Chapter 3 : General Geology, Lithology and Field
Total Page：16
File Type：pdf, Size：1020Kb
CHAPTER 3 : GENERAL GEOLOGY, LITHOLOGY AND FIELD OBSERVATIONS GENERAL GEOLOGY, LITHOLOGY AND FIELD OBSERVATIONS This chapter is devoted to field observations, locality wise giving a detailed account of the geological set up of the area covered in the present work. Based on these observations several 1ithosections were plotted to know the facies variation within individual stratigraphic members. The marine Quaternary rocks of Holocene age are exposed along the west coast of Maharashtra and Goa. These rocks are friable to well cemented and constitute quartz sand, molluscan shell rubbles, trap pebbles, along with foraminifera 1 tests, bryozoan fragments, ostracod shells and minor amounts of coral fragments. They usually occur in the intertidal zone invariably extending up to 400-500 m in the supratidal region and develop a maximum thickness of 10 m. Ordinarily, these rocks are masked by soil, beach sand and sand dunes. Therefore, the sections exposed along the coasts, small creeks, tidal inlets and dug-wells provide valuable information. Especially, the sections exposed in the wells are useful in the lithological correlation. Beach rocks are persistent in their lithologic characters and have a great lateral extent. Sometimes, these rocks also occur as patches dipping seaward and parallel to the existing coast. 46 The plastic mud, and sandy clays are overlain by the beach rocks. These soft lithologies are distinct from the modern sand and mud. The surface exposures of plastic mud are scarce being mostly hidden by sandy soil. These are also rich in a variety of well preserved marine fauna. However, so far geologists and palaeontologists have not paid much attention towards these rocks. 3.1 NOMENCLATURE : The subrecent marine coastal calcareous sediments are commonly known as Beach rock or "littoral concrete" (Buist 1851, Carter 1852, Pascoe 1964). In Indian Geology, the term "littoral concrete" was hardly used by geologists, geomorphologists or sedimentologists except for particular context. Only Ahmed (1973) used this term due to their close resemblance to beach sandstone. Also in the Gazetteer of Maharashtra State (1973m Vol.l) the term "littoral concrete" has been retained. These rocks are also described as "calc arenite" composed predominantly of wave worked shell debris and at some places it is foraminifera 1 or oolitic having mixed aragonite and calcite and inorganically cemented under conditions of intertidal exposures with an aragonite cement that rapidly inverts to calcite (Ginsberg 1953, Setty and Wagle, 1971). 47 Guzder (1980) used the colloquial term "karal", which is common in Konkan to describe cemented sand and shell deposits of these kinds irrespective of differences in composition and location. Whereas Badve et a/. (1984) used the term ”she 11 - 1 imestone” for the beach rocks composed of coarse to fine grain sand along with megascopic and microscopic shells cemented together in a fine grain carbonate matrix and the source is being used in the present work. Moreover, Badve et al. (1984) coined the term "mudstone” for brownish to greenish-grey coloured, soft and highly calcareous mud. However, in the present work the author prefers to use "plastic mud” as an informal term due to lack of precise sedimento1ogica1 status. 3.2 STRATIGRAPHIC CLASSIFICATION : Badve et al, (1984) proposed the stratigraphic succession to accommodate these rocks. They have described them as the Raigad Group, comprising the Nagaon Formation and the Dive Agar Formation. The former is dominantly represented by plastic muds and sandy clays, while the latter is mainly shell limestone. The complete succession of Raigad group is as follows : 48 Format ion Li tho Iogy Type Thickness locality Dive Agar Moderately soft Dive agar 8-10 m Format ion to hard, massive to thinly bedded, grey coloured, medium, grained, fossi1 iferous limestone with gritty and conglomeratic bands. Nagaon Moderately soft, grey Nagaon 2.5 m Format ion to greenish-grey, highly calcareous plastic mud, loose sands and clays of semi-plastic nature. Base not seen (After Badve et a/. 1984) 3.3 NAGAON FORMATION : The Nagaon Formation constitutes the basal member of the Raigad Group which represents plastic muds and sandy clays. In some observed sections it is unconformably overlain by the shell limestone of the Dive agar Formation. It is greenish grey or brown to greyish brown in colour. Therefore, it shows superficial resemblance to recent mud occurring along the tidal inlets. The plastic mud at Nagaon village along the shore shows the development of mud cracks and ellipsoidal to spindle shaped mud balls formed due to wave action. Embedded 49 in these mud balls are gastropod, bivalve shells and microfossils such as foraminifera, ostracoda and bryozoa. The exposed thickness of this plastic mud at Nagaon is about 1 m. The plastic mud at Sion, Kurla is sandy in nature. Whereas at Koparkhairna this fine plastic grey mud occupies the capacious valves of the oyster bed exposed in a foundation pit. The plastic mud at Revdanda is brownish coloured and moderately soft in nature. All of these plastic mud localities show fairly well preserved mega as well as microfossil assemblages. 