stratigraphy composition and form of the deccan basalts western ghats india in the western ghats between latitudes 18 20 n and 19 15 n 7000 km2 of deccan basalt have been mapped with the primary objective of establishing a flow stratigraphy as a guide to the volcanic history of the flood basalts using over 70 measured vertical sections major and trace element analyses of nearly 1200 samples and rare earth and87sr 86sr determinations for over 60 samples we divide the basalt into three subgroups and ten formations in this paper we describe the seven principal formations in the area and the most prominent individual flows

tectonic setting and timing of the final deccan flood basalt eruptions the role of extensional tectonics in the generation of basaltic melt and eruption of continental flood basalts is controversial and yet it remains crucial to understanding the origin and cause of these large volume eruptions establishing the timing of lithospheric extension continental rifting and the onset of continental flood basalt volcanism is therefore of importance to petrogenetic models detailed mapping along the mumbai coast india reveals three chemically distinct sets of dikes which together with high precision 40 ar 39 ar dating demonstrate the temporal relationship between lithospheric extension and deccan volcanism the east west extension evident along the west coast of india which led to the separation of the seychelles from the indian plate only began during the final phases of the basalt eruptions and cannot have initiated the large volume eruptions of the deccan flood basalt province

microchondrules in three unequilibrated ordinary chondrites we report on a suite of microchondrules from three unequilibrated ordinary chondrites uocs microchondrules a subset of chondrules that are ubiquitous components of uocs commonly occur in fine grained chondrule rims although may also occur within matrix microchondrules have a variety of textures cryptocrystalline microporphyritic radial glassy in some cases their textures and in many cases their compositions are similar to their larger host chondrules bulk compositions for both chondrule populations frequently overlap the primary material that composes many of the microchondrules has compositions that are pyroxene normative and is similar to low ca pyroxene phenocrysts from host chondrules primary material rarely resembles olivine or plagioclase some microchondrules are composed of feo rich material that has compositions similar to the bulk submicron fine grained rim material these microchondrules however are not a common compositional type and probably represent secondary feo enrichment microchondrules may also be porous suggestive of degasing to form vesicles our work shows that the occurrence of microchondrules in chondrule rims is an important constraint that needs to be considered when evaluating chondrule forming mechanisms we propose that microchondrules represent melted portions of the chondrule surfaces and or the melt products of coagulated dust in the immediate vicinity of the larger chondrules we suggest that through recycling events the outer surfaces of chondrules were heated enough to allow microchondrules to bud off as protuberances and become entrained in the surrounding dusty environment as chondrules were accreting fine grained rims microchondrules are thus byproducts of cyclic processing of chondrules in localized environments their occurrence in fine grained rims represents a snapshot of the chondrule forming environment we evaluate mechanisms for microchondrule formation and hypothesize a potential link between the emergence of type ii chondrules in the early solar system and the microchondrule bearing fine grained rims surrounding type i chondrules

wildfires in the late palaeozoic of central europe an overview of the rotliegend upper carboniferous lower permian of the saar nahe basin sw germany abstract fossil charcoal from a number of different sedimentary settings e g lakes floodplains tuffs tuffites is reported from the rotliegend late carboniferous early permian of the saar nahe basin in sw germany it is one of the largest carboniferous permian basins which developed during the late variscan in europe the sedimentary deposits encompass the carboniferous permian boundary thus enabling us to examine the development of fire regimes parallel to the reconstructed drop in atmospheric o 2 across this boundary the majority of the identified macroscopical charcoal remains belongs to the conifers or other gymnosperms maybe pointing to a more fire prone vegetation in the drier hinterland preferentially inhabited by these plants most wildfires especially those which took place during the deposition of the lower rotliegend late carboniferous have probably been ignited by lightning whereas in the upper rotliegend early permian an intense volcanism may have also acted as an important additional source of ignition in the lower rotliegend charcoals occur in nearly all sedimentary facies types whereas in the upper rotliegend charcoal has so far only been found in some volcanically influenced deposits the absence of charcoal in most sediments from the upper rotliegend may be explained by i low atmospheric oxygen levels leading to reduced fire frequencies ii a generally sparse vegetation cover iii the mechanical breakdown of macroscopic charcoal under the prevailing semiarid conditions

metal veins in the kernouve h6 s1 chondrite evidence for pre or syn metamorphic shear deformation abstract kernouve is an h6 chondrite that experienced a very small degree of late stage shock loading s1 however kernouve contains fe ni metal vein like structures whose formation have been attributed to an early impact event to establish the formation conditions of metal veins in kernouve we examined the three dimensional 3d arrangement of metal vein like structures and typical metal grains in two samples of kernouve with x ray microtomography ct at resolutions of 11 m voxel we additionally investigated the 3d structure of the porosity present in kernouve using ct at two different resolutions 3 and 11 m voxel these data and optical microscopy support the hypothesis that kernouve has experienced little post metamorphic shock however the moderate 5 8 vol porosity of kernouve is in the form of intergranular voids rather than cracks which indicates any cracking that may have existed in the relatively brittle silicate grains was annealed we estimate that 70 80 of the primordial porosity in kernouve was removed by impact related compaction moreover we found no collective orientation of metal grains so high metamorphic temperatures following compaction erased any common orientation of metal grains due to compaction we propose that the metal vein structures can be explained as a pre or syn metamorphic shock induced process which we infer was primarily shear deformation with some uniaxial compaction also occurring the coarse metal veins probably formed by accumulation of ductile metal grains along shear zones a process that would have been facilitated by having an already warm h chondrite parent body when shock occurred i e syn metamorphic shock the complexity of shape including numerous tendrils expanding from the primary structure of the veins in kernouve is likely due to the coalescence of metal by metamorphic growth after the shear event the metal veins in kernouve thus appear to record evidence for early shock induced metal mobilization and the maximum shock pressure experienced by kernouve may have been 21 gpa our study suggests that collision induced segregation of metal occurred at an early stage in low gravity planetesimals consistent with the idea that this process could have been important for the differentiation of such objects

global trends of pco2 across the cretaceous paleogene boundary supported by the first southern hemisphere stomatal proxy based pco2 reconstruction reliable reconstructions of atmospheric carbon dioxide concentrations pco2 are required at higher resolution than currently available to help resolve the relationship between mass extinctions and changes in palaeo pco2 levels such reconstructions are needed 1 at a high temporal resolution for constraining the pre and post extinction atmospheres and 2 at a sufficient spatial resolution to constrain potential inter hemispheric differences here we estimate pco2 based on fossil lauraceae leaf cuticle specimens derived from three localities with strata spanning the latest cretaceous to the mid paleocene including a new cretaceous paleogene boundary k pg locality in new zealand we use two independent methods of stomatal density based pco2 reconstructions a transfer function calibrated using herbarium material and the stomatal ratio method producing three calibration sets our results based on the mean values of each of the three calibration methods indicate pco2 ranging between ca 460 and 650 ppm during the latest cretaceous falling precipitously to average values between ca 360 and 430 ppm across the k pg boundary and further to ca 305 320 ppm in the mid paleocene a spike of extremely high pco2 at the k pg could not be confirmed but our results are nonetheless consistent with previously published pco2 records from the northern hemisphere and show that stomatal density worldwide was responding to significant changes in pco2 across the k pg

