μxanes study of iron redox state in serpentine during oceanic serpentinization serpentinization of ultramafic rocks at mid ocean ridges generates significant amounts of h2 ch4 and supports specific biological communities the abiotic h2 production is attributed to the reduction of h2o during serpentinization which balances oxidation of ferrous iron contained in primary minerals mainly olivines and pyroxenes to ferric iron contained in secondary minerals mainly serpentines and magnetite magnetite has thus far been considered as the sole fe3 carrier for estimating bulk h2 production notably because the valence of iron in serpentine minerals and its relationship with both magnetite abundance and serpentinization degree are usually not determined we show that the serpentine contribution to the fe and fe3 budget has a significant effect on h2 production we performed xanes analysis at the fe k edge on thin sections of peridotites with various degrees of serpentinization from odp leg 153 mark region 23 n fe3 fetot in oceanic serpentines is highly variable from 0 2 to 1 at the thin section scale and it is related non linearly to the local degree of serpentinization a typical value of 0 7 is observed above 60 serpentinization the highest values of fe3 fetot observed within or close to late veins suggest that the fe3 fetot in serpentine record the local water rock w r ratio as previously proposed from thermodynamic modeling we estimate that the w r ratio increased from 0 6 to 25 during serpentinization at mark and locally reached 100 in veins mass balance calculations combining all mineral and bulk rock analyses provide the distribution of fe and fe3 as serpentinization progresses serpentine dominates the fe3 budget of the rock over magnetite during the first 75 of serpentinization contributing up to 80 of the total fe3 at later stages serpentine contribution to the fe3 budget decreases down to 20 while magnetite formation exponentially increases iron transfer from serpentine to magnetite balances the bulk fe3 content of the rock that increases almost linearly with the advance of the reaction formation of serpentine accounts for the majority of fe3 and h2 production at early stages of serpentinization at a depth 2 km at mark where the concentration of h2 can reach more than 100 mm according to the low w r h2 production values and depths can vary from one site to another depending on the evolution of the temperature w r ratio inlet fluid composition and favored formation of serpentine vs magnetite at mark fe3 in serpentine represents 15 27 of the total fe contained in a rock serpentinized to more than 80 and accounts for 25 of the total h2 production that is estimated at 325 335 mmol kg of rock the absence of magnetite does not necessarily mean a negligible h2 production even at low t conditions 150 200 c under which the fe and fe3 richest serpentines have been observed serpentine minerals are important fe3 carrier in the altered ocean lithosphere and may affect mantle redox state while dehydrating at depth in subduction zones

exceptionally preserved microbially induced sedimentary structures from the ediacaran post glacial successions in the kimberley region northwestern australia abstract a set of exceptionally preserved microbially induced sedimentary structures miss is described from the post glacial yurabi formation of late ediacaran age in the kimberley region northwestern australia these miss comprise oriented grains bamboo leaf like cracks polygonal cracks worm like cracks sinuous cracks gas domes pyrite concentrated laminae and micrite laminae most of the yurabi miss are morphologically comparable with their ancient and modern counterparts micrometre or nannometre scale bacteria or biofilm like inorganic objects are detected from most miss and are interpreted as bacterial bodies or remains that have been replaced with inorganic minerals upon fossilization oriented grains indirectly support microbial control on their accumulation both pyrite conformable laminae and micrite laminae suggest degradation and decay of original microbial mats upon diagenesis sinuous cracks were likely generated in a soft sediment deformation process whereas other sand cracks bamboo leaf like cracks worm like cracks polygonal cracks could have formed from sediment surface layer microbial organics shrinkage upon desiccation gas domes may have formed as the result of anoxic redox reactions and being filled with gases and sediments transported from below two proxies bedding plane coverage percentage and accumulative thickness of single miss bearing beds are developed to semi quantitatively evaluate miss abundance on bedding plane and vertical outcrop respectively the yurabi miss have substratum preference to fine sandstone facies and intertidal settings the yurabi formation yields the most abundant and diverse post glacial miss assemblages during the late ediacaran worldwide both kimberley and global fossil records indicate that miss may have proliferated in shallow marine ecosystems soon after the retreat of ediacaran gaskiers glaciation during late neoproterozoic

subaqueous shrinkage cracks in the devonian of scotland reinterpreted abundant lenticular straight to curving cracks on bedding planes are commonly interpreted as subaqueously formed shrinkage cracks syneresis cracks the two most influential examples being those in the green river formation picard 1966 and the devonian orcadian basin of scotland donovan and foster 1972 cracked surfaces in the orcadian basin deposits show all gradations between lenticular cracks and well developed polygons of subaerial origin some lenticular cracks are inferred to have formed during subaerial desiccation many of the simplest lenticular cracks also have the shapes of gypsum crystals and originated as pseudomorphs the gypsum crystals were often important in providing nuclei for later desiccation cracks more complex crack patterns of incipient to fully formed desiccation polygons have arisen from desiccation of surfaces containing gypsum crystals or pseudomorphs resulting from gypsum crystal dissolution cracks of all types are preserved by sand infills introduced by wind transport across dried up lake floors reassessment of the literature reporting subaqueous cracks in rocks shows that many of these cracks are likely to have originated in a similar way partly as gypsum crystals and partly by limited subaerial desiccation the remaining examples have either already been reinterpreted in other ways or are insufficiently documented to rule out a subaerial origin there is no clear evidence for subaqueously formed cracks being preserved in rocks

multiple negative carbon isotope excursions during the carnian pluvial episode late triassic abstract the carnian pluvial episode was a phase of global climatic change and biotic turnover that occurred during the early late triassic in marine sedimentary basins the arrival of huge amounts of siliciclastic sediments the establishment of anoxic conditions and a sudden change of the carbonate factory on platforms marked the carnian pluvial episode the sedimentary changes are closely associated with abrupt biological turnover among marine and terrestrial groups as for example an extinction among ammonoids and conodonts in the ocean and a turnover of the vertebrate fauna and the flora on land multiple negative carbon isotope excursions were recorded during the carnian pluvial episode in both organic matter and marine carbonates suggesting repeated injection of 13c depleted co2 into the ocean atmosphere system but their temporal and causal links with the sedimentological and palaeontological changes are poorly understood we here review the existing carbon isotope records and present new data on the carbon isotope composition of organic carbon in selected sections of the western tethys realm that record the entire carnian pluvial episode new ammonoid conodont and sporomorph biostratigraphic data were collected and coupled to an extensive review of the existing biostratigraphy to constrain the age of the sampled sections the results provide biostratigraphically constrained composite organic carbon isotope curves for the carnian which sheds light on the temporal and causal links between the main carbon isotope perturbations and the distinct environmental and biotic changes that mark the carnian pluvial episode the carbon isotope records suggest that a series of carbon cycle perturbations possibly recording multiple phases of volcanic activity during the emplacement of the wrangellia large igneous province disrupted carnian environments and ecosystems repeatedly over a remarkably long time interval of about 1 million years

reaction induced fracturing during replacement processes replacement processes are common transformation mechanisms in minerals and rocks at a variety of conditions and scales the underlying mechanisms are in general poorly understood but both mechanical and chemical processes are thought to be important replacement of leucite kalsi2o6 by analcime naalsi2o6 h2o is common in silica poor igneous rocks a 10 increase in volume is associated with the replacement process and this generates stresses that eventually cause fracturing of the reacting leucite experimentally reacted leucite samples display characteristic fracturing patterns that include both spalling of concentric onion skin like layers near the reacting interface and the formation of cross cutting often hierarchically arranged sets of fractures that divide the remaining leucite into progressively smaller domains these structures may explain the patchy alteration patterns observed in natural leucite samples and similar so called mesh textures associated with the serpentinization of olivine grains during hydration of mafic or ultramafic rocks they are also strikingly similar to larger scale patterns formed during spheroidal weathering processes a simple discrete element model illustrates the mechanics that control the formation of such systems and shows how these replacement processes may be accelerated due to the generation of new reactive surface area by hierarchical fracturing processes

crustal structure of the southwest indian ridge at 66 e seismic constraints the southwest indian ridge represents a slow spreading end member of the global mid ocean ridge system and as such its structure places important constraints on models of melt supply and delivery from the mantle at ridges we present results from a wide angle seismic experiment conducted at the ridge axis at 66 e in a region that has comprehensive swath bathymetric gravity and magnetic data coverage and where the full spreading rate is 12 mm yr 1 based on these data the experiment traversed four spreading segments crustal thickness and velocity structure were determined along three intersecting profiles each 100 km long using shots from a 10 gun 71 l tuned airgun array towed at 15 m depth and fired at 40 s intervals recorded on three ocean bottom hydrophones on each profile obh data show high amplitude arrivals from oceanic layer 2 lower amplitude arrivals from layer 3 and wide angle reflections from the moho forward modelling and inversion of traveltime picks from these data show that the crust consists of a 1 5 2 5 km thick layer 2 with a high velocity gradient and a 0 5 3 0 km thick layer 3 with a low velocity gradient and a crustal thickness of 2 2 5 4 km additional constraints on the models come from 2 d modelling of gravity data along the profiles corrected for 3 d effects of off line bathymetry along axis the thickness of layer 2 varies little but layer 3 is thick at segment centres and very thin at segment boundaries along a flowline profile crustal thickness varies by up to 75 per cent from its minimum value in 3 myr the reduced crustal thickness is consistent with observations from very slow spreading ridge axes elsewhere and may be explained by conductive cooling of the upwelling mantle the large along axis variations in layer 3 thickness indicate that magmatic accretion is focused at segment centres and melt is delivered to segment ends perhaps only by lateral dyke propagation flowline variations in crustal thickness may result from episodicity of melt supply on timescales of 3 myr and by tectonic extension during amagmatic periods velocities at the top of layer 2 are poorly correlated with crustal age based on magnetic anomalies suggesting also that episodicity is decoupled between adjacent segments

calculation of oxygen isotope fractionation in hydroxyl bearing silicates abstract the modified increment method has been applied to the calculation of oxygen isotope fractionation factors for hydroxyl bearing silicate minerals the order of 18o enrichment obtained in common rock forming minerals is pyrophyllite kaolinite tourmaline talc prehnite topaz illite phengite lepidolite muscovite staurolite epidote glaucophane serpentine chlorite tremolite hornblende phlogopite biotite humite norbergite ilvaite hydroxyl bearing silicates are enriched in 18o relative to hydroxyl groups but depleted in 18o relative to anhydrous counterparts three sets of self consistent fractionation factors between quartz and the hydroxyl bearing silicate minerals between calcite and the silicate minerals and between the silicate minerals and water have been calculated for a temperature range of 0 1200 c the fractionation factors calculated for mineral pairs are applicable to isotopic geothermometry in igneous metamorphic and sedimentary petrology they can be used as a test of isotopic equilibrium or disequilibrium in natural mineral assemblages over all temperature ranges of geological interest the difference in oxygen isotope composition between the hydroxyl bearing mineral and the oh group is quantitatively demonstrated to be temperature dependent and therefore can be used as a single mineral geothermometer

propagation of a melting anomaly along the ultraslow southwest indian ridge between 46 e and 52 20 e interaction with the crozet hotspot summary regional axial depths mantle bouguer anomaly values geochemical proxies for the extent of partial melting and tomographic models along the southwest indian ridge swir all concur in indicating the presence of thicker crust and hotter mantle between the indomed and gallieni transform faults tfs 46 e and 52 20 e relative to the neighbouring ridge sections accreted seafloor between these tfs over the past 10 myr is also locally much shallower 1000 m and corresponds to thicker crust 1 7 km than previously accreted seafloor along the same ridge region two large outward facing topographic gradients mark the outer edges of two anomalously shallow off axis domains on the african and antarctic plates their vertical relief 2000 m locally and their geometry parallel to the present day axis along a 210 kmlong ridge section suggest an extremely sudden and large event dated between 8 magnetic anomaly c4n and 11 ma magnetic anomaly c5n asymmetric spreading and small ridge jumps occur at the onset of the formation of the anomalously shallow off axis domains leading to a re organization of the ridge segmentation we interpret these anomalously shallow off axis domains as the relicts of a volcanic plateau due to a sudden increase of the magma supply this event of enhanced magmatism started in the central part of the ridge section and then propagated along axis to the east and probably also to the west however it did not cross the gallieni and indomed tfs suggesting that large offsets can curtail or even block along axis melt flow we propose that this melting anomaly may be ascribed to a regionally higher mantle temperature provided by mantle outpouring from the crozet hotspot towards the swir

