Marine Geoscience Unit

Cape Town, South Africa

Marine Geoscience Unit

Cape Town, South Africa
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Green A.N.,University of KwaZulu - Natal | Cooper J.A.G.,University of KwaZulu - Natal | Cooper J.A.G.,University of Ulster | Salzmann L.,University of KwaZulu - Natal | Salzmann L.,Marine Geoscience Unit
Geo-Marine Letters | Year: 2017

On the SE African shelf, a submerged shoreline at a depth of 60 m is examined and its attributes compared between two shelf sectors with different morphologies, yet similar energy regimes. The aim is to assess the controls of antecedent conditioning on shoreline development and later preservation from transgressive ravinement. Using a combination of multibeam bathymetry and single-channel seismic profiles, the stratigraphy and morphology of the shoreline is investigated. Low-gradient bedrock examples reveal several distinctive seismic facies, including onlapping chaotic reflector packages which are interpreted as calcarenite rubble fields. These palaeo-shorelines possess planform equilibrium morphologies, including parabolic dunes and blowout forms along with relict shore platforms. They are strongly associated with incised valleys of last glacial maximum age which underlie the shoreline locations; these provide wide, back-barrier accommodation space during transgression. In contrast, palaeo-shorelines on the steeper-gradient shelf have a simpler stratigraphic arrangement. They are not as well preserved, are generally covered by thick drapes of sediment, and lack the elaborate planform morphologies of their lower-shelf gradient equivalents. Isolated incised valleys and the steep bedrock gradient limit accommodation space. The comparison indicates that antecedent bedrock slope and available accommodation are amongst the dominant controls on overstepping, and thus potential preservation, of palaeo-shorelines on the shelf. Lower-gradient shelves not only promote rapid shoreline translation but, together with wide, sandy back-barrier accommodation, also foster larger barrier volumes. In suitable climates such as in the Mediterranean and other sub-tropical areas, the ensuing shoreline stability promotes rapid and effective cementation of the barrier. In comparison, steep bedrock profiles with limited back-barrier accommodation have much lower preservation potential. Transgressive ravinement is more focussed on steep slopes, effectively removing more material during the ravinement process. The more dynamic environment may also reduce the effectiveness of diagenesis. The potential of beachrock and aeolianite palaeo-shorelines as submerged sea-level indicators may be optimal in low-gradient settings in Mediterranean to subtropical environments. © 2017 Springer-Verlag GmbH Germany

Marean C.W.,Institute of Human Origins | Marean C.W.,Nelson Mandela Metropolitan University | Anderson R.J.,Fisheries Branch | Anderson R.J.,University of Cape Town | And 11 more authors.
Evolutionary Anthropology | Year: 2015

Paleoanthropologists (scientists studying human origins) universally recognize the evolutionary significance of ancient climates and environments for understanding human origins. Even those scientists working in recent phases of human evolution, when modern humans evolved, agree that hunter-gatherer adaptations are tied to the way that climate and environment shape the food and technological resource base. The result is a long tradition of paleoanthropologists engaging with climate and environmental scientists in an effort to understand if and how hominin bio-behavioral evolution responded to climate and environmental change. Despite this unusual consonance, the anticipated rewards of this synergy are unrealized and, in our opinion, will not reach potential until there are some fundamental changes in the way the research model is constructed. Discovering the relation between climate and environmental change to human origins must be grounded in a theoretical framework and a causal understanding of the connection between climate, environment, resource patterning, behavior, and morphology, then move beyond the strict correlative research that continues to dominate the field. © 2015 Wiley Periodicals, Inc.

