SE India Research Articles

Petrological and geochemical characterization of Proterozoic ophiolitic mélange, Nellore–Khammam schist belt, SE India

The ophiolitic mélange occurs close to a major fault within the metavolcanics of Kanigiri area in the western margin of the Khammam–Nellore schist belt, Andhra Pradesh, India. The unique feature of Kanigiri mélange is the occurrence of ophiolitic bodies of various sizes, most of which display petrological and geochemical characteristics typical of supra-subduction zone oceanic crust. The ophiolitic mélange is composed mainly of blocks of variably deformed and metamorphosed pillow basalts, serpentinized ultramafic rocks, meta-gabbros, cherts and pelagic sediments in a pervasive scaly argillite matrix. The mélange has a tectonic contact with a granite intrusion and numerous thrust faults cut across the mélange exposures. Primary magmatic features such as drainage cavities in pillow basalts, magmatic layering in gabbros, and clinopyroxene cumulates in ultramafic rocks are well preserved in low-strain domains. Mafic components enclosed in the mélange are characterized by low pressure secondary mineral assemblages retrograded from amphibolite down to greenschist facies as commonly observed in subducted and exhumed orogens. All amphiboles in the studied fragments are calcic and cluster in the range from winchite, actinolite to hornblende. Amphibole compositions are characterized by Al 2O 3 (2.4–13.8%), FeO (5–13%), K 2O (0.04–0.7%) and X Mg (0.67–0.77). The metabasalt and wherlite samples exhibit light rare earth element (LREE) – enriched patterns and negative high field strength element (HFSE: Zr and Ti) consistent with a suprasubduction zone geodynamic setting. A notable feature of both metabasalt and wherlite samples is their pronounced negative Ce anomaly and a positive Eu anomaly as observed in a number of modern island arc suites. As the petrological features and the geochemical characteristics of the remnant fragments we analyzed in this study are reminiscent of their Phanerozoic ophiolite counterparts, we suggest that these rocks were derived from a Proterozoic oceanic plate belonging to a suprasubduction ophiolite assemblage. Subduction zone geochemical signatures and ophiolite-like crustal lithologies in the study region are consistent with the accretion of juvenile oceanic fragments to the pre-existing continental blocks through a forearc-continent collision.

Methane and nitrous oxide fluxes in the polluted Adyar River and estuary, SE India

We measured dissolved N 2O, CH 4, O 2, NH 4 + , NO 3 - and NO 2 - on 7 transects along the polluted Adyar River–estuary, SE India and estimated N 2O and CH 4 emissions using a gas exchange relation and a floating chamber. High NO 2 - implied some nitrification of a large anthropogenic NH 4 + pool. In the lower catchment CH 4 was maximal (6.3 ± 4.3 × 10 4 nM), exceeding the ebullition threshold, whereas strong undersaturation of N 2O and O 2 implied intense denitrification. Emissions fluxes for the whole Adyar system ∼2.5 × 10 8 g CH 4 yr −1 and ∼2.4 × 10 6 g N 2O yr −1 estimated with a gas exchange relation and ∼2 × 10 9 g CH 4 yr −1 derived with a floating chamber illustrate the importance of CH 4 ebullition. An equivalent CO 2 flux ∼1–10 × 10 10 g yr −1 derived using global warming potentials is equivalent to total Chennai motor vehicle CO 2 emissions in one month. Studies such as this may inform more effective waste management and future compliance with international emissions agreements.

Main Deccan volcanism phase ends near the K–T boundary: Evidence from the Krishna–Godavari Basin, SE India

Recent studies indicate that the bulk (80%) of the Deccan trap eruptions occurred over less than 0.8 m.y. in magnetic polarity C29r spanning the Cretaceous–Tertiary (K–T) boundary. Determining where within this major eruptive phase the K–T mass extinction occurred has remained problematic. For this reason, models estimating the biotic and environmental consequences have generally underestimated the rate and quantity of Deccan gas emissions by orders of magnitude leading to conclusions that volcanism could not have been one of the major causes for the K–T mass extinction. In this study we report that the most massive Deccan trap eruption occurred near the K–T mass extinction. These results are based on sedimentologic, microfacies and biostratigraphic data of 4–9 m thick intertrappean sediments in four quarry outcrops in the Rajahmundry area of the Krishna–Godavari Basin of southeastern India. In this area two Deccan basalt flows, known as the Rajahmundry traps, mark the longest lava flows extending 1500 km across the Indian continent and into the Bay of Bengal. The sediments directly overlying the lower Rajahmundry trap contain early Danian planktic foraminiferal assemblages of zone P1a, which mark the evolution in the aftermath of the K–T mass extinction. The upper Rajahmundry trap was deposited in magnetic polarity C29n, preceding full biotic recovery. These results suggest that volcanism may have played critical roles in both the K–T mass extinction and the delayed biotic recovery.

