Mu. Ramkumar

Stable isotope analyses of belemnites from the Kachchh Basin, western India: paleoclimatic implications for the Middle to Late Jurassic transition

Stable isotope analyses of 61 diagenetically unaltered belemnite rostra from the Middle to Late Jurassic of the Kachchh Basin of western India suggest stable paleotemperatures across the Callovian-Oxfordian boundary (~14°C). Only at the end of the Middle Oxfordian water temperatures drop for more than 3°C before reaching again higher values during the Kimmeridgian (~12.3°C). The data do not support polar glaciations proposed for the Middle to Late Jurassic transition, which necessarily would have led to a global temperature and sea-level minimum at the boundary. Callovian to Oxfordian rocks in the Kachchh Basin point to a gradual shallowing corresponding to a slight fall in relative sea level. However, the magnitude of this regression is comparatively small, and the sea-level minimum is reached in the late Early Oxfordian and not close to the boundary. Results from the Kachchh Basin therefore, imply almost stable climatic conditions during the Middle to Late Jurassic transition and do not show any evidence for polar glaciations.

Landscape response to progressive tectonic and climatic forcing in NW Borneo: Implications for geological and geomorphic controls on flood hazard

Empirical models have simulated the consequences of uplift and orographic-precipitation on the evolution of orogens whereas the effects of these forcings on ridgelines and consequent topography of natural landscapes remain equivocal. Here we demonstrate the feedback of a terrestrial landscape in NW Borneo subject to uplift and precipitation gradient owing to orographic effect, and leading to less-predictable flooding and irreversible damages to life and property. Disequilibrium in a large catchment recording the lowest rainfall rates in Borneo, and adjacent drainage basins as determined through χ, a proxy for steady–state channel elevation, is shown to result in dynamic migration of water divide from the windward-side of the orogen towards the leeward-side to attain equilibrium. Loss of drainage area in the leeward-side reduces erosion rates with progressive shortening resulting in an unstable landscape with tectonic uplift, gravity faults and debris flows. 14C dating of exhumed cut-and-fill terraces reveal a Mid–Pleistocene age, suggesting tectonic events in the trend of exhumation rates (>7 mm a−1) estimated by thermochronology, and confirmed by morphotectonic and sedimentological analyses. Our study suggests that divide migration leads to lowered erosion rates, channel narrowing, and sediment accretion in intermontane basins on the leeward-side ultimately resulting in enhanced flooding.

Toward Standardization of Terminologies and Recognition of Chemostratigraphy as a Formal Stratigraphic Method

Abstract Sediments are reliable records of changes in physical, chemical, and biological conditions that take place before, during, and after their deposition and express the changes through constituent mineralogical and thus geochemical compositions. Individual sedimentary events create more or less homogeneous bulk chemistry of sediments at varying temporal and spatial scales. Distinguishing these homogeneities and for classification of stratigraphic records and correlation of the strata at varying spatiotemporal scales is emerging to be a reliable method of stratigraphy and is termed as chemostratigraphy a la chemical stratigraphy. This method helps stratigraphic correlation with ease where other formal stratigraphic methods have limitations or fail to achieve required spatiotemporal resolution. The study of geochemical variations in stratigraphic context has gained importance since the 1980s. Chemostratigraphy is, thus relatively a younger branch of geosciences. Attempts on distinguishing depositional units at varying spatiotemporal scales (from local to global and from tidal cycles to few tens of millions of years) have been influenced to a larger extent by the sequence stratigraphic concepts. Contemporaneous developments in sophisticated instrumentation for fast, accurate, and less expensive geochemical analyses have also contributed to the popularity and applications of chemostratigraphy. From a humble beginning of identification of similar geochemical values and similar pattern of geochemical profile, chemostratigraphy has traveled a long way. Currently, a wide variety of techniques and data from other subdisciplines of geosciences are used for distinction/recognition and correlation chemozones/geochemically distinguishable depositional units. Yet, chemostratigraphy consists of vaguely defined and often misleading and/or overlapping terminologies. Through an extensive review of published literature, this chapter attempts to enlist these terminologies namely, chemostratigraphy, chemical stratigraphy, geochemical fingerprinting, geochemical signature, geochemical fingerprint, geochemical marker, geochemical proxy, excursion, shift, fluctuation, perturbation, anomaly, trend, chemostratigraphic index, chemozone, chemochron, resolution, and scale of correlation and provides definitions/explanations. This attempt is made for initiating discussion among the practitioners that may lead to consensus on definitions and standardized usage. Despite, fulfilling the criteria required for any standard stratigraphic method and finding its applications in many different fields, this method/tool remains to be formally given its due. Elucidation of the traits and enlisting the terminologies of chemostratigraphy with the criteria for formal recognition prescribed by International Stratigraphic Commission suggests that chemostratigraphy deserves to be formalized as an independent stratigraphic method.

