# Neolithic cultural sites and extreme climate related channel avulsion: Evidence from the Vaigai River Basin, southern India

Mu. Ramkumar, K. Kumaraswamy, K. Balasubramani, R. Nagarajan, M.

Santosh, S. Abdul Rahman, Kumar Arun Prasad, K.J. Juni, Al. Fathima, N.A. Siddiqui, et al.

<figure><img src="image_1.png" /></figure>

## ► To cite this version:

Mu. Ramkumar, K. Kumaraswamy, K. Balasubramani, R. Nagarajan, M. Santosh, et al., Neolithic cultural sites and extreme climate related channel avulsion: Evidence from the Vaigai River Basin, southern India. Journal of Archaeological Science: Reports, 2021, 40, pp.103204. 10.1016/j.jasrep.2021.103204. hal-03506131

HAL Id: hal-03506131

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Submitted on 29 Jul 2025

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<figure><img src="image_2.png" /></figure>
# Neolithic cultural sites and extreme climate related channel avulsion: Evidence from the Vaigai River Basin, southern India

Ramkumar Mu <fnref n="1" />,*, Kumaraswamy K. <fnref n="2" />, Balasubramani K. <fnref n="3" />, Nagarajan R. <fnref n="4" />, Santosh M. <fnref n="5" />,<fnref n="6" />,
Rahman S. Abdul <fnref n="2" />, Prasad Kumar Arun <fnref n="3" />, Juni K. J. <fnref n="1" />, Fathima A. L. <fnref n="1" />, Siddiqui N. A. <fnref n="7" />, Mathew M. J. <fnref n="8" />,
Menier David <fnref n="9" />, Sautter B. <fnref n="7" />, Sharma Rajveer <fnref n="10" />, Kumar Pankaj <fnref n="10" />, Chopra Sundeep <fnref n="10" />, Jegankumar R. <fnref n="2" />

## Abstract :

We document evidence for the role of fluvial dynamics-stream channel avulsion and flooding, tectonics and climatic conditions on the occurrence, shifting and burial of ancient cultural sites in the Vaigai River Basin in southern India. Systematic regional mapping, geophysical investigations and archaeological excavations in the flood plains of the Vaigai River led to the unearthing of an ancient culture that thrived at least since 752 BCE (2435 +/- 34 BP). Evidence for early human activities include sites of ancient and now buried industries that were manufacturing steel, glass bead, dye, gold jewelry and earthenware, in addition to dwelling-habitation, sites of religious sacrificial ceremonies, and places of worship, and burial. These information together with the occurrences of microlithic age stone tools in the vicinity led to the proposition of continued habitation of the Vaigai River Basin until today. Radiometric dating shows oldest age of 2435 +/- 34 BP (752 BCE) obtained at 2.5 m depth in a pit excavated at Keezhadi, a location situated far from the present day channel course whereas the youngest age of 689 +/- 24 BP is obtained at Agaram, a location situated nearest to the present day channel course. The bone fragments were dated as 1115 +/- 39 BP (778 CE - 1016 CE) and 1229 +/- 30 BP (689 CE - 883 CE). The organic carbon/peat layers recovered between flood deposits at the upper reaches of the Vaigai River are dated as 830 +/- 30 BP and 1250 +/- 30 BP. The natural levee deposits show evidences of habitation-related disturbances in the internal sedimentary structure, texture and packing. In addition to occurrences of hearth etc., recurrences of overbank-channel deposits over natural levee-habitation surfaces indicate habitation-

<footnote n="1">¹ Department of Geology, Periyar University, Salem, India</footnote>

<footnote n="2">² Department of Geography, Bharathidasan University, Tiruchirappalli, India</footnote>

<footnote n="3">³ Department of Geography, Central University of Tamil Nadu, Thiruvarur, India</footnote>

<footnote n="4">⁴ Department of Applied Geology, Curtin University Sarawak, Malaysia</footnote>

<footnote n="5">⁵ School of Earth Sciences and Resources, China University of Geosciences Beijing, Beijing, China</footnote>

<footnote n="6">⁶ Department of Earth Sciences, University of Adelaide, Adelaide, SA, Australia</footnote>

<footnote n="7">⁷ Department of Petroleum Geoscience, Universiti Teknologi Petronas, Malaysia</footnote>

<footnote n="8">⁸ Laboratoire Géosciences Océan, IUEM, Université de Brest, UMR 6538 CNRS/UBO/UBS,
RueDumont D'Urville, 29280, Plouzané, France</footnote>

<footnote n="9">⁹ Laboratoire Géoscience Océan, UMR CNRS 6538, Université Bretagne Sud, 56017 Vannes, France</footnote>

<footnote n="10">¹⁰ Inter University Accelerator Centre, New Delhi, India</footnote>

<footnote>* Corresponding author : Mu Ramkumar, email address : muramkumar@yahoo.co.in</footnote>
colonization over relatively highgrounds along river bank, followed by marooning/burial under overbank-
channel deposit and abandon -ment of habitation site and/or shifting of habitat. The same sequence of
events occurred again, showing two habitation surfaces that are buried under surface currently. Together
with facies characteristics and other artifact occurrences, the ages obtained in our study indicate shifting
of the Vaigai River from prior to 2435 BP and colonization of ancient population, which might have been
affected by a catastrophic flood and stream avulsion at about 1250-830 BP. These resulted in the ancient
habitation getting marooned, industries and other structural infrastructure destroyed, and were
transported and deposited as interbeds in flood deposits. Following a reduction in the stream energy, the
point bar-natural levee environmental setting paved way for the deposition of organic carbon layer at
Agaram at about 690 BP. This was followed by lateral shifting of the channel from Agaram towards the
present-day course.

**Graphical abstract**

<figure><img src="image_3.png" /></figure>

**Highlights**

► Diverse civilization sites are found buried under Vaigai River paleoflood deposits. ► Episodic extreme climate related channel avulsion and burial of habitation sites. ► Diversity of religious-industrial-dwelling evidences suggest thriving civilization.

**Keywords** : Ancient settlement, Geoarchaeology, Paleoflood, South India, Vaigai civilization
71 INTRODUCTION

72 Unearthing of ancient cultural sites along buried paleochannels (for example, Indus - Giosan et al. 2012; 2018) suggested floodplains and natural levees were among the preferred locations of human 73 settlements (Singh et al. 2017). Marooning and or burial of cultural sites by various causes including, but 74 not limited to climate change (Mehrotra et al. 2019), sea-level variation (Day et al. 2007; Chen et al. 75 2008), natural hazards such as tsunami (Nigam et al. 2016), coastal storm (Nigam and Chaturvedi, 2006), 76 neomorphotectonics and paleoseismicity (Roy and Sethumadhav, 2014; Similox Tohon et al. 2006; 77 Valdiya, 2008; Roy et al. 2017) and catastrophic floods (Rao, 1991; Kale et al. 2010, Ramkumar et al. 78 2018) are well-known. Examination of paleolithic-historic archaeological sites in Tamil Nadu, southern 79 India suggests that, all the river deltas, especially at the deltahead region contain archaeological sites, 80 and are covered by catastrophic flood deposits (Ramkumar et al. 2016, 2018, 2019). These catastrophic 81 floods were either controlled by periods of intense climate change and/or superimposed on paleo- 82 present seismically active zones. Few of the flood wrecked havoc and associated destruction of ancient 83 habitats found their mention in cave carvings/paintings, stone inscriptions, inscriptions on palm leaves, 84 and copper plates, and Sangam literature (11000-2000 years BP). Tracing the evidence of 85 paleoflood/seismicity and using them as clues to locate and excavate archaeological sites is a potential 86 approach, especially in the rivers of Peninsular India (Ramkumar et al., 2018). The most recent 87 archaeological finds in the Cauvery, the Thamirabarani and the Vaigai river plains are no exception. 88 Thus, the recently excavated Keezhadi site is used as an example to investigate the tectono-climatic 89 factors in forcing the ancient civilization habitants to abandon their habitats, and to evaluate the habitat 90 shifts with climate-tectonic dynamics through applying multi-proxy approach. In this work, we 91 demonstrate that the very landforms that supported human activities and sustenance, wrecked havoc 92 and forced the settled communities to shift or vanish from the habitat due to geohazards such as 93 earthquakes, landslides or flooding (Menier et al., 2017), that buried and/or marooned the settlements 94 causing loss of life and infrastructure. This paper reports the results from our recent studies in 95 unearthing of ancient habitations including, residential, industrial, burial, religious-worship sites, buried 96 under flood deposits of the Vaigai River, southern India. Our study documents the relationships between 97 climatic reversals-sudden and catastrophic flooding, stream avulsion and survival-shifting of ancient 98 human settlements. 99

100

101

102
103 LOCATION AND REGIONAL GEOLOGICAL SETTING

104 The southern Indian Shield is a mosaic of several crustal blocks with a variety of geological
105 signatures ingrained during different times ranging since Achaean (GSI, 1975; Santosh et al., 2020). The
106 Vaigai River (Fig.1) flows on an antecedent valley (Fig.1A), and the directional change is unique, which in
107 turn, was imparted by episodic tectono-morphological events under the influence of inherited
108 Proterozoic structure (Ramkumar et al., 2016, 2019). The regional geology, geomorphology, spatial
109 distributions of soil, slope, land use and rainfall presented in the Figure 2 show that unlike other basins
110 of south India, the Vaigai River Basin maintains its narrow and linear morphology. However, the location
111 of deltahead is at/or nearer to the tectonic line, similar to other rivers (Ramkumar et al. 2016) and
112 suggests a shared/common mechanism for this basin (Ramkumar et al. 2016; 2019) also. Incision of
113 paleo-alluvial fills, development of Miocene-Pliocene Kankar/paleosol, recycling of paleosol and
114 development of Holocene-Recent soil, and Recent-historic changes of river courses are all observed in
115 this basin as a response of episodic tectonic events and short-long term climatic reversals (Ramkumar et
116 al., 2016, 2019). The basin is covered by crystalline rocks of Achaean on the western part, and Tertiary
117 and Quaternary sedimentary sequences on the eastern part (Fig.2). The Quaternary Period witnessed
118 widespread recycling of sediments in this basin. The younger alluvium occurs on either side of the Vaigai
119 River, deposited during Quaternary-Recent floods. The soil of the basin is brown, sandy, with a low
120 fraction of clay and humus. The delta plain is mostly covered by black cotton soil which is admixed with
121 Fe and Ca nodules sourced from paleosols. The upper portion of the basin is covered by structural hills,
122 pediments and valley fills. The lower part of the basin, especially east of the major tectonic line, is
123 covered by paleodeltatic plains, flood plains, paleo and present tidal flats and tanks
124 (https://www.nationalwatermission.gov.in). The Vaigai River Basin lies within the tropical monsoon
125 climate. The average annual rainfall is about 775 mm. The Southwest (SW) monsoon period spanning
126 from June to September and Northeast (NE) monsoon period spanning from October to December bring
127 seasonal rainfall to the basin. However, the Vaigai Basin receives maximum amount of rainfall in a short
128 spell (about 40 days in a year) during NE monsoon season.
129

130 Despite recurrent reports in the mainstream media regarding unearthing of archaeological
131 remains buried under shallow depths the locations of which are strewn all along the Vaigai River course,
132 systematic studies by professional organizations (Archaeological Survey of India and Tamil Nadu State
133 Archaeology department) have begun only recently. In addition, the results have not yet been revealed
134 and or published in any scientific journals. According to commonly prevailing perception, the Vaigai
135 River Basin is one of the examples of continually habitated regions, evidenced by microlithic-historic-
recent settlements, hunting-modern industries, agricultural and other structures. However, these evidences are not systematically recorded, interpreted, scientifically validated, analyzed causative mechanisms/factors that supported origin, sustenance and destruction of habitation sites and corroborated with other known contemporary civilizations elsewhere. This paper reports the initial findings from selected sites in the Vaigai River Basin in terms of shifting of habitation sites and channel avulsion as a result of climate change and natural catastrophes.

Fig.1.

Fig.2.

## 143 METHODS AND MATERIALS

### 144 Delineation of areas of interest through remote sensing and field survey

For effective prospecting, archaeological research takes into consideration the results of the interactions between environmental conditions, morphology, water proximity, slope, soil type, land use, etc., and human activities over time (Brivio et al., 2000). As the geophysical methods are dependent on the terrain conditions such as lithology and types and depths of artifact occurrence, integrating remote sensing techniques with geophysical methods and conventional field mapping is an effective approach (Brivio et al., 2000; Ramkumar et al., 2019) and the same is followed in this study.

A geological map in the scale of 1:50,000, seismo-tectonic and geochronology maps published by Geological Survey of India (http://bhukosh.gsi.gov.in/) were used to prepare structural and geological maps of the basin. Then the thematic layers on various aspects including geomorphology, land use/ land cover and Normalized Difference Vegetation Index (NDVI) were generated with the help of multi-temporal satellite images (IRS-AWIFS, IRS-LISS III and LANDSAT TM) and Geographic Information System software package. SRTM (Shuttle Radar Topography Mission) digital elevation data were used to generate a slope map and drainage networks of the basin. A long-term rainfall data (1980-2014) collected from India Water Portal (www.indiawaterportal.org), were used to generate average rainfall map of the basin. Soil series maps published by National Bureau of Soil Survey and Land Use Planning (NBSS and LUP) were used to prepare soil texture map of the basin. Based on these thematic maps, a narrow strip (Fig.1) was chosen for detailed image processing. It was followed by selection of probable sites for geophysical sounding (Vertical Electrical Sounding -VES and Ground Penetrating Radar -GPR). The survey of India toposheets in the scale of 1:50,000 (58K01 and 58K05) were used to prepare detailed base map for the region of interest. The combined inferences on these helped identify locations wherein excavations could be made.
Using long-term archived satellite data to delineate paleochannel pathway based on long-term vegetation dynamics and image decomposition techniques has been attempted in the present study. The study used surface vegetation characteristics (distribution, and health) as the indicator of presence of paleochannels as a strong correlation exists between vegetation distribution and water availability (Mehdi et al., 2016; van Dijk et al., 2016). The multi-technique seasonal approach using the available range of 30 years (1983-2012) of Landsat data was adopted. The data were divided season-wise (dry and wet months) to calculate Seasonal Multi-Temporal Vegetation Indices (SMTVI), Seasonal Tasselled Cap Transformation (TCT), Seasonal Principal Component Analysis (PCA), and Normalized Difference Vegetation Seasonality Index (NDVSI). The RGB composite generated from three principal components (PCA), three axes of Tasselled Cap Transformed images (TCT) of all bimonthly images, and Seasonal Multi-Temporal Vegetation Index (SMTVI) were used for visual interpretation to delineate paleochannels. The use of long temporal span helped to overcome the errors induced due to seasonal crop-pattern, and land use dynamics that prevailed in the study area. These multi-temporal image derivatives were visually interpreted based on our ground knowledge to derive probable pathways of paleochannels. All these processing were done in Google Earth Engine, a cloud-based computing infrastructure for bulk processing of satellite image data. Through visual interpretation of these datasets, the probable paleochannels and their courses were delineated using vector-based Arc GIS platform. This was followed by field surveys along selected transects that criss-crossed the Vaigai River Basin and the identified paleochannel courses (Fig.1) to delimit the regions of interest for detailed survey and exploration.

## Excavation and sampling

After affirming the presence of man-made structures and artifacts in the sub-surface, through conventional, geophysical and visual interpretation and image processing of remotely sensed data, systematic excavations were conducted at selected locations for the examination of natural and anthropogenic structures, lithofacies characteristics and sediment/soil/artifact sampling. After photographing and logging, sediment samples were collected at 20cm intervals, in addition to artifacts and archaeological remains (Plate I). A total of 30 sediment samples were collected for granulometric, geochemical, mineralogical analyses and C¹⁴ dating.

## Granulometric analysis

Approximately 1 kg of sample was collected from each stratigraphic location, packed in airtight PVC bags, labeled and transported to the laboratory. Granulometric analysis of these sediments and computation of textural parameters were attempted following the procedures detailed in Ramkumar et
al., (2000) and Ramkumar (2001, 2007, 2015b). The samples were air dried, thoroughly mixed, homogenized and cone and quartered to obtain approximately 50 gm of sediment subsample. Large pieces of shell fragments, organic material or pebble, if present, were removed. These subsamples were transferred to a pre-cleaned glass beaker. The samples were cleaned through three stages of treatments by hydrogen-per-oxide, 10 % v/v HCL and quasi-distilled water to remove the organic matter, shell calcite and other contaminants. These subsamples were then dried in an air-oven overnight at 60°C and weighed and dry-sieved in an automated Ro-tap sieve-shaker with ASTM test sieve sets of ½ Ø intervals for about 20 min. The separated size classes of sediments were then weighed and tabulated. These data were converted into ¼ Ø intervals with a computer algorithm (Ramkumar et al., 2000, Ramkumar (2001, 2007, 2015b), and then cumulative percent according to Ø interval were calculated and plotted in a semi-log graph sheet for generating requisite data to compute graphic mean (Mz), standard deviation (SD), skewness (SK) and kurtosis (KG). In addition, the input data required for few bivariate and other discriminant plots of environmental, energy and transport mode interpretations were also generated from the plots of cumulative percent of the sediment samples.

## Plate I

### Geochemical analysis

The sediment samples were dried in an oven at ~60° C and the dried samples were homogenized followed by reduction of sample size through cone and quartering technique. Later, a selected portion of the samples were pulverized to <63 µm size using agate mortar and pestle. The powdered samples were sent to Bureau Veritas for geochemical analysis using complete characterization of sediments using multiple methods. The proportions of major oxides were determined using X-Ray spectrometer after fusion of samples with Lithium borate. A known amount of sample was heated to determine the loss on ignition (LOI) by measuring the weight loss. A second split of sample was dried at 105° C and then sample was fused in a platinum-gold crucible with a commercial lithium tetraborate flux. The molten material was cast in a platinum mold. Fused discs were analyzed by XRF. The prepared samples were mixed with LiBO₂/Li₂B₄O₇ flux and crucibles were fused in a furnace. The cooled bead was dissolved in ACS grade nitric acid and analyzed by ICP and/or ICP-MS. Total Carbon and Sulphur were determined by the Leco method (TC003). The LF202 package includes an additional 14 elements from an aqua-regia digestion AQ200 to provide Au concentration. The CRMs i.e. Std SO-19 was used to check the accuracy of the measurement and replication analysis was also performed to ensure the precision of the results. Only the most relevant geochemical compositional values and ratios are used in this paper.
230 Establishing chronology through C¹⁴ analyses

231 All the 30 sediment samples were measured for total organic carbon, in addition to a pot shred
232 and few bones collected from the dug pits, based on which, 10 samples were chosen for radiocarbon
233 dating. While AMS (Accelerator Mass Spectrometry) radiocarbon dating of two samples (Table 1:
234 Varusanadu 1 and Varusanadu 2) was attempted at Beta Analytic, U.S.A., measurements of 8 samples
235 were performed at IUAC (Inter University Accelerator Centre), New Delhi, using procedures described in
236 Sharma et al. (2019). In brief, sediment samples, pottery and organic carbon samples were first
237 physically examined for visible contaminations. These samples were chemically pretreated using ABA
238 (Acid-Base-Acid) protocol. Samples were mixed with 10 mL 0.5 M HCl and kept overnight on a thermo-
239 shaker at 60°C and 700 rpm to remove carbonates. After acid treatment, samples were cleaned with
deionized (DI) water upto pH 7. To remove humic acid, samples were treated with 10mL 0.1 N NaOH at
240 60°C and 700 rpm for 3 hours. Again, after cleaning with DI water, samples were pretreated with 0.5M
241 HCl at 60°C and 700 rpm for 1 hour to remove any absorbed CO₂ during base treatment. After cleaning
242 with DI water, samples were freeze dried overnight. Freeze dried samples were graphitized using
243 Automated Graphitization Equipment (AGE) in the graphitization laboratory at IUAC.
244

245 Bone samples were first cleaned using ultrasonic, dried and then powdered. Powdered samples
246 were de-mineralized using 0.2M HCl and stored for four days in a refrigerator. Samples were monitored
247 for effervescence continuously and acid was replaced with fresh acid daily. After demineralization,
248 samples were cleaned with DI water and mixed with 10mL DI water and 3-4 drops of 0.5 M HCl to make
249 solution pH 3 and kept for gelatinization at 80°C for 48 hours (Longin, 1971). Gelatinized (collagen)
250 samples were freeze dried and graphitized using AGE. Graphitized samples were measured using XCAMS
251 (The compact Carbon Accelerator Mass Spectrometer extended for ¹⁰Be and ²⁶Al) and sample ratios
252 (¹⁴C/¹²C) were converted to radiocarbon ages as described in Stuvior and Polach (1977). All the
253 radiocarbon ages (BP) were calibrated using OxCal software.
254

254 RESULTS

255 Paleochannels

256 The paleochannel courses recognized are shown in the Figure 1. They follow a parallel-sub-
257 parallel alignment and show fanning-out character from Silaiman-Sakkudi.

