Geomorphic and Sedimentological Evolution of Khagrachari-Mahalchari hills of Bangladesh

Khagrachari and Mohalchari Upazilas (sub-district) of the Khagrachari hill district are located and characterized by a series of parallel hills that tend to be NNW-SSE, situated in the central part of the eastern fold belt of Bangladesh. The central part of the hills features a series of medium high mountains, including the mysterious Alutila cave of Matiranga area of Khagrachari A cave formed in an around 1000 m high hill named as Alutila (potato hill) or Arbari hill. A vast network of streams and valleys flows down the slopes of the hills and represents a well-developed drainage system in the study area. The drainage system in the study area possess dendritic pattern. The Chengi River of the study area is running almost E-W trend cutting across the hill ranges of Khagracahri. There are also a large number of streams (chara or chari in local term) and large lake named ‘Mataipukhiri (Debota Pukur)’ which was naturally formed above 491 msl. The surficial deposit in the study area has been divided into: Dihing Formation, Undivided Dihing and Dupitila Formation, Dupitila Formation, Girujan Clay Formation, Tipam Sandstone Formation, Bokabil Formation and Bhuban Formation. According to laboratory analysis, Dihing sandstone is composed of very fine to medium-sized sand and is poorly sorted. Dupitila sandstone is composed of medium to fine sand and is moderately sorted. Tipam sandstone layer consisting of fine sand. Bokabil sandstones range from fine sands to very fine sand. Various types of sedimentary structures have been observed for example, cross-stratification, cross-lamination, wavy stratification, clay gall, calculus and nodular structure. The concentration of heavy minerals in the sedimentary rocks of the Khagrachari and Mohalchari areas are not significant. However, the heavy mineral concentration gradually increases from the Ding Formation (1.74 to 2.14)%, the Dupitila Formation (1.81 to 2.67)%, the Tipam Formation (1.55 to 3.92)%, and the Bokabil Formation (3.58 to 4.01)%. The identified minerals are quartz, muscovite, biotite, augite, hornblende, garnet, epidote, rutile, tourmaline, actinolite, tremolite, zircon, monazite, kyanite, sillimanite, apatite and other opaque minerals. The analyzed samples have very high abundance of augite, hornblende, and kyanite. The presence of white clay and glass sand has been investigated at Manikchari area in the Kagrachari hill area. The results of the chemical analysis show the percentages of SiO₂ (63.4-82.0)%, Al₂O₃(5.78-18.51)%, Fe₂O₃(0.45-17.93)%, Na₂O (0.160.80)%, K₂O (0.403.24)%, MnO is (0.162)%, CaO is (0.03-0.93)%, and ignition loss is (1.60-7.58)%. The existence of quartz sand (silica enriched) and white clay had also been confirmed adjacent to the surveyed area. Investigations of lithology, minor sedimentary structures and grain size parameters infers that the overall paleoenvironment was a transition from shallow waters condition prevailed during the early sedimentation phase of the area.