Geological inheritance and its role in the geomorphological and sedimentological evolution of bedrock-hosted incised valleys, lake St Lucia, South Africa

Dladla, N.N.; Green, Andrew N.; Cooper, J. Andrew G.; Humphries, Marc

doi:10.1016/j.ecss.2019.04.013

Cited by 20 publications

(20 citation statements)

References 52 publications

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“…LGM-age incised valleys recognised on the Durban shelf by Green et al (2013a), and on the coastal plain to the north of the study area by Benallack et al (2016) and Dladla et al (2019). These are revealed to be lagoonal materials that overtop the interfluves due to prolonged sea level stability and overfilling of the valley form (Green et al, 2013a).…”

Section: Lgm Lowstand and Subsequent Incised Valley/interfluve Depositionmentioning

confidence: 92%

“…Following Zecchin and Catuneanu (2013) we consider this surface and its associated incised valleys to be a subaerial unconformity. Given the stratigraphic position as the uppermost incised surface, together with its links to a regional network of incised valleys (Green et al, 2013a;Dladla et al, 2019;Pretorius et al, 2019), it is interpreted as the subaerial unconformity related to the…”

Section: Lgm Lowstand and Subsequent Incised Valley/interfluve Depositionmentioning

confidence: 99%

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

et al. 2020

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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.

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Section: Lgm Lowstand and Subsequent Incised Valley/interfluve Depositionmentioning

confidence: 92%

Section: Lgm Lowstand and Subsequent Incised Valley/interfluve Depositionmentioning

confidence: 99%

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

et al. 2020

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“…Similar facies associations develop in systems where there is high littoral sediment supply (e.g., mainland-attached baymouth bar and spit systems), or where large transgressive barrier dunes front coastal plains backed by coastal waterbodies. In many instances, these facies associations evolve from barrier islands either through spit growth and eventual closure of embayments, or they amalgamate to form a contiguous mainland attached sand body (e.g., the coastal waterbodies of SE Africa; Wright et al, 2000;Benallack et al, 2016;Dladla et al, 2019). In contrast, strandplains lack backbarriers (Galloway, 1986), and instead, fluvial and coastal plain facies are the correlative proximal time-equivalent facies to strandplain shoreface deposits (Clifton, 2006).…”

Section: Proximity To Backbarrier Depositsmentioning

confidence: 99%

When Is a Barrier Island Not an Island? When It Is Preserved in the Rock Record

2021

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Existing barrier island facies models are largely based on modern observations. This approach highlights the heterogeneous and dynamic nature of barrier island systems, but it overlooks processes tied to geologic time scales, such as multi-directional motion, erosion, and reworking, and their expressions as preserved strata. Accordingly, this study uses characteristic outcrop expressions from paralic strata of the Upper Cretaceous Straight Cliffs Formation in southern Utah to update models for barrier island motion and preservation to include geologic time-scale processes. Results indicate that the key distinguishing facies and architectural elements of preserved barrier island systems have very little to do with “island” morphology as observed in modern systems. Four facies associations are used to describe and characterize these barrier island architectural elements. Barrier islands occur in association with backbarrier fill (FA1) and internally contain lower and upper shoreface (FA2), proximal upper shoreface (FA3), and tidal channel facies (FA4). Three main architectural elements (barrier island shorefaces, shoreface-dominated inlet fill, and channel-dominated inlet fill) occur independently or in combination to create stacked barrier island deposits. Barrier island shorefaces record progradation, while shoreface-dominated inlet fill records lateral migration, and channel-dominated inlet fill records aggradation within the tidal inlet. Barrier islands are bound by lagoons or estuaries and are distinguished from other shoreface deposits by their internal facies and outcrop geometry, association with backbarrier facies, and position within transgressive successions. Tidal processes, in particular, tidal inlet migration and reworking of the upper shoreface, also distinguish barrier island successions. In sum, this study expands barrier island facies models and provides new recognition criteria to account for the complex geometries of time-transgressive, preserved barrier island deposits.

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“…During the Last Glacial Maximum (LGM), sea‐level was 100 to 130 m below the present level (Cooper & Green, 2018), which led to the subaerial exposure of the adjoining shelf (Green, 2009). Several river valleys incised the main dune cordon to the north and south of the study area (Benallack et al ., 2016; Dladla et al ., 2019). With the ensuing transgression of the Holocene, these channels formed sites for inlets in the then stabilized Holocene shoreline that had formed against the pre‐existing mainland dunes (Cooper, 1991b).…”

Section: Regional Settingmentioning

confidence: 99%

Breaking up and making up – reworking of Holocene calcarenite platform into rapidly‐forming beachrock breccias on a high energy coastline (St. Lucia, South Africa)

et al. 2021

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Beachrocks are a common characteristic of tropical and subtropical coastlines. It is known that they have a substantial influence on beach morphodynamics and they are commonly utilised as indicators of palaeo-sea levels. At the same time, facies variability in beachrocks is understudied and their effect on shoreline evolution is poorly understood. At Mission Rocks on the KwaZulu-Natal coastline of South Africa a narrow beach with isolated sand patches occupies low points of an otherwise continuous 3 m thick, raised shore platform of sandy and gravelly beachrocks. These beachrocks are in the process of disintegrating into megagravel deposits through chemical and mechanical weathering in a wave-dominated, high-energy setting. The breakdown is potentially slowed by a contemporary, fast-forming beachrock facies, that blankets the surface and fills fractures and potholes within the older platform. The accumulation and cementation of this recent beachrock is the focus of this study. The beachrock is dated by historical evidence to post-World War II. Data from field observations, petrographic and geochemical methods reveal that the cementing agents of the beachrock were precipitated from marine water in a phreatic setting despite its position above the intertidal zone. Not only does this facies have implications for the interpretation of palaeobeachrock as a sea-level indicator, it also raises further questions regarding modelling of coastal erosion of beaches associated with outcrops of beachrock.

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Geological inheritance and its role in the geomorphological and sedimentological evolution of bedrock-hosted incised valleys, lake St Lucia, South Africa

Cited by 20 publications

References 52 publications

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

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

When Is a Barrier Island Not an Island? When It Is Preserved in the Rock Record

Breaking up and making up – reworking of Holocene calcarenite platform into rapidly‐forming beachrock breccias on a high energy coastline (St. Lucia, South Africa)

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