PICCOLI, LEONARDO, Maraven S. A., Caracas, Venezuela

ABSTRACT: Microfacies and Depositional Environments of Lower Cretaceous Carbonates in Western Venezuela

Petrographic analyses of several hundred sections from western Maracaibo basin cores and outcrops, reveal about 30 microfacies typical of two main settings: ramp and platform (carbonate and mixed siliciclastic-carbonate), deposited from early Barremian to early Cenomanian ("Cogollo Group").

Ramp facies show two different trends in sequential organization: (1) fining-upward mudstones-wackestones containing bivalves, echinoderms, Choffatella sp., ostracods, charophyte fragments and quartz grains organized into an overall regressive unit on an inner ramp; (2) coarsening upward wackestones-packstones rich in bivalves and echinoderms (shoals and outer ramp) deposited as part of a regional transgressive unit. The ramp succession is overlain by pelagic black shales, representing an anoxic event caused by rapid sea level rise.

Platform facies are more varied and their sequential organization is more complex. Facies include ooidal, peloidal and bioclastic/foraminiferal pack-grainstones deposited within the middle platform; localised coral boundstone and related sediments on the middle platform; and calcareous green algae, mollusks and echinoderms wack-packstones typical of middle and inner platform. Intraclast rich facies are common; intraclasts include reworked ooids and peloids, and are associated with larger foraminifera and red or green algae, depending on the depositional environment (inner platform, bank margin, etc...). Further landward (inner platform-continental), carbonate facies are interbedded with siliciclastic sediments, including glauconitic fine sandstones and shales.

AAPG Search and Discovery Article #90987©1993 AAPG Annual Convention, New Orleans, Louisiana, April 25-28, 1993.