Mineral chemistry of Ti-rich biotite from pegmatite and metapelitic granulites of the Kerala Khondalite Belt (southeast India): Petrology and further insight into titanium substitutions

Precise chemical composition, including Fe 3+ and H, of biotite from a pegmatite dike and its host granulite from the Kerala Khondalite Belt of SE India has been determined using a multi-technique approach involving EMP, SIMS, Mössbauer, and C-H-N elemental analysis. Biotite in these rocks formed at T > 800-850 °C and P = 5 ± 1 kbar. The full analyses were normalized on the basis of [O 12-(x+y+z) (OH) x Cl y F z ]. Biotite in the pegmatite is Ti-, F-, and Cl-rich (0.33, 0.46, and 0.16 apfu, respectively), H 2 O-poor (OH = 0.86 pfu), has XMg = 0.49 and Fe 3+ /Fe tot ≤ 3%. The low octahedral vacancies (0.06 pfu) and the high oxygen content in the hydroxyl site (OH + F + Cl = 1.49 pfu) confirm the role of the Ti-oxy substitution as a major exchange vector in these high-T biotites. In the host granulite, fine-grained biotite is Fe 3+ -free, has low Cl (0.03 apfu), and more variable composition, with Ti, F, and X Mg in the ranges 0.26-0.36, 0.52-0.67, and 0.67-0.77, respectively. The number of octahedral vacancies is relatively large (0.10-0.18 pfu) and the sum of volatiles (OH + F + Cl) varies from 1.71 to 2.06 pfu. Systematic variations of X Mg are a function of the microstructural position and are in agreement with retrograde exchange reactions: biotite included in or in contact with garnet has the maximum values, whereas crystals in the matrix have the minima. Titanium has systematic negative correlations with F, X Mg , and (OH + F + Cl), whereas Al and octahedral vacancies are virtually constant. These trends indicate that the Ti-vacancy, along with substitutions involving Al, cannot explain the observed short-scale variations. Conversely, the Ti-oxy exchange appears to be active, resulting from combination of two vectors: the more conventional hydroxylation Ti 4+ + 2O 2- = (Fe,Mg) 2+ + 2OH - and the “fluorination” Ti 4+ + 2O 2- = (Fe,Mg) 2+ + 2F-. The systematic retrograde redistribution involves not only Fe and Mg as commonly observed, but also Ti, F, and H, in a way such to eliminate the primary Ti-oxy component of biotite.

Petrogenesis of the Kunavaram alkaline complex and the tectonothermal evolution of the neighboring Eastern Ghats Belt granulites, SE India

Geochemical and geochronological study of the Kunavaram alkaline rocks near the craton–Eastern Ghats Belt (EGB) contact in SE India indicates that alkaline magma was emplaced at ∼1384Ma along a NE–SW Mesoproterozoic rift at the cratonic margin. This paleo-rift can be delineated by the several Mesoproterozoic alkaline intrusives at the cratonic margin (e.g. Khariar, Kunavaram, Jojuru, Elchuru and others). The geochemical variations in the Kunavaram alkaline suite can be explained by an assimilation-fractional crystallization (AFC) process dominated by the fractionation of clinopyroxene–amphibole, apatite, titanite, zircon and allanite and the simultaneous assimilation of bulk upper crustal material from the craton. The alkaline complex and other craton margin rocks and the marginal granulites of the EGB were overprinted by an amphibolite facies tectonothermal event during the Neoproterozoic–Paleozoic (611–484Ma) when Pan-African tectonism thrust the EGB granulites westward over the cratonic foreland along a number of northwest-vergent shear zones. The Pan-African thrust stacking has the cratonic boundary and the alkaline rocks in the footwall and the EGB granulites in the hanging wall. The continuous nature of the Pan-African shear fabric across the craton–EGB contact near Kunavaram implies that the two crustal segments shared a common boundary during the Neoproterozoic–Paleozoic. Prior to the Pan-African event, the evolutionary history of the EGB is characterized by the presence of an early Neoproterozoic (1017–1087Ma) granulite-facies tectonometamorphic overprint during the Grenvillian orogeny, evidence for which is lacking in the alkaline and other craton margin rocks. The craton–EGB contact thus represents a rifted Mesoproterozoic margin deformed during Pan-African collisional tectonics.

