WAS THERE AN INTRACONTINENTAL RIFT BETWEEN INDIA AND SRI LANKA? by P.K. Agrawal and 0.P Pandey, Jour. Geol. Soc. India, v.54, 1999, pp.237-249.
MeV. Narasimha Chari, Regional Computer Centre, ONGC, 6th Floor (W), CMDA Towers, #8, Gandhi Irwin Road. Egrnore, Chennai - 600 008 comments:
The conclusion of the authors that "no intracontinental rift could ever develop between India and Sri Lanka" seems to have been hastily arrived at, overlooking geoscientilic data and conclusions published elsewhere. Also, the question as to what is the nature of the basin between India and Sri Lanka, if it is not an intracontinental rift, is left unanswered.
1. The authors have reviewed models proposed by earlier workers in the Introduction. But the authors have not brought out the flaws in the earlier model of Katz (1978), which necessitate the postulation of an alternative model. The perceived drawbacks pointed out in Discussion of Results ignore the findings of otl~er workers.
2. The Bouguer gravity anomaly map (1;ig.S) shows a total absence of data in the offshore area between India and Sri Lanka. Data pertaining to this area, which has been published by Verma et al. (1993) and Jain (1996) seems to have been totally ignored. These authors have also demonstrated a correlation between the anomalies and the horst-graben architecture of Cauvery basin.
3. Seismic and borehole data acquired by ONGC Ltd. during its exploration activities has been summarised by Sastri et al. (198 I), Kurnar (1 983) and Prabhakar and Zutshi (1 993), among others. Prabhakar and Zutshi (1993) have presented maps (Figs. 5 and 6) showing the basin architecture comprising horst-graben morphology with steep basin margin faults, and the thickness of the sediments deposited i n the basin.
4. The contention of the authors that India and Sri Lanka have always been apart cannot explain why we encounter only Early Cretaceous and younger sediments and no older sediments. Upper Gondwana sediments of Late Jurassic to Early Cretaceous age have only been encountered in limited troughs near the basin margins.
5. Mechanisms and models for the formation of extensional basins are available in the excellent textbook by Allen and Allen ( 1 99 I ) . Rifting can be classified as either active or passive. In the former, surface deformation is associated with the impingement of a mantle plume at the base of the lithosphere leading to doming and volcanism. In the latter, tensional stresses in the continental lithosphere cause it to fail, allowing hot mantle material to penetrate the lithosphere. In this class, doming and volcanism follow rifting.
DISCUSSION I f I
be classified as an intracontinental rift or aulacogen analogous to the North Sea, though in the latter lithosphere stretching was larger in magnitude and occurred over a longer duration.
P.K. Agrawal and O.P. Pandey, Theoretical Geophysics Group, National Geophysical Research Institute, Uppal Road, Hyderabad - 500 007 reply:
We would like to thank Dr. Narasimha Chari for taking keen interest in our paper and for his comments. Perhaps he has not been able to appreciate the potential of geophysical tools, particularly the gravity and magnetic fields, which provide an unmistakably distinct and characteristic signature over a fault or a rift structure due to contrast in density or susceptibitity and depth across the upthrown and downthrown blocks. Obviously, in a 2-D map these features are reflected in the form of high gradient contours. We have clearly stated as to why we differ with the proposition of the existence of a Proto Boundary Fault (PBF) on the SE coast of India and NW of Sri Lanka, as proposed by Crawford (1974), Katz (1978a, b) and Owen (1976). Our main contention was to examine whether any intracontinental rift could ever develop between these two regions encompassing Gulf of Mannar and Palk Strait regions (Fig.]) after their break up from Gondwanaland in early Cretaceous and not the origin of Krjshna, Godavari and Cauvery basins, as Dr. Chari has pointed out.
PONDICHERRY
To the best of our knowledge, no geophysical data seem to be available in the area of Gulf of Mannar and Palk Strait. However, on the advice of Prof. Kevin Burke who had earlier reviewed our manuscript, we contacted the Oil and Natural Gas Corporation Ltd. (ONGC) to send us the available data over these areas. After repeated requests we were provided the maps over Krishna- Godavari and Cauvery basins which Dr. Narasimha Chari has quoted. These maps do not appear to be pertinent to the problem on hand.
