Sedimentology, geochemistry and OSL dating of the

Geomorphology 297 (2017) 1–19

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Geomorphology journal homepage: www.elsevier.com/locate/geomorph

Sedimentology, geochemistry and OSL dating of the alluvial succession in the northern Gujarat alluvial plain (western India) - A record to evaluate the sensitivity of a semiarid fluvial system to the climatic and tectonic forcing since the late Marine Isotopic Stage 3 Falguni Bhattacharya a,⁎, Anil D. Shukla b, R.C. Patel c, B.K. Rastogi a, Navin Juyal b a b c

Institute of Seismological Research, Gandhinagar, Gujarat 382009, India Geoscience Division, Physical Research Laboratory, Ahmedabad 380009, India Department of Geophysics, Kurukshetra University, 136118, India

a r t i c l e

i n f o

Article history: Received 26 May 2017 Received in revised form 23 August 2017 Accepted 23 August 2017 Available online 06 September 2017 Keywords: Semiarid fluvial system Optical chronology Geochemistry Sedimentology Indian summer monsoon

a b s t r a c t The alluvial successions in the northern Gujarat alluvial plain (western India) have been investigated for reconstructing the climatic fluctuations during the last 40 ka. Alluvial architecture and geochemical proxies indicate prevalence of a strengthened Indian Summer Monsoon (ISM) with fluctuations between the late Marine Isotopic Stage 3 (MIS 3; 37 ka) to the early MIS 2 (27 ka). A gradual onset of aridity (declining ISM) after 27 ka with peak aridity at ~ 22 ka is observed. A gradual strengthening of ISM at around 18 and N 12 ka followed by a short reversal in ISM intensity between 12 and 11 ka, is attributed to the Younger-Dryas (YD) cooling event. The aeolian sand sheet dated to 6 and 3.5 ka represents the onset of regional aridity. Following this, a short-lived humid phase was observed after ~2 ka, which includes the Medieval Warm Period (MWP). The study suggests that the variability in the ISM to the latitudinal migration of the Inter Tropical Convergence Zone was caused by insolation-driven cooling and warming events in the North Atlantic. The incision of the valley fill alluvium occurred in two distinct phases. The older incision phase occurred after 11 ka and before 6 ka, whereas the younger incision phase that led to the development of present day topography is bracketed between 3.5 ka and before 1 ka. The older incision phase is ascribed to the early to mid-Holocene enhanced ISM (climatically driven), whereas the younger incision seems to be modulated by the activation of basement faults (tectonically driven). © 2017 Elsevier B.V. All rights reserved.

1. Introduction Dryland alluvial sequences, owing to their sensitivity to minor climatic perturbations that result in significant changes in flow regime, sediment transport, and associated channel style (Nanson and Tooth, 1999), provide great potential for paleoenvironmental reconstructions (Reid et al., 1998; Nanson and Tooth, 1999). Considering that semiarid regions cover a large proportion of Earth's land surface (Graf, 1988) and the ephemeral fluvial system constitutes one of the important geomorphic agents (Reid and Frostick, 1997), it is therefore important to understand river responses to climate variability. Conventionally, it is assumed that morphological and sedimentological evidence are strongly coupled; hence, fluvial sequences can be used to infer information on past hydrological conditions (climate). However, climatic interpretation is not straightforward because of the variability in flow regimes, disequilibrium between channel forms and associated processes, and tectonics (Graf, ⁎ Corresponding author. E-mail address: [email protected] (F. Bhattacharya).

http://dx.doi.org/10.1016/j.geomorph.2017.08.046 0169-555X/© 2017 Elsevier B.V. All rights reserved.

1983; Hereford, 1984). Therefore, Jain and Tandon (2003) suggest that along with facies architecture, climatic inferences need to be supported by climatic proxies and secured chronologies to avoid circular reasoning. With the inherent constraints, dryland alluvial successions nevertheless provide relatively long continental records relevant to understand river response to long-term (103 yr scale) climatic variability covering the entire glacial-interglacial cycle (Nanson et al., 1992; Page et al., 1996; Juyal et al., 2006). The palaeoclimatic potential of the Quaternary alluvial succession of the Gujarat alluvial plain, which overlies the faulted Cenozoic basement (Maurya et al., 1995; Merh and Chamyal, 1997), was recognized in early 1950s by Zeuner (1950) (Fig. 1A). Subsequent studies based on detailed sedimentology and chronometric data indicate that paleohydrological processes in the Gujarat alluvial plain were modulated by spatiotemporal changes in the ISM (Tandon et al., 1997; Juyal et al., 2000, 2006; Srivastava et al., 2001). According to Jain and Tandon (2003), the fluvial system in western India responded to global climate changes so that fluctuations in the ISM-intensity modulate the relative changes in discharge and sediment supply. Juyal et al. (2006), using the marine

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F. Bhattacharya et al. / Geomorphology 297 (2017) 1–19

Fig. 1. Map showing locations of (A) Gujarat alluvial plain (GAP) and the drainages flowing from the Aravalli Hills towards the Little Rann and Gulf of Cambay. BR = Banas River, SR = Saraswati River, RR = Rupen River, SBR = Sabarmati River, MR = Mahi River, NR = Narmada River, OR = Orsang River. (B) DEM of Gujarat alluvial plain and drainages. (C) Structural configuration of the Cambay basin of Gujarat alluvial plain dominated by ridges and basins. (D) Cambay rift and subsidiary normal and transfer faults traversing through the Banas and Saraswati River basins (after Kundu and Wani, 1992). Note that the Banas and Saraswati rivers occupy the Patan sub-basin. EMCF = Eastern Margin Cambay Fault, WMCF = Western Margin Cambay Faults, and JBL = Jaipur Barwani Lineament. Field locations are shown along the Banas and Saraswati rivers (IQB = Iqbalgarh, DW = Dantiwada, MA = Moti Akhol, JD = Juna-Deesa, OG = Goliya, GN = Gotnath, and SD = Siddhpur).

proxy from the upwelling region of the Arabian Sea, demonstrated that the fluvial and aeolian successions of the Gujarat alluvial plain responded in accordance with variability in the ISM. Numerous studies exist that pertain to the pattern of sedimentation and its correlation with ISM variability during the late Quaternary period from the

subhumid to semiarid alluvial plains of Gujarat (Sareen et al., 1993; Tandon et al., 1997; Juyal et al., 2000, 2004, 2006; Srivastava et al., 2001; Bhandari et al., 2005; Sridhar et al., 2013). In contrast, virtually no data exists from the climatically sensitive transitional zone (semiarid south and arid north), namely the Saraswati and Banas River valleys


(Fig. 1). The geomorphic processes in the transitional climatic zones are known to respond in an amplified scale to even small changes in temperature and precipitation conditions (Lancaster, 1979; Juyal et al., 2003, 2006). The present study investigates the fluvial and aeolian sequences of the Banas and Saraswati River valleys with the following objectives: (i) to document the pattern of sedimentation to ascertain the nature of fluvial-aeolian response to climate variability, (ii) to compare the fluvial and aeolian record with that of semiarid and subhumid rivers to assess the influence of rainfall gradient on sedimentation and postdepositional changes, and (iii) to document the evolution of alluvial sequences in the transitional zone and their relationship with the semiarid and subhumid fluvial system. 2. Study area This study was undertaken in the river valleys carved by the ephemeral Banas and Saraswati rivers (Fig. 1). Channel sand and dunes obstruct a large part of the Banas and Saraswati River valleys, therefore it is rare to find exposed alluvial stratigraphic sections for investigation and sampling. Consequently, we could only locate a couple of sections in the Banas River valley, and only one section in the Saraswati River. The Banas River delimits the northern extent of the Gujarat alluvial plain. A progressive northward decrease in the precipitation can be observed from the Narmada River basin in the south to the Banas River in the north (Fig. 1A). The mean annual precipitation in the region surrounding the study area is ~550 mm/yr (source: Indian Meteorological Department), suggesting that the study area lies in the arid to semiarid transitional zone. 2.1. Banas basin The Banas River has a catchment area of ~8674 km2 and originates from the Aravalli Hills, which are dominated by the Erinpura granite and Sirohi volcanics (Fig. 1C). The river flows about ~200 km and traverses three geomorphic zones, the rocky upland, the middle piedmont zone, and the lower alluvial plain, before merging with the Little Rann (Fig. 1). The Jabalpur-Barwani Lineament (JBL) and the Eastern Margin Cambay Fault (EMCF) mark the boundary between the piedmont and alluvial zone (Figs. 1C, D and 2A). The ravines and gullies observed below the piedmont zone occur preferentially northwest of the Banas River (Fig. 3B). Iqbalgarh (IQB) and Dantiwada (DW) are located in the piedmont zone (Fig. 3B, C and D). Farther downstream in the alluvial plain, the Banas River is dominated by sand, although gullies continue to occur and are largely confined towards the northwestern flank (Fig. 3E). Three of the study sites are located in the alluvial plain: Moti Akhol (MA), Juna-Deesa (JD) and Goliya (OG) (Figs. 3F–I). The river has carved discontinuous terraces at places, particularly along the eastern flank (Fig. 3E). The alluvial cliffs are located between eastern and western margin Cambay faults. Downstream towards the Little Rann, the river has poorly defined riverbanks (Fig. 3K) except near Gotnath where a localized small cliff is exposed (Fig. 3J). Compared to all other sections, Juna-Deesa provides near complete stratigraphy of the Banas River basin (Figs. 3E and 4). 2.2. Saraswati basin The Saraswati River valley is located to the south of the Banas River. The Saraswati originates in the Aravalli Hills and merges in the lower alluvial plain near Punasan village (Fig. 5A) proximal to the EMCF (Fig. 5A, inset). The Saraswati River (~ 370 km2) flows through the same geomorphic zones as the Banas River before disappearing into the distal alluvial plain. Unlike the wide Banas River valley, the Saraswati River has a narrow course that is covered with a stabilized and active sand sheet. Banks with good vertical exposures of alluvium are virtually non-existent along the Saraswati River because of congestion of the river by sand. As a result, we used a stratigraphic sequence that was exposed by construction activities in this study (Fig. 5).

