deep marine origin of equant spar cements in bahama escarpment limestones early and middle cretaceous shallow water limestones exposed on the bahama escarpment by late cretaceous and tertiary erosional processes have abundant petrographic evidence of shallow diagenesis particularly cloudy fine coarse crystalline isopachous bladed fibrous and radiaxial fibrous cements similar petrographic evidence is observed in coeval rocks buried in eastern texas isotopic and sr compositions of micrite and isopachous radiaxial fibrous cement separates support petrographic observations delta 18 o values range from 3 1 per thousand to 0 1 per thousand and delta 13 c from 0 6 per thousand to 3 4 per thousand similar to those of original marine components of coeval strata in eastern texas original shallow marine components in bahama escarpment limestones also have relatively high sr ca values 2 0 10 1 x 10 super 4 177 884 ppm significant cementation in shallow environments was however limited to the shallow marine phreatic zone cementation in vadose and meteoric phreatic environments was rare cements from shallow diagenetic environments only partially fill syndepositional primary porosity fenestrae vugs molds and intra and interparticle pores and are completely absent in secondary porosity principally fractures resulting from erosion of the bahama escarpment abundant clear medium coarse crystalline equant spar in both primary and secondary porosity apparently precipitated either at the seafloor or in deep burial environments from fluids with compositions similar to deep marine waters adjacent to the bahamas isotopic compositions of equant spar in both primary and secondary pores trend from 0 6 per thousand delta 18 o and 1 9 per thousand delta 13 c to 2 0 per thousand delta 18 o and 3 6 per thousand delta 13 c despite partial overlap of the original shallow marine field equant spar is generally 18 o enriched relative to original shallow marine components consistent with precipitation in equilibrium with colder deep marine waters delta 18 o values for bahama escarpment equant spar are also 18 o enriched relative to equant spar in coeval strata in eastern texas reflecting substantially different deep burial fluids bahamian deep burial environments have remained open continually invaded by deep marine water coeval strata in eastern texas however were overlain by impermeable beds as the gulf coast shelf prograded and subsurface fluids evolved in restricted diagenetic environments

u pb trace element and lu hf properties of unique dissolution reprecipitation zircon from uhp eclogite in sw sulu terrane eastern china abstract a unique dissolution reprecipitation zircon has been identified for the first time in a qtz and phe bearing eclogite from the drill hole ccsd mh southwestern sulu uhp terrane by using laser raman and cathodoluminescence cl imaging combined with trace element analyses and u pb and lu hf isotope data relic inherited domains of partially dissolved zircon show magmatic features and preserve mineral inclusions of cpx pl qtz ap whereas newly formed zircons crystallized around or along cracks of the inherited magmatic domains and contain mineral inclusions of coe grt omp phe ap typical for uhp metamorphism in addition primary fluid inclusions were identified in the coesite bearing zircon domains characterized by a single h2o phase or co2 h2o phase indicating the presence of fluids during uhp conditions strongly dissolved inherited magmatic zircons yielded shrimp 206pb 238u ages of 782 588 ma with an upper intercept age of 780 14 ma new recrystallized or metamorphic zircons yielded consistent triassic uhp ages around 238 225 ma with a weighted mean of 229 2 2 3 ma zircon trace element and hf isotope compositions and mineral inclusions classify the protolith of the sulu eclogite as an intermediate to mafic neoproterozoic igneous rock which was subducted to mantle depth and experienced uhp metamorphism in middle triassic times the juvenile ehf 780 ma value of 7 4 2 9 in inherited magmatic zircon points to a significant input from the depleted mantle to the dabie sulu uhp terrane during middle neoproterozoic new uhp zircon domains have low lu hf th u and significantly higher 176hf 177hf ratios than the inherited magmatic zircons consistent with formation during recrystallization and fractionation of the lu hf system during uhp metamorphism

facies analysis of the lo valdes formation tithonian hauterivian of the high cordillera of central chile and the palaeogeographic evolution of the andean basin

geochemical characteristics and hydrocarbon generation modeling of the jurassic source rocks in the shoushan basin north western desert egypt abstract the shoushan basin is an important hydrocarbon province in the western desert egypt but the origin of the hydrocarbons is not fully understood in this study organic matter content type and maturity of the jurassic source rocks exposed in the shoushan basin have been evaluated and integrated with the results of basin modeling to improve our understanding of burial history and timing of hydrocarbon generation the jurassic source rock succession comprises the ras qattara and khatatba formations which are composed mainly of shales and sandstones with coal seams the toc contents are high and reached a maximum up to 50 the toc values of the ras qattara formation range from 2 to 54 wt while khatatba formation has toc values in the range 1 47 wt the ras qattara and khatatba formations have hi values ranging from 90 to 261 mghc gtoc suggesting types ii iii and iii kerogen vitrinite reflectance values range between 0 79 and 1 12 vrr rock eval t max values in the range 438 458 c indicate a thermal maturity level sufficient for hydrocarbon generation thermal and burial history models indicate that the jurassic source rocks entered the mature to late mature stage for hydrocarbon generation in the late cretaceous to tertiary hydrocarbon generation began in the late cretaceous and maximum rates of oil with significant gas have been generated during the early tertiary paleogene the peak gas generation occurred during the late tertiary neogene

petroleum origin and accumulation in basin evolution a quantitative model basin data geologic geophysical geochemical hydrodynamic and thermodynamic can be combined for quantified hydrocarbon prediction a three dimensional deterministic dynamic basin model can be constructed to calculate all the measurable values with the help of mass and energy transport equations and equations describing the physical and or physicochemical changes in organic matter as a function of temperature input data consist of heat flux initial physical and thermal properties of sediments paleobathymetric estimates sedimentation rate and amount and type of organic matter subsequently the model computes pressure temperature physical and thermal properties of sediments maturity of organic matter and the hydrocarbon potential of any source rock as a func ion of space and time thus the complex dynamic processes of petroleum formation and occurrence in a given sedimentary basin can be quantified for example hydrocarbon potential maps for any given source rock and any geologic time slice of the basin evolution can be provided as computer printouts the computer model can be applied to any stage of an exploration campaign and updated as more information becomes available

controls on the siting and style of volcanogenic massive sulphide deposits abstract volcanogenic massive sulphide vms deposits form in subaqueous environments from circulating hydrothermal fluids heated by volcanic activity these deposits form as sulphide mounds stratiform exhalative and or replacive bodies and commonly have stockwork vein mineralization in their immediate footwall these various styles are essentially facies of mineralization each one being the product of a particular set of conditions that control the ore forming processes and the consequent geometry and architecture style of the deposits these controls include the physical and or chemical nature of the host rocks the temperature and composition of the hydrothermal fluids and the redox state of the depositional environment the style of exhalative deposits is controlled by the salinity of the vented fluids and the redox state at the seafloor hydrothermal fluids with salinities less than twice that of seawater that vented into open oxic oceanic environments typically formed small mound and chimney complexes unless they were rapidly covered by sediments or volcanic rocks the massive sulphides were rapidly oxidized and partly dissolved by seawater in contrast stratiform sheet like deposits are typically formed in anoxic bottom waters anoxic marine conditions were periodically of global extent particularly prior to 2 4 ga or of a regional nature local anoxic conditions can also be self induced by the exhalation of saline and reducing hydrothermal fluids that ponded in bathymetric depressions such as second or third order basins to form a brine pool these exhalative systems may have been initiated as chimney vent complexes and subsequently overlain by stratiform sulphides formed under the self induced anoxic conditions deposits formed in anoxic environments can be significantly larger than those in oxic settings and this is attributed to several factors that include longer lived hydrothermal circulation more efficient sulphide precipitation and reduced or inhibited oxidation thereof replacement of volcanic and sedimentary strata by sulphide typically occurs within the feeder zones beneath the exhalative mineralization however successions with abundant porous permeable and or reactive rocks such as glassy and or pumiceous volcaniclastic rocks and in some cases limestone favoured the development of large replacive deposits that may have had little surficial expression on the sea floor vms deposits at spreading centres within oceanic crust formed almost exclusively as mounds most of them have not been preserved likely due to oxidation of the sulphides in the prevailing oxic environment and or destruction of oceanic crust during subsequent subduction intra continental rifts arc rifts and back arc rifts commonly have more complexity in their structure and facies architecture and environments and can host all styles of vms mineralization in these settings early extension favoured the formation of restricted basins with ideal conditions for the onset of hydrothermal activity and development of anoxic bottom waters whereas in mature rifts the conditions were less conducive for the formation of regionally extensive anoxic environments formation of replacive deposits was permissible in all settings with porous or reactive subsea floor strata replacive mineralization is the most likely to be preserved in the geological record due to the sulphides being physically shielded from oxidative weathering and mechanical erosion at the seafloor the various styles of vms mineralization can rarely be distinguished using a single criterion in most cases multiple criteria are required mound style mineralization is distinguished by a mound or lens shaped morphology b presence of chimney fragments c presence of abundant sedimented sulphide breccias d location on a stratigraphic boundary ore horizon and e association with a thin horizon or thicker stratigraphic interval of fine grained clastic rocks e g shale mudstone that accumulated at slow sedimentation rates stratiform exhalative mineralization is distinguished by a sheet like morphology prior to deformation b presence of fine grained clastic host rocks that accumulated at relatively slow rates e g mudstone c presence of local or extensive planar stratification replacive mineralization is characterized by a irregular geometry and distribution of sulphide bodies b gradation from massive sulphides to semi massive sulphides and disseminated mineralization with relict textures of the host rock and c originally pumiceous glassy or reactive host rocks emplaced at high depositional rates mass flow deposits lavas carbonate altered mass flow deposits or limestone one deposit or district may comprise two or more of these main styles of mineralization in many cases the main styles of vms mineralization present in a particular region can be predicted from examination of the facies architecture and depositional environments of the host succession recognition of the style s of mineralization that occur in a particular basin or mineral belt enables exploration models to be improved and should influence the strategy of exploration for vms deposits

diagenetically enhanced bedding in argillaceous platform limestones stratified cementation and selective compaction a field and petrographic study has been made of 34 massive beds in argillaceous limestones of open marine platform facies in the u k lithologies include grainstones packstones wackestones and lime mudstones the rocks are of silurian carboniferous and jurassic ages additional information was obtained from other limestones in the u k the u s a canada and continental europe the beds are parts of sequences composed of couplets of strata fissile limestones alternating with hard limestones in the fissile limestones the effects of mechanical compaction and pressure dissolution have been concentrated whereas in the hard limestones they are minimal or absent bedding planes visible in outcrop are diagenetic in origin and lie in the middle parts of the fissile limestones where compaction has been most severe the features produced by pressure dissolution are dissolution seams and fitted fabric there are no stylolites the original carbonate sediments were bioturbated and any structures produced by flowing water were destroyed the vertical distribution of the bedding planes bears no relation to primary depositional bedding planes which are rare or absent it is inferred that the strata which were to become the hard limestones were selectively cemented before mechanical compaction had been completed thenceforth mechanical compaction and then pressure dissolution were concentrated in the less cemented strata these became the fissile limestones pressure dissolution acted late in the diagenetic history and provided only an insignificant part if any of the carbonate for cementation it is concluded that the orientation of beds couplets is parallel to successive sea floors and that the sediments that eventually became single beds accumulated synchronously similar couplets in platform limestones of the mississippian and pennsylvanian of the u s a extend over thousands of square kilometres the signal that controlled the initial selective cementation must have been widespread and synchronous and also syndepositional in its timing but otherwise cannot be further defined on the basis of the data so far collected the presumed order of events was 1 accumulation of carbonate sediment terrigenous clay and organic matter 2 hydrodynamic reworking and bioturbation the latter finally overprinting the former 3 selective cementation of the more carbonate rich strata yielding couplets each consisting of a relatively well cemented stratum and a poorly cemented stratum 4 mechanical compaction concentrated in the less cemented strata 5 pressure dissolution concentrated in the same strata

geology of el domo deposit in central ecuador a vms formed on top of an accreted margin el domo is a cu au zn ag volcanogenic massive sulfide deposit vms within the paleocene eocene submarine arc rocks of the macuchi formation in central ecuador the mineralization includes massive and semi massive sulfides located at the interface between a submarine rhyodacite dome complex and overlying mafic volcanic and volcaniclastic rocks including chaotic and unsorted debris flow deposits whole rock geochemical analyses show that the felsic rocks belong to a calc alkaline sequence whereas the mafic assemblages have a tholeiitic affinity reflecting a magmatic association typically found in many vms districts this geochemical change temporally coincides with the formation of the massive sulfides new zircon hf isotopic data collected from a hanging wall rhyodacite suggest that the presence of an over thickened crust of oceanic plateau affinity may have influenced the composition of the magmatic rocks the timing of massive sulfide formation is constrained by an 40ar 39ar age of 41 49 0 37 ma 2 from the volcaniclastic sequence which hosts the massive sulfides and a u pb zircon age date of 42 13 0 54 ma 2 from the footwall rhyodacite both ages are interpreted to provide minimum and maximum ages for mineralization the massive sulfides are zoned and contain variable proportions of pyrite sphalerite and chalcopyrite minor accumulations of galena bornite tennantite stromeyerite and proustite are also present the latter two minerals are the major carriers of silver whereas gold typically occurs as minute free grains or in solid solution with silver ore minerals are intergrown with variable proportions of chlorite barite quartz anhydrite and smectite the footwall rhyodacite hosts an irregular stringer zone which is rich in pyrite and gypsum and is characterized by intense quartz sericite pyrite alteration the hanging wall volcaniclastic lithologies exhibit pervasive but vertically limited 8 10 m silicification chloritization and argillization both felsic and mafic glass rich rocks located near the contact between rhyodacites and mafic volcaniclastic rocks are replaced by massive to disseminated mineralization likely controlled by their originally reactive and porous nature replacement appears to have taken place at very shallow depths 10 20 m below the seafloor as shown by 1 the presence of massive sulfide clasts within the uppermost volcaniclastic horizons and 2 the absence of mineralization and alteration in the overlying debris flows no evidence of exhalation on the seafloor has been found and replacement below the sea floor was probably an efficient mechanism for preserving the vms within such a highly dynamic volcanic system regionally mineralization occurred within an extensional submarine arc setting whereby massive sulfides were preferentially controlled by the reactivation of nne ssw crustal scale strike slip faults created during an earlier plate collision local stress regimes resulted in the formation of pull apart basins within a regional transpressive setting and provided conduits for magmatic and hydrothermal discharge the onset of volcanism and local extension coincided with an increase in the oblique convergence of the farallon and the south american plate

