THE CURVATURE OF CARBONATE AND SILICICLASTIC CLINOFORMS

Curve fitting on the first-order morphology of seismic, submarine slope profiles resulted in the classification of three basic types of slope curvature: sigmoidal, exponential, and linear profiles. For carbonate platform flanks the distribution of these three types is dominated by exponential (55%) and linear (35%) profiles; sigmoidal profiles (10%) are a minority. Siliciclastic continental margins are mostly sigmoidal (55%) and exponential (35%); siliciclastic deltas are mostly exponential (60%) and contain not many sigmoidal profiles (5%).

A possible explanation of the origin of the three types is that the boundary between the shelf and slope domain is narrow, the shelf edge sharp, and the curvature exponential if sedimentary base level remains stationary during progradation. Sigmoidal slope profiles develop where shelfbreaks are rounded off due to erosion and redistribution of sediment during storms and sea-level fluctuations. A straight slope segment develops where during progradation the slope steepens to the angle of repose. High rates of progradation, coupled with small fluctuations of base level, thus explain exponential and linear profiles.

Several observations agree with the above explanation of the three basic types of slope curvature. For examples, glaciated continental margins where the shelfbreak is fixed by grounding ice-sheets and carbonate platform flanks where fast progradation is coupled with a vertically stationary margin both explain steep, exponential and linear slope profiles. On the other hand, half of the profiles from siliciclastic continental margins are sigmoidal. This is attributed to their generally modest rate of progradation and their long depositional history during which they were exposed to numerous fluctuations of base level.