a serpentinite hosted ecosystem the lost city hydrothermal field the serpentinite hosted lost city hydrothermal field is a remarkable submarine ecosystem in which geological chemical and biological processes are intimately interlinked reactions between seawater and upper mantle peridotite produce methane and hydrogen rich fluids with temperatures ranging from 40 to 90 c at ph 9 to 11 and carbonate chimneys 30 to 60 meters tall a low diversity of microorganisms related to methane cycling archaea thrive in the warm porous interiors of the edifices macrofaunal communities show a degree of species diversity at least as high as that of black smoker vent sites along the mid atlantic ridge but they lack the high biomasses of chemosynthetic organisms that are typical of volcanically driven systems

interstratal dewatering origin for polygonal patterns of sand filled cracks a case study from late proterozoic metasediments of islay scotland sand filled cracks from the lower fine grained quartzite of dalradian late proterozoic age on the island of islay western scotland may be divided into two main types both of which form orthogonal and non orthogonal closed patterns on bedding surfaces type 1 cracks are short and lenticular in cross section contain sand which had been injected downwards and are found on the bottoms of cross laminated sandstone beds type 2 cracks cut several beds and preserve evidence of upward flow of water saturated sand both types of crack developed through the interstratal intrusion of water saturated sand into shrinkage cracks in mud or muddy sand not as previously thought as a result of sub aerial desiccation or sub aqueous cracking of the sediment surface synaeresis these cracks likely resulted from layer parallel contraction caused by compaction of mudstone layers during burial seismic shock may have provided the trigger for the preferential development of polygonal crack patterns in these layers instead of the more usual small scale dewatering structures from a detailed comparison with published descriptions of filled cracks from a number of different geological environments it is concluded that interstratal cracking is a mechanism which rivals sub aerial desiccation in importance and is more common in the geological record than is currently realized

syneresis cracks subaqueous shrinkage in argillaceous sediments caused by earthquake induced dewatering abstract syneresis cracks often confused with subaerial desiccation phenomena are traditionally ascribed to subaqueous shrinkage whereby salinity changes caused deflocculation of clay this and other previously proposed mechanisms fail to account for their occurrence in low energy typically non evaporitic facies stratigraphically sporadic distribution intrastratal formation under shallow burial depths variation in morphology degree of contraction generation of sedimentary dikelets as crack fills and deformation of dikelets and enclosing layers instead it is suggested that ground motion from strong synsedimentary earthquakes caused argillaceous sediments to dewater interbedded sands and silts to be almost simultaneously liquefied and injected into the resulting fissures and then these dikelets to be distorted comparative rarity of syneresis cracks in phanerozoic versus precambrian marine strata is considered to be primarily an evolutionary consequence of theological changes caused by increased organic binding of clay flocs in the water column greater input of organic matter into the sediment and the diversification of sediment dwelling bacteria and meiofauna in phanerozoic deposits

facies and depositional environment of the holocene evaporites in the ras shukeir area gulf of suez egypt the holocene evaporite sequence in the ras shukeir area conformably overlies marine shell banks and cross bedded to graded bedded beach sands and gravels the evaporite sequence is represented by gypsum anhydrite layers that are interbedded with mudstone layers field and petrographic investigations of the evaporite deposits revealed two facies types laminated evaporite facies primary and nodular to enterolithic anhydrite facies diagenetic the laminated evaporite facies is subdivided from the bottom to top into regular laminated evaporite chevron gypsum algal micrite laminations and wavy algal laminated evaporite based on their textures and fabrics the regular and wavy laminated evaporite facies are interpreted as primary deposits in a coastal lagoon and salina environment the chevron gypsum algal micrite facies formed by the growth of chevron gypsum at the sediment water interface within a shallow subtidal lagoonal environment that was characterized by extensive benthic algal mats the nodular to enterolithic anhydrite facies is secondary and formed diagenetically within a siliciclastic supratidal sediment some of the laminated evaporite facies have been diagenetically altered in a supratidal sabkha environment as evidenced by the following 1 the partial formation of nodular evaporite instead of laminated evaporite 2 disruption of gypsum laminations by plant roots and rootlets as well as by precipitation of lenticular gypsum on the root wall and 3 partial dissolution of halite laminae and the formation of wavy anhydrite laminae consequently the holocene evaporites in the ras shukeir area were deposited in a shallow semi closed to closed basin that was separated from the gulf of suez trough changing sea level led to progradation of the evaporite facies from subtidal to intertidal lagoon and salina to a supratidal sabkha

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

sand injectites at the base of the coconino sandstone grand canyon arizona usa abstract in the grand canyon large tabular and wedge shaped sand filled cracks commonly occur at the base of the coconino sandstone penetrating downward into the coarse siltstones of the hermit formation all previous workers have casually identified the vertical sand filled cracks as desiccation cracks until now they have never been studied cracks and their associated features were found and examined at thirty locations and it was found that they have characteristics difficult to explain using desiccation mud cracks or large playa cracks as a model instead it was found the cracks have features commonly found in clastic dikes and sand injectites some lateral sand bodies associated with the cracks have clastic sill like characteristics liquefaction and injection of the basal coconino into the hermit is indicated by 1 macroscopic and microscopic banding flow structures within the cracks 2 bedded sandstone clast breccias in structureless sandstone lenses at the base of the coconino 3 lateral sand bodies which are connected to the vertical cracks 4 a zoned depth distribution of cracks about the bright angel fault zone 5 insufficient clay mineralogy and particle size for the hermit to crack by desiccation 6 preferred orientation of the cracks roughly perpendicular to the bright angel fault zone and several other features caution should be exercised when interpreting sand filled cracks as desiccation features i e mud cracks even if the interpretation fits well with accepted paleoenvironmental models

high frequency cyclicity in the latest messinian adriatic foreland basin insight into palaeoclimate and palaeoenvironments of the mediterranean lago mare episode abstract late messinian lago mare deposits show high frequency cyclicity in the whole mediterranean basin both millimeter and centimeter scale cyclicities have been observed in several odp sites as well as in stratigraphic sections from the mediterranean borderland we have analyzed a well exposed late messinian lago mare section from the adriatic side of the central apennines italy at the fonte dei pulcini section se majella mts millimeter and centimeter scale white and dark couplets have been observed in the field a 50 cm regular spaced sampling has been performed in the uppermost 53 m of the late messinian lago mare clays on the 107 collected samples geochemical caco 3 content mineralogical xrd analyses and micropalaeontological investigations have been performed in addition sem and microprobe investigations as well as mineralogical and micropalaeontological analyses have been carried out on single lamina from a 34 cm thick interval of millimeter scale laminites besides the 10 3 cycles m and 10 2 cycles m frequencies observed in the field spectral analyses performed on the caco 3 data set indicated other high frequency cyclicities 0 47 cycles m 0 35 cycles m and 0 17 cycles m taking into account the estimated sedimentation rate these frequencies correspond respectively to periodicities of 1 year 10 years 2 1 kyr 2 8 kyr and 5 6 kyr these sub milankovitch cyclicities have been related to annual and sunspot solar activity the millimeter scale couplets are interpreted as varves sedimented in an ephemeral water environment marginal to a perennial brackish water lagoonal basin these varved sediments reflect a marked seasonality characterized by the alternation of arid and more humid climatic phases the presence of high values of smectite 60 80 in the clay minerals of the analyzed samples could be a consequence of these climatic oscillations from drier to moister conditions the climatic scenario suggested in this paper for the late messinian lago mare episode is in agreement with the monsoon system that according to some authors starting from 5 8 ma developed in the mediterranean area

cambrian stratigraphy and depositional history of the northern indian himalaya spiti valley north central india recent work on himalayan tectonics indicates that prior to the cenozoic collision of india and asia an enigmatic cambrian ordovician event may have strongly infl uenced the regional geology of the himalaya stratigraphic and sedimentological analyses of well preserved cambrian deposits are critical for understanding the nature of this early tectonic event and its infl uence on the later tectonic evolution of the himalaya the parahio formation defi ned herein of the parahio valley spiti region in the tethyan himalaya of india is the best biostratigraphically resolved section of cambrian strata in the entire himalaya this formation consists of 1350 m of dominantly siliciclastic deltaic deposits the formation ranges from uppermost lower cambrian lungwangmiaoan stage to middle middle cambrian hsuchuangian stage representing a time span of 5 10 m y it contains numerous mediumscale shoaling cycles that range from storminfl uenced offshore deposits to thick trough cross bedded fl uvial facies many thin carbonate beds with abundant trilobite fossils directly overlie the fl uvial facies and represent transgressive systems tract deposits the cycles are interpreted to have resulted from delta lobe switching based on a lack of systematic stratigraphic changes in cycle or facies thicknesses this paleoenvironmental reconstruction contradicts previous interpretations of this unit that range from deep sea fl ysch to shallow marine tidalites in addition our paleoenvironmental analysis and paleocurrent data suggest that the uppermost lower to middle cambrian deposits of the lesser and tethyan himalaya are parts of the same ancient northward prograding fl uvial deltaic depositional system of the paleo tethys margin of india an angular unconformity with overlying ordovician conglomeratic rocks has considerable local relief with meter scale scours and a valley fi ll 100 m thick the scours have northeast southwest orientations which parallel both the paleocurrents in the underlying parahio formation as well as published paleocurrent readings from the coarse red beds of the overlying ordovician strata the cambrian ordovician unconformity is of regional extent and our recent biostratigraphic database indicates that the minimum hiatus associated with the unconformity in spiti is 15 m y our sedimentological analysis and associated paleocurrent data from the parahio formation along with additional data from units both above and below the unconformity indicate that published models portraying foreland basin development at this time with southward directed thrusting are problematic an alternate possibility that uplift took place south of the tethyan himalaya is also problematic because no published stratigraphic or structural evidence exists for such an uplift to the south for either the greater or the lesser himalaya lithotectonic zones

a tale of two extinctions converging end permian and end triassic scenarios the end permian c 252 ma and end triassic c 201 ma mass extinction events are commonly linked to the emplacement of the large igneous provinces of the siberia traps and central atlantic magmatic province respectively accordingly scenarios for both extinctions are increasingly convergent and cross fertilization of ideas has become important here we present a synthesis of extinction scenarios based on a critical assessment of the available palaeontological sedimentological geochemical and geophysical evidence how similar were the extinction events what gaps exist in our understanding and how can a comparison of the events enhance our understanding of each event individually our focus is on the most important proximate kill mechanisms including climate change and atmospheric pollution increased soil erosion weathering and runoff forest dieback and the spread of pathogens and ocean temperature changes anoxia and acidification there is substantial evidence to suggest that very similar kill mechanisms acted upon late permian as well as late triassic ecosystems strengthening the hypothesis that the ultimate causes of the mass extinction events were similar

microbial mats implicated in the generation of intrastratal shrinkage synaeresis cracks intrastratal shrinkage often termed synaeresis cracks are commonly employed as diagnostic environmental indicators for ancient salinity stressed transitional fluvial marine or marginal marine depositional environments despite their abundance and use in facies interpretations the mechanism of synaeresis crack formation remains controversial and widely accepted explanations for their formation have hitherto been lacking sedimentological ichnological petrographic and geochemical study of shallow marine mudstone beds from the ordovician beach formation of bell island newfoundland has revealed that crack development cf synaeresis cracks on the upper surface of mudstone beds is correlated with specific organic geochemical and sedimentological parameters contorted sinuous sand filled cracks are common at contacts between unbioturbated mudstone and overlying sandstone beds cracks are absent in highly bioturbated mudstone and are considered to pre date firmground assemblages of trace fossils that include planolites and trichophycus the tops of cracked mudstone beds contain up to 2 1 wt total organic carbon relative to underlying mudstone beds that contain around 0 5 wt total organic carbon high resolution carbon isotope analyses reveal low 13corg values 27 6 on bed tops compared with sandy intervals lacking cracks 24 4 to 24 9 cracked mudstone facies show evidence for microbial matgrounds including microbially induced sedimentary structures on bedding planes and carbonaceous laminae and tubular carbonaceous microfossils in thin section non cracked mudstone lacks evidence for development of microbial mats microbial mat development is proposed as an important prerequisite for intrastratal shrinkage crack formation both microbial mats and intrastratal shrinkage cracks have broad palaeoenvironmental distributions in the precambrian and early phanerozoic in later phanerozoic strata matgrounds are restricted to depositional environments that are inhospitable to burrowing and surface grazing macrofauna unless evidence of synaeresis i e contraction of clay mineral lattices in response to salinity change can be independently demonstrated the general term intrastratal shrinkage crack is proposed to describe sinuous and tapering cracks in mudstone beds

