10
1.3 Tectonics and Basin-Filling
Although basin-generating tectonic movements and
basin-fllling depositional processes generally interact,
one can distinguish three different modes (partially
end members) of this relationship (Fig. 1.5, based on
Selley 1985a):
Pre-depositional basins. Rapid tectonic movements
predate significant sediment accumulation and create a
morphological basin, which is filled later by post-tectonic sediments (Fig. 1.5a). The water depth in the
basin decreases with time, although some syndepositional subsidence due to sediment loading is
likely (Sect. 8.1). Sediment transport as well as vertical
and lateral facies associations are substantially influenced by the basin morphology.
Syn-depositional basins. Sediment accumulation is
affected by syn-depositional tectonic movements, e.g.,
differential subsidence (Fig. 1.5b). If the sedimentation rate is always high enough to compensate for
subsidence, the direction of transport and the sedimentary facies largely remain unchanged, but the thicknesses of certain time slices varies. In Figure l.5b they
increase toward the center of the basin. In this case,
the basin structure is syn-depositional, but there was
hardly a syn-depositional morphological basin controlling the sedimentary facies of the basin. If sedimentation is too slow to fill up the subsiding area, a morphological basin will develop. Then, the distribution and
facies of the succeeding sediments will be affected by
the morphology of the deepening basin (transition to
the situation shown in Fig. 1.5a).
Post-depositional basins. The deposition of sediments largely predates tectonic movements forming a
distinct basin structure. Hence, there is no or little
relationship between the transport, distribution, and
facies of these sediments and the later evolved basin
structure (Fig. 1.5c). In most cases, however, some
relationship between a syn-depositional subsidence
phase and the subsequent tectonic overprint cannot be
excluded.
Of course, there are transitions between these simplified models of the interaction between basin-generating tectonics and basin-filling processes (see Chap.
12). Certain basins may show a complex history and
therefore contain sediments affected by both pre- and
syn-tectonic movements.
Chapter 1 Basin Classification
1.4 Basin Morphology and Depositional
Environments
1.4.1 General Aspects
The geometry of an ultimate basin fill is controlled
mainly by basin-forming tectonic processes, but the
morphology of a basin defmed by the sediment surface
is the product of the interplay between tectonic movements and sedimentation. Therefore, as already mentioned, a purely tectonic classification of sedimentary
basins is not sufficient for characterizing depositional
areas. It is true that a sedimentary basin in a particular
tectonic setting often experiences a specific development and subsidence history (Chaps. 8 and 12), but its
morphology, including water depth, may be controlled
largely by other factors, such as varying influx and distribution of sediment from terrigenous sources (Chap.
11 ).
For example, a fluvial depositional system ean develop and
persist for eonsiderable time on top of subsiding ernst in various tectonic settings (Miall 1981). Fluvial deposits are known
from eontinental graben struetures, passive continental margins, foreland basins, foreare and baekarc basins, pull-apart
basins, ete. Fluvial sediments aeeumulate as long as rivers
reaeh the depositional area and supply enough material to
keep the subsiding basin filled. Although the basin-forming
processes and subsidenee histories of these examples differ
fundarnentally from each other, the sedimentary faeies of their
basin fills display no or only minor differences. In order to
distinguish between these varying tectonic settings, one has to
take into aceount the geometry of the entire basin fill, as well
as vertieal and lateral facies ehanges over long distances, including paleoeurrent direetions and other eriteria.
Syndepositional tectonic movements manifested by variations
in thiekness, small diseonformities, or faults dying out upward
(cf. Fig. 1.5b) may indicate the nature of the tectonic proces ses involved.
The erosional base level and hydrographie regime
within a basin are additional important factors controlling sediment dispersal and modifying basin morphology. They largely determine the development of special
sedimentary facies as demonstrated in the elementary
model of Fig. 1.6. In a fluvial environment, sediments
cannot accumulate higher than the base level of erosion
and the elevation added by the gradient of the stream. If
there is more influx of material into the depositional
system than necessary for compensation of subsidence,
the sediment surplus will be carried farther downstream
into lakes or the sea.
This signifies that the level up to whieh a basin ean be filled
with sediments may depend on the geographie position ofthe
basin in relation to the erosionaI base. In Tibet, for example,
the floors of present-day fluviaI basins (intramontane basins
and graben structures) are eIevated higher than 3000 m in
eomparison to the coastal fluvial plains elsewhere.
