Nivedita Chakraborty

Siliciclastic–carbonate mixing modes in the river-mouth bar palaeogeography of the Upper Cretaceous Garudamangalam Sandstone (Ariyalur, India)

Abstract Mixed siliciclastic–carbonate rocks constitute the Upper Cretaceous Garuda-mangalam Sandstone Formation, Ariyalur (India), and offer an opportunity to look into the broad spectrum of mixing of compositionally and genetically different components. The palaeogeographic reconstruction indicates that deposition in the nearshore zone differed strongly in energy and active processes operatives due to the presence of a shore-parallel river-mouth bar. The western wing of the Mississippi bird-foot delta is considered to be a present-day analogon. Facies analysis in combination with petrography clearly shows the variability in palaeoenvironmental characteristics, both biogenic and non-biogenic. It also indicates diagenetic uptake of carbonate that filled empty spaces and actively replaced original components. Chemical staining followed by limited application of cathodoluminescence and energy dispersive X-ray analysis (EDAX) hint at intricacies in mixing arising from the compositional variations in the carbonate components. A model of siliciclastic–carbonate sediment mixing, including both the depositional and diagenetic developments, is presented; it is aimed at generating a better overview of, and a deeper insight into, the physical and chemical mechanisms involved.

Chapter 6 Proterozoic Vindhyan Basin: configuration and evolution

Abstract This chapter attempts an understanding of the Proterozoic Vindhyan Basin history in the broad framework of central India. Although the entire Vindhyan Supergroup is within the scope of this work, particular attention is paid to the little-known northwestern fringe exposures. Distinctive facies assemblages and diverse palaeocurrents in these exposures of the Lower Vindhyan play a pivotal role in the interpretation. Analysis of outcrop and subsurface data that extend under the Gangetic alluvium to the north of the Vindhyan outcrops further supports the hypothesis that an east–west-elongated basement ridge initially separated the master Vindhyan Basin from smaller contemporary basins to the north. Deposition took place in isolated lacustrine and fluvial basins north of the divide and largely in a marine realm south. Dextral shear accompanying rifting generated ridges that criss-crossed the Lower Vindhyan seafloor to the south. The uniform character of the Upper Vindhyan throughout, nevertheless, testifies to later drowning of the divide and unification of all of the basins as a consequence of regional tilt northward. However, the extended Vindhyan Sea was restricted by a second east–west-elongated ridge from merger with the contemporary Proterozoic sea further north, disparate sediments of which have been encountered in a few drill cores only.

Indigenous siliciclastic and extraneous polygenetic carbonate facies in the Albian-Turonian Karai Shale, Cauvery Basin, India

The Albian-Turonian marine Karai Shale, a mixed siliciclastic–carbonate formation in the pericratonic Cauvery rift basin, India, arouses interest as its evenly laminated siliciclastic beds appear indigenous, settling from suspension, while the calcarenite beds are made of materials that were deposited elsewhere and then shifted by agents that occasionally invaded the present depositional site. Three calcarenite facies recognized differ in body geometry, sedimentary structures, recurrence and in association. One of the two calcarenite facies enclosed by grey shale is most widely distributed, characterized by tabular geometry, internal wave-cum current structures, overall graded nature and gutters at base, It is inferred as product of meteorological storms, The other calcarenite facies overlapping in occurrence constitutes a single amalgamated bed package that is characterized by highest degree of dilution by siliciclastics in coarsest fraction, selectively incorporating pebble-sized fragments of the granitic basement and wood. Its beds are internally massive or cross-stratified and interpreted as products of successive pulses of ebb of a very strong event may be a megastorm or tsunami. The third calcarenite facies is represented by a couple of trough cross-stratified sheets embedded within greenish grey shale enriched in phosphate nodules and glauconite pellets and completely devoid of storm beds. Near-orthogonal diversion of palaeocurrent between the two associations invokes ocean bottom current to impinge upon the shelf-margin during maximum transgression for the third calcarenite faces. The δ13C ratio in the TST part is consistent with deposition on open marine shelf, but in the HST part they indicate influx of meteoric water.

Aseismic Tectonism-Induced Soft-Sediment Deformation in a Tranquil Palaeogeography: Chikkshelikere Limestone Member, Proterozoic Kaladgi Basin, Southern India

The wide spectrum of synsedimentary deformation products occurring almost at all stratigraphic levels within the ~40 m-thick section of the Chikkshelikere Limestone Member of tentative Mesoproterozoic age in India is evaluated for its origin. Among the two principal facies components, both carbonate, of this Member the dark micritic facies generally underwent brittle deformation, and the light microsparry facies responded in ductile fashion to the same deformational stress. Breccia patches, hardly having any boundary, abound at almost every stratigraphic level within the Chikkshelikere Limestone Member. The third facies constituting less than 3% by volume of the Member is of laterally persistent carbonate intraclastic conglomerate beds. The dark facies is of massive micrite, while the light facies is made up of interlocking microspar crystals, but bears minor wave-current structures, and rare minute erosional features at its base. Non-luminiscent character of the former under CL is reminiscent of oxidizing basin-floor environment, while the bright orange luminescence of the latter testifies pervasive burial recrystallization. The dark micritic facies is interpreted as indigenous and the light microsparry facies as allochthonous, possibly laid by highly energy-depleted storm wave-cum-current. Mineralogical as well as geochemical analyses indicate preferred dolomitization and carbon enrichment in the dark micritic facies. Selective pyritization is also observed along the base of the same facies. These features collectively suggest selective microbial mat proliferation within this facies. Despite early induration being the rule for carbonate sediments, microbial mat growth apparently enhanced its rate within the dark micritic facies and caused a viscosity contrast between the two principal facies. Both the facies underwent ductile deformation in slump folds generated on the sediment surface, but then acquired and accentuated viscosity contrast led them later to respond differently to similar deformational stress. However, in micro-graben structures both the facies underwent brittle deformation indicating pervasive cementation with longer residence time. Breccia patches, where present, elicit liquefaction, occasionally followed by fluidization, rarely the porewater is poured out on the sediment surface. The intraclastic conglomerate beds are massflow products. Indentation on the bed-roofs and shear fold on their tops elicit subsurface occurrence of the flows. Only those intraclastic beds without indentation on their roofs, but with eroded bases were possibly surficial products. Frequent liquefaction and fluidization without any stratigraphic selectivity and lateral continuity is hardly attributable to seismicity. Aseismic tectonism such that relates to geoidal tilt possibly accounts for the small-scale SSDs distributed all over the Chikkshelikere Limestone Member better. Slow warping of strata engendered frequent pore-water overpressuring that caused the synsedimentary deformation including intraclastic conglomerate beds emplaced under and above the sediment surface.