Milankovitch Climate Cyclicity and its Effect on Relative Sea Level Changes and Organic Carbon Storage, Late Cretaceous Black Shales of Colombia and Venezuela

Abstract

The Late Cretaceous Villeta Group and La Luna Formation shows remarkable depositional cyclicity attributable to Milankovitch climate cycles. Each 30-60 cm thick hemicycle is composed of a basal gray shale, a medial black, organic-rich shale, and an upper gray shale with a dense argillaceous limestone cap. Fourier time-series analysis revealed peak frequencies of 500, 100, and 31 ka (blending 21 and 42 ka data). ThiS cyclicity reflects possibly wet cooler (shale) to dry, possibly warm (limestone) climatic changes and their influence on relative sea level, sedimentation rates/patterns, productivity, water chemistry and stratification. Wet/cool hemicycles may produce slight lowering of sealevel, increased rates of clay sedimentation, diminished carbonate production, water stratification, increased productivity among noncalcareous marine plankton, and increased Corg production and storage. Dry/warm hemicycles may produce a slight rise in sealevel, and return to normal marine conditions with low Corg storage. Source rock quality may depend upon the predominance of wet over dry climatic phases. Differences between climate-forced cyclicity and random facies repetition, are shown by contrasting observed lithological patterns and geochemical signals with litho- and chemostratigraphy generated from random models. Accomodation space plots (Fischer plots) for cyclically interbedded black shale-pelagic limestone sequences, allowed prediction of facies behavior, shoreline architecture,more » and quantitative analysis of relative sea level. The synchroneity of Milankovitch cycles and changes in hemicycle stacking patterns, were tested against a new high-resolution event-chronostratigraphic and biostratigraphic framework for NW South America. Geochemical spikes and hemicycle stacking patterns occur consistently throughout the sections measured, supporting the correlation potential of cyclostratigraphy.« less