mode and timing of terrane accretion in the forearc of the andes in ecuador the volcanic basement of the ecuadorian western cordillera pallatanga formation and san juan unit is made up of mafi c and ultramafi c rocks that once formed an oceanic plateau radiometric ages from these rocks overlap with a hornblende 40 ar 39 ar plateau age of 88 1 6 ma age obtained for oceanic plateau basement rocks of the pinon formation in coastal ecuador and with ca 92 88 ma ages reported for oceanic plateau sequences in the caribbean and western colombia these results suggest that the oceanic plateau rocks of the western cordillera and fl at forearc in ecuador are derived from the late cretaceous caribbean colombia oceanic plateau intraoceanic island arc sequences rio cala group overlie the plateau in the western cordillera and yield crystallization ages that range between ca 85 and 72 ma the geochemistry and radiometric ages of island arc lavas from the rio cala group combined with the age range and geochemistry of their turbiditic volcaniclastic products indicate that the arc was initiated by westward subduction beneath the caribbean plateau they are coeval with island arc rocks of coastal ecuador las orquideas san lorenzo and cayo formations and colombia ricaurte arc these island arc

classification of dolomite rock textures abstract dolomite rock textures can be classified according to crystal size distribution and crystal boundary shape the classification scheme presented here is largely descriptive but carries genetic implications because size distribution is controlled by both nucleation and growth kinetics and crystal boundary shape is controlled by growth kinetics size distributions are classified as unimodal or polymodal crystal boundary shapes are classified as planar or nonplanar if the evidence permits a complete classification includes a description of recognizable allochems matrix and void filling allochems and preexisting cements may be unreplaced partially replaced replaced mimically or replaced nonmimically allochems may be dissolved leaving molds matrix can be unreplace partially replaced or replaced by a unimodal or polymodal size dolomite unimodal size distributions generally indicated a single nucleation event on a unimodal substrate polymodal sizes can be formed by multiple nucleation events on a unimodal or polymodal substrate or differential nucleation on an originally polymodal substrate planar crystal boundaries develop when crystals undergo faceted growth and nonplanar boundaries develop when crystals undergo nonfaceted growth nonplanar boundaries are characteristic of growth at elevated temperature 50 c and or high supersaturation both planar and nonplanar dolomite can form as a cement replacement of caco3 or neomorphism of a precursor dolomite

a palaeoclimate model for the kimmeridgian abstract most attempts at using numerical general circulation model gcm to simulate mesozoic climates have studied the late cretaceous and have focused on the enigmaof apparently equable global climates however this climate regime lasted throughout the mesozoic and it is of interest to examine other periods we present the results of an integration for the late jurassic using a relatively detailed and high resolution model there is generally good agreement between the model climate and the existing geological data in particular the simulation predicts no permanent ice cover near either pole arid conditions over the southwestern usa seasonally arid climate over southern europe and winter storminess over europe and australia however the surface temperatures over siberia and southeastern gondwanaland drop below zero for a significant part of the winter

facies related diagenesis and multiphase siderite cementation and dissolution in the reservoir sandstones of the khatatba formation egypt s western desert in the fluvio deltaic quartz arenitic sandstones of the jurassic khatatba formation in the salam field egypt9s western desert diagenesis and sedimentary facies control reservoir quality fluvial channel sandstones have the highest porosities 10 15 and permeabilities 100 600 md in part because of siderite cementation which 1 inhibited compaction and quartz cement and 2 was later dissolved creating intergranular secondary porosity 1 4 of total porosity fluvial crevasse splay and marine sandstones have the lowest reservoir quality because of an abundance of depositional kaolin matrix and pervasive shallow burial fe dolomite cement respectively siderite precipitation was multiphase and separated by distinct dissolution events the earliest siderite precipitated near surface within suboxic tropical coastal swamps containing predominantly meteoric waters some influence of marine waters is indicated by local enrichments in mg and ca the next major siderite generation shows a trend to decreasing mn and ca contents and is of shallow burial origin the last major siderite phase is mg rich and interpreted as deeper burial in origin some dissolution occurred during shallow burial related to climatically controlled meteoric water fluxing under unconformities the most important dissolution however occurred during deep burial resulting in 1 a major corrosion surface predating the last mg rich zone 2 selective dissolution of some earlier zones and 3 secondary porosity this burial dissolution is interpreted to have been caused by cooling of compactional waters expelled from the basin along major faults other diagenetic phases observed include early diagenetic pyrite kaolin quartz bitumen and late diagenetic barite and illite kaolinite precipitated at shallow depths 130 c during and or after rapid late cretaceous burial quartz developed in two phases separated by oil migration this paper illustrates that in fluvio deltaic quartz arenites deposited under the influence of humid tropical climate reservoir quality can be largely controlled by the contrasting pathways of carbonate diagenesis followed by the different sedimentary facies this paper also documents a case in which siderite dissolution generated significant secondary porosity in reservoir sandstones and where both siderite cementation and dissolution took place in multiple phases during different diagenetic stages including early shallow burial and deep burial diagenesis the present study also shows that in multiphase siderite cements the earlier growth zones can be selectively dissolved and replaced by later siderite zones i e recrystallized during burial diagenesis these findings contrast with the general thought that siderite cements are not susceptible to generation of significant secondary porosity by dissolution and that earlier formed siderites are essentially stable during diagenesis

source rock characteristics and hydrocarbon generation modelling of upper cretaceous mukalla formation in the jiza qamar basin eastern yemen abstract the upper cretaceous mukalla coals and other organic rich sediments which are widely exposed in the jiza qamar basin and believed to be a major source rocks were analysed using organic geochemistry and petrology the total organic carbon toc contents of the mukalla source rocks range from 0 72 to 79 90 with an average toc value of 21 50 the coals and coaly shale sediments are relatively higher in organic richness consistent with source rocks generative potential the samples analysed have vitrinite reflectance in the range of 0 84 1 10 ro and pyrolysis t max in the range of 432 454 c indicate that the mukalla source rocks contain mature to late mature organic matter good oil generating potential is anticipated from the coals and coaly shale sediments with high hydrogen indices 250 449 mg hc g toc this is supported by their significant amounts of oil liptinite macerals are present in these coals and coaly shale sediments and py gc s 2 pyrograms with n alkane alkene doublets extending beyond nc 30 the shales are dominated by type iii kerogen hi one dimensional basin modelling was performed to analysis the hydrocarbon generation and expulsion history of the mukalla source rocks in the jiza qamar basin based on the reconstruction of the burial thermal maturity histories in order to improve our understanding of the of hydrocarbon generation potential of the mukalla source rocks calibration of the model with measured vitrinite reflectance ro and borehole temperature data indicates that the present day heat flow in the jiza qamar basin varies from 45 0 mw m 2 to 70 0 mw m 2 and the paleo heat flow increased from 80 ma to 25 ma reached a peak heat flow values of approximately 70 0 mw m 2 at 25 ma and then decreased exponentially from 25 ma to present day the peak paleo heat flow is explained by the gulf of aden and red sea tertiary rifting during oligocene middle miocene which has a considerable influence on the thermal maturity of the mukalla source rocks the source rocks of the mukalla formation are presently in a stage of oil and condensate generation with maturity from 0 50 to 1 10 ro oil generation 0 5 ro in the mukalla source rocks began from about 61 ma to 54 ma and the peak hydrocarbon generation 1 0 ro occurred approximately from 25 ma to 20 ma the modelled hydrocarbon expulsion evolution suggested that the timing of hydrocarbon expulsion from the mukalla source rocks began from 15 ma to present day

origin and cretaceous tectonic history of the coastal ecuadorian forearc between 1 n and 3 s paleomagnetic radiometric and fossil evidence the presence of at least one block pallatanga block derived from the caribbean colombian oceanic plateau has been proposed in the western cordillera of ecuador new 40ar 39ar dating of the basement of the coastal blocks 90 87 ma and fossil ages in the overlying calentura fm 89 84 ma argue for late turonian coniacian extrusion of the basement of the pinon block paleomagnetic inclinations indicate that the mafic basement of the pinon and san lorenzo blocks extruded at equatorial low southern latitudes no subsequent significant latitudinal drift can be observed during the cretaceous rapid changes in paleomagnetic declination between 73 and 70 ma in both the pinon and the san lorenzo blocks indicate that the large igneous province which originally hosted the coastal blocks experienced a major tectonic event during late campanian times similar late turonian coniacian basement ages and late campanian tectonic events are reported in the pallatanga block of the western cordillera we propose that the pinon san lorenzo and pallatanga blocks all derive from a common large igneous province the caribbean colombian oceanic plateau and were incorporated into the south american margin by collision during the late campanian 73 70 ma

petroleum source rock characterisation and hydrocarbon generation modeling of the cretaceous sediments in the jiza sub basin eastern yemen non source to good source rocks were identified in the studied cretaceous rocks oil and gas generation were expected from cretaceous source rocks biomarkers reflect marine and deltaic depositional environments for cretaceous source rocks oil generation from qishn formation was began in late cretaceous

the milankovitch fingerprint on the distribution and thickness of bedding parallel veins beef in source rocks bed parallel mineralized fractures are common in source rocks and generally consist in mm to cm thick veins developed parallel to bedding known as beef or bedding parallel veins considering they can form a dense network of mechanical discontinuities the prediction of beef distribution is a major issue impacting shale reservoir production beef distribution is predominantly controlled by the lithological characteristics of source rocks and we here decipher the relation between mineralogical and chemical proxies controlled by orbital parameters and distribution of the beef along a late jurassic section of the well known vaca muerta formation source rock in the neuquen basin using multiple proxies collected along the beef rich huncal section we show that milankovitch cycles rule the mineralogical evolution and beef distribution in these organic rich mudrocks cycles inferred from the statistical treatment of sedimentary magnetic susceptibility elemental and mineralogical ratios biogenic total organic carbon and diagenetic beef distribution and thickness signals revealed indeed the influence of an astroclimatic fingerprint in sediments and on processes controlling mineralized fracture generation and distribution the astroclimatic memory recorded in many source rocks worldwide is therefore envisaged as a suitable proxy for the prediction of mineralized fracture distribution

crustal growth along a non collisional cratonic margin a lu hf isotopic survey of the eastern cordilleran granitoids of peru abstract an extensive in situ lu hf isotopic study of zircon from the eastern cordilleran batholiths of peru using laser ablation multi collector inductively coupled mass spectrometry la mc icpms reveals a systematic covariance between granitoid magma sources and tectonic regimes which shaped the proto andean margin of central western amazonia since 1 15 ga the hf isotope systematics are characterised by a range in initial 176hf 177hf compositions for a given intrusive event suggesting mixing of material derived from the paleoproterozoic crustal substrate and variable addition of juvenile sources from neoproterozoic to cenozoic time intrusives associated with phases of regional compressive tectonism correspond to mean initial ehf values of 6 73 2 43 1 57 for the ordovician famatinian carboniferous permian and late triassic respectively suggesting a minimum crustal contribution between 40 and 74 by mass the average initial hf systematics from granitoids associated with periods of regional extension such as the middle neoproterozoic permian triassic and cenozoic andean back arc plutonism are consistently shifted towards positive values mean initial ehf 0 7 to 8 0 indicating systematically larger inputs of juvenile magma 25 to 38 of ancient crust by mass in the absence of evidence for significant lateral accretion of exotic crust the time integrated hf record from the central proto andean margin of western amazonia suggests crustal reworking was the dominant process during episodes of arc magmatism implying that most crustal growth took place vertically via crustal underplating of isotopically juvenile mantle derived melts during intervals of crustal extension

dolomitization of deep water sediments cow head group cambro ordovician western newfoundland abstract dolomite in the cow head group occurs as 1 dolomitic siltstone interbedded with dolomitized or undolomitized shale 2 prominent tan weathering argillaceous partings between bedded limestones 3 disseminated crystals in limestones and 4 as a component of the interparticle matrix in conglomerates dolomite crystals are present as detrital cores cement or replacement of limestone and shale crystals are commonly characterized by inclusion rich cores inclusion poor rims and late iron enrichment many inclusion rich cores in disseminated dolomite crystals and dolomitic siltstones are detrital most inclusion rich cores however probably originated from replacement of peloids and possibly other allochems these detrital and authigenic cores are overgrown by inclusion poor authi enic rims other authigenic crystals lack an obvious core region and consist entirely of inclusion poor dolomite the authigenic dolomite originated in uncompacted or partially compacted sediments during shallow burial below the sediment seawater interface as indicated by 1 the occurrence of dolomitic siltstone and shale clasts in polymictic limestone conglomerates 2 sharp erosive contacts between dolomitic siltstone and shale and overlying undolomitized shale or conglomerate and 3 spatial association with columnar calcite cement interpreted to be a shallow burial precipitate in clast supported conglomerates the dolomite crystals continued to enlarge rather than recrystallize during deeper burial as indicated by variable and complex cathodoluminescence zoning an organic signature is not apparent in their carbon isotopic compositions the oxygen isotopic composition of these crystal reflects the deeper burial conditions of higher temperatures more evolved pore waters or both the cow head group also contains minor late diagenetic dolomites which occur as cement and a replacement of limestone shale and earlier dolomite this dolomite postdates tectonic stylolites and is spatially associated with joints and faults

