472
10 SEDIMENTARY BASINS
topographic axis of the basin, or merely a lateral progradation of the main locus of deposition across an essentially stable basin floor. The danger of using an isopach map
as an measure of subsidence is particularly acute in carbonate basins. As discussed in
Chapter 6, carbonate sedimentation rates are relatively slow inshore and in the open
sea, and are at their fastest in between, forming an accretionary ramp. Thus carbonate
depocenters are not coincident with basin axis.
Basins are separated one from another by raised linear areas where the sediment
cover is thin or absent. These are variously termed arches, paleo-highs, schwelle, axes of
uplift, or positive areas. Similarly, major basins are commonly divisible into sub-basins,
troughs, and embayments by smaller positive features.
10.1.3 Basin-Forming Mechanisms
Accommodation space may be created by three tectonic processes (Stoneley, 1969).
Subsidence may occur where subcrustal displacement of the mantle leads to downdragging and compressional warping of the crust. This occurs principally at what are
called zones of subduction, linear features that are the site of extensive sedimentation. Sedimentation may also occur on a large scale where changes in the mantle cause
foundering and subsidence of the crust. This process is responsible for intracratonic basins. Conversely these changes can cause the crust to dome. Thick volcanic and sedimentary sequences may form in crestal rift basins. Finally, thick sedimentary sequences
may form where the weight of the sediment itself causes isostatic depression of the
crust. This process obviously requires an outside mechanism to create an initial crustal
void, since it poses the old problem of which came first, the hen or the egg? The most
likely place for such a process is the continental margin, where a whole ocean basin waits
to be infilled. Sedimentation at the foot of the continental slope may cause isostatic depression of the crust (Drake et al., 1968).
Considerable attention has been paid to the geomechanics of basin subsidence, a
problem requiring geophysics and structural analysis for its elucidation (e.g., McKenzie,
1978; Neugebauer, 1987; Allen and Allen, 1990; Busby and Ingersoll, 1995; Kearey and
Vine, 1996; Condie, 1997; Lillie, 1999).
Once upon a time geology students were taught that the continents were composed
largely of silica and alumina (sial) and floated isostatically on denser oceanic crust composed largely of silica and magnesia (sima). Mountain chains occurred where continents bumped together pushing up folded belts of sediments deposited in the troughs
between the continents. The compression of the deposits of the Tethys Ocean to form
the Alpine mountain chain was the classic example, the motive power in this case being
the convergence of the European and African shields (Pfiffner et al., 1998). Mountain
chains such ~s the Appalachians and the Andes were hard to fit into so simple a scheme,
as half of the vice was absent. One explanation offered was that the continent had foundered on the oceanic side of such mountain chains, in apparent defiance of the principles of isostasy.
An alternative proposal was that continents could drift horizontally across the face
of the earth. The close geographic and geologic fit of the circum-Atlantic continents
was the keystone of this thesis (Wegener, 1924; du Toit, 1937). These ideas, rejuvenated
10 SEDIMENTARY BASINS
topographic axis of the basin, or merely a lateral progradation of the main locus of deposition across an essentially stable basin floor. The danger of using an isopach map
as an measure of subsidence is particularly acute in carbonate basins. As discussed in
Chapter 6, carbonate sedimentation rates are relatively slow inshore and in the open
sea, and are at their fastest in between, forming an accretionary ramp. Thus carbonate
depocenters are not coincident with basin axis.
Basins are separated one from another by raised linear areas where the sediment
cover is thin or absent. These are variously termed arches, paleo-highs, schwelle, axes of
uplift, or positive areas. Similarly, major basins are commonly divisible into sub-basins,
troughs, and embayments by smaller positive features.
10.1.3 Basin-Forming Mechanisms
Accommodation space may be created by three tectonic processes (Stoneley, 1969).
Subsidence may occur where subcrustal displacement of the mantle leads to downdragging and compressional warping of the crust. This occurs principally at what are
called zones of subduction, linear features that are the site of extensive sedimentation. Sedimentation may also occur on a large scale where changes in the mantle cause
foundering and subsidence of the crust. This process is responsible for intracratonic basins. Conversely these changes can cause the crust to dome. Thick volcanic and sedimentary sequences may form in crestal rift basins. Finally, thick sedimentary sequences
may form where the weight of the sediment itself causes isostatic depression of the
crust. This process obviously requires an outside mechanism to create an initial crustal
void, since it poses the old problem of which came first, the hen or the egg? The most
likely place for such a process is the continental margin, where a whole ocean basin waits
to be infilled. Sedimentation at the foot of the continental slope may cause isostatic depression of the crust (Drake et al., 1968).
Considerable attention has been paid to the geomechanics of basin subsidence, a
problem requiring geophysics and structural analysis for its elucidation (e.g., McKenzie,
1978; Neugebauer, 1987; Allen and Allen, 1990; Busby and Ingersoll, 1995; Kearey and
Vine, 1996; Condie, 1997; Lillie, 1999).
Once upon a time geology students were taught that the continents were composed
largely of silica and alumina (sial) and floated isostatically on denser oceanic crust composed largely of silica and magnesia (sima). Mountain chains occurred where continents bumped together pushing up folded belts of sediments deposited in the troughs
between the continents. The compression of the deposits of the Tethys Ocean to form
the Alpine mountain chain was the classic example, the motive power in this case being
the convergence of the European and African shields (Pfiffner et al., 1998). Mountain
chains such ~s the Appalachians and the Andes were hard to fit into so simple a scheme,
as half of the vice was absent. One explanation offered was that the continent had foundered on the oceanic side of such mountain chains, in apparent defiance of the principles of isostasy.
An alternative proposal was that continents could drift horizontally across the face
of the earth. The close geographic and geologic fit of the circum-Atlantic continents
was the keystone of this thesis (Wegener, 1924; du Toit, 1937). These ideas, rejuvenated