In the foIlowing we clarify each point raised by Dr. Narasimha Chari:
I . The proposition of a common PBF, separation of Sri Lanka from India, rotation and movevent of Sri Lanka are some of the available studies which are primarily based on geoIogic correlation and paleomagnetic studies (Katz, 2978a, b; Crawford, 1974; Yoshida et al. 1992). However, these inferences are not validated by geophysical, geornorpl~ological, Landsat imagery and seismicity data as detailed in our paper. For example, if there was a PBF separating the SE coast of India and NW coast of Sri Lanka, its gravity signature would have been distinct. Besides this, with our proposed position of Sri Lanka, there is an excellent
DISCUSSION f 13
conformity between the gcotectonic, geologic and geophysical data available over segments of East Gondwanaland asse~nbly (Pandey and Agrawal, 1999). It is worth mentioning here that a similar controversy based exclusively on geologic and paleomagnetic arguments for the paleoposition of Madagascar within Gondwanaland was resolved by us using potential field data sets (Agrawal et al. 1992).
2. As stated above, Vermaet al. (1993), Jain (1996) and Kumar (1983) have publishecl gravity data pertaining to Krishna-Godavari and Cauvery basins which are not relevant to thc present discussion. The paper by Sastri et al. (1981) is not accessible to us. Kumar's (19831 [nap (Fig.2) shows no gravity data on the offshore of St-i Lanka. Even his gravity dala of the Cauvery basin suggests smooth continuation of the contours across the East coast of India which indicate non-breakup or non-existence of PBF on SE coast of India and NW of Sri Lanka. Such a scenario i n the offshore Palk Strait also suggests continental nature of the crust in this region. It may also be interestirig to note that these anomalies smoothly close across the international boundary between India and Sri Lan ka similar to the one reported by us in Fig.5 of our paper. The magnitude of the anomaly A in our paper (-75 mgal) conforms well to -80 mgal of Fig.:! (Kumar, 1983). In order to reassess our conclusion on the available Bouguer gravity data, we also examined the recent Seasat-derived free-air gravity anomaly distribution over the area (Haxby, 1987) (Fig.3). Here again, we observe anomalous gravity pattern almost perpendicular to East coast of India and NW coast of
(up LO -100 rngal) along the Eastern margin of India which coincides with the shelf-edge. It
may be pointed out here that the long linear gravity contours coinciding 'with the shelf edge turn towards the east of Sri Lanka rather than passing through Palk Strait and Gulf of Mannar region lying between SE coast of India and NW coast of Sri Lanka (Fig.2). These patterns confirm no rifting episode and non-oceanic nature of the crirst between the two landmasses as suggested by us.
Prabhakar and Zutshi (1 993) also attempted to understand the tectonic origin and evolution of Krishna-Godavari, Penner, Palar and Cauvery basins over the East co;ist of India. These authors themselves have concluded the absence of any major volcanic activity along the Cauvery basin as mainly due to abortion of the tectonic cycle along the hypothesized Sri Lanka-Cauvery rift which support our contention.
Further, we do not dispute the existence o f horst-graben structure and localized faulting in the Cauvery basin, as pointed out by Dr. Chari. Our finding is that the intracratonic rift associated with hypothesized PBF on India and Sri Lanka is absent. The observed scdiment thickness in the Gulf of Mannar and Palk Strait is attributed to the post thermal subsidence induced by cooling.
3. The reason why we encounter only early Cretaceous and younger sediments and not the oIder sediments can be easily explained by our model that thermal remobilization beneath the Palk Strait and adjoining regions commenced only after early Cretaceous. It seems the major subsidence and sedimentation i n the Palk Strait was followed as a result of thermal influxing of this region associated with rifting processes on the eastern margin of greater India (- 130 Ma) and Marion plume activity (-100-80Ma). Cooling induced subsiclence would explain the accumulation of thick sedimentary column in the Gulf of Mannar region also.
