Dhiraj M. Banerjee

Sequence Stratigraphy of the Neoproterozoic Infra Krol Formation and Krol Group, Lesser Himalaya, India

A BSTRACT: A sequence stratigraphic study of terrigenous and carbonate rocks of the Infra Krol Formation and Krol Group in the Lesser Himalaya fold and thrust belt of northern India was undertaken as part of a broader investigation of the significance of carbon isotope data in Neoproterozoic successions. Eight regional stratigraphic discontinuities were traced over a distance of nearly 300 km, and interpretations were anchored in a series of local studies involving the mapping of key beds and the measurement of closely spaced sections. Three of the regional surfaces are interpreted as sequence boundaries on the basis of (1) locally developed incised valleys , 60 m deep; (2) paleokarstic depressions with , 50 m of mappable relief; (3) subaerial dissolution and weathering products (breccias and calcrete) filling vertical fissures, dikes, cavities, and shallow de- pressions in underlying carbonate rocks; and (4) small-scale evidence for sub- aerial exposure at an erosion surface. The remaining five discontinuities are regional flooding surfaces identified on the basis of either facies changes with an abrupt upward deepening across the surface or transitions in facies stacking patterns, typically from forestepping to backstepping. A glacio-eustatic origin is permitted, although not required, for the three sequence boundaries, but no evidence has been found for marked lowering of sea level at other horizons. A mismatch between the stratigraphic location of sequence boundaries and car- bon isotope minima suggests that local diagenetic alteration or oceanographic phenomena unrelated to glaciation may be in part responsible for observed isotopic variation, and that small ice sheets may have existed during apparently nonglacial times without producing either cap carbonates or negative carbon isotope excursions.

The sulfur isotopic composition of Neoproterozoic to early Cambrian seawater-evidence from the cyclic Hanseran evaporites, NW India

The sulfur isotopic composition has been determined for terminal Neoproterozoic to early Cambrian evaporites from the Hanseran Group in NW India. δ34S values, ranging from +27.5‰ to +39.7‰ vs. Canyon Diablo Troilite (CDT) (n=26), further substantiate the strongly positive sulfur isotope signature of contemporaneous seawater. Distinct stratigraphic variations in δ34S within this succession suggest secular variations in seawater composition which might be of global importance. Overall, this documents major pertubations of the sulfur cycle in response to paleooceanographic changes trailing tectonic, climatic and/or biological rearrangements during the Neoproterozoic and early Paleozoic.

Rare-earth elements and Stable Isotope Geochemistry of early Cambrian chert-phosphorite assemblages from the Lower Tal Formation of the Krol Belt (Lesser Himalaya, India)

Abstract The Neoproterozoic–Cambrian transitional sequence in the Mussoorie and Garhwal Hills of the Lesser Himalaya is represented by dolomites of the Upper Krol Formation and chert-phosphorite assemblages of the lowermost Tal Formation. These rocks provide valuable information regarding the assumed existence of a stratified ocean during the terminal Neoproterozoic and beginning of earliest Cambrian (Nemakit-Daldynian). The isotopic compositions of carbonate and organic carbon in the transition profile can be best interpreted as reflecting an oceanic anoxic event (OAE) prior to the phase of early Cambrian phosphate formation. The chert-phosphorite beds record negative Ce and positive Eu anomalies. While Ce is an important paleooceanic redox indicator, Eu is rather immobile under normal diagenetic conditions. The observed Eu anomaly is linked to the high Ba content in these phosphorites and is indicative of an anoxic (or sulfate reducing) diagenetic environment. The suggested stratification of the ocean during the Late Neoproterozoic and Nemakit-Daldynian times would have been linked to the reduction of oxygen supply to the zone below the redoxcline, causing 12 C -and P-rich organic matter previously produced in the photic zone to be trapped in large quantities within the deeper anoxic zone. This would, in turn, increase the 13 C content of the water in the photic zone. Moreover, in the deeper anoxic waters, reduction of Ce4+ to Ce3+ would result in an enrichment of dissolved Ce (as Ce3+). Upwelling of this anoxic bottom stratum would bring the 12 C -and Ce-enriched waters back to the oxic marine realm. However, on entering the highly oxic productive zone, the excess Ce is expected to be removed from the water by oxidation to insoluble Ce4+ followed by precipitation or preferential scavenging. Due to these oxidation reactions, the waters of the shallow oxic zone would tend to become rapidly depleted in Ce. In other words, ocean mixing and upwelling during the early Cambrian were ultimately responsible for the transport of 12 C , P and Ce to the oxic shelves. Sharp drops in both δ 13 C carb and δ 13 C org support the existence of oceanic anoxia followed by oceanic mixing at this juncture of Earth history. The large spread of the δ 34 S values of early diagenetic pyrites within the phosphorite bearing sequence in contrast to the limited spread of δ 13 C carb values suggests that phosphatisation took place under exclusively suboxic diagenetic non-sulfate reducing condition and pyritisation began only when phosphate layers entered the zone of sulfate reduction.

