Page 176 - Sedimentology and Stratigraphy

11

The Marine Realm:

Morphology and Processes

The oceans and seas of the world cover almost three-quarters of the surface of the planet
and are very important areas of sediment accumulation. The oceans are underlain by
oceanic crust, but at their margins are areas of continental crust that may be flooded by
seawater: these are the continental shelves. The extent of marine flooding of these
continental margins has varied through time due to plate movements and the rise and
fall in global sea level related to climate changes. The sedimentary successions in these
shallow shelf areas provide us with a record of global and local tectonic and climatic
variations. There is considerable variety in the sedimentation that occurs in the marine
realm, but there are a number of physical, chemical and biological processes that are
common to many of the marine environments. Physical processes include the formation
of currents driven by winds, water density, temperate and salinity variations and tidal
forces: these have a strong effect on the transport and deposition of sediment in the
seas. Chemical reactions in seawater lead to the formation of new minerals and the
modification of detrital sediment. The seas also team with life: long before there was life
on land organisms evolved in the marine realm and continue to occupy many habitats
within the waters and on the sea floor. The remains of these organisms and the evidence
for their existence provide important clues in the understanding of palaeoenvironments.

11.1 DIVISIONS OF THE MARINE from the ridges the water depth increases as the older
REALM crust cools and subsides, and most of the ocean floor
is between about 4000 and 5000 m below sea level.
The bathymetry, the shape and depth of the sea The deepest parts of the oceans are the ocean
floor (Fig. 11.1), is fundamentally determined by the trenches created by subduction zones, where water
plate tectonic processes that create ocean basins by depths can be more than 10,000 m. At the ocean
sea-floor spreading. The spreading ridges are areas margins the transition from ocean crust to continental
of young, hot basaltic crust that is relatively buoyant crust underlies the continental rise and the
and typically around 2500 m below sea level. Away continental slope, which are the lower and upper