PG 4 Sem Geomorphology Majuli
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(Lecture Note) GEOMORPHOLOGY AND GEOMORPGIC EVOLUTION OF MAJULI ISLAND Dr. P. K. Roy Dept. of Geography Bhattadev University INTRODUCTION and it constituted mainly the shelf part of the basin (Das Gupta and Biswas, 2000). The Brahmaputra, considered as one of the Although basins undergoing active tectonic top ten large anabranching mega-rivers of adjustments are not considered suitable for the world (Latrubesse, 2008), is the seventh hydrocarbon prospects (Fielding, 2000), the largest tropical river (Hovius, 1998; Brahmaputra valley, in spite of intense Latrubesse et al., 2005; Tandon and Sinha, seismic activities, has provided excellent 2007) in terms of mean annual discharge hydrocarbon reservoirs. The upper reach of (20,000m3/s in Bangladesh). It passes the Brahmaputra (Fig. 1) trends northeast- through three populous countries, China, southwest extending from the confluence of India and Bangladesh. The mega-river acts three rivers, the Siang, the Dibang and the as a conduit for transporting a very high Lohit, to the stretch adjacent to the Mikir sediment flux (852.4 t/km2 year in Hills. One of the most diagnostic features Bangladesh) (Singh, 2006; Singh et al., of the alluvial reaches of the Brahmaputra 2006; Latrubesse, 2008) from a source is the presence of large alluvial islands and representing broadly the active zone of several of them are more than a century old continent-continent collision between the and inhabited as well. One such island, Indian and the Eurasian plates (Brookfield, Majuli, located in the upper reach of the 1998). The exceptionally high sediment Brahmaputra valley is the focus of this flux of the Brahmaputra has been attributed paper. Majuli is one of the largest riverine to erosion of actively uplifting mountains of islands in the world and the largest in Asia the Himalayas, slope erosion of the with a population of 0.16 million people Himalayan foothills and movement of and the site of ~64 Vaishnavite spiritual alluvial deposits stored in the Assam valley centres called ‘Satras’ (Fig. 1B-C). The (Thorne et al., 1993; Garzanti et al., 2004). literal meaning of ‘Majuli’ is the land The influence of the Himalayan orogeny locked between two rivers. The present and large influx of the eroded materials length of the Majuli Island is ~64 km and from the hinterland on fluvial dynamics of the maximum width is ~20 km. This place, the Brahmaputra River has been studied by considered as a world heritage several workers (Mathur and Evans, 1964; site that needs preservation, is under the Coleman, 1969; Bhandari et al., 1973; Das threat of total extinction due to massive Gupta and Nandy, 1982; Goswami, 1985; land erosion. The Majuli Island differs from Ahmed et al., 1993; Das Gupta and Biswas, other sandbars in the sense that the latter 2000; Kent and Das Gupta, 2004; Sarma, develop directly as the consequence of the 2005). Available data also suggest that the sediment load redistribution whereas the present-day Brahmaputra valley, a NE-SW former represents the remnant floodplain trending intermountain alluvial relief, was after sudden channel diversions and earlier a part of the Assam-Arakan basin, anabranching (Latrubesse, 2008). Formation of Majuli-like landforms is thus exposed, are located close to the south- a part of river dynamics (Takagi et al., western boundary. The stratigraphy 2007) that might be related either purely to deciphered from different deep wells close the variability in the sediment dispersal to the Majuli Island shows the oldest pattern or neotectonic influences and of sediment of Paleocene-Eocene age (Ranga course there might be interplay of both. Rao, 1983). The geo-tectonic setting of the Apart from Majuli, there are a few other Majuli Island (Fig. 2) shows that the place islands in the Brahmaputra system is situated between the Bouguer gravity including one in the upstream reaches close anomaly contours 220–240 mGal and first to the old confluence of order basement depth contours 3.6- 5.0 km Siang, the Dibang and the Lohit. Locally (Narula et al., 2000). It clearly shows a called ‘new Majuli’, this new island has prominent ‘Low’ in the NW part of the developed during the last two decades only Majuli Island. As per the first order tectono- and about 300 km2 of forested area (Dibru- geomorphic zonation of the intermontane Saikhoa Reserve Forest) has now become valley into central uplift, slope and an island (Fig. 1B). This has resulted in an depression (Lahiri and Sinha, 2012), the unprecedented increase in the width of the Majuli Island falls into the lower part of the channel belt of the Brahmaputra in this ‘central uplift’ zone. Seismological reach and has also impacted the evidence suggests that the eastern morphodynamics of the tributaries. This Himalaya to the west and the Indo-Burma paper puts forward a critical review of the thrust areas to the east of the Majuli Island available hypothesis for the evolution of the have recorded several large earthquakes Majuli Island and presents new insights (greater than magnitude 4) during 1964– based on geophysical evidence. An 1993 (Narula et al., 2000). However, the alternative mechanism for the evolution of Brahmaputra valley area where the Majuli the Majuli Island taking into consideration Island is located is practically aseismic. its morpho-tectonic setting and fluvial This observation is in line with the study of processes. 