Geology Reference
In-Depth Information
7.3.3.1 Gravity-Driven Sediment Transport
Widespread occurrence of mud deposits and active
mud accretion on the middle of continental shelves
has long drawn speculation as to the mechanisms
responsible for their emplacement (Swift et al.
1972
) .
General observations of focused, rapid accumulation
imply an association of these deposits with sediment-
laden density currents, which are near-bottom fl uid
fl ows that are denser than the overlying water column
because of a high concentration of suspended sedi-
ment. Turbidity currents are an example of gravity-
driven transport, but the gradient of the shelf is typically
too low to sustain the high fl ow velocities needed to
maintain continuous sediment suspension and the
downslope propagation of such gravity fl ows. Not only
are shelf gradients low, but very few rivers discharge
sediment plumes that are hyperpycnal (i.e., denser than
the ambient coastal seawater), and this is especially
true of the larger, relatively dilute rivers offshore of
which shelf mud deposits are most prevalent.
In the past two decades, though, repeated synoptic-
scale observations of seabed and water column dynam-
ics during storms and high-discharge fl ood events have
demonstrated that gravity-driven near-bed density
fl ows are a common mode of cross-shelf mud transport
(Wright and Friedrichs
2006
) . The controlling pro-
cesses and boundary conditions can vary widely, but
the fundamental requirements are hyperpycnal near-
bed sediment concentrations and a mechanism for
maintaining sediment suspension on the low-gradient
shelf, typically accomplished by waves and/or tidal
currents. These specialized requirements are most typ-
ically met when rivers are discharging peak sediment
loads onto an energetic shelf, which arguably occurs
with the greatest regularity along tide-dominated del-
taic margins (Harris et al.
2004
). It is uncertain whether
this assertion is true because gravity-driven transport is
recognized in many margin systems, but it can be said
that gravity-driven transport has been documented in
all tide-dominated deltas with adequate observations
(Wright and Friedrichs
2006
) .
from investigations of tide-dominated and tide-
infl uenced deltas in the 1980s (e.g. Amazon, Huanghe),
when it became clear that these systems supported
actively accreting subaqueous deltas that are located
substantial distances offshore of, and separate from,
their better recognized subaerial landforms (Fig.
7.5
;
Nittrouer et al.
1986
; Prior et al.
1986
) . The presence
of well-developed subaqueous deltas has also
been documented for the tide-dominated Ganges-
Brahmaputra, Indus, and Changjiang river deltas
(Chen et al.
2000
; Kuehl et al.
1997
; Giosan et al.
2006
). In these systems the subaerial clinoform
includes primarily the lower delta plain and advancing
shoreline that form at the convergence of onshore-
directed marine processes and river discharge,
whereas the subaqueous clinoform develops at the
boundary between shallow-water and deep-water
processes (i.e., wave-tide-current transport vs. gravity-
driven transport; Swenson et al.
2005
) .
7.3.4
Sediment Budgets
Tide-dominated deltas are commonly large sediment
dispersal systems controlled both by high-energy
coastal processes and high-discharge rivers. Their sed-
iment load is widely dispersed with active deltaic sedi-
mentation occurring tens to hundreds of kilometers
across and along the continental margin. Therefore,
developing sediment budgets for these systems is
inherently useful in understanding how they respond
to external forcings (e.g., climate, sea level) and how
their fl uvial, coastal, and marine reaches interact.
One of the fi rst budgets developed for a tide-dominated
delta was in the Fly River system, where Harris et al.
(
1993
) could only account for about half (55 ± 20%)
of the annual sediment load of ~85 × 10
6
metric
tons within the tide-dominated portion of the delta
(note: load estimate prior to construction of the Ok
Tedi mine). Of the sediment that could be located,
roughly equal volumes were apportioned to the lower
delta plain (i.e., subaerial clinothem) and deltafront/
prodelta system (i.e., subaqueous clinothem).
Subsequent work has shown that most of the 'missing'
fraction is split between deposition on the Fly's vast
lowland river fl oodplain (Swanson et al.
2008
) and the
actively growing alongshelf clinothem (Slingerland
et al.
2008
). A similar distribution of sediment was
7.3.3.2 Compound Clinoform Development
As gravity-driven transport is generally associated
with high-discharge and high-energy conditions, so
too is the development of a compound-clinoform
morphology in delta systems (Fig.
7.3
; Swenson et al.
2005
). The concept of compound clinoforms emerged
