delta or coastal plain with an example of the controversy from the middle jurassic of yorkshire summary deltaic and coastal plain deposits can be distinguished from each other in the rock record where enough data are available to interpret the three dimensional distribution of facies and unconformities over a wide area this distinction is based on the distribution of facies rather than purely on a marine to alluvial transition as has been the case in the past the distinction between deposits of the two environments is often very difficult or impossible and if based on limited data may lead to inaccurate and even incorrect models that can cause problems with prediction of reservoir rocks or coal location in the subsurface models of the gross facies distribution for tectonically eustatically and autocyclically controlled coastal plains are seen as the end members of a continuum between coastline depositional styles the middle jurassic sedimentary rocks of the yorkshire basin are used to illustrate the controversy that can arise from imprecise definitions and the application of incorrect facies models the middle jurassic ravenscar group is interpreted here as the deposits of a tectonically and eustatically influenced coastal plain repeated fluctuations between shallow marine and non marine conditions are interpreted as mainly caused by vertical tectonic movements and sea level changes rather than by autocyclic processes

palaeoseismology review of the state of the art abstract historical seismicity provides only a partial indication of the seismotectonic character i e earthquake potential of a given region even in italy where a relatively long historical record exists available catalogues are generally unsatisfactory for this purpose fortunately strong earthquakes often leave geological evidence herein referred to as seismites that are mainly related to surface faulting folding or other deformation and occasionally to secondary ground failures e g landslides liquefaction features etc these features may be preserved and therefore palaeoseismological studies may detect and date them thereby permitting extension of knowledge beyond the local historical threshold up to 2000 years in the most fortunate cases this paper provides a summary of criteria which may aid in the identification of areas suitable for palaeo earthquake investigation historical seismicity and the presence of well developed tectonic geomorphic features are recognized as the most reliable criteria for identifying active faulting however lack of these characteristics cannot be considered conclusive evidence for lack of activity palaeoseismological techniques aim at identifying not only the obvious effects of surface faulting such as offset of young geologic units but also the more subtle tectonic or non tectonic features including liquefaction structures gravity features and coseismic uplift and or subsidence finally we discuss some questions that pertain to palaeoseismicity evaluation techniques among these are the reliability of the quaternary dating techniques the evaluation of the seismic energy released minimum magnitude and principally the unequivocal identification of a seismic origin for the considered geological structures

estimates of the amount and rate of sea level change across the rhaetian hettangian and pliensbachian toarcian boundaries latest triassic to early jurassic an attempt is made to determine the amount and rate of sea level change for two major events in the latest triassic to early jurassic both of which are associated with mass extinctions using the best available facies evidence and time scale the amount of marine deepening across the pliensbachian toarcian boundary based on analysis of the excellent yorkshire coast section is likely to have been a few tens of metres at a rate of between 1cm 1 2 ka 1 and 1cm 0 4 ka 1 the change across the rhaetian hettangian triassic jurassic boundary was different in that the sea level rise was immediately preceded by a sea level fall effectively a 9regression transgression9 couplet the earliest jurassic sea level rise is likely to have been much more rapid than that across the pliensbachian toarcian boundary at a rate of at least 1 cm 0 2 ka 1 whereas the speed of pliensbachian toarcian change is consistent with the growth of oceanic ridges the appreciably more rapid triassic jurassic change is suggestive of a different tectonoeustatic mechanism involving stress induced changes in plate density

early hettangian benthic planktonic coupling at doniford sw england palaeoenvironmental implications for the aftermath of the end triassic crisis abstract this paper presents a high resolution micropalaeontological study integrated with geochemical and sedimentological analyses of the triassic jurassic tr j transition at the doniford section sw england our analyses indicate the renewal of surface carbonate production in the blue lias formation early hettangian after the end triassic mass extinction based on the high abundance of surface carbonate producers schizosphaerellids which occurred in phase with development at sea floor of calcareous hyaline benthic foraminifers we deduce that a benthic pelagic coupling resulted in both the initial restoration of the biological carbonate pump and also a more efficient export production the presence of monospecific phytoplankton assemblages and benthic foraminiferal assemblages typical of poorly oxygenated environments suggest that the environmental stress continued after the end triassic maximal perturbation complete environmental recovery was prevented by a succession of alternating dysaerobic and anoxic phases that favoured the deposition of black shale sediments the hypothesis of a methane release mechanism as a trigger for the global carbon cycle perturbation and greenhouse conditions during the early jurassic is discounted based on evidence from the decoupled carbon isotope records of carbonate and organic matter apart from local control of a high amplitude sea level change across the tr j boundary the succession and nature of events at doniford are consistent with a scenario of global environmental and biotic changes triggered by the physical and chemical effects of the central atlantic magmatic province camp volcanism a super greenhouse climate in the aftermath of the end triassic mass extinction appears to be the results of the combination of the long term co2 accumulation in the atmosphere system due to the camp volcanic activity as well as the inefficiency of the biological pump due to reduced export production

correlating the end triassic mass extinction and flood basalt volcanism at the 100 ka level new high precision u pb geochronology from volcanic ashes shows that the triassic juras sic boundary and end triassic biological crisis from two independent marine stratigraphic sections correlate with the onset of terrestrial fl ood volcanism in the central atlantic mag matic province to 150 ka this narrows the correlation between volcanism and mass extinc tion by an order of magnitude for any such catastrophe in earth history we also show that a concomitant drop and rise in sea level and negative 13 c spike in the very latest triassic occurred locally in 290 ka such rapid sea level fl uctuations on a global scale require that global cooling and glaciation were closely associated with the end triassic extinction and potentially driven by central atlantic magmatic province volcanism

palaeoenvironments and faunal recovery after the end permian mass extinction abstract strata of the lower triassic werfen formation dolomites northern italy record deposition in a wide range of palaeoenvironments from peritidal to outer ramp settings this enables accurate assessment of faunal recovery following the end permian mass extinction by eliminating facies related effects extinction of the shelly taxa appears to be rapid and occurs in the latest permian tesero oolite horizon disappearance of the various trace fossil taxa appears to be more gradual following the extinction the lower and middle griesbachian are characterised by laminated sediments containing low diversity bedding plane assemblages ichnological geochemical and sedimentological data show evidence of widespread low oxygen conditions normally oxygenated waters return in the late griesbachian and there is a moderate increase in tiering above and below the surface macrofaunal and ichnofossil diversity decrease markedly during the smithian with the appearance of more terrigenous strata possibly reflecting an increased run off and more brackish conditions complete recovery begins in the basal spathian with an increase in tiering above and below the substrate crinoids and ammonoids appear for the first time in the werfen formation and the ichnogenus rhizocorallium re appears global correlation is hindered by poor biostratigraphy in the dolomites region however on a broad scale it appears as though many of the palaeoenvironmental changes are repeated worldwide current available data suggest that the re appearance of some trace fossil taxa e g diplocraterion rhizocorallium may be globally synchronous

biotic recovery after the end triassic extinction event evidence from marine bivalves of the neuquen basin argentina abstract we analyze the late triassic extinction and early jurassic recovery of bivalve faunas within marine environments in the atuel river area of the neuquen basin argentina data were collected from a hundred samples with invertebrates in a well exposed uppermost triassic to lower jurassic section in the neuquen basin southern mendoza province argentina and allow a high resolution reconstruction of the local diversity dynamics the nearly continuous presence of marine stenohaline major taxa such as cnidarians rhynchonelliform brachiopods echinoderms and cephalopods indicates normal salinity throughout all bivalve species were identified and each occurrence was recorded in meters above the base to analyze the systematic diversity trends diversity curves were calculated on the basis of the first and last occurrence data for each bivalve species and both total diversity and boundary crossers diversity were used as a result four main phases were identified a triassic equilibrium phase rhaetian with relatively high origination and extinction rates b extinction phase latest rhaetian to earliest hettangian with high extinction rates and low origination rates c recovery phase late early to early late hettangian with high origination rates and almost null extinction rates and d jurassic equilibrium phase late hettangian sinemurian again with similar and relatively high origination and extinction rates the extinction and recovery phases are separated by a gap of about 135 m without identifiable benthonic invertebrates but with early hettangian ammonites on the other hand bivalve palaeoecologic diversity seems to have been more homogeneous along the section being dominated by attached epifaunal species though before the extinction epifaunal habits were slightly surpassed by infaunal ones slight differences observed include a shallow burrowers were more diverse during the rhaetian than during the earliest jurassic and b epifaunal free lying and semi infaunal attached bivalves were more diverse after the rhaetian extinction

shock induced resuspension deposits from a pleistocene proglacial lake kleszczow graben central poland abstract several glaciolacustrine successions forming part of the quaternary fill of the kleszczow graben contain massive and widespread fine grained units that are abnormally thick and homogeneous field studies of such a unit indicate that it is the result of rapid settling of particles from a sediment cloud consisting of material that had once settled on the lake bottom but were resuspended the geological setting makes it likely that the trigger mechanism responsible for the sediment cloud in the lake water was a shock and that this shock most probably an earthquake was caused by tectonic activity in the graben