widespread bedding parallel veins of fibrous calcite beef in a mature source rock vaca muerta fm neuquen basin argentina evidence for overpressure and horizontal compression in the neuquen basin of argentina beef bedding parallel veins of fibrous calcite is widespread within late jurassic black mudstones a typical vein consists of two grey inner zones and two white outer zones the inner zones contain inclusions of wall rock and hydrocarbons calcite fibres are perpendicular to the margins in the outer zones the angle between fibre and margin varies from about 45 at the vein tips to 90 in the centre imprints of fossils are offset proving that the fibres have grown antitaxially we infer that the veins opened in two phases during phase 1 the opening was vertical against gravity during phase 2 the veins resisted tectonic shortening so that shear stresses acted at the margins the senses of shear account for the fibre angles at outcrop igneous intrusive rocks have cut and metamorphosed the veins from burial curves maturity calculations growth strata and ages of igneous intrusions we estimate that the inner zones of the beef formed in the oil window during the aptian to albian and that the outer zones formed in the gas window during the cenomanian to campanian we infer that the beef is evidence for fluid overpressure during hydrocarbon generation

a new evaluation of palaeo heat flows and eroded thicknesses for the carboniferous ruhr basin western germany palaeo heat flows and thicknesses of eroded carboniferous overburden in the coal bearing ruhr basin western germany were estimated using one dimensional basin modelling techniques thermal and burial histories for 11 localities were calibrated by comparing measured and calculated vitrinite reflectance data based on the assumption of pre tectonic maturation in contrast to former studies the kinetic easy r0 approach applied for the calculation of vitrinite reflectance lopatin s tti algorithm was used in addition for one well in order to demonstrate the impact of the selection of the algorithm for vitrinite reflectance calculation on the simulation results simulated palaeo heat flows for the time of maximum burial most probably in the stephanian range from 64 to 83 mw m2 and decrease towards the south these heat flows are lower than values computed during earlier studies but are still relatively high in comparison to recent foreland basins the regional distribution of palaeo heat flows indicates that the crust beneath the ruhr basin was relatively thin and thickened towards the south at the end of the carboniferous after the stephanian 2200 3500 m of carboniferous overburden were eroded an observed southward increase in eroded thicknesses is caused by an uplift of the area studied which decreased towards the north at the end of the variscan folding large thicknesses of deposited and later eroded carboniferous sediments have to be postulated to explain the maturity data even along the present southern margin of the ruhr basin this indicates that the late carboniferous sediments originally extended far to the south geological arguments suggest that the deposition of carboniferous sediments in the entire ruhr basin continued until the stephanian

relative motion of the nazca farallon and south american plates since late cretaceous time by combining reconstructions of the south american and african plates the african and antarctic plates the antarctic and pacific plates and the pacific and nazca plates we calculated the relative positions and history of convergence of the nazca and south american plates despite variations in convergence rates along the andes periods of rapid convergence averaging more than 100 mm a between the times of anomalies 21 49 5 ma and 18 42 ma and since anomaly 7 26 ma coincide with two phases of relatively intense tectonic activity in the peruvian andes known as the late eocene incaic and mio pliocene quechua phases the periods of relatively slow convergence 50 to 55 30 mm a at the latitude of peru and less farther south between the times of anomalies 30 31 68 5 ma and 21 and between those of anomalies 13 36 ma and 7 correlate with periods during which tectonic activity was relatively quiescent thus these reconstructions provide quantitative evidence for a correlation of the intensity of tectonic activity in the overriding plate at subduction zones with variations in the convergence rate

depositional sequences in deep shelf environments a response to sea level changes and shallow platform carbonate productivity oxfordian germany and spain abstract oxfordian deep shelf deposits of southern germany and southern spain are characterized by marl limestone alternations that are stacked into small scale medium scale and large scale depositional sequences the german sections contain autochthonous sponge reefs and associated fragments of microbialites whereas the sections studied in spain display tempestites composed of autochthonous deeper water and allochthonous shallow water particles the facies of the german limestones and marl layers has been analyzed in detail condensed intervals implied by glauconitization of particles abundant cephalopods intense bioturbation and generally more marls are also enriched in sponge reefs and associated particles and in nannofossils limestone rich intervals however contain fewer sponges and fewer nannofossils neither bioerosion of sponge reefs nor nannofossil blooms can thus explain the abundance of carbonate mud that forms the limestones consequently it is suggested that most of the carbonate mud is exported from shallow platform areas where carbonate productivity is high the clay fraction was derived from weathering of massifs in the hinterland the observed depositional sequences can be correlated between the studied sections in germany and spain situated in different paleotectonic and paleoclimatic domains and also between deeper water sections and platform sections this suggests that they formed through allocyclical processes comparision with published time scales implies that the small scale and medium scale sequences formed in tune with the 100 kyr and 400 kyr orbital eccentricity cycles respectively however the number of marl limestone alternations is not always consistent with the expected number of 20 kyr precessional cycles 5 per 100 kyr cycle the large scale sequences reflect long term third order sea level changes the observed marl limestone alternations are interpreted to have formed through cyclically varying export of carbonate mud from the platform towards the deep shelf the variations being controlled by climatically induced high frequency sea level fluctuations enhanced marl deposition on the deep shelf can be related to sea level fall causing exposure of the shallow platform and reducing the area of carbonate production or by rapid sea level rise maximum flooding leading to partial or total drowning of the platform and or to retrogradation of facies belts enhanced carbonate deposition occurs during transgression when large production areas are created on the platform or during late highstands when progradation is forced and carbonate mud export enhanced depending on the long term trend of sea level change on which the high frequency fluctuations are superimposed one high frequency 20 kyr sea level cycle can thus create one or two marl limestone alternations or only marly deposits consequently one 100 kyr eccentricity cycle may be formed of a variable number of marl limestone alternations commonly 2 to 8 the studied deeper water depositional sequences thus are strongly linked to the history of the adjacent shallow carbonate platforms

basin modeling of the late miocene zeit source rock in the sudanese portion of red sea basin implication for hydrocarbon generation and expulsion history abstract the late miocene zeit formation is exposed in the red sea basin of sudan and represents an important oil source rock in this study five 5 exploratory wells along red sea basin of sudan are used to model the petroleum generation and expulsion history of the zeit formation burial thermal models illustrate that the red sea is an extensional rift basin and initially developed during the late eocene to oligocene heat flow models show that the present day heat flow values in the area are between 60 and 109 mw m2 the variation in values of the heat flow can be linked to the raise in the geothermal gradient from margins of the basin towards offshore basin the offshore basin is an axial area with thick burial depth which is the principal heat flow source the paleo heat flow values of the basin are approximately from 95 to 260 mw m2 increased from oligocene to early pliocene and then decreased exponentially prior to late pliocene this high paleo heat flow had a considerable effect on the source rock maturation and cooking of the organic matter the maturity history models indicate that the zeit formation source rock passed the late oil window and converted the oil generated to gas during the late miocene the basin models also indicate that the petroleum was expelled from the zeit source rock during the late miocene 7 ma and it continues to present day with transformation ratio of more than 50 therefore the zeit formation acts as an effective source rock where significant amounts of petroleum are expected to be generated in the red sea basin

modelling petroleum generation of late cretaceous dabut formation in the jiza qamar basin eastern yemen abstract the jiza qamar basin in eastern yemen is still undergoing hydrocarbon exploration and hydrocarbon potential has not been assigned yet in this study subsurface samples from two onshore and offshore wells were collected to characterize the source rocks of dabut formation and to incorporate into basin modelling in order to know and determine the timing of petroleum generation and expulsion of the dabut source rocks based on organic geochemical analysis the dabut sediments have variable toc content in the range of 0 5 2 5 wt indicating fair to good source rock generative potential the organic matter of dabut source rock is dominated by type iii kerogen with minor mixed iii ii kerogen contributions and is thus considered to be mainly gas prone and some oil prone this is supported by hi values in the range of 50 212 mg hc g toc vitrinite reflectance in the range of 0 63 0 88 ro and pyrolysis tmax in the range of 427 454 c generally indicate that the dabut source rocks contain mature organic matter one dimensional basin modelling was performed to analyse the hydrocarbon generation and expulsion history of the dabut source rocks in the jiza qamar basin based on the reconstruction of the burial and thermal maturity histories in order to improve our understanding of the hydrocarbon generation potential calibration of the model with measured vitrinite reflectance ro and borehole temperature bht data indicates that the paleo heat flow was constant during the late cretaceous paleogene but increased during the late paleogene neogene and then decreased exponentially from neogene to present day the modelled maturity history predicts that the oil generation 0 5 ro in the dabut source rock began from about 56 ma to 42 ma and the peak hydrocarbon generation 0 86 ro occurred approximately from 14 ma to 13 ma these models also indicate that the main phase of oil generation was at 56 ma to 25 ma whereas the main phase of gas generation began from about 25 ma to 14 ma and still generated in the present day

organic geochemical characteristics and depositional environment of the tertiary tanjong formation coals in the pinangah area onshore sabah malaysia article i nfo article history the tertiary tanjong formation coals exposed in the west middle block of the pinangah coalfield central part of southern sabah were analyzed and their depositional environments were interpreted the tertiary tanjong coals are humic and generally dominated by vitrinite with significant amounts of liptinite and low amounts of inertinite macerals total organic carbon contents toc of the coals range from 51 2 to 77 7 wt and yield of bitumen values ranging from 57 300 to 140 000 ppm which meet the standard as a source rock with good hydrocarbon generative potential in support good liquid hydrocarbons generation potential can be expected from the tanjong coals based on significant liptinitic content 15 this is supported by their high hydrogen index up to 300 mg hc g toc consistent with type ii and mixed type ii iii kerogens and py gc s2 pyrograms with n alkane alkene doublets extending beyond c30 the coal samples have vitrinite reflectance values in the range of 0 42 0 66 ro indicating immature to early mature stage for hydrocarbon generation these vitrinite reflectance values also indicated that the tanjong coals are with sub bituminous b a and high volatile bituminous c rank tmax values ranging from 419 to 451 c are good agreement with the vitrinite reflectance data this is supported by biomarker maturity parameters as suggested by the c32 homohopane the saturated fraction of the tanjong coals are characterized by dominant odd carbon numbered n alkanes n c23 to n c33 high pr ph ratios 8 18 high tm ts ratios 6 28 and pre dominant of regular sterane c29 all biomarker parameters clearly indicate that the organic matter was de rived from terrestrial inputs and deposited under oxic condition

geochemical characterization of solid bitumen migrabitumen in the jurassic sandstone reservoir of the tut field shushan basin northern western desert of egypt abstract asphaltene rich solid bitumen was identified within the pore spaces of the middle jurassic khatatba sandstone reservoir in the tut field shoushan basin egypt despite good reservoir properties have been detected the solid bitumen forms barriers in the reservoir which prevents economic petroleum production petrographical and organic geochemical techniques were applied in order to characterize the source rock of the bitumen and its formation mechanisms organic geochemical results confirm the presence of actively migrating non biodegraded hydrocarbons within the khatatba sandstones biomarker data suggest that the solid bitumen originated from a marine shale source rock that was deposited in mildly anoxic to suboxic conditions biomarker oil source rock correlation with the organic rich sediments of the khatatba formation indicates that the shale and coaly shale within the khatatba formation are an effective source rock in the area the natural deasphalting is the predominant mechanism for the formation of the asphaltene rich solid bitumen in the khatatba sandstones adsorption of asphaltenes onto authigenic kaolinite played a major role in the fixation of bitumen furthermore gas deasphalting caused by gas injection may have contributed to a phase change in the reservoir fluid and deposition of solid bitumen the injected gas is generated from khatatba type iii kerogen or older source rocks in the shoushan basin during the mature stage of hydrocarbon generation

thermal maturity history and petroleum generation modelling for the upper jurassic madbi source rocks in the marib shabowah basin western yemen abstract widely exposed in the marib shabowah basin the early upper jurassic madbi formation is divided into two units lam and meem youngest to oldest believed to be major source rocks the madbi source rocks are characterized by relatively high total organic carbon contents toc ranging from 0 5 to 10 0 wt meeting the standard as a fair to excellent source rock generative potential they contain type ii kerogen grading to mixed type ii iii kerogens and type iii kerogen with hi values between 40 and 620 mg hc g toc vitrinite reflectance values range from 0 41 to 2 61 r o indicating sufficient thermal maturity for both oil and gas generation burial thermal history models indicate that the upper jurassic madbi source rocks passed the peak of oil generation stage and convert the oil to dry gas in the late jurassic to late cretaceous time oil generation began immediately after deposition in the late jurassic time 150 145 ma and maximum rates of oil with significant gas have been generated during late cretaceous time 110 65 ma the peak gas generation occurred during the early tertiary the modelled hydrocarbon expulsion evolution suggests that the timing of hydrocarbon expulsion from the madbi source rocks began in the early cretaceous 140 ma persisted to present day the modelling results also indicate that the peak hydrocarbon generation and expulsion from madbi source rocks in the studied area occurs well after deposition of the seal rock and formation of traps ensuring entrapment and preservation of migrated hydrocarbons

time and temperature in petroleum formation application of lopatin s method to petroleum exploration n v lopatin in the soviet union has developed a method for taking both time and temperature into account as factors in thermal maturation of kerogen lopatin s time temperature index of maturity tti values correlate with the thermal regimes corresponding to generation and preservation of hydrocarbons because such information is potentially of great interest for oil exploration a calibration and evaluation have been made of lopatin s method within the limitations of the data presently available the following statements can be made 1 the rate of the chemical reactions involved in thermal maturation of organic material appears to double with every 10 c 18 f rise in temperature 2 threshold values of lopatin s time temperature index of maturity tti are 15 onset of oil generation 75 peak oil generation 160 end oil generation 500 40 oil preservation deadline 1 000 50 oil preservation deadline 1 500 wet gas preservation deadline 65 000 dry gas preservation deadline 3 tti values calculated from lopatin reconstructions consistently agree with other maturation parameters commonly used by petroleum geochemists potential applications of lopatin s method for oil exploration include timing of oil generation calculation of volume of hydrocarbons generated within a basin and determination of economic deadlines

