I
Distinguishing Tectonic from Climatic Control
1. Large variations in flu vial discharge produce re cog nizable effects in the facies record, such as the
interbedding of ve ry coarse and very fine facies,
bu t the time scale of varia tion may not be obviou s
and the climatic implications therefore unclear.
Seasonal effects, such as those arising from the
freeze-thaw cycle, monsoonal varia tions, and
those tha t occu r in arid environme nts cha racterize d by occasional violent flash floods, months or
even years apart, all generate the same wide va riation in discharge and consequent wide variation
in facie s, bu t the climatic interpretation of the
re su lting fa cie s may be qu ite ambiguou s ( considei:ations of time scale are discussed at le ngth in
Chap. 3).
2. The clima t. e of the se diment source area and that
of the depositional basin may not be the same . For
exa mple , rivers flowing southward from the Alw
pineMHimalaya n mountain chain are all strongly
sea sonal, re flecting the alpine climate of their
mountainous source areas, bu t they enter ve ry
differe nt climates in the plains to the south. Europe an rivers, such as the Rhone and Po, have
coastal plains in the subtropical Mediterranean
re gion. The Tigris and Eu phrates ente r the highly
arid Mesopotamian basin. The fluvia l plains of
northern India are located in a tropical climate
where se asonal monsoonal rainfa ll strongly influences discharge cha racteristics (see also Sect.
11.3.1 for othe r characteristics of "big rivers'' ).
3. Re late d to point 2 above is the fact that flow
hydraulics of a river, and the refore the facies and
architecture of the be dload deposits, are determined large ly by the climate of the source area,
whereas floodplains and their finer-graine d deposits are influence d primarily by the climate of
the de positional basin.
4. Topographic effe cts ma y generate local climates
tha t complicate the depositional re cord. For
example , mountain range s rising in the path of
pre vailing humid wi nds may generate hig h orographic rainfa lls on their upwind slopes and rain
shadows downwind. The topographic diversion
of air ma sse s is an important general effect. Very
high pla te aus, such as Tibe t, are also a significant
source of he at, which has its own effect on air
te mperature s and circulation. The possible wide r
linkages betwee n tectonics and climate are discu sse d in Sect. 12.11.
5. Even within a flu vial basin, le ve ls of water saturation (the wa ter ta ble) and redox conditions may
va ry from channe l to floodplain and be twee n the
proximal, possibly more elevated basin-margin
423
rivers and the topographica lly lower basin-ce nte r
channe l systems.
6. Climate is commonly re corded in the sedime ntary record by vege tation, by the fossil re ma ins of
vegetation itself, and by the effects vege ta tion has
on se diment erodibility, sediment yield, cha nnel
style, and so on. However, the styles of vegetation
that characterize the earth today have evolved
with time and were very differe nt in the geological
past. The deve lopme nt of large land plants in
the Devonian, of plants that could survive seaM
sonal climate changes in the Me sozoic, and of
grasses in the Miocene all brought about cha nge s
in fluvial hydrology and channel style (S chumm
1968a). Modern analogue s therefore have a limited applicability to the distant pa st, pre- Devonian
landscape s probably havi ng fu nctione d sedimentologically much as do arid re gions of the present
da y, even where rainfall wa s abundant (Fig . 8.2).
Correct identifica tion of these various effe cts and
separation of them from the sometime s ve ry simi lar
effects of tectonic control re qu ire me ticulous observation and very careful deductive re asoning.
12.3 Distinguishing Tectonic
from Climatic Control
The most obvious effects of te ctonic controls are the
change s tha t tectonism induce s in sedimentary accommodation space and in the qua ntity and ca liber
of the sediment load. Upward coarsening and fining
of alluvial successions, in the form of cycles, sequ ences, and megasequences, have long be en interprete d in tectonic terms (S te el et al. 1977; Heward
J978a), although the nature of the rela tionship be
twee n tectonic moveme nt and the se dime ntary reM
sponse is cu rrently undergoing debate, as discusse d
in Sect. 11.3.6 . Howeve r, similar sedime ntary effects
could be generated by climate cha nge . Langbein and
Schumm (195 8) showed tha t in the United States
sediment yield is relate d to rainfall, va rying from a
minimum in arid climates, to a maximum in subhu mid climates, to a low in hu mid climate s whe n the
effectiveness of vegeta tion in binding soil and re
tarding erosion re aches a maximum. Howe ver, late r
work has shown tha t this is not a si mple relationship
tha t applies to all regions of the earth. For example,
increased sea sonality of the climate also increases
sediment yield, be cause the vege tation cove r is limite d by the constraints of the dry sea son, so that
Distinguishing Tectonic from Climatic Control
1. Large variations in flu vial discharge produce re cog nizable effects in the facies record, such as the
interbedding of ve ry coarse and very fine facies,
bu t the time scale of varia tion may not be obviou s
and the climatic implications therefore unclear.
