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Bed roughness and grain sorting-an experimental study over fine to medium sand beds

International Journal of Sediment Research 2017年第3期32卷 384-400页

作者： Barendra Purkait Dipanjan Das Majumdar rajat mazumder Geological Studies Unit Indian Statistical Institute Department of Geology University of Calcutta Department of Remote Sensing& GIS Vidyasagar University School of Biology Earth and Environmental Sciences University of New South Wales Dept. of Applied Geology Faculty of Engineering and Science Curtin University malaysia

The relation between grain-size distribution of the bed and in suspension was critically examined under a uniform flow velocity of 50 cm/s over two beds: one of mainly fine sands and the other of medium sands. Two sections-one 2.85 m downstream and the other 6.35 m downstream in the experimental channel-were selected for sampling to study the grain-sorting pattern in the vertical direction along the direction of transport. The shape and type of the grain-size distribution pattern were critically studied with height above the bed. The change in the distribution pattern has been attributed to the change of local bed roughness causing scouring against the protruded relatively coarse grains on the bed. Such trends are important to predict the nature of river bed topography. The sand of Bed-1 initially exhibits a log-skew-Laplace distribution at different heights of suspension. The distribution pattern, however,changes but this changing pattern is not consistent along the upstream side. For Bed-2, which initially exhibits a log-normal distribution, the same pattern persists from the height of suspension at 5 cm up to20 cm. Such consistency in log-normality is also observed at the downstream points of measurement. It is generally expected that the mean grain-size would reduce with increases of suspension height but the results of the experiments, in some occasions, differ significantly from the gradual fining upward *** result has been attributed to local changes of bed roughness arising from the protruded relatively coarse grains causing eddies, scouring, and turbulent phenomena which moves coarse particles higher in suspension adding a coarse tail to the distribution increasing the mean grain-size.

关键词： Flume experiments Suspension height Uniform flow velocity Bed roughness Grain-size distribution Bed scouring Turbulent flow

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The response of stromatolites to seismic shocks： Tomboliths from the Palaeoproterozoic Chaibasa Formation, E India

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Journal of Palaeogeography 2016年第4期5卷 381-390页

作者： A.J. (Tom) van Loon rajat mazumder Shuvabrata De Geocom Consultants Valle del Portet 17 03762 Benitachell Spain Department of Applied Geology Faculty of Science and Engineering Curtin University Sarawak CDT 250 98009 Miri Sarawak Malaysia Indian Institute of Science Education and Research Department of Earth and Environmental Sciences Bhopal By-pass Road Bhopal 462066 Madhya Pradesh India

It is demonstrated here for the first time how Palaeoproterozoic stromatolites survived seismic disturbance of their substrate. The stromatolites under study could have been cyanobacteria or any other photo-autotrophic microbes, which formed mats that covered a substrate of very fine-grained sandstones and mudstones of the Chaibasa Fm. in eastern India. The sediments represent a shelf environment. The local abundance of the stromatotites suggests that the tow-energy environment formed a suitable habitat. The common phases of tectonic quiescence were, however, occasionally interrupted by seismic shocks. These were sufficiently strong to deform the mat layers, the tower parts of which might already have been （semi-） consolidated. The mats became partly folded, partly faulted, and already consolidated parts of the stromatotite layers broke off. This can be deduced from the angular shapes of part of the broken-off fragments. It appears, however, that part of these fragments were stilt sufficiently soft to become rounded and deformed by rotting over the seafloor, probably under the influence of tidal currents. When come to rest, these fragments served as a new substrate for new generations of the microorganisms. These micro-organisms thus survived by continued growth on the reworked fragments and built up new stromatolites that may show an ‘angular disconformity＇ with the stromatotites of their substrate. It thus is shown that stromatotites have an adequate response to a sudden disturbance of their habitat, and that they survive earthquakes by colonization of broken-off fragments. We call the ‘healed＇ fragments ‘tombotiths＇（tumbled stones）.

关键词： Stromatolites Seismic shocks Soft-sediment deformation structures Pataeoproterozoic Singhbhum craton Tombotiths India

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“The response of stromatolites to seismic shocks: Tomboliths from the Palaeoproterozoic Chaibasa Formation,E India”: Reply

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Journal of Palaeogeography 2017年第3期6卷 235-241页

作者： A.J. (Tom) van Loon rajat mazumder Shuvabrata De College of Earth Science and Engineering Shandong University of Science and Technology Geocom Consultants Department of Applied Geology Faculty of Science and EngineeringCurtin University Sarawak Geological Studies Division CSIR-National Geophysical Research Institute

Our contribution about a newly recognized sedimentological feature, which we called tombolith,has raised some comments on the etymology of the term, the assumed lack of field data, the usage of a term that has a special meaning in stratigraphy, the origin of the seismic shocks that played a role, so-called basic tenets, the origin of soft-sediment deformation structures, and the role of liquefaction. We argue that most comments result from an insufficiently thorough reading of our text, whereas some other remarks concern aspects that we did not deal with. Finally, several remarks are based on outdated literature and from insufficient knowledge of the regional geology.

关键词： Stromatolites Seismic shocks Palaeoproterozoic Singhbhum Craton Tomboliths India

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A review of the～1600 Ma sedimentation,volcanism,and tectono-thermal events in the Singhbhum craton,Eastern India

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Geoscience Frontiers 2013年第3期4卷 277-287页

作者： Priyanka Chatterjee Shuvabrata De Marinah Ranaivoson rajat mazumder Makoto Arima Department of Geology University of Calcutta35 B.C.RoadKolkata 700019India Geological Studies Unit Indian Statistical Institute203 B.T.RoadKolkata 700108India Graduate School of Environment and Information Science Yokohama National University79-7TokiwadaiHodogaya Yokohama 240-8501Japan School of Biology Earth and Environmental SciencesUniversity of New South WalesKensingtonSydney NSW 2052Australia

The Palaeoproterozoic-Mesoproterozoic transition （ -1600 Ma） is a significant event in the Earth history as a global thermal perturbation affected the pre-1600 Ma landmasses. Like other cratonic blocks of the world, lithospheric thinning, sedimentation, magmatism, metamorphism and crustal melting/anatexis are associated with this significant geological event in the Singhbhum cratonic province of India. This paper is a review of sedimentological, magmatic and tectono-thermal events in the Singhbhum craton at 1600 Ma. The Palaeo-Mesoproterozoic sedimentation and volcanism in the Singhbhum craton took place in a terrestrial intracontinental rift setting. The available geochronological data are indicative of late Palaeoproterozoic to Neoproterozoic tectono-thermal events in the Chhotanagpur Granite Gneissic Complex （CGGC）, an east-west trending arcuate belt of granite gneisses, migmatites and metasedi- mentary rocks. A detailed multidisciplinary geo-scientific investigation of the Dalma volcanic belt and the area to its north （Chandil Formation） and further north in CGGC will enable us to constrain the extant surface processes and crust-mantle interactions, the collision events between the North and South Indian cratonic blocks, and the position of India in the Columbia supercontinent.

关键词： Late Palaeoproterozoic Mesoproterozoic Sedimentation Volcanism Geochronology Singhhhum

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