Controls from Siliciclastic Mudstone Deposition and the Preservation of Organic Matter

Sarah Davies¹, Joe Macquaker², and Melanie Leng^3
1Dept of Geology, University of Leicester, Leicester, United Kingdom
²School of Earth, Atmospheric and Environmental Sciences, University of Manchester, Manchester, United Kingdom
³NERC Isotope Geoscience Laboratory, British Geological Survey, Keyworth, United Kingdom

The need for persistent bottom water anoxia as a necessary pre-requisite for organic matter preservation has been overstated. However, existing models still propose that organic matter is most likely to be preserved in low energy settings where bioturbation is restricted and sediment accumulation rates are optimised to ensure that organic matter is buried before it can be oxidized by organisms using any bioavailable oxidants. The controls on organic carbon preservation and lithofacies variability are investigated in ancient, organic-rich siliciclastic mud-dominated strata (<15 m thick, Pennsylvanian, UK) using a combined approach of field, optical, electron optical, stable isotope and geochemical methods.

These techniques reveal stacked successions of thin-beds (<10-2 m thick) that are composed of three main lithofacies types that also have a distinctive geochemical signature. These include: (1) thin-bedded, upward-fining silt-bearing clay-rich mudstones, (2) pelleted, microfossil-bearing, clay-rich mudstones and (3) intraclast-bearing clay-rich mudstones. Bioturbation has modified the primary sedimentary structures to a greater or lesser extent in all lithofacies.

These data suggest two main mechanisms were responsible for sediment delivery. The coarser silt-bearing clay-rich mudstones were deposited from hyperpycnal flows in distal pro-delta environments. The pelleted clay-rich mudstones were deposited as “marine snow” in regions where clastic dilution was reduced and primary productivity was enhanced. The intraclast-rich unit was deposited in a lower shoreface setting as a transgressive lag on a ravinement surface. Once deposited most sediment was subject to biological reworking. The overall process responsible for these systematic facies changes was probably relative changes in sea-level.

AAPG Search and Discovery Article #90078©2008 AAPG Annual Convention, San Antonio, Texas