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The present research work deals with the lithofacies analysis along with the sequence stratigraphy of the Dupi Tila Formation, Lalmai Hills, Comilla to infer the depositional environment and paleogeography. The Dupi Tila Formation consists of alternation of yellow to light brown medium to fine grained sandstones and siltstone, grayish black to bluish gray shale to siltyshale and mudstone with local conglomerate. Different sedimentary facies are genetically grouped into coarse grained facies association (FAC), tide dominated facies association (FAT), heterolithic facies association (FAHL) and shallow marine facies association (FASM). The study of sequence stratigraphy indicates a lowstand systems tract for the basal part, a transgressive systems tract for the middle to lower horizon of upper part and highstand systems tract for the uppermost part. The whole Dupi Tila Formation is a Type-I sequence, as the upper boundary with the Modhupur Clay Formation is erosive and unconformable and lower contact with the underlying Girujan Clay Formation is also erosive and unconformable, though not exposed here. The study of the facies, facies relationship, facies association, paleocurrent analysis and sequence stratigraphy indicates that the depositional environment was extended from shallow marine shore face to intertidal flat through estuarine to tidal channel except the basal part, where deposition was confined to the piedmont alluvial set up of alluvial fan to braid plain. At the latest time, the area was inundated by sea when black shale and black mudstone with Sll facies and glauconite were deposited. Tripura-Sylhet Hills in the east to northeast and Shillong Massif in the north acted as sources of sediment derivation. [ABSTRACT FROM AUTHOR]

The petrographic study of the Dupi Tila Sandstone, Lalmai Hills, Comilla reveals the presence of both the detrital and authigenic (chemical) components. Detrital framework grains are sand-sized particles of either monomineralic (i.e., quartz, feldspar, mica, etc.) or polymineralic, i.e., rock fragments. Quartz includes both the mono- and polycrystalline types. Both orthoclase and microcline are common. Cementing materials are mainly argillaceous and ferruginous. Glauconite is also present. The sandstones are classified as arkosic wackes. The Qt-F-RF plot indicates that the detritus of the studied sandstones are derived from continental block provenance. The sandstones are characterized by high quartz content, low to moderate feldspar content and low rock fragments/lithic grains. The ternary plate of Qt-F-RF suggests that the sandstones mostly (more than 80%) derived from semi-humid climate and lesser amount (less than 20%) came from metamorphic source under the humid and semi-humid climatic conditions. [ABSTRACT FROM AUTHOR]

The Surma group of rocks are well-exposed in the steeper and faulted western flank as well as gentle and wider eastern flank of the asymmetric doubly plunging Sitakund anticline under the Chittagong district from Bariyadhala in the north to Barabkund in the south, which trends regionally in the NNW-SSE directions. Lithostratigraphically, the Bhuban formation consists of alternation yellow to yellowish gray medium to grained sandstone with black shale and siltstone, Boka Bil formation constitutes the medium grained yellow to yellowish gray sandstone-siltstone with bluish gray shale. Eleven facies have been identified with the lithosuccession of this group, which are grouped into turbidite facies association, deep marine basin plain and tide-generated facies association. The sub facies association of tide-dominated delta, estuarine, tidal creek/small estuarine and mud-dominated tidal flat constitute the tide generated facies association. Five parasequences are delineated. The overall depositional environments of the study area vary from deep marine to submarine fan in the Bhuban formation and tidal delta, estuarine channel and tidal flat in the Boka Bil formation at various phases during deposition due to relative sea level rise and fall, sediment supply and basin tectonics. [ABSTRACT FROM AUTHOR]

The Upper Bhuban Member of the Surma Group is well-exposed in the asymmetric doubly plunging Sitakund anticline from Bariyadhala to Barabkund area of the Sitakund upazilla, Chittagong district. The mineralogical and petrographical studies indicate that sandstones were composed of quartz (42.90%), matrix (31.38%), rock fragments (15.62%), mica (4.88%), feldspars (1.97%), heavy minerals (1.81%), cement (1.28%) and trace amounts of unidentified minerals. The sandstones are lithic graywacke in nature and of recycled orogen in provenance type, which were deposited under the humid and subhumid climatic conditions. The XRD analysis indicates the presence of illite, kaolinite, chlorite and glauconite in clay fraction and quartz and mica in non-clay fraction of the shale. The overall clay mineral assemblage and graywacke nature of the sandstone suggest marine depositional environment. [ABSTRACT FROM AUTHOR]