Tectonic Control of Alluvial Stratigraphy
limits to the ability of rivers to change in this way. At
high sinuosities the incidence of chute and neck
cutoffs increases (reducing the sinuosity again), and
bank erosion and incision ensue. Reduction in slope
may lead to meandering rivers becoming flooded
and developing anastomosed patterns, as was predicted by Smith and Smith (1980). At synclinal and
anticlinal axes, respectively, aggradation and degradation occur. These ideas are of considerable importance in connection with the effects of base-level
change on rivers and are discussed further in Sect.
1 1.2.2.
Recent applications of . these ideas include
the work of Marple and Talwani (1993), who
employed all these indicators in a study of neotectonic deformation of the coastal plain of South
Carolina. Upstream from a region of uplift, rivers
are anastomosed and swampy. In addition, some
rivers flow along one side of their fl oodplains, suggesting lateral migration due to tectonic tilting.
Mather (1993) documented the uplift of a small
fluvial basin in Spain, which was undergoing active alluvial-fan sedimentation in the Pliocene and
now is characterized by incised streams, with the
courses of the fan fe eder channels truncated by river
capture.
These observations based on neotectonic and experimental studies and observations of the Recent
may be applied to analyses of the more ancient
record, where examples abound of the effects of
syndepositional, within-basin tectonic deformation.
Some of the most spectacular examples occur in the
Himalayan foreland basin of India and Pakistan,
where fluvial deposits have been accumulating con�
tinuously since the mid-Cenozoic, and major trunk
rivers such as the Indus, Ganges, and Brahmaputra
have occupied the axis of the basin for millions of
years. These rivers show many changes in channel
patterns (e.g., Holmes 1968; Wells and Dorr 1987),
at least some of which are telated to fault movement and block tilting within the sedimentary basin
(Morgan and Mcintire 1959; McDougall 1989;
Srivastava et al. 1994). For example, the Indus River
has been diverted eastward a lateral distance of 10
km along the western side of the Potwar Plateau by
uplift on a major transpressional fault during the
past 100-500 ka. The former course of the river can
be traced across uplifted fault blocks, where terrace
remants are now as much as 630 m above their
former elevation. Lateral migration of the Indus laid
down a sheet of gravel which is now being drained by
small consequent streams flowing across the newly
tilted block (Fig. 11.9).
351
Earlier changes in the course of the Indus River in
the Potwar plateau area of Pakistan were reconstructed by Mulder and Burbank (1993), based on an
extensive synthesis of facies, petrographic, and
paleocurrent data within a stratigraphic fr amework
that has been rigorously dated using magnetostratigraphy. Several studies have shown that the
Indus River flowed eastward into the Ganges system
prior to about 5 Ma (Fig. 11.10). Its deposits are
characterized by a distinctive white sandstone in the
form of channel bodies bearing a characteristic vari�
ety of hornblende in the heavy-mineral suite. These
beds are interbedded with a variegated assemblage
of brown sandstones showing much greater lithologic variablity and generally smaller-scale crossbed structures and thinner channel units. The brown
units are interpreted as the deposits of smaller
streams draining uplifts of the Himalayas and
the Salt Ranges to the south of the Potwar Plateau.
The structural emergence of the Salt Ranges can
be determined on the basis of the appearance of
brown sandstone units draining the emerging uplift and accompanying shifts in paleocurrent patterns and, later, by the appearance of a distinctive
granite clast type known only from Salt Range
sources. Initially, the uplift of the Salt Ranges shifted
the Indus River system northward, and continued
uplift then deflected it westward to near its present
course (Fig. 11.10). Minor drainages of the )helum
system captured the runoff from the emerging structures.
This picture of tectonic control of fluvial paleogeography, and the sedimentological evidence from
which it can be reconstructed, is typical of many
basins marginal to fo ld-thrust belts, particularly
"piggyback" or "satellite" basins, as described in
Sect. !1.3.
The development of coal is sensitive to tectonic
setting. Given the appropriate climate for peat formation, the thickness and quality of a peat unit
depend on subsidence rate and clastic influx.
