ice age at the middle late jurassic transition a detailed record of sea surface temperatures in the northern hemisphere based on migration of marine invertebrate fauna ammonites and isotopic thermometry 18o values of shark tooth enamel indicates a severe cooling at the middle late jurassic transition mljt about 160 ma ago the magnitude of refrigeration 1 3 c for lower middle latitudes and its coincidence in time with an abrupt global scale fall of sea level documented through sequence stratigraphy are both suggestive of continental ice formation at this time ice sheets may have developed over the high latitude mountainous regions of far east russia the drastic cooling just post dated the middle late callovian widespread deposition of organic rich marine sediments e g northwestern europe central atlantic and arabian peninsula this thermal deterioration can thus be ascribed to a downdraw in atmospheric co2 via enhanced organic carbon burial which acted as a negative feedback effect i e the inverse greenhouse effect the glacial episode of the mljt climaxed in the late callovian lasted about 2 6 myr and had a pronounced asymmetrical pattern composed of an abrupt 0 8 myr temperature fall opposed to a long term 1 8 myr stepwise recovery the glacial conditions at the mljt reveal that atmospheric co2 levels could have dropped temporarily to values lower than 500 ppmv during mesozoic times

ammonite faunal turnover tracing sea level changes during the jurassic betic cordillera southern spain ammonite faunal turnover has been used to trace changes in sea level during the jurassic in the betic cordillera southern spain i e westernmost tethys major turnovers recorded in the jurassic succession are interpreted as extinction phenomena and as appearance diversification and radiation bioevents of the various ammonite associations they are explained as being due to falls and rises in sea level respectively the ten most abrupt diversification and radiation bioevents have been detected in the ammonite associations associated with pelagic jurassic sedimentary rocks all of these bioevents can be linked with sea level rises most coinciding with the eustatic increases of the same age expressed in the eustatic curves of hallam and haq et al the relative sea level falls including eustatic ones coinciding with extinctions are recorded as stratigraphic breaks and stratigraphic condensation phenomena in the betic jurassic succession the seven most significant extinction events recognized in the betic jurassic are also largely attributable to eustatic falls we conclude that the proposed faunal turnover curve is predominantly the expression of global changes in sea level and it can be considered as a basis for plotting a jurassic eustatic curve

condensation processes in shallow platforms abstract sequence stratigraphy allows elucidation of the time space relationships of deposits in sedimentary basins however the relative distribution of unconformities and condensed sections can neither be used as diagnostic criteria for particular palaeogeographic settings nor as indicators of maximum regional transgressions condensation processes also occur in the shallowest portions of platforms and during maximum regression episodes three separate processes of condensation are here distinguished stratigraphic sedimentary and taphonomic these three categories of condensation can be used to analyze patterns of sediment distribution and also to estimate lateral and vertical variations in sedimentation rates contrary to the usual interpretation the maximum values of rate of sedimentation and degree of accommodation of sediments as well as the highest relative sea level occur in the lowest levels of the shallowing upward sequences the distinction between these condensation processes allows one to predict that the degree of sedimentary and stratigraphic condensation will be higher towards the distal portions of the platforms whereas the stratigraphic condensation processes without sedimentary condensation will show the maximum intensity and frequency in the shallowest portions of the platforms

palaeoclimate reconstructions of the middle jurassic of kachchh western india an integrated approach based on palaeoecological oxygen isotopic and clay mineralogical data abstract the middle jurassic sedimentary succession of the kachchh basin shows significant changes in lithology faunal content and diversity across the bathonian callovian boundary the change in lithology from predominantly carbonates in the middle and late bathonian to siliciclastics in the callovian is accompanied by a drop in species diversity from high to moderate characteristic faunal elements of the bathonian corals sponges the bivalves eligmus and members of the opinae are absent or rare in the callovian in contrast callovian sediments contain comparatively high proportions of nuculid bivalves the clay mineral assemblage reveals prominent variations in the smectite vs kaolinite abundance especially during the bathonian to callovian transition from the bajocian to the middle bathonian smectite becomes the dominant clay mineral and is nearly exclusively present in the late bathonian towards the middle callovian its content gradually decreases and kaolinite becomes the dominant clay mineral illite generally is only a minor component throughout the middle jurassic based on these results the bajocian to middle bathonian time interval is interpreted to represent subtropical climatic conditions with seasonal droughts and a moderate supply of terrigenous clastics to the basin the late bathonian was a period with a semi arid climate hot seasonal droughts and a minor input of terrigenous clastics the increase in kaolinite contents in the callovian suggests a subtropical humid climate with less prominent seasonal droughts and a higher input of siliclastics into the basin accompanied by a higher nutrient influx oxygen isotope ratios measured on bathonian to early callovian brachiopod shells give palaeotemperatures of 19 24 c callovian to oxfordian paleotemperatures calculated from oxygen isotopes measured on belemnite rostra range from 11 to 21 c the lower palaeotemperatures estimated from the oxygen isotopic composition of belemnite calcite are explained by assuming that the belemnites spent most of their life span in colder regions of the malagassy gulf and migrated into the warmer subtropical waters of the kachchh basin combining these various lines of evidence the changes in lithology and faunal composition taking place in the kachchh basin during the middle jurassic appear to reflect largely a change towards increasing humidity rather than a distinct decrease in water temperature

