sone megafan a non himalayan megafan of craton origin on the southern margin of the middle ganga basin india abstract researchers in the last few decades have focused on the fluvial megafans at the base of himalayan foothills in the ganga basin no major effort has so far been made to delineate any such large depositional environment at the base of the northern indian peninsular craton at the distal parts of the basin in this work for the first time we document a megafan unusually created by the cratonic sone river in the marginal plains south of the ganga the geomorphology of the megafan surface distribution of palaeochannels sedimentology and areal extent of the megafan are described the study also reconstructs the tectonic and palaeoclimatic conditions which might have helped in sediment supply from the cratonic catchment areas of the sone we suggest tectonic control over the sone river channel dynamics and development of the megafan the oval and fan shaped craton derived sediment body covers an area of around 12 000 km2 in the marginal plains and about 9000 km2 in the central alluvium north of the ganga river the megafan is around 190 km long and 240 km wide the mean slope of the megafan surface is 0 03 an onlap of himalayan sediments has occurred over the megafan at its toe parts caused by southward encroachment of the ganga during the holocene the megafan sediments comprise brownish yellow fine to coarse sand and gravels with an admixture of carbonaceous nodules kankars the thickness of the megafan sediment which directly overlies the precambrian basement varies from only a few meters in the south near the peninsular craton to 1000 m at the north in the central alluvial plain the granular zones form prolific aquifer systems in an otherwise poor hydrogeological setting of the marginal plains

electrical conductivity of the continental crust geophysical measurements indicate that the earth s continental lower crust has a high electrical conductivity for which no simple cause has been found explanation usually relies on either saline fluids saturating the pores or interconnected highly conducting minerals such as graphite fe ti oxides and sulphides providing conducting pathways attempts in the laboratory to clarify the problem have hitherto been unable to recreate conditions likely to be present at depth by controlling the confining pressure and pore fluid pressure applied to a rock saturated with saline fluids at temperatures between 270 c and 1000 c here we report conductivity data obtained using a cell designed to make such measurements on rocks saturated with saline fluids our results show that the conductivity of saturated samples of acidic rocks is explicable entirely in terms of conduction through the pore fluid whereas the conductivity of saturated basic rocks requires the presence of an additional conduction mechanism s we have used the experimental data to construct electrical conductivity depth profiles for the continental crust which when compared with profiles obtained from magnetotelluric observations demonstrate that a mid to lower crust composed of amphibolite saturated with 0 5 m nacl shows electrical conductivities sufficient to explain conductivity depth profiles for the continental crust inferred from geophysical measurements

deep crustal electromagnetic structure of central india tectonic zone and its implications magnetotelluric data at 45 locations along the mahan khajuria kalan profile in the central india tectonic zone are analysed this 290 km long profile yields data in the period range 0 001 1000 s across the tectonic elements of the study region bounded by purna fault gavligarh fault tapti fault narmada south fault and narmada north fault multi site multi frequency analysis suggests n70 e as the geoelectric strike direction data rotated into the n70 e strike direction are modelled using a non linear conjugate gradient scheme with error floors of 10 for both apparent resistivity and phase components two dimensional magnetotelluric model yields conductors that correlate with known faults in the study region and regional seismicity presence of a 30 mgal gravity high together with the observed conductive bodies less than 20 ohm m in the deep crust beneath the purna graben and tapti valley is explained by the process of magmatic underplating the conductive bodies beneath the mahakoshal rift belt and vindhyans accompanied by regional gravity lows of the order 70 mgal are attributed to the presence of deep crustal fluids following the re activation model proposed for the entire region the conductors 20 ohm m at various depth levels correspond to mafic magmatic and or fluid intrusions controlled by deep seated faults that seem to tap reservoirs beyond the crust mantle boundary the shallow depth localized faults also seem to have facilitated further upward movement of these underplated material and fluids release during this process

diagenetic and catagenetic products of isorenieratene molecular indicators for photic zone anoxia a wide range of novel diagenetic and catagenetic products of the diaromatic carotenoid isorenieratene a pigment of the photosynthetic green sulphur bacteria chlorobiaceae has been identified in a number of sedimentary rocks ranging from ordovician to miocene compound identification is based on nmr mass spectrometry the presence of atropisomers and stable carbon isotopes atropisomers contain an axially chiral centre which in combination with other chiral centres results in two or more diastereomers that can be separated on a normal gc column chlorobiaceae use the reverse tca cycle to fix carbon so that their biomass is enriched in 13c high 13c contents of isorenieratene derivatives therefore support their inferred origins isorenieratene derivatives include c 40 c 33 and c 32 diaryl isoprenoids and short chain aryl isoprenoids with additional aromatic and or s containing rings c 33 and c 32 compounds are diagenetic products of c 33 and c 32 carotenoids formed from isorenieratene during early diagenesis through expulsion of toluene and m xylene respectively cyclisation of the polyene acyclic isoprenoid chain can proceed via an intramolecular diels alder reaction followed by aromatisation of the newly formed ring sulphurisation is also an important process during early diagenesis competing with expulsion and cyclisation sulphur bound isorenieratane is released during progressive diagenesis due to cleavage of relatively weak ss and cs bonds cleavage of c c bonds during aromatisation of newly formed rings and during catagenesis yields short chain compounds the inherent presence of a conjugated double bond system in carotenoids implies that similar diagenetic and catagenetic reactions can occur with all carotenoids chlorobiaceae live at or below the oxic anoxic boundary layer and require both light and h 2s the presence of isorenieratene or its diagenetic and catagenetic products in ancient sedimentary rocks and crude oils is therefore an excellent indication for photic zone anoxia in the depositional environment diagenetic and catagenetic products of isorenieratene are expected to find applications in reconstruction of palaeoenvironments and in oil oil and oil source rock correlation studies their presence in several petroleum source rocks suggests that anoxia is an important environmental parameter for the preservation of organic matter

electrical conductivity models for the continental crust based on laboratory measurements on high grade metamorphic rocks summary deep electrical conductivity soundings are increasingly being used as an additional source of information regarding the nature of the lower continental crust however a lack of relevant laboratory based conductivity measurements makes interpretation of such soundings difficult laboratory measurements have been made on saturated and unsaturated samples of possible lower crustal rock types subjected to confining pressures up to 0 4 gpa temperatures up to 300 c and with variable pore fluid pressure up to the confining pressure extrapolation of these results suggests that the surprisingly high conductivities deduced for depths of approximately 20 km in certain stable continental areas may result from a combination of basic rock type and high pore fluid pressures for whereas the conductivities measured in acid rock types can be explained in terms of conduction through the pore fluid alone the conductivities measured in basic rock types imply enhanced conduction presumably through the matrix or along grain boundaries the lower conductivities deduced from field experiments for the upper crust may be due to more acid rock types and or lower pore fluid pressures perhaps due to hydration reactions in areas where the high conductivity layer is coincident with a low velocity layer an explanation in terms of changing pore fluid pressure i e low to high with increasing depth seems more likely

palaeoceanography of lower cretaceous alpine platform carbonates shallow water carbonate platforms evolve in an interplay of local regional and global dynamics thereby generating a detailed record of environmental conditions platform sediments constitute important archives of past environmental change and as such they are increasingly analysed for the information they contain on wider scale patterns the integration of this information with palaeoceanographic data obtained from deeper water sediments provides a powerful tool to develop a more diverse and complete image of past environmental change in this contribution the remains of the northern tethyan carbonate platform of early cretaceous age cropping out in the western swiss jura and the helvetic alps are used to illustrate the advantage of a palaeoceanographic approach in their interpretation the tools applied are biostratigraphy and chemostratigraphy carbon and strontium isotopes in order to obtain a precise age model for correlation with hemi pelagic sediments in adjacent basins sequence stratigraphy as a means to reconstruct the impact of sea level change phosphorus stratigraphy as a proxy for trophic levels facies and microfacies analyses as a gateway to gain better insight into the carbonate factory and the detailed analysis of platform drowning unconformities as an indicator of wider scale palaeoenvironmental change it is shown that changes from periods of predominantly photozoan to predominantly heterozoan carbonate production near the berriasian valanginian boundary and middle early aptian coincided with substantial increases in phosphorus burial rates and major carbon isotope excursions furthermore the majority of the platform drowning episodes occurred just prior to and during episodes of major palaeoenvironmental change and last but not least the platform itself may have influenced the chemistry of adjacent basins and the carbon isotope composition of associated carbonates by the export of periplatform ooze

deep electrical imaging of the narmada tapti region central india from magnetotellurics abstract next to himalayas the narmada son lineament nsl zone is characterized tectonically as a highly disturbed zone in indian geology magnetotelluric mt study is carried out across nsl zone along a 190 km long n s trending traverse the present study region extends from balapur 20 40 34 in the south to andharwadi 22 20 49 in the north the 2d modeling results for shallow crustal section has brought out a moderately resistive layer 50 200 m representing the exposed deccan traps overlying almost similar resistive layer 20 100 m inferred to be subtrappean sediments underlain by high resistive basement the deccan trap thickness varies from around a few tens of meters to a maximum of 1 5 km along the traverse a subtrappean sedimentary basin like feature is delineated in the middle of the traverse with a thickness amounting to as much as 2 km the resistive 1000 m upper crust is relatively thin towards the southern and northern part of the traverse and tends to become thicker in the middle a moderately conductive 50 500 m layer is traced from mid crust towards south and at lower crust upper mantle towards north it is characterized by enhanced heat flow high density and high seismic velocity this layer is interpreted as a mafic intrusive layer presumably associated with magmatic underplating of the crust perhaps due to reported asthenospheric upwelling due to tectonic activity

