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A Mississippian black shale record of redox oscillation in the Craven Basin, UK

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posted on 30.04.2020, 15:25 by J Emmings, S Poulton, C Vane, S Davies, G Jenkin, M Stephenson, M Leng, A Lamb, V Moss-Hayes

Early diagenetic redox oscillation processes have been rarely recognised in the ancient rock record but potentially exert an important control on mineral authigenesis, hydrocarbon prospectivity and supply of metals and/or reduced S as part of associated mineral systems. The upper unit of the Mississippian Bowland Shale Formation is a candidate record of diagenetic redox oscillation processes because it was deposited under a relatively high sediment accumulation rate linked to a large delta system, and under dominantly anoxic and intermittently sulphidic bottom-water conditions. In order to characterise the syngenetic and early diagenetic processes, sedimentological and geochemical data were integrated through the Upper Bowland Shale at three sites in the Craven Basin (Lancashire, UK). Organic matter (OM) comprises a mixture of Type II, II-S, II/III and III OM. ‘Redox zones’ are defined by patterns of Fe-speciation and redox-sensitive trace element enrichment and split into two groups. ‘Sulphidic’ zones (EUX, AN-III, AN-I and AN-IT) represent sediments deposited under conditions of at least intermittently active sulphate-reduction in bottom-waters. ‘Non-sulphidic’ zones (OX-RX, OX-F and OX) represent sediments deposited under non-sulphidic (oxic to ferruginous anoxic) bottom-waters. Operation of a shelf-to-basin ‘reactive Fe’ (FeHR) shuttle, moderated by sea level fluctuation and delta proximity, controlled the position and stability of redoxclines between zones of Fe and sulphate reduction, and methanogenesis. Early diagenetic redoxclines were capable of migration through the shallow sediment column relatively quickly, in response to sea level fluctuation. Preservation of syngenetic and early diagenetic geochemical signals shows redoxclines between Fe and sulphate reduction, and the upper boundary of sulphate-methane transition zone, were positioned within decimetres (i.e., 10 s cm) of seabed. Falling sea level and increasing FeHR supply is recognised as a switch from zones EUX (high sea level), AN-III and ultimately AN-I and AN-IT (low sea level). Zone AN-I defines the operation of ‘redox oscillation’, between zones of Fe and sulphate reduction in shallow porewaters, associated with enhanced degradation of OM and complete dissolution of primary carbonate. Preservation of OM and carbonate, in this system, was a function of changing bottom and pore water redox processes. Redox oscillation operated in a siliciclastic, prodeltaic environment associated with a relatively high sediment accumulation rate and high loadings of labile organic matter and metal oxides. These findings are important for understanding Late Palaeozoic black shales in the context of hydrocarbon and mineral systems.

Funding

This study was funded by the Natural Environment Research Council (NERC) [including grant no. NE/L002493/1, within the Central England Training Alliance (CENTA)]. The study also received CASE funding from the British Geological Survey. n-alkane biomarkers were part-funded by the AAPG Foundation Donald F. Towse Memorial Grant 2016. SWP acknowledges support from a Royal Society Wolfson Research Merit Award and a Leverhulme Research Fellowship. Nick Riley (Carboniferous Ltd) is thanked for sharing biostratigraphic expertise and assistance. Nick Marsh and Tom Knott are thanked for providing assistance during geochemical analyses. Chris Kendrick is thanked for production of the organic carbon isotope data. Hilary Sloane and Jack Lacey (BGS) are thanked for production of the carbon and oxygen (carbonate) isotope data. Pete Sadler (University of California) is thanked for compilation and provision of the background sediment accumulation rate dataset. We gratefully acknowledge the valuable comments by Nicolas Tribovillard and an anonymous reviewer which helped shape this paper. Published with permission of the Director of the British Geological Survey.

History

Citation

Palaeogeography, Palaeoclimatology, Palaeoecology Volume 538, 15 January 2020, 109423

Author affiliation

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

Version

VoR (Version of Record)

Published in

Palaeogeography, Palaeoclimatology, Palaeoecology

Volume

538

Pagination

109423

Publisher

Elsevier

issn

0031-0182

Acceptance date

23/10/2019

Copyright date

2019

Available date

05/11/2019

Publisher version

https://www.sciencedirect.com/science/article/pii/S0031018219307527

Notes

All supporting data and method statements are provided in the accompanying supplementary files. The dataset is also available via the National Geoscience Data Centre (NGDC) open-access repository (Emmings et al., 2019c) [https://dx.doi.org/10.5285/c14ed206-bc46-4f61-938b-171b16205c44].

Spatial coverage

Craven Basin, UK

Language

en

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