ultra shallow marine anoxia in an early triassic shallow marine clastic ramp spitsbergen and the suppression of benthic radiation lower triassic marine strata in spitsbergen accumulated on a mid to high latitude ramp in which high energy foreshore and shoreface facies passed offshore into sheet sandstones of probable hyperpycnite origin more distal facies include siltstones shales and dolomitic limestones carbon isotope chemostratigraphy comparison allows improved age dating of the boreal sections and shows a significant hiatus in the upper spathian two major deepening events in earliest griesbachian and late smithian time are separated by shallowing upwards trends that culminated in the dienerian and spathian substages the redox record revealed by changes in bioturbation palaeoecology pyrite framboid content and trace metal concentrations shows anoxic phases alternating with intervals of better ventilation only dienerian early smithian time witnessed persistent oxygenation that was sufficient to support a diverse benthic community the most intensely anoxic usually euxinic conditions are best developed in offshore settings but at times euxinia also developed in upper offshore settings where it is even recorded in hyperpycnite and storm origin sandstone beds an extraordinary facet of spitsbergen s record the euxinic phases do not track relative water depth changes for example the continuous shallowing upwards from the griesbachian to lower dienerian was witness to several euxinic phases separated by intervals of more oxic bioturbated sediments it is likely that the euxinia was controlled by climatic oscillations rather than intra basinal factors it remains to be seen if all the anoxic phases found in spitsbergen are seen elsewhere although the wide spread of anoxic facies in the smithian spathian boundary interval is clearly a global event

photic zone euxinia in the central rhaetian sea prior the triassic jurassic boundary abstract shortly after the deposition of black shales in the rhaetian sea central european basin ceb the triassic jurassic tr j boundary witnessed one of the big five mass extinction events in earth s history aiming at a better understanding of paleoenvironmental changes in the geochemically less well known middle to late rhaetian german subdivision we studied samples from a cored borehole from the central rhaetian sea in nw germany hebelermeer 2 biomarkers palynomorphs and bulk geochemistry all support a marine brackish setting with inputs from terrestrial plants dinosteranes were found throughout the core most likely suggesting the spread of dinoflagellates it is widely accepted that volcanic exhalations during the development of the central atlantic magmatic province camp had a major impact on marine and terrestrial environments in the earliest jurassic e g the development of photic zone euxinia and h2s poisoning of benthic life there is evidence from the studied core however that comparable conditions already thrived in the central rhaetian sea during the middle rhaetian a first indication of euxinia anoxia is evident from low to very low total organic carbon versus total sulfur toc ts ratios

glacial sediments and erosional pavements near the cambrian precambrian boundary in western henan province china exceptionally well preserved glaciated pavements and a variety of glacigenic sediments comprising the luoquan formation in henan province central china are believed to have formed close to the cambrian precambrian boundary they may therefore be younger than the better known sinian tillites representing the changan and nantuo ice ages they perhaps belong to a distinct glacial epoch that is recorded elsewhere in china and in other continents but for which there is less evidence than for earlier late proterozoic glacial periods lithofacies represented in the luoquan formation are massive diamictite signifying deposition as lodgement or melt out till bedded diamictite waterlain till or glaciomarine glaciolacustrine sediments with dropstones rhythmites with dropstones glaciolacustrine or fjord deposits possibly varves tabular conglomerates and sandstones outwash deltaic or sub aqueous mass flow deposits lens shaped breccia deposits sub aqueous mass flow deposits and gravel sand wedges frost contraction crack infillings the glaciated pavements reveal well preserved striations grooves often intersecting 9plastically moulded9 forms crescentic gouges and steps on a regional scale the linear features are of variable orientation but they indicate a dominant ice flow direction towards the southeast all this evidence points to upland glaciation with erosion followed by deposition in a terrestrial glacial and glaciolacustrine environment

evaluating compositional turnover of brachiopod communities during the end triassic mass extinction northern calcareous alps removal of dominant groups recovery and community reassembly abstract this study highlights the role of large scale physical perturbations in mediating biotic replacements and shows that an environmental disturbance at the triassic jurassic t j boundary correlates with abrupt and substantial changes in the composition of brachiopod communities disturbance changed the phylogenetic structure of early jurassic brachiopod communities owing to a removal of higher taxa that were abundant in the late triassic a replacement of brachiopod communities through the rhaetian in the kossen basin northern calcareous alps related to a combination of habitat tracking and immigration local extinction events indicates a high compositional turnover this turnover is of local nature only because early rhaetian communities migrated or tracked their habitats beyond the kossen basin and persisted through to the late rhaetian in other regions a siliciclastic interval that is several meters thick with rare brachiopods dated as the earliest hettangian marks the extinction survival interval this interval is coeval with a negative carbon isotope anomaly implying a correlation with global perturbation of carbon cycle a rapid brachiopod recovery is indicated by a presence of several distinct communities in late early and middle hettangian that show onshore offshore differentiation and beta diversity comparable to pre extinction levels analyses of similarities demonstrate that 1 the compositional turnover of brachiopod communities on generic level at the t j boundary r 0 83 is substantially higher than turnovers between the rhaetian zones r 0 28 0 57 and between the hettangian zones r 0 28 0 53 and 2 the turnover at superfamily level at the t j boundary accounts for differential composition of rhaetian and hettangian communities a global extinction of athyridoid spondylospiroid and dielasmatoid superfamilies characterized by high community level abundances during the late triassic led to a new assembly of jurassic brachiopod communities from surviving superfamilies in addition to persisting rhynchonellids and zeillerioids hettangian brachiopod communities were dominated by terebratuloids spiriferinoids and pennospiriferinoids these superfamilies were characterized in the late triassic by low community level abundance we argue for tracking the phylogenetic structure of communities across mass extinction events because a measurement of the turnover in community level abundance of higher taxa can be highly relevant for estimating the ecologic impact of mass extinctions taxonomic extinction rate metrics or diversity measures can be depressed by surviving taxa that do not re attain their pre extinction community level abundance

uranium isotope evidence for an expansion of marine anoxia during the end triassic extinction the end triassic extinction coincided with an increase in marine black shale deposition and biomarkers for photic zone euxinia suggesting that anoxia played a role in suppressing marine biodiversity however global changes in ocean anoxia are difficult to quantify using proxies for local anoxia uranium isotopes 238u in caco3 sediments deposited under locally well oxygenated bottom waters can passively track seawater 238u which is sensitive to the global areal extent of seafloor anoxia due to preferential reduction of 238u vi relative to 235u vi in anoxic marine sediments we measured 238u in shallow marine limestones from two stratigraphic sections in the lombardy basin northern italy spanning over 400 m we observe a 0 7 negative excursion in 238u beginning in the lowermost jurassic coeval with the onset of the initial negative 13c excursion and persisting for the duration of subsequent high 13c values in the lower middle hettangian stage the 238u excursion cannot be realistically explained by local mixing of uranium in primary marine carbonate and reduced authigenic uranium based on output from a forward model of the uranium cycle the excursion is consistent with a 40 100 fold increase in the extent of anoxic deposition occurring worldwide additionally relatively constant uranium concentrations point toward increased uranium delivery to the oceans from continental weathering which is consistent with weathering induced eutrophication following the rapid increase in pco2 during emplacement of the central atlantic magmatic province the relative timing and duration of the excursion in 238u implies that anoxia could have delayed biotic recovery well into the hettangian stage

multiple sulfur isotopic evidence for a shallowly stratified ocean following the triassic jurassic boundary mass extinction the cause of the triassic jurassic tr j boundary biotic crisis one of the big five mass extinctions of the phanerozoic remains controversial in this study we analyzed multiple sulfur isotope compositions 33s 34s and 36s of pyrite and spy toc ratios in two tr j successions mariental mingolsheim from the european epicontinental seaway ees in order to better document ocean redox variations during the tr j transition our results show that upper rhaetian strata are characterized by 34s enriched pyrite low spy toc ratios and values of 33spy i e the deviation from the mass dependent array lower than that estimated for contemporaneous seawater sulfate suggesting an oxic suboxic depositional environment punctuated by brief anoxic events the overlying hettangian strata exhibit relatively 34s depleted pyrite high 33spy and spy toc values and the presence of green sulfur bacterial biomarkers indicate a shift toward to euxinic conditions the local development of intense marine anoxia thus postdated the tr j mass extinction which does not provide support for the hypothesis that euxinia was the main killing agent at the tr j transition sulfur and organic carbon isotopic records that reveal a water depth gradient i e more 34s 13c depleted with depth in combination with spy toc data suggest that the earliest jurassic ees was strongly stratified with a chemocline located at shallow depths just below storm wave base shallow oceanic stratification may have been a factor for widespread deposition of black shales a large positive shift in carbonate 13c values and a delay in the recovery of marine ecosystems following the tr j boundary crisis

sequence architecture of ancient rocky shorelines and their response to sea level change an early jurassic example from south wales uk the sea cliffs of ogmore by sea in glamorgan south wales expose a succession of early jurassic nearshore carbonates that drape rocky palaeoplatforms a sequence stratigraphic interpretation allows the recognition of four retrogradationally stacked parasequences the internal facies architecture of each of which is controlled by the bathymetry and geometry of the underlying shore platforms preserved facies are interpreted as representative of foreshore shoreface and offshore environments supratidal facies are absent but representative storm terrace clast assemblages are preserved in all lithofacies as a result of the cannibalization of the backshore during periods of shoreline retreat driven by sea level rise biostratigraphical calibration allows the reconstruction of sea level history and the correlation of regionally important flooding surfaces in the rhaetian sinemurian of southern britain with the position of the contemporary shoreline this suggests that platform incision in south wales occurred several million years before the onset of major deposition superseding deposits above ancient rock shore platforms may therefore not be representative of the environment in which coastal incision took place as one of the best examples of its kind the rockshore at ogmore acts as a deposystem model for ancient rocky shores worldwide

