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Depositional controls on mudstone lithofacies in a basinal setting; implications for the delivery of sedimentary organic matter

Sven F. Koenitzer, Sarah J. Davies, Michael H. Stephenson and Melanie J. Leng
Depositional controls on mudstone lithofacies in a basinal setting; implications for the delivery of sedimentary organic matter
Journal of Sedimentary Research (March 2014) 84 (3): 198-214

Abstract

Deep-water mudstones from ancient epicontinental settings are significant repositories for organic matter, but the detailed temporal variations of, and controls on, the abundance and type of organic matter (OM) is little studied. Using micro-petrographic and geochemical data from late Mississippian mudstones of the Widmerpool Gulf, UK, the processes that delivered fine-grained sediment to this basin during a glacioeustatic sea-level cycle are interpreted from detailed lithofacies analysis. Seven primary lithofacies are identified from core, which show specific and systematic variations in total organic carbon (TOC) content and bulk carbon isotope composition of organic material (delta (super 13) C (sub org) ). During sea-level highstands, thin-bedded carbonate-bearing mudstones are the dominant facies deposited, contain up to 6.6% TOC (average 4.6 + or - 1.3%), and have mean delta (super 13) C (sub org) of -28.5 + or - 0.9 per mil. During phases of lower sea level, thin-bedded silt-bearing clay-rich mudstones with up to 4.1% TOC (average 2.3 + or - 0.8%; mean delta (super 13) C (sub org) : -28.2 + or - 1.0 per mil) were interbedded with more organic-lean graded silt-bearing mudstones and sand-bearing silt-rich mudstones (average TOC: 1.7 + or - 0.6%) derived from turbidity currents. The latter (mean delta (super 13) C (sub org) : -26.2 + or - 0.7 per mil) are closely linked to significant proportions of terrestrial plant material, while some rare plant debris- and sand-bearing mudstones produced from debris flows have more than 7.0% TOC and delta (super 13) C (sub org) > or = -26.0 per mil. The delta (super 13) C values of wood fragments ranged from -27.1 per mil to -24.0 per mil and therefore the delta (super 13) C (sub org) is interpreted as a function of the ratio of marine and terrestrial organic matter. More negative values in the carbonate-bearing and the clay-rich mudstones indicate marine planktonic algae whereas the least negative values reflect greater contribution of terrestrial plant material. The data suggest that the marine conditions prevailed and supported marine planktonic algae throughout different sea-level stages. Marine OM was delivered to the sea floor by continuous hemipelagic settling whereas terrestrial OM was delivered by sediment density flows. Variations in bioproductivity and dilution by siliciclastics influenced the burial rate of marine OM. Organic-rich mudstones preserved in these marine basins are potential hydrocarbon source rocks, especially as unconventional (shale gas) reservoirs. Detailed microtextural and compositional analysis coupled with rigorous geochemical parameters as used in this study are important for the understanding of the source-rock potential of basinal mudstones, and of fine-grained organic-rich sediments in general.


ISSN: 1527-1404
EISSN: 1938-3681
Serial Title: Journal of Sedimentary Research
Serial Volume: 84
Serial Issue: 3
Title: Depositional controls on mudstone lithofacies in a basinal setting; implications for the delivery of sedimentary organic matter
Affiliation: University of Leicester, Department of Geology, Leicester, United Kingdom
Pages: 198-214
Published: 201403
Text Language: English
Publisher: Society for Sedimentary Geology, Tulsa, OK, United States
References: 85
Accession Number: 2014-036963
Categories: Sedimentary petrology
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. 3 plates, 2 tables, sketch map
N52°49'60" - N52°49'60", W01°04'00" - W01°04'00"
Secondary Affiliation: British Geological Survey, GBR, United Kingdom
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2017, American Geosciences Institute. Reference includes data from GeoScienceWorld, Alexandria, VA, United States. Reference includes data supplied by SEPM (Society for Sedimentary Geology), Tulsa, OK, United States
Update Code: 201423
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