distinguishing between in situ and accretionary growth of continents along active margins abstract active continental margins represent both sites of crustal loss by tectonic erosion and gain by juvenile magmatism and oceanic arc plateau accretion estimating the gain component depends on distinguishing between growth of margins by either accretion of oceanic crust or juvenile magmatic addition during extension distinguishing between these allochthonous and autochthonous models is frequently ambiguous due to difficulties in interpreting the tectonic origin of mafic rocks we present a detailed geochronological and isotopic study constraining the evolution of the continental margin of northwest south america during 190 113 ma semi continuous slab rollback resulted in crustal thinning juvenile magmatism and ultimately the removal of continental slivers negating previous hypotheses that the margin evolved by the accretion of an island arc however basic crustal growth estimates along with comparisons between rocks exposed in northern peru and the northern andes imply that uneven losses of continental crust occurred along northwest south america for mesozoic exposures and that the accretion of the caribbean large igneous province clip onto nw south america probably played a crucial role in maintaining and preserving mesozoic continental crust north of the huancabamba deflection

the gold content of volcanogenic massive sulfide deposits volcanogenic massive sulfide deposits contain variable amounts of gold both in terms of average grade and total gold content with some vms deposits hosting world class gold mines with more than 100 t au previous studies have identified gold rich vms as having an average gold grade expressed in g t exceeding the total abundance of base metals expressed in wt however statistically meaningful criteria for the identification of truly anomalous deposits have not been established this paper presents a more extensive analysis of gold grades and tonnages of 513 vms deposits worldwide revealing a number of important features in the distribution of the data a large proportion of deposits are characterized by a relatively low gold grade 2 g t with a gradual decrease in frequency towards maximum gold grades defining a log normal distribution in the analysis presented in this paper the geometric mean and geometric standard deviation appear to be the simplest metric for identifying subclasses of vms deposits based on gold grade especially when comparing deposits within individual belts and districts the geometric mean gold grade of 513 vms deposits worldwide is 0 76 g t the geometric standard deviation is 2 70 g t au in this analysis deposits with more than 3 46 g t au geometric mean plus one geometric standard deviation are considered auriferous the geometric mean gold content is 4 7 t au with a geometric standard deviation of 26 3 t au deposits containing 31 t au or more geometric mean plus one geometric standard deviation are also considered to be anomalous in terms of gold content irrespective of the gold grade deposits with more than 3 46 g t au and 31 t au are considered gold rich vms a large proportion of the total gold hosted in vms worldwide is found in a relatively small number of such deposits the identification of these truly anomalous systems helps shed light on the geological parameters that control unusual enrichment of gold in vms at the district scale the gold rich deposits occupy a stratigraphic position and volcanic setting that commonly differs from other deposits of the district possibly due to a step change in the geodynamic and magmatic evolution of local volcanic complexes the gold rich vms are commonly associated with transitional to calc alkaline intermediate to felsic volcanic rocks which may reflect a particularly fertile geodynamic setting and or timing e g early arc rifting or rifting front at the deposit scale uncommon alteration assemblages e g advanced argillic aluminous strongly siliceous or potassium feldspar alteration and trace element signatures may be recognized e g au ag as sb bi hg te suggesting a direct magmatic input in some systems

mesozoic arc magmatism along the southern peruvian margin during gondwana breakup and dispersal abstract a high resolution u pb zircon geochronological study of plutonic units along the south peruvian margin between 17 and 18 s allows the integration of the geochemical geodynamic and tectonic evolution of this part of the andean margin this study focuses on the composite jurassic early cretaceous ilo batholith that was emplaced along the southern peruvian coast during two episodes of intrusive magmatism a first period between 173 and 152 ma with a peak in magmatic activity between roughly 168 and 162 ma and a second period between 110 and 106 ma emplacement of the jurassic part of the composite ilo batholith shortly post dated the accumulation of the volcanosedimentary succession it intruded chocolate formation which allows to estimate a subsidence rate for this unit of 3 5 km ma the emplacement of the main peak of jurassic plutonism of the ilo batholith was also closely coeval with widespread and repeated slumping during deposition of the cachios formation in the back arc region suggesting a common causal link between these phenomena which is discussed in the context of an observed 100 km trenchward arc migration at 175 ma and the relation with extensional tectonics that prevailed along the central andean margin during pangaea break up

thermochronology of allochthonous terranes in ecuador unravelling the accretionary and post accretionary history of the northern andes the western cordilleras of the northern andes north of 5 s are constructed from allochthonous terranes floored by oceanic crust we present 40ar 39ar and fission track data from the cordillera occidental and amotape complex of ecuador that probably constrain the time of terrane collision and post accretionary tectonism in the western andes the data record cooling rates of 80 2 c my from temperatures of 540 c during 85 to 60 ma in a highly tectonised melange pujili unit at the continent ocean suture and in the northern amotape complex the rates were highest during 85 80 ma and decelerated towards 60 ma cooling was a consequence of exhumation of the continental margin which probably occurred in response to the accretion of the presently juxtaposing pallatanga terrane the northern amotape complex and the pujili unit may have formed part of a single regional scale tectonic melange that started to develop at 85 ma part of which currently comprises the basement of the interandean depression cooling and rotation in the allochthonous continental amotape complex and along parts of the continent ocean suture during 43 29 ma record the second accretionary phase during which the macuchi island arc system collided with the pallatanga terrane distinct periods of regional scale cooling in the cordillera occidental at 13 and 9 ma were synchronous with exhumation in the cordillera real and were probably driven by the collision of the carnegie ridge with the ecuador trench finally late miocene pliocene reactivation of the chimbo toachi shear zone was coincident with the formation of the oldest basins in the interandean depression and probably formed part of a transcurrent or thrust system that was responsible for the inception and subsequent growth of the valley since 6 ma

palaeocene oceans and climate an isotopic perspective abstract the early cenozoic was a time of climatic and oceanographic transition from the cretaceous greenhouse world to the icehouse world of the neogene 18 o measurements shed light on ocean temperature and possible polar ice fluctuations during this interval while 13 c measurements monitor fluctuations in ocean productivity deep water circulation and atmospheric co 2 the major features from 18 o analysis of the early cenozoic are general cooling of surface waters with some evidence for transient cooling across the k t boundary surface water temperatures were at a cenozoic maximum in the early eocene whereas deep waters cooled then warmed during the palaeocene the 13 c of bulk carbonates is at a minimum at the start of the cenozoic due to the profound crisis in ocean surface water productivity associated with the extinctions of marine plankton at the cretaceous tertiary boundary thereafter 13 c values increase in bulk carbonates as well as planktonic and benthonic foraminifera to their cenozoic maximum in the late palaeocene c 60 ma after which time they again decrease over an interval of c 4 5 m y to a cenozoic low in the early eocene the increase in 13 c values characteristic of the palaeocene period is probably related to a combination of increasing surface water productivity and accelerated burial of organic carbon conversely the decline in 13 c into the early eocene is probably related to a decrease in ocean productivity and a deceleration in the rate of organic carbon burial benthic 13 c comparisons suggest that deep waters appear to have been predominantly formed in the high southern latitudes with the exception of a short lived interval near the palaeocene eocene boundary possibly associated with a transient climatic anomaly the late palaeocene thermal maximum the thermal change of palaeocene deep waters may be related to the changing productivity of surface waters by controlling atmospheric co 2 flux in a similar way to that proposed for the control of the pleistocene glacial cycles

sedimentology and sequence stratigraphy of a tithonian valanginian carbonate ramp vaca muerta formation a misunderstood exceptional source rock in the southern mendoza area of the neuquen basin argentina abstract the vaca muerta formation early tithonian early valanginian is a rhythmic succession of marls and limestones cropping out in the neuquen basin west central argentina this lithostratigraphic unit was traditionally interpreted as basinal to slope deposits detailed facies analysis allows to differentiate seven facies associations representing basinal to middle ramp facies of a homoclinal ramp system prograding westward from the eastern margin and slope facies attributed to a distally steepened ramp system that progrades eastward from the andean volcanic arc in the west two sequence hierarchies are recognized five third order depositional sequences and fifteen fourth order high frequency sequences fluctuations in organic matter content within the vaca muerta formation suggest relationship with depositional sequences finding the highest values associated with transgressive system tracts this work represents an important advance in the understanding of the sedimentary and stratigraphic evolution of this exceptional unconventional reservoir our sequence stratigraphic approach contributes to the understanding of the relationship between organic matter facies and sea level changes

allochthonous terranes in northwestern ecuador abstract formations and lithostratigraphic units of the ecuadorian northwestern cordillera can be divided into three major groups the first group comprises lower cretaceous e morb basalts conformably overlain by tholeiitic to boninitic components of an ensimatic island arc the second group consists mainly of tholeiitic to calc alkaline volcanic and volcano sedimentary island arc deposits unconformably on top of the first group and of eocene to early oligocene age the third group is represented by calc alkaline continental volcanics ranging from late oligocene to recent two main tectonic events occur during early eocene and middle oligocene times the first event can be interpreted as an accretion of a slab of oceanic basalt with its overlying arc material the suture zone is marked by slices of morb type material delineating the southward extension of the colombian cauca patia fault zone the second event is related to a major rearrangement of plate tectonic regimes in the eastern pacific during oligocene times its nature is most probably transpressional accompanied by thrusting

sequence stratigraphy of a tidally dominated carbonate siliciclastic ramp the tithonian early berriasian of the southern neuquen basin argentina the tithonian berriasian vaca muerta carrin cura and picun leufu formations in the southern neuquen basin were deposited on a tidally dominated mixed carbonate siliciclastic ramp basinal outer middle shallow and back ramp facies associations are recognized and a sequence stratigraphic analysis reveals that the ramp record consists of three shallowing upwards sequences ti1 ti2 and ti3 set within a lower order progradational cycle a higher order of cyclicity is superimposed on to the middle ti2 sequence the majority of the ramp facies belong to the transgressive and highstand systems tracts however at the base of ti2 a lowstand systems tract is identified characterized by a basal unconformity and an abrupt basinward shift of the shallow marine lithofacies transgressive systems tracts were characterized by slow sedimentation rates and rapid sea level rises that affected carbonate productivity highstand systems tracts show the greatest carbonate productivity and an increased progradation rate on account of a reduction in accommodation space generation palaeogeography played a major role in the development of the depositional systems partial isolation from the pacific ocean reflecting the growth of the andean magmatic arc and geographic restriction due to tectonic inversion in the central part of the basin resulted in a meso macrotidal regime that produced a tidally dominated sedimentary record in the shallow and back ramp environments coeval anoxic conditions in the central part of the neuquen basin favoured distal ramp and basinal black shale deposition during episodes of relative sea level rise

hydrothermal alteration and volatile element halos for the rosebery k lens volcanic hosted massive sulfide deposit western tasmania a detailed study of alteration mineralogy mineral chemistry and lithogeochemistry in the host rocks surrounding the a b and k lenses at the north end of the rosebery mine has revealed a series of overlapping alteration halos with characteristic mineralogy and geochemistry the study involved logging and sampling from nine drill holes spaced at varying distances from the a b and k lenses the stratiform zn pb cu ore lenses have a sheetlike morphology and are hosted by a mixed sequence of rhyolitic to dacitic massive mediumgrained quartz porphyritic pumice breccia black mudstone and crystal rich volcaniclastic sandstone overlying a thick homogeneous sequence of rhyolitic pumice breccia the major alteration minerals at rosebery are arranged in a complex series of zones passing away from the deposit quartz sericite zone mn carbonate zone chlorite zone and outer sericite zone the chlorite zone is best developed in the immediate footwall below the copper pyrite rich ore lenses whereas the strongest mn carbonate alteration occurs in the immediate hanging wall volcanics or lateral to the ore lenses the outermost visible sericitic alteration extends about 60 to 100 m into the footwall 10 to 20 m into the hanging wall and over 500 m along the upper contact of the footwall pumice breccias thallium and antimony form the most extensive trace element halos related to the mineralization thallium forms a halo that extends 200 to 300 m into the overlying volcanics and 60 to 100 m into the footwall anomalously high thallium also occurs over 500 m along strike marking the contact between the footwall pumice breccias and the overlying volcaniclastic sandstones proximal to the ore lenses tl and sb values range from 10 to 100 ppm compared to the halo zone where they vary from around 1 to 10 ppm studies of alteration mineral chemistry at rosebery have revealed some important relationships that may assist exploration the mn content of alteration carbonate increases toward ore both along strike and across strike close to ore alteration carbonates contain 20 mole percent mnco3 kutnahorite rhodochrosite mn siderite and mn ankerite whereas at distances of 40 to 60 m across strike the mole percent mnco3 in carbonate drops to below 10 percent at greater than 80 m the carbonates are mn poor calcites and are commonly located in synmetamorphic structures white mica composition changes with stratigraphy and alteration assemblages and may be related to the mineralizing event although this has not been convincingly demonstrated proximal white mica contains minor ba substituting for octahedral al however except for their ba content these phengetic white micas are similar to those found in nonmineralized areas of the mount read volcanics sodic white mica with up to 0 35 na na k and a low phengite content 0 5 fe mg cations occurs in a zone of volcanic sandstones and black slates overlying the ore deposit this research has lead to the development of a series of proximal medial and distal vectors useful for both regional and mine scale exploration the most useful vectors listed from proximal to distal include zn ba content of white mica na2o k2o ishikawa alteration index ai s na2o ba sr mn content of carbonate tl and sb

