Indian Journal of Marine Sciences Vol. 28, December 1999, pp. 449-454

The depositional history of late Quaternary sediments around , west coast of

B R Manjunatha Department of Marine Geology, , Mangalagangothri 574 199, Kamataka, India and K Balakrishna Physical Research Laboratory, Navrangpura, 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 . 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 . 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

(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 .. 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 • . The sedimentation rate of this sequence similar value (18 m/kyr) on the basis of computation has been calculated on the basis of duration of rapid of Holocene sea level curve. This rapid rise has sea level rise during the early Holocene and thickness resulted in the retreat of the coast, drowning the river of the sediment sequence as O.33mmlyr to 4.33mmlyr channels, as well as the submergence of the marshy for the entire study area. The sea level was close to land, thereby displacing estuaries landwards. This the current level around 7-6 kyr B.P. , but has 7 disequilibrium in estuaries serve as effective filter for fluctuated thereafter . " . High sea levels are 452 INDIAN J. MAR. SCI., VOL., 28, DECEMBER 1999

Table I-Lithologic succession, sedimentation rate of New Mangalore port sediments and strati graphic relations. Unit Lithology/ Depth Thickness Age of Sea level Sedimentation Stratigraph y sediment range below range corresponding rise rate type surface (m) (m) sequence (kyr) (crn/yr) (mrn/yr)

6 Sands with! without shells 7 0.03-0.08 0.14-1.14 Late Holocene Onshore 0-11 0-5 Offshore 0-8 1-8

5 Soft, medium and 10-7 1.8-2 0.33-4.33 Early Holocene stiff clay with wooden pieces, leaves and occasionally shells Onshore 0-20 1-12 Offshore 0-22 1-13

4 Cobble, pebble, 14.5-12.5 1.0 0.5-5.0 Late Pl eistocene secondary laterite coarse sand but no shell Onshore 0-27 1-9 Offshore 9-25 1-10

3 Primary and 1.6-65 ma* Telt iary and lateritic clay Onshore 0-25 1-7 Offshore 10-25 10-25

2 Soft/weathered rock Precambrian Onshore 7-30 1-16 peninsul ar Offshore 1-16 1-6 gneiss

Hard/fresh rock 3200 rna ----do---- Onshore >7 Offshore >30 ma=million years; - no data represented by occurrence of raised beaches along the Geological and geophysical studies of coastline west coast of India4,14, 15. Radiometric ages of these between NMPT and Netravati - Gurpur river mouth 4 ridges suggest that the Holocene transgression indicate that the Gurpur river has four submerged reached its maximum around 2000-2800 yr B.P. paleo valleys of which the southern most is coinciding l The top most sequence of fine sand with shells i.e. , the present river mouth ? and the northern most is unit 6 has been accumulated since late Holocene. below the NMPT. However, in contrast to a number Radiometric ages of Holocene shell sequence in the of paleovalJeys of Gurpur river, there is only ' one' sediments of west coast of Indiall range from 1400 to palaeovalley of Netravati river traced on land as 3410 yr B.P. However, as per the recently updated sea shown in the Fig. I , but there is none reported bel ow level curve for the west coast of India suggests that the seabed. The paleochannel deposits of th e the sea level has fluctuated little since 7 kyr. The rise Netravati river is observed in a number of we ll s in sea level since the late Holocene has been very consisting essentially of fluvial sediments but no less?· 16 which has not varied beyond 0.3 - 0.8mmlyr. marine Imarsh/or estuarine sequences as observed in During this period, the top sequence of sediments has NMPT area. Fluvial sediments consisted of iron­ been accumulated at a slower rate of 0.14-1 .14mmlyr. stained, coarse, medium and fine grained sand with The accumulation of fine sand with shells in the top rock fragments and occasionall y pebbles. sequence clearly suggests that it has been deposited The southerly shifting of Gurpur river mouth to a 2 due to marine acti vity thereby shifting the Gurpur R. greater distance (8 km long, 200-800m wide; Fig. I ) southwards, wh ereas Netravati river mouth shifting in relation to Netravati ri ver mouth could be due to northwards. (1) the net movement of sediment through littoral MANJUNATHA & BALAKRISHNA: DEPOSITIONAL HISTORY OF SEDIMENTS 453