40ar 39ar dating of mineral separates and whole rocks from the western ghats lava pile further constraints on duration and age of the deccan traps abstract there has been an ongoing debate for almost two decades on the age and duration of deccan trap volcanism some of the best available ages have been determined using the 40 ar 39 ar method but differences in sample location laboratory procedure type of analyzed material and ages of monitors may have been responsible for much of the apparent spread in the data we have collected and analyzed samples from close to the bottom and top of the main lava pile in the western ghats near bombay we confirm previous findings that alteration and ar recoil strongly affect these rocks particularly measurements made on whole rocks when mineral separates mostly plagioclase are analyzed the spread is much reduced we find a mean age of 65 4 0 7 ma for five flows near the base of the section jahwar and igatpuri formations and 65 2 0 4 ma for a dyke cross cutting the poladpur formation below the topmost mahabaleshwar formation this implies that at least 1800 m out of the 2500 m composite trap section were erupted in a short interval less than 1 ma long and close to 65 5 ma i e close to the cretaceous tertiary boundary we find that older ages near 67 ma found by several authors are essentially based on whole rock samples which display disturbed spectra when this can be checked i e when the spectra are published

testing for causal relationships between large pyroclastic volcanic eruptions and mass extinctions the permo triassic mass extinction the most extensive of all phanerozoic extinctions erwin 1990 has been correlated with both massive pyroclastic eruptions yang and yin 1987 yin et al 1989 jinwen 1989 and flood basalts rampino and strothers 1988 more tentatively the end cretaceous mass extinction has also been linked with massive volcanic events officer et al 1987 rampino and strothers 1988 however the nature of the connection between volcanism and either mass extinction remains uncertain rather than study volcanic events at known extinction horizons another approach is to identify extremely large 1000 km3 pyroclastic eruptions with well constrained age determinations and evaluate the fossil record for evidence of significant regional or global extinctions we selected the largest best constrained pyroclastic events known toba tuff huckleberry ridge tuff tuff of blacktail and the elkhorn mountain volcanics neither global marine diversity nor global or regional terrestrial vertebrate diversity exhibit any biological effect from these extremely massive pyroclastic eruptions we conclude that massive pyroclastic eruptions are unlikely to have been an effective cause of mass extinctions

palynofloral palynofacies and carbon isotope of permian coal deposits from the godavari valley coalfield south india insights into the age palaeovegetation and palaeoclimate abstract palynological studies of the present permian sediments play a vital role in biostratigraphic and palaeoecological interpretations additionally the palynofacies analysis total organic carbon toc content and 13c value in the present study focus largely for the palaeoenvironmental and palaeoclimatic reconstruction and also provides information about processes responsible for sedimentary deposits along with their lithological counter parts the palynological investigation of 306 m deep borehole msm 25 from the somavaram area of the godavari valley coalfield has led to infer five distinct palynofloral assemblages the artinskian palynofloral assemblage i and ii guadalupian palynofloral assemblage iii and iv and lopingian palynofloral assemblage v ages have been assigned to the palynofloral assemblages based on the palynofloral evidence the distinct stable carbon isotope ratios also comprehend these ages with extreme climatic events the artinskian age 22 4 is characterized by relatively higher average 13c values whereas lowest average 13c values are observed at the lopingian age 24 0 with intermediate average 13c value 23 2 during guadalupian age in the present study the permian triassic p t transition is marked by the distinct negative carbon isotope excursion followed by a distinct recovery during the early triassic time period the relative variability of palynofacies components such as palynomorphs structured terrestrial phytoclasts opaque phytoclasts degraded organic matter om and amorphous om is related to hydrodynamic perturbations during the deposition time based on the dominance and sub dominance of the palynofacies three distinct depositional phases have been identified representing differential environmental settings the palynomorphs and structured terrestrial phytoclasts dominate the palynofacies assemblage a suggesting forest swamp environment while in the palynofacies assemblage b the dominance of degraded om and amorphous om characterize reed marshes lakeshore setting and the abundance of opaque phytoclasts in the palynofacies assemblage c indicate oxidized swamp environment

40ar 39ar ages of bombay trachytes evidence for a palaeocene phase of deccan volcanism we present 40ar 39ar ages of 60 4 0 6 ma and 61 8 0 6 ma 2 for deccan trap trachytes from manori and saki naka bombay situated in the tectonized panvel flexure zone along the western indian rifted continental margin these ages provide clear evidence that i these trachytes are of palaeocene age and therefore substantially younger than the lower part of the main flood basalt sequence exposed in the western ghats which precedes the k t boundary in age and ii the formation of the panvel flexure along the west coast must have been subsequent to 60 ma considering early alkaline deccan rocks previously dated at 68 5 ma the total duration of deccan volcanism was at least 8 my

experiments in waterlogging and sedimentology of charcoal results and implications abstract fossil charcoal has a sporadic occurrence in sedimentary rocks since devonian time it is moderately common as a component of the organic material found in a wide variety of facies but there are also some notable concentrations which occur locally these occurrences have considerable palaeobotanical and palaeoecological value because the process of charring may result in excellent preservation of plant tissue if assemblages of charred material are to be used to interpret palaeoenvironments it is important to understand the behaviour of charcoal during transport from the site of the fire charcoal is an unusual sedimentary material because most fresh material floats but with prolonged immersion becomes waterlogged and sinks we carried out a series of waterlogging experiments on uncharred material and charcoal from a range of different plant types and tissues these show considerable variations in the rates of waterlogging of different charred and uncharred plant tissues and it is suggested that buoyancy contrasts are likely to result in separation during transport and deposition furthermore experiments in a flume tank have shown that the process of deposition of charcoal in sand is controlled by the rate of migration of bedforms which is in turn determined by depth flow velocity and sediment supply allochthonous assemblages of charred and uncharred plant debris will most probably be biased and not fully representative of the contemporaneous plants or vegetation the presence of charcoal concentrations may be used as an aid in interpreting palaeohydraulic conditions

characterization of various stages of calcitization in porites sp corals from uplifted reefs case studies from new caledonia vanuatu and futuna south west pacific abstract this study focuses on the processes of calcitization under meteoric conditions based on observations and analyses of a series of subaerial fossil corals of the genus porites collected from emerged holocene and pleistocene reefs in the pacific ocean we can establish two types of low magnesian calcite lmc precipitates according to their textural characteristics after their transformation from the initial aragonitic skeleton to the calcitized corals in the first variety lmc1 the calcite contains remnants of the original exoskeleton structure in situ insoluble organic tissues pieces of aragonite needles while in the second variety lmc2 filling the primary inter skeletal macro pores such relicts are not present such textural segregation is faithfully reflected in the elemental data on some parts of calcitized corals lmc1 is characterized by mg2 sr2 and mn2 compositions inherited from the aragonite precursor indicating a formation in a semi closed intra skeleton diagenetic environment in disequilibrium with the meteoric bulk aquifer water lmc2 has a chemistry indicative of equilibrium with ambient bulk meteoric water these chemical characteristics can be likened to a two fold mechanism a fine scale process neomorphism typified by the concomitant dissolution of intra fabric aragonite and re precipitation of the lmc1 resulting from the biogenic skeleton and marine waters followed by the cementation of the inter fabric macro voids by an allochthonous subaerial and meteoric lmc2 in the other parts of calcitized corals lmc1 and lmc2 have similar trace element contents in mg2 and sr2 the chemical data are consistent with formation by partial skeletal dissolution followed eventually by cementation of primary voids lmc2 and secondary voids lmc1 by calcite in equilibrium with meteoric bulk aquifer water these two mechanisms of the calcitization of skeletal aragonite mainly differ in the space and time length scales of the intervening transport step and are commonly found juxtaposed in the same thin section taking into account such a heterogeneity is a real challenge for the development of predictive models assessing the effects of diagenetic products on paleoclimatic reconstructions

