The Importance of Near-Bed Sediment Concentration on Shelf Bed-Load Sedimentary Structures
Recently it has been shown that much of the fine-grained sediment (very fine sand to clay) on continental shelves is moved and ultimately deposited by tractional, or bed-load transport processes. Specifically, these processes relate to concentrated near-bed sediment dispersions that are formed and in large part maintained by storm-wave agitation. Once formed these dispersions form a bottom-hugging current that superficially resembles much better known gravity-driven turbidity currents. Sediment deposited by these shelf currents is typically planar or wavy laminated, and much less commonly cross-stratified. However, ripples, whether current ripples or combined-flow ripples, are the lowest energy bed states in a range of sediment sizes formed under unidirectional or combined flow, respectively. In addition, high-angle cross-stratification related to their migration is generally well developed in both kinds of ripples, and hence the ubiquity of planar lamination in shelf mudstones is puzzling.
Bed forms that rise well above the general bed level (i.e. excluding plane bed) grow initially from a bed-surface defect. Defects, in turn, are thought to be initiated by spatial variations in bed-surface sediment transport related to a hydrodynamic instability formed between the sediment-concentrated bed-load layer and much lower concentrated basal part of the flow. In high-concentration flows, like high-energy turbidity currents, high sediment concentrations in the lower part of the flow inhibits the development of the hydrodynamic instability needed to form angular bed forms like ripples and dunes, and hence plane bed remains stable, even at low flow speeds. Shelf gravity currents, however, are certainly much less concentrated in their near-bed region. Moreover, the near-bed sediment load is probably made up of significantly smaller particles that in many cases are capable of flocculating. Here it is argued that instead of the defect forming instability being too poorly formed, because of too much sediment in the lower part of the flow, it is not formed because there is too little contrast between the bed-load layer and the lower part of the flow. Now the bed-load layer consists of abundant flocculated particles whose high porosity significantly reduces its bulk density, and hence its density contrast with the overriding current. This, therefore, inhibits the formation of the instability, in turn the bed-surface defects, and accordingly plane bed remains stable.
AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013