Indian Journal of Marine Sciences Vol. 28, December 1999, pp. 449-454 The depositional history of late Quaternary sediments around Mangalore, west coast of India B R Manjunatha Department of Marine Geology, Mangalore University, Mangalagangothri 574 199, Kamataka, India and K Balakrishna Physical Research Laboratory, Navrangpura, Ahmedabad 380 009, India Received 15 June 1998. revised 14 September 1999 Netravati and Gurpur are relatively large rivers drain the hinterland near Mangalore, form a common estuary before they debouching into the Arabian Sea. Lithologic successions observed in a number of bore holes and dug wells indicate that Netravati and Gurpur rivers drained into the Arabian Sea independently during the last glacial period when the sea level was about 100-138 m below the presenrlevel. In contrast to Netravati, the lower course of Gurpur has migrated southerly in four stages for a distance of 8 km and at presenC forms a common estuary with the Netravati river. This is because of drowning of the river channel due to rapid rise in sea level during the early Holocene and growth of barrier spit under the strong influence of southerly littoral currents during late Holocene when the sea level was relatively stabile. The rate of in­ filling of alluvial and marshy sediments during late Pleistocene to early Holoc<tne (0.5-5.0 mmlyr and 0.33-4.33 mmlyr respectively) is about two-three times slower than that for barrier spit sands accumulation (0. I 4-1.14 mmlyr) during the late Holocene. -, West coast of India is characterized by both emergent west coast of India, particularly Konkan, Karnataka 7 and submergent features because of long term and Kerala coasts, is relatively tectonically stable . changes in sea level, climate, lithology, structure and Therefore, sea level changes observed here are due to l local tectonic movements . Nevertheless, submergent glacio- eustacy whose impact on Quaternary features like estuaries, marshes, tidal swamps, mud sedimentation processes can be best understood by flat deposits, spits and bars, tombolos etc. dominate the study of sediments in these coasts. l 4 the coastline - • The Maharashtra coast is dominated The major part of the Karnataka coast comprises of 7 by ria-type estuaries into which seawater intrudes up sandy beaches , which vary in width from a few to 30-45 km inland and river mouths are widened into meters to 200 m. Beaches of the Uttara Kannada 2 4 broad creeks • • Estuaries of the Karnataka coast are district occur in between estuaries and rocky shallow, characterized by narrow river mouths and headlands, whereas that along Dakshina Kannada are seawater intrusion can barely be traced beyond 15-20 almost straight, interrupted by a number of estuaries 8 km inland. The Kerala coast is characterized by without headlands . Therefore, the study of the estuaries and lagoons'. Dakshina Kannada coast is ideal for understanding The Karnataka coastline is generally straight, long-term depositional history of coastal sediments. 5 trending NNE-SSW of the Precambrian (F2) folding . The study area includes the Netravati-Gurpur The coastal tract can be demarcated into the northern estuary and the adjacent landmass (Fig. I). Netravati Uttara Kannada and the southern Dakshina Kannada and Gurpur rivers originate in the Western Ghats, districts_ The former has a coastline of 150 km, flow for 148 and 87 km respectively covering a total 2 2 covering an area of 8000 km , whereas the latter has area of 4257 km • These two rivers unite to form a 2 140 km long coastline and an area of 8440 km . The common estuary before debouching into the Arabian coastal zone is characterized by heavy rain fall Sea. However, observation of lithologic slIccession in (>3000 mmlyr) and generally high temperature a number of drilled boreholes and dug welis indicate (>30°C). The coastal tract is highly dissected and the that these rivers had independent confluence points landscape is the result of extensive Tertiary with the Arabian Sea. The objectives of this paper are 6 denudation because of effect of tropical climate . The to: (I) identify reasons for shifting of the river course, 450 INDIAN 1. MAR. SCI., YOL., 28, DECEMBER 1999 (2) correlate the coastal sediments of the study area with others along the west coast of India, and (3) characterize and quantify sedimentation rates in the land- ocean boundary around Mangalore since the late Quaternary. Materials and Methods This part includes reconnaissance survey of the area to tentatively reconstruct the stratigraphy on the basis of lithologic successions observed in a number 'l.n.