chemostratigraphy of the posidonia black shale sw germany ii assessment of extent and persistence of photic zone anoxia using aryl isoprenoid distributions aquatic depositional environments where anoxic conditions extend from bottom waters into the photic zone occurred frequently during the geologic past periods of photic zone anoxia pza can be recognized by the presence of specific lipids produced exclusively by the green sulphur bacteria chlorobiaceae that are preserved in the sedimentary record chlorobiaceae perform anoxygenic photosynthesis that requires light penetration into h2s saturated waters sediment samples integrate paleoenvironmental conditions over time intervals ranging from at best a few decades to more commonly several millennia photic zone anoxia recognized in paleoenvironmental analyses is thus defined as episodic with no further interpretation on the duration and persistence of the anoxic events we provide here a high resolution multiproxy chemostratigraphy study for the toarcian posidonia black shale from the dotternhausen section in sw germany in order to assess duration and seasonal fluctuation of photic zone anoxia based on the relative abundance of derivatives of chlorobiaceae lipids we assess the variability in the degree and persistence of photic zone anoxia using an aryl isoprenoid ratio air obtained by calculating the proportion of the short chain c13 17 versus the intermediate chain c18 22 aryl isoprenoids higher relative abundance of the short chain analogues is interpreted as indicating more intensive aerobic degradation of aryl isoprenoids air varies between values of 0 5 indicative of persistent pza phases and 3 0 for short termed episodic pza events absolute concentration of aryl isoprenoids is negatively correlated with the air which is in agreement with a progressive diagenetic breakdown of aryl isoprenoids the air decreases are validated using independent palecological and geochemical redox indicators for the posidonia shale in sw germany d 2004 elsevier b v all rights reserved

grain boundary graphite in rocks and implications for high electrical conductivity in the lower crust the origin of zones of high electrical conductivity in the lower continental crust is a long standing mystery possible explanations include the presence of brines1 2 partial melt3 serpentine4 and graphite5 when discussing the occurrence of graphite in the crust many petrologists have considered phase relations as they would have existed at the peak of metamorphism or during igneous emplacement6 10 here we show that a fine film of graphite is present on the grain boundaries in three rocks from the laramie anorthosite complex in two of these rocks graphite was stable during igneous crystallization but in the other it was not we maintain that in all of the rocks the grain boundary graphite precipitated from a co2 rich fluid during cooling the chemical processes that produced the grain boundary graphite in these rocks are likely to operate in many lower crustal rocks we therefore contend that because the films we observe are capable of producing the high conductivity that is seen in the lower crust grain boundary graphite should be considered as a possible cause for some conductivity anomalies

crustal density structure across the central indian shear zone from gravity data abstract the central indian shear zone is a distinct tectonic feature in the central part of india that separates the central indian tectonic zone from the bastar craton the complete bouguer anomaly map of the region encompassing central indian shear zone is characterised by a broad relative gravity high over central indian tectonic zone as compared to the bastar craton the paired gravity anomaly across the two crustal domains signifies that the central indian shear zone is a locus of density discontinuity along which the two crustal domains accreted horizontal gravity gradient analysis further demonstrates the northward subduction of the bastar craton beneath the bundelkhand craton 2 d gravity modelling along mungwani rajnandgaon profile constrained by seoni kalimati seismic section delineates a thick crust beneath the bastar craton as compared to the central indian tectonic zone northward dipping contact of the two crustal domains when projected on the surface coincides with the central indian shear zone with a well defined moho offset and crustal density discontinuity central indian shear zone represents a suture zone that separates the bastar craton from the central indian tectonic zone a low velocity 6 4 km s density 2 90 g cm 3 layer at the base of the crust and relatively lower density 3 21 g cm 3 subcrustal mantle may be the imprint of thermal remobilization beneath the central indian tectonic zone

reactivated basement structures affecting the sedimentary facies in a tectonically quiescent epicontinental basin an example from nw switzerland abstract the jurassic deposits in the southern part of central europe accumulated in a shallow epicontinental sea and their deposition has usually been believed to have corresponded to a phase of tectonic quiescence neither on conventional seismic records nor in outcrops obvious indications of synsedimentary tectonics were found nonetheless subtle variations in lithofacies and thickness occur above faults in the crystalline basement in fact preexisting structures became reactivated in the jurassic during major extensional phases when the tethys and the north atlantic opened this reactivation led to differential subsidence and or to rotation of fault bounded blocks but the sediments were deformed mainly flexurally as vertical movements in the basement were dissipated by triassic salt thus the depositional area was morphologically differentiated into swells and depressions in siliciclastic muddy environments swells were characterized by hardbottoms and hiatus beds depressions were filled by distal tempestites and gravity deposits in carbonate settings reefs nucleated on swells marls and muds were deposited in depressions many of them gravity deposits reactivation of faults occurred only during short episodes and was not synchronous throughout the study area and reactivation of individual faults was episodic and probably controlled by the palaeostress field and the faults orientation reactivation of deep rooted faults is also documented by hydrothermal activity which led to vein mineralization and alteration of minerals today exposed in nearby basement units the chronostratigraphic ages of the hydrothermal processes coincide with phases of enhanced subsidence during the sinemurian aalenian bajocian bathonian and oxfordian in turn facies changes of the sedimentary cover should be useful to predict basement structures when no seismic records are available

tidal circulation in an ancient epicontinental sea the early jurassic laurasian seaway we model tides and associated bed shear stress in the early jurassic laurasian seaway of northwest europe sensitivity tests with different water depths highlight those regions that are least affected by paleobathymetric uncertainty results show that although the vast seaway was largely microtidal the tides were still capable of affecting sediment transport more than 2000 km from the open ocean flow constriction associated with shallow platforms and straits produced elevated bed shear stresses a direct proxy for the entrainment and transport of sediment that were decoupled from tidal range and were capable of transporting sand areas of increased bed shear stress broadly correlate with published geological data varying the water depth in the basin provides an insight into the effects of an idealized transgression increasing the water level leads to a net reduction in the bed shear stress at the shelf edge but promotes further penetration of tidal energy into the basin interior although it is still focused into areas of flow constriction further drowning of the seaway eventually widens the straits to the point that they fail to constrict the flow and bed shear stress is reduced increasing water level counterintuitively leads to an increase in bed shear stress in a small number of locations

mapping of tectonic corridors through hidden parts of the greater dharwar terrane southern india abstract the dharwar craton india has been divided into western and eastern domains by a tectonic boundary between the closepet granite and chitradurga schist belt it has been noted that only the central portion of the dharwar craton is exposed its northern projection is buried beneath the deccan traps and its southern margin merges with a high grade metamorphic transition of late archean age resolution of structure and extent of the greater dharwar terrane gdt beneath its northern and southern end the deccan traps metamorphic transition zone and cuddapah basin thus remains unsolved results of deep geophysical probings have been examined along with associated geological and tectonic data the analysis leads to a delineation of palaeo accretionary corridors extending under the deccan trap cover along which two damaging stable continental region earthquakes koyna and latur occurred veeraswamy and raval 2005 b it is seen that the latur earthquake seem to lies within an unsuspected tectonic corridor which bifurcates the eastern dharwar craton edc into eastern dharwar craton 1 and eastern dharwar craton 2 passing through the cuddapah basin c possibility of the palghat cauvery shear zone connecting to the 2 5 ga metamorphic karimnagar granulite belt through the interface between napier nc and rayner rc complexes of east antarctica and d the indication that the bavali moyar bhavani mettur and palghat cauvery shear zones form the southern margins of the western dharwar craton eastern dharwar craton 1 and eastern dharwar craton 2 respectively

hydrogen sulphide poisoning of shallow seas following the end triassic extinction the end of the triassic period was marked by a mass extinction biomarkers in black shales that formed at the time suggest that the repeated poisoning of shallow seas by hydrogen sulphide delayed the early jurassic recovery

c16 c29 homologous series of monomethylalkanes in the pyrolysis products of a holocene microbial mat preparative open pyrolysis products of kerogen isolated from a holocene microbial mat collected in abu dhabi united arab emirates contain homologous series of monomethylalkanes mmas c16 c29 the isomer and carbon number distributions of these branched alkanes closely match those of mmas in ancient sediments and oils it is proposed that monomethylalkanes in oils and in thermally mature ancient sediments derive from non extractable organic matter produced by one or more members of the cyanobacterial assemblage eubacterial primary producers and or heterotrophic bacteria 2000 elsevier science ltd all rights reserved

molecular organic matter in speleothems and its potential as an environmental proxy abstract organic matter preserved in speleothems has considerable potential to record changes in the surrounding environment particularly in the overlying vegetation here we review three types of organic matter analysis relevant to speleothems organic fluorescence lipid biomarker analysis and amino acid racemisation organic matter luminescence provides a useful non destructive and rapid method for assessing dissolved organic matter quantity and quality while biomarker analysis amino acids and lipids has the potential to provide a more detailed signal related to specific parts of the surrounding ecosystem such as the dominant vegetation regime and bacterial activity amino acid analysis has yet to prove demonstrably useful in stalagmites due to the inability to characterise the sources of proteinaceous matter however the small but increasing body of work on lipid biomarker analysis in stalagmites has shown that a wide variety of recognisable biomarkers are preserved over long periods of time 100 ka can be recovered at temporal resolutions of

terminal proterozoic mid shelf benthic microbial mats in the centralian superbasin and their environmental significance abstract a combined sedimentological and biogeochemical study has been conducted on several terminal proterozoic mid shelf microbial mat facies from the centralian super basin isotopic and organic geochemical analysis of the bitumen and kerogen indicated that two sources of organic matter from planktonic and benthic microbial mat populations contributed to the sediment the planktonic source provided a suite of n alkanes with c20 whereas the benthic source contributed an overlay of n alkanes c20 with a strong even preference together with mid chain methyl alkanes kerogen and biomarkers derived from the microbial mat were found to be depleted in 13c relative to planktonic material pyrite in the micorbial mats was also found to be depleted in 34s compared to surrounding facies the combination of these observations suggested that the mats may have been at least partly composed of sulfide oxidising bacteria these organisms have specific environmental tolerances that set limits on palaeo environment their requirement for oxygen indicates that the water column above the mid shelf could not have been anoxic accordingly from the results and age determinations reported here it would appear that mid shelf environments of the centralian superbasin of australia were seeing significant levels of oxygen through the ediacarian

