Physiography of Dhemaji District _______________________________
The Brahmaputra Valley is of the nature of a “ramp” valley developed during the simultaneous upheaval of the Himalayas on the North and North East and the Patkai ranges on the South and South East. The region is prone to earthquake and shocks of various intensities are felt from time to time. The great earthquake of 1950 brought a trail of devastation causing loss to both life and property. The frequency of earthquakes in Assam is closely related to the geology of the region.
The geological setting of Arunachal Himalaya and evolution of Tertiary Himalaya:
Tectonically the Himalayan extension of northeast India is being divided into Himalayan mountain ranges in the north and the Arakan Yoma range in the east. The mountain belts merge together at its eastern extremity through an arcuate bend (syntaxial bend) where the NE-SW trend of the Himalaya seem to have ridden over the NNE-SSW trending Arakan Yoma orogen. The northern limit of the Himalaya is marked by a lineament along the westerly flowing Upper Indus and easterly flowing Tsangpo (Upper Brahmaputra). This lineament is known as Indus Suture. The Outer Himalaya, also called as Foredeep Folded Belt or Siwalik Range, mostly and extensively covers the western extremity of the range, but appears as a narrow strip on the Southern Arunachal Pradesh. This Foredeep Folded Belt in the West of Arakan Yoma Range comprises the low-lying hills of Mizoram, Tripura and Manipur and comprises of Neogene sediments. The Indus Suture Zone is highly deformed and is characterized by the presence of nearly vertical thrust faults. According to Plate Tectonic theory, the Indus Suture Zone constitutes the subduction zone along which the Indian Plate collides with the Tibetan Plate giving rise to the formation of the Himalayan mountains.
About 70 million years ago, there was a sea in the North of Peninsular India which connected the Arabian Sea and the Bay of Bengal across the northern parts of the subcontinent. The sea started to recede towards the west and east during the early Eocene. This trend of marine regression continued till the final emergence of the Himalayan Mountains by the end of the Tertiary Period. The rise of the Himalayas was accomplished in a series of five or more impulses, intervened by intervals of comparative quiescence. The collision of the Indian Plate with the Asian Plate retarded the pace of northerly drift of the Indian Plate. The drift direction slightly changed. The Indian Plate began its rotational movement, giving rise to the formation of the syntaxial bend at the northeastern extremity of the Himalayan Mountains. The Arakan Mountains in eastern India and their continuation into the Andaman Nicobar Island were also formed during the Tertiary diastrophism. Six pulses of diastrophic movement having a widespread impact on the sedimentation pattern in the Assam Arakan basin have been recognized.
From a tectonic point of view, Assam-Arakan basin is classified as Foreland basin. The Assam-Arakan basin is located between two thrust belts. The Tertiary sediments of the Upper Assam belong to two distinct depositional facies – a relatively shallow water shelf facies and a deep-water geosynclinal facies. Between the Himalayas and the Naga-Disang thrust complex occurs an autochthonous zone, the Foreland Spar containing sediments ranging in age from Eocene to Pliestocene. These are intersected by a number of gravity faults. Further east in the Patkai Range Tertiary sedimentation took place under deeper water conditions punctuated by slight emergence at some places. The sea receded southwards towards the end of the Oligocene. Miocene sedimentation took place mostly under fresh water to brackish conditions. Oligo-Miocene was also the time when the deposits of Assam-Arakan basin were overthrusted towards the northwest over the northeastern extension of the Indian Shield. The outermost of this thrust, the Naga thrust belt consist of a succession of six thrust sheets (Evans, 1932; Berger et al, 1983). The whole discussion leads to the conclusion that the whole of northeast India is a tectonically active zone due to presence of active thrusts, fault planes and very fragile loose sediments, steep slope angle leading to frequent landslides in the hilly areas. The courses of rivers are also influenced by these active tectonic lineaments.
The district is in a strategic location where steep slope of Eastern Himalayas abruptly drop forming a narrow valley, which widens towards the western side. Numerous drainage systems originating from the hills of Arunachal Pradesh flow through this narrow valley ending at the mighty river Brahmaputra. In general the slope of the triangular district drops from northern and eastern corners towards south and western sides. After the confluence the three mighty rivers i.e. Dihing, Dibang and Lohit from their hilly course to the valley exert tremendous impact of peak runoff at the eastern most corner of Dhemaji district, making the district vulnerable to annual flooding. After the great earthquake in 1950 the Brahmaputra riverbed is rising continuously due to deposition of sand carried down from upstream. This has led to the formation of a saucer shaped low-lying zone in the plains of the district.
Physiographically, the area can broadly be divided into three district units:
· Piedmont zone: The foothill zone near the northern and eastern parts adjacent to Arunachal Pradesh
· Active flood plain: Near the river Bramhaputra and other major tributaries.
· Low-lying alluvial belt: Covering the middle plain zone i.e. the saucer shaped built up zone. Innumerable beels and swampy areas are common features.
Flood in Dhemaji