A detailed analysis of depositional history of Miocene sediments and various effects which are governed for creation of accommodation space as well as the processes of sedimentation inherent to the depositional system at that period is described in this work. The early Miocene clastic sediments are deposited in prograding environment where sediment supply exceeds the accommodation space available. The accommodation space created due to basin subsidence and source area upliftment due to local and regional tectonic activity in the basin. In the early Miocene time, the Assam shelf major transgression occurred and several minor transgression followed. There was wide spread deposition of the fluvial Tipam sandstones. In Miocene time due to thrust loading and flexure subsidence, accommodation space was created for deposition of the sediments. The Tipam Sandstone is deposited by cyclic deposition of fining upward sequence in a fluvial to brackish water environment of braided river processes. The mechanism of braided rivers is also discussed in which it laterally expanded, leaving sheet like or wedge -shaped deposits of channel and bar complexes preserving only minor amounts of flood plain material.
The Talchir Gondwana basin houses the Talchir Formation at the base, succeeded by the Damuda Group and the Kamthi Formation in upward progression. The present study is an attempt to determine the provenance of the Damuda Group and its variability in terms of location, climate and tectonism through time from the composition of sandstone grains and detritus of the constituent Karharbari, Barakar and Barren Measures formations. The Damuda sandstones are composed of variable amounts of monocrystalline undulatory, nonundulatory and polycrystalline quartz grains, potash and plagioclase feldspars as well as metamorphic and sedimentary rock fragments in addition to heavy accessories. Palaeocurrent studies suggest that the Eastern Ghats Supergroup lying to the south of the basin served as the source area of the Damuda sediments. Plots of sandstone composition in tectonic setting discrimination diagrams suggest derivation of the detritus from craton interior, continental block and recycled orogen provinces. Statistical analyses indicate significant differences in the detrital modes of the sandstones of the Karharbari, Barakar and Barren Measures formations, which may be attributed to temporal and spatial variation of the provenance coupled with climate change in commensurate with Damuda sedimentation.
The cyclic arrangement of lithofacies of the Karharbari Formation of the Damuda Group from a part of the Talchir Gondwana basin has been examined by statistical techniques. The lithologies have been condensed into five facies states viz. coarse-, medium-, fine-grained sandstones, shale and coal for the convenience of statistical analyses. Markov chain analysis indicates the arrangement of Karharbari lithofacies in form of fining upward cycles. A complete cycle consists of conglomerate or coarse-grained sandstone at the base sequentially succeeded by medium-and finegrained sandstones, shale and coal at the top. The entropy analysis categorizes the Karharbari cycles into the C-type cyclicity, which is essentially a random sequence of lithologic states. Regression analysis undertaken in the present study indicates the existence of sympathetic relationship between total thickness of strata (net subsidence) and number and average thickness of sedimentary cycle and antipathic relationship between number and average thickness of sedimentary cycle. These observations suggest that cyclic sedimentation of the Karharbari Formation was controlled by autocyclic process by means of lateral migration of streams activated by intrabasinal differential subsidence, which operated within the depositional basin and the channels carrying coarse grade clastic sediments, which make the cycles thicker, tend to be more common in the areas of maximum subsidence. Clastic sediments issued from the laterally migrating rivers interrupted the cyclic sedimentation of the Karharbari Formation in many instances.
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