J. Earth Syst. Sci. (2018) 127:20 c Indian Academy of Sciences https://doi.org/10.1007/s12040-018-0929-0 An approach of understanding acid volcanics and tuffaceous volcaniclastics from field studies: A case from Tadpatri Formation, Proterozoic Cuddapah basin, Andhra Pradesh, India Sukanta Goswami1,*, P K Upadhyay1, Sangeeta Bhagat1, Syed Zakaulla1,AKBhatt1, V Natarajan1 and Sukanta Dey2 1Atomic Minerals Directorate for Exploration and Research, Bengaluru 560 072, India. 2Indian Institute of Technology (ISM), Dhanbad 826 004, India. *Corresponding author. e-mail: [email protected] MS received 27 January 2017; revised 12 July 2017; accepted 13 July 2017; published online 6 March 2018 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 works in 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 fundamental characterization and categorization of field-based features diagnostic of silica-rich volcanic activities in the Tadpatri Formation. Keywords. Acid volcanics; Cuddapah basin; Tadpatri Formation; tuff. 1. Introduction However, acid volcanics are not addressed in detail so far. The volcanic facies study in this context The detailed study of the mafic emplacements over has provided information on facies characteris- Vempalle and Tadpatri Formations have brought tics evident at outcrop scale in the field, such out important information on age relationships, as structures, internal organisation and geome- tectonics, magma evolution and other geochemi- try. After having the idea on facies characters, cal characterisations (Chatterjee and Bhattacharji interpretation is attempted for genetic aspects to 2001; Sesha Sai 2011; Chakraborty et al. 2016). have information on eruption and emplacement 1 0123456789().,--: vol V 20 Page 2 of 21 J. Earth Syst. Sci. (2018) 127:20 processes for primary volcanic and volcaniclastic thin intercalated quartzite and volcanogenic deposits and on subsequent erosion, transport and sediments overlying the Pulivendula Quartzite in redepositional processes for resedimented and vol- the Chitravati Group. canogenic sedimentary deposits. The textures in The importance of associated volcanic and vol- these volcanic rocks are divided on the basis canogenic rocks in compilation of geological record of timing as: (1) Original textures by eruption is apparent due to their characteristic chemistry, and emplacement processes; (2) Modified original relatively fast accumulation and great variety that texture by syn-volcanic processes like oxidation, provide recognizable facies diversity for recon- degassing, hydration, vapour-phase alteration, structing not only the volcanic processes but high-temperature devitrification, and hydrother- also the past eruptive environment of volcanism. mal alteration; (3) Textures formed due to mod- Nagaraja Rao et al. (1987) have given a detailed ification by post-volcanic processes like hydration, description of stratigraphy of the Tadpatri For- devitrification, hydrothermal alteration, diagene- mation which consists of shale with quartzite sis, metamorphism, deformation and weathering. intercalations, shale with limestone/dolomite inter- Since the area consists of volcaniclastics and tuffa- calations, shale with ash fall/flow and tuffs, shale ceous shale and non-volcanic sediments, from a with stromatolitic dolomite intercalations from methodological point of view, studies and interpre- bottom to top. The basic flows and sills are also tations of volcaniclastic rocks are almost similar to present along with acid volcanics in the Tadpatri those performed on clastic non-volcanic sediments. Formation. The tuffs are mainly acidic in com- However, there are characteristic differences that position. The area from Tadpatri to Tonduru is make this volcaniclastic study complicated but a NW–SE trending rectangular block (figure 1), interesting. Such differences are taken into account where the lithounits are striking NW–SE and while defining the lithofacies categories of the area. dipping around 100 towards NE and basic flows The field geologists working for the purpose are very common along the same trend. Parallel of general mapping in such terrains with ancient beds of acid and basic volcanic flow and vol- volcanism can use this field-based