Sedimentology and Stratigraphy of the Southern Miocene-Pliocene Bouse Formation

Mindy B. Homan and Rebecca J. Dorsey
Department of Geological Sciences, University of Oregon, 1272 University of Oregon, Eugene, OR 97403

The ~6.0(?)-4.8 Ma Bouse Formation, exposed along the lower Colorado River, contains a debated record of river integration. This study uses sedimentologic and stratigraphic analysis of the southern Bouse Formation in California and Arizona to interpret depositional processes, water depth, stratal architecture, and basin-filling history. Data collected include measured sections, facies descriptions, paleocurrents, bedding orientations, and field mapping of major contacts/units. Basinward bedding dips of 3-8° are typically steeper than modern washes, thus exerting primary control on distribution of lithofacies in some areas. Five lithologically distinct units, from oldest to youngest are: (1) basal cobble lag with overlying high-energy calcarenite, fossil hash, and conglomerate with abundant marine or estuarine fossils; (2) fine-grained, lowenergy 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 interfinger laterally and are gradationally abrupt deepening-up flooding surfaces that define a transgressive systems tract. The southern Bouse thus preserves sequence-stratigraphic architecture comprising transgressive, highstand, and lowstand systems tracts that record one-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 are 2-5 times thicker than equivalent intervals in the west, and may record syn-basinal eastward tilting toward a prominent basin-bounding normal fault. Future work will seek to refine facies architecture and resolve changes in depositional environment through time.

AAPG Search and Discovery Article #90181©2013 AAPG/SEG Rocky Mountain Rendezvous, University of Wyoming, Laramie, Wyoming, September 27-30, 2013