypsum units may be present beneath Quaternary sediments in the of two distinct sedimentary basins that correspond to east-tilted half gra- northeastern part of the Las Vegas basin but have yet to be documented by bens. The western half graben has been referred to as the Mormon basin and drill holes. the eastern graben as the Mesquite basin (Bohannon et al., 1993). West-dip- Geochronologic data indicate that the limestone and gypsum in the Nellis ping listric normal fault systems bound the half grabens on the east. Late and Boulder basins are late Miocene (ca. 8.5–5.5 Ma) and temporally correlate Miocene–early Pliocene sediments locally cover these faults, but the Pied- with the regionally extensive Muddy Creek Formation. For example, 40Ar/39Ar mont fault cuts Quaternary alluvium on the east side of the Mesquite basin and K/Ar data from previous studies in a variety of units indicate that strata be- (Bohannon et al., 1993), and Quaternary scarps have been observed along neath the gypsum range from ca. 11.6 to 8.5 Ma (e.g., Bohannon, 1984; Feuer- much of the southeastern margin of the basin (Fig. 3; U.S. Geological Survey, bach et al., 1991; Harlan et al., 1998; Castor et al., 2000). Moreover, a tephra 2010). A pronounced negative gravity anomaly (~60–70 mGal) suggests that intercalated in redbeds ~20 m beneath the limestone geochemically correlates basin-fill sediment in the northern part of the Mesquite basin reaches thick- with the 5.62 ± 0.11 Ma tuff of Wolverine Creek (e.g., Perkins et al., 1998; Peder­ nesses of 8–10 km (Langenheim et al., 2001a), nearly double that of other son et al., 2001), as initially described by Castor and Faulds (2001) but with deep basins in the region. The Mormon basin is as much as 4.5–5 km deep, analytical data reported herein (Table 2). This datum provides a maximum age consistent with gravity data and seismic-reflection interpretations (Bohan- for the overlying limestone. non et al., 1993). 87Sr/86Sr values from the late Miocene limestone average 0.7108, similar to The upper part of the basin fill in the Virgin River depression is dominated those of the Bouse Formation and substantially lower than any values obtained by the late Miocene to early Pliocene Muddy Creek Formation composed pri- from the Hualapai Limestone to the east. Roskowski et al. (2010) interpreted the marily of fine- to medium-grained fluvial sandstone with subordinate interca- limestone in the Nellis basin as accumulating in a paleolake fed by Colorado lated conglomerate and siltstone, some of which is lacustrine (e.g., Williams River water and representing the upstream end of the Bouse lake system. This et al., 1997; Forrester, 2009; Dickinson et al., 2014). The age of the Muddy Creek paleolake was reported to have an elevation up to ~675 m and is referred to as Formation in this area is poorly constrained but probably ranges from ca. 10 Lake Las Vegas after Spencer et al. (2013), but we note that the Nellis limestone to 4 Ma (see summary in Dickinson et al., 2014). Seismic-reflection and well and associated marl crop out at elevations as high as 720–747 m asl (Fig. 3B). data indicate 1–2 km of late Miocene to early Pliocene sediments of the Muddy The late Miocene depositional environment of the presumably connected Creek Formation, including gypsiferous units, in the Virgin River depression Boulder, Nellis, and Las Vegas basins resembled that of other basins in the (Bohannon et al., 1993). Only the upper ~250 m of the Muddy Creek Formation Lake Mead region but with some key differences. Similarities include mar- are exposed in the area (Forrester, 2009). However, the Mobil Virgin River 1A ginal alluvial fan complexes and a continental playa setting, where gypsum well (Fig. 3), the only deep drill hole in the depression, penetrated 884 m of the accumulated. However, significant gypsum deposition probably did not begin Muddy Creek Formation in the Mormon basin. The upper 671 m in this well until ca. 8.5 Ma, and limestone deposition initiated shortly after ca. 5.6 Ma, consist primarily of siltstone, whereas abundant gypsum was observed from both somewhat later than in basins to the east. Details are lacking from the 671 to 884 m depth (90–303 m below sea level in elevation; Bohannon et al., Las Vegas­ ­basin, but late Miocene deposits in the Nellis basin are truncated by 1993). The lacustrine sediments (gypsum and siltstone) are more abundant faults bounding Las Vegas basin (Fig. 3), suggesting that at least some of the to the south in the Mormon basin, but these beds are subordinate in surface sedimentary fill within the northeastern Las Vegas basin correlates with that ex- outcrops (Muntean, 2012). Detrital zircon data preclude paleo–Colorado River posed in the Nellis basin. The lacustrine limestone within the Nellis basin sug- sediment in at least the upper part of the Muddy Creek Formation in the Virgin gests that the corresponding lake had an outlet, possibly to the west or south. River depression (Dickinson et al., 2014). Thick halite and other evaporites have not been documented in the Virgin Virgin River Depression River depression, but well control and resistivity information are lacking in deeper parts of the basins. If the Mesquite basin is filled with sediments of the The Virgin River depression is a large northeast-trending basin that covers same density as halite, the thickest parts of the sedimentary section would be >1500 km2 in southeastern Nevada, northwestern Arizona, and the southwest ~5 km, more compatible with other basins in the region. Such a great thick- corner of Utah (Fig. 3A). The tectonic development and overall architecture of ness of halite, however, would result in basement gravity values under the this basin have been assessed through geologic and geophysical investigations, basin that are 20–30 mGal higher than any measured on basement outcrops including seismic-reflection and gravity surveys (Carpenter, 1989;Bohannon ­ surrounding the depression. Although this is possible, the correlation of un- et al., 1993; Langenheim et al., 2001a). The basin is bound on the west by the usually high basement gravity values with the basin is suspicious. Halite as Mormon Mountains and northern Muddy Mountains and on the east by thick as 1 km, however, is possible, given the effects on the basement gravity the ­Virgin and Beaver Dam Mountains. The Virgin River, a major tributary of the and constraints from seismic-reflection data, as well as considering the areal Colorado River, flows through the central part of the depression (Fig. 3). extent and size of the Mesquite basin.

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Although deep well data are lacking, the late Miocene depositional envi- and Sharp, 1998; Faulds et al., 2010), as well as the correlative basalt flows at ronment of the Virgin River depression was probably characterized by a conti­ Snap Point and Nevershine Mesa adorning the Grand Wash Cliffs (Figs. 3, 11, nental playa, significant fluvial input in some areas, and surrounding alluvial and 12). In contrast, tilt fanning brackets major extension in the western Lake fans. On the basis of detrital zircon data from the upper parts of the Muddy Mead region to between ca. 13 and 9 Ma (Duebendorfer and Simpson, 1994; Creek Formation, fluvial input probably emanated from the northeast along an Castor et al., 2000). Latest Miocene to early Pliocene strata in the central to incipient Virgin River (Dickinson et al., 2014). Siltstone, gypsum, and possibly western Lake Mead region are also locally tilted and faulted, as exemplified in some halite accumulated in the playa. The upward progression from gypsum both the Overton Arm and Nellis basins (Figs. 3 and 14B; Beard et al., 2007). to siltstone noted in the Mobil Virgin River 1A well may record a trend toward These relations indicate that the extensive late Miocene lacustrine and an increase in water input in the late Miocene similar to many other basins in evaporite deposits in the Lake Mead region are primarily late synexten- the region. However, it is noteworthy that late Miocene carbonate has not been sional and largely postdate the major episode of extension. For example, the documented in the Virgin River depression, suggesting lack of a long-standing, Hualapai Limestone in the eastern to central Lake Mead region is constrained relatively stable fresh-water lake. to ca. 12–7.5 Ma in the Grand Wash Trough and ca. 10 to <6 Ma in the Temple Bar area (Figs. 3 and 16; Table 3). The thick halite sequence in the Hualapai DISCUSSION basin is likely ca. 12 to <5.6 Ma, similar in age to the Hualapai Limestone. Thus, these deposits postdate the ca. 16–13 Ma episode of major extension. Simi­ Our analysis of the distribution, age, and origin of late Miocene lacustrine larly, generally subhorizontal late Miocene gypsum and limestone deposits in and evaporative deposits in major basins of the Lake Mead region provides the western Lake Mead region (Boulder and Nellis basins) are bracketed be- insights into the paleogeographic evolution of the region, particularly with re- tween ca. 8.5 and 5 Ma, which immediately postdates the 13–9 Ma period of spect to the relationships of such deposits to major episodes of extension and major extension. The general younging trend to the west of the late Miocene development of the Colorado River. Large-magnitude middle to late Miocene lacustrine and evaporite deposits in the Lake Mead region is consistent with extension generated a vast system of half grabens and grabens within which both the apparent westward progression of extension and proposed down- multiple playas and lakes developed immediately prior to arrival of the Colo- stream filling and spilling of lakes inferred for the evolution of the lower Colo- rado River. Unusually thick nonmarine halite sequences accumulated in some rado River (e.g., House et al., 2008; Spencer et al., 2008). of these basins. In this section, we review (1) the impacts of Miocene extension The late synextensional timing for extensive lacustrine and evaporite depo- on the late Miocene paleogeography, (2) depositional model immediately prior sition is also consistent with the structural and paleogeographic evolution to development of the Colorado River, (3) potential economic implications of of an actively extending region. Significant accommodation space must be the unusually thick halite deposits, and (4) possible regional and global ana- generated in half grabens to host such deposits (Fig. 1A), and thus it would logues to the late Miocene setting in the Lake Mead region. Representative be unlikely to form thick sequences during the early stages of extension. Fur- middle Miocene to Pliocene stratigraphic columns, as well as the approximate thermore, significant drainage evolution is needed to develop broad enough elevations of the major lacustrine and evaporite units in major basins in the catchments to supply sufficient groundwater and/or surface water over pro- Lake Mead region, are shown in Figure 16. longed time periods to accumulate appreciable sequences of lacustrine and evaporite deposits. High rates of extension would promote continued isolation Paleogeographic Implications of Middle to Late Miocene Extension of individual basins and hinder regional drainage and groundwater flow-path development, especially in arid regions. Once extension wanes, basins can As demonstrated by tilt fanning in the dated deposits in southern Grand more easily fill and drainage networks can progressively integrate originally Wash Trough and similar relationships in nearby areas (e.g., Faulds et al., 1992, isolated basins into broader regional drainage systems. In addition, lower 2010; Duebendorfer and Simpson, 1994; Beard, 1996; Duebendorfer and Sharp, overall topographic relief during the waning stages of extension will reduce 1998), major extension generated basins in the region from ca. 16 to 9 Ma steep-gradient fluvial environments and thus the dominance of alluvial fan (Fig. 9). Late Miocene to early Pliocene strata (ca. 12–5 Ma) are locally faulted ­facies while inducing development of broad, low-gradient plains, shallow and tilted (<5°) in the eastern Lake Mead region (Fig. 8; Wallace et al., 2005; ­water bodies, and associated fine-grained deposits. This, in turn, would pro- Howard et al., 2010), but the primary episode of extension and basin devel- mote a high surface-to-volume ratio for late synextensional water bodies and opment occurred from ca. 16–13 Ma in the Grand Wash Trough and Hualapai thus greater evaporative surfaces and resulting evaporite deposits. Broader basin. The general physiography of the eastern Lake Mead region, including water bodies would also increase the likelihood of groundwater connections the abrupt western margin of the Colorado Plateau, series of deep basins in through interbasinal sills and possibly some mountain blocks, aided in both the hanging wall of the Grand Wash fault zone, and Grand Wash Cliffs, were cases by preexisting faults. Additionally, continued mild extension and asso- established by ca. 9 Ma, as evidenced by the age of basin-fill deposits in the ciated relative subsidence of basins would be critical in preserving thick halite southern Grand Wash Trough (Figs. 8 and 9) and White Hills (Duebendorfer by allowing for its subsidence below the stagnant point in the phreatic zone.