paper as a caniclastic sediments and ash fall tuff beds are guide for interpreting volcanic processes and the overlain by Gandikota Quartzite with gradational resulting deposits and resedimentation of volcani- conformable contact along the Gandikota range clastics. The objective of the paper is to introduce (GSI 1981; Nagaraja Rao et al. 1987). The Tad- basic field volcanology in Tadpatri Formation. This patri volcaniclastic sequence is often overlain by paper encompassing volcanic facies analysis is one sub-horizontal quartzite beds of Banganapalle For- of the main goals of studying volcanic rocks in the mation of Kurnool Group with angular unconfor- field and this has been utilised for palaeoenviron- mity at above 500 m RL as flat hill top capping in mental reconstructions. the study area. 2. General geology 3. Volcanology of Tadpatri Formation Cuddapah Basin, one of the largest Proterozoic Volcanic rocks that are significantly fragmented are basins in India, consists of sedimentary and asso- important from a stratigraphic point of view and ciated volcanic rocks of about 12 km thickness, thus they are used to study palaeoenvironments. ranging in age from late Paleoproterozoic to Neo- Bimodal volcanism is remarkable in the area. Mafic proterozoic (Nagaraja Rao et al. 1987; Bhaskar Rao volcanism is well documented and more widely et al. 1995; Ramam and Murty 1997; Zachariah studied compared to felsic counterpart and thus et al. 1999). A detail description of stratigraphy, felsic rocks are studied in detail. Evidences of frag- structure, igneous activities, mineral potentiality mentation, a process which occurs when bubbles and basin evolution is summarised by Nagaraja become over-pressured and burst, is noteworthy in Rao et al. (1987). The stratigraphic sequence the area. Stratigraphically, the study area consists initiated by Gulcheru Quartzite, nonconformably of upper part of the Tadpatri Formation domi- overlying the Archaean granitoids and greenstone nated by volcaniclastic rocks. Presence of coherent belts, is followed up by Vempalle Formation which rocks (simply solidified from the melt) and volcani- together forms part of the Papaghni Group. Tad- clastic debris (formed through a wide range and patri Formation hosts argillaceous sediments with combinations of different style of fragmentation, J. Earth Syst. Sci. (2018) 127:20 Page 3 of 21 20 Figure 1. Geological map of the studied area from Tadpatri to Tonduru. The blue circular numbers represent sample locations. transportation and deposition processes) in the as tuffaceous sandstone, tuffaceous siltstone and upper part of the stratigraphic column is promi- tuffaceous shale. However, since >80% area is of nent. As far as bedding characteristics are con- finer than 2 mm, the ash flow/fall tuff must be cerned, massive or thinly laminated units and the most appropriate term that should be used for moderate to well-defined beds with different tex- this sequence. Genetically, most common effusive tural features carry important information about and rarely mild explosive eruption phenomenon the transport mechanisms, such as physical aspects can be explained from lava flow and pyroclastic fall of flow including rheology and particle concentra- and surge deposits in the field. Three major types tion. The pyroclastic materials within the Tadpatri of transportation mechanisms (mass-flow, traction, Formation are noted mainly as lapilli (2–10 mm), and suspension) are noted on the basis of textural ash (<2 mm) and mixture of pyroclastic and epi- features. Small sized lapilli are frequently seen as clastic (non-volcanic) rocks. These are categorized more or less uniformly embedded in ash/tuffaceous 20 Page 4 of 21 J. Earth Syst. Sci. (2018) 127:20 Tuff bed 1600 /200 700 Figure 2. Tuffaceous bed with small circular embedded lapilli. ab Figure 3. (a) Spherulites and (b) lithophysae in filled with secondary materials. shale (figure 2). As far as basic characteristics of area) compared to other places (e.g., Beduduru, lavas and syn-volcanic intrusions are concerned, the Burzupalle, Mallella, Bondladinne, Thimmapuram, coherent facies is characterised by porphyritic tex- and Ahobilapuram) where flow and surges are ture and aphanitic