1.3 Tectonics and Basin-Filling
Although basin-generating tectonic movements and
basin-fllling depositional processes generally interact,
one can distinguish three different modes (partially
end members) of this relationship (Fig. 1.5, based on
Selley 1985a):
Pre-depositional basins. Rapid tectonic movements
predate significant sediment accumulation and create a
morphological basin, which is filled later by post-tectonic sediments (Fig. 1.5a). The water depth in the
basin decreases with time, although some syndepositional subsidence due to sediment loading is
likely (Sect. 8.1). Sediment transport as well as vertical
and lateral facies associations are substantially influenced by the basin morphology.
Syn-depositional basins. Sediment accumulation is
affected by syn-depositional tectonic movements, e.g.,
differential subsidence (Fig. 1.5b). If the sedimentation rate is always high enough to compensate for
subsidence, the direction of transport and the sedimentary facies largely remain unchanged, but the thicknesses of certain time slices varies. In Figure l.5b they
increase toward the center of the basin. In this case,
the basin structure is syn-depositional, but there was
hardly a syn-depositional morphological basin controlling the sedimentary facies of the basin. If sedimentation is too slow to fill up the subsiding area, a morphological basin will develop. Then, the distribution and
facies of the succeeding sediments will be affected by
the morphology of the deepening basin (transition to
the situation shown in Fig. 1.5a).
Post-depositional basins. The deposition of sediments largely predates tectonic movements forming a
distinct basin structure. Hence, there is no or little
relationship between the transport, distribution, and
facies of these sediments and the later evolved basin
structure (Fig. 1.5c). In most cases, however, some
relationship between a syn-depositional subsidence
phase and the subsequent tectonic overprint cannot be
excluded.
Of course, there are transitions between these simplified models of the interaction between basin-generating tectonics and basin-filling processes (see Chap.
12). Certain basins may show a complex history and
therefore contain sediments affected by both pre- and
syn-tectonic movements.
Chapter 1 Basin Classification
1.4 Basin Morphology and Depositional
Environments
1.4.1 General Aspects
The geometry of an ultimate basin fill is controlled
mainly by basin-forming tectonic processes, but the
morphology of a basin defmed by the sediment surface
is the product of the interplay between tectonic movements and sedimentation. Therefore, as already mentioned, a purely tectonic classification of sedimentary
basins is not sufficient for characterizing depositional
areas. It is true that a sedimentary basin in a particular
tectonic setting often experiences a specific development and subsidence history (Chaps. 8 and 12), but its
morphology, including water depth, may be controlled
largely by other factors, such as varying influx and distribution of sediment from terrigenous sources (Chap.
11 ).
For example, a fluvial depositional system ean develop and
persist for eonsiderable time on top of subsiding ernst in various tectonic settings (Miall 1981). Fluvial deposits are known
from eontinental graben struetures, passive continental margins, foreland basins, foreare and baekarc basins, pull-apart
basins, ete. Fluvial sediments aeeumulate as long as rivers
reaeh the depositional area and supply enough material to
keep the subsiding basin filled. Although the basin-forming
processes and subsidenee histories of these examples differ
fundarnentally from each other, the sedimentary faeies of their
basin fills display no or only minor differences. In order to
distinguish between these varying tectonic settings, one has to
take into aceount the geometry of the entire basin fill, as well
as vertieal and lateral facies ehanges over long distances, including paleoeurrent direetions and other eriteria.
Syndepositional tectonic movements manifested by variations
in thiekness, small diseonformities, or faults dying out upward
(cf. Fig. 1.5b) may indicate the nature of the tectonic proces ses involved.
The erosional base level and hydrographie regime
within a basin are additional important factors controlling sediment dispersal and modifying basin morphology. They largely determine the development of special
sedimentary facies as demonstrated in the elementary
model of Fig. 1.6. In a fluvial environment, sediments
cannot accumulate higher than the base level of erosion
and the elevation added by the gradient of the stream. If
there is more influx of material into the depositional
system than necessary for compensation of subsidence,
the sediment surplus will be carried farther downstream
into lakes or the sea.
This signifies that the level up to whieh a basin ean be filled
with sediments may depend on the geographie position ofthe
basin in relation to the erosionaI base. In Tibet, for example,
the floors of present-day fluviaI basins (intramontane basins
and graben structures) are eIevated higher than 3000 m in
eomparison to the coastal fluvial plains elsewhere.