magnetotelluric investigations in tuwa godhra region gujarat india magnetotelluric mt data have been acquired at 40 locations in tuwa and its surrounding region 200 km east of ahmedabad and 15 km north northwest of godhra in the mainland gujarat with an average station spacing of 1 5 km mt impedance tensors have been estimated in the period range of 0 001 100 s the data have been modeled using non linear conjugate gradient scheme taking both apparent resistivity and phase into account from the 2d models of the mt data the weathered granite with quaternary sediments with resistivity of 700 m have been inferred up to a depth of 500 m followed by godhra granite having resistivity up to 105 m with a thickness 6 5 km the aravalli supergroup has been inferred below godhra granite the lunavada group of rocks have been inferred in the eastern part of the study area having resistivity value ranging from 103 to 104 m separated from the godhra granite by a contact zone the comparatively very low resistivity rocks 400 m of udaipur formation followed by paleoproterozoic carbonate rocks with fluid have been inferred below 8 10 km depth the percolation of water from the surface through the contact zone of lunavada and champaner groups has been suggested the presence of hot water springs in 10 km sw from the center of the study area at the contact zone of godhra granite and basalt might be due to the western trending lithostratigraphic slope hydrostatic pressure generated due to heat produced from interaction of water with the carbonate rocks at deeper depth and high subsurface temperature due to high geothermal gradient the segmented nature of himmatnagar fault hnf is identified in the central portion of the study area

carbon isotopic compositions of prokaryotic lipids as tracers of carbon cycling in diverse settings recent work has highlighted the ubiquity of bacteria and archaea and continues to affirm their significance in diverse biogeochemical processes the 13c values and distributions of diagnostic lipids derived from such microorganisms can provide insights into prokaryotic processes in modern settings and are one of the few means by which past prokaryotic processes in depositional settings can be elucidated here we present a brief review of prokaryotic lipids commonly observed in sediments and the controls on their carbon isotopic compositions we then present two case studies one of mediterranean cold seeps at which anaerobic oxidation of methane occurs and one of a holocene peat deposit in which a variety of aerobic and anaerobic microorganisms govern the degradation of organic matter these two studies illustrate the potential utility of compound specific carbon isotope analyses to deconvolute pathways of carbon flow in microbial communities

electrical structure of the crust below the deccan flood basalts india inferred from magnetotelluric soundings summary the indian shield has experienced major stable continental region scr earthquakes in the recent past which occurred in the deccan volcanic province until now the deeper crust below the deccan traps where thick basaltic layers cover large areas of the indian shield remains poorly understood a magnetotelluric mt study covering a broad period range was conducted along a n s running profile of 330 km length to achieve more insights into the nature of the crustal electrical structure below the flood basalts after dimensionality analysis and decomposition 2 d inversions for te and tm modes were carried out the basalt cover characterized by at first glance surprisingly low electrical resistivities is found to have an average thickness of 400 m with the exception of central parts of the profile where it reaches up to 700 m the crust is in general highly resistive but several subvertical zones of enhanced conductivity were delineated in the middle to lower crust which are tentatively explained as images of hidden partly reactivated faults fractures of the precambrian basement sensitivity analysis indicates a high resolution of these features an alternative approach incorporating macro anisotropy is additionally considered and corresponding 2 d anisotropic forward modelling confirms the existence of successions of conductive dykes in the deep crust

magnetotelluric studies in the central india tectonic zone implications for intraplate stress regimes and generation of shallow earthquakes abstract the central india tectonic zone citz dissects the indian peninsula into the northern and southern crustal blocks the citz has been a seismically active region since the precambrian whereas the relatively deep crustal earthquakes near the narmada faults in the eastern part of the citz have been well investigated the mechanisms for the shallow earthquakes in the western part remain unknown here we present results from a new magnetotelluric study to derive the crustal structure and to understand its implications our data show a thick and highly resistive 500 ohm m crust in the south of tapti river as against a less resistive one in the north these results in conjunction with heat flow values indicate that the crust below the southern part has stable continental cratonic signatures on the northern side of the tapti river we infer the ascent of basaltic magmas from the mantle into the shallow crust and crystallization into layered intrusions these mafic ultramafic bodies could be a potential cause for the shallow earthquakes in the western part of the citz the mafic ultramafic bodies below the crust of citz would locally modify the intraplate stresses which in turn would facilitate the occurrence of earthquakes due to reactivation of pre existing faults thus the large accumulation of strain energy in the deep crust beneath the region can be attributed to the presence of high stress bodies emplaced at depth during the deccan volcanic activity

molecular and isotopic characterization of biomarkers in the frick swiss jura sediments a palaeoenvironmental reconstruction on the northern tethys margin abstract molecular and stable carbon isotope compositions of source specific hydrocarbons have been used to reconstruct palaeoenvironmental conditions during deposition of the middle hettangian to upper sinemurian sediments on the northern epicontinental tethys margin frick swiss jura increasing algal cyanobacterial and phytoplanktonic i e dinoflagellate contributions associated with the 13 c enrichment of cyanobacteria derivatives i e hopanes and monomethylalkanes suggest enhanced primary productivity upsection this is related to the 13 c enrichment of dissolved co 2 in the upper layers and the progressive increase of depth and oxygenation of the water column in the middle hettangian shallow water environments lagoon the occurrence of green sulfur bacteria chlorobiaceae derivatives indicates that the lower part of the water column was strictly anoxic and rich in h 2 s since these bacteria require very low light intensity to grow these euxinic conditions may be extended up to the photic zone allowing for anaerobic photosynthesis light penetration depth is most likely reduced by high productivity and or turbidity in the photic zone in these sediments 13 c depleted hopanoids 39 5 are most likely associated with phototrophic purple sulfur bacteria utilizing isotopically light organic carbon at the base of the aerobic zone these purple sulfur bacteria may have consumed the h 2 s used by chlorobiaceae in the deeper layers and thus sustained the algae and cyanobacteria productivity in the upper layers the 13 c depleted carbonate 13 3 may be partially related to the anaerobic oxidation of the organic matter during bacterial sulfate reduction

geochemical study of macromolecular organic matter from sulfur rich sediments of evaporitic origin messinian of sicily by chemical degradations abstract selective chemical degradations have been performed on sulfur rich macromolecular organic matter from sediments of the messinian of sicily upper miocene italy the biomarkers released upon different desulfurization experiments are much more abundant than the free lipids and display completely different distributions which reflect the selective trapping of poly functionalized molecules by inorganic sulfur species at an early stage of diagenesis the bound biomarker distributions reveal a dominant phytoplanktonic input originating from an upper oxic water layer and a moderate bacterial reworking of the organic matter a contribution from cyanobacteria to the organic matter cannot be excluded the presence of carotenoids typical from photosynthetic sulfur bacteria attests to a stratified water body where an anoxic photic zone occurred lialh4 hydrogenolysis of sulfur rich macromolecules revealed that polysulfide bridges contribute to a large extent in the cross linking the results of deuterium labelling experiments are discussed in terms of sulfur incorporation on functionalized lipids

electrical imaging of narmada son lineament zone central india from magnetotellurics the narmada son lineament nsl zone is the second most important tectonic feature after himalayas in the indian geology magnetotelluric mt studies were carried out in the nsl zone along a 130 km long nne ssw trending profile the area of investigation extends from edlabad 20 46 16 7 5 59 05 in the south to khandwa 21 53 51 7 6 18 05 i n the north the data shows in general the validity of a two dimensional 2d approach besides providing details on the shallow crustal section the 2d modeling results resolved four high conductive zones extending from the middle to deep crust spatially coinciding with the major structural features in the area namely the gavligarh tapti barwani sukta and narmada south faults the model for the shallow section has brought out a moderately resistive layer 30 150 m representing the exposed deccan trap layer overlying a conductive layer 10 30 m inferred to be the subtrappean gondwana sediments the latter resting on a high resistive basement upper crust the deccan trap thickness varies from around a few hundred meters to as much as 1 5 km along the traverse a subtrappean sedimentary basin like feature is delineated in the northern half of the traverse where a sudden thickening of subtrappean sediments amounting to as much as 2 km is noticed the high resistive upper crust is relatively thick towards the southern end and tends to become thinner towards the middle and northern part of the traverse the lower crustal segment is conductive over a major part of the profile considering the generally enhanced heat flow values in the nsl region coupled with characteristic gravity highs and enhanced seismic velocities coinciding with the mid to lower crustal conductors delineated from mt presence of zones of high density mafic bodies intrusives with fluids presumably associated with magmatic underplating of the crust in the zone of major tectonic faults in nsl region are inferred 2004 elsevier b v all rights reserved

