The lower Pliocene Bouse Formation in the lower Colorado River Valley (southwestern USA) consists of basal marl and dense tufa overlain by siltstone and fi ne sandstone. It is locally overlain by and interbedded with sands derived from the Colorado River. We briefl y review 87 Sr/ 86 Sr analyses of Bouse carbonates and shells and carbonate and gypsum of similar age east of Las Vegas that indicate that all of these strata are isotopically similar to modern Colorado River water. We also review and add new data that are consistent with a step in Bouse Formation maximum elevations from 330 m south of Topock Gorge to 555 m to the north. New geochemical data from glass shards in a volcanic ash bed within the Bouse Formation, and from an ash bed within similar deposits in Bristol Basin west of the Colorado River Valley, indicate correlation of the two ash beds and coeval submergence of both areas. The tuff bed is identifi ed as the 4.83 Ma Lawlor Tuff derived from the San Francisco Bay region. We conclude, as have some others, that the Bouse Formation was deposited in lakes produced by fi rst-arriving Colorado River water that entered closed basins inherited from Basin and Range extension, and estimate that fi rst arrival of river water occurred ca. Ma. If this interpretation is correct, addition of BristolBasin to the Blythe Basin inundation area means that river discharge was suffi cient to fi ll and spill a lake with an area of ~10,000 km 2 . For spillover to occur, evaporation rates must have been signifi cantly less in early Pliocene time than modern rates of ~2-4 m/yr, and/or Colorado River discharge was signifi cantly greater than the current ~15 km 3 /yr. In this lacustrine interpretation, evaporation rates were suffi cient to concentrate salts to levels that were hospitable to some marine organisms presumably introduced by birds.

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Geologic investigations of late Mioceneearly Pliocene deposits in Mohave and Cottonwood valleys provide important insights into the early evolution of the lower ColoradoRiver system. In the latest Miocene these valleys were separate depocenters; the fl oor of Cottonwood Valley was ~200 m higher than the fl oor of Mohave Valley. When Colorado River water arrived from the north after 5.6 Ma, a shallow lake in Cottonwood Valley spilled into Mohave Valley, and the river then fi lled both valleys to ~560 m above sea level (asl) and overtopped the bedrock divide at the southern end of Mohave Valley. Sediment-starved water spilling to the south gradually eroded the outlet as silici clastic Bouse deposits filled the lake upstream. When sediment accumulation reached the elevation of the lowering outlet, continued erosion of the outlet resulted in recycling of stored lacustrine sediment into downstream basins; depth of erosion of the outlet and upstream basins was limited by the water levels in downstream basins. The water level in the southern Bouse basin was ~300 m asl (modern elevation) at 4.8 Ma. It must have drained and been eroded to a level <150 m asl soon after that to allow for deep erosion of bedrock divides and basins upstream, leading to removal of large volumes of Bouse sediment prior to massive early Pliocene Colo rado River aggradation. Abrupt lowering of regional base level due to spilling of a southern Bouse lake to the Gulf of California could have driven observed upstream river incision without uplift. Rapid uplift of the entire region immediately after 4.8 Ma would have been required to drive upstream incision if the southern Bouse was an estuary. PREVIOUS WORKThe Bouse Formation was fi rst thoroughly described and defi ned by seminal geohydrology studies conducted in the 1960s (Metzger, 1968;. The studies correlated Bouse outcrops within and between basins based on similar physical characteristics, including basal carbonate For permission to copy, contact editing@geosociety.org