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Holocene valley incision during sea level transgression under a monsoonal climate
Sedimentary Geology 179 (2005) 295 – 303 www.elsevier.com/locate/sedgeo
Holocene valley incision during sea level transgression under a monsoonal climate Aniruddha S. Khadkikar *, C. Rajshekhar Agharkar Research Institute,G.G. Agarkar Road, Pune 411 004, India Received 26 October 2004; received in revised form 9 June 2005; accepted 10 June 2005
Abstract The late Quaternary stratigraphic record of the Mahi River, western India records the response of a macrotidal monsoonal estuary to base level and climate change. The timing of the valley incision is constrained based on chronology on deposits that constitute the banks of the Mahi River and on inset valley fill terraces. These results show that valley incision took place in midHolocene, a period of sea level transgression. The reason for valley incision appears to be related to the increase in seasonality of the rivers, which imparts a high work capacity to them due to climatic amelioration. The post-incision stratigraphic record of the Mahi is opposite to that in Type 1 sequence records with respect to grain size trends and facies distribution. This may thus prove useful in identifying transgressive valley incision in pre-Quaternary deposits. Moreover in light of the dichotomy of the timing of valley incision with respect to base level position, maximum flooding surfaces may be more reliable in demarcating sequence boundaries, particularly in monsoonal settings. D 2005 Elsevier B.V. All rights reserved. Keywords: Holocene; Sequence stratigraphy; Quaternary; Monsoon
1. Introduction Estuaries and incised valleys play a key role in the interpretation of sequence stratigraphy of paralic deposits (Miall, 1997). Incised valleys are commonly used to mark sequence boundaries associated with the lowstand systems tracts (Dalrymple et al., 1994; Posamentier, 2001). While valley incision has been attributed primarily to base level lowering, the possibility * Corresponding author. E-mail address: [email protected] (A.S. Khadkikar). 0037-0738/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.sedgeo.2005.06.004
that it may be unrelated to base level has also been raised. Also, special cases in which base level lowering does not result in valley incision have been described by Posamentier (2001). A few examples relating valley incision to non-base level parameters exist (Jain and Tandon, 2003; Srivastava et al., 2001) from the monsoonal regions. Estuaries on the other hand mark transgressive events in the conventional sequence stratigraphic framework (Miall, 1997). Such estuaries form in drowned valleys during periods of sea level rise (e.g. Allen and Posamentier, 1993). As the recognition of sequence boundaries relies heavily on the dregressiveT model of valley incision, it is prudent to ask if alterna-
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tive possibilities exist in nature or not? Were valley incision, governed by other parameters, to be more frequent, then the sequence stratigraphic interpretation would be complicated. The differentiation of incised valleys related to base level lowering and non-base level controls then becomes crucial for sequence stratigraphy. The study of Quaternary depositional systems is important for sequence stratigraphy as it provides an opportunity to understand the response of depositional systems with respect to sea level changes in a geochronologically constrained framework (Blum and To¨rnquist, 2000; Goodbred et al., 2003). The purpose of the present study is to illustrate using remote sensing based geomorphological studies, stratigraphy and sedimentology; an example of climate-controlled valley incision that is co-incident with a Holocene transgression. The timing of these events is well constrained on the basis of luminescence, electron spin resonance, radiocarbon and archaeological ages.
2. Area of study The region in which the river Mahi is located falls in the State of Gujarat (Fig. 1), western India which
experiences rainfall seasonally in the monsoon months of June, July and August. During these months, the river Mahi experiences an abrupt change in discharge from b 100 m3 s1 to as high as 10,000 m3 s1 (Fig. 2). This large magnitude change governs channel geometry and results in a box like channel geometry that appears to be typical to monsoon rivers (Kale, 2002). The Mahi River has incised into late Quaternary deposits which today stand exposed as 30–40 m cliffs along its banks (Fig. 3). The tidal regime in the region is macrotidal and one of the largest in the world with amplitudes exceeding 10 m. Consequently, the present study characterizes the response of a monsoonal macrotidal depositional system within a sequence stratigraphic framework to base level and climate change.
3. Methodology Multispectral Indian Remote Sensing (IRS) 1DLISS 3 satellite data was used to understand the geomorphology of the region. The 4-band data was transformed using the decorrelation stretch method to obtain maximum visual contrast. Based on the pro-
Fig. 1. A) Location of the Mahi estuary in India. B) IRS1D LISS3 multispectral data showing key geomorphic elements. C) Geomorphology of the Mahi estuary based on IRS-1D multispectral satellite data. The spatial extent of terraces mimics the extent of modern bank attached bars.
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Fig. 2. Monthly discharges of the Mahi River between 1988 and 1991 (Data source: Water Resources Division, Government of Gujarat). The changes span several orders of magnitude from summer/winter months to monsoon months and show marked seasonality that is typical to rivers of India.
cessed imagery, river/estuary morphology and terraces were mapped. Field studies were carried out to validate the geomorphic units. The geomorphic units were mapped using photomosaics to judge lateral facies variations. These results were then integrated with earlier geochronological studies on the late Quaternary to Holocene deposits. Sediment samples from the terraces were analyzed for foraminifera following standard procedures involving pretreatment with H2O2 followed by wet sieving.
4. Results The Mahi River estuary has a wide funnel whose width decays exponentially from the mouth (Fig. 1). At the mouth the estuary has widths of about 10 km whereas in the fluvial reach the valley widths are under 2 km. Throughout the lower reaches the Mahi River shows alternating bank attached terraces (Fig. 1). These bank attached terraces abut against older late Quaternary deposits that form steep banks resulting in
Fig. 3. River bank and terrace stratigraphies. The river bank stratigraphy is from Dabka and shows a red soil at the base covering an age range from 50 to 25 ka. Overlying the palaeosol are sheetflood fluvial deposits that formed during the Last Glacial Maximum and were overlain by loess. During the Holocene a thick soil developed over these deposits (vertisol). The terrace stratigraphies are similar, beginning with muds showing bioturbation and wavy bedding that contain foraminifera. The capping deposits are estuarine/fluvial sands.