occurrence and origin of mono di and trimethylalkanes in modern and holocene cyanobacterial mats from abu dhabi united arab emirates n alkanes highly branched isoprenoids monomethylalkanes mmas dimethylalkanes dmas and trimethylalkanes tmas are the most abundant components in the hydrocarbon fractions of extracts of four modern and two holocene cyanobacterial mats t1500 and 5110 170 y bp collected in abu dhabi united arab emirates the homologous families of mmas dmas and tmas were identified by comparison of mass spectral and relative retention time data with published data dmas were also identified by synthesis of authentic standards 3 9 dimethyltricosane 5 9 dimethyltricosane and 11 15 dimethylheptacosane mmas dmas and tmas of the cyanobacterial mats can be separated into two groups on the basis of their distribution patterns and structures mmas and dmas in the c16 c22 range are characterized by methyl substituents mainly located at c 6 or 6 and c 7 or 7 and are derived from cyanobacteria they are relatively abundant components in the modern cyanobacterial mats but with increasing age of the mats they become much less abundant on the contrary mmas dmas and tmas in the c24 c45 range are exclusively found in the holocene cyanobacterial mats their longest chains mainly contain an odd number of carbon atoms and they always carry the methyl substituents at odd numbered carbon atoms the similarity in composition of this very specific group of branched alkanes with that encountered in insect epicuticular waxes suggests that these sedimentary hydrocarbons originate from insects which probably grazed on the cyanobacterial mats

possible algal origin of long chain odd n alkanes in immature sediments as revealed by distributions and carbon isotope ratios a pliocene oil shale pula hungary a c3 plant triticum aestivum and a c4 plant zea mays were compared using isotopic composition of bulk organic matter along with distributions and individual carbon isotope ratios of n alkanes from organic extracts the microalga botryococcus braunii a race was thus shown to be the main source of the predominant 27 29 and 31 n alkanes of pula sediment therefore the dominance of odd carbon numbered n alkanes in the range c25 35 in extracts from immature sediments shall not be systematically assigned to higher plant contribution but algal input is also possible in fact the long chain n alkanes with an odd predominance previously observed in extracts of various immature sediments are likely to be derived at least partially from algae

paleoreconstruction by biological markers abstract during diagenesis and conversion of the original lipid fraction of biological systems to petroleum hydrocarbons the following four basic events needed for paleoreconstruction can be monitored by biological markets 1 sourcing 2 maturation 3 migration and 4 biodegradation actual cases of applying biological markers to petroleum exploration problems in different parts of the world are demonstrated cretaceous and phosphoria sourced oils in the wyoming thrust belt can clearly be distinguished from one another by high quality source fingerprinting of biomarker terpanes using gas chromatography mass spectrometry identification of recently discovered biological markers head to head isoprenoids allows subtle source differentiation between oils from sumatra the degree of crude oil maturation in basins from california alaska russia wyoming and louisiana can be assessed by specific novel biomarker ratios 20s 20r sterane epimers field evidence from such interpretation is confirmed by laboratory pyrolysis of the rock an example of immature sumatra rock is given extensive migration is documented by biomarkers in several oils the effect of biodegradation on biomarker steranes and terpanes is exemlified in a california basin all biological marker results are consistent with the geological setting and add a new dimension in assisting the petroleum explorationist toward paleo reconstruction

evidence from borehole samples for the role of accessory minerals in lower crustal conductivity the high electrical conductivity of the lower continental crust as inferred from geophysical measurements1 has defied a simple explanation both pore saturating brines2 and interconnected carbon films3 5 have been proposed as conducting pathways but their relative importance remains uncertain and may vary from place to place6 so far attempts to address this question in the laboratory have had to rely on samples of metamorphic rock collected from surface exposures7 10 although these rocks almost certainly originated in the lower crust chemical alteration during their transport to and residence at the surface is likely to have affected their conducting properties here we report conductivity data for pristine crustal rocks recovered from depths of up to 4 6 km in the ktb boreholes in southern germany our results show that the electrical conductivity of accessory phases such as fe ti oxides and sulphides can enhance or even surpass the high conductivity produced by saline fluids

how quick was marine recovery after the end triassic mass extinction and what role did anoxia play oxygen restricted conditions were widespread in european shelf seas after the end triassic mass extinction event and they are reported to have hindered the recovery of marine benthos here we reconstruct the redox history of the early jurassic blue lias formation of southwest britain using pyrite framboid size analysis and compare this with the recovery of bivalves based on field and museum collections results suggest widespread dysoxia punctuated by periods of anoxia in the region with the latter developing frequently in deeper water settings despite these harsh conditions initial benthic recovery occurred rapidly in the british jurassic especially in shallowest settings and shows no relationship with the intensity of dysoxia a stable diversity was reached by the first recognised ammonite zone after the end triassic mass extinction this contrasts with the deeper water more oxygen poor sections where the diversity increase was still continuing in the earliest sinemurian stage considerably longer than previously reported similar recovery rates are seen amongst other groups brachiopods and ammonites oxygen poor conditions have been suggested to delay recovery after the permo triassic mass extinction but this is not the case after the end triassic crisis we suggest that this was because the european dysoxia was only a regional phenomenon and there were plenty of well ventilated regions available to allow an untrammelled bounce back

detection of a collision zone in south indian shield region from magnetotelluric studies magnetotelluric mt investigations were carried out along a profile in the greenschist granulite transition zone within the south indian shield region sisr the profile runs over a length of 110 km from kuppam in the north to bommidi in the south it covers the transition zone with 12 mt stations using a wide band 1 khz 1 ks data acquisition system the mettur shear zone mtsz forms the ne extension of moyar bhavani shear zonethattraverses alongthetransitionzone theregionalgeoelectric strikedirectionofn40 eidentifiedfromthepresent studyisconsistentwiththe strike direction of the mtsz in the center of the profile the 2 d conductivity model derived from the data display distinct high electrical resistivity character 10 000 m below the archaean dharwar craton and less resistive b3000 m under the southern granulite terrain located south of the mtsz the mtsz separating the two regions is characterized by steep anomalous high conductive feature at lower crustal depths the deep seismic sounding dss study carried out along the profile shows dipping signatures on either side of the shear zone the variation of deep electrical resistivity together with the dipping signature of reflectors indicate two distinct terrains namely the archaean dharwar craton in the north and the proterozoic granulite terrain towards south they got accreted along the mtsz which could represent a possible collision boundary 2006 international association for gondwana research published by elsevier b v all rights reserved

magnetotellurics study to identify subsurface resistivity structure in the eastern part of kachchh little rann area of gujarat india a magnetotelluric mt survey has been conducted in the eastern part of the kachchh region of gujarat state crossing little rann starting from adesar village in the north to enjar village in the south where limited geophysical data is available the profile has been acquired in the n s direction having a total length of 52 km with the interstation spacing of 3 4 km from two dimensional 2d inversion of the mt data four conductive zones are identified the analysis reveals that the depth of the basement varies from 1 2 km in the south to 1 8 km in the north the first conductive zone is found at a distance of 3 km in the south of adesar and is identified at the location of the kanmer fault kf the eastern extension of south wagad fault swf and might be the kf the second conductive zone is found at a distance of 42 km in the south of adesar and 13 km north of enjar village and is located at the contact zone of kachchh and saurashtra peninsula at the location of the north kathiawar fault nkf it is inferred as the nkf the third conductive zone at a distance of 27 km in the south of adesar is found between the first and second conductor this is interpreted as a step fault formed during the rifting process of kachchh the lower crust below 20 km is found conductive 120 m and might indicate the presence of fluid filled mafic ultramafic rocks in the lower crust

organic geochemistry of the san vicente zinc lead district eastern pucara basin peru abstract isotopic and chemical studies of organic matter of the san vicente mississippi valley type zn pb district hosted in the dolomitic units of the eastern pucara basin peru provide insight into the source of organic matter and of the organic matter ore relationship a large scatter exists in the 13c and 15n values of the highly mature kerogens these bulk data were combined with 13c covariations of the saturated and aromatic fractions of the rock extracts to indicate a contribution of continental higher plant lipids to the marine algal source of the organic matter disseminated in the host carbonate rocks an enrichment in 15n of the kerogens of mineralized samples compared to barren samples suggests that isotopically heavy nitrogen compounds were introduced to the ore fluid the fluid likely leached these compounds from the underlying clastic rocks and other detrital units of the pucara the extracts from mineralized samples are enriched in 13c compared to the associated kerogens this suggests that the rocks in the mineralized areas were stained by a mixture of extrinsic and indigenous bitumens by the use of novel instrumentation such as a gas chromatograph gc coupled to a stable isotope ratio mass spectrometer irms through a combustion c interface gc c irms stable carbon isotope analyses of individual hydrocarbons were also obtained both the molecular and isotopic signatures of the individual hydrocarbons are related to mineralization the variability in the organic molecular parameters and the 13c values of the individual n alkanes and isoprenoids indicate mixing of indigenous hydrocarbons with extrinsic 13c depleted microbially derived hydrocarbons the extrinsic source hydrocarbons were introduced to the mineralization site by the ore fluid or later fluids using the same porosity network oxidative alteration of the migrating hydrocarbons was involved in the thermochemical reduction of sulfate to h2s and precipitation of pyrobitumen large differences 12 in the carbon isotope compositions of long chain c20 n alkanes can be utilized in distinguishing sources of organics in the barren and mineralized samples

molecular and isotopic biogeochemistry of the miocene clarkia formation hydrocarbons and alcohols the miocene lacustrine deposit at clarkia northern idaho u s a is well known for its rich and exceptionally well preserved higher plant leaf fossils in the present study the distributions abundance and isotopic compositions of aliphatic hydrocarbons and alcohols isolated from two sediment samples and from two associated fossils magnolia latahensis and platanus dissecta are investigated the general similarity of these characteristics in the sediments suggests a relatively uniform depositional environment and consistent terrestrial input the sediment hydrocarbons are comprised mainly of compounds derived from higher plants and bacteria whereas the alcohols reflect contributions from both higher plants and algae a notable feature of the higher plant derived terpenoid hydrocarbons is the prevalence of des a triterpenes largely in the absence of their pentacyclic precursors indicating the dominance of a ring degradation for these pentacyclic triterpenoids the des a triterpenes include several novel compounds namely 19 h and 19 h 28 nor des a lup 17 ene and des a oleana 9 13 18 diene the sediment samples contain high abundances of c24 c32 long chain alkanediols consisting of a series of diols 1 diols 1 15 diols and an unusual series of 1 3 diols both fossil leaves show clear differences in their compound distributions and concentrations from the associated sediments which suggests that they retain a composition distinct from the sediment groundmass the 13c values of c25 c33 n alkanes and c24 c32 n alcohols in sediments and fossils are consistent with an origin from terrestrial c3 higher plants algal lipids in the two sediments are represented by dinostanol 24 6 and 25 9 inherited from dinoflagellates and long chain 1 15 alkanediols 26 4 to 29 7 derived from microalgae the long chain 1 and 1 3 alkanediols are more enriched in 13c 21 8 to 26 4 suggesting an origin from sources other than higher plants probably aquatic photoautotrophic organisms the 13c depletion of some hopanoids 35 9 to 61 0 attests to contributions from methanotrophic bacteria and suggests an active methane cycle at the time of deposition