GEOSPHERE | Volume 12 | Number 3 Faulds et al. | Late Miocene lacustrine and nonmarine evaporite deposits, Lake Mead region Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/12/3/721/3335088/721.pdf 749 by guest on 24 September 2021 on 24 September 2021 by guest Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/12/3/721/3335088/721.pdf Research Paper basin (eastern part). N and S for Detrital basin and Grand Wash Trough represent northern and southern parts of these basins, respectively. boxes denote thickness of basin fill based on gravity data. For Virgin River depression, W is Mormon basin (western part) and E is Mesquite elevations and relative thicknesses of documented (solid lines) and inferred (dashed lines) late Miocene lacustrine and evaporite deposits. Gray Figure 16. (A) Generalized middle Miocene to Pliocene stratigraphic columns for major Neogene basins in the Lake Mead region. (B) Approximate B A 4 km –2. 0 –1. 0 –3. 0 Elevation (km) Million Years 14 12 Dashed – Inferred Solid – Documented 10 1. 0 0. 0 2 8 4 6 (Basin Depth) Basin-Fill Lake Las V Las Ve Las Ve ? Representative Stratigraphic Columns, Major Basins in Lake Mead Region ? 4 km ?? ?? ?? Approximate Elevations of Late Miocene Lacustrine-Evaporite Deposits Lake Mead ga s ga s ? Thicknes s egas Leve ? <2 km Nellis Nellis ?? ?? l Boulder Boulder Recent Sediment <1 km Latest Miocene– Basalt Flow Vi Vi Colorado River rgin River rgin River E: 8 km W: s ?? ?? 5 km s Overton Overton 4 km Bouse Limeston Sediment s Colorado River Arm Arm Arrives Detrital N: <2 km Detrital S: 3 km Lake Hualapai Level ?? e Te Te <1.5 km mple Bar mple Bar Anhydrit e Halite Gypsum - Hualapai Hualapai 4 km ?? Lake Grand Wa Lake Mead Grand Wa Grand Wa Middle to Late Miocene Hualapai Limestone Sediments/V N: >4 km S: <1 km sh Level sh sh olcanic

4 14 12 10 8 6 2 –2. 0 1. 0 –1. 0 0. 0 –3. 0

ears Y Million Million s (km) Elevation

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In addition to rates of extension, climatic conditions are crucial in the drain- ble that some halite may also occupy the axial basins (i.e., proximal to the age development of any extended region. Our inferences on the relations be- Colorado River), but significant gravity lows do not coincide with these areas tween tectonism and drainage development in the Lake Mead region may best (Fig. 4), suggesting that thick halite is unlikely. Moreover, large-magnitude and apply to a region that experienced (1) a major pulse of extension with relatively areally extensive gravity lows imply that thick halite may lie in the subsurface high strain rates followed by significantly reduced rates in the waning stages of several other satellite basins to the Colorado River in the region, including and (2) a relatively persistent arid climate. The latter appears to have char- the northern Grand Wash, southern Detrital, northeastern Las Vegas, Mormon acterized the late Miocene in the southwestern United States (e.g., Chapin, Mesa, and Mesquite basins (Fig. 3), in all cases, tens of kilometers away from 2008). In fact, the mean annual temperature in the late Miocene may have the axial drainage and associated late Miocene limestone deposits. been 3–8 °C warmer than the contemporary climate, as evidenced by carbon- Although some faulting has occurred in the region since the late Miocene, ate clumped-isotope thermometry from the Bouse Formation and Hualapai broadly similar elevations (Fig. 16B) of limestone and evaporite (gypsum and Limestone (Huntington et al., 2010; Wernicke, 2011). halite) deposits and similar Sr isotopic values in the Hualapai Limestone (Ros- kowski et al., 2010; Spencer et al., 2013) in various basins suggest an intercon- nected hydrologic system in the eastern to central Lake Mead region during at Late Miocene–Early Pliocene Depositional Model least part of the late Miocene. Limestone elevations range from ~500–912 m asl in the Grand Wash Trough and 367–720 m asl in the Temple Bar area (Table 3). The spatial distribution of late Miocene lacustrine limestone and evaporite Gypsum elevations range from ~370–750 m asl in the Grand Wash Trough and deposits in the Lake Mead region reflects a broad depositional system of inter- ~390–440 m asl in the Temple Bar and Detrital basins. We note that down-to- connected lakes and continental playas (Figs. 1, 3, and 17). This system initi- the-west displacement along the Wheeler Ridge and Lost Basin Range faults ated at ca. 12–11 Ma in the Grand Wash Trough and continued to ca. 5.6–5.3 Ma since the late Miocene is at least partly responsible for the higher elevations in much of the Lake Mead region, including the Hualapai basin in the east, in the Grand Wash Trough. Halite ranges from less than ~2000 m bsl to 418 m Virgin-Detrital trough in the central part of the region, and Boulder and Nellis­ asl in the Hualapai basin, less than ~170–414 m asl in the Detrital basin, and basins in the west. This system of lakes and playas immediately predates less than 706 m bsl to 163 m asl in the Overton Arm basin. These elevations ­arrival of the Colorado River (Fig. 16).The late Miocene–early Pliocene lacus- demonstrate the progression from gypsum to limestone up-section in several trine deposits (e.g., Hualapai Limestone and Bouse Formation) and associated of the basins, as well as the lower elevations of halite in some of the satellite lakes have long been recognized in the region (e.g., Longwell, 1936; Lucchitta, basins, compared to limestone in axial basins. Coeval deposition of limestone 1966; Wallace et al., 2005; House et al., 2008; Spencer et al., 2008, 2013), but our in axial basins and halite in satellite basins suggests some groundwater and/or review of the regional extent and temporal relationships of evaporite deposits surface water input from standing lakes to adjacent playas. The lower eleva- (e.g., halite and gypsum) documents that extensive playas were developing in tions of halite in the Hualapai and Overton Arm basins further imply that some concert with the lakes. of the satellite basins were subsiding more rapidly and essentially deeper than The most extensive late Miocene limestone and gypsum deposits in the axial basins, as supported by depth-to-basement calculations (Fig. 3). How- Lake Mead region crop out within ~25 km of the subsequently developed Colo­ ever, continued tectonism may have induced continued subsidence since the rado River. For example, most of the Hualapai Limestone in the Grand Wash late Miocene in some areas, such as the Overton Arm basin. Trough and Temple Bar basin lies within ~10–25 km of the present course of the Elevations of limestone (~595–675 m) and gypsum (~450–595 m) farther river (Fig. 3). The most extensive gypsum deposits crop out in the northern part west in the Nellis basin are also broadly similar with those of limestone and of the southern Grand Wash Trough and along or directly south of the present gypsum to the east (Fig. 16B; Table 3), but Sr isotopic values of the Nellis lime- course of the Colorado River in the central Temple Bar and northern Detrital stone differ significantly from those from the Hualapai Limestone (Roskowski basins. In the western Lake Mead region, exposures of gypsum and limestone et al., 2010), suggesting that the hydrologic system in the western Lake Mead in the Boulder and Nellis basins are concentrated ~10–25 km northwest of the region differed from that to the east. Nonetheless, the late Miocene strati- current path of the Colorado River, with the limestone extending westward to graphic succession from gypsum to limestone in the Boulder and Nellis basins the faulted eastern margin of the deep northeastern lobe of the Las Vegas basin. is similar to that in basins to the east, although thick halite has not been docu- Notably, late Miocene limestone consistently overlies gypsum throughout the mented in the Boulder, Nellis, and Las Vegas basins. Lake Mead region, albeit the lower part of the limestone locally interfingers These spatial and temporal patterns of lacustrine and evaporite depo- with the upper section of gypsum, and interbeds of both occur in the other. sition suggest that the late Miocene to earliest Pliocene paleogeogra- In contrast to the limestone and gypsum, thick sequences of halite have phy of the Lake Mead region was initially characterized by a system of been documented in basins ~20–50 km to both the south and north of the axial playas, within which silts and gypsum were deposited, with halite­ Colorado River, including the Hualapai, northern Detrital, and Overton Arm accumulating in broad salt pans within satellite basins to the north basins (Figs. 3 and 16A) but not proximal to the Colorado River. It is possi- and south, at least in the eastern to central Lake Mead region (Fig. 17).