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an inset character for the terraces. The bank attached terraces themselves show steep banks due to modern incision and erosion due to the lateral migration of the Mahi River. Terraces are narrow in the upstream reaches but expand significantly in the lower reaches around Kothiyakhad, a pattern seen even today (Fig. 1). These changes in terrace extent reflect incremental tidal influence towards the mouth. The terrace stratigraphy is observed to be broadly similar throughout the region (Fig. 3). It usually begins with wavy laminated silty clay to black mud and coarsens upwards. The black mud at some sites shows intensive bioturbation and presence of mud flakes. The wavy laminated sediments consist of cyclical silt/clay alternations (Fig. 3), which are seen
at Tithor. Upwards the sediments become massive and less distinctly bedded. These sediments also show the presence of diffuse calcretes. While deposits towards the mouth show wavy lamination those away from the funnel show the presence of massive muds implying a control of coastal processes on sedimentary facies. The sediment succession is interpreted to represent a lower estuary to fluvial transition. The lower estuary (downstream of meanders; Fig. 3) deposits show a rich to moderate assemblage of benthic and planktonic foraminifera. The benthic assemblage is dominated by Quinqueloculina sp., Elphidium oceanicum, E. excavatum, Ammonia beccarii and Nonion boueanum, which is typical to estuarine regions (Cann et al., 2002). At Tithor (Fig. 3) the foraminiferal abundance
Fig. 4. A) The timing of various depositional phases in relation to the global sea level curve of Chappell and Shackleton (1986) which show clearly that valley incision took place during the Holocene transgression. B) The same data plotted with respect to a local sea level curve (Hashimi et al., 1995) after calibrating the radiocarbon dates. Luminescence ages other than those marked Dabka are from Rayka (Fig. 1). Loess stabilization in the southern regions (Dabka) was much earlier than towards the north (Rayka). This comparison again shows that valley incision coincided with rising sea levels.
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progressively decreases as the sediment succession changes towards fluvial deposits. Through the profile the degree of preservation of foraminifera also reduces with the frequency of corroded and abraded forms increasing towards the top. The presence of planktonic foraminifera such as Globigerinoides and Globigerina reflects the macrotidal environment of the Mahi estuary. This assemblage is comparable to earlier studies at Kothiyakhad (Rachna and Chamyal, 1998). Radiocarbon dates that are available for the deposits at Kothiyakhad (Chamyal et al., 2003) show an age range from 3660 F 90 14C yr BP to 1760 F 80 14 C yr BP and correspond to calibrated ages 3560– 3776 cal yr BP (1r error) to 1565–1738 cal yr BP. Calibrated ages from Mujhpur (2772–2356 cal yr BP), Sultanpura (3451–2875 cal yr BP) and Tithor (4826– 3836 cal yr BP) suggest that terrace sedimentation is at least older than 4500 yr BP. A radiocarbon age of 6500 14C yr BP from the base of an upstream terrace (Chamyal et al., 2003) indicates that it may have reached the present level about 7000 cal yr BP. This also puts an upper limit on cessation of incision around this time frame (6 F 1 ka). The late Quaternary deposits which have been incised into by the modern Mahi, have a stratigraphy that differs from the terrace stratigraphy. The deposits usually begin with calcrete conglomerates which show trough and planar cross bedding (Khadkikar et al., 1999) and are associated with calcic vertisols (Figs. 3,4). These deposits are associated with sandy channel deposits hosting groundwater calcretes. The age of these deposits has been constrained using electron spin resonance ages of calcretes (Khadkikar et al., 1999) and luminescence dates (Juyal et al., 2000) to the last interglacial period (Fig. 4). These deposits also show evidences of tidal influence in the presence of corroded foraminifera (Chamyal et al., 2003), similar to that observed today in the modern Mahi river about 40 km inland of the river mouth. Overlying these deposits, which have been interpreted by Khadkikar et al. (1999), to represent seasonal rivers under a subhumid climate occur deposits of semi-arid rivers. These deposits show laterally extensive horizontal bedding with faint internal lamination. Within these deposits a laterally extensive red soil bed acts as a marker whose age may be bracketed between 50 and 25 ka BP (Khadkikar et al., 1999) (Figs. 3,4). This second style of sedimentation culminates in the
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deposition of sandy loess deposits that range in age from 22 to 10 ka (Juyal et al., 2000; Juyal et al., 2003) on the basis of luminescence and radiocarbon ages (Fig. 4) and in the distribution of Microlithic sites throughout the region (Khadkikar et al., 1999).
5. Discussion The Mahi estuary and its incised valley provide an alternative possibility of the responses to allogenic controls. The chronology of the late Quaternary and Holocene deposits has some interesting repercussions on the science of sequence stratigraphy. Sequences are defined as genetically related packages bound by unconformities or their correlative conformities (Miall, 1997). In the classical Type 1 sequence, erosional sequence boundaries owe their origin to lowstand incision due to a fall in relative sea level. Recent studies confirm that this may only be one of several possibilities (Dalrymple et al., 1994). That, unconformity development may not be unambiguous, was demonstrated recently by To¨rnquist et al. (2003) based on their well dated cores from the Rhine– Muese delta. The present study provides a vaster outlook based on outcrops. Luminescence dates on the late Quaternary stratigraphic record show punctuated sedimentation till about 7 ka. In the outcrops no definite evidence of incision related to any of the palaeosols were observed. The capping deposits of sandy loess have a deep soil profile, vertic in character suggesting cessation of aeolian deposition after 6 F 1 ka the youngest date obtained on sand dune aggradation in Gujarat northwest of Mahi (Khadkikar et al., 1999; Tandon et al., 1997). As the valley incises the loess also, it post-dates its sedimentation. There remains a possibility of continued aeolian deposition after the valley incision during subsequent arid periods but such situations are rare as seen in the sandy loess chronology (Juyal et al., 2003) and restricted to regions much north of the Mahi basin. Thus, based on regional stratigraphic coherence and chronology, it is observed that incision postdated the bulk of sandy loess deposition. Valley incision may have been gradual or instantaneous occurring in the latter case within centuries. The time duration of the valley incision and also its timing is confirmed on the basis of