dolomitization and dolomite neomorphism trenton and black river limestones middle ordovician northern indiana u s a abstract the trenton and black river limestones are dolomitized extensively along the axis of the kankakee arch in indiana with the proportion of dolomite decreasing to the south and southeast of the arch planar and nonplanar dolomite replacement textures and rhombic type 1 and saddle type 2 void filling dolomite cements are present three stages of dolomitization involving different fluids are inferred on the basis of petrographic and geochemical characteristics of the dolomites nonferroan planar dolomite has relatively high 18o values 1 8 to 6 1o oo pdb and has 87sr 86sr ratios 0 70833 to 0 70856 that overlap those of middle ordovician seawater petrography geochemistry and the geometry of the dolomitized body suggest that the planar dolomite was formed in middle and late ordovician seawater during the deposition of the overlying maquoketa shale ferroan planar and nonplanar dolomite occurs in the upper few meters of the trenton limestone confined to areas underlain by planar dolomite this dolomite contains patches of nonferroan dolomite with cathodoluminescence cl characteristics similar to underlying planar dolomite ferroan dolomite has relatively low 18o values 5 1 to 7 3o oo pdb and has slightly radiogenic 87sr 86sr ratios 0 70915 to 0 70969 similar to those obtained for the overlying maquoketa shale these data indicate that ferroan dolomite formed by neomorphism of nonferroan planar dolomite as fluids were expelled from the overlying maquoketa shale during burial the absence of ferroan dolomite at the trenton maquoketa contact in areas where the earlier formed nonferroan planar dolomite also is absent indicates that the fluid expelled from the overlying shale did not contain enough mg2 to dolomitize limestone type 1 dolomite cement has isotopic compositions similar to those of the ferroan dolomite suggesting that it also formed from shale derived burial fluids cl growth zoning patterns in these cements suggest that diagenetic fluids moved stratigraphically downward and toward the southeast along the axis of the kankakee arch type 2 saddle dolomite cements precipitated late their low 18o values 6 0 to 7 0o oo pdb are similar to those of the type 1 dolomite cement however fluid inclusion data indicate that the saddle dolomite was precipitated from more saline basinal fluids and at higher temperatures 94 to 143 c than the type 1 cements 80 to 104 c a trend of decreasing fluid inclusion homogenization temperatures and salinities from the michigan basin to the axis of kankakee arch suggests that these fluids emerged from the michigan basin after precipitation of type 1 cement

oblique plate convergence slip vectors and forearc deformation slip vectors from thrust earthquakes at subduction zones where convergence is oblique to the trench often point between the directions of relative plate convergence and normal to the trench axis suggesting that oblique convergence is taken up by partial decoupling decoupling means that a component of arc parallel motion of the leading edge of the upper plate results in less oblique thrusting at the trench partial decoupling is modeled by partitioning of oblique convergence into slip on thrust and strike slip faults that are parallel to the trench and to each other and starting with a force equilibrium condition a relationship between the obliquity and the earthquake slip vector orientation is derived assuming that either fault slips when shear stress on it reaches a yield stress oblique slip parallel to the plate vector should occur on the thrust fault when obliquity is smaller than a critical angle for obliquity at or greater than this angle the stress on the strike slip fault is large enough to start it slipping and when both faults are active the arc parallel motion of the forearc deflects the slip vector back toward the trench normal if we assume that continued slip on either fault occurs at constant stress but the two faults can be at different stresses the slip vector will maintain a constant angle relative to the trench normal even when obliquity is larger than the critical angle this limiting angle of the slip vector called max measured relative to the trench normal is simply the arcsine of the ratio of the shear forces resisting slip on the strike slip and thrust faults a consequence is that when the obliquity exceeds max the slip vectors on the thrust fault are sensitive only to the thrust fault orientation and contain no information about the convergence direction between the plates slip vectors at the java trench southwest of sumatra show the relationship clearly with max 20 5 while slip vectors at the aleutian trench show the relationship less clearly with max 25 to 45 the greater angle at the aleutian trench suggests that the upper plate is stronger in the aleutian arc relative to the thrust fault than in the sumatran arc consistent with the sumatran arc being continental and having a well developed strike slip fault while the aleutian arc is oceanic and without a clear transcurrent fault slip vectors at the philippine trench which like sumatra has a large strike slip fault inboard of it tend to stay within 25 of the trench normal when obliquity is as large as 50 if obliquity exceeds max and continues to increase along a subduction zone the rate of motion of the forearc relative to the upper plate will vary with obliquity in which case the forearc sliver should extend or contract parallel to the arc from the geometry of modern island arcs arc parallel extension should be the more common and has been hypothesized for both sumatra and the aleutians on the basis of earthquake slip vectors and for these and other arcs from geological observations from estimates of max and the arc parallel gradients in obliquity arc parallel strain rates are estimated to be 1 to 3 10 8 yr for the sumatran forearc 2 to 6 10 8 yr for the aleutian forearc and 0 3 to 3 10 8 yr for the philippine forearc oblique convergence and subsequent arc parallel extension if accompanied by crustal thinning may provide an important yet little appreciated mechanism for bringing high grade metamorphic rocks to the surface of subduction complexes

low grade metamorphism and geotectonic setting of the macuchi formation western cordillera of ecuador the cretaceous eocene basic to intermediate marine volcanic rocks of the mucuchi formation constitute the western cordillera in northern ecuador their chemical features mostly correspond to those of tholeiitic basalts with some calc alkaline affinities and suggest an oceanic island arc setting the macuchi rocks are affected by low grade non deformative metamorphism characterized by zeolite prehnite pumpellyite and lower greenschist facies assemblages depth zonation is suggested by the downward mineral sequence i laumontite pistacitic epidote pumpellyite prehnite ii pumpellyite prehnite pistacitic epidote iii actinolite biotite pistacitic epidote chlorite this broad zonation and the chemistry of individual minerals point to an interaction between the volcanic rocks and sea water under a moderate to high thermal gradient 75 c km alteration appears to have been dependent primarily on fluid control volume pressure composition temperature and reaction kinetics which together partly overshadow the role of load pressure compositional variations of a mineral species at the scale of a contiguous flow or even at the scale of a thin section show that intensity of alteration was spatially uneven depending on rock permeability and consequently metastable equilibrium commonly exists however a progressive approximation to equilibrium as a result of p t control is shown by the mineralogy a high fo2 of the fluid phase is evident from the mineral chemistry the metamorphism of the macuchi volcanics is similar to the hydrothermal burial type produced during the development of a volcanic arc where lavas and volcanoclastics accumulated in a shallow marine environment however some of its characteristics point to a transition toward systems defined by a higher t p ratio such as those found in ocean floor metamorphism a model is proposed in which the macuchi volcanics are assigned to an oceanic island arc generated contemporaneously with a marginal basin which has opened as the outcome of progressive north south attenuation of the continental crust due to mantle diapirism

oxygen and carbon isotopic composition of marine carbonate concretions an overview abstract compilation of stable isotope data 18o and 13c for marine calcite siderite and dolomite concretions from a large number of studies reveals consistent and unanticipated isotopic trends although many of the dolomites have 18o values that are compatible with precipitation from unaltered seawater at low temperatures most of the calcites and siderites are far too depleted in 12o for such an origin this is surprising as most of the concretions appear to have formed very early in the diagenetic history of the sediments often within several meters of the sediment water interface the most likely cause of the anomalously depleted oxygen values is a decrease in pore water 18o values during early diagenesis a result perhaps of a significant influx of meteoric water into the marine shelf environment or water mineral interaction e g precipitation of 18o enriched minerals the carbon isotopic field for dolomite is quite different from that for calcite and siderite highly positive 13c values are fairly common only for dolomites this indicates that while dolomite concretions often form in sediments where methanogenesis is rapid precipitation of calcite and siderite concretions is restricted to sediments where either n methanogenesis takes place or where methanogenesis is not extensive enough to generate highly positive pore fluid 13c values this variation in carbon isotopic composition among the different minerals suggests that the rates of organic carbon oxidation and sedimentation may control the mineralogy of carbonate concretions high organic carbon oxidation rates favor dolomite and lower oxidation rates favor calcite or siderite

petroleum generation modeling of the organic rich shales of late jurassic early cretaceous succession from mintaq 01 well in the wadi hajar sub basin yemen late jurassic early cretaceous shales of the naifa safer and madbi formations were studied to evaluate source rock characterization the results of the source rock were then incorporated into basin modeling to understand the timing of hydrocarbon hc generation and expulsion the late jurassic early cretaceous shales have low to high organic matter with total organic carbon toc values in the range of 0 50 28 01 indicating fair to excellent source rock potential main oil and gas are anticipated to be generated from the naifa safer and lam shale samples with types i and or ii and types ii iii kerogens in contrast the meem samples are dominated by type iii kerogen hydrogen index hi 200 mg hc g toc and are thus considered to be gas prone the late jurassic early cretaceous shale samples have temperatures of maximum pyrolysis yield tmax in the range of 337 515 c consistent with immature to post mature stages the tmax data also indicate that the safer and madbi shale samples h

australian volcanic hosted massive sulfide deposits features styles and genetic models the cambro ordovician period has yielded the major development of volcanic hosted massive sulfide vhms deposits in australia resulting in about 12 million metric tons of contained metal copper lead and zinc concentrated in the mount read volcanics tasmania and the mount windsor volcanics queensland the archean 4 million metric tons of metal and the silurian 3 5 million metric tons of metal constitute the next important episodes while the devonian and permian have produced isolated deposits the deposits range from cu type to zn cu type to zn pb cu type the zn cu type deposits are restricted to the archean and silurian whereas the cu type and zn pb cu type occur sporadically throughout the time span from early archean to permian in general terms the major vhms bearing districts are calc alkaline in character and display a thick basal pile of rhyolitic volcanics 1 3 km thick including lavas epiclastics and subvolcanic intrusions which is overlain by a polymodal sequence containing various proportions of rhyolite dacite andesite basalt and sediments the major deposits are commonly located at the top contact of the rhyolite pile or within the lower part of the overlying polymodal sequence there is a wide range in variability of styles of australian vhms deposits including mounds pipes sheets layered deposits stacked deposits stockwork and disseminated deposits distal reworked deposits and cyclic layered deposits although these various styles show a range of features there is a consistent theme across the spectrum of metal zonation alteration mineralogy alteration chemistry sulfur isotopes macrotextures microtextures and host volcanic relationships which strongly suggests that they all belong to the one genetic group of ore deposit the classic mound style deposits such as hellyer have a series of key features subsets of which are represented in the other deposit styles the mound deposits are considered to form from deposition of metal sulfides on the sea floor immediately around the hydrothermal vent growth of the mound occurs by upward replacement of sulfide assemblages stable at higher fluid temperatures leading to zone refining and lead zinc silver gold enrichment in the outer and upper parts of the mound departure from the classic mound style of deposit is related principally to three key factors the chemistry of the ore fluid salinity temperature f02 and ah2s the nature permeability and chemistry of the volcanic pile and the sea hoor environment sea hoor topography and seawater depth these factors control the aspect ratio of the deposit the extent of stringer zone development the degree of subsea floor replacement mineralization the nature and spatial development of footwall alteration and the development and style of related distal mineralization sulfur isotope studies of australian vhms deposits indicate that reduced seawater sulfate is a major source of sulfur in the deposit whereas lead and strontium isotope results are compatible with the metals being derived by seawater convection and leaching from the volcanic pile and basement rocks this conclusion is supported by the application of various leaching models which demonstrate the availability of an adequate source of both base and precious metals in the footwall volcanic sequences the input of metals directly from volcanic magma chambers is not precluded by the available data and one likely scenario involves the contribution of gold and copper from a magmatic vapor plume rising to mix with convective seawater fluids which contribute lead zinc silver and gold leached from the footwall volcanics and basement rocks the relative importance of the magmatic input compared with the seawater convective input may help to explain the spectrum of deposit styles arid their spatial relationship to volcanic centers or adjacent sedimentary basins

displacive calcite syntaxial overgrowths in open marine limestones some calcite syntaxial overgrowths in the upper greensand cretaceous of southern england and the onondaga limestone devonian of new york state had a displacive mode of formation precipitation of the overgrowths in both of these open shelf marine limestones occurred in a phreatic environment two displacive crystal growth patterns are recognized by cathodoluminescence 1 unimpeded displacive precipitation in which obstructions such as adjacent grains and matrix offer little resistance to crystal growth and 2 impeded displacive precipitation in which obstructions significantly decrease the calcite precipitation rate at overgrowth obstruction contacts impeded displacive precipitation can be recognized by an irregular luminescence zonation reflecting precipitation perpendicular to overgrowth obstruction contacts rather than along rational crystal faces

multi proxy approach to screen the hydrocarbon potential of the jurassic succession in the matruh basin north western desert egypt abstract this paper discusses the hydrocarbon potential of the jurassic succession in a mature basin in egypt as an important element of a larger petroleum system study prior to any organic petrographic and organic geochemical analyses a palynological age dating of the originally undifferentiated jurassic sequence was carried out to identify different formations under investigation this was based on key bioevents of some recovered dinoflagellate cysts and the identified lithology palynological toc rock eval pyrolysis including modified rock eval methods and vitrinite reflectance vro data from a total of 14 samples taken from the uppermost lower upper jurassic sequence represented by the wadi natrun toarcian aalenian khatatba late bathonian callovian and masajid oxfordian formations in the abu tunis 1 well are presented in addition two samples from the abu tunis 1 well and the proximal 32 km to the east siqueifa 1 well having the highest remaining hydrocarbon potential s2 yields were analyzed and their results were compared using modified pyrolysis programs although the data showed a good correlation between the tai of the palynomorph assemblage vitrinite reflectance and tmax from rock eval pyrolysis in the shallower intervals the correlation between vro and tmax was poor in the lower half of the studied succession middle khatatba and wadi natrun formations the very low tmax values indicate immature om while vro and tai indicated middle stage of oil window to past peak oil generation the reason for this discrepancy is that the deeper samples are reservoir rocks not source rocks and the majority of the organic matter is not composed of reactive kerogen but consists of migrated hydrocarbons and nso compounds contamination due to oil based mud obm was eliminated because the abu tunis 1 well was drilled in 1969 prior to the extensive use of obm in drilling this study showed that a multi proxy approach is the best way to screen the hydrocarbon potential in a thick succession that contains interbedded source and reservoir rocks

