High Sediment-Supply Shelf and Shelf-break Morphology; Implications for Interpretation of Ancient Shelf Margins

Cornel Olariu and Ronald J. Steel
University of Texas at Austin, Austin, TX

Modern sea-floor bathymetry indicates that the continental-shelf and shelf-break morphology has some unique and predictable characteristics in areas with high sediment supply. Using a global bathymetry dataset, we looked at the open shelf areas in front of rivers that discharge over 30*x106 tons/ year. The shelf break, constrained to 50-300 m water depth, was defined as the point where the gradient ratio between that point and basinward (for 30 km) and the shoreline to that point, is maximum. The shelf surface was characterized by a “rugosity” term, defined as a coefficient that takes into consideration changes in gradient sign (from positive to negative and viceversa). Two distinct morphology characteristics are observed for high sediment-supply shelves compared with adjacent lower-supply areas: (1) there is an increase in the ratio shelf gradient/ slope gradient, and (2) higher sediment-supply shelves have lower rugosity. These observations are at ‘seismic' scale and have direct implications for the recognition and positioning of principal supply fairways on ancient shelf/shelf margins. Cases of higher shelf/slope gradient ratios, where the shelf-break will be more prominent and easier to recognize on seismic data, predict areas with high sediment supply. Lower rugosity in shelf areas with high sediment supply suggest that seismic reflectors will be more continuous and have less variability. We are searching for criteria to distinguish current, wave and tide components in the shelf morphology.