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Cooper, et al.
Vertical stacking of multiple highstand shoreline deposits from the
Cretaceous to the present: facies development and preservation
J.A.G. Cooper†‡, A.N. Green‡, A.M. Smith‡
†Coastal Research Group, University of
Ulster, Coleraine BT52 1SA, Northern
Ireland, UK
[email protected]
‡ Discipline of Geological Sciences,
School of Agricultural, Earth and
Environmental Sciences, University of
KwaZulu-Natal, Westville campus,
Durban, South Africa
www.cerf-jcr.org
ABSTRACT
www.JCRonline.org
Cooper, J.A.G., Green, A.N. and Smith, A.M. 2013. BARDEX II: Vertical stacking of multiple highstand shoreline
deposits from the Cretaceous to the present: facies development and preservation. In: Conley, D.C., Masselink, G.,
Russell, P.E. and O’Hare, T.J. (eds.), Proceedings 12th International Coastal Symposium (Plymouth, England), Journal
of Coastal Research, Special Issue No. 65, pp. 1904-1908, ISSN 0749-0208.
A sequence of vertically stacked shoreline facies exposed by unprecedented water level lowering in Lake St Lucia,
South Africa, records multiple occupation of the same shoreline (5-6m amsl) on at least eight occasions since the late
Cretaceous. The sequence involves a basal wave-cut surface that is the outcrop of a regional unconformity cut into Late
Cretaceous siltstone with occasional borings, representing a hardground (Facies 1). This is succeeded by a limestone
unit indicative of sedimentation in a region of low terrigenous input quite different to today. This commences with a
10cm-thick unit comprising corals and giant clams that colonised the hardground as a shallow reef (Facies 2). The reef
has an erosional upper surface that is overlain by a 30-50cm thick coquina (Facies 3) with characteristic sand-lined
branching burrows, representing a coarse clastic beach unit. This is equated with the Uloa Formation of
Miocene/Pliocene age. This unit has in turn been colonised by a patchy development of a coral reef of a single species,
representing a renewed phase of reef development (Facies 4). The reef and the underlying Facies 3 have been waveplaned and eroded to form an erosional rocky shoreline with small potholes on a shore platform. The potholes are
encrusted with barnacles and oysters to form a distinctive unit (Facies 5). The oysters and barnacles are encrusted
with red algae suggesting a slight subsequent rise in sea level which is also associated with the formation of an
erosional notch and a higher level shore platform with several small erosional gullies (Facies 6). These gullies are in
turn encrusted by thick accumulations of serpulid worm tubes (Facies 7) into which two subsequent notches have been
cut by wave action.
The shorelines preserved represent a succession of sea level highstands within a few metres of the contemporary sea
level since Late Cretaceous times. They survived intervening sea level lowerings and fluvial incision by virtue of their
location on an interfluve between adjacent incised valleys. Early cementation would have also been key to their
preservation. Each shoreline facies was in turn influenced by the antecedent conditions imparted by the preceding
shoreline as well as the contemporaneous conditions of sediment supply, sea level change and the surrounding
palaeogeography. The presence of limestone and the absence of clasts or storm beach deposits suggests a protected
coastline. The intermittent occurrence of coral and the reduced coral assemblage suggests that the water may not always
have been fully marine.
ADDITIONAL INDEX WORDS: Interfluve, coral reef, South Africa, Lake St Lucia
INTRODUCTION
With their high porosity, stacked shoreline sequences in the
geological record are clear targets for hydrocarbon exploration
(e.g. Walker and Eyles, 1988). Some unconsolidated or semiconsolidated Tertiary and Quaternary examples are also
economically important as heavy mineral placers (e.g. Hou et al.,
2008). Deposition and preservation of shoreline sediments within
the highstand systems tract is, however, inhibited by limited
accommodation space at the highstand and preferential fluvial
incision during subsequent base level falls (Catuneanu, 2006),
and/or erosion by ravinement during subsequent transgressions.
Consequently, the preservation of multiple, stacked shoreline units
____________________
DOI: 10.2112/SI65-322.1 received 07 December 2012; accepted 06
March 2013.
