Bhandara Craton

The Bhandara craton is located in the north-central part of the Indian subcontinent, bounded on the east by the Eastern Ghats, the Godavari rift and Dec-can basalts on the southwest, the Aravalli craton under the Deccan basalts and younger sediments to the northwest, and the singhbhum craton to the northeast. The craton consists largely of granites and gneisses with many inclusions of older sedimentary and volcanic rocks, overlain by several Late Proterozoic basins, including the Chhattisgarh and Bastar basins. The groups of volcanic and sedimentary rocks include the Dongargarh, sakoli, sausar, Bengpal, sukma, and Bailadila Groups.

The satpura orogenic belt cuts east-west across the craton, disrupting the dominantly north-south strike of structures in other parts of the region. The Bhandara craton is known for its rich sedimentary manganese ores that are especially abundant in the sausar Group.

Granites and gneisses of the Bhandara craton are abundant and intrude older continental shelf-type sediments, although no extensive older continental-type basement has been identified. Some of the gneisses are broadly similar to the peninsular gneisses of the Western Dharwar craton, but their ages have not been determined. Metamorphism of the gneisses occurred at 1.5 billion years ago.

The sedimentary assemblages engulfed in the granites and gneisses include mainly the Dongargarh, Sakoli, and Sausar Assemblages. The Dongargarh Supergroup includes quartz-feldspar-biotite gneisses with minor amounts of basalt, metamorphosed to amphibolite facies in the 2.3-billion-year-old Amgaon Orogeny. Younger rhyolites, sandstones, shales, and tuffaceous rocks are approximately 2.2 billion years old. The Dongargarh Supergroup is intruded by granites dated to be 2.27 billion years old, so the rocks are older than this and likely Early Proterozoic or Archean. Rocks of the Dongargarh Supergroup are deformed by three major fold sets, the first of which produced isoclinal folds in the Amgaon orogeny, and the second and third of which produced tight folds during the Nandgaon and Khairagarh Orogenies.

The Sakoli Group includes metapelitic rocks disposed in a large, synclinal structure in the western part of the craton. The rocks are metamorphosed to lower amphibolite facies and deformed by an early generation of isoclinal folds and a later group of more open upright folds.

The Sausar Group forms a thin, elongate belt of sandy, shaly, and calcareous metamorphosed sedimentary rocks along the northern part of the cra-ton and is one of the main manganese-producing units in India. The lack of volcanic material in the Sausar Group has made it difficult to determine the age of this sequence, but it is known that the rocks were deformed in the Satpura Orogeny at 1.53 billion years ago. The structural geology of the Sausar Group is interesting and unusual. The southern part of the belt is deformed into isoclinal folds, many of which are overturned and show nappe-style movements toward the north. These are bordered on the north by gneisses of the Tirodi suite and have many inclusions of the sedimentary rocks. This gneissic belt may represent the core of the orogen. To the north in the Satpura Ranges, the rocks are disposed in a series of south-directed nappes and thrust sheets.

Despite much work on the structural geology of the Sausar Group, details of the Satpura orogeny that affected these rocks are vague. The orogeny was a Middle Proterozoic event, with some metamorphic ages of circa 1.5 billion years. The main tectonic transport direction in the orogeny was likely toward the south, although the belt of northward-directed nappes south of the crystalline core of the orogen is enigmatic, and few other orogens show tectonic movement toward the center of the belt. Some fold interference or strike-slip motions in the core of the orogen may have eluded detection.

Continue reading here: Singhbhum Craton

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