Elsevier

Marine Geology

Volume 441, November 2021, 106608
Marine Geology

Bayhead delta evolution in the context of late Quaternary and Holocene sea-level change, Richards Bay, South Africa.

https://doi.org/10.1016/j.margeo.2021.106608Get rights and content

Highlights

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    Stratigraphic evolution of incised valley systems underlying Richards Bay Harbour.

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    Aggrading, prograding and backstepping bayhead delta into underlying incised valleys.

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    Pulses of rapidly rising sea levels — 8.2 ka event and Meltwater pulse (MWP)-1d.

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    Low topographic gradients sensitive to abrupt sea level rises.

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    Small Glacial Isostatic Adjustments (GIA) during postglacial transgression.

Abstract

Richards Bay is part of a back-barrier lagoon fronted by high coastal dunes on the NE, Indian Ocean coast of South Africa. In the early 1970s, a berm was constructed, dividing the original Mhlathuze Estuary into two separate systems; the Richards Bay Harbour and the new Mhlathuze Estuary. This study investigates the stratigraphic evolution of the incised valley system and bayhead delta in the Richards Bay Harbour segment. Seven seismic units (Units 1–7) were imaged. A single regionally developed sequence boundary (SB) along with two tidal ravinement surfaces (tRS1 and tRS2) were identified. Surface SB is associated with the LGM lowstand which developed when sea levels were ~ 130 m below present, until ~18,000 year BP. Cretaceous age siltstones (Unit 1) form the basement. Transgressive material overlying SB (Unit 2) reflects the filling of an incised valley located in the middle segment of a wave-dominated back-barrier system. It is overlain by a bayhead delta (Unit 3), the geometry and seismic signature of which indicate alternating periods of aggradation/progradation and backstepping. The behaviour is attributed to episodic jumps in sea-level, and is tentatively (on the basis of elevations in relation to the regional sea-level curve) linked to periods of rapidly rising sea-level (8.2 ka event and Meltwater Pulse (MWP)-1d). These intervals of rapidly rising sea-level, combined with relatively low gradient settings facilitated backstepping of the delta. Fills (Unit 4) occur within minor incisions along the delta top. These are interpreted as distributary channels that fed sediment to the seaward edge of the bayhead delta system. Elongated mounds on the seafloor (Unit 5) are interpreted as spoil from contemporary port dredging. Slump deposits (Unit 6) along the delta front are attributed to a combination of oversteepening of the delta by dredging, as well as deposition of modern sediments brought into the system by tidal currents. The system is capped by fine-grained, tidally redistributed and deposited sediments (Unit 7) which were possibly sourced from older organic material of an indeterminate source. This site is especially sensitive to episodic rates of sea-level change due to the relatively small Glacial Isostatic Adjustments (GIA) during the postglacial transgression and the flat antecedent gradients of both the subaerial unconformity and the overlying tidal ravinement.

Introduction

Bayhead deltas are defined as fluvially-dominated deltas that prograde into a semi-enclosed body of marine water (Nichol et al., 1997). On a transgressive coastline bayhead deltas form where the rate of sediment input from a fluvial source surpasses the rate of sea-level rise (Aschoff et al., 2018). As such, bayhead deltas are an integral component of most classic wave-dominated lagoon/embayment systems (Dalrymple et al., 1992). They provide a link between the fluvial and central basin depositional environments of many incised valley systems (Simms and Rodriguez, 2015). However, they are not restricted to incised valley systems (Bhattacharya and Giosan, 2003) and also form in fjords, structural basins, interdistributary bays, and other backbarrier environments (see Simms et al., 2018).

In most cases, fluvial systems associated with bayhead deltas provide the majority of the sediment and freshwater input to the upper portions of these systems (Smith et al., 2013). They are also an important part of the rock record, providing vital information for sequence stratigraphic models, and play host to important hydrocarbon reservoirs worldwide (Simms et al., 2018). Many ports and coastal cities are also partly constructed on bayhead deltas, underlining the importance of these shallow and flat-lying areas for land reclamation.