Genesis of upper Proterozoic−Cambrian phosphorite deposits of India: isotopic inferences from carbonate fluorapatite, carbonate and organic carbon

Carbon and oxygen isotope analyses performed on sedimentary carbonates, organic carbon (kerogen) and structurally bound CO2−3 groups of carbonate fluorapatite (francolite) from four widely separated Indian phosphorite provinces have shown that each of the deposits is characterized by specific (basically facies-dependent) distribution patterns of δ 13Ccarb, δ 13Ocarb, δ 13Corg and δ 13Capat Comparison of the carbon isotope composition of apatite-CO2 with that of the coexisting sedimentary carbonate has served to further constrain the genetic type of the individual phosphorite deposit investigated, augmenting relevant geological evidence by an additional geochemical discriminator. As a whole, the results confirm the importance notably of carbon isotope work for elucidating the processes involved in sedimentary phosphorite formation.

Petrology and geochemistry of greywackes from the Aravalli Supergroup, Rajasthan, India and the tectonic evolution of a Proterozoic sedimentary basin

The Proterozoic Aravalli Supergroup in south-central Rajasthan, western India is predominantly composed of coarse, low-grade meta-arenites, carbonates, greywackes, interbedded fine-grained silty arenites and metapelites resting on an Archaean basement complex of gneisses, schists and high-grade metasediments. We have differentiated four lithofacies (A, B, C and D) within the greywacke suite, each with characteristic QFL composition, grain size and major, trace and rare earth element concentrations. Tectonic setting for each facies type could only be discriminated using QmFLt plots. Greywackes of facies A and D were derived from a recycled orogen and facies B was derived from arc-related andesitic source rocks. A continental block provenance is indicated for the silty arenites of facies C. This petrofacies-based interpretation shows good agreement with environmental reconstructions based on lithofacies and sedimentary structure analyses. The only exception is facies B, whose andesitic parentage as deduced from petrofacies analysis could not be demonstrated at its outcrop. These provenance discriminations were tested through tectonic setting interpretations based on major, trace and rare earth elements. The geochemical data broadly confirmed the tectonic interpretations made on the basis of the petrological attributes. Use of these laboratory-based methods in conjunction with field measurements of primary sedimentary structures allow us to discriminate provenance and tectonic setting fairly precisely. A Proterozoic plate model with subduction to the west has been proposed to explain the sedimentological variation in the Aravalli Supergroup.

Regional variations in the carbon isotopic composition of phosphorite from the Early Cambrian Lower Tal Formation, Mussoorie Hills, India

Abstract Early Cambrian Lower Tal phosphorites of the Mussoorie Syncline have been analyzed for their carbon isotopic composition of apatite-bound carbonate and organic carbon. Two mine excavations at Maldeota and Durmala, located on two limbs of the syncline and ∼14-km apart were subjected to detailed sampling along selected stratigraphic profiles. Field and microscopic evidence suggest a shallow marine environment at the site of phosphorite formation characterized by small-scale fluctuations in the sedimentation milieu. Isotope data have been utilized to characterize differing redox conditions prevalent during phosphorite formation at Durmala and Maldeota. In addition, a marked negative carbon isotope excursion has been recorded at the dolomite–phosphorite contact defined by the Krol–Tal stratigraphic boundary. It is thought to reflect the influx of 12 C- and P-rich waters from a deeper part of the sedimentary basin. Based on the isotopic composition of carbon in the phosphorite, sulfur in the associated pyrite and petrological data, it has been inferred that the Durmala phosphorite reflects deposition under a suboxic diagenetic environment and the Maldeota phosphorite formed within suboxic to anoxic sulfate-reducing conditions.