electrical conductivity of molten basalt and andesite to 25 kilobars pressure geophysical significance and implications for charge transport and melt structure electrical conductivities of molten hawaiian tholeiite and crater lake andesite were measured between 1200 c and 1400 c at atmospheric pressure and at pressures up to 17and 25 kbar respectively isobaric plots of log versus 1 t is electrical conductivity are linear with the exception of the zero pressure tholeiite melt data conductivities decrease with increasing pressure in both melts with the andesitic melt exhibiting a greater pressure dependence between 5 and 10 kbar abrupt decreases in the slopes of isothermal log versus p plots i e decreases in activation volume are observed for both rock melts this discontinuity probably reflects changes in melt structure as opposed to changes in conduction mechanism in each pressure range the data for each rock melt can be described reasonably well by an equation of the form 0 exp ea p v kt where 0 is a preexponential constant ea is the activation energy and v is the activation volume a qualitative model involving depolymerization of the melt with increased pressure leading to increased efficiency of packing can explain the observed discontinuity in activation volume as well as the observed pressure dependences of other melt physical properties such as viscosity and density conductivity versus melt fraction curves for partially molten peridotite are reevaluated using high pressure tholeiitic melt conductivities and crystalline conductivity values recently determined by other workers minimum melt fraction estimates of 5 10 are required to explain upper mantle regions of anomalously high electrical conductivity in terms of a partial melting hypothesis

dinoflagellate origin for sedimentary 4α methylsteroids and 5α h stanols biological marker compounds provide useful tools for evaluating the depositional environments of recent and ancient sediments and petroleums1 largely through established relationships between the organic matter of sediments and source organisms1 2 such relationships are best tested and explored by investigating the lipid components of natural populations of autochthonous biota that can be clearly recognized as contributors to underlying bottom sediments in this context we have studied the lipids of the freshwater dinoflagellate peridinlum lomnickii woloszynska order peridiniales class dinophyceae collected from the waters of priest pot3 a eutrophic lake in the english lake district its distributions of both 4 methylsterols and 4 methylstanones closely resemble those of the underlying bottom sediments demonstrating that dinoflagellates are important contributors of these sedimentary compounds the 4 methylsteroidal hydrocarbons found in ancient sediments and petroleums4 6 are presumably diagenetic products of such 4 methylsteroids and therefore reflect dino flagellate inputs to the original depositional environments furthermore the prominence of 5 h cholestan 3 ol in p lomnickii suggests that dinoflagellates may be the long sought direct biological source of sedimentary 5 h stanols

the significance of hiatus beds in shallow water mudstones an example from the middle jurassic of switzerland abstract limestone beds formed in nearly carbonate free shallow water mudstones by discontinuous sedimentation and erosion are called hiatus beds anaerobic oxidation of organic matter by microbes provided excess alkalinity inducing carbonate precipitation a multiphase history of such beds is documented from the swiss jurassic by several cementation and dissolution phases four cement types occur micrite as earliest cement 13c 10 to 20o oo stellate calcite between micrite cemented parts 13c 5 to 10o oo fibrous calcite cement in dissolution cavities 13c 30o oo and blocky calcite in remaining pores 13c 5o oo except for the late blocky cement all cements contain pyrite indicating carbonate precipitation within the sulfate reduction zone after early cementation by micrite the beds moved relatively upwards into a shallower geochemical zone and some even to the seafloor because of erosion cavities formed during reburial by dissolution in the upper part of the sulfate reduction zone and in the upper part of the methanogenic zone strongly reduced sedimentation rates provided the requisite stable geochemical conditions for at least several thousands of years which permitted precipitation and dissolution of carbonate by biochemical processes and diffusion this happened on short lived swells caused by differential subsidence and rotation along listric faults when basement structures became reactivated during the extensional stress regime from opening of the tethys during the jurassic and cretaceous breakup of pangea the shelf area increased and differential subsidence on these newly formed shelves was the main factor responsible for the observed post paleozoic maximum in hiatus beds and hiatus concretions

precambrian continental crust of india and its evolution the indian precambrian continental crust exhibits a variety of geological features fashioned at different times by different geotectonic processes the bulk of this crust was formed prior to 2600 m y ago and remobilized at least twice between 2600 2000 m y ago early proterozoic mobile belt epmb and 2000 1500 m y ago middle proterozoic mobile belt mpmb three early precambrian nucleii karnataka kn jeypore bastar jbn and singhbhum sn appear to have survived in the craton and are characterized by low grade supracrustals and tonalitic trondhjemite gneisses formed 3800 2600 m y ago the epmb event involved sedimentation amphibolite granulite facies metamorphism and co 2 k metasomatism and produced amphibolite facies rocks and k granites in the north and charnockite and other granulite facies rocks in the south gold sporadically distributed in the supracrustal rocks of the craton was remobilized during the epmb event k granites form a garland around the central dharwar craton sugg

mid chain branched mono and dimethyl alkanes in hot spring cyanobacterial mats a direct biogenic source for branched alkanes in ancient sediments hot spring cyanobacterial mats from yellowstone national park u s a and the orakei korako thermal area new zealand have been found to contain suites of monomethyl alkanes compounds frequently reported in cultured and natural populations of cyanobacteria the complete range of possible structural isomers for mid chain branching for c17 c18 and c16 c18 alkanes occurs in the two mats respectively several c19 dimethyl and c20 multibranched alkanes were also observed as major hydrocarbons of the orakei korako mat the recognition of series of mid chain branched alkanes in modern cyanobacterial mats suggests that their occurrence in ancient sediments may reflect direct biogenic contributions rather than formation by diagenetic processes

partial melt or aqueous fluid in the mid crust of southern tibet constraints from indepth magnetotelluric data summary the indepth project has applied modern geophysical techniques to the study of the crustal structure and tectonic evolution of the tibetan plateau in the lhasa block seismic reflection surveys in 1994 detected a number of bright spots at 15 20 km depths that indicate zones of crustal fluids aqueous fluids or partial melt coincident magnetotelluric mt data collected in 1995 detected a major zone of high electrical conductivity at the same depth as the brightspots using constrained inversion the mt data require a minimum crustal conductance of 6000 s this abnormally high electrical conductance can be best explained by a layered model with fluids partial melt aqueous fluids or a combination of partial melt and aqueous fluids the non uniqueness of the mt method means that a wide range of melt fraction thickness combinations for the above models could all explain the 6000 s conductance to distinguish between these three models other geophysical and geological data are required reflection seismic data suggest that a high fluid content 15 per cent is present at the top of the layer the amplitude versus offset data suggest that the top of this layer may be aqueous fluids rather than partial melt passive seismic data imaged a 20 km thick layer of lower fluid content that is probably partial melt petrological studies suggest that concentrations of aqueous fluids above 0 1 per cent at mid crustal depth cannot be sustained taken together these data show that the high conductivity in southern tibet is most probably the result of a relatively thin layer of aqueous fluids 100 200 m overlying a thicker zone of partial melt 10 km

factors controlling 13c contents of sedimentary organic compounds principles and evidence the carbon isotopic composition of any naturally synthesized organic compound depends on 1 the carbon source utilized 2 isotope effects associated with assimilation of carbon by the producing organism 3 isotope effects associated with metabolism and biosynthesis and 4 cellular carbon budgets these factors are reviewed and quantitative considerations summarized particularly with regard to active and passive modes of carbon assimilation by phytoplankton and the existence of discernible regularities in isotope effects associated with lipid biosynthesis it is concluded that n alkyl lipids are in general depleted in 13c by about 1 5 relative to polyisoprenoid lipids produced by the same organism the effects of biological reworking on isotopic compositions of organic carbon are examined and it is suggested that enrichment of 13c in sedimentary organic carbon may result from loss of ch4 from zooplanktonic gut communities isotopic methods for estimation of ancient co2 levels are considered and a hyperbolic form favored for the relationship between concentrations of dissolved co2 and isotopic fractionation

lithospheric electrical imaging of the deccan trap covered region of western india 1 we present here the results of magnetotelluric mt studies carried out along a 330 km long ne sw trending traverse from daulatabad at the northeastern end to koyna on the southwestern end across the deccan volcanic province dvp in western india the 2 d mt modeling results obtained along this traverse together with those from other mt traverses in this area are examined and integrated with the results from deep seismic sounding 3 d teleseismic tomography and heat flow studies the deep geoelectric section as deduced from mt models point out to a two layered lithospheric mantle structures with an upper very high resistive layer several thousands of ohm m and a bottom relatively conductive one a few tens of ohm m to a few hundred ohm m the lithospheric electrical structure is inferred to be linked to compositional changes associated with a depleted lithospheric upper mantle in the dharwar craton an impressive feature that the study has brought out is the presence of a well defined lithospheric upper mantle conductor in the depth range of 80 120 km limited to the middle segment of the mt traverse from sangole to partur over the dvp the electrical characterization of the region including the presence of upper mantle conductor shows close similarities with that of other similar cratonic regions like the slave and superior cratons in northwestern canada