258 Lithofacies Characteristics

259 A composite litholog, covering the natural exposures and dug pits was constructed and
260 presented in the figure 3. It shows the lithological information, contact relationships and the
261 stratigraphic locations of collected sediment and artifact samples. The lithofacies succession logged as
262 observed in the field is as described herein.

263 Fig.3.

264 The present-day channel runs on barren rocky surface in the upstream/northern regions of the
265 basin. From there on, the channel runs on paleosol/kankar (Quaternary-Recent). The ~2m thick paleosol
266 is typical of consolidated sediments of paleochannel (Fig.3f, i, j). This sediment layer contains pink-
267 reddish, pebble-boulder rich, massive, very coarse sandstone with scattered and recycled human bone
268 remains (Fig.3j; Plate I. o and q) and occasional-very frequent burial urns (Fig.3j; Plate I. o, p, t). At the
269 top of this layer, very-frequent occurrences of transported and recycled bone remains are observed to
270 occur and typically these are all bleached. Regions in which this paleosol forms the substrate were
271 utilized extensively as burial grounds (Fig.3j; Plate I. o) of the ancient populations. This paleosol is
272 overlain by fluvial channel and / or natural levee sediments. The contact between the kankar/paleosol
273 and the fluvial channel deposit is sharp, and erosional, while the contact with the natural levee sediment
274 is gradational. The fluvial channel deposits are composed of loose, unconsolidated, unsorted to poorly
275 sorted, angular to subrounded quartz rich sand with scarce admixture of granule to very-fine pebbles
276 and rare to regionally varying well-rounded gravel to cobble clasts. The natural levee deposit is massive,
277 highly compact, mostly brownish and wet clayey, silty to sandy loam typical of natural levee (Fig.3f, i, j;
278 Plate I. a, b, i). Pot shreds, quartz and quartzite clasts are scarce in this layer. This layer contains insitu,
279 large (65cm dia) concave up pots (Plate I. e) and bowels (Plate I. i) at two levels. The fluvial channel
280 deposit and the natural levee deposit might be lateral facies variations. At Pasiyapuram, paleo-wells
281 with clay outer linings occur. Successive excavations have exposed horizontally aligned pipelines (Plate I.
282 k) with filtering system (Plate I. h) connected to large storage bowels/pots.

283 This natural levee deposit is overlain by overbank deposit (~55cm) or fluvial channel deposit
284 (~2m) (Fig.3). The overbank deposits are highly compact, mostly dark brownish colored and wet clay,
285 silt, sandy loam with less frequent pebbles. The pot shreds are relatively bigger and are randomly
286 oriented within the bed (Fig.3f). There is a 12cm thick pot-shred interbed within this loamy soil layer.
287 The shreds are all concave-up oriented, except the very large shreds (>25cm) which are randomly
288 oriented. In addition, there is an organic matter rich layer within this massive bed. The upper and lower
289 contacts of this overbank deposit are gradational. Whereas, the lower contact of the pot-shred
290 interlayer is sharp and erosional. The fluvial channel deposit beds are composed of loose,
291 unconsolidated, unsorted to poorly sorted, angular to subrounded quartz rich sand with scarce
admixture of granule to very fine pebble. The overbank deposit and the fluvial channel deposit might be
lateral facies variations.

Towards top, the overbank deposit that houses many paleo-wells (Plate I. a, b, c) is overlain by
180cm thick, typical natural levee deposit of dusty brown–greyish colored, massive, compact, silty loam
and contains regionally varying quantities of pot shreds. This bed is immediately overlain by two beds of
floodplain deposits that are composed of massive, structureless loamy soil with subrounded-subangular
quartz and quartzite grains, pot shreds and evidences of recycled sediments. A paleohearth, filled with
highly disturbed sediments and artifacts are also identified. Towards top, this floodplain deposit is
overlain by overbank deposit, with an intervening, sharp, undulatory surface contact. The 40cm thick
overbank deposit is dusty brown colored, massive, loamy soil with pebble-sized pot shreds. The bed
looks massive; however, fining upward character is discernible. A gradual decrease of sizes (cobble-
gravel-pebble) of pot shreds from bottom to top is observable. At top, this bed is overlain by a dusty
brown colored, 15cm thick silty loam with parallel, even bedded, mm-scale fine laminations/very thin
beddings that regionally shows enrichments of organic carbon. Towards top, it is followed by dusty
brown colored, silty loamy soil with abundant pebble-gravel sized pot shreds and polycrystalline,
granule-pebble sized, translucent, pale yellow and red-tinted, angular-subangular quartz and quartzite
grains. This layer is highly anthropogenically disturbed due to cultivation.

## Textural properties

The results of granulometric analyses were utilized to compute mean size, standard distribution,
skewness and kurtosis and the data were plotted against stratigraphic positions (Fig.4). In addition, the
data were plotted on various established discrimination diagrams (Fig.5) to interpret the provenance,
affinity, modes of sediment transport, depositional energy and environments. Ubiquitous affinity with
fluvial environments, deposition in channel and adjoining regions, temporal variations in energy
conditions, i.e., normal and flood flow conditions, are all depicted by these granulometric characteristics
and the discriminant diagrams.

Fig.4.

Fig.5.

## Chronology

Table 1 lists the chronological estimates obtained from ¹⁴C analyses of 10 samples. Oldest age
estimate of 2435 ± 34 BP (752 BCE) is obtained from 2.5m depth at Keezhadi, a location situated far
from the present-day channel course and the youngest age estimate of 689 ± 24 BP is obtained at
Agaram, a location situated nearest to the present day channel course. The bone fragments were dated
324 to be 1115 ± 39 BP (778 CE – 1016 CE) and 1229 ± 30 BP (689 CE – 883 CE). The organic carbon/peat
325 layers recovered between flood deposits at upper reaches of the Vaigai River are dated to be -830 ± 30
326 BP and -1250 ± 30 BP.

327 Table 1.

328 DISCUSSION

329 Habitat shifts due to tectonics and depositional setting

330 Examination of the lithofacies succession, textural characteristics and sediment size
331 characteristics in the light of established discriminant diagrams show that the studied sedimentary
332 lithological succession represents a low-moderate slope fluvial channel sequence (channel-overbank-
333 natural levee-flood plain). This sequence documented the stratigraphic records of climatic extremes and
334 / or episodic overflow/shifting of channel that caused shifting of associated environmental niches. From
335 the configurations and geographic locations of paleochannel courses with reference to the present-day
336 channel and the locations of excavated sites of ancient, now buried industries that once manufactured
337 steel, glass bead, dye, gold jewelry (Plate I. d, h, I, j, k, v) and earthen pot, in addition to dwelling-
338 habitation, sites of religious sacrificial ceremonies (Plate I. r), places of worship (Plate I. x, y, z, ai, aii, aiii
339 and aiv) and burial sites (Plate I. o), it is perceivable that the paleochannels were ephemeral and flowing
340 on a Quaternary Kankar/paleosol (Fig.3I and 3J) from south of Silaiman (Fig. 1B) where, the tectonic line
341 (Fig.2A) crosses the Vaigai River. The low-lying area and the Vaigai River follow a strike fault. E-W aligned
342 fault-controlled river course and its N-S tilting events shifted the active delta lobe north and south of the
343 modern channel course. The stream courses of the Vaigai River were changing historically and during
344 pre-historic times, which is revealed from previous studies (Ramkumar et al., 2016, 2018, 2019), as well
345 as geomorphic information garnered from image processing (Fig.1C, D, E, and F; Fig. 2C, ) conducted by
346 the present study. From the occurrences and alignments of the paleochannels and the sites of
347 archaeological finds, the following are discerned: a) the avulsion of channels commences from east of
348 the tectonic line, as could be found in any other delta of the Peninsular India, b) from the deltahead
349 located near Sakkudi-Silaiman-Manaloor, the paleochannels are aligned parallel-tangent to the present
350 day course, forming a fanning-out character, c) the oldest channel is located at farthest (southwest of
351 the figures 1B, C, D, E, and F) with a progressive younging nature toward present day course and d)
352 collectively the relative and absolute ages of the paleochannels and present day channel course suggest
353 tilt-rotational movement of the deltaic region of the Vaigai River from SW toward NE. The paleochannel
354 courses, as identified (Fig.1C, D, E, and F) during the present study and verified in the field (Fig.3j; Plate
355 I. c, q) also corroborate with the fact that all of these shifted/changed courses in a pendulum
356 fashion/high-sinuosity channels from east of the tectonic line (south of Silaiman-Fig.2A), and occupied
tangent directions at different times forming a pendulum fashion reinforcing the previous inferences (Ramkumar et al., 2016, 2018, 2019) of influences of this tectonic structure over location and morphology of landscape development in the region. Stream avulsion has been considered a common phenomenon that made the settlements of Indus Civilization to be abandoned and recolonized (Singh et al., 2017) and similar abandonment of ancient settlements that are aligned/located along the paleochannels of the Vaigai River are identified (Fig. 1C, D, E, and F).

The paleochannels were shallow incised channels flown over Quaternary kankar/paleosol (Fig.3f, i, j; Plate I. o). They received only ephemeral/seasonal flows, that necessitated establishment of dugwells (Plate I. a, b, c) in every habitation units (i.e., in every house and the industrial site) for domestic and other uses. The high density of dug wells could be a result of two factors: One, the river was ephemeral and the second is prevalent arid climate (Ponton et al., 2012). Field observations, type, nature and characteristics of man-made structures, artifacts exposed during excavation suggest: there were multiple habitation sites, steel, dye, ceramic and jewelry industries, burial /cremation sites, and religious ritualistic sacrificial sites, besides scattered occurrences of buried-not fully exposed stone (Plate I. x, y, z, ai, aii, aiii, aiv) and polymetallic statues of deities that necessitated establishment of sporadic-extremely dense arrangements of backed clay ringed and clay lined dug wells (Plate I. a, b, c). Owing to the shallow morphology and seasonal nature, there might have been overbank flows episodically, especially during significant climatic extremes (i.e., highly different from normal flow), that occurred at least twice as indicated by linear arrangements of archaeological sites, following paleochannels and also as decipherable in the lithofacies characteristics and shifts of habitation sites/surfaces. From the lithological succession, it is evident that the river channel deposits were overlain repeatedly by natural levee and overbank deposits that irrefutably indicate marooning of ancient habitation sites/surfaces and their burial under channel deposits. Occurrence of pot shred interbed within overbank-natural levee deposit (Fig.3f) indicates perhaps a sudden flooding-related erosion and resultant transportation of pot materials from ceramic industry and deposition of eroded material in aqueous medium (as they are all oriented concave-up except the very large shreds that are randomly oriented). While the natural levee deposits show evidence of habitation-related disturbances/churning/recycling/mixing in the internal sedimentary structure, texture and packing, in addition to occurrences of hearth, etc., recurrences of overbank-channel deposits over natural levee-habitation sites/surfaces indicate habitation-colonization over relatively high-grounds along river bank, followed by marooning/burial under overbank-channel deposit and abandonment of habitation site and/or shifting of habitat. The same sequence of events occurred again, showing two habitation
surfaces that are buried under surface. Together with facies characteristics and other artifact occurrences, these dates indicate shifting of the Vaigai Channel from prior to 752 BCE (2435 ± 34 BP), and colonization of ancient population, which could have been affected by a catastrophic flood that might have occurred along with stream avulsion at 1250-830 BP, in which the ancient habitation might have been marooned, the industries and other structural infrastructure destroyed, transported and deposited as interbeds in flood deposits. This high-energy condition was reduced leading to the formation of point bar-natural levee environmental setting that paved way for the deposition of organic carbon layer at Agaram at about 689 ± 24 BP. Further shifting of the channel from Agaram toward present day course might have occurred since then. According to the available age constraints, the first habitation-colonization might have been prior to about 752 BCE (2435 ± 34 BP), the second habitation-colonization might have been during 641 CE (1348 ± 27 BP). The third and most recent habitation-colonization could have been during post 689 ± 24 BP and the topsoil is intensively disturbed anthropogenically, including that of gold enrichment in the soils. The geochemistry of the top soil shows enrichment of P₂O₅, Ag and Au and depletion of Na₂O, Mo, Cs, Sn and Ti.

All the sediment samples fall in the riverine fields (Figs.5A, B) without exception. However, depositional environments varied from braided bar, point bar (Fig.5C) and show poor-moderate sorting character, suggestive of excessive sediment supply within short distance and short duration that could have been a response of high-intensity, short-duration rainfall in the catchments. The energy conditions were also variable, not typical of normal fluvial environment, as indicated by plot of all the samples in region next to river process, but away from quite-water and wave process fields (Fig.5D), suggestive of high-energy, unidirectional flow (Ramkumar, 2007; Ramkumar et al., 2000, 2015b). The scattered nature of the sample fields might suggest highly-variable depositional energy conditions. Almost all the samples fall within the field of 'rolling and suspension' (Fig.5E) except few near the C-S line. While the latter represents the fluidized alluvial fill capping the residual basement rock and consists of immature sediments, cemented with calcareous material, the former represents deposition of the entire sequence during periodic floods, during which sediments were brought essentially under turbulent flow and deposited when peak flows dwindle/recede (Sridhar, 2007; Ramkumar, 2009; Ramkumar et al., 2015b), during which rolling (coarser sediments) and suspension (finer sediments) modes of transport prevail and ultimately lead to deposition. This bimodality is also reflected in the sediment size distribution. These observations based on sediment granulometry and sediment size parameter-based discrimination diagrams broadly follow the inferences on environmental setting and depositional conditions inferred from lithofacies and sedimentary structural characteristics. All these further indicate climatic extremes-
forced abandonment of ancient habitation sites and shift to newer higher altitudes and or regions not
affected by flooding/marooning, but near the channel course, of course, indeed a newer channel course.

The depositional conditions inferred are also supported by the geochemical characteristics as
detailed herein. This is further reinforced by the variations of sand and silt-size sediments based on their
SiO₂ and Al₂O₃ contents. Sand dominated facies are enriched in SiO₂ and Silt/clay dominated facies are
enriched Al₂O₃ unless the sediments are rich in Fe-oxides or calcareous cement (i.e., calcrete at the
bottom of the section). Silicon to Al ratio (Si/Al) can reflect the proportion of quartz to clay minerals (e.g.
Cole et al., 2009) and the ratios vary from 3.1 to 10.7. The lower values are reported from the top of the
section which indicates enrichment of fine particles with clay minerals and the higher ratios recorded at
the bottom of the section which represents enrichment of quartz with the sand and silt fractions. The
sediments are low to moderately mature and can be chemically classified as shale, wacke and litharenite
(Herron, 1988). This is further supported by the ICV values (> 0.8), which represent that the sediments
mainly consist of rock forming minerals than the clay minerals (e.g. Cox et al., 1995). The chemical
weathering had less influence in the modification of the sediment composition and physical weathering
might be the dominant process in the study area. This is confirmed by the CIA values (52-69) and the
biplot of CIA vs. Al/Na (Fig.6).

Fig.6.

437
438
439 **Geochemical proxies of climate induced fluvial dynamics and habitat shifts**

The stream avulsion characteristics and location-alignments of paleochannels of the Vaigai
River, including the general nature of the Peninsular Rivers, including the Vaigai River that experience(d)
short-duration, high-intensity rainfall, annually, might have enhanced the flood vulnerability of the
region. This general character, when superimposed over the catastrophic flooding events, that could
have occurred episodically during historic and pre-historic times, as inferred through the facies
succession (Fig.3) and textural properties (Figs.4 and 5) suggests the plausibility of prevalent climate
reversal and or significant deviation from normalcy. Sridhar (2007) reported flooding as a common
phenomenon in all the Indian rivers during peak monsoon. Extreme floods caused either by known or
unknown return period flow can alter channel hydraulics and geomorphic effects (Heritage et al., 2004).
Changes in rainfall patterns in catchments that are potentially linked with climate changes are
considered as first-order controls (Wulf et al., 2010) on topographic evolution (Sahu et al., 2010; Menier
et al., 2017) on short and long-term erosion/deposition patterns in fluvial systems (Menier et al., 2017;
Ramkumar et al., 2019). Given cognizance to these, it is perceivable that the channel shifting, marooning
and burial of ancient habitats might have been related to extreme floods and associated changes in
channel courses of the Vaigai River. The characteristics of sedimentary successions are controlled by three main factors: the sediment supply from the eroding source region, the grain size distribution of that sediment supply, and the area available for sediment accumulation in the downstream regions. The interplay of these factors can help interpret climatic-tectonic signals (Armitage et al., 2011). The geochemical proxies are examined in the light of these observations and to draw support for prevalent significant climatic changes.

The single element mobility was calculated for selective mobile elements during incongruent weathering by comparing non-mobile elements (in this case Al; Garzanti et al., 2013) as Al has similar magmatic compatibility (Gaillardet et al., 1999; Garzanti et al., 2013). The bulk sediment mobility in the present study is observed as: αAlCs>αAlNa>αAlRb>αAlMg>αAlCa>αAlK>αAlSr>αAlFe>αAlBa. This sequence is different from the general observations made elsewhere. High mobility is recorded for Cs and Na than other smaller cations such as Ca and Sr. According to Nesbitt et al. (1980), Rb and Cs have large ionic radii and thus are retained in weathered product during chemical weathering. Ca and Na also behave similar during chemical weathering. In addition, other divalent cations, i.e., Mg, Ba and Sr become mobile during chemical weathering (Nesbitt et al., 1980; Middelburg et al., 1988; Zhao and Zheng, 2015). However, in the present study, the immobile element (i.e. Cs) and mobile elements (i.e. Na) are depleted. In particular, Na and Cs show higher mobility in specific depth (Cs at K24, K26-K27 and K1; River channel and modern flood plain environment). It means, specific time intervals were climatically highly different than other durations. It is noted that Rb and Mg behaved similar and moderate mobility is recorded at both top and bottom parts of the studied lithosection. Majority of the elements showed moderate to high mobility in the bottom portion of the section (K1-K4). Cesium is strongly adsorbed in clay particles and clay containing substances (Jolin and Kaminski, 2016) and its depletion may indicate removal of Ce-bearing clay sediments during the flood events. In addition, the enrichment of Ca can be post-depositional effect. Contrary to their nature, the Ca and Na show inverse trend at the bottom of the section, which may indicate mass conservation during the chemical weathering at the source region, as enrichment of immobile elements may be balanced by depletion of mobile elements (Zhao and Zhend, 2015).