geochemistry and petrology of a middle tithonian limestone marl rhythmite in the neuquen basin argentina depositional and burial history

cretaceous and tertiary terrane accretion in the cordillera occidental of the andes of ecuador new field geochronological geochemical and biostratigraphical data indicate that the central and northern parts of the cordillera occidental of the andes of ecuador comprise two terranes the older pallatanga terrane consists of an early to late cretaceous oceanic plateau suite late cretaceous marine turbidites derived from an unknown basaltic to andesitic volcanic source and a tectonic melange of probable late cretaceous age the younger macuchi terrane consists of a volcanosedimentary island arc sequence derived from a basaltic to andesitic source a previously unidentified regionally important dextral shear zone named the chimbo toachi shear zone separates the two terranes regional evidence suggests that the pallatanga terrane was accreted to the continental margin the already accreted cordillera real in campanian times producing a tectonic melange in the suture zone the macuchi terrane was accreted to the pallatanga terrane along the chimbo toachi shear zone during the late eocene probably in a dextral shear regime the correlation of cretaceous rocks and accretionary events in the cordillera occidental of ecuador and colombia remains problematical but the late eocene event is recognised along the northern andean margin

basinal marine dolomicrite from the pennsylvanian wolfcampian horquilla formation new mexico abstract a 6 m thick sequence of black dolomicrite occurs within a shaley basinal portion of the pennsylvanian wolfcampian horquilla formation new mexico deep water lithologic associations absence of hypersaline and freshwater indicators elevated na and sr contents and marine derived 18o values all indicate that the dolomite precipitated from marine derived water during burial beneath the sediment water interface trace and major element contents and oxygen isotopic compositions of the dolomites exhibit significant covariance sr na and 18o all covary positively with excess ca in the dolomite structure and fe and mn covary negatively with excess ca in the dolomite structure such variations are suggestive of a diagenetic alteration trend ranging from primary marine compositions preserved in calcian dolomite to diagenetic compositions in more stoichiometric dolomite moreover dolomite which preserves primary 18o values exhibits only minor variation in 13c this is in contrast to the highly variable 13c values observed in tertiary deep water dolomites where sulfate reduction during organic diagenesis plays a key role in promoting dolomitization thus depletion of sulfate was not a prerequisite for dolomitization of basinal carbonates of the hor uilla formation rather dolomitization was promoted by the metastability of the precursor mineralogy while such a mechanism has been postulated for dolomitization of marginal slope facies of oceanic atolls this study extends its application to burial marine dolomitization of deep water micritic sequences

the formation and fate of large oceanic igneous provinces abstract large igneous provinces are conspicuous features of late phanerozoic geology and include continental flood basal rifted continental margin volcanic sequences and oceanic plateaus the latter are formed in an environment which typically recycles back into the mantle on a time scale of 200 m y but because comparisons have recently been made between oceanic plateaus and precambrian greenstone belt sequences new questions arise about their formation their fate and their preservation here we review some critical aspects of three oceanic plateaus ontong java kerguelen and the caribbean colombian abducted plateau and comment on their make up and the factors governing their preservation with particular relevance to ancient terranes many large igneous provinces can be linked to mantle plumes where plumes ascend beneath spreading ridges their energy is transformed into a large melt volume producing over thickened plateau crust where the spreading rate is low in relation to magma supply the plateau may become subaerial e g iceland but with fast spreading the plateau remains submarine thicker lithosphere may result in plume incubation before magma extrusion and there are many intermediate situations where plumes could readily break through thin lithosphere oceanic or continental because magma supply exceeds extension rate plateaus may be characterised by thick sequences of flows and sills rather than the sheeted dykes typical of phanerozoic ophiolites precambrian greenstones could represent imbricated oceanic plateaus or plumes penetrating thin continental lithosphere the initial high temperature and the buoyant nature of the depleted refractory keel of plateaus contributes to their preservation relative to normal oceanic crust when they collide with active margins they choke the subduction zone causing subduction flip or backstep and the development of extensive talc alkaline arc volcanism on top of the plateau sequences however after x 100 m y they are potentially negatively buoyant so if fluids become available to promote tmnsfotmation of the deeper zones to eclogite they may be able to spontaneously subduct

modeling of gas generation from the alam el bueib formation in the shoushan basin northern western desert of egypt the shoushan basin is an important hydrocarbon province in the northern western desert egypt but the burial thermal histories for most of the source rocks in the basin have not been assigned yet in this study subsurface samples from selected wells were collected to characterize the source rocks of alam el bueib formation and to study thermal history in the shoushan basin the lower cretaceous alam el bueib formation is widespread in the shoushan basin which is composed mainly of shales and sandstones with minor carbonate rocks deposited in a marine environment the gas generative potential of the lower cretaceous alam el bueib formation in the shoushan basin was evaluated by rock eval pyrolysis most samples contain sufficient type iii organic matter to be considered gas prone vitrinite reflectance was measured at eight stratigraphic levels jurassic cretaceous vitrinite reflectance profiles show a general increase of vitrinite reflectance with depth vitrinite reflectance values of alam el bueib formation range between 0 70 and 0 87 vrr indicating a thermal maturity level sufficient for hydrocarbon generation thermal maturity and burial histories models predict that the alam el bueib source rock entered the mid mature stage for hydrocarbon generation in the tertiary these models indicate that the onset of gas generation from the alam el bueib source rock began in the paleocene 60 ma and the maximum volume of gas generation occurred during the pliocene 3 2 ma

mineral deposits of canada a synthesis of major deposit types district metallogeny the evolution of geological provinces and exploration methods wayne d goodfellow editor 1 068 pp geological association of canada mineral deposits division special publication no 5 2007 hardcover isbn 13 978 1 897095 24 9 price members cdn 60 00 non members cdn 80 00 following in the footsteps of other landmark reviews of major ore producing countries such as australia hughes 1990 solomon and groves 1994 and south africa anhaeusser and maske 1986 this substantial 1 000 pages monograph is a great achievement that should be in every geologic research library in the world in many ways it sets a new standard most notably in that all figures and photographs are in color and that the entire volume is available in electronic form both as an included dvd and mostly on the gac website it is common in such compendia to have a spectacular cover photo or frontispiece to catch the reader s attention and this book does not disappoint the frontispiece photo is of a 61 8 kg sample from the dome mine timmins ontario and is stated to contain 20 wt percent gold at today s prices the gold alone would be worth 1 5 million dollars

paleoceanographic model of neogene phosphorite deposition u s atlantic continental margin the neogene stratigraphic section of the southeastern u s continental shelf coastal plain system is characterized by i a series of major regional phosphogenic episodes ii a strong spatial relationship between the structural or topographic framework and phosphate deposition and iii distinct cyclical and regional patterns of deposition of the terrigenous carbonate and phosphate lithofacies the complex depositional patterns are explained by a paleoceanographic model based upon the interaction of glacial eustatic sea level fluctuations associated changes in climate and the dynamics of the gulf stream in response to the bathymetric configurations of the continental margin during the past 20 million years

geochemical evidence for suppression of pelagic marine productivity at the cretaceous tertiary boundary the normal biologically productive ocean is characterized by a gradient of the 13c 12c ratio from surface to deep waters here we present stable isotope data from planktonic and benthic micro fossils across the cretaceous tertiary boundary in the north pacific which reveal a rapid and complete breakdown in this biologically mediated gradient the fluxes of barium a proxy for organic carbon and caco3 also decrease significantly at the time of the major marine plankton extinctions the implied substantial reduction in oceanic primary productivity persisted for 0 5 myr before the carbon isotope gradient was gradually re established in addition the stable isotope and preservational data indicate that environmental change including cooling began at least 200 kyr before the cretaceous tertiary boundary and a peak warming of 3 c occurred 600 kyr after the boundary event

subsea floor replacement in volcanic hosted massive sulfide deposits abstract recent research on volcanic hosted massive sulfide vms deposits indicates that syngenetic subsea floor replacement ores form an important component of many deposits in the context of vms deposits subsea floor replacement can be defined as the syn volcanic formation of sulfide minerals within pre existing volcanic or sedimentary deposits by infiltration and precipitation in open spaces fractures inter and intra granular porosity as well as replacement of solid materials there are five criteria for distinguishing subsea floor replacement in massive sulfide deposits 1 mineralized intervals are enclosed within rapidly emplaced volcanic or sedimentary facies lavas intrusions subaqueous mass flow deposits pyroclastic fallout 2 relics of the host facies occur within the mineral deposit 3 replacement fronts occur between the mineral deposit and the host lithofacies 4 the mineral deposit is discordant to bedding and 5 strong hydrothermal alteration continues into the hanging wall without an abrupt break in intensity criteria 1 3 are diagnostic of replacement whereas criteria 4 and 5 may suggest replacement but are not alone diagnostic because clastic sulfide ores contain accessory rock fragments collected by the parent sediment gravity flow s during transport criteria 2 can only be applied to massive semi massive disseminated or vein style deposits and not clastic ores the spectrum of vms deposit types includes deposits that have accumulated largely subsea floor and others in which sedimentation and volcanism were synchronous with hydrothermal activity and precipitation of sulfides occurred at and below the sea floor over the life of the hydrothermal system deposits that formed largely subsea floor are mainly hosted by syn eruptive or post eruptive volcaniclastic facies gravity flow deposits water settled fall autoclastic breccia however some subsea floor replacement vms deposits are hosted by lavas and syn volcanic intrusions sills domes cryptodomes burial of sea floor massive sulfide by lavas or sediment gravity flow deposits can interrupt sea floor mineralization and promote subsea floor replacement and zone refining the distance below the sea floor at which infiltration and replacement took place is rarely well constrained with published estimates ranging from less than 1 to more than 500 m but mainly in the range 10 200 m the upper few tens to hundreds of metres in the volcano sedimentary pile are the favoured position for replacement as clastic facies are wet porous and poorly consolidated in this zone and at greater depths become progressively more compacted dewatered altered and less amenable to large scale infiltration and replacement by hydrothermal fluids furthermore sustained mixing between the upwelling hydrothermal fluid and cold seawater is regarded as a major cause of sulfide precipitation in vms systems and this mixing process generally becomes less effective with increasing depth in the volcanic pile the relative importance of subsea floor replacement in vms systems is related principally to four factors the permeability and porosity patterns of host lithofacies sedimentation rate the relative ease of replacement of host lithofacies especially glassy materials and early formed alteration minerals during hydrothermal attack and physiochemical characteristics of the hydrothermal fluid

the origin of dolomites in tertiary sediments from the margin of great bahama bank abstract based on an integrated geochemical and petrographic investigation of dolomites from two cores drilled on great bahama bank we have determined three different environments of formation for the dolomites that are common throughout the pliocene and miocene parts of these cores the first environment of dolomitization occurs in association with development of nondepositional surfaces dolomite typically forms below each of these surfaces the concentration and extent of which is governed by the length of the period of nondeposition these dolomites are recognized by their association with the nondepositional surfaces characteristic positive 18o values indicative of formation from cold bottom waters and 18o and sr profiles with depth that suggest formation in the presence of diffusive temperature and sr gradients the second environment of dolomitization occurs in pore fluids where the cation and anion profiles are governed by diffusive processes the dolomite forming here is termed background dolomite this is a microsucrosic dolomite and forms both by recrystallization of the existing sediment and precipitation directly into void space dolomitization by this mechanism uses a local source of mg2 and consequently the dolomite never constitutes more than between 5 and 10 of the sediment this type of dolomite is characterized by extremely high sr concentrations which reflect concentrations of sr2 in the pore fluids from which it formed the high concentrations of sr2 in the pore fluids arise through the continued recrystallization of metastable aragonite and high mg calcite to dolomite and low magnesium calcite driven by oxidation of organic material by sulfate not only does sulfate reduction provide an additional thermodynamic drive for recrystallization but because the absolute concentration of sr2 in the pore fluids is governed by the solubility of celestite srso4 removal of sulfate allows the sr2 ca2 ratio of the interstitial fluid to become much higher than normally encountered the final type of environment of dolomitization is associated with coarse grained reefal sediments the pervasive nature of the dolomitization and the relatively normal sr concentrations suggest the circulation of normal marine water in a relatively open system

the nature and provenance of accreted oceanic terranes in western ecuador geochemical and tectonic constraints western ecuador consists of a complex tectonic melange of oceanic terranes accreted to the continental margin from late cretaceous to eocene time new geochemical data from these accreted terranes arising from a 5 year british geological survey mapping programme indicate that they comprise rocks from a variety of oceanic tectonic settings from thickened and relatively unsubductable oceanic plateau basalts through island arc tholeiites with occasional more calc alkaline lavas to back arc basin basalt sequences this study has enabled us to construct a new geodynamic model for the cretaceous tertiary evolution of the northern andes and has placed important new constraints on the extent of oceanic plateau sequences in colombia and around the caribbean the age and nature of sediments combined with evidence for the age of peak metamorphism suggests that a prolonged 15 20 ma accretionary event occurred in late cretaceous time and involved the collision of an oceanic plateau represented by the pallatanga unit with the continental margin this accreted unit can be correlated with similar oceanic plateau sequences from the western cordillera of colombia and those within and around the caribbean region the naranjal and macuchi island arcs and the associated la portada back arc basin developed along the accreted margin from late campanian to eocene time and these arcs accreted to the continental margin along with oceanic plateau material represented by the pinon unit and pedernales esmeraldas sequences during eocene time the development of island arcs which separate the two accretionary events implies that the most westerly coastal oceanic plateau sequences both in ecuador pinon and pedernales esmeraldas and in colombia gorgona and serrania de baudo cannot belong to the caribbean colombian oceanic plateau ccop it therefore appears that at least two different oceanic plateaux are preserved within the accreted oceanic terranes of the northern andes it is possible that the ccop formed over the galapagos hotspot as previously proposed but the more westerly coastal plateau was derived from a more southerly hotspot source region such as sala y gomez in the se pacific

