Flume Experiments on the Co-Deposition of Organic Matter and Clays in Muddy Flows

Our understanding of mudstone sedimentology has entered a period of rapid change, largely driven by recent experimental studies and the boom in shale gas exploration. A key milestone was the recognition that many seemingly quiet deposited mudstones may actually have been deposited by currents that carried flocculated clays in bedload. Multiple ancient black shales show sedimentary features that indicate accumulation from bottom currents rather than simply settling of organic matter and detritus from the water column, and many contain organic-rich sand-size mixed aggregates of clays and organic matter. The latter have previously been interpreted as marine snow aggregates that settled rapidly through the water column and enhanced rapid delivery of organic matter to the sediment water interface. An alternative formative process is the mingling of organic marine matter (degrading organic debris) with flocculating clays in muddy bottom currents. We are conducting flume experiments to compare organo-clay textures in bottom current and still-water settling mode, in order to evaluate the impact of these competing processes on organic matter preservation. Initial results show that degrading organic particles form sand-size aggregates with clays in muddy flows, and that these aggregates form ripples and accrete into laminated appearing deposits. Still-water settled muds generally show a more "open" pore structure with dispersed clumps of OM and clays, whereas flow deposited OM-clay mixtures show indications of roll-aggregation, small scale OM-bridges between clay particles, and "coatings" of OM and/or clays. These fabric differences are also reflected in the water contents of the accumulating sediments and suggest a fundamentally different pore structure. Roll-aggregated material dewaters more rapidly upon burial than still-water settled material, even though roll-aggregated surface sediments (top 2 cm) have higher initial water contents than comparable still-water settled sediments. Our observations suggest that there is likely a pronounced difference in carbon preservation potential between still water vs flow deposited marine organic-rich muds.