EVIDENCE FOR AN OFF-LAP SEQUENCE BETWEEN THE BOUSE FORMATION AND BULLHEAD ALLUVIUM IN SOUTHERN BLYTHE BASIN, ARIZONA AND CALIFORNIA

In southern Blythe Basin within the lower Colorado River corridor, an enigmatic sequence of mixed bioclastic and siliciclastic deposits mapped as the upper bioclastic unit (UBU), originally “Unit A” by Metzger et al. (1973), is stratigraphically bound between the underlying latest Miocene and early Pliocene Bouse Formation transgressive sequence and the overlying early Pliocene Bullhead Alluvium aggradation sequence. Based on extensive field mapping and well data, we present the following unique stratigraphic and sedimentological relationships between the UBU and its bounding units: 1) The UBU spans the interval of time between Bouse carbonate and siliciclastic deposition and Bullhead aggradation, 2) deposits are derived from adjacent ranges and eroded from Bouse carbonate and underlying units, 3) bioclastic content is higher near bedrock margins at higher elevations and decreases basinward and toward lower elevations, 4) surface exposures and subsurface well data show this unit is 30 to 35 m thick near bedrock margins and is 5 to 10 m thick the near valley axis, 5) UBU paleoslopes dip basinward at angles similar to modern alluvial slopes, and grade to elevations near the modern valley axis except in areas subsequently deformed by subsidence, 6) UBU disconformably overlies lower Bouse carbonate deposits between 170 and 290 m-asl at gradients similar to modern washes, 7) UBU is interbedded with Bouse siliciclastic sand and mud in a mixed subaerial and subaqueous environment at elevations between about 130 and170 m-asl, 8) UBU is unconformable with Bouse siliciclastic deposits between about 130 to <80 m-asl, 9) UBU deposits coarsen upward and grade into overlying fanglomerate deposits, and 10) fanglomerate deposits interpreted to be syn- to post-UBU deposition are interbedded with early Bullhead Alluvium.

Based on these findings we provide a new interpretation of the transitional UBU deposits as a sequence-stratigraphic unit bound by unconformities but with conformable end-members. We infer that UBU deposits represent a retrograde or off-lap sequence of tributary-sourced alluvial margin sediments derived from and deposited across a piedmont covered with Bouse carbonate and siliciclastic deposits that was exposed as the water level in Blythe basin declined prior to aggradation of the Bullhead Alluvium.