Unravelling the fabrics preserved inside early diagenetic concretions: Insights for the distribution, accumulation and preservation of organic-rich mud in the interior of epicontinental basins

Fine-grained sedimentary rocks generally undergo severe mechanical compaction during burial, which compli-cates the recognition of primary mudstone fabrics and associated sedimentary features. Early diagenetic concre-tions, however, provide a rare glimpse of primary fabrics because cement filling the pore space prevents the collapse of original grain arrangements. Hand specimens of concretions collected from the basal condensed sec-tion of the Late Jurassic-Early Cretaceous Vaca Muerta Formation (Neuquen Basin, Argentina), allow for analysis of sedimentary processes responsible for the dispersal, accumulation and burial of organic carbon-rich sediment in an epicontinental sea. Representative samples from central basin depositional localities were examined by op-tical, scanning electron microscopy and energy dispersive X-ray spectroscopy. Petrographic observations were complemented with palynological and organic geochemical analyses. Close examination of uncompacted fabrics reveals a significantly more complex and dynamic depositional scenario than previously assumed (suspension settling). Although many of the component grains in the studied samples were originally delivered to the sediment-water interface by suspension settling processes (i.e., marine snow, hypopycnal plumes, pumice rafts), there is substantial evidence of episodic sedimentation controlled by punctuated events of seafloor distur-bance and erosion. The common presence of muddy intraclasts indicate that the seafloor was frequently reworked by bottom currents that caused the widespread distribution of organic carbon-rich sediment across distal basin depositional environments. Bottom current circulation supplied oxygen to the sediment-water inter-face and created suitable conditions for benthic life, contravening the assumption of bottom water anoxia as a prerequisite for organic carbon preservation. The excellent preservation state of freshwater algae (Pediastrum complex) suggests that organic matter contained inside mud composite particles can travel long distances before being deposited in distal depositional settings. Encapsulation protects organic components from mechanical/bio-genic degradation and provides an anoxic microenvironment for preventing the oxidation of the organic matter contained inside of mud composite grains. The study shows that organic carbon encapsulation may be an impor-tant mechanism for organic carbon preservation in relatively energetic and non-anoxic settings, calling for a crit-ical reappraisal of the processes responsible for the sequestration of organic carbon from the biosphere and its long-term storage in organic-rich mudstone successions.(c) 2022 Elsevier B.V. All rights reserved.

Automated summary

This study provides insights into the sedimentary processes of mudstone and the preservation mechanisms of organic carbon-rich sediment in non-anoxic environments. The analysis of concretions from the Late Jurassic-Early Cretaceous Vaca Muerta Formation reveals a complex and dynamic depositional scenario, with evidence of episodic sedimentation and bottom current reworking. Encapsulation of organic matter within mud composite particles plays a crucial role in the long-term storage of organic carbon.