The Cuddapah basin consists of generally well-preserved Palaeoproterozoic-Neoproterozoic sedimentary and associated volcanic rocks. The detailed lithological studies of sedimentary rocks of Vempalle Formation from the narrow linear belt of 15 km, in the southern margin, show the occurrence of layered gypsum in the cherty dolostone-shale intercalated facies, red shale and phosphatic dolostone facies of the Vempalle Formation. The petromineralogical studies reveal that gypsum is in close association with anhydrites. Microscopically, three different types of gypsum and anhydrite are identified, viz., lath-shaped, equant-shaped and anhedral-shaped grains. The equant variety corresponds to a granular gypsum, whereas the anhedral grains of gypsum exist as the granular and fibrous variety as seen in the hand specimen. The presence of gypsum/anhydrite has been confirmed by the petromineralogical, X-ray diffraction and chemical analytical data. The phosphatic dolostone is the host rock for stratabound type of uranium deposit at Tummalapalle, Cuddapah district, Andhra Pradesh, which is one of the most unique types of uranium mineralisation in the world. Abundant pseudomorphs of gypsum and anhydrite relicts and discontinuous gypsum layers within these dolostones, nodules of chert and gypsum indicate the interrelationship between the diagenesis and genesis of uranium mineralisation which indicates the carbonate precipitation in the sulphate-rich hypersaline environments.
Sedimentological studies along Horin nala section of Umthongkut area in the lower Mahadek sediments have enabled the characterization of channel and resulting sandstone body. Nine lithofacies repeat to generate multistoried sandstone bodies deposited in channel belts. Lithofacies types identified were grouped into facies assemblages and into three different architectural elements. Based on the architectural elements the depositional palaeo-environment, resemble multichannel, low sinuosity braided system, considered as the product of the complex evolution of a fluvial fan segment. Palaeo-current data analysis indicates a low sinuosity river system with consistent current directions pointing toward south. The provenance for the sandstone type uranium mineralisation is primarily related to the basement crystalline rocks like granite and granite-gneisses, which apparently supplied the uranium. As the channel fills tend to have more porosity due to difference in the sizes of grains and clasts and with the tendency of concentration of more organic matter along these loci, the interaction of mineralizing fluids along with the reductants caused the precipitation of the uranium.
Plate tectonic activity has played a critical role in the development of petrotectonic associations in the Kadiri schist belt. The calc alkaline association of basalt, andesite, dacite and rhyolite (BADR) is the signature volcanic rock suite of the convergent margin. The N‐S belt has gone below the unconformity plane of Cuddapah sediments. In the northern part geochemical and structural attributes of the Kadiri greenstone belt is studied along with microscopic observations of selected samples. Harker diagram plots of major elements generally indicate a liquid line of descent from a common source, such that BADR rocks are derived from a common parent magma of basaltic to andesitic composition. These calc‐alkaline volcanic rocks are formed at convergent margins where more silicic rocks represent more highly fractionated melt. All the litho‐units of this greenstone belt indicate crush and strain effects. The stretched pebbles in the deformed volcanic matrix with tectonite development along with associated greenschist facies metamorphism, alteration and hydration is remarkable. Flow foliation plane with N‐S strike and very low angle (5° to 10°) easterly dip and N‐S axial planar schistosity formed due to later phase isoclinal folding can be clearly identified in the field. Basic intrusives are quite common in the surrounding area. All the observations including the field setting and geochemistry clearly demonstrate ocean‐continent subduction as the tectonic environment of the study area.
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