Coastal Asymmetry, Geomorphology, and Tidal Forcing
69
the flow. The consequences of all this for the location of shallow-water fronts between
tidally mixed and stratified water was discussed, as you will recall, in Chapter 3.
The form of continental terraces reflects the movement of the continents themselves:
trailing-edge terraces along the eastern coast of the Americas differ strongly in profile
from the “collision” or leading-edge terraces along the western coastline (see, for example,
Eisma, 1988). Because mountain building occurs preferentially along leading-edge coasts,
rivers opening there are short and carry little organic material; in contrast, great rivers
tend to flow to trailing-edge coasts and carry larger sediment loads, richer in humus.
Note, however, that the present-day sediment loads carried to the sea may be very
different from the pristine state: the Amazon carries only about half as much sediment
as the smaller Ganges/Brahmaputra system, because of the relative intensity of human
development in the two drainage basins.
We shall expect these facts to have major consequences for the ecology of each
type of continental terrace. Thus, “upwelling” coasts associated with eastern boundary
currents have narrow, steep-to continental terraces, whereas coasts associated with western
boundary currents tend to have broader terraces with a less steep slope. On geologically
active, collision coasts the continental crust is thinly covered with sediments, whereas
on trailing-edge shelves the sediment cover may become very deep. In extreme cases, on
prograding shelves along trailing-edge coasts, the crustal topography becomes covered
with a thick blanket of sediments within which bottom currents, themselves heavily
loaded with sediments, may cut deep canyons. Slumping and folding of sediments may
also occur by local tectonic activity. It is also helpful to consider the relative ages, or
maturity, of continental shelves, depending on the era in which the large continental
blocks separated. Young shelves are characteristic of the eastern Asian region, from New
Zealand to the Sea of Okhotsk, with the exception of those around the South China Sea.
More mature shelves are characteristic of much of the Atlantic coastline.
The relative strength of tidal streams on the shelf has, of course, a second significance
for ecological geography. This has to do with the transport, sorting, and redistribution
of terrigenous sediments, which largely determine the location of characteristic faunas of
benthic invertebrates and demersal fish. It may therefore be useful to use a little space
in a discussion of how shelf deposits were constituted and came to be distributed as we
observe them to be.
Recall that the continental shelves were largely dry land during the recent glaciations
and that their superficial geology is no more than an extension of that of the continents,
partially overlain by terrigenous and pelagic sediments, and calcareous biogenic material
of benthic origin. Therefore, present-day bottom types range from rocky ridges, cobbles,
and water-worn gravel, through sand and shell sand, to organic muds, in addition to
biogenic reefs of calcareous skeletal material. During glacial periods, coarse sediments
were preferentially deposited near the actual shelf edge, and this pattern still dominates
the pattern of sedimentation on many shelves. Along eastern coasts of both the North
American and Asian continents, relic sediments from glacial epochs still dominate much
of the shelf, and modern sand and silt lies preferentially shallower, closer to the coast,
dominating in bights and gulfs where sedimentation from suspension is induced.
It was originally supposed that there would be a simple particle-size gradient toward the
shelf edge and that a “mud line” would be found everywhere at about 50 m, shoaler than
which wave action would prevent the deposition of the smallest organic particles. This
approximately describes the situation off Western Europe, where the earliest investigations
were made, but it is not a useful generalization.
Sedimentation is not, as it might appear to be from the resultant arrangement of
sediments, a continuous process, but is rather episodic and the arrangement of sediments
is evolutionary. Episodic settlement of fine-grained material occurs when tidal streams
(or other sources of turbulence) weaken. The deposit at first tends to form patches,
69
the flow. The consequences of all this for the location of shallow-water fronts between
tidally mixed and stratified water was discussed, as you will recall, in Chapter 3.
The form of continental terraces reflects the movement of the continents themselves:
trailing-edge terraces along the eastern coast of the Americas differ strongly in profile
from the “collision” or leading-edge terraces along the western coastline (see, for example,
Eisma, 1988). Because mountain building occurs preferentially along leading-edge coasts,
rivers opening there are short and carry little organic material; in contrast, great rivers
tend to flow to trailing-edge coasts and carry larger sediment loads, richer in humus.
Note, however, that the present-day sediment loads carried to the sea may be very
different from the pristine state: the Amazon carries only about half as much sediment
as the smaller Ganges/Brahmaputra system, because of the relative intensity of human
development in the two drainage basins.
We shall expect these facts to have major consequences for the ecology of each
type of continental terrace. Thus, “upwelling” coasts associated with eastern boundary
currents have narrow, steep-to continental terraces, whereas coasts associated with western
boundary currents tend to have broader terraces with a less steep slope. On geologically
active, collision coasts the continental crust is thinly covered with sediments, whereas
on trailing-edge shelves the sediment cover may become very deep. In extreme cases, on
prograding shelves along trailing-edge coasts, the crustal topography becomes covered
with a thick blanket of sediments within which bottom currents, themselves heavily
loaded with sediments, may cut deep canyons. Slumping and folding of sediments may
also occur by local tectonic activity. It is also helpful to consider the relative ages, or
maturity, of continental shelves, depending on the era in which the large continental
blocks separated. Young shelves are characteristic of the eastern Asian region, from New
Zealand to the Sea of Okhotsk, with the exception of those around the South China Sea.
More mature shelves are characteristic of much of the Atlantic coastline.
The relative strength of tidal streams on the shelf has, of course, a second significance
for ecological geography. This has to do with the transport, sorting, and redistribution
of terrigenous sediments, which largely determine the location of characteristic faunas of
benthic invertebrates and demersal fish. It may therefore be useful to use a little space
in a discussion of how shelf deposits were constituted and came to be distributed as we
observe them to be.
Recall that the continental shelves were largely dry land during the recent glaciations
and that their superficial geology is no more than an extension of that of the continents,
partially overlain by terrigenous and pelagic sediments, and calcareous biogenic material
of benthic origin. Therefore, present-day bottom types range from rocky ridges, cobbles,
and water-worn gravel, through sand and shell sand, to organic muds, in addition to
biogenic reefs of calcareous skeletal material. During glacial periods, coarse sediments
were preferentially deposited near the actual shelf edge, and this pattern still dominates
the pattern of sedimentation on many shelves. Along eastern coasts of both the North
American and Asian continents, relic sediments from glacial epochs still dominate much
of the shelf, and modern sand and silt lies preferentially shallower, closer to the coast,
dominating in bights and gulfs where sedimentation from suspension is induced.
It was originally supposed that there would be a simple particle-size gradient toward the
shelf edge and that a “mud line” would be found everywhere at about 50 m, shoaler than
which wave action would prevent the deposition of the smallest organic particles. This
approximately describes the situation off Western Europe, where the earliest investigations
were made, but it is not a useful generalization.
Sedimentation is not, as it might appear to be from the resultant arrangement of
sediments, a continuous process, but is rather episodic and the arrangement of sediments
is evolutionary. Episodic settlement of fine-grained material occurs when tidal streams
(or other sources of turbulence) weaken. The deposit at first tends to form patches,