Here we document the stratigraphic evolution of a bayhead delta and incised valley system on which one of the busiest ports in Africa, Richards Bay Harbour, is situated. The purpose of this study is to: (1) provide a detailed and complete stratigraphic framework of the underlying incised valley network of the palaeo-Mhlatuze Estuary, now the site of Richards Bay Harbour; (2) describe the stratigraphic evolution of the backbarrier system and its bayhead delta and (3) determine the process and controls on bayhead delta backstepping in the local context of sea-level change.

Section snippets

Physiography

The Richards Bay Harbour is situated on the subtropical northeast coast of South Africa (approximately 28°47′55.15" S, 32°03'26.71" E) and lies adjacent to the Mhlathuze Estuary (Fig. 1). Before the harbour was developed, the two systems comprised a single large lagoonal system (Weerts and Cyrus, 2002) connected to the Indian Ocean via a narrow inlet roughly in the location of the modern harbour entrance (Jerling, 2003) (Fig. 1).

The original estuary system had five rivers flowing into it, the

Seismic reflection data

This study focuses on a series of incised valleys and sedimentation related to the palaeo-Mhlatuze River entrance to the Richards Bay backbarrier system. High resolution, single-channel seismic data were collected using a Design Projects Boomer and a 20-element hydrophone array at a power level of 175 J. Data were collected along coastal strike with a line spacing of 25–50 m. Several tie lines were collected down-dip with a line spacing of ≤200 m totalling approximately 180 line kilometres (Fig

Bathymetry

The study area encompasses a shallow, low-gradient intertidal-subtidal bayhead delta of sediment adjacent to the entry point of the palaeo-Mhlatuze River into the Richards Bay Harbour. The platform is fringed landward by mangroves and is modified along its southeastern and northeastern margins by regular seafloor dredging (Fig. 2). A clear, planar morphology is evident, with the platform widening seaward (Fig. 2). Platform elevations range from +1 m MSL along the mangrove fringes in the

Acoustic basement and LGM lowstand (Unit 1 and Surface SB)

Unit 1 forms the acoustic basement to the study area. This unit is intersected by numerous boreholes in the region (Maud and Orr, 1975) and represents the Cretaceous age siltstones that have been widely recognised along the shelf and underlying the coastal water bodies of the east coast of South Africa (Green and Garlick, 2011; Green et al., 2013; Benallack et al., 2016; Dladla et al., 2019). A series of incised valleys, represented by Surface SB, are cut into the Cretaceous siltstones. This

Conclusion

The backstepping of bayhead deltas into underlying incised valleys is a global phenomenon. The prograding and eventual backstepping of these bayhead deltas may be attributed to a number of different factors. These include: (1) the amount of sediment brought into the system by rivers, (2) the rate at which this sediment comes into the system, (3) the rate of accommodation creation, (4) rapidly rising sea levels, (5) the gradient of the palaeo-landscape surface, etc. For the Richards Bay Harbour

Data availability

The data used for the research described in this article are proprietary and were released to us. They can be made available on request to Anchor Energy (Pty)Ltd.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

This work was supported by the German Federal Ministry of Education and Research [grant numbers: 03F0798A, 03F0798B, 03F0798C] and is part of project TRACES (Tracing Human and Climate impact in South Africa) within the SPACES II Program (Science Partnerships for the Assessment of Complex Earth System Processes). The field work support by George Best is gratefully acknowledged. We acknowledge Dr. Peter Ramsay and Anchor Energy (Pty) Ltd. for permission to use the geophysical data sets. Doug

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      Citation Excerpt :

      Unit B, which forms the incised valley fills, onlaps and drapes the incised valley walls. This thickly-developed unit (up to 18 m thick) is characteristic of the incised valley systems in coastal waterbodies along the north-east coast of South Africa (Benallack et al., 2016; Dladla et al., 2019, 2021) identified as the central basin deposits of a wave-dominated estuary (Zaitlin et al., 1994) or mixed wave- and tide-dominated estuary (Allen and Posamentier, 1994). Where Unit B is penetrated by KB2, the core comprises organic-rich fines that date to at least 2890 ± 30 cal.

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