Influence of Sedimentation on the Geomorphology of the Northwestern ...

Marine Geodesy, 23:259– 265, 2000 Copyright ° C 2000 Taylor & Francis 0149-0419 /00 $12.00 + .00

In uence of Sedimentation on the Geomorphology of the Northwestern Continental Margin of India ONKAR S. CHAUHAN F. ALMEIDA JANGAM SUNEETHI National Institute of Oceanography Dona Paula, Goa, India Based upon the analysis of about 10,000 line km of echosounding and bathymetric data and variations in mass accumulation rate along the NW continental margin of India (between Kori creek and Mumbai), we have deduced that in the northern region (in the vicinity of the River Indus) the shelf-break occurs at a shallower depth and the slope is shallower, has the steepest gradient (< 1:20), is smoother with no major features, and has reduced width (slope edge at 1450 m; width 19 km than off Mumbai). The width and depth of the slope edge gradually increases southwards, and is at maximum off DamanMumbai (slope edge depth 2900 m; width 84 km). The intensity of the occurrence of physiographic features also increases southwards. The shelf edge off Saurashtra is undulating and on the slope, regional notches and benches (the most prominent at 560 m) are observed. Further southwards off Khambhat-Mumbai , the slope is characterized by the presence of bathymetric highs and lows. We have also observed numerous features on the shelf, with a variable depth of the shelf-break. The gradient of the continental slope is also reduced from the northern region to the southern region. The variations in the gradient of the slope and the presence of distinct physiographic features in this area are examined vis-a-vis uvial supply of the sediments into the region. Keywords Indian NW continental margin, NW Indian Ocean uvial sources, River Indus sedimentation

As a result of regional tectonic events associated with the intricate movements of the Indian Plate after its breakup from Gondwana (Biswas 1982), the northwestern (NW) continental margin of India is characterized by numerous, complex, tectonic and physiographic features (Bhattacharya and Subramanyam 1986; Chauhan and Almeida 1993). In the northernmost region of the study area, the River Indus discharges 400 million tons of sediments annually. There are other uvial sources (Figure 1) along the study area; prominent among them are the Tapi, Narmada, Mahi, and Sabarmati Rivers in the Khambhat region. The regional physiography of the slope off the NW continental margin of India (Gujarat– Maharashtra), particularly the in uence of uvial input on physiography, is poorly understood, partly due to the fact that the studies (Wagle 1979; Chauhan and Vora 1990; Wagle and Veerayya 1996) in this area until now broadly reported the localized physiographic subprovinces of the areas on the shelf or inland bays. No attempt has so far been made Received 23 February, 2000; accepted 26 June 2000. The authors are thankful to the Director, National Institute of Oceanography, Dona Paula, Goa for providing facilities. Thanks are also due to Dr. R. R. Nair for fruitful discussions and comments on the manuscript. This article was originally presented as a paper at IGCP 369, Goa. Address correspondenc e to Dr. Onkar S. Chauhan, National Institute of Oceanography, Dona Paula, Goa 403 004, India. E-mail: [email protected] n

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FIGURE 1 Location of the survey tracks in the study area. to separate out the in uences of uvial discharge on the regional physiographic changes, particularly on the slope, which is reported to be the depositional site of the Indus sediments (Rao and Rao 1995). The present work, which is based upon study of over 10,000 line km of closely spaced bathymetric data, describes the detailed geomorphology of the continental shelf edge and slope between Kori Creek and south of Mumbai along the NW continental margin of India, and it attempts to separate out the in uences of sedimentation on the regional physiography of this region. Particular emphasis is made to determine the in uence of Indus sediment input on reshaping the physiographic subunits of the continental slope.

Methodology The study area lies between 18±– 23±N latitude and 65±– 72±E longitude (Figure 1). Over 10,000 line km of new bathymetric data together with other published bathymetric data (Bhattacharya and Subramanyam 1986), and over 600 station depth data of INS Darshak and RV Gaveshani from this area (Figure 1) have been used to determine the geomorphologi c features. The digitized continuous echosounding data merged with other sounding and station data were interpolated on a Norsk Data 570 Computer using Geco Mapping System (GMS) software, and a contour map of the area was prepared. To identify and describe the shelf-break and other subphysiographi c features, the methodology and terminology of Wear, Stanley, and Boula (1974), McKelvey and others (1969), and Dingle (1977) have been

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FIGURE 2 Bottom topography of the study area derived from the survey lines shown in Figure 1. Contours are in m (uncorrected depths). used, and a composite geomorphological map was prepared. The rate of mass accumulation along the slope is derived from available Carbon-14 dates, and from biostratigraphy of the cores in the earlier studies (Agarwal, Kusumgar, and Yadav 1991; Borole 1988; Chauhan and Almeida 1993).

