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Geomorphology, tectonism and sedimentation in the Nal region, western India
Geomorphology 25 Ž1998. 207–223
Geomorphology, tectonism and sedimentation in the Nal region, western India Sushma Prasad, K. Pandarinath, S.K. Gupta
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Physical Research Laboratory, Post Box No. 4218, NaÕrangpura, Ahmedabad 380 009, India Received 28 July 1997; revised 16 March 1998; accepted 26 March 1998
Abstract The low lying Nal region in western India, linking the Gulf of Kachchh with the Gulf of Khambhat through the Little Rann and Nal Sarovar is barely 15 m above msl and lacks surface exposures. The evolutionary history of the Nal region using remote sensing data and sub-surface lithological correlation indicated that late Quaternary sedimentation in the Nal region was governed by changes in sea level and by tectonism in the region of Cambay Graben. The geomorphic evidence for changes in sea level was found in the form of inland palaeo-deltas and old mud flats. Abrupt changes in lithological data in the vicinity of Nal region pointed to the role of tectonism. Contrary to the earlier view, a shallow sea linked the Gulf of Kachchh to the Gulf of Khambhat only in a time period around Marine Isotope Stage 5. Our studies also suggest that the Nal region itself may not have witnessed any major uplift Žbeyond ; 10 m. during late Quaternary. q 1998 Elsevier Science B.V. All rights reserved. Keywords: Nal region; western India; Quaternary sedimentation; tectonism; geomorphology
1. Introduction The Quaternary sediments of Gujarat, in western India, preserve a record of a complex interplay of eustatic, climatic and tectonic changes. In the past few decades a concerted attempt has been made to decipher their evolutionary history through a study of exposed sections along river courses ŽSridhar, 1995; Tandon et al., 1997.. The low lying areas like the Nal region lack exposed sections and have remained unstudied or have received only cursory attention. The Nal region, links the salt encrusted flat land of the Little Rann of Kachchh to the Gulf of Khambhat through Nal Sarovar ŽFigs. 1 and 2.. It )
Corresponding author.
had been surmised earlier that the entire Nal region was a sea link, of which Nal lake is a remnant. This link was thought to have existed until ; 2 ka ŽMerh, 1992. and was believed to have been progressively filled due to fluvial input from N–NE and west ŽSukeshwala, 1948; Allchin et al., 1978.. However, the topographic low of Nal region does not coincide with the Cambay Graben, located to the east of Nal, where the Deccan traps have been downthrown by more than 2000 m ŽMathur et al., 1968.. Considering, Ži. the magnitude of Holocene eustatic sea level rise on the west coast of India, variously quoted at q2 to q6 m ŽGupta, 1976., ; q3 m ŽPant and Juyal, 1993; Hashimi et al., 1995., and Žii. the absence of any reported evidence of subsequent uplift for the region, a sea link at the time of Holocene
0169-555Xr98r$ - see front matter q 1998 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 5 5 5 X Ž 9 8 . 0 0 0 4 2 - 7
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Fig. 1. Map of the Nal and surrounding regions showing location of Nal Sarovar, geology and the various faults.
transgression seems unlikely. The close proximity of the location of Nal region to the tectonically active region of Cambay Graben in the east, makes it likely that both tectonism and eustasy may have played a role in the geomorphic evolution of this region. Located almost in the middle of the Nal region is a large Ž; 120 km2 ., shallow Ždepth ; 2 m. lake called Nal Sarovar. Its location within the climatically sensitive region of palaeo-Thar margin ŽGoudie
et al., 1973. makes it a potential site for detailed palaeoclimatic studies for reconstruction of late Quaternary variations of SW monsoon in India. This has become increasingly important as Indian summerr SW monsoon is a significant component of the global atmospheric circulation system and the GCM studies so far conducted have yet to produce an acceptable simulation of monsoon rainfall over India ŽSikka, 1997.. The other large lakes of NW India are located
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Fig. 2. Map of India and satellite imagery of Gujarat showing the location of Nal Sarovar and surrounding region.
several hundred km north in extra-tropical region in Rajasthan and the palaeoclimatic records from these lakes are influenced by the western disturbances during winter–spring ŽWinstanley, 1973.. It has been suggested that the winter–spring rainfall caused by western disturbances in Rajasthan can influence the precipitation efficiency of the atmosphere during the period of SW monsoon through biogeophysical feedbacks from vegetation, dust, albedo and soil moisture changes ŽDas, 1995; Gupta and Prasad, in press.. Since the Nal region located ; 400 km south, is much less influenced by the western disturbances, the palaeoclimatic data from this region are expected to provide important inputs for calibrating numerical atmosphere circulation models. However, towards this end, it is essential to understand the evolutionary
history of this region. Additionally, this site would give an insight into the various processes involved in the closure of a basin by sediment infilling and the role played by eustasy and tectonism in the development of Quaternary landforms. It was with this purpose that the present study was undertaken.
2. Area of investigation The study area ŽFig. 1. lies between the Gulfs of Kachchh and Khambhat between 22815X –23815X N and 71830X –72830X E. The entire region is low lying, with an average elevation of ; q15 m msl and the brackish water lake, Nal Sarovar, lies in the centre of
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this depression. Presently, no major river flows into the Nal lake and most of the input is in the form of shallow surface runoff during the summer monsoon season from the adjoining areas of Surendranagar and Ahmedabad districts, to the west and east respectively of Nal region. However, south and north of the Nal lake, small streams drain into the Nal region. The study area is bounded in the west by basaltic trap rocks of the Saurashtra and in the north-west by Jurassic–Cretaceous sandstone. To the extreme north-east of Nal are the pre-Cambrian igneous and metamorphic rocks of the Aravallis. In the immediate vicinity, to the east, lies the western boundary of Cambay Graben ŽWCBF. and the Quaternary alluvial plains occupying the Cambay Graben ŽFig. 1.. These show evidence of tectonism in the form of entrenched streams, cliffy sections and fault controlled river courses ŽGhosh, 1952; Sareen et al., 1993; Sridhar, 1995.. To the west ŽAAX . and south ŽAX BX . of Nal region are faults that manifest themselves as abrupt lithological boundaries between the alluvium and the Deccan basalts. Sridhar Ž1995. has also inferred a rotational fault ŽCCX . passing through Nal Sarovar ŽFig. 1..
3. Methodology Owing to the inherent limitations created by low lying nature of this area and the absence of adequate surface exposures, a combination of spatial data from remote sensing studies and stratigraphic sub-surface correlation from borehole lithologs has been employed to study the landform development. Geomorphological studies including palaeochannel identification, were carried out using Indian Remote Sensing Satellite 1A, False Colour Composite ŽIRS FCC. imagery ŽRow: 32 Path: 52. at 1:250,000 scale and Survey of India toposheets Ž1:250,000 and 1:50,000 scale.. For palaeochannel identification, 1:50,000 IRS FCC imageries of selected areas were also used. The major geomorphic units were demarcated, and selected field checks carried out. Subsurface lithological data from Nal and surrounding regions were collected from Government agencies, namely, Gujarat Water Resources Development ŽGWRDC., Central Ground Water Board ŽCGWB., Gujarat Water Supply and Sewerage Board
ŽGWSSB.. These lithologs were used to construct NS and NE–SW transects through the Nal region. The data from these studies were combined with the data on sedimentological, mineralogical and chronological Žradiocarbon and luminescence. measurements on a 54-m long core raised from the Nal Sarovar. The entire information was then synthesised with the available data on the eustatic sea level changes and a scenario for the evolution of Nal region was developed.
4. Results and discussion 4.1. Surface eÕidence 4.1.1. Geomorphology and drainage pattern Geomorphic features representing three distinct depositional environments, namely, Ži. marine, Žii. fluvial, and Žiii. aeolian, could be identified from the satellite data and field studies. These are ŽFig. 4.: Ži. inland palaeo-deltas, Žii. old mud flats, Žiii. alluvial plains, and Živ. stabilised dunes. 4.1.1.1. Palaeo-deltas. On the Saurashtra side, west of Nal region, fan shaped features were observed on the satellite imageries. These appear to coalesce in their lower part ŽFig. 5.. Sood Ž1983. had identified these features as deltas based on remote sensing alone. These, however, could also represent coalescing alluvial fans. During field checks, in a section upstream of Bhogava river ŽFig. 3., east of Limbdi, occurrence of basalt dominated coarse sand, showing foreset beds was observed. Underlying these, in some of the wells dug in the river bed, clayey sediments, lithologically similar to the clayey sediments found in the sub-surface in the Nal region were seen. The occurrence of forest beds, underlying a layer of surface siltrsand together with the occurrence of clayey sediments in the dug wells Žpossibly representing bottomset deposits. would suggest these fan shaped features to be deltas of the Gilbert type ŽReineck and Singh, 1980.. It must, however, be emphasised here that the region does not have adequate surface exposures and this study, together with that of Sood Ž1983. can at best be indicative of a deltaic origin. However, studies on a 54-m long core raised from the Nal region Ždiscussed subsequently.
