We explored the recent cementation of modern beachrock on the seaward margin of the Durban Bluff, central KwaZulu-Natal. The low latitude and
subtropical climatic setting is a unique context compared to the more commonly documented contemporary beachrock formation in the tropics.
Geological field mapping was carried out and here we present results based on sedimentary facies of a clastic shoreline and carbonate diagenesis
of interstitial cements using transmitted light microscopy. The beachrock was cemented by micrite and aragonite, and iron oxide infilled voids. The
presence of human artefacts within the deposit showed evidence for cementation within the last century. The elevation (at Mean Low Water) and
correlation to rates of sea level change for the east coast of South Africa showed that the beachrock is less than 72 years in age. In contrast to
older local Pleistocene deposits, beachrocks have cemented along this stretch of coast during successive sea level highstands with similar climatic
regimes – the last Interglacial, the Holocene High and the present. Here we report the most southerly documentation of modern beachrock in
KwaZulu-Natal, which, to our knowledge, represents the youngest deposit reported in southern Africa.
Beachrocks are consolidated coastal sedimentary formations, which consist of beach material that is bonded together relatively rapidly
by in-situ precipitated carbonate cements (calcite and/or aragonite),1,2 and are
commonly found along warm equatorial-tropical
coasts.3,4,5,6 Constituent particles include clastic,
biogenic and authigenous sands and gravels, as well as human artefacts at some
Although lithification occurs in the intertidal and/or supratidal zone, either on the beach surface or beneath a thin veneer of unconsolidated
sediment,7,8 the significance of beachrock as a reliable sea level indicator has been
questioned.9 In addition, beachrock
formation and resulting outcrops have a significant impact on beach morphodynamics, altering longshore and cross-shore sediment transport and
Although carbonate cemented Pleistocene beachrocks are common along the South African
modern beachrocks are
rare.17 Here we describe and report the occurrence of a modern beachrock formation at present Mean Low Water level.
This report is the
most southerly documentation of modern beachrock in central KwaZulu-Natal and, to our knowledge, represents the youngest deposit reported in
southern Africa. The beachrocks have developed along a coastline that comprises older Pleistocene and Holocene carbonate cemented beachrocks and
aeolianites, which form the seaward margin of the Bluff extending along a 16-km stretch from Durban Harbour to
(Figure 1). The area lies at 29°52’S and experiences a subtropical climate with warm wet humid summers and dry moderate
winters.18 The coastline is a high-energy, wave-dominated microtidal or low mesotidal
system10 with a mean spring tidal range of 1.72 m and a mean neap tidal range of 0.5 m.
(a) Regional context of the locality of the study area. (b) Enlarged area of interest in the vicinity of Durban. The study area lies on the seaward
margin of the Durban Bluff, central KwaZulu-Natal. The Bluff Ridge, Isipingo and the whaling station are indicated.
The modern beachrock has developed seaward of a pipeline support structure that formed part of whaling station operations between 1908 and 1975 and
extends seaward of an embayment of a ‘Type 2’ intertidal platform19 of older beachrock (Figure 2).
The exposure has a maximum
coast-parallel extent of 40 m that extends seaward for approximately 6 m and is estimated to be no more than 30 cm thick. The site is only exposed
at spring low tides and is normally submerged and covered by unconsolidated beach sand. The rocks have developed within the breaker to swash zone
at the foreshore–shoreface transition, corresponding to the Mean Low Water level.
(a) Photograph of the whaling station on the seaward flank of the Bluff sometime between 1930 and 1950 (courtesy of W. Kidwell). The view is from
the top of
the Bluff Ridge, towards the south-east. Whale oil storage drums are in the foreground, the administration building for the Union Whaling Company
lies adjacent to the
drums and the processing plant forms the white building in the background. The pipeline used for dumping whaling debris is indicated. (b) The
building and supports for the pipeline which remain on the beach. The view is towards the south. (c and d) Modern beachrock containing human
artefacts. The material
consists of harpoon heads, bricks and other objects. The arrows from (e) indicate the position of the artefacts. (e) Geological association of
the modern beachrock deposit
in front of the whaling station with the relatively older unit of shoreline beachrock. (f) A photograph of a World War II grenade, cemented into
the modern beachrock.
The sedimentary facies comprises a poorly sorted, locally pebbly, very coarse to coarse sand. A matrix of quartz, feldspar and bioclastic sediment
supports pebbles of older beachrock and aeolianite as well as lithic fragments of lydianite and sandstone, and most importantly an assortment of
waste from the whaling station and military dump. This waste comprises harpoon heads, bricks and unexploded hand grenades and is linked to the
activities of the whaling station and military disposal (Figure 2). The human artefacts date the deposit to the last century of active whaling
between 1908 and 1975.20
The beachrock cement forms bladed isopachous rims of micrite and prominent aragonite crystals around sediment grains that radiate into voids
(Figures 3b and 3d). Aragonite and micrite are volumetrically the most abundant cement phases and the remaining interstitial porosity of the deposit
appears either empty or is infilled by iron oxide (Figure 3a). Carbonate precipitation on grains commenced with micrite followed by a generation of
acicular aragonite. The regular distribution and isopachous nature of the two cement generations suggests precipitation in a marine phreatic
environment for beachrock formation2 where sea water is conducted into sediments by tides, waves and
currents.21 The regular
orientation and uniform size of rims are the result of simultaneous crystal nucleation at a magnitude of sites on the grain
Inner portions of isopachous rims are more equant in form, enveloped by an outer prismatic variety. The absence of organic structures, such as
microbial filaments, suggests that the beachrock cementation was an inorganic process.