GEOSPHERE | Volume 12 | Number 3 Faulds et al. | Late Miocene lacustrine and nonmarine evaporite deposits, Lake Mead region Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/12/3/721/3335088/721.pdf 751 by guest on 24 September 2021 Research Paper

A N. Grand Wash Trough Colorado Plateau ~12–7.5 Ma Lake Grand Wash Hualapai Basin

Paleo-Damso Dam Damss S. Grand Wash Trough

Figure 17. Google Earth images showing L ake Mead the inferred sequential development of late Miocene lakes and regional drainage in the Lake Mead region, including major exposures of late Miocene limestone and B ~10–5.6 Ma gypsum, as well as deposits of known or inferred subsurface halite. Hualapai Lime- Mesquite Basin stone is shown in purple, late Miocene Overton Arm Basin gypsum in light pink, and Nellis limestone in dark pink. Possible paleospring loca- ? tions and basin depths (same contours Lake Hualapai as in Fig. 3) are also shown. Scale is the same in each panel. The present-day ? Detrital Basin Lake Mead is the black body in the cen- Paleo-Dams tral part of the images, as labeled in A Temple Bar Basin Lake Mea and B. See text for further discussion. Paleo-Dam (A) Lake Grand Wash occupied the Grand d Wash Trough and neighboring basins from ca. 12 to 7.5 Ma, with probable terminal sinks in the Hualapai basin and northern Grand Wash Trough. Some water in Lake Grand Wash may have also drained west- ward into Temple Bar and Detrital basins. (B) Lake Hualapai developed and occu- C GrandG Canyon ~5.6–5.0 Ma pied at least the Temple Bar and northern Detrital basins beginning as early as ca. 10 Ma and continuing to at least 5.6 Ma. Lake Hualapai may have also drained into deeper basins to both the south and north (e.g., Overton Arm, Mormon, and Mes- quite to the north and Hualapai and south- ern Detrital to the south). (C) Lake Las ­Vegas developed shortly after ca. 5.6 Ma and may have persisted to ca. 5.0 Ma, Black Canyon resulting in deposition of the Nellis lime- stone (dark pink). Although post-5 Ma Lake Las faulting has affected the area, elevations Paleo-Dams Vegas of the Nellis limestone suggest that this lake had maximum levels relatively similar

N to that of Lake Hualapai. 5 km

in Inferred Paleo-Spring Location s Basin ke Las Vegas Las Vega La Known Thick Inferred Thick Halite Halite

Inferred Surface Inferred Ground Water Flow Path Water Flow Path

GEOSPHERE | Volume 12 | Number 3 Faulds et al. | Late Miocene lacustrine and nonmarine evaporite deposits, Lake Mead region Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/12/3/721/3335088/721.pdf 752 by guest on 24 September 2021 Research Paper

These playas were probably largely fed by groundwater discharge, as evi- face drainage through the late Miocene. These axial basins were consistently or denced by textures within the halite, interbedded muds, and apparent scar- at least episodically connected, either through surface spillways or groundwater,­ city of fluvial sediments. To the north, however, greater fluvial input appears to broad salt pans in continental playas in satellite basins (Figs. 17 and 18). The to have characterized the Virgin River depression. Nonetheless, the influx of interconnectedness of the axial basins in the eastern to central Lake Mead region groundwater and/or surface water appears to have increased region-wide is supported by relatively similar Sr isotopic values in the scattered exposures during the late Miocene, because the initial axial playas gave way to lakes, of the Hualapai Limestone (Roskowski et al., 2010; Spencer et al., 2013). Some of marshes, and expanding fluvial systems, as marked by the Hualapai Lime- the satellite basins with extensive salt pans (e.g., northern Detrital basin) appear stone in the eastern to central Lake Mead region, fluvial-dominated sediments to have been directly connected to axial basins (e.g., Temple Bar) and essen- in the upper part of the Muddy Creek Formation in the Virgin River depression, tially represent the more distal parts of large composite basins. In other cases, and Nellis limestone in the western part of the region. however, relatively low topographic sills likely separated the satellite basins On the basis of the lateral distribution, elevation, age, and geochemistry of from the axial basins. For example, a low topographic sill composed primarily the lacustrine carbonate deposits, three sequential late Miocene paleolakes have of alluvial fan deposits probably separated the Hualapai basin from axial basins previously been defined in the Lake Mead area: (1) Lake Grand Wash, (2) Lake and lakes to the north. Permeable alluvial fan sediments may have permitted Hualapai, and (3) Lake Las Vegas (Spencer et al., 2013; Pearthree and House, significant groundwater influx from both Lake Grand Wash and Lake Hualapai 2014; Figs. 16 and 17). Lake Grand Wash (ca. 12–7.5 Ma, 912 m) inundated the into the Hualapai basin, facilitating deposition of the unusually thick sequence Grand Wash Trough and Hualapai basin and was contained by inferred paleo- of halite. Our hypothesized interconnectedness between the axial and satellite dams between Gregg Basin and Temple Bar on the west and between the Grand basins (e.g., Hualapai, Overton Arm, and others) could be tested through iso- Wash Trough and Virgin River depression on the northwest. Deposits of the topic analysis of late Miocene gypsum and anhydrite deposits exposed in the Hualapai Limestone reach a maximum elevation of 912 m in the Grand Wash associated basins and available from core in the Hualapai basin. Trough (Wallace et al., 2005; Beard et al., 2007). Lake Hualapai (ca. 10–5.6 Ma, The Lake Mead region was integrated into the Colorado River between ca. 720 m) covered the Temple Bar area and backed up into northern Detrital Valley 5.3 and 4.9 Ma, as evidenced by the ca. 6 Ma upper Hualapai Limestone in and Grand Wash Trough but probably did not occupy the Hualapai basin. De- the Temple Bar basin, ca. 6 Ma uppermost halite in the Overton Arm basin, posits of the Hualapai Limestone reach a maximum elevation of 720 m between <5.6 Ma limestone in the Nellis basin, <5.6 Ma uppermost halite in the Hualapai Detrital Valley and the Temple Bar area and provide the basis for this lake level basin, and 4.5 Ma Sandy Point basalt intercalated with Colorado River ­gravels elevation (e.g., Beard et al., 2007; Felger and Beard, 2010). Based on the western in the Grand Wash Trough (Fig. 16). The Colorado River debouched into the and northwestern extent of the Hualapai Limestone, Lake Hualapai may have Blythe area by ca. 4.9 Ma (Spencer et al., 2013) and possibly into the Gulf of been dammed ~10–15 km west and north of Temple Bar (Fig. 17). Notably, the California as early as ca. 5.3 Ma (Dorsey et al., 2007). Because there is no evi­ more than 275 m of displacement along the Wheeler Ridge fault since the late dence that any of the lacustrine and evaporite deposits in the Lake Mead re- Miocene can account for the difference in elevation between Lake Hualapai and gion are any younger than ca. 5.3–4.9 Ma, it follows that the Lake Mead region Lake Grand Wash. Identical in upper elevation to Lake Hualapai, Lake Las Vegas was integrated into the Colorado River between ca. 5.3 and 4.9 Ma. (ca. 5.6–5 Ma, 720 m) covered the entire Lake Mead area up to at least ~720 m, The approximate coincidence in the timing of cessation of halite deposi- as defined by the upper elevation of the Nellis Limestone. Our estimate for the tion in satellite basins (shortly after 5.6 Ma), termination of lacustrine depo­ upper elevation of Lake Las Vegas at 720 m is significantly higher than the 650 m sition in axial basins, and regional development of a through-going Colorado level surmised in previous analyses (e.g., Spencer et al., 2013; Pearthree and River suggests that deposition of both the late Miocene lacustrine deposits House, 2014). Lake Las Vegas was probably dammed in the Black Canyon area and evaporites in the Lake Mead region was directly linked to immediate pre- (Fig. 17), where it eventually spilled over to form paleo–Lake Mohave and initiate cursors of the Colorado River. This begs the question as to whether a proto– deposition of the Bouse formation (e.g., Pearthree and House, 2014). Colorado River fed the lakes and playas, and if so, from where did it originate. The isotopic geochemistry of the Hualapai and Nellis limestone deposits However, a lack of fluvial sediments either intercalated with or underlying the suggests hydrochemically distinct basins persisted in the western versus the limestone and evaporite deposits (e.g., Lucchitta, 1989), as well as the isotopic eastern-central Lake Mead region up to ca. 6 Ma. The Grand Wash and Hualapai geochemistry of the Hualapai Limestone (Roskowski et al., 2010; Crossey et al., lakes were probably initially fed by springs with an increasing influx of meteoric 2015), appear to preclude a major river system emptying into the Lake Mead water from ca. 8–6 Ma (Crossey et al., 2002, 2006, 2015), which may have been a region prior to ca. 5.6 Ma. The interpretation of Lovejoy (1980) that an early direct precursor to the Colorado River (Faulds et al., 2001c). As the axial basins Colorado River entered into the Grand Wash Trough, turned sharply south, transitioned to more permanent lakes and wetlands, halite deposition appears and flowed into the Hualapai basin, thus producing the thick halite, is not sup- to have continued in at least some of the satellite basins (Hualapai and Overton ported by the Sr isotopic values in the Hualapai Limestone in the Grand Wash Arm basins), persisting to ca. 6 to <5.6 Ma. Thus, we envision a system of axial Trough and lack of extensive fluvial and deltaic deposits in the Grand Wash playas progressively replaced by lakes and becoming more integrated by sur- Trough and Hualapai basin.

