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

Utpal Bose ¹, Fareeduddin ¹, M. S. Reddy ¹

Affiliations
1 Geological Survey of India, Regional Geology Division II (Operation: Rajasthan), Jhalana Dungri, Jaipur 302 004, IN

Abstract

The Proterozoic South Delhi Fold Belt (SDFB) is known to have a dominant volcanic component along the western flank of the Aravalli Range. Earlier studies have indicated both bimodal and unimodal characters of these volcanics in different parts of the SDFB. The Shyopura-Narpura area in north-central part of the Aravalli Range has, on the other hand, volcanics of intermediate and ultrabasic composition, interlayered with basic and felsic volcanics. This polymodal suite of secondary greenstone character is interpreted to have erupted in a rifted back-arc setting.

Keywords

Geochemistry, Delhi Fold Belt, Volcanism, Rajasthan.

Full Text

     

P. Gupta ¹, K. Mukhopadhyay ¹, Fareeduddin ¹, M. S. Reddy ¹

Affiliations
1 Geological Survey of India, Jaipur 302004, IN

Abstract

The Delhi rocks are deposited in two sub-basins separated by a median inlier of Pre·Delhi rocks. The Barotiya and Sendra Fonnations occupying the western sub-basin contain significant basic flows and tuffs with associated felsic volcanics. The eastern sub-basin recording a diachronous evolutionary history is the repository of the Rajgarh and Bhim sediments.

Striking similarities are noted in respect of lithology, structure and metamorphism amongst the gneisses and schists of the Pre-Delhi rocks flanking the Delhi Supergroup and also of the median inlier. Structural imprints and contact relationship indicate that these gneisses and schists fonn the basement for the Delhi sediments. Migmatization related to orogeny suggests reactivation of a sialic basement, which along with the tectonic emplacement of basement slices in the Delhi stratigraphy argue in favour of an intra-cratonic development of the Delhi basin.

The western sub-basin shows bimodal volcanism and the various geochemical discriminants suggest an IATor MORB setting, but fail to make a choice between the two. Bimodal volcanism in association with shallow clastic sediments signify ensialic origin in a rifted mobile belt. Structural studies indicate large-scale dextral vergence with steep 'S' surfaces which are characteristic of oblique shear transpressive mobile belt.

Keywords

Delhi Fold Belt, Tectona-Stratigraphy, Rajasthan, Aravalli.

Full Text

     

Fareeduddin ¹, M. S. Reddy ¹, U. Bose ¹

Affiliations
1 Geological Survey of India, Jhalana Dungari, Jaipur-302004, IN

Abstract

The supracrustals occurring in the northern sector of the South Delhi Fold Belt (SDFB; main Delhi Synclinorium of Heron, 1953) between Ajmer-Pushkar valley and Sarnbhar Lake in north-central Rajasthan are classified into Anasagar migmatites and the overlying Ajmer Formation comprising Taragarh quartzites and Kalyanipura arkose-pelite-greywacke sequence. The Anasagar migmatites and paragneisses are intruded by Iatero genic granite(1600 Ma) and post orogenic ultramafic and alkaline rocks. Four folding and two shearing movements characterize these rocks. Three metamorphic zones could be identified in these supracrustals with grade increasing towards west: staurolite-kyanite grade (middle arnphibolite facies) in the eastern sector, sillimanite-muscovite grade (upper amphibolite facies) in the central sector, and orthopyroxene-plagioclase and orthopyroxene-sillimanite grade (granulite facies) in the western sector.

The Anasagar and Ajmer supracrustals are deposited in an Early Proterozoic ensialic basin in the Archaean BGC (Banded Gneissic Complex) protocontinent and differ from the other tectono-stratigraphic units of the SDFB in respect of lithological organisation, metamorphism and magmatism besides the latter being decisively of Upper Proterozoic age. The authors, therefore suggest a separate and older stratigraphic status to these supracrustals and delink from the rest of the SDFB stratigraphy.

Keywords

Stratigraphy, Delhi Supergroup, Ajmer, Sambhar, Rajasthan.

Full Text

     

I. V. Reddy ¹, M. S. Reddy ¹, G. Satyanarayana ¹, V. K. K. kalluraya ¹

Affiliations
1 Geological Survey of India, Hyderabad, IN

Abstract

New finds of oligomictic conglomerate at the base of Culcheru Quartzite near Venganamala and at the base of Nagari Quartzite at Gadikota are reported. It is proposed that the Nagari Quartzite and Pullampet Formation arc unconformably overlain by Bairenkonda Quartzite and Cumbum Formation, thereby necessitating a stratigraphic revision of thc Nallamalai Group.

Keywords

Conglonierate Horizon, Stratigraphy, Cuddapah Basin, Andhra Pradesh.

Full Text