Seismic, geochemical and sedimentological characteristics of storm deposits from the Durban continental shelf, South Africa.
Dixon, Shannon Louise.
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The Durban shelf is a wave-dominated, high energy setting, characterised by submerged shorelines at depths of ~60 m, backed to landward by low-relief backbarrier depressions. The exposure at the seafloor of a back-barrier/lagoon complex, coupled with the general high wave energy of the shelf makes for a unique opportunity to examine records of marine storminess, preserved as tempestites in the shelf stratigraphy. This thesis examined a variety of ultra-high resolution seismic data, coupled with multibeam bathymetry, core and sedimentological data. High resolution XRF, XRD and other provenance proxies were also examined in order to reveal cycles of storminess that have impacted the lower shoreface offshore Durban. These are integrated with a rigorous geochronological framework. The seismic sections revealed two distinct packages which comprise the unconsolidated shoreface (Unit A and B). These are further subdivided into an upper five packages (Sub-unit B1-B5 and sub-unit A-Ai). Cores that penetrated this sediment enabled a further correlation with that of the bounding surfaces, sediment compositions and the nature of individual sub units. Unit A and Ai are considered incised valley fills corresponding to organic-rich fine sand of the central basin and flood tide delta deposits with a distinctly higher terrigenous sediment signature in comparison to the overlying sediment packages. The tidal ravinement surface (tRS) is restricted to the incised valley where it separates unit A and Ai. The wave ravinement surface (WRS) truncates the incised valley fills and is overlain by unconsolidated material of unit B. Sub-units B3 and B4 are storm associated deposits which are of particular interest to this study. Sub-unit B3 comprises a number of high energy deposits, namely mudballs; these deposits consist of organic rich material indicative of storm winnowing of an exposed muddy backbarrier (such as presently occurring along sectors of the adjoining coastal plain). This is corroborated by the geochemical analyses of the mudballs which displays significantly higher concentrations of terrigenous elements (Si, Al, K, Ti and Rb), in comparison to the surrounding sediment, indicative of a terrigenous sediment origin. The centre of the mudball was dated at 9 850 ± 50 cal yrs BP. The outer veneer dates to 3 835 ± 35 cal yrs BP and represents the final phase of deposition in the lower shoreface. The mudballs are encased in coarse sediment, dominated by Ca and Sr elemental concentrations, suggestive of a marine origin. Sub-unit B4 consists of alternating horizons of storm generated gravel horizons displaying increased marine elemental signatures, interbedded with finer sediment with increased terrigenous concentrations indicative of fair-weather conditions. It was found that horizons of coarser material had higher elemental signatures of Ca and Sr indicating a predominantly marine input into the system. These horizons are intercalated with finer material with distinctly higher concentrations of elements associated with terrigenous source material that represents fair-weather suspension settling of terrestrial materials. Based on modelling of the largest experienced contemporary marine storm (Hs = 8.5 m), it is clear that storm waves do not significantly rework gravelly sediment on the lower shoreface, especially in the areas of smooth seafloor where the cores are situated. Bathymetry of the area shows no contemporary evidence for storm scour or gravel deposition. As the palaeo-tempestites date to a time when sea level occupied a similar position to that of today, it is logical to assume these represent much larger storm events than are commonly experienced. This study shows for the first time a period of increased storminess in the Indian Ocean between 6 480 ± 40 cal yr BP to 4 595 ± 35 cal yr BP; a time linked to a strongly positive Indian Ocean Dipole (IOD) anomaly and increased sea surface temperatures (SST). With further global warming, it appears that Durban may be more vulnerable to large marine storms as the associated changes in atmospheric circulation patterns and oceanic currents promote the formation of positive IOD phenomena.