shell beds as tools in basin analysis the jurassic of kachchh western india skeletal concentrations are ubiquitous in bathonian oxfordian shallow water sediments of the pericratonic basins of kachchh and rajasthan western india by analysing the biofabric of the concentrations and the taphonomic signatures of individual components it is possible to distinguish between the final concentration process recorded by the biofabric and events taking place prior to final deposition recorded by taphonomic signatures the skeletal concentrations are grouped in nine genetic types ranging from fair weather wave concentrations to storm concentrations and condensed concentrations they mainly reflect biological and hydraulic processes and in some instances permit estimates of their duration and show a rough zonation along a bathymetric gradient plotted against the stratigraphic sections the skeletal concentrations show in accordance with other sedimentological data three orders of bathymetric trends the first two orders are shallowing deepening cycles which possibly correspond to eustatic changes in sea level the third order represents parasequences with a strongly asymmetric sedimentary record skeletal concentrations are a useful additional tool in basin analysis but are best used in combination with other data

ecologic and taxonomic diversification in the mesozoic brackish water bivalve faunas in japan with emphasis on infaunalization of heterodonts abstract mesozoic brackish water bivalve faunas in japan diversified in three steps at the beginning of the early jurassic early and late cretaceous the hettangian niranohama fauna in northeastern honshu represents the establishment of a heterodont dominated brackish water fauna that persisted until the early late cretaceous no similar composition is known from the triassic the infauna consists mostly of non siphonate and some short siphonate heterodonts while the epifauna is represented by diverse pteriomorphian families in the early cretaceous tetori group in central honshu the long siphonate heterodonts tetoria corbiculidae and the semi infaunal soft bottom oyster crassostrea appeared the evolutionary diversification of the latter known as the most important element of modern brackish water faunas may thus originate at that time in the early late cretaceous cenomanian of the goshoura and mifune groups in west kyushu several euryhaline deep burrowing heterodont families such as veneridae and tellinidae further diversified in the brackish and marine environments the late cretaceous is characterized by massive shell biolithic beds in which large crassostrea species are common a feature common for cenozoic brackish water faunas the long term changes in the composition of the brackish water faunas in japan represents thus an evolutionary record irrespective of the severe physiological and environmental conditions imposed on the highly conservative nature of the fauna

palaeoecology and evolution of marine hard substrate communities abstract marine organisms have occupied hard substrates since the archaean shells rocks wood and sedimentary hardgrounds offer relatively stable habitats compared to unconsolidated sediments but the plants and animals which inhabit them must develop means to gain and defend this premium attachment space hard substrate communities are formed by organisms with a variety of strategies for adhering to and or excavating the substrates they inhabit while mobile grazers organically attached and even soft bodied organisms may leave evidence of their former presence in ancient hard substrate communities a superior fossil record is left by sessile encrusters with mineralised skeletons and by borers which leave trace fossils furthermore encrusters and borers are preserved in situ retaining their spatial relationships to one another and to the substrate spatial competition ecological succession oriented growth and differential utilisation of exposed vs hidden substrate surfaces can all be observed or inferred hard substrate communities are thus excellent systems with which to study community evolution over hundreds of millions of years here we review the research on modern and ancient hard substrate communities and point to some changes that have affected them over geological time scales such changes include a general increase in bioerosion of hard substrates particularly carbonate surfaces through the phanerozoic this is at least in part analogous to the infaunalisation trends seen in soft substrate communities encrusting forms show an increase in skeletalisation from the palaeozoic into the mesozoic and cenozoic which may be a response to increasing levels of predation hard substrate communities considering borers and encrusters together show a rough increase in tiering through the phanerozoic which again parallels trends seen in soft substrate communities this extensive review of the literature on living and fossil hard substrate organisms shows that many opportunities remain for large scale studies of trends through time at the community and clade levels palaeontologists will especially benefit by closer integration of their work with that of neontologists particularly in aspects of ecology such as larval recruitment competition and succession