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radiocarbon dates between 7000 and 4800 cal yr B.P. on basal terrace deposits in the Mahi valley. The uncertainties in the luminescence and 14C ages allow the interpretation of rapid valley incision between 7 and 5 ka (early–mid-Holocene). 6 ka is a most interesting period in the earth’s history and referred to as the mid-Holocene optimum, owing to the worldwide hot and humid climate state. It also was a period of relative sea level rise (Hashimi et al., 1995). In the Indian context, local sea level curves also reflect global sea level tendencies (Fig. 4) and show a sea level of + 2 m during the mid-Holocene optimum. Thus valley incision in the Mahi coincided with a major transgression. Valley incision during the Holocene appears to be widespread in the Indian region and has been observed by Pratt et al. (2002) in the Himalayan rivers and in western India too (Srivastava et al., 2001; Jain and Tandon, 2003). All these studies suggest that valley incision occurred due to enhancement of the Indian monsoon broadly between 12 and 5 ka BP. This monsoon intensification during the early Holocene is well reflected in palaeorainfall proxy records and peaked at 5.6 ka (Thamban et al., 2001). The youngest terrace ages are about 2 ka which suggests that the Mahi valley aggraded for about 5000 years. This was indeed followed by another incision, probably within the past millennium. But the exact cause of this incision is uncertain (i.e. base level vs. monsoon intensification). How can we possibly reconcile a transgressive incised valley, which contradicts our current understanding of the responses of rivers to base level change (Miall, 1997)? It should be noted that in the earlier laboratory studies (e.g. Koss et al., 1994) major emphasis was allocated towards understanding river responses to base level change, neglecting changes in river discharge. These studies showed how rates of base level change affected the nature of delta progradation and valley incision. Valley incision occurred when the base level fell below the shelf break and migrated upstream through headward erosion. Schumm (1993) however also highlighted earlier that apart from base level change, significant aberrations to the graded river concept (followed by sequence stratigraphers) could occur due to climate change. This appears to be particularly important in the monsoonal regions. Monsoonal rivers show a dramatic annual discharge pattern with b100 m3 s1
in winter/summer months (in W. India) to as high as 60,000 m3 s1 in the monsoon (Kale, 2002). This imparts to the rivers a very high work capacity. The change from loess deposits which form in desert margins under mean annual rainfalls of ~250 mm to valley incision and river rejuvenation (mean annual rainfalls equivalent to today of 500–700 mm) coincides with the Holocene optimum (Jain and Tandon, 2003). This is supported by various proxy records of the Indian monsoon which demonstrate intensification during the Holocene. The extreme seasonality of monsoonal rivers (Fig. 2) also enables them to negate base level effects in their lower reaches as exemplified by the Mahi record. Moreover during this period, an amelioration of climate also led to the formation of a vertisol over the loess deposits (Khadkikar et al., 1999). This may have led to reduced sediment availability and imparted a greater erosive power to the Mahi River during the Holocene optimum enabling valley incision. Since valley incision is possible under two situations, one due to base level change and other due to climatic amelioration, there may arise a problem in relating unambiguously, the sequence boundary to base level change. An alternative approach in sequence stratigraphy is to define sequence boundaries based on maximum flooding surfaces (Gibling and Bird, 1994; Miall, 1997) which are necessarily related to sea levels. This dichotomy in the timing of valley incision with respect to base level change shows that the approach of Gibling and Bird (1994) in using maximum flooding surfaces for demarcating sequence boundaries also holds true when studying deposits of seasonal rivers. In the Mahi estuary, the maximum flooding surface may be demarcated between the tidally influenced facies at the base of the terrace deposits and the overlying alluvial facies (Fig. 5). The identification of the MFS can best be considered tentative as it is assumed that future rise in sea level due to global warming will not exceed the last rise in sea level. The MFS can be clearly demarcated till the upper estuary, upstream of which the facies is entirely fluvial cobble rich sand. Sediments dating to the Last Glacial Maximum (~20 ka) in the Mahi region lie at a stratigraphic level of about 10–20 m from the modern river bed (Fig. 5). Thus the geomorphic surface on which rivers flowed during the lowstand systems tract at 20 ka was much higher. Hence valley incision during the Holo-
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Fig. 5. Summary model of along valley changes in sedimentary facies of the Holocene terraces of the Mahi estuary. Lower, middle and upper refer to the position within the estuary. Also shown is a typical across valley section (A–B) that illustrates the relationship between the late Quaternary stratigraphic succession, its chronology and relationship to the terrace succession. Terraces are the small wedge in the cross section labelled 7 ka which reflects the initiation of terrace sedimentation. The maximum flooding surface (MFS) may be demarcated at the boundary between tidally influenced facies and fluvial sand/cobbly sand facies assuming that future sea level rise due to global warming will not exceed the last rise in sea level. Details of the late Quaternary stratigraphic record, and regional correlations, based on which the typical across valley section is drawn are given in Khadkikar et al. (1999). The lowermost part of the estuary towards the mouth does not host any terraces. Hence facies changes are shown from the region where terrace stratigraphies are observed.
cene due to climatic amelioration exceeded the depths at which incision if any may have occurred due to base level fall during the Last Glacial Maximum. The region is tectonically active and falls within the Cambay graben (Chamyal et al., 2003) which could affect river response if such activity affected the base level. However, there are no evidences suggesting regional subsidence in the early to mid-Holocene. In light of the absence of any strong indicators of regional subsidence (which could promote incision by altering base level), this parameter appears to be insignificant. The stratigraphy of transgressive valley incision fills appears to differ from the valley fill record of the base level-change related valley fill (Dalrymple et al., 1992; Allen and Posamentier, 1993). In the latter case, the deposits show a change from fluvial sandstones/sands to gradually fining intertidal to estuarine facies as the valleys get drowned. This leads to a fining upward sedimentary record. In the present case the stratigraphic record is exactly the opposite as foraminifera bearing deposits are overlain by flu-
vial sands. This leads to a general coarsening upward deposit after valley incision. However, there are significant differences in facies assemblages in different parts of the estuary. In the lower estuary (Fig. 5) the transition is easy to map due to facies contrast (tidal rhythmites to fluvial sands). In the middle estuary (Fig. 5), where clay deposition is at maximum, bioturbated clays are found at the base which are succeeded upsection by interlayered fluvial sands and clays, suggesting progradation. In the upper estuary there is a transition from silty rhythmites to fluvial sands (Fig. 5). So in the entire estuary a clear transition from a lower tidally influenced facies to an upper fluvially influenced facies is observed. This facies sequence is thus opposite to the drowning valley model where the sequence is capped by tidally/marine influenced facies. These contrasts in the stratigraphic record may be useful in discriminating between the two modes of valley incision, which may be aided by the analysis of the distribution of foraminifera in the deposits.
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There are not too many radiocarbon dates on the terrace sediments to comment on the synchrony of the Holocene sediments through the valley. The chronological data available only allows conclusions to be reached regarding the timing of valley incision with respect to a major sea level change around the last glacial maximum.
