PROCESS SEDIMENTOLOGY OF THE SOUTHERN BOUSE FORMATION

The latest Miocene to early Pliocene Bouse Formation preserves an enigmatic record of Colorado River integration. Ongoing debate is focused on whether the southern Bouse was deposited in an isolated lake or a marine estuary linked by a narrow connection to the ocean. This study examines the sedimentology of the southern Bouse Formation near Cibola, Arizona, to interpret depositional processes and paleoenvironments. In this study we identify 9 major lithofacies (with interpretations): (1) basal cobble lag (transgressive ravinement surface); (2) charophyte-rich matted calcareous muddy sand (low-energy shoreline and lagoonal charophyte meadows); (3) well-sorted cross-bedded gravel with rounded local clasts (wave-worked beach gravels and detached nearshore bars); (4) moderately to well-sorted calcarenite with common gravel lenses and pebble beds (proximal bioclastic sandy bottomsets of planar-wedge and cross-bedded gravel bedforms); (5) silty flaggy marl (distal bottomsets of the cross-bedded gravel bedforms) (6) chalky clay marl (distal carbonate deposited by suspension settling in nearshore lagoonal and offshore low-energy settings); (7) well-sorted bioclastic barnacle hash interfingering laterally with chalky clay marl (nearshore to shoreface planar-wedge and bioclastic bars); (8) Colorado River derived claystone, mudstone, and trough cross-bedded sandstone (pro-delta, delta front, and possible fluvial channel); and (9) mixed carbonate-siliciclastic calcarenite (upper limestone) grading upwards into siliciclastic-dominated calcareous conglomerate (standing body of water with prograding local alluvial fan deltas). This unit is conformable to angular-unconformable with underlying facies across the basin.

Abundant pebble layers and hummocky cross-stratification suggest storm-waves were an important mechanism for dispersing sediment across and along a carbonate shelf. Common wave-ripple and flaser-ripple lamination, wavy bedding, lenticular bedding, and herringbone cross-bedding suggest but do not require that tides exerted an important influence on depositional processes. These sedimentary structures are common, but not exclusive to marine environments. Thus, the lake versus marine-estuary interpretation for the Bouse Formation remains unresolved.