Mesoproterozoic rift-related alkaline magmatism at Elchuru, Prakasam Alkaline Province, SE India

The Elchuru alkaline complex in the Prakasam igneous province represents one occurrence of several alkaline bodies within the craton–Eastern Ghats Belt contact zone in Peninsular India. Nepheline syenites and associated mafic rocks intruded the cratonic crust at ≈1321 Ma and were deformed–metamorphosed to amphibolite facies condition during Pan-African times. Trace element compositions and Sr, Nd and Pb isotopic systematics indicate that the alkaline magma was derived from an enriched mantle source in the sub-continental lithosphere. The adjacent crusts of the Eastern Dharwar craton and the Eastern Ghats Belt were not involved either as source or as contaminants. The enriched mantle source was at least 1.9–2.1 Ga old as seen from the depleted mantle model ages of the rocks. The primary parent magma was a basanitic liquid that fractionated ferrokaersutite and clinopyroxene in the mantle, lowering the density sufficiently for the residual melt to intrude the crust. Magmatic differentiation in the suite can be explained by a two stage fractional crystallization model involving the removal of amphibole, clinopyroxene, allanite, titanite, apatite and zircon. The rift-related intra-continental setting of the complex indicates that alkaline magmatism represents the manifestation of a Mesoproterozoic continental breakup. Rifting along the cratonic margin may have led to the formation of several cratogenic basins (e.g., Chattisgarh basin, Indravati basin etc.) where stable shelf-type sediments could have been deposited on the passive margin during the Proterozoic. It could also have opened an ocean where some of the sediments of the Eastern Ghats Province may have been deposited.

Mesoproterozoic rifting and Pan-African continental collision in SE India: evidence from the Khariar alkaline complex

The suture zone between the Bhandara craton and the granulite-facies rocks of the Eastern Ghats Province in SE India contains a number of deformed alkaline and tholeiitic intrusives. The Khariar alkaline complex is one of the several occurrences which intruded in the Mesoproterozoic (1,480±17 Ma, 2σ) and was deformed during the Pan-African tectonothermal event. The geochemical signatures indicate a rift-related setting for the magmatic activity. The nepheline syenite parent magma may have been produced by in-mantle fractionation of clinopyroxene and Ti-rich amphibole from a basanitic primary magma derived from an enriched spinel lherzolite mantle source in the sub-continental lithosphere. Geochemical variations in the Khariar alkaline suite can be modeled by the fractionation of clinopyroxene, amphibole, titanite, zircon, apatite and allanite. The Mesoproterozoic alkaline magmatism at Khariar marks the initiation of a NE-SW rift which formed several craton margin basins and opened an ocean towards the south. The sediments of the cratogenic basins and the Eastern Ghats Province were deposited in these rift-related basins. A K-Ar age of 1,330±53 Ma from glauconites in sandstone suggests that the NW-SE trending Godavari–Pranhita graben formed at approximately the same time as the rift at the craton margin. If the two are related, the Godavari–Pranhita graben may represent the failed arm of a rift system in which the NE-SW arm was the active segment. The granulite-facies deformation and metamorphism of the Eastern Ghats Province sediments may be related to an episode of Grenvillian basin inversion. The Mesoproterozoic rifting and Grenvillian basin closure may thus represent two well-defined parts of a Wilson cycle i.e. the opening and closure of an ocean. The Khariar and other alkaline bodies were, however, deformed during a Pan-African collisional event associated with the westward thrusting of the Eastern Ghats Province granulites over the cratonic foreland.

Allegories of Alienation and Politics of Bargaining: Minority Subjectivities in Mani Ratnam's Dil Se

This article examines Mani Ratnam's film Dil Se (India, 1998, Tamil and Hindi), as an allegory of the relationship between dominant and dominated groups within the Indian nation-state. By reading its narrative of doomed love against its counter-narratives of cinematography and soundtrack, I argue that Dil Se exposes the libidinal economy of the federal democratic framework, urging groups currently alienated within that framework to work towards a successful politics of bargaining in order to claim their rightful place within the nation, and initiates processes of mourning for cultural losses sustained through homogenising majoritarian discourses, including those of Bollywood itself. By invoking Gilles Deleuze and Felix Guattari's theory of ‘minor literatures’, I reveal the minority subjectivities of those involved in the film's making as the key unlocking the therapeutic, even redemptive potential of its radical political messages, that, despite the film's commercial failure, circulate subliminally through its popular soundtrack.