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Fig. 3. A composite drainage map of the Nal and surrounding regions showing the present and palaeo-channels Žbroken lines..
also lent support to our surmise of these fan shaped features being palaeo-deltas. The inferred palaeo-deltas are presently 30 km inland, at an average elevation slightly over 15 m. In this region several small channels are seen which terminate abruptly and the morpho-boundary is located at ; q15 m msl ŽFig. 3.. A portion of the satellite image with our interpretation of the palaeodelta and the zone of terminating streams is shown in Fig. 5. Since we surmise, based on the satellite and field studies, that these fan shaped features represent
palaeo-deltas, the morpho-boundary would then be a palaeo-strandline. The rivers to the west of this palaeo-strandline have wide valleys that appear incompatible with the maximum seasonal flow in both pre- and post-monsoon seasons imageries. It is likely that these wide valleys were formed during a substantially wetter period in the past, when the rivers had higher erosional potential and larger load carrying capacity. Field studies also indicated limited river entrenchment of about 4 m in the palaeo-delta region.
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Fig. 4. Geomorphological map of the Nal and surrounding regions showing inland palaeo-delts, mudflats, dunes and alluvial plains.
According to Thornbury Ž1985., two conditions are necessary for the formation of deltas—the availability of sediments and sheltered conditions at the site of deposition in a body of water. The data from satellite and field observations, namely, Ži. valleys incompatible with the present seasonal flow; Žii. limited river entrenchment Žiii. presence of palaeostrandline at ; 15 m and Živ. absence of evidence Ždiscussed subsequently. of significant tectonic uplift of the Saurashtra region during late Quaternary, would suggest that the palaeo-deltas were formed at a time when eustatic level of sea was relatively higher so that it could reach as much as 30 km inland, and the rivers draining Saurashtra carried substantially larger sediment load.
4.1.1.2. Old mud flats. At a lower elevation, to the east of palaeo-deltas and south of Nal Sarovar ŽFigs. 4 and 5., two sets of older than present mud flats were identified between ; q14 m and ; q10 m msl on the basis of tonal and colour changes. The mud flat M1, with a darker tone is at ; q14 m msl and was mapped on both the eastern and western side of the Gulf of Khambhat. The mud flat M2, at a slightly lower elevation, had a lighter tone and is noticeable only on the western side. Present day mud flats were also mapped from the satellite images at elevations of upto ; q8 m msl. During field observations, wide expanse of flat, sparsely vegetated, at times salt encrusted land with silty clay type of soils was seen at the locations of M1, M2 and the present
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Fig. 5. Satellite imagery and interpretation showing inland deltas, zone of terminating streams and old mud flats south of Nal Sarovar.
day mud flats. There was also some agricultural activity both on M1 and M2 but not on present day flats. The ancient port town of Lothal Žsee Fig. 3., belonging to the Indus Valley Civilisation lies within M1, south of Nal Sarovar. The archaeological remains of this port town have been dated by radiocarbon in the interval 4.3–3.6 ka B.P. ŽThapar, 1993.. The region of mud flats also showed deranged drainage pattern ŽFigs. 3 and 5.. This drainage pattern may perhaps be indicative of the recent development of drainage in a region ŽThornbury, 1985.. Maximum entrenchment of streams is only 2 m in mud flats, on the western side, indicating that since their formation, there has been no major uplift in this region. The present elevation of these old mud flats ŽM1 and M2. may not be taken as an indication of the extent of eustatic sea level rise since the region
close to Gulf of Khambhat is subjected to tidal surges as high as 12 m. As mentioned earlier, in this region, present day mud flats are being formed at elevations up to q8 m above msl. Therefore, a sea-level rise of even a few meters in the past, accompanied by tidal surges would be enough to form the older mud flats. 4.1.1.3. AlluÕial plains. These are found to the east and north of Nal Sarovar. No surficial expression of Pleistocene sea level change was found. Instead, these sediments show cliffs, entrenched streams and fault controlled river courses, reflecting regional tectonism. The general slope of the region is towards southwest. Satellite images also indicate the presence of several abandoned palaeo-channels to east of Nal region
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ŽFigs. 3 and 6.. These occur as broad sinuous stretches exhibiting sharp tonal contrasts with their surroundings. These generally appear as whitish with brown patches, indicating the dominance of sandy material, and patchy vegetation. These palaeochannels appear to be broadly oriented in NE–SW direction. Some of the mapped palaeochannels were also checked in the field where a variety of related features were observed. At Phangdi, the pre-existing channels exist in patches as small seasonal streams that terminate abruptly. Near Bavla, it was observed that the shallow wells in the vicinity of the palaeochannels had relatively sweet water as compared to the surrounding regions. Ox-bow lakes caused by channel avulsion are also present. Thus, it appears that the presently barren area to the east of
Nal had several channels flowing through it which in the past may have been responsible for bringing sediments into this region. The rivers became inactive either due to a changeover to a drier climate andror due to tectonism which disrupted the drainage pattern in this region. The abandoned river courses lie in the Cambay Graben, known to have been tectonically active during late Quaternary ŽGhosh, 1952; Sareen et al., 1993.. 4.1.1.4. Dunes. Stabilised relicts of barchans, parabolic, longitudinal and obstacle dunes are present north-northeast of Nal Sarovar ŽFigs. 3 and 6.. Patel and Desai Ž1988. have attributed the formation of these dunes and those present in other areas of north Gujarat, to periods of stronger aeolian activity
Fig. 6. Satellite imagery and interpretation showing location of palaeochannels Ždashed lines., palaeo-dunes and alluvial plains north of Nal Sarovar.
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during periods of regression following the Flandrian sea level rise. However, TL data from north of Ahmedabad, indicated the dune building activity to have begun as early as ; 20 ka ŽWasson et al., 1983. with intervening periods of stabilisation. Considering that Ži. the dunes overlie the fluvial sediments, Žii. the wide variety of dune forms observed, and Žiii. the differences in wind regime required for the formation of such forms, it is not unlikely that the dunes represent several periods of aeolian activity, probably pre-dating 20 ka.
4.1.2. Discussion: surface eÕidence Summarising, the salient points of remote sensing and field studies, mentioned above, are: Ži. rivers on the western Saurashtra side have wide valleys inconsistent with the present seasonal flow; Žii. palaeo-deltas Želevation slightly over 15 m. and a palaeostrandline Žat ; 15 m. are also seen on Saurashtra side; Žiii. evidence of Holocene transgression, in the form of old mud flats M1 and M2, showing deranged drainage pattern, was observed south of Nal Sarovar, at a lower topographic position than the palaeo-deltas. The ancient port town of Lothal is located in M1; Živ. both in the palaeo-delta and in the old mud flats only limited Ž; 2–4 m. river entrenchment is observed suggesting the absence of significant tectonic uplift subsequent to their formation. The presence of palaeo-deltas and old mud flats would also point to at least two episodes of transgression. The evidence of relative topographical position, archaeology and deranged drainage pattern pointed to the mud flats ŽM1 and M2. having been formed during the most recent, possibly Holocene, transgression. This also implies that the inland palaeo-deltas were formed during an earlier transgression of the sea. It is possible to explain the formation of various geomorphic features by considering the possible role of eustatic sea-level changes and tectonism during the late Quaternary. In the following we consider the available evidence of sea-level changes and tectonism from the Saurashtra coast. The evidence for sea-level change is available from two kinds of radiometrically dated deposits: Ži. miliolites, which are found in coastal as well as inland regions, and Žii. non-miliolitic material Žoysters and corals. found
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in coastal areas only. In this paper only non-miliolite deposits are considered as the miliolite ages are embroiled in controversies owing to the debate about their origin and chronology ŽBaskaran et al., 1989; Gupta, 1991; Bruckner, 1987; Merh, 1995.. Corals, from NW Saurashtra, Želevation - 10 m. give 234 Ur238 U ages that fall in two age brackets: 6–7 ka and 118–176 ka B.P. ŽGupta and Amin, 1974; Somayajulu et al., 1985.. The spread of ages between 118 and 176 ka, could well be due to the post depositional chemical alteration of the sample ŽSomayajulu et al., 1985.. A similar age range has also been obtained by 230 Thr234 U dating of oysters Želevation- 12 m. from southern part of Saurashtra coast ŽJuyal et al., 1995.. These data on sea-level change along Saurashtra, coupled with the eustatic ; 7 m rise in sea level during last major interglacial transgression Ž; 125 ka. ŽKu et al., 1974; Cronin et al., 1981; Chappel and Shackleton, 1986. would clearly imply the absence of large scale Žbeyond 10 m. uplift in these regions during late Quaternary. This would also suggest that the palaeo-deltas, located slightly over 15 m elevation, were formed during the last major interglacial transgression of the sea at ; 125 ka. This would require only a small Ž- 10 m. tectonic uplift subsequent to their formation—consistent with the field observations of limited river entrenchment Ž; 4 m. on the Saurashtra side. This is contrary to the view held by Baskaran et al. Ž1989., who have indicated an uplift of ) 50 m for the Saurashtra peninsula, in this region, during late Quaternary on the basis of miliolite ages. If the suggested age assignment of ; 125 ka for the formation of palaeo-deltas is correct, it is reasonable to expect that the low lying Nal region was covered by a shallow sea around that time. Therefore, a study of sub-surface lithology and chronology should help to corroborate the above hypothesis. 4.2. Subsurface eÕidence 4.2.1. Lithological correlation from borehole data Lithological data from several boreholes Ždepth of sections 60–200 m. drilled for ground water exploitation were collected. Two transects in NE–SW direction and one in NS direction across the Nal region were constructed ŽFig. 7.. The lithological
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Fig. 7. Map showing the location of bore-hole sites and transects taken. Also shown are the eastern and western margins of Cambay Graben. The approximate boundaries of the Nal region are shown by dotted lines.