matrix micrite δ13c and δ18o reveals synsedimentary marine lithification in upper jurassic ammonitico rosso limestones betic cordillera se spain abstract matrix micrites are a commonly used carbonate archive for the reconstruction of past environmental parameters but one that is submitted to known limitations main reasons for the often ambiguous value of many micrite based isotope data sets are the unknown origin of the micrite components and their poorly resolved diagenetic history here we present carbon and oxygen isotope data retrieved from oxfordian to tithonian ammonitico rosso nodular micrites sampled from three sections in the betic cordillera southern spain all three sections were correlated and sampled using a rigorous biostratigraphic framework a noteworthy feature is that analyzed matrix micrites are more conservative in terms of their isotopic composition than other carbonate materials commonly considered to resist diagenetic alteration under favourable circumstances remarkably this refers not only to 13 c ratios which reflect the typical late jurassic global trend but also to 18 o ratios that range around 0 3 the 18 o enriched oxygen isotope ratios are considered to represent diagenetic stabilization of carbonate ooze under the influence of marine porewaters within the sediment water interphase i e the immature sedimentary section usually submitted to biogenic activity this interpretation agrees with the very early lithification of micrite nodules with cements precipitated from marine porewaters enriched by the dissolution of aragonite skeletals i e ammonite shells according to the model proposed low sedimentation rates as well as rapid early marine differential cementation under the influence of currents and seawater pumping affected the sediment water interphase of epioceanic swells where deposition resulted in early lithified ammonitico rosso facies the data obtained show that special care must be taken to prevent oversimplified interpretations of carbonate archives particularly in the context of epioceanic settings

perturbation of the carbon cycle at the middle late jurassic transition geological and geochemical evidence a compilation of new and published stratigraphic paleontological and geochemical data is used to detect the reciprocal influences of carbon cycling and global environmental changes in the jurassic a major perturbation of the surface carbon cycling accompanied by pronounced climate and sea level fluctuations waxing and waning of continental ice affected earth history around the middle late jurassic transition mljt we establish the respective timing of changes of carbonate and organic matter sedimentation and global fluctuations of sea surface temperatures paleobiogeography and o isotope paleothermometry and sea level sequence stratigraphy so that causative mechanisms and feedback effects can be considered it is apparent that the global sea level rise and warming initiated in the late bathonian led to a constriction of carbonate platforms to low latitudes and enhanced marine organic deposition sea level and temperature optima were achieved several million years later during the middle callovian a detailed record of sea surface temperatures in the northern hemisphere based on migration of marine fauna and isotopic thermometry indicates that a drastic climatic decline set in during the early late callovian just post dating the increased capture of organic matter by marine sediments this decline in temperature is interpreted in terms of an inverse greenhouse effect triggered by drawdown of co 2 consequent upon excess carbon burial the magnitude of refrigeration and its coincidence in time with an abrupt global scale fall of sea level are both suggestive of continental ice formation at this time carbonate sedimentation was jeopardized at the mljt as a result of both global cooling and presumed p c o 2 lowering and resumed abruptly during the middle oxfordian by spreading again over mid latitude zones salient conclusions are that 1 the pattern of excess carbon burial coincident with elevated temperature but followed by climatic deterioration supports the general hypothesis that a major control on mesozoic climate was the abundance of atmospheric co 2 2 significant masses of continental ice may have formed during this part of the jurassic and correlatively high co 2 levels were certainly not sustained throughout this period 3 the global carbonate sedimentation budget correlated with the surface temperature and sea level but the latitudinal spreading of type tropical carbonates was not simply related to the thermal status of seawaters 4 on a global scale the c o r g and c i n o r g burial rates were coupled apparently through the correlation existing between the co 2 level and surface temperature

