Paleogeographic implications of late Miocene lacustrine and nonmarine evaporite deposits in the Lake Mead region: Immediate precursors to the Colorado River

摘要: Thick late Miocene nonmarine evaporite (mainly halite and gypsum) related lacustrine limestone deposits compose the upper basin fill in half grabens within Lake Mead region of Basin Range Province directly west Colorado Plateau southern Nevada northwestern Arizona. Regional relations geochronologic data indicate that these are synextensional to postextensional (ca. 12–5 Ma), with major extension bracketed between ca. 16 9 Ma abrupt western margin established by Ma. Significant accommodation space allowed for deposition sediments. Concurrently, waning promoted integration initially isolated basins, progressive enlargement drainage nets, development broad, low gradient plains shallow water bodies extensive clastic, carbonate, and/or sedimentation. The continued subsidence basins under restricted conditions also preservation particularly thick, localized sequences prior through-going River. spatial temporal patterns increasing amounts freshwater input during 12–6 Ma) immediately preceding arrival River 5.6 4.9 In axial along proximal present course River, transitioned progressively from east west, beginning 12–11 Grand Wash Trough shortly after region. several satellite both north south was more extensive, thick (>200 m 2.5 km thick) accumulating Hualapai, Overton Arm, northern Detrital basins. Gravity magnetic lows suggest may lie Wash, Mesquite, Detrital, northeastern Las Vegas New tephrochronologic part Hualapai is Ma, rates ∼190–450 m/m.y., assuming ceased approximately coincidental A 2.5-km-thick sequence have accumulated ∼5–7 m.y. or which coincides near distribution similar age a system lakes continental playas salt pans. pans were probably fed groundwater discharge evaporation lakes, as evidenced sedimentary textures. elevated terrain likely source playas. physical relationships evaporites be accumulate relatively large catchments developing river systems broad terranes. Other adjacent lower downstream Mead, such Dutch Flat, Blythe-McCoy, Yuma contain deposits.