Allozyme and morphological variation throughout the geographic range of the tropical shad, hilsa Tenualosa ilisha

The population structure of the shad, hilsa Tenualosa ilisha, was investigated with both allozymes and morphometric analysis. Fish samples were collected from nine sites within Bangladesh and compared with samples from four other countries that covered the entire species range. Five polymorphic loci, Idh-l, Idh-m, Mdh-l, Mdh-m and Pgm, were interpretable in liver and muscle with starch–gel electrophoresis, although levels of variation were low. No significant differences in allele frequencies were detected within Bangladesh or within the Bay of Bengal (SE India and Myanmar) samples. Significant differences in allele frequencies occurred between Kuwait, Bangladesh and Indonesia. Nine morphological measurements and two meristic counts were compared among the same fish that were collected for genetic analysis. Discriminant function analyses (DFA) of transformed measurement ratios varied widely between sites, even those separated by short distances (<100 km). The results showed that hilsa vary greatly in shape and this showed little geographic concordance. Indonesian and Indian hilsa were more distinctive than the most distant sample from Kuwait. Cross-validation of the classification revealed that sites in the middle reaches of the Meghna River in Bangladesh (Chandpur, Goalando, Bhairob Bazar) and Cox’s Bazar had less than half the fish correctly classified to their origin. Fish from other countries were much more distinctive. We suggest the morphological results reflect local environmental conditions rather than any population-level differences. The allozyme results suggest that there is substantial gene flow between groups of hilsa within the Bay of Bengal. This means that there is a need for Bangladesh to take into account actions and management decisions in India and Myanmar in order to sustainably manage their fishery.

Evolution in Basin Fill Style During the Mesozoic Gondwana Continental Break-up in the Godavari Triple Junction, SE India

Consistent changes are observed in the Mesozoic sedimentary lithofacies and palaeocurrents during the transition from the continental Gondwana (NW-SE trending Chintalpudi Graben - CG) to the coastal Gondwana (NE-SW trending Krishna Godavari Coastal Basin - KGCB) in the Godavari Triple Junction (GTJ). The Mesozoic continental Gondwana sediments include the Kamthi (early Triassic), Kota (Jurassic) and Gangapur (early Cretaceous) Formations. Coastal Gondwana succession comprising the Raghavapuram (early Cretaceous) and the Tirupati (Middle to late Cretaceous) Formations transversely superimpose the continental Gondwana. The Continental Gondwana is mainly cross-stratified sandstone and fossiliferous ( Glossopteris and Ptilophyllum flora) clay assemblage of alluvial environment. Palaeocurrent is towards NW in the Kamthi Formation, WNW in the Kota Formation and centripetal in the Gangapur Formation. The Raghavapuram Formation (KGCB) comprises of grey/carbonaceous clay-sandstone association of SE flowing delta plain distributories with inter-distributory swamps and fossiliferous white clay (a mix of Ptilophyllum flora and invertebrate fauna) of prodelta / shallow marine environment. The succeeding sandstone- conglomerate facies of the Tirupati Formation (Middle to late Cretaceous) with SE palaeocurrent indicates delta progradation whereas the infratrappean clay-limestone beds (mainly invertebrate fauna of Maastrichtian age) point to delta abandonment, prior to the K/T boundary volcanism. Tectono-sedimentary model presented in this communication incorporates that the change from the continental Gondwana to coastal Gondwana had occurred in four stages. During stage-I (late Carboniferous to early Jurassic) the sediments were deposited by a north-westerly flowing alluvial system whose distal source was located in southeast beyond the east coast of India. Stage -II (Middle to late Jurassic) is marked by westerly shift in palaeocurrents attributed to an obstruction caused by an upliftment of the Mailaram high parallel to the NE-SW Eastern Ghat trend. Stage-III (early Cretaceous) is heralded by the onset of deltaic sedimentation in SE due to separation of landmass that supplied detritus to the CG until the end of the Jurassic. During stage-IV, the delta progradation is accentuated by the basement uplift (Eastern Ghat Mobile Belt). It is suggested that the change over from continental Gondwana to coastal Gondwana had occurred over a time span of about 70 my between late Jurassic to Cretaceous and controlled by phase-wise upliftment of the basement as a corollary to the Gondwana continental break-up.

