Basin circulation affecting sediment partitioning in a fine-grained carbonate–siliciclastic, subaqueous clinoform: the Upper Jurassic–Lower Cretaceous Vaca Muerta Formation, Neuquén Basin, Argentina

Abstract

Fine-grained sediments deposited under oxygen-deficient conditions are enriched in organic matter and hold considerable economic interest as unconventional reservoirs. The present analysis integrates sedimentological, ichnological and geochemical datasets to understand depositional processes in the Upper Jurassic–Lower Cretaceous Vaca Muerta Formation (Argentina), the most important economic unconventional play outside North America. For this purpose, cores from eight wells and one outcrop were examined. The succession comprises 13 facies associations corresponding to marginal-marine, basin, drift and slope environments, deposited in a mixed carbonate–siliciclastic, shelf-margin, subaqueous clinoform system. Estuarine circulation during warm and humid climates produced a stable stratification. Sediment delivery was dominated by pelagic processes in the bottomset, and hemipelagic processes and fluid mud flows in the foreset. However, the typical estuarine circulation changed to weakened estuarine or anti-estuarine during cool and arid climates, triggering wind-and thermohaline-driven contour (bottom) currents that reworked and oxygenated the complete clinoform system. The analysis suggests that distal–proximal trends are of lesser importance for the delineation of sedimentary environments in settings with contrasting basin circulation regimes and provides several insights to understand sediment partitioning in fine-grained clinoform systems.