Lithofacies and Granulometric Characteristics of the Kallamedu Formation, Ariyalur Group, South India: Implications on Cretaceous-Tertiary Boundary Events

The Kallamedu Formation, deposited during Late Maastrichtian is the thickest and largest sprawling non-marine/coastal stratigraphic unit of the Ariyalur Group, Cauvery basin, South India. However, it had attracted scanty attention from geoscientists owing to its poorly fossiliferous and weathered nature besides the paucity of good exposures. Understanding the depositional conditions of the Kallamedu Formation was necessitated due to the recent finding of continuous exposure of Maastrichtiah-Danian stratigraphic records namely, the Kallankurichchi Formation, the Kallamedu Formation and the Niniyur Formation near Niniyur and presumption of sauropod nesting site there. In this chapter, we report the textural properties of Kallamedu Formation sandstones and draw inferences on depositional and climatic conditions prevalent. The results indicate that the Kallamedu Formation was deposited under coastal plain environment that was periodically inundated by freshwater overflown from ephemeral river channels. The facies characteristics indicate deposition in river channel, flood plain, and overbank micro-environments located adjoining coastal region. Sediment availability was scarce and thus older sedimentary rocks were recycled through erosion-deposition during flood seasons. Predominance of riverine origin and transport of sediments by rolling and suspension mode are indicated by textural properties. Climatic conditions prevalent were inferred to be dry-humid alternations, principally influenced under seasonal/monsoonal conditions. Massive, monotonous, thick to very thick beds and other contact relationships together with mineralogical and textural properties of the Kallamedu Formation suggest the prevalence of flash floods under the influence of anomalous climatic conditions, that might have destabilized the environmental conditions and destructed the ecological niches, contributing towards dwindling and extinction of taxa during end-Cretaceous.

Calcareous Nannofossils from the Ottakoil Formation, Cauvery Basin, South India: Implications on Age and Late Cretaceous Environmental Conditions

A more or less complete Upper Cretaceous – Palaeocene age sequence is preserved in the Ariyalur- Pondicherry depression of the Cauvery basin. The Cretaceous-Tertiary boundary (KTB) has been recognized in this basin. Geochemical and isotopic anomalies with sequence stratigraphic framework from this 236 m thick Maastrichtian-Danian section in this basin showed the occurrences of double peaked nature of 87Sr/86Sr, stable isotopic, Barium and other trace elemental anomalies preceding K/T boundary. However, owing to patchy occurrences of fossils, precise dating of these anomalies could yet not be made, thwarting correlation of this section with coeval strata elsewhere. Hence, this composite section was subjected to nannofossil study. Out of 55 samples representing several formations and members in this sequence, only one sample (157B) representing upper part of the Ottakoil Formation yielded Late Maastrichtian age calcareous nannofossils. Though limited in diversity, the assemblage shows exceptional preservation for most of the forms. Most of the studied samples contain abundant organic matter and calcite mineral providing clue on prevalent destruction of nannofossils due to diagenetic transformation/dissolution-precipitation. The calcareous nannofossil taxa recorded are: Ahmuellerella octoradiata, Arkhangelskiella cymbiformis, Braarudosphaera bigelowii, Ceratolithus aculeus, Chiastozygus litterarius, Cyclagelosphaera deflandrei, Cribrosphaerella ehrengergii, Cribrosphaera sp., Eiffelithus gorkae, E. parallelus, E. turriseiffeli, Microrhabdulus undosus, Micula decussata, M. staurophora, M. swastika, Petrobrasiella? Bownii, Prediscosphaera cretacea, P. spinosa, Stradneria crenulata, Staurolithites crux, Zygodiscus minimus, Z. spiralis. Common occurrence of large sized A. cymbiformis (the LAD of which marks the base of CC25a) and the presence of A. octaradiata (the LAD of which indicates top of UC20aTP) permitted placement of the assemblage in CC 25 Arkhangelskiella cymbiformis Zone equivalent to UC 19 nannofossil Zone of Late Maastrichtian age. These observations and zonal placement of the studied rocks have helped to date the geochemical and isotopic anomalies recorded previously which in turn may help correlate the interpretations made with those anomalies with that of comparable stratigraphic sections elsewhere.