Sm–Nd and Rb–Sr dating of amphibolite from the Nellore–Khammam schist belt, SE India: constraints on the collision of the Eastern Ghats terrane and Dharwar–Bastar craton

The Nellore–Khammam schist belt, SE India, is sandwiched in between the Proterozoic Eastern Ghats terrane and the Archaean Dharwar–Bastar craton. We report Sm–Nd and Rb–Sr mineral isochron ages of amphibolite from the schist belt. The Sm–Nd and Rb–Sr ages are 824±43 Ma and 481±16 Ma, respectively. The Sm–Nd age indicates the timing of peak metamorphism, whereas the Rb–Sr age indicates the Pan-African thermal overprint. The peak metamorphism was related to collision of the Eastern Ghats terrane with the Dharwar-Bastar craton, which occurred during early Neoproterozoic time.

Can otolith chemistry detect the population structure of the shad hilsa Tenualosa ilisha? Comparison with the results of genetic and morphological studies

The stock structure of the valuable tropical shad Tenualosa ilisha ('hilsa') has been stud- ied in Bangladesh and India by analysing morphometric and genetic data. However, these studies had a narrow geographic scope and their results conflict. We made a comprehensive study of the stock structure of hilsa with otolith microchemistry in conjunction with complementary genetic and morphometric studies of the same fish. We examined the trace-element composition of the otolith cores of hilsa with laser-ablation inductively coupled plasma mass spectrometry. The otoliths of fish from 19 collections at 13 sites in Bangladesh and 6 collections at 4 sites from elsewhere within the species' range (Kuwait, SE India, Myanmar and Sumatra) were analysed for 8 trace elements. Sam- ples were collected from Bangladesh mainly during 2 comprehensive surveys (1996 and 1997). When these data were analysed separately, there were significant differences in otolith composition among sites. However, when both years' data were analysed together, there were few significant differences among sites, and some sites separated by hundreds of kilometres that were sampled in different sea- sons and years had very similar compositions. This was in spite of both large seasonal intra-site and between-site differences in water chemistry. Repeat samples from 5 sites (4 in Bangladesh) showed that differences in otolith composition at a single site were significant and of similar magnitude to that found among sites. Our results support the conclusion from allozyme studies that there is extensive movement and mixing of hilsa throughout Bangladesh, and therefore the population should be man- aged as a single stock. Genetic and otolith data both showed that hilsa from SE India and Myanmar were not significantly different from fish collected in coastal areas of Bangladesh, and suggest that hilsa in the Bay of Bengal were a single stock. Both methods also separated fish from Sumatra and Kuwait from other sites, providing strong evidence of separate stocks in those regions. In contrast, morphometric studies separate fish from several nearby sites in Bangladesh, but these differences are likely to be largely due to phenotypic variability and are unlikely to be geneticallybased. Our results suggest that otolith microchemistry may be a good proxy for genetic structure at large scales where differences in water chemistry are highly likely. However, for sedentary species and those without distinct spawning and non-breeding areas, it requires both comprehensive and repeated sampling at finer scales before any confidence should be placed in the results.

Neoproterozoic Deformation at a Boundary Zone Between the Nellore-Khammam Schist Belt and Pakhal Basin, SE India: Strain Analysis of Deformed Pebbles

In the Kinnerasani area in southeastern India, the terrain boundary between the Archean Nellore-Khammam Schist Belt and the Proterozoic Pakhal Supergroup overlying the Dharwar-Bastar cratons can be observed. We analyzed the mesoscopic and microscopic structural features of the highly deformed pebbles in the basal conglomerate bed of the Pakhal Supergroup that occurs at the terrain boundary. The results of the analysis of the pebbles suggest that: 1) deformation of pebbles resulted from ductile deformation during peak metamorphism 2) the mode of strain is plane strain to constrictive and maximum elongation located to be vertical and 3) the apparent stretch of the pebbles is up to ∼ 300%. In the Nellore-Khammam Schist Belt, quartz grains constituting the quartz layer of the feldspathized gneiss folded by the last-phase deformation also show vertical maximum stretching in constrictive strain. This observation suggests that the deformational features, at least the mode of strain, during the last-phase deformation is comparable to the deformation forming elongated pebbles of the Pakhal conglomerate. The last-phase deformation structures of the Nellore-Khammam Schist Belt are well observed near the terrain boundary. This indicates that the Pakhal deformation overprinted the rocks of the Nellore-Khammam Schist Belt near the boundary, and that their tectonic juxtaposition occurred during or before this deformation period. Because the Pakhal deformation took place during or soon after the peak metamorphism of the Pakhal Supergroup, which is known to be ∼1000 Ma, and the last metamorphism of the Nellore-Khammam Schist Belt in the Khammam area were reported to be ∼1100 Ma. The tectonic juxtaposition between the Pakhal Supergroup and Nellore-Khammam Schist Belt was around 1000∼1100 Ma.

