S Ramasamy, A Ramachandran, K Velmurugan, David Chawngthu, S Bhuvaneswari, M Suresh Gandhi
2012 International Journal of Geology, Earth and Environmental Sciences   unpublished
The Kallamedu sedimentary sequence, one of the least investigated sedimentary formations in South India (Ariyalur District, Tamil Nadu State), is well known for the occurrence of dinosaur remains. For the present study, detailed field and laboratory investigations have been conducted for understanding the depositional environment of Kallamedu Formation. The field studies comprise of collection of representative samples, measuring litho-sections and recording sedimentary physical structures such
more » ... as parallel beddings and laminations besides planar and trough cross beddings. It has been observed in the field that dominant lithologies comprise of fine grained siltstones and fine laminated clays with rare occurrence of coarser sandstones and polished pebbles in the fine sandstone units. The dinosaur remains are mostly fragments of bones, and we rarely observed well preserved animal remains in the exposed litho-units. At places calcrete formation and vertically grown calcite crystals over the substratum of siltstones are observed. Thin section petrography revealed the presence of argillaceous siltstones, wackes, calcretes and minor arenites. Most of the petrographic types contain considerable amount of argillaceous material. The framework grains are angular quartz grains of silt and fine sand sizes. The grain size data of selected sand-dominated samples of the Kallamedu Formation reveal that the presence of coarse-to fine-grained, moderate to poorly sorted, fine skewed and platykurtic with bimodality. Clay mineralogy reveals the dominance of kaolinite followed by illite, smectite and chlorite. Perhaps the granitic gneissic terrain released feldspars which altered to kaolinitic clays. Illitic clays have been formed in a diagenetic environment which altered the kaolinite. The sediments have experienced a moderate to deep burial diagenesis. The high amount of organic carbon attests the existence of many swamp basins, which are ideal for organic matter accumulation due to restricted oxygen circulation with luxuriant growth of plants around them. Most of the trace elements (Co, Pb, Cu, Zn and Ni) are related to detrital phase and as such are correlatable. SEM studies of fine silty clayey sediments reveal low undisturbed energy level existed during the deposition of clays excepting flocculation which resulted in a juxtaposed arrangement of clay plates. However bioturbation is not evident during/after deposition in the fine sediments. The sand grains surface texture indicate angular to subangular shape with thick secondary precipitation of argillaceous/calcium carbonate over the irregular undulated grains topography. Thus most of the clastics did not suffer much textural alteration and were derived from nearby source. The present study points out that the Kallamedu clastic rich sediments have developed in a moderate energy level, most likely in a terminal fluvial setting where coarse materials with cross bedding features formed in channels and fine silts and clays deposited vertically over the flood plains. The dinosaur remains preserved in the fine clastics perhaps point out the existence of a depositional system ranging from fluvial marshy to estuarine environments.