The lower stratigraphic part of the Cuddapah basin is marked by mafic and felsic volcanism. Tadpatri Formation consists of a greater variety of rock types due to bimodal volcanism in the upper part. Presence of bimodal volcanism is an indication of continental rift setting. Various genetic processes involved in the formation of such volcanic sequence result in original textures which are classified into volcaniclastic and coherent categories. Detailed and systematic field worksin Tadpatri–Tonduru transect of SW Cuddapah basin have provided information on the physical processes producing this diversity of rock types. Felsic volcanism is manifested here with features as finger print of past rhyolite-dacite eruptions. Acid volcanics, tuffs and associated shale of Tadpatri Formation are studied and mapped in the field. With supporting subordinate studies on geochemistry,mineralogy and petrogenesis of the volcanics to validate field features accurately, it is understood that volcanism was associated with rifting and shallow marine environmental condition. Four facies (i.e., surge, flow, fall and resedimented volcaniclastic) are demarcated to describe stratigraphic units and volcanic history of the mapped area. The present contribution focuses on the fundamentalcharacterization and categorization of field-based features diagnostic of silica-rich volcanic activities in the Tadpatri Formation.

Bimodal volcanism in the Cuddapah basin is associated with a cratonic rift setting. The Cuddapah basin consists of five sub-basins (viz., Papaghni, Nallamalai, Srisailam, Kurnool and Palnad) and a total thickness of $\sim$12 km sediments and associated bimodal volcanics. The oval-shaped gravity high observed over the Papaghni sub-basin is due to lopolithic intrusions along listric faults. A basin evolution model is prepared in this context with signatures of active rifting.Mapping and geochemical sampling along the Tadpatri–Tonduru tract along with petrographic observations additionally supports the proposed model. The model presents the mechanism of bimodal volcanism during rifting and sedimentation. Basin evolution with tectonic modifications revealed a link with global tectonic events (e.g., $\sim$1.8 Ga Hudsonian orogeny, $\sim$1.3 Ga Grenville orogeny, $\sim$0.9 Ga Enderbia docking). The stratigraphic disposition of the surge, flow, fall and volcaniclastic deposits in this old Proterozoic terrane indicate the magma history and eruption conditions. The felsic volcanic rocks are classified as rhyolite and rhyodacite. The mafic volcanics are mainly basaltic. Primordial mantle normalized trace element plots indicate enrichment of large ion lithophile elements (Rb, Th and K) along with negative Sr, P and Ti anomalies. The chondrite normalized REE patterns are characterized by LREE enrichment, negative Eu anomaly and flat HREE pattern. These features indicate origin of felsic volcanics through shallow crustal melting with plagioclase either as a residual or a fractionating phase. The mafic rocks of the area are product of shallow mantle melting related to asthenospheric upwelling followed by decompression melting and generation of basaltic magma. This was also associated with lithosphereic stretching, rifting and initiation of sedimentation. The less viscous mafic magma was probably channelized along the rift-related faults. The underplating and intraplating of hot mantle-derived magma supplied heat into the crust. The attendant partial melting of continental crust produced the felsic magma. Different sub-basins within the Cuddapah basin indicates a combined mechanism of rifting and orogenic events.