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

latest permian to middle triassic redox condition variations in ramp settings south china pyrite framboid evidence a detailed 10 m y redox history of changhsingian to anisian latest permian to middle triassic oceans in ramp settings is reconstructed based on framboidal pyrite analysis from south china the result shows that the well established phenomenon of intense ocean euxinia anoxia is faithfully recorded in pyrite framboid data three major euxinia anoxia episodes namely the end changhsingian to end smithian middle to late spathian and early to middle anisian have been recognized from the ramp successions the first reducing episode is subdivided into four subepisodes permian triassic boundary griesbachian dienerian boundary earliest smithian and end smithian redox variations broadly track other oceanographic proxies euxinia anoxia episodes coincide with positive excursions of conodont ce anomalies negative excursions of 34scas carbonate associated sulfate increases in sea surface temperature and negative excursions of 13c in most cases however euxinia anoxia near the dienerian smithian boundary coincided with positive excursions of 13c and a general cooling period this exception may be the result of locally developed water column anoxia the permian triassic boundary subepisode witnessed two ephemeral euxinia anoxia events separated by a dysoxic to oxic period the former together with a rapid increase in sea surface temperature up to 8 c may have been responsible for the biodiversity crisis while the latter anoxic event destroyed ecosystem trophic structures in addition to the permian triassic boundary euxinia anoxia event which spread over habitats in all oceans the spathian and anisian euxinia anoxia episodes also prevailed in global oceans variation of the oxygen minimum zone are suggested as the driving mechanism that facilitated the movement of oxygen poor water columns in various paleogeographic settings over this critical period

interaction between faulting and sedimentation during dinantian extension of the solway basin sw scotland the ene wsw trending north solway fault acted as a northern marginal fault to the solway basin during dinantian sedimentation structural sedimentological and diagenetic relationships suggest syn sedimentary dinantian movement was dip slip and normal a later carboniferous phase of dextral transtension is also indicated allokinetic deformation features increase in abundance and intensity towards the north solway fault their presence and nature being strongly facies dependent syn sedimentary fault deformation of the hanging wall sediments also increases towards this fault both distributed and localized shearing characterizes these early faults and is thought to be related to the water content of beds at the time of deformation local minor thrusting and reverse fault movement also occurred within the sediments during basin extension due to dextral transtensional movement on the north solway fault n s and nw se trending faults appear to show no evidence for syn sedimentary dinantian movement and are interpreted as the products of 9hercynian9 basin shortening and mesozoic basin formation

body size trends and recovery amongst bivalves following the end triassic mass extinction abstract fossils in the immediate aftermath of mass extinctions are often of small size a phenomenon attributed to the lilliput effect temporary size reduction of surviving species there has been little attempt to study size trends during subsequent recovery intervals nor has the relationship between size diversity and environmental controls been evaluated here we examine the recovery following the end triassic mass extinction amongst bivalves of the british lower and middle lias three distinct phases of size change are seen that are independent of other recovery metrics initially bivalves are small but the lilliput effect is a minor factor the majority of small taxa belong to new species that undergo a later within species size increase the brobdingnag effect throughout the subsequent hettangian stage new species that appeared during the hettangian were also progressively larger and cope s rule size increase between successive species is seen notably amongst ammonites the size increase was reversed during the sinemurian stage when bivalves once again exhibited small body sizes during the pliensbachian stage another phase of size increase occurred with further evidence of the brobdingnag effect these three phases of size change are seen across all suspension feeding ecological guilds of bivalve but are not expressed among deposit feeders local environmental conditions explain some aspects of size patterns but factors such as temperature marine oxygenation and sea level do not correlate with the long term size trends the brobdingnag effect may reflect increased availability quality of food during the recovery interval a factor that controlled bivalve size but not evolution

late pleistocene depositional systems of metropolitan toronto and their engineering and glacial geological significance the municipality of metropolitan toronto area 480 km2 population 2 15 million is centrally located on the late pleistocene sedimentary infill of the laurentian channel a broad bedrock low up to 115 km wide connecting the huron and ontario basins this channel forms part of a relict late tertiary drainage network the laurentian river modified by pleistocene glacial erosion and infilled by over 100 m of glacial and interglacial sediments the subsurface stratigraphy of the channel fill below metropolitan toronto has been established from many different data sources and is depicted in this paper as a series of cross sections with a total length of nearly 105 km the subsurface stratigraphy has been divided provisionally into five depositional complexes which have been mapped in the subsurface along several transects these are 1 a glacial complex of illinoian age 2 a lacustrine complex of sangamon interglacial and earliest wisconsinan sediments 120 000 75 000 bp 3 a glaciolacust

hemipelagic shelf sedimentation and climatic cycles the basal jurassic blue lias of south britain abstract the sedimentology cyclicity and hiatuses of the blue lias formation basal jurassic were investigated at five localities in south britain most carbonate mud which is the dominant carbonate component has been neomorphosed to microspar however carbonate mud was apparently supplied by coccoliths in zooplankton faecal pellets suggesting hemipelagic sedimentation clay is assumed to have been supplied by rivers the three sediment types involved reflect cyclic changes in bottom water oxygenation levels combined with changes in the clay to carbonate mud ratio walsh power spectra have been applied for the first time to measured sections in order to investigate the control of cyclicity this application is analogous to palaeoclimatological studies of pleistocene oxygen isotope ice volume records the spectra show that 1 or 2 sedimentary cycles of constant thickness occur in four out of five sections each of which represents several million years the maximum possible duration of these cycles is too short for their generation by sea level changes their regularity stability and order of duration tens of thousands of years are consistent with the hypothesis of sedimentary cyclicity controlled by climatic change itself forced by orbital changes in insolation milankovitch theory the duration of both cycles recognised is less than 93 000 years 93 ka and may record changes in orbital precession 21 ka and obliquity 41 ka assuming that one ammonite zone represents 1 ma comparison of the longest possible with the implied durations of the cycles suggests that the sections are at best 20 40 complete loss of sediment may have occurred during winter storms or hurricanes a model relating the sedimentary cycles to changes in the volume of runoff is proposed

comments and reply on sedimentation in a large lake a reinterpretation of the late pleistocene stratigraphy at scarborough bluffs ontario canada reply

sedimentation in an active strike slip basin southeastern turkey the modern structural drainage and depositional systems of the lake hazar basin in southeastern turkey exhibit an instantaneous picture of faulting and sedimentation within an actively evolving pull apart basin a general sedimentation model is proposed which consists of two types of drainage and depositional systems streams entering the basin laterally cross numerous oblique slip faults and are short steep associated with small drainage basins and characterized by profiles with many knickpoints they build alluvial fan deltas which are small steep and composed of coarse grained debris flow and fluvial deposits fluvial deposits consist of braided channel and bar sheetflood and overbank facies streams entering the basin longitudinally flow along the surface trace of single faults and are long gently sloping associated with large drainage basins and characterized by smooth profiles they build large gently sloping alluvial fan deltas composed of fluvial and lacustrine deposits lacustrine

the occurrence and preservation of ammonites in the blue lias formation lower jurassic of devon and dorset england and their palaeoecological sedimentological and diagenetic significance abstract more than two thirds of beds in the lowest jurassic blue lias formation lack ammonites which are commonly preserved in irregular or planar bedded bioturbated limestones very rarely in laminated limestones and almost never in laminated black shales ammonites are preserved in 3d in nodular and planar bedded limestones and at any orientation to bedding co occurrence with macrobenthos and absence from beds without benthos suggest that blue lias ammonites were nektobenthonic scour structures and imbrication of ammonites in the best bed imply presence of traction currents lack of epifauna on large cephalopod shells and other fossils implies rapid deposition in event beds blue lias deposition was episodic not slow and continuous as the fine grain size implies undistorted trace fossils uncrushed ammonites and stable isotope values all suggest early cementation of limestone beds from pore waters of a similar composition to contemporary jurassic sea water a clear diagenetic trend exists with limestones having least and laminated black shales most modified stable isotope values contrast between trace fossil fills and host sediment demonstrates that blue lias rhythms are primary but limestone beds have been diagenetically cemented