The Rb/Sr ratios in the sediments have been widely used as indicators of weathering intensity and it is found that the Rb/Sr ratios of the bulk surface sediments are closely related to the terrigenous input from the catchment area (i.e., Xu et al., 2010). Rb is mainly associated with K in the minerals K-Feldspar and biotite etc. Whereas, the Sr is mainly associated with Ca-Minerals such as limestones and Ca-bearing silicates (hornblende, plagioclase and picrate) (Desch, 1969). Among these, Ca-Minerals
easily breakdown as compared to K-bearing minerals which leads to fractionate the K-Rb and Ca-Sr pairs
during the geological processes (Liu et al., 1999; Xu et al., 2010). Thus, the concentrations of K-Rb get
enriched in the residue than the source rocks by increasing of weathering intensity (Brass, 1975; Jin et
al., 2001; Cole et al., 2009). The CIA value of the Upper Continental Crust (UCC) is ~47 and the CIA values
within a range of 45 to 55 indicate absence of chemical weathering in the source area. Upon intensive
weathering, the CIA values of sediments can reach to 100, due to removal of alkalis and alkaline earth
metals (McLennan, 1993; McLennan et al., 1993; Nesbitt and Young, 1982; 1984). In the present study,
the CIA values ranges from 52 to 69 indicating low to moderate weathering in the source region. In
addition, Rb/Sr ratio also follows the CIA trend and ranges between 0.06 and 0.31. Overall, four
increments are noticed in CIA values at various depth intervals (i.e. sample number K3, K13, K22, K29
belong to overbank deposits, River channel and Floodplain environment). The influence of paleoclimates
was assessed by using Al₂O₃+Na₂O+K₂O content against SiO₂ (Suttner and Dutta, 1986). Accordingly, the
sediments of the present study fall on arid climatic zone (Fig.7) except the samples from floodplain-relict
alluvium and calcrete, which is comparable to the prevalent regional climatic variations of south India
(Ramkumar et al. 2019). This further confirms the inference that the sediments are chemically less
matured and were transported through mass erosion-transport event such as major flood.

## 502 Granulometric proxies of fluvial dynamics and habitat shifts/abandonment

Earth's continental landscape is largely shaped by the rivers (Mathew et al., 2016). However, the river patterns that evolve through natural selection and the spatial arrangements of channels are controlled by lithology, slope and structure. Unless influenced and forced by multiple factors including, but not limited to, tectonics, climate and anthropogenic interventions, the river channels persist (Twidale, 2004). There might be changes in channel patterns, including the shifting of locations are on the order of few tens to few thousands of years on a temporal scale (for example, Ramkumar et al., 2018, 2019) to which ancient settlements were also subjected to change. Changes on regional climatological conditions most often reflects on geomorphology in a linear scale (Harrison et al., 2019). The topographic evolution of southern Peninsular India, with its attendant Western Ghat Mountains, in tectonic quiescence since at least ca. 65 Ma was examined by Mandal et al., (2015), who evinced that the steady-state likely reflects the balance between erosion and isostatically driven uplift of the southern Peninsular India. Based on the landscape evolution histories of three major river basins of southern India, Ramkumar et al., (2019), concluded that the region evolved through five stages of tectono-geomorphic changes, among which the Holocene-Anthropocene is the recent. Building on this, the present study has documented further finer-scale changes in the ancient landscape, on a millennial
scale. In this light, the absence of significant trends in the stratigraphic variations of granulometric parameters, except few short-episodic intervals suggest that there were no drastic changes in the provenance and/or sediment supply; rather, the abandonment of habitation-colonization surfaces might have been due to overbank deposition and burial due to sudden, catastrophic floods, perhaps related to out-of-cycle event(s) that were significantly different from annual-seasonal flows. In regions of short-duration, high-intensity rains such as Peninsular India, where the Vaigai River is also part, frequent bank breach and overbank deposition are normal and frequent occurrences (Ramkumar et al., 2015b).

Fig.7.

Sediment transport dynamics, landscape evolution and resultant habitat shifts/abandonments

Clastic sediments are transported to the ocean from the catchments at variable speeds that span from days or weeks, to >10⁵–10⁶ years, because of storage and reactivation *en route* (Clift and Giosan, 2013). Nevertheless, many of the Asian river systems have shown relatively constant mass flux into the ocean during the past 2 Ma due to buffering of sediments in the flood plains (Metivier and Gaudemer, 1999). The sedimentary response to erosion of an upland source region and down-system aggradation occurs at a range of temporal and spatial scales. Over geological timescales, the architecture and granulometry of fluvial successions is dependent on the prevailing tectonic and climatic conditions; however, inverting the sedimentary record for these key parameters is complex (Armitage et al., 2011). Alluvial riverbed elevation responds to the balance between sediment supply and transport capacity, which is largely dependent on climate and its translation into fluvial discharge (Slater and Singer, 2013). In this regard, occurrences of flood deposits slightly-significantly above the current bed-levels of the modern river channel in the Keezhadi and Varusanadu sites unequivocally indicate two paradigms: first, the modern Vaigai River had incised vertically its own alluvial deposit, and second, there were basin-scale downward sediment transport at least thrice namely, post 752 BCE (2435 ± 34 BP), colonization event, 641 CE (1348 ± 27 BP) colonization event and post organic carbon layer deposition event (689 ± 24 BP) among others, and shifting of channel courses to which the ancient settlements were subjected to burial, that are being exhumed by lateral channel shift in Varusanadu and excavation at Keezhadi. The Early Holocene was a period of rapid sediment flux to South Asian deltas (Goodbred and Kuehl, 2000; Giosan et al., 2006). However, it is not clear to what extent this sediment was freshly eroded bedrock rather than older fluvial sediments reworked from the floodplains and terraces in the mountains. Recycling and sorting can also influence the sediment composition which can be predicted based on Th and Zr to Sc ratios. The Th/Sc ratios show minimal variations than Zr/Sc ratios, where selected samples show higher Zr/Sc ratio, which might be indicating heavy mineral enrichment
(i.e. zircon) due to recycling and/or sorting (McLennan et al., 1993). Recycling/sorting had little effect on the river channel sediments (i.e. sample numbers K20, K21 and K23), which is well supported by the enrichment of heavy minerals (i.e., zircon) and thus, enrichment of Zr concentration with higher Zr/Sc ratios (i.e., Zr/Sc = >50). However, on the whole, the sediments are not affected by sedimentary sorting or recycling process and low to moderately affected by weathering.

Previous studies have established the relationship between enhanced erosion and influx of excessive sediments into the trunk channel due to changes in landuse (Ramkumar et al., 2015a, c). These excessive sediments can be flushed out of the channel during extreme flooding (Ramkumar, 2000; 2003) or forming the cause of bank breach and channel course shift (Ramkumar et al., 2015b). Giosan et al., (2012) showed that the monsoonal-fed rivers were active there during the Holocene. As the monsoon weakened, monsoonal rivers gradually dried or became seasonal, affecting habitability along their courses. Our observations in terms of episodic stream avulsion, marooning of ancient habitation sites/surfaces under flood deposits, and subsequent abandonment of habitation sites also evince strong relationship between tectono-climatic factors on the river systems and thus, on the survival-abandonment of ancient habitation sites, especially in the plains of the Vaigai River. The ancient city of Madurai in southern India has been under habitation for many tens of centuries. This city found its mention in the writings of Magasthenes (who visited the city during 300 BCE), Pliny the Younger (61-112 BCE), and Ptolemy (90-168 BCE) and Indian chroniclers such as Kautila's Arthashastra (370-283 BCE), and Sangam Literature (1200-100 BCE). Since then, various parts of the Madurai continue to be inhabited along the Vaigai River until Recent and our study attests to the continuous habitation since microlithic age (Plate I). The Sangam literature also mentions about the havoc caused on agricultural fields and ancient settlements by periodic flooding of the Vaigai River and resultant mitigation efforts and shifting of settlements of kings of yore.

Flood chronologies developed across the world have shown that even minor changes in the climatic patterns can be associated with more frequent occurrences of extreme flood events (Kaleet al., 2010; Knox, 2000). Independent studies of Kale et al., (2010), Ramkumar et al., (2018) and Goswami et al., (2019) documented the flood deposits of the Cauvery River and found distinct flood clusters occurred during the times of major shifts in the monsoon climate, from fluvial dormancy to sudden outburst of monsoons (~2 ka), from warmer to colder (onset of 'Little Ice Age' (LIA) ~ 14th century), from colder to warmer (end of LIA ~ 19th century) and ~20th century, indicating that climatic pattern can be associated with more frequent occurrences of extreme flood events. The study also shows that the two major flood events of the 20th century reported from the upper Cauvery were produced by high intensity
short duration storms, suggesting that not all wet phases are associated with major floods and all dry phases with low floods/droughts. Documentation of flood deposits in the studied sites located in the Vaigai River, from upper to downstream regions and associated shifts in river channel affirm the dominance of climatic reversals on the development and destruction of ancient settlements of the Vaigai Civilization, as could be found elsewhere including India (eg. Ponton et al. 2012; Giosan et al. 2012, 2018; Giesche et al. 2019).

On a geological, pre-historic and historic scale, the Vaigai River was a dryland river, the dynamics of which might be different from that of rivers flowing on humid regions. According to Tooth (2000), the dryland rivers have unique fluvial environments, hillslope and channel hydrological and sediment transport processes, river pattern and geometry, temporal and spatial aspects of channel change, sedimentary structures and bedforms that contrast with more humid fluvial environments. The dryland rivers experience extreme temporal and spatial variability of rainfall, runoff and sediment transport, and poor integration between tributary and trunk channels. Above all, large floods control the channel morphology of the dryland rivers. This uniqueness might have played a significant role in channel avulsion in the downstream reaches of the Vaigai River and played spoilsport on ancient settlements. The occurrences of flood events at about 200-300 year intervals as indicated by flood layers post 752 BCE (2435 ± 34 BP) colonization event, 641 CE (1348 ± 27 BP) colonization event and post organic carbon layer deposition event (689 ± 24 BP) concur with the findings of Zhao et al., (2015) who have documented variations in low-latitude monsoon precipitation driven by shifts in the mean position of the intertropical convergence zone (ITCZ), which is further mediated by solar activity and tropical SSTs. These authors have also stated that the Asian Southern Monsoon has 220-year cyclicity concordant with the dominant cycle of solar activity (Zhao et al., 2015). However, in order to ascertain this presumption, very-high resolution age control is required, which in turn is in progress.

## CONCLUSIONS

According to the available age constraints, the first habitation-colonization in the Vaigai River Basin might have been prior to about 752 BCE (2435 ± 34 BP), the second habitation-colonization might have been during 641 CE (1348 ± 27 BP). The third and most recent habitation-colonization might have been during post 689 ± 24 BP and the top soil is intensively disturbed anthropogenically, including that of gold enrichment in the soils. Among the floods and channel avulsions recognized by the present study, the second flood might have been catastrophic and sudden, as indicated by marooning, destruction and burial of ancient ritualistic sacrificial site and ceramic industry. On a related note, the dates provided by the present estimates with an oldest age of 752 BCE, relatively at shallower depths
614 indicate well-entrenched and mature urban, industrial, social and religious culture in the Vaigai River
615 Civilization, comparable with Indus Civilization. Urbanism in the Vaigai archaeological sites are indicated
616 by the graffiti in pot shreds, gold ornaments, glass beads of all hues, chess coins and other play-
617 paraphernalia, utensils for weaving, dye industry, ceramic industry, steel industry, multiple burial sites
618 and ritual methods, systematic and dense arrangements of backed clay-lined wells, settlements, all of
619 which occur along the Vaigai River channel strewn over hundreds of km², including that of port city. It
620 can be safely concluded that the civilization at Vaigai commenced during the microlithic age (at about
621 10000 BP) and continued to evolve and developed settlements, industries, temples/sites of worships of
622 various kinds, burial grounds, structured cities and commercial places over time. These developed along
623 the contemporary banks of the Vaigai River, as the river had shown avulsion under the influences of
624 episodic tectonic activism, climatic reversal, associated flow conditions and resultant variations in
625 sediment influx, transport, changes in depositional sites and topography. Thus, the Vaigai River
626 Civilization could be considered as one of the oldest and continually inhabited sites starting from the
627 mid-channel until confluence and also that requires extensive, large scale, systematic study and
628 documentation. Occurrences of similar sites in other adjacent river basins have also been documented
629 through this study, that reinforce the premise of control of climate-tectonics on a regional scale and
630 potential use of fluvial dynamics and ages of climate reversals to precisely explore buried archaeological
631 sites.

632 ACKNOWLEDGEMENTS

633 Authors thank the editor of this journal and two anonymous reviewers for their inquisitive,
634 critical comments and constructive criticisms that helped to improve the manuscript. The authorities of
635 Periyar University and the Department of Archaeology, State Government of Tamil Nadu are thanked for
636 according necessary administrative permission to conduct field investigations and sampling campaigns.
637 Few of the photographs of artifacts are recovered by the archaeological department personnel who
638 accompanied senior authors during field survey. The MoU's signed between the Periyar University and
639 the University of South Brittany, France, Curtin University Sarawak, Malaysia and Universiti Teknologi
640 Petronas facilitated collaborative work and the authors are thankful for the same. Authors are also
641 thankful to IUAC for extending AMS facility, an institution funded by the Ministry of Earth Sciences,
642 Government of India.

643 DECLARATION OF INTEREST STATEMENT

644 The authors declare no conflict of interest.

645
646 REFERENCES

647 Armitage, J.J., Duller, R.A., Whittaker, A.C. and Allen, P.A., 2011 Transformation of tectonic and climatic
648 signals from source to sedimentary archive. Nature Geoscience 4,231-235.

649 Brivio, P.A., Pepe, M. and Tomasoni, R., 2000 Multispectral and multiscale remote sensing data for
650 archaeological prospecting in an alpine alluvial plain. Journal of Cultural Heritage 1, 155-164.

651 Chen, Z., Zong Y., Wang Z., Wang, H. and Chen, J., 2008 Migration patterns of Neolithic settlements on
652 the abandoned and Yellow and Yangtze River deltas of China. Quaternary Research 70, 301-314.

653 Clift, P.D. and Giosan, L., 2013 Sediment fluxes and buffering in the post-glacial Indus Basin. Basin
654 Research 25, 1-18.

655 Day, J.W., Gunn, J.D., Folan, W.J., Yanez-Arancibia, A. and Horton, B.P., 2007 Emergence of complex
656 societies after sea level stabilized. EOS, Transactions, American Geophysical Union 88, 169-170.

657 Friedman, G.M., 1967 Dynamic processes and statistical parameters compared for size frequency
658 distribution of beach and river sands. Journal of Sedimentary Research 37, 327-354.

659 Giesche, A., Staubwasser, M., Petrie, C.A. and Hodell, D.A., 2019 Indian winter and summer monsoon
660 strength over the 4.2 ka BP event in foraminifer isotope records from the Indus River delta in the
661 Arabian Sea. Climatic of the Past 15, 73-90.

662 Giosan, L., Clift, P.D., Blusztajn, J., Tabrez, A., Constantinescu, S. and Filip, F., 2006 On the control of
663 climate- and human-modulated fluvial sediment delivery on river delta development: the Indus. EOS
664 Trans. American Geophysical Union 87, 14-24.

665 Giosan, L., Clift, P.D., Macklin, M.G., Fuller, D.Q., Constantinescu, S., Durcan, J.A., Stevens, T., Duller,
666 G.A.T., Tabrez, A.R., Gangal, K., Adhikari, R., Alizai, A., Filip, F., Van Laningham, S. and Syvitski, J.P.M.,
667 2012 Fluvial landscapes of the Harappan civilization. Proceedings of the National Academy of
668 Sciences 109, E1688-E1694.

669 Giosan, L., Orsi, W.D. Coolen, M., Wuchter, C., Dunlea, A.G., Thirumalai, K., Munoz, S.E., Clift, P.D.,
670 Donnelly, J.P., Galy, V. and Fuller, D.Q., 2018 Neoglacial climate anomalies and the Harappan
671 metamorphosis. Climatic of the Past Discuss 1-30. https://doi.org/10.5194/cp-2018-37

672 Glaister, R.P., Nelson, H.W., 1974 Grain size distribution: an aid in facies classification. Bulletin of
673 Canadian Petroleum Geology 22, 203-240.

674 Goodbred, S.L. and Kuehl, S.A. 2000 The significance of large sediment supply, active tectonism and
675 eustasy on margin sequence development; Late Quaternary stratigraphy and evolution of the
676 Ganges-Brahmaputra delta. Sedimentary Geology 133, 227-248.

677 Goswami, K., Rawat, M., Jaiswal, M.K. and Kale, V.S., 2019 Luminescence chronology of late-Holocene
678 palaeofloods in the upper Kaveri basin, India: An insight into the climate-flood relationship. The
679 Holocene 29, 1094-1104. https://doi.org/10.1177/0959683619831436

680 GSI, 1975 Geology of parts of Madurai, Nilakkottai and Tirumangalam Taluks, Madurai District, Tamil
681 Nadu. SROGSI 5428, Geological Survey of India.

682 Harrison, S., Mighall, T., Stainforth, D.A., Allen, P., Macklin, M., Anderson, E., Knight, J. and Mauquoy, D.,
683 2019 Uncertainty in geomorphological responses to climate change. Climatic Change 156, 69-86.
684 https://doi.org/10.1007/s10584-019-02520-8

685 Heritage, G.L., Large, A.R.G., Moon, B.P. and Jewitt, G., 2004 Channel hydraulics and geomorphic effects
686 of an extreme flood event on the Sabie River, South Africa. Catena 58, 151-181.

687 Kale, V.S., Achyuthan, H, Jaiswal, M.K. and Sengupta, S., 2010 Palaeoflood records from upper Kaveri
688 River, southern India: evidence for discrete floods during Holocene. Geochronometria 37, 49-55.

689 Knox, J.C., 2000 Sensitivity of modern and Holocene floods to climate change. Quaternary Science
690 Reviews 19, 439-457.

691 Longin, R. 1971 New Method of Collagen Extraction for radiocarbon Dating. Nature 231, 241-242.
692 Mandal, S.K., Lupker, M., Burg, J., Valla, P.G., Haghipour, N. and Christl, M., 2015 Spatial variability of
693 $^{10}$Be-derived erosion rates across the southern Peninsular Indian escarpment: A key to landscape
694 evolution across passive margins. Earth and Planetary Science Letters 425, 154-167.

695 Mathew, M.J., Menier, D., Siddiqui, N. A., Ramkumar, M., Santosh, M., Kumar, S. and Hassan, M., 2016
696 Drainage Basin and Topographic Analysis of a Tropical Landscape: Insights into Surface and Tectonic
697 Processes in northern Borneo. Journal of Asian Earth Sciences 124, 14-27.

698 Mehdi, S.M., Pant, N.C., Saini, H.S., Mujtaba, S.A.I. and Pande, P., 2016 Identification of Palaeochannel
699 configuration in the Saraswati river basin in parts of Haryana and Rajasthan, India, through digital
700 remote sensing and GIS. Episodes 39, 29-38. http://doi.org/10.18814/epiiugs/2016/v39i1/89234

701 Mehrotra, N., Shah, S.K., Basavaiah, N., Laskar, A.H. and Yadava, M.G., 2019 Resonance of the '4.2ka
event' and terminations of global civilizations during the Holocene, in the palaeoclimate records
702 around PT Tso Lake, Eastern Himalaya. Quaternary International 507, 206-216.

703 Menier, D., Manoj, M.J., Pubellier, M., Sapin, F., Delcaillau, B., Siddiqui, N., Ramkumar, M. and Santosh,
704 M., 2017 Landscape response to progressive tectonic and climatic forcing in NW Borneo:
705 Implications for geological and geomorphic controls on flood hazard. Nature Geoscience Reports
706 7.http://doi.org/10.1038/s41598-017-00620-y

708 Mètivier, F. and Gaudemer, Y., 1999 Stability of output fluxes of large rivers in South and East Asia
during the last 2 million years; implications of floodplain processes. Basin Research 11, 293-303.

710 Moiola, R.J. and Weiser, D. (1968) Textural parameters: an evaluation. Journal of Sedimentary Research
711 38, 45-53.

712 Nigam, R. and Chaturvedi, S.K., 2006 Do inverted depositional sequences and allochothonous
713 foraminifers in sediments along the coast of Kachchh, NW India, indicate palaeostorm and/or
714 tsunami effects? Geo-Marine Letters 26, 40-52.

715 Nigam, R., Dubey, R., Saraswat, R., Sundaresh, A., Gaur, A.S. and Loveson, V.J., 2016 Ancient Indians
716 (Harappan settlement) were aware of tsunami/storm protection measures: a new interpretation of
717 thick walls at Dholavira, Gujarat, India. Current Science 111, 2040-2043.

718 Orengo, H.A. and Petrie, C.A., 2017 Large-scale, multi-temporal remote sensing of palaeo-river
719 networks: A case study from Northwest India and its implications for the Indus civilisation. Remote
720 Sensing 9, 9070735. http://doi.org/10.3390/rs9070735

721 Passega, R., 1957 Texture as characteristic of clastic deposition. The American Association of Petroleum
722 Geologists Bulletin 41, 1952-1984.