GEOSPHERE | Volume 12 | Number 3 Faulds et al. | Late Miocene lacustrine and nonmarine evaporite deposits, Lake Mead region Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/12/3/721/3335088/721.pdf 753 by guest on 24 September 2021 on 24 September 2021 by guest Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/12/3/721/3335088/721.pdf Research Paper B –1 A C Lake Hualapai (red); Las P 0 1 2 North olygon outlines – Lake Grand –1 A We 2 0 Elevation (km1 ) B st Salt Pa n Las Ve N. Grand (terminal playa) Evaporation

gas Basin

s ga Ve Las

n Basi In 50 km Te B (gypsum, lesser halite) Wa (carbonate deposition) Through- owing lakes termediate lakes/playa

N rminal lakes/pla (halite sh Tr Lake Las Ve , gypsum) ough V

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flow

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asin

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sh Tr Arizon a Spill poin ough T

N. Detrital Basi Detrital N. Surface flow emple Bar Basi

Temple Bar-

Lake Hualapai Basin Detrital S. Ov t er ton Mormon Arm Basin Groundwate Basin flow su rf Lake Grand su rf Undened – Groundwater (dotted); su rf Lake Las V su rf Lake Hualapai – Groundwater (dotted);

n ace ow (dashed) n ace ow (dashed) ace ow (dashed) ace ow (dashed) r (terminal playa ) Hualapai Basi n s egas – Groundwater (dotted); Salt Pa n

Mesquite W Evaporatio

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B′ W i a Arizon Wa sh h Salt Pa n Tr Utah oug Nor sh h thern Gran A r d sh A′ Wash T rough a Sout h

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1 2 –1 0 ) Elevation (km Elevation is the same as that in Figure 16. major rock types shown in cross sections Colorado River water. Color scheme for Vegas resembles the Bouse Formation and whereas limestone deposited in Lake Las for Lakes Grand Wash and Hualapai, indicate primarily groundwater sources 87 carbonate deposition in these axial basins. minor halite) were followed by lacustrine quences (mainly gypsum with possibly the older lake basins. Early evaporite se - the lake that presumably backed up into lines corresponding to that portion of ing the central part of the lake and dashed extent of each lake, with solid lines denot - lines above the cross section show lateral lakes are shown in cross section. ­ from east to west. Maximum elevations of into through-flowing lakes sequentially containing terminal playas that evolved west cross section of axial basins, each gypsum-anhydrite). (C) Schematic east- quences accumulated (mainly halite and Hualapai basin, where thick evaporite se - of the northern Grand Wash Trough and trating into the deeper terminal basins groundwater probably spilling and infil ­ a through-flowing lake, with surface and southern Grand Wash Trough evolved into early intermediate lake and/or playa in the prised Lake Grand Wash ca. 12–7.5 Ma. An Hualapai basin), which collectively com - basins (northern Grand Wash Trough and Wash Trough) and neighboring deeper va ­ south cross section showing relative ele ­ east (see Fig. 17). (B) Diagrammatic north- which are time transgressive from west to ing to individual through-flowing lakes, water flow paths are color coded accord - tion model. Inferred drainage and ground- terminal playas overlain on digital eleva - through-flowing lakes and interconnected region. (A) Map view of region showing Miocene drainage networks in Lake Mead Figure 18. Major elements of evolving late Sr/ tions of axial basin (southern Grand 86 Sr ratios in the Hualapai Limestone Colored

GEOSPHERE | Volume 12 | Number 3 Faulds et al. | Late Miocene lacustrine and nonmarine evaporite deposits, Lake Mead region 754 Research Paper

The late Miocene lakes and playas in the Lake Mead region were probably Nellis limestone suggest that this lake had maximum levels similar to that of fed instead by a complex system of springs and lesser amounts of surface Lake Hualapai, although late Miocene to recent tectonism, especially in the ­water (e.g., Crossey et al., 2015) derived primarily from the Colorado Plateau. Frenchman Mountain-Nellis basin area, has probably altered these elevations. The Colorado Plateau is implicated as the main source by the spatial associa- Nonetheless, it is possible that Lake Las Vegas backed up into the same region tion of the thickest and apparently oldest late Miocene limestone and halite in as Lake Hualapai, leading to continued groundwater sapping and/or spillover basins (Grand Wash Trough and Hualapai basin) directly adjacent to its west- into several satellite basins, such as the Overton Arm and Hualapai basins. ern margin. The springs were probably associated with extensive and complex It seems likely that the deep satellite basins distributed tens of kilometers carbonate aquifers in the southwestern Colorado Plateau (e.g., Huntoon, 1996, to the north and south of the present course of the Colorado River, such as 2000; Crossey et al., 2006, 2011; Hill and Polyak, 2014). This further implies that the Hualapai, northern Grand Wash, Mesquite, and Overton Arm basins (Fig. Permian redbeds on the Colorado Plateau were a primary source of Na and 17), had to fill to a certain threshold before the drainage could expand signifi- Cl for the late Miocene halite deposits. The proposed link between evaporite cantly and spillover into the next set of downstream catchments. For example, deposits in the Lake Mead region with groundwater derived from the Colorado the relatively deep (~4 km) Hualapai basin and northern Grand Wash Trough Plateau could also be tested through isotopic analysis (e.g., Sr) of late Miocene probably served as initial terminal sinks for an incipient drainage system that gypsum and anhydrite deposits. fed Lake Grand Wash in the eastern Lake Mead region. Significant filling of The transition from evaporitic to normal lacustrine deposition in the axial these basins probably occurred prior to appreciable spillover into the Tem- basins of the Lake Mead region indicates an increasing input of fresh water ple Bar basin. Similarly, it is likely that the deeper basins in the Virgin River from this groundwater system and possibly some surface water during the depression and Virgin-Detrital trough served as sinks farther west and north. late Miocene. This may reflect a combination of a progressive increase in the Both Lake Hualapai and Lake Las Vegas may have initially drained to the north size of the catchment, resulting in part from waning tectonism, and possibly into the Overton Arm and Mesquite basins, as well as southward, possibly evolving climatic effects. As axial lakes filled with fresh water, the regional through groundwater sapping, into the Hualapai and southern Detrital basins. groundwater table rose, and a prolonged period of groundwater discharge and However, such interpretations should be considered tentative, with available evaporation ensued in satellite basins (Figs. 1, 17, and 18), ultimately produc- data, due to the vagaries of erosion and variable local uplift and subsidence ing some of the thickest known nonmarine halite deposits. resulting from subsequent tectonism and sediment loading. Lake Grand Wash was the first in the series of lakes to develop in the Grand It is noteworthy that the Nellis limestone and underlying gypsum ­deposits Wash Trough and the neighboring Gregg and Hualapai basins from ca. 12 to in the western Lake Mead region lie well to the northwest of the present course 7.5 Ma (Fig. 17A). The drainage may have emanated from a series of springs in of the Colorado River, with no intervening bedrock barriers to the deep north- the southern Grand Wash Trough, where water ponded in lakes and marshes eastern lobe of Las Vegas basin (Fig. 3). It is therefore probable that Lake Las leading to accumulation of the Hualapai Limestone. The lake in the southern Vegas connected and possibly emptied westward into the Las Vegas basin (Fig. Grand Wash Trough probably drained to both the north and south into conti­ 17). Interestingly, the Nellis limestone has similar Sr isotopic ratios to that of nental playas in the deeper northern Grand Wash Trough and Hualapai ba- the Bouse Formation, suggesting that it was associated with an incipient Colo­ sin, respectively, within which thick halite and/or gypsum accumulated. Some rado River rather than with the Hualapai Limestone to the east (Roskowski of the water in Lake Grand Wash may have also drained westward into Lake et al., 2010; Spencer et al., 2013). We therefore suggest that the incipient Colo- Hualapai in the Temple Bar and Detrital basins. rado River in the Lake Mead region briefly emptied into Las Vegas basin prior Lake Hualapai ponded in at least the Temple Bar and northern Detrital ba- to spilling to the south through Black Canyon and becoming integrated with sins (Fig. 17B) beginning as early as ca. 10 Ma and continuing to at least 5.6 Ma, basins to the south of Lake Mead. Such a model is compatible with evaporites thus partially overlapping in time with Lake Grand Wash. Lake Hualapai proba- partially accounting for the significant gravity low in the northeastern part of bly drained into deeper basins to both the north and south (e.g., Overton Arm, Las Vegas Valley. Well logs indicate that the late Miocene section within the Mormon, and Mesquite to the north and Hualapai and southern Detrital to the Las Vegas basin contains relatively thick gypsum (Taylor et al., 2008). As a south) through a combination of groundwater sapping and possibly surface proto–Colorado River in the Lake Mead region initially spilled over into a sys- runoff and spillovers. Sr isotopic data from the Hualapai Limestone deposited tem of downstream lakes and eventually emptied into the Gulf of California in both Lake Grand Wash and Lake Hualapai indicate no connection with the as a through-going fluvial system by ca. 5.3–4.9 Ma (e.g., Dorsey et al., 2007; Colorado River (Spencer and Patchett, 1997; Roskowski et al., 2010; Spencer ­Spencer et al., 2013), the corresponding loss of axial lakes in the Lake Mead et al., 2013). region combined with the likely channeling of groundwater systems in the Lake Las Vegas was the last in the series of late Miocene lakes to develop southwestern part of the Colorado Plateau to the Grand Canyon would have in the Lake Mead region (Fig. 17C). An initial playa setting formed by ca. 8 Ma collectively induced a significant decrease in groundwater discharge in satel- and was followed by lacustrine carbonate deposition (Nellis limestone) shortly lite basins and ultimately terminated significant evaporite deposition even in after ca. 5.6 Ma, which may have persisted to ca. 5.0 Ma. Elevations of the areas, such as the Hualapai basin, that remained internally drained.

