JOURNAL GEOLOGICAL SOCIETY OF INDIA Vol.67, May 2006, pp.649-657 Late Holocene Geomorphic Evolution of Cauvery Delta, Tamil Nadu SM. RAMASAMY, J. SARAVANAVEL and R. SELVAKUMAR Centre for Remote Sensing, School of Geosciences, Bharathidasan University, Tiruchirappalli - 620 023 Email: [email protected], [email protected] Abstract: The Cauvery delta is a major bird-foot shaped delta with its apex located east of Tiruchirappalli with the mother channel Cauvery flowing along Thanjavur– Kumbakonam – Kaveripoompattinam. The distributary drainages are radiating over an arc towards east from its apex with a fan shape and finally debouch into the sea from south of Tiruthuraipoondi in the south to Kaveripoompattinam in the north. All these distributary drainages are now seen as palaeochannels including the mother channel Cauvery, which is now called as “Palam Cauvery”. The present day flow of Cauvery river is confined to the northernmost rim of the delta as river Coleroon. On the basis of the archaeological, epigraphic and historical data analysed earlier and the14C dating carried out recently, the present authors infer that the age of the delta may vary from 2300 Y.B.P. (Years Before Present) to 750 Y.B.P. and the age of the palaeochannels are also younging from 2300 Y.B.P. in the south to 750 Y.B.P. in the north. In the southern part of the Cauvery delta, the Mio-Pliocene Sandstone of Pattukottai-Mannargudi area is undergoing upliftment along N-S faults. On the basis of the rapid land progradation during 6085±233 Y.B.P. - 1020±80 Y.B.P. in Vedaranniyam coast, it is visualized that the Mio-Pliocene Sandstone must be undergoing upliftment significantly since 6100 Y.B.P. onwards. On the basis of the occurrence of older palaeochannels in the south, the gradual younging palaeochannels towards the north and the land emergence in the south of the Cauvery delta in Pattukottai – Mannargudi area, it is concluded that the distributary drainages of river Cauvery would have become sequentially defunct / shifted from south to north and finally the mother channel Cauvery also would have been pushed towards north to reach the present tract of Coleroon river. The thinner palaeochannels in the south and the gradual wider palaeochannels in the north of the delta further indicate that as the land was already undergoing emergence in the south even before the river Cauvery had entered into Thanjavur plains, the river could not comfortably spread its distributaries in the south. Hence, the southern distributaries, after a brief flow, would have started drying sequentially from south to north during 2300–750 Y.B.P. thereby the floodwater of each of the southern distributary channel would have been pushed to its successive northern counterpart, i.e. the southern distributary (say 1), pushing its water to its immediate northern counterpart (say 2), then the combined flow of 1 and 2 pushing the water to 3 and so on. The anomalous eyed drainage and its modifications during the last 50–60 years and compressed meanders in Coleroon river, deflected drainages in Palam Cauvery, truncation and punctuation of beach ridges by the E–W faults along the N–S coast and such faults acting as pathways for the sub parallel creeks indicate vibrant Late Holocene geological history predominated by the tectonic movements in Cauvery delta. Keywords: Remote Sensing, Geomorphic Anomalies, Post Deltaic Geological Events, Late Holocene Tectonics of Cauvery Delta. INTRODUCTION (1991) and Govindan (1993) for elucidating the Quaternary The Cauvery delta, covering an area of approximately fluvio-marine interactive processes; Kumar (1983) and 4000 sq.km in Tiruchirappalli – Thanjavur region, is unique Agarwal and Mitra (1991) for its hydrocarbon prospects. In in its morphology and architecture with its apex located addition, the groundwater problems and prospects have just east of Tiruchirappalli and the outermost arc passing also attracted many geoscientists and agencies to study through Tiruthuraipoondi–Karaikal (Fig.1). This delta has this delta. been studied in detail by many and the significant amongst From the tectonic point of view, Raiverman et al. (1966) them are the studies by Raiverman et al. (1966), Balakrishnan have observed evidences for the tectonic alertness of the and Sharma (1981) and Prabhakar and Zutshi (1993) for N-S, NE-SW, NW-SE, E-W and ENE-WSW trending understanding the neotectonics; Meijerink (1971), Babu lineaments / faults in the Cauvery delta and amongst which (1975), Sambasiva Rao (1982), Babu (1991), Ramasamy the N-S and the ENE-WSW ones remained active from 0016-7622/2006-67-5-649/$ 1.00 © GEOL. SOC. INDIA 650 SM. RAMASAMY AND OTHERS Fig.1. Study area. Late Cretaceous till date and controlled the evolution of that the Cauvery delta provides a complex story, but at the this delta. Sambasiva Rao (1982) has traced major same time possesses enough records on the evolutionary lineaments in NE-SW and ENE-WSW directions in the history including sedimentation, isostatic – eustatic interplay, delta. Prabhakar