Stratigraphy of Cuddapah Basin

What is Cuddapah Basin?

The crescent-shaped, convex towards the west, easterly concave, and NS trending Cuddapah basin covers an area of 44,000 sq. km in the east-central part of the Dharwar craton. The Cuddapah Basin is an arcuate north-plunging, low-amplitude asymmetrical synclinorium. Cuddapah sedimentation commenced in the interval 1700–1800 Ma. The basin extends for a length of about 450 km along the arcuate eastern margin with a mean width of 150 km and is the second-largest Purana basin of Peninsular India, after the great Vindhyan basin.

Structural Features of Cuddapah Basin

The basin is divided into two halves by the Rudravaram Fault line:

1. The undeformed western half.
2. The Nallamalai thrust-fold belt.

The arcuate north, south, and western boundary of the Cuddapah basin marks the profound unconformity (called Eparchean Unconformity in early literature) on basement granites enclosing the Eastern greenstone belts of Kadiri, Veligallu, and Tsundupalle in the south, and Raichur, Gadwal, and Peddavuru in the north.

Eparchean Unconformity

Between Nagri Quartzite (1600 Ma) and Archean Granite (2100 Ma) almost 500 Ma old hiatus. At this location, the boundary is between sedimentary rock of the Cuddapah Supergroup, 1600 million years in age, and Archaean rocks comprising granites, gneisses, and dolerite dykes that are more than 2100 million years old. This unconformity gives an indication of the enormous time gap in the creation of the earth’s crust.

Boundary Features of Cuddapah Basin

The arcuate eastern margin is marked by a prominent boundary thrust, which is parallel to the Nellore schist belt, Eastern Ghats mobile belt, and the East Coast. A giant basic dyke swarm underlies the basin in the west from Chittoor in the south to Mahbubnagar in the north. The Cuddapah basin is characterised by quartzite-carbonate-shale cycles having an aggregate thickness that is estimated to vary between 6 and 12 km. The early sediments of the basin are interspersed with basic volcanics and sills, similar to the small Palaeoproterozoic basins of Abujhmar, Bijawar, and Gwalior.

Sub-basins of Cuddapah Basin

The western half of the basin is undeformed and consists of 4 sub-basins: the Papaghni, Kurnool, Srisailam, and Palnad. The Papaghni and Kurnool sub-basins are geographically interlinked but were sites of deposition at different times, the Papaghni sub-basin containing lower Cuddapah sediments, and the Kurnool sub-basin containing the younger Kurnool sediments that overlie the Cuddapah sequence with a major unconformity.

Fossils and Volcanics of Cuddapah Basin

Life-stromatolites are abounding, carbonaceous microfossils like Tawuia, Chuaria, trace fossils, and calcareous algae indicate the Neoproterozoic age. Felsic volcanic and tuffs are intercalated with sediments at many horizons.

Mineral Potential of Cuddapah Basin

The basin is well known for its mineral potential in the form of limestones and dolomites, bedded and vein barytes, chrysotile asbestos, and steatite, besides occurrences of base metals, diamond, phosphorite, uranium, and abundant building and ornamental stones.

Geological Age of Cuddapah Basin

• Pre-Cuddapah basement dykes are of two prominent ages at 2100 and 2400 Ma.
• Rb-Sr dating of dolerite and picrite sills of Pulivendla Formation (Chitravati group) gives an age of approximately 1800 Ma.
• K-Ar ages of some dolerite dykes underlying the basin are also similar at 1800 Ma.
• The age of galena and micas in Cumbum shales (Nallamalai group) is 1350-1450 Ma.
• Rb-Sr age for dolerite dyke intruding the Kurnool basin is 980 Ma, suggesting Mesoproterozoic age for the Kurnool group.
• Paleoproterozoic Age of Papaghni Group and Mesoproterozoic of the rest of Cuddapah supergroup.
• Neoproterozoic of Kurnool Group.

Stratigraphy of Cuddapah Basin

Group	Subgroup	Formation/Member	Lithology
Kurnool Group (200-450)	Kunderu Subgroup (75-150)	Nandyal Shale	Shale, partly calcareous
		Koilkuntla Limestone	Siliceous, shaly limestone with quartzite interbeds
		Paniam Quartzite	Massive and pinnacled quartzite, siliceous shale, basal pebble bed
	Unconformity
	Jammalamadugu Subgroup (120-260)	Auk Shale	Laminated shale, mostly ochreous with siltstone interbeds
		Narji Limestone	Flaggy grey limestone, glauconitic sandstone interbeds
		Banganapalle Quartzite	Oligomict conglomerate, ‘grit’, quartzite, shale
	Unconformity
Cuddapah Supergroup		Srisailam Quartzite	Glauconitic ferruginous quartzite with shale
	Unconformity
	Nallamalai Group (3500-6000)	Cumbum Formation	Mainly slate and phyllite, chert, dolomite
		Pullampet Formation	Shale, felsic tuff, barytes, dolomite, quartzite
		Bairenkonda Quartzite	Quartzite, quartz wacke, shale
		Nagari Quartzite	Quartzite, quartz wacke, shale, basal conglomerate
	Angular Unconformity
	Chitravati Group (4900-5000)	Gandikota Quartzite	Shale-quartzite alternations, glauconitic quartzite interbeds
		Tadpatri Formation	Shale, felsic tuff, basic sills and flows, chert, stromatolitic dolomite, quartzite
		Pulivendla Quartzite	Quartzite and conglomerate
	Papaghni Group (2100)	Vempalle Formation	Stromatolitic dolomite, chert, quartzite, basic sills and flows
		Gulcheru Quartzite	Basal conglomerate, arkose, quartzite with shale interbeds
Crystalline basement of Dharwar craton

Economic Importance of Cuddapah Basin

Uranium

• Found in the Vempalle Formation of the Papaghni Group, the host rock is phosphatic dolostone, promising mineralisation at Tummalapalle, Rachakuntapalli, and Gadankipalli.
• Uranium minerals with U3O8 concentration up to 1.44% form lensoid bodies in a 2-km-long belt near Bandi within the Gulcheru Quartzite of the Papaghni Group. There are disseminations of uraninite and coffinite in chlorite–sericite matrix of the fractured quartzite cut by E–W trending faults.

Barytes

• Found in Pullampet Formation of Nallamalai Group at Mangampeta, forms the single largest deposit of bedded barytes in the world, and has been declared a National Geological Monument.
• Granular and rosette forms of barytes with BaSO4 content as high as 85% constitute India’s 90% (and the world’s 25%) deposits.

Diamond

• Found as placer deposits in Banganapalle quartzite of Kurnool Group.

Other Minerals

• Limestones and Dolomites.
• Chrysotile (White asbestos – Serpentine group) – asbestos and steatite (soapstone – Talc schist) – Vempalle Formation of Papaghni Group.
• Ornamental stones like jasper, chert, etc.
• Pb-Zn deposit – Hosted by dolomite, found in Pullampet Formation of Nallamalai Group at Agnikundala, Zangamrajupalle-Varikunda.

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