CONTROLS ON LACUSTRINE DELTA STRATIGRAPHY: INFLUENCES OF WATER DENSITY CONTRAST AND BOTTOM GRADIENT AT A RIVER MOUTH

Contrasting water densities and lake margin gradient at river outlets control sediment dispersal, transport, and deposition on a delta. These controls are typically unrecognizable in lacustrine stratigraphic records due to the dampening effect caused by rapid lake level fluctuations that are characteristic of lakes. The Bisbee Basin in SE Arizona held a saline rift lake (Lake Tohono) in the early Cretaceous where density and gradient controls are apparent in the deltaic stratigraphic architecture, recorded by the lower Shellenberger Canyon Formation (SCF). The lower SCF was deposited as the lake was filled from an axial margin drainage network feeding a delta on a shallow-ramp margin. Lake Tohono was not characterized by dramatic lake level changes in the period of SCF deposition, and the sedimentary effects of salinity-induced density contrast and gentle lake margin gradient are recognizable in the stratigraphy. Density differences between saline lake water and fresh influent limited basal traction currents by causing the distributary plume to overflow. Gently inclined deltaic slope restricted foreset development and the dimensions of sub-lacustrine channels. Paleo-lake Tohono provides a unique look at how the interplay between offshore slope and inflow versus lake water density affect stratigraphic architecture in a lacustrine delta setting.