microbially induced sedimentary structures in the paleoproterozoic upper huronian supergroup canada abstract the paleoproterozoic gordon lake and bar river formations huronian supergroup contain a variety of sedimentary structures in the flack lake area of ontario canada that have been considered of debatable origin we identify these structures as microbially induced sedimentary structures miss the preserved miss are related to microbial mat destruction and decay and include sand cracks mat chips remnant gas domes pyrite patches and iron laminae a biological origin for the fossil structures is supported by their similarities to modern and ancient documented examples of miss the sand dominated nature of the substrate in which they are preserved and key microtextures identified in thin section microtextures include curled frayed and layered mat chips carbonaceous laminae oriented grains and concentrated heavy minerals on outcrop scale the presence of desiccation cracks flaser and lenticular bedding and ripples in association with the types of miss identified in the gordon lake formation support the interpretation of a tidal flat depositional environment the gordon lake formation contains a greater number and diversity of miss than the overlying bar river formation as a result of lower energy deposition in the former the quartz arenite of the bar river formation contains fine grained to pebbly granulestone characterized mainly by tangential and planar cross beds which is consistent with a tidal channel or sand shoal setting although fossil evidence of life is rare in the rocks of the huronian supergroup identification of miss in the flack lake area provides a significant and convincing indication of microbial colonization at the time of deposition

intra and intercrystalline oxygen isotope variations in minerals from basalts and peridotites igneous phenocrysts commonly exhibit zoning in major and trace element composition reflecting and potentially constraining the differentiation and or mixing histories of their parent melts to date little work has been done characterizing zonation of oxygen isotopes in minerals from mafic and ultramafic rocks we present 259 ion probe cameca ims 1280 measurements of 18 o in 34 natural magmatic and mantle olivines and pyroxenes from five hand samples from diverse igneous environments we compare 18 o variations with zonation in other elements especially p analyzed by electron microprobe analysis empa and nano secondary ionization mass spectrometry nanosims there is generally a close average within 0 1 0 2 agreement between average 18 o values of olivines measured by sims standardized against san carlos olivine and independently known values for bulk separates from the same samples measured by laser fluorination these data demonstrate that current ion microprobe techniques are not only precise but also accurate enough for study of sub per mil oxygen isotope variations in silicates within 0 2 provided samples are prepared and analyzed following strict guidelines all but one of the 34 studied grains are homogeneous in 18 o within a small multiple of analytical precision estimated 0 2 1 for most data poorer for a subset of measurements made on small 5 m spots this population of isotopically homogeneous grains includes some with oscillatory micrometer scale p banding the lack of 18 o variations suggests that whatever factors lead to this common mode of trace element zonation have no detectable effect on melt crystal partitioning of oxygen isotopes large 2 oxygen isotope variations are observed in one olivine glomerocryst from mauna kea hawaii this glomerocryst contains p rich domains that are either equant or skeletal or feathery in outline and these p rich domains are systematically low in 18 o compared with adjacent later grown p poor olivine this unusual oxygen isotope zonation pattern might reflect a kinetic fractionation during nucleation and growth of the cores of some olivine phenocrysts we tested this hypothesis through measurements of 18 o distributions in synthetic olivines grown at a range of rates and exhibiting diverse patterns of p zoning these synthetic olivines are homogeneous in 18 o within the limits of our analyses 0 3 0 4 in this case and show no connection between p zonation and oxygen isotope heterogeneity we therefore think it more plausible that unusual o isotope zonation in the mauna kea glomerocryst reflects addition of a low 18 o component to some hawaiian magmas just before nucleation of olivine more generally this study demonstrates the utility of modern sims techniques for in situ study of the subtle 1 range oxygen isotope variations characteristic of common mafic and ultramafic rocks

seismic structure across the rift valley of the mid atlantic ridge at 23 20 mark area implications for crustal accretion processes at slow spreading ridges the results from a 53 km long wide angle seismic profile across the rift valley of the mid atlantic ridge south of the kane transform near 23 20 n mark area provide new constraints on models of tectonic extension and magmatic accretion along slow spreading mid ocean ridges anomalously low middle and lower crustal p wave velocities beneath the neovolcanic snake pit ridge are consistent with elevated axial temperatures and with the presence of 4 1 partial melt evenly distributed within the lower crust in preferentially oriented elongated thin films if the melt inclusions have larger aspect ratios melt fractions can be up to 17 3 this and other geological observations suggest that the study area is presently in a magmatically active period the igneous crust is anomalously thin beneath both flanks of the median valley 2 3 2 5 km thus the mantle rocks observed along the western rift valley wall at pink hill were probably emplaced at shallow levels within the valley floor during a period of very low magma supply and were later exposed on the valley walls by normal faulting the crust within the eastern rift valley and flanking rift mountains is seismically heterogeneous with igneous crustal thickness variations of 2 2 km over horizontal distances of 5 km this heterogeneity indicates that the magma supply in the area has fluctuated during the last 2 m y thus magmatic and amagmatic periods at slow spreading ridges may alternate over much shorter temporal scales that previously inferred from sea surface gravity data

sedimentary structures sorby and the last decade henry clifton sorby pioneered in the last century the description and especially the hydrodynamic interpretation of sedimentary structures together with their use as palaeocurrent indicators research completed since the last syntheses were published a decade ago shows that work along these lines continues to be necessary and relevant particularly as regards the physical explanation of structures and to present significant challenges and opportunities perhaps the most pressing needs are for a better understanding of 1 bedforms in gravels silts and carbonate sediments 2 tidal and especially sand wave bedding 3 hummocky and swaley cross stratification and 4 soft sediment and dewatering structures in turbidites many sedimentary structures present a little exploited opportunity to quantify process rates and define short time periods from the rock record

depositional environments and facies of the late triassic abu ruweis formation jordan abstract the abu ruweis formation is composed of carbonates evaporites and mudstones with some locally developed pelletic oolitic and stromatolitic limestones the lateral persistence of bedding the purity of the evaporite rocks the alternating arrangement of marine carbonates and evaporites indicates periodic deposition in subaqueous conditions salina petrographic investigations x ray diffraction analysis as well as chemical analysis have shown that the outcropping evaporite beds are mainly composed of secondary gypsum with rare anhydrite relics five microfacies of gypsum were recognized according to their fabrics porphyroblastic and granoblastic gypsum showing polarization texture gypsum pseudomorph after anhydrite laths and satin spar gypsum the textures they display indicate a hydration origin of precursor anhydrite which is in turn rehydrated from primary gypsum some of these anhydrites were formed as a result of replacement processes of the carbonate sediments associated with the evaporites as evidenced from the textural relationships of the carbonate and sulfate minerals the o18 content ranges from 1 45 to 8 38 pdb and the c13 content ranges from 1 52 to 4 73 pdb trace elements analysis has shown that the abu ruweis dolomites are rich in strontium up to 600 ppm and sodium up to 835 ppm the isotope composition and trace elements content as well as the petrographic characteristics point to a penecontemporaneous hypersaline dolomitization origin for the abu ruweis dolomites the evaporites were deposited during a regressive lowstand systems tract whereas the carbonates were deposited under shallow water marine conditions during a highstand systems tract the abu ruweis succession represents a relatively stable arid climate within a rapidly subsiding basin restricted conditions were provided by the development of beach barriers

diastasis cracks mechanically generated synaeresis like cracks in upper cambrian shallow water oolite and ribbon carbonates upper cambrian limestones and dolostones of western newfoundland canada display conspicuous synsedimentary mud cracks cracks occur in carbonate mudstone interbedded with ooid and peloid grainstone unwashed oolite and ribbon rock lithofacies the traditional interpretation is that these are desiccation cracks the weight of evidence supports an alternative e explanation cracks resulted from the differential mechanical behaviour under stress of stiff mud interlayered with loose ooid peloid sand the processes envisaged to cause such diastasis cracks may be applicabl e to a wide variety of both carbonate and terrigenous clastic deposits composed of interlayered sediments of contrasting material properties and may be a viable alternative to synaeresis

serpentinization of oceanic peridotites 2 kinetics and processes of san carlos olivine hydrothermal alteration 1 the kinetics of the reaction mg fe olivine h 2 o serpentine magnetite brucite h 2 were investigated at 500 bars in the 250 350 c range using natural olivine san carlos fo 91 with grain sizes between 1 and 150 mm and for run durations up to 514 d the amount of magnetite produced which directly relates to reaction progress was accurately monitored using up to 24 time resolved magnetic measurements per experiment eighty percent of serpentinization was achieved after 60 d for olivine grain sizes of 5 15 mm and after 500 d for grain sizes of 50 63 mm serpentinization kinetics were found to be inversely proportional to the geometrical surface area of the starting olivine grains they were one or two orders of magnitude slower than serpentinization kinetics commonly used for modeling serpentinization related processes the nature of the serpentine mineral product depended on the initial olivine grain size igs for igs in the 5 150 mm range lizardite formed and olivine dissolution was the rate limiting process at igs below 5 mm chrysotile crystallized instead of lizardite and the relationship between olivine surface area and reaction kinetics no longer held we infer that for such small olivine grain sizes dissolution is no longer the rate limiting process serpentinization in our experiments was associated with the creation of new reactive surface area according to two cooperative processes etch pits formation associated with dissolution and grain fracturing for igs above 20 mm interestingly fractures and etch pits with similar geometry and sizes were also observed for residual olivine with a typical grain size of 50 mm in serpentinized peridotite samples from the southwest indian ridge this suggests that the processes governing olivine serpentinization kinetics in our experiments are similar to those prevailing in natural systems we therefore suggest that the new kinetic data set that we present here which encompasses a range of olivine grain sizes and reaction temperatures is relevant to the serpentinization of olivine in the oceanic crust insofar as water is available

continuous exhumation of mantle derived rocks at the southwest indian ridge for 11 million years the global mid ocean ridge system where tectonic plates diverge is traditionally thought of as the largest single volcanic feature on the earth yet wide expanses of smooth sea floor in the easternmost part of the southwest indian ridge in the indian ocean lacks the hummocky morphology that is typical for submarine volcanism at other slow spreading ridges the sea floor can extend by faulting the existing lithosphere along only one side of the ridge axis however the smooth sea floor in the easternmost southwest indian ridge also lacks the corrugated texture created by such faulting instead the sea floor is smooth on both sides of the ridge axis and is thought to be composed of altered mantle derived rocks here we use side scan sonar to image the sea floor and dredge samples to analyse the composition of two sections of the southwest indian ridge between 62 050 e and 64 400 e where the sea floor formed over the past 11 million years we show that the smooth floor is almost entirely composed of seawater altered mantle derived rocks that were brought to the surface by large detachment faults on both sides of the ridge axis faulting accommodates almost 100 of plate divergence and the detachment faults have repeatedly flipped polarity we suggest that this tectonic process could also explain the exhumation of mantle derived rocks at the magma poor margins of rifted continents

a 2 3 million year lacustrine record of orbital forcing from the devonian of northern scotland detailed sedimentological examination of onshore sections through cyclic lacustrine deposits of the middle devonian succession of northern scotland has been augmented with offshore data to allow the construction of a continuous 2 3 myr record of orbital forcing these data provide an important record of climatic variance through the devonian which by comparison with other records from this time can significantly improve our understanding of the devonian global climatic system precessional and eccentricity cycles are shown to be dominant by the direct measurement and extrapolation of depositional rates in well exposed sections as well as the analysis of ratios between primary and modulating cycles spectral analysis has confirmed the presence of regular cycles throughout the offshore data which provide a far longer record of orbitally forced cyclicity than previously available the presence of a half precession cycle is also suggested similar up section variations in cycle thicknesses are recorded from both the onshore and offshore succession these variations and those in cycle symmetry are related to a trend from an underfilled to a balanced fill basin state of further importance in controlling cycle thickness is the period of each cycle during which lacustrine conditions and therefore lake level controlled accommodation existed

