492
10 SEDIMENTARY BASINS
genetically distinct from all other rift basins save in their tensional origin. Typically they
are infilled by continental clastics and volcanics. The three other types of rift basin all
develop at zones of upwelling of the mantle which may ultimately become belts of sea
floor spreading. More or less continuous lines of rift valleys are found along the midocean ridge systems of the world and can be traced into the continental crust of Africa
and Europe. The sedimentary basins of the mid-ocean ridge rifts are filled by sediment
interbedded with the volcanic assemblage of new oceanic crust which is generated in
these zones of intense igneous and seismic activity.
Rifts are often initiated by an updoming of the crust. The fracture system that results
consists of a triradiate rift system whose node is referred to as a triple-rift (or colloquially triple-R) junction. Some rifts, such as the Red Sea, show straight subparallel faults.
Recent detailed studies of the East African rifts show, however, that they are not composed of two parallel normal faults. They are, in fact, made up of an alternating series
of listric faults dipping in opposing directions. The actual rift is formed by an antithetic
fault that dips toward the opposing master fault. Thus though the rift is linear, it is asymmetric in cross-section, the asymmetry tilting from side to side along the length of the
basin (Frostick et aL, 1987).
When a zone of sea floor spreading develops beneath a rift the crust separates and
the floor of the rift subsides to be infilled by a regular sequence of sedimentary facies
(Schneider, 1972). Initially the rift lie above sea level, so it is infilled with coarse fluvial
and lacustrine sediments, often associated with volcanics. When the rift floor reaches
sea level it provides a shallow restricted trough which (given the right climate) favors
evaporite development, followed by restricted marine sedimentation as the rift subsides
permanently below the sea. This favors oil source rock formation. With continued subsidence and separation the rift becomes infilled with open marine sediments, clastic or
carbonate.
As the two sides of the new zone of sea floor spreading move apart, two of the rift
arms develop into incipient oceans, each flanked with half-graben sedimentary basins.
The third branch of the triple-R junction will not separate, though it may be infilled by
sediments similar in facies and thickness to those in the other two rifts. This is referred
to as an aborted rift or failed arm (Fig. 10.18). The North Sea rift is a fine example of a
failed arm (Woodland, 1975; Illing and Hobson, 1981; Parker, 1993; Glennie, 1997).
Thus rift basins form a continuous spectrum related to the progressive breaking up
and lateral drift of continental margins. The various basin types are now described in
more detail.
10.2.4.1 Intracontinental Rift Basins
The first phase of rift development is seen within cratonic areas of continental crust.
Localized updoming generates crestal tensional rifts. Triradial rift valley systems commonly diverge from the culmination of the dome. The rifts become infilled with continental fluviolacustrine deposits shed from the faulted basin margins. Igneous activity
contributes lavas and volcaniclastic detritus to the basin fill. The best known of these
intracontinental rift systems are those of Lake Baikal in Russia, of the Rhine Valley in
Germany, and of East Africa.
10 SEDIMENTARY BASINS
genetically distinct from all other rift basins save in their tensional origin. Typically they
are infilled by continental clastics and volcanics. The three other types of rift basin all
develop at zones of upwelling of the mantle which may ultimately become belts of sea
floor spreading. More or less continuous lines of rift valleys are found along the midocean ridge systems of the world and can be traced into the continental crust of Africa
and Europe. The sedimentary basins of the mid-ocean ridge rifts are filled by sediment
interbedded with the volcanic assemblage of new oceanic crust which is generated in
these zones of intense igneous and seismic activity.
Rifts are often initiated by an updoming of the crust. The fracture system that results
consists of a triradiate rift system whose node is referred to as a triple-rift (or colloquially triple-R) junction. Some rifts, such as the Red Sea, show straight subparallel faults.
Recent detailed studies of the East African rifts show, however, that they are not composed of two parallel normal faults. They are, in fact, made up of an alternating series
of listric faults dipping in opposing directions. The actual rift is formed by an antithetic
fault that dips toward the opposing master fault. Thus though the rift is linear, it is asymmetric in cross-section, the asymmetry tilting from side to side along the length of the
basin (Frostick et aL, 1987).
When a zone of sea floor spreading develops beneath a rift the crust separates and
the floor of the rift subsides to be infilled by a regular sequence of sedimentary facies
(Schneider, 1972). Initially the rift lie above sea level, so it is infilled with coarse fluvial
and lacustrine sediments, often associated with volcanics. When the rift floor reaches
sea level it provides a shallow restricted trough which (given the right climate) favors
evaporite development, followed by restricted marine sedimentation as the rift subsides
permanently below the sea. This favors oil source rock formation. With continued subsidence and separation the rift becomes infilled with open marine sediments, clastic or
carbonate.
As the two sides of the new zone of sea floor spreading move apart, two of the rift
arms develop into incipient oceans, each flanked with half-graben sedimentary basins.
The third branch of the triple-R junction will not separate, though it may be infilled by
sediments similar in facies and thickness to those in the other two rifts. This is referred
to as an aborted rift or failed arm (Fig. 10.18). The North Sea rift is a fine example of a
failed arm (Woodland, 1975; Illing and Hobson, 1981; Parker, 1993; Glennie, 1997).
Thus rift basins form a continuous spectrum related to the progressive breaking up
and lateral drift of continental margins. The various basin types are now described in
more detail.
10.2.4.1 Intracontinental Rift Basins
The first phase of rift development is seen within cratonic areas of continental crust.
Localized updoming generates crestal tensional rifts. Triradial rift valley systems commonly diverge from the culmination of the dome. The rifts become infilled with continental fluviolacustrine deposits shed from the faulted basin margins. Igneous activity
contributes lavas and volcaniclastic detritus to the basin fill. The best known of these
intracontinental rift systems are those of Lake Baikal in Russia, of the Rhine Valley in
Germany, and of East Africa.