GEOSPHERE | Volume 12 | Number 3 Faulds et al. | Late Miocene lacustrine and nonmarine evaporite deposits, Lake Mead region Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/12/3/721/3335088/721.pdf 755 by guest on 24 September 2021 Research Paper

Available geochronologic data place some constraints on rates of deposi- ­liquid ­ gas (Spencer, 2005). Clearly, the thick and widespread halite tion of halite in the Lake Mead region. For example, 79 m of halite and 136 m ­deposits in the Lake Mead region represent an important economic resource of halite and anhydrite accumulated in the Hualapai basin above the 5.6 Ma with many potential industrial applications (e.g., mineral resources and natural tephra prior to integration of the region into the Colorado River by 5.3–4.9 Ma. gas storage).­ It should be noted, however, that water use and brine disposal This suggests depositional rates of evaporites of ~0.19–0.45 m/1000 yr (or are key challenges for developing salt cavern storage in the Neogene basins ~190–450 m/m.y.), assuming that evaporite deposition ceased between ca. 5.3 of this arid region. and 4.9 Ma. If the Lake Mead region was integrated into downstream lakes Nonetheless, our synthesis indicates that the Lake Mead region may con- earlier than 5.3 Ma, evaporite depositional rates would be higher. Such rates tain as much as ~700 km3 of halite. Because most halite in the region accumu- are compatible with documented rates of halite deposition (e.g., Schreiber and lated in half grabens and is partly synextensional, lenticular wedge-shaped Hsü, 1980; Roveri et al., 2008; Manzi et al., 2009). These rates are also com- bodies probably characterize the geometry of the halite deposits, similar to patible with the entire 2.5-km-thick sequence of halite and capping anhydrite that observed in the Hualapai basin (e.g., Faulds et al., 1997). These geometries in the Hualapai basin accumulating in ~5–7 m.y. (from ca. 12 Ma to 5 Ma), or can be modeled as quarter ellipsoids for purposes of estimating volumes (Fig. roughly coincidental with deposition of the ca. 12–6 Ma Hualapai Limestone 19). The largest halite deposits in the Lake Mead region appear to be hosted by in the region. the Hualapai and Mesquite basins, where halite volumes probably approach and possibly exceed 200 km3.

Economic Implications Regional and Global Analogues Evaporite , such as halite and gypsum, are important industrial minerals that have many societal applications. For example, late Miocene gyp- Although the southwestern United States contains dozens of Cenozoic ba- sum is currently mined in the western Lake Mead area and processed into sins (Figs. 2 and 3), thick halite has been documented in relatively few (e.g., wallboard by PABCO Gypsum. Miocene–Pliocene halite in the Luke basin­ Mannion, 1963; Peirce, 1976; Faulds et al., 1997). However, most Cenozoic west of Phoenix (Fig. 2) has also been mined for its mineral content. In addi­ ­halite deposits in this region, as well as many elsewhere, are buried and do tion, man-made caverns within the Luke salt body are used for storage of not crop out (e.g., Peirce, 1976; Faulds et al., 1997). In such cases, the regional

A B

E

x t

e Volume (km3) = n

t Thickness (km) Length radius (km)Width (km) 1/4(4/3π*a*b*c)

Basin

o

f

Lengt H Hualapai 2.5 9.5 8 200 Thi ckness a h li Northern Grand Wash 1.5 10 6 94 Width te D e Mesquite 1 15 14 220 po sit Mormon 1 7.5 4 31 ion Overton Arm 1.5 12.5 5 98 Northern Detrital 0.8 4 6 20 Southern Detrital 1.5 10 3 47 Las Vegas 0.3 10 3 9 Total Volume 719

Figure 19. (A) Schematic wedge-shaped, synextensional halite deposit modeled as a quarter ellipsoid for purposes of calculating volume of deposit (modified from Faulds et al., 1997). (B) Table shows estimated volumes for documented (bold larger font) or presumed (italic) halite-bearing basins in the Lake Mead region. Total halite may approach 700 km3 in the region.

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setting and geophysical techniques, particularly gravity and magnetics, can be ponding of the developing Colorado River would presumably result in thinner utilized to determine the most likely locations for subsurface deposits and to evaporative sequences. However, once the Colorado River had evolved to the design other geophysical surveys, such as electrical, seismic reflection, and point of progressively cascading southward in a series­ of lakes toward the Gulf even heat flow, to test whether the gravity and magnetic lows are caused by of California, it probably had a much larger catchment area compared to ear- halite, rather than other low-density fill, and to ultimately determine the depth lier drainage and groundwater systems emptying into the Lake Mead region. and thickness of potential halite deposits. Considering the potential high rates of evaporite deposition (e.g., Schreiber The spatial and temporal relations in the Lake Mead region between lacus­ and Hsü, 1980) and large volumes of water emptying into regional, albeit short- trine and evaporative sequences and development of the Colorado River pro- lived sinks, it is possible that some of the basins proximal to the Colorado River vide a conceptual model for examining the potential for thick halite and related south of Lake Mead contain relatively thick sequences of halite. These include evaporites in downstream reaches of the Colorado River south of Lake Mead the Dutch Flat, Sacramento, Chemehuevi, Blythe-­McCoy, Yuma, and ­Mohawk and in major tributaries of the Colorado River in the arid southwestern United basins (Fig. 2). Thick halite may be more likely in the Blythe-McCoy­ and Yuma States, especially the Gila River drainage in southern Arizona (Fig. 2). For ex- basins due to possible late Miocene–early Pliocene marine incursions in this ample, lacustrine carbonate facies are more common in the Lake Mead region area (McDougall, 2008; McDougall and Martinez, 2014). Nonetheless, docu- within the axial basins proximal to the subsequent Colorado River, with thick mentation of appreciable halite in these basins awaits comprehensive analysis evaporite sequences (particularly halite) characterizing nearby satellite basins­ of well and geophysical data. that presumably served as terminal sinks to segments of the developing drain- The overall extent of thick Neogene evaporite deposits in the southwestern age system. Some of these basins served as regional sinks for prolonged United States, stretching from central Arizona to the Lake Mead region, marks periods (>5 Ma) and thus accumulated thick (1–2.5 km) evaporite sequences. a belt of evaporites that rivals those in other parts of the world, including the These relationships indicate that position within a drainage system should be central Andes (Alonso et al., 1991), southern (Rodríguez-Fernández and factored into estimating the potential for thick halite and related evaporites, as Sanz de Galdeano, 2006; García-Veigas et al., 2015), and many passive conti­ a general correlation may exist between this position, time interval of ponding, nental margins (Tankard and Balkwill, 1989). Some of the basins in central and thickness of evaporite sequences. and southern Spain may be broadly analogous to those in the southwestern Such a model has relevance to understanding both the evolution of these United States. For example, although the Granada Basin began as part of a drainage systems as well as for guiding exploration models for identifying much larger and more extensive marine basin from ca. 8.5 Ma to 7.2 Ma, it thick, economically viable halite deposits. For example, assuming that the Gila gradually became cut off from the ocean and continued to widen while remain- River (Fig. 2) evolved similar to the Colorado River through a chain of pro- ing deep but nonmarine (Rodríguez-Fernández and Sanz de Galdeano, 2006; gressive downstream sapping and spilling of lakes, the thickest halite would García-Veigas et al., 2013, 2015; Navarro-Hervas et al., 2014). During a final pe- be expected in the more upstream reaches, still within the Basin and Range riod of cut-off from marine water sources (end Tortonian), marine halite depo-

Province but proximal to the highlands of the Colorado Plateau. In such areas, sition transitioned into nonmarine halite with added CaCl2 (hydrothermal). The large quantities of fresh water could be derived from the Colorado Plateau, top of the section accumulated as a shallow nonmarine gypsum-anhydrite either through surface drainage or groundwater systems, large deep basins deposit. Although the initial phase of marine deposition in the Granada basin would facilitate prolonged ponding of waters and contain sufficient accom- clearly differs from the sequence of events in the Lake Mead region, the overall modation space for accumulation of thick evaporative sequences, and a warm extent of nonmarine evaporites in southern Spain is comparable to those in arid climate would afford rapid evaporation rates (Figs. 1 and 2). Notably, thick the Lake Mead region and southwestern United States. Moreover, the relation- halite has been documented within this general setting in the Gila River drain- ships between the thick evaporite sequences and chronology of both regional age network, including the Luke and Picacho basins in south-central Arizona extension and drainage development in the southwestern United States may (Fig. 2; Peirce, 1976, 1981; Rauzi, 2002c; Spencer, 2005). elucidate the evolution of other nonmarine evaporative belts in extensional All basins proximal to the lower Colorado River to the south of Lake Mead settings throughout the world and help to guide exploration efforts for thick also have significant potential for containing thick halite deposits. However, the subsurface halite deposits. lower reaches of the Colorado River were integrated relatively rapidly through sequential, downstream spillovers (e.g., House et al., 2005, 2008; Spencer et al., 2008, 2013). Thus, compared to the Lake Mead region, where some lakes and CONCLUSIONS associated playas persisted for as long as ca. 5 Ma (e.g., Lake Grand Wash with apparent terminal sinks in Hualapai basin and northern Grand Wash Trough; The late Miocene landscape in the Lake Mead region contained a series Figs. 3B and 17), lakes downstream from Lake Mead, associated with develop- of lakes, wetlands, and playas, which stretched from the mouth of the Grand ment of the Colorado River, were probably more transient features, possibly Canyon in the Grand Wash Trough westward to the Las Vegas basin. Thick lasting for only a few hundred thousand years or less. Shorter increments­ of late Miocene nonmarine evaporite (primarily halite and gypsum) and related