the exaerobic zone a new oxygen deficient marine biofacies classical biofacies models1 2 for reconstructing palaeoenvironments of strata deposited in oxygen deficient marine settings define three principal facies aerobic 1 0 ml 1 1 o2 dysaerobic 1 0 to 0 1 ml 1 1 o2 and anaerobic 0 1 ml 1 1 o2 zones these models have postulated a decrease in organism size and degree of calcification as well as a drastic reduction in the relative percentage of fauna possessing calcified skeletons as the dysaerobic anaerobic boundary is approached through use of evidence independent of that provided by body fossils we demonstrate here that in portions of the monterey formation miocene california the bivalve anadara montereyana occurs in situ almost exclusively in strata deposited at the dysaerobic anaerobic boundary the occurrence of fossils of large well calcified benthic macroinvertebrates at this redox boundary contradicts classical biofacies models1 2 and there fore provides the basis for definition of a new oxygen related biofacies the exaerobic zone the exaerobic zone concept when adjusted for differences in basin configuration and palaeoceano graphic conditions provides a potentially useful model for explain ing occurrences of shelly benthic fossils within laminated organic rich strata of other phanerozoic marine sequences

an early jurassic oligohaline ostracod assemblage within the marine carbonate platform sequence of the venetian prealps ne italy boomer i whatley r bassi d fugagnoli a loriga c 2001 an early jurassic oligohaline ostracod assemblage within the marine carbonate platform sequence of the venetian prealps ne italy palaeogeography palaeoclimatology palaeocology 166 3 4 331 344

taphonomy and palaeoecology of high stress benthic associations from the upper jurassic of asturias northern spain abstract the late kimmeridgian terenes formation exposed on the coast of asturias northern spain displays a complex pattern of directed changes of grain size carbonate content and skeletal concentrations in its upper part here investigated the formation represents a protected shelf lagoon in which four major facies types are distinguished the nanogyra virgula mudstone 1 and the corbulomima concentrations 2 are characterized by concentrations of small bivalves the carbonate mudstone 3 contains pseudomorphs after gypsum crystals and thin crusts of gypsum occasionally in connection with thin microbial layers finally the silty to fine sandy marlstone and micrite and marly silt 4 is highly bioturbated and contains a moderately diverse benthic macrofauna these facies indicate a generally quiet environment punctuated by brief episodes of high water energy two low diversity macrobenthic assemblages can be recognized each of them strongly dominated by a single bivalve taxon the nanogyra virgula assemblage exhibits a higher diversity than the near monospecific corbulomima assemblage the former lived in well aerated waters of slightly reduced salinity and the latter in dysoxic waters of more strongly reduced salinity the environmental stress responsible for the extremely low species richness and evenness is thought to be multifactorial produced by reduced salinity dysoxic conditions and a soft substrate and resulted in simple food chains the eurytopic opportunist corbulomima was the only element of the shelly macrobenthos that was able to thrive in the shelf lagoon under these conditions it occurs in countless mm to cm thick pavements and shell beds which show evidence of winnowing influence of weak currents and occasionally of distal storms as can be deduced from the orientation pattern of shells these rhythmic corbulomima concentrations are explained as reflecting small scale climatic fluctuations between wetter stormier conditions leading to mixing of the water masses and enabling colonization of the lagoonal floor by the bivalve and drier more tranquil conditions the latter resulted in a stratified water mass and anoxia at the bottom superimposed on this rhythmic alternation are three higher orders of cycles which are partly climatic controlled partly reflect changes in relative sea level

subsidence and sedimentation on jurassic passive continental margin southern alps italy the southern alps of italy preserve a tectonically intact array of jurassic facies that record the evolution of a part of the margin of the apulian plate from its ancestral beginnings in a complex of permian and triassic rifted continental basins through the initial stages of breakup and stepwise foundering of a carbonate platform breakup was accompanied first by the rapid accumulation of thick prisms of carbonate turbidites in newly formed fault troughs then as the new ligurian oceanic basin began to open farther west and as subsidence gradually slowed accumulation of a succession of slowly deposited biogenous pelagic sediments recorded not only the increasing depths of the seafloor but also fluctuations in oceanographic conditions of fertility carbonate dissolutio levels and the strength of bottom currents estimates of the history of seafloor depths based on a simple subsidence law of the form subsidence k age 12 provide a basis for the construction of a set of curves showing the changing depths of significant carbonate dissolution surfaces during the jurassic in this region the rapid 1 km deepening of the compensation depth for calcite during the late jurassic may be due to a change in regional oceanic vertical circulation patterns from upwelling fertile silica rich carbonate dissolving to downwelling less fertile silica poor carbonate preserving

