nortonthe2nd, the Lake Missoula Flood actually gives us a good example of how floods shape the landscape and supports the idea of a global flood. The continental margin does not make sense from a slow and gradual viewpoint. Uniformatarian processes would favor a gradual descent to the ocean depths with no continental shelf or slope.
A good example for the rapid Flood deposition along the continental margin is in the Lake Missoula flood. The floodwater rushed through the Columbia Gorge and then slowed as it came to the wide mouth of the gorge in the Portland and Vancouver area. This wandering current then deposited a giant sand and gravel bar, called the Portland Delta, over 350 ft. thick covering an area greater than 200 miles. "Such deposition in waning currents would be similar to the formation of the continental shelf during the Flood. Later, the Portland Delta was dissected, where the Columbia and Willamette Rivers now flow, probably as the Lake Missoula flood subsided and the currents became more channelized. This later dissection would mimic the formation of the submarine canyons* after the deposition of the Portland Delta."-- Michael Oard, Flood by Design.
*Many hypotheses for the origin of the submarine canyons have been proposed, but geologists have rejected most of them. "Submarine Canyons have long been a geological enigma."--Von Der Borch. Michael Oard has a new hypotheses in his book based on the evidence left by other catastrophic occurrences such as the Lake Missoula Flood.
Another interesting hypothesis. However, there are a few problems with it:
*Where did the water come from and where did it go to give that particular pattern?
*How were igneous and metamorphic rocks formed if the continents were formed by sedimentation?
*How come mountains still exist?
*Finally, how were smaller features such as canyons formed? These demonstrably require tens or hundreds of thousands of years to form according to the rate of erosion we see in modern rivers.
With regards to your first point, the continental shelves are mostly a product of plate tectonics, not erosion. Continental plates are much higher than oceanic plates, causing a steep drop where they meet. This drop is then softened into a softer slope through millions of years of sedimentary deposition. The Great Rift Valley in Africa is an example of this process in the early stages.