PITTELLI, ROSINA, and VALENTI RULL,* Maraven, S.A., Caracas, Venezuela
ABSTRACT: High Resolution Biostratigraphy of Oligo-Miocene Leon and Chama Formations: An Integrated Approach for Sequence Stratigraphy Analysis
Based on foraminiferal and palynological high resolution studies, a set of Sequence Boundaries (SB) and Maximum Flooding Surfaces (MFS) were identified for the Venezuelan Northwestern Andean Foothills Leon and Chama formations. Changes in abundance patterns of foraminifera and palynomorphs, in faunal/floral composition and distribution, together with quantitative studies
of particulate organic matter allowed picking of SB 30, 21 and 15.5 (very low values of fossil abundance and faunal discontinuities, recognized by a rapid stratigraphic change in biofacies and faunal assemblage and the associated mineralogical contents) and MFS 18.5, 16 and 15 (abundant fossils). The Leon Formation represents coastal plain and swamp deposits with some minor fluctuations in the seawater level. The top of the Leon Formation shaly unit is bounded by SB 30, according to Haq B.U. et al (version 1992), marked by a decrease in fossil abundance. An increase in faunal/floral content close to the top of the section coincides with the MFS 18.5. The upper part of the formation is marked by a sandy unit with SB 16.5, characterized again by a decrease in fossil abundance at its top The Chama Formation was deposited in a transitional environment, with minor seawater level fluctuations. Climate type was tropical humid, with seasonal precipitations, except at the Early Miocene arid or semiarid phase. Mangrove and rain forest vegetation dominated throughout the Early to Middle Miocene. MFS 16 was picked on the basis of a high abundance of microforams and glauconite.
AAPG Search and Discovery Article #90988©1993 AAPG/SVG International Congress and Exhibition, Caracas, Venezuela, March 14-17, 1993.