Results and Discussions The regional contour map, bottom topography, and composite physiography of the area are presented in Figures 2 to 4, respectively. It is deduced from these results that in the northernmost region, the shelf-break occurs at rather shallow depth (117 – 123 m) and the shelf edge has a NW– SE regional trend. However, off Dwarka at about 22±000 N, 68±300 E, there is an abrupt change in its orientation. Here, the shelf-break turns initially E-W, then NW-SE, and NNE-SSW, before returning to the original NW-SE trend (Figure 4). The shelf point of in exion deepens to 150 m in this region (Figure 4). At about 21±000 N, 69±000 E the orientation of shelf edge is altered again to a N-S direction, then to WNW-ESE, and nally to NW-SE. The depth of the shelf-break also varies at these locations (some 126 – 140 m; Figure 4). In the southern region, off the Gulf of Khambhat, the shelf edge is deeper relative to the northernmost end (between 122 – 140 m depths) and the morphological characteristics of the shelf-break also vary. The shelf-break in the northern region is much sharper compared to the south, except at a few locations off Saurashtra, where the shelf edges changes its orientation (Figure 4). The width and the gradient of the slope also vary considerably (Figure 4). In the northern region close to the mouth of the River Indus off Kachchh, the slope width is minimum (19 km) and the slope edge is observed at 1450 m water depth (Figure 4). The gradient

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FIGURE 3 Regional geomorphological features of the study area. The Holocene rate of sedimentation (cm ky ¡ 1 ) is shown in circles. of the slope is also steep (1:13). Its physiography remains smooth, and the only features observed in this area are some regional notches between 500 and 800 m on Transects 1 – 3 (Figure 3). Several depressions and elevations with a general relief of 100 – 300 m (occasionally >300 m) were also observed at the base of the slope. An isolated bathymetric high with relative elevation of about 460 m, and width of about 18 km, was delineated on Transect 2 (Figure 3). The width of the slope gradually increases southwards, and is a maximum in the southern extreme (39 km off Dwarka, 50 km off Porbandar, 60 km off Daman, and 84 km off Mumbai; Figure 4). There is also a signi cant change in the regional gradient of the slope (Figure 4). Based on the variations in the gradient, the area may be divided into two broad zones, with the slope NW of 21±N, 69±E having steeper slopes (1:9) than that to the SE (1:25– 1:35, Figure 4). Signi cantly, the upper slope off Saurashtra also has numerous complex regional physiographic features. The slope in this region is incised with benches and terraces between 180 m and 230 m depth on Transects 7 – 10. These features widen southeastwards from 2 km on Transect 7 to about 10 km at Transect 10, and their linear N-S extension is 150 km. The second deeper break in the slope was recorded off Porbandar at a depth of 560 m on Transect 13. This break deepens southwards to about 900 m on Transect 16. A steep scarp occurs at 145 m depth on Transect 12 (an increase of over 50 m in one km). A bathymetric high (about 15 km wide), with a relief of about 400 m is also delineated on Transect 13 (Figure 3). On the lower slope, two prominent benches were observed at 1900 – 2000 m and 2300 – 2340 m on Transects 15 – 17. These features have a NW-SE extent of around 60 km (Figure 3). An undulating bottom topography with high and low (relief 100 – 400 m) is also observed at the foot of the slope on Transects 11 – 16.


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FIGURE 4 Variations in the regional gradient, and depth and orientation of the shelf and slope edge within the study area. Oscillation in the shelf edge and change in the gradient of the slope north of 21±N, 69±E off Shaurashtra are conspicuous features. The physiography in the areas under intense in uence of the Indus sedimentation is schematically demarcated. Further southwards off the Gulf of Khambhat (Daman-Mumbai) the slope has its maximum width and is much gentler than in the northern region. A number of breaks are also noticed between 200 and 600 m water depths (Transects 22 – 26, Figure 3). The crests of these features extend for about 150 km N-S and are characterized by rugged topography. In a few instances, they also exhibit at-topped structures. A bench/terrace (width 3 – 4 km) is also observed between 600 – 780 m depth. At the base of the slope, topographic highs and lows (with relief around 100 – 300 m) are conspicuously present. The crests of these features have been mapped for over 120 km along the slope (Figure 3). The geomorphology of a continental margin is a re ection of tectonic structures of a region and their subsequent modi cation by morphodynamic processes (Bhattacharya and Subramanyam 1986; Biswas 1982; Chauhan and Almeida 1993; Wagle 1979). The regional variations in the physiography of the study area are therefore evaluated in terms of sedimentary processes in the area, and manifestation of the tectonics in the region. The sediment contribution to the study area is variable. The River Indus, located in the north, discharged 400 million tons of sediments annually prior to the construction of dams on the river (Nair, Hashimi, and Rao 1982). The Rivers Mahi, Sabarmati, Narmada, and Tapi located in the Khambhat region are relatively less signi cant (discharge 59 million m2 ). Based upon the sedimentological studies on the shelf and slope (Chauhan 1994; Rao and Rao 1995; Nair 1974), it is deduced that sediments of the River Indus disperse alongslope. Also, from the existence of carbonate sand over the sizable area off the Gulf of Khambhat on the outershelf—shelf edge with a rugged physiography comprised of sand ridges (Nair 1974, 1975; Wagle and Veerayya 1996)—it is also deduced that the detritus of the Rivers

