ABSTRACT: Toe-of-Slope Depositional Processes and Stratal Relationships within the Permian Lamar Limestone, McKittrick Canyon, Guadalupe Mountains, Texas

BROWN, ALTON A., and ROBERT G. LOUCKS, ARCO Oil and Gas Co., Plano, TX

Decrease in paleoslope at the transition from the Capitan foreslope to the Lamar basin deposits is associated with a change in grain size and a change in depositional processes. Lower foreslope, reef-derived, boulder conglomerates were deposited by debris flows and modified grain flows. Upper and medial toe-of-slope intraclastic and skeletal packstones were deposited by debris flows and high-density turbidity currents, whereas lower toe-of-slope skeletal wackestones were deposited by low-density turbidity currents.

Units dominated by steeply dipping boulder conglomerates downlap onto gently dipping wackestone units, and wackestone units onlap steeply dipping boulder conglomerate units. This bedding termination pattern is caused by differences in depositional processes. Matrix-poor boulder conglomerates were deposited on steep slopes because gravity flows could transport these coarse-grained sediments only on steep slopes. Mud-rich, low-density turbidity current and debris flow deposits had low angles of repose, so they were remobilized if the slope became too steep.

The systematic bedding terminations are related to changes in the types of sediment supplied to the basin margin, which was a function of sea level. Matrix-poor boulder conglomerate units were sourced by fringing reefs when the shelf was emergent (early transgression). The carbonate mud in toe-of-slope wackestones was derived when the shelf was flooded (highstand). Boulder conglomerates with lime-mud matrixes formed after the Capitan reef had caught up with the sea-level rise (late highstand).

The sea level history interpreted from sediment supply is almost the opposite of that indicated by siliciclastic sequence stratigraphy criteria. This example demonstrates the need to understand the cause of bedding termination patterns before using them to interpret sea level history.

AAPG Search and Discovery Article #91012©1992 AAPG Annual Meeting, Calgary, Alberta, Canada, June 22-25, 1992 (2009)