EVOLUTION OF THE REGIONAL HYDROGEOLOGIC UNITS OF THE GREAT INDIAN SEDIMENTARY BASIN IN RELATION TO PREVAILING TECTONIC MOVEMENTS

ÉVOLUTION DES SECTIONS HYDROGÉOLOGIQUES RÉGIONALES DU GRAND BASSIN SEDIMENTABLE INDIEN PAR RAPPORT AUX MOUVEMENTS TECTONIQUES

M. BAUDHAN International Hydrologieal Programme, Council of Scientific and Industrial Research, Rafi Marg, New Delhi-110001, India RÉSUMÉ L'auteur a étudié le rôle des mouvements tectoniques dans la formation des princi­ pales sections hydrogéologiques régionales du Grand Bassin Sédimentaire Indien, situées dans l'ouverture entre la chaîne montagneuse de l'Himalaya au Nord et le rempart péninsulaire au Sud. En s'appuyant sur les données géohistoriques et les résultats des récentes explorations géologiques, il propose une nouvelle hypothèse expliquant l'origine du bassin. Il démontre que les mouvements tectoniques ont exercé une influence décisive sur la détermination de la configuration, de la structure, de la stratigraphie, de la litho­ logie et même des caractères physico-chimiques des quatre différentes sections géologiques du bassin, à savoir, les zones de Siwalik, Bhabar, Terai et Alluviale. L'auteur étudie également les principales caractéristiques hydrogéologiques de ces sections en vue de souligner leurs traits distinctifs.

Introduction

The Great Indian Sedimentary Basin, drained by the Indus—Ganges — Brahmaputra river system, is one of the largest and most productive ground­ water provinces of the world. It is flanked by the Great Himalaya in the north and the Deccan Shield in the south (Fig. 1). The history of development of this basin and its sedimentary units is closely related to the orogenic and neotectonic events of the late and post Himalayan times. These events are traced back to assess their impact on the formation of the basin and its filling materials. For this purpose, reliance is placed on the geohistorical and recent geotechnical informations. It is shown that there had been both direct and indirect influences of these movements in shaping the basin as well as in deciding the distribution and nature of the sediments. The existing ideas pertaining to the origin of the basin are discussed and a new hypothesis is proposed, which appears to better explain the present structural and hydro- geologic features of the basin as well as the history of sedimentation leading to the development of the regional hydrogeologic units. 287

Fig. 1. Distribution of regional hydrogeologic units in the Great Indian Sedimentary- Basin

Description of the basin and its hydrogeologic units

The basin runs a length of over 2400 km from Punjab in the west to Assam in the east. Its width is variable, the maximum being over 400 km and the minimum as small as 25 km (Fig. 1). The floor of the basin appears to be highly disturbed by NE —SW trending cross faults, as have been located at Patna, Lucknow, Moradabad and other places in the basin, giving rise to a good number of horsts and grabens (MITHAL and SRIVASTAVA, 1959). While 288

the configuration of the basin floor has not been worked out in detail, it can be reasonably inferred from the available information that the presence of these horsts and grabens together with the existence of a large number of apophyses of Peninsular shelf margins, is liable to render the topography of the basin bottom highly rugged and irregular. This situation should lead to the uneven spatial distribution of thickness of the sediments in the basin, so also the groundwater regime, particularly in the deeper zones. It is further postulated that there exist at least five to six deeper sub-basins along the northern margin of the basin with different groundwater regimes (MITHAL, 1966). The basin is filled with four distinct sedimentary units designated as the Siwalik, Bhabar, Terai and Alluvial formations (MEDLICOTT, 1873), which are disposed in a nearly parallel fashion between themselves as well as with the Himalayan range. Though the major part of the Siwalik formation is now present as the Siwalik hills along the northern border of the basin and consi­ dered as a separate geologic system, from a tectonic angle it can be considered as an integral part of the Great Indian Sedimentary Basin, as will be clear from the discussions to be taken up later. In the following the relevant salient features of the four units of the basin are presented to facilitate further treat­ ment of the subject matter at hand.