723 Ramkumar, M., 2000 Recent changes in the Kakinada spit, Godavari delta. Journal of Geological Society
of India 55, 183-188.

725 Ramkumar, M., Sudharani, P, Gandhi, M.S., Pattabhiramayya, M., Rajanikumari, V., Bhagavan, K.V.S. and
726 Swamy, A.S.R., 2000 Textural characteristics of coastal sedimentary environments of the modern
727 Godavari delta.Journal of Geological Society of India 56, 471-487.

728 Ramkumar, M., 2001 Sedimentary environments of the modern Godavari delta: characterization and
729 statistical discrimination towards computer assisted environment recognition scheme. Journal of
730 Geological Society of India 57, 49-63.

731 Ramkumar, M., 2003 Progradation of the Godavari delta: A fact or empirical artifice? Insights from
732 coastal landforms. Journal of Geological Society of India 62,290-304.

733 Ramkumar, M., 2007 Spatio-temporal variations of sediment texture and their influence on organic
734 carbon distribution in the Krishna estuary. Indian Journal of Geochemistry 22,143-154.

735 Ramkumar, M., 2009 Flooding - A manageable geohazard. In: Ramkumar, M., (Ed.) Geological hazards:
causes, consequences and methods of containment. New India Publishers, 177-190.

737 Ramkumar, M., Kumaraswamy, K. and Mohanraj, R., (Eds.) 2015a Environmental management of river
basin ecosystems. Springer-Verlag 761. http://doi.org/10.1007/978-3-319-13425-3.
739 Ramkumar, M., Kumaraswamy, K., Arthur James, R., Suresh, M., Sugantha, T., Jayaraj, L., Mathiyalagan, Saraswathi, M. and Shyamala, J., 2015b Sand mining, channel bar dynamics and sediment textural properties of the Kaveri River, South India: Implications on flooding hazard and sustainability of the natural fluvial system. In: Ramkumar, M., Kumaraswamy, K. and Mohanraj, R., (Eds.) Environmental management of river basin ecosystems. Springer-Verlag14. 283-318. http://doi.org/10.1007/978-3-319-13425-3.

745 Ramkumar, M., Kumaraswamy, K. and Mohanraj, R., 2015c Land use dynamics and environmental management of River Basins with emphasis on deltaic ecosystems: Need for integrated study based development and nourishment programs and institutionalizing the management strategies. In: Ramkumar, M., Kumaraswamy, K. and Mohanraj, R., (Eds.). Environmental management of River Basin ecosystems. Springer-Verlag, Heidelberg. DOI 10.1007/978-3-319-13425-3-1. pp.1-20.

750 Ramkumar, M., Menier, D., Manoj, M.J., and Santosh, M., 2016 Geological, geophysical and inherited tectonic imprints on the climate and contrasting coastal geomorphology of the Indian peninsula. Gondwana Research 36, 52-80.

753 Ramkumar, M., Kumaraswamy, K., Balasubramani, K., Abdul Rahaman, S., Jegankumar, R., Kumara Ganesh, U., Manikandan, E., Balasundareshwaran, A., Prithiviraj, G., Ayyanthurai, R., Santosh, M., Nagarajan, R., Siddiqui, N.A., Menier, D., Mathew, M.J., Ramachandran, C., Sathyaseelan, M. and Sivakumar, P., 2018 Discovery of buried historical structures through integrated geological and geophysical techniques in the Kaveri-Kollidam interfluve, India: Implications for morpho-tectonics and archeological exploration. Archaeological Prospection 26, 58-73.

759 Ramkumar, M., Santosh, M., Abdul Rahaman, S., Balasundareshwaran, A., Balasubramani, K., Mathew, M.J., Sautter, B., Siddiqui, S.N., Usha, K., Sreerhishiya, K., Prithiviraj, G., Nagarajan, R., Thirukumaran, V., Menier, D. and Kumaraswamy, K., 2019 Tectono-morphological evolution of the Cauvery, Vaigai and Thamirabarani river basins: Implications on timing, stratigraphic markers, relative roles of intrinsic and extrinsic factors and transience of Southern Indian landscape. Geological Journal 54, 2870-2911. https://doi.org/10.1002/gj.3520.

765 Rao, S.R., 1991 Dawn and devolution of the Indus civilization. Aditya Prakashan, 392.

766 Roy, A.B. and Sethumadhav, M.S., 2014 The ‘mystic’ sand dune-covered temples of Talakad, Mysore district, Karnakata: evidence of earthquake-related destruction. Current Science 107, 246-254.

768 Roy, A.B., Bhu, H., Sharma, P. and Vaishnav, K., 2017 Deserted nineteenth century Paliwal villages around Jaisalmer, western Rajasthan, India: historical evidence of palaeoseismicity. Current Science 112, 402-405.

771 Sahu, S., Raju, N.J. and Saha, D., 2010 Active tectonics and geomorphology in the Sone-Ganga alluvial tract in mid-Ganga Basin, India. Quaternary International 227, 116-126.

773 Santosh, M., Tsunogae, T., Yang, C.X., Han, Y.S., Hari, K.R., Prasanth, M.M. and Uthup, S., 2020. The Bastar craton, central India: A window to Archean-Paleoproterozoic crustal evolution. Gondwana Research, 79, pp.157-184.

776 Similox-Tohon, D., Sintubin, M., Muchez, P., Verhaert, G., Vanneste, K., Fernandez, M., Vandycke, S., Vanhaverbeke, H. and Waelkens, M., 2006 The identification of an active fault by a multidisciplinary study at the archaeological site of Sagalassos (SW Turkey). Tectonophysics 420, 371-387.

780 Sharma, R., Umapathy, G.R., Kumar, P., Ojha, S., Gargari, S., Joshi, R., Chopra, S. and Kanjilal, D., 2018 AMS and upcoming geochronology facility at Inter University Accelerator Centre (IUAC), New Delhi, India. Nuclear Instrumentation Methods B, 438, 124-130.

783 Singh, A., Thomsen, K.J., Sinha, R., Buylaert, J.P., Carter, A., Mark, D.F., Mason, P.J., Densmore, A.L., Murray, A., Jain, M., Paul, D. and Gupta, S., 2017 Counter-intuitive influence of Himalayan river morphodynamics on Indus civilization urban settlements. Nature Communications 8, http://doi.org/10.1038/s41467-017-01643-9.
787 Slater, L.J. and Singer, M.B., 2013 Imprint of climate and climate change in alluvial riverbeds: Continental

788 United States, 1950-2011. Geology 41, 595-598.

789 Sridhar, A., 2007 A mid-late Holocene flood record from the alluvial reach of the Mahi river, Western

790 India. Catena 70, 330-339.

791 Stewart, H.B. Jr., (1958) Sedimentary reflections on depositional environment in San Migne lagoon, Baja

792 California, Mexico. The American Association of Petroleum Geologists Bulletin 42, 2567-2618.

793 Stuiver, M. and Polach, H.A., 1977 Discussion: reporting of C data. Radiocarbon, 19, 355-363.

794 Tooth, S., 2000 Process, form and change in dryland rivers: A review of recent research. Earth-Science

795 Reviews 51, 67-107.

796 Twidale, C.R., 2004 River patterns and their meaning. Earth-Science Reviews 67, 159-218.

797 Valdiya, K.S., 2008 Sinking of ancient Talakad temples on the Kaveri Bank, Mysore Plateau, Karnataka.

798 Current Science 95, 1675-1676.

799 Van Dijk, W.M., Densmore, A.L., Singh, A., Gupta, S., Sinha, R., Mason, P.J. and Rai, S.P., 2016 Linking the

800 morphology of fluvial fan systems to aquifer stratigraphy in the Sutlej-Yamuna plain of northwest

801 India. Journal of Geophysical Research: Earth Surface 121, 201-222.

802 http://doi.org/10.1002/2015JF003720.

803 Wulf, H., Bookhagen, B. and Scherler, D., 2010 Seasonal precipitation gradients and their impact on

804 fluvial sediment flux in the Northwest Himalaya. Geomorphology 118, 13-21.

805 Zhao, K., Wang, Y., Edwards, R.L., Cheng, H., Liu, D. and Kong, X., 2015 A high-resolved record of the

806 Asian summer monsoon from Donggecave, China for the past 1200 years. Quaternary Science

807 Reviews 122, 250-257.

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833 **Fig.1. Location of the study area and the recognized paleochannel courses.** A: Location of the study area within the Vaigai River Basin, embedded on a Digital Elevation Model. It also depicts the inherited nature of the river basin morphology. B: Inset shows the location of the region within the Vaigai River Basin that was subjected to image processing to recognize paleochannels and the locations wherein natural exposures were examined and excavations were made. Yellow lines show the paleochannels recognized through conventional image interpretation. C: Panels showing the paleochannel courses and the location/region selected for image processing, geophysical exploration and excavation. D: E: and F: Panels showing processed images and the paleochannel courses.

842 **Fig.2. Thematic information on the Vaigai River Basin and the studied sites.** A: Regional geology and structure. The tectonic line is aligned NE. As could be observed in all other rivers flowing east in the Peninsular India, the deltaic plains of the Vaigai River are also located on the east of this tectonic line. This tectonic line and other regional structures are Proterozoic structures and are active until recent (Ramkumar et al., 2016, 2017, 2018). B: Soil types present in the Vaigai River Basin and their spatial distribution. Note the occurrences of loamy soils only in the upper part of the basin and their relative absence in the middle and lower parts of the basin. C: Regional geomorphic features and their distribution within the basin. Note the occurrences of relict landforms that are inherited by the modern river channel. D: Regional slope characteristics of the Vaigai River Basin. The map depicts clearly the inherited landscape by the modern river channel. E: Landuse/landcover characteristics of the Vaigai River Basin. F: Rainfall distribution within the Vaigai River Basin.

854 **Fig.3. Composite litholog, lithofacies and depositional characteristics of the studied sections.** Based on the field logging of natural exposures and excavated pits, a composite litholog was constructed. The lithofacies characteristics, contact relationships and other criteria including depositional environments were interpreted and are presented in this figure. From the bottom, the litholog depicts the initiation of channel incision over former relict alluvium and its transition into river channel and into overbank deposit and later into sandy loam-flood plain (lithofacies types k-e). Again, sandy loam-flood plain deposits is followed by overbank deposit and finally modern flood plain (lithofacies types d-a) occurs. While the natural levee/overbank deposits were habitation sites/colonized by the ancient human population, the overbank deposits marooned them under sediments and forced shifting of settlements, as a consequence of stream avulsion events.

864 **Fig.4. Stratigraphic variations of textural properties, selected geochemical composition, ratios and indices of the sediments.** Mz: Mean size. SD: Standard Deviation. SK: Skewness. KG: Kurtosis. SiO₂ and TiO₂ are in wt. %; WIP: Weathering index of plagioclase, expressed in %; CIA: Chemical index of alteration, expressed in %. Note the ubiquitous monotony in all the textural parameters, despite representation of few thousand years of depositional history, punctuated with stream avulsion (note the general fining trend at the bottom of mean size and corresponding changes in all other textural characteristics). The overland flow followed by significant change in grain size is distinct and is also represented fairly in standard deviation. Also note that though the absolute values of textural characteristics show changes, the long-term trend represented by the linear
873 curve is more or less stable/steady signifying stable tectonic setting of the depositional sites. The
874 changes and trends observable in absolute value curves and the polynomial curves may, at the
875 best, reflect the climatic changes in the basin.

876 **Fig.5. Discriminant diagrams of depositional environment, energy, setting and transport modes.**
877 A: Discriminant diagram of River-Beach environments. The figure shows that all the samples fall
878 far away from both the discriminant lines of Moiola and Weiser (1968) and Friedman (1967), yet
879 in a scattered fashion. This suggests ubiquitous riverine nature of the sediments, however under
880 variable physical environmental settings and or energy conditions. B: Discriminant diagram of
881 River-Wave-Quite environments based on Stewart (1958). This figure shows that almost all the
882 samples fall in a region between River process and slow deposition from quite water, perhaps
883 suggestive of ephemeral nature of the river flow, variable energy conditions of transport and
884 deposition, an observation that is in conformity with the observations on stratigraphic variations
885 of textural properties, especially mean size (Fig.4A) and standard deviation (Fig.4B). C: Distinction
886 of depositional environments based on Glaister and Nelson (1974) shows the sediment samples
887 studied belong to middle deltaic plain, showing moderate maturity, and are deposited in braided
888 bar (an affirmation of stream avulsion character), point bar etc. D: Discrimination of modes of
889 sedimentary transport based on Passega (1957). Deposition of majority of the sediment samples
890 studied through rolling and suspension mode, a mode of transport normally possible during
891 flooding, in combination with the repetitive inferences based on lithofacies characteristics that
892 indicated natural levee and overbank deposit. Together, these suggest episodic flooding as a
893 result of off-seasonal and or catastrophic flooding due to climatic changes and consequent stream
894 avulsion events.

895 **Fig.6. Plot of Al/Na Vs CIA for interpreting weathering climate/intensity.** A range of weathering
896 characteristics are indicated by this plot as the samples show a spread across low-intermediate
897 silicate weathering, perhaps under the influence of dry-arid climate prevalent.

898 **Fig.7. Climatic interpretation based on SiO₂ Vs Al₂O₃+K₂O+Na₂O.** This plot corroborates with the
899 inference of shallow, monsoon-influenced ephemeral channels that received flows only for short
900 durations and also that they shifted their courses whenever extreme climatic conditions were
901 prevalent that forced habitats to get marooned/buried/shifted as all, if not most of the samples
902 fall within the arid climate field and show a range of chemical maturity.

903
904
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906
907 TABLE
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**Table 1. Radiocarbon ages represented in before present (BP) and calibrated ages (in CE and BCE).** The
serial nos. 1-8 were measured at AIMS facility in New Delhi and serial nos. 9 and 10 were
measured at Beta Analytic, U.S.A.
915 PLATES

916 Plate I.

917 a. Field photograph showing the backed clay ring lined dug well, backed brick paved floor and wall
918 in a pit excavated at Keezhadi. b. Close-up view of a backed clay ring lined dug well found in a pit
919 excavated at Keezhadi. Very close spacing between successive wells, variously sized diameters of
920 the wells and variously shaped and sized backed clay rings used for lining the dug wells suggest,
921 individual ownership and usages of these wells, shallow depth of water table, and individualistic
922 designs and productions of clay rings, custom-design and/or manufacturer-design or both
923 indicative of market-availability of a variety of produce which in turn point to a mature market,
924 producer-customer relationship. c. Dug well located in flood-paleochannel deposit at
925 Pasiyapuram. While all the dug wells found in Keezhadi and elsewhere were lined with backed
926 clay rings that have had perfect fitting shape one over other, the gaps between successive rings
927 were found filled with loamy clay at this location perhaps to enforce impermeability. This could
928 also mean that while in other locations, the dug wells were found dug in overbank and natural
929 levee sediments that are relatively clay rich and cohesive, the Pasiyapuram well was dug in loose,
930 unconsolidated channel and flood deposit, which might have been leaky. At this instance, while
931 the backed clay rings could have provided protection against well collapse, but not impermeability
932 and or contamination of percolating water. In order to avoid these, the well might have been
933 plastered with 25cm thick clay in the gaps between successive rings. This observation suggests the
934 knowledge level of the humans inhabited at this location. d. A globular shaped backed pot with
935 very narrow (1.5cm dia) opening perhaps produced for a specific industrial purpose. The gold
936 enrichment in surface soils at this level is multifold, suggestive of usage of this ceramic for
937 industry. e. A large, intact, perfectly and aesthetically crafted pot, perhaps used for storage. f. and
g. Black and red colored backed clay pots, with fish markings engraved on them and a Tamizh
938 brahmi name "kuviran atha" written on them. The fish insignia is the official insignia of the
939 Pandiya dynasty that had ruled the entire south India. Having insignia and name inscribed on pots
940 could indicate individual ownership and or official purposes to distinguish the vessels from others.
941 This practice also suggests the mature culture prevalent. h. Filters of various sorts and sizes made
942 out of backed clay, perhaps used to filter water and or industrial purposes. i. Bottom half of a pot
943 with largest diameter (65cm) size found ever in this part of the country. In addition to its size, its
944 character being black and Red colored ware adds value to this pot and might have been used for
945 storage, however, not for regular domestic purpose. j. A collapsed hearth with evidences of
946 burned soil and pot shreds. k. A series of horizontally arranged conical/tapered shaped backed
947 clay pipeline aligned over as series of horizontally arranged larger barrel shaped backed clay
948 pipeline, culminating at larger vertically aligned larger vessel made of backed clay. The
949 conical/tapered shaped backed clay pipeline has backed clay filters as shown in "h'. By all
950 probability, this arrangement might be for a dye industry and or gold jewelry industry, as the
951 coeval soil samples show multifold increase in Au enrichment than the background values. l. m. n.
952 o. p. s. and t. Burial urns were found with aesthetic epirelief features. The urns are multifold
953 thicker than all other pot shreds and contain three different types of human bone remains viz.,
954 charred/burned bone remains without any skull and or complete skeletal remains, bone remains
955 with other utensils/artifacts, perhaps used by the person who might have used and complete
956
957 bone remains including skull, jaw apparatus, etc. These three types of burial characters indicate at
958 least three different cultural traits and or cultural groups that inhabited the region, almost
959 simultaneously, as these are found at similar stratigraphic levels. q. Vertebrate bone remains
960 found in a flood deposit. The sediments along with the bones might have been eroded during
961 upstream regions and deposited in downstream overbank/flood deposit during stream avulsion
962 associated with catastrophic flooding. r. Bone remains of bovine along with partially
damaged/broken horn. Taphonomic arrangements of the bones and their broken edges suggest
963 sacrificial ceremony and offering to a local deity of yore. It is a custom being followed since
964 prehistoric time, Sangam era (to which the findings belong) and until today by certain
965 communities to offer bovine sacrifice to ‘ugra devata’ (ferocious deities, such as Kali).
966 u. Microlithic stone tool, perhaps used as a small handy blade found at Maranadu. This finding,
967 together with the carbon isotope dates and existences of temples at Sakkudi (aged close to a
968 thousand years ago), habitation sites at Agaram (aged about 600-700 years BP) and existence of
969 modern city of Madurai, suggest continuous habitation in the Vaigai River plains since at least
970 10000 yr BP until today. v. Glass beads of various colors found everywhere in Maranadu,
971 Keezhadi, Kondagai, Pasiyapuram, etc. The sizes of these beads are all uniform <1.5mm and all the
972 beads have a uniform sized hole in them, suggestive of they were made to make
973 garland/necklace, etc. This also suggest the mastery of technology that existed to make multiple
974 colored glass beads, as they required various raw materials containing different metal oxides. w. A
975 piece of bangle made of elephant tusk. This may indicate the prevalence of wealthy society. x. y.
976 z. and ai. Statues and parts of sculpted (mythical creatures “yali”) pillars of a temple that
977 remained buried under second flood deposit vide Fig.3 and now excavated. This temple is located
978 at Sakkudi, near Silaiman, east of the Tectonic line vide Fig.2A. aii. Statue partly exposed at
979 Agaram, a location wherein buried temple and associated houses aligned in rows are detected
980 and excavations are yet to begin. aiii. Statue of a Jain deity/high-priest, found at Maranadu. Initial
981 assessment of age of this statue is about 1400 yr BP. A buried temple might be at the place this
982 statue is found and needs affirmation through excavation. aiv. Statue of a local deity (Sasta with
983 his consorts?), found at Keezhadi, perhaps worshiped by the ancestors and is being continued
984 until today.
985
<figure><img src="image_4.png" /></figure>

1 NEOLITHIC CIVILIZATION-CULTURAL SITES AND EXTREME CLIMATE RELATED CHANNEL AVULSION:

2 EVIDENCE FROM THE VAIGAI RIVER BASIN, SOUTHERN INDIA

3 Mu.Ramkumar¹, K.Kumaraswamy², K.Balasubramani³, R.Nagarajan⁴, M.Santosh⁵,⁶, S.Abdul

4 Rahman², K.Arun Prasad³, Juni, K.J¹., AL Fathima¹, N.A. Siddiqui⁷, M.J.Mathew*⁸, D.Menier⁹,

5 B. Sautter⁷, Rajveer Sharma¹⁰, Pankaj Kumar¹⁰, Sundeep Chopra¹⁰, R.Jegankumar²

6 ¹Department of Geology, Periyar University, Salem, India.

