Accommodation Space
The problem with the last sentenc e in this quotation
is discussed in Sect. 11.2.2. Apart from that, the
quote succinctly explains the importance of the
equilibrium profile. Accommodation spac e in fluvial
environments can be defined as the space between
an existing graded profile and a new graded profile
to which the river adjusts in reponse to changes in
ba se level, tec tonic movements, or hydraulic conditions (discha rge, sediment supply). Accommoda
tion space is positive, and sedimentation oc curs,
under the following conditions (Fig. 13. 2):
1. At the coast, when the graded profile moves upward in response to a rise in sea level. Most sedimentation occ urs nea r the mouth of the river, and
there may be little change in accommodation
space in upper reaches.
2. At the coast, when the graded profile moves upward in response to coastal regression and
prograda tion, under conditions of sea-level
stillstand. This effect is small, and in the case of
prograda tion by lobate deltas it is limited by the
tendency for avulsive delta abandonment when
the profile of the ma in channel is ra ised by aggradation.
3. Inland, when the graded profile moves upward in
response to uplift of the source area, for example
along a basin-ma rgin fault. New accommodation
space is generated adjacent to the zone of uplift
and, given adequate sediment supply, progradation may shift the equilibrium profile basinward.
4. Throughout the profile when, because of tec tonic
or climatic changes in the source area, there is an
increase in discharge and/or sed iment supply.
Ac commodation space is negative, and incision oc
curs, under the reverse of these conditions: lowering
of sea level, reduction in source-area elevation, e.g.,
by regiona l erosion, and reduction in discharge or
sediment supply.
All these allogenic controls fluctuate constantly
in all sedimentary ba sins. Sea level ha s been rising and falling eustatica lly at least throughout the
Pha nerozoic (see review by Miall 1990, Chap. 8).
Tectonic subsidence and uplift, driven by crustal
stresses and subcrustal thermal changes, is in a continua l state of dynamic balanc e with uplift driven by
erosional downc utting of uplifted areas and consequent isostatic buoyancy. Climatic changes, driven
by orbital forcing and by the drift of continental
pla tes through latitudina l climatic zones, bring
about changes in discharge and sediment yield. All
these changes have been occ urring throughout geologic time over a wide range of time sca les, ra nging
457
from a few thousand years to hundreds of millions of
years. Several of these processes of change ma y be
occ urring simultaneously at more tha n one time
sc ale. Few rivers are ever, therefore, in a state of true
equilibrium. The concept of the graded profi le is a
geomorphic one, which is appropriate for short time
sc ales (tens to hundreds of years), but over geologic
time sc ales the concept of dynamic equilibrium is
more appropriate and useful. Local conditions must
alwa ys be taken into account. For example, along
muc h of the present-da y coastal plain of South
Island, New Zealand, a slow eustatic rise and
shelf subsidence are underway. This is causing a
tra nsgression, but one tha t is being recorded not by
deposition, but by valley incision and erosional
shoreface retreat, because of high wave energies
(Lec kie 1994).
For marine shoreline and nearshore deposits predictions of sequence architecture in response to sealevel change are easy to make. When sea level rises
the coast is flooded, and transgression ta kes place.
Sea-level fall is accompanied by regression and
progradation. Such predictions were the ba sis for the
first sequence models in the modern sense, those of
Fra zier (1974), followed by Vail et al. (1977). However, in the case of rivers, the response to change is
more complex. For example, as discussed in Sect.
11.2.2, a lowering of sea level and the extension of a
river profile out to a new mouth, lower out and
further down on the continental shelf, may lead to
incision, progradation, or simple changes in sinuosity with neither enhanced aggradation or erosion,
depending on a variety of fac tors, of which the slope
of the newly exposed shelf is one of the most critical.
The development of sequence models for fluvial systems ha s therefore lagged behind tha t for marine
environments, and the subj ec t could be said to ha ve
arrived at a state of ma turity on ly recently, with the
publication of the review by Shanley and McCabe
(1994).
In the case of tectonic control,_the effects of uplift,
subsidence, and erosion at a fa ulted ba sin margin
have long been known [see the discussion of the
importa nt paper by Heward (1978a) in Sect. 11. 2.1.].
Only in recent yea rs, however, ha ve we begun to
appreciate the complexities of tectonism as a sedimentary controL For exa mple, in foreland basins it
now seems likely tha t regional crustal shortening
and load ing on the one hand, and the movement of
ind ividua l thrust or nappe plates, on the other ha nd ,
may generate pulsed, irregular subsidenc e on both
long and short time sca les at the same time (S ects.