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3. Methodology 3.1. Geomorphology and field stratigraphy Geomorphological mapping of the Banas and Saraswati valleys was carried out using the Survey of India (SOI) topographic maps (1:50,000) and SRTM satellite data (30 m resolution). The data was processed in GIS environment (ArcGIS 10.1) in order to generate a Digital Elevation Model (DEM). Stratigraphy of the sedimentary succession was documented using conventional sedimentological and stratigraphic techniques. Based on the lithology, color, composition, textural variations, and degree of weathering, the alluvial successions were differentiated into different lithostratigraphic units (Figs. 3–6). Paleoflood deposits were identified using the conventional sedimentological criteria as suggested by Kale et al. (1994) and Kochel and Baker (1982), which involves the identification of flood couplets/events (i.e., upward fining sand layers capped by a fine silty clay layer). We recorded sediment assemblages at each subunit for detailed facies analysis (Supplementary Table 1) and OSL chronology (Tables 1 and 2). The stratigraphic sections were laterally correlated based on the facies assemblages. Further, to strengthen our database and intrabasin correlation of the alluvial sequences, we used ~60 published optical dates from the Gujarat alluvial record (Supplementary Table 2).

3.2. Geochemistry Major and trace element concentrations were estimated using the X-Ray Fluorescence (XRF) technique in an automated Philips AXIOS X-ray Spectrometer fitted with a Rh X-ray tube, operated at 50 kV and 55 mA, of 4 kW power. Samples (~ 2 g) were finely powdered and mixed homogeneously with 0.5 g wax binder in an agate pestle and mortar. The sample pellets were made by transferring the mixture into the standard aluminum cups (37 mm) and subjecting them to a pressure of 150 kN using a hydraulic press for about 1 min. The analytical precision carried out at the two sigma level for major oxides and trace elements was better than 5% (Shukla, 2011). The temporal change in climate variability was ascertained using weathering proxies such as the Chemical Index of Alteration (CIA) along with the Plagioclase Index of Alteration (PIA) and the Chemical Index of Weathering (CIW). The CIA is defined by Nesbitt and Young (1982) using the molecular proportions [Al 2O 3 / (Al 2O 3 + *CaO + NaO + K2O)] × 100. The *CaO is the amount of CaO incorporated in the silicate fraction (therefore, we decalcified the samples to remove Ca from the carbonate fraction). The CIA measures the proportion of Al2O3 versus more labile oxides (i.e., CaO, K2O and NaO) and reflects the relative amount of feldspars and clay minerals in the sediment (Minyuk et al., 2014). The PIA indicates the weathering intensity of plagioclase and is given by (Fedo et al., 1995) as [(Al 2 O3 − K 2O) / (Al2 O3 + CaO + Na 2 O*K 2 O)] × 100. The CIW reflects the ratio of immobile Al2O3 to the labile CaO and NaO and is calculated using the equation (Harnois, 1988) [Al2O3 / (Al2O3 + CaO + NaO)] × 100. Here *K is not considered in the estimation as it may be leached or accumulated in the residual weathering products. In addition to this, the SiO2/Al2O3 ratio, also known as Ruxton ratio (Ruxton, 1968), is used to determine temporal changes in detrital particles. The Zr/Al and Ti/Al ratios are used to identify periods of enhanced and weak surface runoff (i.e., relatively wet and dry phases) (Wehausen and Brumsack, 1999; Deplazes et al., 2014), which in this study is modulated by the monsoon variability. 3.3. Chronology Chronology of the alluvial succession was obtained using the optically stimulated luminescence (OSL) dating technique on quartz mineral extract. The technique relies on the principle that prior to deposition,

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Fig. 2. (A) The Banas River emerges from the Aravalli upland and drains into the Little Rann. Locations of the field photographs are marked as follows: (B) IQB = Iqbalgarh, (C) DW = Dantiwada, (D) MA = Moti Akhol, (E) OG = Goliya, and (F) GN = Gotnath. The stratigraphic units are marked in roman numbers and OSL sample locations are shown by red boxes. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Fig. 3. (A) Banas River. (B) Geomorphology around IQB = Iqbalgarh and DW = Dantiwada. Stratigraphic sequences at (C) IQB and (D) DW. (E) Geomorphology around MA = Moti Akhol, JD = Juna Deesa, and OG = Goliya. Stratigraphic sequences at (F) MA, (G) JD, (H) Juna-Deesa floodplain (depth is shown in cm × 10) and (I) OG. (J) and (K) Stratigraphy and geomorphology around GN = Gotnath respectively. Stratigraphic sequence at GN. (OSL sampling positions are shown with red boxes in the stratigraphic sections; OSL ages: IQB: 5.0 ± 0.3 ka, DW-1: 26 ± 1.6 ka, DW-2: 5.5 ± 0.3, MA: 25 ± 2 ka, JD-1: 37 ± 2 ka, JD-2: 22 ± 2 ka, JD-3A: 18 ± 2 ka, JD-5: 12 ± 1 ka, JD-6: 10.5 ± 1 ka, JD-7: 10.5 ± 1 ka, JD-8: 3.5 ± 0.1 ka, JD-fl-plain: 1.0 ± 0.1 ka, OG-1: 37 ± 2 ka, OG-2: 31 ± 3 ka, GN-2: 1 ± 0.1 ka). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)


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Fig. 4. Field photograph of the alluvial sequence at (A) JD = Juna-Deesa along with detailed stratigraphy. OSL sample locations are indicated by red boxes. (B) Floodplain deposit at JD. (C) Point bar deposit at JD. (D) Schematic cross section between the left bank alluvial cliff and point bar deposit at JD (Banas River). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

the geological luminescence is reduced to a near zero residual value owing to daylight exposure during weathering and transport. Once buried (concealed from sun light), the sediment starts accumulating luminescence. The ages thus obtained gives the burial ages of the sediment (Aitken, 1998; Juyal et al., 2000; Singhvi et al., 2001). Samples were collected in opaque pipes from the freshly exposed alluvial sequences. In order to assess the internal consistency of the ages and to discern the continuity/breaks in sedimentation, multiple samples in a single profile at different depths were collected. Pure quartz was extracted through sequential pretreatment using 1 N HCl to remove carbonates and 30% H2O2 to remove organic matter. The 90–150 μm grain size samples were then separated using a Frantz magnetic separator and etched with 40% HF for 80 min followed by treatment with 12 N HCL for 30 min with constant magnetic stirring to remove the outer alpha irradiated skin (~ 20 μm). Luminescence measurements were made using an automated Risø TL-OSL reader (TL/OSL-DA-20). The samples were stimulated using blue diode (470 ± 20 nm) and detected with EMI 9835QA photomultiplier tube coupled with a 7.5 mm Hoya U-340 filter (emission 330 ± 35 nm). Beta irradiations were carried out using a 90Sr/90Y beta source with a dose rate of 7 Gy/min. The purity of the quartz grains in terms of feldspar contamination was checked by infrared stimulated luminescence (IRSL). Equivalent Dose (De) was determined by the sensitivity corrected single aliquot regenerative dose (SAR) protocol of Murray and Wintle (2000). Typically, around 70 aliquots were analyzed out of which 20–40 aliquots satisfied the criterion of a recycling ratio of 0.90–1.1. We used a preheat temperature of 240 °C for 10 s and a cutheat temperature of 200 °C for 0 s. The dose recovery test was carried

