Journal of Geological Society of India (Online archive from Vol 1 to Vol 78)

Mrinal Kanti Roy ¹, Md. Abdullah-Al-Mahbub ¹, Partha Jit Roy ¹

Affiliations
1 Department of Geology and Mining, University of Rajshahi, Rajshahi-6205, BD

Abstract

The Mio-Pliocene Tipam Sandstone Formation is well exposed in the Sitapahar Hill Range in Chittagong Hill Tracts, Bangladesh. Stratigraphically the rock sequence along the Chandraghona to Boraichari road-cut section consists of alteration of yellow to brownish yellow, very fine sandstone, siltstones,silty shale and gray to bluish gray shale with occasionally decomposed layers in varying proportion. Two facies associations recognized in the area are (a) medium to fine grained sandy-silty facies association FAS and (b) the silt-clay facies association or heterolithic facies association, FAHL. The facies characteristics suggest that the deposition was restricted within estuary to tidal channels in FAS and very shallow water condition as well as mainly tidal flat to tidal marsh environment in FAH. The dominantly bi-directional and bi-polar paleocurrent charactenstics strongly support presence of tidaI action in the Tipam Sandstone Formation in the study area.

Keywords

Sedimentary Facies and Paleoenvironments, Tipam Sandstone Formation, Sitapahar Chittagong Hill Tracts, Bangladesh.

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Partha Jit Roy ¹, Mrinal Kanti Roy ¹, Debasish Shome ², Md. Abdullah-Al-Mahbub ¹, Dhiman Kumar Roy ¹, Sudip Saha ¹

Affiliations
1 Department of Geology and Mining, University of Rajshahi, Rajshahi-6205, BD
2 Department of Geological Sciences, Jadavpur University, Kolkata, IN

Abstract

The rock sequences of the Sylhet Limestone Formation, Dauki River section, Gwainghat, Sylhet were studied to identify their various petrological units, non-clastic minerals and fossil content. Limestones are of two varieties crystalline in lower horizon and fossiliferous in upper horizon. The various petrological units identified in the non-clastic portion of the Sylhet Limestone Formation include micrites, dismicrites, intramicrites, pelrmicrites, spantes, intrasparites, pelsparites, and oosparites. Calcite, dolomite and fossils with minor amount of clay, mica, chert, quartz, zircon, tourmaline, rutile, and opaque minerals constitute the limestones. Fossiliferous limestone consists of smaller and larger foraminiferal genera, such as Nummulites, Discocyclina Alveolina, Operculina, Assilina etc. with some Bryozoans and Brachiopod shells. The various petrological units mght have deposited in shallow marine environment with nonreducing as well as reducing condition by tidal, winnowing and non-agitating currents. Algal growth aided the various units in formation.

Keywords

Sylhet Limestone Formation, Dauki River, Gwainghat,Northeastern Sylhet, Bangladesh.

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