Temporal Trends of Geochemistry, Relative Sea Level, and Source Area Weathering in the Cauvery Basin, South India

The lithofacies characteristics, geochemical signals, and the trends of weathering indices of the Barremian-Danian strata of the Cauvery Basin, South India, record a synchronicity between relative sea level fluctuations as enforced by climatic cycles of varying durations, and the alternations of calcareous-siliciclastic deposition in tune with weathering in the source area. Occurrences of all the six third-order global sea level peaks, higher order cycles within these third-order cycles, absence of tectonic influence over depositional pattern except during Santonian, are all observed in this basin. Analyses of the stratigraphic variations of elemental concentrations and the trends of stable isotope, weathering indices, and other elemental ratios have revealed the prevalence of the coldest climate during Campanian and the warmest climate during Albian and Maastrichtian. Influence of alternating warm and cold climatic conditions over the weathering pattern in the provenance region and the influx of sediments to the depositional sites were also inferred from these data. However, aberrations and absence of perfect coupling between these alternations are also observed that owe allegiance to tectonic and other causes too. These inferences suggest that caution has to be exercised while interpreting chemostratigraphic signals from the long-duration strata.

Depositional and Diagenetic Environments of the Dhosa Oolite Member (Oxfordian), Kachchh Basin, India: Implications for the Origin and Occurrence of the Ooids and Their Correlation with the Global Fe-Oolite Peak

Based on bio and lithofacies characteristics and occurrences of hardground surfaces, oolites, iron crusts and shell lags, the Dhosa Oolite member (DOM) of Kachchh basin, India is considered to be equivalent of Fe-oolitic deposits of many European sections that formed during the Callovian-Oxfordian eustatic sea-level maximum. Lithofacies and textural and diagenetic characteristics revealed that the ooids of DOM were originally calcitic and, after initial deposition, experienced diagenesis under reducing conditions followed by exhumation, multiple episodes of transport and final burial in low-energy areas. High-frequency sea-level cycles occurred during the deposition of the Dhosa Oolite member that exposed the oolitic and other sediments to diagenesis in shallower marine regions and with significant dissolution-precipitation in marine-phreatic, burial, meteoric-phreatic and vadose zones. Owing to the synsedimentary lithification at the final burial site and low-magnesian calcitic precursors of bioclasts and ooids, the micritic matrix and marine cement spars preserved their morphologic integrity until dissolution in the meteoric-vadose zone. Subaerial exposure of the sediments following sea-level retreat had subjected them to intense meteoric-phreatic zone diagenesis that had morphologically and mineralogically transformed the susceptible carbonate components. Change in groundwater table had exposed these carbonate components to meteoric-vadose and oxygenated waters of the meteoric-phreatic zone, caused oxidation, subjected them to leaching of Fe and silicification, that might have resulted in distinct color to the rocks, iron crusts, and iron oxide coating. Independent analyses of selected ooids under EPMA compositional mapping also revealed the original calcareous nature of the ooids, enrichment of Fe and latter Si in selective portions and layers of ooids. These observations suggest precipitation of low-magnesian calcitic ooids on shallow, coastal ooid shoals → multiple episodes of transport → deposition, lithification, Fe enrichment in susceptible parts of ooids and bioclasts, and iron-mineral replacement under sub-oxic condition → exhumation, offshore transport, deposition in low-energy areas → neomorphic alteration in the marine-phreatic zone → selective dissolution and stabilization of calcite in the marine-burial zone → complete dissolution in the meteoric-vadose zone, development of mouldic porosity and its infilling by ferroan equant calcitic spar in the meteoric-phreatic zone → exposure to oxidizing waters in the meteoric-vadose and -phreatic zones, destructive neomorphism, selective silicification of portions of ooids, oxidation of iron in ferroan calcitic spars and iron-rich clay present in the rock → iron oxide coating → Ca-Fe-Si-ooid formation as the paragenetic sequence. The results suggest that these ooids should not be correlated with worldwide occurrences of Fe-oolites per se for two reasons, namely, the complete erosion of the coeval stratigraphic record of the worldwide Fe-oolite peak, and the originally calcite nature of the ooids. It is also brought to light that while the ooids were formed during a transgressive phase, final burial of ooids occurred during regression. However, the sea-level rise resumed towards the top of the DOM resulting in formation of shell lags, hardground surfaces, and/or maximum flooding surfaces, etc. The top of the DOM is represented by karstic surfaces, dissolution-related collapse structures, and mixing of younger sediments.