volcanological and tectonic control of stratigraphy and structure in the western deccan traps many of the world s flood basalt provinces form elevated plateaux at the margins of continents although in most cases their present large elevation is not the result of mountain building processes several explanations have recently been put forward to explain such occurrences of epeirogeny the deccan trap basalt province forms one such elevated plateau and results are presented here showing how the epeirogenic uplift in this region combined with crustal subsidence probably associated with the rifting of the indian continental margin has affected the structure of the basalt sequence trace element analytical data are used for samples from numerous vertical sections through the deccan traps lava series along and around the western ghats ridge in india the results reinforce the previously defined stratigraphy of the mahabaleshwar area and extend it over a region covering some 36 000 km2 reaching as far south as belgaum and the trap basement contact these results show that the lava pile is not flat lying but forms a very low amplitude anticlinal fold structure plunging southwards by up to 0 3 over most of the area although in the south there is evidence of a reversal of this plunge the fold is interpreted as being the result of two tilting processes 1 westward tilting near the coast due to the foundering of the passive continental margin and 2 epeirogenic uplift along the whole west coast of india producing the observed topography and the peninsula wide drainage patterns and also the easterly component of dip variations in the magnitude of the latter effect along the western continental margin may also be important in generating the plunge of the fold although the possibility of some component of depositional dip may also be important this latter possibility can be modelled using a simple computer program the results of this modelling show that a migrating linear volcanic edifice fits the observations best

palynofacies prediction of distance from sediment source a case study from the upper cretaceous of the pyrenees quantitative variations in the size shape and preservation state of microscopic plant debris assemblages correlate with the actual distance of the sample site from the coastal siliciclastic sediment source a case study from a shelf to basin transect in the upper cretaceous of the spanish pyrenees reveals that 1 the diameter of equant black wood phytoclasts 2 the ratio of equant to lath shaped black wood phytoclasts and 3 several other parameters show predictable changes over a 55 km onshore offshore transect these parameters primarily reflect sedimentological sorting based on particle size and density rather than paleoecological gradients when only samples from the transgressive systems tract are considered multiple regression models predict the distance from the source with a standard error of 5 km r 2 0 84 0 93 these parameters also allow the maximum flooding surface to be more precisely located within mudstone dominated sections by applying the lateral regression relationship to stratigraphic data a quantitative estimate can also be made for the scale of lateral facies shifts through depositional sequences here 10 km

a statistical study of trees damaged by the 22 november 1994 eruption of merapi volcano java indonesia relationships between ash cloud surges and block and ash flows abstract on 22 november 1994 after 2 years of dome growth merapi volcano generated pyroclastic flows and surges this eruption destroyed the south southwest flank that had been intact for several decades about 9 5 km 2 of forest were blown down a study of tree damage correlated with geological and eyewitness observations is used here to reconstruct the dynamics of the pyroclastic surges and block and ash flows of the 22 november eruption the main results are summarised in the form of maps showing blown down trunk directions and zones indicating the percentage of broken and downed trees these maps provide evidence for various aspects of the emplacement of the 1994 flows 1 pyroclastic flow generation from gravitational collapse of part of the active lava dome without significant initial explosion 2 the control of topography cliffs hills river channels on damage distribution and range 3 the genesis and decoupling of ash cloud surges from block and ash flows

the gao guenie impact melt breccia sampling a rapidly cooled impact melt dike on an h chondrite asteroid the gao guenie h5 chondrite that fell on burkina faso march 1960 has portions that were impact melted on an h chondrite asteroid at 300 ma and through later impact events in space sent into an earth crossing orbit this article presents a petrographic and electron microprobe analysis of a representative sample of the gao guenie impact melt breccia consisting of a chondritic clast domain quenched melt in contact with chondritic clasts and an igneous textured impact melt domain olivine is predominantly fo80 82 the clast domain contains low ca pyroxene impact melt grown pyroxene is commonly zoned from low ca pyroxene in cores to pigeonite and augite in rims metal troilite orbs in the impact melt domain measure up to 2 mm across the cores of metal orbs in the impact melt domain contain 7 9 wt of ni and are typically surrounded by taenite and ni rich troilite the metallography of metal troilite droplets suggest a stage i cooling rate of order 10 c s 1 for the superheated impact melt the subsolidus stage ii cooling rate for the impact melt breccia could not be determined directly but was presumably fast an analogy between the ni rim gradients in metal of the gao guenie impact melt breccia and the impact melted h6 chondrite orvinio suggests similar cooling rates probably on the order of 5000 40 000 c yr 1 a simple model of conductive heat transfer shows that the gao guenie impact melt breccia may have formed in a melt injection dike 0 5 5 m in width generated during a sizeable impact event on the h chondrite parent asteroid

microchondrule bearing clast in the piancaldoli ll3 meteorite a new kind of type 3 chondrite and its relevance to the history of chondrules in the piancaldoli ll3 chondrite we found a mm sized clast containing 100 chondrules 0 2 64 m in apparent diameter much smaller than any previously reported that are all of the same textural type radial pyroxene fs1 17 this clast like other type 3 chondrites has a fine grained ferich opaque silicate matrix sharply defined chondrules abundant low ca clinopyroxene and minor troilite and si and cr bearing metallic fe ni however the very high modal matrix abundance 63 8 vol unique characteristics of the chondrules and absence of microscopically observable olivine indicate that the clast is a new kind of type 3 chondrite most chondrules have feo rich edges and chondrule size is inversely correlated with chondrule core feo concentration the first reported correlation of chondrule size and composition chondrules acquired fe by diffusion from fe rich matrix material during mild metamorphism possibly before final consolidation of the rock microchondrules those chondrules 100 m in diameter are also abundant in another new kind of type 3 chondrite clast in the rio negro l chondrite regolith breccia in other type 3 chondrite groups microchondrule abundance appears to be anticorrelated with mean chondrule size viz 0 02 0 04 vol in h and co chondrites and 0 006 vol in l ll and cv chondrites microchondrules probably formed by the same process that formed normal sized droplet chondrules melting of pre existing dustballs because most compound chondrules in the clast and other type 3 chondrites formed by collisions between chondrules of the same textural type we suggest that dust grains were mineralogically sorted in the nebula before aggregating into dustballs the sizes of compound chondrules and chondrule craters which resulted from collisions of similarly sized chondrules while they were plastic indicate that size sorting of dustballs occurred before chondrule formation probably by aerodynamic processes in the nebula we predict that other kinds of type 3 chondrites exist which contain chondrule abundances size ranges and proportions of textural types different from known chondrite groups

main deccan volcanism phase ends near the k t boundary evidence from the krishna godavari basin se india abstract 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