depositional environments and iron ooid formation in condensed sections callovian oxfordian south eastern paris basin france carbonate platforms across western europe were superseded at the middle upper jurassic callovian oxfordian boundary either by alternating marl limestone and widespread marl deposits or by condensed sections containing iron ooids the characteristics of marine condensed sections in the south eastern part of the paris basin france and their distribution pattern are examined here and a model of iron ooid formation is developed iron ooids are found from the shoreface to the offshore zone they are most abundant in the median to distal offshore transition zone where they originally formed they also occur commonly albeit often as reworked grains in the proximal offshore zone to which they were transported the contemporaneous thick predominantly marl sections that occur laterally are devoid of iron ooids and were deposited in deeper settings distal offshore zone the iron ooids are composed of goethite typically they have a nucleus made up of a clump of goethite crystals and a laminated cortex three distinctive nanostructures are identified in the cortex laminae i a nanograined crystalline structure typical of primary goethite ii a secondary nanoflaked structure thought to have formed mechanically by reorientation of the goethite crystals and iii a coalesced structure acquired by subsequent diagenetic recrystallization the iron ooids formed successively i by lamina growth when goethite precipitated in the surface layer of the sediment nanograined structure and ii by interruption of growth when the ooids were remobilized by hydrodynamic agents as reflected by the flaked nanostructure iii these two nanostructures were sometimes transformed into a coalesced structure by recrystallization when ooids were buried

mineralogical characterization and genesis of hydrothermal mn oxides from the flank of the juan the fuca ridge several sites of active hydrothermal flow have been found on the eastern flank of the juan de fuca ridge these sites are typically located along the edge of basaltic outcrops where sediment is thin we present data on mn oxides formed on such outcrops zona bare and grinin bare these oxides are either black layered crust or soft micro concretions found in partially altered sediments x ray diffraction xrd and scanning electron microscopy sem analyses of mn crusts indicate the presence of well crystallized todorokite and birnessite encrusting detrital minerals and replacing siliceous fossil transmission electron microscopy tem and energy dispersive x ray spectroscopy edx analyses were used to identify amorphous and poorly crystallized mn rich phases in partially altered sediments and crusts tem of impregnated samples showed textural evidence suggesting that amorphous mn oxides are incrusting cellular structures that could be bacteria the valence state of mn in these oxides was determined by parallel electron energy loss spectroscopy peels results indicate that todorokite and birnessite have an average valence state of about 3 7 whereas the poorly crystallized mn rich phases have a lower valence state these data suggest that the formation of hydrothermal mn concretions occurs in several steps the initial step is the adsorption or precipitation of mn fe and si around cell wall bacteria extracellular polymers and siliceous fossil remains these mineralizations are poorly crystallized phyllomanganates which progressively increase in size and crystallinity to give the final birnessite and todorokite products all of these mn rich phases are the result of interactions between hydrothermal fluid and sediments and formed in areas where hydrothermal fluids discharge through the sediment

limestone concretion growth documented by trace fossil relations a concretionary limestone bed and subjacent black laminated shales examined within the upper part of the jurassic posidonienschiefer in the holzmaden area of southern germany contain trace fossils that are derived exclusively from an overlying bioturbated green claystone bed that reflects a short episode of oxygenation the character and systematic distribution of trace fossil assemblages which include chondrites thalassinoides and rhizocorallium provide a record of depth and timing of syngenetic concretionary limestone growth that is more precise than growth histories provided by more traditional physical and geochemical criteria ichnologic evidence indicates that concretion growth began at isolated nuclei at or just below the sediment water interface just prior to or contemporaneous with the onset of the oxygenation episode concretion growth progressed downward and laterally from these nuclei during and after minor compaction but was essentially complete before the termination of the oxygenation event at a depth less than 1 m below the sea floor the temporal association of this concretion growth with the oxygenation episode implies a genetic relation that warrants further investigation of the significance of bottom water oxygenation fluctuations for the formation of other concretionary limestones in the stratigraphic record

growth of ferruginous oncoliths in the bajocian middle jurassic of europe limonitic oncoliths from the bajocian of europe widely known as snuff boxes grew by accretion of bundles of laminae on one side followed by overturning and accretion of laminae on the other side a diverse hard substrate encrusting fauna grew interleaved with the ferruginous laminations the palaeoecological preferences of this fauna indicate that its growth and hence the accumulation of the laminations took place on the gloomy undersides of the oncoliths this upsidedown growth is supported by characteristics of the lamination bundles themselves and by the predominant present day orientations of the snuff boxes the abundance of the associated fauna and the lack of carbonate dissolving macroborers cutting the laminae suggest an original ferric composition precipitation was probably under the control of non photosynthetic microorganisms that inhabited the microenvironments represented by the oncoliths lower surfaces

