Carbonate Depositional Styles Controlled by Siliciclastic Influx and Relative Sea-Level Change--Lower Cretaceous, Central Lake Maracaibo, Venezuela

AZPIRITXAGA, IZASKUN, and DON G. BEBOUT, University of Texas at Austin, Austin, TX

The Aptian-Albian Cogollo Group in the Maracaibo basin is represented by two major depositional styles--upward-fining and upward-coarsening cycles. The upward-fining cycles consist of burrowed skeletal, orbitolinid, and peloid grainstones at the base that grade into shaly mudstone and skeletal and peloidal wackestones at the top; firmgrounds occur in the mudstones at the top of several of these cycles. The overlying upward-coarsening cycles consist of skeletal wackestones at the base that grade upward into laminated and cross-bedded ooid and skeletal grainstones at the top.

All of the carbonates of the Cogollo Group were deposited in shallow water. The upward-filling cycles resulted from deposition under subtidal conditions and represent a dominantly muddy sea floor with lag deposits of carbonate sand at the base. The upward-fining trend to these cycles is thought to be the consequence of periodic influx of siliciclastics at the top of the cycles, which results in lower carbonate production, and rise in relative sea level, which results in drowning episodes over the large carbonate platform. The overlying upward-coarsening cycles represent more open-water deposits that accumulated as broad subtidal to intertidal shoal complexes.

A Fischer plot of this carbonate section suggests, in general, a gradational increase in water depth represented by the thick section of upward-fining cycles of the lower and middle parts of the Cogollo. The thin sequence of upward-coarsening cycles at the top of the Cogollo represents a brief return to shallow-water, high-energy conditions prior to abrupt deepening of water and subsequent deposition of the overlying Upper Cretaceous La Luna carbonates.

AAPG Search and Discovery Article #91004 © 1991 AAPG Annual Convention Dallas, Texas, April 7-10, 1991 (2009)