new constraints on carbonation associated with brecciation in hyperextended margins example of iberia and newfoundland margins the sequestration of co2 occurs naturally in ultra mafic rocks by carbonation processes and is commonly noted in areas of the seafloor where mantle lithologies are exhumed as well as carbonation mantle exhumation is also responsible for rock brecciation the relationship between carbonation and brecciation is not well constrained a temporal evolution from syn to post tectonic carbonation and brecciation is proposed in line with progressive mantle exhumation using a petrological study of brecciated material from iodp drill cores of the iberia newfoundland conjugated margins we relate crack seal veins to tectonic brecciation authigenic calcite with scalenohedral structure to hydraulic brecciation and reworked clasts within cement to tectono sedimentary processes oxygen isotope compositions reveal late staged 50 c carbonate generation in the proximal part of the ocean continent transition which have followed an earlier phase of sub seafloor carbonate generation the results are crucial to understand co2 exchange within the reworked sub seafloor in passive margins and oceanic systems

deciphering the sedimentological imprint of paleoseismic events an example from the aptian codo formation northern brazil abstract a stratigraphic horizon bearing several styles of soft sediment structures occurs sandwiched within entirely undisturbed deposits in the lacustrine codo formation aptian located in the eastern part of the grajau basin northern brazil these deposits show a variety of sedimentary features distinguished according to their deformation style as follows from bottom to top a zone 1 consisting of widespread molar tooth structures i e fine grained spar filled cracks that are interconnected with small scale faults fissures and stylolites inclined at a high angle to bedding b zone 2 represented by complex convolute folds with lateral gradation of smooth synclines and anticlines into isoclinic folds and internal layers that retain the normal bedding features recumbent folds nappe folds with associated thrust faults pseudonodules and mound and sag structures c zone 3 consisting of normal faults and fissures that are vertical to near vertical show ragged morphology with small delicate peaks taper both downward and upward after a few centimeters and are associated with large up to 2 5 m long collapsed boulders and d zone 4 characterized by shales with irregular convolute folds these deformation zones are interpreted to reflect sediment rheology and alternating periods of sediment accumulation and deformation under the influence of synsedimentary seismic events as suggested mostly by 1 occurrence between undeformed deposits 2 lateral correlation for several kilometers of distance 3 similarity with other features formed in areas affected by seismic activity 4 presence of structures attesting to sediment deformation in an unconsolidated to semi consolidated state and 5 complex association of brittle and ductile deformation structures hence zone 1 records small scale cracking and faulting of early cemented cohesive deposits zone 2 is dominated by ductile deformation structures formed on water saturated unconsolidated sediments probably as a reflex of compressional stress zone 3 records a change in tectonic style to dominantly distension as evidenced by the horizon with normal faults finally zone 4 shows structures attributed to ductile deformation of water saturated muds which was probably formed during the same episode of deformation responsible for the faults and fissures in zone 3 this succession of deformation events appears to be consistent with transtensional and compressional shear stresses during the sedimentary evolution of the grajau basin

paleoproterozoic microbially induced sedimentary structures from lagoonal depositional settings in northern china abstract microbially induced sand cracks crack fills occur extensively on the top surface of fine sandstone beds of the paleoproterozoic zhaojiazhuang formation changcheng group 1 7 ga around the cangyan mountain hebei province northern china detailed field and microscopic petrographical evidence reveal that these sand cracks crack fills possibly resulted from dehydration and desiccation of microbial mats the age and peculiar morphology of these microbially induced sedimentary structures do not allow comparison with trace fossils or purely physical desiccation cracks the fine sandstone beds on the surfaces of which these microbially induced sedimentary structures formed were deposited in a lagoon brackish depositional setting marine to non marine transitional setting with episodic injection of marine water as such these microbially induced sedimentary structures suggest the colonization of the marine to non marine transitional settings in the paleoproterozoic period and that the search for evidence of early microbial life should be sought in the marine to non marine transitional settings these broad habitats suggest these microbes could have been eurytopic organisms capable of adapting to varying extents of salt and oxygen content variations like their modern counterparts

the lower kaskapau formation cenomanian a multiple frequency retrogradational shelf system alberta canada the cenomanian to earliest turonian cretaceous kaskapau formation in northwestern alberta and adjacent british columbia contains three shallow marine shelf sandstone bodies encased in shale the doe creek pouce coupe and howard creek members these sandstone bodies were deposited in a retrogradational pattern subparallel to the paleoshoreline of the underlying dunvegan formation they represent distal deltaic sedimentation during sea level lowstands or stillstands that punctuated the overall transgression represented by the shale of the kaskapau formation a thin slightly to nonbioturbated finely interbedded sandstone siltstone and shale facies containing syneresis cracks and no benthic foraminifera facies 4 may be important paleoenvironmentally representing th leading edge of a progradational wedge the sandstone bodies are bounded by disconformities the lower disconformity is erosive perhaps the result of wave scour associated with a minor fall in relative sea level the upper disconformity is a flooding surface formed as relative sea level subsequently rose within the doe creek member the overlapping i n and a reservoir sandstone bodies suggest an oscillatory shoreline caused by high frequency relative sea level fluctuations during the overall transgression as such the succession represents a transgressive high frequency sequence set the howard creek member is directly overlain by the second white speckled shale turonian which represents a condensed section having good source rock potential the latter unit was deposited durin the peak of the marine transgression corresponding to an eustatic highstand

changing lake dynamics and sequence stratigraphy of synrift lacustrine strata in a half graben an example from the triassic ischigualasto villa union basin argentina well exposed triassic rift strata from the ischigualasto villa union basin nw argentina include a 80 to ca 515 m thick lacustrine dominated package that can be correlated across a half graben using key stratigraphic surfaces sequence boundaries lacustrine flooding surfaces and forced regressive surfaces the characteristics of the synrift lacustrine fill in different parts of the half graben have been examined and the mechanisms controlling sedimentation inferred a variety of sedimentary environments are recognized including volcaniclastic floodplain mildly saline lake and playa lake offshore lacustrine delta front to fluvial dominated and wave dominated deltas distributary and fluvial channel and interdistributary bay the succession can be divided into four stratigraphic sequences ss1 to ss4 the oldest of which ss1 contains volcaniclastic fluvial and saline lake deposits it is thickest close to the western border fault zone reflecting more rapid subsidence here accommodation exceeded sediment and water input during ss1 the second and third sequences ss2 and ss3 mark the onset of widespread lacustrine sedimentation reflecting a balance between accommodation creation and water and sediment fluxes sequences ss2 and ss3 are represented by offshore meromictic lacustrine and deltaic deposits the latter mostly sourced from the flexural and southern axial margins of the half graben the presence of stacked parasequences bound by lacustrine flooding surfaces is related to climatically induced lake level fluctuations superimposed on variable rates of subsidence on the controlling rift border fault zone the youngest sequence ss4 is represented by the deposits of littoral lacustrine and shallow shelf deltas distinguished by a change in lithofacies palaeocurrents and sandstone composition suggesting a switch in sediment supply to the footwall margin to the nw the change in the sediment source is related to reduced footwall uplift the possible presence of a relay ramp and or supply from a captured antecedent drainage network during ss4 the rate of creation of accommodation was exceeded by the sediment and water discharge the stratigraphic evolution of lacustrine strata in the half graben was mainly controlled by tectonic processes including subsidence rate and the growth and evolution of the border fault zone but changing climate inducing changes in water balance and lake level and autocyclic processes delta lobe switching were also important

metamorphic record of high pressure dehydration of antigorite serpentinite to chlorite harzburgite in a subduction setting cerro del almirez nevado filabride complex southern spain

along axis variations in seafloor spreading in the mark area recent investigations with the manned submersible alvin and the angus deep towed camera sled greatly extended the known range of variations in the style of seafloor spreading along the axis of the mid atlantic ridge five transects of the spreading centre at intervals of10 20 km south of the kane fracture zone at 24 n latitude demonstrate dramatic changes in the style and magnitude of tectonic extension development of the neovolcanic zone expression of hydrothermal venting types oflithologic exposures and morphology of the median valley

pressure temperature estimates of the lizardite antigorite transition in high pressure serpentinites serpentine minerals in natural samples are dominated by lizardite and antigorite in spite of numerous petrological experiments the stability fields of these species remain poorly constrained this paper presents the petrological observations and the raman spectroscopy and xrd analyses of natural serpentinites from the alpine paleo accretionary wedge serpentine varieties are identified from a range of metamorphic pressure and temperature conditions from sub greenschist p 20 kbar t 460 c along a subduction geothermal gradient we used the observed mineral assemblage in natural serpentinite along with the tmax estimated by raman spectroscopy of the carbonaceous matter of the associated metasediments to constrain the temperature of the lizardite to antigorite transition at high pressures we show that below 300 c lizardite and locally chrysotile are the dominant species in the mesh texture between 320 and 390 c lizardite is progressively replaced by antigorite at the grain boundaries through dissolution precipitation processes in the presence of sio2 enriched fluids and through a solid state transition in the cores of the lizardite mesh above 390 c under high grade blueschist to eclogite facies conditions antigorite is the sole stable serpentine mineral until the onset of secondary olivine crystallization at 460 c

microbial mat mediated structures in the ediacaran sonia sandstone rajasthan india and their implications for proterozoic sedimentation abstract a strong influence of microbial mats on the physics of neoproterozoic sedimentation is explored within a 60 m thick stratigraphic interval within the sonia sandstone jodhpur group western india this marine interval is bounded by two terrestrial units its base being marked by a transgressive lag and the top by an unconformity progradation from upper neritic above fair weather wave base to supralittoral settings was later terminated by a transgression deposits of lower shoreface upper shoreface transition thus overlie supralittoral aeolian sandstones the basal contact of the former deposits being marked by another transgressive lag a wide spectrum and abundant examples of microbial mat or mat derived structures supports unusual cohesiveness within granular sand deposited in a high energy marine environment the cohesiveness is manifested in abundant preservation of several delicate primary structures and also in their replication in overlying beds even after they were subjected to high energy currents a low rate of sedimentation and severely restricted sediment reworking resulted from prolific mat growth and consequent depletion in the sediment budget was manifested in bedform migration and evolution these factors could also have influenced the sequence building pattern in a distinctive way as is already reported from several meso and neoproterozoic successions ubiquitous mat growth thus had the potential to impart significant distinctions to proterozoic clastic sedimentary successions

upper permian lacustrine organic facies evolution southern junggar basin nw china abstract the jingjingzigou lucaogou and hongyanchi formations of the southern junggar basin together comprise over 1000 m of organic rich lacustrine facies they record an overall evolution from relatively shallow evaporative lakes to freshwater lakes associated with fluvial systems the jingjingzigou formation is characterized by low toc and hi and biomarker features such as abundant carotane consistent with a specialized saline or hypersaline biota biomarker distributions in jingjingzigou formation extracts most closely resemble oils from the giant karamay oilfield the lucaogou formation represents one of the richest and thickest lacustrine source rock intervals in the world yet it contradicts conventional lacustrine source rock models in at least two important aspects first deposition occurred at middle paleolatitudes 39 43 n rather than in the tropics second limited nutrient supply in a drainage basin dominated by intermediate volcanic rocks appears to have caused low to moderate primary productivities stable salinity stratification and low inorganic sedimentation rates in a deep lake nonetheless resulted in deposits with up to 20 toc and hi near 800 the overlying hongyanchi formation has 1 5 toc but low hi and was deposited in freshwater oxic to sub oxic lakes

from slow to ultraslow a previously undetected event at the southwest indian ridge at ca 24 ma changes in plate motion are thought to be recorded in the trend of fracture zones even though fracture zones provide no information about the spreading rate using newly compiled published and unpublished magnetic data from the southwest indian ridge we calculated fi nite rotation poles for a13 a8 and a6 from which we determined a 50 decrease in spreading rate from slow to ultraslow at ca 24 ma not accompanied by a signifi cant change in spreading direction this spreading rate decrease is concurrent with changes in plate motions at only two of the four adjoining plate boundaries finally we discuss the possible relationships of this event with other absolute or relative plate motion events that occurred at ca 24 ma at the global scale