information is based on drillers’ logs and electrical resistivity log data. The NE–SW transects ŽYYX and ZZX in Fig. 8. indicated that, in the eastern side of Nal, the lithology is dominated by sand with intercalations of clayrsilty clay. However, in the vicinity of Nal Sarovar and along the axis of the low lying region extending from near the Little Rann in the north to Dholera in south near the Gulf of Khambhat, a change in lithology to a layer of sand underlain by a thick Ž40–55 m. sequence of clayrsilty clay Žsimilar to that found in dug wells in the inland delta region. is seen ŽFig. 8.. The lithologs of Vekria, Ranagadh and Phulwadi, which also lie in the Nal region, also show a layer of sand underlain by a thick sequence of clayrsilty clay. The thickness of the silty-clay horizon in the borehole sections varied between 40– 55 m in the NS transect and elevation in relation to msl of the top of this layer varies from q3 m in north to y30 m in the south, in the Nal region. The top part of the silty clay horizon in the basin thus appear to be sloping towards the south.
The presence of a thick sequence of clayrsilty clay indicates a similarity in depositional environment in the entire low lying region, extending from near the Gulf of Khambhat to Little Rann of Kachchh, in a narrow corridor about ; 25 km wide as shown in Fig. 7 by the two dashed lines. 4.2.2. EÕidence from Nal SaroÕar core A 54-m long core ŽFig. 9. was raised from the Nal Sarovar bed for laboratory studies Žsedimentological, clay and heavy mineral, d 13 C and CrN on organic matter, radiocarbon and luminescence dating.. The coring was done using rotary type drilling rig with a tungsten carbide bit, with a 5 cm internal diameter of the core tube without using drilling fluid Ždry drilling.. After about every 1.5 m of drilling, the core was raised and the sample removed. Only a summary of studies on the Nal Sarovar core is given here. The details have been discussed elsewhere wPandarinath, K., Prasad, S., Gupta, S.K. Late Quaternary palaeoclimate and sea-level changes inferred from sedimentology and clay minerals: Nal Sarovar,
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Fig. 8. Lithological variation along the transects shown in Fig. 7. Note similarity in lithological sequence in the Nal region.
Western India Žcommunicated.; Prasad et al., 1997x. Three lithounits were recognised in the core on the basis of texture and mineralogy.
Ø Horizon-1 Ž0–3 m. comprised clayey silt with organic matter. In this horizon clay fraction varied between 3–80% and was dominate by illite Ž74–
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Fig. 9. Lithology of the Nal Sarovar core.
81%., followed by smectite Ž7–13%.. An uncalibrated radiocarbon age of 6.75 " 0.13 ka was obtained on organic fraction from the base of this horizon. Ø Horizon-2 Ž3–18 m. was dominantly sandy. Occasional subrounded basalt fragments Ž2–4 mm size. were also found in this horizon. The clay
fraction varied from 3–17%, and was dominated by smectite Ž43–74%., followed by illite Ž17–38%.. As the clay fraction in this horizon was small, heavy minerals were used to determine the provenance of this horizon. The heavy mineral assemblage was dominated by opaques Žmagnetite and ilmenite. followed by garnet, epidote, staurolite, hornblende,
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monazite and rutile indicating its source to be from the igneous andror metamorphic rocks from the east and north east. To identify the depositional environment for this horizon, grain size parameters were computed and plotted on a sorting vs. skewness bivariate plot ŽFig. 10. using the technique of Friedman Ž1961.. However, since it has been indicated that the usefulness of grain size parameters in identifying depositional environment, by themselves, is limited ŽSolohub and Klovan, 1970., we have relied on a comparison of grain size parameters of Nal core sediments with those obtained from core samples raised from the present day Sabarmati riverbed on the bivariate plot. This approach was chosen because the Sabarmati is the nearest large river originating in the north-east and traversing the Cambay Graben to flow into the Gulf of Khambhat ŽFig. 3.. The late Quaternary sediments in the Cambay Graben are overwhelmingly fluvial ŽMerh and Chamyal, 1993.
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and are believed to have been deposited by stronger fluvial regimes during Quaternary ŽSridhar et al., 1994.. It is, therefore, not unlikely that the sediments in the sandy horizon of Nal Sarovar were deposited by this older fluvial regime of which Sabarmati formed a part. Samples from both the sites fall within the field of fluvial sediments ŽFig. 10.. Additionally during the luminescence dating of the Nal core samples, it was found that the sediments from Horizon-2 indicated inadequate pre-depositional zeroing of the geological Thermally Stimulated Luminescence ŽTL. signal ŽPrasad, 1996., not an unusual situation in fluvial sediments ŽBerger, 1988; Berger and Easterbrook, 1993; Forman and Ennis, 1991.. For this reason, the Optically Stimulated Luminescence ŽOSL. method, in which only a few minutes of sun exposure is sufficient to zero the geological OSL signal, was used for dating of these sediments. A radiocarbon age estimate of ; 7 ka at
Fig. 10. Skewness vs. Sorting in sediments of Nal Sarovar core ŽHorizon-2. compared with sediments obtained from the cores raised from the present day Sabarmati river bed near Ahmedabad.
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3 m depth and an OSL age estimate of ; 65 ka at 18 m depth was obtained for this horizon. The age estimates were constrained by measured radiocarbon and luminescence ages of 6.75 " 0.13 ka at 3 m depth and 69 " 9 ka at ; 24 m depth. No fossils were found in this horizon. Ø Horizon-3 Ž18–) 54 m. comprised clayey silt and silty clay with occasional sand lenses and subrounded basalt fragments. In this horizon, the dominant clay mineral was smectite Ž53–97%. upto 31 m depth, below which the clay became monominerallic Ž) 99% of smectite.. A luminescence age range of 69 " 9 ka at ; 24 m depth and ; 101 " 10 ka at 54 m depth ŽPrasad, 1996. was obtained for this horizon. The base of this lithounit was not reached in the core. No fossils were found in this horizon either. The dominance of smectite in this horizon is indicative of its origin from a basaltic terrain. It was possibly derived from the basaltic rocks on the Saurashtra side or brought in from the Gulf of Khambhat due to action of tidal currents. Mineralogically and sedimentologically, the sediments in the inner shelf near the Gulf of Khambhat are similar to the Horizon-3 of the core ŽRao, 1991; Rao and Rao, 1995.. Interestingly, a yellowish clay layer, ŽRL y7 m., comprising 80–90% smectite, devoid of shells and having an abrupt contact with the overlying gritty sand layer, has also been reported from the Little Rann of Kachchh ŽGupta, 1975.. This layer may be correlated with Horizon-3 of the Nal core. The contact between Horizon-2 and Horizon-3 is abrupt whereas Horizon-2 grades into Horizon-1.