out for all the samples and the recovered dose ratio was within 10% of unity. Considering that fluvial sediments usually suffer from inhomogeneous bleaching, more than one equivalent dose (De) population exists. Therefore, in order to obtain the best estimate on the De values for age computation, statistical models suggested by Galbraith and Laslett (1993) and Galbraith et al. (1999) were applied. In the present study we employed Minimum Age Model (MAM) for those samples where the over dispersion (OD%) was N 40%. Otherwise, the Centralized Age Model (CAM) of Galbraith et al. (1999) was employed. The uranium, thorium, and potassium concentrations were measured using high purity Germanium detector (HPGe). The samples were sealed in plastic boxes and kept for ~15 d to attain radioactive equilibrium. The concentrations were estimated based on the characteristic gamma rays using a standard basalt reference source (107). The errors of measurement (systematic and statistical uncertainties) are b 5% (Shukla et al., 2002). An average water content of 10 ± 2% was used and cosmic ray contributions in dose rate were calculated using the method suggested by Prescott and Hutton (1994). 4. Results 4.1. Banas basin Details of the field stratigraphy, facies assemblages, and associated depositional environment were documented at six sites in the piedmont zone (Iqbalgarh, Dantiwada), the lower alluvial plain (Moti-Akhol, Juna-Deesa, Goliya), and the river mouth (Gotnath) of the Banas River valley. The facies assemblages were characterized using the lithofacies


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Fig. 5. Stratigraphic sequences investigated along the Saraswati River. (A) Inset shows DEM of the Saraswati River basin traversing across major structures. (B) stratigraphy of upper units (Unit IV to Unit IX). (C) stratigraphy of lower units (Unit I to Unit V). (OSL sampling positions are shown with red boxes in the stratigraphic sections; OSL ages: SDS-1A: 32 ± 1.5 ka, SDS 3A: 27 ± 2 ka, SDS-3B: 20 ± 1, SDS-4B: 3 ± 0.1 ka, SDS-5: 2 ± 0.1 ka, SDS-7: 230 ± 12 a). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

classification suggested by Miall (1977, 1985, 1988). The facies and sediment characteristics of these locations are given in Supplementary Table 1. 4.1.1. Iqbalgarh Iqbalgarh (24°21′50.6″N, 72°32′23.5″E) is located in the piedmont zone of the Banas River. At this location, a ~ 7 m thick, fluvio-aeolian alluvial sequence is preserved (Figs. 2B and 3C). The sequence commences with the deposition of a 1.5 m thick layer of SW dipping, cross-stratified gravels embedded in a gritty matrix (Gm; gravel bars and bedforms; Unit I). This horizon consists of angular to subrounded quartzite lithoclasts along with subrounded calcretes. This is overlain by a 1.1 m thick, pedogenised clayey silt containing diffused carbonate (Fm, P; overbank fines; Unit II). Above this is a 1.2 m thick, matrix supported unit, with crudely laminated, angular to subrounded lithoclasts dominated by quartzites and basic rocks (Gm; gravel bars

and bedforms; Unit III). Finally, the uppermost Unit IV consists of a 3 m thick unit of buff colored, massive aeolian micaceous sands (Ae1; dunes). Presence of cross-stratified gravels at the bottom indicates a high energy episodic fluvial discharge suggesting deposition under braided river conditions (Hooke, 1967; Miall, 1985). Geomorphologically, the site is located in the piedmont zone; such areas are usually subjected to higher stream power caused by the over-steepening of the river gradient (Holbrook and Schumm, 1999). The cross-stratified beds dipping towards the SW conforms to the regional gradient of the Gujarat alluvial plain. Overlying this unit is the pedogenised clayey silt unit, which indicates periods of non-deposition and subaerial exposure of the sediment that led to the pedogenesis. The overlying crudely laminated sandy matrix indicates improved hydrological regime. Finally, presence of aeolian sand capping the succession points towards dwindling hydrological conditions.

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Fig. 6. Stratigraphy and chronology of the alluvial sequences in the Banas River. (A) and (B) pediment zone upstream, (C) to (F) middle alluvial plain (for Juna Deesa flood plain sequence, depth is shown in cm × 10), and (G) lower alluvail plain. (H) stratigraphy and chronology of the alluvial sequences at Siddhpur (Saraswati River). (I) Composite lithostratigraphy and chronology of the alluvial sequences in Banas and Saraswati rivers.


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Table 1 Showing radioactivity, equivalent dose and ages for the samples from the Banas river. Ages shown in bold are used for interpretation and discussion. Sample no.

Depth (m)

U (ppm)

Th (ppm)

K%

ED (CAM) Gy

ED (MAM) Gy

Dose rate

OD%

Age (CAM) ka

Age (MAM) ka

JD-1 JD-2 JD-3A JD-5 JD-6 JD-7 JD-8 JD-ch-bar JD-fl-plain DW-1 DW-2 GN-2 OG-1 OG-2 IQB MA

10 8.5 7.5 4.5 3.0 2.5 1.8 1.5 1.0 0.3 3.2 2.5 7.5 9 3.0 3.8

7.74 ± 0.38 5.04 ± 0.25 6.41 ± 0.32 8.52 ± 0.42 11.8 ± 0.59 6.05 ± 0.30 13.6 ± 0.68 10.9 ± 0.54 8.41 ± 0.42 9.81 ± 0.49 12.1 ± 0.60 2.09 ± 0.10 2.06 ± 0.1 1 ± 0.05 2.42 ± 0.12 0.94 ± 0.05

1.47 ± 0.07 0.68 ± 0.03 0.75 ± 0.37 1.26 ± 0.63 1.19 ± 0.06 0.68 ± 0.03 0.94 ± 0.05 1.22 ± 0.06 3.02 ± 0.15 0.995 ± 0.05 0.82 ± 0.04 9.25 ± 0.46 13.1 ± 0.65 5.24 ± 0.3 16.74 ± 0.83 6.59 ± 0.32

1.34 ± 0.03 1.09 ± 0.02 1.27 ± 0.02 1.55 ± 0.03 2.11 ± 0.05 1.03 ± 0.02 2.22 ± 0.05 1.68 ± 0.03 1.04 ± 0.02 1.75 ± 0.03 2.14 ± 0.05 1.1 ± 0.02 1.45 ± 0.03 0.8 ± 0.02 2.61 ± 0.05 0.62 ± 0.01

111 ± 4 92 ± 4 93 ± 6 80 ± 4 84 ± 2 27 ± 2 17 ± 0.3 3 ± 0.4 7 ± 0.3 98 ± 4 25 ± 1 2.2 ± 0.1 98 ± 2 82 ± 4 37 ± 1 62 ± 2

109 ± 6 48 ± 4 47 ± 5 41 ± 3 47 ± 4 13 ± 1 17 ± 0.3 3±1 3 ± 0.4 56 ± 4 25 ± 1 2 ± 0.1 97 ± 2 43 ± 4 20 ± 1 33 ± 2

3 ± 0.1 2 ± 0.1 2.6 ± 0.1 3.3 ± 0.1 4.5 ± 0.2 2.3 ± 0.1 5 ± 0.2 4 ± 0.1 2.9 ± 0.1 3.7 ± 0.2 4.5 ± 0.2 2 ± 0.1 2.7 ± 0.1 1 ± 0.06 4 ± 0.1 1.3 ± 0.06

35 47 47 40 41 52 10 50 55 25 33 56 25 45 40 40

37 ± 2 42 ± 3 36 ± 3 24 ± 1.6 19 ± 1 12 ± 1 3.5 ± 0.1 0.8 ± 0.1 2.3 ± 0.1 26 ± 1.6 5.5 ± 0.3 1 ± 0.1 37 ± 2 59 ± 4 9 ± 0.4 48 ± 3

36 ± 3 22 ± 2 18 ± 2 12 ± 1 10.5 ± 1 5.6 ± 0.5 3.5 ± 0.1 0.8 ± 0.2 1.0 ± 0.1 15 ± 1 5.5 ± 0.3 1 ± 0.1 36 ± 2 31 ± 3 5.0 ± 0.3 25 ± 2