The climate of Madras during the eighteenth century

Archives of a German–Danish missionary foundation, including two weather diaries for Madras covering the years 1732–1737 and 1789–1791 and annual reports and letters, are used to reconstruct aspects of the climate of the Madras region of SE India during the eighteenth century. Records of rainfall occurrence and diurnal variations in wind direction in the diaries are used to derive the number of raindays and wind direction frequencies at 08:00 and 17:00 h for each month of record. Modern data on rainfall per rainday are used to convert rainday data to estimates of monthly rainfall. Wind direction data at times of land breeze (08:00 h) and sea breeze (17:00 h) are used as indices of the strength of the NE monsoon (in the October–December rainy season) and summer SW monsoon, respectively. Results suggest that annual rainfall in the period 1732–1737 (and probably also the 1720s) was about 27% below the 1813–1991 average, with very low NE monsoon (−41%) but near-average SW monsoon rainfall. The wind direction data suggest that both the SW monsoon and NE monsoon were stronger than normal. In contrast, estimated annual rainfall in 1789–1791 was near-average, but with SW monsoon rain 54% below average and NE monsoon rain 22% above average. This appears to be linked to less strong NE and SW monsoon winds than in 1732–1737. The period 1740–1788 appears to have been considerably wetter than the 1720s and 1730s. Tropical cyclone frequency was low except in the 1740s and 1780s. The 1730s dry period coincided with a period of anomalously warm winters in Europe. Copyright © 1999 Royal Meteorological Society

The climate of Madras during the eighteenth century

Archives of a German–Danish missionary foundation, including two weather diaries for Madras covering the years 1732–1737 and 1789–1791 and annual reports and letters, are used to reconstruct aspects of the climate of the Madras region of SE India during the eighteenth century. Records of rainfall occurrence and diurnal variations in wind direction in the diaries are used to derive the number of raindays and wind direction frequencies at 08:00 and 17:00 h for each month of record. Modern data on rainfall per rainday are used to convert rainday data to estimates of monthly rainfall. Wind direction data at times of land breeze (08:00 h) and sea breeze (17:00 h) are used as indices of the strength of the NE monsoon (in the October–December rainy season) and summer SW monsoon, respectively. Results suggest that annual rainfall in the period 1732–1737 (and probably also the 1720s) was about 27% below the 1813–1991 average, with very low NE monsoon (−41%) but near-average SW monsoon rainfall. The wind direction data suggest that both the SW monsoon and NE monsoon were stronger than normal. In contrast, estimated annual rainfall in 1789–1791 was near-average, but with SW monsoon rain 54% below average and NE monsoon rain 22% above average. This appears to be linked to less strong NE and SW monsoon winds than in 1732–1737. The period 1740–1788 appears to have been considerably wetter than the 1720s and 1730s. Tropical cyclone frequency was low except in the 1740s and 1780s. The 1730s dry period coincided with a period of anomalously warm winters in Europe. Copyright © 1999 Royal Meteorological Society

Foraminiferal recovery after the mid-Cretaceous oceanic anoxic events (OAEs) in the Cauvery Basin, southeast India

Abstract The Cauvery Basin, SE India, is one of the best exposed late Mesozoic to Tertiary basins in India. The study of foraminiferal assemblages from the core samples obtained from two 120m deep wells in the basin records the occurrence of two mid-Cretaceous anoxic events in the basin. Abrupt increases in planktonic: benthonic ratios and reductions in benthonic diversity are recorded in the late Albian and in the late Cenomanian-early Turonian. These events coincide with worldwide oceanic anoxic events (OAEs). The OAEs had an impact on the microfauna of the basin, with approximately 35–45% of the benthonic species not surviving and major morphological changes occurring in the planktonic community. The late Cenomanian-early Turonian event was more significant than the late Albian event, with considerable readjustments occurring in the planktonic foraminifera. The genus Rotalipora disappeared and the genus Hedbergella was largely replaced by Whiteinella. Marginotruncana appeared for the first time and the Dicarinella population expanded considerably. Praeglobotruncana was the most tolerant genus, undergoing least change. The planktonic foraminifera evolved from small, weakly ornamented forms with poorly developed keels, into robust, well ornamented forms with well developed keels. The pattern of evolution of planktonic foraminifera suggests a recolonization of deeper water environments after the late Cenomanian-early Turonian anoxic event.