climatic ups and downs in a disturbed jurassic world the tropical warm and equable climate of the jurassic world is regularly challenged by geoscientists especially since oxygen isotopes 18 o of fossil hardparts have been used to reconstruct the paleotemperature history of seawater by applying the innovative sizer sig nifi cant zero crossings of the derivatives statistical approach to a newly compiled 18 o data base for the jurassic we demonstrate the occurrence of major and multiscale 18 o changes mainly related to climate disturbances for the fi rst time two long term anomalies in 18 o are identifi ed during the toarcian and the late jurassic in conjunction with intensive volcanism in large igneous provinces these results support a strong infl uence of repeated volcanic pulses on the modulation of pco 2 temperatures and polar ice cap volumes over protracted periods at shorter time scales 13 relatively rapid 0 5 1 m y and signifi cant warming and cooling events are identifi ed the causes of which include transient fl uctuations in greenhouse gas con centrations related to still debated mechanisms

trophic regimes of benthic foraminiferal assemblages in lower jurassic shallow water carbonates from northeastern italy calcari grigi trento platform venetian prealps abstract benthic foraminiferal distribution patterns in the lower jurassic rotzo member trento platform northeastern italy are analysed in terms of trophic gradients to provide a better understanding of the benthic communities during the time of the initial diversification of lower jurassic large foraminifera in the southern alps the rotzo member deposited on the trento platform records a complex mosaic of distinct shallow water ecosystems the changes in the foraminiferal assemblages of the rotzo member are considered to be primarily a reflection of gradients in trophic resources and oxygen levels the comparative analysis of benthic foraminiferal assemblages characteristic taxa number of species test size reproduction strategy the associated sediments lithofacies and the macrofossil and microfossil content allow three trophic regimes to be identified during the time of deposition of the rotzo member a eutrophic b mesotrophic and c oligotrophic a the eutrophic regime marked by episodically dysaerobic bottom conditions and high organic matter supply occurs stratigraphically below and within the orbitopsella spp assemblage sinemurian pro parte carixian gradual change in the benthic foraminiferal assemblage may have been caused by a combination of reduced nutrient supply and increased bottom water circulation which led to a mesotrophic regime b the latter which has been recognised in the basal and middle part of the rotzo member alternating with the eutrophic assemblage is characterised by a foraminiferal assemblage dominated by orbitopsella sp a major foraminiferal faunal turnover across the carixian domerian boundary corresponding to the disappearance of orbitopsella spp indicates a significant environmental change an oligotrophic regime c became established towards the upper part of the rotzo member new taxa appeared and diversity of the foraminiferal faunas was higher than in the previous ecosystems this part of the member characterised by the maximum lateral and vertical development of large lithiotis bivalve mounds reflects low and more stable nutrient supply which provided optimal conditions for the more specialised benthic communities evolution of these benthic foraminiferal faunas from the late pliensbachian onwards was obliterated by events that led to the drowning of the trento platform which is marked by a regional unconformity