the geological history of northwestern south america from pangaea to the early collision of the caribbean large igneous province 290 75ma abstract northwestern south america preserves a record of the assembly of western pangaea its disassembly and initiation of the far western tethys wilson cycle subsequent pacific margin magmatism and ocean plateau continent interaction since the late cretaceous numerous models have been presented for various time slices although they are based on either spatially restricted datasets or dates that are inaccurate estimates of the time of crystallisation here we review a very large quantity of geochronological geochemical thermochronological sedimentological and palaeomagnetic data that collectively provide tight constraints for geological models these data have been collected over a trench pacific parallel distance of 1500 km colombia and ecuador and reveal important temporal trends in rifting and subduction the temporal framework for our model constraints are obtained from robust concordant zircon u pb ages of magmatic rocks during 290 75 ma the late cretaceous thermal history of the margin

contemporaneous paleogene arc magmatism within continental and accreted oceanic arc complexes in the northwestern andes and panama abstract the basement rocks of the northwestern andes are composed of accreted fault bound slivers of the cretaceous caribbean large igneous province clip previous geologic mapping geochronological and geochemical data show that paleogene arc like units make up part of the accreted clip in sw colombia timbiqui complex nw colombia santa cecilila la equis complex and at the boundary between colombia and panama acandi region similar paleogene igneous units have been reported in ecuador silante and macuchi arc and panama chagres bayano arc these arc like rocks are commonly interpreted to have formed as part of an intra oceanic arc before accretion to the continental margin alternative explanations suggest that the paleogene igneous rocks in ecuador were formed within a continental magmatic arc we compare paleogene igneous rocks in the san blas complex chagres bayano arc and acandi region santa cecilia la equis complex and the sw complex timbiqui complex silante and macuchi arcs using geochemical major and trace elements geochronological zircon u pb and isotopic zircon hf and whole rock pb sr nd isotopes data zircon u pb analyses yield ages of 46 42 ma for the acandi region 59 51 ma for the santa cecilia la equis complex and 44 42 ma for the timbiqui complex major and trace element data show low k to calc alkaline compositions with negative nb anomaly and lree hree enrichment typically attributed to arc like signatures for the three complexes pb sr and nd whole rock and zircon lu hf isotopic data for the santa cecilia la equis and the san blas complexes are consistent with a depleted mantle source mixed with pacific sediments however the presence of inherited proterozoic zircons elevated 207pb 204pb ratios in some evolved igneous rocks and crystallization under low pressure conditions suggest assimilation of continental derived sediments these data indicate that the santa cecilia la equis and the san blas complexes likely originated in an intra oceanic island arc setting in a close proximity to the south american margin before accretion to the andean margin in the case of the sw complex the absence of eu anomaly slightly depleted hree as well as increased sr y values with fractionation indicate that these magmas formed in a moderately mature arc within a mid shallow crust level moreover radiogenic pb and sr isotopic values for the less evolved igneous rocks of the sw complex evolving to less radiogenic values suggest involvement of continental sediments in the parental magma source and assimilation of the mafic basement crust this indicates that the paleogene magmatic rocks of the sw complex originated in a continental arc like setting within previously accreted slivers of the clip

periplatform ooze in a mixed siliciclastic carbonate system vaca muerta formation argentina abstract thick carbonate mud successions and marl limestone alternations are often found in periplatform environments the sources of these fine grained sediments and the processes that govern their accumulation and distribution in the basin are frequently a matter of debate this case study from a mixed siliciclastic carbonate succession in the neuquen basin argentina relates the origin of the carbonate dominated intervals to shelfal carbonate production the tithonian early valanginian basinal strata vaca muerta formation contains several carbonate rich intervals at different stratigraphic levels the thickest of these intervals is the mid late tithonian los catutos member in sierra de la vaca muerta this wedge shaped package of micritic limestones and intercalated marls thins basinwards proximal distal variations and cyclic partitioning in facies and geochemical proxies indicate highstand shedding and a periplatform origin for the los catutos member the proposed source is a coeval 20 m thick pure carbonate interval located 50 km south of the sierra de la vaca muerta within the mixed siliciclastic carbonate shelf facies of the picun leufu formation younger carbonate rich intervals in the center of the basin puerta curaco area show similar characteristics to the distal portion of the los catutos member and are also interpreted as periplatform deposits but sourced from carbonate shelves farther east the high sedimentation rates of the los catutos member require an efficient mechanism other than tides and winds to deliver significant amounts of fine grained carbonate sediment to the basin this mechanism is envisioned to be sediment charged hyperpycnal flows cascading from the platform to the basin analogous to what is observed in modern carbonate systems the occurrence of periplatform ooze in different stratigraphic levels of the siliciclastic dominated vaca muerta formation demonstrates that off shelf transport and distribution of carbonate mud in the basin are also common processes in mixed clastic carbonate environments

preservation of delta 18 o and delta 13 c in belemnite rostra from the jurassic early cretaceous successions stratigraphic collection of 263 belemnite rostra guards from jurassic to lower cretaceous sediments in the moscow area of russia northern germany england new zealand and morocco were analyzed to determine temporal variations in their delta 18 o and delta 13 c as a proxy for isotopic composition of surface seawater internal structures of the rostra were studied using cathodoluminescence and sem techniques their trace element fe mn mg and sr and the stable carbon and oxygen isotope compositions the average stratigraphic resolution is about 1 ma these samples represent a broad range of paleolatitudes and paleoceanographic settings ranging from 42 degrees n to 82 degrees s covering boreal to subtropical brackish to open marine shelf paleoenvironments the rostra originally contained domains enriched in organic matter that were later replaced by luminescent diagenetic calcite thereby generating an impression of seasonal growth rings diagenetic calcite accounts for up to 6 8 percent of the rostrum the trace element content of the rostra range from 227 to 3935 ppm for mg 379 to 1514 ppm for sr and up to 305 and 3662 ppm for mn and fe respectively however selective drilling of only primary calcitic domains resulted in 90 percent of all analyzed samples to fall within the range of trace element values typical for modern low mg calcite shells the chemical and textural data suggest that the primary calcite of the rostra is well preserved as is its isotopic signature the delta 13 c and delta 18 o measurements define oscillating temporal bands of about 3 to 4 per mil width around more or less modern marine values of 2 and 1 per mil respectively oxygen isotopes in the well preserved rostra most likely reflect temperature variations within a approximately 14 or 8 degrees c range the observed 3 to 4 per mil spread of values between contemporaneous specimens which is comparable to the scatter of data reported for modern low mg calcitic brachiopod shells probably reflects the diversity of local ecosystems although some of the spread may be due to real short term temporal variations within single biozones the delta 13 c values of the rostra define a band similar to that of the oxygen isotopes and the temporal oscillations of both isotope curves are commonly in phase the above dataset complemented by the results of jones jenkyns and hesselbo 1994 and jones and others 1994 yields a composite delta 18 o and delta 13 c variations in belemnite guards for the entire jurassic early cretaceous interval for delta 18 o the oscillating band indicates that the most negative values occur at times of independently postulated global and or regional anoxic events such as those in the toarcian and kimmeridgian as well as the aptian these oaes appear to be characterized by up to 10 degrees c higher seawater temperatures than the intervening background times the delta 13 c record shows 13 c depletion for the kimmeridgian event but enrichment for the other two suggesting that the delta 13 c temporal oscillations and oaes are not simply a reflection of surficial productivity due to operation of the biological pump but rather of changing oceanic circulation patterns due to variations in latitudinal surface temperature gradients

subaerial exposure and deposition of shallowing upward sequences evidence from stable isotopes of purbeckian peritidal carbonates basal cretaceous swiss and french jura mountains purbeckian lowermost cretaceous peritidal carbonates are characterized by open marine lagoonal intertidal and lacustrine facies arranged in milankovitch type shallowing upward sequences shallowing upward sequences typically consist of 2 6 individual beds the sequences may be i complete ii incomplete or ii pedogenetically overprinted reflecting the duration of subaerial exposure and or the extent of erosion and pedogenetic modification at the cycle tops the stable isotopic composition of the peritidal micrites reveals homogenous 6 o values attributed to diagenetic stabilization in a meteoric water buffered system carbon isotopes show three distinctly different carbon isotope patterns dependent on the completeness of the shallowing upward sequences complete shallowing upward sequences consist of l6 individual carbonate beds the carbon isotope values show a facies dependent pattern open marine carbonate muds record enriched 6i3c values of 0 28 while lagoonal 0 82 intertidal 2 46 10 and lacustrine micrites 2 96 are increasingly depleted this distinct pattern is explained by carbonate mud deposition in environments of differing salinity and marine influence incomplete sequences 2 5 carbonate beds are characterized by depleted 6 c values below subaerial exposure surfaces that become progressively enriched in c with increasing depth pedogenetically overprinted sequences 1 3 carbonate beds show strong i3c depletion throughout the sequence with little variation in the carbon isotopic composition the depleted values 4 51 10 of the pedogenetically altered micrites suggest that modification during subaerial exposure was associated with equilibration with meteoric solutions enriched in isotopically light soil gas coz the duration of subaerial exposure is the most crucial factor determining the extent of pedogenetic alterations the completeness of the shallowing upward sequences and the carbon isotope pattern the recorded patterns clearly illustrate that micrites have a good potential for the preservation of their primary carbon isotopic composition if the duration of subaerial exposure is rather brief otherwise the recorded carbon isotope patterns may support sequence stratigraphic analysis by providing a refinement of the time stratigraphic interpretation

cyclostratigraphy of an orbitally driven tithonian valanginian carbonate ramp succession southern mendoza argentina implications for the jurassic cretaceous boundary in the neuquen basin we conducted detailed sedimentological sequence stratigraphic and cyclostratigraphic analyses on four sections of the vaca muerta formation this unit is characterized by a decimetre scale rhythmic alternation of marls and limestones it consists of five facies associations which represent basin to middle carbonate ramp deposits the rhythmic vertical organization shows a well ordered hierarchy of cycles where elementary cycles bundles of cycles and superbundles within the milankovitch frequency band are recognized cyclostratigraphic data allowed us to build a floating orbital scale for the lower tithonian lower valanginian interval in the neuquen basin orbital calibration of these sections is consistent with riccardi s biostratigraphic scheme which places the jurassic cretaceous boundary within the substeueroceras koeneni ammonite zone equivalent to durangites spp jacobi grandis and initial occitanica standard zones

the jurassic succession in west central argentina stratal patterns sequences and paleogeographic evolution abstract jurassic rocks in west central argentina are predominantly marine and marginal marine siliciclastics associated with prominent but volumetrically subordinate carbonates and evaporites facies developments were ruled by paleogeographic persistence within the southern infratropical latitude belt within 40 50 s and by siliciclastic derivation from the patagonian hinterland to the southwest minor volcanic and volcaniclastic supply arrived from the west out of a magmatic belt related to circum pacific convergence timing of the main marine flooding events and general correspondence with the high rank stratal packaging recorded in western north america suggest that global eustasy was also a factor in controlling the local stratigraphic record early jurassic sedimentation occurred within a series of semi isolated depocentres linked to fault bounded triassic troughs the sinemurian toarcian deposits record depocentre expansion and coalescence these trends were coeval with progressively more widespread marine invasions from the northwest and west leading to an elongate marine seaway which connected central patagonia with the pacific domain during the aalenian bajocian the region was subject to a more subdued tectonism and the foreland side of the basin became fringed by an extensive clastic embankment bathonian and early callovian were times when coarse clastics prograded into the basin while the marine embayment shrank as a result of stepwise forced regressions during late callovian to oxfordian globally rising sea level the depocentre witnessed the appearance of cosmopolitan invertebrates and a stratal pattern of basin widening and depositional underbalance that promoted cratonward onlap and inception of widespread carbonate deposition ooidal coralline carbonate development was terminated after a relatively sudden messinian style event that desiccated a large tract of the andean basin and favored massive precipitation of anhydrite in the course of the kimmeridgian the evaporite basin was largely flooded by siliciclastics and turned into a broad and featureless mudflat salina complex linked to a widespread erg and to an ephemeral drainage system by the tithonian at a period of peak oceanic stand marine connection was reinstated and recorded as the most widespread jurassic transgression across the neuquen aconcagua embayment shelfal deposition consisted of molluscs and ooid dominated carbonate terraces that grew in pulses tuned to eustatic fluctuations like in other prolific petroleum provinces around the world the tithonian basinal strata involve widespread euxinic deposits featuring unusually high organic content

modification of an oceanic plateau aruba dutch caribbean implications for the generation of continental crust the generation of the continental crust may be connected to mantle plume activity however the nature of this link and the processes involved are not well constrained an obstacle to understanding relationships between plume related mafic material and associated silicic rocks is that later tectonic movements are liable to obscure the original relationships particularly in ancient greenstone belts studies of younger analogous regions may help to clarify these relationships on the island of aruba in the southern caribbean a sequence of partly deformed mafic volcanic rocks intruded by a predominantly tonalitic batholith is exposed the mafic lavas show geochemical and isotopic affinities with other basaltic picritic and komatiitic rocks that crop out elsewhere in the caribbean these are well documented as belonging to an 88 91 ma plume related oceanic plateau which is allochthonous with respect to the americas and is thought to have been formed in the pacific region the 85 to 82 ma tonalitic rocks share some geochemical characteristics high sr and ba low nb and y with archaean tonalite trondhjemite granodiorite ttg suites field relationships suggest that deformation of the plateau sequence possibly related to collision with a subduction zone was synchronous with intrusion of the aruba batholith new incremental heating 40ar 39ar dates combined with existing palaeontological evidence show that cooling of the batholith occurred shortly after eruption of the plateau basalt sequence sr nd isotopic data for both rock suites are uniform 87sr 86sri 0 7035 endi 7 whereas pb isotopes are more variable plateau sequence 206pb 204pb 18 6 19 1 207pb 204pb 15 54 15 60 208pb 204pb 38 3 38 75 aruba batholith 206pb 204pb 18 4 18 9 207pb 204pb 15 51 15 56 208pb 204pb 38 0 38 5 this suggests that there has been a minor sedimentary input into the source region of the batholith however the limited time interval between basaltic and tonalitic magmatism makes a normal subduction related origin for the tonalites improbable instead models involving derivation of tonalite from partial melting of the plateau sequence or alternatively genesis in an unusual subduction zone environment are investigated

