| Coquina Horizon detrital limestone
horizon composed wholly or chiefly of mechanically sorted fossil debris
that experience abrasion and transport before reaching the depositional
site and that is poorly to moderately cemented but not completely indurated.
st
martin's island exposes a sequence of marine sedimentary
rocks ranging in age from Late Miocene (around 10 million years
ago) to Recent. The fossiliferous marine horizon, the Dakshinpara
Formation, is overlain by the Holocene (0.1 million years before
present till today) coquina bed, which is part of St Martin's
Limestone Formation.
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Coquina
Horizon,
St Martin's Island, Cox's Bazar |
With the gradual rise of the sea
level dead shell fragments were thrust toward the shore of
the island by wave action and finally heaped up as a narrow ridge of coquina
horizon along the present southeast border of the island. Coquina bed
is also known as shelly limestone. It is best exposed in a 6.5m cliff
of 166m length along the eastern coast of Dakshinpara. It is composed
entirely of broken and crushed shells of mollusca, ostracoda, foraminifera,
corals held together by a calcareous cement; tiny shells are often found
unbroken, brown with grey weathered surface, massive and cross-bedded,
loose and friable. Many of the shells are similar to those currently found
strewn over the beach. Micropalaeontological investigation of the samples
of coquina reveals the presence of Elphidium crispum, Rotalia
tectoria and Amphistegina radiatavar.
Radiocarbon dating of a 3m notch of the coquina limestone
cliff (photograph) located in the central coast of Dakshinpara, St Martin's
Island, indicates an age of about 450 years at the base and 292 years
at the top. From this coquina cliff the present mean uplifting rate of
the island can be calculated as 19 mm/year.
[Sifatul Quader Chowdhury]
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