dynamic control on serpentine crystallization in veins constraints on hydration processes in oceanic peridotites deformation and hydration processes are intimately linked in the oceanic lithosphere but the feedbacks between them are still poorly understood especially in ultramafic rocks where serpentinization results in a decrease of rock density that implies a volume increase and or mass transfer serpentinization is accompanied by abundant veining marked by different generations of vein filling serpentines with a high variety of morphologies and textures that correspond to different mechanisms and conditions of formation we use these veins to constrain the role of deformation and mass transfer processes during hydration of oceanic peridotites at slow spreading ridges we have selected a representative set of veins from ocean floor serpentinites of the mid atlantic ridge near kane transform fault 23 n and characterized these in detail for their microstructures and chemistry by coupling optical and electron microscopy sem tem with electron microprobe analyses four main veining episodes v1 to v4 accompany the serpentinization the first episode identified as vein generation v1 is interpreted as the tectonically controlled penetration of early seawater dominated fluid within peridotites enhancing thermal cracking and mesh texture initiation at 3 4 km up to 8 km depth and at t 300 350 c the two following vein stages v2 and v3 formed in a closed diffusive system and accommodate volume expansion required to reach almost 50 serpentinization of the protolith the cracks exploited by these veins were caused by the progressive unroofing at depths of 4 to 2 km along a detachment fault degree and rate of serpentinization seem to be controlled by the capacity of the system to create space and to drive the mass transfer needed for ongoing serpentinization and this capacity is in turn linked to the exhumation rate and local tectonics during this period water consumed by hydration may prevent the establishment of convective hydrothermal cells the onset of an open hydrothermal system in the shallow lithosphere 2 km where brittle fracturing and advective transfer dominate and enable the completion of serpentinization is marked by the last vein generation v4 these results show a complete history of alteration with the crystallization of different types of serpentine recording different tectonic events chemical conditions and modes of hydrothermal alteration of the lithosphere

vertebrate paleoecology of the chinle formation late triassic of the southwestern united states abstract the late triassic chinle formation is an extensive series of fluvial and lacustrine sediments exposed mostly in the colorado plateau region fossil vertebrates occur in local concentrations throughout the vertical and horizontal extent of the formation although the most diverse accumulations occur within the petrified forest member in arizona and new mexico successive faunas are known from the lower and upper units of the petrified forest member and can be subdivided into terrestrial and aquatic communities faunal turnover occurs among herbivores phytosaurs and metoposaurs between the lower and upper units no major change is observed in fish faunas throughout chinle deposition mass accumulations of vertebrate fossils in the chinle occurred on floodplains in bogs and small ponds and in or near fluvial channels paleontological and sedimentological evidence supports the interpretation of chinle paleoclimates as having significant precipitation but being strongly seasonal vertebrate occurrences do not clearly reflect the gradual aridification at the end of chinle deposition suggested by sedimentological study

comparison of clay mineral stratigraphy to other proxy palaeoclimate indicators in the mesozoic of nw europe this paper reviews the opportunities and pitfalls associated with using clay mineralogical analysis in palaeoclimatic reconstructions following this conjunctive methods of improving the reliability of clay mineralogical analysis are reviewed the mesozoic succession of nw europe is employed as a case study this demonstrates the relationship between clay mineralogy and palaeoclimate proxy analyses may be integrated with clay mineralogical analysis to provide an assessment of aridity humidity contrasts in the hinterland climate as an example the abundance of kaolinite through the mesozoic shows that while interpretations may be difficult the mesozoic climate of nw europe was subject to great changes in rates of continental precipitation we may compare sedimentological facies mineralogy geochemistry indicators of palaeoprecipitation with palaeotemperature estimates the integration of clay mineralogical analyses with other sedimentological proxy indicators of palaeoclimate allows differentiation of palaeoclimatic effects from those of sea level and tectonic change we may also observe how widespread palaeoclimate changes were whether they were diachronous or synchronous how climate sea level and tectonics interact to control sedimentary facies and what palaeoclimate indicators are reliable

negligible microbial matground influence on pre vegetation river functioning evidence from the ediacaran lower cambrian series rouge france supported by shell international exploration and production b v under research framework agreement pt38181

structure and dating errors in the geologic time scale and periodicity in mass extinctions structure in the geologic time scale reflects a partly paleontological origin as a result ages of cenozoic and mesozoic stage boundaries exhibit a weak 28 myr periodicity that is similar to the strong 26 myr periodicity detected in mass extinctions of marine life by raup and sepkoski radiometric dating errors in the geologic time scale to which the mass extinctions are stratigraphically tied do not necessarily lessen the likelihood of a significant periodicity in mass extinctions but do spread the acceptable values of the period over the range 25 27 myr for the harland et al time scale or 25 30 myr for the dnag time scale if the odin time scale is adopted acceptable periods fall between 24 and 33 myr but are not robust against dating errors some indirect evidence from independently dated flood basalt volcanic horizons tends to favor the odin time scale

in situ oxygen isotope analyses in serpentine minerals constraints on serpentinization during tectonic exhumation at slow and ultraslow spreading ridges abstract peridotites exhumed along detachment faults at slow and ultraslow spreading ridges undergo variable and complex serpentinization here we present in situ oxygen isotope analyses in serpentine minerals from a textural sequence identified in samples drilled at the atlantis massif mid atlantic ridge mar 30 n and dredged along the easternmost southwest indian ridge swir 62 65 e the textural sequence is similar at both locations and involves mesh texture and bastite formation after olivine and orthopyroxene mesh texture recrystallization into chrysotile and antigorite dominated textures and banded and fibrous veins the 18o in serpentine decreases with the textural sequence which we interpret to record an increase in the time integrated water rock ratio under a nearly constant temperature while mesh texture development starts at stoichiometric water rock ratios and creates an isotopic variability at a scale of 100 m recrystallized textures indicate serpentinization temperatures on the order of 260 290 c seawater dominated fluid hypothesis or 320 360 c hydrothermally altered seawater hypothesis the subsequent banded veins record the evolution towards colder and seawater dominated fluids potentially present in shallower levels of the detachment footwall serpentinization leading to the mesh textures appears faster and more heterogeneous in atlantis massif samples than in swir samples we propose that such features are influenced by the amount of gabbros intruded in the peridotite in addition to influencing fluid pathways gabbros also provide a source for observed si enrichments in some atlantis massif samples and control the ph which in turn influences the mobility of al

episodic hydrothermal alteration recorded by microscale oxygen isotope analysis of white mica in the larderello travale geothermal field italy abstract microscale oxygen isotope analysis 18o 16o of minerals can identify distinct events of fluid rock interaction this method is however still limited to a few major and accessory minerals of which most are anhydrous minerals we present the systematic study of oxygen isotope distribution in white mica by secondary ion mass spectrometry sims texturally and chemically distinct white mica populations in granitic and contact metamorphic rocks from the larderello travale geothermal field ltgf italy record stages of magmatic crystallization metamorphic hydrothermal replacement and fluid rock interaction the large range in intra and inter grain white mica 18o values between 1 and 14 reflects varying protoliths and degrees of fluid mineral interaction at variable temperatures 180 450 c present day temperatures p d t this variability reflects the large scale circulation of both 1 magmatic syn intrusive to early contact metamorphic hydrothermal fluids with high 18o values and 2 meteoric fluids with 18o values of 7 during a post intrusive late hydrothermal stage metasedimentary rocks from the upper reservoir contain distinct white mica populations occurring in close proximity m scale including detrital grains 18o 12 14 high na low mg partially altered white mica 18o 8 9 and late hydrothermal white mica 1 6 low na mid mg the late hydrothermal white mica has similar 18o values to other secondary minerals and is in equilibrium with meteoric dominated fluids with a 18o of 6 to 0 5 which circulated in the late hydrothermal stage downhole towards the lower reservoir white mica from two contact metamorphic micaschist samples shows either 1 homogeneous 18o values of ca 9 likely due to recrystallization in the contact metamorphic hydrothermal stage t ca 600 c or 2 a large spread in 18o from 2 to 12 within and across grains of variable texture and chemistry in the host rock and a cross cutting quartz white mica vein ca 300 c present day temperature hereafter p d t this contrasting 18o signature of white mica is also recorded in granite cored at up to 4 6 km depth the carboli granite contains white mica with a homogeneous magmatic 18o of 10 0 6 whereas older granite samples from radicondoli have magmatic to hydrothermal white micas that vary in 18o from 4 to 10 a pronounced intra grain 18o variability of up to 6 occurs in white mica domains with higher fe mg ti halos around inclusions of chloritized biotite as a result of interaction with dominantly meteoric fluids that infiltrated to depths of at least 4 6 km in the porto azzurro granite on elba italy altered white mica has 18o values of 2 6 down from 10 in unaltered grains the distribution of oxygen isotope ratios in white mica is thus firstly a result of pervasive versus selective fluid alteration at depth sample and grain scale secondly the actual preservation of these m scale variabilities indicates that volume diffusion is not detectable at microscale at p d t at or below 350 c where most of the heterogeneous white mica is found selective sample and grain scale fluid penetration occurs episodically and anisotropically on micro and megascale along faults fractures and cleavages producing lower 18o white mica at various times in zones of higher secondary permeability and active hydrothermal fluid circulation

the rainbow vent fluids 36 14 n mar the influence of ultramafic rocks and phase separation on trace metal content in mid atlantic ridge hydrothermal fluids fluids were collected from the rainbow vent field 36 14 n on the mid atlantic ridge mar during the 1997 diving flores cruise this vent field in ultramafic rocks at a depth of 2300 m is composed of 10 black smokers emitting acidic ph 2 8 fluids at 365 c the low ph of the hot temperature rainbow fluids likely results from seawater ultramafic rock interaction that releases h ions into reducing hydrothermal fluids fluid chemistry is strongly influenced by phase separation generating cl rich brines clem 750 mm strongly enriched with mn fe co ni cu zn ag cd cs pb y and rare earth elements ree ree and transition metal abundance particularly fe and mn in the rainbow fluids is dramatically higher than in other mar fluids the abundance of trace element and ree enrichment is due to the greater solubility of these elements that is strongly favored by cl complexation at low ph and high temperature conditions chondrite normalized ree patterns show strong lree enrichment with evidence of the typical eu anomaly this ree partitioning suggests that either 1 ultramafic rocks represent only a part of rocks leached during hydrothermal alteration and or 2 that the unique rainbow fluid temperature ph and redox state issued from the ultramafic character of leached substratum can produce unique ree partitioning

mat related sedimentary structures in neoproterozoic peritidal passive margin deposits of the west african craton anti atlas morocco abstract proterozoic inliers in the central anti atlas mountains expose predominantly siliciclastic sedimentary successions deposited in peritidal zones along the neoproterozoic continental margin of the west african craton wac the low grade metamorphic and modestly deformed sediments contain a wealth of sedimentary structures related to the former presence and activities of microbial mats and respective physicobiological processes the well preserved structures include wrinkle structures erosion marks microbial sand chips spindle shaped and subcircular microbial shrinkage cracks and possibly gas domes and cabbage head structures thin sections exhibit mat fragments and dispersed grains of hematite limonite after pyrite in fine grained quartzitic storm deposits post storm layers frequently consist of matrix supported sand sized to silt sized grains and are overlain by argillaceous veneers including isolated silt sized grains and black carbonaceous laminae the muddy veneers are considered to represent compacted stacks of microbial mats biolaminites which colonized and biostabilized storm and post storm layers and thus prevented them from eroding in the absence of grazing and burrowing organisms and at suitable depositional and hydrodynamic conditions it may be expected that proterozoic microbial mats extensively grew from the supratidal to the intertidal zones and occasionally e g behind protective barriers in the subtidal zone and beyond mat related structures however need specific conditions for their formation and preservation wrinkle structures erosion marks and microbial sand chips require tractional currents and soon deposition and burial respectively such conditions are preferably met in intertidal and supratidal zones spindle shaped and subcircular cracks require mat shrinkage due to either desiccation or syneresis crack propagation implies progressive shrinkage while superposition of crack generations indicates repeated alternation between mat exposure and flooding respective conditions prevail in the upper intertidal zone gas domes and cabbage head structures are related to the production of gas from decaying organic material beneath a sealing cohesive mat they may be some of the first features formed during genesis of petee structures in the intertidal to lower supratidal zones mat related structures may serve as sensitive facies indicators once their modes of formation are revealed

