Influence of point-source sediment-supply on modern shelf-slope morphology: implications for interpretation of ancient shelf margins

Present sea-floor bathymetry indicates that the continental-shelf and shelf-break morphology have some unique and predictable characteristics in areas with and without high sediment supply. Using a global bathymetry dataset in open shelf areas in front of rivers that discharge over 25 × 10⁶ tons of sediment per year, five distinct accretionary types of shelf-break are distinguished based on along-shelf gradient variability and inferred shelf-break trajectory. Morphological characteristics of river-mouth shelves (compared with adjacent areas lateral to the immediate fairway of the river) are: (1) an overall lower gradient and greater width, and (2) a relatively high slope gradient/shelf gradient ratio. The exceptions are shelves with active shelf-edge deltas; these are narrower, steeper and have an attenuated shelf break in front of rivers. These observations are at seismic scale and have direct implications for the recognition and positioning of principal cross-shelf, supply fairways on ancient shelves or shelf margins, and therefore the potential by-pass routes for deepwater sands. Higher slope/shelf gradient ratios in areas of actively accreting margins, where the shelf-break is more prominent and easier to recognize on seismic data compared with adjacent areas, predict areas with high sediment supply. Along-strike morphological changes on supply-dominated shelves suggest that identification of the sediment-feed route and depocenter relative to the shelf break during a relative sea level cycle are critical for understanding/predicting the 3-D architecture of the shelf-slope-basin floor clinoform.