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Tapi, Narmada, Mahi, and Sabarmati is mostly trapped on the inner shelf (Nair 1974, 1975). Attesting to this thesis is the clay mineral distribution in which, illite and chlorite, the characteristic clays of the River Indus, are dominant species on the shelf and slope region off Kori Creek in the vicinity of the mouth of the River Indus (Chauhan 1994). There is a southerly decreasing trend in the content of these clay minerals along the slope (Rao and Rao 1995; Ramaswamy and Nair 1989). From the above results it may be deduced that the main source of terrigenous detritus along the NW continental slope is from the Himalayan River Indus, with negligible supply from the peninsular rivers, particularly on the outer-shelf/upper-slope regions of the southern area between Daman and Mumbai. The results of the present study show that the shelf edge in the northern region occurs at a shallow depth (117 – 123 m). The slope edge also occurs at the shallower depth in this region. The steep (gradient < 1 : 20) and smooth slope is narrower (19 km) than in the southern region (60 – 84 km). We have deduced high sedimentation rates from sedimentological, clay mineralogy, and high mass accumulation rates into this area by the River Indus (Figure 3). Evidently, this leads to the piling up of the sediments on the slope, narrowing its width; it also lls the shelf and buries the physiographic features. High terrigenous supply is also found to induce the mass/gravity transfer due to piling and resuspension of sediments on the shelf edge (Almogar and Wiseman 1982). The presence of 100 – 300 m undulations at the base of the slope, which are identi ed as the surfaces of slump deposits (Naini and Kolla 1982), further exhibits the in uence of mass/gravity induced sedimentation on reshaping the physiography in this region. The southern region, between Okha and the Gulf of Khambhat and further southwards, is reported to have many deep-seated tectonic structures (Bhattacharya and Subramanyam 1986; Biswas 1982; Gosh and Zutshi 1989). Regional anticlinal structures extending along Mumbai and Saurashtra are reported from these areas (Bhattacharya and Subramanyam 1986; Biswas 1982; Harbison and Bassinger 1970). The presence of benches and terraces in this zone, therefore, points towards a structural control. The points of in exion of the shelf edge also correlate with the Saurashtra Anticline and with the strike slip faults of the Narmada Graben located off the Gulf of Khambhat. These features suggest a tectonic origin for these physiographic features. Further, the shelf edge in this area is covered with relict carbonate sand (Nair 1974, 1975), and therefore, it appears unlikely that there is signi cant input of sediments to the slope regions from peninsular uvial sources, bypassing this relict facies. The Holocene sedimentation rates off the mouth of the River Indus are 18– 15 cm ky ¡ 1 which reduces to 4– 6 cm ky ¡ 1 in the southern region (Agarwal et al. 1991; Borole 1988; Chauhan and Almeida 1993; Figure 3). Due to negligible sediment input in the southern region, the geomorphic features, which exist on the slope, do not get buried or modi ed by sedimentation. Therefore, the existence of the complex geomorphological features off Saurashtra and further southwards on the slope are manifestations of the tectonic events associated with the intricate movement of the Indian Plate (Bhattacharya and Subramanyam 1986; Biswas 1982). As deduced from the sedimentation rates, the supply of the terrigenous detritus to the southern area is low, and it appears inadequate to bury or modify the sur cial signature of the deep-seated tectonic features. It may therefore, be concluded that the supply and dispersal mechanism of the detritus of the River Indus plays an important part in reshaping the physiography of the NW continental margin of India. Based upon the gradient of the slope, its width, and occurrence of exposed physiographic features, the areas which are under the intense in uence of Indus sedimentation have been schematically demarcated on Figure 4.

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