organic geochemistry across the permian triassic transition at the idrijca valley western slovenia abstract bulk and molecular stable c isotopic compositions and biomarker distributions provide evidence for a diverse community of algal and bacterial organisms in the sedimentary organic matter of a carbonate section throughout the permian triassic p tr transition at the idrijca valley western slovenia the input of algae and bacteria in all the upper permian and lower scythian samples is represented by the predominance of c15 c22 n alkanes odd c number alkylcyclohexanes c27 steranes and substantial contents of c21 c30 acyclic isoprenoids the occurrence of odd long chain n alkanes c22 c30 and c29 steranes in all the samples indicate a contribution of continental material the decrease of corg and ccarb contents increase of rock eval oxygen indices and 13c enrichment of the kerogen suggest a decrease in anoxia of the uppermost permian bottom water the predominance of odd c number alkylcycloalkanes c27 steranes and c17 n alkanes with 13c values 30 and 13c enrichment of the kerogens in the lowermost scythian samples are evidence of greater algal productivity this increased productivity was probably sustained by a high nutrient availability and changes of dissolved co2 speciation associated to the earliest triassic transgression a decrease of corg content in the uppermost scythian samples associated to a 13c depletetion in the carbonates up to 4 and individual n alkanes up to 3 4 compared to the upper permian samples indicate lowering of the primary productivity algae cyanobacteria and or higher degradation of the organic matter

novel cyclised and aromatised diagenetic products of β carotene in the green river shale abstract a suite of novel diagenetic products of carotene has been identified in an extract of a sample from the green river shale identification is mainly based on comparison of mass spectra with those of diagenetic products of another carotenoid isorenieratene these compounds were formed by i cyclisation and aromatisation of the polyene isoprenoid chain ii explusion of toluene and m xylene from the polyene isoprenoid chain and iii aromatisation of the 1 1 5 trimethylcyclohex 5 enyl moieties resulting in 1 2 dimethylbenzene end groups reduction of the double bonds of carotene is the most important diagenetic process however as judged from the high relative amount of carotane 63 of quantified diagenetic products of carotene these results indicate that the fossilisation potential of carotenoids is greater than previously thought

electrical conductivity temperatures and fluids in the lower crust we have compiled both laboratory and worldwide field data on electrical conductivity to help understand the physical implications of deep crustal electrical profiles regional heat flow was used to assign temperatures to each layer in regional electrical conductivity models we avoided those data where purely conductive heat flow suggested temperatures more than about 1000 sup 0 c substantially higher than solidus temperatures and outside the range of validity of heat flow models the resulting plots of log conductivity sigma versus 1 t demonstrate that even low conductivity layers lcl have conductivities several orders of magnitude higher than dry laboratory samples and that the data can be represented by straight line fits in addition tectonically active regions show systematically higher conductivities than do shield areas because volatiles are usually lost in laboratory measurements and their absence is a principal difference between laboratory and field conditions these materials probably account for the relatively higher conductivities of rocks in situ in the crust free water in amounts of 0 01 0 1 in fracture porosity could explain crustal conductivities other possibilities are graphite hydrated minerals in rare instances and sulfur in combination with other volatiles as most of the temperatures are less than 700 sup 0 c partial melting seems likely onlymore in regions of highest heat flow where the conductive temperature profiles are inappropriate another result is that at a given temperature crustal high conductivity layes hcl are more conductive by another order of magnitude and show more scatter than do lcl s less

resistivity structure of the central indian tectonic zone citz from multiple magnetotelluric mt profiles and tectonic implications the central indian tectonic zone citz is a major tectonic feature extending across the indian subcontinent it was formed in the paleoproterozoic when the bastar craton and the bundelkhand craton were sutured together this region is recognized in the geological record as a persistent zone of weakness with many tectonothermal events occurring over geologic time the weakness of this region may have caused the late cretaceous early tertiary deccan volcanism to have been localized in the citz the zone is still tectonically active as evidenced by sustained levels of seismic activity this paper presents the first systematic investigation of the resistivity structure of the citz using multiple magnetotelluric mt transects two dimensional 2d resistivity models were generated for five north south profiles that cross the citz and encompass an area of 60 000 km2 the models were based on the joint inversion of transverse electric te transverse magnetic tm and tipper hz data all the profiles showed a low resistive 10 80 m middle to lower crust beneath the citz with a crustal conductance of 300 800 s the presence of an interconnected fluid phase and or hydrous metallic minerals appears to be the most likely explanation for the elevated conductivity that is observed beneath the citz the presence of fluids is significant because it may indicate the cause of persistent weakness at crustal depths a northward dip of both the crustal conductive layer and coincident seismic reflections favor a northward polarity of the subduction process associated with the formation of the citz

composition and sources of lipid compounds in speleothem calcite from southwestern oregon and their paleoenvironmental implications we analyzed speleothem calcite from the oregon caves national monument southwestern oregon to determine the preservation distribution concentrations and sources of aliphatic lipid compounds preserved in the calcite maximum speleothem growth rate occurs during interglaciations and minimum during glacial intervals concentrations of the total lipid compounds range from 0 5 to 12 9 g g 1 they increase at times of low speleothem growth rate suggesting dilution whereas the apparent accumulation rate of lipid compounds tends to be highest during times of fastest speleothem growth rate such increased accumulation generally corresponds to times of warm interglacial climate suggesting either a greater source of organic materials during interglacial times and or greater efficiency of compound capture during more rapid calcite growth aliphatic lipid compounds include homologous n alkanoic acids n alkanols and methyl n alkanoates and sterols with concentrations ranging from 0 3 to 7 8 g g 1 0 4 to 1 1 g g 1 0 5 to 9 6 g g 1 and 0 1 to 2 7 g g 1 respectively minor amounts of branched methyl n alkanoates and dimethyl n alkanedioates are also present the high concentrations of methyl n alkanoates are the result of esterification reactions of free fatty acids in alkaline solutions with high ph values associated with the dripping cave waters the distribution patterns and geochemical parameters and indices indicate that the major sources of the aliphatic lipids involved leaching from higher plants and microbial residues derived from the soil zone above the cave system the estimated percentage of microbial inputs ranged from 42 to 90 of the total lipids and also showed an increase in accumulation during warm climates these well preserved lipid compounds in speleothem calcite could be used as biomarkers for paleoenvironmental study

chemical and carbon isotopic evolution of hydrocarbons during prograde metamorphism from 100 c to 550 c case study in the liassic black shale formation of central swiss alps abstract hydrocarbon distributions and stable isotope ratios of carbonates 13ccar 18ocar kerogen 13cker extractable organic matter 13ceom and individual hydrocarbons of liassic black shale samples from a prograde metamorphic sequence in the swiss alps were used to identify the major organic reactions with increasing metamorphic grade the studied samples range from the diagenetic zone 17 n alkanes suggests the occurrence of cracking reactions of high molecular weight compounds the isotopically heavier up to 5 6 c 13 n alkanes with metamorphism suggests progressive thermal release of kerogen linked fatty acid precursors and degradation of n alkanes changes of the steroid and terpenoid distributions are clearly related to increasing metamorphic temperatures the absence of 18 h 22 29 30 trisnorneohopane ts the occurrence of 17 h trisnorhopane 17 h 21 h hopanes in the c29 to c31 range and 5 h 14 h 17 h 20r c27 c29 steranes in the low diagenetic samples the occurrence of specific hydrocarbons e g n alkylcyclopentanes cadalene hydrogenated aromatic compounds in metamorphic samples points to kerogen degradation reactions most probably occurring in the presence of water and under reducing conditions the changes of hydrocarbon distributions and carbon isotopic compositions of n alkanes related to metamorphism suggest that the organic geochemistry may help to evaluate the lowest grades of prograde metamorphism

new high resolution geochemistry of lower jurassic marine sections in western north america a global positive carbon isotope excursion in the sinemurian abstract recognising variations in the carbon isotope compositions of marine organic rich sedimentary rocks can provide insight into changes in ocean chemistry throughout geological time further identification of global excursions in the carbon isotope record has proved to be valuable as a chronostratigraphic correlation tool this investigation presents new high resolution organic carbon isotope data c org 13 for marine sediments from 2 regions in north america last creek british columbia canada and five card draw nevada usa the carbon isotope profiles demonstrate that there were significant differences between the carbon reservoirs at five card draw and last creek notably in the upper part of the leslei zone the c org 13 values show a gradual positive cie 2 at last creek in the upper part of the leslei zone this corresponds to a coeval positive cie of similar duration in dorset uk upper turneri zone jenkyns and weedon 2013 suggesting that this may be a global marine carbon isotope signature and likely reflects a widespread increase in primary productivity during the early sinemurian in addition a brief negative cie is observed in the uppermost lower sinemurian at last creek this negative excursion is not recorded in the dorset section suggesting localised upwelling of 12c rich bottom waters at last creek further the signals identified at last creek are not present in coeval sections at five card draw thus highlighting a significant difference between these localities osmium os isotope data initial 187os 188os values provide a quantitative determination of the contrasting depositional environments of five card draw and last creek at least partially restricted with high levels of continental inundation and open ocean respectively this demonstrates that basinal restriction may act as a major factor that controls isotopic stratigraphic signatures thus preventing the identification of global or widespread regional excursions