Titheridge (1993) described the effects of faulting
and consequent local basinal tilting on the distribution and thickness of coals. He showed that the
slightly increased subsidence rate on the downthrown block of a half-graben leads to an increase in
seam thickness and also to a greater frequency of
seam splitting. The difference between true low-ash
coal and carbonaceous shale depends on the clastic
influx into the coal swamp. McCabe (1984) demonstrated that low-ash coal may reflect the development of raised peat islands, which deflect drainage
channels and their contained clastic influx away
limits to the ability of rivers to change in this way. At
high sinuosities the incidence of chute and neck
cutoffs increases (reducing the sinuosity again), and
bank erosion and incision ensue. Reduction in slope
may lead to meandering rivers becoming flooded
and developing anastomosed patterns, as was predicted by Smith and Smith (1980). At synclinal and
anticlinal axes, respectively, aggradation and degradation occur. These ideas are of considerable importance in connection with the effects of base-level
change on rivers and are discussed further in Sect.
1 1.2.2.
Recent applications of . these ideas include
the work of Marple and Talwani (1993), who
employed all these indicators in a study of neotectonic deformation of the coastal plain of South
Carolina. Upstream from a region of uplift, rivers
are anastomosed and swampy. In addition, some
rivers flow along one side of their fl oodplains, suggesting lateral migration due to tectonic tilting.
Mather (1993) documented the uplift of a small
fluvial basin in Spain, which was undergoing active alluvial-fan sedimentation in the Pliocene and
now is characterized by incised streams, with the
courses of the fan fe eder channels truncated by river
capture.
These observations based on neotectonic and experimental studies and observations of the Recent
may be applied to analyses of the more ancient
record, where examples abound of the effects of
syndepositional, within-basin tectonic deformation.
Some of the most spectacular examples occur in the
Himalayan foreland basin of India and Pakistan,
where fluvial deposits have been accumulating con�
tinuously since the mid-Cenozoic, and major trunk
rivers such as the Indus, Ganges, and Brahmaputra
have occupied the axis of the basin for millions of
years. These rivers show many changes in channel
patterns (e.g., Holmes 1968; Wells and Dorr 1987),
at least some of which are telated to fault movement and block tilting within the sedimentary basin
(Morgan and Mcintire 1959; McDougall 1989;
Srivastava et al. 1994). For example, the Indus River
has been diverted eastward a lateral distance of 10
km along the western side of the Potwar Plateau by
uplift on a major transpressional fault during the
past 100-500 ka. The former course of the river can
be traced across uplifted fault blocks, where terrace
remants are now as much as 630 m above their
former elevation. Lateral migration of the Indus laid
down a sheet of gravel which is now being drained by
small consequent streams flowing across the newly
tilted block (Fig. 11.9).
351
Earlier changes in the course of the Indus River in
the Potwar plateau area of Pakistan were reconstructed by Mulder and Burbank (1993), based on an
extensive synthesis of facies, petrographic, and
paleocurrent data within a stratigraphic fr amework
that has been rigorously dated using magnetostratigraphy. Several studies have shown that the
Indus River flowed eastward into the Ganges system
prior to about 5 Ma (Fig. 11.10). Its deposits are
characterized by a distinctive white sandstone in the
form of channel bodies bearing a characteristic vari�
ety of hornblende in the heavy-mineral suite. These
beds are interbedded with a variegated assemblage
of brown sandstones showing much greater lithologic variablity and generally smaller-scale crossbed structures and thinner channel units. The brown
units are interpreted as the deposits of smaller
streams draining uplifts of the Himalayas and
the Salt Ranges to the south of the Potwar Plateau.
The structural emergence of the Salt Ranges can
be determined on the basis of the appearance of
brown sandstone units draining the emerging uplift and accompanying shifts in paleocurrent patterns and, later, by the appearance of a distinctive
granite clast type known only from Salt Range
sources. Initially, the uplift of the Salt Ranges shifted
the Indus River system northward, and continued
uplift then deflected it westward to near its present
course (Fig. 11.10). Minor drainages of the )helum
system captured the runoff from the emerging structures.
This picture of tectonic control of fluvial paleogeography, and the sedimentological evidence from
which it can be reconstructed, is typical of many
basins marginal to fo ld-thrust belts, particularly
"piggyback" or "satellite" basins, as described in
Sect. !1.3.
The development of coal is sensitive to tectonic
setting. Given the appropriate climate for peat formation, the thickness and quality of a peat unit
depend on subsidence rate and clastic influx.
Titheridge (1993) described the effects of faulting
and consequent local basinal tilting on the distribution and thickness of coals. He showed that the
slightly increased subsidence rate on the downthrown block of a half-graben leads to an increase in
seam thickness and also to a greater frequency of
seam splitting. The difference between true low-ash
coal and carbonaceous shale depends on the clastic
influx into the coal swamp. McCabe (1984) demonstrated that low-ash coal may reflect the development of raised peat islands, which deflect drainage
channels and their contained clastic influx away