4. Based on the mechanis~ns and models for the formation of extensional basins by various workers including his own model of the evoiutjon of the Cauvery basin, Dr. Narasimha Chari has classified Cauvery basin as an intracontinental rift or aulacogen. As explained above, rifting would have produced distinct geophysical signature which is missing in the Cauvery basin region. Also,-as reported in our paper, south of latitude 1 1
"
there are very few or almost no basic dykes present on the Eastern margin of Indian peninsula (in contrast to western margin) and a combined influence of both plate and plume tectonics seen-rs to have contributed to the evolution of the Cauvery basin. However, we would like to re-emphasize here that the origin of Cauvery basin is not what we have aimed to address in our paper. The basic issue is that of non-separation between SE coast of India and NW coast of Sri Lanka which is evidenced by multiparametric geoscientific data including Seasat derived free-air gravity data (Fig.3).In view of above, we reaffirm our contention of non-existence of PBF and non-separation of the Indian and Sri Lankan landmasses after their breakup from Gondwanaland in the early Cretaceous.
References
AGRAWAL, P.K., PANDEY, O.P. aid NEGI, J.G. (1992). Madagascar: A continental fragment of ihc paleo-super Dhanvar craton of India. Geology, v.20, pp.543-546.
.ALLEN, P.A. and ALLEN, J.R. (1991). Basin Analysis: Principles and Applications, Blackwell Scientific, Oxford. CRAWFORD, A.R. (1974). Tndo-Antarctica, Gondwanala~~d and distortion of a granulite belt. Tectonopliysics. v.22.
pp.14L-157.
DISCUSSION I IS
HAXQY, W. (1987). Gravity Field of the World Oceans, National Geoptiysical Data Centre, Colorado, USA.
JAIN, B.K. (1996). Composite gravity maps of Krishnn-Godavari and Cauvery basins. Proc. Second Internat. Sen~inar and Exhibition on "Geophysics Beyond 2 0 0 0 , Assoc. Expl. Geophys., Hyderabad,ppp.429-432.
KATZ, M.B. (1978a). Sri Lanka in Gondwanaland and the evolution of the lndian Ocean. Geol. Mag., v. f 15, pp.237-244.
KAIZ, M.B. (1978b). Tectonic evoIution of the Archaean granulite facies belt of Sri Lanka-South India. Jour. Geol. Soc. India, v. 19, pp. 185-205.
KUMAR, S.P. (1 983). Geology and hydrocarbon prospects of Krishna-Godavari and Cauvery baxins. Petroleum Asia Jour., v.6, pp.57-65.
MCKENZIE, D.P. (1978). Some remarks on the development of sedimentary basins. Earth Planet. Sci. Lett., v.40, pp.25-32.
NARASIMI-IA C H A R I , M.V.? SAEIIJ, J.N., BANERJEE, B., ZUTSEII, P,L. and CHANDRA, K . (1995). Evolu~ion of Cauvery basin, India, from subsidence modelling. Marine and Petroleuln Geol., v. 12, pp.667-675.
OWEN, H.G, (1976). Continental displacement and expansion of the earth during the Mcso~oic and Cenozoic. Phil. Trans. Roy. Soc., London, v.A28 1 , pp.223-291.
PANDEY, O.P. and AGRAWAL, P.K. (1999), Madagascar, Sri Lanka and Enderby Land (Antnrclic;~): Paleocontinental segments of a super south Indian shield. Geol. Jour. (communicaled).
PRABHAKAR, K.N. and ZUTSHI, P.L. (1993). Evolution o f southern part of Indian Ensr coast basins. Jour. Geol. Soc. India, v.4 1, pp.2 1 5-230.
ROYDEN, L. and KEEN, C.E. (1980). Rifting process and thermal evolution of the continental margin of eastern Canada determined from subsidence curves. Earth Planet. Sci. Lett., v.5 1, pp.343-36 1.
SASTKI, V.V., VENKATACHALA, B.S. and NARAYANAN, V. (1981). The evolution of the east coast of India. Paleogeogr. Paleoclimatol. and Paleoecol., v.36, pp.23-54.