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lacustrine limestone deposits accumulated in lakes and playas throughout the Lake Las Vegas are similar to the Bouse Formation and may reflect earlyarrival ­ region. 40Ar/39Ar and tephrochronologic data, regional relationships, and a pro- of the Colorado River in the region (Roskowski et al., 2010; Spencer et al., gressive upward decrease in tilt in some basins indicate that these deposits 2013). All of the late Miocene lakes in the region overlapped spatially and/or are late synextensional, ranging from ca. 12 to 5 Ma. The uppermost deposits temporally (Figs. 17 and 18). The thickest and most long-lived late Miocene immediately predate the arrival of the Colorado River. The late synextensional lacustrine and evaporite deposits thus far documented in the region reside setting allowed for development of sufficient accommodation space in half in half grabens­ in the eastern Lake Mead region, suggesting that the Colo- grabens and deposition of relatively thick sequences of lacustrine and evapo- rado Plateau was a major source of groundwater and possibly some surface rite sediments. Continued subsidence of basins also allowed for the preserva- water to the basins. Isotopic analyses of late Miocene gypsum and anhydrite tion of thick sequences of halite below the stagnant level in the phreatic zone. throughout the region are needed, however, to test this hypothesis, as well Furthermore, waning rates of extension facilitated development of drainage as our models for a series of interconnected basins and evolving hydrologic networks large enough to supply appreciable groundwater and some surface regimes in the late Miocene (Figs. 17 and 18). water to lakes and playas. Lower overall topographic relief in the late syn­ The relations between the thick evaporite sequences and chronology of extensional setting also induced development of broad, low-gradient plains both regional extension and drainage development in the Lake Mead region and shallow water bodies, which, in turn, promoted high surface-to-volume may elucidate the evolution of other nonmarine evaporite sequences in the ratios for late synextensional water bodies and thus greater evaporative sur- southwestern United States and elsewhere, helping to guide exploration ef- faces and resulting evaporite deposits. forts for thick subsurface halite deposits. Our conceptual model suggests that The distribution, age, and composition of late Miocene deposits in the Lake the most extensive nonmarine evaporite sequences will be late synextensional Mead region suggest a broad depositional system involving axial ­playas and and occur in basins with large catchments proximal to developing river sys- subsequent lakes (along and proximal to the eventual course of the Colorado tems and/or broad elevated terranes (e.g., Colorado Plateau). River), with extensive playas and salt pans in satellite basins within ~50 km of the axial basins. In the axial basins, gypsum deposition transitioned to ACKNOWLEDGMENTS limestone accumulation as fresh-water input increased during the late Mio- cene. Evaporite deposition dominated many of the satellite basins, with thick This work was funded by a variety of sources over several years, including grants awarded to Faulds from the National Science Foundation (EAR99-10977 and EAR04-09913) and EDMAP pro- halite (~200–2500 m) accumulating in the Hualapai, Detrital, and Overton gram of the U.S. Geological Survey (Cooperative agreement #1434-HQ-97-AG-07146). In addition, Arm basins. Large-magnitude negative gravity anomalies indicate that thick Unocal and LK Energy partially funded some of the research on the halite deposits. The U.S. Geo- undiscovered halite may comprise a significant part of the fill within several logical Survey in Las Vegas kindly provided a field vehicle for substantial amounts of this work, for which we thank Gary Dixon and Peter Rowley. We also thank the National Park Service at the Lake other satellite basins, including the northern Grand Wash, Mesquite, southern Mead National Recreation Area for extensive logistical support over many field seasons, including ­Detrital, and northeastern Las Vegas basins (Fig. 3). Total halite volume in the boat access into remote areas. Kent Turner and Darlene Carnes with the Lake Mead National Recre- region probably exceeds 700 km3. On the basis of the age and distribution of ation Area were especially helpful. We also greatly appreciate Mark Odegard, Grizzly Geosciences, Inc., and Bill Cathey, Earthfield Technology, for drawing our attention to and demonstrating the lacustrine limestone, it would appear that late Miocene fresh-water lakes first utility of gravity and magnetic data in delineating salt bodies in these basins. This research has also formed ca. 12 Ma in the Grand Wash Trough adjacent to the Colorado Plateau benefited from fruitful discussions with Jon Spencer, Sue Beard, Kyle House, Keith Howard, Gary (Lake Grand Wash) and then progressed westward to the central Lake Mead Dixon, and Mark Wallace. We also thank David Davis at the Nevada Bureau of Mines and Geology for discovering obscure reports describing drill holes in the Lake Mead area and Holly McLachlan area (Lake Hualapai) ca. 10 Ma, and finally to the Las Vegas area by ca. 5.6 Ma for assistance with preparing figures portraying 3D perspectives of the wells. Reviews by Melissa (Lake Las Vegas). Sr isotopic values from the <5.6 Ma limestone deposited in Lamb, Dave Miller, Karl Karlstrom, and an anonymous individual greatly improved this manuscript.

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APPENDIX A: 40Ar/39Ar DATA

TABLE A1. SUMMARY OF 40Ar/39Ar AGE RESULTS AND ANALYTICAL METHODS Age Sample Number L# min Irrad (Ma)±MSWD%39Ar n/n H JF99-459 60967gm NM-125 8.93 0.03 1.11100 7/7 I JF99-460 60968gm NM-125 8.94 0.05 2.19 90.7 6/7 F1 JF-98-155 9966san NM-101 13.720.610.19NA15/15 F2* JF-98-155* 9966san NM-101 13.26* 0.06 2.52 NA 7/15 B† MW-98-36† 9951 san NM-101 7.52† 0.11 NA NA 1/28 G JF-98-308 9952san NM-101 15.510.041.57NA23/27 E JF-97-144 9953san NM-101 11.200.130.54NA23/25 Notes: See Figure 7 and Table 1 for locations of samples. All errors at 1 sigma and include error in J. Error in decay constant not included. Sanidine separated by standard heavy liquid and magnetic techniques. Abbreviations: L#—Lab number; sanidine n/n—number of grains providing preferred age and/or number of grains dated; min—material dated; gm—groundmass concentrate; san—sanidine; NA—Not applicable; Irrad—Irradiation package; %39Ar—percentage of total 39Ar comprising the plateau steps. MSWD—mean square of weighted deviates. *Maximum eruption age based on assuming sample is 100% radiogenic. †Youngest apparent age represents maximum eruption age. Sample preparation and irradiation. Groundmass concentrates were prepared by crushing, ultrasonic washing in deionized water (DI) and hand-picking fragment free of visible phenocrysts. Samples were loaded into a machined Al disc and irradiated in two batches (NM-101 and NM-125) for 7h in D-3 position, Nuclear Science Center, College Station, Texas. Neutron flux monitor Fish Canyon Tuff sanidine (FC-1). Assigned age= 28.201 Ma (Kuiper et al., 2008). Instrumentation. Mass Analyzer Products 215-50 mass spectrometer on line with automated all-metal extraction system. Detector: Johnston electron multiplier operated at 2.1kV. Basalt samples: Samples step-heated in Mo double-vacuum resistance furnace. Heating duration 8 min. Reactive gases removed during heating by reaction with 1 SAES GP-50 getter operated at ~450 °C and for 6 min in a second stage by reaction with 2 SAES getters (1 at 450 °C; 1 at 20 °C). Gas also exposed to a W filament operated at ~2000 °C. Sanidine samples: Crystals were fused by

a 10 W Synrad CO2 laser. Reactive gases removed during a 1.5 min reaction with 2 SAES GP-50 getters, one operated at ~450 °C and one at 20 °C. Gas also exposed to a W filament operated at ~2000 °C and a cold finger operated at ~140 °C. Analytical parameters. System sensitivity 1.63 × 10–16 moles/pA, furnace and 1.82 × 10–16 moles/pA, laser. Basalt: Total system blank and background for the furnace averaged 480, 5.0, 0.24, 1.7, 1.9× 10–18 moles at masses 40, 39, 38, 37, and 36, respectively for temperatures <1300 °C. Sanidine: Total system blank and background averaged 250, 3.5, 0.4, 2.4, 1.6× 10–18 moles for mass 40, 39, 38, 37, 36, respectively. J-factors determined to a precision of ±0.1%

by CO2 laser-fusion of four single crystals from each of four or six radial positions around the irradiation tray. Correction factors for interfering nuclear reactions were determined using K-glass and CaF2 and are as follows: 40 39 36 37 39 37 NM-101: ( Ar/ Ar)K = 0.00020 ± 0.0003; ( Ar/ Ar)Ca = 0.00026± 0.00002; and ( Ar/ Ar)Ca = 0.00070± 0.00005. 40 39 36 37 39 37 NM-125: ( Ar/ Ar)K = 0.00020 ± 0.0003; ( Ar/ Ar)Ca = 0.00028± 0.000011; and ( Ar/ Ar)Ca = 0.00089± 0.00003.