3.4 D I V E A G A R FORMATION : The Dive Agar Formation mainly comprises shell limestone which is the upper member of the Raigad Group. Outcrops of the shell limestones show laminated, gritty or conglomeratic nature at the base while massive, compact nature towards the top. The boulders and pebbles of both basalt and laterite make up the ungraded conglomeratic bands. Mostly these boulders and pebbles are perfectly rounded indicating that they have undergone a long transport. Moreover, they also signify that the laterite in the coastal Maharashtra is older in age than these limestones. The shell limestone is usually grey to buff white in colour, but brownish red tinge is seldom visible. It 50 is moderately hard to soft, fine grained or coarse grained, well cemented and partly laminated limestone. It is also rich in a variety of well preserved megascopic and microscopic invertebrate marine fauna. 3.5 FIELD OBSERVATIONS : 3.5.1. PLASTIC MUD LOCALITIES - BORDI SECTION : At Bordi (Plate A, Fig.l; Text Fig. 2k), an extensive mud flat is developed in the intertidal zone. During the low tide water recedes to expose mud flat having a width more than 1 km. The mangroves inhabit in this zone where the Deccan Trap is exposed. A site was selected to the south-west of Bordi village approximately 1.5 km inside the sea within the high tide zone. The hand auger was operated in the swampy area where undisturbed mangrove vegetation is thriving. The auger penetrated in the mud upto 1.5 m to reveal the following section. This mud is greenish grey and highly plastic containing the microfossils like foraminifers, few bryozoan fragments and palynoflora and also fragments of molluscs. SION-KURLA SECTION : The exposures of the Sion-Kurla (Text Fig. 2F) plastic mud occur near the railway station on Lai Bahadur Shastri Marg, in the trenches excavated for drainage 51 pipeline and sewage by the Bombay Municipal Corporation. All the four excavations were about 9 m deep and ceparated from each other by 5 to 6 ra. The basement for all excavations was the Deccan Trap which was overlain by 2.5 to 3ra thick of sandy clay consisting of medium grained, angular to subangular fragments of Traps, quartz and molluscan shells admixed with green coloured, fine plastic mud. This bed was loilowed by 3m thick greyish-green coloured, very fine plastic mud in which well preserved bivalve shells having both valves intact. Microscopic examination revealed Lhe presence of the well preserved bivalves, gastropods, foraminifera 1 tests, bryozoan colonies and ostracods. The uppermost layer was dumped debris having its thickness of about 5 to 6 feet. On Bombay island analogous sections have been observed at Andheri, Dharavi etc. KOPARKHAIRNA SECTION : At Koparkhairna (Text Fig.2G ), following section has been observed in a foundation pit measuring 6x5 m having depth of 3 m. The site is nearly 1.5 km from the tidal zone of Thana creek. The oyster bed is developed on a fairly hard trappean basement attaining a thickness of 0.25 m to 0.30 ra. The oyster species is Saccostrea ct. forskali. Shells are well preserved showing multiple scaly nature. Gaps 52 and crevices in the oyster bed are occupied by mud borer bivalve Trapezium (Neotrapezium) vellicatum. The capacious valves are filled up by fine plastic, grey mud yielding fragments of erect bryozoan and benthic foraminiferal species. The encrusters were found to grow luxuriantly. The oyster bed is overlain by a mixture of clays and terrigenous sediments (sands and pebbles) which has thickness of about 0.2 m. Cerithium cingulata appears abundantly and exclusively in these sediments. Hence, it is called as C. cingulata zone. This zone is followed by 2 meters thick bed of plastic, carbonaceous clays with carbonised wood fragments constituting a rich palynoflora. The upper sequence is of yellow coloured soil containing trap pebbles. It is 0.5 meters thick and totally devoid of marine organisms. NAGAON SECTION : Village Nagaon (Text Fig. 2H) is located along the coast 12 km south of Alibag. The surface exposures of the plastic mud are scarce being concealed within the sandy soil. However, well sections show excellent deposition of the plastic mud. A small 2 meters pit was opened along the coast where the plastic, greenish grey coloured mud occurs below 0.5 m band of highly porous shell limestone. The plastic mud at Nagaon is consistently in contact with tidal waters, so it usually remain wet and soft. This plastic mud 53 CT C c fO ■4-’ u c rtJ o u Q. U1 E E T 7 o D JD u lO E c d CJ u CL fO l_ (V CJI c in XI c o JS Q.