Sedimentology, coral reef zonation, and late Pleistocene coastline models of the Sodwana Bay continental shelf, Northern Zululand

Abstract

This geostrophic current-controlled Zululand/Natal shelf displays a unique assemblage of interesting physical, sedimentological and biological phenomena. The shelf in this area is extremely narrow compared to the global average of 75km, and is characterised by submarine canyons, coral reefs, and steep gradients on the continental slope. A shelf break occurs 2.1km to 4.1km offshore and the shelf can be divided into a northern region and a southern region based on the presence or absence of a defined shelf break. The southern shelf has a poorly-defined shelf break whilst the northern shelf has a well-defined break at -65m. The poor definition of the shelf break on the southern shelf can possibly be attributed to the presence of giant, climbing sand dunes offshore of Jesser Point at depths of -37m to -60m. The northern shelf has a series of coast-parallel oriented patch coral reefs which have colonised carbonate-cemented, coastal-facies sequences. The northern shelf can be divided into three distinct zones: inner-, mid-, and outer-shelf zones. The inner-shelf is defined as the area landward of the general coral reef trend, with depths varying from 0m to -I5m and having an average gradient of 1.1. The mid-shelf is defined by the general coral reef trend, varying from -9m over the shallow central axis of the reefs to -35m along the deep reef-front environments. The outer-shelf is seaward of the coral reefs and occurs at a depth range of -35m to - 65m. Gradients vary from 1° in the south to 2.5° in the northern part of the study area, and are steep compared to world average shelf gradient of 0.116°. Four submarine canyons occur in the study area and are classified as mature- or youthful-phase canyons depending on the degree to which they breach the shelf. The origin of these canyons is not related to the position of modern river mouths but can probably be linked to palaeo-outlets of the Pongola and Mkuze River systems. It is suggested that the canyons are mass-wasting features which were exploited by palaeo-drainage during regressions. The youthful-phase canyons appear to be mass-wasting features associated with an unstable, rapidly-deposited, progradational late Pliocene sequence and a steep upper continental slope. The mature-phase canyons were probably initiated by mass-wasting but have advanced shoreward, breaching the shelf, due to their link with the palaeo-outlets of the Pongola and Mkuze Rivers during late Pleistocene regressions. Evidence of modem canyon growth has been noted on numerous SCUBA diving surveys carried out on the canyon heads. These take the form of minor wall slumps and small-scale debris flows. The canyons are also supplied with large quantities of sand in the form of large-scale shelf subaqueous dunes generated and transported by the Agulhas Current. As these bedforms meet the canyons the sediment cascades down the canyon thalweg and causes erosion and downcutting of the canyon walls and floor thereby increasing the canyon dimensions. Late Pleistocene beachrock and aeolianite outcrops with or without an Indo-Pacific coral reef veneer are the dominant consolidated lithology on the shelf. These submerged, coast-parallel, carbonate cemented, coastal facies extend semi-continuously from -5m to -95m, and delineate late Pleistocene palaeocoastline events. The rock fabric of these high primary porosity lithologies shows grains floating in a carbonate cement with occasional point-contacts. Grains are mostly quartz (80-90%), minor K-feldspar and plagioclase (5-10%), and various lithic fragments. The rocks contain conspicuous organic grains including foraminifera, bivalve, echinoid, bryozoan, red algal, and occasional sponge spicule fragments; these commonly display replacement fabrics or iron-stained rims. The dominant sedimentary structures found in these sandstone outcrops include high-angle planar cross-bedding and primary depositional dip bedding. Palaeocurrent directions sngest a palaeoenvironment dominated by a combination of longitudinal and transverse dunes with wind directions similar to those observed forming the modem dune systems. Erosional features evident on the submerged beachrocks and aeolianites include gullies trending in two different directions and sea-level planation surfaces with or without the presence of potholes. The unconsolidated sediment on the shelf is either shelf sand, composed mainly of terrigenous quartz grains; or bioclastic sediment which is partially derived from biogenic sources. The quartzose sand from the inner-shelf is generally fine-grained, moderately- to well-sorted, and coarsely- to near symmetrically-skewed. Carbonate content is low, and varies between 4-13%. Quartzose sand from the outer-shelf is fine-grained, moderately- to well-sorted, and coarsely- to very coarsely-skewed. The inner-shelf quartzose sand is better sorted than the outer-shelf sand due to increased reworking of this sediment by the high-energy swell regime. Sediment from the shallower areas of the outer-shelf (< -50m) is better sorted than sediment from depths of greater than -50m. Generally wave-reworking of quartzose