GEOSPHERE | Volume 12 | Number 3 Faulds et al. | Late Miocene lacustrine and nonmarine evaporite deposits, Lake Mead region Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/12/3/721/3335088/721.pdf 759 by guest on 24 September 2021 on 24 September 2021 by guest Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/12/3/721/3335088/721.pdf Research Paper Plateau ±1 σ : Plateau ±1 σ : Integrated age ±1 Integrated age ±1 σ :n Plateau ±1 Integrated age ±1 Plateau ±1 σ : Integrated age ±1 σ : H H1 G1 I* I1 G H1 H1 F F9 G1 E E G F9 D D F E8 C C7 E9 D B B D8 ( ( B7 JF99-460 groundmass, wt. JF97-76-2 groundmass, wt. A A5 JF97-76 groundmass, wt . USGS data B JF99-459 groundmass, wt. NMGRL K mean of selected steps. Plateau age error is inverse-variance-weighted (T ID ( Correction factors: (NMGRL) weighted deviates; NMGRL—New Mexico Geochronology Research Laboratory; USGS—U.S. Geological Surve y. (Kuiper et and Jäger (1977). Decay constant ( λ 40 36 39 2 Notes: *Analyses excluded from plateau age calculations. Ar/ Ar/ Ar/ O calculated from 39 37 37 Ar) Ar) Ar) data Integrated age error calculated by quadratically combining errors of isotopic measurements all steps. Plateau is invers e-variance-weighted Te K Ca Ca al., 2008). USGS ages calculated relative to FCT σ = 0.0002 mperatur e = 0.00028 = 0.00089 : 145 0 1650 1050 1075 (°C) 45 01 05 01 65 0 25 02 25 01 950 07 58 850 85 01 800 80 0 97 58 750 80 0 650 65 03 550 70 0 50 75 75 50 00 00 00 50 σ :n σ : 39 ± 0.0003 Ar signal, sample weight, and instrument sensitivity ± 1.1e-05 ± 3e-05 Steps B– Hn Steps F–H Steps F– Hn = 245.5 mg , J= Steps B– In = 44.41 mg , J = 44.52 mg, J= 40 107. 12 167. 52 = 244.9 mg , J= 34.2 0 17.1 2 13.9 92 18.0 17.7 91.3 8 Ar/ 11 1.1 1.051 1.81 4.68 1.236 1. 11 1.558 8.65 21 8.55 6 2.630 2.51 91 2.160 2.36 7 9.97 7 4.648 .171 .026 .024 .364 .142 .5 91 .787 39 17 Ar 72 40 K (total)) ( ( ( Correction factors: (USGS) 40 36 39 37 12.502 16.666 42.5 2 = 0.0007733 0.002697 Ar/ Ar/ Ar/ 0.0007746 1.61 80 5.74 1.434 6.29 35 7.40 9 2.123 2.18 76 1.966 1.77 21 0.9866 1.636 1.99 66 0.977 0.77 7 1.95 3 1.94 13 1.346 1.10 3 2.515 2.50 7 3.41 4 Ar/ n= n= n= 0.002689 .309 .293 .31 .510 .302 .683 = 8 = 3 = 81 = 6 = 7 39 37 37 39 3 7 7 Ar) Ar) Ar) = 5.463e-10/a (Min et 15 Ar K Ca Ca = 0.0057 = 0.00026 = 0.00067 ± 0.50%, D T MSWD MSW D= MSW D MSW D ABLE ± 0.10%, D= ± 0.50%, D ± 0.10%, D= 36 (×10 341. 86 107. 0 540. 90 286. 10 Ar/ -3 Fish Canyon 30.3 7 17.9 3 12.8 0 27.2 5 57.93 56.8 6 1 2.494 3.01 92 0.71 5.52 1.392 1.03 32 1.717 8.22 5 7.82 72 5.222 3.871 4.55 2 6.92 1.67 9.26 01 A2. ± 0.00 4 39 .6570 .550 .217 .147 .715 = 0.29 = 2.19 = 1. 11 –3 Ar ± 1.7e-06 ± 3.7e-06 1.72 ) 10 = 1.0065 40 Ar/ = 1.0065 1.0074 5± 1.0074 5 39 al., 2000). D= Ar DA (×10 1491 . NMGRL 929.7 885. 8 342.9 50 2 351. 60 371.3 215.5 120.1 128. 20 189.9 203. 0 1 11 ± 0.0006, Lab# 90.2 67.3 74.1 87.3 87.3 16.2 44.0 14.5 18.8 12.6 28.6 12.4 63.78 08.9 10.2 69.29 60.2 00 Tu 11 11 39 5.12 5. 9 4. 10 7.65 ± 0.0006, Lab# –1 5 TA Ar .8 6 .1 .4 .41 .323 ff ± 0.00121, NM-125, Lab# mol) sanidine at 28.201 0.00121, NM-125, Lab# K FOR BASAL 1 ages calculated relative to FC-1 Fish Canyon 1 aylo r, AMU discrimination in favor of light isotopes. MSWD—mean square = 436KD7 1982) times square root MSWD, where MSWD >1. We K/Ca 0.50 0.08 0.06 0.60 0.02 0 0.12 0.01 2 0.09 2K 0.68 0.08 1 0.06 9 0.46 0.47 0.23 0.50 0.61 0.39 0.60 0.26 0.34 1.01 1.39 0.26 0.26 0.73 0.98 0.22 0.19 0.39 0.15 = 45KD7 .6 7 .5 2 .8 2 .4 3 T SAMPLES 0.41 0.40 0.36 0.20 Ma (Kuiper et ± 0.30 ± 0.31 ± 0.15 ± 0.10 = 51331-0 1 = 51333-0 1 81.6 78.9 87.5 17.9 86.0 27.2 43.0 74.6 80.9 78.1 73.5 79.0 73.1 73.9 45.3 43.1 77.7 55.4 50.1 45.7 63.7 37.2 28.8 30.6 43.8 40 (%) 7. 61 4. 70 8.0 8. 24 7. 80 Ar* al., 2008). Isotopic abundances after Steiger K2 O= 2O 100.0 100. 04 100. 08 100. 08 62.3 58.9 80.7 00.0 77.1 90.7 90.7 94.1 52.2 57.3 75.6 37.7 41.0 75.7 58.8 29.7 33.3 59.0 26.3 21.6 24.8 26.2 10.9 12.0 11 39 (%) = 0.97 %8 8.9 4.6 Ar 0.83 % .3 .6 .0 .4 Tu ff sanidine at 28.20 1 10.1 11 11 (Ma ) Ag e 4.509 5.02 4.49 4.48 4.91 4.42 9.03 4.454 8.42 8.93 9.00 4.542 4.45 8.88 4 5.65 5.32 8.94 8.93 5.88 5.37 8.95 9.07 5.34 5.34 9.01 9.01 6.60 5.74 8.69 7.09 7.16 .584 .7 9 .9 9 .806 .928 .1 50 80 70 60 80 80 ight percent (Ma ) 0.075 0.23 0.06 9 0.23 0.85 0.13 0.45 0.06 8 0.070 0.18 0.13 0.03 4 0.103 0.06 4 0.30 0.15 0.25 0.16 0. 11 0.14 0.16 0. 11 0.21 0.41 0.23 0. 11 2. 5 0.46 0.71 2. 2 ±1 .052 .074 .052 .069 .041 .045 σ Ma

GEOSPHERE | Volume 12 | Number 3 Faulds et al. | Late Miocene lacustrine and nonmarine evaporite deposits, Lake Mead region 760 on 24 September 2021 by guest Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/12/3/721/3335088/721.pdf Research Paper 16 MW 05 JF-98-155* sanidine, 1–5 crystals per fusion, J 13 JF-98-155 sanidine, 1–5 crystals per fusion, J ID 11 04 10 27 09 17 19 02 26 11 20 08 Maximum eruption age 1 σ :n 15 02 14 18 25 05 03 28 24 22 21 Maximum eruption age 1 09 06 01 07 14 12 13 15 04 01 12 23 05 07 06 13 16 06 02 01 08 11 16 09 03 14 03 08 04 15 12 07 Mean age ±1 σ : † † † † † † † † † † † † † † † † † † † † † † † † † † † † † † † † † † † -98-36 sanidine; 10–35 crystals per fusion, J 126.1 237.6 40 90.30 81.18 78.08 62.01 41.04 30.10 19.72 17.33 13.23 13.02 15.6 9 12.5 90 10.3 0 57.23 22.36 15.50 10.1 8– 10.0 90 12.5 90 10.1 8 10.3 0 10.0 90 15.6 9 1 9.86 60 9.173 8.489 8.725 8.472 6.694 6.229 6.155 6.521 1.36 8.722 9.554 7.835 9.90 8 9.86 90 5.743 9.44 70 9.54 4 9.46 20 9.44 7 9.54 0– 9.55 4 9.54 40 9.75 8 9.55 40 9.90 80 9.69 3 9.86 9 9.75 80 9.86 6 9.69 3 9.46 20 9.54 0– Ar/ 39 Ar σ :n –0.064 8 37 0.0528 0.1 0.1492 0.1 0.1285 0.1435 0.1202 0.1046 0.1 0.1442 0.0866 0.2786 0.0779 0.2473 0.0984 0.0946 0.1 0.0948 0.1058 0.1234 0.1265 0.1812 0. 1111 0.1370 0.1213 0.1 0.064 8 0.1618 0.1929 0.1742 0.2795 0.0918 0.0235 0.0693 0.005 1 0.0269 0.0521 0.0589 0.0906 0.0779 0.0358 0.0589 0.2786 0.005 1 n= Ar/ .0358 .0606 .0785 .0906 .0693 .0374 .0235 .0269 .0606 .2795 .0521 .0785 .0374 156 194 163 152 183 15 39 Ar = 0.0007612 = 0.0007612 MSW D = 0.0007597 –1.083 3 17.9 8 –0.569 4 –0.1978 –0.376 70 –0.070 7 –3.390 00 –0.070 70 –3.800 9 –0.099 50 –1.846 60 –0.376 70 –0.099 50 –1.083 30 –1.846 60 –0.569 40 17.9 80 –3.390 00 –3.800 90 36 3.626 1.254 3.681 3.636 1.967 2.346 1.512 2.979 2.568 2.1 5.236 9.46 60 0.8707 1.354 0.2509 2.844 2.606 1.630 0.3802 0.7187 2.393 1.486 6.484 4.283 4.414 1.615 0.5767 2.637 2.561 0.4270 1.124 0.5663 0.1882 0.5663 0.1882 9.46 60 0.4270 0.5767 0.8707 (×1 0 Ar/ T 12 = 7 = 1 ABLE 39 = 0.19 –3 Ar ) ± 0.10%, D= ± 0.10%, D= A3. SANIDINE ± 0.10%, D MSWD (×1 0 1.0024 1.0024 = 1.0024 0.27 2 1.267 2.010 1.335 2.1 2.810 0.590 0.869 1.579 0.254 1.957 1.488 0.22 5 0.19 56 0.508 0.466 0.901 1.452 0.864 1.431 1.870 1.338 0.422 0.484 0.18 3 2.216 0.752 1.841 0.15 6 0.744 0.403 0.936 0.20 3 0.51 6 2.996 0.27 12 0.25 1 0.30 2 0.30 9 0.34 4 0.17 8 0.13 6 0.17 16 39 .483 .457 .329 .453 .292 .423 .305 .400 .333 .238 .268 .160 .197 .289 .220 –1 5 40 Ar 13 Ar/ mol) = NA K ± 0.001, NM-101, Lab# 39 MSW D= ± 0.001, NM-101, Lab# Ar ± 0.001, NM-101, Lab# ANAL 2.52 YTICAL K/Ca 14.3 13.6 19.0 21.8 19.0 14.3 13.6 9.7 4.4 3.4 4.3 4.0 3.6 4.2 4.9 4.4 3.5 5.9 1. 8 8. 4 2.1 5.2 5.4 4.4 5.4 4.8 4.1 4.0 2.8 4.6 6. 5 3.7 4.2 4.3 3.2 2.6 2.9 1. 8 5. 6 7. 4 5.6 1. 89 7. 41 9. 81 8. 4 1. 8 8. 7 5. 6 9. 81 8. 7 6. 5 1. 8 – DA .5 –1 –1 – .5 TA = 9966 = 9966 = 9951 10 0 10 0 10 0 10 0 10 0 100.8 10 0 10 0 10 0 10 0 10 01 10 0 10 0 101. 21 103. 3 105. 71 101. 7 11 11 40 99.2 99.6 98.7 98.6 99.1 98.3 98.5 95.6 95.7 95.4 88.2 95.9 99.2 90.5 91.0 92.9 98.3 96.7 89.3 96.3 99.2 97.8 94.2 97.0 00 92.0 90.5 94.3 00.3 00 99.4 00.3 77.8 98.9 00 98.3 97.6 66.3 (%) Ar* 8. 31 0. 6 1. 81 166.13 121.00 108.15 104.09 301.5 13.7 0 83.51 55.29 40.80 26.02 22.9 17.47 15.89 13.2 6 21.7 3 17.461 14.2 9 12.19 10.94 10.69 15.15 14.0 0 76.23 29.06 20.78 14.1 4 12.19 12.18 13.7 6 13.700 13.1 13.1 4 13.1 5 13.2 5 13.2 13.6 13.2 6 13.6 13.6 13.4 13.5 5 13.9 13.9 14.1 14.2 14.4 14.5 13.7 2 1 (Ma) Ag e 1.68 8.63 8.50 8.26 8.08 7.52 9.84 7.52 3. 0 3.25 3.86 4. 4. 8 21 18 00 60 ( continued (Ma) 0.13 0.55 0.31 0.35 0.29 0.20 0.58 0.39 0.22 1.3 0.18 0.23 0.06 0.22 0.09 6 0.17 0.70 0.38 0.23 0.18 0.25 0.65 0.37 0.23 0.78 0.1 1.7 0.18 0.23 0.45 0.19 0.20 0.35 0.44 0.82 0.17 0.08 3 0.07 6 0.1 1. 2 0.10 0.74 0.13 1. 1 0. 11 1. 1 0.10 1. 9 0.84 0. 11 2. 5 1. 9 1. 4 2. 1 1. 7 1. 2 1. 5 0.61 ±1 σ .8 0 .1 .2 1 1 1 )

