AZPIRITXAGA, IZASKUN, and BRUNO MURAT, Maraven S.A., Caracas, 1010A Venezuela

ABSTRACT: Types of Cretaceous Carbonate Platforms in Western Venezula

Detailed sedimentological, biostratigraphical and wireline-log analyses of wells in the Maracaibo Basin have revealed the major depositional settings of Early Barremian to Early Cenomanian "Cogollo Group."

Three successive second-order, regressive-transgressive cycles, were controlled by relative sea level and paleomorphology of the basin. A carbonate ramp which formed during the Early Barremian- Early Aptian (stage 1) evolved progressively into a platform with clastic and carbonate build-ups during the Early Aptian to Mid-Late Albian (stage 2), and finally, into a typical carbonate platform at the end of the third stage. The Merida paleohigh is evidenced by the pinch out towards the south-east of the transgressive sediments (first stage), and by the cycle thinning and siliciclastic increase in the same direction (second stage). By the end of the third stage, the paleohigh was ineffective and the type of facies deposited on the platform are more laterally persistent.

Facies are specific to each stage. The first setting consists predominantly of muddy carbonates (limestones and dolomites) deposited along a ramp dipping towards the west and northwest. Lateraly equivalent high-energy facies occur in the southeast, close to a possible shoreline. The second setting is characterized by the deposition of high-energy, bioclastic and oolitic sand bodies, together with nearshore sand bars trending NNW-SSE. The third sequence setting is dominated by lithoclastic glauconitic carbonate sands at the base, followed by retrograding limestones. The Cogollo Group is overlain by organic shales (La Luna Formation) representing the Cenomanian drowning event.

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