12.4 Intracontinental Sag Basins
"Modem" examples of intracratonic sag basins include
the slowly subsiding, continental Chad basin in northern
central Africa and the Eyre basin in Australia (Mitchell and
Reading 1986). The North Sea, the Baltic Sea (part of the
Baltic basin), and Hudson Bay in Canada are old sag basins
inundated by the sea.
12.4.2 Tectonic Evolution and
Subsidence History
The erustal proeesses forming these basins are not
well understood. Most basins laek major extensional
faulting, apart from early rifting as observed in many
eases. Thus, long-term thermal eontraetion and/or
eclogitization of the lowermost erust, or intraplate
stress seem to be the major meehanism in generating
subsidenee.
Lateral tensile deviatoric stress appears to have been insufficient to overcome rock strength and cause brittle fracture
(e.g., Allen and Allen 1990). Instead, intraplate stress is
mentioned by some authors, causing long-wave-Iength
undulations in the continental crust (e.g. in Asia; Nikishin
et al. 1993). This mechanism can also explain irregularities
and shortcomings of thermal models of post-rift basin subsidence (Cloetingh and Kooi 1992). In any case, post-rift
subsidence appears to be much longer than that which can
be attributed to a rift-produced thermal anomaly (Quinlan
1987). In the absence of significant stretching and thinning
(or shortening) of the crust, a slow gabbro-eclogite transformation in the lowermost crust or uppermost mantle
could be the principal mechanism causing subsidence (cf.
Sect. 8.1.3). Other authors (e.g., Finlayson et al. 1989)
emphasize that the earlier history of the crust (prior to the
evolution ofthe sag basin) plays an important role in determining the subsidence and structure of this type of basin.
Klein and Hsui (1987) assume that the North American
cratonic basins resulted from the breakup of a late Precambrian supercontinent similar to the Triassic-Jurassic
breakup ofPangea mentioned in Sect. 12.3.
The initiation and "eyclic" evolution of many
intraeontinental sag basins seem to be assoeiated
with the breakup of super-eontinents (Pan-Africa,
-600 Ma B.P. and Pangea in the Triassic-Jurassie; ef.
Seet. 7.8). Some old, formerly intracontinental basins
were split up by subsequent rifting and ocean spreading and now represent craton margin sag basins. The
North Afriean examples of sag basins are loeated on
a re1atively "mobile" erust and possibly not the best
representatives of this basin type. They display, however, two distinct "super-cycles" in the Paleozoic and
Mesozoie-Cenozoic, each beginning with a rift stage
and ending in a long sag stage (Macgregor 1998).
The tectonic evolution of the various intra-continental basins was not always slow and steady. Many
of them show a multi-phase evolution (Fig. 12.17b)
indieated by the upward termination of major faults
and/or large erosional uneonformities. This development is also refleeted by subsidenee curves display575
ing re1atively short periods of increased subsidenee
altemating with long-term slow rates (Fig. 12.17 d).
Both, maximum total subsidenee and the history of
subsidence ean markedly differ in adjaeent sag basins. Average rates of subsidenee in intraeontinental
sag basins vary from less than 10 to 25 mlMa, including periods of nondeposition and erosion.
The huge western Siberian basin, 2300 km in length, was
initiated by Paleozoic rifting and later affected two times
by moderate tectonism (horst-and-graben structures and
volcanism in the Triassic, further structures in the Neogene; Khain 1994).
The subsidence rates varied markedly at different 10cations
within the basin and with time. The Timan-Pechora basin
in northeastern Europe, located to the west ofthe Ural fold
belt and including part the Barents Sea, accumulated 7-8
km sediment since the early Paleozoic. Here, subsidence
proceeded in several distinct steps (lsmail-Zadeh et al.
1997).
The Parana basin in South America subsided >5 km
since the Silurian in three distinct steps (Zalän 1990). Subsidence in the Permo-Carboniferous may have been accelerated by the ice load of the coevel continental glaciation; a
further pulse of subsidence was probably caused by the
heavy load ofthick and extensive volcanics.
The central parts of the Michigan basin in North America subsided steadily at an average rate of 24 mlMa over a
time span of 200 Ma (Fischer 1975). Differing subsidence
in neighboring basins was observed, for example, in basins
on the Saharan platform in North Africa, to the north ofthe
Precambrian massifs of Tibesti, Hoggar, and Eglab. The
western Erg basin subsided -5 km in -250 Ma in the Paleozoic, the eastern Erg basin subsided in three steps up to
10 km in 500 Ma in the Paleozoic, Mesozoic and Tertiary
(Ricbe and Charpal 1998).
12.4.3 Sediments
Although the intracontinental sag basins are large,
their terrestrial sediment supply and authoehthonous
sediment produetion eould eommonly keep pace with
the slow subsidenee of their basin floor. As a result,
the sediments of sag basins tend to be continental,
shallow-laeustrine, or shallow-marine. Overfilled
basins display pronounced stratigraphie gaps. Many
sag basins were intermittently eonnected with the sea
and therefore reeord major eustatic sea-Ievel ehanges
(Fig. 12.17c), prograding and baekstepping facies
associations (offlapping and onlapping units), and
phases of nondeposition or erosion. Pronounced uneonformities have been used to distinguish several
mega-sequences in these basins and to establish a
stratigraphie eorrelation between basins of different
continents.