VERMA. R.K.. SATYANARAYANA. Y. and RAO, S.C.S. (1993). Gravity field, tectonics and evolution of Krishna-Godavari and Cauvery basins of India. Jour. Petrol. Geol., v.2, pp.39-72.
Yosnru~, M., FUNAKI, M . and VITANAGE, P.W. (1992). Proterozoic to Mesozoic east Gondwana: The juxtaposition of India, Sri Lanka and Antarctica. Tectonics, v. I I, pp.381-391.
CHARACTERISATION OF GROUNDWATER IN THE UNCONFINED AQUIFERS OF CHENNAI CITY, INDIA by Ballukraya and Ravi, Jour. Geol. Soc. India, v.54, 1999, pp. 1-1 1.
N. Lakshmi Narayana, J 8-331 16, MAK Towers, Uppal Road, Hyderabad - 500 039 comments:
I . Based on the EC values (Table I), it was stated that "salinity increases during pre-recharge period (June) at well nos. 8,5,9 1 and 101 (page 6)" But EC values in Table 1 show increase only at well no.5 [7 140(1192) - 11 820 (6/93)] and well no. 101 [5820 (1192) - 7200 (6/93)] and decrease at well 110.8 [ 16000 (1192)
- I4530 (6/93)] and well no.91
[7 160 ( 1 192) - 6930 (6/93)]. Fig.2 shows that well nos. 5 and I01 are situated in sandy clay and well no. 8 and 91 in clay zone. So, grouping of well nos. 5, 8, 91 and 101 may not reflect the statement that "concentration of higher TDS in the groundwater in all the four locations may be due to restricted groundwater movement leading to slow enrichment."3. The chemical analysis of water sample from well no.8 shows high EC, Na and CI during post-recharge (1192) compares well with that of the situation in pre-recharge (6193) and is different from that of the sample of well nos. 2 , S , 42,46,50,65,68,8 1,87,95,99 and 101 which does not conform to the statement that "during the pre-recharge period, areas around we11 nos. 2,5,8,42,50,65,68,8 I , 87,95,99 and 101 fall under the discharge area category." 4. It is indicated that vertical recharge through infiltration of rain water is the dominant mode of groundwater recharge and in areas having clayey formations, the "TDS/EC increases after the rains as a result of dissolution of minerals from the overlying materials by the infiltrating water". The clay material with low permeability may not support the above conclusion as the well nos. (93,92,9 1, 89,86,8 1 and 87) having clay as overburden do not show any uniform pattern (Fig. 1, 2, 4 and Table I ) . To observe the correlation of water leveIs (1192 to 6193) with all parameters like EC, Na, CI etc., (Table I), presentation of water level data would have helped to get a clear picture and to understand the nature and level of recharge through various formations like clay, sand etc.
5. Comparison of Figs. 1 , 2 and 4 with Table 1 shows some mismatch:
Near Avadi, well no.86 show EC values of 990 pS/crn (6193, Table 1 ) i.e. fresh water whereas Fig.2 shows
>
3000 pS/cm i.e. brackish water.At well no.3, the EC value is 950 pS/cm (6193) in Table 1 i.e. freshwater whereas Fig.:! shows the same between 2000-3000 pS/cm i s . brackish water. In this borehole, the EC increases due to rainfall recharge (1192) i.e. 1190 (1/92) from 950 (6193) but Fig.4 reflects the same i n opposite way i.e., EC increases due to lowered groundwater level (6193). From 'Jhble 1, EC values varies from 270 pS/cm (well no.98 in 6193) to 14530 pS1cm (well no.8 in 6/93) but its spatial distribution in Fig.2 reflects EC contours varying from
1000 pS1cm to 6000 pS1cm only
The EC value of water sample at well no.78 is 3900 pS1cm i.e. brackish water (Table 1 ) but Fig.2 show the same around 1000 pSlcm i.e. freshwater.