chlorine isotope behavior during prograde metamorphism of sedimentary rocks abstract chlorine stable isotope compositions of two sedimentary sequences and their metamorphic equivalents were measured in order to study fractionation effects during prograde metamorphism and devolatilization protoliths n 25 were collected from a 50 m section of triassic fluvial and playa lake strata and jurassic liassic marine black shales in a well characterized quarry low greenschist to middle amphibolite facies equivalents n 80 were collected from the glarus alps urseren zone and lucomagno region bulk 37 cl values are constant within individual sedimentary layers but vary from 2 0 to 2 4 in triassic rocks and from 3 0 to 0 in the black shales dolomitic and gypsiferous samples have positive 37 cl values but marls and shales are isotopically negative bulk cl contents show only small declines during the earliest stages of metamorphism metamorphic equivalents of the triassic and liassic protoliths record the same overall ranges in 37 cl as their protoliths samples with highly correlated bulk compositions but different metamorphic grade show no statistically significant difference in 37 cl these data lead to the following conclusions 1 terrestrial and marine sedimentary rocks display large primary heterogeneities in chlorine isotope composition as a result an unambiguous sedimentary signature does not exist in the chlorine stable isotope system 2 no isotopic fractionation is discernable during metamorphic devolatilization even at low temperatures alpine style metamorphism thus has little to no effect on bulk chlorine isotopic compositions despite significant devolatilization 3 cl is largely retained in the rocks during devolatilization contrary to the normally assumed hydrophilic behavior of chlorine continuous release of mixed volatile c o h fluids likely affected cl partitioning between fluid and minerals and allowed chlorine to remain in the rocks 4 there is no evidence for fluid communication across meta sedimentary layers except within the urseren shear zone these data contribute to a relatively closed system view of metamorphic devolatilization from anchizone through mid amphibolite facies conditions

the bolivian altiplano conductivity anomaly 1 a long period magnetotelluric study was carried out in the central andes between latitudes 19 5 s and 21 s along two almost parallel profiles of 220 and 380 km length respectively the investigation area extends from the pacific coast to the southern altiplano plateau in the back arc of the south american subduction zone the main geoelectrical structure resolved is a broad and probably deep reaching highly conductive zone in the middle and deeper crust beneath the high plateau although the data show deviations from two dimensionality a two dimensional approach is justified for large parts of the profiles sensitivity studies were carried out in order to constrain the depth extent another electrically conductive structure was resolved in the middle crust of the chilean forearc thought to be connected with the precordillera fault system the andean continental research program ancorp seismic reflection profile carried out along the same line at 21 s revealed highly reflective zones below the altiplano in good correlation with the upper boundary of the altiplano conductor this highly conductive domain also coincides with low seismic velocities and a zone of an elevated vp vs ratio and although not well resolved with low qp seismic quality factors taking into account the enhanced heat flow and a derived temperature model the most probable explanation lies in the assumption of granitic partial melts the good conductor below the volcanic arc which was found in regions farther south at 22 s gradually vanishes toward the north this is consistent with the results of seismic tomography concerning qp values and a gap of recent volcanism

deep electrical signatures across the achankovil shear zone southern granulite terrain inferred from magnetotellurics abstract two parallel magnetotelluric mt traverses across the achankovil shear zone aksz in southern india using a wide band 1 khz 1 ks data acquisition system reveal the regional geo electric average strike direction of n 40 w consistent with the geological strike direction of the aksz the derived model shows distinct high electrical resistivity 1000 m for the upper crust below the madurai granulite block mgb with a gentle dip towards south and a northerly dip below the kerala khondalite block kkb the lower crust is resistive below both mgb and kkb while it is moderately conductive

3d upper crustal density structure of the deccan syneclise central india a constrained 3ddensity model of the upper crust along a part of the deccan syneclise is carried out based on the complete bouguer anomaly data spectral analysis of the complete bouguer gravity anomaly map of the study region suggests two major sources short wavelength anomalies 100 km produced due to the sources deeper than the upper crust a residual map of the short wavelength anomalies is prepared from the complete bouguer anomaly using butterworth high pass filter 100 km cut off wavelength utilizing the constraints from deep resistivity sounding magnetotellurics and deep seismic sounding studies 2 5d density models have been generated along 39 profiles of this region the mismatch between the calculated response of the a priori 2 5d model with the residual short wavelength gravity anomalies is minimized by introducing high density intrusive bodies 2 81 g cm3 in the basement with these 2 5d density models the initial geometry of our 3d density model which includes alluvium deccan trap mesozoic sediment and high density intrusive bodies in the basement up to a depth of 7 km upper crust is generated in the final 3d model deccan trap extends from 200 m to nearly 1700 m below the 90 150 m thick quaternary sediment further down the sub trappean mesozoic sediment is present at a depth range of 600 3000 m followed by the basement the derived 3d density model also indicates six intrusive bodies of density 2 83 g cm3 in the basement at an average depth of about 4 7 km that best fits the residual gravity anomaly of the study area

the central india tectonic zone a geophysical perspective on continental amalgamation along a mesoproterozoic suture abstract the central india tectonic zone citz is a prominent divide and a major suture zone between the north indian and south indian crustal blocks the resistive upper crust as modeled in the magnetotelluric data from citz suggests a dominant tonalite trdondhjemite granodiorite composition associated with an accretionary complex characterized by mainly felsic rock components the highly conductive bodies in this zone might represent mafic ultramafic layered intrusives derived from a deeper reservoir of underplated basaltic magma related to the formation of the cretaceous deccan flood basalts the uniformly thick mafic lower crust below the cratons on both sides of the suture is interpreted as the accreted remnants of archaean and paleoproterozoic subducted slabs we redefine the nature of deep faults traversing the citz which were described as steep and penetrating the moho by previous workers and classify them as listric faults with gentle dips at depth seismic reflection data from the eastern side of the suture suggest a northwestward subduction of the bhandara craton reflection data from the central part of the citz show northerly dip in the southern part suggesting northward subduction of the dharwar craton however an opposite trend is observed in the northern part of the suture with a southward dip of the bundelkhand craton based on these features and in conjunction with existing magnetotelluric models we propose a double sided subduction history along the citz this would be similar to the ongoing subduction accretion process in the western pacific region which possibly led to the development of paired collision type and pacific type orogens one important feature is the domal structure along the central part of the suture with a thick felsic crust occurring between mafic and intermediate crust the high resistivity felsic domain suggests underplated sediments felsic crust that would have caused the doming our model also accounts for the extrusion of regional metamorphic belts at the orogenic core and the occurrence of high pressure ultrahigh temperature paired metamorphic belts within the suture

carbon isotope evidence for unusual plankton blooms and fluctuations of surface water co2 in strangelove ocean after terminal cretaceous event abstract a decrease in 13c kerogen by 3 and increase in 13c carbonate kerogen by 1 2 has been observed in sediments at the k t boundary substantiating the occurrence of a strangelove ocean wherein dissolved co2 was enriched in 12c because of mass mortality or reduced plankton production a subsequent increase in 13c kerogen by 7 and decrease in 13c carbonate kerogen by 5 4 indicate a dramatic reduction in pco2 this decrease could be the consequence of depletion of co2 in surface waters because of unusual plankton blooms these environmental changes which characterize the strangelove ocean occurred after the terminal cretaceous event during this time rapid fluctuations in dissolved co2 of ocean surface waters may have caused significant perturbations in atmospheric pco2 levels and consequently major instability of the global climate

regional geoelectric structure beneath deccan volcanic province of the indian subcontinent using magnetotellurics understanding deep continental structure and the seismotectonics of deccan trap covered region has attained greater importance in recent years for imaging the deep crustal structure magnetotelluric mt investigations have been carried out along three long profiles viz guhagarh sangole gs sangole partur sp edlabad khandwa ek and one short profile along nanasi mokhad nm the results of gs sp and nm profiles show that the traps lie directly over high resistive basement with thin inter trappean sediments where large thickness of sediments of the order of 1 5 2 0 km has been delineated along ek profile across narmada son lineament zone the basement is intersected by faults fractures which are clearly delineated as narrow steep conducting features at a few locations the conducting features delineated along sp profile are also seen from the results of aeromagnetic anomalies towards the southern part of the profile these features are spatially correlated with kurduwadi rift proposed earlier from gravity studies apart from the kurduwadi rift extending to deep crustal levels the present study indicates additional conductive features in the basement the variation in the resistivity along gs profile can be attributed to crustal block structure in koyna region similar block structure is also seen along nm profile deccan trap thickness based on various geophysical methods varies gradually from 1 8 km towards west to 0 3 km towards the east while this is the general trend a sharp variation in the thickness of trap is observed near koyna the resistivity of the trap is more 150 200 m towards the west as compared to the east 50 60 m indicating more compact or denser nature for the basalt towards west the upper crust is highly resistive 5000 10 000 m and the lower crust is moderately resistive 500 1000 m in the present study seismotectonics of the region is discussed based on the regional geoelectrical structure with lateral variation in the resistivity of the basement and presence of anomalous conductors in the crust 2007 elsevier b v all rights reserved

chlorobiaceae in palaeozoic seas revealed by biological markers isotopes and geology biomarker hydrocarbons in oils and sediments have considerable potential for assessing the nature of biological processes in the past but only a few can be uniquely related to particular organisms reef hosted oils in the silurian of the michigan basin canada and the devonian of western canada have high abundances of 1 alkyl 2 3 6 trimethylbenzenes their precise structures suggest that they are diagenetic products of aromatic carotenoids of the green sulphur bacteria individual components are enriched in 13c by 7 8 relative to the saturated hydrocarbons of the same oil an isotopic anomaly of this magnitude and direction correlates with a component of photosynthetic carbon assimilation by the reductive tricarboxylic acid cycle these results together with data on palaeoenvironments provide compelling evidence for the existence in ancient restricted seas of microbial communities containing chlorobiaceae organisms with distinctive biochemistry which leave morphologically identifiable remains

