Bedload Transport of Mud — A Mechanism for the Infilling of Epicontinental Basins and Lateral Displacement of Source Rocks

Schieber, Juergen; Bennett, Richard; Curry, Kenneth; Schimmelmann, Arndt

Flume experiments show that bedload deposition of mud is feasible under flow conditions that permit transport and deposition of sandy sediments. The flocculated mud responsible for this phenomenon forms ripples and related bedforms that leave behind a laminated sediment as they migrate over the seabed. Although these current deposited muds undergo severe compaction, in multiple ancient shales intrinsic features of cross-lamination, such as basal downlap and top truncation of laminae bear witness to current deposition.

Traditional depositional models suggest that much mud is deposited in basin marginal settings, leaving deep depocenters in distal areas. Yet, probing the record of for example Devonian shales in the eastern US shows numerous erosional truncations that are continuous between multiple depocenters (such as the Illinois, Appalachian and Michigan Basins). This suggests that the synoptic relief of these basins was flat to gently sloping, and that mud was distributed over large areas by bottom currents. In this way areas of more rapid subsidence simply accumulate a thicker mud interval than slower subsiding areas, and the final fill geometry is established without a need for troughs and basins in a depositional sense.

Whereas energetic mud deposition from bottom currents may seem incompatible with enhanced organic matter preservation, many ancient black shales can be shown to have accumulated in comparatively shallow shelf seas. In recent flume experiments simulated marine-snow and clays were deposited from moving suspensions as well as via gravity settling. TEM fabric studies show that flow-deposited OM-clay mixtures, due to roll-aggregation of floccules, have a less porous fabric, whereas still-water settled muds show a more "open" pore structure. We propose that the differences in OM-clay association and pore structure exert a significant difference on the way in which the OM in these sediments will be bacterially processed in surface sediments and on how much of the initial OM is likely to enter the rock record. Intimate OM-clay association in current transported OM-bearing floccules opens the possibility that bottom currents can move organic matter large distances from its site of origination, and lead to source rock accumulation in places that would not be predicted by models of ocean surface productivity.

AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013