4.1.2. Dantiwada This section (24°20′47.5″N, 72°19′33.2″E) is located at ~ 25 km downstream from Iqbalgarh (Figs. 2C and 3D) and consists of ~ 3.0 m thick alluvial successions. The lowermost Unit I consists of a 0.15 m thick unit of crudely laminated, pale yellow, mottled, medium to fine sand containing friable calcretes (Fsc; overbank fines). Overlying this is Unit II, which consists of a 0.2 m thick layer of medium to coarse, crudely laminated sand with dispersed calcretes (Se; minor bars; sandy bedforms) and is followed by Unit III, which is 0.1–0.2 m thick, containing angular assorted gravels dominated by granite lithoclasts (Gm; gravel bars and bedforms). This is overlain by Unit IV, which is 0.3 m thick, consisting of light grey, massive sand with dispersed nodular calcretes (Sh; sandy bedforms). Above this is Unit V (0.2 m thick), pale yellow medium to fine crudely laminated sand in which the upper part contains bedded calcretes (Fl; overbank fines). Following this is Unit VI, which is a 0.4 m thick layer containing pale yellow crudely laminated mottled sands (Fsc; overbank fines). The upper 0.1 m contains bedded calcretes. At the top is Unit VII (1.7 m thick), which is pale yellow to ash grey in color, coarse to medium sands containing nodular calcretes and discrete lithoclasts (Ae1; dunes). The lowermost pale yellow, mottled sandy unit implies deposition in a low energy fluvial environment. Mottling of the sediment implies prolong saturation caused by a raised water table and resulting acidification of groundwater and changes in Eh, which cause the reduction, mobilization, and re-precipitation of iron and manganese sesquioxides (Leenheer, 2002). The overlying crudely laminated sandy unit and assorted gravel horizon suggests gradual strengthening of the hydrological regime. The thinner gravel horizon suggests short-lived ephemeral floods leading to sediment mobilization (Juyal et al., 2006). The overlying light grey sand unit further indicates prevalence of a high energy hydrological regime. The crudely laminated sands and bedded calcretes above this suggest a channel proximal floodplain environment of a meandering river system (Goudie, 1983). The bedded calcretes in the semiarid regions are associated with the fluctuating groundwater table in the distal floodplain environment (Goudie, 1983; McCarthy and Metcalfe, 1990). The high evaporation rate draws up the carbonate

rich groundwater through capillary processes (Goudie, 1983; Slate et al., 1996; Nash and Smith, 1998; Slate, 1998). It has been observed that climate characterized by wet, mild autumn/winters and warm, dry summers are ideal settings for calcrete formation as carbonates are mobilized during the wetter months and re-precipitated during the dry summer months (Yaalon, 1997) with optimum degree of leaching (Candy and Black, 2009). The topmost aeolian sand in the stratigraphic section suggests weakening of hydrological conditions. 4.1.3. Moti Akhol The Moti Akhol site (24°16′36.9″N, 72°10′08.7″E) is located ~8 km downstream from Dantiwada (Figs. 2D and 3F). The lowermost horizon is a ~ 3 m thick, fine sand containing a high concentration of bedded calcretes (Unit I) (F1; overbank fines). This is overlain by a 4 m thick, reddish brown, weathered, mottled silty sand (Unit II) containing both rhizolithic and nodular calcretes (Fsc, P; overbank fines). The overlying 3 m thick, brownish red, pedogenised horizon containing high concentration of nodular calcretes (St, P; sandy bedforms; Unit III) is succeeded by a 1 m thick aeolian sand (Unit IV) (Ae1; dunes). Conspicuous presence of bedded calcretes in the lower units at Moti Akhol suggests channel proximal floodplain deposition in a meandering channel (Goudie, 1983). The overlying weathered silty sand units with rhizoliths and nodular calcretes suggests weathering of the floodplain fines in a lateral migrating alluvial channel (Kraus and Aslan, 1993). The succeeding laterally persistent pedogenised and calcretised fluvial sediment (occurring as distinct bench) suggests a prolonged phase of non-deposition probably caused by lateral migration of the channel that may occur owing to an increase in sediment supply from the upper catchment (Kraus and Aslan, 1993). The uppermost aeolian sand represents the onset of aridity (dwindling hydrological system). 4.1.4. Juna-Deesa The sedimentary sequence at Juna-Deesa (24°13′06.1″N, 72°09′ 03.6″E) is located ~ 5 km downstream from Moti-Akhol (Figs. 3G and 4). At this location, one of the most detailed alluvial successions in the Banas River valley is exposed along the left bank of the Banas

Table 2 Showing radioactivity, equivalent dose and ages for the samples from the Saraswati river. Ages shown in bold are used for interpretation and discussion. Sample no.

Depth (m)

U (ppm)

Th (ppm)

K%

ED (CAM) Gy

ED (MAM) Gy

Dose rate

OD%

Age (CAM) ka

Age (MAM) ka

SDS-1A SDS-OSL-3A SDS-OSL-3B SDS-OSL-4B SDS-OSL-5 SDS-OSL-7

9.0 7.5 4.0 6.0 4.3 1.5

1.57 ± 0.03 1.66 ± 0.04 1.95 ± 0.04 1.6 ± 0.03 1.13 ± 0.03 1.38 ± 0.03

14.52 ± 0.21 10.67 ± 0.20 12.8 ± 0.20 11.89 ± 0.19 7.72 ± 0.15 9.43 ± 0.20

2.1 ± 0.02 1.65 ± 0.02 2.05 ± 0.02 1.90 ± 0.02 1.30 ± 0.01 1.26 ± 0.03

103 ± 3 136 ± 6 119 ± 5 9 ± 0.3 4 ± 0.1 0.49 ± 0.02

57 ± 4 71 ± 6 62 ± 3 9 ± 0.3 4 ± 0.1 0.49 ± 0.02

3.2 ± 0.1 2.6 ± 0.1 3 ± 0.1 2.9 ± 0.1 2 ± 0.1 2 ± 0.1

29 40 40 28 10 20

32 ± 1.5 52 ± 3 38 ± 2 3 ± 0.1 2 ± 0.1 0.2 ± 0.01

18 ± 1 27 ± 2 20 ± 1 3 ± 0.1 2 ± 0.1 0.2 ± 0.01

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Fig. 7. Reconstruction of the palaeoenvironmental changes based on weathering indices at Juna-Deesa (Banas River) CIA = Chemical Index of Alteration, PIA = Plagioclase Index of Alteration, CIW = Chemical Index of Weathering.

River. The lowermost exposed Unit I consists of 0.7 m thick, weathered medium sand (27%) to fine sand (35%) containing dispersed calcretes (F1; overbank fines). This is overlain by Unit II, which is 0.8 m thick with faint parallel and current laminations. Texturally, the horizon contains very coarse sand (61%) with dispersed nodular calcretes, particularly in the upper part, whereas in the lower part the calcretes are bedded in nature (Fig. 3G) (Sh; sandy bedforms). This is overlain by 0.7 m thick, crudely laminated, loose medium sand (49%) containing friable calcretes (Unit III) (Sh; overbank fines). This horizon is succeeded by Unit IV, which is 3 m thick, containing reddish brown pedogenised, medium sand (46%) to fine massive sand (35%) with weakly developed pedfaces and faint indication of clay illuviation (F1, P; overbank fines). At places dispersed nodular calcretes and rhizoliths can be observed. The concentration of nodular calcretes increases in the upper 1.5 m of

Unit IV. Notably, the reddish brown color of Unit IV makes it distinct from all other units. This is overlain by Unit V, which is a 1.4 m thick, medium friable sand (58%) interspersed with nine fluvially reworked, angular to subrounded, rolled calcrete gravels which occur at a rhythmic interval of 0.2–0.1 m (Sh, lateral accretion). Overall, the unit is dominated by well sorted, medium to fine sand. This is followed by the uppermost 3.0 m thick Unit VI, containing fine, light grey aeolian dune sands (47%) with dispersed nodular calcretes (Ae1; dunes) (Fig. 3G). The lowermost unit of medium to fine sand overlain by laminated coarse sand with bedded and nodular calcrete suggests deposition under channel proximal environment of a meandering channel (Juyal et al., 2006 and the references therein). Presence of rhizoliths in the pedogenized horizons signifies that the rooted vegetation created channel bank stability and promoted systematic channel migration

Fig. 8. Reconstruction of the palaeoenvironmental changes based on weathering indices at Siddhpur (Saraswati River). CIA = Chemical Index of Alteration, PIA = Plagioclase Index of Alteration, CIW = Chemical Index of Weathering.


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Fig. 9. The compositional space diagram (A-CN-K) of Juna-Deesa (Banas River) and Siddhpur (Saraswati River) indicating a typical chemical weathering trend.

(Gibling and Davies, 2012) under sustained and enhanced hydraulic discharge (McCarthy and Metcalfe, 1990; Willis and Behrensmeyer, 1994). The overlying pedogenised unit (red soil) indicates periods of oxidation and soil drying, probably associated with a falling water table or good drainage. In general, red palaeosols develop on betterdrained sites reflecting higher elevation and/or more permeable parent material (Kraus and Aslan, 1993). The overlying sandy unit with rolled calcretes suggest erosion of pre-existing alluvial carbonate from adjoining banks by migrating channels (Juyal et al., 2000 and references therein). The overlying aeolian sand sheet points to the onset of aridity and weakening of the fluvial regime.

