Geophysical Evidences of Concealed East Coast Sedimentary Basin, Kavali-Nellore Areas of Andhra Pradesh, South India

East coast of India with a combined strike length of over 2000 km has five main peri-cratonic rift basins bordering the shield areas; Bengal Basin, Mahanadi Basin, Krishna- Godavari Basin, Palar Basin and Cauvery Basin. Possibly these basins are extending in to off-shore areas and these are known for their hydrocarbon resources and have been extensively explored for them. However, the intervening portion between Krishna-Godavari basin and Cauvery-Palar basin of South Indian East Coast is not known for any major sedimentary basin other than discrete exposures of sedimentary rocks here and there. Combined analysis of Gravity and Magnetic anomaly maps constrained with Electrical Resistivity surveys have suggested possibility of a concealed Gondwana/Teritiary sedimentary basin in Kavali-Nellore areas of Andhra Pradesh, which if proven would add the resource potentiality of the area.

National Geophysical Mapping being carried out in the cratonic areas have yielded significant results about the bed rock geology, basement architecture and nature of crustal rocks more particularly in covered areas with Proterozoic/Gondwana/Tertiary/Quaternary sediments. One of such attempts in the areas between the Nellore Schist Belt (NSB) and the transition zone with Eatern Ghat Mobile belt has depicted a well-defined gravity low with discrete closures over a strike length of 70 Km and an average width of 10 Km. This major gravity low occupies the area geologically mapped as Gneissic region belonging to Eastern Dharwar Craton. However, the discrete gravity low closures are typical character of sedimentary basins suggesting the undulations in the basement rocks. Corroborative evidences are obtained from magnetic map with low intensity anomalies suggesting pile of sediments.

Encouraged by the typical signatures of this area, Electrical Resistivity soundings are conducted over the entire length of gravity low zone and they have clearly indicated H, HKH type curves indicating presence of sediments. Depth of basement rocks estimated from these surveys suggest progressive decline in thickness of sedimentary column from North to South ranging from 400m to 40 m. The open end of gravity low at northern part suggests that the possibility of the basin extending towards north. These suggestive evidences are constrained by recent re-interpretation of seismic data by National Geophysical Research Institute - NGRI wherein they have indicated possibility of Gondwana sediments in these areas.