the timing of crustal extension and the eruption of continental flood basalts the association between mantle plumes continental flood basalt eruption and the extension and rifting of the crust is well recognized1 3 but there has always been controversy as to whether the plume or the rifting was the initiating factor4 5 was rifting triggered by doming above the thermal plume2 3 6 or was rifting the driving force preceding the associated volcanicity7 8 here i show that recent flow by flow mapping of two continental flood basalt provinces the columbia river basalt group and the deccan basalt group allows one to infer the relative timing of volcanicity and crustal extension in both provinces eruption of the main tholeiitic phase preceded significant extension and crustal thinning this suggests that the primary event in the formation of these provinces was the upwelling of a mantle plume and that the subsequent crustal extension rifting and decompression was in part a consequence of the initiation of that plume

regional uplift associated with continental large igneous provinces the roles of mantle plumes and the lithosphere the timing and duration of surface uplift associated with large igneous provinces provide important constraints on mantle convection processes here we review geological indicators of surface uplift associated with five continent based magmatic provinces emeishan traps 260 million years ago ma siberian traps 251 ma deccan traps 65 ma north atlantic phase 1 61 ma and phase 2 55 ma and yellowstone 16 ma to recent all five magmatic provinces were associated with surface uplift surface uplift can be measured directly from sedimentary indicators of sea level in the north atlantic and from geomorpholocial indicators of relative uplift and tilting in yellowstone in the other provinces surface uplift is inferred from the record of erosion in the deccan north atlantic and emeishan provinces transient uplift that results from variations in thermal structure of the lithosphere and underlying mantle can be distinguished from permanent uplift that results from the extraction and emplacement of magma transient surface uplift is more useful in constraining mantle convection since models of melt generation and emplacement are not required for its interpretation observations of the spatial and temporal relationships between surface uplift rifting and magmatism are also important in constraining models of lip formation onset of surface uplift preceded magmatism in all five of the provinces biostratigraphic constraints on timing of uplift and erosion are best for the north atlantic and emeishan provinces where the time interval between significant uplift and first magmatism is less than 1 million years and 2 5 million years respectively rifting post dates the earliest magmatism in the case of the north atlantic phase 1 and possibly in the case of siberia the relative age of onset of offshore rifting is not well constrained for the deccan and the importance of rifting in controlling magmatism is disputed in the emeishan and yellowstone provinces in these examples rifting is not a requirement for onset of lip magmatism but melting rates are significantly increased when rifting occurs models that attempt to explain emplacement of these five lips without hot mantle supplied by mantle plumes often have difficulties in explaining the observations of surface uplift rifting and magmatism for example small scale convection related to craton or rift boundaries edge driven convection cannot easily explain widespread 1000 km scale transient surface uplift emeishan deccan north atlantic and upper mantle convection initiated by differential incubation beneath cratons the hotcell

comments on deccan flood basalts at the cretaceous tertiary boundary by v courtillot j besse d vandamme r montigny j j jaeger and h cappetta the age of the flood basalts of the deccan area and the length of the interval during which the outpourings took place are not known precisely the authors have tried to date those basalts and estimate how long the volcanic activity lasted on the basis of new paleomagnetic paleontological and geochronological data of the deccan effusives they came to the preliminary conclusion that the volcanic activity may have lasted less than 1 ma and that the majority of the lavas poured out during chron 29r unfortunately the authors were not familiar with our paper on paleomagnetism and radiomettic age determinations of deccan trap lavas 2 in our paper we presented the paleomagnetic data derived from 223 individual lava flows from 16 sections all over the deccan area and gave the k ar age determinations for 13 selected unaltered lava flows we agree that the enormous volume of deccan extrusives poured out in a relatively short interval and that the volcanic activity was probably concerned with the spreading in the central indian ocean in late cretaceous and early tertiary time

chicxulub crater a possible cretaceous tertiary boundary impact crater on the yucatan peninsula mexico we suggest that a buried 180 km diameter circular structure on the yucatan peninsula mexico is an impact crater its size and shape are revealed by magnetic and gravity field anomalies as well as by oil wells drilled inside and near the structure the stratigraphy of the crater includes a sequence of andesitic igneous rocks and glass interbedded with and overlain by breccias that contain evidence of shock metamorphism the andesitic rocks have chemical and isotopic compositions similar to those of tektites found in cretaceous tertiary k t ejecta a 90 m thick k t boundary breccia also containing evidence of shock metamorphism is present 50 km outside the crater s edge this breccia probably represents the crater s ejecta blanket the age of the crater is not precisely known but a k t boundary age is indicated because the crater is in a thick carbonate sequence shock produced co2 from the impact may have caused a severe greenhouse warming

the deccan tholeiite lavas and dykes of ghatkopar powai area mumbai panvel flexure zone geochemistry stratigraphic status and tectonic significance abstract mumbai city situated on the western indian coast is well known for exposures of late stage deccan pillow basalts and spilites pyroclastic rocks rhyolite lavas and trachyte intrusions these rock units and a little studied sequence of tholeiitic flows and dykes in the eastern part of mumbai city constitute the west dipping limb of a regional tectonic structure called the panvel flexure here we present field petrographic major and trace element and sr nd isotopic data on these tholeiitic flows and dykes best exposed in the ghatkopar powai area the flows closely resemble the mahabaleshwar formation of the thick western ghats sequence to the east in sr nd isotopic ratios and multielement patterns but have other geochemical characteristics e g incompatible trace element ratios unlike the mahabaleshwar or any other formation the flows may have originated from a nearby eruptive center possibly offshore of mumbai two dykes resemble the ambenali formation of the western ghats in all geochemical characteristics though they may not represent feeders of the ambenali formation lavas most dykes are distinct from any of the western ghats stratigraphic units some show partial e g sr nd isotopic similarities to the mahabaleshwar formation and these include several dykes with unusual concave downward ree patterns suggesting residual amphibole and thus a lithospheric source the flows and dykes are inferred to have undergone little or no contamination by lower continental crust most dykes are almost vertical suggesting emplacement after the formation of the panvel flexure and indicate considerable east west lithospheric extension during this late but magmatically vigorous stage of deccan volcanism

trace element distributions within ordinary chondrite chondrules implications for chondrule formation conditions and precursors the concentrations of thirty six lithophile elements in the major silicate phases of chondrules have been determined by ion microprobe two types of porphyritic chondrule olivines normal and incompatible rich have been identified the incompatible rich olivines which have forsterite contents that are greater than fo98 appear to have formed under highly reducing conditions the olivine and low ca pyroxene grains in porphyritic chondrules have relatively unfractionated l chondrite normalized ree abundances compared to expectations from equilibrium distribution coefficients this is consistent with porphyritic chondrules having formed at cooling rates of the order of 1000 c h or faster the alkali metal na k rb and cs to al ratios in chondrule glasses vary by two orders of magnitude the lower values possibly being the result of volatile loss from chondrules no correlation was found between alkali metal depletions in chondrule glasses and enrichments in the surrounding rims but any such correlation may have been destroyed by the diffusive redistribution of these elements in rims that is likely to occur even under mild metamorphic conditions the refractory lithophiles with the exception of mg are all highly incompatible in silicate melts and are as a result concentrated in the chondrule glasses the refractory lithophiles in the glass are in most cases unfractionated from one another suggesting that neither vapor phase nor crystal liquid fractionation played a major role in the formation of chondrules or their precursors based largely on the observation that the refractory lithophiles are concentrated in the chondrule glasses it is suggested that previously reported correlations between these elements in bulk chondrule analyses are due to chondrules themselves as opposed to refractory condensates having been their own immediate precursors a simple monte carlo simulation using the compositions of the major chondrule silicate phases reported here supports this hypothesis it is argued that the upper limit of about one observed in the atomic na al ratios of chondule glasses is not due to an albitic precursor but simply reflects that for na to be incorporated into the structure of the melt it must take part in the coupled substitution naalo2 sio2

