JACQUIN, THIERRY, URA CNRS, Ecole des Mines de Paris, Centre de GEologie, Paris, France, IZASKUN AZPIRITXAGA, BRUNO MURAT, and MARCOS F. ROBERTO, Maraven, Caracas, Venezuela

ABSTRACT: Sequential Evolution of Lower Cretaceous Carbonate Platforms in the Maracaibo Basin, Venezuela

The sequence stratigraphic framework of the Upper Barremian to the Upper Cenomanian series was carried out in detail on cored material and well logs from Perija region and central part of Lake Maracaibo. Sediments are predominantly shallow marine carbonates (muddy lagoons, grainstone bars, pelecypod biostroms, intrabars), with an occasional siliciclastic input (nearshore sand bars, mixed tidal flats).

Three second order cycles representing various stages of the evolution of the Cogollo platform have been identified:

Cycle I (Upper Barremian-Lower Aptian) is a time transgressive unit, bounded at the base by backstepping sandstones (Rio Negro Formation) and at the top by a drowning unconformity (Machiques Member).

Cycle II (Upper Aptian-Middle Albian) is a complete second-order cycle with a regressive phase followed by a long-term transgression. It corresponds mainly to the Apon Formation, and it is limited by two drowning unconformities: the Machiques at the base and the basal Lisure at the top. It is made up of 7 depositional sequences. They are slightly prograding during the regressive phase, aggrading and then backstepping during the transgression.

Cycle III (Upper Albian-Upper Cenomanian) is another complete second-order cycle covering the Lisure, Maraca and La Luna Formations. The drowning unconformity at the top of the Maraca Formation is not the peak transgression, this one is at the top of La Luna Formation at the Cenomanian/Turonian boundary. Cycle III is made up of 15 depositional sequences.

This sequence stratigraphic framework reveals that carbonate reservoirs of the Cogollo group are best developed at the top of highstand systems tracts.

AAPG Search and Discovery Article #90988©1993 AAPG/SVG International Congress and Exhibition, Caracas, Venezuela, March 14-17, 1993.