JOURNAL GEOLOGICAL SOCIETY OF INDIA Vol.67, May 2006, pp.649-657

Late Holocene Geomorphic Evolution of Cauvery Delta,

SM. RAMASAMY, J. SARAVANAVEL and R. SELVAKUMAR Centre for Remote Sensing, School of Geosciences, Bharathidasan University, - 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 – – 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- 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– (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

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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. (7) in Tiruthuraipoondi – Vedaranniyam area (Fig. 2A). Hence, accordingly from the various tectonic and While the older beach ridges encircle the outer rim of the geomorphic features/anomalies observed in IRS 1A/1B arcuate delta in Tiruthuraipoondi region, the succeeding satellite data of this delta, the probable syn- post deltaic younger beach ridges from Tiruthuraipoondi in the northwest geological history or the Late Holocene geological history to Vedaranniyam in the southeast form triangular shaped of the Cauvey delta was arrived at. cuspate features (6, Fig.2A, 2C). One of the older beach ridges observed southeast of Tiruthuraipoondi, dates back to 6085±233 Y.B.P.; the outer and the youngest beach ridge GEOMORPHOLOGY near Kodiyakkarai gave a radiocarbon date of 1020±80 The southern part of the Cauvery delta is prominently Y.B.P. and the intermediate beach ridges gave in between marked by the undissected Mio-Pliocene Sandstone upland dates (Ramasamy et al 1998). To the south of such E-W in Vallam region in the west and its intensely dissected trending beach ridges, the major Vedaranniyam Backwater counterpart in Pattukottai region in the east (1, 2 Fig. 2A, (8, Fig. 2A, 2C) is found with 50-55 km length and 10-12 2B). The N-S aligned contact observed in between these km breadth and this backwater is filled with silt to the tune two Sandstone blocks is very sharp and rectilinear. The river of 60-70 % of its areal extent. The satellite imagery shows a Cauvery has developed a distributary network of drainages spectrum of lineaments/faults with N-S, NE-SW, NW-SE with the main river flowing along Thanjavur – Kumbakonam and E-W orientations (Fig.3A). – Kaveripoompattinam and the distributary branches laid in a broad bird-foot shape forming an overall arcuate delta GEOMORPHIC ANOMALIES AND TECTONIC (Fig.1&2A). But these distributary drainages as well as the FEATURES main Cauvery river are seen as palaeochannels distinctly displaying bundles and branches of rectilinear, curvilinear, The palaeochannels found in the area southeast of ribbon like and loop like features with dark grey tone in Kumbakonam and northeast of Mannargudi show

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Fig.2. IRS 1B imagery – Geomorphology of Cauvery delta. 1 and 2. Vallam undissected and Pattukottai-Mannargudi dissected Mio- Pliocene sandstone uplands. 3. Palaeo channels of Cauvery. 4. Present course of Cauvery as Coleroon. 5. Outer arc of arcuate delta. 6. Beach ridges. 7. Palaeo lagoons/swales. 8. Backwater. 9. Srivanjiam. 10. Malliyam. conspicuous annularity (1-4, Fig. 3A, 3B). In the southern Virankoiltittu area, such rectilinearly flowing Coleroon river part of the delta, in Vallam - Pattukottai - Mannargudi region, displays an eyed drainage (10, Fig. 3A, 3D, 3E). Ramasamy three distinct morphotectonic zones are observed with and Ramesh (1999) have studied the Virankoiltittu area using western undissected Mio-Pliocene Sandstone forming a multidated topographic sheets and satellite data and observed prominant upland (5) in Vallam area, the central highly that the river island and the water spread area of this eyed dissected Mio-Pliocene Sandstone (6) with gulleys and relict drainage of the Coleroon river was in rectangular shape butte landforms in Pattukottai – Mannargudi region and the during 1931 AD and the same has attained a trapezohedran eastern delta (7) in the Tiruthuraipoondi area (Fig.3A, 3C). shape during 1993 AD. These morphotectonic zones are separated by two N-S In the northern rim, the Jayamkondam Mio-Pliocene trending sub parallel lineaments (A, B, Fig. 3A, 3C) and Sandstone (9) is sharply delimited by a major NW-SE these two lineaments are found to extend beyond Cauvery trending lineament (D) which in its southeastern extension delta in the north in Jayamkondam region too. The faults caused Compressed Meandering (11) near Mayavaram in trapped Mio-Pliocene Sandstone block (9) which has the otherwise rectilinearly flowing Coleroon river (Fig. 3A, undergone active sheet erosion while its western counterpart 3D). Further southeast, along this lineament, one of the (8), occurring west of the western fault, remains uneroded palaeochannels (Palam Cauvery) is sharply deflected (12) (Fig. 3A). and in the southeastern end of the lineament, major beach The Coleroon river is distinctly confined along a NE- ridges (13) are truncated abruptly (Fig. 3A, 3D). In the SW trending major lineament (C) and in the coastal zone in southwestern part of the delta, in Pattukottai – Mannargudi

