SEDIMENTOLOGICAL AND HYDROGEOLOGICAL STUDIES OF PARTS OF DURG AND RAIPUR DISTRICTS, M.P., INDIA

Abstract

Rocks of the Chhattisgarh Supergroup exposed in parts of Durg an<i Raipur districts of Madhya Pradesh have been studied in detail towards their sedimentological and hydrogeological aspects. The Chhattisgarh Supergroup of Lower Vindhyan age (1000 - 700 million years ?) comprises two groups - a lower Chandarpur Group which is arenaceous in nature and an upper Raipur Group which is of calcareous and argillaceous character. The Chandarpur Group comprises only one formation, naraaly, the Chandarpur Formation. It is exposed in the southern part of the area. The Chandarpur Formation consists mainly of white to brown, fine to coarse grained, hard and compact sandstones with minor shales and conglomerates. Within the Chandarpur Formation, two lithofacies, namely,A and B, have been identified on the basis of lithologies, size of clasts, and sedimentary structures. The brownish sandstones of the lithofacies a are medium to coarse grained which show abundant cross beds with unidirectional paleoflow towards north, northeast,and southeast. The flesh coloured sandstones of lithofacies B are fine to medium grained, well sorted and exhibit horizontal to wavy laminations. The lithofacies A and B have been interpreted in terms of environmental facies as braided fluvial and beach facies respectively. Overlying the Chandarpur Formation (ii) is the Charmuria Formation of Raipur Group which occupies the central part of the area. The Charmuria Formation consists of white, cream, grey and buff coloured karstic limestones. Pink to brown coloured, fissile shales of the Gunderdehi Formation overlie the Charmuria Formation which are exposed in the northern part of the area. A thin patch of alluvium is present in the southeastern part. The rocks are mostly horizontally disposed and show occasional dips of 2° to 5° towards northeast. A distinct unconformity separates the rocks of the two groups, namely, the Chandarpur and Raipur Groups. A number of faults have been inferred on the basis that the horizontal formations of different ages lie in juxtaposition with each other. Two sets of major faults running NE-SW and NW-SB have also been delineated on the aerial photographs. Textural analysis, based on thin section study, suggest that the beach sediments are better sorted than the fluvial ones. However, the fluvial sediments become progressively better sorted towards north and northeast. Thus the bsach and distal fluvial sediments appear to form relatively better aquifers. The sediments of the two different environments, viz., fluvial and beach, have been differentiated with the help of multivariate linear discriminant function analysis, using various grain size and shape statistics. Petrographically, the sandstones of the Chandarpur Formation are mainly quartz arenite with minor subfeldspathic lithic arenite. Quartz is the dominant constituent among the light minerals. Quartz grains show overgrowth,thereby indicating the well cemented iii nature of these sediments. Such an overgrowth on quartz grains in sandstones diminish their porosity. However, the beach and distal fluvial sediments exhibit poor overgrowth on quartz grains, and as such they form better aquifers. Zircon, tourmaline and rutile are the common heavy minerals. A variety of source rocks, mainly plutonic and metasedimentaries seem to have contributed towards the deposition of these sediments. Further, the occurrence of rock fragments of granite and quartzite in thin sections confirms taat the granites and quartzite exposed to the south of the area are the main source rocks. Sediments of two different facies, namely, fluvial and beach, have been differentiated with the help of multivariate linear discriminant function analysis using modal constituents of these sediments. The limestones of the Charmuria Formation are mainly micritic limestones which are karstic in nature and thus exhibit secondary porosity and permeability. They seem to have been deposited in a shallow water intratidal and supratidal environment. Illite, kaolinite, montmorillonite and chlorite are the major clay minerals of the shales of Gunderdehi Formation. Lateral variation of kaolinite-illite ratio suggests that northern and northeastern parts were 'marine like', whereas the southern and southwestern parts were 'continental like1 during the geological past. Incidentally, the paleocurrent directions of the sandstones of Chandarpur Formation are also towards north, northeast and southeast. The paleoslope of the basin towards north and northeast remained unchanged in this part of the Chhattisgarh basin during iv the deposition of these sediments. In the drought prone area, the main source of ground water are dug and dug-cum-bored wells which tap the shallow aquifers. Dug wells of moderate to shallow depth (5 to 9 m) are in alluvium and limestones; whereas those in shales and sandstones are relatively deep (9 to 16 m). General direction of ground water flow is towards NE and NW on the eastern and western sides respectively of the Tandula Main Canal, which perennially flows through the central part of the area. Photo-hydrogeological interpretation suggests that wells falling on a lineament or on the intersection of lineaments show high yield e.g., in villages, Limora, Bori, Nipani, Surra, Arkar etc. Based on electrical resistivity method, only shallow regime aquifers (5 to 20 mdepth) have been delineated. Aquifer and well characteristics have been determined by analysing the pumping test data. Transmissivity (T) and storativity (S) for a small diameter well tapping the limestones of Charmuria Formation range from 360 to 449 m2/day and 0.44 x 10~4 to 0.21 to 10" respeetively. Transmissivity values for the shallow aquifers in alluvium, limestones and sandstones range from 1000 to 2112, 250 to 566 and 52 to 126 m2/day respectively, as obtained by analysing the pumping test data of large diameter dug wells by Papadopulos-Cooper's (1967) method and Theis' recovery method. Specific capacity of dug wells tapping alluvium, limestones and sandstones range from 698 to 1787, 15 to 159 and 8 to 16 Ipmin./m respectively. In case of shallow tube wells, the productivity values have been subjected to frequency analysis and it is observed that the limestones are the most productive aquifers; the sandstones form the second best aquifer, whereas the shales are poor aquifer. Productivity, in general, increases in the direction of ground water flow. Transmissivity values have also bean computed from specific capacity data using Theis' formula. Within limestone terrain, the transmissivity increases in the direction of ground water flow. Ground waters from various formations, viz., alluvium, shales, limestones and sandstones, have been analysed chemically. Most of the waters are rich in alkaline earths (Ca2+ and Mg2+) and weak acids (EC0~ and CO ~ ) irrespective of the lithology. The most dominant hydrochemical facies in the area, irrespective of lithology,is Ca-Na-Mg-HX-j-Cl-SO . Concentration of major ions, TD3 and hardness as CaCO^ increases in general, in the direction of ground water flow; whereas the ratios S0./C1 and Mg/Ca decrease in the direction of ground water flow. The ground waters are quite suitable for drinking and irrigation purposes. Most of the ground waters from limestones are undersaturated to just saturated with respect to CaCO^. However, some ground waters from areas farther from the recharge zone are oversaturatud with respect to CaCO,. vi The ground waters from different formations have been differentiated on the basis of their chemical characters with the help of multivariate linear discriminant function analysis. Thus, it appears that the lithology has a bearing on the chemical quality of water. In the light of above investigations in parts of Durg and Raipur districts, it is inferred that the grey coloured micritic limestones of the Charmuria Formation, which are karstic also, are the most productive aquifer. Beach and distal fluvial sediments in the northern and northeastern parts of sandstone covered areas are second best aquifer. The shales of Gunderdehi Formation are poor aquifer. However, the calcareous bands within the shales are karstic and thus can be tapped for ground water exploitation.