Carbonate Sediment Gravity Flows and Eustatic Cycles in a Distally Steepened Ramp System: Upper Pennsylvanian of Southeastern California

Lyndon A. Yose, Paul L. Heller

Basin margin deposits of the lower Keeler Canyon Formation and time-equivalent outer platform facies of the Bird Spring Formation record a complex interaction of terrigenous influx, eustatics, and possible tectonic overprinting during their deposition. Outer platform facies alternate between low-energy drowned platform deposits (rhythmically bedded micrites) and higher energy rimmed platform facies (bioclastic wackestones to packstones). Lithologic and facies associations in the adjacent basinal deposits indicate that sediment-gravity flows derived from the platform margin accumulated in a base-of-slope apron system. An alternation between low-energy mudstone intervals (3-7 m thick) and mixed-clastic sediment gravity flow packages (10-30 m thick) indicate that sediment su ply to the apron system was episodic. Megabreccia debris sheets (9-17 m thick) which cap several of the cycles indicate partial collapse of the platform margin.

These depositional cycles in both the outer platform and basin margin environments are attributed to sea level changes of possible glacio-eustatic origin. During highstands the outer platform was distally-steepened (rhythmically bedded micrites) and sediment supply to the basin margin was very low (mudstones). As sea level fell, the outer platform was subjected to higher energy more open marine conditions (bioclastic limestones) and the mass-transport system was activated (gravity flows). In addition, as sea level fell, siliciclastics were transported across the shelf to the platform margin where they, along with the in-situ bioclastic sediments, were incorporated into sediment gravity flows. The capping debris sheets represent lowstand collapses. In this mixed-clastic system, the fre uency and volume of sediment gravity flows are greatest during sea level falls rather than during highstands as proposed for pure carbonate systems such as the Bahamas.

AAPG Search and Discovery Article #91038©1987 AAPG Annual Convention, Los Angeles, California, June 7-10, 1987.