Transmitted light microscopy of the modern cemented beachrock. (a) Iron oxide infilling voids between coarse-grained, rounded to angular quartz
rock fragments. (b) Beachrock cement forming bladed isopachous rims of micrite and prominent aragonite crystals around sediment grains radiating
into voids. These
cements are considered to form early in the diagenetic history. (c) Shell fragments, quartz and lithic fragments with iron oxide precipitated into
pores. (d) Magnified
micrite rims and aragonite crystals.
Two hypotheses are suggested for the recent cementation of this unusual deposit. The first is that modern beachrock is forming along this section
of the central KwaZulu-Natal coastline and is only recognisable at this site because of the presence of human artefacts. The second is that
carbonate precipitation and cementation has occurred at the interface between oxidising runoff that infiltrates through the forebeach from the waste
disposal site and the reducing seawater. The structure of the outcrop, with the erosional embayment on the landward margin, acted as a sediment
trap and allowed the dumped artefacts to accumulate with beach sand from the swash zone. Because iron oxide only infills voids instead of binding
to the grains, it could have only facilitated the cementation, which implies that the beachrock is a true beachrock. In this case, the iron oxide
promoted the release of additional CO2 as described in the Bay of Biscay, Spain.7
The cementation observed, bounded by aragonite and micrite, is as reported for modern beachrocks
elsewhere7,23 and is in contrast to
older Holocene submerged beachrocks mapped offshore of the Bluff24 which are characterised by micrite-only
cements. However, Pleistocene
beachrock deposits along a sea level highstand palaeoshoreline at Isipingo Beach15 display a similar cementation
history to the modern
beachrock. Here, quartz grains of the swash zone facies are fringed by fibrous isopachous calcite cements, which indicate diagenesis in the marine
environment.1,21 These cements are considered to form early in the diagenetic
history,15 originally as aragonite inverting to
calcite after several hundred or thousand years.25 The sedimentology and mode of cementation thus assigns the
modern beachrock described here to an environment of deposition within the swash zone.
Regional significance and applicability to local sea level change
Although the relationship of beachrock to sea level is not suitably resolved, and the potential use of beachrock as a reliable sea level indicator
remains controversial,9,26 beachrock can be used to establish former beach
It is clear that the modern beachrock described here was deposited and cemented within the last century, was formed in the swash zone and is now
at Mean Low Water. The tidal framework can be loosely applied to further constrain the age of the modern beachrock. Contemporary beachrock is
thought to form at an elevation of 0.1 m – 0.2 m above Mean Low Water.27,28
The rate of sea level rise of
+2.74 mm/year29 reported for the South African east coast suggests that the deposit may have formed 36 to 72 years
before the present.
This estimated age range falls within the time frame of whaling on the Bluff, post-dates World War II and is consistent with the distribution of
the in-situ human artefacts observed.
Although the described modern beachrock presents an unusual locality, the sedimentary structures, facies type and carbonate cements are comparable
to those recorded in tropical areas and to the Pleistocene of Isipingo Beach. The steep swash zone, coarse grain size and similarity to the modern
beach system suggest the deposition occurred on a high-energy, wave-dominated coastline equivalent to the modern intertidal zone environment.
The deposit is of significance because of the timing of diagenesis and the mode of cementation. The cementation is similar to the Pleistocene
beachrocks at Isipingo Beach,15 implying that beachrocks formed along this stretch of coast during successive sea
level highstands with
similar climatic regimes (the last Interglacial and the present).
The presence of modern artefacts cemented into the deposit provides evidence for the cementation occurring within the last century, and indicates
that the beachrock described here is the youngest catalogued unit in southern Africa, considerably younger than that described from Vilancoulos,
Mozambique which is aged at 920–910 BP.13
Although the validity of beachrock as a reliable sea level indicator has been questioned, in this case the data available has allowed a broad
correlation to sea level change during the last ~72 years. The migration of the Mean Low Water mark to the position previously occupied by the
swash zone may attest to facies stacking associated with a transgressional regime and the use of these clastic shoreline facies as reliable sea
level indicators in this area. Our data are in agreement with the east coast rate of sea level rise.29
This work was funded by the Council for Geoscience Statutory Programme. We thank Mr Wade Kidwell for sharing his knowledge on whaling in Durban and
for permission to reproduce the photograph presented in Figure 2a.
We declare that we have no financial or personal relationships which may have inappropriately influenced us in writing this paper.
This work formed part of H.C.’s MSc which was carried out through the Council for Geoscience Statutory Programme and the University of
KwaZulu-Natal. R.U. was the academic supervisor of this project. H.C. mapped the deposit and subsequently H.C. and R.U. visited the site for
sampling. H.C. and R.U. wrote the manuscript.
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