7 ²Department of Geography, Bharathidasan University, Tiruchirappalli, India.

8 ³Department of Geography, Central University of Tamil Nadu, Thiruvarur, India.

9 ⁴Department of Applied Geology, Curtin University Sarawak, Malaysia.

10 ⁵School of Earth Sciences and Resources, China University of Geosciences Beijing, Beijing, China.

11 ⁶Department of Earth Sciences, University of Adelaide, Adelaide, SA, Australia.

12 ⁷Department of Petroleum Geoscience, Universiti Teknologi Petronas, Malaysia.

13 ⁸Southeast Asia Carbonate Research Laboratory, Universiti Teknologi Petronas, Malaysia.

14 ⁹Laboratoire Géosciences Marines et Géomorphologie du Littoral, Université de Bretagne Sud, France.

15 ¹⁰Inter University Accelerator Centre, New Delhi, India

16 ¹⁰*Corresponding Author.

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We document evidence for the role of fluvial dynamics-stream channel avulsion and flooding, tectonics and climatic conditions on the occurrence, shifting and burial of ancient civilization-cultural sites in the Vaigai River Basin in southern India. Systematic regional mapping, geophysical investigations and archaeological excavations in the flood plains of the Vaigai River led to the unearthing of an ancient civilizationculture that thrived at least since 752 BCE (2435 ± 34 BP). Evidence for early human activities include sites of ancient and now buried industries that were manufacturing steel, glass bead, dye, gold jewelry and earthenware, in addition to dwelling-habitation, sites of religious sacrificial ceremonies, and places of worship, and burial. These information together with the occurrences of microlithic age stone tools in the vicinity led to the proposition of continued habitation of the Vaigai River Basin until today. Radiometric dating shows oldest age of 2435 ± 34 BP (752 BCE) obtained at 2.5m depth in a pit excavated at Keezhadi, a location situated far from the present day channel course whereas the youngest age of 689 ± 24 BP is obtained at Agaram, a location situated nearest to the present day channel course. The bone fragments were dated as 1115 ± 39 BP (778 CE – 1016 CE) and 1229 ± 30 BP (689 CE – 883 CE). The organic carbon/peat layers recovered between flood deposits at the upper reaches of the Vaigai River are dated as-830 ± 30 BP and -1250 ± 30 BP. The natural levee deposits show evidences of habitation-related disturbances in the internal sedimentary structure, texture and packing. In addition to occurrences of hearth etc., recurrences of overbank-channel deposits over natural levee-habitation surfaces indicate habitation-colonization over relatively highgrounds along river bank, followed by marooning/burial under overbank-channel deposit and abandonment of habitation site and / or shifting of habitat. The same sequence of events occurred again, showing two habitation surfaces that are buried under surface currently. Together with facies characteristics and other artifact occurrences, the ages obtained in our study indicate shifting of the Vaigai River from prior to 2435 BP and colonization of ancient population, which might have been affected by a catastrophic flood and stream avulsion at about 1250-830 BP. These resulted in the ancient habitation getting marooned, industries and other structural infrastructure destroyed, and were transported and deposited as interbeds in flood deposits. Following a reduction in the stream energy, the point bar-natural levee environmental setting paved way for the deposition of organic carbon layer at Agaram at about 690 BP. This was followed by lateral shifting of the channel from Agaram towards the present-day course.

**KEY WORDS:** Ancient settlement, Geoarchaeology, Paleoflood, South India, Vaigai civilization
71 INTRODUCTION

72 Unearthing of ancient *civilizational-cultural* sites along buried paleochannels (for example, Indus
73 - Giosan et al. 2012; 2018); Saraswati—Radhakrishna and Merh, 1999; Tripathi et al. 2004; Valdiya, 2013)
74 suggested floodplains and natural levees were among the preferred locations of human settlements
75 (Singh et al. 2017). Marooning and or burial of *civilizational-cultural* sites by various causes including, but
76 not limited to climate change (Mehrotra et al. 2019), sea-level variation (Day et al. 2007; Chen et al.
77 2008), natural hazards such as tsunami (Nigam et al. 2016), coastal storm (Nigam and Chaturvedi, 2006),
78 neomorphotectonics and paleoseismicity (Roy and Sethumadhav, 2014; Similox Tohon et al. 2006;
79 Valdiya, 2008; Roy et al. 2017) and catastrophic floods (Rao, 1991; Kale et al. 2010, Ramkumar et al.
80 2018) are well-known. Examination of paleolithic-historic archaeological sites in Tamil Nadu, southern
81 India suggests that, all the river deltas, especially at the deltahead region contain archaeological sites,
82 and are covered by catastrophic flood deposits (Ramkumar et al. 2016, 2018, 2019). These catastrophic
83 floods were either controlled by periods of intense climate change and/or superimposed on paleo-
84 present seismically active zones. Few of the flood wrecked havoc and associated destruction of ancient
85 habitats found their mention in cave carvings/paintings, stone inscriptions, inscriptions on palm leaves,
86 and copper plates, and Sangam literature (11000-2000 years BP). Tracing the evidence of
87 paleoflood/seismicity and using them as clues to locate and excavate archaeological sites is a potential
88 approach, especially in the rivers of Peninsular India (Ramkumar et al., 2018). The most recent
89 archaeological finds in the Cauvery, the Thamirabarani and the Vaigai river plains are no exception.
90 Thus, the recently excavated Keezhadi site is used as an example to investigate the tectono-climatic
91 factors in forcing the ancient civilization habitants to abandon their habitats, and to evaluate the habitat
92 shifts with climate-tectonic dynamics through applying multi-proxy approach. In this work, we
93 demonstrate that the very landforms that supported human activities and sustenance, wrecked havoc
94 and forced the settled communities to shift or vanish from the habitat due to geohazards such as
95 earthquakes, landslides or flooding (Menier et al., 2017), that buried and/or marooned the settlements
96 causing loss of life and infrastructure. This paper reports the results from our recent studies in
97 unearthing of ancient habitations including, residential, industrial, burial, religious-worship sites, buried
98 under flood deposits of the Vaigai River, southern India. Our study documents the relationships between
99 climatic reversals-sudden and catastrophic flooding, stream avulsion and survival-shifting of ancient
100 human settlements.

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The southern Indian Shield is a mosaic of several crustal blocks with a variety of geological signatures engrained during different times ranging since Achaean (GSI, 1975; Santosh et al., 2020). The Vaigai River (Fig.1) flows on an antecedent valley (Fig.1A), and the directional change is unique, which in turn, was imparted by episodic tectono-morphological events under the influence of inherited Proterozoic structure (Ramkumar et al., 2016, 2019). The regional geology, geomorphology, spatial distributions of soil, slope, land use and rainfall presented in the Figure 2 show that unlike other basins of south India, the Vaigai River Basin maintains its narrow and linear morphology. However, the location of deltahead is at/or nearer to the tectonic line, similar to other rivers (Ramkumar et al. 2016) and suggests a shared/common mechanism for this basin (Ramkumar et al. 2016; 2019) also. Incision of paleo-alluvial fills, development of Miocene-Pliocene Kankar/paleosol, recycling of paleosol and development of Holocene-Recent soil, and Recent-historic changes of river courses are all observed in this basin as a response of episodic tectonic events and short-long term climatic reversals (Ramkumar et al., 2016, 2019). The basin is covered by crystalline rocks of Achaean on the western part, and Tertiary and Quaternary sedimentary sequences on the eastern part (Fig.2). The Quaternary Period witnessed widespread recycling of sediments in this basin. The younger alluvium occurs on either side of the Vaigai River, deposited during Quaternary-Recent floods. The soil of the basin is brown, sandy, with a low fraction of clay and humus. The delta plain is mostly covered by black cotton soil which is admixed with Fe and Ca nodules sourced from paleosols. The upper portion of the basin is covered by structural hills, pediments and valley fills. The lower part of the basin, especially east of the major tectonic line, is covered by paleodeltatic plains, flood plains, paleo and present tidal flats and tanks (https://www.nationalwatermission.gov.in). The Vaigai River Basin lies within the tropical monsoon climate. The average annual rainfall is about 775 mm. The Southwest (SW) monsoon period spanning from June to September and Northeast (NE) monsoon period spanning from October to December bring seasonal rainfall to the basin. However, the Vaigai Basin receives maximum amount of rainfall in a short spell (about 40 days in a year) during NE monsoon season.

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Despite recurrent reports in the mainstream media regarding unearthing of archaeological remains buried under shallow depths the locations of which are strewn all along the Vaigai River course, systematic studies by professional organizations (Archaeological Survey of India and Tamil Nadu State Archaeology department) have begun only recently. In addition, the results have not yet been revealed and or published in any scientific journals. According to commonly prevailing perception, the Vaigai River Basin is one of the examples of continually habitated regions, evidenced by microlithic-historic-
recent settlements, hunting-modern industries, agricultural and other structures. However, these evidences are not systematically recorded, interpreted, scientifically validated, analyzed causative mechanisms/factors that supported origin, sustenance and destruction of habitation sites and corroborated with other known contemporary civilizations elsewhere. This paper reports the initial findings from selected sites in the Vaigai River Basin in terms of shifting of habitation sites and channel avulsion as a result of climate change and natural catastrophes.

Fig.1.

Fig.2.

## 143 METHODS AND MATERIALS

### 144 Delineation of areas of interest through remote sensing and field survey

For effective prospecting, archaeological research takes into consideration the results of the interactions between environmental conditions, morphology, water proximity, slope, soil type, land use, etc., and human activities over time (Brivio et al., 2000). As the geophysical methods are dependent on the terrain conditions such as lithology and types and depths of artifact occurrence, integrating remote sensing techniques with geophysical methods and conventional field mapping is an effective approach (Brivio et al., 2000; Ramkumar et al., 2019) and the same is followed in this study.

A geological map in the scale of 1:50,000, seismo-tectonic and geochronology maps published by Geological Survey of India (http://bhukosh.gsi.gov.in/) were used to prepare structural and geological maps of the basin. Then the thematic layers on various aspects including geomorphology, land use/ land cover and Normalized Difference Vegetation Index (NDVI) were generated with the help of multi-temporal satellite images (IRS-AWIFS, IRS-LISS III and LANDSAT TM) and Geographic Information System software package. SRTM (Shuttle Radar Topography Mission) digital elevation data were used to generate a slope map and drainage networks of the basin. A long-term rainfall data (1980-2014) collected from India Water Portal (www.indiawaterportal.org), were used to generate average rainfall map of the basin. Soil series maps published by National Bureau of Soil Survey and Land Use Planning (NBSS and LUP) were used to prepare soil texture map of the basin. Based on these thematic maps, a narrow strip (Fig.1) was chosen for detailed image processing. It was followed by selection of probable sites for geophysical sounding (Vertical Electrical Sounding -VES and Ground Penetrating Radar -GPR). The survey of India toposheets in the scale of 1:50,000 (58K01 and 58K05) were used to prepare detailed base map for the region of interest. The combined inferences on these helped identify locations wherein excavations could be made.
Using long-term archived satellite data to delineate paleochannel pathway based on long-term vegetation dynamics and image decomposition techniques has been attempted in the present study. The study used surface vegetation characteristics (distribution, and health) as the indicator of presence of paleochannels as a strong correlation exists between vegetation distribution and water availability (Mehdi et al., 2016; van Dijk et al., 2016). The multi-technique seasonal approach using the available range of 30 years (1983-2012) of Landsat data was adopted. The data were divided season-wise (dry and wet months) to calculate Seasonal Multi-Temporal Vegetation Indices (SMTVI), Seasonal Tasselled Cap Transformation (TCT), Seasonal Principal Component Analysis (PCA), and Normalized Difference Vegetation Seasonality Index (NDVSI). The RGB composite generated from three principal components (PCA), three axes of Tasselled Cap Transformed images (TCT) of all bimonthly images, and Seasonal Multi-Temporal Vegetation Index (SMTVI) were used for visual interpretation to delineate paleochannels. The use of long temporal span helped to overcome the errors induced due to seasonal crop-pattern, and land use dynamics that prevailed in the study area. These multi-temporal image derivatives were visually interpreted based on our ground knowledge to derive probable pathways of paleochannels. All these processing were done in Google Earth Engine, a cloud-based computing infrastructure for bulk processing of satellite image data. Through visual interpretation of these datasets, the probable paleochannels and their courses were delineated using vector-based Arc GIS platform. This was followed by field surveys along selected transects that criss-crossed the Vaigai River Basin and the identified paleochannel courses (Fig.1) to delimit the regions of interest for detailed survey and exploration.

## Excavation and sampling

After affirming the presence of man-made structures and artifacts in the sub-surface, through conventional, geophysical and visual interpretation and image processing of remotely sensed data, systematic excavations were conducted at selected locations for the examination of natural and anthropogenic structures, lithofacies characteristics and sediment/soil/artifact sampling. After photographing and logging, sediment samples were collected at 20cm intervals, in addition to artifacts and archaeological remains (Plate I). A total of 30 sediment samples were collected for granulometric, geochemical, mineralogical analyses and C¹⁴ dating.

## Granulometric analysis

Approximately 1 kg of sample was collected from each stratigraphic location, packed in airtight PVC bags, labeled and transported to the laboratory. Granulometric analysis of these sediments and computation of textural parameters were attempted following the procedures detailed in Ramkumar et
al., (2000) and Ramkumar (2001, 2007, 2015b). The samples were air dried, thoroughly mixed, homogenized and cone and quartered to obtain approximately 50 gm of sediment subsample. Large pieces of shell fragments, organic material or pebble, if present, were removed. These subsamples were transferred to a pre-cleaned glass beaker. The samples were cleaned through three stages of treatments by hydrogen-per-oxide, 10 % v/v HCL and quasi-distilled water to remove the organic matter, shell calcite and other contaminants. These subsamples were then dried in an air-oven overnight at 60°C and weighed and dry-sieved in an automated Ro-tap sieve-shaker with ASTM test sieve sets of ½ Ø intervals for about 20 min. The separated size classes of sediments were then weighed and tabulated. These data were converted into ¼ Ø intervals with a computer algorithm (Ramkumar et al., 2000, Ramkumar (2001, 2007, 2015b), and then cumulative percent according to Ø interval were calculated and plotted in a semi-log graph sheet for generating requisite data to compute graphic mean (Mz), standard deviation (SD), skewness (SK) and kurtosis (KG). In addition, the input data required for few bivariate and other discriminant plots of environmental, energy and transport mode interpretations were also generated from the plots of cumulative percent of the sediment samples.

## Plate I

### Geochemical analysis

The sediment samples were dried in an oven at ~60° C and the dried samples were homogenized followed by reduction of sample size through cone and quartering technique. Later, a selected portion of the samples were pulverized to <63 µm size using agate mortar and pestle. The powdered samples were sent to Bureau Veritas for geochemical analysis using complete characterization of sediments using multiple methods. The proportions of major oxides were determined using X-Ray spectrometer after fusion of samples with Lithium borate. A known amount of sample was heated to determine the loss on ignition (LOI) by measuring the weight loss. A second split of sample was dried at 105° C and then sample was fused in a platinum-gold crucible with a commercial lithium tetraborate flux. The molten material was cast in a platinum mold. Fused discs were analyzed by XRF. The prepared samples were mixed with LiBO₂/Li₂B₄O₇ flux and crucibles were fused in a furnace. The cooled bead was dissolved in ACS grade nitric acid and analyzed by ICP and/or ICP-MS. Total Carbon and Sulphur were determined by the Leco method (TC003). The LF202 package includes an additional 14 elements from an aqua-regia digestion AQ200 to provide Au concentration. The CRMs i.e. Std SO-19 was used to check the accuracy of the measurement and replication analysis was also performed to ensure the precision of the results. Only the most relevant geochemical compositional values and ratios are used in this paper.
230 Establishing chronology through C¹⁴ analyses

231 All the 30 sediment samples were measured for total organic carbon, in addition to a pot shred
232 and few bones collected from the dug pits, based on which, 10 samples were chosen for radiocarbon
233 dating. While AMS (Accelerator Mass Spectrometry) radiocarbon dating of two samples (Table 1:
234 Varusanadu 1 and Varusanadu 2) was attempted at Beta Analytic, U.S.A., measurements of 8 samples
235 were performed at IUAC (Inter University Accelerator Centre), New Delhi, using procedures described in
236 Sharma et al. (2019). In brief, sediment samples, pottery and organic carbon samples were first
237 physically examined for visible contaminations. These samples were chemically pretreated using ABA
238 (Acid-Base-Acid) protocol. Samples were mixed with 10 mL 0.5 M HCl and kept overnight on a thermo-
239 shaker at 60°C and 700 rpm to remove carbonates. After acid treatment, samples were cleaned with
deionized (DI) water upto pH 7. To remove humic acid, samples were treated with 10mL 0.1 N NaOH at
240 60°C and 700 rpm for 3 hours. Again, after cleaning with DI water, samples were pretreated with 0.5M
241 HCl at 60°C and 700 rpm for 1 hour to remove any absorbed CO₂ during base treatment. After cleaning
242 with DI water, samples were freeze dried overnight. Freeze dried samples were graphitized using
243 Automated Graphitization Equipment (AGE) in the graphitization laboratory at IUAC.
244

245 Bone samples were first cleaned using ultrasonic, dried and then powdered. Powdered samples
246 were de-mineralized using 0.2M HCl and stored for four days in a refrigerator. Samples were monitored
247 for effervescence continuously and acid was replaced with fresh acid daily. After demineralization,
248 samples were cleaned with DI water and mixed with 10mL DI water and 3-4 drops of 0.5 M HCl to make
249 solution pH 3 and kept for gelatinization at 80°C for 48 hours (Longin, 1971). Gelatinized (collagen)
250 samples were freeze dried and graphitized using AGE. Graphitized samples were measured using XCAMS
251 (The compact Carbon Accelerator Mass Spectrometer extended for ¹⁰Be and ²⁶Al) and sample ratios
252 (¹⁴C/¹²C) were converted to radiocarbon ages as described in Stuvior and Polach (1977). All the
253 radiocarbon ages (BP) were calibrated using OxCal software.
254

254 RESULTS

255 Paleochannels

256 The paleochannel courses recognized are shown in the Figure 1. They follow a parallel-sub-
257 parallel alignment and show fanning-out character from Silaiman-Sakkudi.

258 Lithofacies Characteristics

259 A composite litholog, covering the natural exposures and dug pits was constructed and
260 presented in the figure 3. It shows the lithological information, contact relationships and the
261 stratigraphic locations of collected sediment and artifact samples. The lithofacies succession logged as
262 observed in the field is as described herein.

263 Fig.3.

264 The present-day channel runs on barren rocky surface in the upstream/northern regions of the
265 basin. From there on, the channel runs on paleosol/kankar (Quaternary-Recent). The ~2m thick paleosol
266 is typical of consolidated sediments of paleochannel (Fig.3f, i, j). This sediment layer contains pink-
267 reddish, pebble-boulder rich, massive, very coarse sandstone with scattered and recycled human bone
268 remains (Fig.3j; Plate I. o and q) and occasional-very frequent burial urns (Fig.3j; Plate I. o, p, t). At the
269 top of this layer, very-frequent occurrences of transported and recycled bone remains are observed to
270 occur and typically these are all bleached. Regions in which this paleosol forms the substrate were
271 utilized extensively as burial grounds (Fig.3j; Plate I. o) of the ancient populations. This paleosol is
272 overlain by fluvial channel and / or natural levee sediments. The contact between the kankar/paleosol
273 and the fluvial channel deposit is sharp, and erosional, while the contact with the natural levee sediment
274 is gradational. The fluvial channel deposits are composed of loose, unconsolidated, unsorted to poorly
275 sorted, angular to subrounded quartz rich sand with scarce admixture of granule to very-fine pebbles
276 and rare to regionally varying well-rounded gravel to cobble clasts. The natural levee deposit is massive,
277 highly compact, mostly brownish and wet clayey, silty to sandy loam typical of natural levee (Fig.3f, i, j;
278 Plate I. a, b, i). Pot shreds, quartz and quartzite clasts are scarce in this layer. This layer contains insitu,
279 large (65cm dia) concave up pots (Plate I. e) and bowels (Plate I. i) at two levels. The fluvial channel
280 deposit and the natural levee deposit might be lateral facies variations. At Pasiyapuram, paleo-wells
281 with clay outer linings occur. Successive excavations have exposed horizontally aligned pipelines (Plate I.
282 k) with filtering system (Plate I. h) connected to large storage bowels/pots.