The stratigraphic chart ofthe Illinois basin in North America, for example, was used by Sloss (1963) to define four
mega-sequences, which are today regarded as 2nd order
sequences (cf. Sect. 7.8). These mega-sequences are separated by prominent unconforrnities (smaller ones are omit-
"Modem" examples of intracratonic sag basins include
the slowly subsiding, continental Chad basin in northern
central Africa and the Eyre basin in Australia (Mitchell and
Reading 1986). The North Sea, the Baltic Sea (part of the
Baltic basin), and Hudson Bay in Canada are old sag basins
inundated by the sea.
12.4.2 Tectonic Evolution and
Subsidence History
The erustal proeesses forming these basins are not
well understood. Most basins laek major extensional
faulting, apart from early rifting as observed in many
eases. Thus, long-term thermal eontraetion and/or
eclogitization of the lowermost erust, or intraplate
stress seem to be the major meehanism in generating
subsidenee.
Lateral tensile deviatoric stress appears to have been insufficient to overcome rock strength and cause brittle fracture
(e.g., Allen and Allen 1990). Instead, intraplate stress is
mentioned by some authors, causing long-wave-Iength
undulations in the continental crust (e.g. in Asia; Nikishin
et al. 1993). This mechanism can also explain irregularities
and shortcomings of thermal models of post-rift basin subsidence (Cloetingh and Kooi 1992). In any case, post-rift
subsidence appears to be much longer than that which can
be attributed to a rift-produced thermal anomaly (Quinlan
1987). In the absence of significant stretching and thinning
(or shortening) of the crust, a slow gabbro-eclogite transformation in the lowermost crust or uppermost mantle
could be the principal mechanism causing subsidence (cf.
Sect. 8.1.3). Other authors (e.g., Finlayson et al. 1989)
emphasize that the earlier history of the crust (prior to the
evolution ofthe sag basin) plays an important role in determining the subsidence and structure of this type of basin.
Klein and Hsui (1987) assume that the North American
cratonic basins resulted from the breakup of a late Precambrian supercontinent similar to the Triassic-Jurassic
breakup ofPangea mentioned in Sect. 12.3.
The initiation and "eyclic" evolution of many
intraeontinental sag basins seem to be assoeiated
with the breakup of super-eontinents (Pan-Africa,
-600 Ma B.P. and Pangea in the Triassic-Jurassie; ef.
Seet. 7.8). Some old, formerly intracontinental basins
were split up by subsequent rifting and ocean spreading and now represent craton margin sag basins. The
North Afriean examples of sag basins are loeated on
a re1atively "mobile" erust and possibly not the best
representatives of this basin type. They display, however, two distinct "super-cycles" in the Paleozoic and
Mesozoie-Cenozoic, each beginning with a rift stage
and ending in a long sag stage (Macgregor 1998).
The tectonic evolution of the various intra-continental basins was not always slow and steady. Many
of them show a multi-phase evolution (Fig. 12.17b)
indieated by the upward termination of major faults
and/or large erosional uneonformities. This development is also refleeted by subsidenee curves display575
ing re1atively short periods of increased subsidenee
altemating with long-term slow rates (Fig. 12.17 d).
Both, maximum total subsidenee and the history of
subsidence ean markedly differ in adjaeent sag basins. Average rates of subsidenee in intraeontinental
sag basins vary from less than 10 to 25 mlMa, including periods of nondeposition and erosion.
The huge western Siberian basin, 2300 km in length, was
initiated by Paleozoic rifting and later affected two times
by moderate tectonism (horst-and-graben structures and
volcanism in the Triassic, further structures in the Neogene; Khain 1994).
The subsidence rates varied markedly at different 10cations
within the basin and with time. The Timan-Pechora basin
in northeastern Europe, located to the west ofthe Ural fold
belt and including part the Barents Sea, accumulated 7-8
km sediment since the early Paleozoic. Here, subsidence
proceeded in several distinct steps (lsmail-Zadeh et al.
1997).
The Parana basin in South America subsided >5 km
since the Silurian in three distinct steps (Zalän 1990). Subsidence in the Permo-Carboniferous may have been accelerated by the ice load of the coevel continental glaciation; a
further pulse of subsidence was probably caused by the
heavy load ofthick and extensive volcanics.
The central parts of the Michigan basin in North America subsided steadily at an average rate of 24 mlMa over a
time span of 200 Ma (Fischer 1975). Differing subsidence
in neighboring basins was observed, for example, in basins
on the Saharan platform in North Africa, to the north ofthe
Precambrian massifs of Tibesti, Hoggar, and Eglab. The
western Erg basin subsided -5 km in -250 Ma in the Paleozoic, the eastern Erg basin subsided in three steps up to
10 km in 500 Ma in the Paleozoic, Mesozoic and Tertiary
(Ricbe and Charpal 1998).
12.4.3 Sediments
Although the intracontinental sag basins are large,
their terrestrial sediment supply and authoehthonous
sediment produetion eould eommonly keep pace with
the slow subsidenee of their basin floor. As a result,
the sediments of sag basins tend to be continental,
shallow-laeustrine, or shallow-marine. Overfilled
basins display pronounced stratigraphie gaps. Many
sag basins were intermittently eonnected with the sea
and therefore reeord major eustatic sea-Ievel ehanges
(Fig. 12.17c), prograding and baekstepping facies
associations (offlapping and onlapping units), and
phases of nondeposition or erosion. Pronounced uneonformities have been used to distinguish several
mega-sequences in these basins and to establish a
stratigraphie eorrelation between basins of different
continents.
The stratigraphic chart ofthe Illinois basin in North America, for example, was used by Sloss (1963) to define four
mega-sequences, which are today regarded as 2nd order
sequences (cf. Sect. 7.8). These mega-sequences are separated by prominent unconforrnities (smaller ones are omit-