Controls on the temporal and spatial distribution of organic matter in siliciclastic mudstones: implications for source rock development in shale gas plays

A multidisciplinary approach combining thin section and whole rock geochemical analyses has been used to investigate and characterise the Bilinguites gracilis marine band (Marsdenian, Carboniferous) and the fine-grained succession that includes this horizon. The lithofacies, geochemical signatures and organic matter distribution (TOCcalc) are examined from proximal (Brough, Cumbria) and distal (Oakamoor, Staffordshire) successions, relative to main sediment source, across the Pennine Basin. Clay-rich homogeneous mudstones dominate the more proximal succession and contain evidence of intense bioturbation which significantly modified the original fabrics. These bioturbated mudstones are characterised by low abundances of redox-sensitive trace elements (e.g. molybdenum (<1.8 ppm) and uranium (<4.5 ppm)), reflecting deposition in an oxygenated environment. Clay-rich mudstones with a lenticular fabric dominate the distal succession and have much higher abundances of redox-sensitive trace elements (e.g. molybdenum (up to 205 ppm) and uranium (up to 25.1 ppm)). The preserved lenticular fabrics, containing faecal pellets from benthic deposit-feeders, are interpreted to result from more rapid deposition and burial, reducing the influence by bioturbation. The fabrics (matrix-supported 3-D preserved goniatites (lower leaf) and sub-millimetre graded beds (upper leaf)) in the Bilinguites gracilis marine band indicate that debris flows (lower leaf)and turbulent flows (upper leaf) were the main depositional mechanisms, contrasting with the traditional interpretation that marine bands were deposited slowly by suspension settling. Proximally TOCcalc is typically less than 2%, due to organic matter degradation in a better oxygenated environment (e.g. clay-rich homogeneous mudstones contain <2%). TOCcalc abundances are higher in the distal location, up to8.9%TOCcalc, which reflects less destruction by bioturbation and enhanced preservation due to more rapid burial(e.g. clay-rich lenticular mudstones and carbonate-rich bedded mudstones contain >2%). Therefore, understanding mudstone characteristics (fabric and geochemical signatures) is important to assessing where the organic matter may be concentrated in a potential shale gas play and why.