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From marine bands to hybrid flows: Sedimentology of a Mississippian black shale

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posted on 25.10.2019, 13:13 by Joseph F. Emmings, Sarah J. Davies, Christopher H. Vane, Vicky Moss-Hayes, Michael H. Stephenson
Organic‐rich mudstones have long been of interest as conventional and unconventional source rocks and are an important organic carbon sink. Yet the processes that deposited organic‐rich muds in epicontinental seaways are poorly understood, partly because few modern analogues exist. This study investigates the processes that transported and deposited sediment and organic matter through part of the Bowland Shale Formation, from the Mississippian Rheic–Tethys seaway. Field to micron‐scale sedimentological analysis reveals a heterogeneous succession of carbonate‐rich, siliceous, and siliciclastic, argillaceous muds. Deposition of these facies at basinal and slope locations was moderated by progradation of the nearby Pendle delta system, fourth‐order eustatic sea‐level fluctuation and localized block and basin tectonism. Marine transgressions deposited bioclastic ‘marine band’ (hemi)pelagic packages. These include abundant euhaline macrofaunal tests, and phosphatic concretions of organic matter and radiolarian tests interpreted as faecal pellets sourced from a productive water column. Lens‐rich (lenticular) mudstones, hybrid, debrite and turbidite beds successively overlie marine band packages and suggest reducing basin accommodation promoted sediment deposition via laminar and hybrid flows sourced from the basin margins. Mud lenses in lenticular mudstones lack organic linings and bioclasts and are equant in early‐cemented lenses and in plan‐view, and are largest and most abundant in mudstones overlying marine band packages. Thus, lenses likely represent partially consolidated mud clasts that were scoured and transported in bedload from the shelf or proximal slope, as a ‘shelf to basin’ conveyor, during periods of reduced basin accommodation. Candidate in situ microbial mats in strongly lenticular mudstones, and as rip‐up fragments in the down‐dip hybrid beds, suggest that these were potentially key biostabilizers of mud. Deltaic mud export was fast, despite the intrabasinal complexity, likely an order of magnitude higher than similar successions deposited in North America. Epicontinental basins remotely linked to delta systems were therefore capable of rapidly accumulating both sediment and organic matter.


This study was funded by the Natural Environment Research Council (NERC), (Grant no. NE/L002493/1), within the Central England Training Alliance (CENTA) consortium. The study received CASE funding from the British Geological Survey. Nick Riley (Carboniferous Limited) is thanked for sharing his expertise and assistance. Charlotte Watts is thanked for providing field assistance. Nick Marsh, Tom Knott, Cheryl Haidon and Annika Burns are thanked for offering expertise and laboratory assistance. Pete Sadler (University of California) is thanked for compilation and provision of the background sediment accumulation rate dataset. We thank all reviewers, Editor Peir Pufahl and Elaine Richardson (Editorial Office) for their valuable comments and advice, which have significantly improved this manuscript.



Sedimentology, 2019

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/Organisation/COLLEGE OF SCIENCE AND ENGINEERING/School of Geography, Geology and the Environment


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Wiley, International Association of Sedimentologists





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