deep sea record of impact apparently unrelated to mass extinction in the late triassic the 34 million year my interval of the late triassic is marked by the formation of several large impact structures on earth late triassic impact events have been considered a factor in biotic extinction events in the late triassic e g end triassic extinction event but this scenario remains controversial because of a lack of stratigraphic records of ejecta deposits here we report evidence for an impact event platinum group elements anomaly with nickel rich magnetite and microspherules from the middle norian upper triassic deep sea sediment in japan this includes anomalously high abundances of iridium up to 41 5 parts per billion ppb in the ejecta deposit which suggests that the iridium enriched ejecta layers of the late triassic may be found on a global scale the ejecta deposit is constrained by microfossils that suggest correlation with the 215 5 mya 100 km wide manicouagan impact crater in canada our analysis of radiolarians shows no evidence of a mass extinction event across the impact event horizon and no contemporaneous faunal turnover is seen in other marine planktons however such an event has been reported among marine faunas and terrestrial tetrapods and floras in north america we therefore suggest that the manicouagan impact triggered the extinction of terrestrial and marine organisms near the impact site but not within the pelagic marine realm

proliferation of miss forming microbial mats after the late neoproterozoic glaciations evidence from the kimberley region nw australia abstract a total of 20 morphological types of microbially induced sedimentary structures miss are recognized from the late neoproterozoic tillite bearing successions of the kimberley northwestern australia which include alpha petees triradiate cracks multidirectional linear ridges millimeter ripples erosional remnants and pockets and so on of these triradiate cracks multidirectional linear ridges and millimeter ripples are reported for the first time from the neoproterozoic worldwide most kimberley miss are morphologically comparable with their ancient and present day counterparts whose biogenicity have been well demonstrated in the kimberley during the interglacial period miss have colonization preference to relatively coarse sandy substrates ranford formation and their abundance decreased towards the younger strata throssell shale in contrast miss abundance increased from the older strata yurabi formation towards the younger strata flat rock formation during the postglacial period however overall both miss abundance and bedding plane coverage percentage increased from the marinoan younger marinoan gaskiers interglacial successions to younger marinoan gaskiers postglacial successions indicating a proliferation of microbial mats after the younger marinoan gaskiers glaciation the increase of microbial mats could have ameliorated marine biogeochemical conditions and ecosystems and thus laid the foundation for the rise of ediacaran biota in australian basins

patterns of family extinction depend on definition and geological timescale the recent claim1 of an approximate 26 myr periodicity in the pattern of mass species extinction over the past 250 myr has already stimulated much astrophysical speculation and debate on causal mechanisms2 13 however as shown here the evidence for that claim is strongly contingent on arbitrary decisions concerning the absolute dating of stratigraphical boundaries14 the culling of the database and the definition of what is mass extinction as opposed to background extinction this evidence becomes insufficient under other plausible geological timescales and other acceptable definitions of mass extinction analysis of the nonculled database shows that the reliability of identification of mass extinctions and their timing is at present extremely limited it also suggests that the apparent periodicity of mass extinctions results from stochastic processes

upper permian lacustrine oil shales southern junggar basin northwest china upper permian organic rich lacustrine mudstones oil shales that crop out in the southern junggar basin rank among the richest and thickest petroleum source rock intervals in the world with maximum toc values reaching 34 and rock eval pyrolytic yields s 2 up to 200 kg hc t rock lacustrine sedimentary facies define an overall transgressive regressive cycle of approximately 2000 m gross thickness which includes approximately 800 m of source rocks averaging 4 1 toc and 26 2 kg hc t rock basinal facies comprise silicic organic rich laminated lacustrine mudstones and interbedded siltstones organic matter contained in the mudstones ranges in composition from type i to type iii basinal facies were deposited in a deep oxygen deficient stratified lake lake margi facies consist of nonlaminated siliciclastic mudstones rippled dolomitic siltstones and sandstones and minor limestones maximum toc values are approximately 6 desiccation cracks are common in the marginal facies but evaporite minerals are rare or absent biomarker correlation parameters measured from rock extracts exhibit significant stratigraphic variability but strongly support the hypothesis that upper permian lacustrine oil shales charge the giant karamay field in the northwestern junggar basin karamay oils are characterized by high relative abundances of beta carotane this characteristic is restricted to desiccated facies in the outcrop sections however we therefore propose that an abundance of beta carotane indicates elevated environmental salinities during deposition of the oil shales orographic precipitation induced by a paleo tian shan range may have provided fresh water to the southern areas of the basin while more evaporative conditions prevailed to the north outcrop oil seeps confirm that the outcrop sections have attained sufficient thermal maturity to generate oil t max values range from 431 to 451 degrees c vitrinite reflectances range from 0 73 to 0 88 and sterane isomerization ratios range from 0 30 to 0 48 20s 20s 20r matoil registered trademark computer modeling of burial history and thermal maturity of the outcrop sections indicates that approximately 4 5 km of triassic jurassic cover was needed to bring the oil shales to their current maturity the major phase of hydrocarbon generation in this area appears to have preceded the creation of traps by neogene thrusting which may account for the lack of discoveries in the southern junggar basin most of the oil discovered so far is trapped in structures which originated duri g the late paleozoic or mesozoic

randomness in the pattern of mass extinctions and waves of origination mass extinctions and waves of origination are defined as peaks in the average probability of family extinction and origination per geologic stage respectively a simple neutral model is proposed assuming only that the average probabilities of family extinction and origination have varied independently and had equal chances of going up and down from one stage to another as a null hypothesis this model is not rejected by the available palaeontologic data on marine animal and nonmarine invertebrate families in the phanerozoic the only discrepancy between the model and the empirical pattern is due to a systematic bias introduced by fossil lagerstatten the alleged periodicity of late phanerozoic mass extinctions follows inevitably from the model

resolving miss conceptions and misconceptions a geological approach to sedimentary surface textures generated by microbial and abiotic processes the rock record contains a rich variety of sedimentary surface textures on siliciclastic sandstone siltstone and mudstone bedding planes in recent years an increasing number of these textures have been attributed to surficial microbial mats at the time of deposition resulting in their classification as microbially induced sedimentary structures or miss research into miss has developed at a rapid rate resulting in a number of misconceptions in the literature here we attempt to rectify these miss misunderstandings the first part of this paper surveys the stratigraphic and environmental range of reported miss revealing that contrary to popular belief there are more reported miss bearing rock units of phanerozoic than precambrian age furthermore miss exhibit a pan environmental and almost continuous record since the archean claims for the stratigraphic restriction of miss to intervals prior to the evolution of grazing organisms or after mass extinction events as well as claims for the environmental restriction of miss appear to result from sampling bias in the second part of the paper we suggest that raised awareness of miss has come at the cost of a decreasing appreciation of abiotic processes that may create morphologically similar features by introducing the umbrella term sedimentary surface textures of which miss are one subset we suggest a practical methodology for classifying such structures in the geological record we illustrate how elucidating the formative mechanisms of ancient sedimentary surface textures usually requires consideration of a suite of sedimentological evidence from surrounding strata resultant interpretations microbial or non microbial should be couched within a reasonable degree of uncertainty this approach recognizes that morphological similarity alone does not constitute scientific proof of a common origin and reinstates a passive descriptive terminology for sedimentary surface textures that cannot be achieved with the current miss lexicon it is hoped that this new terminology will reduce the number of overly sensational and misleading claims of miss occurrence and permit the means to practically separate initial observation from interpretation furthermore this methodology offers a scientific approach that appreciates the low likelihood of conclusively identifying microbial structures from visual appearance alone informing the search for true miss in earth s geological record and potentially on other planetary bodies such as mars

recognising the taghanic crisis in the devonian terrestrial environment and its implications for understanding land sea interactions the late mid devonian eday marl formation from the orcadian basin scotland is a terrestrial equivalent of the marine devonian taghanic crisis the eday marl from orkney contains a high resolution archive of climatic change controlled by the relative strength of the seasonal insolation this includes several distinct and discrete episodes of basin flooding as demonstrated by the deposition of lacustrine laminites bedded evaporites marginal sheet flood sands and marine influenced bioturbated sheet sands these flooding events are intercalated with intense and sustained episodes of aridity indicating that insolation was relatively weak and the monsoon climate was ineffective at drawing in seasonal rainfall recognition of these basin flooding events has enabled the definition of three new units the wha taing roeberry and berstane members the eday marl formation can be correlated offshore and also recognised within marginal facies as an aeolian intercalation an environmental hydrological and climatic interpretation is given for the different events recognised in the eday marl this sequence of events when combined with the palynological age dates enables a detailed correlation to be made with the marine tully formation of new york state this suggests that regressions forced by climatic aridity are significant for understanding the deposition of the shallow water carbonates of the tully formation in what is otherwise a clastic depositional system comparison with the pelagic marine record shows that the latter as yet gives only limited resolution of the taghanic crisis recognising the taghanic as a rapid alternation of cool arid and relatively hotter pluvial events provides a unifying explanation for the associated collapse in the terrestrial vegetation and the parallel faunal changes in the marine realm

hydrothermal alteration of olivine in a flow through autoclave nucleation and growth of serpentine phases a single pass flow through experiment was conducted to induce serpentinization and to relate serpentine mineralogy to reaction progress during open system alteration of san carlos olivine by air saturated water the fluid flowed at 300 c and 300 bars for 1368 hours through crushed olivine contained in a tubular titanium reaction cell chemical analysis of the fluids indicated a steady state composition with respect to mg and ph after 550 hours which was interpreted to reflect saturation with brucite and one or more serpentine group minerals the alteration products were characterized by x ray diffraction xrd scanning electron microscopy sem and high resolution transmission electron microscopy hrtem nucleation and growth characteristics of serpentine phases on the fracture surfaces of olivine were studied by tem using platinum carbon replica techniques after reaction olivine fragments displayed clear signs of dissolution and growth of neoformed phases brucite and magnetite were distributed uniformly through the reaction cell although magnetite was more abundant there is however a distinct zonal distribution of the serpentine phases from the inlet to the outlet of the reaction cell lizardite is the dominant serpentine phase at the inlet whereas fibrous serpentines are predominant near the outlet lizardite crystals are more numerous but smaller in the middle of the cell and are topotactically aligned along a single preferential crystallographic orientation of the olivine cylindrical chrysotile formed predominantly on the surfaces of lizardite conical or polygonal serpentine the morphology and sorting of the serpentine phases through the reaction cell suggest that crystallization of lizardite was controlled by heterogeneous nucleation on olivine under supersaturated conditions and crystallization of chrysotile by nucleation on lizardite at higher levels of supersaturation

sedimentary record of seismic events in the eocene green river formation and its implications for regional tectonics on lake evolution bridger basin wyoming abstract outcrops and cores from the top of the lacustrine tipton member and the base of the wilkins peak member 51 5 ma of the eocene green river formation bridger basin in southwestern wyoming yield a wide variety of sedimentary deformation features many of which are laterally extensive for more than 50 km they include various types of folds load structures pinch and swell structures microfaults breccias and sedimentary dikes in most cases deformation is represented by hybrid brittle ductile structures exhibiting lateral variation in deformation style these occur in low energy profundal organic rich carbonate mudstones oil shales trona beds tuffs and profundal to sublittoral silty carbonate deposited in paleolake gosiute the deformation is not specific to the depositional environment because sedimentary units stratigraphically higher with similar facies show no deformation the studied interval lacks any evidence for possible trigger mechanisms intrinsic to the depositional environment such as strong wave action rapid sediment loading evaporite dissolution and collapse or desiccation so endogenic causes are ruled out thus the deformation features are interpreted as seismites and change in deformation style and inferred increase in intensity towards the south suggest that the earthquakes were sourced from the nearby uinta fault system the 22 levels exhibiting seismites recognized in cores indicate earthquakes with minimum magnitudes between 6 and 7 minimum epicentral intensity mcs of 9 and varying recurrence intervals in the seismic history of the uinta fault system with a mean apparent recurrence period of 8 1 k y using average sedimentation rates and dated tuffs in detail however there are two noticeably active periods followed by relative quiescence the stratigraphic position of these deformed intervals also marks the transition between two distinct stages in lake evolution from the balanced filled tipton member to the overlying underfilled wilkins peak member thus these seismites are evidence for regional scale changes in lacustrine sedimentation of eocene lake gosiute in response to syndepositional tectonic activity analysis of synsedimentary deformation features is therefore a promising yet under utilized tool to trace the tectonic evolution of lacustrine deposits of the green river formation and other tectonically active marine and non marine basins

