Construction and evolution of submerged deltaic bodies on the high energy SE African coastline: The interplay between relative sea level and antecedent controls

Abstract

This paper investigates the interplay between allocyclic controls and antecedent topography in the evolution of submerged coastal landforms, including a back-stepped delta. Using high-resolution tools, we examine the wave-dominated Thukela shelf, and define the major seismic units. Key features identified comprise incised valleys scoured into bedrock, that have overspilled to form lagoons at depths of 50 m. These are in turn overlain by two prograding and backstepped sandy delta systems at 40 m and 32 m depth respectively. The deltas interfinger with muddy prodelta deposits and are truncated by the Holocene ravinement, overlain by the contemporary prodelta of the Thukela River system. A bedrock high separates two physically separate strato-morphological zones; landward a sediment stripped, steep and shallow nearshore zone, and seaward a gentle zone downdip where the deltaic accumulations are sited. Delta development was favoured during sea-level stillstands at−40 m and −32 m respectively. The step-back of the deltas corresponds to sharp increases in the rate of sea-level rise associated with meltwater pulses. The overall gentle palaeobathymetric gradient has moderated erosion associated with rising sea level, preserving a sandy back-stepping delta and a draping mud clinoform. Submerged delta positioning relates to underlying incised valleys, suggesting a synchronous transgressive evolution of the drainage and the delta. Incised valley network positioning is further governed by Late Pliocene aged growth faults in the basement rocks. The geological framework has acted as a recurring primary control to the geomorphic evolution of the area, partitioning accommodation for sediment accumulation and moderating the efficiency of ravinement.

Transgressive Architecture of Coastal Barrier Systems in the Ofanto Incised Valley and Its Surrounding Shelf in Response to Stepped Sea-Level Rise

“…The alternative interpretation would be that unit A l is a second small depositional lobe deposited at (or close to) sea level due to the Ofanto sediment discharge, arranged in a landward backstepping pattern with respect to the greater MOL (Figure 13B,C). This situation is similar to that of the Thukela Shelf (South Africa), where two prograding and backstepped sandy delta systems at depths of 40 m and 32 m have been recognised [76]. Delta development was favoured during sea-level stillstands at −40 m and −32 m, respectively, while the step-back of the deltas corresponded to sharp increases in the rate of sea-level rise associated with meltwater pulses.…”

Lifting back the waters: Marine geophysics provides new insights into the uThukela Banks Marine Protected Area

“…Abutting and surrounding these aeolianite palaeo-shorelines are sandy sediments that become progressively muddier with proximity to the coastline; these constitute the submerged portions of the wavedominated Thukela delta. 20 Despite the large quantities of sediment delivered to the shelf, it is comparatively sediment starved when compared to other shelves of the world 12 and much of the mid shelf has been considered to comprise palimpsest gravels, or muds from palaeolagoons, exposed by erosion during rising sea levels 11 . These create niche habitats for a variety of benthic organisms not found in comparable quantities elsewhere in South Africa.…”

Coastal adaptations on the eastern seaboard of South Africa during the Pleistocene and Holocene? Current evidence and future perspectives from archaeology and marine geology

“…In some places, this is decreased where submarine canyons intersect the shelf break (Flemming, 1981). From Durban to Port Durnford, the shelf widens to a maximum of ~40 km offshore the Thukela River (Martin and Flemming, 1988;Engelbrecht et al, 2020), before narrowing to a minimum of 4 km from Richards Bay to the Mozambique border (Green et al, 2022a). The area between Durban and Port Durnford is known as the Natal Bight, and is underlain by a Kimmeridgian to Cenozoic-aged submerged fan delta complex formed by the Thukela River, termed the Thukela Cone (Hicks and Green, 2016).…”

“…In many of the submerged shorelines, there is strong potential for cave and rockshelter features to form: Overhangs, notches and caves are common in the submerged beachrocks and aeolianiates of the SE African margin (Green and Uken, 2005;Bosman, 2012), and may have provided similar environments as the Pinnacle Point caves of the southern Cape coast and offshore (Cawthra et al, 2018;. Notable examples include offshore the Thukela River and throughout the Aliwal Shoal, where the foresets of aeolianites weather to form overhangs up to 2 m high and several meters wide (Bosman, 2012;Engelbrecht et al, 2020). Based on their relative stability and ages (from the Holocene to at least MIS 3), the now submerged paleo-shorelines and lagoons around modern-day Durban appear to be further attractive possibilities for uncovering submerged MSA and LSA occupations.…”

“…Relict rocky coast paleo-shorelines may provide good shelter and foraging potential, however these need to form and then be stranded by falling sea levels so that the shelter potential is not quickly submerged, and the ensuing occupation record remains preserved from marine erosion. Some of the aeolanites offshore the Sibhudu area are important in this regard (Engelbrecht et al, 2020). Other landforms such as present and paleoriver valleys and mouths constitute another primary target for identifying potential Stone Age settlements along the coasts.…”