Old World Gesneriaceae. IV. Notes on Didymocarpus and Lysionotus

Didymocarpus rodgeri (incl. var. siamensis) is reduced to a synonym of D. aureoglandulosus (SE India, Burma and Thailand); D. subalternans var. curvicapsa (Nepal–Sikkim) is raised to specific rank, and a new species, D. triplotrichus (Darjeeling distr. and Sikkim), is described. Lysionotus himalayensis is redetermined and reduced to synonymy under L. serratus; as a result L. atropurpureus, which name it had recently superseded, is reinstated. Aeschynanthus kingii and A. levipes are both transferred to Lysionotus, while L. wardii and L. gracilipes are reduced to synonymy under L. pubescens. Notes and new records are provided for L. confertus and L. serratus var. pterocaulis.

The Eocene unconformity on Southeast and East Sundaland

The Early Palaeogene landmass of Sundaland extended as far southeast as West Sulawesi. The cratonic nature of the Malay Peninsula and China did not extend into or beyond the Borneo region of Sundaland, where the pre-Eocene outcrops were dominated by Cretaceous rocks. Deep water sediments, melange and ophiolite terrain were characteristic of this non-cratonic southeastern peninsula of Sundaland. By 45 Ma (magnetic anomaly 19), India was in flush collision with Eurasia, seafloor spreading ceased at the NW Wharton Basin, and a new fracture system was initiated, separating Australia from Antarctica, and extending WNW along the SE India Ocean Ridge towards the Red Sea. This event was felt throughout the whole of Southeast Asia, and is recorded as the major Eocene Unconformity, both onland and offshore Sundaland. The NNE directed push of cratonic India into the continental margin of Eurasia resulted in a general clockwise rotation of the Sundaland Peninsula, made possible by major right-lateral shear along the pre-existing structural grain (extrusion tectonics). The Sunda arc-trench system was also initiated at 45 Ma. The oroclinal bending resulted in crustal pull-apart in some places and compression in others. Anti-clockwise rotation and left-lateral shear were imparted to eastern Sundaland by the collision of Australia to the east. Rejuvenated rivers carried an increased load from the uplifted areas, active faulting created river capture and lakes, many of which date back to the Eocene in most places, but were locally delayed to the Oligocene. Marine platform carbonates became widespread and characteristic of E aiid SE Sundaland from Late Oligocene through Miocene, except in the Rajang Group Basin and some rift-related river/delta systems. Late Miocene to Pliocene uplift was important in non-cratonic Sundaland, causing internal subdivision offormerly more extensive Cenozoic basins.

Silica and calcium carbonate replacement of plant roots in tropical dune sands, SE India

Summary The Quaternary coastal sediments of SE India are composed of raised marine deposits capped by aeolianite and red weathered dune sands (Teri). The red weathering associated with these sands is irregular in depth and unusual in the fast rate at which it has developed. Associated at depth with this red weathering is precipitated silica and calcium carbonate which replace plant roots and act as a cement. In places the plant root cells are prefectly replaced to form rhizoliths which occur in the soil profile with and without surrounding cement. Ultimately, the silica source is silicic acid (H 4 SiO 4 ) that is a product of the in situ weathering of unstable minerals in the overlying dune sands during the last 20 000 years. The calcium carbonate is thought to have had a much more localized source. Plants and the litter they produce play at least two possible roles in the silica cycle. First, silica-organic complexing occurs as the Teri sands are leached by organic acids derived from decomposing litter. Secondly, the plant roots create a favourable micro-environment for silica and calcium carbonate precipitation and replacement by altering partial gas pressures and hence the pH of soil solutions through their respiratory activity.

Mangrove Forest of Krusadai Island, SE India, and Its Management

Mangrove Forest of Krusadai Island, SE India, and Its Management