6. Conclusion Integrated geomorphological, sedimentological and chronostratigraphic results on the late Quaternary to Holocene sedimentary record of the Mahi River show that alternatives exist to the Type 1 sequence model of lowstand valley incision. These results demonstrate an instance of valley incision during a period of sea level transgression. Deposits of such incised valleys show stratigraphies that differ from those that overlie Type 1 sequence boundaries. This may thus turn out to be useful for discriminating transgressive and regressive incised valleys. In light of the dichotomy in the timing of valley incision with respect to base level, maximum flooding surfaces appear to be more reliable in demarcating sequence boundaries in monsoonal environments.
Acknowledgments ASK would like to thank DST, New Delhi for financial support. Comments on earlier versions of the manuscript by A. Miall, M. Gibling, T. To¨rnquist and S. Goodbred were useful in developing these ideas.
References Allen, G.P., Posamentier, H.W., 1993. Sequence stratigraphy and facies model of an incised valley fill: the Gironde estuary, France. J. Sediment. Petrol. 63, 378 – 391. Blum, M.D., To¨rnquist, T.E., 2000. Fluvial responses to climate and sea-level change: a review and look forward. Sedimentology 47, 2 – 48. Cann, J.H., Harvey, N., Barnett, E.J., Belperio, A.P., Bourman, R.P., 2002. Foraminiferal biofacies eco-succession and Holocene sea levels, Port Pirie, South Australia. Mar. Micropaleontol. 44, 31 – 55.
Chamyal, L.S., Maurya, D.M., Rachna, R., 2003. Fluvial systems of the drylands of western India: a synthesis of late quaternary environmental and tectonic changes. Quat. Int. 104, 69 – 86. Chappell, J., Shackleton, N.J., 1986. Oxygen isotopes and sea level. Nature 324, 137 – 140. Dalrymple, R.W., Zaitlin, B.A., Boyd, R., 1992. Estuarine facies models: conceptual basis and stratigraphic implications. J. Sediment. Petrol. 62, 1130 – 1146. Dalrymple, R.W., Boyd, R., Zaitlin, B.A., 1994. History of research, types and internal organization of incised-valley systems: introduction to the volume. In: Dalrymple, R.W., Boyd, R., Zaitlin, B.A. (Eds.), Incised-Valley Systems: Origin and Sedimentary Sequences, SEPM special publication, vol. 51, pp. 3 – 10. Gibling, M.R., Bird, D.J., 1994. Late Carboniferous cyclothems and alluvial paleovalleys in the Sydney basin, Nova Scotia. Geol. Soc. Amer. Bull. 106, 105 – 117. Goodbred Jr., S.L., Kuehl, S.A., Steckler, M.S., Sarker, M.H., 2003. Controls on facies distribution and stratigraphic preservation in the Ganges–Brahmaputra delta sequence. Sediment. Geol. 155, 301 – 316. Hashimi, N.H., Nigam, R., Nair, R.R., Rajagopalan, G., 1995. Holocene sea level curve and related climatic fluctuations for western Indian continental margin. An update. J. Geol. Soc. India 46, 157 – 162. Jain, M., Tandon, S.K., 2003. Fluvial response to late quaternary climate changes, western India. Quat. Sci. Rev. 22, 2223 – 2235. Juyal, N., Rachna, R., Maurya, D.M., Chamyal, L.S., Singhvi, A.K., 2000. Chronology of late Pleistocene environmental changes in the lower Mahi basin, western India. J. Quat. Sci. 15, 501 – 508. Juyal, N., Kar, A., Rajaguru, S.N., Singhvi, A.K., 2003. Luminescence chronology of aeolian deposition during the late quaternary on the southern margin of Thar desert, India. Quat. Int. 104, 87 – 98. Kale, V.S., 2002. Fluvial geomorphology of Indian rivers: an overview. Prog. Phys. Geogr. 26, 400 – 433. Khadkikar, A.S., Mathew, G., Malik, J.N., Gundu Rao, T.K., Chowgaonkar, M., Merh, S.S., 1999. The influence of the southwest Indian monsoon on late quaternary continental sedimentation, western India over the past 130 kyrs. Terra Nova 11, 273 – 277. Koss, J.E., Ethridge, F.G., Schumm, S.A., 1994. An experimental study of the effects of base level change on fluvial, coastal plain and shelf systems. J. Sediment. Res., Sect. B Stratigr. Glob. Stud. 64, 90 – 98. Miall, A.D., 1997. The Geology of Sequences. Springer Verlag, Berlin. 433 pp. Posamentier, H.W., 2001. Lowstand alluvial bypass systems: incised vs. unincised. AAPG Bull. 85, 1771 – 1793. Pratt, B., Burbank, D.W., Heimsath, A., Ojha, T., 2002. Impulsive alluviation during early Holocene strengthened monsoons, central Nepal Himalaya. Geology 30, 911 – 914. Rachna, R., Chamyal, L.S., 1998. Microfauna from a Holocene valley fill terrace, lower Mahi basin, Gujarat. J. Palaeontol. Soc. India 43, 55 – 67. Schumm, S.A., 1993. River response to base level change: implications for sequence stratigraphy. J. Geol. 101, 279 – 294. Srivastava, P., Juyal, N., Singhvi, A.K., Wasson, R.J., Bateman, M.D., 2001. Luminescence chronology of river adjustment and
A.S. Khadkikar, C. Rajshekhar / Sedimentary Geology 179 (2005) 295–303 incision of Quaternary sediments in the alluvial plain of the Sabarmati River, north Gujarat, India. Geomorphology 36, 217 – 229. Tandon, S.K., Sareen, B.K., Rao, M.S., Singhvi, A.K., 1997. Aggradation history and luminescence chronology of Late Quaternary semi-arid sequences of the Sabarmati basin, Gujarat, western India. Palaeogeogr. Palaeoclimatol. Palaeoecol. 128, 339 – 357. Thamban, M., PurnachandraRao, V., Schneider, R.R., Grootes,
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P.M., 2001. Glacial to Holocene fluctuations in hydrography and productivity along the southwestern continental margin of India. Palaeogeogr. Palaeoclimatol. Palaeoecol. 165, 113 – 127. To¨rnquist, T.E., Wallinga, J., Busschers, F.S., 2003. Timing of the last sequence boundary in a fluvial setting near the highstand shoreline — insights from optical dating. Geology 31, 279 – 282.
