The bathymetry, sedimentology and seismic stratigraphy of Lake Sibaya- Northern KwaZulu-Natal.
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Date
1998
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Abstract
The morphology of Lake Sibaya is a product of an ancient fluvial system that drained a coastal
landscape dominated by aeolian processes. The sedimentary processes within the lake are driven
by wind generated currents. The dominant sedimentary process is one of lake segmentation,
whereby prograding bedforms isolate the lake into smaller water bodies. The prograding
bedforms include cuspate forelands and sand spits. The size and mobility of these bedforms is a
function of sediment availability and current regime.
The bathymetry of Lake Sibaya is discussed, with emphasis on geomorphic features derived from
the ancient aeolian landscape as well as features related to modern sedimentary processes. The
presence of underwater knickpoints and terraces indicate that lake level fluctuations have been
common in Lake Sibaya. It is during lake highstands that large volumes of sand are eroded from
aeolian dunes which surround the lake and made available for shoreline progradation. Ancient
dune topography is preserved to depths of 20 m below water-level within the lake.
Surface sediment distribution maps were compiled from 515 grab samples and thirteen core
samples. Fine grained, well sorted, coarse skewed quartz sand comprises the majority of the
surface area of the lake floor. Gyttja is the other dominant sediment type and accumulates in
palaeovalleys and depressions on the lake floor. Sediment distribution in Lake Sibaya is discussed
in terms of modern lacustrine processes as well as inherited sedimentary characteristics.
The stratigraphy of the sediments underlying Lake Sibaya was investigated using a Uni-Boom
seismic profiling system. Seismic profiles were compiled by identifying acoustically reflective
surfaces that show regional development. Thirteen seismic overlays were prepared, and are
illustrated as west - east and north - south seismic profiles. Five sequences ranging in age from
late Cretaceous to Holocene were identified from the seismic profiles, and are described in terms
of sequence stratigraphic principles. The seismic sequences were interpreted within a
lithostratigraphic framework and are presented as a series of idealised geological sections.
Thirteen sediment cores were collected from the Lake Sibaya area in order to ascertain the
accuracy of the stratigraphic interpretation of the seismic records, to investigate reflective
horizons identified from seismic records and to collect dateable material. Interpretation of the
sediment cores reveals that a proto Lake Sibaya existed on drowned dune topography, during the
period ± 43500 BP to ± 25500 BP prior to the Last Glacial Maximum. During the early to mid
Holocene the Lake Sibaya site was occupied by a saline lagoon which underwent isolation from
the sea ± 5030 BP. Since the mid-Holocene the lake has evolved to totally freshwater conditions
and has undergone little sedimentation.
The geological evolution of the Lake Sibaya area is discussed in terms of the geometry of the
identified seismic sequences, the sedimentary characteristics of these sequences and the
radiocarbon dates provided from the sediment cores. Palaeo-environmental conditions during the
accumulation of the sedimentary sequences is discussed where fossil remains permit.
Description
Thesis (M.Sc.)-University of Natal, Durban, 1998.
Keywords
Geology--KwaZulu-Natal--Sibaya, Lake., Sibaya, Lake (KwaZulu-Natal), Seismic reflection method., Geology, Stratigraphic., Sediments (Geology)--KwaZulu-Natal--Sibaya, Lake., Sibaya, Lake (KwaZulu-Natal)--Bathymetric maps., Theses--Geology.