Why does the mississipi river valley not look like the entire planet?

In the 1990's a very large area of the mississipi river valley was flooded with the 1000 year flood (i know, i lived there lol). We had water covering everything there, we had water moving very fast, and the flood lasted all summer--not 40 days. Interesting thing is--we don't see seashells in core samples of the river (layers of limestone the planet over have sea shells in them)--we see a gradation in the layering of sediment from the flood (boulders with spaces filled with sand, mud, and organic detritus followed by a layer of smaller stones with the voids once again filled with detritus, followed by sands and silts and muds--looks nothing like for example the geology of the grand canyon? So--if there was a great flood--why wouldn't an area that experienced a great flood have the same features as the entire planet?