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

Joseph F Emmings,Christopher H Vane,Vicky Moss‐Hayes,Sarah J Davies,Michael H Stephenson

doi:10.1111/sed.12642

Joseph F Emmings, Christopher H Vane + Show 3 more

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https://doi.org/10.1111/sed.12642

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Journal: Sedimentology	Publication Date: Sep 27, 2019
Citations: 23	License type: CC BY 4.0

Affiliation: British Geological Survey, University of Leicester

Abstract

AbstractOrganic‐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.

Highlights

Syn-rift to early post-rift epicontinental seaways, such as the Mississippian Rheic-Tethys, were distinct from ‘static’ interior seaways such as the Cretaceous Western Interior Seaway (Schieber, 2016)
This study shows that organic-rich sediments accumulated in a dynamic environment host to pelagic to hemipelagic deposition and a variety of subaqueous density flows, moderated by fourth-order eustatic sea-level fluctuation, delta progradation and slope instability, and under variable bottom water salinity
Sedimentary logs for each basin position indicate that the Bowland Shale comprises limestone, and carbonate-bearing, siliceous and argillaceous mudstones

Summary

Introduction

Syn-rift to early post-rift epicontinental seaways, such as the Mississippian Rheic-Tethys, were distinct from ‘static’ interior seaways such as the Cretaceous Western Interior Seaway (Schieber, 2016). The depositional processes that operated in the Mississippian Rheic-Tethys seaway are poorly understood partly because modern analogues are rare (Nyberg & Howell, 2015) This is especially true for the organic-rich upper unit of the Bowland Shale Formation (Upper Bowland Shale; ‘Bowland Shale’), deposited in the RheicTethys seaway, defining a key transition between the carbonate-dominated Lower Bowland Shale (Newport et al, 2018) and the Millstone Grit Group, a siliciclastic toe of slope fan system (Holdsworth & Collinson, 1988; Aitkenhead et al, 1992; Martinsen et al, 1995; Waters et al, 2009). The Bowland Shale is poorly understood beyond a few regional (Fraser & Gawthorpe, 1990) and basin-specific studies (Davies et al, 2012; Ko€nitzer et al, 2014; Słowakiewicz et al, 2015; Fauchille et al, 2017; Hennissen et al, 2017; Newport et al, 2018)

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