SEDIMENTOLOGICAL AND HYDROGEOLOGICAL STUDIES OF CHORWAD-MADHAVPUR AREA JUNAGADH DISTRICT, GUJARAT, INDIA

Abstract

The present study is an endeavour to understand geohydrological aspects eg. ,hydraulic characteristics and hydro chemistry of the groundwaters in limestones of the GaJ and Miliolite Formations (Tertiary and Quaternary) occurring in the Chorwad- Madhavpur area along the southwest coast of Saurashtra. In addition to this, the stratigraphy of these formations has been worked out as this was not studied in detail by previous workers. Also an attempt has been made to examine the development of porosity due to solution activity and diagenetic changes as groundwater occurrence and movement is influenced by these aspects. Detailed study of the Gaj and Dwarka Formations could not be carried out because of the scarcity of outcrops,whereas a detailed study of the Miliolite Formation has been made. The Quaternary carbonate sediments of the Saurashtra coast hav baffled the geologists ever since the term 'Miliolite' o •Miliolite limestone' was first proposed by Carter in 1849. The early workers described it as an oolitic limestone containing the foraminifera Miliolina. Srivastava (l968) was the first who gave a detailed petrographic classification of these limestones. Mathur and Mehra (1975), proposed a new stratigraphic classification for the Miliolite limestone. They renamed the •Miliolite limestone' as Miliolite Formation, while Sub-Recent deposits were referred as Chaya Formation which were put under re )r

the Porbandar Group. This classification has been followed in the present study also. Based on field observations of vertical sections at different places in the area and petrographic studies, the author in the present study has divided the Miliolite Formatic into three lithofacies, i.e., lithofacies I,II and III. The lithofacies I and II have been again sub-divided into litho facies IA,IB, and IC, and IIA, 113, and IIC respectively. Petrographic study of the limestones beloging to the Miliolite Formation indicates that the rocks include calcarenites, calcirudites, chalk (micrite) , and pelletoid calcarenites. It is not essentially an oolitic limestone as described by earlier workers. Neomorphism is very common. Sparry calcite occurs both as cement and neomorphic grains. Scanning electron microscopic study indicates the presence of micro and macroporosity of both intergranular and intragranular types. Microporosity is generally intragranular. Vuggy and channel porosity is common. The porosity has been developed by the dissolution of carbonate grains and skeletal material. Primary porosity is also not obliterated due to diagenesis. Calcite crystal growth and formation of dolomite are also noticed. Lithofacies I, IIA and IIB of the Miliclite formatic having extensive secondary porosity act as major aquifers. Diagenesis of carbonate rock took place successively in (i; stagnant marine phreatic zone (2) active marine zone (3) mixed freshwater and marine water zone (4) freshwater .on :on (vii) phreatic zone, and (5; freshwater vadose zone mainly during a marine regression. Regarding the environment of deposition, the lithofacies 1A might have been deposited in lagoons and IB and IC by fluviomarine action during a transgressive phase. Lithofacies IIA and IIB have been formed in the littoral zone dominated by strong wave action and transitional zone between littoral and shelf zone respectively. Lithofacies IIC contains abundant foraminifera, i.e., Cibicides, Uvigerina. Bulimina. Cassudilina. Bolivina. Eponides. Nonionella. Fissurina. and Lag en a which suggest relatively deeper water biofacies and most probably an outer shelf marine environment. Lithofacies III is pelletic calcarenite which has been formed by the action of wind by reworking the deposits formed as a thin sheet in lagoons left behind the regressing sea. The hydrogeological studies in Gaj Formation and Miliolite Formation have been carried out in the area under study. Rainfall is one of the most important factors which influence the ground water levels. The small though significant values of correlation coefficient between rainfall and ground water levels indicate that although rainfall affects the water levels, other ifactors such as irrigation, pumping, etc. are also responsible for groundwater fluctuations. A study of the trend surface maps of water-level data indicates that the general groundwater flow is towards i south, southwest, and west from the central part of the area. In the western part, the groundwater flow is from the coast towards inland and a groundwater trough is seen. This might be because of the overdevelopment of groundwater for irrigation purposes. Acomparison of pre-monsoon (March) and post-monsoon (October) groundwater levels in the Gaj Formation and the Miliolite Formation shows that the groundwater fluctuation is -ore in Gad Formation. This indicates difference in the geohydrological characteristics of Gaj Fetation and Miliolite Formation. Anumber of pumping tests were conducted in dug add dugcum- bore wells tapping the Miliclite Formation and the Gaj Formation with a view to study the aquifer characteristics. Ihe drawdown data have been used to determine transmissivity (T) by Papadopulos-Cooper method and Boulton and Streltsova method and storativity (S) by Papadopulos-Cooper method. From recovery data also Twas determinedby using Theis recovery method. It is observed that the I values obtained from Boulton and Streltsova'. method are comparable with those obtained from the later part of recovery data, using Theia equation. The transmissibility of Miliolite Formation is higher as compared with the Gaj Formation. The specific capacity determined from recovery data indicate that it is higher in the Miliolite Formation than in the Gaj Formation. The productivity values as determined from specific capacity data also indicate that the limestones belonging to the Miliolite Formation are mere productive than

the Gaj Formation. A total of 67 groundwater samples were collected from wells tapping the Miliolite Formation, Gaj Formation and Deccan Trap basalts during February, 1979 and 29 samples during October, 1979 for chemical analysis.lt has been observed that the groundwater in the coastal belt within the Miliolite Formation is quite saline. Trend surface maps of various ions (except HCO~) indicate an increase in concentration towards the coast. Hydrochemical sections from inland towards the coast in the direction of ground water flow also show a similar trend. The high Cl"/(CO?" +HC0; ) ratios, increase in salinity with pumping, the total alkalinity/ total hardness ratio which is ^. l,the primary and secondary saline nature of water samples, all indicate mixing of modern sea-water with groundwater. The mixing of sea-water and fresh water is on account of over pumping at isolated places and also due to the ingress of sea water along the creeks and rivulets at the time of high tides. The positive base exchange index values of most of the waters from the Miliolite Formation indicate base exchange of Na+ present in the mixture of groundwater and sea-water,with the Ca 'v present in the clays of Miliolite Formation, This is further evidenced by comparing a hypothetical groundwater with that of sea-water by comparing the percentage reacting values of Na + K + Ca + of groundwater with those of sea water. The plotting of hydrochemical data in Hill-Piper diagram indicates that groundwater from the Gaj Formation is mainly of secondary (x) alkaline type while those from the coastal belt in the Miliolite Formation are of primary saline and secondary saline type. It has also been observed that groundwater further inland in the Miliolite Formation has low salinity. Therefore, a belt of highly saline water is restricted only in the coastal region. Earlier,!.e.,till about ten years back, in this belt groundwater was fresh. All these observations indicate that salinity is due to intrusion of modern sea-water, later modified by various processes. Carbonate equilibria studies show that most of the waters are saturated with respect to calcite. In the Gaj Formation, the waters in the north are undersaturated in nature. In the Miliolite Formation, near to the coast, in many places, the waters are of undersaturated nature. It is seen that the groundwaters from Miliolite Formation are of poor quality and hence not suitable for domestic and irrigation purposes. Even then the people use this water for various purposes because of the non-availability of better quality, water. The groundwater of the Gaj Formations are of better quality and can be used for various purposes.