Cawthra H.C.,Marine Geoscience Unit | Neumann F.H.,University of Witwatersrand | Neumann F.H.,University of Munster | Uken R.,University of KwaZulu - Natal | And 3 more authors.
Marine Geology | Year: 2012

The narrow transform margin of southeast Africa and its associated sedimentological and hydrodynamic setting differs to other documented continental margins. The Durban Bluff continental shelf is extremely narrow and steep (8. km wide with a gradient ranging from 2 to 8°) characterised by a wave- and oceanic current-dominated regime. Seismic Sequence Boundary 2, developed during the Last Interglacial regression, spans the entire shelf separating the Holocene sediment wedge (Seismic Unit H) from underlying Pleistocene deposits. A wave ravinement surface marks the Holocene transgression, comprising a pavement lag of well sorted gravels and bioclastics overlain by inshore reef-derived carbonate rich sediments and offshore by quartzose mid-shelf sands. The shelf sands represent the transgressive Holocene to modern sediment wedge forming a seaward thinning unit stacked against the Pleistocene Blood Reef aeolianite/beachrock substrate. The sediment wedge is dynamic and constantly redistributed by currents associated with the Durban Eddy inshore of the Agulhas Current and bottom surge associated with high swells and marine storm events. These have been instrumental in shaping large-scale shoreface attached and detached sand ridges. The presence of mud lenses in the vicinity of Blood Reef represents deposition from turbid flood waters with preservation facilitated by the morphology of the Durban Bluff and Blood Reef. Palynological results, reflecting the local subtropical vegetation and recently introduced neophytes, together with radiocarbon dates, provide a very recent age for this sediment supporting a terrestrial origin and deposition from a single large flood event. © 2012 Elsevier B.V.

Cawthra H.C.,Marine Geoscience Unit | Cawthra H.C.,University of Cape Town | Compton J.S.,University of Cape Town | Fisher E.C.,Arizona State University | And 3 more authors.
Geological Society Special Publication | Year: 2016

Coastal geomorphic systems have been studied widely to understand the responses of shorelines to fluctuating sea levels. Submerged shorelines, remnant of pleistocene sea-level lowstands, are well preserved on the south african continental shelf. This paper describes work undertaken to better understand offshore coastal environments now submerged by high sea levels off the south african south coast near mossel bay, offshore of the pinnacle point archaeological locality. Multibeam bathymetry and side-scan sonar reveal evidence of past sea-level fluctuations and submerged coastal landscape features on the seabed. These results form the basis of an ongoing palaeoenvironmental reconstruction for this part of the shelf. We describe seven significant geomorphic features that show a submerged environment that differs significantly to the immediate adjacent coastal plain. However, these features are comparable to other stretches of the present south african shoreline that serve as modern analogues.We propose that features on the continental shelf primarily reflect geological substrate, gradients and pleistocene sea-level fluctuations. Early modern humans were likely to have had a different set of resources to use inthis pleistocene landscape compared to those available along the presently exposed coast. © 2016 The Author(s).

Cawthra H.C.,Marine Geoscience Unit | Cawthra H.C.,University of Cape Town | Bateman M.D.,University of Sheffield | Carr A.S.,University of Leicester | And 2 more authors.
Quaternary Science Reviews | Year: 2014

