Seismic-stratigraphic models for late Pleistocene/Holocene incised valley systems on the Durban continental shelf.
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Date
2013
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Abstract
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.
Description
M. Sc. University of KwaZulu-Natal, Durban 2013.
Keywords
Continental shelf--Durban., Earthquake zones--Durban., Geology, Stratigraphic--Holocene., Geology, Stratigraphic--Pleistocene., Valleys--Durban., Sea level--Durban., Theses--Geology.