Organic-rich, fine-grained contourites in an epicontinental basin: The Upper Jurassic-Lower Cretaceous Vaca Muerta Formation, Argentina

Abstract

Over the last few years, recognition and characterization of sedimentary processes in fine-grained successions is receiving considerable attention in part due to the increased importance of unconventional reservoirs. Recent sedimentologic analyses of these successions have revealed abundant traction transport structures that suggest bottom current activity of different origins. The present study constitutes a detailed sedimentologic analysis of the Upper Jurassic-Lower Cretaceous Vaca Muerta Formation (Neuqu´en Basin, Argentina), which comprises organic-rich, fine-grained subaqueous clinoform deposits. Locally, intervals showing traction structures such as parallel lamination, ripple cross-lamination and cross-bedding, and displaying high bioturbation, occur. These intervals can be differentiated into three facies associations, namely crinoidal mixed to calcareous mudstone, mixed to calcareous mudstone, and calcareous to mixed mudstone interbedded with calcareous mudstone. These facies associations were deposited by contour currents showing semi-permanent activity, low sediment concentration, and generating long-term oxygen supply to the seafloor. Locally, some successions display an increasing to decreasing bioturbation intensity pattern that may have a similar origin to the bi-gradational sequence of contourites. The present sedimentologic analysis supports the idea that contour currents are associated with high bioturbation levels due to increased oxygenation, which controls the degree of preservation of traction structures, and consequently the possibility to delineate these processes in fine-grained successions. These currents occurred in relatively shallow water, probably as part of a weakened estuarine or anti-estuarine, basin-wide circulation system intensified by the enhanced action of dense water cascading during arid and cool climate periods.