48 Origin and Morphology of Ocean Margins
a gap, now sinks under the load of sediments and by cooling. As it widens, the sea
invades and the magmatic accretion process asserts itself morphologically, as a young
mid-ocean ridge with an axial valley.
The processes at depth, in the zone where continental crust meets the melts from
the asthenosphere, are hidden from view, and are correspondingly less well understood. Magma intrudes between blocks of continental crust and partly remelts it and
mixes with it. Some of the magma reaches the surface, producing large outpourings
of basalt, as in the Afar desert of Ethiopia. Examples with volcanic units up to 3 or 5
km thickness from Atlantic rifting are "volcanic" margins as in East Greenland and
Norway Voring plateau, see Fig. 2.4). Here deep-sea drilling confIrmed the character
of "dipping reflectors" as ancient lava flows.
Back to the Red Sea (Fig. 2.3 c), where the submerged margins are sinking on the
cooling lithosphere! Thick reef structures can grow on these sinking blocks, building
up a carbonate shelf, and further depressing the crust with their weight. If the Red Sea
were only slightly less open, salt deposits would form - indeed there is evidence from
thick evaporite deposits that this happened in the past.
In sum, the receding margins sink and a rampart of reef carbonate may build up,
and salt deposits also may form in the early phase of rifting.
Ancient salt deposits and reef ramparts are just what we see along many of the
Atlantic margins (Figs. 2.5, 2.6). Salt deposits, of course, also are well known from
the Gulf of Mexico, where they push up as salt domes (diapirs), providing a path for
petroleum migration (see Chap. 10). In the Atlantic proper, good evidence for salt
deposits exists, especially off Angola. The salt in the South Atlantic was laid down,
presumably, when this ocean was narrow, was closed to the north, and had restricted
exchange to the south due to the Walvis Ridge-Rio Grande Barrier (now near 30° S).
Large petroleum reserves may be associated with these salt deposits, because the
South Atlantic also was the site for deposition of organic-rich sediments, during a
long period in the middle Cretaceous.
The kind of material accumulating on sinking continental margins depends on the
geologic setting of the region. In the tropics, and where no large rivers bring sediment
or freshwater, reef carbonates can grow. Elsewhere, mixtures of lagoonal and riverine
sediments may gradually be buried by offshore deposits - mainly hemipelagic mud,
rich in the shells of planktonic (floating) and benthic (bottom-living) organisms. In
places, the sediments can become extraordinarily thick: 10 to 15 km of sediment are
reported from off the Niger, the Mississippi and from other large deltas (Fig. 2.6).
The end-result of rifting, then, is continental margins consisting of thick sediment
stacks piled both on the sinking blocks of a continental edge (Figs. 2.3, 2.6).
We can generalize these conclusions to all margins which originated by rifting and
are riding passively on the moving plate (hence passive margins). Besides the Atlantic margins proper, there are the East African margin, the margins of India, much of
the margin of Australia, and practically all of Antarctic. In the Antarctic, of course,
special conditions prevail with respect to erosion and deposition, at least since the
formation of ice sheets. Thick ice sheets have been present there since the late
Tertiary and possibly earlier.
a gap, now sinks under the load of sediments and by cooling. As it widens, the sea
invades and the magmatic accretion process asserts itself morphologically, as a young
mid-ocean ridge with an axial valley.
The processes at depth, in the zone where continental crust meets the melts from
the asthenosphere, are hidden from view, and are correspondingly less well understood. Magma intrudes between blocks of continental crust and partly remelts it and
mixes with it. Some of the magma reaches the surface, producing large outpourings
of basalt, as in the Afar desert of Ethiopia. Examples with volcanic units up to 3 or 5
km thickness from Atlantic rifting are "volcanic" margins as in East Greenland and
Norway Voring plateau, see Fig. 2.4). Here deep-sea drilling confIrmed the character
of "dipping reflectors" as ancient lava flows.
Back to the Red Sea (Fig. 2.3 c), where the submerged margins are sinking on the
cooling lithosphere! Thick reef structures can grow on these sinking blocks, building
up a carbonate shelf, and further depressing the crust with their weight. If the Red Sea
were only slightly less open, salt deposits would form - indeed there is evidence from
thick evaporite deposits that this happened in the past.
In sum, the receding margins sink and a rampart of reef carbonate may build up,
and salt deposits also may form in the early phase of rifting.
Ancient salt deposits and reef ramparts are just what we see along many of the
Atlantic margins (Figs. 2.5, 2.6). Salt deposits, of course, also are well known from
the Gulf of Mexico, where they push up as salt domes (diapirs), providing a path for
petroleum migration (see Chap. 10). In the Atlantic proper, good evidence for salt
deposits exists, especially off Angola. The salt in the South Atlantic was laid down,
presumably, when this ocean was narrow, was closed to the north, and had restricted
exchange to the south due to the Walvis Ridge-Rio Grande Barrier (now near 30° S).
Large petroleum reserves may be associated with these salt deposits, because the
South Atlantic also was the site for deposition of organic-rich sediments, during a
long period in the middle Cretaceous.
The kind of material accumulating on sinking continental margins depends on the
geologic setting of the region. In the tropics, and where no large rivers bring sediment
or freshwater, reef carbonates can grow. Elsewhere, mixtures of lagoonal and riverine
sediments may gradually be buried by offshore deposits - mainly hemipelagic mud,
rich in the shells of planktonic (floating) and benthic (bottom-living) organisms. In
places, the sediments can become extraordinarily thick: 10 to 15 km of sediment are
reported from off the Niger, the Mississippi and from other large deltas (Fig. 2.6).
The end-result of rifting, then, is continental margins consisting of thick sediment
stacks piled both on the sinking blocks of a continental edge (Figs. 2.3, 2.6).
We can generalize these conclusions to all margins which originated by rifting and
are riding passively on the moving plate (hence passive margins). Besides the Atlantic margins proper, there are the East African margin, the margins of India, much of
the margin of Australia, and practically all of Antarctic. In the Antarctic, of course,
special conditions prevail with respect to erosion and deposition, at least since the
formation of ice sheets. Thick ice sheets have been present there since the late
Tertiary and possibly earlier.