Journal of Geological Society of India (Online archive from Vol 1 to Vol 78)

Pinaki Majumdar ¹, S. Ganapathi ¹

Affiliations
1 Department of Geology, M.S. University of Baroda, Vadodara-390002, IN

Abstract

Environmental significance of grain-size frequency distribution of ancient terrigenous deposits has been examined in the light of applicability as one of the tools of environmental analysis. Synthesis of results obtained from grain-size analysis of 71 subsurface samples of Barren Measures Formation in Bellampalli-Chinnur area of Pranhita-Godavari Lower Gondwana basin points to the interplay of various sedimentary dynamics of diverse microenvironments within a broad fluvial system. Formative processes were controlled by availability of different sizes of particles and competency of flow. Channel pattern deduced from the study conform to the model of "mixed load anabranching river pattern." This study reaffirms the reliability of grain-size analysis of sands in interpreting ancient depositional environment.

Keywords

Sedimentology, Grain-Size Frequency Distribution, Barren Measures Formation, Pranhitagodavari Basin, Andhra Pradesh.

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Pinaki Majumdar ¹, S. Ganapathi ²

Affiliations
1 HOEC, Baroda - 390 002, IN
2 Department of Geology, M S University of Baroda, Vadodara - 390002, IN

Abstract

Petrographic examination of 106 sandstone samples belonging to Lower Gondwana Group of Bellampalli-Chinnur area, Pranhita-Godavari Basin were carried out to evaluate the effects of interstitial water chemistry and burial depth on detrital and authigenic mineralogy of these sandstones. Except Basal Talchir sandstones there was no significant alteration in the framework mineralogy. Minor variation in the Q F R ratio of these sandstones was brought about by variable relief-ratio of the watershed and changing climatic during the Permian. Overall sequence of the mineral authigenesis can he divided into two stages. The early stage was characterised by eogenetic formation of kaolinite and chlorite under the influence of climate-controlled groundwater chemistry. Chlorite was formed in times of relative aridity (as in Talchii Barren Measures and Lower Kamthi times) when cation concentration in groundwater was high. During the times of high precipitation (as during Barakar and Lower Kamthi times) pore water was dilute, resulting in the flushing out of the cations and consequent formation of kaolinite. The later stage of diagenetic sequence involves variation in Eh-pH condition of pore fluids as a function of thermocatalytic degradation of kerogen and depth of burial. This stage is characterised by mica authigenesis, precipitation of calcite and iron Oxide cement and dissolution of early formed calcite. Alteration of potash feldspar into hydromuscovite in the Barakar and Talchir strata shows Signatures of depth-controlled Process of illitization.

Keywords

Sedimentology, Lower Gondwana Sands, Diagenesis, Dentrital Mineralogy, Authigenesis, Pranhita Godavari Basin, Andhra Pradesh.

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