Holocene valley incision during sea level transgression under a monsoonal climate Aniruddha S. Khadkikar *, C. Rajshekhar Agharkar Research Institute,G.G. Agarkar Road, Pune 411 004, India Received 26 October 2004; received in revised form 9 June 2005; accepted 10 June 2005
Abstract The late Quaternary stratigraphic record of the Mahi River, western India records the response of a macrotidal monsoonal estuary to base level and climate change. The timing of the valley incision is constrained based on chronology on deposits that constitute the banks of the Mahi River and on inset valley fill terraces. These results show that valley incision took place in midHolocene, a period of sea level transgression. The reason for valley incision appears to be related to the increase in seasonality of the rivers, which imparts a high work capacity to them due to climatic amelioration. The post-incision stratigraphic record of the Mahi is opposite to that in Type 1 sequence records with respect to grain size trends and facies distribution. This may thus prove useful in identifying transgressive valley incision in pre-Quaternary deposits. Moreover in light of the dichotomy of the timing of valley incision with respect to base level position, maximum flooding surfaces may be more reliable in demarcating sequence boundaries, particularly in monsoonal settings. D 2005 Elsevier B.V. All rights reserved. Keywords: Holocene; Sequence stratigraphy; Quaternary; Monsoon
1. Introduction Estuaries and incised valleys play a key role in the interpretation of sequence stratigraphy of paralic deposits (Miall, 1997). Incised valleys are commonly used to mark sequence boundaries associated with the lowstand systems tracts (Dalrymple et al., 1994; Posamentier, 2001). While valley incision has been attributed primarily to base level lowering, the possibility * Corresponding author. E-mail address: [email protected] (A.S. Khadkikar). 0037-0738/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.sedgeo.2005.06.004
that it may be unrelated to base level has also been raised. Also, special cases in which base level lowering does not result in valley incision have been described by Posamentier (2001). A few examples relating valley incision to non-base level parameters exist (Jain and Tandon, 2003; Srivastava et al., 2001) from the monsoonal regions. Estuaries on the other hand mark transgressive events in the conventional sequence stratigraphic framework (Miall, 1997). Such estuaries form in drowned valleys during periods of sea level rise (e.g. Allen and Posamentier, 1993). As the recognition of sequence boundaries relies heavily on the dregressiveT model of valley incision, it is prudent to ask if alterna-
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tive possibilities exist in nature or not? Were valley incision, governed by other parameters, to be more frequent, then the sequence stratigraphic interpretation would be complicated. The differentiation of incised valleys related to base level lowering and non-base level controls then becomes crucial for sequence stratigraphy. The study of Quaternary depositional systems is important for sequence stratigraphy as it provides an opportunity to understand the response of depositional systems with respect to sea level changes in a geochronologically constrained framework (Blum and To¨rnquist, 2000; Goodbred et al., 2003). The purpose of the present study is to illustrate using remote sensing based geomorphological studies, stratigraphy and sedimentology; an example of climate-controlled valley incision that is co-incident with a Holocene transgression. The timing of these events is well constrained on the basis of luminescence, electron spin resonance, radiocarbon and archaeological ages.
2. Area of study The region in which the river Mahi is located falls in the State of Gujarat (Fig. 1), western India which
experiences rainfall seasonally in the monsoon months of June, July and August. During these months, the river Mahi experiences an abrupt change in discharge from b 100 m3 s1 to as high as 10,000 m3 s1 (Fig. 2). This large magnitude change governs channel geometry and results in a box like channel geometry that appears to be typical to monsoon rivers (Kale, 2002). The Mahi River has incised into late Quaternary deposits which today stand exposed as 30–40 m cliffs along its banks (Fig. 3). The tidal regime in the region is macrotidal and one of the largest in the world with amplitudes exceeding 10 m. Consequently, the present study characterizes the response of a monsoonal macrotidal depositional system within a sequence stratigraphic framework to base level and climate change.
3. Methodology Multispectral Indian Remote Sensing (IRS) 1DLISS 3 satellite data was used to understand the geomorphology of the region. The 4-band data was transformed using the decorrelation stretch method to obtain maximum visual contrast. Based on the pro-
Fig. 1. A) Location of the Mahi estuary in India. B) IRS1D LISS3 multispectral data showing key geomorphic elements. C) Geomorphology of the Mahi estuary based on IRS-1D multispectral satellite data. The spatial extent of terraces mimics the extent of modern bank attached bars.
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Fig. 2. Monthly discharges of the Mahi River between 1988 and 1991 (Data source: Water Resources Division, Government of Gujarat). The changes span several orders of magnitude from summer/winter months to monsoon months and show marked seasonality that is typical to rivers of India.
cessed imagery, river/estuary morphology and terraces were mapped. Field studies were carried out to validate the geomorphic units. The geomorphic units were mapped using photomosaics to judge lateral facies variations. These results were then integrated with earlier geochronological studies on the late Quaternary to Holocene deposits. Sediment samples from the terraces were analyzed for foraminifera following standard procedures involving pretreatment with H2O2 followed by wet sieving.
4. Results The Mahi River estuary has a wide funnel whose width decays exponentially from the mouth (Fig. 1). At the mouth the estuary has widths of about 10 km whereas in the fluvial reach the valley widths are under 2 km. Throughout the lower reaches the Mahi River shows alternating bank attached terraces (Fig. 1). These bank attached terraces abut against older late Quaternary deposits that form steep banks resulting in
Fig. 3. River bank and terrace stratigraphies. The river bank stratigraphy is from Dabka and shows a red soil at the base covering an age range from 50 to 25 ka. Overlying the palaeosol are sheetflood fluvial deposits that formed during the Last Glacial Maximum and were overlain by loess. During the Holocene a thick soil developed over these deposits (vertisol). The terrace stratigraphies are similar, beginning with muds showing bioturbation and wavy bedding that contain foraminifera. The capping deposits are estuarine/fluvial sands.
