Deep-Sea Fans
67
The very distribution of abyssal plains supports the idea that turbidity currents run
along on the floor over extremely long distances. This can be observed in turbidites
off river mouths such as off the St. Laurence, Hudson, Mississippi, and Amazon
Rivers and more than 3000 krn to the south of the Ganges-Brahmaputra Delta.
For such currents, trenches would form an absolute obstacle, and we should not
find abyssal plains beyond them. Once a trench is filled, however, plains can develop
on the other side. This is exactly the distribution seen in the Gulf of Alaska. As the
supply of turbidity deposits run out, far in the deep sea, the abyssal ocean floor starts
showing its typical bumpy morphology: the familiar abyssal hills, which are covered
by pelagic oozes and clay.
It reflects the youth of the science of marine geology that major processes of
sediment erosion, transport, and deposition, which are responsible for much of ocean
margin morphology, were essentially unknown until the 1950s. It took yet another
decade for the tectonics of ocean margins to come into focus, through the theory of
seafloor spreading. The history of ocean margin development is still, in the whole,
poorly understood because of the lack of samples from within the sediment wedges:
recovery by deep drilling is now possible, but is extremely expensive.
While turbidity currents and turbidites are of great importance in helping shape the
morphology of continental margins, they are but one aspect of the large-scale transfer
of material from the continent to the ocean. To appreciate the scope of this subject,
we next discuss marine sediments in general.
Further Reading
Shepard FP, Dill RF (1966) Submarine canyons and other sea valleys. Rand McNally, Chicago
Burk CA, Drake CL (eds) (1974) The geology of continental margins. Springer, Berlin Heidelberg
New York
Dickinson WR, Yarborough H (1981) Plate tectonics and hydrocarbon accumulation. Am Assoc
Petrol Geol, Continuing Education Ser I, revised edn. Tulsa, Okla
Watkins JS, Drake CL (eds) (1983) Studies in Continental Margin Geology. AAPG Mem, 34. Am
Assoc Petrol Geol, Tulsa, Okla
Bally AW (1981) Geology of passive and continental margins: history, structure and sedimentological record. Am Assoc Petrol Geol. Education Course Note Ser 19
67
The very distribution of abyssal plains supports the idea that turbidity currents run
along on the floor over extremely long distances. This can be observed in turbidites
off river mouths such as off the St. Laurence, Hudson, Mississippi, and Amazon
Rivers and more than 3000 krn to the south of the Ganges-Brahmaputra Delta.
For such currents, trenches would form an absolute obstacle, and we should not
find abyssal plains beyond them. Once a trench is filled, however, plains can develop
on the other side. This is exactly the distribution seen in the Gulf of Alaska. As the
supply of turbidity deposits run out, far in the deep sea, the abyssal ocean floor starts
showing its typical bumpy morphology: the familiar abyssal hills, which are covered
by pelagic oozes and clay.
It reflects the youth of the science of marine geology that major processes of
sediment erosion, transport, and deposition, which are responsible for much of ocean
margin morphology, were essentially unknown until the 1950s. It took yet another
decade for the tectonics of ocean margins to come into focus, through the theory of
seafloor spreading. The history of ocean margin development is still, in the whole,
poorly understood because of the lack of samples from within the sediment wedges:
recovery by deep drilling is now possible, but is extremely expensive.
While turbidity currents and turbidites are of great importance in helping shape the
morphology of continental margins, they are but one aspect of the large-scale transfer
of material from the continent to the ocean. To appreciate the scope of this subject,
we next discuss marine sediments in general.
Further Reading
Shepard FP, Dill RF (1966) Submarine canyons and other sea valleys. Rand McNally, Chicago
Burk CA, Drake CL (eds) (1974) The geology of continental margins. Springer, Berlin Heidelberg
New York
Dickinson WR, Yarborough H (1981) Plate tectonics and hydrocarbon accumulation. Am Assoc
Petrol Geol, Continuing Education Ser I, revised edn. Tulsa, Okla
Watkins JS, Drake CL (eds) (1983) Studies in Continental Margin Geology. AAPG Mem, 34. Am
Assoc Petrol Geol, Tulsa, Okla
Bally AW (1981) Geology of passive and continental margins: history, structure and sedimentological record. Am Assoc Petrol Geol. Education Course Note Ser 19