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Fig.3. IRS 1B imagery – Geomorphic anomalies and tectonic features of Cauvery delta. 1– 4. Annular palaeo drainages (Fig.3A, 3B). 5 – 6. Vallam undissected and Pattukottai-Mannargudi dissected Mio-Pliocene sandstone and 7. Delta (Fig.3A, 3C). 8 and 9. Western uneroded and the eastern highly eroded Jayamkondam Mio-Pliocene sandstone upland (Fig.3A). 10. Eyed drainage, 11. Compressed meander. 12. Deflected palaeo drainage. 13. Truncated beach ridges. 14. Spit (Fig.3D, 3E). 15. Dissected beach ridges. A — L. Lineaments (Fig.3A). region, two major NW-SE trending lineaments (E, F Fig. and in the eastern end of this lineament, sinistral shifting of 3A) are found to control the drainage systems in Mio- coastal landforms is observed. Along such sinistrally shifted Pliocene Sandstone. Intensive dissections are also found and projected landmass, a spit (14) is also found at the mouth along these lineaments. of Coleroon river (Fig. 3A, 3E). Ramasamy and Ramesh In the tail end, the Coleroon river is abruptly deflected (1999), on the basis of the analysis of multidated topographic by a major E-W trending lineament (G, Fig. 3A, 3D, 3E) sheets (1931 AD) and IRS 1A satellite data (1993 AD), have

