Journal of Geological Society of India (Online archive from Vol 1 to Vol 78)

S. J. Desai ¹, M. P. Patel ¹, S. S. Merh ¹

Affiliations
1 Geology Department, M. S. University, Baroda-390002, IN

Abstract

The crystalline rocks of Balaram-Abu Road area comprise a varied assemblage of metamorphic and igneous rocks of Precambrian age. Earlier workers have considered the rocks of the area to belong to the Alwar & Ajabgarh Series of Delhi System intruded by mafic rocks and granites. These rocks are petrologically more complex and ate much older-Aravallis or even earlier. The mineralogical assemblages of these rocks suggest metamorphism under pyroxene granulite facies, pointing to anhydrous or dry conditions.

These granulitic rocks are seen involved in Delhi deformation and later invaded by the Erinpura granite. A superimposition of contact metamorphism (due to Erinpura granite) on the granulites, has caused the development of hornfelsic rocks containing andalusite. The rocks of Balaram-Abu Road area thus comprise a polymetamorphic terrain.

The abundance of cordierite and its association with garnet and hypersthene in pelitic granulites, the occurrence of scapolite and diopside in calc-granulites, the coexistence of olivine and plagioclase in basic rocks, absence of garnet in charnockitic rocks and the development of horn felsic rocks with andalusite indicate that the rocks of the area belong to a low pressure granulite province of metamorphism.

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Geeta Desai ¹, M. G. Hardas ², M. P. Patel ¹, S. S. Merh ¹

Affiliations
1 Department of Geology, M. S. University of Baroda, Baroda 390002, IN
2 Regional Laboratories, Oil & Natural Gas Commission, Baroda 390009, IN

Abstract

Miliolites occur within the Kutch highlands as thin horizontal sheets occupying topographic depressions or as obstacle dunes resting against the slopes of big hills. Sheet deposits of the low-lying ground, reveal a marine origin and are seen to consist of pellets and microfossils embedded in a micritic to fine sparry matrix. These valley-fill sheet deposits are characterised by a faunal assemblage typical of warm, clear, shallow, carbonate-rich sea, where terrigenous influx was poor. On the other hand, the obstacle dunes typically comprise aeolinites and appear to have developed during the regression of the sea, the strong winds having lifted the exposed carbonate sands and dumped them against the nearby hillsides. The higher strand line indicated by these marine miliolites could be related to one of the Quaternary transgressions.

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M. P. Patel ¹

Affiliations
1 Department of Geology, M.S. University of Baroda, Vadodara - 390 002, IN

Abstract

Detailed studies of various miliolite occurrences of Saurashtra have revealed fluctuations in the palaeoclirnate witnessed in the course of deposition and consolidation of the dune material during Middle and late Upper Pleistocene. The entire process of formation of miliolite rocks is indicative of alternating arid and humid phases. During protracted arid phases strong wind actions were effective whereas during brief wet phases fluvial erosion & deposition and pedogenesis were brought about. The various evidences in support of palaeoclimatic fluctuations include: (i) the presence of erosional surfaces separating the miliolite deposits by intercalated redsoil horizons, cross-beddedand horizontally bedded miliolite deposits, two cross-bedded miliolite sequences with reversed dips and the two superimposed unidirectional cross-bedded miliolite sequences with different angles of dipin the coastal cliffsections, (ii) the occurrence of intercalated red beds and fluvial debris (sand/silt/gravel) within miliolite sequences as seen in some of the river and well sections, together with slope-wash debris between two miliolite layers substantiating small scale intervening periods of quiescence, (iii) a well defined junction between the two different major cycles of aeolian accumulation marked by the surfaces of non-deposition, erosion and honeycomb weathering at the top of the lower cycle as encothered in some of the well sections, (iv) burial of animal burrows by a younger cycle of miliolites devoid of animal burrows in some exposures and (v) the presence of diagenetically formed coated grains, vadose calcite silt around the ooliths & intraclasts, and its occurrence between two generations of cement in thin section also provide aclue to the minor breaks in diagenetic environment.

Keywords

Palaeoclimate, Miliolite, Quaternary, Gujarat.

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Nilesh Bhatt ¹, M. P. Patel ¹

Affiliations
1 Department of Geology, The M.S. University of Baroda, Vadodara-390 002, IN

Abstract

The high energy coastal segment between Okha and Diu in Western Saurashtra is characterised by typical kind of raised bioclastic shore deposits. These occur up to 15 m (max.) above present day mean sea level and about 5 km inland. Variously described as "coast fringing rocks", "Chaya Formation", "ancient beach rocks", "shell limestones", etc., these possess poorly sorted allochems and lithic fragments of coarse sand to gravel size, mostly cemented by aragonitic micrite and occasionally fibrous aragonitic and high magnesian calcite, their pores being infilled by second generation of low magnesian sparry calcite cement. On the western coast of Saurashtra, they underlie coastal dunes and sheets of rniliolitic limestone and form gently seaward dipping sheets and their surfacial exposures are encountered southward only up to Kadwar, near Diu. However, they occur at the base of deep quarries of miliolitic limestone between Kadwar and Mahuva. Looking at the available reliable Th²³⁰/U²³⁴ and ESR dates of fossil shells from these deposits, they represent a higher sea level during the oxygen-isotope sub-stage 5e. On the South Saurashtra coast, the high coastal cliffs of miliolitic limestone exhibit presence of raised shore platforms and notches which may be related to this high sea after considering the influence of tectonism.

Keywords

Sedimentology, Quaternary, Bioclastic Sediments, Sea-Level, Saurashtra.

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Nilesh Bhatt ¹, M. P. Patel ¹

Affiliations
1 Department of Geology, M. S. University of Baroda, Vadodara - 390 002, IN

Abstract

The miliolites of Saurashtra Peninsula represent reworked bioclastic sand accumulations under the varying palaeoclimatic conditions during the Late Quaternary period. Their origin, whether marine or aeolian, forms a matter of controversy. These rocks comprise medium to fine grained allochems along with minor amount of detrital grains cemented together by non-ferroan, low magnesian sparry calcite cement exhibiting various ultrastructures. The intragranular porosity, geopetal infilling, dripstone-drapstone, meniscus and dogtoothspar cements suggest vadose condition while the presence of isopacholls rim and blocky void filling cements indicate their formation in phreatic freshwater environment.

Keywords

Sedimentary Petrology, Miliolite, Freshwater Diagenesis, Quaternary, Saurashtra.

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