Seismic-stratigraphic models for late Pleistocene/Holocene incised valley systems on the Durban continental shelf.
Dladla, Nonkululeko Nosipho.
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This dissertation examines the Durban continental shelf of the east coast of South Africa from a seismic and sequence stratigraphic perspective. High resolution seismic data reveal eleven seismic Units (A-K) offshore the Durban continental shelf comprising several partially preserved sequences. Unit A is the lower most unit, comprising Permian age shale of the Pietermaritzburg Formation. An early Santonian age is assigned to Unit B. The ages of Units C and D are indeterminate. Unit E is considered late Maastrichtian in age. Units F to I are assigned a late Pliocene age and represent an aggradational progradational shelf-edge wedge. Unit J comprises calcite cemented late Pleistocene/Holocene shoreline deposits which display morphologies similar to planform equilibrium shorelines on modern coasts. Unit K caps the stratigraphy and comprises a seaward thinning, shore-attached wedge of Holocene age. The lower portions of Unit K comprise the fills of an extensive LGM age incised valley network. A widespread network of incised valley systems on the continental shelf offshore Durban was recognised and examined; the evolution of which were compared over time. These incised valleys represent the shelf extension of the main river systems in the area, namely, the Mgeni, Mhlanga and Mdloti rivers as well as those that drain into the Durban Harbour complex. In the study area late Pleistocene/Holocene aged valleys occur together with a subsidiary series of late Pliocene isolated valleys. Valleys of the last glacial maximum (LGM) of ~ 18 Ka BP exhibit simple fills and have intersected and reworked or completely exhumed the late Pliocene incised valleys. Only isolated examples of these late underlying Pliocene valleys are apparent. Twenty five prominent incised valleys are recognised within the study area and occur predominantly in the mid-outer shelf. These valleys mainly incise into Cretaceous age rock, except for a few incisions occurring within Permian age shale of the Pietermaritzburg Formation. Six seismic units (Units 1-6) comprise the infill material within the late Pleistocene/Holocene incised valleys, and on the basis of their architecture are interpreted to correspond with a succession from high energy basal fluvial deposits, low-energy central basin fines, mixed-energy estuarine mouth plug deposits, clay-rich flood deposits through to capping sandy shoreface deposits. The LGM aged fills in particular have volumetrically thick fluvial deposits, the result of increased gradient and stream competence during the LGM. The youngest valleys show a situation of differential evolution along the valley length due to varying rates of sea level rise in the Holocene. Initially, rapid sea level rise caused drowning and overstepping of the outer segment of the incised valley. During the late Holocene, slower rates of sea level rise caused shoreface ravinement of the inner-mid segments of the valley and created an imbalance between accommodation space and sediment supply, producing different facies architectures in the valleys. This differential exposure to accommodation has resulted in a sedimentological partitioning between tide-dominated facies in the outer valley segment and river dominated facies in the inner segment. Due to significantly wider exposed coastal plain during lowstand intervals, the rivers in the study area avulsed and coalesced on this lowstand surface and thus possess no defined drainage patterns. A crenulate shaped subsurface knickpoint occurs at a depth of ~ 50 m, and is considered to have formed by initial slow ravinement processes that graded the antecedent shelf, followed by overstepping and preservation of the knickpoint during meltwater pulse 1B.
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