10.2 SEDIMENTARY BASINS CLASSIFIED AND DESCRIBED
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erally extensive continental margin downwarps. The latter are typical of passive continental margins that develop at the end of the rift-drift sequence, in which context they
will subsequently be described.
Epicratonic embayments, as the first type may be called, tend to be embayed and open
toward the adjacent ocean basin. The axis of an epicratonic basin may plunge to the floor
of the ocean or be interrupted by a sill-like feature at the rim of the continental margin.
Epicratonic basins include those predominantly infilled with clastics, such as the Mississippi Gulf coast, and the Niger delta basin (Weber, 1971; Burke, 1972; Walcott, 1972;
Wilhelm and Ewing, 1972; Evamy et aL, 1978; Reijers et al., 1997), and those with predominant carbonate fill, such as the Sirte basin of North Africa (Salem et al., 1996a,b,c;
Selley, 1997c).
The Mississippi and Niger delta basins are very similar (Salvador, 1991; Reijers et al.,
1997). Both originated toward the end of the Mesozoic Era and continue to be sites of
active clastic sedimentation until the present day. Both basins contain a basal layer of
salt, which diapirically intrudes younger sedimentary rocks. The Louann salt of the Gulf
basin is of Jurassic age. The basal salt of the Niger delta basin is believed to be of AlbianAptian age (Mascle et al., 1973). The Mississippi and Niger delta basins are infilled by
prisms of terrigenous clastics which were deposited in a range of environments. On the
landward sides of the embayments alluvial deposits predominate. These pass basinward
into diverse shoreline facies, which include both barrier and deltaic deposits. These thin
and grade seaward into marine slope muds, with some development of turbidite sand
facies at the base of the slope (Fig. 10.15).
Geophysical data suggest a gradual seaward thinning of the continental crust beneath
both the Mississippi Gulf coast and Niger delta basins (Walcott, 1972, and Burke, 1972,
respectively).
The Sirte basin of North Africa shares many features with the Gulf coast and Niger basins. It, too, is essentially an embayment which opens out to an oceanic basin
(the Mediterranean). The area now occupied by the Sirte basin was a major northsouth uplift that controlled sedimentation throughout the Paleozoic era. This regional
arch collapsed in the middle of the Cretaceous Period to form the Sirte embayment
(Fig. 10.16).
Basal sands and thin evaporites are overlain by deep-water Upper Cretaceous and
Paleocene shales. These are thickest in intra-basinal troughs, while reefal carbonates
were deposited on adjacent horsts. Throughout the Eocene the Sirte embayment was
infilled by nearly a kilometer of interbedded carbonates and evaporites. The final phase
of basin infilling during the Oligocene and Miocene involved terrigenous and carbonate sedimentation in both marine and continental environments. The active history of
the basin was concluded by an epsode of basaltic volcanism in the Pleistocene (Salem
et aL, 1996a,b,c; Selley, 1997c).
These brief reviews of three epicratonic basins show how they differ from the previously described intracratonic examples. Epicratonic basins tend to be much less stable
than intracratonic ones, due to their situation at continent margins. Initial basin subsidence can be rapid, resulting in an early phase of deep-water sedimentation. The floor
of the Sirte basin was extensively faulted and there was some igneous activity. Like intracratonic basins, however, epicratonic embayments can be infilled by both carbonate
483
erally extensive continental margin downwarps. The latter are typical of passive continental margins that develop at the end of the rift-drift sequence, in which context they
will subsequently be described.
Epicratonic embayments, as the first type may be called, tend to be embayed and open
toward the adjacent ocean basin. The axis of an epicratonic basin may plunge to the floor
of the ocean or be interrupted by a sill-like feature at the rim of the continental margin.
Epicratonic basins include those predominantly infilled with clastics, such as the Mississippi Gulf coast, and the Niger delta basin (Weber, 1971; Burke, 1972; Walcott, 1972;
Wilhelm and Ewing, 1972; Evamy et aL, 1978; Reijers et al., 1997), and those with predominant carbonate fill, such as the Sirte basin of North Africa (Salem et al., 1996a,b,c;
Selley, 1997c).
The Mississippi and Niger delta basins are very similar (Salvador, 1991; Reijers et al.,
1997). Both originated toward the end of the Mesozoic Era and continue to be sites of
active clastic sedimentation until the present day. Both basins contain a basal layer of
salt, which diapirically intrudes younger sedimentary rocks. The Louann salt of the Gulf
basin is of Jurassic age. The basal salt of the Niger delta basin is believed to be of AlbianAptian age (Mascle et al., 1973). The Mississippi and Niger delta basins are infilled by
prisms of terrigenous clastics which were deposited in a range of environments. On the
landward sides of the embayments alluvial deposits predominate. These pass basinward
into diverse shoreline facies, which include both barrier and deltaic deposits. These thin
and grade seaward into marine slope muds, with some development of turbidite sand
facies at the base of the slope (Fig. 10.15).
Geophysical data suggest a gradual seaward thinning of the continental crust beneath
both the Mississippi Gulf coast and Niger delta basins (Walcott, 1972, and Burke, 1972,
respectively).
The Sirte basin of North Africa shares many features with the Gulf coast and Niger basins. It, too, is essentially an embayment which opens out to an oceanic basin
(the Mediterranean). The area now occupied by the Sirte basin was a major northsouth uplift that controlled sedimentation throughout the Paleozoic era. This regional
arch collapsed in the middle of the Cretaceous Period to form the Sirte embayment
(Fig. 10.16).
Basal sands and thin evaporites are overlain by deep-water Upper Cretaceous and
Paleocene shales. These are thickest in intra-basinal troughs, while reefal carbonates
were deposited on adjacent horsts. Throughout the Eocene the Sirte embayment was
infilled by nearly a kilometer of interbedded carbonates and evaporites. The final phase
of basin infilling during the Oligocene and Miocene involved terrigenous and carbonate sedimentation in both marine and continental environments. The active history of
the basin was concluded by an epsode of basaltic volcanism in the Pleistocene (Salem
et aL, 1996a,b,c; Selley, 1997c).
These brief reviews of three epicratonic basins show how they differ from the previously described intracratonic examples. Epicratonic basins tend to be much less stable
than intracratonic ones, due to their situation at continent margins. Initial basin subsidence can be rapid, resulting in an early phase of deep-water sedimentation. The floor
of the Sirte basin was extensively faulted and there was some igneous activity. Like intracratonic basins, however, epicratonic embayments can be infilled by both carbonate