Maibi Ruiz¹, Maria A. Lorente¹

(1) PDVSA, Caracas, Venezuela

ABSTRACT: Paleogeography Of Late Cretaceous Upwelling In The Barinas Basin: A Prediction Tool For Potential Source Rock Distribution

New studies show evidence of repetitive upwelling events taking place in the southwestern margin of Thetys from the Turonian through the Maastrichtian. Based on comparative analyses of sedimentological and biotic characteristics, Zones II and III of the upwelling model are interpreted in seven subcropping sections along the northern border of the Barinas Basin, Venezuela.

The presence of laminated dark shales, phosphate pellets, and abundant fish debris, glauconite, diatoms, radiolarian and dinoflagellates, and biogenic chert, support the upwelling model within a continental shelf and with the upwelling center located over the mid-inner shelf.

The following biofacies were established: Planktic Foraminifera Biofacies associated to light yellow glauconitic wackestone-mudstone limestones, suggesting high productivity and well oxygenated marine conditions; Buliminids and Planktic Foraminifera, Diatoms,Radiolaria and Calcareous Nannoplankton Biofacies in dark gray calcareous shales and wackestone-mudstone, Diatoms, Radiolarian and Dinoflagellate Biofacies in dark gray shales and dolomites interstratified with yellow-brown sandstone and black phosphorites and the Fish debris Biofacies.

Low diversity and high abundance assemblages, typical of opportunistic species characterize the microfaunal associations. These assemblages are considered survivors of anoxic to dysoxic conditions due to the upwelling. The subsequent high productivity conditions, associated to high salinity and low oxygen level of the water mass, results in high mortality which is reflected by the presence of fish debris and phosphate nodules.

The biofacies succession shows a tendency of the upwelling to have changed from more distal during the Turonian-Coniacian to a more proximal location during the Santonian-Maastrichtian, this migration path allows to predict the startigraphic and geographic distribution of the richest organic intervals.

AAPG Search and Discovery Article #90906©2001 AAPG Annual Convention, Denver, Colorado