taphonomic windows and molluscan preservation recent studies on silicified fossil biotas have suggested that substantial skewing of the molluscan record resulted from early aragonite dissolution in mid outer carbonate ramp settings if those rare skeletal lagerstatten are representative then the quality and completeness of the molluscan record are thrown into doubt yet database studies suggest that the bivalve fossil record is actually relatively complete if so then biodiversity must be captured by other processes that preserved shells vulnerable to early dissolution and which operated on a relatively high frequency i e less than the species duration for bivalves storm beds shell plasters and submarine hardgrounds are identified as fossil deposits that can preserve the labile aragonitic component of the fauna and thus represent potential taphonomic windows many storm event beds include rich accumulations of shelly benthos differences between storm bed faunas and those of the background facies could reflect transportation effects however some storm bed assemblages are rich in originally aragonitic infaunal bivalves that are not represented in background facies or more proximal shelf equivalents and here rapid burial and removal of organic matter by winnowing may be the keys to aragonite shell preservation despite palaeozoic to cenozoic changes in the thickness and frequency of shell beds that reflect the predominant bioclast producers shallow infaunas are commonly concentrated together with epifauna in such deposits some low energy organic rich mud dominated settings are associated with preservation of aragonitic molluscs infaunal bivalves are a prominent component of shell plasters or pavements in such settings linked to episodic bottom water anoxia decaying algal blooms drew the redox boundary up above the sediment water interface and brought populations of infaunal bivalves to the surface where they died isolated from the oxic taphonomically active zone the shells were not dissolved and were buried as thin shell layers in similar settings aragonitic shells were preserved as moulds through early pyritisation or even through preservation of original shell aragonite in oxic environments bioturbational reworking of surface sediment destroyed moulds of aragonitic shells after early dissolution in some hardgrounds these delicate moulds were preserved due to synsedimentary cementation probably using carbonate released by aragonite dissolution the examples included here come from both intervals of calcite and aragonite seas and it is not possible to assess whether the saturation state with respect to aragonite of the ambient sea water played a role in the selective removal of aragonitic shells while taphonomic windows may have captured the diversity of individual groups it is clear from quantitative data involving skeletal lagerstatten that the scale of loss from early aragonite dissolution has drastically altered the trophic composition of some fossil assemblages commonly used as the basis for reconstructions of past communities

genesis and environmental significance of upper cretaceous shell concentrations from the cauvery basin southern india abstract upper cretaceous shallow marine calcarenites of the upper campanian maastrichtian kallankurichchi formation from the cauvery basin of southeastern india are rich in shell concentrations dominated by large bivalves with minor admixtures of brachiopods bryozoans and larger foraminifera rarely these concentrations occur as lenses or pavements but usually as beds 10 to 200 cm thick depending on the dominant skeletal elements five types of shell concentrations can be distinguished isognomon pycnodonte ceratostreon inoceramid platyceramus cataceramus and polyspecific concentrations these shell concentrations occur in various preservational states which reflect the gradual mixing and mechanical destruction of former communities analysis of the taphonomic signatures of their components and of the biofabric suggests that storm induced waves and currents reduced sediment input the settling behaviour of taxa and a high rate of production of biogenic hardparts were responsible in varying degrees for the formation of the various shell concentrations accordingly they can be classified as storm wave proximal tempestite and primary biogenic concentrations the shell concentrations formed below fair weather wave base in shallow subtropical low stress environments

taphonomy and sedimentology of storm generated continental shell beds a case example from the cretaceous western interior basin abstract an extraordinary continental shell bed is reported from the upper cretaceous kaiparowits formation in southern utah this shell bed referred to as the kaiparowits blues ceratopsian shell bed is highly unusual among fluvial estuarine shell beds for its great thickness surface area and shell density and its geometry it covers 850 m2 ranges from 85 to 280 cm thick and consists of a series of 10 50 cm thick low angle dipping beds the shells are generally undamaged and articulated 75 commonly with valves still closed and strongly oriented normal to the dip orientation of the shell layers the shell bed is interpreted as a lateral accretion bar set that developed in a point bar or midchannel bar setting nearly 45 of shells are encrusted by the brackish water bryozoan conopeum sp indicating that deposition transpired within the upper fluvial to mixed fluvial marine part of an estuarine channel system at least five unionoid unionoidea shell morphotypes are present representative of

