The Shelf Areas
55
Not all margins can be readily classified. Some of these rim the marginal basins
behind island arcs and elsewhere in adjacent seas. Some are originating from a
combination of continent shearing and rift extension, with examples on both sides of
the South Atlantic. Some have an extremely complex history, such as the Californian
margin, which not so long ago used to be dominated by accretionary processes and is
now characterized largely by tectonic shear (San Andreas Fault) and extension
(Southern Californian Borderland). However, the task of the marine geologist is not
so much to devise classifications which fit all possible instances, as to identify those
processes which characterize various types of margins and set them apart from others.
2.7 The Shelf Areas
The submerged part of a continent is the shelf. We have seen earlier that emerged
low-lying regions also can be argued to be part of the shelf. However, here we are
concerned with the actual sea floor, covered by water.
Typically, shelves are flat and are not very deeply submerged. An average depth
for the shelves rimming the Atlantic is near 130 m. Some shelves are quite wide,
especially those on passive margins. These are, on the whole, depositional features,
that is, they are built up by sediments. Narrow and rocky shelves, on the other hand,
are common on active margins (see Fig. 2.7a). Here, erosional processes play an
important role in shaping the shelf.
Some shelves extend deep into continents, and hamor shelf seas such as the
Hudson Bay, the Baltic Sea, or the Persian Gulf. Most of the marine sediments found
on land were originally deposited in shelf seas. To understand these sediments -
which cover the greater portion of the continents - one needs to study sedimentary
processes in modem shelf seas (see Chaps. 3 to 5).
Quite generally, present shelf environments and sediment types show considerable
variety over short distances. In part, this variability stems from the fact that the sea
level stood much lower only 15 000 years ago. Conditions were entirely different
then, and many portions of the present shelf still reflect those conditions in topography and sediment cover. The reason for the low sea level, of course, was the
presence of continental ice sheets which locked up enough water to make the ocean
go down by about 130 m.
We see that the nature of shelves reflects tectonics (active versus passive) and the
recent rise of sea level, on a grand scale. On a regional scale, climatic conditions and
sediment supply are of prime importance. In low latitudes buildup by reef forming
organisms is (or was) important in many places. In high latitudes ice has been an
important agent for the last several million years.
In the northern North Atlantic, for example, shelves everywhere show the effects
of ice. The growth of ice not only led to exposure of the shelves, it also dumped
enormous amounts of debris in places, the moraines. This material still sits on the
shelves, off Newfoundland, and in the North Sea. The ice, moving far out onto the
shelves, also actively carved deep ravines and depressions, which have not yet been
55
Not all margins can be readily classified. Some of these rim the marginal basins
behind island arcs and elsewhere in adjacent seas. Some are originating from a
combination of continent shearing and rift extension, with examples on both sides of
the South Atlantic. Some have an extremely complex history, such as the Californian
margin, which not so long ago used to be dominated by accretionary processes and is
now characterized largely by tectonic shear (San Andreas Fault) and extension
(Southern Californian Borderland). However, the task of the marine geologist is not
so much to devise classifications which fit all possible instances, as to identify those
processes which characterize various types of margins and set them apart from others.
2.7 The Shelf Areas
The submerged part of a continent is the shelf. We have seen earlier that emerged
low-lying regions also can be argued to be part of the shelf. However, here we are
concerned with the actual sea floor, covered by water.
Typically, shelves are flat and are not very deeply submerged. An average depth
for the shelves rimming the Atlantic is near 130 m. Some shelves are quite wide,
especially those on passive margins. These are, on the whole, depositional features,
that is, they are built up by sediments. Narrow and rocky shelves, on the other hand,
are common on active margins (see Fig. 2.7a). Here, erosional processes play an
important role in shaping the shelf.
Some shelves extend deep into continents, and hamor shelf seas such as the
Hudson Bay, the Baltic Sea, or the Persian Gulf. Most of the marine sediments found
on land were originally deposited in shelf seas. To understand these sediments -
which cover the greater portion of the continents - one needs to study sedimentary
processes in modem shelf seas (see Chaps. 3 to 5).
Quite generally, present shelf environments and sediment types show considerable
variety over short distances. In part, this variability stems from the fact that the sea
level stood much lower only 15 000 years ago. Conditions were entirely different
then, and many portions of the present shelf still reflect those conditions in topography and sediment cover. The reason for the low sea level, of course, was the
presence of continental ice sheets which locked up enough water to make the ocean
go down by about 130 m.
We see that the nature of shelves reflects tectonics (active versus passive) and the
recent rise of sea level, on a grand scale. On a regional scale, climatic conditions and
sediment supply are of prime importance. In low latitudes buildup by reef forming
organisms is (or was) important in many places. In high latitudes ice has been an
important agent for the last several million years.
In the northern North Atlantic, for example, shelves everywhere show the effects
of ice. The growth of ice not only led to exposure of the shelves, it also dumped
enormous amounts of debris in places, the moraines. This material still sits on the
shelves, off Newfoundland, and in the North Sea. The ice, moving far out onto the
shelves, also actively carved deep ravines and depressions, which have not yet been