Stratigraphy of the Chanda Limestone of the Proterozoic Penganga Group, Adilabad, Andhra Pradesh: New Light on Depositional Setting and Palaeogeography

摘要

Stratigraphic classification of apparently monotonous lithographic limestone of the Proterozoic Penganga Group leads to the understanding of the depositional setting and palaeogeography of a deep-water limestone-shale succession. The Penganga Group around Adilabad unconformably overlies granitic basement, and includes three formations, namely, the Pranhita Sandstone (c. 25 m), the Chanda Limestone (c. 250 m), and the Sat Nala Shale (>2000 m). The Pranhita Sandstone is a cross-stratified subarkosic sandstone deposited in a shoreface environment. The Chanda Limestone developed as a deep-water, below wave-base platform. It is essentially micritic, and includes a number of slope-related autoclastic limestone mass-flow deposits, a lentil of glauconitic sandstone emplaced as submarine fan deposit (c. 25 m), and two regionally persistent horizons of deep-water bedded chert interstratified with manganese ore. The Sat Nala Shale is a deep-water, brown shale without any siliciclastic coarser than mud. On the basis of stratigraphically-controlled variations in colour, bedding character and composition, the Chanda Limestone can be subdivided into a number of mappable (1:25,000) units, that can be organized into three formal members, the Bhimsari, Bilari and Ramai Members, and one informal Brown heterolithic member. The members were deposited in different parts of the platform, at different stages of its evolution, under varied state of oxidation and reduction. Detailed mapping of the units reveals the presence of two major thrust faults that divide the succession into a southern autochthonous package successively over-ridden by a para-autochthonous package and an allochthonous package, occurring in the northern and northeastern part of the area. The autochthonous package includes sandstone and shale of the Pranhita Sandstone deposited in shoreface and inner shelf environments. The para-autochthonous package is dominated by lithographic limestone of the Bhimsari and Bilari Members, and a thin horizon of the Sat Nala Shale. The allochthonous package is dominated by slope to basinal deposits of the Ramai Member and the Bhimsari Member, and a thick sequence of the Sat Nala Shale. This package also includes the stratified bedded chert and manganese ore indicating open ocean upwelling circulation. The attitude of the thrust planes and the vergence of thrust-related folds suggest southerly propagation of thrust sheets. The allochthonous northern package was transported from a terrain beyond the northern limit of the study area, implying that the Penganga sea was open towards north. Northward opening of the basin along with upwelling-related bedded chert and manganese deposits point to the existence of an open marine basin in the central part of peninsular India, north of the Dharwar craton.