JOUR.GEOL.SOC.INDIA, VOL.67,MAY2006 654 SM. RAMASAMY AND OTHERS observed that the sinistral shift of the land along the coast is Mannargudi Sandstone would have provided a positive much less with no spit in the former year, but the same shift landform around which the littoral currents would have built is more in the latter year along with the northerly protruding the beach ridges along Tiruthuraipoondi around 6100 Y.B.P. longer spit grown in such projected land. Further down, in and when the sandstone kept on rising, the outer and outer the area east of Kumbakonam, a number of E-W trending beach ridges would have formed during 6100 Y.B.P. – 1020 lineaments (H – K) are seen in satellite data (Fig. 3A, 3F) Y.B.P. Ramasamy and Ravikumar (2002) observed that such and these lineaments act as pathways for the palaeodrainages sandbar building is taking place even today. From the same, and have further split the beach ridges (15, Fig. 3A, 3F). it can be surmised that the Pattukottai – Mannargudi Mio- Along these E-W lineaments, chains of subparallel creeks Pliocene Sandstone of the southern part of the Cauvery delta are also found. Another major E-W lineament (L, Fig.3A) might have been in the process of upliftment at least since is found to sharply control the Agniar river flow in the land 6100 Y.B.P. Ramasamy and Balaji (1995) have also made part and delimit the Vedaranniyam backwater in the south similar observations that the N-S faults of Tamil Nadu in in the sea part. general are the Pleistocene extensional faults related to the post collision tectonics. Raiverman et al (1966) have also observed that the N-S faults are active in Cauvery delta since POST DELTAIC GEOLOGICAL HISTORY AND DISCUSSION Post- Cretaceous to date. The observations like The drainages and the tectonic anomalies observed in â the Cauvery’s entry into Tiruchirappalli – Thanjavur Cauvery delta seem to provide a hierarchy of information plains around 2300 Y.B.P. (Ramasamy et al. 1992), on the post deltaic geological history of the area. The â distributary drainages radiating in an arc towards conspicuous morphotectonic blocks observed in the Mio- east from east of Tiruchirappalli onwards with the Pliocene Sandstones of the southern fringe of the delta mother channel Cauvery along Thanjavur – with the western undissected Sandstone in Vallam area (5), Kumbakonam – Kaveripoompattinam during the central dissected Sandstone in Pattukottai – Mannargudi 2316±103 Y.B.P. (Sarma 1991) and area (6) and the eastern Cauvery delta (7) in Tiruthuraipoondi â the present expression of all the distributaries as area along with their sharper separation by N-S lineaments palaeochannels including the mother channel (A, B) show that the central Mio-Pliocene Sandstone must Cauvery (Palam Cauvery) along with the gradual be undergoing upliftments along these lineaments/faults younging of the palaeochannels from south to north (Fig.3A, 3C). The sharper bifurcation of the northern during 2300 – 750 Y.B.P (Ramasamy et al. 1992). Jayamkondam Mio-Pliocene Sandstone by the western fault (A), with western uneroded (8) and the eastern (9) eroded suggest that the river Cauvery would have developed its morphology also confirm such upliftment of the Mio- delta as a bird-foot shaped arcuate delta around 2300 Y.B.P. Pliocene Sandstone block (6&9) trapped between the two with a broader distributary drainage system from south of N-S faults (A, B, Fig.3A). Such intense dissection within Tiruthuraipoondi in the south to Kaveripoompattinam in the the faults-bounded central Pattukottai - Mannargudi Mio- north (Palam Cauvery). But, due to the already ongoing rise Pliocene Sandstone in the south (6) and the sheet erosion of the Pattukottai-Mannargudi Mio-Pliocene Sandstone with within the same faults-bounded Jayamkondam Mio-Pliocene the northerly tilting effect along the N-S hinge faults (Fig. Sandstone block in the north (9) may probably indicate 2A & 3A), Cauvery river could not easily push its southern that these two N-S faults (A and B) must be hinge faults distributaries. As a result, the southern distributaries, after a with their hinge located in the north (Fig.3A). The beach brief flow, would have started becoming defunct sequentially ridges that are rimming the arcuate delta in between from south to north during 2300 – 750 Y.B.P. This process Tiruthuraipoondi and Kodiyakkarai indicate that these of sequential extinction would have followed a pattern that have been built during 6085±233 Y.B.P. — 1020±80 Y.B.P. the floodwater of each of the southern distributaries would (Ramasamy et al 1998). They have further attributed such have been pushed towards their immediate northern land progradation in Vedaranniyam coast to the upliftment counterparts. That is, the southernmost channel (say ‘1’) of Pattukottai – Mannargudi Mio-Pliocene Sandstone. The would have pushed its floodwater to its immediate northern intensive dissection, river rejuvenation and aggressive counterpart (say ‘2’), then the combined flow of 1 and 2 siltation of Vedaranniyam backwater also substantiate the would have pushed its water to its successive northern ongoing emergence of Pattukottai – Mannargudi Mio- counterpart 3 and so on. This must be the reason why each Pliocene Sandstone. It follows that the rise of the Pattukottai- of the northern palaeo distributary is broader than its