bombay gravity high and eruption of deccan flood basalts india from a shallow secondary plume abstract the western margin of india especially the area surrounding the gravity high of bombay is covered by a thick pile of deccan flood basalts the massive eruption is one of the largest continental volcanic events at the cretaceous tertiary biological mass extinction boundary 65 ma ago the area is characterized by several hidden tectonic features and their origin and evolution have remained controversial in the search for magma source structure attention was focused on an unusual oval shaped positive gravity anomaly over the bombay coastal zone the gravity analysis indicates the presence of two subsurface structures at depths of 4 5 0 5 km and 18 2 km these depths correspond to the top of the crustal basaltic layer and an upwarped moho discontinuity respectively the positive gravity anomaly in three dimensional analysis reveals a possible magma conduit structure and the near absence of the granitic layer the unusually shallow moho layer coincides with the region of a broad high heat flow anomaly average heat flow more than 80 mw m2 the geophysical signatures from the neighborhood show that the asthenosphere has welled up considerably and is located at a depth of about 40 km high heat flow and asthenospheric depth estimates suggest melting conditions at very shallow depths it is suggested that the highly differentiated magma associated with the major eruptive phase of deccan traps did not originate at the d layer it is hypothesized that magma was brought to the surface by a major secondary plume at a much shallower depth within the lithosphere and possibly at the crust mantle boundary as a consequence of subcrustal erosion and secondary melting caused by the extreme velocity of india during the late cretaceous and early tertiary substantial melting of the base of the lithosphere due to frictional heating might have resulted in the accumulation of a large quantity of magma below the thinned lithosphere at the pre existing fracture zone near the continental margin the presence of a magmatic conduit structure above the moho derived from the inversion of gravity data and other tectonic conditions suggests a possible triggering of the magma reservoir by a large bolide impact 10 km diameter at the cretaceous tertiary boundary near bombay

thermal and impact history of the h chondrite parent asteroid during metamorphism constraints from metallic fe ni abstract we have studied cloudy taenite metallographic cooling rates and shock effects in 30 h3 6 chondrites to elucidate the thermal and early impact history of the h chondrite parent body we focused on h chondrites with old ar ar ages 4 4 gyr and unshocked and mildly shocked h chondrites as strongly shocked chondrites with such old ages are very rare cooling rates for most h chondrites at 500 c are 10 50 c myr and do not decrease systematically with increasing petrologic type as predicted by the onion shell model in which types 3 5 are arranged in concentric layers around a type 6 core some type 4 chondrites cooled slower than some type 6 chondrites and type 3 chondrites did not cool faster than other types contrary to the onion shell model cloudy taenite particle sizes which range from 40 to 120 nm are inversely correlated with metallographic cooling rates and show that the latter were not compromised by shock heating the three h4 chondrites that were used to develop the onion shell model ste marguerite beaver creek and forest vale cooled through 500 c at 5000 c myr our thermal modeling shows that these rates are 50 higher than could be achieved in a body that was heated by 26al and cooled without disturbance by impact published ar ar ages do not decrease systematically with increasing petrologic type but do correlate inversely with cloudy taenite particle size suggesting that impact mixing decreased during metamorphism metal and silicate compositions in regolith breccias show that impacts mixed material after metamorphism without causing significant heating impacts during metamorphism created portales valley and two other h6 chondrites with large metallic veins excavated the fast cooled h4 chondrites around 3 4 myr after accretion and mixed petrologic types metallographic data do not require catastrophic disruption by impact during cooling

the composition of metal phases in bruderheim l6 and implications for the thermal histories of ordinary chondrites erratum abstract the compositions and textures of metal phases in bruderheim have been examined in detail using nomarski differential interference contrast techniques backscattered electron imaging and quantitative electron microprobe analysis for fe ni and co the results clearly demonstrate substantial inter and intra grain compositional variations and that these persisted through the high temperature thermal event which largely homogenised the silicate phases forming the bulk of the meteorite these results are difficult to reconcile with prograde thermal metamorphism instead accretion at high temperatures is envisaged with homogenisation of silicates occurring at these temperatures relatively sparse metal grains were largely unaffected at these temperatures because they did not form three dimensional networks and hence lattice diffusion of metallic fe and ni was ineffective and because at the temperature of accretion most grains were present as homogeneous particles of the phase the complexities now apparent in metal grains were established first during the decline of temperature and retrograde metamorphism after accretion and then later during relatively low temperature burial metamorphism in asteroidal bodies the reliability and significance of cooling rate calculations based on measurements of ni concentrations in metal grains of such meteorites are questioned

impact winter and the cretaceous tertiary extinctions results of a chicxulub asteroid impact model abstract the chicxulub impact crater in mexico is the site of the impact purported to have caused mass extinctions at the cretaceous tertiary k t boundary 2 d hydrocode modeling of the impact coupled with studies of the impact site geology indicate that between 0 4 and 7 0 1017 g of sulfur were vaporized by the impact into anhydrite target rocks a small portion of the sulfur was released as so3 or so4 which converted rapidly into h2so4 aerosol and fell as acid rain a radiative transfer model coupled with a model of coagulation indicates that the aerosol prolonged the initial blackout period caused by impact dust only if the aerosol contained impurities a larger portion of sulfur was released as so2 which converted to aerosol slowly due to the rate limiting oxidation of so2 our radiative transfer calculations combined with rates of acid production coagulation and diffusion indicate that solar transmission was reduced to 10 20 of normal for a period of 8 13 yr this reduction produced a climate forcing cooling of 300 wm 2 which far exceeded the 8 wm 2 greenhouse warming caused by the co2 released through the vaporization of carbonates and therefore produced a decade of freezing and near freezing temperatures several decades of moderate warming followed the decade of severe cooling due to the long residence time of co2 the prolonged impact winter may have been a major cause of the k t extinctions

some clues to the history of the h group chondrites abstract sem optical and chemical observations have been performed on 12 h3 6 chondrites 9 of them being also studied by other groups morphological features of chondrules and crystals growth steps are shown the significance of the finely crystallised troilite in menow and ambapur nagla is discussed in the light of the discovery that the nife blebs associated with it are ni rich 50 60 ni sulphur should have been mobilized without shock evidence possibly as a result of solar heating pre chondritic relict material is recognized by anomalous or variable mineral compositions and in some cases by the presence of overgrowths on relict cores after short notes on individual chondrites a tentative history of h chondrites is proposed the chondrule forming episode is considered as a remelting of pre existing material the accretion would immediately follow this event for type 6 around 1000 c and would occur at progressively lower temperature for types 5 and 4 type 3 would represent material coming from an extended source region an hypothesis consistent with the broader range composition of the particles and with their cooling before accretion to much lower temperatures below 350 c

sequential palynological changes across the composite cretaceous tertiary k t boundary claystone and contiguous strata western canada and montana u s a the boundary claystone in the western canada basin is composed of three intimately associated layers with a regional distribution a basal brownish grey hackly claystone a middle brownish black sa

