Eparchaean Unconformity
Geological feature in Tirumala, India
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Geography
The Eparchaean Unconformity is exposed along the Ghat section leading to Tirumala. It forms part of Sapthagiri hills in the Seshachalam Range of the Eastern Ghats. It was declared as a National Geological Monument by the Geological Survey of India in 1976. The highest point on the range is located atop the Venkatadri at about 980 m (3,220 ft) above sea level. The Venkateswara Temple at Tirumala, dedicated to Venkateswara, an avatar of the Hindu god Vishnu, is located atop the hill.
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Geology
The Tirumala Hills are composed predominantly of Precambrian sedimentary rocks, particularly quartzites and intercalated shales. These rocks are known as the Nagari Quartzites and form part of the Kadapa Supergroup. The Eparchaean Unconformity in the Tirumala Hills is a major geological boundary that represents a period of no rock deposition and separates the Nagari Quartzites of the Proterozoic Kadapa Supergroup, dating to 1.6 Ga, from the older Archean Peninsular Gneissic Complex, dating to 2.3 Ga. The Peninsular Gneissic Complex consists of granites, gneisses, and dolerite dykes. The unconformity represents a time gap of more than 800 million years. It marks a period of remarkable geological quiescence, with little structural disturbance or igneous activity.
The sedimentary thickness of the Kadapa Basin is of the order of 12 km (7.5 mi) and includes volcanic sequences in the form of sills and dykes, which rest on Archaean peninsular gneissic complex marked by the unconformity. In the Kadapa Basin, the unconformity is represented by the Gulcheru Conglomerate, the earliest sedimentary unit of the Kadapa Supergroup. The initiation of basin sedimentation is generally placed at about 2.0–2.1 Ga. The unconformity is also exposed along the Kurnool–Nandyal road, where about 20 m (66 ft) thick Gulcheru Formation rests unconformably on the Gadwal Schist Belt. An equally thick succession of the Gulcheru Formation is exposed at Parnapalli along the Veldurthi–Pulivendla road, where it rests on Archean granitic gneisses.