Organic Carbon Isotopic Composition of Proterozoic Sedimentary Rocks from India: Preliminary Results

In order to characterize the organic carbon preserved in Proterozoic sedimentary rocks of India, a comprehensive collaborative study has been initiated. The present chapter summarizes the first results from four selected basins: Aravalli-Delhi, Chattisgarh, Vindhyan, and Krol-Tal. Variations in TOC, δ 13Corg and H/C ratios from this study, and additional data from earlier published work, are discussed. In general, the carbon isotopic composition falls within the envelope of previously published whole-rock δ13Corg data for the Middle and Late Proterozoic displaying values in the range — 33 to — 8‰.

Morphometric analysis of Proterozoic stromatolites from India — preliminary report on testing of a new technique

Abstract A recent technique of image analysis of stromatolite laminae has been tested on Proterozoic stromatolitic assemblages in the Aravalli dolomite of Udaipur and Jhabua, the Bhagwanpura limestones of Chittorgarh, the Bilara carbonates of Jodhpur and the Raipur limestones of the Chattisgarh region. The technique involves tracing of the upper lamina boundary of each stromatolite column which permits measurements of different profile shape attributes such as area, length, width, vertical and horizontal intercepts and profile inclination. The numerical data so generated were subjected to statistical analysis. Cluster analysis by the pair group method shows mutually correlatable cluster centres. Dendrograms based thereon have been drawn to depict the nature of correlation. Four major groupings of lamina shape have been recognized: (1) highly convex, (2) flattened convex and irregular shape, (3) erect, highly convex to conical, and (4) horizontal development of flattened laminae. Successful correlation between stromatolites of the Bhagwanpura limestone (Chittorgarh), the Bilara Formation (Jodhpur) and Raipur (Chattisgarh) and negative correlation between the Aravalli stromatolites of Udaipur (Rajasthan) and Jhabua (Madhya Pradesh) clearly indicate the biostratigraphic utility of morphometric analysis.

Diagenetic formation of interlayer-deficient fluorophlogopite as a clay mineral in Early Cambrian phosphorite (Lesser Himalaya, India): The trioctahedral analog of illite

Abstract The occurrence of a trioctahedral analog of illite, the dioctahedral interlayer-deficient K-mica, has long been debated. Due to the inherent difficulties of determining structure and chemical composition of the extremely fine-grained material, earlier descriptions based on separated material are equivocal. Here we describe low-temperature (diagenetic) formation of fluorophlogopite, which is interlayerdeficient and therefore analogous to illite, using high-resolution in situ methods (transmission electron microscopy, TEM, with preparation by focused ion beam milling, combined with wavelength-dispersive analysis by field-emission gun electron microprobe). The average composition is K0.5Mg2.8V0.01Fe0.005 [Si3.15Al0.85O10(OH)0.65F1.35], including minor amounts of NH4 for charge compensation as determined by electron energy loss spectroscopy. The K-deficient Mg-mica occurs in layer packages of ~10 layers, and no indications for interlayering with other sheet silicate layers such as chlorite or vermiculite could be identified with TEM. X-ray powder diffraction patterns of separated material confirm the absence of smectite components. The mineral was identified in phosphorites from the lowermost Cambrian Tal Group, Mussoori Syncline, Lesser Himalayas, India. The rocks are alternating phosphatic mudstones and phosphatic dolostones, at times interbedded with phosphate-poor carbonate layers, which are rich in organic matter. Sedimentary fluorophlogopite occurs in both rock types and in two textural associations; one in vesicles filled with amorphic organic matter, the other as reaction rims around illite, which contains up to 5 wt% V2O3 in its rims. Textural arguments favor an early diagenetic formation of both, V-bearing illite and fluorophlogopite, closely associated with organic matter and linked to dolomitization. The high-F content stabilizes phlogopite to low temperatures. Our findings confirm that the stability field of fluorophlogopite extends from magmatic to metamorphic and sedimentary conditions.