microboring versus recrystallization further insight into the micritization process sem observations of lightly etched thin sections of bahamian sediments reveal an unusual process of micritization that involves carbonate precipitation in microborings concurrent with endolithic activity a coccoid cyanobacterium tentatively identified as solentia sp bores tunnels which initially penetrate just beneath grain surfaces and eventually extend throughout the entire grain these tunnels are filled by radial fibrous aragonite which is precipitated as the microorganism advances extensive multicyclic repetitions of this process result in obliteration of original grain textures with almost complete preservation of grain margins and rare empty bore holes the rapidly filled tunnels cannot be detected by resin cast embedding techniques that are commonly used to study microboring this type of multicyclic boring and concurrent filling of bore holes forms micritized grains that can be difficult or impossible to distinguish from micritized grains formed by recrystallization

multiple plume events in the genesis of the peri caribbean cretaceous oceanic plateau province the oceanic crust fragments exposed in central america in north western south america and in the caribbean islands have been considered to represent accreted remnants of the caribbean colombian oceanic plateau ccop on the basis of trace element and nd sr and pb isotopic compositions we infer that cumulate rocks basalts and diabases from coastal ecuador have a different source than the basalts from the dominican republic the latter suite includes the 86 ma basalts of the duarte complex which are light rare earth element ree enriched and display relative to normal mid ocean ridge basalts nmorb moderate enrichments in large ion lithophile elements together with high nb ta pb and low th contents moreover they exhibit a rather restricted range of nd and pb isotopic ratios consistent with their derivation from an ocean island type mantle source the composition of which includes the himu high 238u 204pb component characteristic of the galapagos hotspot in contrast the 123 ma ecuadorian oceanic rocks have flat ree patterns and relative to nmorb are depleted in zr hf th and u moreover they show a wide range of nd and pb isotopic ratios intermediate between those of ocean island basalts and nmorb it is unlikely on geochemical grounds that the plume source of the ecuadorian fragments was similar to that of the galapagos in addition because of the nne motion of the farallon plate during the early cretaceous the ecuadorian oceanic plateau fragments could not have been derived from the galapagos hotspot but were likely formed at a ridge centered or near ridge hotspot somewhere in the se pacific

dolomitization in chalk from the north sea central graben abstract sem studies of the upper cretaceous tertiary north sea chalk have revealed minor amounts of secondary dolomite occurring mainly as cement but also involving minor small scale neomorphism of some enveloped grains the dolomite content in the studied sequences varies considerably but generally amounts to 2 to 8 weight percent the dolomite consists of limpid crystals 10 to 30 micrometers in diameter occurring as widely scattered single rhombohedra minor aggregates and interlocking rhombohedra fringing stylolites the microtextural pattern observed demonstrates considerable envelopment and partial replacement of the biogenic constituents of the chalk by dolomite minor amounts of cryptograined clay minerals pyrite and silica are also found incorporated within the dolomite crystal x ray diffraction electron probe microanalysis and sem examination indicate a slightly calcium rich dolomite composition strontium concentrations average 1600 ppm the dolomite crystals are slightly depleted in 18o and enriched in 13c in comparison to the host sediment it is believed that the dolomitization resulted from relatively early diagenetic processes in a largely closed system

late jurassic climate and its impact on carbon cycling abstract the climate of the late jurassic has been characterized by high atmospheric co2 levels and by a monsoonal rainfall pattern in this study we traced the evolution of the global carbon cycle through the late jurassic with the help of carbonate carbon isotope stratigraphy the oxfordian tithonian 13c curve is marked by one major positive carbon isotope excursion with an amplitude 13 c 1 0 middle to late oxfordian and a second minor positive excursion with an amplitude 13 c late kimmeridgian the early kimmeridgian and early tithonian 13c values fluctuate around 13 c 2 0 3 and contrast with the less positive 13c values of early oxfordian and late tithonian age 13 c 1 0 5 a comparison of the late jurassic carbonate carbon isotope curve with the occurrence of organic rich sediments suggests that not only fluctuations in organic carbon burial but also in carbonate carbon burial had an impact on the c isotope record the oxfordian c isotope excursion appears to correspond to a time of overall increased organic carbon burial triggered by increased nutrient transfer from continents to oceans during a time of rising global sea level however episodes of enhanced organic carbon burial during the kimmeridgian and early tithonian are not reflected by prominent spikes in the c isotope record favourable conditions for carbonate platform growth at a time of high global sealevel may have resulted in the stabilisation of the c org c carb burial ratio and hence maintained the 13c record at steady but relatively positive values the middle and late tithonian c isotope values drop below 13 c 1 5 a similar shift to less positive c isotope values was recognized in other c isotope records from the tethys and atlantic oceans and reflects a decrease in the c org c carb burial ratio possibly related to a reorganisation of the global climate system globally widespread marine black shale mature quartzose sandstone assemblages suggest that the relative efficiency of the late jurassic carbon pumps was controlled by weathering erosion and runoff causing widespread marine eutrophication eutrophication favoured the organic carbon pump but it diminished the carbonate platform growth potential the monsoonal rainfall distribution pattern may explain why late jurassic carbonate platforms experienced less severe growth crises than early cretaceous carbonate platforms

along strike variations in the thermal and tectonic response of the continental ecuadorian andes to the collision with heterogeneous oceanic crust oblique to strike geological segmentation in the andean chain has been previously recognised at various scales and is commonly attributed to changes in the convergence vectors of the oceanic and continental plates as well as the upperplate expressions of differing along strike subducted slab age strength and composition we present new white mica and biotite 40 ar 39 ar and zircon and apatite fission track data from several traverses across the cordillera real of ecuador in the northern andes that reveal distinct along strike differences in the timing of accelerated crustal cooling during the cenozoic the data record elevated cooling rates from temperatures of v380 c during v65 55 and v43 30 ma from all sampled regions of the cordillera real and at v15 ma and since v9 ma in the northern cordillera real each cooling period was probably driven by exhumation in response to the accretion and subduction of heterogeneous oceanic crust elevated cooling rates of up to v30 20 c myr were initiated during the palaeocene and eocene early oligocene along the entire contemporaneous margin of ecuador and were driven by the accretion of the oceanic pallatanga terrane and pin on macuchi block respectively onto northwestern south america both of these geological provinces originated at the southern parts of the leading and trailing boundaries of the caribbean plateau and accreted onto the margin during the approximately northeastward migration of the plateau into its current position within ecuador the development of higher topography and elevated cooling rates of up to 50 c myr at v15 ma and since v9 ma are restricted to the region north of 1 30ps and is situated above the postulated subducted flatslab section of the aseismic carnegie ridge plate convergence rate calculations suggest the carnegie ridge collided with the ecuador trench at v15 ma which caused the pre existing coastal provinces to displace to the northeast subsequently driving extension and marine ingressions in southern ecuador and compression and uplift in northern ecuador fl 2001 elsevier science b v all rights reserved

black shales and carbon isotopes in pelagic sediments from the tethyan lower jurassic detailed sampling and analysis of jurassic pelagic limestones and marls from italy hungary and switzerland have enabled construction of an isotope stratigraphy across the pliensbachian toarcian boundary with resolution to the zonal level the oxygen isotope record is unremarkable the carbon isotopes however show two positive excursions one relatively minor during the pliensbachian margaritatus zone subnodosus subzone the other more major during the toarcian early falciferum zone where a maximum 13c value of 4 52 pdb is attained these intervals are known to be favoured periods of organic rich sedimentation in diverse parts of the globe and the isotopic excursions are interpreted as a response to abnormally high rates of storage of organic carbon in the sedimentary record a comparable phenomenon has been documented from the cenomanian turonian boundary in the cretaceous where it has been referred to the influence of an oceanic anoxic event some italian sections spanning this lower jurassic interval contain organic rich shales in the falciferum zone the isotopic signatures from their included locally manganiferous carbonate betray a considerable diagenetic overprint and they cannot therefore be incorporated in a composite isotopic curve carbon isotopes from the organic carbon itself are extremely negative falling to 33 pdb and in one section examined in detail correlate with the calcium carbonate content of the shales they may reflect a partial change to a non calcified planktonic biota during deposition of this lime poor interval possibly responding to upwelling and increased fertility of near surface waters the onset of upwelling may have been as early as spinatum tenuicostatum zone time that is at the pliensbachian toarcian boundary

permo triassic anatexis continental rifting and the disassembly of western pangaea crustal anatectites are frequently observed along ocean continent active margins although their origins are disputed with interpretations varying between rift related and collisional we report geochemical isotopic and geochronological data that define an 1500 km long belt of s type meta granites along the andes of colombia and ecuador which formed during 275 223 ma these are accompanied by amphibolitized tholeiitic basaltic dykes that yield concordant zircon u pb dates ranging between 240 and 223 ma a model is presented which places these rocks within a compressive permian arc setting that existed during the amalgamation of westernmost pangaea anatexis and mafic intrusion during 240 223 ma are interpreted to have occurred during continental rifting which culminated in the formation of oceanic crust and initiated the break up of western pangaea compression during 275 240 ma generated small volumes of crustal melting rifting during 240 225 ma was characterized by basaltic underplating the intrusion of tholeiitic basalts and a peak in crustal melting tholeiitic intrusions during 225 216 ma isotopically resemble depleted mantle and yield no evidence for contamination by continental crust and we assign this period to the onset of continental drift dissected ophiolitic sequences in northern colombia yield zircon u pb dates of 216 ma the permo triassic margin of ecuador and colombia exhibits close temporal faunal and geochemical similarities with various crustal blocks that form the basement to parts of mexico and thus these may represent the relict conjugate margin to nw gondwana the magmatic record of the early disassembly of pangaea spans 20 ma 240 216 ma and the duration of rifting and rift drift transition is similar to that documented in cretaceous tertiary rift settings such as the west iberia newfoundland conjugate margins and the taupo lau havre system where rifting and continental disassembly also occurred over periods lasting 20 ma

organic geochemistry and basin modeling of late cretaceous harshiyat formation in the onshore and offshore basins in yemen implications for effective source rock potential and hydrocarbon generation abstract the purpose of this study is to present the source rock characteristics of late cretaceous harshiyat formation source rock in the onshore and offshore basins of yemen i e mukalla sayhut sayun masilah and jiza qamar source rock geochemical characterizations were completed and integrated to perform 1 d basin modeling to back predict source rock thermal maturation oil generation and expulsion the geochemical findings revealed that mudstone intervals within the harshiyat formation comprised favorable source rocks with total organic carbon toc content between 0 50 and 35 10 wt the organic matter intervals in the harshiyat mudstones are mainly types ii iii and iii kerogens with certain amount of type ii and i kerogens based on rock eval hi values ranging from 40 to 923 mg hc g toc mixed kerogen types were consistent with vitrinite and sapropelic organic matters as observed by visual kerogen examination and the distribution of biomarker fingerprints the existence of mixed types i ii and iii organic matter suggests that the harshiyat mudstones could contribute to oil and gas at sufficient thermal maturity levels the biomarker data of the organic matter in the harshiyat mudstones show that the source rock contains a mix marine phytoplanktonic bacteria terrestrial land plant enthronement and were deposited in suboxic anoxic conditions results of chemical and optical maturity indicators led to the conclusion of varying maturity stages from immature to late oil window levels for the mudstones of the harshiyat formation thermal maturation distributions inferred from the analyzed samples indicate that mudstone intervals in the offshore mukalla sayhut basin are extremely mature compatible with the late mature oil window and may have generated significant amounts of oil this interpretation was supported by strong geochemical similarity between the organic matter in the mudstones and those of the oils found in the offshore mukalla sayhut basin and demonstrated by the concentrations of n alkanes and isoprenoids as well as carbon isotopes in the offshore ras ghashwah 1x well burial and thermal models were constructed and used to address the effect of tectonic events in triggering oil generation and expulsion from mature harshiyat source rock in the offshore area kinetic database models for type ii and iii kerogen mixtures indicate that the initial conversion of kerogen to oil occurred since the late oligocene with transformation ratios tr that span between 10 and 50 furthermore the oil was expelled from organic matter intervals in the harshiyat source rock starting from late oligocene and continuing until late miocene with trs of 50 72 from that period up to the present the retained oil was cracked into wet gas in the gas window with an easy ro greater than 1 30 and with peak tr ratios greater than 72

the early interaction between the caribbean plateau and the nw south american plate the determination of accurate and precise ages for the timing of collision between oceanic plateaus and continental crust requires an understanding of how the indenting and buttressing plates respond to the collision we present geochronological thermochronological geochemical and isotopic analyses of magmatic rocks from the ecuadorian andes which relate to the collision of the late cretaceous caribbean plateau and great arc sequence with nw south america the cessation of subduction magmatism during 65 64 ma beneath the eastern edge of caribbean plateau was synchronous with accelerated surface uplift and exhumation within the buttressing continental margin during 75 65 ma we interpret this as the collision of the leading edge of the caribbean plateau and arc sequence with the south american plate at 75 65 ma a u pb zircon shrimp age of 87 10 1 66 2 sigma ma yielded by an accreted fragment of the plateau precludes previous estimates of collision at 85 80 ma if the plateau erupted above the galapagos hotspot less