SEDIMENTOLOGY AND STRATIGRAPHY OF THE SOUTHERN MIOCENE-PLIOCENE BOUSE FORMATION

The ~6.0(?)-4.8 Ma Bouse Formation, exposed along the lower Colorado River, contains a well exposed but debated record of river integration. In this study we use sedimentologic and stratigraphic analysis of the southern Bouse Fm in California and Arizona to interpret depositional processes, relative water depth, stratal architecture, and basin-filling history. Data collected include detailed measured sections, facies descriptions, paleocurrents, bedding orientations, and field mapping of major contacts and units. Basinward bedding dips of 3-8° are typically steeper than the gradient of modern washes and in some areas exert the primary control on distribution of lithofacies. We identify five widespread correlable lithosomes, or lithologically distinct units, from oldest to youngest: (1) basal cobble lag with overlying high-energy calcarenite, fossil hash, and conglomerate with abundant fossils including marine or estuarine molluscs, fish, barnacles, ostracodes, coralline red algae, echinoids, and bryozoans; (2) fine-grained low-energy marl with minor interbedded calcarenite, shell hash, and shale; (3) green claystone locally with interbedded marl; (4) Colorado River sand and gravel; and (5) high-energy reworked calcarenite and conglomerate inset into older Bouse units. Contacts between lithosomes 1-3 are gradationally abrupt deepening-up flooding surfaces that define a transgressive systems tract. Lateral interfingering of calcarenite, marl, and green claystone (lithosomes 1-3) provides further evidence for regional transgression during deposition of the lower Bouse. The southern Bouse Formation thus preserves a systematic sequence-stratigraphic architecture comprising transgressive, highstand, and lowstand systems tracts that record one or two cycles of base level rise and fall. Lacustrine versus estuarine interpretation remains elusive, though the presence of echinoids suggests at least intermittent marine connections. Stratigraphic intervals in the east (Cibola area) are 2-5 times thicker than equivalent intervals in the west (Milpitas Wash), and may record syn-basinal eastward tilting toward a prominent basin-bounding normal fault southeast of Cibola. Future work will seek to refine facies architecture and resolve changes in depositional environment through time.