shelf sand from the Sodwana Bay shelf results in greater sediment maturity than that observed from geostrophic current effects or a combination of geostrophic and wave-reworking. This sediment was derived by reworking of aeolian and beach sediments, deposited on the shelf during the period leading up to the Last Glacial Maximum (15 000 - 18 000 years B.P.) when sea-level was -130m, during the Holocene (Flandrian) transgression. Bioclastic sediment on the Sodwana Bay shelf is defined as having a CaC03 content of greater than 20% and is a mixture of biogeoically-derived debris and quartzose sand. The distribution of bioclastic sediment in the study area is widespread, with reef-derived and outer-shelf-derived populations being evident. This sediment consists of skeletal detritus originating from the mechanical and biological destruction of carbonate-secreting organisms such as molluscs, foraminifera, alcyonaria, scleractinia, cirripedia, echinodermata, bryozoa, porifera. The reef-derived bioclastic population is confined to depths less than -40m in close proximity to reef areas, whereas the shelf-derived bioclastic population occurs at depths greater than -40m and is derived from carbonate-producing organisms on deep water reefs and soft-substrate environments on the shelf. Large-scale subaqueous dunes form in the unconsolidated sediment on the outer-shelf due to the Agulhas flow acting as a sediment conveyor. These dunes are a common feature on the Sodwana Bay shelf occurring as two distinct fields at depths of -35m to -70m, the major sediment transport direction being towards the south. The two dune fields, the inner- and outer subaqueous dune fields, are physically divided by Late Pleistocene beachrock and aeolianites ledges. A bedform hierarchy has been recognised. The larger, outer dune field appears to have originated as a system of climbing bedforms with three generations of bedforms being superimposed to form a giant bedform, while the inner dune field has a less complex construction. The largest bedforms are those of the outer dune field off Jesser Point, being up to 12 m high, 4 km long and 1.2 km wide. A major slip face, with a slope of 8° is present. Bedload parting zones exist where the bedform migration direction changes from south to north. Three bedload parting zones occur in the study area at depths of -60m, -47m and -45m; two in the inner dune field and one in the outer dune field. These zones are invariably located at the southern limits of large clockwise eddy systems. Such eddies appear to be the result of topographically induced vorticity changes in the geostrophic flow and/or the response to atmospheric forcing caused by coastal low-pressure system moving up the coastline. It has been demonstrated that the inner subaqueous dune sediment conveyor is not active all the time but only during periods . of increased current strength when the Agulhas Current meanders inshore. The smaller bedforms in the outer dune field undergo continuous transport due to the current velocity on the shelf edge outer dune field being higher than the velocity experienced on the inner dune field. The very large 2·D dune which forms the outer dune field is probably not active at present: this is inferred due to the shallow angle of the mega-crest lee slope (8°). The very large Sodwana Bay subaqueous dune fields may be compared with the very large, reconstructed, subaqueous dunes which occur in Lower Permian sediments of the Vryheid Formation, northern Natal. These Permian dunes are represented, in section, as a fine- to medium-grained distal facies sandstone with giant crossbeds. These large-scale bedforms are unidirectional, but rare directionally-reversed, climbing bedforms do occur, this directional reversal may be related to bedload parting zones. On the evidence presented in this thesis, it is proposed that these Permian subaqueous dunes may be ancient analogues of the modem subaqueous dune field on the Sodwana Bay shelf. Positive-relief hummocks and negative-relief swale structures are fairly common in the fine-grained, quartzose shelf sand at depths of -30m to -60m. These appear to be transitional bedforms related to the reworking by storms of medium 2-D subaqueous dunes. These hummocky structures may be the modem equivalent of hummocky cross-stratification noted in the geological record, and if so, they are probably the first to have ever been observed underwater. The occurrences of ladderback ripples on the Sodwana Bay shelf at depths of -4m to -17m, suggest that subtidal ladderback ripples may be more common than previously thought. Ladderback ripples are common features of tidal flats and beaches where they form by late-stage emergence run-off during the ebb tide. They are generally considered diagnostic of clastic intertidal environments. The mode of formation on the Sodwana Bay shelf is different from the classic late-stage emergence run-off model of intertidal occurrences, being a subtidal setting. Subaqueous observations indicate that ladderback ripples are not environment-specific, and that additional evidence of emergence is therefore necessary to support an intertidal setting in the rock record: ladderback ripples alone are insufficient to prove an intertidal