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

discussion on palaeoecology of the late triassic extinction event in the sw ukjournal vol 165 2008 pp 319 332 jonathan radley writes the penarth group of late triassic and possibly ranging to early jurassic age of the southern uk marks a marine transgression and the establishment of a shallow epicontinental seaway hallam el shaarawy 1982 warrington ivimey cook 1992 influenced by regressive transgressive pulses and characterized by rapid facies changes hallam wignall 2004 hesselbo et al 2004 a well documented invertebrate macrofauna includes corals brachiopods molluscs and echinoderms swift martill 1999 facies developments range from storm influenced shallow marine mudrocks westbury formation upwards into calcareous mudstones siltstones and essentially fine grained carbonates lilstock formation demonstrating extremely shallow water conditions wave ripples storm beds desiccation and omission surfaces swift 1999 a wignall 2001 hallam wignall 2004 the low diversity macrofauna of the lilstock formation has frequently been attributed to salinities deviating from that of normal seawater within shallow water partly landlocked settings isolated from the open sea hallam el shaarawy 1982 swift 1995 allison wright 2005 however widespread occurrences of stenohaline marine macrofossils such as corals and echinoderms swift martill 1999 suggest that salinity fluctuations were at most slight wignall 2001 mander et al 2008 have presented high resolution benthic macrofaunal data principally from molluscan macrofossils for significant palaeoecological change within the penarth group of two key sections in the sw uk their detailed results demonstrate loss of taxonomic richness within the uppermost westbury formation lower lilstock formation cotham member interval latest rhaetian followed by a poorly fossiliferous interval represented by the latest rhaetian or early hettangian upper lilstock formation upper cotham member lower langport member they attributed this pattern to an extinction event followed by a post extinction dead zone the latter coinciding with the onset of a negative organic carbon isotope excursion and an extinction event recorded in the terrestrial palynoflora the relatively diverse shelly

sea level change and facies development across potential triassic jurassic boundary horizons sw britain the late triassic to early jurassic aged succession of sw britain the penarth and lower lias groups comprises mudstone sandstone and limestone strata deposited in a variety of marine to non marine environments faunal and floral characteristics of these successions have led to the proposal that one location in sw england st audrie9s bay should serve as the global stratotype section and point gssp for the base of the hettangian stage and thus for the triassic jurassic tr j boundary the sections of sw britain have also been used previously to infer sea level change history and relate this to potential kill mechanisms associated with the tr j boundary mass extinction chemostratigraphic biofacies and lithofacies data are used here to suggest alternative models of sea level change in relation to possible tr j boundary horizons in the sections of sw britain a sea level lowstand surface of erosion is inferred to occur within the cotham member of the lilstock formation a unit deposited in an environment that was often subaerially exposed in contrast to previous interpretations the top surface of the overlying langport member here inferred to be deposited on a carbonate ramp of depositional or tectonic origin represents a drowning event of at least regional extent all horizons regarded as plausible levels at which to place the tr j boundary based on fossil distributions lie within strata deposited during relative sea level rise however it is doubtful whether the higher horizons proposed to mark the boundary faithfully record times of true biotic change on a global scale and additionally there is no positive evidence that sea level fall had any relation to the genesis of proposed tr j marker horizons it is unlikely that sea level fall played a significant role in the tr j boundary extinctions in either a local or a global context

nitrogen isotope record of a perturbed paleoecosystem in the aftermath of the end triassic crisis doniford section sw england 1 the triassic jurassic transition tj is characterized by successive perturbations of the carbon cycle during a time of biotic disruption as recorded by the carbon isotopic composition of organic matter 13corg the nitrogen isotopic composition of sedimentary organic matter 15norg constitutes a key parameter to explore the functioning of the ecosystem during carbon cycle perturbations and biological crises because it provide information on seawater redox conditions and or nutrient cycling here we report the first continuous 15norg record across the tj transition at the doniford bay section bristol channel basin uk combined with 13corg kerogen typology and carbon 13cmin and oxygen 18omin isotopic composition of bulk carbonates the end triassic is characterized by a major negative excursion both in 13corg and 13cmin very low toc total organic carbon wt and high 15norg values associated with a sea level lowstand a second 13corg negative excursion occurs during the lower hettangian this interval is characterized by phases of carbonate production increase alternated with phases of exceptional accumulations of type i organic matter up to 12 associated with lower 15norg and 13corg this alternation likely reflects a succession of nutrient input increase to the basin leading to enhanced productivity and eutrophication which promoted a primary production driven by organic walled prokaryotic organisms the following om export increase generates anaerobic conditions within the basin these events occur between periods of relatively good seawater column ventilation and nutrient recycling boosting the carbonate producer recovery ecosystems remain perturbed in the bristol channel basin during the aftermath of the end triassic crisis

massive sandstone facies in the hawkesbury sandstone a triassic fluvial deposit near sydney australia abstract massive sandstone is a common facies in the hawkesbury sandstone a triassic formation attributed to deposition by a large braided river the massive sandstone occurs as sheets and in elongate depressions trending perpendicular to the paleoslope as indicated by the unidirectional mean orientation of abundant cross strata some of the depressions filled with massive sandstone also contain angular mudstone intraclasts up to several meters in length massive sandstone is also associated with deformed cross strata formed by progressive loss of lamination during mass movement down foreset slopes foreset failure is attributed to liquefaction due to falling water level or to collapse of adjacent mud banks the latter mechanism resulted in rapid loading of bedform foresets and introduce large mudclasts into the massive sand sand and mudclasts accumulated in transverse scours at the foot of the foreset slopes but also travelled along the scours or spilled out to form massive sheet sands the abundance of massive sandstone in the hawkesbury sandstone is attributed to deposition in a large river in which flood stage bedforms were probably up to 15 m high because of the size of the bedforms large volumes of liquefied sand were generated when foreset slopes failed

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

liquefaction and fluidization of lacustrine deposits from lahaul spiti and ladakh himalaya geological evidences of paleoseismicity along active fault zone abstract soft sediment deformational structures are important diagnostic features of paleoseismic studies these seismites result from local vigorous fluidization and hydroplastic deformation formed during liquefaction as a direct consequence of permanent deformation of ground surface due to earthquakes of large magnitude the region of eastern ladakh spiti basin and the upper sutlej river basin record a concentration of earthquakes in a well defined ne trending seismic zone sub parallel to the strike of leo pargil horst and the kaurik chango fault zone soft sediment deformational studies of the fluvial and fluvio lacustrine sequences near sumdo village along sangla valley of himachal pradesh and near saspol village of ladakh indicate that the kaurik chango fault zone is a seismogenic fault causing liquefaction and fluidization as the consequence of earthquake activity in the past thermal luminescence tl date of the deformational structures near the sumdo village constrains the paleoseismic activities of large magnitude between 90 000 years third activity to 26 000 years last geological evidence with a recurrence interval of approximately 10 000 years during the quaternary period

milankovitch scale palynological turnover across the triassic jurassic transition at st audrie s bay sw uk abstract a high resolution palynological study of the triassic jurassic boundary in the st audrie9s bay section revealed a palynofloral transition interval with four pronounced spore peaks in the lilstock formation regular cyclic increases in palynomorph concentrations can be linked with periods of increased runoff and correspond to the orbital eccentricity cycle spore peaks can be related to precession induced variations in monsoon strength an implication is that the initial carbon isotope excursion lasted for at least 20 ka emergence during deposition of the cotham member had an influence on one of the peaks which is dominated by spore producing pioneer plants e g horsetails and liverworts there is no compelling evidence of a global end triassic spore spike that by analogy with the k t boundary fern spike could be related to a catastrophic mass extinction event climate change is a more plausible mechanism to explain the increased amount of spores supplementary material an alphabetical list of palynomorphs identified in the st audrie9s bay section is available at http www geolsoc org uk sup18406

earthquakes and deformational structures seismites in holocene sediments from the himalayan andaman arc india soft sediment deformational structures from the himalayan arc caused by episodic earthquake recurrences during the holocene recent are produced by the ongoing tectonic processes these small scale subtle structures occur along some major thrusts in the garhwal himalaya and vary from normal faults in the lower zone to compressional structures in the middle zone along the main central thrust in the tons valley the latter includes isoclinal angular kink and disharmonic folds with discrete displacements along their axial surfaces thrusts with decollement listric and overstep thrusts in duplex style and load slumping the upper zone is marked by very low angle thrusts with left lateral displacement and associated displacement shears d riedel shears r and thrust shears p as well as small scale sporadic kinks three zones reveal changing tectonic character from gravity tectonics in the lower zone to compression in both the middle and upper zones with the maximum principal stress 1 oriented n60 to n110 respectively along the tons thrust post depositional structures include drag along normal fault zone characterized by d r and p shears and conjugate fractures that provide localized orientation of stresses 1 at 75 n60 2 at 12 n280 and 3 at 12 n290 earthquake induced liquefaction and subtle variations in physiomechanical properties of soft sediments have primarily controlled the deformational pattern due to localized tectonic stresses ground and sediment deformations during two earthquakes in the himalayan andaman arc provide additional support for the seismogenic origin of such structures preserved in loose or consolidated sediments

lacustrine ostracodes in the late pleistocene sunnybrook diamicton of southern ontario canada two opposing interpretations have been made on the depositional origin of the late pleistocene sunnybrook diamicton in the metro toronto region of southern ontario the traditional view holds this deposit to be a till but more recent arguments advocate a glaciolacustrine origin by suspension deposition and ice rafting the discovery of a low diversity ostracode fauna consisting of candona subtriangulata candona caudata and darwinula stevensoni in the sunnybrook diamicton suggests a lacustrine origin excellent preservation of thin fragile shells and a mixture of juveniles and adults in sediments of varying texture clayey silt to silty clay indicate that there has been little winnowing or selective sorting and that the ostracodes are in situ indications are that the sunnybrook diamicton was deposited in a large deep and cold lake these new palaeontological observations call for a reassessment of the quaternary stratigraphy of the ontario basin and suggest that the laurentide ice sheet in the eastern

