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

Pradip K. Maulik ¹, Chandan ChaKraborty ¹, Parthasarathi Ghosh ¹, Dhiraj Rudra ²

Affiliations
1 Geological Studies Unit, Indian Statistical Institute, 203, B.T. Road, Calcutta - 700 035, IN
2 Geological Studies Unit, Indian Statistical Institute, 81 T. C. Road, Calcutta- 700 053, IN

Abstract

The Denwa Formation (∼300m thick) representing a part of the unconformity bounded Triassic (Pachmarhi-Denwa) succession of the Satpura Gondwana basin, is characterized by two different fluvial styles marked by contrasting architectural elements within the alluvial system. These fluvial styles are distinct from the underlying Pachmarhi Formation representing an alluvial plain with channel belts characterised by multiple-channel, sandy braided rivers.

The lower part of the Denwa Formation is characterized by an alternation of medium to fine grained, thick, sheet-like, sandstone bodies and red mudstone intervals having centimetre to decimetre scale fine grained sandstone interlayers. The thick sandstone bodies are comprised of several groups of storeys, and individual storeys are made up of macroform strata reflecting frontal, oblique and vertical accretion. It is evident from the overall architecture that the lower part of the Denwa Formation represents' an alluvial plain characterised by sandy, braided channel belts and associated floodplain/basin.

The upper part of the Denwa Formation is characterised by conspicuous absence of thick multistorey sandstone bodies. It is predominantly a mudstone dominated, sand-poor succession characterized by ribbon-shaped bodies encased within pedoturbated mudstones. Ribbon-shaped bodies internally show inclined heterolithic (sand-mud) stratification representing lateral accretion of channel bars of high sinuosity rivers. The upper part of the Denwa Formation is interpreted to represent deposits of a network of meander channels laterally separated by well-developed and stable interfluve and floodplain areas. The changes in the fluvial styles represented in the Denwa succession have been attributed to changes in the aggradation rate, avulsion frequency and climatic setting.

Keywords

Sedimentology, Fluvial Succession, Depositional Environment, Satpura Basin, Gondwana System, Madhya Pradesh.

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Chandan Chakraborty ¹, Subrata Karmakar ¹

Affiliations
1 Department of Geology, J.K. College, Purulia, West Bengal, IN

Abstract

Detailed field studies on the regional structure of the Vindhyan strata in Son valley reveal that the Vindhyan strata define a broad syncline with a gently plunging (westerly), mildly curved fold axis (average trend ENE-WSW) and a southerly inclined axial plane. The southern limb of the broad syncline shows several smaller folds which are conspicuously absent in the gently dipping, homoclinal, northern limb. The mesoscopic folds also conform to the regional structural pattern. Progressive deformation of horizontal strata under asymmetric compression associated with a vertical shear displacement has been simulated with the help of a computer software. The computer-generated structural pattern is remarkably similar to the pattern revealed from field studies indicating that the structural pattern of the Vindhyan strata is the consequence of asymmetric compression with the northerly directed force far exceeding the magnitude of the southerly directed force.

Absence of any northerly directed compressional movement in the Indian peninsula in post-Vindhyan period suggests that the deformation of the Vindhyan had been syn-Vindhyan, perhaps associated with the Satpura orogeny. This leads to the deduction that the origin and evolution of the Vindhyan basin was related to flexural subsidence associated with repeated thrust loading in the south.

Keywords

Structural Geology, Basin Tectonics, Son Valley, Vindhyan Basin, Central India.

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Chandan Chakraborty ¹

Affiliations
1 Department of Geological Sciences, Jadavpur University, Calcutta 700032, IN

Abstract

The Goa coast of southern India is affected by short-term storm events during summer which perturb the fair-weather beach equilibrium considerably. The storm surge ebb at Goa coast moves perpendicular to the shoreline and deposits its load in the form of shore-parallel ridges. Renewal of fair-weather condition throws the storm profile into disequilibrium and exposes part of the storm-induced ridge field. The effort for re-equilibration establishes a new beach profile in which the part of the storm-induced ridge field turns into backshore.

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Gerald M. Friedman ¹, Chandan Chakraborty ²

Affiliations
1 Department of Geology, Brooklyn College and Graduate School of the City University of New York, Brooklyn, New York - 11210, IN
2 Department of Geology, J. K. College, Purulia, West Bengal, IN

Abstract

The carbon and oxygen isotopic compositions of the minerals of carbonate rocks depend on the composition of the precipitating water and the temperature of precipitation. The carbon isotopes suffer little fractionation during precipitation of carbonate minerals and their carbon isotopic composition reflects that of the precipitating water. On the other hand, carbonate minerals become enriched in ¹⁸O with respect to the precipitating water and the degree of enrichment depends on the temperature of precipitation - lower the temperature, higher the enrichment.

Isotopic studies of marine carbonate rock successions have the potential to reveal the temporal trends in the isotopic composition and temperature of seawater. The oxygen isotopic composition of seawater is a function of a number of processes such as, evaporation, rainfall, runoff and mixing of surface and deep ocean water. Evaporation leads to enrichment in heavier isotope whereas the other processes cause enrichment in lighter isotope. The oxygen isotopic composition of seawater also varies between glacial and non-glacial periods. The seawater becomes enriched in heavier isotope during glacial period due to preferential removal of the lighter isotope in glaciers. Melting of glaciers, on the other hand, leads to dilution of the seawater isotopic composition. The oxygen isotopic composition of marine carbonate rocks has been found to be decreasing with increasing geologic ages. This may either indicate that the seawater composition has progressively become heavier or that the seawater temperature has decreased through time. There is a general opinion that the seawater composition did not vary much throughout geological time and the seawater temperature may have been relatively higher in past geological periods.

The carbon isotopic composition of seawater is a function of (1) terrestrial input of dissolved inorganic carbon and particulate organic carbon through rivers, (2) organic carbon productivity in the ocean, and (3) organic carbon deposition and burial in the ocean. The lighter river water dilutes the overall carbon isotopic composition of seawater. On the other hand, increased organic carbon productivity and burial enriches the seawater in heavier isotope. The present day zero permit carbon isotopic composition of seawater is maintained by a balance between the terrestrial input and oceanic output through organic carbon production and burial, and carbonate precipitation. On a long-term scale (10⁹a), the average δ¹³C values of carbonates of Precambrian and Phanerozoic ages have been found to be fairly constant around zero permil. However, δ¹³C values of marine carbonates indeed show short-term (0.2 to 10 Ma) excursions from the zero permil value, both in the positive and negative directions. For a constant terrestrial input of inorganic carbon, two processes can lead to positive excursion ill the isotopic record of shallow marine carbonates: (1) increased rate of preservation of organic matter in the ocean and (2) increased rate of photosynthetic organic carbon production in the ocean. These processes preferentially extract lighter carbon from the seawater and make it isotopically heavier. Negative excursion would occur if the above processes move in the opposite direction.

Isotopic studies of Precambrian-Cambrian boundary interval in different parts of the world have revealed that the transition is marked by a positive excursion close to the boundary and a swing back to more negative-less positive values in the early Cambrian strata. The similarity of the carbon isotopic record across the transition in different boundary sections of the world indicates that the isotopic excursion is of chronostratigraphic significance and may be used as an aid to correlation. The carbon isotope chronostratigraphic marker for the Precambrian-Cambrian boundary has been identified in the Krol section of the Himalayas. The present study reveals that the chronostratigraphic marker is also present in the Vindhyan Basin of central India.

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