Sedimentary Environment

The average continental shelf is about 45 mi (75 km) wide. Shelf sediments generally decrease in grain size with increasing distance from shore. This occurs for two reasons: (1) greater distance from sediment sources and (2) decreasing sediment movement (transport) with increasing water depth.

Shelf sediments vary significantly with latitude. At high latitudes, glacial ice flowing into coastal water generates icebergs, which transport large sediment loads of various sizes out onto the shelf. As icebergs melt, they drop their load. These glaciomarine sediments are generally less sorted and coarser grained than lower latitude deposits. In fact, boulders known as dropstones occur on the sea floor in deep water, hundreds of miles from shore.

Rivers deliver most of the sediments to mid-latitude shelves. Therefore, grain size routinely decreases with distance from shore; sediment sorting also tends to be rather good. Shallow water, nearshore sediments form thick sand blankets with abundant ripple marks. As depth increases and water movement decreases, average grain size decreases, and sand, silt, and clay occur interbedded. In water depths greater than 150-200 ft (45-60 m), even storm waves do not stir the bottom; consequently, silts and clays predominate. Scattered sand deposits are also located on outer shelf margins. During periods of lower sea level, rivers flowing across what is now the inner shelf deposited these so-called relict sediments.

At low latitudes, bottom-dwelling plants and animals secrete large volumes of calcium carbonate, producing thick blankets of carbonate sediment. Perhaps the best known carbonate environment is the coral reef. Corals produce a rigid framework of carbonate rock (limestone), which is also a major source of sediment of various grain sizes. Where stream input is great, terrigenous sediments discourage habitation by carbonate-producing organisms and dilute any carbonate sediment that is produced.