Coastal barrier systems have been widely used to understand the responses of coastal margins to fluctuating Pleistocene sea levels. What has become apparent, particularly with the development of robust chronological frameworks, is that gaps in terrestrial barrier sedimentary records are not uncommon and that they most likely reflect phases of barrier construction on the now submerged continental shelf. Thus, understanding the land-ocean interface through time is critical to fully appreciate the Quaternary archives contained within the barriers and their associated back-barrier deposits. This study uses offshore and lakefloor (back-barrier) seismic profiling from the South African south coast at Wilderness to link the sub-aerially exposed barrier stratigraphy to the currently submerged geological and sedimentological record. A total of eight separate submerged aeolian units are identified at water depths of up to 130m below mean sea level. Their approximate ages are constrained with reference to the eustatic sea-level record and the deepest units are consistent with the estimated magnitude of sea-level lowering during the Last Glacial Maximum (LGM) on the South African coastline. As previously assumed, aeolian sedimentation tracked the shoreline onto the continental shelf during the Late Pleistocene. During sea-level regressions, both the incision of fluvial channels and the deposition of back-barrier systems occurred across the continental shelf. During late low stand/early transgression periods, landward shoreface migration occurred, pre-existing channel incisions were infilled and pre-existing barriers were truncated. Rapid transgression, however, allowed the preservation of some back-barrier deposits, possibly aided by protection from antecedent topography. As sea level neared the present-day elevation, erosion of the mid-shelf sediments resulted in the development of a Holocene sediment wedge, which was augmented by Holocene fluvial sediment supply. The Holocene sand wedge is preserved in the back-barrier lakes and was deposited during the Holocene highstand inundation. Overlying middle to late Holocene terrestrial muds reflect the deposition of river-borne mud onto the shelf. These results clearly demonstrate that within transgressive-regressive sea-level cycles, accommodation space for barriers is controlled by antecedent drainage systems and gradients on the adjacent inner continental shelf. © 2014 Elsevier Ltd.

Cawthra H.C.,Marine Geoscience Unit | Uken R.,University of KwaZulu - Natal | Ovechkina M.N.,University of KwaZulu - Natal | Ovechkina M.N.,Russian Academy of Sciences
South African Journal of Geology | Year: 2012

The narrow, oceanic current-swept shelf of the Durban Bluff is characterised by Pleistocene aeolianites deposited unconformably onto a Cretaceous sequence. Subsequent beachrocks were deposited on and erosional features cut into the aeolianites, tracking a series of palaeocoastlines that extend from the supratidal zone to the outer continental shelf and record sea level fluctuations from the Last Interglacial to the present. High-resolution boomer seismics reveal a detailed sequence stratigraphy for the late Cretaceous drift sequence (Early Santonian to Late Maastrichtian) followed by a likely Miocene/Pliocene sequence preserved on the shelf edge, the first record of these deposits from the Durban continental shelf. Seven seismic units are recognised (Units A to G), bounded by regional sequence boundaries, maximum flooding surfaces and wave ravinement surfaces. Mapping of the Bluff Ridge and adjoining Blood Reef by geophysical surveys and scuba diving enabled subdivision of the Quaternary deposits into three aeolianite units and 13 beachrock units. Caicareous nannoplankton preserved in the aeolianite units indicate a late Pleistocene to Holocene age (Zone NN21, maximaliy 290 ka) for the deposits and the presence of reworked Miocene forms provides further support for the existence of Neogene on the shelf edge. A new Infared Stimulated Luminescence age of 60 ka obtained from offshore aeolianite indicates dune-building during the Marine Isotope Stage 4 glacial period. © 2012 September Geological Society of South Africa.

Cawthra H.,Marine Geoscience Unit | Uken R.,University of KwaZulu - Natal
South African Journal of Science | Year: 2012

We explored the recent cementation of modern beachrock on the seaward margin of the Durban Bluff, central KwaZulu-Natal. The low latitude and subtropical climatic setting is a unique context compared to the more commonly documented contemporary beachrock formation in the tropics. Geological field mapping was carried out and here we present results based on sedimentary facies of a clastic shoreline and carbonate diagenesis of interstitial cements using transmitted light microscopy. The beachrock was cemented by micrite and aragonite, and iron oxide infilled voids. The presence of human artefacts within the deposit showed evidence for cementation within the last century. The elevation (at Mean Low Water) and correlation to rates of sea level change for the east coast of South Africa showed that the beachrock is less than 72 years in age. In contrast to older local Pleistocene deposits, beachrocks have cemented along this stretch of coast during successive sea level highstands with similar climatic regimes - the last Interglacial, the Holocene High and the present. Here we report the most southerly documentation of modern beachrock in KwaZulu-Natal, which, to our knowledge, represents the youngest deposit reported in southern Africa. © 2012. The Authors. Licensee: AOSIS OpenJournals.

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