The Sylhet Limestone Formation and Kopili Formation are exposed in the
Gwainghat area, Sylhet Trough of the Bengal Basin.
The Sylhet Limestone Formation consists of crystalline limestone facies and fossiliferous limestone facies. Fossiliferous
limestone overlies the crystalline limestone, which looks like an assemblage of foraminifera
and other invertebrates. The Kopili Formation overlies the Sylhet Limestone
Formation with drowning unconformal contact that was later faulted. Black laminated shale,
red shale (clay), ripple laminated very fine sandstone-siltstone and parallel laminated
very fine sandstone-siltstone are the sedimentary facies of the Kopili Formation.
The facies associations are: the limestone facies association of epeiric sea to
shallow marine environment and the shale facies association of deep marine
environment. Crystalline and fossiliferous limestones are of epeiric sea and shallow marine
shelf deposit respectively. Black to dark green shale and red shale are designated as
deep marine shale of abyssal to bathyl basin plain. The drowning unconformal
contact indicates the shutdown of the carbonate factory of the Sylhet Limestone
Formation following a rise in relative sea level that onsets the deep abyssal plain environment
of the Kopili Formation in the study area within the short temporal and spatial sense.
Evans (1932) introduced the name Kopili Formation of Eocene age for the Upper unit
of the Jaintia Series after the name of the Kopili river in Assam, India. The Kopili
Formation overlies the Sylhet Limestone Formation with sharp and faulted contact (Jurgan, 1986)
and the upper contact is unconformable with the Barail Group of sediments (Renji
Formation) in the study area. Biswas (1961) stated that it would be desirable to designate
additional typical sections of the Kopili Formation from various portions of the Bengal basin as
reference sections representing different environments. The outcrops along the Dauki river and
the Sangram-Tamabil road-cut section (Sona Tila) are the sole surface representative
of the Kopili Formation within Bangladesh. The exposure of dark grey to black shale
with siltstones and red clay (shale) displays merely a minor portion of the entire sequence
and is apparently bounded by faults (Reimann, 1993; and Uddin, 1995). Now it seems to
be drowning unconformable.
The geological field investigation was carried out along the river bank and road-cut
section by compass-clinometer traverse method. Distances were measured by measuring tape
and stepping method. Attitude of beds was measured by compass-clinometer. Channel
width and depth were measured by measuring tape. Photographs of important features were
taken by camera. Grain size, shape and sorting of the sediments were visually estimated
by comparing these with a pocket rockoner developed by the Department of Geological
Sciences, Jadavpur University, India, followed Folk and Ward (1957), and pocket lense.
Lithologies of different stations were studied by naked eye, pocket lenses and using HCl, and all
the information was clearly recorded in the field notebook. The rocks were identified by
the color, texture, and composition following Pettijohn (1984), Walker (1984), and
Reading (1986). The sedimentary structures were recognized following Reineck and Singh
(1980), and Collinson and Thompson, 1982. Fifteen rock samples were collected. Facies and
its variations were recorded in the field, details of which were refined in the laboratory.
Detailed diagrammatic geologic lithologs with facies notation and facies models were
constructed with a view to distinctly delineate the depositional environment.
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