6. Assuming that Fig.:! represents the EC data furnished in Table 1 (6193) any area showing EC more than 1560pSlcm (> 1000 ppnl TDS) reflects the quality of the water under the category of "brackish wate'i-" (Todd, 1980, p.281 and 3 10). In Fig.2, the area occupied by the EC vatues up to 1560 pSlcm accounts for
<
40% of the total study area of Chennai city. It means >60% of the city has the influence of "brackish water" in the unconfined aquifers. The intensity of brackishness goes up to 14530 pS1cm of EC (well no.8). This shows that significant (Fig.2 is not properly showing the data given i n Table 1 and may vary in light of above observations) part of the Chennai city's top unconfined aquifer water quality comes under "brackish water".7. From Table 1 it is observed that during pre-recharge (6/93), the EC values varies frorn 270 pS1cm to 14530 mslcrn and during post-recharge (1192) EC values varies from 320 pS/cm in well no.76 to 16000 pSlcm i n well no.8. During pre-recharge period 59 out of 101 and in post-recharge period 49 out of 10 1 well samples show EC > 1560 pS1cm i.e.,
>
1000 ppm to TDS. This reflects that during post-recharge and pre-recharge period considerable part of the study area is under the influence of "brackish water".DISCUSSION 117
9. Along the Cooum River (to the extent of 12 km), mostly the water quality of well nos. 48, 24,29, 21, 67, 68, 66, 65 and 64 (except well nos. 26 and 28 which shows increase in EC due to rainfall recharge as per Table 1 and Fig.4 shows the opposite trend i.e., EC increases due to lowered groundwater level) shows increase in EC in pre-recharge period due to Iowered groundwater level. Similarly along the Adayar River (to the extent of 5 km) the well nos. 43, 42, 38, 39, 37, 40 and 36 (except well no.41 which shows increase in EC during post-recharge period) show increase in EC during pre-recharge period. Along these two rivers, if it is to be assumed that the increase i n EC is due to the "tidal influx of seawater into the rivers" (page 1 I ) , it would have been more appropriate to suggest control measures like check dams (check dams help to increase the recharge during rainy season and decrease the tidal influx of seawater into the rivers during summer season) at the river mouth in addition to the one already suggested (page 1 I).
10. In the Chennai city area, it appears that the situation needs proper monitoring measures to enhance the quantum of recharge by employing suitable techniques to have an effective groundwater management system, which finally helps to improve the quality of life with a healthy hydrologic environment.
Reference
TODD, D.K. (1980). Groundwater Hydrology, John Wiley and Sons Inc.
P.N. Batlukraya, Department of Applied Geology, University of Madras, Guindy Campus, Chennai 600 025 replies:
The authors are gratified that N. Lakshmi Narayana has gone through the paper in such detail and brought out certain points for discussion, At the cutset, we would like to say that many of the points raised by him could be related to the fact that we had included only the chemical analysis results of two sezsons, while the paper is based on the results of anlaysis done during four seasons (1192, 6/92, 1/93 and 6/93) and of monthly monitoring in case of some selected wells during the period 7/94 and 7/95, as noted in the paper.
I . The reader points out that in the case of well nos. 8 and 9 I , the EC decreases during pre- recharge period and not increases. He is right if the data in Table 1 only is considered, which is for Jan.92 and June 93. In these wells the EC values for the four seasons are: in well 8 - 16000, 28000, 1 1780 and 14530; and in well 91 - 71 60, 78 10, 3980, 6930 pS/crn for the period 1192,6192, 1 I93 and 6/93. Thus, though there is decrease in EC from 1/92 to 6/93, the values do increase in these wells from 1/92 to 6/92 and also 2/93 to 6/93, i.e., from post- recharge to pre-recharge season. The results of chemical analysis of more periods were not included in the paper, as it would have made the table quite unwieldy.
Well nos. 5 and 8 are located in sandy clay aquifer areas (sand content 25-50%), well 91 in clay area (< 25%) and we11 101 in sandy clay-clayey sand area as per the figure. The figure shows only the regional picture based on strata logs of majority of wells in a given location. In an alluvial area local lithological variations, some time quite sharp, are quite common. The groupings were done on the basis of the lithology of the welIs, which may slightly differ from that of the figure due to reasons explained above. Restricted movement of groundwater is caused by presence of surface depressions in addition to clayey aquifer
3. Same as above.
4. Fig.2 is a map of sand content of the aquifer horizon and not that of overburden. Regarding pattern in reference to Fig.4, it is to be noted that the figure shows only the regional trend which might have tocal variations due to the nature of the sub-surface formation. Tn an alluvial terrain the sand content may vary with in a few metres and it would be almost impossible to show such details in a small scale map. Only a generalisecl map can be prepared which reflects the major trend. I agree that water level maps would improve the presentation and a few representative cases are given in Fig.3 along with geochemical results.