relation of mineralization to wall rock alteration and brecciation mascot jefferson city mississippi valley type district tennessee this study was undertaken to assess the relation of mississippi valley type mineralization to wall rock alteration and brecciation in the mascot jefferson city district the largest part of the east tennessee mississippi valley type ore field the main question of interest was whether the mississippi valley type forming brines created or greatly enlarged the breccia system that hosts the ore or whether the breccia system was a preexisting paleoaquifer that simply controlled movement of the mineralizing brines a secondary and closely related question was whether brine wall rock interaction deposited mississippi valley type ore the breccia system that hosts the east tennessee ore field began as karst breccias which formed in the upper part of the late cambrian early ordovician knox group during middle ordovician emergence brecciation which was most common at the paleosurface and in a limestone rich zone about 200 m below the surface took place when limestone solution caused collapse of primary dolostone layers mississippi valley type mineralization consisting of sphalerite and sparry dolomite fills interstices in the breccias that formed in the limestone rich part of the knox group ore is associated with recrystalline dolomite that replaced limestone and there is an inverse correlation between the original limestone and sphalerite abundance suggesting that the ore forming fluids reacted strongly with limestone wall rock possibly dissolving it where alteration was most intense the assessment of a relation between alteration and mississippi valley type mineralization was based on 3 533 surface drill holes covering the 110 km 2 mascot jefferson city district each of which provided stratigraphic data and quantified estimates of mineralization intensity and alteration intensity these data show clearly that as much as 50 percent of the limestone in the mineralized breccia section was lost over enormous areas that extend far beyond significant mineralization the intensity of this effect clearly decreases downdip toward the east away from the probable source of meteoric karst forming waters these relations combined with isotopic analyses and reaction path calculations suggest that breccia formation and limestone dissolution took place during the original karst breccia formation in contrast later mississippi valley type mineralization was associated with replacement of limestone by recrystalline dolomite the main effect of dolomitization on the chemistry of the mississippi valley type brines an increase in their ca mg ratio would not cause sulfide precipitation thus it appears unlikely that mississippi valley type forming brines created much of their ore hosting breccias or that water rock interaction was a major cause of mississippi valley type ore deposition

hardgrounds reworked concretion levels and condensed horizons in the jurassic of western india their significance for basin analysis jurassic sediments in the shallow pericratonic basins of kachchh and rajasthan western india exhibit numerous signs of reduced sedimentation omission erosion and in situ reworking in combination with synsedimentary cementation hardgrounds developed on carbonate shoals in the bathonian of rajasthan whilst reworked concretion levels are characteristic of offshore siliciclastic sediments of the callovian of kachchh a prominent marker horizon the oxfordian dhosa oolite member occurs throughout much of the kachchh sub basin and is a highly condensed unit characterized by hardgrounds intraformational cobbles reworked concretions stromatolitic iron crusts iron oncoids and shell lags hardgrounds reworked concretion levels and condensation horizons are interpreted as the preserved relicts of transgressive pulses such pulses were possibly controlled by eustatic rise of sea level of at least equal importance however was a tectonic control which is demonstrated by the presence of small neptunian dykes boulder beds derived from small submarine cliffs and rapid lateral facies and thickness changes in the dhosa oolite member these indications of extensional tectonics are thought to be connected to rifting and initial sea floor spreading between africa and india

taphonomy of cephalopod assemblages from middle jurassic hardgrounds of pelagic swells south iberian palaeomargin western tethys abstract taphonomic analysis of cephalopod assemblages together with analyses of ichnologic and sedimentologic features are useful tools in the interpretation of the history of unconformities in jurassic pelagic swells the external subbetic presents a complex evolution during the middle bathonian middle oxfordian interval that includes hardgrounds and condensed levels rich in ammonoids bathonian and callovian hardgrounds separated by a condensed calcarenitic bed can be differentiated in the bathonian hardground ammonoids are preserved with unfilled phragmocone chambers as a consequence of initial fast burial neomorphic calcite in the ammonoids is related to early diagenesis indicating a stop in sedimentation the ichnological features testify to an early diagenesis and progressive cementation of the sea bottom which is corroborated by the record of neptunian dykes subsequently sediment by passing and erosion produced reorientation of the ammonoids and belemnite remains and exhumation of moulds during which faceting and reelaboration took place finally the sedimentation began resulting in the deposition of a condensed calcarenite bed with fe mn macro oncoids the callovian hardground developed at the top of the calcarenite bed where the cephalopods show signs of long exposure on the sea bottom such as colonization by chemosynthetic microbial communities and sessile foraminifera the study proves that the middle bathonian middle oxfordian represents a time interval of strongly reduced and highly discontinuous sedimentation interrupted by periods of non deposition and erosion

