The Colorado River is the terrestrial part of a continental scale sediment routing system that has been evolving and carving the landscape of western North America for at least six million years. This study aims to test models of the geological evolution of the Colorado River in particular and the more general drainage history of the SW US. Several possible routes are proposed for the ancestral Colorado River prior to its integration across the Colorado Plateau and incision of the Grand Canyon. Palynological samples from prodelta deposits of the palaeo-Colorado River delta in the Salton Trough produced reworked pollen and dinoflagellate cysts from the river catchment. These provide strong evidence that the Colorado River was fully integrated across the Colorado Plateau during the early Pliocene, supporting heavy mineral data and U-Pb detrital zircon ages. Detrital zircon U-Pb dating provides accurate information for the source of sediment in the basins. Comparison with known ages of zircons in sedimentary units of the Colorado Plateau as well as local basement rocks in the basinal regions has identified two populations of zircons in the deltaic sediments: one from local Mesozoic plutonic basement and a second from Colorado Plateau stratigraphy. The data support recent work on the timing of integration of the river through the Grand Canyon, proving that the 5.33 Ma Colorado River that fed into the Salton Trough was integrated across the Colorado Plateau at that time and that there had already been a degree of incision of the Grand Canyon. A literature review shows how uplift of the Colorado Plateau and development of the San Andreas transform boundary had significant consequences for evolution of the Colorado River. The San Andreas Fault in southern California is responsible for the dextral lateral migration of the Los Angeles Basin and Salton Trough (both on the Pacific Plate) at least from Middle Eocene through Present. The Colorado River, which drains much of the western part of the North American Plate, crossed this major strike-slip plate boundary prior to deposition of the main part of its sedimentary load. The delta of the Colorado is in the v northern Gulf of California at the present day, but palaeo-reconstructions of lateral displacement along the fault show that the Salton Trough lay adjacent to the point where the Colorado River crossed from the North American Plate (at about 5.33 Ma). It is also possible that at about 18 Ma, at the time of fault initiation, the Los Angeles Basin was at the same point. The study uses heavy mineral analysis (HMA) and associated techniques to test the hypothesis that an ancestral Colorado River supplied sediment both to the Los Angeles Basin and the Salton Trough. Analysis of HMA data suggests a broadly similar source for some of the sediment in the two basins, and for the modern river. The data also indicate changes in the catchment area, suggesting that the Colorado River became fully integrated across the Colorado Plateau by the early Pliocene.

The Fish Creek-Vallecito section is the most stratigraphically complete and structurally intact Neogene exposure in the Salton Trough, and thus provides a useful reference section for regional stratigraphie revision and historical interpretation of the early Gulf of California and Colorado Delta. The section comprises a marine sequence (Imperial Formation) bracketed by nonmarine units (Split Mountain and Alverson Formations below, Palm Spring Formation and Canebrake Conglomerate above). Recognition of distinct suites of locally-derived and Colorado River-derived sediment, combined with sedimentological evidence, led to revision of this sequence in terms of informal members and geneticstratigraphic units: (1) pre-rift braided-stream deposits (2) syn-rift fanglomerates and volcanics, with local pre-marine evaporites; (3) pre-deltaic marine units, deposited primarily as small fan deltas; a progradational sequence of the ancestral Colorado delta, consisting of (4) an upward-shoaling marine sequence, and (5) a nonmarine deltaplain sequence; (6) lacustrine units; and (7) locally-derived basinmargin alluvium that interfingers with (4), (5) and (6). Neogene palinspastic base maps for paleogeographic mapping were based on displacement histories for the Pacific-North American plate boundary and its constituent faults. The tectonic-sedimentary history consists of: (1) early to middle Miocene rifting that propagated southward from southern California to the Gulf mouth; (2) northward marine transgression of the rift basin, reaching southern California by the late Miocene; (3) development of the San Andreas-Gulf of California transform boundary by inboard transfer of intraplate slip; (4) earliest Pliocene initiation of the lower Colorado River and Delta by rapid epeirogenic uplift of the Bouse Embayment; and (5) late Pliocene or Pleistocene transpressive uplift in the western Salton Trough caused by outboard transfer of slip from the San Andreas fault. The stratigraphic succession in the western Salton Trough resulted largely from tectonic transport through a series of paleoenvironments anchored to the North American plate by the entry point of the Colorado River.