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an inset character for the terraces. The bank attached terraces themselves show steep banks due to modern incision and erosion due to the lateral migration of the Mahi River. Terraces are narrow in the upstream reaches but expand significantly in the lower reaches around Kothiyakhad, a pattern seen even today (Fig. 1). These changes in terrace extent reflect incremental tidal influence towards the mouth. The terrace stratigraphy is observed to be broadly similar throughout the region (Fig. 3). It usually begins with wavy laminated silty clay to black mud and coarsens upwards. The black mud at some sites shows intensive bioturbation and presence of mud flakes. The wavy laminated sediments consist of cyclical silt/clay alternations (Fig. 3), which are seen
at Tithor. Upwards the sediments become massive and less distinctly bedded. These sediments also show the presence of diffuse calcretes. While deposits towards the mouth show wavy lamination those away from the funnel show the presence of massive muds implying a control of coastal processes on sedimentary facies. The sediment succession is interpreted to represent a lower estuary to fluvial transition. The lower estuary (downstream of meanders; Fig. 3) deposits show a rich to moderate assemblage of benthic and planktonic foraminifera. The benthic assemblage is dominated by Quinqueloculina sp., Elphidium oceanicum, E. excavatum, Ammonia beccarii and Nonion boueanum, which is typical to estuarine regions (Cann et al., 2002). At Tithor (Fig. 3) the foraminiferal abundance
Fig. 4. A) The timing of various depositional phases in relation to the global sea level curve of Chappell and Shackleton (1986) which show clearly that valley incision took place during the Holocene transgression. B) The same data plotted with respect to a local sea level curve (Hashimi et al., 1995) after calibrating the radiocarbon dates. Luminescence ages other than those marked Dabka are from Rayka (Fig. 1). Loess stabilization in the southern regions (Dabka) was much earlier than towards the north (Rayka). This comparison again shows that valley incision coincided with rising sea levels.
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progressively decreases as the sediment succession changes towards fluvial deposits. Through the profile the degree of preservation of foraminifera also reduces with the frequency of corroded and abraded forms increasing towards the top. The presence of planktonic foraminifera such as Globigerinoides and Globigerina reflects the macrotidal environment of the Mahi estuary. This assemblage is comparable to earlier studies at Kothiyakhad (Rachna and Chamyal, 1998). Radiocarbon dates that are available for the deposits at Kothiyakhad (Chamyal et al., 2003) show an age range from 3660 F 90 14C yr BP to 1760 F 80 14 C yr BP and correspond to calibrated ages 3560– 3776 cal yr BP (1r error) to 1565–1738 cal yr BP. Calibrated ages from Mujhpur (2772–2356 cal yr BP), Sultanpura (3451–2875 cal yr BP) and Tithor (4826– 3836 cal yr BP) suggest that terrace sedimentation is at least older than 4500 yr BP. A radiocarbon age of 6500 14C yr BP from the base of an upstream terrace (Chamyal et al., 2003) indicates that it may have reached the present level about 7000 cal yr BP. This also puts an upper limit on cessation of incision around this time frame (6 F 1 ka). The late Quaternary deposits which have been incised into by the modern Mahi, have a stratigraphy that differs from the terrace stratigraphy. The deposits usually begin with calcrete conglomerates which show trough and planar cross bedding (Khadkikar et al., 1999) and are associated with calcic vertisols (Figs. 3,4). These deposits are associated with sandy channel deposits hosting groundwater calcretes. The age of these deposits has been constrained using electron spin resonance ages of calcretes (Khadkikar et al., 1999) and luminescence dates (Juyal et al., 2000) to the last interglacial period (Fig. 4). These deposits also show evidences of tidal influence in the presence of corroded foraminifera (Chamyal et al., 2003), similar to that observed today in the modern Mahi river about 40 km inland of the river mouth. Overlying these deposits, which have been interpreted by Khadkikar et al. (1999), to represent seasonal rivers under a subhumid climate occur deposits of semi-arid rivers. These deposits show laterally extensive horizontal bedding with faint internal lamination. Within these deposits a laterally extensive red soil bed acts as a marker whose age may be bracketed between 50 and 25 ka BP (Khadkikar et al., 1999) (Figs. 3,4). This second style of sedimentation culminates in the
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deposition of sandy loess deposits that range in age from 22 to 10 ka (Juyal et al., 2000; Juyal et al., 2003) on the basis of luminescence and radiocarbon ages (Fig. 4) and in the distribution of Microlithic sites throughout the region (Khadkikar et al., 1999).
5. Discussion The Mahi estuary and its incised valley provide an alternative possibility of the responses to allogenic controls. The chronology of the late Quaternary and Holocene deposits has some interesting repercussions on the science of sequence stratigraphy. Sequences are defined as genetically related packages bound by unconformities or their correlative conformities (Miall, 1997). In the classical Type 1 sequence, erosional sequence boundaries owe their origin to lowstand incision due to a fall in relative sea level. Recent studies confirm that this may only be one of several possibilities (Dalrymple et al., 1994). That, unconformity development may not be unambiguous, was demonstrated recently by To¨rnquist et al. (2003) based on their well dated cores from the Rhine– Muese delta. The present study provides a vaster outlook based on outcrops. Luminescence dates on the late Quaternary stratigraphic record show punctuated sedimentation till about 7 ka. In the outcrops no definite evidence of incision related to any of the palaeosols were observed. The capping deposits of sandy loess have a deep soil profile, vertic in character suggesting cessation of aeolian deposition after 6 F 1 ka the youngest date obtained on sand dune aggradation in Gujarat northwest of Mahi (Khadkikar et al., 1999; Tandon et al., 1997). As the valley incises the loess also, it post-dates its sedimentation. There remains a possibility of continued aeolian deposition after the valley incision during subsequent arid periods but such situations are rare as seen in the sandy loess chronology (Juyal et al., 2003) and restricted to regions much north of the Mahi basin. Thus, based on regional stratigraphic coherence and chronology, it is observed that incision postdated the bulk of sandy loess deposition. Valley incision may have been gradual or instantaneous occurring in the latter case within centuries. The time duration of the valley incision and also its timing is confirmed on the basis of