paleoclimatic control of biogeographic and sedimentary events in tethyan and peri tethyan areas during the oxfordian late jurassic abstract the paleobiogeographical distribution of oxfordian ammonites and coral reefs in northern and central europe the mediterranean area north and east africa and the middle east and central asia is compared with the distribution in time and space of the most important lithofacies interest in the oxfordian is focused on changes in facies and in biogeographical patterns that can be interpreted as the results of climatic events paleotemperature trends inferred from oxygen isotopes and paleoclimatic simulations are tested against fossil and facies data a late callovian early oxfordian crisis in carbonate production is indicated by the widespread absence of lower oxfordian reefal formations there is a gap hiatus in deposition on epicontinental platforms with middle oxfordian deposits resting paraconformably on upper callovian while shales accumulated in adjacent intracratonic basins simultaneously in mediterranean tethys radiolarites accumulated in deep troughs while rosso ammonitico facies formed on pelagic swells however deposition on swells was also discontinuous with numerous gaps hiatuses and sequences that are much reduced in thickness middle callovian deposits are generally overlain by middle oxfordian limestones the dearth of carbonates is consistent with a cooling event lasting about 1 my by the middle oxfordian a warming leading to greenhouse type conditions is suggested on the basis of both biogeographical mostly coral reef distribution and geochemical data carbonates spread onto an extensive european platform while radiolarites reached a maximum development in the mediterranean tethys two distinct latitudinal belts with seemingly different accumulation regimes are therefore inferred similar latitudinal belts were also present in the late oxfordian when carbonates were widespread the distribution of reefal facies in the late oxfordian early kimmeridgian fits relatively well with gcms simulations that imply low rainfall in the tethyan mediterranean area and slightly higher precipitation in central and northern europe local salinity variations reflecting more arid or humid conditions may bias the paleotemperature signal inferred from 18o values biogeographical and facies distributions combined with 18o values unravel the ambiguity and support a late callovian early oxfordian cooling followed by warming in the later oxfordian

comment on apatite fission track and u th he data from fennoscandia an example of underestimation of fission track annealing in apatite by hendriks and redfield earth planet sci lett 236 443 458 abstract in a recent paper hendriks and redfield question the existence of major episodes of burial and subsequent denudation in old cratonic settings they use fennoscandia as an example a continental shield region once covered by foreland basin deposits related to the caledonian orogen but today characterized by an exposed precambrian basement hendriks and redfield argue against a significant caledonian foreland basin cover referring to a selection of the numerous thermal indicator studies that have been performed in the region furthermore they discern an inconsistency between previously published fission track and u th he results in the region and suggest an alternative interpretation of the apatite fission track data from fennoscandia here we present geological arguments and highlight the numerous studies only briefly mentioned or not referred to at all by hendriks and redfield that strongly support the former existence of thick and extensive deposits on the caledonian foreland furthermore we discuss the alleged inconsistency between the different data sets by examining the data referred to more closely finally we evaluate the significance of the suggested inverse correlation between fission track age and 238u concentration presented by hendriks and redfield there is in fact no published example of an inconsistency between the two methods concerning paleozoic cooling in fennoscandia at present and the inverse relationship stated by the authors is poorly constrained therefore although radiation enhanced lattice recovery may have an influence on the apatite fission track age and should be examined further we conclude that the study by hendriks and redfield is poorly constrained their argumentation weakly and sometimes wrongly founded and that the thermochronology data from fennoscandia indeed do reflect sedimentary loading

