Palynostratigraphy and lithostratigraphy of Upper Cretaceous and Paleogene outcrop sections, Mérida Andes (Maracaibo Basin), Western Venezuela

Abstract

Detailed palynologic and lithostratigraphic studies of outcrop sections on the western flank of the Mérida Andes in Western Venezuela were undertaken to establish a framework for interpreting and correlating regional and local unconformities from the outcrop to subsurface data. The Upper Cretaceous to lower Paleogene section of the Maracaibo Basin consists of laterally continuous rock units attributed to the Mito Juan and Barco/Catatumbo formations, which are characterized by marine sediments deposited relatively close to shore. The continuity of plant and dinoflagellate species across the Cretaceous-Paleogene boundary in this area may have potential significance to K/Pg studies elsewhere. The lower to upper Paleogene Los Cuervos, Mirador, and Carbonera formations were predominantly deposited in non-marine environments, though palynologic and stratigraphic data from each unit show the variable influence of marine and lacustrine conditions. A new chronostratigraphic framework for the study area (the “Venezuelan Composite Standard”) was developed based on first and last appearances of spores, pollen, and dinoflagellates, and three pollen acme events and calibrated to the latest time scale. Graphic correlation plots of the most complete outcrop sections at Río Lobaterita, Río Lora, and Río Chama indicate the presence of significant hiatuses in time, due to erosion and non-deposition (or a combination of the two) reflecting the complex structural and depositional history of the southeastern Maracaibo Basin. A regionally significant upper Paleocene to lower Eocene unconformity, as well as several local and regional intra middle Eocene unconformities appear to have been caused by a drop in base level (sea level plus structural uplift) forming incised valleys over much of the study area. Unconformities are also present in the upper middle Eocene to Oligocene and represent ravinement surfaces caused by transgressive marine reworking of pre-existing units. These surfaces are often marked by the presence of dinoflagellates within the upper Eocene Carbonera Formation. Depositional environments during much of the Upper Cretaceous to Paleogene succession vary from fluvial-floodplain to marginal marine, with newly noted periods of widespread lacustrine deposition in all geologic units. Temporal differences between similar Paleogene lithostratigraphic units in northeastern Colombia and southwestern Venezuela are likely due to the influence and timing of structural events on regional and local depositional environments.