uniquely extensive seismite from the latest triassic of the united kingdom evidence for bolide impact a 2 4 m thick seismite in places overlain by a previously unreported tsunamite can be traced across 250 000 km 2 of the outcrop and subcrop of the latest triassic rhaetian cotham member of the penarth group united kingdom an extent unique for the british phanerozoic its consistent thickness intensity of deformation and preferred orientations of slump fold axes indicate a seismic event of m 10 with an epicenter 600 km w or nw of central britain the magnitude of the event is incompatible with known terrestrial mechanisms fault volcano but is consistent with a major bolide impact a short but unknown interval separates the top of the cotham member seismite from major geochemical and biotic perturbations associated with the end triassic extinction although a direct link between the seismite and these other events remains equivocal the exceptional extent of mega seismites such as this may prove a useful indicator of previously undocumented bolide impacts

facies architecture variations and seismogenic structures in the carboniferous permian saar nahe basin sw germany evidence for extension related transfer fault activity abstract sedimentary and volcanic strata in the late orogenic intermontane saar nahe basin preserve a record of sedimentary responses to motion on contemporaneously active faults studies of such deposits can constrain both concepts for lateral and vertical sedimentary facies development and geometric and kinematic models of intrabasinal extension during carboniferous permian syn rift development the saar nahe basin underwent a marked intrabasinal segmentation generated by the offsets of longitudinal normal faults and dextral oblique strike slip faults which were developed perpendicular to the sw ne basin elongation the latter are interpreted as transtensional transfer faults which compartmentalized the basin into segments of differing structural style bed thickness and facies development the fault zones themselves bound an assemblage of variously sized uplifted and downthrown blocks showing variable magnitudes and senses of displacement their activities are recorded by the contrasting depositional histories of tephrostratigraphically constrained sedimentary units occupying footwall and hanging wall block positions during basin filling the influence of faults on the depositional system is seen in their effect as topographic barriers thus restricting free dispersal and mixing of sediment particular sedimentary facies such as coals and biogenic limestones are confined to the immediate vicinity of the fault zones indicating the isolation of sediment starved areas near downthrown hanging walls other fault generated depressions acted as depositional traps favouring considerable accumulations of basaltic lavas volcaniclastic mass flows and fluvial channel deposits in contrast uplifted footwalls are characterized by the formation of unconformities and pedogenic features the abundance of syn tectonic deformation structures such as sediment filled fissures intraformational breccias series of micro faults clastic dykes and other water escape features within syn rift strata and their distinct spatial linkage to fault zones implies considerable seismically triggered processes associated with faulting the combined occurrence of abrupt variations in facies architecture across faults and seismotectonic structures suggests that the basin subsidence history involved a series of events rather than gradual change

trace fossils in the aftermath of mass extinction events abstract ichnology has great potential to advance our understanding of mass extinction events and yet is currently an underutilized resource in such studies here we review published ichnological studies for the ordovician silurian permian triassic and cretaceous tertiary extinction recovery intervals in addition new information regarding the triassic jurassic ichnological record from england austria and the western usa is presented trace fossils provide important information on the ecological response of the benthic community at such times in the immediate post extinction aftermath the ichnodiversity burrow size depth of bioturbation and ichnofabric index of the sediments are all much reduced there is an increase in all these parameters through the post extinction recovery period in some cases the stepwise reappearance of certain distinctive ichnotaxa e g diplocraterion rhizocorallium and thalassinoides may be of some stratigraphic use evidence from permian triassic studies indicates that recovery took longer at low tropical palaeolatitudes than mid high palaeolatitudes trace fossils also provide important information on palaeoenvironmental change through the extinction recovery interval the application of ichnology to mass extinction studies is in its infancy but should prove a valuable tool in future research

internal architecture of mixed tide and storm influenced deposits an example from the alcântara formation northern brazil abstract the uppermost portion of the itapecuru group is exposed in the eastern margin of the sao luis basin northern brazil where it consists of two units the alcantara formation cenomanian and the cujupe formation late cretaceous early tertiary rossetti d f truckenbrodt w 1997 revisao estratigrdfica para os depositos do albiano terciario inferior na bacia de sao luis ma norte do brasil bol mus paraense emilio goeldi ser cienc terra in press the alc ntara formation which contains the large scale structures discussed in this paper consists of deposits attributed to mid to upper shoreface foreshore tidal channel and lagoon washover environments attributed to a regressive barred shoreline several types of large scale cross bedding i e simple foreset compound mixed undulatory and intricately bounded were recognized in the shoreface facies association these structures are interpreted to record the interaction of storm and tidal processes the storm influence is suggested by a combination of factors mostly including a the genetic association with other storm generated sedimentary structures i e swaley cross stratification and undulating parallel lamination with internal truncations b the deposition on prominent surfaces formed by storm erosion which are defined by large scale either symmetrical or asymmetrical scours arranged in a regular repeating pattern c the sedimentary features formed under combined unidirectional and oscillatory flow processes e g compound mixed bedding with superimposed either swaley cross sets or complexly truncating cross sets with highly undulating boundaries large scale undulatory and intricately bounded cross beddings and d the lateral change in structural styles within short distances which records frequent modification from asymmetrical to symmetrical nearly symmetrical bedform profiles more likely to occur under storm generated combined flows the tidal signature is locally recognized by regularly spaced thick thin sandstone bundles defined by reactivation surfaces and or mud drapes which are attributed to tidal ebb flood cycles the analysis of paleocurrent distribution suggests that vigorous southwestward oriented storm flows interacted with local tidal currents on the shoreface to promote the landward transport of significant volumes of sand which resulted in the large scale cross stratification described in this paper in addition a secondary southeastward directed oblique to shore parallel combined flow would have periodically interacted with the main flow the origin of this oblique to shore parallel flow is attributed to either the refraction of the main storm waves as they approached the paleocoastline or the interference of a separate storm episode which competed with the major landward moving one this complex storm flow pattern can be related to past penecontemporaneous seismic activity i e tsunamis as suggested by the paleogeography of the study area during the cenomanian combined with the structural history of the sao luis basin and the sedimentary features recognized in the alcantara formation

tracking the redox history and nitrogen cycle in the pelagic panthalassic deep ocean in the middle triassic to early jurassic insights from redox sensitive elements and nitrogen isotopes abstract in order to clarify the redox history of the central panthalassic ocean in the middle triassic anisian to early jurassic toarcian we determined the abundances of major trace and rare earth elements and organic carbon and total nitrogen isotopes from well exposed shales interbedded with deep sea cherts from inuyama southwest japan a distinct positive ce anomaly accompanying high mn and weak mo enrichments was detected in the transitional sequence across the tr j boundary which indicates deposition under a more oxic condition than at any other period in the study section the oxic period lasted for 130 170 kyr and coincided with a faunal turnover at the end of the triassic on the other hand strong enrichments of u v mo toc and tn in anisian and toarcian black shales suggest deep ocean anoxia in both intervals highly concentrated mo contents in some black shales indicate euxinia in the pelagic panthalassic deep ocean 15ntn values are low down to 2 0 in such black shales enriched in redox sensitive elements compared with other shales this low 15ntn values during recurrent oceanic euxinic events in the anisian may indicate strong assimilation of ammonium but the possibility of enhanced nitrogen fixation cannot be ruled out especially for the toarcian anoxic event this work provides the first documentation of the long term redox history in the pelagic panthalassic deep ocean from the middle triassic to the early jurassic based on redox sensitive elements in shales and it reveals probable links between oceanic redox changes and biological activity

response of the marine infauna to triassic jurassic environmental change ichnological data from southern england abstract the trace fossil record through the triassic jurassic boundary interval is examined at three sites in central and southern england st audrie s bay somerset pinhay bay devon and long itchington warwickshire the lower pre planorbis beds of the blue lias formation record low ichnotaxonomic diversity low bioturbation intensity small burrow diameters and an absence of deep tier bioturbation the stepwise reappearance of ichnotaxa following this interval is similar at the three sites suggesting similar rates of recovery in the benthic marine ecosystem and highlighting the potential contribution of these trace fossils to stratigraphic correlation mass extinction in the late triassic is increasingly linked to the emplacement of the central atlantic magmatic province camp timing of the onset of camp volcanism in the uk is currently imperfectly known but potentially occurred within either the westbury formation or the lilstock formation if the onset of camp was within the lilstock formation the modestly diverse trace fossil assemblage of the langport member of the lilstock formation suggests that emplacement of camp had little lasting effect on the marine benthos in central and southern england the decline in benthic activity recorded in the pre planorbis beds appears to be unrelated to the onset of camp and any associated environmental change it is more likely related to an episode of marine anoxia an episode of marine anoxia also provides a suitable causal mechanism for the reduction of infaunal tiering

sediment deformation structures and the palaeotectonic analysis of sedimentary basins with a case study from the carboniferous of northern england