JOUR.GEOL.SOC.INDIA, VOL.67,MAY2006 LATE HOLOCENE GEOMORPHIC EVOLUTION OF CAUVERY DELTA, TAMIL NADU 655 southern counterpart (Fig.2B). In this process, at one stage, deflection of Palam Cauvery river (12) and the abrupt the mother channel Cauvery also would have been pushed / truncation of the beach ridges along the coast (13) etc. along shifted towards north to reach the present tract of river the lineament “D” (Fig.3D) indicate definite recent tectonism Coleroon. While the great Tamil literature Periyapuranam along this lineament. Infact, Ramasamy et al (in press) have suggests an age of 900 Y.B.P to the palaeochannel found observed compressed meanders with ”Z” shapes in a number along Tirupurampayam – Achalpuram on the north of Palam of places in and explained the same to be due to Cauvery (Veerasamy 1980), Janakiraman (1971) observed ongoing dextral strike slip movements along NW-SE faults. that the river Coleroon, which is located further north of the In this context, such “Z” shaped compression of the Coleroon above Periyapuranam palaeochannel, came into being as a river (11, Fig.3D) also might indicate the probable recent / result of surplus flow in Cauvery during 750 Y.B.P. Hence, ongoing dextral strike slip movement along this NW-SE it can be said that such sequentially drying southern lineament / fault. Similarly, extensive gulleying along the distributaries might have reached their mother channel other two NW – SE lineaments (E, F Fig. 3A) of Pattukottai Cauvery (now called as Palam Cauvery) before 900 Y.B.P. – Mannargudi area and their extension into the inland parts As the mother channel Cauvery was also under the pressure of Tamil Nadu with dextral strike slip movements of similar northerly push by the rising Pattukottai – (Ramasamy, in press) indicate their active tectonics in the Mannargudi upland in the south and with the additional southern part of the Cauvery delta. floodwater, the mother channel Cauvery has also shifted The anomalous deflection of Coleroon river near its towards north. In this process, the Cauvery would have mouth along an E - W fault (G, Fig.3E), the lesser sinistral reached the Coleroon tract around 750 Y.B.P. after briefly shift of coastal beds along this fault during 1931 AD and flowing along Tirupurampayam – Achalpuram tract around larger shift during 1993 AD observed by Ramasamy and 900 Y.B.P. This must be the possible model for the Ramesh (1999) suggest very recent tectonism along this anticlockwise migration of river Cauvery during 2300 – 750 fault. Further, the growth of such a spit in between 1931 AD Y.B.P. as suggested by the dates collected so far. Further and 1993 AD with northerly growth also confirms the precise dating of the palaeochannels at different depths observations made by Ramasamy (1989) that such projected through modern means would provide still finer and accurate land portions due to sinistral faults act as favourable loci information on the phenomenon of river migration and the for the development of northerly protruding spits in the east related geological history of the delta. coast of Tamil Nadu . Similarly, the E-W set of lineaments The Coleroon river which is the final arm of the Cauvery (H-K, Fig.3F) found in the delta are the reflections of recent river flowing in the northern- most rim of the delta is faults as these not only control the drainages but also strikingly controlled by a NE-SW trending lineament (C, fragmented the beach ridges and further act as pathways for Fig.3D, 3E). Ramasamy and Balaji (1995) have observed the creeks. Elliott et al (1998) have also referred to the E-W that the NE-SW faults of Tamil Nadu are Pleistocene sinistral tectonic weak zones in this part of the delta. faults in nature. Ramasamy (1995) has further observed that While the Mio-Pliocene Sandstone, palaeochannel these faults express clear sinistral displacements of beds in systems and the lineaments provide such a picture on the coast as well as in Laccadives. So one of such active post deltaic / Late Holocene geological history, the annular NE-SW faults found in the northern rim of the delta might palaeo drainages (1-4, Fig.3A&3B) indicate probable have trapped the anticlockwise migrating Cauvery river and morphostructures related to some doming. stabilised it as river Coleroon. The eyed drainage observed in Coleroon river near Virankoiltittu area (10, Fig. 3A, 3D, CONCLUSIONS 3E) while indicates probable land subsidence (Ramasamy and Kumanan 2000), its modifications during 1931 – 1993 The study of various geomorphic and tectonic anomalies AD suggest active sinistral movement along the NE – SW of the Cauvery delta indicates that lineament / fault which is controlling Coleroon river during 1. The Cauvery delta would have been built originally as these years (Ramasamy and Ramesh 1999). These an arcuate delta with well developed bird-foot observations indicate active tectonic movements along this distributary drainages with its mother channel along NE-SW lineament over a period of few decades. Thanjavur – Kumbakonam – Kaveripoompattinam, The conspicuous NW-SE Lineament seen in the delta the probable age of the delta may be around 2300- (D, Fig.3A) with signatures like abrupt truncation of the 750 Y.B.P. Jayamkondam Mio-Pliocene Sandstone to its north, 2. Due to the upliftment of the Mio-Pliocene Sandstone anomalous compressed meander in Coleroon river (11), in the south along N–S trending sub parallel hinge

