continental shelf geology

continental shelf geology The continental shelf includes the gently sloping area from the shoreline to the shelf break where the inclination of sea floor increases with passage to the continental slope. The continental shelves represent about 8 per cent of the area of the oceans and have an area of 29 × 10⁶ km². The majority are shallower than 130 m but some reach depths of 550 m. Continental shelves are generally narrow along convergent plate margins (as around the Pacific Ocean), where they border deep trenches, and wide along intraplate passive margins (as around much of the Atlantic Ocean) where they border a wide continental slope/rise.

Shelves are the most accessible marine areas and they provide rich economic resources such as oil and gas, minerals (aggregate, sand, heavy minerals rich in rare elements such as titanium and chromium, diamonds) while the contemporary sea floor provides the habitat for fish and shellfish which are important food resources for mankind. Their accessibility has led to abuse by man. Marshes and shallow areas are cordoned off to ‘reclaim’ land, which in reality means destruction of marine habitat (almost universally but on a major scale in The Netherlands). Soft sediment is hydraulically removed by civil engineers to build up the adjacent land and leads to the formation of marine deserts in the source areas (for instance along parts of the nearshore of the United Arab Emirates). Continental shelves are also used as a dumping ground for wastes of various kinds (sewage sludge, munitions), and the overlying water is often polluted with industrial effluents and untreated sewage. All these activities are detrimental to the environment. and the marine biota, especially in coastal areas. They have serious consequences for the sustainability of food resources derived from shallow seas.

Continental shelves are the more ephemeral parts of the marine realm because of oscillations of climate and sea level. During the Last Glacial Maximum (LGM) 18 000 years ago, large areas of modern continental shelves were exposed subaerially. Those at high latitudes were ice-covered, and in areas bordered by mountains they are commonly cut by deep ice-gouged valleys that now form fiords, for example, along the margins of the Norwegian-Greenland Sea. There are also large quantities of morainal deposits (as on the shelf off Newfoundland). In middle to low latitudes, the shelves were crossed by rivers that disgorged their sediment load on to what is now the outer shelf and the adjacent continental slope.

Sea level has risen by about 130 m since the LGM. As a consequence, the lower parts of river valleys have become flooded. River-borne sediment has then invariably been deposited close to the point where the river enters the sea. Where the rate of input is low, the flooded valley forms a ria (as along the west coast of Spain). Where the rate of input is moderate, the result is an estuary largely infilled with sediment and bordered by intertidal flats and marshes (as with most rivers entering the North Sea). Where the rate of input is high, a delta progrades across the shelf (as in the Mississippi in the Gulf of Mexico). Since the sediment is then ponded close to the shore, the shelves are largely starved of sediment. The deposits are thus out of equilibrium with present sea level and are partially or entirely relict, relating to deposition in the early stages of the marine transgression. Such deposits cover some 70 per cent of modern shelves. In some instances, former alluvial deposits associated with a previous lower sea level remain exposed on the sea floor. Deposits of this kind off Africa yield alluvial diamonds. These sediments are sucked from the sea floor in order to extract the diamonds. Elsewhere, sand and gravel are extracted in the same way for use in the building industry (as off southern England). Sands containing heavy minerals have been extracted from beaches (as at Redondo Beach, California). The process of extraction has serious consequences for marine life, not only on the sea floor, but also through putting sediment into suspension in the water column, which affects the primary productivity of the plankton.

In some instances, the absence of detrital input has led to the accumulation of carbonate deposits formed from the shells of marine organisms. In middle to high latitudes such deposits lack aragonite mud; they are formed of bioclasts of molluscs, barnacles, bryozoans, and foraminifera and are termed heterozoan. The rates of accumulation are a few centimetres per thousand years. In low-latitude arid climates, such sediments commonly contain aragonite mud; the deposits are formed of bioclasts of molluscs, hermatypic (reef-forming) corals, and calcareous algae and are termed chlorozoan because the hermatypic corals and calcareous algae are light-dependent. The rates of formation here are considerably greater than those in middle to high latitudes. In some instances, large protected shelf lagoons have developed where reef growth on the shelf has formed a barrier that isolates the inner shelf from severe wave attack (for example, the lagoon behind the Great Barrier Reef). Fine-grained sediments accumulate in such settings.

The shelves along passive margins were initiated by rifting associated with the divergence of two lithospheric plates. After the rift phase, during which half-graben basins formed (and were infilled with syn-rift, mainly clastic, sediments), such areas underwent subsidence caused partly through thermal cooling of the lithosphere and partly through loading caused by the deposited sediment. In areas well nourished by clastic sediment (the east coast of the USA), the sediments reach a thickness in excess of 10 km. In areas starved of clastic sediment, subsidence is much less and, although the syn-rift sediments may be clastic, the post-rift sediments are at least partly calcareous (as in the Western Approaches to the English Channel and the Celtic Sea). In the North Atlantic, the initiation of rifting took place during the Triassic. Wide epicontinental shelves may also be underlain by basins that were initiated by rifting (such as that of the North Sea). In this case there are thick pre-Permian shallow marine and continental deposits which formed prior to rifting. Permo-Triassic rifting led to the initiation of the North Sea basin. Deep-water deposits rich in organic matter formed in the central graben, and these provided a source for oil which has accumulated in Jurassic-Palaeogene reservoirs now exploited in off-shore oilfields. In the southern North Sea, natural gas derived from Carboniferous sources has accumulated in Permian sandstones.

Pacific shelves are situated in areas of active tectonism resulting from the convergence of two lithospheric plates. The oceanic plate plunges beneath the continental plate along the line of the trench, which marks the zone of subduction. The shelf may suffer uplift during tectonism and, in very shallow areas, this leads in some instances to emergence as land (as off Alaska). There may also be mass failure of the slope and part of the shelf as a result of oversteepening through compressional tectonics and of build-up sediment. Shelf deposits are then transported either as a fluidized flow or en masse into deeper water. The sediments are partly clastic and partly volcaniclastic because volcanism is active along such margins.

On the eastern sides of oceans where the coast lies to the left of the coast-parallel prevailing wind, Ekman transport of the surface waters is away from the coast (as off Peru). This results in upwelling: the ascent of nutrient-rich water from the thermocline or deeper. These regions of high productivity promote the development of plankton, which feeds planktivorous fish such as anchovies. The latter produce faecal pellets rich in phosphate, which in turn may lead to the accumulation of phosphate-rich sediments. If the supply of organic material to the sea floor is very high, there may be temporary or permanent anoxia of the bottom waters. Under such conditions biogenic carbonate may be replaced by phosphorite (as off the coast of South Africa beneath the Benguela Current).

There is popular concern that global warming may cause sea level to rise above its present level. Geologists are aware that sea level has changed constantly though time so, regardless of whether or not global warming is taking place, there is no reason to suppose that the level of the sea will stay at its present position. What are the consequences of a rise? Clearly the effects will be felt mainly in very shallow areas, but the consequences will not uniform because different ecosystems and organisms respond to change in different ways. Deltas would be more affected by an increase in sea level than an increase in temperature, whereas for corals the opposite is true (increased temperature leads to a higher incidence of disease). Sea level has, however, risen about 130 m in the past 18 000 years and the faunas and environments have managed to adjust to this rapid change. There is therefore no cause to be pessimistic about future changes to the natural environment, even though there may be severe consequences for those countries lying close to present sea level.

John W. Murray

Bibliography

Kennett, J. (1982) Marine geology. Prentice Hall, New Jersey.
Seibold, E. and and Berger, W. H. (1996) The sea floor: an introduction to marine geology (3rd edn). Springer-Verlag, Berlin.