4.1.5. Goliya Goliya (24°12′25.4″N, 72°06′31.6″E) is located ~7 km downstream from the Juna-Deesa (Figs. 2E and 3I) and is exposed along the right flank of the Banas River. The sequence commences with the deposition of a 3 m thick, crudely laminated, indurated, pedogenised sandy horizon with well-developed pedfaces (Unit I) (Sp, P; sandy bedforms). This is overlain by a 5 m thick, calcretised, reddish brown, sandy silt with occasional mud balls, rhizoliths and bedded calcretes (Unit II) (F1, P; overbank fines). The horizon shows weakly developed pedfaces with evidence of pedofaunal activity. Overlying this is a 3 m thick, massive, compact and pedogenised brownish aeolian sand contain a high concentration of nodular calcretes (Unit III) (Ae1, P; dunes). The facies architectures at this location is similar to the one observed at Juna-Dessa with the exception that apart from the presence of a laterally persistent, fluvially modified, pedogenised bench and the red soil, the horizon contains mud balls and bedded calcretes. These are indicative of a low energy meandering river system and the presence of mud balls suggests semi-permanent pools on the floodplains (Kar et al., 2001; Mariott and Wright, 2004).

4.1.6. Gotnath Gotnath (23°45′52.5″N, 71°37′29.0″E) is located ~20 km upstream from the eastern fringe of the Little Rann (Figs. 2F and 3J). At this location, the Banas River virtually merges with the vast saline plain of the Little Rann of Kachchh. The channel appears like a braided sandy sheet, with wide, shallow channels and low relief bank margins. However, along the left bank, a ~ 3.2 m thick, alluvial succession is exposed in which the lower 1.2 m thick Unit I is a light grey, silty clay with a high concentration of dispersed nodular calcretes (Fm, overbank fines). This unit is overlain by a 1 m thick, massive, compact, dark brown to light colored silty clay (Unit II) (Fsc, lateral accretion). Above this is a 0.3 m thick, gritty to coarse convoluted channel sand (Unit III) (Sp, P; channel sand). Finally, the succession terminates with the deposition of a 0.7 m thick, dark brown, pedogenised, silty sand containing brick fragments (Unit IV) (SI, P; sandy bedforms). The location is proximal to the Little Rann of Kachchh, hence dominance of fine sand along with the silty clay indicates a significant decrease in the stream power caused by a decrease in the river gradient. However, presence of gritty to coarse sand within silty clay horizon can be an indication of a short-lived flashy flood condition (Bhattacharya et al., 2014). The upwarping seems to be a syn-sedimentary deformation caused by the loading (i.e., the sediment density gradient). The dominance of fine sand and silty clay suggests deposition under waning flood conditions in a meandering channel. Since the location is proximal to the Little Rann of Kachchh, the incision of the sediment succession may have taken place during low sea level.

4.1.7. Floodplain sequence (near Juna-Deesa) At the distal left bank of the Banas River near Juna-Deesa, a ~100 cm thick unit consisting of seven alternating sand and silty clay layers is exposed (Figs. 3H and 4B). From bottom upwards, a 30 cm thick, massive

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sand horizon is overlain by a 2 cm thick, laminated silty clay layer (Unit I) (Fm). This is succeeded by a 4 cm thick, laminated fine sand capped by a 2 cm thick, convoluted silty clay (Unit II). Overlying this is a 3 cm thick, laminated fine sand overlain by a 1 cm thick, silty clay (Unit III) which in turn is overlain by a 2 cm thick, laminated fine sand and capped by 2 cm thick, laminated sandy clay (Unit IV). This is overlain by a 2 cm thick, laminated fine sand, which is capped by 1 cm thick, silty clay (Unit V). Above this is 2 cm thick, laminated fine sand overlain by a 2 cm thick, sandy clay (Unit VI). Following this is a 2 cm thick, swelling and pinching fine sand that is followed by a 2 cm thick, sand clay (Unit VII). Finally, the succession is terminated with the deposition of a 15 cm thick, trough and cross-stratified coarse to gritty sand. The wide sandy channel of the dryland fluvial system is associated with a narrow floodplain zone (Malmon and Reneau, 2004) that provides the accommodation space for the preservation of sediments corresponding to extreme hydrological events (Sridhar et al., 2013). The seven vertically staked fining upward sediment succession, each one invariably capped by discontinuous clay drapes of cm to mm size, indicates deposition on a low relief floodplain (Baker, 1987; Nehyba and Roetzel, 2015). Thus, the successive seven fining upward layers represent a temporal increase in flood magnitude stages caused by higher discharges (Baker, 1987; Benito and Thorndycraft, 2005). The overlying cross-stratified coarse gritty sand capping the flood sequence indicates mobilization of the source proximal alluvium during the weakening of the hydrological discharge in the trunk river. A composite lithostratigraphy based on the seven alluvial sequences along the Banas River valley discussed above is presented in Fig. 6I. 4.2. Saraswati basin In the Saraswati basin, a ~ 11 m thick alluvial succession is documented near Siddhpur (72.2°E, 23.8°N), which is exposed because of constructional activities on the otherwise flat and monotonous river bed (Fig. 5). The composite stratigraphy was reconstructed by combining a lower sediment succession at location (A) and an upper succession at location (B) (Fig. 5A). At location (B), the sediment succession was deposited over the aeolian sand, which has been incised by channel activity (Fig. 5B) (Supplementary Table 1). At location (A), the lowermost 1.6 m thick, massive, medium sand (46%) is underlain and overlain by ~ 5 cm thick bedded calcretes (Unit I) (Sh; sandy bedforms) (Fig. 5C). Overlying this is a 0.2 m thick, crudely laminated, friable, medium sand (68%) (Unit II) that is laterally persistent (Sh; sandy bedforms). The upper 0.1 m of this unit contains bedded calcretes. This is overlain by a 3.0 m thick, weathered, pale yellow, massive, very fine sand (46%) containing discrete calcrete nodules (Unit III) (SI; lateral accretion). This is followed by a 1.5 m thick, pebbly sand containing rounded to subrounded calcretes (Unit IV) (Sp; lateral accretion). Overlying this is the aeolian sand (Unit V) that shows a relict channel morphology (Ae1; dunes) (Fig. 5B). The lowermost gritty to coarse massive sand suggests flashy hydrological discharge – a characteristic of the unstable climatic conditions in a semiarid environment (Graf, 1988). The overlying laminated sand with bedded calcretes indicates laminar flow under consistent fluvial discharge with seasonality. The bedded calcretes indicate an episodic evaporative condition in a channel proximal floodplain environment. This was followed by the onset of aridity as indicated by the deposition of a 4 m thick, medium to fine pale yellow aeolian sand sheet, which shows evidence of moderate pedogenesis. At location (B), the sediment succession was deposited over the incised (channelized) aeolian sand of the section A (Fig. 5B) and is dominated by the gritty coarse sand containing a significant amount of rolled calcretes (Unit VI) (St; channel sand) (Fig. 5B). Overlying this is a 1 m thick, pedogenised medium to very fine sand (75%) (Unit VII) (Ae1, P; dunes). The upper 0.4 m is rich in humus similar to the Ah horizon of a paleosol. This is overlain by a 3 m thick, trough cross-stratified gravelly sand (Unit VIII) (Se, sandy bedforms) in which buried stone sculptures

