A SEQUENCE-STRATIGRAPHIC INTERPRETATION OF THE UPPER BIOCLASTIC UNIT CAPPING THE BOUSE FORMATION IN THE CIBOLA AREA, ARIZONA AND CALIFORNIA

In the southern Blythe Basin, the stratigraphically highest and youngest unit of the Bouse Formation is an enigmatic combination of siliciclastic and bioclastic deposits. This upper bioclastic unit (Tbz) has a wide range of composition, grain size and clastic texture, and exhibits several intermixed lithofacies generally separated into a calcarenite or bioclastic lithofacies, a quartz arenite lithofacies, and gravel-dominated lithofacies. Vertical and lateral distribution of lithofacies is complex, but where relatively thick sections of these deposits are preserved they generally coarsen upwards. Below ~110 m above sea level (asl) unit Tbz unconformably overlies non-deformed to strongly deformed Bouse siliciclastic deposits (unit Tbs). Between ~110 m and 160 m asl unit Tbz is considered conformable with the underlying Bouse siliciclastic interbedded sand and mud. This is the zone where relatively thick Tbz deposits coarsen upward and grade semi-conformably into overlying siliciclastic post-Bouse fanglomerate. Between ~160 m to ~205 m asl (the highest unambiguous Tbz outcrop), Tbz overlies basal Bouse carbonate deposits over an obvious erosional unconformity. From these relationships, we infer that the upper bioclastic deposits represent an off-lap sequence of tributary-sourced alluvial fan and margin sediments rich in reworked basal Bouse carbonate deposits. Locally, these deposits record a near-shore environment on the margin of a standing body of water. Thus, these deposits were partially derived from and deposited across a piedmont covered with Bouse carbonate deposits that were re-exposed as the base level of a standing body of water declined.