Page 261 - Sedimentology and Stratigraphy

248 Deep Marine Environments

Fig. 16.1 Deep water environments are floored by ocean crust and are the most widespread areas of deposition worldwide.
from river and delta systems and bypasses the shelf. less (11.1). Nevertheless, they are physiographically
There is also intrabasinal material available in ocean significant, as they contrast with the very low gradi-
basins, comprising mainly the hard part of plants ents of continental shelves and the flat ocean floor.
and animals that live in the open oceans, and air- Continental slopes extend from the shelf edge, about
borne dust that is blown into the oceans. These 200 m below sea level, to the basin floor at 4000 or
sources of sediment all contribute to oceanic deposits 5000 m depth and may be up to a hundred kilometres
(Douglas 2003). The large clastic depositional sys- across in a downslope direction. Continental slopes
tems are mainly found near the margins of the ocean are commonly cut by submarine canyons, which,
basin, although large systems may extend a thou- like their counterparts on land, are steep-sided ero-
sand kilometres or more out onto the basin plain, sional features. Submarine canyons are deeply
and the ocean basin plains provide the largest incised, sometimes into the bedrock of the shelf, and
depositional environments on Earth. may stretch all the way back from the shelf edge to
The problem with these deep-water depositional the shoreline. They act as conduits for the transfer of
systems, however, is the difficulty of observing and water and sediment from the shelf, sometimes feeding
measuring processes and products in the present day. material directly from a river mouth. The presence of
The deep seas are profoundly inaccessible places. Our canyons controls the formation and position of sub-
knowledge is largely limited to evidence from remote marine fans.
sensing: detailed bathymetric surveys, side-scan sonar The generally flat surface of the ocean floor is
images of the sea floor and seismic reflection surveys interrupted in places by seamounts, underwater
(22.2) of the sediments. There are also extremely volcanoes located over isolated hotspots. Seamounts
localised samples from boreholes, shallow cores and may be wholly submarine or may build up above
dredge samples. Our database of the modern ocean water as volcanic islands, such as the Hawaiian
floors is comparable to that of the surface of the Moon island chain in the central Pacific. As subaerial volca-
and understanding the sea floor is rather like trying to noes they can be important sources of volcaniclastic
interpret all processes on land from satellite images sediment to ocean basins. The flanks of the volcanoes
and a limited number of hand specimens of rocks are commonly unstable and give rise to very large-
collected over a large area. However, our knowledge scale submarine slides and slumps that can involve
of deep-water systems is rapidly growing, partly several cubic kilometres of material. Bathymetric
through technical advances, but also because hydro- mapping and sonar images of the ocean floor around
carbon exploration has been gradually moving into volcanic islands such as Hawaii in the Pacific and the
deeper water and looking for reserves in deep-water Canary Islands in the Atlantic have revealed the
deposits. existence of very large-scale slump features. Mass
movements on this scale would generate tsunami
(11.3.2) around the edges of the ocean, inundating
16.1.1 Morphology of ocean basins coastal areas.
The deepest parts of the oceans are the trenches
Continental slopes typically have slope angles of formed in regions where subduction of an oceanic
between 28 and 108 and the continental rise is even plate is occurring. Trenches can be up to 10,000 m