-"ro~vo_t_i _R'-j of bore holes and dug wells around NMPT and ""e_ Someswar respectively (Fig. 1). Among 25 bore holes drilled around New Mangalore port9 (NMPT; Figs. 1 and 2), 14 are from the land and remaining ones are from the sea bed. Lithologic succession of the study area has been correlated with other formations along the west coast of India since the samples of the NMPT are quite old and are not available for laboratory work. The depth ranges of each litho units Talapadi and their thickness, age of the corresponding o 12. formation along the west coast of India are given in 5' === Paleochannel Table 1. The sea level fluctuations data have been 7 lI taken from Kale & Rajguru and Hashimi et al. Sedimentation rates have been calculated on the basis of radiocarbon ages 10.11 of the corresponding sequences along the west coast of India and the Fig. I-Map showing the present drainage of Netravati and thickness of the sediments in the area of study. Gurpur rivers, their paleochannels, common estuary and barrier spits. Results and Discussion found on land from surface to 27m depth, ranging in The lithologic succession of 25 bore holes and their thickness from 1 to 9m. They are even extended locations are given in the Fig. 2. The lithologic below the sea bed (9-25 m depth) having thickness sequence of the NMPT area can be categorised into range from 1 to 10 m. This indicates that the last six units (Table 1), of which units 1 and 2 belong to glacial sea level (14.5 kyr)1O was as low as 25-27m Precambrian, unit 3 is Tertiary, and units 4, 5 and 6 below the present sea level in the area of study. are Quaternary. As the prime objective of this study is However, on an average the sea level was as low as to trace the history of Quaternary sedimentation 1DO-138m along the western continental shelf of India processes, units 4, 5 and 6 are discussed. There is no during the last glacial maximum (14.5 kyr) 7.11. As the repetition or cyclicity of the units 4, 5 and 6 in the sea level rose, marine waters invaded the Gurpur above classified lithogenic sequence, which is valley and submerged land resulting in the deposition suggestive of the absence of last interglacial marine of fluvial (unit 4), estuarine/ marsh (unit 5) and formations in the area of study (Fig. 2). Bruckner4 shoreline (unit 6) sediments. The unit 4 has been also has made similar observations that there are no deposited as valley filled deposit due to reduction in Pleistocene marine deposits along the northern part of the gradient of river channel because of the rise in sea the west coast of India. The unit 4 comprises of level. The recent sea level curve proposed for the 7 11 cobble, pebble and coarse sand and is devoid of west coast of India • shows that the sea level was calcareous shell suggesting its origin from fluvial lowered by about 100-138 m at 14.5 kyr B.P. as activity. These are scattered throughout the site of compared to the current level. It rose by 20 m from investigation around NMPT. However they are 14.5 kyr to 12.5 kyr B.P. with an average II rise of concentrated at areas close to the present day 10mlkyr, resulting in the decrease of river flow which shoreline in sandy layers at different depths. They are in tum allowed to accumulate fluvial sediments. The MANJUNATHA & BALAKRISHNA: DEPOSITIONAL HISTORY OF SEDIMENTS 451 95 65 100 78 261 B8 356 354 269 310 349 351 Om LL 1() m 20m 30m N , II 56 15 ~ 200 0 200m 356 354 , I , 12" 12" • - 269 56' • B8 310 • •• 351• 55' 45" 349 " ARABIAN 12 SEA 12· 55' -a7 ·94 244 • 273 ·92 241 • ••B4 • 275 105 276 55' 15" • - 12 " 248... 48'ocf' 74" i48"30· 7( 49' qo" 49'.3 o Sand with Iwithout shells 87 ' 92 94 105 84 239 59 241 244 H8 276 273 275 r===lSoft medium 8, stitt clay with - B wooden pieces leaves & she Us '. - - - - fO:Ol -- - - - --- - ~ Cobble Pebble a. Coarse sand -- - ---- - - -- - -- ~ - - --- - --- --- - --- -- - -- --- L -- [5jLaterite a. Lateritic clay L --- ---- , ~sott rock DHard rock 9 Fig. 2-Locations and lithology of bore holes drilled in the New MangaIore port area . Inset map shows the location of New Mangalore port .. rate of channel filled deposits, i.e. sedimentation rate continental materials '2. This episode can be correlated of cobble, pebble and coarse sand sequence was to the accumulation of soft, medium and stiff clay, deduced from the duration of initial rise of sea level with wooden pieces, leaves and occasional shells in from 14.5 kyr to 12.5 kyr and the thickness of the unit the NMPT area. This unit comprises of the remains of 4 as O.5-5mm1yr. tree branches, wooden pieces and leaves, which is From 12.5 kyr B. P. to 10 kyr B.P., the sea level distributed throughout the NMPT site, except in the 9 was relatively stable and from 10 kyr to 7 kyr B.P., lateritic belt • This sequence is similar to those the Holocene sea level rise was as high as 18- reported for the western continental shelf sediments 7 1l 7 lo 13 20mlkyr • • Similarly Kale & Rajguru have obtained of India • .