Environmental Engineering Reference
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fans commonly consist of coarse-grained sediment (e.g., sand and gravel;
Covault et al., 2007 ).
Finally, the relative hydraulic energy of the coastline affects sedimentation
distribution and sedimentation rates. Fluvially dominated systems may main-
tain a regressive (progradational) character throughout periods of RSL rise
and fall, whereas tidal- and wave-dominated coasts more closely track changes
in RSL with corresponding cycles of transgression and regression. This differ-
ence is mainly a consequence of the ability of basin processes to redistribute
sediment along the coastline.
The above-mentioned relationships notwithstanding, siliciclastic systems do
respond to changes in RSL in predictable ways. During falling RSL (falling
stage), the shoreline is forced to regress. Previously deposited shallow-marine
deposits (5-15 m deep; Roy et al., 1994 ) are eroded by wave action, producing
sharp-based “incised” shorefaces (e.g., Downing andWalker, 1988; Plint, 1988;
Posamentier et al., 1992; Walker and Eyles, 1988 ). Shelf and coastal sediments
deposited during previous highstand or transgression are eroded and transported
across the shelf by rivers. Rivers may incise into the shelf, and deltas are more
prevalent than estuaries. In the offshore, sediment is more easily transported
across the shelf, and turbidite fans receive increasingly more abundant and
generally coarser-grained sediment ( Catuneanu, 2003, 2006 ). Lowstand depos-
its are similar to falling-stage deposits, although these deposits lie the most sea-
ward, commonly extending to the shelf-slope break and typically display
normal regressive or aggradational shoreline trajectories. Fluvial topsets form
during lowstand normal regression, starting the process of aggradation within
incised valleys.
During transgression, the shoreline shifts landward. Coastal successions that
were deposited during RSL fall are cannibalized by wave and tidal processes,
and incised valleys continue to be infilled with fluvial and estuarine successions
(e.g., Dalrymple et al., 1992; Zaitlin et al., 1994 ). Small deltas are flooded and
evolve into estuaries, whereas large deltas may exhibit a punctuated backstepping
pattern, but may prograde across the shelf during intermittent periods of stillstand.
Incremental transgression favors erosive shoreface retreat, and pauses in RSL rise
permit short periods of shoreline and delta progradation across the transgressively
modified SU (e.g., MacEachern et al., 1999a,b; Walker andWiseman, 1995 ). In the
offshore, increased accommodation on the shelf results in sediment being trapped
near the coastline and limits sediment transport to the slope and abyssal plain
( Catuneanu, 2003, 2006 ). Consequently, submarine fans that were sand dominated
during RSL fall commonly revert to a mud-dominated condition during RSL rise.
During highstand, shorelines aggrade and/or prograde. Normal progradation
occurs when sedimentation rates are high, typical of deltaic or delta-influenced
settings. Submarine fans tend to be cut off from sediment supply, particularly in
the case of narrow shelves, as most sediment is trapped in landward positions on
the shelf ( Catuneanu, 2003 ).
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