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Geology
Sediment supply is controlled primarily by
tectonics and climate.
In geologically simple areas, where the
basin is fed directly from the adjacent margins and source-area uplift is
related to basin subsidence, supply considerations are likely to be directly
correlated to basin subsidence and eustasy as the major controls of basin
architecture.
Such is the case where subsidence is yoked
to peripheral upwarps, or in proximal regions of foreland basins adjacent to
fold-thrust belts.
However, where the basin is supplied by
long-distance fluvial transportation, complications are likely to arise.
Where the rate of sediment supply is high,
it may overwhelm other influences to become a dominant control on sequence
architecture.
Many sedimentary basins were filled by river
systems whose drainage area has been subsequently remodeled by tectonism, and
it may take considerable geological investigation to reconstruct their possible
past positions.
For example, stratigraphic successions may
occur that cannot be related to the evolution of adjacent orogens.
In North America, dynamic topographic
processes have generated regional uplifts and continental tilts that have
resulted in deep erosion and large-scale continental fluxes of detrital
sediment.
For example, much of the detritus derived
by uplift and erosion of the Grenville orogen of eastern North America during
the late Precambrian may have ended up contributing to the thick Neoproterozoic
sedimentary wedges on the western continental margin.
Detailed study of detrital zircons from
sedimentary rocks of this age in the western Canadian Arctic indicated that 50%
of them are of Grenville age.
A major west-flowing river system was
established during the late Proterozoic which transported this detritus some
3,000 km across the continental interior.
Much of the thick accumulations of late
Paleozoic and Mesozoic fluvial and eolian strata in the southwestern United
States had been derived from Appalachian sources, and this was confirmed by the
detrital-zircon.
Tertiary river system draining from the
continental interior of North America into Hudson Bay, ultimately delivering
sediment to the Labrador Shelf.
Major river systems may cross major
tectonic boundaries, feeding sediment of a petrographic type unrelated to the
receiving basin, into the basin at a rate unconnected in any way with the
subsidence history of the basin itself.
The modern Amazon river is a good example.
It derives from the Andean Mountains, flows across and between, and is fed from
several Precambrian shields, and debouches onto a major extensional continental
margin.
From the point of view of sequence
stratigraphy, the important point is that large sediment supplies delivered to
a shoreline may overwhelm the stratigraphic effects of variations in sea level.
A region undergoing a relative or eustatic
rise in sea level may still experience stratigraphic regression if large delta
complexes are being built by major sediment-laden rivers.
Effects of upstream controls on the
development of fluvial graded profiles, fluvial style and the development of
nonmarine sequences downstream.
Upstream controls may also be significant in
the case of deep-marine deposits.
Major episodes of submarine-fan
sedimentation in the North Sea and Shetland-Faeroes basins correlate with
pulses of Iceland plume activity, which caused magmatic underplating of the
continental margin, and uplift, erosion, and enhanced sediment delivery to
offshore sedimentary basins.
A significant example of this long-distance
sedimentary control is the Cenozoic stratigraphic evolution of the
Texas-Louisiana coast of the Gulf of Mexico.
This continental margin is fed with
sediment by rivers that have occupied essentially the same position since the
early Tertiary.
The rivers feed into the Gulf Coast from
huge drainage basins occupying large areas of the North American Interior.
Progradation has extended the continental
margin of the Gulf by up to 350 km.
This has taken place episodically in both
time and space, developing a series of major clastic wedges, some hundreds of
metres in thickness.
The major changes along strike of the
thickness of these clastic wedges is also evidence against a control by passive
sea-level change.
Highly suggestive are the correlations with
the tectonic events of the North American Interior; for example, the timing of
the Lower and Upper Wilcox Group wedges relative to the timing of the Laramide
orogenic pulses along the Cordillera.
It seems likely that sediment supply,
driven by source-area tectonism, is the major control on the location, timing
and thickness of the Gulf Coast clastic wedges.
A secondary control is the nature of local
tectonism on the continental margin itself, including growth faulting,
evaporite diapirism and gravity sliding.
Variations in deep-marine sediment
dispersal in the Gulf of Mexico show very similar patterns to the coastal and
fluvial variations.
In arc-related basins volcanic control of
the sediment supply may overprint the effects of sea-level change.
Sediment supply and tectonic activity
overprinted the eustatic effects and enhanced or lessened them.
If large supplies of clastics or uplift
overcame the eustatic effects, deep marine sands were also deposited during
highstand of sea level, whereas under conditions of low sediment input,
thin-bedded turbidites were deposited even during lowstands of sea level.
Other examples of the tectonic control of
major sedimentary units are provided by the basins within and adjacent to the
Alpine and Himalayan orogens.
Sediments shed by the rising mountains
drain into foreland basins, remnant ocean basins, strike-slip basins, and other
internal basins.
But the sediment supply is controlled
entirely by uplift and by the tectonic control of dispersal routes.
For example, the Oligocene Molasse of the
Swiss proforeland basin was deposited by rivers flowing axially along the basin,
and that these underwent reversal in transport directions as a result of
changes in the configuration of the basin and the collision zone during
orogenesis.
The shifting of dispersal routes through
basins and fault valleys within the Himalayan orogen of central and southeast
Asia.
Some of the major rivers in the area
(Tsangpo, Salween, Mekong) are known to have entirely switched to different
basins during the evolution of the orogen.
Much work remains to be done to relate the details of the stratigraphy in these various basins to the different controls of tectonic subsidence, tectonic control of sediment supply, and eustatic sea-level changes.
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