5. Synthesis and interpretational model On the Saurashtra side inland deltas were identified at an elevation of ) 15 m and ascribed to last major interglacial transgression Ž; 125 ka.. In the low lying Nal region to the east of these deltas, an unfossiliferous layer of smectite-dominated clayey siltrsilty clay is found, the top of which had a relative elevation in relation to msl varying from q3 m in north to y30 m in the south in the Nal region. Similar sediments have also been reported in the inner shelf off the Gulf of Khambhat and also at RL y7 m in the little Rann to the north. The OSL
chronology of this layer indicated that this was deposited in the interval ) 65 ka to ) 100 ka. All these would indicate that the Nal region was indeed covered by a shallow sea during the period ) 65–; 100 ka. The base of Horizon-3 was not reached in the core but it is reasonable to expect that the region which received marine sedimentation from ; 65–; 100 ka would have also been covered by sea at ; 125 ka when the sea level was ; 7 m higher than present. The absence of marine fauna in the ) 63 m fraction studied is puzzling. However, the environment visualised here during the deposition of Horizon-3, for the most part, is of a narrow shallow sea corridor with influx of water and sediments from both the land and the sea. It would have been subjected to considerable salinity fluctuations induced by tidal changes, fresh water flux during monsoon months and high rates of evaporation during summer months. Very few species can survive under such stressed environments of wide salinity fluctuations ŽRaup and Stanley, 1985.. An indication of this is also found in a study of present day distribution of planktonic foraminifera in the Arabian sea. Near the Gulf of Khambhat, very low concentrations of foraminifera Ž0–200 specimensr1000 m3 . were found as compared to 7000–40,000 specimensr1000 m3, further south. This has been attributed to arid climate around this region and resulting intense evaporation ŽRao et al., 1991.. The presence of a Ž5–35 m. thick sandy horizon in the bore well sections in the entire Little Rann to Gulf of Khambhat corridor suggests that during its deposition the sea link no longer existed in this region and high energy fluvial sediments derived from east and north east were being deposited in this entire belt. To identify the depositional environment for the sand-dominated Horizon-2 integration of surface as well as sub-surface data was done. The presence of the palaeochannels, the extensive occurrence of fluvial deposits to the east, in the Cambay Graben region, the close agreement of the Nal Sarovar core samples and Sabarmati riverbed samples in the bivariate plots and heavy mineral assemblage, and inadequate pre-depositional zeroing of the luminescence signal indicated that the sediments of Horizon-2 of the Nal core are fluvial in nature. These could have been deposited during advancement of the depositional front of the eastern rivers as a result
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of the combined effect of a eustatic lowering of the sea level and tectonic uplift in the region of Cambay Graben. Presently, this area has a surface elevation of q80 m to q100 m and the low elevation area has shifted to Nal Sarovar which is q13 m to q16 m msl. Evidences of Late Quaternary tectonism in the Cambay Graben region have been reported ŽGhosh, 1952; Sareen et al., 1993.. As a result of the combined influence of Ži. the westward advance of the sedimentation front; Žii. tectonism and; Žiii. the post glacial sea-level rise, the level of the Nal Sarovar came to within few metres of its present elevation at about 7 ka when it became a closed basin. The present Nal Sarovar, therefore, originated as a result of westward advance of the sedimentation front until it could no longer advance due the presence of the high land of Saurashtra. At that time, either due to sedimentation processes alone or aided by tectonism, the west-flowing rivers shifted their courses southwards and presently only the abandoned channels remain.
6. Conclusions Based on the data and discussion presented in the foregoing, the following conclusions may be drawn. 1. The geomorphological features and sub-surface lithological data revealed that the Nal region has had an evolutionary history different from that of the eastern region of Cambay Graben. The sedimentation in this region was linked to the late Quaternary eustatic rise and fall of sea level, the evidence of which is found in the form of palaeo-deltas and mud flats. 2. Mineralogical and chronological studies on the Nal Sarovar core indicated that the sediments deposited during the period ; 65 ka to ) 100 ka were rich in smectite. These fine grained sediments are correlatable lithologically throughout the Nal corridor and are also similar to the present day sediments found in the inner shelf in the Gulf of Khambhat. The low lying Nal region, linking the gulfs of Kachchh and Khambhat through Nal Sarovar, was covered by a shallow sea and clayey silt and silty clay sediments of Horizon-3 were being deposited during this interval.
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3. Combining the evidence of geomorphology and sub-surface lithological data with Ži. the global eustatic sea level curve Žii. the elevation in relation to msl of the top of Horizon-3 in the Nal region Žq3 m and y30 m respectively. Žiii. chronological evidence of raised marine deposits Žwith the exception of the controversial miliolite deposits. along the Saurashtra coast which indicate an absence of large scale tectonic uplift, we suggest that the palaeo-deltas were formed during the last major interglacial transgression of the sea Ž; 125 ka.. There may have been some Ž; 10 m. tectonic uplift subsequent to the formation of inland deltas but our studies rule out any large scale uplift. 4. Subsequently, with the eustatic lowering of sea level, the sea receded from the Nal corridor ; 65 ka B.P. and fluvial sediments were being deposited in the Nal corridor as a result of westward migration of the depositional front of the eastern rivers. The process continued until ; 7 ka B.P. when the sedimentation front could no longer advance westwards. The old mud flats, south of Nal Sarovar, were formed during the Holocene transgression. In this paper we have been able to decipher the major events in the evolutionary history of the Nal region. We have shown that the record in this area spans nearly a complete climatic cycle from the last major interglacial to the present. Further detailed work is needed to clearly decouple the effects of eustasy, climate and tectonism. This would yield important data for the palaeoclimatic reconstruction in the tropical areas of southeast Asia.
Acknowledgements This work was partly supported by DST grant number ESTr44r014r90. The Directorate of Geology and Mining, Gujarat is thanked for raising the core from the Nal Sarovar. The State Govt. Agencies, GWRDC, CGWB and GWSSB are thanked for providing the bore-hole data. Help and assistance of the Centre for Environmental Planning and Technology ŽCEPT. Ahmedabad; and Gujarat Engineering Research Institute ŽGERI. and M.S. University of Baroda in remote sensing studies is gratefully acknowledged. Mr. R.D. Deshpande is thanked for
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making available unpublished grain size data from Sabarmati river bed cores. Prof. A. Gupta and an anonymous reviewer are thanked for their suggestions which helped to improve the manuscript.
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Gupta, S.K., Prasad, S. Holocene Palaeoclimate of NW India. Encyclopedia of Quaternary Sciences, Chapman & Hall Žin press.. Hashimi, N.H., Nigam, R., Rajagopalan, G., 1995. Holocene sea level fluctuations on Western Indian continental margin: An update. Geol. Soc. India 46, 157–162. Juyal, N., Pant, R.K., Bhusan, R., Somayajulu, B.L.K., 1995. Radiometric dating of late Quaternary sea levels of the Saurashtra coast, Western India: an experiment with oyster and clam shells. Memoirs. Geol. Soc. India 32, 372–379. Ku, T.L., Kimmel, M.A., Easion, W.H., O’Neil, T.J., 1974. Eustatic sea level 120,000 years ago on O’ahu, Hawaii. Science 183, 959–962. Merh, S.S., 1992. Quaternary sea level changes along Indian coast. Proc. Indian Natl. Sci. Acad. 58, 461–472. Merh, S.S., 1995. Geology of Gujarat. Publ. Geological Soc. of India. Merh, S.S., Chamyal, L.S., 1993. The Quaternary sediments in Gujarat. Curr. Sci. 64, 823–827. Mathur, L.P., Rao, K.L.N., Chaube, A.N., 1968. Tectonic framework of Cambay basin, India. Bull. ONGC 5 Ž1., 7–28. Pant, R.K., Juyal, N., 1993. Late Quaternary coastal instability and sea level changes: New evidences from Saurashtra coast, Western India. Z. Geomorph N.F. 37, 29–40. Patel, M.P., Desai, S.J., 1988. Environmental significance of fossil sand dunes of north Gujarat. In Proc. Natl. Sem. Recent Quat. Studies in India, 1988, pp. 139–147. Prasad, S., 1996. Late Quaternary palaeoenvironment and evolution of Nal region, Gujarat, India. Unpubl. Ph.D. Thesis. M.S. University of Baroda, India; Prasad, S., Gupta, S.K. Luminescence dating of a 54 m long core from Nal region, Western India—implications Žcommunicated.. Prasad, S., Pandarinath, K., Gupta, S.K., 1997. Late Quaternary evolution of Nal region, Gujarat, India. In: Das, S.N., Thakur, R.S. ŽEds.., Changes in Global Climate due to Natural and Human Causes. Allied Publishers, pp. 176–179. Rao, V.P., 1991. Clay mineral distribution in the continental shelf and slope off Saurashtra, west coast of India. Indian J. Mar. Sci. 20, 1–6. Rao, V.P., Rao, B.R., 1995. Provenance and distribution of clay minerals in the sediments of the western continental shelf and slope of India. Continental Shelf Res. 15 Ž14., 1757–1771. Rao, K.K., Jayalakshmy, K.V., Kutty, M.K., 1991. Ecology and distribution of recent planktonic foraminifera in eastern part of the Arabian Sea. Indian J. Mar. Sci. 20, 25–35. Raup, D.M., Stanley, S.M., 1985. Principles of Palaeontology. CBS publishers and distributors, 2nd edn., 261 pp. Reineck, H.-E., Singh, I.B., 1980. Depositional Sedimentary Environments. Publ. Springer-Verlag, 322 pp. Sareen, B.K., Tandon, S.K., Bhola, A.M., 1993. Slope deviatory alignment, stream network and lineament analysis of the Sabarmati river system—Neotectonic activity in the Mid to Late Quaternary. Curr. Sci. 64, 827–836. Sikka, D.R., 1997. Desert climate and its dynamics. Curr. Sci. 72, 448–459. Solohub, J.T., Klovan, J.E., 1970. Evaluation of grain-size parameters in lacustrine environments. J. Sed. Petrol. 40, 81–101.
