Journal of Coastal Research
SI 56
594 - 598
ICS2009 (Proceedings)
Portugal
ISSN 0749-0258
Late Holocene Beach Evolution: Sediment Starvation under a Falling
Sea Level
J.A.G. Cooper † D.W.T. Jackson† and J.T. Kelley∞
† ∞School of Environmental Sciences
University of Ulster, Coleraine
BT52 1SA, Northern Ireland, UK
[email protected]
∞ Earth Sciences, University of Maine,
Bryand Global Sciences, Orono
ME 04469-5790, USA
[email protected]
ABSTRACT
COOPER, J.A.G JACKSON, D.W.T. and KELLEY, J.T. 2009. Late Holocene beach evolution: sediment starvation
during a falling sea level. Journal of Coastal Research, SI 56 (Proceedings of the 10th International Coastal
Symposium), 594 – 598. Lisbon, Portugal, ISSN 0749-0258
On the paraglacial coast of Northern Ireland the late Holocene sea level history involves a rise from a lowstand
at 13ka to cross the present level by around 7kaBP reaching a few metres above present by 5ka BP and a
subsequent fall to present sea level. ‘Raised’ or ‘stranded’ beaches associated with the late Holocene highstand
are distributed widely around the Northern Ireland Coast but have received little geomorphological or
sedimentological study. In this paper we report on investigations at Millin Bay on the Ards Peninsula where a
raised beach is exposed in a 3m-high scarp at the rear of the modern beach. Sedimentological analysis, coupled
with GPR investigations provide insights into the development and preservation of the raised beach. A
compressed basal peat on which the beach rests, represents a former back-barrier lagoon developed on poorly
drained underlying glacial clay. The raised beach itself comprises sand and gravel with occasional shell
concentrations, similar to modern beaches in the locality and is topped by a relict aeolian dune deposit.
However, the late Holocene beach is over 3m thick and over 50m wide whereas the contemporary beach
comprises a thin (typically< 30cm) veneer overlying bedrock. This points to an abundance of sediment in the
late Holocene compared to the modern sediment-starved condition. We attribute the reduction in sediment
supply to stranding of sediment sources (eroding glacial bluffs) as sea level fell. The implications for
environmental change during future sea level rise are discussed.
ADDITIONAL INDEX WORDS: Forced regression, raised beach, isostatic uplift, sand supply
INTRODUCTION
Beach response to sea-level change has received much attention in
the literature (DEAN and MAURMEYER, 1983; COWELL et al., 1995;
COOPER and PILKEY, 2004) With an ongoing eustatic rise in sea
level, prediction of the future behaviour and morphology of
beaches is of particular importance for planning and management
(BIRD, 1993). Typical scenarios for beaches backed by sea walls
or cliffs are that they will narrow and even disappear through
coastal squeeze. Elsewhere, beaches are likely to migrate upward
and landward, although the nature of these changes is highly sitespecific (LIST et al., 1987; COOPER and PILKEY, 2004). Several
parts of the world have experienced higher than present sea level
during the late Holocene (PIRAZZOLI, 1991). These fall into two
main categories. The first includes the temperate and subtropical
southern hemisphere coasts of South America, Africa and
Australia as well as archipelagoes of the south Atlantic, and IndoPacific region where evidence for late Holocene high sea level is
widespread. The second area is more restricted but includes the
paraglacial areas of the northern hemisphere (northern coasts of
Europe and North America) where isostatic uplift after
deglaciation temporarily outstripped eustatic sea level rise, leading
to now-emergent shorelines. In such regions, the late Holocene
was characterized by minor regression and in some instances,
associated shoreline progradation or stranding of raised shorelines.
Ongoing and future sea-level rise in these areas is likely to bring
sea level close to the former shorelines. This is in marked contrast
to the areas of the world that have not experienced continuing
transgression during the Holocene.
In this paper we examine the changes in beach morphology at a
site in Northern Ireland (Figure 1) from the Mid-Holocene to the
present in relation to sea level change during that period and
speculate on the likely scenarios for beach development under
future rising sea level.