GEOSPHERE | Volume 12 | Number 3 Faulds et al. | Late Miocene lacustrine and nonmarine evaporite deposits, Lake Mead region 761 on 24 September 2021 by guest Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/12/3/721/3335088/721.pdf Research Paper ( ( 22 JF-97-144 sanidine; 5–30 crystals per fusion, J 05 JF-98-308 sanidine; 8–25 crystals per fusion, J= AMU mass discrimination in favor of light isotopes; – —Not detectable ID mean age of interfering reactions. Errors quoted for individual analyses include analytical error onl y, incorporates uncertainty in J factors and irradiation correction uncertainties. ( Correction factors: 27 10 Mean age ±1 σ :n 17 Mean age ±2 σ :n 16 20 09 et 25 04 09 20 02 08 19 23 23 16 01 07 06 24 11 21 14 05 08 13 01 26 25 10 18 19 15 11 13 24 06 22 03 21 03 14 12 12 02 15 07 18 17 04 40 36 39 *Apparent ages calculated assuming all corrected † Notes: Ar/ Ar/ Ar/ † † † † † † al., 2008). Isotopic abundances after Steiger and Jäger (1977). Decay constant ( λ Analyses excluded from mean age calculations. 39 37 37 Ar) Ar) Ar) MSWD—mean square of weighted deviates. Isotopic ratios corrected for blank, radioactive decay K Ca Ca = 0.0002 = 0.00026 = 0.0007 Ta 40 17.54 12.56 12.8 90 12.1 20 12.9 40 10.40 10.23 10.90 11 11 11 11 11 11 11 11 11 11 11 11 11 11 11 11 11 11 11 11 11 11 11 11 11 9.77 30 9.63 30 9.27 30 9.873 9.964 9.356 9.067 9.806 9.927 9.760 9.238 9.047 9.837 9.710 9.626 9.296 9.385 9.788 9.526 Ar/ ylor (1982). Mean age error is weighted of the mean (T .9 70 .9 80 .4 70 .1 40 .0 90 .2 20 .2 70 .3 80 .3 80 .1 40 .2 20 .4 90 .2 40 .4 6 .3 00 .3 1 .4 60 .3 4 .3 80 .4 30 .3 00 .3 60 .3 70 .4 00 .6 90 39 Ar ± 0.0003 ± 5e-05 ± 2e-05 37 0.1839 0.1585 0.2439 0.2660 0.1699 0.1472 0.2049 0.1648 0. 111 0.1684 0.1227 0.1 0. 11 0.2853 0.1709 0.1504 0.1678 0.1657 0.1909 0.1863 0.2636 0.1656 0.2214 0.0857 Ar/ .0981 .1280 .1312 .1623 .2860 .1234 .2027 .0970 .1468 .1507 .2282 .0917 .1407 .1444 .1417 .1462 .1307 .1721 .0998 .1956 .1457 .1408 .1374 .0961 .1981 .1073 .1525 .1326 = 23 = 23 143 39 88 Ar 3 = 0.0007600 T MSWD MSWD 0.0007593 ABLE 17.4 90 36 0.3617 0.7776 7.154 2.107 0.2451 0.8650 7.00 70 1.92 20 6.46 20 0.3784 5.10 30 0.0856 0.4175 6.980 0.5745 0.8988 6.661 0.8945 4.551 0.0575 3.552 0.2702 5.725 1.16 91 6.120 0.2579 5.534 3.696 1.02 42 2.851 0.4984 0.4662 5.500 0.9063 5.012 0.4845 4.649 3.470 0.6127 3.606 0.7758 5.747 0.2846 0.3709 5.108 0.3269 3.451 0.3714 2.294 0.7007 3.539 (×1 0 40 Ar/ Ar is A3. SANIDINE 39 = 0.54 = 1.57 –3 Ar ) 40 Ar* (i.e., 100% radiogenic). ± 0.10%, D= ± 0.10%, D= 37 40 (×1 0 Ar above blank level. MSWD—mean square of weighted deviates. Ar/ 1.0024 1.0024 4.84 2 1.61 4 0.282 0.227 1.80 1 2.89 33 2.90 25 1.19 23 4.36 45 1.014 1.88 43 0.94 6 0.672 1.91 0 0.435 0.95 13 0.568 2.34 63 0.918 0.718 1.60 75 0.251 0.468 0.470 1.77 42 4.30 1 0.274 1.80 7 0.295 1.65 33 0.449 0.381 3.96 03 0.486 2.33 13 0.333 1.91 55 1.07 22 0.205 3.81 04 0.353 1.22 8 0.305 4.28 6 0.297 39 39 aylo r, Ages calculated relative to FC-1 Fish Canyon .736 .981 .293 .655 .502 .640 .757 –1 5 Ar Ar mol) K ANAL 1982), multiplied by the square root of MS WD where MSWD>1, and also ± 0.001, NM-101, Lab# ± 0.001, NM-101, Lab# 40 YTICAL K (total)) without interfering reaction or J u ncertainties. Mean age is weighted DA = 5.643e-10/a (Min et K/Ca TA 5. 2 4. 09 2.8 3.2 3. 99 1. 87 4. 19 2. 5 3. 5 2. 2 2.1 3. 5 1.9 3. 6 3.0 3.5 2.5 3. 09 3.1 4.6 3.0 4. 2 4.5 4. 3 1.8 3. 5 3.0 3.0 3.1 2.7 2.7 1.9 3.1 3. 3 2.3 3. 8 6.0 ( .1 .3 .4 .6 .6 .5 .9 .1 .6 .4 .6 .7 .3 .6 .8 continue d = 9953 = 9952 , and mass discrimination, not co rrected for ) 40 99.2 88.0 95.1 98.0 80.3 99.1 83.9 99.9 60.2 99.0 79.3 98.6 97.8 80.5 97.8 85.8 99.9 88.6 99.4 82.9 81.9 99.4 83.3 88.3 97.4 90.8 98.8 98.9 83.7 97.8 84.9 98.8 85.9 89.1 98.5 88.8 98.1 83.8 99.3 99.2 85.4 99.2 89.7 99.1 93.1 98.3 90.5 (%) Ar* 8. 21 9. 51 9. 01 5. 11 7. 11 al., 2000). NA—Not applicable; D—1 Tu ff sanidine at 28.201 16.422 21.4 16.6 16.4 3 15.5 1 10.7 15.2 7 10.7 15.3 10.8 15.366 10.8 6 15.3 15.3 9 15.4 1 15.4 2 15.4 3 15.4 15.4 6 15.4 15.479 15.5 0 15.5 15.518 15.5 2 12.09 15.5 15.6 0 12.1 15.619 12.18 15.6 4 12.3 15.908 13.7 1 1 1 1 1 1 1 1 1 1 1 1 1 1 (Ma) Ag e 6.27 7.74 0.72 5.10 1.20 1.12 1.13 1.14 1.28 5.44 1.28 1.3 1.32 1.39 1.4 1.4 1.46 1.49 1.56 90 30 10 80 60 60 10 30 Ma (Kuiper (Ma) 0.21 0.07 9 0.19 1.2 1.5 0.12 0.46 0.34 0.13 0.04 1. 2 0.12 0.08 4 0.33 0.36 0.51 0.18 0.78 0.35 0.59 0.15 0.37 0.21 0.48 1.3 0.71 0.13 0.71 0.08 3 1.2 0.19 1.2 0.74 0.89 0.09 0 0.69 0.15 1.00 0.31 1.7 0.09 6 0.95 0.27 1.1 0.08 4 1.1 ±1 σ .5 2 .2 8 .6 7 .1 8 .2 1 .1 9 .2 1 .1 8

GEOSPHERE | Volume 12 | Number 3 Faulds et al. | Late Miocene lacustrine and nonmarine evaporite deposits, Lake Mead region 762 Research Paper

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