are found. This is succeeded by a massive buff colored, fine aeolian sand (56%) (Ae1; dunes) (Unit IX). The incised (channelized) aeolian sand is plugged with a ~5 m thick crudely bedded, fining upward gritty sand suggesting deposition under flashy hydrological conditions. Gravelly lithoclasts dominate the lower part of this horizon, followed by crudely laminated gritty to coarse sand implying a gradual decline in the flash flood intensity (Juyal et al., 2000). This overlying aeolian sand indicates temporary onset of aridity, whereas the presence of a humus rich layer above the aeolian sand indicates landscape stability (in terms of aeolian sedimentation). Occurrence of crudely laminated planer to cross-stratified sand towards the upper part of the succession indicates a renewed phase of fluvial activity. 4.3. Chronology 4.3.1. Banas basin The chronology is obtained from six alluvial successions exposed between the piedmont and the lower alluvial plain in the Banas River (Fig. 3 and Table 1). In the piedmont zone, at Iqbalgarh, the middle assorted gravel (Unit II) is dated to 5 ± 0.3 ka (Fig. 3C). At Dantiwada, the lowermost crudely laminated mottled sand (Unit I) is dated to 26 ± 2 ka, whereas the uppermost assorted gravel (Unit VII) is dated to 6 ± 0.3 ka (Fig. 3D). In the middle alluvial plain, at Moti Akhol, the calcretized sandy Unit II is dated to 25 ± 2 ka (Fig. 3F). Farther downstream at Juna-Deesa, which has preserved the most detailed sediment succession, a total of eight samples were dated from different lithostratigraphic units (Fig. 3G). The lowermost coarse to medium friable sand is dated to 37 ± 2 ka (Unit I). The coarse sand containing friable calcretes is dated to 22 ± 2 ka (Unit III). The pedogenised massive sand (reddish brown) is dated to 18 ± 2 ka (IV). The medium to fine friable sand horizon interspersed with nine fluvially reworked rolled calcrete gravels is dated to 12 ± 1 ka to 11 ± 1 ka (Unit V). The upper aeolian sand (Unit VI) is dated to 6 ± 0.5 ka to 3.5 ± 0.1 ka (Fig. 3G). Floodplain units adjoining the valley flank at Juna-Deesa are dated to 1 ± 0.1 ka (Fig. 3H). In the downstream segment, near Goliya village, the crudely laminated and indurated, pedogenised sandy horizon is dated to 37 ± 2 ka (Unit I), whereas the overlying highly calcretised reddish brown pedogenised sandy silt is dated to 31 ± 3 ka (Unit II) (Fig. 3I). In the lower alluvial plain at Gotnath, the upper dark brown pedogenised silty sand is dated to 1 ± 0.1 ka (Unit III) (Fig. 3J). Table 1 shows the detail of the radioactivity, equivalent dose, dose rates and ages obtained. Radial plots are shown in Supplementary Figs. 1–3 and typical shine down (decay) and growth curves are shown in Supplementary Fig. 4. The chronology of the composite lithostratigraphy suggests that the lowermost coarse to medium sand with bedded and nodular calcretes (Unit I) was deposited ~37 ka (Fig. 6I and Table 1). The fine sand dominated by moderately pedogenised, reddish brown horizons containing a high concentration of nodular calcretes can be bracketed between 31 ka and 25 ka (Unit II). The overlying unweathered, crudely laminated, medium sand containing friable calcretes is dated to 22 ka (Unit III). The succeeding pedogenised horizon is dated to 18 ka (Unit IV), whereas the overlying medium friable sand containing fluvially reworked rolled calcrete gravels is dated between 12 ka and 11 ka (Unit V). The uppermost aeolian sand along with the intervening cross-stratified gravels (Unit VI) was deposited between 6 ka and 3.5 ka (Fig. 6A). Additionally, the paleoflood sediment and the riverbed alluvium (channel sand) is dated to ~1 ka (Fig. 6A). 4.3.2. Saraswati valley In the Saraswati River valley near Siddhpur, six optical ages are obtained from different lithostratigraphic units (Fig. 6I and Table 2). The lowermost gritty to coarse, massive sand with bedded calcretes is dated to 32 ± 2 ka (Unit I). The age of the pale yellow, medium to fine sand (Unit III) is dated to 27 ± 2 ka, whereas the upper part of


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Fig. 10. (A) NW-SE subsurface cross section of the Quaternary alluvium in the Cambay basin (between Luni and Sabarmati rivers). The alluvial succession is differentiated into the basal clay, gravel, pedogenic mud, red soil, coarse sand, aeolian silt, and dune sand. Subsurface faults demarcating horst and graben structures are shown (modified after Thiagarajan et al., 2001). (B) and (C) The alluvial sequences exposed from incision correspond to the upper few meters that are investigated in the present study in Banas (B) and Saraswati rivers (C). (D) Section across the Cambay basin (between Tharad and Gandhinagar, subsurface topography is represented by the horst structures (Diodar and Mehsana) at 5 km depth bordering the Patan basin. The heat flow profile (Thiagarajan et al., 2001) and Vp velocity profiles (Kaila et al., 1990) indicate that the surface heat dips above the sub-surface horst/ridges at Diyodar and Mehsana and is accentuated above the Patan basin. Likewise, the presence of horst and graben structures are corroborated by the Vp velocity profile. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

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this unit is dated to 20 ± 1 ka (Fig. 5A). The gritty cross-stratified channel sand (Unit V) is dated to 3 ± 0.1 ka (Fig. 5B). The humus rich pedogenised aeolian sand (Unit VII) gave an age of 2 ± 0.1 ka, which is similar to the radiocarbon age obtained on humus (2 ± 0.2 cal. yr B·P). The uppermost buff colored aeolian sand is dated to 0.2 ± 0.01 ka (Unit IX) (Table 2).

indicate a marginal increase in the weathering intensity between 3 ka and 0.2 ka with a corresponding decrease in aeolian proxies (Fig. 9).

4.4. Geochemistry

Field stratigraphy, sedimentology, geochemistry, and optical chronology suggest that the alluvial succession in the Banas and Saraswati rivers has preserved the record of the Indian Summer Monsoon (ISM) variability spanning 37 ka to 200 yr. Climatically, the period spans between the late Marine Isotopic Stage 3 (MIS 3) and late MIS 1 (Fig. 11). The composite lithostratigraphy of the Banas River (Fig. 6I) suggests that during the late MIS 3 (37 ka) until the early MIS 2 (25 ka) (Juyal et al., 2006 and reference therein), the Banas River experienced a relatively enhanced and rather consistent hydrological discharge that can be attributed to the strengthened ISM with fluctuations. In the Saraswati River, similar inferences can be drawn for the lower part of the alluvial succession (dated to 32 ka), which is dominated by weathered fluvial sand containing bedded calcretes (Figs. 5 and 6H). Comparing this evidence with that of the southern margin of the Thar Desert (i.e., the Sabarmati and Mahi River valleys) (Fig. 1A) shows regional floodplain aggradation and pedogenesis during the strengthened ISM corresponding to MIS 3 (Juyal et al., 2006; Thokchom et al., 2017). Similar evidence accrues from the core region of the Thar Desert located to the northwest of the study area (Andrews et al., 1998; Jain and Tandon, 2003), in western India (Kale and Rajaguru, 1987), and in central India (Singh et al., 1999; Srivastava et al., 2001). Therefore, considering the regional nature of the floodplain aggradation, Juyal et al. (2006) suggested that MIS 3 was probably similar to or wetter than present. Reconstructions based on the compilation of 75 continental palaeoclimatic records for the palaeomoisture evolution in central Asia suggest that during the middle and late MIS 3, the moisture regime fluctuated between moderately dry and wet conditions. However, the most favourable moisture conditions were suggested between 43 and 37.6 ka with the weakening in moisture conditions after 25.5 ka and the moisture minimum observed between 21.3 and 19.8 ka BP (Herzschuh, 2006). The inference regarding the enhanced hydrological conditions followed by gradual decline are broadly in agreement with the increase in weathering indices and the lowered ratios in the aeolian proxies (Figs. 7 and 8). It has been observed that the position of the intertropical convergence zone (ITCZ) dictates the intensity of the ISM (Sirocko et al., 1993; Prasad and Enzel, 2006), which in turn is modulated by cooling and warming events in the North Atlantic (Broccoli et al., 2006). According to Juyal et al. (2006), the ITCZ shifted northwards, beyond the southern margin of the Thar Desert during MIS 3. As a consequence, the southern margin of the Thar Desert witnessed regional floodplain aggradation and pedogenesis (Andrews et al., 1998; Jain and Tandon, 2003; Juyal et al., 2006). However, deposition of the overlying moderately weathered sand dated between 27 ka to 20 ka indicates gradual weakening in hydrological conditions (Fig. 11). The sedimentological observations (discussed earlier) supported by the weathering indices and aeolian proxies point towards weakening in moisture conditions with a corresponding increase in aridity ~ 20 ka, a period corresponding to the Last Glacial Maximum (LGM) (Figs. 7 and 8). Presence of pedogenised/weathered (red soil) in Banas River valley and the laterally correlatable (undated) truncated red soil (Unit IV; Fig. 4) in the Saraswati River valley suggests gradual strengthening of the ISM after LGM. A broad framework of monsoon variability reconstructed from the Himalaya, Gangetic Plain, and Thar Desert (western India) shows major climatic excursions after the LGM. For example, basin-wide aggradation of valley fill in the central Himalaya (Juyal et al., 2009) and the western Himalaya (Sharma et al., 2016) is ascribed to the post-LGM strengthening of the ISM.