palaeoecology of the late triassic extinction event in the sw uk a high resolution palaeoecological study of the shelly invertebrate macrofauna across two marine triassic jurassic boundary sections in the uk st audrie9s bay and lavernock point is presented loss of taxonomic richness occurs in the upper westbury formation to lower lilstock formation late rhaetian but if sample size is taken into account there is little convincing evidence of a catastrophic marine extinction there is however good evidence for significant palaeoecological change in the benthic marine ecosystem at this time the immediate post event recovery interval in the upper lilstock formation is characterized by assemblages of low abundance low diversity high dominance and low evenness body sizes of taxa that survived the event and originated afterwards were low until the later hettangian recovery to higher abundance higher diversity and higher evenness is recorded in the psiloceras planorbis zone recovery of the benthic ecosystem in the aftermath of the late triassic event was disrupted by marine anoxia and shows additional similarities to the much slower recovery that followed the late permian event the pattern of body size changes recorded in the shelly fossil record closely matches that of the trace fossil record shell thickness trends do not support a biocalcification crisis during the late triassic biotic event

geochronological constraints on post extinction recovery of the ammonoids and carbon cycle perturbations during the early jurassic abstract this paper presents the first quantitative study of the early jurassic recovery of ammonoids after the end triassic mass extinction based on detailed u pb id tims isotope dilution thermal ionization mass spectrometry geochronology from ash bed zircons placed within a clear phylogenetical and biochronological framework at the subzonal and species level this study was triggered by the discovery of a rich peruvian succession of ammonites deposited concomitantly with an unusually large number of ash beds two major phases of rediversification are observed during the psiloceras spelae and angulaticeras zones that correspond to positive peaks in the 13corg curve providing a possible link between biodiversity and the global carbon cycle in the case of the post extinction recovery the development of the earliest hettangian ammonites occurs within the genus psiloceras which begins with the occurrence of p spelae and then explodes into worldwide development of smooth psiloceratids of the psiloceras planorbis group s l this rapid biodiversification likely occurred less than 100 ka after the end triassic crisis the genus psiloceras occupied all the possible ecological niches worldwide from the pacific deep waters to the nw european shallow deposits and also in some rare tethyan occurrences like at germig in tibet this global dispersion allowed the differentiation of the group in several major phyla the schlotheimiidae discamphiceratinae arietitidae and lytocerataceae which were the roots of all other jurassic and cretaceous ammonites

structural controls on upper palaeozoic sedimentation in south east wales marked thickness changes in early dinantian courceyan chadian sediments occur across the cardiff cowbridge anticline a major east west variscan fold in the vale of glamorgan they resulted from differential subsidence over an active concealed basement fault zone the vale of glamorgan axis which is coincident with the hinge of the anticline the latter formed in response to inversion on this basement fault a caledonian ancestry for the vale of glamorgan axis is indicated by pronounced upper old red sandstone overstep across it across the severn estuary between south wales and the bristol mendips region there is a significant offset of isopachs and facies belts in both the courceyan chadian and arundian sequences it is suggested that these result from intra carboniferous dextral strike slip along a major fault zone underlying the severn estuary the severn estuary fault zone and that the vale of glamorgan axis was a synthetic structure developed in conjunction with this strike slip movement by inference the severn estuary fault zone shares a common history with the vale of glamorgan axis extending back to the devonian and possibly earlier late dinantian holkerian facies distribution reflects uplift and emergence on the north south usk anticline and malvern lineament continued uplift is recorded in silesian sequences in south east wales and the bristol mendips area the orientation of structures bordering the severn estuary is consistent with their development in a right lateral strike slip zone

a two phase end triassic mass extinction abstract the end triassic mass extinction was one of the big five crises of the fossil record it affected diverse marine groups including bivalves brachiopods ostracods calcareous algae radiolarians and tetrapods and coincided with major changes in palynological assemblages the rapidity of the event has long been debated and here we show that it can be resolved into two distinct short lived extinction pulses separated by a several hundred thousand year interlude phase detailed collecting in the british isles shows the first extinction in the lower cotham member eliminated many bivalves and ostracods this event has been previously considered as the end triassic mass extinction horizon however a second extinction is also seen at the top of the langport member after a phase of recovery marked by diversity increase this younger crisis caused the loss of further bivalves ostracods and the last of the conodonts the two phases of marine extinctions coincide with marked turnovers amongst palynofloral assemblages with the interlude phase manifest as a fern spore dominated interval the polypodiisporites polymicroforatus abundance interval correlation of triassic jurassic boundary sections and their associated extinction record in europe has been controversial the most parsimonious correlation scheme assumes that a brief high amplitude negative excursion of 13corg values seen at the level of the first extinction phase in the uk is manifest in other european sections in the same way alternative correlation schemes require complex scenarios with extinction occurring in some areas as recovery is happening elsewhere in europe the first extinction coincides with regression whilst the second occurs at a flooding surface marked by the spread of anoxia the extinction history of the terminal triassic crisis is remarkably similar to that seen during other extinction events end ordovician permo triassic toarcian and suggests that if the ultimate driver of these crises large igneous province eruptions is the same then they show the same double punch eruption history separated by a more benign interval of a few hundred thousand year duration

geological evidence for intra jurassic faulting in the wessex basin and its margins geological observations on jurassic outcrops close to major faults in the wessex basin mendip area reveal the local presence of ammonite and brachiopod bearing sediments penetrating underlying strata toarcian and bajocian neptunian dykes and particularly sills are associated with the eypemouth fault and bajocian sills with the bride fault and mere fault in the mendip area numerous neptunian dykes of hettangian sinemurian pliensbachian and bajocian ages cross cutting carboniferous limestone are recorded typically also associated with major basement faults e g cranmore and leighton faults these periods of assumed sediment injection are taken as indicating times of displacement along the faults in question variations in facies hettangian sinemurian toarcian bajocian uppermost oxfordian kimmeridgian spatially linked to faults are documented from some areas and boreholes reveal considerable fault controlled thickness changes in hettangian sinemurian bajocian and kimmeridgian sediments the timing of jurassic faulting in the wessex basin mendip area thus polarizes into two intervals hettangian bajocian and latest oxfordian onwards correlating with the early rifting phases of the central and north atlantic respectively

impacts of small scale faults on continental to coastal plain deposition evidence from the realgrunnen subgroup in the goliat field southwest barents sea norway abstract in this study we synthesise sedimentological fault and amplitude versus angle ava analysis and propose that the fruholmen and tubaen formations realgrunnen subgroup are syn kinematic deposits that record a previously undocumented early phase of mesozoic rifting on the troms finnmark fault complex and within the hammerfest basin the realgrunnen subgroup hosts one of two triassic reservoirs currently being produced in the goliat field here the subgroup sits unconformably on top of the storfjorden subgroup carnian snadd formation away from the goliat field which is characterised by a periclinal anticline the realgrunnen subgroup also comprises the lower middle jurassic nordmela and sto formations sedimentological analysis of six exploration wells reveals that the fruholmen formation was deposited in a prodelta to delta plain environment where tide influenced and fluvial dominated distributary channels are represented by clay siltstones and very fine grained sandstones the overlying tubaen formation is characterised by medium to very coarse grained deposits locally conglomeratic and represents a widespread braid plain with localised alluvial fans displacement profiles of faults and along fault thickness variations demonstrate that an immature fault system was active during deposition of the realgrunnen subgroup a series of unconnected fault segments hosted isolated sub basins and erosional catchment areas in their hanging and footwalls respectively an ava attribute map generated from a 10 ms interval of the uppermost part of the subgroup reveals gross sand prone depositional bodies i e individual and amalgamated channels some of which show meandering geometries ox bow lakes and alluvial fans sand bodies frequently show elongate geometries parallel to faults indicative of syn depositional fault related subsidence driving mechanisms responsible for the norian to rhaetian event may relate to contemporaneous rejuvenation of the fennoscandian hinterland development of the novaya zemlya fold and thrust belt and or the early cimmerian tectonic phase in northern europe

models of glaciomarine sedimentation and their application to the interpretation of ancient glacial sequences abstract this paper argues that glaciomarine environments can be regarded as special glacially influenced types of continental margin environments e g continental shelf slope rise and basin plain knowledge of the stratigraphic architecture and typical sedimentary sequences of non glacial margins is becoming well known but remains limited for those that have been glacially modified the principal influences on sedimentation in these environments relate to the glacial sediment input controlled by relief of basin margin glacier thermal regime and ice flow dynamics and depositional environments influence of traction currents substrate relief and proximity to nearby ice margins typical ranges of sedimentation rates can be established for glacially influenced continental margin environments and these may provide a framework for ancient sequences starvation of sediment supply to glacially influenced continental margins is common the nature of sub ice shelf sedimentation a model that has been applied to many ancient glacial sequences is critically reviewed the significance of such sedimentation in the rock record has probably been exaggerated because of oversimplistic interpretations of diamictite sequences existing process models of glaciomarine sedimentation derived from study of modern environments are sometimes difficult to employ in investigation of ancient sedimentary sequences because simple lithofacies criteria and typical vertical profiles are not available to aid in interpretation in addition compositional data emphasized by many workers for distinguishing glaciomarine from continental glacial diamict ite s frequently fingerprint sediment source and not mode of deposition the importance of facies analysis methods for isolating depositional environments is illustrated by three examples of ancient glaciomarine sequences these are the early proterozoic gowganda formation 2 3 ga of northern ontario canada the late proterozoic port askaig formation 670 ma of scotland and ireland and the late cenozoic yakataga formation 20 ma to recent of the gulf of alaska these examples illustrate the significance of detailed genetic studies of ancient glacial rocks in the interpretation of palaeogeographic and tectonic settings diamictite units in ancient glaciomarine sequences cannot be easily interpreted in terms of climatic or ice advance retreat cycles because of the varied controls on diamict accumulation and diamictite preservation in marine basins