S. Prasad et al.r Geomorphology 25 (1998) 207–223 Somayajulu, B.L.K., Broecker, W.S., Goddard, J., 1985. Dating Indian corals by U-decay-series method. Quaternary Res. 24, 235–239. Sood, R.K., 1983. Geomorphology of the Kathiawar Peninsula and Environs Using Remote Sensing Techniques. Unpubl. Ph.D thesis. Sridhar, V.S., Chamyal, L.S., Merh, S.S., 1994. North Gujarat rivers: remnants of a super fluvial system. J. Geological Soc. India 44, 427–434. Sridhar, V.S., 1995. Sequential Stratigraphy and Palaeoclimatic Studies on the Quaternary Deposits of Lower Luni and Sabarmati basins of Western India. Unpubl. Ph.D thesis, M.S. Univ. Baroda. Sukeshwala, R.N., 1948. Geological evolution of Maha-Gujarat. Maha Gujarat Series. Publ. Gujarat Res. Soc. Monograph 3, 1–28. Tandon, S.K., Sareen, B.K., Rao, M.S., Singhvi, A.K., 1997.
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Aggradation history and luminescence chronology of Late Quaternary semi-arid sequences of the Sabarmati basin, Gujarat, Western India. Palaeogeog. Palaeoclim. Palaeoeco. 128, 339–357. Thapar, B.K., 1993. The Harappan civilization: Some reflections on its environments and resources and their exploitation. In: Possehl, G.L. ŽEd.., Harappan Civilization. American Institute of Indian Studies and Oxford and IBH Publishing, New Delhi, India, pp. 3–13. Thornbury, W.D., 1985. Principles of Geomorphology, 2nd edn. Wiley Eastern, pp. 170–173. Wasson, R.J., Rajaguru, S.N., Misra, V.N., Agrawal, D.P., Dhir, R.P., Singhvi, A.K., 1983. Geomorphology, Late Quaternary stratigraphy and palaeoclimatology of the Thar dune field. Z. Geomorph. N F. 45, 117–151. Winstanley, D., 1973. Recent rainfall trends in Africa, the Middle East and India. Nature 243, 464–465.
Geomorphology, tectonism and sedimentation in the Nal region, western India Sushma Prasad, K. Pandarinath, S.K. Gupta
)
Physical Research Laboratory, Post Box No. 4218, NaÕrangpura, Ahmedabad 380 009, India Received 28 July 1997; revised 16 March 1998; accepted 26 March 1998
Abstract The low lying Nal region in western India, linking the Gulf of Kachchh with the Gulf of Khambhat through the Little Rann and Nal Sarovar is barely 15 m above msl and lacks surface exposures. The evolutionary history of the Nal region using remote sensing data and sub-surface lithological correlation indicated that late Quaternary sedimentation in the Nal region was governed by changes in sea level and by tectonism in the region of Cambay Graben. The geomorphic evidence for changes in sea level was found in the form of inland palaeo-deltas and old mud flats. Abrupt changes in lithological data in the vicinity of Nal region pointed to the role of tectonism. Contrary to the earlier view, a shallow sea linked the Gulf of Kachchh to the Gulf of Khambhat only in a time period around Marine Isotope Stage 5. Our studies also suggest that the Nal region itself may not have witnessed any major uplift Žbeyond ; 10 m. during late Quaternary. q 1998 Elsevier Science B.V. All rights reserved. Keywords: Nal region; western India; Quaternary sedimentation; tectonism; geomorphology
1. Introduction The Quaternary sediments of Gujarat, in western India, preserve a record of a complex interplay of eustatic, climatic and tectonic changes. In the past few decades a concerted attempt has been made to decipher their evolutionary history through a study of exposed sections along river courses ŽSridhar, 1995; Tandon et al., 1997.. The low lying areas like the Nal region lack exposed sections and have remained unstudied or have received only cursory attention. The Nal region, links the salt encrusted flat land of the Little Rann of Kachchh to the Gulf of Khambhat through Nal Sarovar ŽFigs. 1 and 2.. It )
Corresponding author.
had been surmised earlier that the entire Nal region was a sea link, of which Nal lake is a remnant. This link was thought to have existed until ; 2 ka ŽMerh, 1992. and was believed to have been progressively filled due to fluvial input from N–NE and west ŽSukeshwala, 1948; Allchin et al., 1978.. However, the topographic low of Nal region does not coincide with the Cambay Graben, located to the east of Nal, where the Deccan traps have been downthrown by more than 2000 m ŽMathur et al., 1968.. Considering, Ži. the magnitude of Holocene eustatic sea level rise on the west coast of India, variously quoted at q2 to q6 m ŽGupta, 1976., ; q3 m ŽPant and Juyal, 1993; Hashimi et al., 1995., and Žii. the absence of any reported evidence of subsequent uplift for the region, a sea link at the time of Holocene
0169-555Xr98r$ - see front matter q 1998 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 5 5 5 X Ž 9 8 . 0 0 0 4 2 - 7
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Fig. 1. Map of the Nal and surrounding regions showing location of Nal Sarovar, geology and the various faults.
transgression seems unlikely. The close proximity of the location of Nal region to the tectonically active region of Cambay Graben in the east, makes it likely that both tectonism and eustasy may have played a role in the geomorphic evolution of this region. Located almost in the middle of the Nal region is a large Ž; 120 km2 ., shallow Ždepth ; 2 m. lake called Nal Sarovar. Its location within the climatically sensitive region of palaeo-Thar margin ŽGoudie
et al., 1973. makes it a potential site for detailed palaeoclimatic studies for reconstruction of late Quaternary variations of SW monsoon in India. This has become increasingly important as Indian summerr SW monsoon is a significant component of the global atmospheric circulation system and the GCM studies so far conducted have yet to produce an acceptable simulation of monsoon rainfall over India ŽSikka, 1997.. The other large lakes of NW India are located
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Fig. 2. Map of India and satellite imagery of Gujarat showing the location of Nal Sarovar and surrounding region.
several hundred km north in extra-tropical region in Rajasthan and the palaeoclimatic records from these lakes are influenced by the western disturbances during winter–spring ŽWinstanley, 1973.. It has been suggested that the winter–spring rainfall caused by western disturbances in Rajasthan can influence the precipitation efficiency of the atmosphere during the period of SW monsoon through biogeophysical feedbacks from vegetation, dust, albedo and soil moisture changes ŽDas, 1995; Gupta and Prasad, in press.. Since the Nal region located ; 400 km south, is much less influenced by the western disturbances, the palaeoclimatic data from this region are expected to provide important inputs for calibrating numerical atmosphere circulation models. However, towards this end, it is essential to understand the evolutionary
history of this region. Additionally, this site would give an insight into the various processes involved in the closure of a basin by sediment infilling and the role played by eustasy and tectonism in the development of Quaternary landforms. It was with this purpose that the present study was undertaken.
2. Area of investigation The study area ŽFig. 1. lies between the Gulfs of Kachchh and Khambhat between 22815X –23815X N and 71830X –72830X E. The entire region is low lying, with an average elevation of ; q15 m msl and the brackish water lake, Nal Sarovar, lies in the centre of
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this depression. Presently, no major river flows into the Nal lake and most of the input is in the form of shallow surface runoff during the summer monsoon season from the adjoining areas of Surendranagar and Ahmedabad districts, to the west and east respectively of Nal region. However, south and north of the Nal lake, small streams drain into the Nal region. The study area is bounded in the west by basaltic trap rocks of the Saurashtra and in the north-west by Jurassic–Cretaceous sandstone. To the extreme north-east of Nal are the pre-Cambrian igneous and metamorphic rocks of the Aravallis. In the immediate vicinity, to the east, lies the western boundary of Cambay Graben ŽWCBF. and the Quaternary alluvial plains occupying the Cambay Graben ŽFig. 1.. These show evidence of tectonism in the form of entrenched streams, cliffy sections and fault controlled river courses ŽGhosh, 1952; Sareen et al., 1993; Sridhar, 1995.. To the west ŽAAX . and south ŽAX BX . of Nal region are faults that manifest themselves as abrupt lithological boundaries between the alluvium and the Deccan basalts. Sridhar Ž1995. has also inferred a rotational fault ŽCCX . passing through Nal Sarovar ŽFig. 1..
