Emplacement and Evolution of Debris Deposits on Coastal
Platforms, Jamaica
E. ROBINSON, S. A. KHAN, D-A. C. ROWE1, R. COUTOU, M. JOHNSON
Marine Geology Unit, U. of the West Indies, Jamaica, mgu@uwimona.edu.jm
1
Dept. of Geography & Development Studies, U. of Chester, UK, d.rowe@chester.ac.uk
Holocene debris deposits occur on a
number of raised Pleistocene limestone
terraces along the Jamaican coastline, at
elevations up to 12 metres above sea-level
and typically where the island shelf is
narrow. Three main kinds of deposit are
found. Towards the rear of the platforms
occur ridges of unsorted carbonate sand,
coral debris and boulders. Nearer the
shoreline perched beaches of modern coral
rubble may be found. Large isolated
boulders composed of the platform
limestone are frequently strewn over the
platform surface. Sand-sized material also
occurs on the platform as a discontinuous
veneer, mainly in depressions in the
phytokarst, or as perched beaches.
Except where there has been human
interference, the ridges define the contact
between the forested interior and the
karstified platform with sparse, low
vegetation, and they are themselves
vegetated. The perched coral beaches are
well-developed, with imbrication, only on
low (1-2 metre high) platforms. On higher
platforms coral debris occurs but is
scattered. The isolated boulders vary in
size, but at any one location the larger
boulders are usually closer to the cliff
edge, so that the debris ridges contain
more of the relatively small blocks. There
is a strip of the platform, adjacent to the
cliff edge that is always free of debris and
vegetation, except for a few boulders.
We have data on movements of many
boulders during recent hurricanes. These
include displacement of 2 m of an 80
tonne boulder, located 55 m from a cliff
edge 12 metres above current sea-level,
and emplacement onshore of boulders up
to 25 tonnes on 4 m high platforms during
the passage of hurricanes Ivan (2004) and
Dean (2007).
We conclude that the ridges result from
the progressive emplacement and movement, throughout the past four to five
millennia, of storm and/ or tsunami
generated debris over the platform into
zones where the energy level of the
impacting waves or surge is reduced and
forest cover impedes further transport.
Here clasts accumulate, evolving
into
ridges that increasingly provide a barrier
to further incursion of debris inland. The
large, isolated boulders are torn off the
platform bedrock and are either in-transit
towards the ridge deposits, or following
initial emplacement on the platform, are
too big to be moved by subsequent waveforming events. In the latter case initial
emplacement may be from tsunami. The
perched beach deposits are essentially
ephemeral, formed during the passage of
storms, and modified or destroyed by later
wave events.
For coastal management practice, the
position of the debris ridge is critical,
separating the vulnerable, seaward part of
the platform, from the sheltered area
behind the ridge (ROWE et al. in press).
Destruction of the ridge during
development projects may increase
vulnerability to storm impacts of those
areas originally protected by the ridge.
Literature
ROWE, D.-A.C., KHAN, S.A. & ROBINSON, E. (in
press). Hurricanes or tsunami? Comparative analysis of extensive boulder
arrays along the southwest and north
coasts of Jamaica: implications for coastal
management. Chapter in MACGREGOR, D.,
DODMAN, D., BARKER, D. (eds) Global
Change
&
Caribbean
Vulnerability:
Environment, Economy & Society at Risk?
UWI Press, Kingston, Jamaica.