linking early diagenesis and sedimentary facies to sequence stratigraphy on a prograding oolitic wedge the bathonian of western france aquitaine basin to improve the understanding of the distribution of reservoir properties along carbonate platform margins the connection between facies sequence stratigraphy and early diagenesis of discontinuities along the bathonian prograding oolitic wedge of the northeastern aquitaine platform was investigated eight facies are distributed along a 50 km outcropping transect in 1 toe of slope 2 infralittoral prograding oolitic wedge 3 platform margin shoal 4 open marine platform interior 5 foreshore and 6 terrestrial settings the transition from shallow platform to toe of slope facies is marked in the field by clinoforms hundred of meters long carbonate production was confined to the shallow platform but carbonates were exported basinward toward the breakpoint where they cascaded down a 20 25 slope ooid to intraclast grainstones to rudstones pass into alternating marl limestone deposits at an estimated paleodepth of 40 75 m three sea level falls of about 10 m caused the formation of discontinuities corresponding to sequence boundaries along these discontinuities erosional marine hardgrounds formed in a high hydrodynamic environment at a water depth of less than 10 m displaying isopachous fibrous cements and meniscus type cements the cements pass landward into meniscus and microstalactitic forms along the same discontinuities which are characteristic of subaerial exposure during the deposition of transgressive systems tracts carbonate accumulation remained located mostly on the shallow platform energy level increased and carbonates were exported during the deposition of highstand systems tracts forming the infralittoral prograding oolitic wedge during the deposition of lowstand systems tracts carbonate production fell to near zero and intraclast strata derived from the erosion of hardgrounds on the shallow platform prograded basinward early diagenetic cements are related exclusively to discontinuities that are not found within the prograding wedge because of the continuous high sedimentation rate under lower hydrodynamic conditions this absence of early cementation within the infralittoral prograding oolitic wedge was conducive to porosity conservation making such features good targets for carbonate reservoir exploration this study proposes a novel sequence stratigraphy model for oolitic platform wedges including facies and early diagenesis features

the middle late jurassic forced regression and disconformity in central portugal eustatic tectonic and climatic effects on a carbonate ramp system successions across the middle upper jurassic disconformity in the lusitanian basin west central portugal are highly varied and were probably developed on a large westward inclined hangingwall of a half graben the disconformity is preceded by a complex forced regression showing marked variations down the ramp and provides an example of the effects of rapid relative sea level falls on carbonate ramp systems in the east middle jurassic inner ramp carbonates candeeiros facies are capped by a palaeokarstic surface veneered by ferruginous clays or thick calcretes in the west mid outer ramp marls and limestones brenha facies are terminated by two contrasting successions 1 a sharp based carbonate sandbody capped by a minor erosion surface overlain by interbedded marine lagoonal deltaic deposits with further minor erosion exposure surfaces 2 a brachiopod rich limestone with a minor irregular surface overlain by marls lignitic marls with marine and reworked non marine fossils and charophytic limestones with further minor irregular surfaces and capped by a higher relief ferruginous erosional surface the age ranges from late bathonian in the east to late callovian in the west this disconformity assemblage is succeeded by widespread lacustrine lagoonal limestones with microbial laminites and evaporites cabacos facies attributed to the middle oxfordian over the whole basin increasingly marine facies were deposited afterwards in middle jurassic inner ramp zones in the east the overall regression is marked by a major exposure surface overlain by continental sediments in middle jurassic outer ramp zones to the west the regression is represented initially by open marine successions followed by either a sharp marine erosion surface overlain by a complex sandbody or minor discontinuities and marginal marine deposits in both cases capped by the major lowstand surface reflooding led to a complex pattern of depositional conditions throughout the basin from freshwater and brackish lagoonal to marginal and shallow marine settings additional complications were produced by possible tilting of the hangingwall of the half graben the input of siliciclastics from westerly sources and climate change from humid to more seasonally semi arid conditions the middle late jurassic sea level fall in the lusitanian basin is also recorded elsewhere within the iberian and other peri atlantic regions and matches a transgressive to regressive change in eustatic sea level curves indicating that it is related in part to a global event

abrasion surfaces on internal moulds of ammonites as palaeobathymetric indicators abstract some abrasion surfaces displayed by internal moulds of ammonites such as annular abrasion furrows formed on the venter surface and ellipsoidal abrasion facets preferentially developed on the last third part of the preserved whorl are diagnostic criteria of taphonomic reworking these taphonomic criteria indicate that the internal moulds have been exhumed and moved before their final burial we explain the origin of these abrasion surfaces by the action of directional non oscillatory currents in very shallow water most probably in intertidal environments the distribution of ammonite moulds displaying such preservational features is correlated with the changes in the facies and the trend of the sedimentary sequences taphonomic analysis of jurassic ammonites of the iberian range underlines the importance and usefulness of taphonomic gradients in sequence stratigraphy and palaeogeographic reconstructions

jurassic guyots on the southern iberian continental margin a model of isolated carbonate platforms on volcanic submarine edifices during the middle jurassic on the southern iberian continental margin at the westernmost end of the northern tethys isolated carbonate platforms developed over volcanic edifices forming guyots the volcanic edifices were composed of k rich pillow lavas and pyroclastic rocks with a radiometric age 170 myr such phenomena have not been described until now in this continental margin nor in other passive continental margins of alpine domains the presence of a shallowing upward megasequence bajocian bathonian with hummocky cross stratification strata below oolitic shoals shows that very shallow isolated carbonate platforms developed above the volcanic edifices with similar facies to those recognized in other guyots but with a different age and geodynamic context

