Abstract: Chemostratigraphy: Applications in the Tachira Depression, Tachira State, Venezuela
Rock analyses (40 chemical parameters) from densely sampled Cretaceous sections from La Luna, Navay, Aguardiente and Escandalosa formations in southwestern Venezuela, combined with multivariate statistical analysis, allowed statistical characterization of formations, environmentally meaningful clustering of correlated geochemical variables, as well as calculation of chemical accumulation rates.
Chemofacies, defined from accumulation rates, were used for correlation and depositional environment definition. Organic matter maturity does not affect NiN ratios, but causes homogenization in Ca/Sr ratios. Th/V ratios clearly separate shelf vs. oceanic fades, closely matching those from biofacies analysis. Cycles in Th/V repeat every 60 m (Navay) to 20 m (La Luna), corresponding to Milankovitch climatic cycles. Shales (high Th/V) represent cooler-shallower sedimentation periods, and lodestones (low Th/V) represent warmer-deeper ones.
Differences in accumulation rates were used for establishing changes in: source rock (La-Ce), either due to tectonism or variations in marine current pattern; redox conditions (Ni-V); clay mineralogy (Al-K); energy level (Al-Si); and salinity (Mg-Na). Finally, chemostratigraphy, combined with reliable structural and stratigraphical field data, has proven to be a powerful tool for unravelling tectono-stratigraphical histories.
AAPG Search and Discovery Article #90951©1996 AAPG International Conference and Exhibition, Caracas, Venezuela