Geology of the Julakallu Area, Palnad Sub-Basin, Cuddapah Basin, Guntur District, Andhra Pradesh

Recent geological studies have brought out the presence of hitherto unknown Cumbum shale/Phyllite in the Julakallu-Pinneli area, Guntur district, Andhra Pradesh within the Palnad Sub-Basin of the Cuddapah basin. Cumbum shale/Phyllite and quarzite was found to rest above the Narji Limestone at southeast of Petasannigandla. This has necessitated northward shift of thrust boundary separating the Cumbum Formation and Narji Limestone from earlier known location. Subsurface data have confirmed larger thickness of the Narji Limestone than that reported in the literature. Geochemical characteristics of the Cumbum shale/Phyllite have indicated its deposition as first cycle of sediments in a tectonically active setting. This is supported by the depletion of immobile oxide Al₂O₃ and enrichment of mobile oxide CaO in the Cumbum shale. The A-CN-K plot indicates that provenance for shale/Phyllite could be granodiorite to granite in composition, which is also supported by petrological observations. The field, petrological and geochemical interpretations have brought out much new information on this part of the basin, which necessitates, revision of the geology of area.