LACUSTRINE OFF-LAP SEQUENCE IN BOUSE PALEOLAKE BLYTHE: A PROXY FOR UPSTREAM LOCO BASINS AND OTHER SIMILAR CLOSED-TO-OPEN BASIN TRANSITIONS

The southern Blythe basin in the lower Colorado River corridor (LOCO) has a distinctive sequence of mixed bioclastic and siliciclastic deposits stratigraphically between the underlying Bouse Formation lake-filling transgressive sequence and the earliest through-flowing Colorado River deposits of the Bullhead Alluvium. These informally named Trigo sediments are predominantly tributary siliciclastic deposits with several lithofacies in subaerial and subaqueous alluvial margin environments, including subordinate nearshore lacustrine bioclastic-rich facies. Trigo sediments rest unconformably on Bouse basal carbonate between ~300 and 160 meters above sea level (masl), are interbedded with Bouse siliciclastic deltaic sand and mud between ~160 and 80 masl, and are interbedded with early Bullhead Alluvium below ~110 masl. Thus, Trigo sediments are interbedded with the uppermost Bouse Formation and lowermost Bullhead Alluvium and represent a key transitional and gradational unit that bridges space and time between the two.

Based on extensive mapping and field observations, we interpret the Trigo sediments as an off-lap sequence of tributary-sourced deposits derived from and deposited across piedmonts covered with Bouse deposits. Trigo deposition was linked initially to declining water levels and interacted with lacustrine deposition at short-lived lake-stands as the lake diminished, and later to erosional incision in the Blythe basin associated with the development of the through-flowing Colorado River. Paleolake Blythe was larger, longer-lived and more fossiliferous than smaller upstream basins in LOCO that were filled with Colorado River water and sediment. These smaller upstream basins also have siliciclastic wedges of retrograde fanglomerates and fan delta deposits between Bouse and Bullhead formations we interpret as similar off-lap sequences but lacking the bioclastic facies, likely due to shorter lake lifespans. Without aggradation of the Bullhead Alluvium, a record of an off-lap sequence would likely never have been preserved. Given the large number of paleolakes in the Basin and Range, we present sedimentary and stratigraphic patterns diagnostic of off-lap sequences related to fill-and-spill integration that are largely understudied or not recognized.