Sand Mining, Channel Bar Dynamics and Sediment Textural Properties of the Kaveri River, South India: Implications on Flooding Hazard and Sustainability of the Natural Fluvial System

The Kaveri River, the fourth largest river in India, undergoes the onslaught of urbanization and extensive construction activities within, along and adjoining its channel. In addition to its dwindling natural flow due to the failing monsoonal supply, and constructions of major, medium and minor dams, the extensive mining of sand from its channel bed causes severe stress on its natural fluvial processes. Reduction of carrying capacity of the channel, extensive vertical accretion of sediments within the channel, development of channel-in-channel physiography, and alteration of stream configuration and textural parameters of the stream bed sediments have contributed towards deterioration of the environmental integrity of this important river and exacerbated the flood hazard in the adjoining regions. This paper is an attempt to document the deterioration of natural fluvial dynamics due to the anthropogenic intervention and lack of required data for proper understanding for environmental management and sustenance of the fluvial system. The textural and geomorphic characteristics and the mechanism of mid-channel bar formation and stabilization documented through this study suggest that the whole of the river channel of the Kaveri River behaves like a braided bar/flood plain, which means the prevalence of slow abandonment of the fluvial processes, that could only be observed in the flood plain region of mature and or old stage of a river and/or in the event of shifting of channel course. Occurrence of such characteristics at the upper deltaic region and the observation that the channel area gets converted into mid-channel bars (in terms of textural-geomorphic traits), at a rate of 1.08 km2/year warrant immediate measures for the restoration of natural fluvial processes.

Environmental and Climatic Conditions during the K–T Transition in the Cauvery Basin, India: Current Understanding Based on Chemostratigraphy and Implications on the KTB Scenarios

The environmental and climatic conditions prevalent during the transition from Cretaceous–Tertiary boundary (KTB) that caused extinction of marine and terrestrial fauna have been among the intensively studied geological phenomena. Cataclysmic meteoritic impact, Deccan volcanism, sea level fluctuation, acid rain, ocean acidification, contamination of marine and terrestrial ecosystems and atmosphere etc., by sulfur and methane are proposed to be the cause(s) of the extinction. In addition, the pattern of extinction itself, namely, sudden extinction due to extra-terrestrial catastrophic event and stepwise extinction due to multiple events and processes, is debated by the geoscientists. The Cauvery Basin, South India, contains more or less complete stratigraphic record of Barremian–Danian and thus has been studied by many researchers for the cause(s) of extinction pattern across K–T. This chapter presents the initial results of recently identified continuous K–T section located near Niniyur village in the Cauvery Basin and discusses the inferences in the light of regional and global models proposed as to the cause(s) of the extinction pattern. Results of the study indicate the predomination of influence of sea level fluctuations (which in turn might have been influenced by Deccan volcanism) over the depositional pattern of the Cauvery Basin. In addition, prevalence of many events namely off-seasonal floods, sharp, and sudden climatic reversals, destabilization of gas hydrates, and release of methane, all of which predating KTB and coincidental with similar events in geographically widely separated stratigraphic sections as indicated by geochemical anomalies are also recorded by the present study. Thus, present analysis supports the interpretation of regional and global scale, multicausal stepwise extinction pattern across KTB.

Chemostratigraphy of the Dhosa Oolite Member (Oxfordian), Kachchh Basin, Western India: Implications for Completeness of the Stratigraphic Record and Correlation with Global Oolite Peak

Abstract Based on bio and lithofacies characteristics and occurrences of hardground surfaces, iron-rich ooids, iron crusts, and shell lags, the Dhosa Oolite member deposited during Oxfordian in the Kachchh Basin of western India is traditionally considered to be equivalent of Fe-oolitic deposits formed as a result of eustatic sea-level maximum during Callovian-Oxfordian in southern Poland, Madagascar, France, and England. The regionally extensive and unique lithological characteristics of the Dhosa Oolite member made this member a regional marker interval. This member is also in use for correlation as marker interval in adjacent basins too. Yet, correlation of this member with regional and global equivalents is often mired with skepticism due to its condensed nature, occurrences of omission and hardground surfaces, mixed fauna typical of many biozones, and resedimented nature of sediments. In order to address this issue, this paper documents chemostratigraphic characteristics of this member to identify synchronicity of chemical signatures with regional and global signals. The geochemical profiles constructed with the present study recognized a major sea-level cycle spanning from upper part of the Dhosa Sandstone member that reached its zenith near the top of the Dhosa Oolite member. Individual short-term fluctuations represented by positive excursions of P and negative excursion of Ba were recognized within the studied sections. These excursions are regionally recognizable, and sedimentation pattern in the basin was found to be coupled with climate-sea-level fluctuations (as indicated by corresponding variations in source area weathering and detrital influx). However, correlating the Oxfordian oolite deposit of the Kachchh Basin with coinciding Fe-oolite deposits elsewhere requires adequate precautions.