Seismic stratigraphy of the northern KwaZulu-Natal upper continental margin.
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Date
1998
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Abstract
This study presents the interpretation of Edo-Western and Sparker seismic
geophysical data acquired on the northern KwaZulu-Natal upper continental margin
by various organisations since 1981.
Five seismic sequences are recognised and these are traceable across the entire
length of the study area. The oldest is interpreted as a late Cretaceous marine
sequence (Sequence A), probably the offshore equivalent of the St. Lucia Formation
exposed onshore. This sequence is overlain by a progradational, probable late
Tertiary shelf sequence (Sequence B) onlapping in places against the underlying
marine sequence. The outer portion of this sequence on the upper continental slope
is characterised by complicated reflection termination patterns indicating the
possible presence of discreet sequences within this shelf and slope unit. These
shelf and slope sediments are overlain by a thin (less than 20m) reworked and
eroded Pleistocene shelf unit (Sequence C), itself overlain by linear Pleistocene
aeolianites (Sequence D) in places. The youngest sequence observed is the
Holocene unconsolidated sediment wedge (Sequence E) on the inner shelf,
attaining thicknesses of greater than 20m in places. The various sequences were
mapped out and sediment isopach maps were produced (wherever possible) as well
as an overall geological subcrop map of the study area.
150 kilometres of shallow penetration Edo Western seismic records acquired off the
Sodwana Bay continental shelf were interpreted. Two sediment types are
recognised, namely consolidated beach rock/aeolianite and unconsolidated
Quaternary shelf sand/bioclastic reef derived sediment. In places, accumulations of
bioclastic sediment in subaqueous dune troughs which have been subsequently
buried by migrating bedforms manifest themselves on seismic records as dark semi-continuous
reflectors beneath the migrating bedform. Close inshore, seismic records
show prominent reflectors interpreted as consolidated sediment beneath varying
thicknesses of unconsolidated sediment. Close to the shelf break (occurring at
approximately -60m), seismic interpretation indicates that thin beach rock
developments perch directly upon unconsolidated shelf sand, with the beach rock
having been eroded through in places to expose unconsolidated sediment beneath.
A sediment thickness map for this area was compiled from the seismic data. The
limited penetration of the Pinger system necessitated "greater-than" values being
used in many areas. Greatest sediment thicknesses occur in subaqueous dune
fields where unconsolidated sediment thickness is at least 11 m. In inshore areas
absent of subaqueous dune fields, sediment thicknesses are typically low, varying
between 1 and 3m. A prominent submerged dune ridge close inshore limits
substantial unconsolidated sediment build-up to landward of this feature. On the
seaward side substantial build-up is limited by the action of the Agulhas Current
which is actively transporting sediment into the head of submarine canyons which
incise the continental shelf at Sodwana Bay. This study shows that on the northern
KwaZulu-Natal continental shelf where there is a dearth of unconsolidated
Quaternary sediment, the Edo Western seismic system is a useful tool for
discerning thin veneers of unconsolidated sediment less than 4m thick. When
considering the overall low volumes of unconsolidated sediment present on the
shelf, this hitherto unconsidered volume of sediment constitutes an important part of
the shelf sediment budget.
Submarine landslide features observed on sparker seismic records are described
and discussed. Submarine landslides are present which affect a) Sequences A and
B, b) Sequence B only and c) Sequence A only, ages of these sediment failures can
thus be inferred as being either post- Late Cretaceous or post- Late Tertiary.
Offshore Kosi Bay, submarine landslide features affecting Sequence A are buried by
unaffected Sequence B sediments, indicating a post- Late Cretaceous to pre- Late
Tertiary age of occurrence. Style of failure tends towards mass flow in those
submarine landslides in which Sequence B only sediments are affected, while those
in which Sequence A is affected exhibit some slide features indicating a greater
degree of internal coherency of these sediments compared to Sequence B. Slope
stability analysis of a submarine landslide feature offshore St. Lucia Estuary Mouth
indicates the failed sediment mass would have been stable under static conditions
and that external dynamic forces such as storm waves or seismic activity would
have been necessary to induce failure. It is demonstrated that the Zululand
earthquake of 1932 would have exceeded the intensity necessary to induce
sediment failure and this event should therefore be considered as a possible cause.
Seismic evidence of fluvial incision/subaerial exposure at the boundaries between
Sequences A and B and C and E are further evidence of lowered sea-levels
probably during the Oligocene and Late Pleistocene. The position of the incision
into Sequence C relative the present course of the Mkuze River indicates the
possibility that this incision could represent the palaeo-outlet of this river.
Seismic expression of 3 submarine canyons in the study area indicate that they are
currently undergoing active headward erosion, independent of any direct modern
fluvial influence. In the case of Ntabende Canyon, a nearby continental shelf
incision postulated to be the palaeo-Mkuze outlet indicates that provision of
terrigenous material to this portion of the continental shelf could well have
accelerated mass wasting processes within the canyon itself. This submarine
canyon could therefore have progressed more rapidly to a relatively mature phase
of development. Subsurface structure indicates the lack of any post- Late Tertiary
fault features beneath the canyons, thus excluding faults active in post- Late
Tertiary times as a developmental factor.
It is shown that the overall, external morphology of the KwaZulu-Natal upper
continental margin is strongly influenced by seismic stratigraphic relationships, with
the main influencing factors being outcrop position of the various sequences and
depositional angle of sediments of which a sequence is comprised. External
morphology has also been greatly modified in places by mass-wasting processes. It
is demonstrated also that relating the observed seismic stratigraphy to onshore
geological cross sections is problematic due to the distances involved and lack of
confident offshore dates for the seismic sequences observed.
Seismic relationships observed contribute to an understanding of relative sea-level
movements since the Late Cretaceous and the overall geological evolution of the
northern KwaZulu-Natal upper continental margin, details of which are discussed.
Description
Thesis (M.Sc.)-University of Natal, Durban, 1998.
Keywords
Geology--KwaZulu-Natal., Geology, Stratigraphic., Continental margins--KwaZulu-Natal., Isotope geology--KwaZulu-Natal., Seismic reflection method., Theses--Geology.