Seascape Evolution on Clastic Continental Shelves and Slopes

Abstract

The morphology of clastic continental margins directly reflects their formative processes. These include interactions between plate movements and isostasy, which establish the characteristic stairstep shape of margins. Other factors are thermal and loading-induced subsidence, compaction and faulting/folding, which create and/or destroy accommodation space for sediment supplied by rivers and glaciers. These processes are primary controls on margin size and shape. Rivers and glaciers can also directly sculpt the margin surface when it is subaerially exposed by sea-level lowstands. Otherwise, they deposit their sediment load at or near the shoreline. Whether this deposition builds a delta depends on sea level and the energy of the ocean waves and currents. Delta formation will be prevented when sea level is rising faster than sediment supply can build the shoreline. Vigorous wave and current activity can slow or even arrest subaerial delta development by moving sediments seaward to form a subaqueous delta. This sediment movement is accomplished in part by wave-supported sediment gravity flows. Over the continental slope, turbidity currents are driven by gravity and, in combination with slides, cut submarine canyons and gullies. However, turbidity currents also deposit sediment across the continental slope. The average angle of continental slopes (~4°) lies near the threshold angle above which turbidity currents will erode the seafloor and below which they will deposit their sediment load. Therefore, turbidity currents may help regulate the dip of the continental slope. Internal waves exert a maximum shear on the continental-slope surface at about the same angle, and may be another controlling factor.

Space-for-time substitution and the evolution of a submarine canyon–channel system in a passive progradational margin

“…Valley evolution in our study area appears to be most active when sediment influx to the slope is greatest, which coincides with sea-level lowstands. Features 5 and 6, for example, have likely gone through several phases of erosion and deposition as sea-level changed and shelf-edge depocentres shifted.These patterns have been widely reported in the literature (Daly, 1936; Felix and Gorsline, 1971;Twichell et al, 1977;Vail et al, 1977;Stanley et al, 1984;Posamentier et al, 1988;Bertoni and Cartwright, 2005;Pratson et al, 2009). Submarine channel inception via a series of scours has been documented in many recent studies (Pirmez and Imran, 2003; Fildani and Normark, 2004; Fildani et al, 2006;Normark et al, 2009;Kostic, 2011; Covault et al, 2012; Fildani et al, 2013), as has the development of channel-levée systems in the channel marginal regions (Fildani et al, 2006(Fildani et al, , 2013Armitage et al, 2012).…”

“…First, canyon elongation is generally higher downslope, which contrasts with the predominant headward development of submarine canyons reported in numerous models (e.g. Pratson et al, 1994Pratson et al, , 2009). Second, the canyon goes through an early stage of rapid incision, where the rate of change of depth is much higher than that of the width, and where wall erosion is minimal.…”

“…First, submarine canyons play an important role in margin development globally and act as sinks and conduits for sediment particles and associated nutrients, organic carbon and pollutants (Shepard, 1981;Normark and Carlson, 2003;Puig et al, 2004;Allen and Durrieu de Madron, 2009;Harris and Whiteway, 2011;Sànchez-Vidal et al, 2012;Canals et al, 2013). In spite of their relevance and widespread distribution, the genesis and evolution of submarine canyons continues to be debated (Bertoni and Cartwright, 2005;Pratson et al, 2009). We attribute this to three reasons: (i) complexity and diversity of canyon topographies from different continental margins (Harris and Whiteway, 2011); (ii) difficulties faced by geoscientists in observing contemporary sediment transport and canyon formation processes directly on site (see review by Puig et al, 2014); and (iii) the fact that present-day processes may provide limited understanding of canyon evolution because many submarine canyons could be relict features that may only bypass sediment from the continental shelf to the deep basins (Emery and Uchupi, 1972;Vail et al, 1977;Burger et al, 2001).…”

“…A second mechanism is spring sapping, which involves the discharge of groundwater or another type of fluid from the face of the continental slope at a specific depth or stratigraphic level (Orange et al, 1994;Pratson et al, 2009). Apart from explaining the occurrence of numerous gullies at a similar water depth and distance from the shelf break, this process would also account for the smooth, undisturbed seafloor observed upslope of the gully heads.…”

“…Californian (Paull et al, , 2005(Paull et al, , 2013; Gulf of Lions (Sultan et al, 2007); and others (e.g., Normark and Carlson, 2003;Pratson et al, 2009)). Our inferred valley evolution model for the south Ebro Margin best corresponds to the downslope erosion model.…”