The morphology of clastic continental margins directly reflects their formative processes. These include interactions between plate movements and isostasy, which establish the characteristic stairstep shape of margins. Other factors are thermal and loading-induced subsidence, compaction and faulting/folding, which create and/or destroy accommodation space for sediment supplied by rivers and glaciers. These processes are primary controls on margin size and shape. Rivers and glaciers can also directly sculpt the margin surface when it is subaerially exposed by sea-level lowstands. Otherwise, they deposit their sediment load at or near the shoreline. Whether this deposition builds a delta depends on sea level and the energy of the ocean waves and currents. Delta formation will be prevented when sea level is rising faster than sediment supply can build the shoreline. Vigorous wave and current activity can slow or even arrest subaerial delta development by moving sediments seaward to form a subaqueous delta. This sediment movement is accomplished in part by wave-supported sediment gravity flows. Over the continental slope, turbidity currents are driven by gravity and, in combination with slides, cut submarine canyons and gullies. However, turbidity currents also deposit sediment across the continental slope. The average angle of continental slopes (~4°) lies near the threshold angle above which turbidity currents will erode the seafloor and below which they will deposit their sediment load. Therefore, turbidity currents may help regulate the dip of the continental slope. Internal waves exert a maximum shear on the continental-slope surface at about the same angle, and may be another controlling factor.

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[1] Submarine canyons are shaped by turbidity currents flowing down the continental slope. But canyon morphology also depends on the patterns of sediment deposition that drive long-term outbuilding of continental margins. Relating the importance of each to the shape of canyon long profiles provides a tool for inferring process from observed (modern and buried) canyon morphologies. Here we present a morphodynamic model that predicts the equilibrium long-profile curvature of a canyon affected by turbidity currents and background sedimentation, with the latter defined by the average sigmoidal shape of many clastic margin clinoforms. The model includes the effects of margin progradation (i.e., seaward advance through time) and the down canyon evolution of turbidity currents. We contrast predictions for equilibrium long-profile shape under three sets of conditions. In the absence of background sedimentation and progradation, the graded canyon long profile is concave and described by a simple power law slope-distance relationship that arises from down canyon increases in discharge due to flow evolution. Similar slope-distance predictions exist for rivers where discharge instead increases from tributary input. Adding background sedimentation can generate graded convex long-profile segments in a manner analogous to rivers experiencing uplift. The curvature of an equilibrium long profile that progrades basinward with constant form depends on the relative importance of turbidity-current deposition and background sedimentation. To illustrate and quantify model predictions in the field, we present examples of canyons from modern continental margins thought to reasonably approximate each of the three cases.

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