11.3.6, 11.4.6). Uplift and subsidence ma y also be
The problem with the last sentenc e in this quotation
is discussed in Sect. 11.2.2. Apart from that, the
quote succinctly explains the importance of the
equilibrium profile. Accommodation spac e in fluvial
environments can be defined as the space between
an existing graded profile and a new graded profile
to which the river adjusts in reponse to changes in
ba se level, tec tonic movements, or hydraulic conditions (discha rge, sediment supply). Accommoda
tion space is positive, and sedimentation oc curs,
under the following conditions (Fig. 13. 2):
1. At the coast, when the graded profile moves upward in response to a rise in sea level. Most sedimentation occ urs nea r the mouth of the river, and
there may be little change in accommodation
space in upper reaches.
2. At the coast, when the graded profile moves upward in response to coastal regression and
prograda tion, under conditions of sea-level
stillstand. This effect is small, and in the case of
prograda tion by lobate deltas it is limited by the
tendency for avulsive delta abandonment when
the profile of the ma in channel is ra ised by aggradation.
3. Inland, when the graded profile moves upward in
response to uplift of the source area, for example
along a basin-ma rgin fault. New accommodation
space is generated adjacent to the zone of uplift
and, given adequate sediment supply, progradation may shift the equilibrium profile basinward.
4. Throughout the profile when, because of tec tonic
or climatic changes in the source area, there is an
increase in discharge and/or sed iment supply.
Ac commodation space is negative, and incision oc
curs, under the reverse of these conditions: lowering
of sea level, reduction in source-area elevation, e.g.,
by regiona l erosion, and reduction in discharge or
sediment supply.
All these allogenic controls fluctuate constantly
in all sedimentary ba sins. Sea level ha s been rising and falling eustatica lly at least throughout the
Pha nerozoic (see review by Miall 1990, Chap. 8).
Tectonic subsidence and uplift, driven by crustal
stresses and subcrustal thermal changes, is in a continua l state of dynamic balanc e with uplift driven by
erosional downc utting of uplifted areas and consequent isostatic buoyancy. Climatic changes, driven
by orbital forcing and by the drift of continental
pla tes through latitudina l climatic zones, bring
about changes in discharge and sediment yield. All
these changes have been occ urring throughout geologic time over a wide range of time sca les, ra nging
457
from a few thousand years to hundreds of millions of
years. Several of these processes of change ma y be
occ urring simultaneously at more tha n one time
sc ale. Few rivers are ever, therefore, in a state of true
equilibrium. The concept of the graded profi le is a
geomorphic one, which is appropriate for short time
sc ales (tens to hundreds of years), but over geologic
time sc ales the concept of dynamic equilibrium is
more appropriate and useful. Local conditions must
alwa ys be taken into account. For example, along
muc h of the present-da y coastal plain of South
Island, New Zealand, a slow eustatic rise and
shelf subsidence are underway. This is causing a
tra nsgression, but one tha t is being recorded not by
deposition, but by valley incision and erosional
shoreface retreat, because of high wave energies
(Lec kie 1994).
For marine shoreline and nearshore deposits predictions of sequence architecture in response to sealevel change are easy to make. When sea level rises
the coast is flooded, and transgression ta kes place.
Sea-level fall is accompanied by regression and
progradation. Such predictions were the ba sis for the
first sequence models in the modern sense, those of
Fra zier (1974), followed by Vail et al. (1977). However, in the case of rivers, the response to change is
more complex. For example, as discussed in Sect.
11.2.2, a lowering of sea level and the extension of a
river profile out to a new mouth, lower out and
further down on the continental shelf, may lead to
incision, progradation, or simple changes in sinuosity with neither enhanced aggradation or erosion,
depending on a variety of fac tors, of which the slope
of the newly exposed shelf is one of the most critical.
The development of sequence models for fluvial systems ha s therefore lagged behind tha t for marine
environments, and the subj ec t could be said to ha ve
arrived at a state of ma turity on ly recently, with the
publication of the review by Shanley and McCabe
(1994).
In the case of tectonic control,_the effects of uplift,
subsidence, and erosion at a fa ulted ba sin margin
have long been known [see the discussion of the
importa nt paper by Heward (1978a) in Sect. 11. 2.1.].
Only in recent yea rs, however, ha ve we begun to
appreciate the complexities of tectonism as a sedimentary controL For exa mple, in foreland basins it
now seems likely tha t regional crustal shortening
and load ing on the one hand, and the movement of
ind ividua l thrust or nappe plates, on the other ha nd ,
may generate pulsed, irregular subsidenc e on both
long and short time sca les at the same time (S ects.
11.3.6, 11.4.6). Uplift and subsidence ma y also be