6.8: Continental Margin Sediments

Because most biogenous, hydrogenous, and cosmogenous sediments consist of silt- or clay-sized particles, they do not readily accumulate on continental shelves where currents are comparatively swift. Consequently, continental shelves are covered with lithogenous sediment of larger grain sizes, except in those areas where currents are slow or production of biogenous particles is very high.

On many continental shelves, lithogenous sediments accumulate continuously as rivers and coastal erosion supply new sediment. The particles are sorted by grain size, and grain size generally decreases with distance from shore. However, on some continental shelves, the supply of lithogenous material is very limited. For example, along the Mid- and North Atlantic coasts of the U.S., most river-borne sediment and sediment eroded from the shore is trapped in estuaries and coastal lagoons. Consequently, little lithogenous sediment is transported offshore beyond the longshore drift system (Chap. 11). If the shelf is wide and currents are generally swift on the outer part of the shelf, little or no new sediment is supplied to the seafloor. Such areas of the continental shelf are not bare rock, but are covered by generally coarse-grained sediments, known as relict sediments, that were deposited under a different set of ocean conditions (Fig. 6-18).

Figure 6-18. Relict sediments. (a) Sediments characteristic of shallow water were deposited on the outer continental shelf when sea level was lower. (b) As sea level rose, the relict sediments were not buried by more recent sediments, probably because of the rapid rise of sea level and the trapping of lithogenous sediments in newly submerged river valleys. (c) Shallow-water sediments formed close to the new shoreline as it migrated inland. (d) As sea level rose more slowly, the sediments close to the now slowly retreating shoreline were buried by terrigenous and shallow water biogenous sediments. (e) The history of sea-level change during the past 30,000 years shows the rapid rise between 19,000 and 4000 years ago and the much reduced rate of rise in the past 5000 years. (f) Locations of the Atlantic coastline of North America 15,000 years ago and today. Relict sediments are found primarily near the coastline location of 15,000 years ago.

Relict sediments, including those on the outer continental shelf of the U.S. East Coast, contain terrestrial fossils and shells of organisms, such as oysters, that live only in water less than a few meters deep. These materials are too big to have been moved by currents, but are now under 100 m or more of water. The reason is that the relict sediments were deposited several thousand years ago when the sea level was lower, and the coastline was near what is now the edge of the continental shelf.

Sea level determines the distribution and age of relict sediments. Much of the detailed history of recent sea-level change has been learned through studies of the location of relict sediments. Sea level has been rising from its most recent low point about 19,000 years ago to the present day, although not at a uniform rate.