Environmental controls on trace-fossil distribution in the Upper Jurassic-Lower Cretaceous Vaca Muerta Formation (Argentina): Implications for the analysis of fine-grained depositional systems

Abstract

Over the past years, the interest in organic-rich, fine-grained depositional systems has been growing due to the increasing relevance of unconventional shale reservoirs and several breakthroughs in the study of mud. However, the paleoenvironmental application of ichnologic analyses of these systems is still hindered by the scarcity of discrete trace fossils, small burrow size, and difficulty in obtaining fresh rock samples. The Upper Jurassic-Lower Cretaceous, Vaca Muerta Formation is the most important fine-grained unconventional reservoir in South America, and hence, contains a large density of cores that provide fresh cross-section views with detailed information on mudstone. An ichnologic analysis was carried out on cores and outcrop to develop an integrated ichnologic-sedimentologic model and to understand environmental controls on trace-fossil distribution. The Vaca Muerta Formation shows indistinct bioturbation represented by biodeformational structures generating irregular-laminated and massive mudstone fabrics, and discrete bioturbation and biodeposition structures mostly associated with tuff and heterolithic successions. Trace fossils were grouped into nine ichnocoenoses that illustrate organism responses to environmental controls. Abundant biodeformational structures in mudstone are associated with soupy substrates, whereas a change to softgrounds or loosegrounds occurs during coarser-grained sedimentation (associated with contourite drift, fluid mud flow, slope hemipelagic, and volcanic ash sedimentation), enhancing preservation of discrete traces. Locally, higher-energy events associated with exhumation of consolidated sediment generated firmgrounds colonized by the producers of the Thalassinoides isp. ichnocoenosis. In terms of oxygen, increased bioturbation index, penetration depth, burrow size, and ichnodiversity record an increase in bottom and interstitial water oxygenation. Moreover, an overall food-rich environment precluded specialized feeding and the development of tiered communities. The present study provides a clear picture of the relationship between substrate and oxygen controls in trace-fossil composition and distribution, contributing to the understanding of taphonomic controls and paleoecologic dynamics in fine-grained depositional systems, and providing an additional tool to refine sedimentologic and sequence stratigraphic analyses in similar mudstone successions elsewhere.