PALEOGEOGRAPHIC IMPLICATIONS OF LATE MIOCENE LACUSTRINE AND NONMARINE EVAPORITE DEPOSITS IN THE LAKE MEAD REGION: IMMEDIATE PRECURSORS TO THE COLORADO RIVER

Thick late Miocene nonmarine evaporites and related lacustrine limestone compose the upper basin fill in half grabens in the Lake Mead region directly west of the Colorado Plateau in southern Nevada and NW Arizona. These deposits are late synextensional to post-extensional (~12 to 5 Ma), with major extension bracketed between ~16 and 9 Ma and the abrupt margin of the Colorado Plateau established by ~9 Ma. Significant accommodation space in half grabens allowed for lacustrine and evaporite deposition. Waning extension promoted integration of initially isolated basins, enlargement of drainage networks, development of low-gradient plains with greater evaporative surfaces, continued basin subsidence, preservation of thick halite sequences, and ultimately development of the through-going Colorado River.

Freshwater input increased during the late Miocene (~12 to 6 Ma) immediately preceding development of the Colorado River ~5.6 and 4.9 Ma. In axial basins near the course of the river, evaporite deposition (mainly gypsum) transitioned to lacustrine limestone from east to west ~12 to 5.6 Ma. In satellite basins to the north and south, evaporite deposition was more extensive, with thick halite (>0.2 to 2.5 km thick) accumulating in the Hualapai, Overton Arm, and northern Detrital basins. Gravity and magnetic lows suggest that thick halite is possible within the northern Grand Wash, Mesquite, southern Detrital, and northeast Las Vegas basins. A 2.5 km thick halite in the Hualapai basin accumulated from ~12 to 5 Ma, coincidental with lacustrine limestone deposition near the present Colorado River.  Depositional rates are constrained to ~190 to 450 m/Ma in the upper part of the halite.

The distribution and similar age of the limestone and evaporites suggest a system of late Miocene axial lakes and surrounding continental playas and salt pans. The elevated Colorado Plateau was likely a major source of water. The evaporites were fed by groundwater discharge and evaporation in shallow lakes. These relationships suggest that thick nonmarine evaporites are more likely to accumulate in late synextensional basins with relatively large catchments proximal to developing river systems and/or broad elevated terranes. Other basins adjacent to the lower Colorado River downstream of Lake Mead may also contain thick halite deposits.