283 This natural levee deposit is overlain by overbank deposit (~55cm) or fluvial channel deposit
284 (~2m) (Fig.3). The overbank deposits are highly compact, mostly dark brownish colored and wet clay,
285 silt, sandy loam with less frequent pebbles. The pot shreds are relatively bigger and are randomly
286 oriented within the bed (Fig.3f). There is a 12cm thick pot-shred interbed within this loamy soil layer.
287 The shreds are all concave-up oriented, except the very large shreds (>25cm) which are randomly
288 oriented. In addition, there is an organic matter rich layer within this massive bed. The upper and lower
289 contacts of this overbank deposit are gradational. Whereas, the lower contact of the pot-shred
290 interlayer is sharp and erosional. The fluvial channel deposit beds are composed of loose,
291 unconsolidated, unsorted to poorly sorted, angular to subrounded quartz rich sand with scarce
admixture of granule to very fine pebble. The overbank deposit and the fluvial channel deposit might be
lateral facies variations.

Towards top, the overbank deposit that houses many paleo-wells (Plate I. a, b, c) is overlain by
180cm thick, typical natural levee deposit of dusty brown–greyish colored, massive, compact, silty loam
and contains regionally varying quantities of pot shreds. This bed is immediately overlain by two beds of
floodplain deposits that are composed of massive, structureless loamy soil with subrounded-subangular
quartz and quartzite grains, pot shreds and evidences of recycled sediments. A paleohearth, filled with
highly disturbed sediments and artifacts are also identified. Towards top, this floodplain deposit is
overlain by overbank deposit, with an intervening, sharp, undulatory surface contact. The 40cm thick
overbank deposit is dusty brown colored, massive, loamy soil with pebble-sized pot shreds. The bed
looks massive; however, fining upward character is discernible. A gradual decrease of sizes (cobble-
gravel-pebble) of pot shreds from bottom to top is observable. At top, this bed is overlain by a dusty
brown colored, 15cm thick silty loam with parallel, even bedded, mm-scale fine laminations/very thin
beddings that regionally shows enrichments of organic carbon. Towards top, it is followed by dusty
brown colored, silty loamy soil with abundant pebble-gravel sized pot shreds and polycrystalline,
granule-pebble sized, translucent, pale yellow and red-tinted, angular-subangular quartz and quartzite
grains. This layer is highly anthropogenically disturbed due to cultivation.

## Textural properties

The results of granulometric analyses were utilized to compute mean size, standard distribution,
skewness and kurtosis and the data were plotted against stratigraphic positions (Fig.4). In addition, the
data were plotted on various established discrimination diagrams (Fig.5) to interpret the provenance,
affinity, modes of sediment transport, depositional energy and environments. Ubiquitous affinity with
fluvial environments, deposition in channel and adjoining regions, temporal variations in energy
conditions, i.e., normal and flood flow conditions, are all depicted by these granulometric characteristics
and the discriminant diagrams.

Fig.4.

Fig.5.

## Chronology

Table 1 lists the chronological estimates obtained from ¹⁴C analyses of 10 samples. Oldest age
estimate of 2435 ± 34 BP (752 BCE) is obtained from 2.5m depth at Keezhadi, a location situated far
from the present-day channel course and the youngest age estimate of 689 ± 24 BP is obtained at
Agaram, a location situated nearest to the present day channel course. The bone fragments were dated
324 to be 1115 ± 39 BP (778 CE – 1016 CE) and 1229 ± 30 BP (689 CE – 883 CE). The organic carbon/peat
325 layers recovered between flood deposits at upper reaches of the Vaigai River are dated to be -830 ± 30
326 BP and -1250 ± 30 BP.

327 **Table 1.**
328

329 **DISCUSSION**
330

329 **Habitat shifts due to tectonics and depositional setting**

330 Examination of the lithofacies succession, textural characteristics and sediment size
331 characteristics in the light of established discriminant diagrams show that the studied sedimentary
332 lithological succession represents a low-moderate slope fluvial channel sequence (channel-overbank-
333 natural levee-flood plain). This sequence documented the stratigraphic records of climatic extremes and
334 / or episodic overflow/shifting of channel that caused shifting of associated environmental niches. From
335 the configurations and geographic locations of paleochannel courses with reference to the present-day
336 channel and the locations of excavated sites of ancient, now buried industries that once manufactured
337 steel, glass bead, dye, gold jewelry (Plate I. d, h, I, j, k, v) and earthen pot, in addition to dwelling-
338 habitation, sites of religious sacrificial ceremonies (Plate I. r), places of worship (Plate I. x, y, z, ai, aii, aiii
339 and aiv) and burial sites (Plate I. o), it is perceivable that the paleochannels were ephemeral and flowing
340 on a Quaternary Kankar/paleosol (Fig.3I and 3J) from south of Silaiman (Fig. 1B) where, the tectonic line
341 (Fig.2A) crosses the Vaigai River. The low-lying area and the Vaigai River follow a strike fault. E-W aligned
342 fault-controlled river course and its N-S tilting events shifted the active delta lobe north and south of the
343 modern channel course. The stream courses of the Vaigai River were changing historically and during
344 pre-historic times, which is revealed from previous studies (Ramkumar et al., 2016, 2018, 2019), as well
345 as geomorphic information garnered from image processing (Fig.1C, D, E, and F; Fig. 2C, ) conducted by
346 the present study. From the occurrences and alignments of the paleochannels and the sites of
347 archaeological finds, the following are discerned: a) the avulsion of channels commences from east of
348 the tectonic line, as could be found in any other delta of the Peninsular India, b) from the deltahead
349 located near Sakkudi-Silaiman-Manaloor, the paleochannels are aligned parallel-tangent to the present
350 day course, forming a fanning-out character, c) the oldest channel is located at farthest (southwest of
351 the figures 1B, C, D, E, and F) with a progressive younging nature toward present day course and d) collectively the relative and absolute ages of the paleochannels and present day channel course suggest
352 tilt-rotational movement of the deltaic region of the Vaigai River from SW toward NE. The paleochannel
353 courses, as identified (Fig.1C, D, E, and F) during the present study and verified in the field (Fig.3j; Plate
354 I. c, q) also corroborate with the fact that all of these shifted/changed courses in a pendulum
355 fashion/high-sinuosity channels from east of the tectonic line (south of Silaiman-Fig.2A), and occupied
356
tangent directions at different times forming a pendulum fashion reinforcing the previous inferences (Ramkumar et al., 2016, 2018, 2019) of influences of this tectonic structure over location and morphology of landscape development in the region. Stream avulsion has been considered a common phenomenon that made the settlements of Indus Civilization to be abandoned and recolonized (Singh et al., 2017) and similar abandonment of ancient settlements that are aligned/located along the paleochannels of the Vaigai River are identified (Fig. 1C, D, E, and F).

The paleochannels were shallow incised channels flown over Quaternary kankar/paleosol (Fig.3f, i, j; Plate I. o). They received only ephemeral/seasonal flows, that necessitated establishment of dugwells (Plate I. a, b, c) in every habitation units (i.e., in every house and the industrial site) for domestic and other uses. The high density of dug wells could be a result of two factors: One, the river was ephemeral and the second is prevalent arid climate (Ponton et al., 2012). Field observations, type, nature and characteristics of man-made structures, artifacts exposed during excavation suggest: there were multiple habitation sites, steel, dye, potceramic and jewelry industries, burial /cremation sites, and religious ritualistic sacrificial sites, besides scattered occurrences of buried-not fully exposed stone (Plate I. x, y, z, ai, aii, aiii, aiv) and polymetallic statues of deities that necessitated establishment of sporadic-extremely dense arrangements of backed clay ringed and clay lined dug wells (Plate I. a, b, c). Owing to the shallow morphology and seasonal nature, there might have been overbank flows episodically, especially during significant climatic extremes (i.e., highly different from normal flow), that occurred at least twice as indicated by linear arrangements of archaeological sites, following paleochannels and also as decipherable in the lithofacies characteristics and shifts of habitation sites/surfaces. From the lithological succession, it is evident that the river channel deposits were overlain repeatedly by natural levee and overbank deposits that irrefutably indicate marooning of ancient habitation sites/surfaces and their burial under channel deposits. Occurrence of pot shred interbed within overbank-natural levee deposit (Fig.3f) indicates perhaps a sudden flooding-related erosion and resultant transportation of pot materials from potceramic industry and deposition of eroded material in aqueous medium (as they are all oriented concave-up except the very large shreds that are randomly oriented). While the natural levee deposits show evidence of habitation-related disturbances/churning/recycling/mixing in the internal sedimentary structure, texture and packing, in addition to occurrences of hearth, etc., recurrences of overbank-channel deposits over natural levee-habitation sites/surfaces indicate habitation-colonization over relatively high-grounds along river bank, followed by marooning/burial under overbank-channel deposit and abandonment of habitation site and/or shifting of habitat. The same sequence of events occurred again, showing two habitation
surfaces that are buried under surface. Together with facies characteristics and other artifact occurrences, these dates indicate shifting of the Vaigai Channel from prior to 752 BCE (2435 ± 34 BP), and colonization of ancient population, which could have been affected by a catastrophic flood that might have occurred along with stream avulsion at 1250-830 BP, in which the ancient habitation might have been marooned, the industries and other structural infrastructure destroyed, transported and deposited as interbeds in flood deposits. This high-energy condition was reduced leading to the formation of point bar-natural levee environmental setting that paved way for the deposition of organic carbon layer at Agaram at about 689 ± 24 BP. Further shifting of the channel from Agaram toward present day course might have occurred since then. According to the available age constraints, the first habitation-colonization might have been prior to about 752 BCE (2435 ± 34 BP), the second habitation-colonization might have been during 641 CE (1348 ± 27 BP). The third and most recent habitation-colonization could have been during post 689 ± 24 BP and the topsoil is intensively disturbed anthropogenically, including that of gold enrichment in the soils. The geochemistry of the top soil shows enrichment of P₂O₅, Ag and Au and depletion of Na₂O, Mo, Cs, Sn and Ti.

All the sediment samples fall in the riverine fields (Figs.5A, B) without exception. However, depositional environments varied from braided bar, point bar (Fig.5C) and show poor-moderate sorting character, suggestive of excessive sediment supply within short distance and short duration that could have been a response of high-intensity, short-duration rainfall in the catchments. The energy conditions were also variable, not typical of normal fluvial environment, as indicated by plot of all the samples in region next to river process, but away from quite-water and wave process fields (Fig.5D), suggestive of high-energy, unidirectional flow (Ramkumar, 2007; Ramkumar et al., 2000, 2015b). The scattered nature of the sample fields might suggest highly-variable depositional energy conditions. Almost all the samples fall within the field of 'rolling and suspension' (Fig.5E) except few near the C-S line. While the latter represents the fluidized alluvial fill capping the residual basement rock and consists of immature sediments, cemented with calcareous material, the former represents deposition of the entire sequence during periodic floods, during which sediments were brought essentially under turbulent flow and deposited when peak flows dwindle/recede (Sridhar, 2007; Ramkumar, 2009; Ramkumar et al., 2015b), during which rolling (coarser sediments) and suspension (finer sediments) modes of transport prevail and ultimately lead to deposition. This bimodality is also reflected in the sediment size distribution. These observations based on sediment granulometry and sediment size parameter-based discrimination diagrams broadly follow the inferences on environmental setting and depositional conditions inferred from lithofacies and sedimentary structural characteristics. All these further indicate climatic extremes.
forced abandonment of ancient habitation sites and shift to newer higher altitudes and or regions not
affected by flooding/marooning, but near the channel course, of course, indeed a newer channel course.

The depositional conditions inferred are also supported by the geochemical characteristics as
detailed herein. This is further reinforced by the variations of sand and silt-size sediments based on their
SiO₂ and Al₂O₃ contents. Sand dominated facies are enriched in SiO₂ and Silt/clay dominated facies are
enriched Al₂O₃ unless the sediments are rich in Fe-oxides or calcareous cement (i.e., calcrete at the
bottom of the section). Silicon to Al ratio (Si/Al) can reflect the proportion of quartz to clay minerals (e.g.
Cole et al., 2009) and the ratios vary from 3.1 to 10.7. The lower values are reported from the top of the
section which indicates enrichment of fine particles with clay minerals and the higher ratios recorded at
the bottom of the section which represents enrichment of quartz with the sand and silt fractions. The
sediments are low to moderately mature and can be chemically classified as shale, wacke and litharenite
(Herron, 1988). This is further supported by the ICV values (> 0.8), which represent that the sediments
mainly consist of rock forming minerals than the clay minerals (e.g. Cox et al., 1995). The chemical
weathering had less influence in the modification of the sediment composition and physical weathering
might be the dominant process in the study area. This is confirmed by the CIA values (52-69) and the
biplot of CIA vs. Al/Na (Fig.6).

Fig.6.

Geochemical proxies of climate induced fluvial dynamics and habitat shifts

The stream avulsion characteristics and location-alignments of paleochannels of the Vaigai
River, including the general nature of the Peninsular Rivers, including the Vaigai River that experience(d)
short-duration, high-intensity rainfall, annually, might have enhanced the flood vulnerability of the
region. This general character, when superimposed over the catastrophic flooding events, that could
have occurred episodically during historic and pre-historic times, as inferred through the facies
succession (Fig.3) and textural properties (Figs.4 and 5) suggests the plausibility of prevalent climate
reversal and or significant deviation from normalcy. Sridhar (2007) reported flooding as a common
phenomenon in all the Indian rivers during peak monsoon. Extreme floods caused either by known or
unknown return period flow can alter channel hydraulics and geomorphic effects (Heritage et al., 2004).
Changes in rainfall patterns in catchments that are potentially linked with climate changes are
considered as first-order controls (Wulf et al., 2010) on topographic evolution (Sahu et al., 2010; Menier
et al., 2017) on short and long-term erosion/deposition patterns in fluvial systems (Menier et al., 2017;
Ramkumar et al., 2019). Given cognizance to these, it is perceivable that the channel shifting, marooning
and burial of ancient habitats might have been related to extreme floods and associated changes in
channel courses of the Vaigai River. The characteristics of sedimentary successions are controlled by three main factors: the sediment supply from the eroding source region, the grain size distribution of that sediment supply, and the area available for sediment accumulation in the downstream regions. The interplay of these factors can help interpret climatic-tectonic signals (Armitage et al., 2011). The geochemical proxies are examined in the light of these observations and to draw support for prevalent significant climatic changes.

The single element mobility was calculated for selective mobile elements during incongruent weathering by comparing non-mobile elements (in this case Al; Garzanti et al., 2013) as Al has similar magmatic compatibility (Gaillardet et al., 1999; Garzanti et al., 2013). The bulk sediment mobility in the present study is observed as: αAlCs>αAlNa>αAlRb>αAlMg>αAlCa>αAlK>αAlSr>αAlFe>αAlBa. This sequence is different from the general observations made elsewhere. High mobility is recorded for Cs and Na than other smaller cations such as Ca and Sr. According to Nesbitt et al. (1980), Rb and Cs have large ionic radii and thus are retained in weathered product during chemical weathering. Ca and Na also behave similar during chemical weathering. In addition, other divalent cations, i.e., Mg, Ba and Sr become mobile during chemical weathering (Nesbitt et al., 1980; Middelburg et al., 1988; Zhao and Zheng, 2015). However, in the present study, the immobile element (i.e. Cs) and mobile elements (i.e. Na) are depleted. In particular, Na and Cs show higher mobility in specific depth (Cs at K24, K26-K27 and K1; River channel and modern flood plain environment). It means, specific time intervals were climatically highly different than other durations. It is noted that Rb and Mg behaved similar and moderate mobility is recorded at both top and bottom parts of the studied lithosection. Majority of the elements showed moderate to high mobility in the bottom portion of the section (K1-K4). Cesium is strongly adsorbed in clay particles and clay containing substances (Jolin and Kaminski, 2016) and its depletion may indicate removal of Ce-bearing clay sediments during the flood events. In addition, the enrichment of Ca can be post-depositional effect. Contrary to their nature, the Ca and Na show inverse trend at the bottom of the section, which may indicate mass conservation during the chemical weathering at the source region, as enrichment of immobile elements may be balanced by depletion of mobile elements (Zhao and Zhend, 2015).

The Rb/Sr ratios in the sediments have been widely used as indicators of weathering intensity and it is found that the Rb/Sr ratios of the bulk surface sediments are closely related to the terrigenous input from the catchment area (i.e., Xu et al., 2010). Rb is mainly associated with K in the minerals K-Feldspar and biotite etc. Whereas, the Sr is mainly associated with Ca-Minerals such as limestones and Ca-bearing silicates (hornblende, plagioclase and picrate) (Desch, 1969). Among these, Ca-Minerals
easily breakdown as compared to K-bearing minerals which leads to fractionate the K-Rb and Ca-Sr pairs
during the geological processes (Liu et al., 1999; Xu et al., 2010). Thus, the concentrations of K-Rb get
enriched in the residue than the source rocks by increasing of weathering intensity (Brass, 1975; Jin et
al., 2001; Cole et al., 2009). The CIA value of the Upper Continental Crust (UCC) is ~47 and the CIA values
within a range of 45 to 55 indicate absence of chemical weathering in the source area. Upon intensive
weathering, the CIA values of sediments can reach to 100, due to removal of alkalis and alkaline earth
metals (McLennan, 1993; McLennan et al., 1993; Nesbitt and Young, 1982; 1984). In the present study,
the CIA values ranges from 52 to 69 indicating low to moderate weathering in the source region. In
addition, Rb/Sr ratio also follows the CIA trend and ranges between 0.06 and 0.31. Overall, four
increments are noticed in CIA values at various depth intervals (i.e. sample number K3, K13, K22, K29
belong to overbank deposits, River channel and Floodplain environment). The influence of paleoclimates
was assessed by using Al₂O₃+Na₂O+K₂O content against SiO₂ (Suttner and Dutta, 1986). Accordingly, the
sediments of the present study fall on arid climatic zone (Fig.7) except the samples from floodplain-relict
alluvium and calcrete, which is comparable to the prevalent regional climatic variations of south India
(Ramkumar et al. 2019). This further confirms the inference that the sediments are chemically less
matured and were transported through mass erosion-transport event such as major flood.

## 502 Granulometric proxies of fluvial dynamics and habitat shifts/abandonment

Earth's continental landscape is largely shaped by the rivers (Mathew et al., 2016). However, the river patterns that evolve through natural selection and the spatial arrangements of channels are controlled by lithology, slope and structure. Unless influenced and forced by multiple factors including, but not limited to, tectonics, climate and anthropogenic interventions, the river channels persist (Twidale, 2004). There might be changes in channel patterns, including the shifting of locations are on the order of few tens to few thousands of years on a temporal scale (for example, Ramkumar et al., 2018, 2019) to which ancient settlements were also subjected to change. Changes on regional climatological conditions most often reflects on geomorphology in a linear scale (Harrison et al., 2019). The topographic evolution of southern Peninsular India, with its attendant Western Ghat Mountains, in tectonic quiescence since at least ca. 65 Ma was examined by Mandal et al., (2015), who evinced that the steady-state likely reflects the balance between erosion and isostatically driven uplift of the southern Peninsular India. Based on the landscape evolution histories of three major river basins of southern India, Ramkumar et al., (2019), concluded that the region evolved through five stages of tectono-geomorphic changes, among which the Holocene-Anthropocene is the recent. Building on this, the present study has documented further finer-scale changes in the ancient landscape, on a millennial
scale. In this light, the absence of significant trends in the stratigraphic variations of granulometric parameters, except few short-episodic intervals suggest that there were no drastic changes in the provenance and/or sediment supply; rather, the abandonment of habitation-colonization surfaces might have been due to overbank deposition and burial due to sudden, catastrophic floods, perhaps related to out-of-cycle event(s) that were significantly different from annual-seasonal flows. In regions of short-duration, high-intensity rains such as Peninsular India, where the Vaigai River is also part, frequent bank breach and overbank deposition are normal and frequent occurrences (Ramkumar et al., 2015b).

