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

H. N. Siddiquie ¹

Affiliations
1 Geological Survey of India, Calcutta, IN

Abstract

A comparative study of ,the Ba,rmer and the adjacent Jaisalmer basins brings out striking differences in the stratigraphy, palaeontology and heavy mineral suites of the sediments.

Sedimentation in the Jaisalmer basin started with the deposition of the PermoCarboniferous tiliites and continued with breaks through (?) Triassic, Jurassic, Cretaceous and Eocene. The total computed thickness of sediments appears to be more than 7000 feet in the centre of the basin; and there are evidences of a thickening northwestwards. 

Sedimentation in the Barmer basin started with the Lathi formation (? Triassic); and after a major break during the Jurassic, continued intermittently from Cretaceous through Eocene to the Late Tertiary. The total thickness of the sediments in the Barmer basin is only of the order of 1500 feet, very much less than in the Jaisalmer basin.

The Jaisalmer sediments were deposited on a platform dominantly under marine conditions. On the other hand, the Barmer sediments were deposited in a narrow embayment under changing conditions, viz., fresh, marine or brackish water. Throughout the entire sedimentary column the Jaisalmer sediments are characterised by the presence of staurolite, garnet, and kyanite. Staurolite is markedly absent in the Barmer basin sediments which have hornblende, garnet and kyanite.

Subsurface and surface geological data show that the two basins were separated by a ridge formed by the Malani Igneous suite and Vindhyan sediments. The ridge acted as a barrier preventing the intermigration of the fauna, and also as a watershed separating the two distinct provenances of the Barmer and Jaisalmer basins.

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H. N. Siddiquie ¹, S. M. Karisiddaiah ¹, V. Subrahmanyam ¹

Affiliations
1 National lnstitute of Oceanography, Dona Paula, Goa 403004, IN

Abstract

The total magnetic intensity map of the area shows that the inner shelf to a depth of 50m is marked by a zone of magnetic closures. The occurrence of offshore ortho-amphibolites and the qualitative interpretation of the magnetic map indicate the extension of onshore rocks (or the amphibolites and associated rocks of Precambrian (?) and intrusions of Deccan Trap age) under a thin cover of sediments to the offshore to a depth of about 50 m.

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H. N. Siddiquie ¹, R. N. Sukheswala ²

Affiliations
1 National Institute of Oceanography, IN
2 Geology Department, St. Xavier's College, Bombay, IN

Abstract

A study of thin sections from the lower and middle parts of Unit V (Paleocene) from Site 219shows that these largely consist of acidic or rhyolitic tuffs. The overlying limestones in Unit V (Paleocene) and Unit IV (Lower Eocene) also contain tuffaceous material in varying proportions. Albite/oligoclase forms the phenocrysts as also needles in the groundmass of these tuffs and tuffaceous material in limestones. The euhedral fresh felspars suggest that the source of activity was nearby and possibly most of the activity was subaqueous rather than subaerial. Similar acidic tuffs in the Deccan Traps have been reported particularly from the West Coast of India. The acidic tuffs at site 219 are of the same age as the Deccan Traps and may represent a late phase of Deccan Trap activity on the Laccadive Ridge before its subsidence and separation from peninsular India.

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