Biwalik system

The Siwalik system derived its name from the Siwalik hills lying in the Hardwar region of Uttar Pradesh. It continuously extends along the southern foot of Himalaya from the Brahmaputra valley in the east, to the Potwar Plateau and Bannu plaines in the west. Its equivalents are seen in Sindh, Baluchistan, Assam and Burma. The Siwaliks comprise consolidated and semi-consolidated rocks namely sandstones, grits, conglomerates, pseudo- conglomerates, clays and silts. They have the character of deposits formed by torrential streams and floods in shallow and fresh-water basins. The fossils found in them indicate that the earlier beds were deposited in a somewhat brackish environment. The Siwalik system is divided into three major divisions namely the Lower, Middle and Upper Siwaliks, ranging in age from Middle Miocene to Lower Pleistocene. While there are no marked unconformities within a system, there are indications that the Upper Siwaliks were deposited on the Middle Siwaliks after severe tectonic disturbances resulting in folding and uplift. So far the Siwaliks are very little exploited for groundwater. However, due to high elevation, in general the groundwater conditions in them appear to be difficult except in rare, wider longitudinal valleys.

Bhabar formation

The Bhabar formation comprising boulders cobbles and gravels as pied­ mont deposits occurs all along the southern slope of Siwaliks as a distinct belt, varying in width between 3 and 24 km. The formation occurs as an accumulation of talus materials and coalescent alluvial cones built by the hill streams. Groundwater is known to stay in this formation as unçonfined. The 289

water table is generally deep but varies between 5 and 90 m below ground level. The ground slope is high and towards the south in the range it descends 8 to 17 m per km. The individual alluvial cones in the Bhabar belt have their own set of aquifers and the adjacent cones appear to have poor inter­ connection (SAH, 1966). The water table contours generally reflect upon the surface topography. Yearly fluctuation of water table is rather high and a figure of 8 m is quite common. In general the ground water conditions are considerably better along the southern portion of the belt, both in respect of depth of occurrence and quantum of availability. Most of the surface streams in the Bhabar zone remain dry except during monsoon (July to September), though they may be perennial both upstream in the Siwalik hills,and down­ stream in the Terai zone, respectively. This is due to the absorption of the flow of these streams in the highly porous materials constituting the Bhabar.

Terai formation

Immediately following the Bhabar belt on its south is the Terai belt, composed of alternate layers of clay and sand-pebble beds. A spring line is usually seen to separate the Bhabar from the Terai. These sand beds, except the topmost one, usually form artesian aquifers, in which the piezometric level lies at 0.3 to 1.5 m above ground surface. The pressure head shows a tendency to decrease from the north to the south. The confined aquifers occur at varied depths, but more often below 60 m. The general slope of ground is towards south averaging about 0.4 m per km. The sand-clay ratio in the Terai formation is of the order of 25:75. The granular beds, mostly confined to the stream channels, appear as tongue-like projections into thick clays, often maintaining a lithological continuity with Bhabar in the north. The permeability of the granular zone in the Terai formation show large spatial variation due to the presence of highly irregular textural gradation. In this zone the streams are perennial, and many of them receive discharge from the spring-line intervening the Bhabar and Terai.

Alluvial Plain deposits

On its south, the Terai belt is followed by the vast alluvial plain comprising of sand and clay with kankar. The sand beds constitute highly rich aquifers. It is interesting to note that in the northern half of the Plain the aquifers maintain a continuity in the N—S direction, whereas in the southern half an E — W continuity is exhibited. On a regional scale the aquifers are unconfined but subartesian conditions have developed locally (SINGHAL and GUPTA, 1966). However, flowing wells are practically absent in the alluvial Plain. The depth of water table or piezometric surface lies within about 4 to 12 m from the terrain level. The aquifers commonly show a lenticular character indicating that the sand and gravel layers were deposited in the channel beds whereas the silt and clay beds were formed in the flood plains (MITHAL et al., 1973). It is estimated that the alluvial plain together with the Terai and Bhabar zones, covering an area of about 1,048,500 sq.km holds a groundwater reserve of approximately 9.08-1013 cu.m within a depth of 300 m from the terrain level (MITHAL, 1966).