30 000 years of hydrothermal activity at the lost city vent field strontium carbon and oxygen isotope data and radiocarbon ages document at least 30 000 years of hydrothermal activity driven by serpentinization reactions at lost city serpentinization beneath this off axis field is estimated to occur at a minimum rate of 1 2 10 4 cubic kilometers per year the access of seawater to relatively cool fresh peridotite coupled with faulting volumetric expansion and mass wasting processes are crucial to sustain such systems the amount of heat produced by serpentinization of peridotite massifs typical of slow and ultraslow spreading environments has the potential to drive lost city type systems for hundreds of thousands possibly millions of years

iron partitioning and hydrogen generation during serpentinization of abyssal peridotites from 15 n on the mid atlantic ridge aqueous dihydrogen h2 aq is produced in copious amounts when seawater interacts with peridotite and h2o oxidizes ferrous iron in olivine to ferric iron in secondary magnetite and serpentine poorly understood in this process is the partitioning of iron and its oxidation state in serpentine although both impose an important control on dihydrogen production we present results of detailed petrographic mineral chemical magnetic and mosbauer analyses of partially to fully serpentinized peridotites from the ocean drilling program odp leg 209 mid atlantic ridge mar 15 n area these results are used to constrain the fate of iron during serpentinization and are compared with phase equilibria considerations and peridotite seawater reaction path models in samples from hole 1274a mesh rims reveal a distinct in to out zoning from brucite at the interface with primary olivine followed by a zone of serpentine brucite magnetite and finally serpentine magnetite in the outermost mesh rim the compositions of coexisting serpentine mg 95 and brucite mg 80 vary little throughout the core about 30 50 of the iron in serpentine brucite mesh rims is trivalent irrespective of subbasement depth and protolith harzburgite versus dunite model calculations suggest that both partitioning and oxidation state of iron are very sensitive to temperature and water to rock ratio during serpentinization at temperatures above 330 c the dissolution of olivine and coeval formation of serpentine magnetite and dihydrogen depends on the availability of an external silica source at these temperatures the extent of olivine serpentinization is insufficient to produce much hydrogen hence conditions are not reducing enough to form awaruite at t 330 c hydrogen generation is facilitated by the formation of brucite as dissolution of olivine to form serpentine magnetite and brucite requires no addition of silica the model calculations suggest that the iron distribution observed in serpentine and brucite is consistent with formation temperatures ranging from 150 to 250 c and bulk water to rock ratios between 0 1 and 5 these conditions coincide with peak hydrogen fugacities during serpentinization and are conducive to awaruite formation during main stage serpentinization the development of the common brucite rims around olivine is either due to an arrested reaction olivine brucite serpentine brucite or reflects metastable olivine brucite equilibria developing in the strong gradient in silica activity between orthopyroxene talc serpentine and olivine serpentine brucite

the pliensbachian and tithonian extinction events the claim by raup and sepkoski1 that an approximately 26 myr mass extinction periodicity can be recognized on a global scale at family level over the past 250 myr has excited much attention and provoked speculation about an ultimate extraterrestrial cause2 10 while the purported periodicity can be challenged on grounds of both the inaccuracy of the geological timescale11 and the method of extinction analysis12 there can be no doubting the reality of at least some of the 12 proposed extinction events such as those at or near the end of the permian triassic and cretaceous others are less widely acknowledged further analysis by raup and sepkoski13 confirms the importance of two of these events within the jurassic at the end of the pliensbachian and tithonian stages i demonstrate here that marine invertebrate mass extinctions at approximately these times occurred on a regional not a global scale they can be related to severe reductions in habitat area provoked either by regression of epicontinental sea or by widespread anoxia

the end triassic bivalve extinction event one of the most important episodes of mass extinction of animal groups in the whole of the phanerozoic took place in the latter part of the triassic but the time of major faunal turnover of the vertebrates preceded that of marine invertebrates by several million years as shown by ammonites brachiopods and conodonts decline from a carnian early norian diversity peak was followed by a significant extinction episode at the end of the period this pattern is confirmed at generic and subgeneric level by the bivalve molluscs probably the most abundant and diverse macroinvertebrate group this paper analyses the bivalves in detail and attempts to seek a relationship of the extinction event to environmental changes as recorded by late triassic facies stratigraphy of the youngest triassic is briefly discussed following a recent recommendation the rhaetian stage is dropped and the upper norian sevatian subdivided into three new ammonite zones it is suggested that the classic northwest european rhaetic correlates with the kossen and zlambach beds of the alps and corresponds at least approximately with the upper two of these zones it is estimated that nearly half the existing bivalve genera and nearly all the species failed to survive the end triassic extinction event the trigonioid unionoid and hippuritoid orders were especially strongly affected but most of the pholadomyoid genera survived three major ecological categories are distinguished a shallow neritic group to which most taxa belong a euryhaline group that could tolerate marginal marine and lagoonal conditions and a deep neritic group notably halobia and monotis all groups were strongly affected by the extinction but species of the deep neritic group were apparently the most susceptible seven biogeographic regions are distinguished for the purpose of analysis and it is shown that provinciality is slight but some important taxa are endemic to the tethyan and pacific regions endemic genera prove to have been much more subject to extinction than cosmopolitans there are very few places in the world with a more or less continuous marine sequence of deposits across the triassic jurassic boundary details are given of sections in the alps northwest europe and nevada and briefer mention made of others with regard to late triassic palaeogeographic changes those involving climate were neglibible but there was a more or less progressive regression of epicontinental seas following a ladinian early carnian transgressive maximum with a corresponding shallowing in the more continuously marine tethyan region that this first order effect was due to a eustatic lowering of sea level is fairly certain but the situation is less clear for second order transgressive regressive cycles the late late norian rhaetian cycle is probably however the results of a minor eustatic event by the end of the triassic the sea had regressed substantially and the mass extinction event is though to be bound up partly with this phenomenon and partly with the phase of widespread anoxicity that occurred during the succeeding hettangian transgression

more than one event in the late triassic mass extinction the recent hypothesis that mass extinctions are discrete phenomena that have occurred with great regularity during the history of life1 2 is testable in several ways two essential elements of the hypothesis are 1 that each extinction event represents a significant departure from normal or background rates of extinction and 2 that such mass extinction events are spaced equally in time the analyses of the cyclicity of mass extinctions so far have concentrated on the past 250 myr with the first event occuring at the permian triassic boundary 245 myr ago1 5 the second event followed 26 33 myr later1 6 in the late triassic here i present a detailed analysis of the fossil record of marine and non marine life during the late triassic which suggests that there were at least two phases of mass extinction during that time separated by 12 17 myr

milankovitch cyclicity recorded from devonian non marine sediments hydrographically closed basins are sensitive to environmental change at all scales the influence of climate being particularly strong some devonian basins contain non marine successions which appear to represent the deposits of such basins climatic fluctuations may be recorded by sediments in the form of milankovitch band cyclicity two novel records of climatically related cyclicity are discussed well 12 13 1 in the north sea penetrates a fine grained old red sandstone succession which is thought to be comparable to the largely lacustrine middle devonian deposits of the orcadian basin which outcrop onshore the gamma ray log through the devonian interval reveals periodic large peaks which correlate with u rich fine grained intervals deposited during periods of greatest lake expansion the peaks are cyclical power spectrum of the gamma ray data reveal periodicities of c 8 and 40 m which are thought to correlate with the 21 000 yr precession and 100 000 yr eccentricity milankovitch cycles the sherkin sandstone formation of the munster basin sw ireland is a thick 1000 m succession of largely alluvial deposits which were deposited by a terminal fan network detailed analysis of channel body sandstones and sedimentary structures contained within them reveals a cyclical variation in the dimensions of the bedforms camed by the channels the same cyclicity is evident in the proportion of channel deposits this variation reflects fluctuations in palaeodischarge probably related to climatic variation the main cycles are approximately 130 m thick spectral analysis reveals two further strong periodicities at approximately 36 and 55 m estimated sediment accumulation rates suggest that the cycles represent periods of the order of lo4 years it is proposed that the longer cycles reflect the 412 000 yr orbital eccentricity cycle and that the shorter cycles are either harmonics or components of the 100 000 yr eccentricity cycle

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

palaeogeography of neogene red bed sequences along the aksa ata river in the parkent nurekata intermontane basin tien shan mountains uzbekistan with special reference to the magnetic susceptibility of siliciclastic rocks abstract siliciclastic red beds formed by stream action in the parkent nurekata intermontane basin of the tien shan mountains e uzbekistan the sedimentary rocks were cross sectioned in the field and subject to heavy mineral clay mineral and thin section analyses in the laboratory in addition to these conventional methods a survey into the variation of the magnetic susceptibility was conducted during the paleogene calcareous rocks unit a developed in a subtidal through supratidal environment of deposition subsequently tectonic disturbances tilted the paleogene carbonate rocks and triggered the formation of an unconformity during the oligocene the miocene to pliocene red beds have the hallmarks of a delta like wedge actively prograding over near shore marine deposits two progradational stages units b and c were substituted for by a more regressive event unit d during deposition and burial the siliciclastics were percolated by strongly alkaline pore fluids under semi arid climatic conditions the joint action of rapid uplift in the hinterland and the afore mentioned hydrological processes were instrumental for heavy minerals such as epidote and staurolite to survive and some peculiar phyllosilicates such as smectite and palygorskite to precipitate debris from metamorphic and igneous rocks were delivered from the rising hinterland the petrographic and magnetic records of red beds are a mirror image of the unroofing of a magmatic edifice in the provenance area between 11 and 3 ma

detachment shear zone of the atlantis massif core complex mid atlantic ridge 30 n near bottom investigations of the cross section of the atlantis massif exposed in a major tectonic escarpment provide an unprecedented view of the internal structure of the footwall domain of this oceanic core complex integrated direct observations sampling photogeology and imaging define a mylonitic low angle detachment shear zone dsz along the crest of the massif the shear zone may project beneath the nearby corrugated upper surface of the massif the dsz and related structures are inferred to be responsible for the unroofing of upper mantle peridotites and lower crustal gabbroic rocks by extreme localized tectonic extension during seafloor spreading over the past 2 m y the dsz is characterized by strongly foliated to mylonitic serpentinites and talc amphibole schists it is about 100 m thick and can be traced continuously for at least 3 km in the tectonic transport direction the dsz foliation arches over the top of the massif in a convex upward trajectory mimicking the morphology of the top of the massif kinematic indicators show consistent top to east toward the mar axis tectonic transport directions foliated dsz rocks grade structurally downward into more massive basement rocks that lack a pervasive outcrop scale foliation the dsz and underlying basement rocks are cut by discrete anastomosing normal slip shear zones widely spaced steeply dipping normal faults cut all the older structures and localize serpentinization driven hydrothermal outflow at the lost city hydrothermal field a thin few meters sequence of sedimentary breccias grading upward into pelagic limestones directly overlies the dsz and may record a history of progressive rotation of the shear zone from a moderately dipping attitude into its present gently dipping orientation during lateral spreading and uplift

serpentinization of mantle derived peridotites at mid ocean ridges mesh texture development in the context of tectonic exhumation at slow spreading ridges axial detachment faults exhume mantle derived peridotites and hydrothermal alteration causes serpentinization in a domain extending more than 1 km next to the fault at the microscopic scale serpentinization progresses from a microfracture network toward the center of olivine relicts and forms a mesh texture we present a petrographic study sem ebsd and raman of the serpentine mesh texture in a set of 278 abyssal serpentinized peridotites from the mid atlantic and southwest indian ridges we show that serpentinization initiated along two intersecting sets of microfractures that have consistent orientations at the sample scale and in at least one studied location at the 100 m scale we propose that these microfractures formed in fresh peridotites due to combined thermal and tectonic stresses and subsequently served as channels for serpentinizing fluids additional reaction induced cracks developed for serpentinization extents 20 the resulting microfracture network has a typical spacing of 60 m but most serpentinization occurs next to a subset of these microfractures that define mesh cells 100 400 m in size apparent mesh rim thickness is on average 33 19 m corresponding to serpentinization extents of 70 80 published laboratory experiments suggest that mesh rims formation could be completed in a few years i e quasi instantaneous at the plate tectonic timescale the depth and extent of the serpentinization domain in the detachment fault s footwall are probably variable in time and space and as a result we expect that the serpentine mesh texture at slow spreading ridges forms at variable rates with a spatially heterogeneous distribution