Fig.7.

Sediment transport dynamics, landscape evolution and resultant habitat shifts/abandonments

Clastic sediments are transported to the ocean from the catchments at variable speeds that span from days or weeks, to >10⁵–10⁶ years, because of storage and reactivation *en route* (Clift and Giosan, 2013). Nevertheless, many of the Asian river systems have shown relatively constant mass flux into the ocean during the past 2 Ma due to buffering of sediments in the flood plains (Metivier and Gaudemer, 1999). The sedimentary response to erosion of an upland source region and down-system aggradation occurs at a range of temporal and spatial scales. Over geological timescales, the architecture and granulometry of fluvial successions is dependent on the prevailing tectonic and climatic conditions; however, inverting the sedimentary record for these key parameters is complex (Armitage et al., 2011). Alluvial riverbed elevation responds to the balance between sediment supply and transport capacity, which is largely dependent on climate and its translation into fluvial discharge (Slater and Singer, 2013). In this regard, occurrences of flood deposits slightly-significantly above the current bed-levels of the modern river channel in the Keezhadi and Varusanadu sites unequivocally indicate two paradigms: first, the modern Vaigai River had incised vertically its own alluvial deposit, and second, there were basin-scale downward sediment transport at least thrice namely, post 752 BCE (2435 ± 34 BP), colonization event, 641 CE (1348 ± 27 BP) colonization event and post organic carbon layer deposition event (689 ± 24 BP) among others, and shifting of channel courses to which the ancient settlements were subjected to burial, that are being exhumed by lateral channel shift in Varusanadu and excavation at Keezhadi. The Early Holocene was a period of rapid sediment flux to South Asian deltas (Goodbred and Kuehl, 2000; Giosan et al., 2006). However, it is not clear to what extent this sediment was freshly eroded bedrock rather than older fluvial sediments reworked from the floodplains and terraces in the mountains. Recycling and sorting can also influence the sediment composition which can be predicted based on Th and Zr to Sc ratios. The Th/Sc ratios show minimal variations than Zr/Sc ratios, where selected samples show higher Zr/Sc ratio, which might be indicating heavy mineral enrichment
550 (i.e. zircon) due to recycling and/or sorting (McLennan et al., 1993). Recycling/sorting had little effect on
551 the river channel sediments (i.e. sample numbers K20, K21 and K23), which is well supported by the
552 enrichment of heavy minerals (i.e., zircon) and thus, enrichment of Zr concentration with higher Zr/Sc
553 ratios (i.e., Zr/Sc = >50). However, on the whole, the sediments are not affected by sedimentary sorting
554 or recycling process and low to moderately affected by weathering.

555 Previous studies have established the relationship between enhanced erosion and influx of
556 excessive sediments into the trunk channel due to changes in landuse (Ramkumar et al., 2015a, c).
557 These excessive sediments can be flushed out of the channel during extreme flooding (Ramkumar, 2000;
558 2003) or forming the cause of bank breach and channel course shift (Ramkumar et al., 2015b). Giosan et
559 al., (2012) showed that the monsoonal-fed rivers were active there during the Holocene. As the
560 monsoon weakened, monsoonal rivers gradually dried or became seasonal, affecting habitability along
561 their courses. Our observations in terms of episodic stream avulsion, marooning of ancient habitation
562 sites/surfaces under flood deposits, and subsequent abandonment of habitation sites also evince strong
563 relationship between tectono-climatic factors on the river systems and thus, on the survival-
564 abandonment of ancient civilization-habitation sites, especially in the plains of the Vaigai River. The
565 ancient city of Madurai in southern India has been under habitation for many tens of centuries. This city
566 found its mention in the writings of Magasthenes (who visited the city during 300 BCE), Pliny the
567 Younger (61-112 BCE), and Ptolemy (90-168 BCE) and Indian chroniclers such as Kautila's Arthashastra
568 (370-283 BCE), and Sangam Literature (1200-100 BCE). Since then, various parts of the Madurai continue
569 to be inhabited along the Vaigai River until Recent and our study attests to the continuous habitation
570 since microlithic age (Plate I). The Sangam literature also mentions about the havoc caused on
571 agricultural fields and ancient settlements by periodic flooding of the Vaigai River and resultant
572 mitigation efforts and shifting of settlements of kings of yore.

573 Flood chronologies developed across the world have shown that even minor changes in the
574 climatic patterns can be associated with more frequent occurrences of extreme flood events (Kaleet al.,
575 2010; Knox, 2000). Independent studies of Kale et al., (2010), Ramkumar et al., (2018) and Goswami et
576 al., (2019) documented the flood deposits of the Cauvery River and found distinct flood clusters
577 occurred during the times of major shifts in the monsoon climate, from fluvial dormancy to sudden
578 outburst of monsoons (~2 ka), from warmer to colder (onset of 'Little Ice Age' (LIA) ~ 14th century), from
579 colder to warmer (end of LIA ~ 19th century) and ~20th century, indicating that climatic pattern can be
580 associated with more frequent occurrences of extreme flood events. The study also shows that the two
581 major flood events of the 20th century reported from the upper Cauvery were produced by high intensity
short duration storms, suggesting that not all wet phases are associated with major floods and all dry phases with low floods/droughts. Documentation of flood deposits in the studied sites located in the Vaigai River, from upper to downstream regions and associated shifts in river channel affirm the dominance of climatic reversals on the development and destruction of ancient settlements of the Vaigai Civilization, as could be found elsewhere including India (eg. Ponton et al. 2012; Giosan et al. 2012, 2018; Giesche et al. 2019).

On a geological, pre-historic and historic scale, the Vaigai River was a dryland river, the dynamics of which might be different from that of rivers flowing on humid regions. According to Tooth (2000), the dryland rivers have unique fluvial environments, hillslope and channel hydrological and sediment transport processes, river pattern and geometry, temporal and spatial aspects of channel change, sedimentary structures and bedforms that contrast with more humid fluvial environments. The dryland rivers experience extreme temporal and spatial variability of rainfall, runoff and sediment transport, and poor integration between tributary and trunk channels. Above all, large floods control the channel morphology of the dryland rivers. This uniqueness might have played a significant role in channel avulsion in the downstream reaches of the Vaigai River and played spoilsport on ancient settlements. The occurrences of flood events at about 200-300 year intervals as indicated by flood layers post 752 BCE (2435 ± 34 BP) colonization event, 641 CE (1348 ± 27 BP) colonization event and post organic carbon layer deposition event (689 ± 24 BP) concur with the findings of Zhao et al., (2015) who have documented variations in low-latitude monsoon precipitation driven by shifts in the mean position of the intertropical convergence zone (ITCZ), which is further mediated by solar activity and tropical SSTs. These authors have also stated that the Asian Southern Monsoon has 220-year cyclicity concordant with the dominant cycle of solar activity (Zhao et al., 2015). However, in order to ascertain this presumption, very-high resolution age control is required, which in turn is in progress.

## CONCLUSIONS

According to the available age constraints, the first habitation-colonization in the Vaigai River Basin might have been prior to about 752 BCE (2435 ± 34 BP), the second habitation-colonization might have been during 641 CE (1348 ± 27 BP). The third and most recent habitation-colonization might have been during post 689 ± 24 BP and the top soil is intensively disturbed anthropogenically, including that of gold enrichment in the soils. Among the floods and channel avulsions recognized by the present study, the second flood might have been catastrophic and sudden, as indicated by marooning, destruction and burial of ancient ritualistic sacrificial site and *potceramic* industry. On a related note, the dates provided by the present estimates with an oldest age of 752 BCE, relatively at shallower depths
614 indicate well-entrenched and mature urban, industrial, social and religious culture in the Vaigai River
615 Civilization, comparable with Indus Civilization. Urbanism in the Vaigai Civilization-archaeological sites
616 are indicated by the graffiti in pot shreds, gold ornaments, glass beads of all hues, chess coins and other
617 play-paraphernalia, utensils for weaving, dye industry, potceramic industry, steel industry, multiple
618 burial sites and ritual methods, systematic and dense arrangements of backed clay-lined wells,
619 settlements, all of which occur along the Vaigai River channel strewn over hundreds of km², including
620 that of port city. It can be safely concluded that the civilization at Vaigai commenced during the
621 microlithic age (at about 10000 BP) and continued to evolve and developed settlements, industries,
622 temples/sites of worships of various kinds, burial grounds, structured cities and commercial places over
623 time. These developed along the contemporary banks of the Vaigai River, as the river had shown
624 avulsion under the influences of episodic tectonic activism, climatic reversal, associated flow conditions
625 and resultant variations in sediment influx, transport, changes in depositional sites and topography.
626 Thus, the Vaigai River Civilization could be considered as one of the oldest and continually inhabited
627 sites starting from the mid-channel until confluence and also that requires extensive, large scale,
628 systematic study and documentation. Occurrences of similar sites in other adjacent river basins have
629 also been documented through this study, that reinforce the premise of control of climate-tectonics on
630 a regional scale and potential use of fluvial dynamics and ages of climate reversals to precisely explore
631 buried habitation/civilizationarchaeological sites.

632 ACKNOWLEDGEMENTS

633 Authors thank the editor of this journal and two anonymous reviewers for their inquisitive,
634 critical comments and constructive criticisms that helped to improve the manuscript. The authorities of
635 Periyar University and the Department of Archaeology, State Government of Tamil Nadu are thanked for
636 according necessary administrative permission to conduct field investigations and sampling campaigns.
637 Few of the photographs of artifacts are recovered by the archaeological department personnel who
638 accompanied senior authors during field survey. The MoU's signed between the Periyar University and
639 the University of South Brittany, France, Curtin University Sarawak, Malaysia and Universiti Teknologi
640 Petronas facilitated collaborative work and the authors are thankful for the same. Authors are also
641 thankful to IUAC for extending AMS facility, an institution funded by the Ministry of Earth Sciences,
642 Government of India.

643 DECLARATION OF INTEREST STATEMENT

644 The authors declare no conflict of interest.

645
646

647 REFERENCES

648 Armitage, J.J., Duller, R.A., Whittaker, A.C. and Allen, P.A., 2011 Transformation of tectonic and climatic
649 signals from source to sedimentary archive. Nature Geoscience 4,231-235.

650 Brivio, P.A., Pepe, M. and Tomasoni, R., 2000 Multispectral and multiscale remote sensing data for
651 archaeological prospecting in an alpine alluvial plain. Journal of Cultural Heritage 1, 155-164.

652 Chen, Z., Zong Y., Wang Z., Wang, H. and Chen, J., 2008 Migration patterns of Neolithic settlements on
653 the abandoned and Yellow and Yangtze River deltas of China. Quaternary Research 70, 301-314.

654 Clift, P.D. and Giosan, L., 2013 Sediment fluxes and buffering in the post-glacial Indus Basin. Basin
655 Research 25, 1-18.

656 Day, J.W., Gunn, J.D., Folan, W.J., Yanez-Arancibia, A. and Horton, B.P., 2007 Emergence of complex
657 societies after sea level stabilized. EOS, Transactions, American Geophysical Union 88, 169-170.

658 Friedman, G.M., 1967 Dynamic processes and statistical parameters compared for size frequency
659 distribution of beach and river sands. Journal of Sedimentary Research 37, 327-354.

660 Giesche, A., Staubwasser, M., Petrie, C.A. and Hodell, D.A., 2019 Indian winter and summer monsoon
661 strength over the 4.2 ka BP event in foraminifer isotope records from the Indus River delta in the
662 Arabian Sea. Climatic of the Past 15, 73-90.

663 Giosan, L., Clift, P.D., Blusztajn, J., Tabrez, A., Constantinescu, S. and Filip, F., 2006 On the control of
664 climate- and human-modulated fluvial sediment delivery on river delta development: the Indus. EOS
665 Trans. American Geophysical Union 87, 14-24.

666 Giosan, L., Clift, P.D., Macklin, M.G., Fuller, D.Q., Constantinescu, S., Durcan, J.A., Stevens, T., Duller,
667 G.A.T., Tabrez, A.R., Gangal, K., Adhikari, R., Alizai, A., Filip, F., Van Laningham, S. and Syvitski, J.P.M.,
668 2012 Fluvial landscapes of the Harappan civilization. *Proceedings of the National Academy of
669 Sciences 109, E1688-E1694.

670 Giosan, L., Orsi, W.D. Coolen, M., Wuchter, C., Dunlea, A.G., Thirumalai, K., Munoz, S.E., Clift, P.D.,
671 Donnelly, J.P., Galy, V. and Fuller, D.Q., 2018 Neoglacial climate anomalies and the Harappan
672 metamorphosis. Climatic of the Past Discuss 1-30. https://doi.org/10.5194/cp-2018-37

673 Glaister, R.P., Nelson, H.W., 1974 Grain size distribution: an aid in facies classification. Bulletin of
674 Canadian Petroleum Geology 22, 203-240.

675 Goodbred, S.L. and Kuehl, S.A. 2000 The significance of large sediment supply, active tectonism and
676 eustasy on margin sequence development; Late Quaternary stratigraphy and evolution of the
677 Ganges-Brahmaputra delta. Sedimentary Geology 133, 227-248.

678 Goswami, K., Rawat, M., Jaiswal, M.K. and Kale, V.S., 2019 Luminescence chronology of late-Holocene
679 palaeofloods in the upper Kaveri basin, India: An insight into the climate-flood relationship. The
680 Holocene 29, 1094-1104. https://doi.org/10.1177/0959683619831436

681 GSI, 1975 Geology of parts of Madurai, Nilakkottai and Tirumangalam Taluks, Madurai District, Tamil
682 Nadu. SROGSI 5428, Geological Survey of India.

683 Harrison, S., Mighall, T., Stainforth, D.A., Allen, P., Macklin, M., Anderson, E., Knight, J. and Mauquoy, D.,
684 2019 Uncertainty in geomorphological responses to climate change. Climatic Change 156, 69-86.
685 https://doi.org/10.1007/s10584-019-02520-8

686 Heritage, G.L., Large, A.R.G., Moon, B.P. and Jewitt, G., 2004 Channel hydraulics and geomorphic effects
687 of an extreme flood event on the Sabie River, South Africa. Catena 58, 151-181.

688 Kale, V.S., Achyuthan, H, Jaiswal, M.K. and Sengupta, S., 2010 Palaeoflood records from upper Kaveri
689 River, southern India: evidence for discrete floods during Holocene. Geochronometria 37, 49-55.

690 Knox, J.C., 2000 Sensitivity of modern and Holocene floods to climate change. Quaternary Science
691 Reviews 19, 439-457.

692 Longin, R. 1971 New Method of Collagen Extraction for radiocarbon Dating. Nature 231, 241-242.
693 Mandal, S.K., Lupker, M., Burg, J., Valla, P.G., Haghipour, N. and Christl, M., 2015 Spatial variability of
694 $^{10}$Be-derived erosion rates across the southern Peninsular Indian escarpment: A key to landscape
695 evolution across passive margins. Earth and Planetary Science Letters 425, 154-167.

696 Mathew, M.J., Menier, D., Siddiqui, N. A., Ramkumar, M., Santosh, M., Kumar, S. and Hassan, M., 2016
697 Drainage Basin and Topographic Analysis of a Tropical Landscape: Insights into Surface and Tectonic
698 Processes in northern Borneo. Journal of Asian Earth Sciences 124, 14-27.

699 Mehdi, S.M., Pant, N.C., Saini, H.S., Mujtaba, S.A.I. and Pande, P., 2016 Identification of Palaeochannel
700 configuration in the Saraswati river basin in parts of Haryana and Rajasthan, India, through digital
701 remote sensing and GIS. Episodes 39, 29-38. http://doi.org/10.18814/epiiugs/2016/v39i1/89234

702 Mehrotra, N., Shah, S.K., Basavaiah, N., Laskar, A.H. and Yadava, M.G., 2019 Resonance of the '4.2ka
event' and terminations of global civilizations during the Holocene, in the palaeoclimate records
703 around PT Tso Lake, Eastern Himalaya. Quaternary International 507, 206-216.
704

705 Menier, D., Manoj, M.J., Pubellier, M., Sapin, F., Delcaillau, B., Siddiqui, N., Ramkumar, M. and Santosh,
706 M., 2017 Landscape response to progressive tectonic and climatic forcing in NW Borneo:
707 Implications for geological and geomorphic controls on flood hazard. Nature Geoscience Reports
708 7.http://doi.org/10.1038/s41598-017-00620-y

709 Mètivier, F. and Gaudemer, Y., 1999 Stability of output fluxes of large rivers in South and East Asia
during the last 2 million years; implications of floodplain processes. Basin Research 11, 293-303.

711 Moiola, R.J. and Weiser, D. (1968) Textural parameters: an evaluation. Journal of Sedimentary Research
712 38, 45-53.

713 Nigam, R. and Chaturvedi, S.K., 2006 Do inverted depositional sequences and allochothonous
714 foraminifers in sediments along the coast of Kachchh, NW India, indicate palaeostorm and/or
715 tsunami effects? Geo-Marine Letters 26, 40-52.

716 Nigam, R., Dubey, R., Saraswat, R., Sundaresh, A., Gaur, A.S. and Loveson, V.J., 2016 Ancient Indians
717 (Harappan settlement) were aware of tsunami/storm protection measures: a new interpretation of
718 thick walls at Dholavira, Gujarat, India. Current Science 111, 2040-2043.

719 Orengo, H.A. and Petrie, C.A., 2017 Large-scale, multi-temporal remote sensing of palaeo-river
720 networks: A case study from Northwest India and its implications for the Indus civilisation. Remote
721 Sensing 9, 9070735. http://doi.org/10.3390/rs9070735

722 Passega, R., 1957 Texture as characteristic of clastic deposition. The American Association of Petroleum
723 Geologists Bulletin 41, 1952-1984.

724 Radhakrishna, B.P. and Merh, S.S., 1999 Vedic Saraswati—Evolutionary history of a lost river of
725 northwestern India. Geological Society of India, Bangalore. 329p.

726 Ramkumar, M., 2000 Recent changes in the Kakinada spit, Godavari delta. Journal of Geological Society
of India 55, 183-188.

728 Ramkumar, M., Sudharani, P, Gandhi, M.S., Pattabhiramayya, M., Rajanikumari, V., Bhagavan, K.V.S. and
729 Swamy, A.S.R., 2000 Textural characteristics of coastal sedimentary environments of the modern
730 Godavari delta. Journal of Geological Society of India 56, 471-487.

731 Ramkumar, M., 2001 Sedimentary environments of the modern Godavari delta: characterization and
statistical discrimination towards computer assisted environment recognition scheme. Journal of
732 Geological Society of India 57, 49-63.

734 Ramkumar, M., 2003 Progradation of the Godavari delta: A fact or empirical artifice? Insights from
coastal landforms. Journal of Geological Society of India 62, 290-304.

736 Ramkumar, M., 2007 Spatio-temporal variations of sediment texture and their influence on organic
carbon distribution in the Krishna estuary. Indian Journal of Geochemistry 22, 143-154.

738 Ramkumar, M., 2009 Flooding - A manageable geohazard. In: Ramkumar, M., (Ed.) Geological hazards:
causes, consequences and methods of containment. New India Publishers, 177-190.
740 Ramkumar, M., Kumaraswamy, K. and Mohanraj, R., (Eds.) 2015a Environmental management of river
basin ecosystems. Springer-Verlag761. http://doi.org/10.1007/978-3-319-13425-3.

742 Ramkumar, M., Kumaraswamy, K., Arthur James, R., Suresh, M., Sugantha, T., Jayaraj, L., Mathiyalagan, Saraswathi, M. and Shyamala, J., 2015b Sand mining, channel bar dynamics and sediment textural properties of the Kaveri River, South India: Implications on flooding hazard and sustainability of the natural fluvial system. In: Ramkumar, M., Kumaraswamy, K. and Mohanraj, R., (Eds.) Environmental management of river basin ecosystems. Springer-Verlag14. 283-318. http://doi.org/10.1007/978-3-319-13425-3.