3. Methodology Owing to the inherent limitations created by low lying nature of this area and the absence of adequate surface exposures, a combination of spatial data from remote sensing studies and stratigraphic sub-surface correlation from borehole lithologs has been employed to study the landform development. Geomorphological studies including palaeochannel identification, were carried out using Indian Remote Sensing Satellite 1A, False Colour Composite ŽIRS FCC. imagery ŽRow: 32 Path: 52. at 1:250,000 scale and Survey of India toposheets Ž1:250,000 and 1:50,000 scale.. For palaeochannel identification, 1:50,000 IRS FCC imageries of selected areas were also used. The major geomorphic units were demarcated, and selected field checks carried out. Subsurface lithological data from Nal and surrounding regions were collected from Government agencies, namely, Gujarat Water Resources Development ŽGWRDC., Central Ground Water Board ŽCGWB., Gujarat Water Supply and Sewerage Board
ŽGWSSB.. These lithologs were used to construct NS and NE–SW transects through the Nal region. The data from these studies were combined with the data on sedimentological, mineralogical and chronological Žradiocarbon and luminescence. measurements on a 54-m long core raised from the Nal Sarovar. The entire information was then synthesised with the available data on the eustatic sea level changes and a scenario for the evolution of Nal region was developed.
4. Results and discussion 4.1. Surface eÕidence 4.1.1. Geomorphology and drainage pattern Geomorphic features representing three distinct depositional environments, namely, Ži. marine, Žii. fluvial, and Žiii. aeolian, could be identified from the satellite data and field studies. These are ŽFig. 4.: Ži. inland palaeo-deltas, Žii. old mud flats, Žiii. alluvial plains, and Živ. stabilised dunes. 4.1.1.1. Palaeo-deltas. On the Saurashtra side, west of Nal region, fan shaped features were observed on the satellite imageries. These appear to coalesce in their lower part ŽFig. 5.. Sood Ž1983. had identified these features as deltas based on remote sensing alone. These, however, could also represent coalescing alluvial fans. During field checks, in a section upstream of Bhogava river ŽFig. 3., east of Limbdi, occurrence of basalt dominated coarse sand, showing foreset beds was observed. Underlying these, in some of the wells dug in the river bed, clayey sediments, lithologically similar to the clayey sediments found in the sub-surface in the Nal region were seen. The occurrence of forest beds, underlying a layer of surface siltrsand together with the occurrence of clayey sediments in the dug wells Žpossibly representing bottomset deposits. would suggest these fan shaped features to be deltas of the Gilbert type ŽReineck and Singh, 1980.. It must, however, be emphasised here that the region does not have adequate surface exposures and this study, together with that of Sood Ž1983. can at best be indicative of a deltaic origin. However, studies on a 54-m long core raised from the Nal region Ždiscussed subsequently.
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Fig. 3. A composite drainage map of the Nal and surrounding regions showing the present and palaeo-channels Žbroken lines..
also lent support to our surmise of these fan shaped features being palaeo-deltas. The inferred palaeo-deltas are presently 30 km inland, at an average elevation slightly over 15 m. In this region several small channels are seen which terminate abruptly and the morpho-boundary is located at ; q15 m msl ŽFig. 3.. A portion of the satellite image with our interpretation of the palaeodelta and the zone of terminating streams is shown in Fig. 5. Since we surmise, based on the satellite and field studies, that these fan shaped features represent
palaeo-deltas, the morpho-boundary would then be a palaeo-strandline. The rivers to the west of this palaeo-strandline have wide valleys that appear incompatible with the maximum seasonal flow in both pre- and post-monsoon seasons imageries. It is likely that these wide valleys were formed during a substantially wetter period in the past, when the rivers had higher erosional potential and larger load carrying capacity. Field studies also indicated limited river entrenchment of about 4 m in the palaeo-delta region.
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Fig. 4. Geomorphological map of the Nal and surrounding regions showing inland palaeo-delts, mudflats, dunes and alluvial plains.
According to Thornbury Ž1985., two conditions are necessary for the formation of deltas—the availability of sediments and sheltered conditions at the site of deposition in a body of water. The data from satellite and field observations, namely, Ži. valleys incompatible with the present seasonal flow; Žii. limited river entrenchment Žiii. presence of palaeostrandline at ; 15 m and Živ. absence of evidence Ždiscussed subsequently. of significant tectonic uplift of the Saurashtra region during late Quaternary, would suggest that the palaeo-deltas were formed at a time when eustatic level of sea was relatively higher so that it could reach as much as 30 km inland, and the rivers draining Saurashtra carried substantially larger sediment load.
4.1.1.2. Old mud flats. At a lower elevation, to the east of palaeo-deltas and south of Nal Sarovar ŽFigs. 4 and 5., two sets of older than present mud flats were identified between ; q14 m and ; q10 m msl on the basis of tonal and colour changes. The mud flat M1, with a darker tone is at ; q14 m msl and was mapped on both the eastern and western side of the Gulf of Khambhat. The mud flat M2, at a slightly lower elevation, had a lighter tone and is noticeable only on the western side. Present day mud flats were also mapped from the satellite images at elevations of upto ; q8 m msl. During field observations, wide expanse of flat, sparsely vegetated, at times salt encrusted land with silty clay type of soils was seen at the locations of M1, M2 and the present
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Fig. 5. Satellite imagery and interpretation showing inland deltas, zone of terminating streams and old mud flats south of Nal Sarovar.
day mud flats. There was also some agricultural activity both on M1 and M2 but not on present day flats. The ancient port town of Lothal Žsee Fig. 3., belonging to the Indus Valley Civilisation lies within M1, south of Nal Sarovar. The archaeological remains of this port town have been dated by radiocarbon in the interval 4.3–3.6 ka B.P. ŽThapar, 1993.. The region of mud flats also showed deranged drainage pattern ŽFigs. 3 and 5.. This drainage pattern may perhaps be indicative of the recent development of drainage in a region ŽThornbury, 1985.. Maximum entrenchment of streams is only 2 m in mud flats, on the western side, indicating that since their formation, there has been no major uplift in this region. The present elevation of these old mud flats ŽM1 and M2. may not be taken as an indication of the extent of eustatic sea level rise since the region
close to Gulf of Khambhat is subjected to tidal surges as high as 12 m. As mentioned earlier, in this region, present day mud flats are being formed at elevations up to q8 m above msl. Therefore, a sea-level rise of even a few meters in the past, accompanied by tidal surges would be enough to form the older mud flats. 4.1.1.3. AlluÕial plains. These are found to the east and north of Nal Sarovar. No surficial expression of Pleistocene sea level change was found. Instead, these sediments show cliffs, entrenched streams and fault controlled river courses, reflecting regional tectonism. The general slope of the region is towards southwest. Satellite images also indicate the presence of several abandoned palaeo-channels to east of Nal region
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ŽFigs. 3 and 6.. These occur as broad sinuous stretches exhibiting sharp tonal contrasts with their surroundings. These generally appear as whitish with brown patches, indicating the dominance of sandy material, and patchy vegetation. These palaeochannels appear to be broadly oriented in NE–SW direction. Some of the mapped palaeochannels were also checked in the field where a variety of related features were observed. At Phangdi, the pre-existing channels exist in patches as small seasonal streams that terminate abruptly. Near Bavla, it was observed that the shallow wells in the vicinity of the palaeochannels had relatively sweet water as compared to the surrounding regions. Ox-bow lakes caused by channel avulsion are also present. Thus, it appears that the presently barren area to the east of
Nal had several channels flowing through it which in the past may have been responsible for bringing sediments into this region. The rivers became inactive either due to a changeover to a drier climate andror due to tectonism which disrupted the drainage pattern in this region. The abandoned river courses lie in the Cambay Graben, known to have been tectonically active during late Quaternary ŽGhosh, 1952; Sareen et al., 1993.. 4.1.1.4. Dunes. Stabilised relicts of barchans, parabolic, longitudinal and obstacle dunes are present north-northeast of Nal Sarovar ŽFigs. 3 and 6.. Patel and Desai Ž1988. have attributed the formation of these dunes and those present in other areas of north Gujarat, to periods of stronger aeolian activity
Fig. 6. Satellite imagery and interpretation showing location of palaeochannels Ždashed lines., palaeo-dunes and alluvial plains north of Nal Sarovar.