environment. The coral patch reefs of the northern Natal coast are unique, being the most southerly reefs in Africa, and totally unspoilt. The Zululand reefs are formed by a thin veneer of Indo-Pacific type corals which have colonised submerged, late Pleistocene beachrocks and aeolianites. Two-Mile Reef at Sodwana Bay has been used to develop a physiograpbic and biological zoning model for Zululand coral reefs, which has been applied to other reefs in the region. Eight distinct zones can be recognised and differentiated on the basis of physiographic and biological characteristics. The reef fauna is dominated by an abundance of alcyonarian (soft) corals, which constitute 60-70% of the total coral fauna. The Two-Mile Reef zoning model has been successfully applied to larger reefs such as Red Sands Reef, and smaller patch reefs (Four-Mile and Seven-Mile Reefs) in the same general area. In this thesis extensive use has been made of Hutton's uniformitarian principles. Hutton's doctrine is particularly relevant to the study of depositional processes and relict shorelines. Coastal processes and weather patterns during the late Pleistocene were broadly similar to modem conditions enabling direct comparisons to be made. A computer-aided facies analysis model has been developed based on textural statistics and compositional features of carbonate-cemented coastal sandstones. Many attempts have been made to distinguish different ancient sedimentary depositional environments, most workers in this field having little success. The new method of facies reconstruction is based on: (1) underwater observations of sedimentary structures and general reef morphology; (2) a petrographic study of the reef-base enabling flve facies: aeolianite, backbeach, forebeach, swash, and welded bar facies to be recognised, which control the geomorphology of Two-Mile Reef; (3) cluster and discriminant analysis comparing graphic settling statistics of acid-leached reef-base samples with those of modem unconsolidated dune/beach environments. The results of this analysis demonstrated that the beachrocks and aeolianites on the shelf formed during a regression and that late Pleistocene coastal facies are similar to modem northern Zululand coastal environments, which have been differentiated into aeolian, backbeach, forebeach, swash, & welded bar. A late Pleistocene and Holocene history of the shelf shows that during the late Pleistocene, post Eemian regressions resulted in deposition and cementation of coast-parallel beachrocks and aeolianites, which define a series of four distinct palaeocoastline episodes with possible ages between 117 000 and 22 000 years B.P. The beachrock/aeolianites formed on the shelf during stillstands and slow regressions, and the gaps between these strandline episodes represent periods of accelerated sealevel regression or a minor transgressive phase which hindered deposition and cementation. The formation of these lithologies generated a considerable sediment sink in the nearshore zone. This reduced sediment supply and grain transport in the littoral zone during the Holocene, and probably enhanced landward movement of the shoreline during the Flandrian transgression. Prior to the Last Glacial Maximum, the beachrock/aeolianite sedimentary sequence was emergent and blanketed by shifting aeolian sands. The Pongola River, which flowed into Lake Sibaya, reworked the unconsolidated sediments on the shelf, and exploited the route of least resistance: along White Sands and Wright Canyon axes. The erosion resulting from fluvial denudation in Wright Canyon has caused this canyon to erode some of the beachrock/aeolianite outcrops which form palaeocoastline episode 2 and entrench the canyon to a deeper level; this eroded the shelf to a distance of 2km offshore. During the Flandrian transgression the unconsolidated sediment cover was eroded, exposing and submerging the beachrock/aeolianite sequence. Flandrian stillstands caused erosional features such as wave-planed terraces, potholes, and gullies to be incised into beachrock and aeolianite outcrops; these are seen at present depths of -47m, -32m, .26m, -22m, -17m to -15m, and -12m. High energy sediment transfers, in an onshore direction, resulted in the deposition of sand bars across the outlet of Lake Slbaya's estuary and the development of a 130m + coastal dune barrier on a pre-existlng, remnant Plelstocene dune stub. Sea-level stabilised at its present level 7 000-6 000 years B.P. and coral reef growth on the beachrock/aeolianite outcrops probably started at 5 000 years B.P. A minimum age for the formation of the northern Zululand coral reefs has been established at 3780 ± 60 years B.P. A mid Holocene transgression relating to the Climatic Optimum deposited a + 2m raised beach rock sequence. This transgression eroded the coastal dune barrier and caused a landward shoreline translation of approximately 40m. A minor transgression such as this can be used as a model for coastal erosion which will result from the predicted 1.5m rise in sea-level over the next century. This rise in sea-level could result in a 30m landward coastline translation of the present coastline, ignoring the influence that storms and cyclones will have on the coastline configuration.