and Zutshi (1993) have recognized three fluvial and marine interactive mechanisms, land-ocean distinct episodes of evolution in the Cauvery basin viz. rift interactive phenomenon etc. Hence, detailed studies of phase, pull apart phase and shelf sag/tilt phase. The this delta would provide strong knowledge base on the sedimentational history of this delta was punctuated by four overall evolution of the delta, which in turn would provide major unconformities correlatable to Mid Mesozoic, Early clear information on the hydrocarbon and water resources Tertiary, Late Tertiary and Pleistocene erosional surfaces potential and further on the disaster proneness of the (Babu 1975). The sedimentation and progradation of the delta too. However, while the overall sedimentational and delta was estimated to be at the rate of 10 km per 1000 tectonic histories of this delta have been studied years by Sambasiva Rao (1982) again. In contrast, Govindan considerably, the syn - post deltaic geological history has (1993) has inferred pulsatory marine transgressions and not been given much attention, though the same substantially regressions since the onset of Cretaceous. Kumar (1983) controls the resources, environment, geohazards etc. has related the oil accumulation in the Cauvery basin to local Hence, an attempt has been made here to study and add and regional unconformity surfaces. Agarwal and Mitra to the syn - post deltaic geological history of the Cauvery (1991) have observed positive correlation between NW-SE delta on the basis of the visibly seen geomorphic anomalies lineaments and the hydrocarbon locales. and the tectonic features from remotely sensed data. Foote Thus, the state of our knowledge, as on date, indicates (1916) had suspected major fluvial changes in Chennai JOUR.GEOL.SOC.INDIA, VOL.67,MAY2006 LATE HOLOCENE GEOMORPHIC EVOLUTION OF CAUVERY DELTA, TAMIL NADU 651 region in the past. Vaidyanadhan (1971) has observed that black and white and reddish tone in FCC satellite images the river Cauvery has earlier flowed northeasterly and met (3, Fig.2A, 2B). Amongst these palaeochannels, the channels the sea north of Chennai and migrated southerly from found on the southern part of the delta are narrow, whereas its original course due to the capture of the lower part of the palaeochannels of the northern part of the delta are wider. its course, by headward erosion of a consequent stream However, over these palaeochannels, a number of streams flowing due east into Bay of Bengal. But, Radhakrishna viz: Vennar, Vettar, Arasalar, Kodamuruti etc., are flowing (1992) has observed that the upliftment of Bilgirirangan now as misfit streams. The present day Cauvery river hill ranges along a set of N-S faults would have forced the flowing in the northern edge of the delta (4, Fig. 2A, 2B) is river Cauvery to migrate from its original northeasterly river Coleroon and the Thanjavur – Kumbakonam – Hognekkal-Chennai flow towards south to Stanley Kaveripoompattinam tract which was the original course of reservoir - Tiruchirappalli – Thanjavur tract. Ramasamy river Cauvery is now called as “Palam (old) Cauvery”. et al (1992), on the basis of the amalgamation of remote Whereas Ramasamy et al (1992) have indicated the sensing revealed palaeo courses of river Cauvery, with northerly migration of the Cauvery river in its deltaic regime archaeological, historical and epigraphical data, have as during 2300 – 750 Y.B.P., on the basis of archaeological, explained that the river Cauvery might have originally epigraphic, Tamil literature and historical data, the 14C dating flowed in Hogenekkal-Chennai tract from 500000 to carried out recently gave an age of 1320±100 Y.B.P. to the 3000 Y.B.P. (Before present), migrated and flowed along palaeochannels of the Central part of the delta in Srivanjiyam Stanley reservoir – Harur - present day Ponnaiyar delta region (9) and 960±110 Y.B.P. to the palaeochannels of the during 2700 – 2300 Y.B.P and finally, further migrated northern part of the delta in Malliyam area (10, Fig.2A) and southerly and entered into Tiruchirappalli – Thanjavur plains thus overall corroborating with the anticlockwise migration after 2300 Y.B.P. They have further observed that the inferred by Ramasamy et al (1992). However, the oldest Cauvery river which has entered into Thanjavur region radiocarbon date (2316±103 Y.B.P) available for the around 2300 Y.B.P. has initially flowed in the southern part palaeochannels of Cauvery delta is that of Palam Cauvery of the delta in Mannargudi – Vedaranniyam region and later river, west of Kaveripoompattinam (Sarma 1991). The IRS on gradually migrated in an anticlockwise fashion to reach 1B satellite data vividly displays the arcuate morphology of the present day tract of Coleroon river around 750 Y.B.P. this delta (5), the outer arc of which is rimmed by bundles From these it is visualized, in the present study, that the of beach ridges (6) intervened by palaeo lagoons and swales age of the Cauvery delta must probably be of Late Holocene.