Figure 1. Location of study area at Millin Bay on the outer
Ards Peninsula, Northern Ireland
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Late Holocene Beach Evolution
STUDY AREA
The formerly glaciated coast of Northern Ireland has
experienced a complex sea level history during and since the last
deglaciation as a result of the interplay between glacio-isostatic
loading and offloading and eustatic sea level change (CARTER,
1982; KELLEY et al., 2006; MCCABE et al., 2007; ORFORD et al.,
2003). Of particular importance to this paper is a late Holocene
sea level highstand of a few (3-5) metres above present at ca 7000
BP, that was followed by a fall to the present level. Recent tide
gauge records show that while sea level has been essentially stable
during the past 50 years (Orford et al., 2006), the past 15 years or
so show a distinct upward trend. This latter trend, while covering
only a short period and therefore not being statistically significant,
is interpreted as an indication that eustatic sea level rise might
now be more rapid than rates of isostatic uplift (during the
historical period the two had been essentially equal, creating
conditions of no relative sea level change). Projections of future
sea level scenarios (ORFORD et al., 2007) suggest a rise in sea
level in Northern Ireland consistent with global eustatic trends as
the eustatic signal begins to dominate the isostatic.
The coast of Northern Ireland is largely composed of bedrock
of a wide variety of lithologies. There are some local outcrops of
unconsolidated Quaternary sediment, particularly in the southeast
on the Irish Sea coast. There, sand beaches are normally confined
to coastal re-entrants between rock headlands, where mobile
sediment (sand and gravel) accumulates under contemporary wave
action.
The study area, Millin Bay on the Ards Peninsula (Figure 1) is
typical of much of the east coast of Northern Ireland. There, a
650m-wide embayment flanked by low rocky headlands contains a
fine sand beach that is up to 100m wide at low tide. The sand,
however, forms only a thin veneer (typically <0.5m thick) that
overlies folded and faulted Palaeozoic bedrock. Occasional
bedrock outcrops protrude through the thin and discontinuous
sand cover (Figure 2).
The contemporary coast is sediment starved, since there is little
supply of sand from relict offshore sediments, little active erosion
of soft cliff lithologies landward of beaches, and minimal fluvial
sediment input from the low, vegetated hinterland. Beaches along
the coast have developed a swash-aligned planform and longshore
sediment transport between adjacent embayments is believed to be
minimal.
Figure 2. The contemporary beach at Millin Bay. This consists of a thin intertidal sand veneer overlying bedrock which occasionally
protrudes through the sand cover. The sediment volume is small and the sediment supply is negligible.
constructed on the ridge and this, plus worked early Neolithic
flints recovered from the dune sediments surface indicates that it
MILIN BAY RIDGE
had been deposited by early Neolithic times. Although it is steep,
At the rear of the contemporary beach is a vegetated steep scarp
the scarp is well vegetated and map evidence reveals it to have
cut into a ridge composed of unconsolidated sand and gravel
been stable in historical times (at least since 1836). The face is
deposits (Figure 3). This deposit extends as a continuous ridge
partly lithified in places, which further attests to its stability.
along the backshore of Millin Bay and its crest reaches a
maximum elevation of 12m above MSL.
An important
archaeological site (a Neolithic Mortuary Monument), was
Journal of Coastal Research, Special Issue 56, 2009
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Cooper et al.
Figure 3. The seaward limit of the raised beach deposit exposed
in a scarp at the back of the contemporary beach at Millin Bay.
Figure 5. Thin peat outcrop on the foreshore of Millin Bay,
directly underlying the sand and gravel deposit.
Agricultural activity in the fields that cover the ridge surface
show its upper layers to be composed of fine sand that is notably
free of pebbles. This fine sandy sediment is also exposed at the top
of the scarp where it overlies coarser sand and gravel. Exposures
in the scarp show the sediments to comprise alternating medium to
coarse sand and gravel in variable proportions (Figure 4A and B).
The units are quite well bedded with marked textural variability
between adjacent beds. Within the sediment sequence are
occasional periwinkle and limpet shells that reveal the marine
nature of the deposit.
At its base, the sand and gravel unit overlies a thin peat deposit
that is exposed on the modern intertidal beach (Figure 5). The
peat contains well-preserved plant remains including leaves and
stems and rests on a thin impermeable clay layer which probably
impeded drainage and prompted wetland development.
The full sedimentary sequence comprises a basal peat,
overlain by up to 4m of interbedded sand and gravel containing
transported littoral mollusc shells. This is capped by up to 3m of
fine grained, well-sorted sand
Preliminary investigations were conducted using GPR to
assess the internal structure of the ridge. A shore-normal profile
(Figure 6) shows a basal reflector overlain by a transparent unit
that coincides with the gravel and sand unit exposed in outcrop.