the interface scale mechanism of reaction induced fracturing during serpentinization peridotite serpentinization has first order effects on geochemical and petrophysical processes in the lithosphere this process induces intensive fracturing generating fluid pathways to facilitate the hydration of vast amounts of originally impermeable rocks but the mechanism linking interfacial reaction processes with fracture propagation has not been understood by combining microstructural characteristics of olivine lizardite serpentinization with fundamental aspects of interface coupled dissolution precipitation and crack growth theory we propose a microstructurally consistent self propagating fracturing mechanism fracturing is driven by stress generated from the growth and transformation of a metastable amorphous proto serpentine phase where stress is localized within surface perturbations etch pits and coalesced etch pits that originate from the anisotropic dissolution of olivine water migration into fractures reiterates the process resulting in hierarchical olivine grain segmentation our results indicate that the advancement of serpentinization at the grain scale is independent of solid state diffusion and does not rely on external forces

hydrogen generation from mantle source rocks in oman hydrogen gas associated with ca2 oh rich alkaline groundwaters ph 10 12 is at present emanating from ultramafic rocks of the oman ophiolite isotopic and chemical evidence indicates that hydrogen is formed by low temperature redox reactions in a closed groundwater environment this is normally a cryptic process in the hydrosphere but is fortuitously revealed by the unusual hydrogeological conditions in oman where atmospheric oxygen is totally assimilated it is suggested that hydrogen generation in mantle source rocks at depth and in the early archaean surface environment may be more widespread than has hitherto been realised

formation of the axial relief at the very slow spreading southwest indian ridge 49 to 69 e the comparison of segment lengths relief and gravity signature along the very slow spreading southwest indian ridge swir between 49oe and 69oe suggests that the marked change in segmentation style that occurs across the melville transform 60o45 e reflects a change in the modes of formation of the axial topography we propose that the axial relief east of melville is largely due to volcanic constructions that load the axial lithosphere from above by contrast the axial relief in segments west of the melville fracture zone appears to be primarily due as proposed for segments of the faster spreading mid atlantic ridge to along axis changes in the depth of the axial valley and to partial compensation of negative loads thicker lower crust and or lighter upper mantle acting within the plate or at the bottom of the plate in terms of geology this means that the contribution of the uppermost effusive part of the crust to along axis crustal thickness variations may be greater east of melville than in other regions of the study area regional axial depths suggest that the ridge east of melville is also characterized by a low melt supply and is underlain by cold mantle a simple model of mantle melting and regional isostatic compensation suggests that differences in mantle temperature and in melt thickness between this deep eastern ridge region and the shallower region west of the gallieni transform 52o20 e are of the order of 80oc and 4 km respectively

subsidence structures associated with subaerial desiccation crack piping and their role in drainage evolution on a drained proglacial lake bed hagavatn iceland abstract a range of subsidence structures associated with desiccation crack piping are described from the drained proglacial lake bed of hagavatn iceland subsurface drainage along desiccation cracks leads to the formation of small scale subsidence structures these subsidence structures include linear fissures located along the limbs of desiccation cracks and crater like depressions that form at crack intersections the controls on subsurface piping are discussed and the role of these subsidence structures in determining the pattern of surface drainage on the former lake bed is identified preservation potential of these sedimentary structure is considered to be high given renewed lacustrine sedimentation the potential for similar structures to be found in the geological record is considered likely and consequently these structures may be of importance in the interpretation of crack networks in the geological record as being of subaerial as opposed to subaqueous origin

serpentine mineral replacements of natural olivine and their seismic implications oceanic lizardite versus subduction related antigorite we report on microstructural data obtained by optical microscopy and transmission electron microscopy concerning the crystallographic relationships of serpentine minerals with their host olivine in two contrasting situations in the first case mesh textured lizardite liz is developed in a standard 60 serpentinized oceanic harzburgite from the oman ophiolite where olivine converts to columnar lizardite the joined columns are perpendicular to the basal plane 001 liz corresponding to the pseudofibres observed optically the plane 001 liz is locally parallel to the narrow boundary ol liz thus column orientations register the interface of serpentinization the ol liz relationships are not strictly topotactic but reflect preferred cracking orientations in olivine parallel to 010 ol in the second case antigorite atg develops in a rare sample of antigorite schist in a kimberlite from moses rock colorado plateau representative of a suprasubduction zone mantle wedge high resolution transmission electron microscopy hrtem images along 010 atg show domains of very regular modulation with a 43 center dot 5 a wavelength m 17 where m is the number of silicate tetrahedra along the wave with few defects indicative of hp ht antigorite and also heavily kinked regions as fingerprints of strong tectonic shear tem imaging and electron diffraction patterns reveal two topotactic relationships between antigorite and olivine 100 atg 010 ol and atg ol the planes in contact are 001 atg 100 ol and 001 atg 010 ol respectively the 010 atg 001 ol and antigorite lamellae are parallel to the forsterite b axis in both cases the topography of olivine serpentine interfaces is controlled by open fluid pathways along microcracks oriented according to the anisotropy of the olivine aggregate in the cases studied the serpentine aggregate exhibits a preferred orientation inherited from that of the peridotite these results have some relevance to the seismic anisotropy of serpentinized mantle anisotropy of propagation of seismic waves as a result of the olivine fabric is maintained and reinforced with the development of lizardite conversely the development of antigorite produces a trench parallel fast s wave polarization and an anisotropy that is lowered at low degrees of serpentinization and then increased with increasing serpentinization

segmentation and melt supply at the southwest indian ridge the segmentation of mid ocean ridge axes and its variation with spreading rate provide constraints on models of mantle upwelling and the supply of basaltic melt to the crust we present seismic refraction results from three segments of the very slow spreading southwest indian ridge from the absence of oceanic layer 3 beneath segment boundaries and from significant variations in maximum crustal thickness between segments we infer that melt generation and or delivery is focused at segment midpoints and varies between segments anomalously thin crust 2 0 2 5 km beneath the nontransform segment boundaries and 3 5 6 0 km beneath the segment mid points is caused by restricted melt generation resulting from conductive heat loss from the upwelling mantle under the spreading center

reaction induced porosity and onset of low temperature carbonation in abyssal peridotites insights from 3d high resolution microtomography abstract in a drillcore sample of serpentinized harzburgite from the uppermost oceanic crust mid atlantic ridge odp leg 209 site 1270 we demonstrate using high resolution 3d microtomography that micron sized open cavities are present the development of porosity is interpreted to result from dissolution of brucite and or olivine petrographic observations indicate that voids are integrated in a network of carbonate veins the formation of which is linked to changing alkalinity in conjunction with dissolution reactions partial carbonate filling of pore spaces indicates that under static conditions low temperature carbonation leads to clogging of fluid pathways and thus to a reduction in permeability electron microprobe analyses show that the inner walls of open voids are lined with fe rich precipitates we propose that the iron in those phases was released by brucite or olivine dissolution and was subsequently oxidized and precipitated as ferric hydroxide thermodynamic computations show that this process may be a potential source of catabolic energy for microorganisms inhabiting serpentinites the proposed carbonation mechanism implies that carbonate precipitation may start soon after exposure of the abyssal peridotites when dissolution of brucite and weathering of olivine begin and continue until the phases become inaccessible to seawater predicting carbonation rates of abyssal peridotites will hence require understanding of permeability reactions

periodicity of extinctions in the geologic past deterministic versus stochastic explanations the temporal spacing and the magnitude of major extinctions over the past 250 and 570 million years based on the use of different metrics of extinction probability are analyzed by comparing deterministic and stochastic explanations the best fitting time series model is a stochastic autoregressive model that displays a pseudoperiodic behavior with a cycle length of 31 million years for the past 250 million years regardless of the metric of extinction probability the periodicity lengthens and weakens when the analysis is extended to the entire phanerozoic the history of the probability of extinction for the entire phanerozoic based on time series analysis does not support the reported bipartite distribution of van valen rather the probability of extinction has decreased uniformly over phanerozoic time whereas the inertia or stability of the biotic system after the late permian crisis has increased

climatic and biotic change in the late triassic generally arid conditions that pervaded much of europe and north america during the late triassic were interrupted by a wet monsoonal climatic phase during middle and late carnian times extensive fluviatile sandstones deposited at this time throughout the region occur within a thick sequence of playa lake mudstones the sandstones occasionally contain kaolinite suggesting a humid climate an extreme 13 c depletion in a shallow marine sequence of this age in israel has been interpreted as evidence for an influx of freshwater a widespread change from carbonates to clastics in marine sequences at this time may also be climate related water course cave systems in limestone areas exposed during the late triassic indicate high levels of runoff during the middle and late carnian the marine invertebrate fauna shows a significant turnover at the end of the early carnian the terrestrial fauna and flora were relatively unaffected at this time but subsequently diversified prior to a major biotic turnover at the carnian norian boundary these periods of biotic change appear to be synchronous with the onset and cessation of a carnian humid phase the change to a monsoonal climate during this interval has been documented over more than 90 of longitude between 5 and 50 north of the triassic equator it may have been caused by rising atmospheric co 2 levels due to volcanism associated with the incipient dispersal of pangaea

magnetic properties of variably serpentinized abyssal peridotites we have compiled new and published data on the magnetic properties of 245 serpentinized abyssal peridotites from seven deep sea drilling project and ocean drilling program sites the magnetic susceptibility k of these samples does not increase linearly with the degree of serpentinization s instead k remains modest in partially serpentinized samples s 75 peridotites in the oceanic crust moderately serpentinized peridotites s 75 have nrm values 5 a m and k 0 05 si low temperature oxidation of magnetite is found to lower the nrm and k values of seafloor samples and of cataclastically deformed drilled samples

aluminum speeds up the hydrothermal alteration of olivine the reactivity of ultramafic rocks under hydrothermal conditions controls chemical fluxes at the interface between the internal and external reservoirs of silicate planets on earth hydration of ultramafic rocks is ubiquitous and operates from deep subduction zones to shallow lithospheric environments where it considerably affects the physical and chemical properties of rocks and can interact with the biosphere this process also has key emerging societal implications such as the production of hydrogen as a source of carbon free energy to date the chemical model systems used to reproduce olivine hydrothermal alteration lead to the formation of serpentine with sluggish reaction rates here we use in situ diamond anvil cell experiments and show that the presence of aluminum in hydrothermal fluids increases the rate of olivine serpentinization by one to two orders of magnitude aluminum increases the solubility of olivine and enhances the precipitation of aluminous serpentine after two days olivine crystals were fully transformed to aluminous serpentine under conditions typical for natural hydrothermal environments i e 200 and 300 c 200 mpa this result motivates a re evaluation of the natural rates of olivine serpentinization and of olivine hydrolysis in general in a wide range of settings this discovery also opens the potential of the serpentinization reaction for industrial scale production of hydrogen at economically feasible timescales and temperature

devonian marine incursions into the orcadian basin scotland bp chevron well 14 6 1 drilled in the east orkney basin has penetrated devonian sedimentary rocks which can be dated palynologically as within the latest givetian to early frasnian interval this microflora contains the first substantiated record in western europe of archaeoperisaccus a spore hitherto regarded as restricted to northern laurasia three grey green intervals within the well contain marine microfossils which provide the first direct evidence for the movement of marine waters into the northern part of the old red sandstone continent onshore an eday marls section in the bay of berstane orkney contains a marine microfauna which demonstrates the presence of a marine incursion of mid late givetian age this is the first record of a devonian marine incursion in scotland combination of this palaeontological data with sedimentological studies of the eday marls and upper old red sandstone suggests that a wide flat sabkha plain extended eastwards from the onshore orcadian basin during the later stages of its infill and was subjected to episodic marine incursions these incursions are tentatively linked with marine transgressions into the argyll area of the central north sea two orders of cyclicity occur within the dated devonian interval of 14 6 1 and are interpreted as resulting from orbital cyclicity at periodicities of 39 5 and 413 9 ky this indicates that the control on the devonian sea level maxima may have been orbital cyclicity these marine incursions are speculatively correlated to high stands on the devonian sea level curve the bay of berstane marine incursion representing the taghanic onlap whilst those in 14 6 1 are the rhinestreet middlesex and genundewa transgressions reconsideration of the palaeogeography of the northern margin to the marine devonian indicates that the sea entered the orcadian basin from the east along the tornquist zone at the margin of the fenno scandian high