Major elements and its derivatives have been used to infer relative changes in post-depositional weathering and the associated paleoenvironmental conditions. Towards this end, we analyzed 43 samples from Juna-Deesa (Banas River; Fig. 7) and 21 samples from the Siddhpur (Saraswati River; Fig. 8). The weathering proxies in Juna-Deesa show wide fluctuations across the stratigraphic succession. For example, the SiO2/Al2O3 (Ruxton ratio) varies from 6.0–8.8, Rb/Sr (0.61–0.91), CIA (72.5–85.6), PIA (64.1–84.1), and CIW (81.7–94.4). The geochemical ratios representing the relative changes in aridity, such as the Ti/Al2O3 ratio, varies from 0.035–0.052, and the Zr/Al2O3 ratio varies from 15.9–42.6 (Supplementary Table 3). Similarly, in the Saraswati River valley, the alluvial succession at Siddhpur shows a comparative change in the geochemical ratios. For example, the SiO2/Al2O3 (Ruxton ratio) is 5.0–9.4, Rb/Sr is 0.63–1.44, CIA is 73.2–82.5, PIA is 64.1–80.7, and CIW is 82.5–94.0, whereas the geochemical ratios representing the relative changes in aridity such Ti/Al2O3 varies from 0.037–0.056, and Zr/Al2O3 varies from 18.2–54.5 (Supplementary Table 4). The compositional space diagram of A-CN-K (Nesbitt and Young, 1982) for Juna-Deesa (Banas River) and Siddhpur (Saraswati River) shows a typical chemical weathering trend (Fig. 9). This we ascribe to post-depositional changes for the following reasons. (i) Modern alluvial successions in the river valley do not show an appreciable weathering profile under present climatic conditions. (ii) This could be caused by rapid sediment evacuation and transport in ephemeral rivers compared to their perennial counterparts (Graf, 1987; Laronne and Reid, 1993; Reid et al., 1998). (iii) The sediments thus evacuated from the catchment are partitioned into two distinct components, a coarse fraction occupying the channel bed, whereas the fine fraction settles onto the floodplain. Since the channel deposits are frequently mobilized by discrete flash floods compared to their floodplain counter parts (Malmon and Reneau, 2004), they are chemically immature. (iv) The relatively high weathering is associated with distal channel facies (floodplain) is caused by the longer residence time of the floodplain sediment (Malmon and Reneau, 2004). However, at Siddhpur (Saraswati River) few samples deviate from the weathering trend line (Fig. 9), which could be associated with the change in sediment provenance and/or contamination from unweathered sediments. The temporal variability in the weathering and aridity proxies are plotted alongside the alluvial stratigraphy of Juna-Deesa (Banas River) and Siddhpur (Saraswati River). In Juna-Deesa the pattern of variability in the geochemical proxies suggests two events of relatively high weathering (with fluctuations) around 37 ka and between 18 ka and N 12 ka, punctuated by low weathering events at ~ 22 ka and between 12 ka and 11 ka (Fig. 7). Although the aeolian proxies, i.e., Ti/Al2O 3 and Zr/Al2O 3 ratios, show a relative decrease around 37 ka, after ~ 22 ka the ratios fluctuate between low to high. This can be interpreted as the terrain experiencing intermittent aeolian activity after 22 ka (Fig. 9). At Siddhpur, in the Saraswati River valley, the pattern of variability in the weathering proxies indicates a gradual decline in the weathering intensity from 32 ka to N 20 ka (Fig. 8). Similarly, the aeolian proxies indicate an overall decreasing trend until 27 ka and beyond, with a conspicuous increase around 20 ka which could be caused by an increase in aeolian contributions. Considering that after 20 ka, deposition of Unit IV and V continued, the channel activity can be bracketed sometime after 20 ka and before 3 ka (Fig. 6H, I). The geochemical proxies

5. Discussion 5.1. Palaeoenvironment

F. Bhattacharya et al. / Geomorphology 297 (2017) 1–19 Fig. 11. Chronologically constrained composite lithostratigraphy of the Banas and Saraswati River valleys and the inferred geomorphic processes. Plotted alongside are the Indian Summer Monsoon (ISM) proxies, i.e., the oxygen isotopic data from the Guliya ice core (Thompson et al., 1997) Enriched values indicate periods of strengthened ISM and vice versa. The Total Organic Carbon (TOC) is a proxy for productivity and hence an indicator of monsoon induced upwelling from the northern Arabian Sea. The moisture evolution in Central Asia is based on 75 palaeoclimatic record (Herzschuh, 2006) and indicates wet conditions during the middle and late MIS 3, and dry conditions during the Last Glacial Maximum (LGM). The phases of fluvial aggradation are shown by blue bars, whereas aeolian sedimentation is indicated by the orange bar. Note that the fluvial aggradation (shown by blue bars) during the middle and late MIS 3 in the study area is well represented in the (1) Sabarmati and Mahi rivers and the Thar Desert (Juyal et al., 2006). Similarly, the post LGM aggradation phase (2, 3, and 4) is reported from the southern Gujarat alluvial plain, the Thar Desert, the Ganga plain, and the western and central Himalaya (Kale and Rajaguru, 1987; Singh et al., 1999; Kotlia et al., 2000; Srivastava et al., 2001; Jain and Tandon, 2003; Juyal et al., 2009; Wasson et al., 2013; Sharma et al., 2016). However, there was no fluvial aggradation in the study area during the intensified ISM in the early Holocene, which is contrary to the observation made in the 5: Kachchh region (Bhattacharya et al., 2013, 2014), 6: Luni (Kale et al., 2000), and 7 Sabarmati (Sridhar et al., 2014). Similar observations are also made in the central and western Himalaya (Juyal et al., 2009; Sharma et al., 2016). The aeolian accumulation began after around 6 ka and persisted intermittently until around 0.2 ka (represented by the orange bar). The study identifies two major phases of incision. The older phase (I) occurred during the intensified ISM (b11 ka and N6 ka), whereas the youngest phase (II) that was initiated during b3.5 ka to N1 ka is modulated by the activity of the basement faults in the Cambay basin (tectonically driven). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

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Sinha et al. (2005), using oxygen isotopic variation in a stalagmite collected from Timta cave in the western Himalaya, showed multidecadal monsoon variability after the LGM. The friable medium sand containing fluvially reworked rolled calcretes dated between 12 ka and 11 ka in the Banas River valley are interpreted as short-lived, local mobilization of the sediments during an overall weak ISM (Juyal et al., 2000). Weathering proxies show a prominent decrease during the above period and aeolian proxies show a marginal increase (Fig. 7). Therefore, together with the sedimentology and geochemistry, we suggest that there was a temporary reversal in the post-LGM strengthening of the ISM between 12 ka and 11 ka (Fig. 11). With the large age uncertainty, it can be speculated that the period corresponds to the post-LGM cooling associated with the Younger Dryas event (YD) (Bond et al., 1993; Adams et al., 1999; Alley, 2000). The continental record of the YD cooling event is scanty; however, there is evidence from lake records in the monsoon dominated Himalaya (Juyal et al., 2004), Ganga Plain (Sharma et al., 2004; Wasson et al., 2013), and fluvial records from the southern margin of the Thar Desert (Kar et al., 2001) indicating significant weakening in the ISM. At the regional scale, the YD cooling event was implicated for the reduction in African and Asian monsoon strength (Street-Perrott and Perrott, 1990; Gasse and van Campo, 1994). The continental records from across monsoon influenced regions (Zhou et al., 1996; Enzel et al., 1999; Fleitmann et al., 2003) and the bordering oceans (Sirocko et al., 1993; Wang et al., 1999) indicate that following the YD, intensification of the ISM occurred during ~ 11.5 ka. The high resolution marine record from the Arabian Sea suggested the strongest ISM during the first half of the Holocene (~ 11.5–6 ka) (Sirocko et al., 1993; Overpeck et al., 1996). However, fluvial records corresponding to the intensified ISM so far are illusive from both the Banas and Saraswati rivers. Similar observations were made in the Orsang, Mahi, and Sabarmati rivers where fluvial sediments corresponding to strengthening of the ISM in the early Holocene are missing from the sections exposed in high channel banks (Juyal et al., 2000, 2006). In semiarid regions, any long-term change in precipitation results in changes in the amount of erosion and consequently the sediment yield. Studies suggest that a change in climate from drier to wetter conditions causes increased sediment delivery and downstream aggradation as the vegetation response lags behind the precipitation increase. At a later stage, as vegetation cover becomes denser the fluvial regime would gradually change to an erosive one (Eriksson et al., 2006). This could, therefore, be the reason for the absence of sediments pertaining to the intensified monsoon (b11 ka and N 6 ka) because the sediment supply was limited owing to vegetation cover and the river was dominantly involved in incising its own sediments. Lacking additional data, this suggestion remains speculative. Two major channel sand bodies separated by a pedogenised dune sand represent the youngest alluvial succession in the Saraswati River. The onset of the channel activity (Unit VIII; Fig. 6B) corresponds to the late Holocene intensification of the ISM that seems to have persisted intermittently until ~2 ka. After this, the second phase of channel activity (Unit IX) is bracketed between b2 ka and N 0.2 ka (Fig. 11). In the Banas River the channel sand on the river bed at depth 1.5 m gave an age of 1 ± 0.1 ka, whereas the paleoflood sediments also gave an identical age that broadly coincides with the Medieval Warm Period (MWP). Similar evidence of large floods associated with the MWP are reported from the Luni River (Thar Desert) (Kale et al., 2000) and the Sabarmati River (Sridhar et al., 2014). The continental record of the moist climatic conditions during the MWP is well represented in the fluvial record of the Kachchh Peninsula (Bhattacharya et al., 2013, 2014), the pollen record from the northeastern Himalaya (Bhattacharyya et al., 2011), and western and central India (Farooqui, 2013). In the marine record, this period is represented by the appreciable increase in the G. bulloides in the eastern Arabian Sea suggesting a short-lived increase in the ISM (Gupta et al., 2003). Thus, the chronology of the younger alluvial