compositions and textures of relic forsterite in carbonaceous and unequilibrated ordinary chondrites several percent of the olivine in the c2 c3 and unequilibrated ordinary chondrites uoc can be distinguished by blue cathodoluminescence cl and an unusual composition for forsterite this olivine has the following textural features 1 1 forms cores in single olivine grains 2 2 shows subhedral to euhedral boundaries against rim olivine 3 3 rarely contains inclusions 4 4 has embayments containing olivine like that of the rim 5 5 occurs within chondrules especially in uoc meteorites the blue olivine is always fe poor 0 25 feo 1 0 and shows the following average and maximum values al2o3 0 25 0 5 tio2 0 05 0 09 cao 0 5 0 8 cr2o3 0 15 0 5 and mno 0 02 0 15 vanadium is present within a single olivine and within all blue olivines al ca and ti are strongly positively correlated as are mn fe and cr in olivine surrounding the blue the blue cores are not zoned but each element shows a marked change at the boundary of the blue with al showing the most rapid change these are interpreted as diffusion profiles between rim and core olivine textures suggest initial free growth probably from a gas and later addition of olivine by liquid crystallization to form single crystals or chondrules the unusual olivine composition indicates high temperature growth from a refractory rich reservoir with al entering olivine in tetrahedral coordination vapor growth is suggested as the process allowing the high minor element levels the occurrence of blue olivine in all primitive meteorites indicates that it is relic material which was widespread prior to chondrule and hence meteorite formation similarities in composition exist between this relic olivine and olivine of cosmic dust and deep sea particles pointing to this olivine being a common component in all primitive extraterrestrial material

meteorites evidence for the interrelationships of materials in the solar system of 4 55 ga ago abstract members of the five major chemical groups of chondrites are samples of rocks which have remained essentially unaltered for 4 55 ga the ordinary chondrites are often regarded as primitive in both chemical and temporal senses however the presence of noritic and anorthositic inclusions in a few ordinary chondrites of great age indicates that at least some unequilibrated ordinary chondrites are not primitive in a temporal sense the eucrites are differentiated basaltic rock types which have ages indistinguishable from those of the oldest chondrites members of three groups of iron meteorites too appear to be equally old it is concluded that the period of formation of ordinary chondrites continued until after differentiation had occurred within some planetary bodies such early melting may have been caused by the decay of 26 al some of the differentiated materials were probably incorporated as plastic rocks liquid chondrules and solid clasts into the accreting parent bodies of the ordinary chondrites

coastal ecosystem responses to late stage deccan trap volcanism the post k t boundary danian palynofacies of mumbai bombay west india abstract the deccan trap continental flood basalt eruptions of india occurred c 67 63 ma thus spanning the cretaceous tertiary boundary 65 ma deccan eruptions were coeval with an interval of profound global environmental and climatic changes and widespread extinctions and this timing has sparked controversy regarding the relative influence of deccan volcanism upon end cretaceous catastrophic events if deccan trap activity was capable of affecting global ecosystems evidence should be present in proximal indian sedimentary facies and their palaeontological contents the impact of late stage deccan volcanism upon biota inhabiting mumbai bombay island s post k t boundary lagoonal systems is documented here sediments or intertrappeans which accumulated within these lagoons are preserved between trap lavas that characterise the closing stages of this flood basalt episode mumbai island formation intertrappean faunal and floral communities are conspicuously distinct from those common to many pre k t boundary late maastrichtian intertrappeans across the deccan province the latter sedimentary intercalations mostly developed in cognate semiarid palustrine ecosystems by contrast those around mumbai evolved in sheltered peripheral marine settings within subsiding continental margin basins unique to this late deccan stage and under an increasingly humid danian climate geochemical analyses reveal that mumbai sedimentation and diagenesis were intimately related to local explosive volcanic and regional intrusive activity at c 65 63 ma although tectonic and igneous events imprinted their signatures throughout these sedimentary formations organisms usually sensitive to environmental perturbations including frogs and turtles thrived critically palynofacies data demonstrate that whilst plant material deposition was responsive to environmental shifts there were no palpable declines in floral productivity following mumbai pyroclastic discharges therefore it is implausible that this late stage explosive volcanism influenced major ecosystem collapses globally

stable carbon and oxygen isotope compositions of invertebrate carbonate shells and the reconstruction of paleotemperatures and paleosalinities a case study of the early pleistocene of rhodes greece abstract the coastal sediments of rhodes in the eastern mediterranean have recorded transgression regression cycles that took place during the early pleistocene the sedimentary deposits from the kritika member of the rhodes formation consist in conglomerates sandstones siltstones and clays deposited in brackish to shallow marine environments faunal associations are dominated by molluscs and reveal rapid ecological changes carbon and oxygen isotope ratios of aquatic skeletal carbonates show that these ecological changes were most likely driven by large salinity changes while water temperature remained rather constant at about 22 0 1 5 c the tectonic activity of the island rather than glacio eustatic variations of climatic origin is advocated to be responsible for the ecological and salinity changes and sea level variations recorded in the sedimentary sequence

tholeiitic dykes in the seychelles and the original spatial extent of the deccan we report geochemical data from c deccan age tholeiitic dykes from praslin island in the seychelles a continental fragment which lay adjacent to the coast of india before sea floor spreading started in the arabian sea at 64 ma the dykes have compositions very similar to the distinctive bushe magma type from the deccan and hence constitute the first traces of deccan activity recognized on the seychelles fitting the seychelles and india back together to reconstruct the original extent of the deccan we find that the present estimate of 1 million km 2 needs to be revised upwards by c 25 to account for the deccan magmatism in the seychelles if dipping reflectors found on seismic lines to the northwest of the seychelles are also deccan age flows this would increase still further the area and volume estimates

paleomagnetic constraints on the structure of the deccan traps abstract new paleomagnetic results from two widely separated areas of the deccan traps allow one to test the simple normal reversed normal nrn magnetostratigraphic model previously proposed the new data from the mandla region to the northeast of the main outcrop of the deccan traps together with earlier apparently conflicting results are all compatible with the nrn stratigraphy provided the flows are not strictly horizontal but have a synform antiform structure with 500 m amplitude and 150 km wavelength the structure parallels the paleozoic narmada son rift system with slopes never exceeding 0 5 sampling in rajahmundry a remote southeastern outlier of the deccan with well constrained age allowed identification of the rn boundary in an area where only normal polarities had been reported it has also been possible to correlate the detailed geochemical stratigraphy in the western ghats with the nrn magnetic stratigraphy a compilation of 35 magnetic studies together with the above findings confirms the general applicability of the nrn model and reveals the large scale topography of the rn boundary which is an isochron over most of the deccan the rn isochron surface displays a boomerang shape with topography ranging from sea level to above 1600 m this observed topography is consistent with a simple model in which doming and renewed rifting and dyke intrusion resulted from impingement 70 65 ma ago of the reunion plume under the indian lithosphere the narmada son rift was rejuvenated and a continental rift system with minor extension with a triple junction in the cambay graben resulted and was subsequently covered by the deccan flows although in places tectonic faulting and volcanism continued to interfere the rajahmundry outcrops constrain the original extent and southeastward dip away from the triple junction of much of the traps a significant part of which has now been removed by erosion and rifting away of the seychelles

