ネパール中央西部,アルン川地域のチュリア(シワリク)層群

The uplift and erosion of the Himalayas has resulted in a vast pile of terrestrial sediments called the Siwalik Group deposited at the frontal deep of the mountains. The Churia (Siwalik) Group of the Arung Khola area in the Churia Hills was studied in 1982, 1984 and 1985. The Group is bounded by the Main Boundary Thrust in the north and Frontal Churia Thurst in the south. It consists of a north and south belt separated by the Central Churia Thrust. The group is divided into the Arung Khola Formation (2,100 m in thickness), Binai Khola Formation (2,800 m), Chitwan Formation (700 m) and Deorali Formation (450 m) in ascending order. As a whole, the group comprises a coarsening-upward sequence, reflecting the rising Himalayas. The paleomagnetic polarity stratigraphy of the Churia Group has been successfully established.Detailed mapping and sampling made it possible to draw a paleomagnetic iso-polarity zone map. Magnetostratigraphically the Al and Am Members of the Arung Khola Formation are correlated to Chron 10-15, the Au Member of the Formation to around Chron 9, the Binai Khola Formation to Chron 8-Gilbert Reversed Polarity Chron, and the Chitwan and Deorali Formations to Matuyama Reversed Polarity Chron. Lithostratrgraphic nomenclature of "Lower, Middle and Upper Siwaliks" is different chronologically between the Siwalik Group of the type area in Pakistan and the Churia Group in Nepal. Sedimentation rate is calculated to be between 0.2 to 0.5 mm/yr in the Churia Group. Sedimentary studies have clarified the development of fluvial fan-river systems and their hinterlands. The Great Himalayas began their bigorous upheaval movements at about 10 Ma, which is reflected well in frequent intercalations of "Pepper-and-Salt Sandstones". Many freshwater molluscs have been discovered in the present area and were identified to the generic level. Most geologic structures of the present area are still active now, and are reflected well by geomorphologic features. Syn-sedimentary tectonic movement or syn-tectonic sedimentation characterises the Sub-Himalayan foothills, which may be interpreted by underthrusting of the Sub-Himalayan terrane against the Lesser Himalayan terrane .