Facies and Depositional Environment of the Dupitila Formation, Dupitila Hill Range, Jaintiapur, Sylhet, Bangladesh

摘要

The Mio-Pliocene Dupitila Formation is exposed in its type area in a sub-latitudinal hill range known as Dupiiila Hill Range, located at the foothills of the Shillong plateau within Jaintiapur Police Station, Sylhet, Bangladesh. These sediments are exposed at the southern margin of the homoclinal fold that is separated from the Shillong plateau by the Dauki fault. Lithostratigraphically the Dupitila sediments can be conveniently divided into a sand dominant lower Dupitila member and clay dominant upper Dupitila member. The sandstones are poorly sorted and strongly fine skewed and show a dominant unimodal distribution with mostly angular to subangular grains. Low ZTR (Zircon, Tourmaline, and Rutilc) index indicates a nearby source. The lower Dupitila sediments are represented by lithofacies Gm, Gt, Gp, St, Sp, Sr, Sh, Ss and Fl which form an association resembling fine gravel mid fan, whereas Fl and Fm with facies P and Ss dominating the upper clay rich member suggesting alluvial plain deposits associated with mid fan (Rust, 1984a). This facies association is also similar to medial to distal facies association of alluvial fan of Walker (1984) and McGowen and Groat (1971). The stratigraphic cross section of this formation has a convex up profile that resembles alluvial fan profile of Blair and McPhcrson (1994). The radial length of this alluvial fan is assumed to be 12km and the plunges of channel axes are relatively high (1Q°-30°) towards south. Palaeoflow expansion angle distinctly varies from 120° to 248° making a variance angle of 128° which may be upto 180° at the maximum. Cross profile relief of lower Dupitila fan is 292 metres at its central part and that of upper Dupitila is 240 metres. Radial slope value ranges from 16° to 20°. An alluvial fan environment during the Dupitila sedimentation (Late Miocene to Middle Pliocene) at the southern piedmont slope of the Shillong plateau is suggested.