a geological map of the southern deccan traps india and its structural implications a geological map of an area of over 125 000 km 2 within the southern deccan traps india is presented which is based upon variations in the basalt geochemistry it represents a synthesis of new data and earlier work the use of large data sets has allowed stratigraphic boundaries to be established with greater accuracy over a much larger area than before the geochemical sections demonstrate that the southward overstepping stratigraphic style encountered along the ghats ridge has an essentially north to south polarity and that there appears to be detectable overstep in an east to west sense however the stratigraphy defines a pronounced coastward dipping monoclinal flexure across the western ghats ridge between 16 n and 18 n the data are consistent with two models of eruption the first considers the deccan eruptive focus to have been fixed throughout the volcanic episode producing an essentially annular flow pattern the second considers the flow pattern to have been generated from a southerly migrating eruptive centre resulting from the northerly drift of india over a fixed hotspot both hypotheses are consistent with recent quantitative models which suggest that rapid and voluminous flood basalt eruptions are the result of continental rifting events associated with plume generated thermal 9highs9 until a causal link between established plume hotspot trails and impact sites can be satisfactorily demonstrated we find it unnecessary to invoke a cataclysmic impact event as an explanation for the deccan eruptions

upb systematics of phosphates from equilibrated ordinary chondrites u pb systematics were determined from fifteen phosphate separates from equilibrated ordinary chondrites and from small bulk fragments of the same meteorites the high 238u204pb ratios of thirteen of these phosphate separates lead to extremely radiogenic pb whose 206pb204pb ratios range from 250 up to 3500 the pb pb model ages for these phosphates range from 4 563 to 4 502 ga with an analytical precision of 106 y and the u pb system is apparently concordant the time interval observed 60 106 y reflects the thermal processing of the equilibrated chondrites and is consistent with that previously derived from the rb sr k ar and pu chronologies the pb pb ages of the phosphates from the seven h chondrites show a negative correlation versus their metamorphic grade this is the first clear relationship ever observed between a long lived chronometer and the intensity of metamorphism as reflected by metamorphic grade assuming that the pb pb age indicates the accurate u pb closure time in phosphates the pb pb chronology is compatible with the model of a layered h chondrite parent body however this interpretation of the u pb systematics is not unique it postulates a slow cooling of the equilibrated materials at high temperature in apparent conflict with petrological observations except for the h chondrites which agree rather well with pu systematics comparison of the pb pb chronology with published radiochronometric data does not reveal simple correlations in the present debate concerning the thermal history of chondrites the chronometric information derived from each isotope system is interpreted as the time of its thermal closure however this basic assumption may not be correct for all the radiochronologies and must be evaluated before the radiochronometric data can be applied as compelling time constraints for the period of 4 56 4 4 ga of proto planetary history

unshocked equilibrated h chondrites a common low temperature record from orthopyroxene iron magnesium ordering a suite of eight unshocked equilibrated h chondrites ranging from petrographic type 4 to 6 la villa raguli fro 90076 miami fro 90049 estacado fro 90156 and acfer 314 was selected for a systematic study of orthopyroxene cation ordering temperatures the closure temperatures of the fe mg exchange lie within the 384 48 c to 480 28 c range regardless of the petrographic type data suggests that metamorphosed h chondrites share a similar thermal evolution close to 380 480 c indeed this indicates that h4 h5 and h6 chondrites were located in petrogenetic environments characterized by similar temperature time conditions when cooling through the above temperature interval

petrogenesis of rhyolites and trachytes from the deccan trap sr nd and pb isotope and trace element evidence trachytes and rhyolites from salsette island north of bombay have distinctive trace element and isotope features which mark them out from typical crustal melts their highly incompatible trace element and sr nd and pb isotope ratios are similar to those of the associated deccan flood basalts thus the rhyolites and trachytes are closely related to the basalts and a striking compositional gap between 50 and 65 sio2 suggests that the high sio2 rocks evolved by 10 15 partial melting followed by variable amounts of fractional crystallisation the source material could have been basalt within the deccan trap or related gabbroic rocks in deep crustal sill complexes the rhyolites yield an rb sr whole rock age of 61 5 1 9 ma with a slightly high initial 87sr 86sr 0 7085 18 it is argued that crustal extension provides a suitable regime for the generation of acid magmas by partial melting of associated basic rocks

a possible k t boundary bolide impact site offshore near bombay and triggering of rapid deccan volcanism abstract the temporal coincidence of a major biological mass extinction including dinosaurs the well known iridium excess anomaly at the cretaceous tertiary k t boundary and the eruption of deccan flood basalts at about 65 ma has aroused global interest among geologists and biologists it is widely debated whether the mass extinction and iridium anomaly are due to an asteroid impact or the massive outpouring of extensive deccan volcanism an oval shaped unusual positive gravity anomaly 10 000 km 2 in area near bombay has attracted our attention during a search for an impact site near deccan basalts a detailed gravity interpretation indicates the presence of a fossil conduit structure of 12 km height extending from a shallow crust mantle boundary at 18 km to an approximate depth of 6 km from the surface the conduit structure with a maximum diameter of about 35 km at its base may originate from cracking of a weak pre deccan trap shallow upwarped mantle the structure may have been caused by a bolide impact which triggered the eruption of massive flood basalts deccan traps on the western margin of the fast moving indian plate an impact in this locality can explain the sudden detachment of the arcuate seychelles block from india as well as the large scale reorganisation of plate boundaries in the indian ocean our hypothesis of impact triggered volcanism at 65 ma advocates a bimodal cause for the mass extinction at the k t boundary extraordinary geothermal and structural conditions of the nearby region are also discussed as circumstantial evidence to support the twin cause mechanism by weakened features and the presence of partial melt at subcrustal depth

tertiary palaeosurfaces of the sw deccan western india implications for passive margin uplift two genetically distinct lateritized palaeosurfaces of different ages are recognized in the southwest deccan traps region of western india using a combination of geochemical topographical and satellite image data the deccan traps were erupted at the cretaceous tertiary boundary c 65 ma and comprise a huge area of originally near horizontal basalt lavas covering much of northwest peninsular india and topographically forming the coast parallel western ghats escarpment and elevated maharashtra plateau to the east remnants of the older palaeosurface currently exist as a series of isolated laterite capped plateaux forming the highest elevations along the western ghats 15 30 18 15 n this surface is of late cretaceous early tertiary age and originally developed upon flows which lay at or near to the top of the lava sequence this lateritization phase was terminated by a period of uplift and extensive erosion in lower to mid tertiary times during which the low lying low relief coastal konkan plain developed through the eastward recession of the ghats scarpline a second phase of lateritisation occurred upon this coastal pediplain during mid to late tertiary times since the earlier uplift had gently deformed the lava pile prior to the development of the pedimented surface the low level konkian laterite lies with marked angular unconformity upon the lava stratigraphy both surfaces have been subject to further large scale distortion resulting from continuing uplift effects development and evolution of these deccan palaeosurfaces is important since together they provide a record of uplift effects in western india moreover they offer a datum against which the uplift erosional history may be further constrained and demonstrate that uplift effects have acted upon the indian margin throughout the tertiary since such longevity of uplift is difficult to reconcile with the commonly cited thermal and dynamic post rift mechanisms known to act upon passive margins the morphological and structural evolution of the rifted deccan margin is better described in terms of denudational isostasy

