All chalk is a fine-grained pelagic sediment derived from calcareous microplankton and nannoplankton. However, "shelf-sea" chalk (deposited in hundreds of meters of water) differs from "deep-sea" chalk (deposited in thousands of meters of water) in several paleontologic and sedimentologic respects. Unlike deep-sea chalk, shelf-sea chalk characteristically contains abundant megafossils, flints, omission surfaces, hardgrounds, borings and early diagenetic mineralization (pyrite, glauconite and phosphate) of trace fossils and clasts. Trace fossil associations are dominated by Thalassinoides and other crustacean burrows in most shelf-sea chalk sections; trace fossil associations in deep-sea chalk cores are dominated by Chrondrites, Planolites and Zoophycos.