THE TRIGO SEDIMENTS -- AN EARLY PLIOCENE OFFLAP-SEQUENCE RECORDING THE DEMISE OF PALEOLAKE BLYTHE, LOWER COLORADO RIVER CORRIDOR, SOUTHWEST USA

Transitional deposits referred to as Trigo sediments (Tts) occur between the Bouse Formation and Bullhead Alluvium in the Blythe basin of southern Arizona and California. Recent investigation of these deposits shed new light on processes involved in the decline of the southernmost lake fed by the downward-integrating Colorado River in the early Pliocene. Tts consists primarily of reworked detritus from Bouse basal carbonate deposits (Tbc) and locally-derived sand and gravel. Relationships between Tts and Bouse deposits are complex. In southern Blythe basin, Tts disconformably overlies Tbc in middle and upper piedmont areas between elevations of ~300 m (a few tens of m lower than the highest Tbc) to ~150 m; are interbedded with Bouse siliciclastic deposits (Tbs) between ~150-80 m; and rest unconformably on Tbs in lower piedmont areas between ~130-90 m. Near the Palo Verde Mtns (CA), Tts that disconformably overlie Tbc in the upper piedmont can be traced down-dip into a conformable relationship with Tbc below and Tbs above, and multiple Tts layers are interbedded with Tbs up section into delta-top cross-bedded sands. In northern Blythe basin near Mesquite Mtn, widespread tributary gravels correlated with Tts unconformably overlie Tbs and are overlain by Bullhead Alluvium.

We interpret these relations as evidence for rapid lake level lowering due to spillover and erosion of the southern paleodivide in the Chocolate Mtns, and redistribution of sediment in the lake as it waned. Lake level decline due to divide spillover after Lawlor Tuff deposition at 4.9 Ma exposed large areas of Tbc in the south and Tbs (delta deposits) in the north to erosion. In the south, reworked Tbc material was redeposited across piedmonts by tributary drainage systems and into the declining lake as density flows. Tts deposition in the southern part of the lake began just before and continued through Tbs deposition; newly exposed delta deposits in northern Blythe basin were recycled and the delta prograded southward as the downstream sill eroded and the dwindling lake filled with sediment. Unconformable relationships between Tts and Tbs near the basin axis in the south record post-lake erosion that occurred roughly synchronous with the first arrival of Colorado River sand to the northern Gulf of California in the early Pliocene.