© Coastal Education & Research Foundation 2013
at a single location requires particular circumstances. Stacked
shorelines have, however, been reported in the geological record
from a variety of locations. Palmer and Scott (1984) described a
240m-thick Cretaceous sequence of deltaic and strandplain
sediments that accumulated as a result of a gross balance between
subsidence, sea level change and sediment supply. An unusual
series of eight stacked shelf/shoreline sequences (each up to 12m
thick) was described by Walker and Eyles (1988) from the
Cretaceous of Alberta.
In the Tertiary Eucla Basin of South Australia (Hou et al., 2003,
2008), a sequence of highstand shorelines was attributed to
differential vertical movements and high rates of sedimentation
leading to shoreline progradation. In this paper we describe an
occurrence from KwaZulu-Natal, South Africa, of a stacked
shoreline sequence that is less than 2m in vertical extent but which
records eight shoreline and sea/water level positions between 3.4
Journal of Coastal Research, Special Issue No. 65, 2013
Stacked Shoreline Sequence
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and 1 m above present sea level (Botha, 1997, p41) from the
Cretaceous to the present.
ENVIRONMENTAL SETTING
The study area is at Lister’s Point on the western shore of
Lake St Lucia, a coastal lagoon on the East coast of South Africa
(Figure 1). Previous accounts of the coastal plain geomorphology
(Maud, 1968; Orme, 1973; Hobday, 1979) identify a Cretaceous
basement (St Lucia Formation, Wolmarans and Du Preez, 1986.)
overlain by Tertiary and Quaternary sediments. A distinctive
Mio-Pliocene coquina (the Uloa Formation) has been ascribed to
the Miocene (Frankel, 1966) or Miocene-Pliocene (Stapleton,
1977). Distinct coastal barriers are marked by large aeolian dune
accumulations of which the most distinctive are those of the
Nibela Peninsula and the contemporary coastal barrier which is a
composite feature comprising Holocene sands banked against a
pre-existing Quaternary dune. Porat and Botha (2008) presented
the first detailed chronology of dune development on the coastal
plain, identifying post-Cretaceous sediments of Mid-Miocene to
Pliocene age mantled by Quaternary sediments (mostly aeolian
sand). Those authors dated the core of the complex barrier on the
modern coast to MIS 5 and 4 and the Nibela Peninsula ridge to the
middle Pleistocene (MIS 11-7).
Relevant to the present paper is the changing coastal setting at
Listers Point which in Late Cretaceous and Tertiary times was an
open ocean shoreline subject to direct oceanic wave influences.
The Nibela Peninsula formed a barrier to direct wave action since
at least the Middle Pleistocene when its covering aeolian sands
were accumulated, rendering the Listers Point area a low energy
marine embayment thereafter. This was likely also the case
during MIS5.
An incised valley was cut under the northern
section of Lake St Lucia during the LGM (Wright et al., 2000) and
this marked the site of a tidal inlet until about 3000 BP when it
was closed by aeolian deposition following a sea level fall from a
mid-Holocene highstand of +3.5m (Ramsay and Cooper, 2002).
Since closure of that inlet, Listers Point has been a sheltered
lagoonal shoreline far from direct marine influences (the
contemporary tidal inlet is more than 50km distant), but subject to
locally generated windwaves across a maximum fetch of 15km.
The salinity in Lake St Lucia is temporally highly variable as a
result of changes in freshwater and marine inflow and evaporation
and has varied from zero to 110ppm (Whitfield et al., 2006). In
April 2012, a period of prolonged inlet closure and high
evaporation lowered lake water levels to such an extent that
previously submerged sections of the Listers Point geological
exposure were visible. These outcrops are the subject of this
paper. The aim is to describe the newly exposed section, (which
comprises a series of stacked shoreline deposits) and to interpret
Figure 1. The study area at Listers Point is in a large
embayment in the contemporary Lake St Lucia lagoon complex
in eastern South Africa. It is currently isolated from the sea by
Quaternary barrier/dune complexes at Nibela (mid Pleistocene)
and the modern coastal barrier (MIS5-Holocene) (after Orme,
1973).
its depositional conditions and preservation in terms of changes in
sea level, and antecedent conditions.
