Coasts and shallow seas are marine areas that lie adjacent to land. Coasts include a broad range of environments, including deltas, strandplains, barrier islands, beaches, estuaries, tidal inlets, tidal flats, lagoons, and rocky cliffs. Shallow seas (water depths up to 200 m) include continental shelves adjacent to the deep ocean and partly enclosed seas (epicontinental seas) such as the North Sea, Hudson Bay, and the Arabian Gulf. “Seas” that are completely enclosed by land, such as the Black Sea and the Caspian Sea, are discussed in Chapters 14 and 16 dealing with lakes. Continental shelves are commonly on the order of 100 km across (range 2–1,000 km) and nearly all have a distinct break in slope at their oceanward edge (known as the shelf–slope break). The oceanward edge of a shelf may be marked by sand shoals or by organic reefs. The depth of the shelf–slope break ranges from 0 m (at shelf margins with organic reefs) to 200 m, but averages 125 m. Thus, most continental shelves have slopes on the order of 1 in 1,000.
Sediments of coasts and shallow seas are mainly terrigenous in origin, with varying amounts of biogenic, chemogenic, and volcanogenic sediment. However, biogenic carbonate sediment can be dominant in areas where the input of terrigenous sediment is minimal, such as distant from the mouths of large rivers.