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radiocarbon dates between 7000 and 4800 cal yr B.P. on basal terrace deposits in the Mahi valley. The uncertainties in the luminescence and 14C ages allow the interpretation of rapid valley incision between 7 and 5 ka (early–mid-Holocene). 6 ka is a most interesting period in the earth’s history and referred to as the mid-Holocene optimum, owing to the worldwide hot and humid climate state. It also was a period of relative sea level rise (Hashimi et al., 1995). In the Indian context, local sea level curves also reflect global sea level tendencies (Fig. 4) and show a sea level of + 2 m during the mid-Holocene optimum. Thus valley incision in the Mahi coincided with a major transgression. Valley incision during the Holocene appears to be widespread in the Indian region and has been observed by Pratt et al. (2002) in the Himalayan rivers and in western India too (Srivastava et al., 2001; Jain and Tandon, 2003). All these studies suggest that valley incision occurred due to enhancement of the Indian monsoon broadly between 12 and 5 ka BP. This monsoon intensification during the early Holocene is well reflected in palaeorainfall proxy records and peaked at 5.6 ka (Thamban et al., 2001). The youngest terrace ages are about 2 ka which suggests that the Mahi valley aggraded for about 5000 years. This was indeed followed by another incision, probably within the past millennium. But the exact cause of this incision is uncertain (i.e. base level vs. monsoon intensification). How can we possibly reconcile a transgressive incised valley, which contradicts our current understanding of the responses of rivers to base level change (Miall, 1997)? It should be noted that in the earlier laboratory studies (e.g. Koss et al., 1994) major emphasis was allocated towards understanding river responses to base level change, neglecting changes in river discharge. These studies showed how rates of base level change affected the nature of delta progradation and valley incision. Valley incision occurred when the base level fell below the shelf break and migrated upstream through headward erosion. Schumm (1993) however also highlighted earlier that apart from base level change, significant aberrations to the graded river concept (followed by sequence stratigraphers) could occur due to climate change. This appears to be particularly important in the monsoonal regions. Monsoonal rivers show a dramatic annual discharge pattern with b100 m3 s1
in winter/summer months (in W. India) to as high as 60,000 m3 s1 in the monsoon (Kale, 2002). This imparts to the rivers a very high work capacity. The change from loess deposits which form in desert margins under mean annual rainfalls of ~250 mm to valley incision and river rejuvenation (mean annual rainfalls equivalent to today of 500–700 mm) coincides with the Holocene optimum (Jain and Tandon, 2003). This is supported by various proxy records of the Indian monsoon which demonstrate intensification during the Holocene. The extreme seasonality of monsoonal rivers (Fig. 2) also enables them to negate base level effects in their lower reaches as exemplified by the Mahi record. Moreover during this period, an amelioration of climate also led to the formation of a vertisol over the loess deposits (Khadkikar et al., 1999). This may have led to reduced sediment availability and imparted a greater erosive power to the Mahi River during the Holocene optimum enabling valley incision. Since valley incision is possible under two situations, one due to base level change and other due to climatic amelioration, there may arise a problem in relating unambiguously, the sequence boundary to base level change. An alternative approach in sequence stratigraphy is to define sequence boundaries based on maximum flooding surfaces (Gibling and Bird, 1994; Miall, 1997) which are necessarily related to sea levels. This dichotomy in the timing of valley incision with respect to base level change shows that the approach of Gibling and Bird (1994) in using maximum flooding surfaces for demarcating sequence boundaries also holds true when studying deposits of seasonal rivers. In the Mahi estuary, the maximum flooding surface may be demarcated between the tidally influenced facies at the base of the terrace deposits and the overlying alluvial facies (Fig. 5). The identification of the MFS can best be considered tentative as it is assumed that future rise in sea level due to global warming will not exceed the last rise in sea level. The MFS can be clearly demarcated till the upper estuary, upstream of which the facies is entirely fluvial cobble rich sand. Sediments dating to the Last Glacial Maximum (~20 ka) in the Mahi region lie at a stratigraphic level of about 10–20 m from the modern river bed (Fig. 5). Thus the geomorphic surface on which rivers flowed during the lowstand systems tract at 20 ka was much higher. Hence valley incision during the Holo-
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Fig. 5. Summary model of along valley changes in sedimentary facies of the Holocene terraces of the Mahi estuary. Lower, middle and upper refer to the position within the estuary. Also shown is a typical across valley section (A–B) that illustrates the relationship between the late Quaternary stratigraphic succession, its chronology and relationship to the terrace succession. Terraces are the small wedge in the cross section labelled 7 ka which reflects the initiation of terrace sedimentation. The maximum flooding surface (MFS) may be demarcated at the boundary between tidally influenced facies and fluvial sand/cobbly sand facies assuming that future sea level rise due to global warming will not exceed the last rise in sea level. Details of the late Quaternary stratigraphic record, and regional correlations, based on which the typical across valley section is drawn are given in Khadkikar et al. (1999). The lowermost part of the estuary towards the mouth does not host any terraces. Hence facies changes are shown from the region where terrace stratigraphies are observed.
cene due to climatic amelioration exceeded the depths at which incision if any may have occurred due to base level fall during the Last Glacial Maximum. The region is tectonically active and falls within the Cambay graben (Chamyal et al., 2003) which could affect river response if such activity affected the base level. However, there are no evidences suggesting regional subsidence in the early to mid-Holocene. In light of the absence of any strong indicators of regional subsidence (which could promote incision by altering base level), this parameter appears to be insignificant. The stratigraphy of transgressive valley incision fills appears to differ from the valley fill record of the base level-change related valley fill (Dalrymple et al., 1992; Allen and Posamentier, 1993). In the latter case, the deposits show a change from fluvial sandstones/sands to gradually fining intertidal to estuarine facies as the valleys get drowned. This leads to a fining upward sedimentary record. In the present case the stratigraphic record is exactly the opposite as foraminifera bearing deposits are overlain by flu-
vial sands. This leads to a general coarsening upward deposit after valley incision. However, there are significant differences in facies assemblages in different parts of the estuary. In the lower estuary (Fig. 5) the transition is easy to map due to facies contrast (tidal rhythmites to fluvial sands). In the middle estuary (Fig. 5), where clay deposition is at maximum, bioturbated clays are found at the base which are succeeded upsection by interlayered fluvial sands and clays, suggesting progradation. In the upper estuary there is a transition from silty rhythmites to fluvial sands (Fig. 5). So in the entire estuary a clear transition from a lower tidally influenced facies to an upper fluvially influenced facies is observed. This facies sequence is thus opposite to the drowning valley model where the sequence is capped by tidally/marine influenced facies. These contrasts in the stratigraphic record may be useful in discriminating between the two modes of valley incision, which may be aided by the analysis of the distribution of foraminifera in the deposits.
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There are not too many radiocarbon dates on the terrace sediments to comment on the synchrony of the Holocene sediments through the valley. The chronological data available only allows conclusions to be reached regarding the timing of valley incision with respect to a major sea level change around the last glacial maximum.