early diagenesis of marine organic matter alteration of the carbon isotopic composition in two distal carbonate turbidites labelled a and e from the madeira abyssal plain oxidation of organic matter om has been accompanied by a reduction in hydrogen index hi and change in the isotopic composition of carbon since the emplacement of turbidite 0 5 ka ago 35 of the om in the upper part has been oxidised by oxygen and nitrate diffusing from sea water into the sediment there is an accompanying decrease of 0 3 in 13c om and a decrease of 25 in hi turbidite e was emplaced about 73 ka and was exposed to bottom water for 12 3 ka before being buried beneath later turbidites during exposure oxygen and nitrate diffused into its upper part and oxidised 50 of the toc without an accompanying change in isotopic composition further oxidation of the more refactory organic components decreased toc by a further 25 and decreased 13c by 2 5 oxidation also reduced hi by 50 from originally low values unless corrected for the isotopic changes accompanying extended oxic and suboxic degradation of organic matter in sediments will give rise to erroneous estimates of marine terrestrial om ratios

the tectonic evolution of western india and its impact on hydrocarbon occurrences an overview abstract the largest hydrocarbon accumulations of india were localized in western india by a unique convergence of tectonic events mesozoic rifting along the structural trends of proterozoic mobile belts formed the passive margin basins of the indian coast during the campanian rifting of madagascar from india north south striking normal faults propagated into the area of the cambay graben late maastrichtian doming over the incipient deccan reunion hotspot imparted extensional stresses to the northwestern indian coast and formed the fault block that became the bombay high after eruption of the deccan flood basalts in early paleocene extension continued in the weakened crust this resulted in enhanced subsidence of the cambay graben and surat danahu depression rifting of the seychelles microcontinent from india and reactivation of normal faults on the emergent bombay high the cambay graben and the surat danahu depression filled with organic rich source shales as they underwent thermal subsidence shallow water eocene to miocene carbonates developed on the slowly subsiding bombay platform and sea level oscillations fostered secondary porosity development late tertiary maturation of the surat danahu depression shales generated hydrocarbons that migrated into the carbonate reservoirs on the bombay high konkan kerala and the older basins of the east coast of india were unaffected by the post campanian tectonic events and lack the favorable play elements that characterize the giant fields

a post k t boundary early palaeocene age for deccan type feeder dykes goa india basaltic dykes exposed along the coast of goa represent the youngest phase of a number of different mafic suites intruding the complex precambrian terrain that forms the pre deccan basement of peninsular india these dykes crop out 50 80 km beyond the southern limits of the deccan continental flood basalt cfb province four of seven sampled dykes were dated using the whole rock 40ar 39ar step heating technique three 40ar 39ar determinations yield palaeocene plateau and isochron ages that clearly indicate an early post cretaceous i e danian age and as a weighted mean average of all four samples yields an age of 62 8 0 2 ma these intrusions are clearly significantly younger than the current estimates for the age range of deccan traps volcanism 65 69 ma the trace element signatures rare earth element patterns and 87sr 86sr ratios of these dykes are consistent with those of the nearby deccan basalts more specifically their geochemical signatures are similar to those of the basalt flows composing the volumetrically important ambenali mahabaleshwar formations which form the uppermost succession wai subgroup of the sw deccan these geochemical and age data demonstrate that deccan type magmatism continued to affect western india for at least 1 2 my after the k t boundary 65 0 0 1 ma and therefore their emplacement places important constraints upon the timing duration and evolution of this much debated cfb province the importance of these dykes is examined within the context of the deccan eruptions and the associated rifting of the western indian margin

the panvel flexure along the western indian continental margin an extensional fault structure related to deccan magmatism abstract on the basis of field observations aided by remote sensing data a new model for the western indian panvel flexure is proposed the model involves normal motion on east dipping faults and westward tilting of fault blocks the flexuring occurred by attenuation and foundering of continental crust due to crustal thinning and loading by a dyke swarm following a period of massive magmatic discharge accompanying the continental break up and opening of indian ocean during early tertiary times

records of the influence of deccan volcanism on contemporary sedimentary environments in central india deccan volcanism with a duration of 5 myr represents a major continental flood basalt province in which 2 million km3 of lavas were formed such voluminous and rapid volcanism in phanerozoic earth history has commonly been advanced as a cause for mass extinctions despite this significance few studies have attempted to understand the influence of continental flood basalt volcanism on contemporary sedimentary environments thin sedimentary sequences associated with the deccan volcanics i e the infra and inter trappean sequences offer an opportunity to assess the influence of deccan volcanism on contemporary sedimentary environments in central india the lameta formation mainly constitutes a well characterised regolith regionally consisting of calcretes and palustrine facies which may in places be associated with ephemeral sandy braided river and sheetflood facies an assessment of the influence of deccan volcanism on contemporary surficial environments reveals a an input of basaltic degradational products in the basal fluvial facies of the lameta formation b upward changes in facies assemblages from lacustrine palustrine to subaerial dominated followed eventually by burial by the first lavas locally c significant changes in the palaeoflow characteristics of fluvial channel facies respectively under and overlying the main calcareous regolith of the lameta formation d that most freshwater lacustrine taxa survived the initial effects of the deccan volcanism without undergoing any drastic change in addition to the role of constructional topography resulting from lava emplacement and associated surface uplift the mode of preservation of sedimentary packets significantly affects changes in the vertical motifs of coeval sedimentary sequences because of the problems of time resolution on 105 106 year time scales in the continental infra and inter trappean sequences detailed cause volcanism effect surficial environments relationships at the event level are not well understood also the lack of data from multiple stratigraphic levels of the inter trappean beds precludes any serious assessment of the directions and rates of changes that took place in co2 concentrations of the palaeoatmospheres of that period 65 3 myr

biodiversity during the deccan volcanic eruptive episode this paper gives a detailed overview of biotic assemblages recovered from the deccan trap intercalated sedimentary sequences infra and intertrappean beds of peninsular india as a result of extensive research done during the last 20 years the infra and intertrappean beds contain remnants of gondwanan forms such as myobatrachinae frogs pelomedusid turtles dinosaurs i e titanosaurids and abelisaurids and mammals apart from these gondwanan elements the infra and intertrappean beds also contain forms of laurasian affinity though recently doubt has been cast on such relationships based on previous fossil records laurasiatic forms were considered to be represented by a great variety of micro and megavertebrate assemblages such as discoglossid and pelobatid frogs anguid lizards alligatorid crocodiles palaeoryctid mammals charophytes and ostracodes the biotic assemblages show a remarkable similarity between the infra and intertrappean beds indicating a short time period for the deposition of these deccan volcano sedimentary beds the recovered biotic assemblages strongly indicate a maastrichtian age for the initiation of deccan volcanic activity and the sedimentary beds associated with it the cretaceous tertiary boundary as such remains to be defined in any known sections in sedimentary sequences in so far investigated localities of peninsular india what have been identified are maastrichtian age beds in the east central and western narmada river region on the basis of pollens vertebrate assemblage and planktonic foraminiferas in infratrappean offshore sequences a palaeocene intertrappean bed at lalitpur uttar pradesh that is among those lacking dinosaurian remains but having palynological assemblages similar to those from well established palaeocene sequences suggests the presence of palaeocene intertrappeans but the k t boundary is yet to be properly defined