sequence indicates the sensitivity of the dryland fluvial system to the late Holocene short-lived monsoon instability. 5.2. Incision The Gujarat alluvial plain with a regional slope towards west/ southwest is flanked by the Aravalli Hills in the east and the Gulf of Cambay and the Little Rann in the west (Fig. 1). Although laterally migrating, the Banas, Rupen, and Saraswati rivers (Zankhna and Thakkar, 2014), follow the regional slope, in contrast to the Sabarmati and Mahi rivers that flow towards the southeast (Fig. 1). The southeast deflection of the rivers is ascribed to the activity along the numerous NNE-SSW trending fractures that developed consequent to the formation of the Cambay graben (Sareen et al., 1993; Agrawal et al., 1996) during the late Quaternary and early Holocene (Tandon et al., 1997; Srivastava et al., 2001). Multiple factors could be responsible for incision in the alluvial plain such as changes in discharge and sediment supply (Jain and Tandon, 2003), eustatic lowering of sea level (Leigh and Feeney, 1995; Rao and Wagle, 1997; Juyal et al., 2006; Martins et al., 2010, 2017; Cunha et al., 2012), or the tectonic movement along the river bed (Schumm, 1993; Bhattacharya et al., 2014). A conservative estimate suggested that in alluvial rivers incision caused by eustatic sea level lowering is limited to ~80 km inland (Leigh and Feeney, 1995). The progradation of fluvial deposits can be ascribed to sea level lowering and corresponding upstream migration of incision (Cunha et al., 2017). This implies that the lower reaches of rivers are more susceptible to sea level changes (Merritts et al., 1994; Blum and Törnqvist, 2000; Lewis et al., 2004). More recently, studies have suggested that incision migrating upstream from the river mouth and the difference in the upper and lower reaches of the river profiles (differential uplift) were attributed to sustained crustal uplift superimposed on long term sea level adjustments (Cunha et al., 2017). Studies pertaining to the sea level fluctuations in the western coast of India (Arabian Sea) suggested that during the early Holocene the sea level was stabilized temporarily between − 40 m to −20 m (Hashmi et al., 1995). Considering the gentle sloping shelf of the Arabian Sea, it is likely that rivers would have extended their course into the shelf during a low sea stand and the incision would have progressively migrated upstream (Rao and Wagle, 1997; Juyal et al., 2006). Because the incised middle alluvial plain of the Banas River is located ~200–250 km inland from the Gulf of Cambay, we do not believe the incision to be a eustatically-governed early Holocene low sea stand. In such a case the incision would have been continuous along the river longitudinal profile, with areas proximal to the coastline showing drowned valley topography as observed in the Mahi River (Juyal et al., 2006). Therefore, we suggest that the incision was governed by a combination of a strengthened ISM modulated by hinterland tectonics during the Holocene. Inferences to the contributory role of tectonics accrue from the fact that the subsurface topography between Tharad (north of the study area) to Mehsana (south of the study area) shows presence of horst structures (basement highs) (Fig. 10D). The Banas and Saraswati River valleys are located in the Diyodar ridge and the Patan basin, respectively. Considering the structural configuration, it is reasonable to assume that these valleys (particularly the Banas River) are influenced by basement uplift and the reactivation of secondary intrabasinal faults. This is further supported by the Vp velocity (Kaila et al., 1990) and surface heat flow profiles (Thiagarajan et al., 2001) plotted along the SW-NE transect that clearly shows the presence of ridge structures below 200 m representing a horst structure (Fig. 10D). The horst and the associated graben structures, as reported by Maurya et al. (1995) along the NW-SE transect in the Gujarat alluvial plain, are the post-rift intrabasin faults developed after Tertiary sedimentation (Maurya et al., 1995) and are similar to the Type 4 subrift basins of Schlische et al. (2003). It has been suggested that the intrabasin structures not only dictated the Quaternary sedimentation pattern, controlled


sediment transport pathways, and provided accommodation space, but also modulated alluvial plain topography (Maurya et al., 1995; Merh and Chamyal, 1997). The preferential occurrence of discontinuous alluvial cliffs (Supplementary Fig. 5) and presence of gullies and ravines mostly in the northwestern part of the study area between eastern and western margin Cambay faults could be the geomorphic expression of localized tectonic activity. 6. Conclusion The alluvial succession preserved in the study area indicates that geomorphic processes responded sensitively to the millennial and centennial scale climate variability since late MIS 3. Facies architecture, along with the variations in geochemical proxies, indicate overall strengthened ISM with fluctuations between 37 ka to 27 ka that is attributed to the northward shift in the Inter Tropical Convergence Zone (ITCZ) during warming events in the North Atlantic (insolation driven). Textural attributes and geochemical indices suggest a declining ISM after 25 ka with maximum decrease inferred during the LGM. This is followed by a renewed phase of fluvial aggradation dated to 18 to N12 ka, suggesting re-strengthening of the ISM after the LGM. However, a short-term reversal in ISM intensity can be suggested between 12 ka and 11 ka, which we ascribed to the Younger Dryas (YD) cooling event. The onset of a regional phase of aridity is dated between 6 ka and 3.5 ka. This was followed by short-lived, centennial scale humid phases after 2 ka that includes the Medieval Warm Period (MWP) when the fluvial system experienced episodic flooding. This study suggests that incision of the middle alluvial plain in the Banas River occurred in pulses. The older phase of incision occurred during the intensified ISM (b 11 ka and N 6 ka) (climatically driven), whereas the younger incision phase (b3.5 ka to N1 ka) was modulated by the activity of basement faults in the Cambay basin (tectonically driven). Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.geomorph.2017.08.046. Acknowledgement We are thankful to two anonymous reviewers for their critical comments and valuable suggestions. We also want to express our gratitude towards the editor Professor Scott Lecce for painstaking corrections, which significantly improved readability of the manuscript. F.B. acknowledges support and encouragement by Dr. M. Ravi Kumar (D.G., ISR) and Dr. S. Chopra (Director ISR). Dr. P.S. Thakkar is thanked for many help in the fieldwork and Dr. M.G. Yadava of PRL for providing radiocarbon age. References Adams, J., Maslin, M., Thomas, E., 1999. Sudden climate transitions during the Quaternary. Prog. Phys. Geogr. 23, 1–36. Agrawal, R.P., Dotiwala, S., Mitra, D.S., Bhoj, R., 1996. The palaeodelta of the “Proto” Vatrak and “Proto” Mahi rivers of northeastern Gujarat, India: a remote sensing interpretation. Geomorphology 15, 67–78. Aitken, M.J., 1998. An Introduction to Optical Dating. Oxford University Press, Oxford (267 p.). Alley, R.B., 2000. The Younger Dryas cold interval as viewed from central Greenland. Quat. Sci. Rev. 19, 213–226. Andrews, J.E., Singhvi, A.K., Kailath, J.A., Kuhn, R., Dennis, P.F., Tandon, S.K., Dhir, R.P., 1998. Do stable isotope data from calcrete record late Pleistocene monsoonal climate variation in the Thar desert of India? Quat. Res. 50, 240–251. Baker, V.R., 1987. Paleoflood hydrology and extraordinary flood events. J. Hydrol. 96 (1–4):79–99. http://dx.doi.org/10.1016/0022-1694(87)90145-4. Benito, G., Thorndycraft, V.R., 2005. Paleoflood hydrology and its role in applied hydrological sciences. J. Hydrol. 313, 3–15. Bhandari, S., Maurya, D.M., Chamyal, L.S., 2005. Late Pleistocene alluvial plain sedimentation in Lower Narmada Valley, Western India: palaeoenvironmental implications. J. Asia Earth Sci. 24, 433–444. Bhattacharya, F., Rastogi, B.K., Ngangom, M., Thakkar, M.G., Patel, R.C., 2013. Late quaternary climate and seismicity in the Katrol Hill Range, Kachchh, Western India. J. Asian Earth Sci. 73, 114–120.

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