combined sea level and climate controls on limestone formation hiatuses and ammonite preservation in the blue lias formation south britain uppermost triassic lower jurassic

cyclostratigraphy stratigraphic gaps and the duration of the hettangian stage jurassic insights from the blue lias formation of southern britain the lithostratigraphic characteristics of the iconic blue lias formation of southern britain are influenced by sedimentation rates and stratigraphic gaps evidence for regular sedimentary cycles is reassessed using logs of magnetic susceptibility from four sites as an inverse proxy for carbonate content standard spectral analysis including allowing for false discovery rates demonstrates several scales of regular cyclicity in depth bayesian probability spectra provide independent confirmation of at least one scale of regular cyclicity at all sites the frequency ratios between the different scales of cyclicity are consistent with astronomical forcing of climate at the periods of the short eccentricity obliquity and precession cycles using local tuned time scales 62 ammonite biohorizons have minimum durations of 0 7 to 276 ka with 94 of them

aseismic controls on in situ soft sediment deformation processes and products in submarine slope deposits of the karoo basin south africa intervals of soft sediment deformation features including vertical fluid escape and load structures are common and well exposed in permian lower slope deposits of the tanqua depocentre karoo basin the structures mainly comprise elongated flames and load structures associated with ruptured sandstones and structureless siltstones observed over a range of scales the presence of an upper structureless siltstone layer linked to the flames interpreted as a product of the debouching of fine grained material transported through the flame onto the palaeo seabed together with the drag and upward folding of lower sandstone layers is evidence that the flames were formed in situ by upward movement of sediment rich fluids flames are oriented parallel to the deep water palaeoslope in lateral splay deposits between two major slope channel complexes statistical correlation and regression analyses of 180 flame structures from seven stratigraphic intervals suggest a common mechanism for the deformation and indicate the importance of fluidization as a deformation mechanism importantly deformation occurred in an instantaneous and synchronous manner liquefaction and fluidization were triggered by incremental movement of sediment over steeper local gradients that were generated by deposition of a lateral splay on an inherited local north west facing slope seismic activity is not invoked as a trigger mechanism because of the restricted spatial occurrence of these features and the lack of indications of earthquakes during the time of deposition of the deep water succession the driving mechanisms that resulted in the final configuration of the soft sediment deformation structures involved a combination of vertical shear stress caused by fluidization development of an inverse density gradient and a downslope component of force associated with the local slope ground penetrating radar profiles confirm the overall north east orientation of the flame structures and provide a basis for recognition of potential larger scale examples of flames in seismic reflection data sets

towards accurate numerical calibration of the late triassic high precision u pb geochronology constraints on the duration of the rhaetian numerical calibration of the late triassic stages is arguably the most controversial issue in mesozoic stratigraphy despite its importance for assessing mechanisms of environmental perturbations and associated biologic consequences preceding the end triassic mass extinction here we report new chemical abrasion isotope dilution thermal ionization mass spectrometry zircon u pb dates for volcanic ash beds within the aramachay formation of the pucara group in northern peru that place precise constraints on the age of the norian rhaetian boundary nrb and the duration of the rhaetian the sampled ash bed bearing interval is located just above the last occurrence of the bivalve monotis subcircularis placing this stratigraphic sequence in the uppermost norian perhaps ranging into the earliest rhaetian zircon u pb dates of ash beds constrain the deposition age of this interval to be between 205 70 0 15 ma and 205 30 0 14 ma providing precise constraints on the age of the nrb combined with previously published zircon u pb dates for ash beds bracketing the triassic jurassic boundary we estimate a duration of 4 14 0 39 m y for the rhaetian this ends a prolonged controversy about the duration of this stage and has fundamental implications for the rates of paleoenvironmental deterioration that culminated in the end triassic mass extinction

seismically induced soft sediment deformation structures seismites around sumdo in the lower spiti valley tethys himalaya abstract soft sediment deformation structures were observed in 120 m thick fluvio lacustrine sediments exposed in the sumdo area in the lower spiti valley tethys himalaya himachal pradesh india these structures vary in their morphology and pattern and occur at eight stratigraphic levels s1 s8 spaced at irregular intervals the deformation structures include over steepened stratification diapiric flame like intrusions of clayey material or complex type recumbent folding of flame like nature pseudonodules large scale soft sediment folding faulting small scale associated with folding and sand dykes these structures are restricted to single stratigraphic layers separated by undeformed beds and can be correlated over a large distance the deformation of the soft sediments is attributed to the combined effects of fluidization liquefaction and shear stress and can be related to past seismic activity in the area recently on 19 january 1975 a major earthquake kinnaur earthquake of magnitude 6 8 caused extensive damage including ground rupturing in the sumdo kaurik area this earthquake has been related to the activation of the kaurik chango fault the deformed layers seismites reported here lie adjacent to this fault zone and can be related with episodic seismicity along this fault it is evident from this study that the kaurik chango fault has been re activated a number of times and that the area suffered at least nine major earthquakes with a magnitude of more than 6 during late pleistocene holocene time including the 1975 kinnaur earthquake

elevated pco2 leading to late triassic extinction persistent photic zone euxinia and rising sea levels the late triassic mass extinction event is the most severe global warming related crisis to have affected important extant marine groups such as scleractinian corals and offers potential insights into climate change scenarios here we present evidence from chlorobi derived biomarkers of episodic and persistent photic zone euxinia from biomarkers and stable carbon isotopes we present evidence of rapid mixing of atmospheric and oceanic carbon reservoirs global versus regional trends are resolved in kerogen organic matter type carbonate 13c and bulk and pyrite 34s this suite of data demonstrates for the first time a comprehensive organic and stable isotope geochemical reconstruction of events leading up to the late triassic extinction event and its aftermath the cascade of events prior to during and after the extinction is remarkably similar to those reported for the late permian extinction the largest extinction event of the phanerozoic we predict that similar conditions will have occurred during all past episodes of rapid global warming and biotic crisis that are associated with similar rises in p co2

pyrite framboid study of marine permian triassic boundary sections a complex anoxic event and its relationship to contemporaneous mass extinction size analysis of pyrite framboids has been undertaken on epicontinental permian triassic boundary sections throughout the world in order to evaluate the intensity and duration of anoxia mid paleolatitude sections from the margins of the boreal spitsbergen greenland and neotethyan oceans western australia reveal intense anoxia throughout the permian triassic boundary interval with euxinic conditions frequently developing and dysoxia encountered even in relatively shallow water settings above storm wave base at equatorial paleolatitudes weakly oxygenated dysoxic conditions are widely developed in a broad range of water depths including those shallow enough to produce oolite deposition although euxinia was rare western and eastern tethyan locations re veal a complex and unstable redox history anoxia in the hindeodus praeparvus zone was replaced by oxygenated facies in the permian triassic boundary interval h changxingensis to h parvus zones oxygen poor deposition returned during the succeeding isarcicella isarcica zone the more persistent and intense development of oxygen restriction in cooler water mid paleolatitude sections argues against warming and dissolved oxygen decline as the key cause of permian triassic bound ary anoxia in higher paleolatitudes the ben thic invertebrate extinctions occurred during a prolonged phase of oxygen poor deposition while in equatorial tethyan locations benthic losses occurred at the end of the fi rst anoxic phase in the late h praeparvus zone

missing molluscs field testing taphonomic loss in the mesozoic through early large scale aragonite dissolution as a field test for cherns and wright9s previously published hypothesis based on paleozoic faunas that the paucity of certain molluscan groups in the fossil record may be largely a taphonomic not ecologic effect a typical mesozoic molluscan dominated fauna is compared with its early silicified lagerstatten equivalent from a similar depositional setting a lower jurassic offshore carbonate ramp facies in south wales comprises alternating thin limestones and organic rich shales the former are largely diagenetic and carry marine isotopic signatures their typical fauna is mainly well preserved calcitic and bimineralic calcite aragonite epifaunal and semiinfaunal bivalves evidence of aragonitic shells is normally restricted to molds common in inner ramp settings in high energy skeletal grainstones but mostly poorly preserved and restricted to deep burrowing bivalves in offshore deposits however locally early silicification in offshore facies has preserved much more diverse faunas with abundant formerly aragonitic molluscs including many more infaunal burrowers it appears that major aragonite dissolution normally distorted both apparent diversity 65 decrease in bivalve diversity and the trophic structure of the offshore facies providing aragonite that probably sourced the diagenetic carbonates we suggest that aragonitic shells were selectively dissolved in the upper sediment column in lower energy settings where high organic contents favored microbial decay and acidity such early dissolution was absent from the higher energy facies that originally had low organic contents taphonomic loss through early skeletal aragonite dissolution was an equally important process in mesozoic offshore shelf environments and although still leaving depleted molluscan dominated faunas resulted in a massive distortion of diversity

