Palynology and Chronostratigraphy of the Upper Cretaceous in the Subsurface of the Barinas Area, Western Venezuela

Abstract

Palynological and well-log analyses of Cretaceous strata from southeastern Venezuela allow the recognition of third-order eustatic cycles and are the basis for a chronostratigraphic framework with a datum approximately every 0.8 m.y. This definition permits the correlation of events in the local geologic evolution with regionally important sedimentological and tectonic events. An Early Cretaceous marine transgression reached areas north of the Andes before 113 Ma; however, according to our framework, the transgression extended into the Barinas area shortly after 98 Ma. An interval of relatively low sea level, from 95.5 Ma to 91.5 Ma, is represented by oil-bearing sandstones and dolomitic limestones. A second marine transgression reached the area at approximately 91.5 Ma and resulted in the deposition of organic-rich clastics that are correlatable with source rocks from areas to the north. A major hiatus (Santonian-Campanian) encompasses approximately 11 m.y. and probably is related to the emplacement of oceanic rocks in western Colombia. A younger major hiatus (Maastrichtian-Eocene) encompasses approximately 26 m.y. and probably is related to the emplacement of the Lara nappes in northern Venezuela. Two other minor intra-Turonian and intra-Maastrichtian hiatuses are recognized and encompass approximately 1.5 m.y. each.