deposition of upper jurassic fine grained limestones in the western subalpine basin france abstract the fine grained carbonate sequence on the western margin of the french subalpine basin provides information about the depositional conditions of the late jurassic nodular lithofacies which closely resemble the age equivalent ammonitico rosso of the mediterranean and central atlantic domains a variety of rock textures can be distinguished including lumpy nodular restricted sense and conglomeratic the formation of these textures is referred to different sedimentological processes cyclic marl limestone deposition bioturbation winnowing early lithification gravity displacement etc in addition the development of deep water cryptalgal carbonate constructions which include stromatolites and small mud mounds is noted outer shelf deep marine palaeoenvironments 100 s m are inferred from the sedimentary structures biotic associations and facies distributions the callovian oxfordian discontinuity is believed to have resulted from a sediment starved environment part of the overlying fine grained carbonate material was then presumably derived from an adjacent platform oscillations in nature and quantity of carbonate and clay input onto the margin probably reflect cyclic climatic and sea level changes

time averaging overcompleteness and the geological record time averaging makes events that happened at different times appear to be synchronous in the geological record it can generate overcompleteness by concentrating objects e g fossils clasts from a longer time interval into a unit that formed during a shorter interval time averaging results both from extrinsic agents e g vertical mixing condensation analytical pooling of data and the intrinsic properties of the fossilized objects preservational potential and abundance variation in intrinsic and extrinsic factors results in disharmonious time aver aging different taxa preserved together may be time averaged over different time scales because extrinsic and intrinsic factors vary in time and space the geological record is characterized by vertical and lateral variations in time averaging the importance of time averaging depends on the time aver aging threshold which is determined by the time scale of the process of interest significant time aver aging occurs when the scale of temporal mixi

composite ichnofabrics and tiering of burrows infaunal communities in marine environments typically are tiered that is different taxa live at different depths within the sediment tiered suites of biogenic structures yield complex biogenic sedimentary fabrics ichnofabrics with the traces of deep burrowing organisms overprinted on those of shallow burrowing organisms careful analysis of crosscutting relationships of burrows in such composite ichnofabrics allows reconstruction of the tiered nature of fossil endobenthic communities it is important to recognize that the best preserved and most prominently displayed trace fossils in most assemblages usually represent the deepest tier thus they were farther removed from the sea floor and therefore less indicative of actual sea floor conditions than the more poorly preserved traces of the shallower tiers on which the deeper traces are juxtaposed

mesozoic climates general circulation models and the rock record abstract general circulation models gcms use the laws of physics and an understanding of past geography to simulate climatic responses they are objective in character however they tend to require powerful computers to handle vast numbers of calculations nevertheless it is now possible to compare results from different gcms for a range of times and over a wide range of parameterisations for the past present and future e g in terms of predictions of surface air temperature surface moisture precipitation etc gcms are currently producing simulated climate predictions for the mesozoic which compare favourably with the distributions of climatically sensitive facies e g coals evaporites and palaeosols they can be used effectively in the prediction of oceanic upwelling sites and the distribution of petroleum source rocks and phosphorites models also produce evaluations of other parameters that do not leave a geological record e g cloud cover snow cover and equivocal phenomena such as storminess parameterisation of sub grid scale processes is the main weakness in gcms e g land surfaces convection cloud behaviour and model output for continental interiors is still too cold in winter by comparison with palaeontological data the sedimentary and palaeontological record provides an important way that gcms may themselves be evaluated and this is important because the same gcms are being used currently to predict possible changes in future climate the mesozoic earth was by comparison with the present an alien world as we illustrate here by reference to late triassic late jurassic and late cretaceous simulations dense forests grew close to both poles but experienced months long daylight in warm summers and months long darkness in cold snowy winters ocean depths were warm 8 c or more to the ocean floor and reefs with corals grew 10 of latitude further north and south than at the present time the whole earth was warmer than now by 6 c or more giving more atmospheric humidity and a greatly enhanced hydrological cycle much of the rainfall was predominantly convective in character often focused over the oceans and leaving major desert expanses on the continental areas polar ice sheets are unlikely to have been present because of the high summer temperatures achieved the model indicates extensive sea ice in the nearly enclosed arctic seaway through a large portion of the year during the late cretaceous and the possibility of sea ice in adjacent parts of the midwest seaway over north america the triassic world was a predominantly warm world the model output for evaporation and precipitation conforming well with the known distributions of evaporites calcretes and other climatically sensitive facies for that time the message from the geological record is clear through the phanerozoic earth s climate has changed significantly both on a variety of time scales and over a range of climatic states usually baldly referred to as greenhouse and icehouse although these terms disguise more subtle states between these extremes any notion that the climate can remain constant for the convenience of one species of anthropoid is a delusion although the recent rate of climatic change is exceptional

