The role of shelf morphology and antecedent setting in the preservation of palaeo-shoreline (beachrock and aeolianite) sequences: the SE African shelf

Green, Andrew N.; Cooper, J. Andrew G.; Salzmann, Leslee

doi:10.1007/s00367-017-0512-8

Cited by 36 publications

(36 citation statements)

References 38 publications

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“…However, tests with the model also suggest that smaller barriers with very low shoreface response rates could allow for autogenic partial overstepping on steeper slopes. Such a relationship is intriguing, as recently Green et al (2018), studying the role of antecedent topography on overstepped deposits in South Africa, suggest that barrier volume is inversely related to shelf slope.…”

Section: Discussionmentioning

confidence: 95%

Interaction of Sea-Level Pulses With Periodically Retreating Barrier Islands

2019

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Submerged barrier deposits preserved on continental shelf seabeds provide a record of paleocoastal environmental change from the last glacial maximum through the Holocene. The formation of these offshore deposits is often attributed to intermittent periods of rapidly rising sea levels, especially glacial meltwater pulses, which are expected to lead to partial or complete drowning-overstepping-of migrating barrier islands. However, recent cross-shore modeling and field evidence suggests that even for constant sea-level rise and shelf slope, the internal dynamics of migrating barriers could plausibly drive periodic retreat accompanied by autogenic partial overstepping and deposition of barrier sediment. We hypothesize that the interaction of periodic retreat with changes in external (allogenic) forcing from sea-level rise may create novel retreat responses and corresponding relict barrier deposits. Specifically, we posit that autogenic deposits can be amplified by an increased rate of relative sea-level rise, while in other cases internal dynamics can disrupt or mask the production of allogenic deposits. Here, we model barriers through a range of autogenic-allogenic interactions, exploring how barriers with different inherent autogenic periods respond to discrete, centennial-scale sea-level-rise pulses of variable magnitude and timing within the autogenic transgressive barrier cycle. Our results demonstrate a diversity of depositional signals, where production of relict sands is amplified or suppressed depending on both the barrier's internal dynamic state and the pulse magnitude. We also show that millennial-scale autogenic periodicity renders barriers vulnerable to complete drowning for relatively low pulse rates of rise (<15 mm/year).

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Section: Discussionmentioning

confidence: 95%

Interaction of Sea-Level Pulses With Periodically Retreating Barrier Islands

2019

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“…The barriers found on the shelf were rapidly cemented. Both their large volume and early cementation resulted in these features being overstepped during rising sea levels and thus ultimately preserved in the stratigraphic record (Green et al, ). When coupled with coastal retreat, and in hand with the continuing back‐barrier aggradation due to washover accumulations, the smaller barrier‐spits would rollover until no available accommodation space exists.…”

Section: Discussionmentioning

confidence: 99%

Overwash‐dominated stratigraphy of barriers with intermittent inlets

Green

Pillay

Cooper³

et al. 2019

Earth Surf Processes Landf

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The morphodynamics and structure of barriers with persistent tidal inlets have been well studied. In contrast the stratigraphy and functioning of barrier systems with ephemeral inlets is poorly understood. This article examines the barrier-inlet systems of two intermittently closed open lagoons or temporarily open closed estuaries on the east coast of South Africa. Multiple geophysical surveys using ground-penetrating radar (GPR) were correlated with exposed sections of the barrier where inlet formation revealed the internal stratigraphy. Stratigraphic observations were placed in the context of the contemporary wave dynamics and mesoscale geomorphic evolution. The integrated databases reveal an absence of migrating channel features. Instead the stratigraphy is dominated by landward dipping sheets of alternating high-and low-amplitude reflectors. These correlate with gravel, shell debris and heavy mineral-lined beds formed by overwash processes. Where ephemeral inlet structures are preserved in the stratigraphy, their fills comprise aggrading, high-amplitude reflectors, linked to washover infilling of the inlet mouth. Multiple small channels in the more distal portions of the barrier in georadar stratigraphy are related to channelized washover flow.These barriers often breach during high swell and are subsequently sealed during fairweather wave conditions. Time series analysis of waves and satellite imagery shows a link between storms from the south and breach events. This is consistent with the truncations in subsurface images and inferred barrier lowering by overwash channelling. These barriers experience quasi-stable oscillations in their landward and seaward shore position, punctuated by periods of barrier rollover associated with the most intense storms. As overwash is responsible in part for both the constructive and destructive phases of the barrier, these barriers have low preservation potential. Persistent rollover driven by overwashing will terminate once accommodation space is eliminated and the barriers are eroded by storm activity.

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“…Favourable conditions for barrier preservation during overstepping include early cementation (e.g. Gardner et al, 2005Gardner et al, , 2007Green et al, 2013aGreen et al, , 2014Green et al, , 2018, long relaxation times, as in gravel and boulder barriers (e.g. Forbes et al, 1991;Hartstein and Dickinson, 2000), an increase in back-barrier accommodation (linked to a low topographic gradient) (Storms et al 2008), large sediment volume in the barrier (Orford et al, 1995) and low wave energy .…”

Section: Profile Modelsmentioning

confidence: 99%

Geological constraints on mesoscale coastal barrier behaviour

Cooper

Green

Loureiro

2018

Global and Planetary Change

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Barrier/lagoon systems occupy a significant part of the world's coast. They are diverse in size, morphology, geological and oceanographic setting, and morphodynamic behaviour. Understanding the behaviour of barriers at 10 1 to 10 2 year and 10 1 to 10 2 kilometre scales (mesoscale) is an important scientific and societal goal, not least because of the preponderance of intensive coastal development in a time of global climate change. Such understanding presents significant challenges. Challenges in describing mesoscale system behaviour relate largely to the incomplete evidence base of (i) morphological change in system components, (ii) dynamic and internal forcing factors (drivers) and (iii) geological constraints. These shortcomings curtail the development of baseline datasets against which to test models. Understanding observed changes and thereby predicting future behavioural patterns demands assumptions and simplifications regarding the linkages between initial state, dynamic drivers, system feedbacks and a multiplicity of geological constraints that are often location-specific. The record of mesoscale change is improving with the acquisition of long-term morphological datasets. Advances in technology and chronological control mean that geological investigations can now provide decadal to century-scale temporal resolution of morphological change. In addition, exploratory modelling is improving understanding of the influence of various dynamic and geological factors. most cases unquantified and are usually disregarded when conceptualizing and modelling barrier evolution. Consideration of the geological influences is, however, essential in efforts to predict future behaviour at mesoscale (management) timescales.

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The role of shelf morphology and antecedent setting in the preservation of palaeo-shoreline (beachrock and aeolianite) sequences: the SE African shelf

Cited by 36 publications

References 38 publications

Interaction of Sea-Level Pulses With Periodically Retreating Barrier Islands

Interaction of Sea-Level Pulses With Periodically Retreating Barrier Islands

Overwash‐dominated stratigraphy of barriers with intermittent inlets

Geological constraints on mesoscale coastal barrier behaviour

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