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faults, probably since 6100 Y.B.P., the southern 2670±90 Y.B.P. for the upper flood plain of the distributaries might have become sequentially Cauvery, north of Tiruchirappalli, they have also pushed from south to north and the main river Cauvery overall observed that the Cauvery river was flowing would have also subsequently been pushed towards along Stanley reservoir – present tract of Ponnaiyar north to reach the present Coleroon tract during 2300- river during 2700 – 2300 Y.B.P. and might have entered 750 Y.B.P. and thus the delta building and the channel into Tiruchirappalli – Thanjavur plains only after migration would have concurrently taken place. 2300 Y.B.P. Raiverman (1969), however, observed that 3. The confinement of the Coleroon river along a prior to Cauvery’s flow in Thanjavur delta there flowed NE-SW fault and the eyed drainages and their a river called “Proto Amaravati” and one of its modifications in Coleroon river indicate active sinistral tributaries has captured Cauvery through headward movements along this fault. On the contrary, the erosion along a N–S fracture from Hogenekkal. Again, compressed meanders of Coleroon river show dextral Babu (1975) has observed unconformities in Cauvery movement along a NW-SE lineament / fault. The E-W delta related to various erosional surfaces from fault with sinistral movements a few decades ago and Mesozoic to Pleistocene. the growth of the spit in Coleroon’s mouth and also the 6. Hence, it is a matter to be resolved as to whether the fragmentation of the beach ridges and the opening up Thanjavur delta is built exclusively by Cauvery river of the creeks in the entire N – S segment of the coast or by the combination of earlier rivers and Cauvery. indicate active tectonism along this set of lineaments/ However, tectonic activities in the southern Pattukottai faults in the very recent years. area, northerly migration and the sequential shift of 4. While all such geomorphic anomalies and the tectonic the distributary channels and other tectonic features features strongly advocate such Syn - Post deltaic revealed by the geomorphic anomalies etc. indicate geological history predominated by the tectonic definite ongoing geological processes aided by tectonic movements, the age of the delta becomes significant. activity during Late Holocene. Hence, these warrant Infact, if the Cauvery river has entered into detailed studies both from the point of hydrocarbon Tiruchirappalli – Thanjavur plains during 2300 Y.B.P. and groundwater resources and from the point of then these geological events must be younger to it. ongoing tectonic activity of this region as well. But, however some of the geomorphic features /ages/ anomalies indicate a very young age of few tens to few Acknowledgment: The senior author acknowledges hundred years. Hence, these geological phenomenon ISRO, Dept. of Space, Bangalore which has sanctioned must be related to Late Holocene period. Project “RIMIGTA” (River Migrations of Tamil Nadu) 5. However, the date of entry of river Cauvery into through which the river migration studies were carried out Thanjavur delta has been visualized to be around and the DST, New Delhi for Seismotectonics project 2300 Y.B.P. on the basis of observations made by “SEISTA” (Seismo Tectonic Mapping of Tamil Nadu) Sarma (1991) and Ramasamy et al (1992). But, while which has helped the authors to study the active tectonic Ramasamy et al (1992) have again reported the date of features.

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