Seasonal effects, such as those arising from the
freeze-thaw cycle, monsoonal varia tions, and
those tha t occu r in arid environme nts cha racterize d by occasional violent flash floods, months or
even years apart, all generate the same wide va riation in discharge and consequent wide variation
in facie s, bu t the climatic interpretation of the
re su lting fa cie s may be qu ite ambiguou s ( considei:ations of time scale are discussed at le ngth in
Chap. 3).
2. The clima t. e of the se diment source area and that
of the depositional basin may not be the same . For
exa mple , rivers flowing southward from the Alw
pineMHimalaya n mountain chain are all strongly
sea sonal, re flecting the alpine climate of their
mountainous source areas, bu t they enter ve ry
differe nt climates in the plains to the south. Europe an rivers, such as the Rhone and Po, have
coastal plains in the subtropical Mediterranean
re gion. The Tigris and Eu phrates ente r the highly
arid Mesopotamian basin. The fluvia l plains of
northern India are located in a tropical climate
where se asonal monsoonal rainfa ll strongly influences discharge cha racteristics (see also Sect.
11.3.1 for othe r characteristics of "big rivers'' ).
3. Re late d to point 2 above is the fact that flow
hydraulics of a river, and the refore the facies and
architecture of the be dload deposits, are determined large ly by the climate of the source area,
whereas floodplains and their finer-graine d deposits are influence d primarily by the climate of
the de positional basin.
4. Topographic effe cts ma y generate local climates
tha t complicate the depositional re cord. For
example , mountain range s rising in the path of
pre vailing humid wi nds may generate hig h orographic rainfa lls on their upwind slopes and rain
shadows downwind. The topographic diversion
of air ma sse s is an important general effect. Very
high pla te aus, such as Tibe t, are also a significant
source of he at, which has its own effect on air
te mperature s and circulation. The possible wide r
linkages betwee n tectonics and climate are discu sse d in Sect. 12.11.
5. Even within a flu vial basin, le ve ls of water saturation (the wa ter ta ble) and redox conditions may
va ry from channe l to floodplain and be twee n the
proximal, possibly more elevated basin-margin
423
rivers and the topographica lly lower basin-ce nte r
channe l systems.
6. Climate is commonly re corded in the sedime ntary record by vege tation, by the fossil re ma ins of
vegetation itself, and by the effects vege ta tion has
on se diment erodibility, sediment yield, cha nnel
style, and so on. However, the styles of vegetation
that characterize the earth today have evolved
with time and were very differe nt in the geological
past. The deve lopme nt of large land plants in
the Devonian, of plants that could survive seaM
sonal climate changes in the Me sozoic, and of
grasses in the Miocene all brought about cha nge s
in fluvial hydrology and channel style (S chumm
1968a). Modern analogue s therefore have a limited applicability to the distant pa st, pre- Devonian
landscape s probably havi ng fu nctione d sedimentologically much as do arid re gions of the present
da y, even where rainfall wa s abundant (Fig . 8.2).
Correct identifica tion of these various effe cts and
separation of them from the sometime s ve ry simi lar
effects of tectonic control re qu ire me ticulous observation and very careful deductive re asoning.
12.3 Distinguishing Tectonic
from Climatic Control
The most obvious effects of te ctonic controls are the
change s tha t tectonism induce s in sedimentary accommodation space and in the qua ntity and ca liber
of the sediment load. Upward coarsening and fining
of alluvial successions, in the form of cycles, sequ ences, and megasequences, have long be en interprete d in tectonic terms (S te el et al. 1977; Heward
J978a), although the nature of the rela tionship be
twee n tectonic moveme nt and the se dime ntary reM
sponse is cu rrently undergoing debate, as discusse d
in Sect. 11.3.6 . Howeve r, similar sedime ntary effects
could be generated by climate cha nge . Langbein and
Schumm (195 8) showed tha t in the United States
sediment yield is relate d to rainfall, va rying from a
minimum in arid climates, to a maximum in subhu mid climates, to a low in hu mid climate s whe n the
effectiveness of vegeta tion in binding soil and re
tarding erosion re aches a maximum. Howe ver, late r
work has shown tha t this is not a si mple relationship
tha t applies to all regions of the earth. For example,
increased sea sonality of the climate also increases
sediment yield, be cause the vege tation cove r is limite d by the constraints of the dry sea son, so that