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

OTHARÁN, GERMÁN; ZAVALA, CARLOS; SCHIEBER, JUERGEN ; OLIVERA, DANIELA; MARTÍNEZ, MARCELO A.; DÍAZ, PABLO; YAWAR, ZALMAI ; AGÜERO, LUIS

SEDIMENTARY GEOLOGY

ELSEVIER SCIENCE BV

Lugar: Amsterdam; Año: 2022 vol. 440 p. 1 - 21

0037-0738

Fine-grained sedimentary rocks generally undergo severe mechanical compactionduring burial, which complicates the recognition of primary mudstone fabrics andassociated sedimentary features. Early diagenetic concretions, however, provide a rareglimpse of primary fabrics because cement filling the pore space prevents the collapseof original grain arrangements. With the purpose of analyzing the sedimentaryprocesses responsible for the dispersal, accumulation and burial of organic carbon-richsediment in epicontinental seas, hand specimens of concretions were collected fromthe basal condensed section of the Late Jurassic-Early Cretaceous Vaca MuertaFormation, Neuquén Basin, Argentina. Representative samples from central basindepositional localities were examined by optical, scanning electron microscopy andenergy dispersive X-ray spectroscopy (EDS). Petrographic observations werecomplemented with palynological and organic geochemical studies. The closeexamination of uncompacted fabrics reveals a significantly more complex and dynamicdepositional scenario than previously assumed (suspension settling paradigm).Despite the fact that component grains in the studied samples include abundantmaterial originally delivered to the sediment-water interface by suspension settlingprocesses (i.e., marine snow, hypopycnal plumes, pumice rafts), there is substantialevidence of an episodic sedimentation controlled by punctuated events of seafloordisturbance and erosion. The common presence of muddy intraclasts indicate that theseafloor was frequently reworked by bottom currents that caused the widespreaddistribution of organic carbon-rich sediment across distal basin depositionalenvironments. Bottom current circulation supplied oxygen to the sediment-waterinterface and created suitable conditions for benthic life, contravening the assumptionof bottom water anoxia as a prerequisite for organic carbon preservation. The excellentpreservation state of freshwater algae ( Pediastrum complex) suggests that organicmatter contained inside mud composite particles is capable of traveling for longdistances before being deposited in distal depositional settings. Besides protectingorganic components from mechanical/biogenic degradation, encapsulation would alsoprovide an anoxic microenvironment for preventing the oxidation of the organic mattercontained inside of mud composite grains. The central findings of this study show thatorganic carbon encapsulation seems to be an important mechanism for organic carbonpreservation in relatively energetic and non-anoxic settings, calling for a critical reappraisal of the processes responsible for the sequestration of organic carbon fromthe biosphere and its long-term storage in organic-rich mudstone successions.