the age fauna and palaeoenvironment of the late triassic fissure deposits of tytherington south gloucestershire uk important vertebrate faunas occur in fissure deposits of late triassic jurassic age in sw britain although the faunas are well described their age and palaeoenvironment remain poorly understood one such fissure system was documented in detail during quarrying operations at tytherington and has yielded in situ palynomorphs that add much information concerning its age and palaeoenvironment significantly the tytherington fauna is of the sauropsid type that has generally been dated as norian or pre penarth group transgression and was also regarded as representing a distinct upland fauna the palynomorphs which include a significant marine component demonstrate that the tytherington triassic fissures are infilled with late triassic rhaetian sediments that match specific levels in the westbury formation in addition many of the tytherington solutional fissures probably formed during the rhaetian and are consistent with a fluctuating saline to freshwater environment there is no prima facie evidence of solutional formation and infilling of the reptile bearing deposits before the rhaetian trangression the fissure reptile fauna which includes the early dinosaur thecodontosaurus inhabited a small fire swept limestone island in the rhaetian sea the features of the herpetofauna are entirely consistent with this island model which has quaternary analogues

primary versus diagenetic origin of blue lias rhythms dorset uk evidence from sulphur geochemistry the question of a primary versus diagenetic origin for the limestone shale rhythms of the blue lias has been addressed through a study of pyrite abundance and isotopic composition pyrite is relatively abundant and isotopically light in the central portions of the bioturbated limestones as compared to adjacent less calcareous sediment the abundance of pyrite shows that the limestones were a focus for prolonged sulphate reduction and pyrite formation the isotopic data indicate that bioturbation oxidized some pyrite to produce isotopically light sulphate part of which was subsequently reduced back to pyrite before preservation by burial acidity generated by pyrite oxidation was buffered in the limestones by carbonate dissolution hence supersaturation of sulphides could be maintained by contrast in adjacent less calcareous sediments carbonate dissolution was unable to buffer acidity and bioturbational oxidation of pyrite formed iron rich pore solutions continued sulphate reduction in the limestones acted as a sink for iron from the adjacent sediments and with burial below the zone of bioturbation the alkalinity so generated caused cementation of the limestones diagenetic cementation would be enhanced during an hiatus in sedimentation an event which might be related to a milankovitch forcing mechanism but which would not be recorded in bioturbated less calcareous sediment thus leaving an imperfect record only cyclicity in pre diagenetic sedimentation patterns may be safely related to a milankovitch forcing mechanism as proposed by weedon

pillow beds a new type of seismites an example from an oligocene turbidite fan complex alicante spain a new type of soft sediment deformed beds termed pillow beds originally a packet of calcarenitic turbidites show sand in sand loading with plane lower and upper surfaces the pillow beds have some resemblance to ball and pillow structures and they are considered to have formed as a result of dewatering the pillow beds are oligocene in age and are part of turbidite lobes in a submarine fan system which was fed through a canyon along the southern border of the ancient iberian continent three hypotheses may explain their occurrence overloading sliding or seismic shock a seismic origin is preferrred on the basis of the flat undeformed lower surface of the pillow beds excluding dewatering of and loading into the previously deposited beds furthermore the pillow bed structures are related to grain size and may show repetitions thus excluding sliding the scale and nature of the pillow beds suggest they could be the result of an earthquake with a magnitude of 6 7 on the richter scale

discussion on sea level change and facies development across potential triassic jurassic boundary horizons sw britain tony hallam paul wignall write we welcome the paper by hesselbo et al 2004 for its thorough description and careful evaluation of the environmental significance of an important group of system boundary strata but beg to take issue on a number of key points 1 most importantly while we agree with the evidence of a regressive phase within the cotham member we maintain in marked contrast to their opinion that there was also a sharp regression followed by a rapid transgression at the junction of the langport member of the lilstock formation and blue lias it is pertinent before discussing the evidence in sw england further to take into account the broader european context hallam wignall 1999 with many relevant references cited therein the evidence is especially clear in germany where there was extensive shallowing in the latest rhaetian marked by progradation of sandstone over shales in northern frankonia bavaria fluvial sandstones infill channels incised into late rhaetian marine strata and are overlain by marine hettangian the sea level rise in the earliest hettangian planorbis zone was evidently rapid with the limit of marginal marine sandstones in the eastern part of southern germany being pushed back at the expense of fully marine shales to the maximum extent achieved during the whole hettangian stage both in the north and south of europe a similar pattern of successive sea level fall and rise can be inferred a clear end triassic regressive pulse can be recognized in the danish basin while both in southern sweden and nw poland the upper rhaetian is missing and there is an unconformity at the base of the jurassic in the northern calcaerous alps of austria widespread emergence at the end of the triassic is recognized with the creation of karst surfaces on emergent reef complexes while

rapid biotic rebound during the late griesbachian indicates heterogeneous recovery patterns after the permian triassic mass extinction new fossil data from two early triassic griesbachian to dienerian sections from south china show unusually high levels of both benthic and nektonic taxonomic richness occurring in the late griesbachian in total 68 species including 26 newly originated species representing mollusks brachiopods foraminifers conodonts ostracods and echinoderms occur in the late griesbachian indicating well established and relatively complex marine communities furthermore the nekton shows higher origination rates than the benthos analyses of the sedimentary facies and size distribution of pyrite framboids show that this high diversity interval is associated with well oxygenated environments in contrast to the previously suggested scenario which inferred that persistently harsh environmental conditions impeded the biotic recovery during the early triassic our new findings combined with recent work indicate a fitful regional recovery pattern after the permian triassic crisis resulting in three main diversity highs late griesbachian early dienerian early middle smithian and spathian the transient rebound episodes were therefore influenced by both extrinsic local e g redox condition temperature and intrinsic e g biological tolerances origination rates parameters

soft sediment deformations in the kleszczow graben central poland abstract a wide variety of soft sediment deformation structures is found in the quaternary overburden of a giant open cast browncoal mine near belchatow sw of od central poland situated within a graben the deformation structures range in size from tens of meters to only millimeters it is remarkable that their size is often related to a specific type of genesis most large scale deformations appear to result from tectonic activity other structures commonly of intermediate size result from exogenic processes such as glaciotectonism and overburden induced diapirism penecontemporaneous and other early diagenetic deformational processes among which bioturbation and cryoturbation are commonly of small size only they are nevertheless important because their analysis allows a reconstruction of not only the depositional environment but also the deformational history of the sedimentary succession in spite of the different types of genesis many of the soft sediment deformation structures are in one way or another related to the graben activity since the tectonic conditions in the quaternary deposits within the graben are in many respects comparable to those in rift valleys the deformation structures in the graben might be considered as representative for rift valleys as well the analysis of the structures resulting in an improved insight into their genesis may thus contribute to a better understanding of the conditions to which sediments in rift valleys can be exposed

episodic photic zone euxinia in the northeastern panthalassic ocean during the end triassic extinction severe changes in ocean redox nutrient cycling and marine productivity accompanied most phanerozoic mass extinctions however evidence for marine photic zone euxinia pze as a globally important extinction mechanism for the end triassic extinction ete is currently lacking fossil molecular biomarker and nitrogen isotopic records from a sedimentary sequence in western canada provide the first conclusive evidence of pze and disrupted biogeochemistry in neritic waters of the panthalassic ocean during the end triassic increasing water column stratification and deoxygenation across the ete led to pze in the early jurassic paralleled by a perturbed nitrogen cycle and ecological turnovers among noncalcifying groups including eukaryotic algae and prokaryotic plankton if such conditions developed widely in the panthalassic ocean pze might have been a potent mechanism for the ete

siderite concretions indicators of early diagenesis in the gammon shale cretaceous the upper cretaceous gammon shale in the northern great plains contains abundant siderite concretions that formed during the early compactional history of the fine grained marine sediments the relative depth of siderite precipitation can be estimated on the basis of textural mineralogical and isotopic evidence siderite concretions that formed at shallow depths in the sediment column contain a large volume percent carbonate 75 85 preserve uncompacted sediment textures and have oxygen isotope ratios similar to those of carbonates precipitated in equilibrium with seawater delta 18 o nearly equal e 0 to 2 per thousand pdb in contrast lower volume percent carbonate values 55 75 and relative decreases in 18 o 16 o values characterize concretions that formed longer after deposition and or at greater depths in the accumulating sediments both volume percent carbonate and delta 18 o values are strongly correlated with independently determined sediment accumulation rates concretions in rapidly deposited sediments formed at shallow depths 200 m later in the burial history the gammon served as both source rock and reservoir for accumulations of biogenic methane and like the siderite concretions the methane was generated during early diagenesis both siderite precipitation and methane accumulation require interstitial pore waters that have sufficiently high bicarbonate activities and that are depleted in dissolved sulfur and free oxygen because of chemical requirements siderite did not precipitate until after the onset of methane generation therefore if the depth of siderite formation is known the minimum depth for the onset of methane accumulation can be inferred the presence of abundant pyrite in slowly accumulated gammon sediments suggests by analogy to data from recent sediments that the depth of siderite precipitation and methane accumulation was controlled by the sedimentation rate in slowly accumulating sediments most organic matter was probably consumed by aerobic organisms and sulfate reducing anaerobic bacteria whereas in areas of more rapid deposition large quantities of organic matter were available for methane generation