LISTERS POINT SECTION
A number of facies were identified in the field during a period
of exceptional low waters in Lake St Lucia. These occur over a
shore-parallel distance of 50m on an outcrop that is ca 1m in
thickness and located between +1 and +3.4 m asl.
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Cooper, et al.
Figure 2. Features of the various sedimentary facies at Listers Point. A. The planed upper surface of the Cretaceous siltstone (Cr) is
overlain by a thin coquina (U), tentatively ascribed to the Uloa Formation (Mio-Pliocene). B. Facies 2, comprises corals growing
directly on the eroded upper Cretaceous surface. C. Facies 3, a coquina with abundant pectinid shells and containing lined burrows of
Thalassinoides. D. The upper surface of the coral-encrusted coquina has been eroded and numerous small potholes developed. The
pothole surfaces are themselves encrusted first with barnacles and then red algae. E. A slight lowering of water level has formed a
second erosional surface (marked by where the people are standing) backed by a microcliff. This erosional surface is also present in
Figure 2D and the large gully in that image is linked to this phase of erosion. F. Encrusting serpulid worm growth on the seaward face
of the outcrop. This suggests a change to brackish water conditions. Note the distinct notch cut during a subsequent phase of lower
water level.]
Journal of Coastal Research, Special Issue No. 65, 2013
Stacked Shoreline Sequence
The individual facies are described below and are illustrated in
Figure 2.
Facies 1
The Basal facies (Figure2A) comprises a fossiliferous
grey/black silty sandstone of the Upper Cretaceous St Lucia
Formation (Wolmarans and Du Preez, 1986). It has a planar upper
erosional surface that has been bored by Lithophaga. This surface
forms a regional unconformity that has been mapped offshore
(Green, 2009).
Facies 2
Directly overlying the Cretaceous siltstone is a >10 cm-thick
unit comprising a diverse assemblage of corals including Favia
and Favites that are attached (in growth position) to the underlying
siltstone(Figure 2B). These corals are associated with fairly
common giant clams (Tridachna sp).
Facies 3
Facies 3 lies disconformably on Facies 2. It comprises a loosely
packed but highly cemented coquina with abundant disarticulated
pectinids 3-10 cm in diameter, (tentatively assigned to
Aequipecten uloa), articulated and disarticulated oysters and
subordinate transported coral fragments (~ 2% of the clasts). The
degree of exposure of this unit is variable along strike. Within the
unit are a number of sub-horizontal burrows lined with sand that
are assigned to the Thallasinoides ichnofacies (Fig 2C). Overall,
however, the unit is sand-poor and dominated by allochtonous
shell valves. No lithic fragments were identified.
Facies 4
This facies comprises a thin coral veneer < 20 cm thick growing
on the surface of the underlying coquina. It comprises a
discontinuous and patchy outcrop on the upper surface of Facies 3.
It is unclear whether this is an erosional surface or not. The corals
are all of a single species, suggesting stressed environmental
conditions.
Facies 3 and 4 are truncated by a well-developed erosional
platform (Figure 2D) on which abundant potholes are developed.
Facies 5
This facies comprises an erosional surface and potholes. The
potholes are encrusted with barnacles and oysters that indicate
quiet water conditions after the erosional phase associated with
pothole formation.
Facies 6
The barnacles and oysters of Facies 5 in turn are encrusted by
calcareous red algae (Facies 6), pointing to a change in
environmental conditions. The upper surface of this facies is
truncated on the seaward side by a slightly lower (20cm lower)
erosional platform and associated large erosional embayments that
are 2-3 m in diameter (Figure 2E).
Facies 7
The near- vertical seaward faces of the outcrop have serpulid
worm encrustations (Figure 2F). These encrustations are about 0.5
m in vertical extent and up to 0.4 m thick. They are themselves
encrusted at a high level by oysters and have been notched,
subsequent to their formation.
