Research Paper THEMED ISSUE: CRevolution 2: Origin and Evolution of the Colorado River System II GEOSPHERE Paleogeographic implications of late Miocene lacustrine and nonmarine evaporite deposits in the Lake Mead 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 Geology, University of Nevada, 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, Houston, Texas 77098, USA CITATION: Faulds, J.E., Schreiber, B.C., Langen- 5New Mexico Bureau of Geology and Mineral Resources, New Mexico Tech, Socorro, New Mexico 87801, USA heim, V.E., Hinz, N.H., Shaw, T.H., Heizler, M.T., 62025 E. White Circle, Salt Lake City, Utah 84109, USA Perkins, M.E., El Tabakh, M., and Kunk, M.J., 2016, 7154-78 71st Avenue, Queens, New York 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 gypsum) 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 rela tively small and limit the extent and thickness of evaporite deposition. © 2016 Geological Society of America GEOSPHERE | Volume 12 | Number 3 Faulds et al. | Late Miocene lacustrine and nonmarine evaporite deposits, Lake Mead region Downloaded from https://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/12/3/721/3335088/721.pdf 721 by University of Nevada Reno user on 11 February 2019 Research Paper 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 Gulf of Mexico (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 Dead Sea (Niemi et al., 1997). Depending on water composition and evaporation rates, evaporites 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 United States (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-sulfate deposits (anhydrite 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;