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during periods of regression following the Flandrian sea level rise. However, TL data from north of Ahmedabad, indicated the dune building activity to have begun as early as ; 20 ka ŽWasson et al., 1983. with intervening periods of stabilisation. Considering that Ži. the dunes overlie the fluvial sediments, Žii. the wide variety of dune forms observed, and Žiii. the differences in wind regime required for the formation of such forms, it is not unlikely that the dunes represent several periods of aeolian activity, probably pre-dating 20 ka.
4.1.2. Discussion: surface eÕidence Summarising, the salient points of remote sensing and field studies, mentioned above, are: Ži. rivers on the western Saurashtra side have wide valleys inconsistent with the present seasonal flow; Žii. palaeo-deltas Želevation slightly over 15 m. and a palaeostrandline Žat ; 15 m. are also seen on Saurashtra side; Žiii. evidence of Holocene transgression, in the form of old mud flats M1 and M2, showing deranged drainage pattern, was observed south of Nal Sarovar, at a lower topographic position than the palaeo-deltas. The ancient port town of Lothal is located in M1; Živ. both in the palaeo-delta and in the old mud flats only limited Ž; 2–4 m. river entrenchment is observed suggesting the absence of significant tectonic uplift subsequent to their formation. The presence of palaeo-deltas and old mud flats would also point to at least two episodes of transgression. The evidence of relative topographical position, archaeology and deranged drainage pattern pointed to the mud flats ŽM1 and M2. having been formed during the most recent, possibly Holocene, transgression. This also implies that the inland palaeo-deltas were formed during an earlier transgression of the sea. It is possible to explain the formation of various geomorphic features by considering the possible role of eustatic sea-level changes and tectonism during the late Quaternary. In the following we consider the available evidence of sea-level changes and tectonism from the Saurashtra coast. The evidence for sea-level change is available from two kinds of radiometrically dated deposits: Ži. miliolites, which are found in coastal as well as inland regions, and Žii. non-miliolitic material Žoysters and corals. found
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in coastal areas only. In this paper only non-miliolite deposits are considered as the miliolite ages are embroiled in controversies owing to the debate about their origin and chronology ŽBaskaran et al., 1989; Gupta, 1991; Bruckner, 1987; Merh, 1995.. Corals, from NW Saurashtra, Želevation - 10 m. give 234 Ur238 U ages that fall in two age brackets: 6–7 ka and 118–176 ka B.P. ŽGupta and Amin, 1974; Somayajulu et al., 1985.. The spread of ages between 118 and 176 ka, could well be due to the post depositional chemical alteration of the sample ŽSomayajulu et al., 1985.. A similar age range has also been obtained by 230 Thr234 U dating of oysters Želevation- 12 m. from southern part of Saurashtra coast ŽJuyal et al., 1995.. These data on sea-level change along Saurashtra, coupled with the eustatic ; 7 m rise in sea level during last major interglacial transgression Ž; 125 ka. ŽKu et al., 1974; Cronin et al., 1981; Chappel and Shackleton, 1986. would clearly imply the absence of large scale Žbeyond 10 m. uplift in these regions during late Quaternary. This would also suggest that the palaeo-deltas, located slightly over 15 m elevation, were formed during the last major interglacial transgression of the sea at ; 125 ka. This would require only a small Ž- 10 m. tectonic uplift subsequent to their formation—consistent with the field observations of limited river entrenchment Ž; 4 m. on the Saurashtra side. This is contrary to the view held by Baskaran et al. Ž1989., who have indicated an uplift of ) 50 m for the Saurashtra peninsula, in this region, during late Quaternary on the basis of miliolite ages. If the suggested age assignment of ; 125 ka for the formation of palaeo-deltas is correct, it is reasonable to expect that the low lying Nal region was covered by a shallow sea around that time. Therefore, a study of sub-surface lithology and chronology should help to corroborate the above hypothesis. 4.2. Subsurface eÕidence 4.2.1. Lithological correlation from borehole data Lithological data from several boreholes Ždepth of sections 60–200 m. drilled for ground water exploitation were collected. Two transects in NE–SW direction and one in NS direction across the Nal region were constructed ŽFig. 7.. The lithological
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Fig. 7. Map showing the location of bore-hole sites and transects taken. Also shown are the eastern and western margins of Cambay Graben. The approximate boundaries of the Nal region are shown by dotted lines.
information is based on drillers’ logs and electrical resistivity log data. The NE–SW transects ŽYYX and ZZX in Fig. 8. indicated that, in the eastern side of Nal, the lithology is dominated by sand with intercalations of clayrsilty clay. However, in the vicinity of Nal Sarovar and along the axis of the low lying region extending from near the Little Rann in the north to Dholera in south near the Gulf of Khambhat, a change in lithology to a layer of sand underlain by a thick Ž40–55 m. sequence of clayrsilty clay Žsimilar to that found in dug wells in the inland delta region. is seen ŽFig. 8.. The lithologs of Vekria, Ranagadh and Phulwadi, which also lie in the Nal region, also show a layer of sand underlain by a thick sequence of clayrsilty clay. The thickness of the silty-clay horizon in the borehole sections varied between 40– 55 m in the NS transect and elevation in relation to msl of the top of this layer varies from q3 m in north to y30 m in the south, in the Nal region. The top part of the silty clay horizon in the basin thus appear to be sloping towards the south.
The presence of a thick sequence of clayrsilty clay indicates a similarity in depositional environment in the entire low lying region, extending from near the Gulf of Khambhat to Little Rann of Kachchh, in a narrow corridor about ; 25 km wide as shown in Fig. 7 by the two dashed lines. 4.2.2. EÕidence from Nal SaroÕar core A 54-m long core ŽFig. 9. was raised from the Nal Sarovar bed for laboratory studies Žsedimentological, clay and heavy mineral, d 13 C and CrN on organic matter, radiocarbon and luminescence dating.. The coring was done using rotary type drilling rig with a tungsten carbide bit, with a 5 cm internal diameter of the core tube without using drilling fluid Ždry drilling.. After about every 1.5 m of drilling, the core was raised and the sample removed. Only a summary of studies on the Nal Sarovar core is given here. The details have been discussed elsewhere wPandarinath, K., Prasad, S., Gupta, S.K. Late Quaternary palaeoclimate and sea-level changes inferred from sedimentology and clay minerals: Nal Sarovar,
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Fig. 8. Lithological variation along the transects shown in Fig. 7. Note similarity in lithological sequence in the Nal region.
Western India Žcommunicated.; Prasad et al., 1997x. Three lithounits were recognised in the core on the basis of texture and mineralogy.
Ø Horizon-1 Ž0–3 m. comprised clayey silt with organic matter. In this horizon clay fraction varied between 3–80% and was dominate by illite Ž74–
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Fig. 9. Lithology of the Nal Sarovar core.
81%., followed by smectite Ž7–13%.. An uncalibrated radiocarbon age of 6.75 " 0.13 ka was obtained on organic fraction from the base of this horizon. Ø Horizon-2 Ž3–18 m. was dominantly sandy. Occasional subrounded basalt fragments Ž2–4 mm size. were also found in this horizon. The clay
fraction varied from 3–17%, and was dominated by smectite Ž43–74%., followed by illite Ž17–38%.. As the clay fraction in this horizon was small, heavy minerals were used to determine the provenance of this horizon. The heavy mineral assemblage was dominated by opaques Žmagnetite and ilmenite. followed by garnet, epidote, staurolite, hornblende,
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monazite and rutile indicating its source to be from the igneous andror metamorphic rocks from the east and north east. To identify the depositional environment for this horizon, grain size parameters were computed and plotted on a sorting vs. skewness bivariate plot ŽFig. 10. using the technique of Friedman Ž1961.. However, since it has been indicated that the usefulness of grain size parameters in identifying depositional environment, by themselves, is limited ŽSolohub and Klovan, 1970., we have relied on a comparison of grain size parameters of Nal core sediments with those obtained from core samples raised from the present day Sabarmati riverbed on the bivariate plot. This approach was chosen because the Sabarmati is the nearest large river originating in the north-east and traversing the Cambay Graben to flow into the Gulf of Khambhat ŽFig. 3.. The late Quaternary sediments in the Cambay Graben are overwhelmingly fluvial ŽMerh and Chamyal, 1993.
