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Spatial Organization of Sedimentary Organic Matter: The Importance of Scale Variant Approaches to Understanding Non-steady Remineralization Pathways of Organic Carbon in Early Ordovician Black Shales (Bell Island, Newfoundland, Canada)

Harazim, Dario *1; McIlroy, Duncan 1; Schneider, Celine 2
(1) Earth Sciences, Memorial University, St. John's, NF, Canada.
(2) C-CART NMR Facility, Memorial University, St. John's, NF, Canada.

Modern mud-dominated shelves (e.g., the Amazon shelf) are highly dynamic depositional environments, where more than 60% of all terrestrially fluxed organic debris is re-mineralized and incorporated into the rock record. Ancient counterparts of mud-dominated shelves host important unconventional reservoirs, with stark fluctuating fabric-dependant organic matter qualities. Our process-sedimentological understanding on organic matter production and preservation pathways in these systems is patchy because grain size variability, sedimentary structures, and kerogen quality are rarely described or measured at the optimal scale and resolution. Within this study we investigate the most likely controls on organic matter production and preservation on a Lower Ordovician, mud-dominated shoreline on Bell Island, Newfoundland. The studied section exposes well-bioturbated, sandstones and mudstones that are interbedded with decimeter-thick packages of unbioturbated shales. We have studied these facies using a combined sedimentological, ichnological, and geochemical approach at multiple scales. Unbioturbated shales are low-angle cross-stratified and composed of thin (<1 cm thick) stacked beds with erosional bases. Very small (mm-sized) burrows are common within mudstones, including: Planolites and Gyrolithes burrows; structures resembling fluid sediment swimming; and a three-dimensional network of pyritized burrows and tubes. Mudstones investigated in this study contain on average 0.5 % TOC, with peak values of up to 3.4% occurring close to sandstone-mudstone interfaces. Organic-rich zones are found to be characterized by very light δ13Corg (-29.5‰) values. Preliminary analyses of extracted bitumen using solid-state NMR spectroscopy (13C{1H} CPMAS NMR) reveal a high ratio of aromatic/aliphatic hydrocarbons in units dominated by in situ microbial productivity, whereas shales resulting from sediment gravity flows exhibit a more homogeneous composition, purely dominated by aromatic (C=C) compounds. The variable organic matter composition on storm-dominated, muddy shelves, most likely mirrors the spatial juxtaposition of several microbial metabolic strategies in mud, while the preservation potential of sedimentary organic matter appears to be a function of exposure time and reworking frequency of bottom-sediment. In this study, salinity stress and bottom-water anoxia have been found to play a subordinate role in governing organic matter quality.

 

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California