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DISCUSSION
The sequence preserved appears to record a series of minor sea
level fluctuations and environmental changes. The erosional
surface on the upper Cretaceous siltstones represents a regional
subaerial unconformity produced by regression at the end of the
Cretaceous. This was colonised by a thin veneer of corals at some
time in the Tertiary during renewed transgression, forming in an
environment characterised by low sediment input. The succeeding
coquina (which contains some reworked coral clasts) probably
represents a shoreline or nearshore sequence associated with a fall
in sea level from conditions that promoted reef growth.
Its
similarity to the Uloa Formation (Miocene-Pliocene) prompts
correlation with that unit. Frankel (1966) recorded three units – a
lower ‘nodule bed’ (1m thick) overlain by the ‘Pecten bed’ (5m
thick), and finally a 6m-thick unit of alternating fine and coarse
calcarenites. That outcrop is some distance seaward of the one
described in this paper and it is possible that the Listers Point
Tertiary facies are the lateral facies equivalents of those described
by Frankel. They are much thinner which is consistent with their
implied more coast-proximal position, but are also much lower in
elevation.
Renewed growth of corals on the surface of the coquina records
renewed transgression and reef-forming conditions during low
sediment input. The coral assemblage of Facies 4 is, however,
notably monospecific, suggesting that it formed under more
stressed conditions, perhaps associated with restricted connection
with the open ocean. This in turn suggests that at least the Nibela
barrier (Figure1) (and possibly the main coastal barrier) was in
place and circulation was restricted.
Subsequent modification of the sequence was largely
accomplished by erosion or biological encrustation, pointing to a
scarcity of sediment supply. The upper surface was subject to an
energetic phase of erosion (including pothole formation) at a
higher water level than present. The subsequent encrustation of
the potholes by barnacles and oysters points to subsequent
transgression during which the potholes became less active and
subject to biological colonisation. The encrustation of these
barnacles by red algae points to further transgression as deeper
water encrusting organisms replaced shallower water ones.
However the fauna (particularly the red algae) points to fully
marine conditions at this time. A subsequent reduction in water
level by 10-20 cm is marked by a phase of erosion of the frontal
edge of the platform during which large gullies were formed
and/or enlarged.
A change in conditions to brackish water is suggested by
substantial encrustation of the leading edge of the outcrop by
serpulid worms. These must have grown during a high water level
as their elevation exceeds that of the post-pothole eroded surface.
The final indication of a change in water level is the cessation of
serpulid growth and the formation of an erosional notch in the
mid-point of the serpulid encrustation.
A number of stacked shoreline sequences have been
documented from areas of subsidence and high sedimentation
rates thus leading to shoreline progradation (Palmer and Scott,
1984; Walker and Eyles, 1988; Hou et al., 2008). The sequence
described here is in an area that has been tectonically stable and
has experienced low rates of sedimentation during each highstand
from the Tertiary to the present. It therefore appears to represent a
series of ‘non-accretionary’ transgressive systems tracts (HellandHansen and Martinsen, 1996). This is reflected in the very thin
nature of the individual shoreline sequences (<<1m) compared to
the 12m-thick units described by Walker and Eyles (1988).
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Preservation of this series of shorelines despite several intervening
regressions and prolonged periods of subaerial erosion, as well as
the potential for erosion during highstand conditions is highly
fortuitous.
The limestone dominance indicates little or at least less
terrigenous input than that of today. The absence of boulders
indicates that the environment was probably protected and not
exposed to the open ocean wave climate. We ascribe its
preservation to its presence on an interfluve (see Figure 1), early
diagenesis and development of sheltered conditions through
barrier development during the mid-Pleistocene. The close (1 m)
vertical stacking of the successive shoreline positions alludes to
the inheritance of older shoreline topography in successive stages
of shoreline positioning during transgressive sea level cycles
(Ramsay and Cooper, 2002).
CONCLUSION
A series of shallow marine and shoreline sediments is preserved in
a contemporary lagoonal shoreline setting. The sediments record
several successive transgressions since the Cretaceous and differ
in sedimentology as a result of changing environmental conditions
with each transgression. Their preservation in spite of erosional
processes during transgression, and incision and subaerial
weathering during regression, over several million years is
ascribed to their location on an interfluve, rapid cementation and
development of sheltered lagoonal conditions during the mid to
late Pleistocene.
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