748 Ramkumar, M., Kumaraswamy, K. and Mohanraj, R., 2015c Land use dynamics and environmental management of River Basins with emphasis on deltaic ecosystems: Need for integrated study based development and nourishment programs and institutionalizing the management strategies. In: Ramkumar, M., Kumaraswamy, K. and Mohanraj, R., (Eds.). Environmental management of River Basin ecosystems. Springer-Verlag, Heidelberg. DOI 10.1007/978-3-319-13425-3-1. pp.1-20.

753 Ramkumar, M., Menier, D., Manoj, M.J., and Santosh, M., 2016 Geological, geophysical and inherited tectonic imprints on the climate and contrasting coastal geomorphology of the Indian peninsula. Gondwana Research 36, 52-80.

756 Ramkumar, M., Kumaraswamy, K., Balasubramani, K., Abdul Rahaman, S., Jegankumar, R., Kumara Ganesh, U., Manikandan, E., Balasundareshwaran, A., Prithiviraj, G., Ayyanthurai, R., Santosh, M., Nagarajan, R., Siddiqui, N.A., Menier, D., Mathew, M.J., Ramachandran, C., Sathyaseelan, M. and Sivakumar, P., 2018 Discovery of buried historical structures through integrated geological and geophysical techniques in the Kaveri-Kollidam interfluve, India: Implications for morpho-tectonics and archeological exploration. Archaeological Prospection 26, 58-73.

762 Ramkumar, M., Santosh, M., Abdul Rahaman, S., Balasundareshwaran, A., Balasubramani, K., Mathew, M.J., Sautter, B., Siddiqui, S.N., Usha, K., Sreerhishiya, K., Prithiviraj, G., Nagarajan, R., Thirukumaran, V., Menier, D. and Kumaraswamy, K., 2019 Tectono-morphological evolution of the Cauvery, Vaigai and Thamirabarani river basins: Implications on timing, stratigraphic markers, relative roles of intrinsic and extrinsic factors and transience of Southern Indian landscape. Geological Journal 54, 2870-2911. https://doi.org/10.1002/gj.3520.

768 Rao, S.R., 1991 Dawn and devolution of the Indus civilization. Aditya Prakashan, 392.

769 Roy, A.B. and Sethumadhav, M.S., 2014 The ‘mystic’ sand dune-covered temples of Talakad, Mysore district, Karnakata: evidence of earthquake-related destruction. Current Science 107, 246-254.

771 Roy, A.B., Bhu, H., Sharma, P. and Vaishnav, K., 2017 Deserted nineteenth century Paliwal villages around Jaisalmer, western Rajasthan, India: historical evidence of palaeoseismicity. Current Science 112, 402-405.

774 Sahu, S., Raju, N.J. and Saha, D., 2010 Active tectonics and geomorphology in the Sone-Ganga alluvial tract in mid-Ganga Basin, India. Quaternary International 227, 116-126.

776 Santosh, M., Tsunogae, T., Yang, C.X., Han, Y.S., Hari, K.R., Prasanth, M.M. and Uthup, S., 2020. The Bastar craton, central India: A window to Archean–Paleoproterozoic crustal evolution. Gondwana Research, 79, pp.157-184.

779 Similox-Tohon, D., Sintubin, M., Muchez, P., Verhaert, G., Vanneste, K., Fernandez, M., Vandycke, S., Vanhaverbeke, H. and Waelkens, M., 2006 The identification of an active fault by a multidisciplinary study at the archaeological site of Sagalassos (SW Turkey). Tectonophysics 420, 371-387.

783 Sharma, R., Umapathy, G.R., Kumar, P., Ojha, S., Gargari, S., Joshi, R., Chopra, S. and Kanjilal, D., 2018 AMS and upcoming geochronology facility at Inter University Accelerator Centre (IUAC), New Delhi, India. Nuclear Instrumentation Methods B, 438, 124-130.

786
787 Singh, A., Thomsen, K.J., Sinha, R., Buylaert, J.P., Carter, A., Mark, D.F., Mason, P.J., Densmore, A.L.,
788 Murray, A., Jain, M., Paul, D. and Gupta, S., 2017 Counter-intuitive influence of Himalayan river
789 morphodynamics on Indus civilization urban settlements. Nature Communications 8,
790 http://doi.org/10.1038/s41467-017-01643-9.

791 Slater, L.J. and Singer, M.B., 2013 Imprint of climate and climate change in alluvial riverbeds: Continental
792 United States, 1950-2011. Geology 41, 595-598.

793 Sridhar, A., 2007 A mid-late Holocene flood record from the alluvial reach of the Mahi river, Western
794 India. Catena 70, 330-339.

795 Stewart, H.B. Jr., (1958) Sedimentary reflections on depositional environment in San Migne lagoon, Baja
796 California, Mexico. The American Association of Petroleum Geologists Bulletin 42, 2567-2618.

797 Stuiver, M. and Polach, H.A., 1977 Discussion: reporting of C data. Radiocarbon, 19, 355-363.

798 Tooth, S., 2000 Process, form and change in dryland rivers: A review of recent research. Earth-Science
799 Reviews 51, 67-107.

800 Tripathi, J.K., Bock, B., Rajamani, V. and Eisenhauer, A., 2004 Is River Ghaggar, Saraswati? Geochemical
801 constraints. Current Science 87, 1141-1145.

802 Twidale, C.R., 2004 River patterns and their meaning. Earth-Science Reviews 67, 159-218.

803 Valdiya, K.S., 2008 Sinking of ancient Talakad temples on the Kaveri Bank, Mysore Plateau, Karnataka.
804 Current Science 95, 1675-1676.

805 Valdiya, K. S., 2013 The River Saraswati was a Himalayan-born river. Current Science 104, 42-54.

806 Van Dijk, W.M., Densmore, A.L., Singh, A., Gupta, S., Sinha, R., Mason, P.J. and Rai, S.P., 2016 Linking the
807 morphology of fluvial fan systems to aquifer stratigraphy in the Sutlej-Yamuna plain of northwest
808 India. Journal of Geophysical Research: Earth Surface 121, 201-222.
809 http://doi.org/10.1002/2015JF003720.

810 Wulf, H., Bookhagen, B. and Scherler, D., 2010 Seasonal precipitation gradients and their impact on
811 fluvial sediment flux in the Northwest Himalaya. Geomorphology 118, 13-21.

812 Zhao, K., Wang, Y., Edwards, R.L., Cheng, H., Liu, D. and Kong, X., 2015 A high-resolved record of the
813 Asian summer monsoon from Donggecave, China for the past 1200 years. Quaternary Science
814 Reviews 122, 250-257.

815
816
817
818
819
820
821
822
823
824
825
826
827
828
829
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834 **Fig.1. Location of the study area and the recognized paleochannel courses.** A: Location of the study area within the Vaigai River Basin, embedded on a Digital Elevation Model. It also depicts the inherited nature of the river basin morphology. B: Inset shows the location of the region within the Vaigai River Basin that was subjected to image processing to recognize paleochannels and the locations wherein natural exposures were examined and excavations were made. Yellow lines show the paleochannels recognized through conventional image interpretation. C: Panels showing the paleochannel courses and the location/region selected for image processing, geophysical exploration and excavation. D: E: and F: Panels showing processed images and the paleochannel courses.

843 **Fig.2. Thematic information on the Vaigai River Basin and the studied sites.** A: Regional geology and structure. The tectonic line is aligned NE. As could be observed in all other rivers flowing east in the Peninsular India, the deltaic plains of the Vaigai River are also located on the east of this tectonic line. This tectonic line and other regional structures are Proterozoic structures and are active until recent (Ramkumar et al., 2016, 2017, 2018). B: Soil types present in the Vaigai River Basin and their spatial distribution. Note the occurrences of loamy soils only in the upper part of the basin and their relative absence in the middle and lower parts of the basin. C: Regional geomorphic features and their distribution within the basin. Note the occurrences of relict landforms that are inherited by the modern river channel. D: Regional slope characteristics of the Vaigai River Basin. The map depicts clearly the inherited landscape by the modern river channel. E: Landuse/landcover characteristics of the Vaigai River Basin. F: Rainfall distribution within the Vaigai River Basin.

855 **Fig.3. Composite litholog, lithofacies and depositional characteristics of the studied sections.** Based on the field logging of natural exposures and excavated pits, a composite litholog was constructed. The lithofacies characteristics, contact relationships and other criteria including depositional environments were interpreted and are presented in this figure. From the bottom, the litholog depicts the initiation of channel incision over former relict alluvium and its transition into river channel and into overbank deposit and later into sandy loam-flood plain (lithofacies types k-e). Again, sandy loam-flood plain deposits is followed by overbank deposit and finally modern flood plain (lithofacies types d-a) occurs. While the natural levee/overbank deposits were habitation sites/colonized by the ancient human population, the overbank deposits marooned them under sediments and forced shifting of settlements, as a consequence of stream avulsion events.

865 **Fig.4. Stratigraphic variations of textural properties, selected geochemical composition, ratios and indices of the sediments.** Mz: Mean size. SD: Standard Deviation. SK: Skewness. KG: Kurtosis. SiO₂ and TiO₂ are in wt. %; WIP: Weathering index of plagioclase, expressed in %; CIA: Chemical index of alteration, expressed in %. Note the ubiquitous monotony in all the textural parameters, despite representation of few thousand years of depositional history, punctuated with stream avulsion (note the general fining trend at the bottom of mean size and corresponding changes in all other textural characteristics). The overland flow followed by significant change in grain size is distinct and is also represented fairly in standard deviation. Also note that though the absolute values of textural characteristics show changes, the long-term trend represented by the linear
874 curve is more or less stable/steady signifying stable tectonic setting of the depositional sites. The
875 changes and trends observable in absolute value curves and the polynomial curves may, at the
876 best, reflect the climatic changes in the basin.

877 **Fig.5. Discriminant diagrams of depositional environment, energy, setting and transport modes.**
878 A: Discriminant diagram of River-Beach environments. The figure shows that all the samples fall
879 far away from both the discriminant lines of Moiola and Weiser (1968) and Friedman (1967), yet
880 in a scattered fashion. This suggests ubiquitous riverine nature of the sediments, however under
881 variable physical environmental settings and or energy conditions. B: Discriminant diagram of
882 River-Wave-Quite environments based on Stewart (1958). This figure shows that almost all the
883 samples fall in a region between River process and slow deposition from quite water, perhaps
884 suggestive of ephemeral nature of the river flow, variable energy conditions of transport and
885 deposition, an observation that is in conformity with the observations on stratigraphic variations
886 of textural properties, especially mean size (Fig.4A) and standard deviation (Fig.4B). C: Distinction
887 of depositional environments based on Glaister and Nelson (1974) shows the sediment samples
888 studied belong to middle deltaic plain, showing moderate maturity, and are deposited in braided
889 bar (an affirmation of stream avulsion character), point bar etc. D: Discrimination of modes of
890 sedimentary transport based on Passega (1957). Deposition of majority of the sediment samples
891 studied through rolling and suspension mode, a mode of transport normally possible during
892 flooding, in combination with the repetitive inferences based on lithofacies characteristics that
893 indicated natural levee and overbank deposit. Together, these suggest episodic flooding as a
894 result of off-seasonal and or catastrophic flooding due to climatic changes and consequent stream
895 avulsion events.

896 **Fig.6. Plot of Al/Na Vs CIA for interpreting weathering climate/intensity.** A range of weathering
897 characteristics are indicated by this plot as the samples show a spread across low-intermediate
898 silicate weathering, perhaps under the influence of dry-arid climate prevalent.

899 **Fig.7. Climatic interpretation based on SiO₂ Vs Al₂O₃+K₂O+Na₂O.** This plot corroborates with the
900 inference of shallow, monsoon-influenced ephemeral channels that received flows only for short
901 durations and also that they shifted their courses whenever extreme climatic conditions were
902 prevalent that forced habitats to get marooned/buried/shifted as all, if not most of the samples
903 fall within the arid climate field and show a range of chemical maturity.

904
905
906
907
908 TABLE
909

910 **Table 1. Radiocarbon ages represented in before present (BP) and calibrated ages (in CE and BCE).** The
911 serial nos. 1-8 were measured at AIMS facility in New Delhi and serial nos. 9 and 10 were
912 measured at Beta Analytic, U.S.A.

913
914
915
916 PLATES

917 Plate I.

918 a. Field photograph showing the backed clay ring lined dug well, backed brick paved floor and wall
919 in a pit excavated at Keezhadi. b. Close-up view of a backed clay ring lined dug well found in a pit
920 excavated at Keezhadi. Very close spacing between successive wells, variously sized diameters of
921 the wells and variously shaped and sized backed clay rings used for lining the dug wells suggest,
922 individual ownership and usages of these wells, shallow depth of water table, and individualistic
923 designs and productions of clay rings, custom-design and/or manufacturer-design or both
924 indicative of market-availability of a variety of produce which in turn point to a mature market,
925 producer-customer relationship. c. Dug well located in flood-paleochannel deposit at
926 Pasiyapuram. While all the dug wells found in Keezhadi and elsewhere were lined with backed
927 clay rings that have had perfect fitting shape one over other, the gaps between successive rings
928 were found filled with loamy clay at this location perhaps to enforce impermeability. This could
929 also mean that while in other locations, the dug wells were found dug in overbank and natural
930 levee sediments that are relatively clay rich and cohesive, the Pasiyapuram well was dug in loose,
931 unconsolidated channel and flood deposit, which might have been leaky. At this instance, while
932 the backed clay rings could have provided protection against well collapse, but not impermeability
933 and or contamination of percolating water. In order to avoid these, the well might have been
934 plastered with 25cm thick clay in the gaps between successive rings. This observation suggests the
935 knowledge level of the humans inhabited at this location. d. A globular shaped backed pot with
936 very narrow (1.5cm dia) opening perhaps produced for a specific industrial purpose. The gold
937 enrichment in surface soils at this level is multifold, suggestive of usage of this *potceramic* for
938 industry. e. A large, intact, perfectly and aesthetically crafted pot, perhaps used for storage. f. and
939 g. Black and red colored backed clay pots, with fish markings engraved on them and a Tamizh
940 brahmi name "kuviran atha" written on them. The fish insignia is the official insignia of the
941 Pandiya dynasty that had ruled the entire south India. Having insignia and name inscribed on pots
942 could indicate individual ownership and or official purposes to distinguish the vessels from others.
943 This practice also suggests the mature culture prevalent. h. Filters of various sorts and sizes made
944 out of backed clay, perhaps used to filter water and or industrial purposes. i. Bottom half of a pot
945 with largest diameter (65cm) size found ever in this part of the country. In addition to its size, its
946 character being black and Red colored ware adds value to this pot and might have been used for
947 storage, however, not for regular domestic purpose. j. A collapsed hearth with evidences of
948 burned soil and pot shreds. k. A series of horizontally arranged conical/tapered shaped backed
949 clay pipeline aligned over as series of horizontally arranged larger barrel shaped backed clay
950 pipeline, culminating at larger vertically aligned larger vessel made of backed clay. The
951 conical/tapered shaped backed clay pipeline has backed clay filters as shown in "h'. By all
952 probability, this arrangement might be for a dye industry and or gold jewelry industry, as the
953 coeval soil samples show multifold increase in Au enrichment than the background values. l. m. n.
954 o. p. s. and t. Burial urns were found with aesthetic epirelief features. The urns are multifold
955 thicker than all other pot shreds and contain three different types of human bone remains viz.,
956 charred/burned bone remains without any skull and or complete skeletal remains, bone remains
957 with other utensils/artifacts, perhaps used by the person who might have used and complete
bone remains including skull, jaw apparatus, etc. These three types of burial characters indicate at least three different cultural traits and or cultural groups that inhabited the region, almost simultaneously, as these are found at similar stratigraphic levels. q. Vertebrate bone remains found in a flood deposit. The sediments along with the bones might have been eroded during upstream regions and deposited in downstream overbank/flood deposit during stream avulsion associated with catastrophic flooding. r. Bone remains of bovine along with partially damaged/broken horn. Taphonomic arrangements of the bones and their broken edges suggest sacrificial ceremony and offering to a local deity of yore. It is a custom being followed since prehistoric time, Sangam era (to which the findings belong) and until today by certain communities to offer bovine sacrifice to 'ugra devata' (ferocious deities, such as Kali). u. Microlithic stone tool, perhaps used as a small handy blade found at Maranadu. This finding, together with the carbon isotope dates and existences of temples at Sakkudi (aged close to a thousand years ago), habitation sites at Agaram (aged about 600-700 years BP) and existence of modern city of Madurai, suggest continuous habitation in the Vaigai River plains since at least 10000 yr BP until today. v. Glass beads of various colors found everywhere in Maranadu, Keezhadi, Kondagai, Pasiyapuram, etc. The sizes of these beads are all uniform <1.5mm and all the beads have a uniform sized hole in them, suggestive of they were made to make garland/necklace, etc. This also suggest the mastery of technology that existed to make multiple colored glass beads, as they required various raw materials containing different metal oxides. w. A piece of bangle made of elephant tusk. This may indicate the prevalence of wealthy society. x. y. z. and ai. Statues and parts of sculpted (mythical creatures "yali") pillars of a temple that remained buried under second flood deposit vide Fig.3 and now excavated. This temple is located at Sakkudi, near Silaiman, east of the Tectonic line vide Fig.2A. aii. Statue partly exposed at Agaram, a location wherein buried temple and associated houses aligned in rows are detected and excavations are yet to begin. aiii. Statue of a Jain deity/high-priest, found at Maranadu. Initial assessment of age of this statue is about 1400 yr BP. A buried temple might be at the place this statue is found and needs affirmation through excavation. aiv. Statue of a local deity (Sasta with his consorts?), found at Keezhadi, perhaps worshiped by the ancestors and is being continued until today.
<figure><img src="image_5.png" /><figcaption>Figure1</figcaption></figure>
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<table><thead><tr><td>S. No.</td><td>Sample Name</td><td>Radiocarbon Age (BP)</td><td>Calibrated Age (95.4% Probability)</td></tr></thead><tbody><tr><td>1.</td><td>Organic Carbon</td><td>-689 ± 24</td><td>NA</td></tr><tr><td>2.</td><td>Human Bone 3</td><td>1115 ± 39</td><td>778 CE – 1016 CE</td></tr><tr><td>3.</td><td>Human Bone 2</td><td>1229 ± 30</td><td>689 CE – 883 CE</td></tr><tr><td>4.</td><td>K 10</td><td>1348 ± 27</td><td>641 CE – 764 CE</td></tr><tr><td>5.</td><td>K 12</td><td>1921 ± 40</td><td>20 BCE – 214 CE</td></tr><tr><td>6.</td><td>K 15</td><td>2314 ± 29</td><td>412 BCE – 237 BCE</td></tr><tr><td>7.</td><td>Pot Shred</td><td>2369 ± 30</td><td>539 BCE – 388 BCE</td></tr><tr><td>8.</td><td>K 17</td><td>2435 ± 34</td><td>752 BCE – 406 BCE</td></tr><tr><td>9.</td><td>Varusanadu-1</td><td>-830 ± 30</td><td>NA</td></tr><tr><td>10.</td><td>Varusanadu-2</td><td>-1250 ± 30 BP</td><td>NA</td></tr></tbody></table>
AUTHOR CONTRIBUTION STATEMENT

MR planned, drafted and finalized the work and the manuscript. MR, KK and KB planned, coordinated and executed the field work and subsequent laboratory analyses work. In addition to interpretation of data and drafting, RN, MS performed/provided access to geochemical analyses. SAR, KJJ, AF and RJ took part in the field survey and data analyses. KAP performed GIS analyses and interpretations. NAS, MJM and BS prepared figures, helped in interpretations and drafting text. RS, PK, and SC performed carbon dating and interpretations. All the authors corresponded, corrected, modified, and improved the text of the manuscript
DECLARATION OF INTEREST STATEMENT

The authors declare no conflict of interest.
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