EVIDENCE FOR MARINE RE-FLOODING OF THE LOWER COLORADO RIVER VALLEY AFTER THE RIVER FIRST RAN THROUGH IT: UPPER BIOCLASTIC MEMBER (UBM) OF THE SOUTHERN BOUSE FORMATION

The late Miocene to early Pliocene Bouse Formation south of Blythe, Calif., contains a record of initiation of the Colorado River (CR) for which two models have been proposed: (i) deposition of Bouse basal carbonate in a large inland lake isolated from the sea, filling of the lake with CR-derived delta sediment, and final lake lowering with arrival of the earliest through-flowing river gravels (Bullhead Alluvium); or (ii) deposition of Bouse basal carbonate in a marine tidal seaway, arrival of earliest through-flowing CR river channel sands, re-flooding of the southern valley by shallow marine water during a hiatus in CR sediment output, and final resumption of the through-flowing river (Bullhead). In model #1 the upper bioclastic member (UBM) is a nonmarine deposit composed of sediment reworked from older Bouse carbonates and other local sources during final lowering of the Bouse lake. In model #2 the UBM is a shallow marine unit that overlies older Bouse carbonates and river sands along a regional unconformity, recording a complex history of secular changes in relative sea level and CR sediment discharge. The UBM contains 3 main facies: (1) fine-grained well sorted sandy carbonate with horizontal stratification and wave-ripple lamination, locally interbedded with salty CR red mudstone with desiccation cracks; (2) coarse-grained sandy and pebbly bioclastic carbonate with common flat-based wave-formed gravelly ripples (wavelengths 0.5-3.0 m); and (3) calcareous-matrix conglomerate transitional to alluvial-fan gravels. Process sedimentology requires deposition of facies 1 and 2 by strong wave currents in shallow water. New results of paleontologic study indicate a marine origin for this unit. Fossils in the UBM include distinctive upward-branching segmented coralline-like red algae with no known modern relative but definitely a form of marine calcareous red algae, commonly preserved as reworked clasts and rarely seen in life position. We also document uncommon echinoderm spines, whole articulated barnacles, shallow marine foraminifers Ammonia beccarii, and nondiagnostic mollusks and ostracodes. These and related data indicate that shallow marine waters inundated the lower Colorado River valley shortly after the river first ran through it, though the underlying controls on these events remain poorly understood.