grain boundary graphite in kapuskasing gneisses and implications for lower crustal conductivity surface electromagnetic measurements commonly reveal that the intermediate and lower continental crust has an appreciable electrical conductivity much higher than that found for laboratory analogues of deep crustal rocks or for upper crustal rocks either formed at shallow depths or uplifted from deeper levels conducting films at grain boundaries are thought to play an important part in enhancing deep crustal conductivity with brines and graphite being the main candidates1 at present however there is a lack of direct evidence for such films frost et al 2 have reported an enhancement in conductivity of igneous lower crustal rocks from the la ramie complex apparently owing to the presence of graphite films but the possible role of brines was not clear from this work here we present a study of the grain surface composition of metamorphic and one igneous rocks exhumed from 20 km depth by the kapuskasing uplift of the canadian shield3 we expect these samples to be representative of metamorphic and igneous rocks of the deep archaean crust auger spectroscopy provides evidence for graphite films at grain boundaries which can account for the present electrical signature of the kapuskasing crust4 6 we also detect traces of chlorine sulphur and iron which suggest that brines and solid conductors other than graphite may have helped to enhance the conductivity but their abundance suggests at best a minor role relative to graphite

application of 3d inversion to magnetotelluric profile data from the deccan volcanic province of western india abstract using synthetic data siripunvaraporn et al 2005b demonstrated possible advantages of interpreting single profile mt data with a three dimensional 3d inversion program here we explore this idea further using real mt data from two profiles on the indian subcontinent the first profile 330 km long cuts across the deccan volcanic province of peninsular india the second 130 km long is in the narmada son lineament zone approximately 100 km further north using the data space occam inversion code of siripunvaraporn et al 2005a 3d inversion is carried out on each of these profiles independently and results are compared with previously published two dimensional 2d interpretations in addition to inversion of the full impedance tensor we consider 3d inversion of only the off diagonal components we also experiment with variants on the model covariance in particular allowing for longer smoothing length scales along the geoelectric strike not surprisingly the 3d inversion finds models that fit the data better than had been possible with the 2d programs many of the features inferred from these previous 2d interpretations are also present in the 3d inverse solutions but the positions and amplitudes of individual conductive features are in some cases changed the 3d models suggest substantial non uniqueness in the single profile data even without explicit or special treatment we find that the relatively modest near surface distortion effects in these datasets were well fit by the 3d inversion by inserting small scale conductive and resistive features in surface layers mostly off profile

organic matter sources and early diagenetic alterations in arctic surface sediments lena river delta and laptev sea eastern siberia ii molecular and isotopic studies of hydrocarbons abstract the hydrocarbons extracted from three surface sediment samples from the delta of the lena river and adjacent areas of the laptev sea were examined to derive information on their source s natural vs anthropogenic terrestrial vs marine the abundance nature distribution and individual stable carbon isotope ratios of these hydrocarbons were determined it thus appeared that the laptev samples are characterized by i a negligible level of oil pollution due to anthropogenic and or natural petrogenic inputs ii an almost exclusive occurrence of long chain predominantly odd carbon numbered n alkanes and iii some differences in the level of bacterial alteration with a relatively higher degradation of the n alkanes in the seaward sediment a non terrestrial origin is proposed for these n alkanes based on the present results and on previous observations on carboxylic acid moieties and the pyrolysis products of the three surface sediments such hydrocarbons are probably not derived from the epicuticular waxes of higher plants but chiefly correspond to autochthonous products likely of algal origin these observations added to recent results on samples from other types of marine environments stress that an abundance of long chain predominantly odd n alkanes in extracts should not be systematically considered alone as evidence for a major contribution of higher plant waxes the relatively low contribution of terrestrial n alkanes in the laptev sediments should result from the combination of i a major dilution of the terrestrial input due to high primary productivity in summer promoted both by a large influx of nutrients provided by the lena river and by the specific conditions in such an arctic environment and ii the high level of degradation of the organic material transported by the lena river thus the latter probably provided a low input of lipids from terrestrial sources to the laptev samples

partial melting below the magmatic arc in the central andes deduced from geoelectromagnetic field experiments and laboratory data abstract magnetotelluric and geomagnetic deep soudings in northern chile revealed a pronounced high conductivity zone hcz below the western cordillera which constitutes the present magmatic arc with active volcanism of the south american continental margin conductivities in the range of 1 s m are observed the anomalously high conductivities in a broad depth range from approximately 20 km to at least 60 km are interpreted in terms of partial melting other geophysical observations such as a zone of low seismic velocities lvz at similar depths high heat flow values 100 mw m2 and a pronounced negative anomaly in the residual gravity field are also considered impedance spectroscopic laboratory experiments up to and in the temperature range of partial melting were performed under controlled oxygen fugacities at sub solidus temperatures electrical behavior is described by defect electrons with an activation energy of 1 34 ev and a conductivity of 2 5 ms m at 900 c model calculations using a modified brick layer model mbl were compared with experimental observations a good agreement between calculations and experiments is achieved with an electrical resistivity of the melt phase of 7 s m at 1250 c assuming an activation energy of 1 ev the same mbl model is used to calculate melt proportions beneath the western cordillera between 14 and 27 vol of interconnected melt are necessary to explain the observed hcz the stability of the melt rich crust is explained by a dynamic melting crystallisation behavior during crustal anatexis and by magma filled dikes

deep crustal structure in central india using magnetotelluric studies summary magnetotelluric studies over the damoh jabalpur mandla anjaneya profile in central india have delineated vindhyan sediments which are about 5 km thick in the damoh katangi region the crust below the vindhyan sediments shows the characteristics of lower crust as observed from the relatively lower resistivity of about 200 v m and high seismic velocities p wave velocities of 6 5 km s x1 compared with 5 8 6 2 km s x1 in the surrounding region it is conjectured that the upper crust may have been completely eroded in the uplift and erosion process and thus the vindhyan sedimentation has occurred directly over the lower crust an anomalous conductivity is observed at depths of 10 12 km in the vindhyan crust the conductance of more than 1200 s observed here may be due to either the serpentinization of the mafic and ultramafic rocks or the presence of grain boundary graphites the thickness of the deccan traps is about 100 m near jabalpur and decreases near mandla on the south of mandla the archaean crust is exposed two crustal conductors are delineated below the deccan volcanics with a resistivity of about 30 v m the first on the immediate south of jabalpur seems to mark the southern boundary of the jabalpur horst block the second conductor was delineated about 40 km southeast of jabalpur coincident with a positive gravity anomaly of about 30 mgal deep seismic sounding studies do not show any significant density contrast associated with this conductive feature it is proposed that the gravity high may be due to the upwarp of the moho the high electrical conductivity is attributed to the fluids in the upper crust

nature of continental crust along the narmada son lineament inferred from gravity and deep seismic sounding data abstract the narmada son lineament nsl is a very prominent feature on the tectonic map of india and divides the indian peninsula into two parts the northern part characterised by lower elevations and more positive bouguer gravity anomalies as compared to the southern part the lineament is characterised by a gravity high practically all along its length four published deep seismic sounding dss profiles across the nsl give vital information concerning its origin an analysis of the bouguer gravity field along these profiles has been carried out using all available geological and velocity density information the analysis reveals that the areas immediately south of the nsl are characterised by higher density crust in comparison to the areas north of it this increase in density could have taken place during mesozoic to tertiary times when major tectonic volcanic activity took place over peninsular india the presence of deep seated faults sediments and anomalous crust shows that the lineament has some characteristics of a rift valley

evidence for middle triassic to miocene dual subduction zones beneath the shan thai terrane western thailand from magnetotelluric data abstract kanchanaburi province western thailand is in the middle of shan thai terrane paleomagnetic and geological data support the hypothesis of dual subduction zones beneath the shan thai terrane during the late triassic there was a westward subduction in the east where the lampang chiang rai block subducted beneath the shan thai terrane in addition in the early tertiary the western burma terrane subducted underneath the shan thai in the west resulting in an eastward subduction a pioneer survey of this region was therefore conducted using magnetotelluric mt techniques with the aim of finding deep structures associated with these ancient subduction zones 39 mt stations were deployed to cover most of the province resistivity structures are obtained from the 3 d inversion the near surface resistivity structures correspond well with the surface geology the mid and lower crusts are conductive and this is interpreted as the crusts being composed of mafic granulites of 3 porosity in which the remnant dehydrated fluid was accumulated during the subduction in addition the westward and eastward subduction zones generated mafic ultramafic intrusions which appear as two conductive zones on the east and west respectively these two conductors and the conductive lower crust support the hypotheses of the middle triassic to miocene dual subduction zones

provenance and paleoenvironment of organic matter within the fika sediments in chad bornu basin northeastern nigeria an integrated organic geochemical and palynofacies approach abstract the upper cretaceous fika sediments in the chad bornu basin were characterised to define paleoenvironment and source of the organic matter using an integration of organic geochemical analyses and palynofacies observations the present day toc contents of the fika sediments are in the range of 0 41 2 37 wt while the hydrogen index hi values are largely below 200 mg hc g toc suggesting that the sediments generally have fair to good generative potential and contain mainly type iii and type iv kerogens this is supported by palynofacies observations whereby the kerogen is characterised by large amounts aom and phytoclasts the fika sediments have vitrinite reflectance and t max values in the range of 0 70 1 34 and 429 465 c respectively indicating early mature to late mature stage of oil window the level of organic maturation in fika sediments was further supported by the several biomarker maturity indicators biomarker distributions and palynofacies data provide evidence for a mixed aquatic algae and terrigenous organic matter input deposited in a marginally marine moderate salinity proximal setting with a strong influx of terrestrial plant under environmental conditions that were mostly suboxic compound specific isotopic analysis of n alkanes fraction were also used for the paleoenvironmental inferences the flat portion pattern of the n alkane profile between n c 21 and n c 26 in fika samples is an indication of marine environment and it shows that fika sediments consist of both terrestrial and marine organic matter deposited in fluvial deltaic to shallow marine settings