5. Regarding the mismatching of values in relation to the contours in Fig.2, it is inevitable that such minor errors enter a contour map when sharp variations in groundwater quality are present in an area. For example, the EC values for well 86 is 990 pS/cm. It is surrounded by zones having groundwater with ECs of 4520 (87); 3920 (78); 6920 (9 I) etc. Thus, a general trend is shown in the figure as otherwise too many contours will have to be drawn close to each other, which wiil mask the overall trend and also clutter the figure. In the case of well no.3, it is located very close to the sea shore and it would be difficult to show the 1000 pS/
cm contour there, in view of the small scale of the map. Well no.78 falls in the region of 3000 pS/cm in Fig.:! and not near 1000 pS/cm EC contour as noted by the reader.
While drawing contours to reflect spatial variation in the EC, certain averaging is necessary to remove the effects of spikes in isolated locations arid this is what the authors have done. 6. I agree that the groundwater quality in Chennai is mostly brackish (to saline in a few instances). That Fig.2 does not include contours of values up to 14,000 pS/cm is due to the small scale of the map 3s already discussed.
7. I agree with the reader that the groundwater in a significantly large area of Chennai remains brackish both during the post- and pre-recharge seasons.
8. The study on quality in the year 1995 showed that after a good monsoon, when groundwater IeveIs recovered fully in the area, the TDS in groundwater came back to their original levels and that there was no significant change over a hydrological year.
9. Tidal influx in the two rivers is limited about 2-3 km only from their mouth. Thus the increase in TDS upstream cannot be said to be caused by sea water intrusion as suggested by the reader.
10. I fully agree that the situation needs close monitoring iind also that recharge measures are to be taken up. Several organisations in Chennai are already advocating/constructing rainwater harvesting structures i n major apartment complexes and even i n individual houses to augment groundwater recharge.
HYBRID ACID XENOLITHS IN DOLERITE DYKES INTRUDING DECCAN
FLOOD BASALTS, PUNE-AHMEDNAGAR REGION, WESTERN INDIA by R.K. Sharrna, M.K. Pandit and S. Warrier, Jour. Geol. Soc. India, v.54, 1999, pp.303-308
Dr. S . Viswanathan, 10, Bapuji Aptd., R.P. Road, Dombivili (E), Mumbai 421 201 comments:
I . How do the authors account for only rhyolite bodies in dyke A and only granitic in B when the dykes are spatially close?
DISCUSSION I19
3. Hybridisation is a reciprocal phenomenon and as such chemical variations across a single xenolith into the dolerite body would provide a realistic picture.
4. H,O is very high in PV12, PV13, PV/4. Also Na,O (5.00) is greater than K , 0 (1 -08) in PVl4. Can the authors throw light?
R.K. Sharma, M.K. Pandit and S. Warrier reply:
We are thankful to Dr. S. Viswanathan for his keen inter-c~l ill c11t1. paper. The reply to his
comments is as under:
1. We have described only one dyke where B is in strike continuity of A. The rhyolitic and granitic xenoliths show spatial distribution within the same dyke; the rhyolitic ones are confined to the southern part and granitic to the northern part of the dyke. This preferential spatial distribution of the xenoliths has also been mentioned i n the paper (p.304). We have described the xenoliths as they occur in the field.
2. No significant ferromagnesian mineral is present in the granite except minor biotite flakes, 3 . . We agree that the hybridization is a reciprocal phenomenon. However, the matter of monitoring and evaluating the elemental dispersion across the interface requires a detailed geochemical investigation. This was not taken up as the paper was intended to report and provide the first hand information on the occurrence of 'acid xenoliths' which are rather rare in the dolerite dykes of Deccan Flood Basalt Province.
