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Are We There Yet? Geometric Modeling of the Controls on the Transit Time of River Deltas to the Shelf Edge


Deepwater deposition of sand outside of relative sea level lowstand events is classically attributed to narrow continental shelves or the proximity of submarine canyons. A growing body of work, including geologic investigations, computational and physical experiments, has shown that shelf-edge deltas—and their associated deepwater sedimentation—can occur even in the absence of these factors. We developed a simple 2D geometric model to investigate the relationship between shelf transit time and the shelf slope and width, delta topset slope, sediment supply, and relative sea level. We show that transit times for river deltas across a geologically plausible range of shelf widths (50 km to 150 km), shelf slopes (5E-4 to 5E-3), topset slopes (1E-5 to 1E-4), and sediment supply scenarios (100 km2/yr to 1000 km2/yr) can range from <1 kyr to >500 kyr. Delta transit times have a power-law relationship with shelf width, are linearly related to shelf slope, are quadratically related to topset slope and are inversely proportional to sediment flux. These simple analyses demonstrate that fluviodeltaic systems have a complex but predictable shelf transit time behavior and are able to reach shelf edges on relatively short geologic timescales to deliver coarse-grained sediment beyond the edge of the continental shelf.