6. Conclusion Integrated geomorphological, sedimentological and chronostratigraphic results on the late Quaternary to Holocene sedimentary record of the Mahi River show that alternatives exist to the Type 1 sequence model of lowstand valley incision. These results demonstrate an instance of valley incision during a period of sea level transgression. Deposits of such incised valleys show stratigraphies that differ from those that overlie Type 1 sequence boundaries. This may thus turn out to be useful for discriminating transgressive and regressive incised valleys. In light of the dichotomy in the timing of valley incision with respect to base level, maximum flooding surfaces appear to be more reliable in demarcating sequence boundaries in monsoonal environments.
Acknowledgments ASK would like to thank DST, New Delhi for financial support. Comments on earlier versions of the manuscript by A. Miall, M. Gibling, T. To¨rnquist and S. Goodbred were useful in developing these ideas.
References Allen, G.P., Posamentier, H.W., 1993. Sequence stratigraphy and facies model of an incised valley fill: the Gironde estuary, France. J. Sediment. Petrol. 63, 378 – 391. Blum, M.D., To¨rnquist, T.E., 2000. Fluvial responses to climate and sea-level change: a review and look forward. Sedimentology 47, 2 – 48. Cann, J.H., Harvey, N., Barnett, E.J., Belperio, A.P., Bourman, R.P., 2002. Foraminiferal biofacies eco-succession and Holocene sea levels, Port Pirie, South Australia. Mar. Micropaleontol. 44, 31 – 55.
Chamyal, L.S., Maurya, D.M., Rachna, R., 2003. Fluvial systems of the drylands of western India: a synthesis of late quaternary environmental and tectonic changes. Quat. Int. 104, 69 – 86. Chappell, J., Shackleton, N.J., 1986. Oxygen isotopes and sea level. Nature 324, 137 – 140. Dalrymple, R.W., Zaitlin, B.A., Boyd, R., 1992. Estuarine facies models: conceptual basis and stratigraphic implications. J. Sediment. Petrol. 62, 1130 – 1146. Dalrymple, R.W., Boyd, R., Zaitlin, B.A., 1994. History of research, types and internal organization of incised-valley systems: introduction to the volume. In: Dalrymple, R.W., Boyd, R., Zaitlin, B.A. (Eds.), Incised-Valley Systems: Origin and Sedimentary Sequences, SEPM special publication, vol. 51, pp. 3 – 10. Gibling, M.R., Bird, D.J., 1994. Late Carboniferous cyclothems and alluvial paleovalleys in the Sydney basin, Nova Scotia. Geol. Soc. Amer. Bull. 106, 105 – 117. Goodbred Jr., S.L., Kuehl, S.A., Steckler, M.S., Sarker, M.H., 2003. Controls on facies distribution and stratigraphic preservation in the Ganges–Brahmaputra delta sequence. Sediment. Geol. 155, 301 – 316. Hashimi, N.H., Nigam, R., Nair, R.R., Rajagopalan, G., 1995. Holocene sea level curve and related climatic fluctuations for western Indian continental margin. An update. J. Geol. Soc. India 46, 157 – 162. Jain, M., Tandon, S.K., 2003. Fluvial response to late quaternary climate changes, western India. Quat. Sci. Rev. 22, 2223 – 2235. Juyal, N., Rachna, R., Maurya, D.M., Chamyal, L.S., Singhvi, A.K., 2000. Chronology of late Pleistocene environmental changes in the lower Mahi basin, western India. J. Quat. Sci. 15, 501 – 508. Juyal, N., Kar, A., Rajaguru, S.N., Singhvi, A.K., 2003. Luminescence chronology of aeolian deposition during the late quaternary on the southern margin of Thar desert, India. Quat. Int. 104, 87 – 98. Kale, V.S., 2002. Fluvial geomorphology of Indian rivers: an overview. Prog. Phys. Geogr. 26, 400 – 433. Khadkikar, A.S., Mathew, G., Malik, J.N., Gundu Rao, T.K., Chowgaonkar, M., Merh, S.S., 1999. The influence of the southwest Indian monsoon on late quaternary continental sedimentation, western India over the past 130 kyrs. Terra Nova 11, 273 – 277. Koss, J.E., Ethridge, F.G., Schumm, S.A., 1994. An experimental study of the effects of base level change on fluvial, coastal plain and shelf systems. J. Sediment. Res., Sect. B Stratigr. Glob. Stud. 64, 90 – 98. Miall, A.D., 1997. The Geology of Sequences. Springer Verlag, Berlin. 433 pp. Posamentier, H.W., 2001. Lowstand alluvial bypass systems: incised vs. unincised. AAPG Bull. 85, 1771 – 1793. Pratt, B., Burbank, D.W., Heimsath, A., Ojha, T., 2002. Impulsive alluviation during early Holocene strengthened monsoons, central Nepal Himalaya. Geology 30, 911 – 914. Rachna, R., Chamyal, L.S., 1998. Microfauna from a Holocene valley fill terrace, lower Mahi basin, Gujarat. J. Palaeontol. Soc. India 43, 55 – 67. Schumm, S.A., 1993. River response to base level change: implications for sequence stratigraphy. J. Geol. 101, 279 – 294. Srivastava, P., Juyal, N., Singhvi, A.K., Wasson, R.J., Bateman, M.D., 2001. Luminescence chronology of river adjustment and
A.S. Khadkikar, C. Rajshekhar / Sedimentary Geology 179 (2005) 295–303 incision of Quaternary sediments in the alluvial plain of the Sabarmati River, north Gujarat, India. Geomorphology 36, 217 – 229. Tandon, S.K., Sareen, B.K., Rao, M.S., Singhvi, A.K., 1997. Aggradation history and luminescence chronology of Late Quaternary semi-arid sequences of the Sabarmati basin, Gujarat, western India. Palaeogeogr. Palaeoclimatol. Palaeoecol. 128, 339 – 357. Thamban, M., PurnachandraRao, V., Schneider, R.R., Grootes,
303
P.M., 2001. Glacial to Holocene fluctuations in hydrography and productivity along the southwestern continental margin of India. Palaeogeogr. Palaeoclimatol. Palaeoecol. 165, 113 – 127. To¨rnquist, T.E., Wallinga, J., Busschers, F.S., 2003. Timing of the last sequence boundary in a fluvial setting near the highstand shoreline — insights from optical dating. Geology 31, 279 – 282.