I7 18 21 drift that is from north to south . , (2) Gurpur is a and (2) wave activity is also higher during 22 small river and soon becomes dry during the summer SW monsoon than that during NE monsoon . as compared to the Netravati, therefore, the river Lithologic successions observed in a number of mouth could have been blocked due to accumulation bore holes and dug wells around Mangalore can be of sand due to littoral drift; and (3) the net longshore concluded that the Netravati and Gurpur had an sediment transport in the VlIal spit is as high as independent exit to Arabian Sea during the last 4.1x105m3/yr compared to the Bengrel9 spit which is 5 3 Glacial period. In contrast, the submergence of river O.44x10 m /yr 18 which may not induce the faster channels and growth of 8.0 km long barrier spit under growth rate of the VlIal spit further northwards. The the strong influence of southerly littoral currents are river mouth of Netravati-Gurpur is quite narrow (Fig. responsible for unification of Ourpur river with the 1) due to the predominance of marine agency in an Netravati before they drained into the Arabian Sea estuarine dominated coast like the west coast of India. during the late Holocene. However, the river mouth remains open throughout the year. In small river-estuaries along the west coast References of India, ego Talapadi Lagoon will be closed by the I Wagle B G, Geomorphology and evolutioll of the central and barrier spit during the non-monsoon seasons due to offshore areas of Maharastra alld , India , Ph D thesis. insignificant river discharge. University of Bombay, Bombay, 1987. 2 Ahmed E, Coastal geomorphology of India , (Ori ent Similarly, there are a number of barrier spits Longman, Bombay) 1972, pp. 222. developed along the west coast of India. Of these, the 3 Nair M, J Geol Soc India , 29 (1987) 450. longest one is along the Kerala coast ( 55 km long, 10 4 Bruckner H, in Late Qllatemary correlation alld km wide) formed between Alleppey and Cochin, applications, edited by D B Scott, P A Pirazzoli & C A 2 Honing, (Kluwer Academic Publishers, London) 1989, 169. enclosing the Vembanad lake • Barrier spits are long, narrow sand accumulations that are generally parallel 5 Ramasamy S M, in Recent geosciellt(fic studies ill the 20 Arabian Sea off India, Spec Publ No 24. (Geol Surv India, to the mainland and formed due t0 (1) littoral Calcutta) 1989,333. transport of sand along a barrier spit connecting one 6 Subramanian S K & Rao D P, Landuse ill relation to headland to another and then migrating landward or landforms - A case study of Goa and Kamataka, Tech Rep seaward, (2) emergence of offshore sand bars in a 0915, (NRSA ) 1984, pp. 10. stable /rising sea level condition, (3) rising sea level 7 Kale V S & Rajguru S N, Bull Deccan College Res In5t , 44 (1985) 153. and drowning shore - parallel feature such as coastal 8 Manjunatha B R, in Geo-Kamataka : Geological dunes and (4) mUltiple causality combining elements Department Centenary Volume, edited by B M Ravindra & of the other three processes. N Ranganathan (Department of Mipes and Geology, ) 1994, 327. The probable reasons for the formation of barrier 9 Cheryan P P, Geological and geotechlJical study of boring spit along the west coast of India, particularly in the profiles at M Tech thesis, University of Mysore , area of study is due to littoral transport of sediments India, 1968. particularly during the stable sea level condition. 10 Nambiar A R, Rajagopalan G & Rao B R J. Curr Sci, 61 (1991) 353. Further, the significant growth of the spit in II Hashimi N H, Nigam R, Nair R R & Rajagopalan G, J Geol relation to VlIal spit (Fig. 1) is because of the Soc India, 46 (1995) 157. prominent southerly transport of sediments. In the 12 Fracer G S, Clastic depositional sequences: Processes of coastal Arabian Sea, the seasonal reversal of surface evolution and principles of interpretation, (Prentice Hall , water currents is the characteristic feature of the Englewood Cliff, New Jersey) 1989, pp. 459. 13 Manjunatha B R, Geochemistry and magnetic susceptibility monsoon systems. of riverine, estuarine and marine environments around This is further evidenced by seasonal variations in Mangaiore, west coast of India , Ph. D. thesis, Mangalore University, India, 1990. physical oceanographic processes like (1) the southerly moying currents those are much stronger 14 Wagle B G, Geo Mar Lett, 10 (1990) III. (mean speed 50crn!sec) during southwest monsoon 15 Subrahmanya K R & Bhat H G, in Remote sensing = applications and geographic information systems: Recent than the northerly moving currents those are weaker trends. edited by I. V. Muralikrishn a, (Tata McGraw Hill , (mean speed = 1Scm/sec) during the northeast New ) 1992, 3 14. 454 INDIAN J. MAR. SCI., VOL., 28, DECEMBER 1999

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