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Evolution of the basin The history of development of the basin is interlinked with the formation of Himalaya and hence with the past existence of the great mediterranean sea or Tethys geosyncline. The basin emerged as a by-product of the tectonic process that moulded the sediments of the Indian portion of the Tethys geosyncline into the Himalaya mountains. The movements associated with this tectonic process acted intermittently throughout the Tertiary period in 5 to 6 major stages, beginning at the end of Cretaceous and culminating in the Early Pleistocene. The basin appears to have begun to form in the Late Eocene and attained the fullest development in the Middle Miocene, during the third and most violent Himalayan upheavel, at the end of which the Tethys basin disappeared. While the major tectonic movements built the initial basic framework of the basin, finishing touches towards its final shaping were given by the neotectonic movements. There are at least three different opinions regarding the type of tectonic movements responsible for the emergence of the Great Indian Sedimentary Basin (KRISHNA», 1960), as discussed below. According the EDTJARD STJESS, the basin is a "fore —deep" formed due to compression exerted on the Tethys sediments by the moving Central Asian mass in the north, against the stable Indian Peninsular mass (Deccan Shield). But SIR SYDNEY BTTRRARD considered the basin as a rift valley formed by parallel faulting on both sides, that is, along its northern and southern borders. The third view is that the basin is a sag in the crust in front of the rising Himalaya, caused by the northward drift of the Indian sub-continent against the Tethys sediments. However, taking indications from the recent geotechni- cal, especially geophysical data, a new hypothesis for the origin of the basin can be proposed. In the opinion of this author the basin is the remanent southern continental shelf zone of the old Tethys sea which refused uplift during Hima­ layan orogenesis with the support of the resistant Deccan Shield. The following observations tend to back up this hypothesis. Firstly, recent gravity anomaly data (QUERSHY, 1964) give evidence that in the north the Deccan Shield forms a wide continental shelf margin under the Indo-Gangetic alluvium of which the apophyses appear to extend even below the Siwaliks. Secondly, the Siwalik sediments indicate that water of the basin of deposition was first brackish and then it became increasingly fresh with time. Thirdly, recent deep geophysical survey and drilling activities have proved that contrary to the earlier belief, the thickness of the sediments in the basin is not of the high order of 12,000 to 15,000 m but much lower. As for example, in Ujhani, Kasganj and Tilhar in Uttar Pradesh the thickness " of alluvium averages only 400 m. Forthly, it appears from the study of the general shape of the depression of the basin that it is deepest within a few kilometres of the Himalayan foothills, and progressively shelves up towards the Deccan Shield in the south.

Development of the sedimentary units The tectonic movements were not only responsible for shaping the basin, as already discussed, but dictated the terms of sedimentation and the nature of the materials deposited therein. The tectonic forces acted in three principal 291 directions namely upward, downward, and lateral as will be further discussed later. At the outset of this section, it should be mentioned that while the long-term sedimentation sequences were principally controlled by the paleo- tectonic and neotectonic movements. The short sequences were mainly deter­ mined by the seasonal climatic cycles. This is true of all the four sedimentary units of the basin, but conspicuously traceable in the case of Siwalik system. Hence this aspect will be discussed in greater detail with reference to the Siwalik formations. The basin took its root in the Middle Miocene, that is after the third episode in mountain building, and since then the sedimentation history became prominent facilitating the deduction of the tectonic and climatic environ­ ments then prevailing. The sandstones of the Siwalik system are coarse­ grained and mostly ungraded, indicating that they were borne by torrential streams. The great lateral extension of the sedimentary beds suggests that the basin of deposition was practically continuous from Assam in the east to Punjab in the west. Further, the sediments are extraordinarily similar over long stretches along the strike of the basin signifying that they were derived from the same or similar source rocks. It appears that the prevailing tectonic conditions helped in deriving most of the sediments from the north due to the denudation of the newly risen Himalaya, offering terrain with steep slopes to the south. The coarse sediments may have been derived in the wet flood season. The presence of the ancient stable Deccan Shield in the south offered low relief to the streams originating from it, which appears to have contributed mainly the finer ferruginous materials. The character of the sedimentary sequences and the high rate of deposi­ tion of coarse materials reflects upon the presence of monsoon climate during the greater part of the Siwalik time. This climate might have favoured high rainfall, being induced by the orographic effect caused by the establishment of Himalaya. The abundance of fossils of plants, molluscs, fishes, reptiles and mammals in some areas in Siwalik also testifies to the presence of favourable climate and hydrologie conditions during that time. The thickness of the Siwalik sediments is 5000—6000 m, while they are mostly coarse. Such large thickness course of shallow water deposits gives evidence of the interplay of neotectonic movements during the period of sedi­ mentation. It leads one to think that with an increasing accumulation of the sediments there had been gradual downward movement or sinking of the basin. Further, the intermittent activity of the lateral tectonic forces may be inferred from the fact that the basin of deposition has shifted in steps towards the south, particularly at the end of Middle Siwalik time. Probably these shifts were associated with the pulses of uplifts. The evidence of the occurrence of the upward movement, even late after the creation of the basin is given by the fact that while the sediments are thrown up as the Siwalik hills along the northern border of the basin, to the further south the Siwalik formation continues below the younger sediments of Bhabar, Terai and even Alluvial Plain. This phenomenon of asymmetrical uplift of the basin made the earlier-deposited sediments, along the northern part of the basin, the source rocks for the later sediments. Hence, it is seen that the tectonic move­ ments have greatly influenced the order and pace of sedimentation as well as the location of the source rocks. Thus the stratigraphy and the texture of the formations in the basin were influenced by the tectonic movements.