This is overlain by a distinctive unit corresponding to fine sand
in outcrop that is revealed by GPR to contain steeply dipping
foresets.
Figure 4. Detail of the sand and gravel deposits that constitute the
Millin Bay ridge. A. (above) shows a sand-dominated section with
subordinate gravel layers. B (below) shows a gravel-dominated
section.
Figure 6. Shore-normal ground penetrating radar profile through
the ridge. Seaward is to right. The sequence is interpreted by
comparison with the exposed scarp as a sequence of sand and
gravel (beach) overlain by fine sand (dune) which at its distal end
overlies possible back-barrier deposits. The dashed line probably
represents a water table.
Journal of Coastal Research, Special Issue 56, 2009
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Late Holocene Beach Evolution
this relict cliff once formed a sediment source for the development
of the large ridge at Millin Bay.
The fall in sea level that stranded the beach ridge, also stranded
the cliffline (Figure 8) and cut off the sediment supply. The cliff
is now fronted by a wide supratidal rocky platform and protected
from wave action. Consequently, the modern beach has a limited
sand volume and forms only a veneer on bedrock.
Any future rise in sea level will have important
geomorphological implications at this site and others like it. A
rise in sea level would enable waves to reach to base of the ridge
and the former cliffline once again. The ridge in particular
contains a large volume of sediments that are suitable for beach
building and the future scenario for such an area would be that the
beach would become more sediment-rich through reworking of the
raised beach deposits and their incorporation into the
contemporary beach (providing no coastal defence structures are
constructed).
A similar scenario is likely to exist at many sites in the southern
hemisphere and paraglacial regions of the northern hemisphere
where a late Holocene sea-level highstand existed. Millin Bay is a
particularly good example because of the stranding of the
Holocene beach deposit and lack of any ongoing sediment supply.
Figure 7. The geomorphological setting of Millin Bay, showing a
relict scarp cut in Quaternary sediments south of the bay and the
ridge crest (dotted line). It is proposed that a fall in sea level cut
off the sediment supply and left the ridge stranded and isolated
above contemporary wave action. Consequently, the modern
beach is sediment-starved.
The combination of sedimentology, geomorphology and GPR
data point to the Millin Bay ridge as a coastal barrier that
comprises a lower unit of interbedded medium to coarse sand and
gravel with occasional littoral shells that accumulated as a beach.
This is overlain by a unit of fine, well-sorted sand with steeply
dipping internal bedding which is consistent with a sand dune.
The GPR profile reveals an additional radar facies underlying the
landward margin of the dune facies. This is not exposed at the
surface but its position in a low-lying area toward the rear of the
ridge suggests it may represent back-barrier deposits that may be
the lateral equivalent of the basal peat unit that underlies the beach
facies.
DISCUSSION AND CONCLUSIONS
At Millin Bay, a large coastal barrier is present that comprises
large volumes of sand and gravel deposited as a beach ridge under
conditions of sediment abundance compared to the present
sediment-starved situation. The ridge is interpreted as having
formed during the Holocene transgression and to have migrated
across back-barrier deposits preserved as a peat that is now
exposed on the foreshore. Stabilisation and growth of the ridge
including aeolian deposition at the top of the unit then took place
during conditions of sediment abundance during the late Holocene
sea-level maximum. A reduction in sediment supply clearly
followed the ridge emplacement so that instead of a regressive set
of beach ridges being formed as at neighbouring Dundrum Bay
(Figure 1; ORFORD et al., 2003), the solitary ridge was stranded
above sea level.
To the south of Millin Bay is a raised cliff cut into
unconsolidated Quaternary deposits (Figure 7). The Quaternary
deposits themselves rest upon a bedrock outcrop that now
constitutes the modern shoreline. At the Holocene highstand,
however, this cliff would have been actively eroded by wave
action. It is south (updrift) of Millin Bay and it is proposed that
Figure 8. The relict scarp south of Millin Bay that is now isolated
from wave action and hence inactive since the late Holocene fall
in sea level
LITERATURE CITED
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ACKNOWLEDGMENTS
We thank Lisa Rodgers for drawing the diagrams and Robert
Stewart and Anna Serra for assistance with fieldwork.
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