2. The upper Brahmaputra valley the Coda waves (Hazarika et al., 2009) for and the Majuli Island In the present study, the smaller earthquakes (Magnitudes focuses on the upper Brahmaputra valley varying from 1.2 to 3.9). A quality index of situated in the extreme north east corner of the coda waves ‘Qc’ is supposed to have India in the foothills of eastern higher values for the lesser decay. Himalayas (Fig. 1A). This part of the valley Unconsolidated material highly fractured or belongs to the Assam Arakan foreland otherwise, is supposed to cause greater basin system (DeCelles and Giles, 1996) degree of attenuation of the waves and that and the Majuli Island and the adjacent areas is why itwill show lower values of ‘Qc’. are a part of the foredeep having some of The eastern Himalayan side nearer to the the thickest depocenters. The upper Majuli Island represents highly Brahmaputra valley is 280 km long and 80 unconsolidated materials thereby causing a km wide (Fig. 1B) sandwiched between higher degree of seismic energy attenuation NE-SW bound Himalayan Frontal Thrust in all ranges of frequencies (1–18 Hz). (HFT) and the Naga-Patkai Thrust (NPT). TheMishmi hills, belonging to the syntaxial zone of the Himalayas mark the northeastern boundary and the Mikir hills, having basement metamorphic rocks Figure 1:Geotectonic setting of the Majuli Island and the surrounding area with Bouguer gravity contours in mGal and basement depths are shown in km. Deeper the basement, thicker is the sediment thickness and more negative Bouguer anomaly. GEOMORPHOLOGY AND (1915– 975) to 4.40 km2/yr (1975–2005). EROSIONAL HISTORY OF THE The length of MAJULI ISLAND this island has also reduced greatly from 79.7 km in 1915 to 75.16 km in 1975 and The spindle-shaped Majuli Island, in spite then 63.33 km in 2005 (about 20.5% of the recent surge in the rate of erosion, is reduction compared to 1915). Our initial a fairly steady landmass within the channel observations suggest significant spatial belt of the Brahmaputra River. Unlike ther variability in terms of erosion of the Majuli smaller islands located in the adjacent reas, Island in the upper, middle and lower parts. the Majuli never submerges completely Although it is often difficult to define the even during maximum flooding. Majuli is thalweg line on maps and images, mapped also a relic island because it is older than the median path of the widest channel to the Brahmaputra River (Sarma and Phukan, represent the temporal variability of 2004). Geomorphic mapping of the Majuli thalweg line. In the upper aswell as the Island and the surrounding reaches of the middle parts of the Majuli, the thalweg line Brahmaputra river using multi-date satellite of the Brahmaputra shows an eastward shift images and toposheet covering a period of away from the island during the period about 90 years reflects a highly dynamic 1915–2005. In the lower parts of the regime and a very high rate of erosion of the Majuli, the thalweg line moved closer to the island. Between 1915 and 1975, the surface Majuli. Further, the Subansiri, a major area of the island reduced from 787.87 km2 tributary of the Brahmaputra, shifted SW to 640.5 km2 (18.7% reduction) and then to after the 1950 earthquake and now joins the 508.2 km2 by 2005 (35.5% reduction as Brahmaputra close to the western edge of compared to the 1915). The average rate of the Majuli. Bankline shift along the right erosion in the last thirty plus years has and left banks shows different and at places increased considerably from 2.46 km2/yr opposite trends with respect to erosion of Majuli. The right bankline is very close to around the Majuli Island. There is a the western edge of Majuli. Over the years, remarkable shift in the relative trend of due to human interventions and migration channel area (CH) and the braid bar area of the Subansiri River further downstream, (BB) changes. During 1915–1975, CH and channel flow along the right bank has been BB often show opposite trends although fairly stable and erosion has also decreased CHB follows a trend similar to that of BB. Figure 2: Reach scale planform changes in the Also, the average amplitude of change in Brahmaputra and Majuli island during 1915, CH was higher than that of BB and the CHB.