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and are believed to have been deposited by stronger fluvial regimes during Quaternary ŽSridhar et al., 1994.. It is, therefore, not unlikely that the sediments in the sandy horizon of Nal Sarovar were deposited by this older fluvial regime of which Sabarmati formed a part. Samples from both the sites fall within the field of fluvial sediments ŽFig. 10.. Additionally during the luminescence dating of the Nal core samples, it was found that the sediments from Horizon-2 indicated inadequate pre-depositional zeroing of the geological Thermally Stimulated Luminescence ŽTL. signal ŽPrasad, 1996., not an unusual situation in fluvial sediments ŽBerger, 1988; Berger and Easterbrook, 1993; Forman and Ennis, 1991.. For this reason, the Optically Stimulated Luminescence ŽOSL. method, in which only a few minutes of sun exposure is sufficient to zero the geological OSL signal, was used for dating of these sediments. A radiocarbon age estimate of ; 7 ka at
Fig. 10. Skewness vs. Sorting in sediments of Nal Sarovar core ŽHorizon-2. compared with sediments obtained from the cores raised from the present day Sabarmati river bed near Ahmedabad.
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3 m depth and an OSL age estimate of ; 65 ka at 18 m depth was obtained for this horizon. The age estimates were constrained by measured radiocarbon and luminescence ages of 6.75 " 0.13 ka at 3 m depth and 69 " 9 ka at ; 24 m depth. No fossils were found in this horizon. Ø Horizon-3 Ž18–) 54 m. comprised clayey silt and silty clay with occasional sand lenses and subrounded basalt fragments. In this horizon, the dominant clay mineral was smectite Ž53–97%. upto 31 m depth, below which the clay became monominerallic Ž) 99% of smectite.. A luminescence age range of 69 " 9 ka at ; 24 m depth and ; 101 " 10 ka at 54 m depth ŽPrasad, 1996. was obtained for this horizon. The base of this lithounit was not reached in the core. No fossils were found in this horizon either. The dominance of smectite in this horizon is indicative of its origin from a basaltic terrain. It was possibly derived from the basaltic rocks on the Saurashtra side or brought in from the Gulf of Khambhat due to action of tidal currents. Mineralogically and sedimentologically, the sediments in the inner shelf near the Gulf of Khambhat are similar to the Horizon-3 of the core ŽRao, 1991; Rao and Rao, 1995.. Interestingly, a yellowish clay layer, ŽRL y7 m., comprising 80–90% smectite, devoid of shells and having an abrupt contact with the overlying gritty sand layer, has also been reported from the Little Rann of Kachchh ŽGupta, 1975.. This layer may be correlated with Horizon-3 of the Nal core. The contact between Horizon-2 and Horizon-3 is abrupt whereas Horizon-2 grades into Horizon-1.
5. Synthesis and interpretational model On the Saurashtra side inland deltas were identified at an elevation of ) 15 m and ascribed to last major interglacial transgression Ž; 125 ka.. In the low lying Nal region to the east of these deltas, an unfossiliferous layer of smectite-dominated clayey siltrsilty clay is found, the top of which had a relative elevation in relation to msl varying from q3 m in north to y30 m in the south in the Nal region. Similar sediments have also been reported in the inner shelf off the Gulf of Khambhat and also at RL y7 m in the little Rann to the north. The OSL
chronology of this layer indicated that this was deposited in the interval ) 65 ka to ) 100 ka. All these would indicate that the Nal region was indeed covered by a shallow sea during the period ) 65–; 100 ka. The base of Horizon-3 was not reached in the core but it is reasonable to expect that the region which received marine sedimentation from ; 65–; 100 ka would have also been covered by sea at ; 125 ka when the sea level was ; 7 m higher than present. The absence of marine fauna in the ) 63 m fraction studied is puzzling. However, the environment visualised here during the deposition of Horizon-3, for the most part, is of a narrow shallow sea corridor with influx of water and sediments from both the land and the sea. It would have been subjected to considerable salinity fluctuations induced by tidal changes, fresh water flux during monsoon months and high rates of evaporation during summer months. Very few species can survive under such stressed environments of wide salinity fluctuations ŽRaup and Stanley, 1985.. An indication of this is also found in a study of present day distribution of planktonic foraminifera in the Arabian sea. Near the Gulf of Khambhat, very low concentrations of foraminifera Ž0–200 specimensr1000 m3 . were found as compared to 7000–40,000 specimensr1000 m3, further south. This has been attributed to arid climate around this region and resulting intense evaporation ŽRao et al., 1991.. The presence of a Ž5–35 m. thick sandy horizon in the bore well sections in the entire Little Rann to Gulf of Khambhat corridor suggests that during its deposition the sea link no longer existed in this region and high energy fluvial sediments derived from east and north east were being deposited in this entire belt. To identify the depositional environment for the sand-dominated Horizon-2 integration of surface as well as sub-surface data was done. The presence of the palaeochannels, the extensive occurrence of fluvial deposits to the east, in the Cambay Graben region, the close agreement of the Nal Sarovar core samples and Sabarmati riverbed samples in the bivariate plots and heavy mineral assemblage, and inadequate pre-depositional zeroing of the luminescence signal indicated that the sediments of Horizon-2 of the Nal core are fluvial in nature. These could have been deposited during advancement of the depositional front of the eastern rivers as a result
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of the combined effect of a eustatic lowering of the sea level and tectonic uplift in the region of Cambay Graben. Presently, this area has a surface elevation of q80 m to q100 m and the low elevation area has shifted to Nal Sarovar which is q13 m to q16 m msl. Evidences of Late Quaternary tectonism in the Cambay Graben region have been reported ŽGhosh, 1952; Sareen et al., 1993.. As a result of the combined influence of Ži. the westward advance of the sedimentation front; Žii. tectonism and; Žiii. the post glacial sea-level rise, the level of the Nal Sarovar came to within few metres of its present elevation at about 7 ka when it became a closed basin. The present Nal Sarovar, therefore, originated as a result of westward advance of the sedimentation front until it could no longer advance due the presence of the high land of Saurashtra. At that time, either due to sedimentation processes alone or aided by tectonism, the west-flowing rivers shifted their courses southwards and presently only the abandoned channels remain.
6. Conclusions Based on the data and discussion presented in the foregoing, the following conclusions may be drawn. 1. The geomorphological features and sub-surface lithological data revealed that the Nal region has had an evolutionary history different from that of the eastern region of Cambay Graben. The sedimentation in this region was linked to the late Quaternary eustatic rise and fall of sea level, the evidence of which is found in the form of palaeo-deltas and mud flats. 2. Mineralogical and chronological studies on the Nal Sarovar core indicated that the sediments deposited during the period ; 65 ka to ) 100 ka were rich in smectite. These fine grained sediments are correlatable lithologically throughout the Nal corridor and are also similar to the present day sediments found in the inner shelf in the Gulf of Khambhat. The low lying Nal region, linking the gulfs of Kachchh and Khambhat through Nal Sarovar, was covered by a shallow sea and clayey silt and silty clay sediments of Horizon-3 were being deposited during this interval.
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3. Combining the evidence of geomorphology and sub-surface lithological data with Ži. the global eustatic sea level curve Žii. the elevation in relation to msl of the top of Horizon-3 in the Nal region Žq3 m and y30 m respectively. Žiii. chronological evidence of raised marine deposits Žwith the exception of the controversial miliolite deposits. along the Saurashtra coast which indicate an absence of large scale tectonic uplift, we suggest that the palaeo-deltas were formed during the last major interglacial transgression of the sea Ž; 125 ka.. There may have been some Ž; 10 m. tectonic uplift subsequent to the formation of inland deltas but our studies rule out any large scale uplift. 4. Subsequently, with the eustatic lowering of sea level, the sea receded from the Nal corridor ; 65 ka B.P. and fluvial sediments were being deposited in the Nal corridor as a result of westward migration of the depositional front of the eastern rivers. The process continued until ; 7 ka B.P. when the sedimentation front could no longer advance westwards. The old mud flats, south of Nal Sarovar, were formed during the Holocene transgression. In this paper we have been able to decipher the major events in the evolutionary history of the Nal region. We have shown that the record in this area spans nearly a complete climatic cycle from the last major interglacial to the present. Further detailed work is needed to clearly decouple the effects of eustasy, climate and tectonism. This would yield important data for the palaeoclimatic reconstruction in the tropical areas of southeast Asia.
Acknowledgements This work was partly supported by DST grant number ESTr44r014r90. The Directorate of Geology and Mining, Gujarat is thanked for raising the core from the Nal Sarovar. The State Govt. Agencies, GWRDC, CGWB and GWSSB are thanked for providing the bore-hole data. Help and assistance of the Centre for Environmental Planning and Technology ŽCEPT. Ahmedabad; and Gujarat Engineering Research Institute ŽGERI. and M.S. University of Baroda in remote sensing studies is gratefully acknowledged. Mr. R.D. Deshpande is thanked for
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making available unpublished grain size data from Sabarmati river bed cores. Prof. A. Gupta and an anonymous reviewer are thanked for their suggestions which helped to improve the manuscript.
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