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This is not all. In the following it will be discussed how the structural, hydro- physical and even the hydrochemical characteristics of the sediments were influenced by the tectonic movements. The intermittent southward shift of the basin coupled with the progressive rise of Himalaya helped the Bhabar to overlap the southern portion of the Terai and further the Terai, in turn, to come over the alluvium of the Plain. This process of overlapping, engineered by the tectonic movements, made the boundaries between these formations irregular and confusing. Further, this process is perhaps responsible for the creation of some perched aquifers in the Bhabar and Terai belts. A significant impact brought forth by the tectonic movements is the frequent change in the drainage pattern of the basin which induced far-reaching geohydrological changes, such as rapid spatial variation in thickness, particularly of the Plain. As a result of the frequent shift of the drainage channels the disposition of the individual aquifers is rendered highly complex, so much so that on a regional scale the various aquifers in the basin are interconnected, although locally they may exhibit semi-confined and leaky conditions (SINGHAL and GTJTTA, 1966). The frequent dislocation of the stream courses and change in their gradient, caused by the tectonic movements, also influenced the stratigraphy, lithology and configuration of the alluvial cones in the Bhabar zone, such as the rivers Gola, NIhal, Nandhaur, Bhakra, Kosi etc. Regarding the influence of the tectonic movements on the quality of water in the basin the following observations can be made. Due to the uplift of the northern portion of the basin into the Siwalik hills a high ground slope is established along its northern border, which is gradually moderated in the direction of south, towards the Plain. This created the condition for maintain­ ing a favourable hydrochemical regime in the northern three hydrogeologic units of the basin, namely the Siwalik, Bhabar and Terai. The high ground slope induced steep hydraulic gradient in the aquifers of these units, thereby facilitating good flushing and quick subsurface drainage. As a result, the groundwaters in these units are generally fresh with low content of dissolved solids. However, the alluvial deposits in the Plain, occupying low elevation, hold groundwater of variable chemical quality. While in the well-drained areas of the Plain the groundwater is fresh, in clayey and stagnated pockets brackish and even saline water is seen to occur. It may be noted, however, that in the Terai belt while the confined aquifers are generally fresh, the top water table aquifer is often brackish. This may be due to the accumulation of salt in this aquifer owing to evaporation from the groundwater table, which is quite shallow in this belt. Further, it would appear resonable to say that the hydrochemical regime of the basin, particularly along its northern part, has undergone intermittent changes due to the redisfrietion of the drainage network from time to time. This situation leaves the possibility of existence of some subterranean fossil hydrochemical regimes in some areas of the basin.

REFERENCES

KKISHAN, M. S. I960: Geologj- of India and Burma, Higginbotharns (Private) Limited, Madras, India. MEDLICOTT, H. B. 1873: Rec. Geol. Survey of India, Vol. VI. 293

MITHAL, R. S. 1966: A reappraisal of groundwater distribution and provinces of India, Proo. Sympo. on groundwater studies in arid and semi-arid regions, University of Roorkee, India. MITHAL, R. S.—SINGHAL, B. B. S.—BAJPAI, I. P. 1973: Groundwater conditions in the Gangetic Alluvium of Western Utter Pradesh, Proo. Intnl. Sympo., CSIR —IHD — UNESCO, Madras, India, Vol. 3, V. MITHAL, R. S. — SEIVASTAVA, L. S. 1959: Geotectonic position of Gango — Brahmaputra region, 1st Sympo. in Earthquake Engineering, Roorkee. QUBESHY, M. N. 1964: Geologic analysis of Bouguer anomaly map of peninsular India, Proc. Natl. Instt., Soi., India., Pt. A, Vol. 30. SAH, D. L. 1966: Geohydrology of Bhabar and Terai belts of Naital district, U.P., Indian Geohydrology, Vol. II. No. I. SINGHAL, B. B. S.— GUPTA, B. L. 1966: Analysis of pumping test data from a well in the Indo-Gangetic alluvium of India and its bearing on the aquifer characteristics, Journal of Hydrology, Vol. 4.