Abstract: Carbonate Megabreccias: Depositional Processes and Stratal Geometries (Cenomanian, South-Central Pyrenees, Spain)

Peter A. Drzewiecki, J.A. (Toni) Simo

The lower slope of the Cenomanian Santa Fe platform reflects three distinct depositional processes: (1) pelagic deposition, (2) slumping of slope

deposits, and (3) collapse and redeposition of the margin-derived clasts. Slumps, resulting from instability on a high angle (10-12°) fine-grained slope, and progradation of the margin over slope sediments resulted in the episodic collapse and redeposition of slope and margin sediments. Sea level fluctuations appear to have had a negligible effect on the Santa Fe slope deposits.

Lower slope sediments of the Santa Fe platform contain two breccia types encased in fine-grained pelagic wackestones: (1) clast-supported megabreccias (<20% of slope volume) composed of angular, margin-derived, packstone-grainstone blocks (<7m), and (2) matrix-supported megabreccias composed of contorted and reworked beds of slope origin. Individual beds of the clast-supported breccia are <30m thick and have erosive bases and planar tops. They represent single debris flow events, and are either confined to shallow channels (<300m wide) or form laterally continuous sheets up to 500m wide. These beds downlap down-dip and onlap up-dip. The matrix-supported breccias (>50% of the lower slope deposits) form extensive sheets up to 10m thick and 500m wide. The upper slope facie is thinner, lacks megabreccia, and is interpreted as a bypass slope.

Carbonate slope megabreccias are difficult to interpret only in a sequence stratigraphic framework because they often represent deposits formed by a number of erosional and depositional processes. Understanding these processes, lithologies, and geometries provides valuable information on the relative effects of sea level, tectonics, and platform morphology on carbonate slopes.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994