A PERSPECTIVE ON LAKE FILLING, SPILLING, RIVER INCISION AND AGGRADATION, AND DEFORMATION DURING THE EARLY PLIOCENE IN THE BLYTHE BASIN, ARIZONA AND CALIFORNIA

We have proposed a model for early development of the lower Colorado River (LCR) through a series of downstream-spilling lakes fed by the river water and sediment. Bouse Formation accumulated in upstream lakes as spillover erosion lowered lake levels prior to their demise. Deep incision followed in each lake basin as the river adjusted to a new lower base level, recycling stored sediment into the lower lakes and ultimately, the Gulf of California. Massive river aggradation (Bullhead Alluvium, BA) then filled post-Bouse erosional topography. Diverse models have been proposed for the southernmost Blythe basin, but general facies and stratigraphic relationships are consistent with the lake filling/spilling model and subsequent river aggradation in the early Pliocene, with syn- and post-depositional subsidence in the LCR valley axis.

Basal Bouse carbonate deposits (Tbc) overlie alluvial fan deposits on valley flanks and extend higher in the landscape over erosional bedrock topography on many basin margins, recording a rising water level to ~300m asl at ~4.8 Ma in widely separated areas. Tbc are much thicker in the southern part of the basin, which may reflect late arrival of river-supplied fine siliciclastic deposits (Tbs). Tbs are very thick in outcrop, and the base is well below sea level in the axis of the Blythe Basin. They occur up to >300m asl in northern outcrops, where they were likely deposited up to about the maximum lake surface, but are no higher than ~160m asl in the far south, where they may have filled the diminishing lake. Bio- and siliciclastic-rich deposits in the south that erosionally overlie Tbc on the valley flanks may record progressive lowering of the lake level after spillover. BA deposits are inset into Tbs and occupy large swaths of valleys where Tbs are not exposed; they are also interbedded with alluvial fan deposits that erosionally overlie Bouse deposits. Thus river aggradation began after substantial post-Bouse incision that could only have occurred if the region was above sea level. The BA/Tbs contact is now ~100m below sea level in the basin axis near Blythe, requiring substantial post-BA subsidence, but maximum levels of BA on the basin margins rise from ~250 to 300m asl from south to north. Thus, we infer that most syn- and post-Bouse vertical deformation was restricted to the LCR valley axis.