vascular plant biomarkers as proxies for palaeoflora and palaeoclimatic changes at the dogger malm transition of the paris basin france abstract vascular plant biomarkers are used here as a chemostratigraphic tool for tracing both palaeoflora and palaeoclimatic changes during the callovian and oxfordian in the paris basin the retene cadalene ratio shows low values in claystones from the middle callovian to the end of the lower oxfordian then increases drastically from the end of the lower oxfordian cordatum zone synchronously with the progressive development of the oxfordian carbonate platform the relative proportion of retene a conifer biomarker is strongly correlated with the abundance of some of its diagenetic precursors and with saturated diterpanes some of these are very common and abundant in the pinaceae family while specific markers for non pinaceae conifers are not detected thus the increase in the retene cadalene ratio around the cordatum zone reflects the rising proportion of pinaceae on the london brabant massif which was the nearest land from the locations studied as pinaceae show many morphological adaptations to dry climate compared to the other plants reported in the jurassic taphofloras of the paris basin their increasing proportion indicates an increasing aridity this climatic change is also supported by published geochemical palaeobiogeographical palynological palaeobotanical and mineralogical data a comparable increase in the relative proportion of retene was also described for the oxfordian of western australia which could be an argument for a worldwide increase in aridity at the end of the lower oxfordian

temporal and bathymetric resolution of nautiloid death assemblages in stratigraphically condensed oozes new caledonia cephalopod shells can be affected by postmortem transport and biostratigraphic condensation but direct estimates of the temporal and spatial resolutions of cephalopod assemblages are missing amino acid racemisation calibrated by 14c demonstrates a centennial scale time averaging 500 years of nautilus macromphalus in sediment starved epi and mesobathyal pelagic environments the few shells that are thousands of years old are highly degraded the median occurrence of dead shells is at 445 m depth close to the 300 400 m depth where living n macromphalus are most abundant therefore dead shells of this species accumulate at a centennial temporal resolution and with excellent bathymetric fidelity dead nautilus shells exist for only a few hundred years on the seafloor in contrast to the biostratigraphically condensed mixture of extant foraminifers and foraminifers that went extinct during the pleistocene cephalopod shells that do not show any signs of early diagenetic cementation are unlikely to be biostratigraphically condensed

fossil assemblages lithofacies taphofacies and interpreting depositional dynamics in the epicontinental oxfordian of the prebetic zone betic cordillera southern spain abstract the study of six sections within the external prebetic pozo canada fuente alamo chinchilla de montearagon and riogazas chorro ii and the internal prebetic navalperal and rio segura comprises the first results of a taphonomic study to be incorporated into the ecostratigraphic interpretation of the oxfordian in the prebetic zone betic cordillera southern spain a taphonomic analysis was made of 9350 specimens including fragments of macroinvertebrate fossils ammonoids 64 benthic organisms 26 and belemnoids and nautiloids 10 the taphonomic features analysed were a state of preservation ratio of moulds with without shell and type of mould filling b fossil size c position within the bed d corrasion see brett c e baird g c palaios 1 1986 207 227 e fragmentation f epibionts and encrustment g disarticulation h uncoupling and i deformation analysis of the relative significance of these features and their relation to the lithofacies allows us to characterise and distinguish three major taphofacies a taphofacies i characterised by the greater average size of the specimens recorded higher values of corrasion encrustation and colonisation and by the fact that a high proportion of the specimens are in a quasi horizontal position b taphofacies ii characterised by a higher proportion of smaller specimens and a greater variety of azimuthal orientation although the quasi horizontal position is always predominant and corrasion values are low c taphofacies iii with predominantly small specimens the highest proportion of quasi horizontal orientation and fragmentation and no evidence of corrasion the taphofacies are stratigraphically and palaeogeographically distributed in interpreting the taphofacies the exposure time of skeletals is an essential parameter and is related to the controlling effect of the sedimentation rate this latter depends on factors such as the tectonic eustatic dynamics the relative distance from the shore and the bottom physiography