bivalve pavements from shallow water black shales in the early jurassic of northern italy a record of salinity and oxygen depleted environmental dynamics abstract thin shelled bivalve pavements which constitute the eomiodon horizon occur in a distinctive black shale unit in the lowermost part of a lower jurassic carbonate platform sedimentary system of the southern alps italy the eomiodon horizon is characterised by laminated mudstone and fissile dark grey and black organic rich marlstone and claystone yielding abundant bivalve pavements with monotypic to paucispecific assemblages dominated by eomiodon an opportunistic brackish shallow infaunal veneroid bivalve litho and biofacies indicate environmental conditions ranging from nearshore euhaline waters to oxygen depleted brackish lagoon with a high input of terrestrial organic matter culminating in marsh and terrestrial conditions these pavements were analysed using a microstratigraphic approach while considering their taxonomic composition the geometry of the beds their biofabric criteria size sorting abundance fragmentation orientation and disarticulation origin and time averaging these characteristics led to reconstruction of the following palaeoenvironmental dynamics 1 the event concentrations represented by mass mortality of stunted and juvenile individuals caused by anoxia are indicative of deeper and predominantly stagnant parts of the lagoon 2 an increase in shell sizes higher growth rate and a decrease in size sorting and winnowing illustrate higher and more continuous bottom oxygenation and enhanced life in oxygen fluctuated environmental conditions 3 poorly sorted highly fragmented and bioturbated bivalve accumulations indicate the eomiodon habitat here large sized eomiodon specimens record a decrease in water depth and sedimentation rate and almost constantly oxygenated bottom conditions 4 the appearance of byssally attached bivalves lithioperna marks the transition from the muddy soupy substrate of the eomiodon habitat to firm hard bottom conditions this new substrate allowed the settlement of lithioperna a byssate isognomonid which records a shallower and landward marsh setting the eomiodon horizon testifies to a short term warm humid climatic phase and yielded lithioperna the first genus of the lower jurassic lithiotis fauna to appear in southern alps this fauna is composed of large and aberrant bivalves which are spread throughout the lower jurassic tethyan and tropical panthalassian carbonate platforms as a result the eomiodon horizon and its salinity and oxygen depleted environmental setting represent a possible progenitor incubator for the lithiotis fauna

water mass exchange and variations in seawater temperature in the nw tethys during the early jurassic evidence from neodymium and oxygen isotopes of fish teeth and belemnites article i nfo oxygen and neodymium isotope analyses performed on biostratigraphically well dated fish remains recovered from the hettangian to toarcian of the paris basin were used to reconstruct variations of early jurassic seawater temperature and to track oceanographic changes in the nw tethys our results indicate a strong correlation between 18 o trends recorded by fish remains and belemnites confirming the paleoenvironmental origin of oxygen isotope variations interestingly temperatures recorded by pelagic fishes and nektobenthic belemnites and bottom dwelling fishes are comparable during the late pliensbachian sea level lowstand but gradually differ during the early toarcian transgressive episode recording a difference in water temperatures of 6 c during the bifrons zone this could suggest that the surface bottom water temperature difference was not large enough during regressive phases to be recorded by organisms living near the lower and upper part of the water column the globally unradiogenic nd budget of euro boreal waters through the early jurassic suggests that these waters were strongly affected by continental neodymium input from surrounding emerged areas and that exchange with more radiogenic waters from the tethys and panthalassa oceans remained limited this supports the existence of a southward directed current in the euro boreal area for most of the early jurassic the only exception is observed at the early late pliensbachian transition where a positive end excursion is recorded suggesting northward influx of low latitude tethyan or panthalassan waters which may have contributed to the warming of nw tethyan seawater recorded at this time the absence of a marked negative excursion in end concomitant with a negative 18 o shift recorded during the falciferum zone exaratum subzone argues against the influence of less radiogenic arctic water influxes with low 18 o values during this interval instead we suggest that

environmental control and dynamics of lower jurassic bivalve build ups in the trento platform southern alps italy abstract bivalve molluscs show a short and peculiar early jurassic evolutionary phase characterized by the lithiotis sensu lato fauna mainly represented by the three monospecific genera lithiotis cochlearites and lithioperna these bivalves with stick like or flattened large shells are mostly restricted to the pliensbachian they have a wide geographical distribution occurring both in tethyan and panthalassian carbonate platforms where considerable biogenic accumulations formed they were common on the trento platform southern alps where they occur in the rotzo formation calcari grigi group this formation was deposited in a tropical lagoon which was closed seawards by oolitic shoals and bars and bordered landwards by marshes the lithiotis s l fauna of the trento platform seems to be restricted to the upper pliensbachian it appeared after a eutrophic phase with recurrent low oxygen and brackish conditions and flourished in meso to oligotrophic waters the stratigraphic and geographic distributions of each genus are compared to the environmental development of the rotzo formation in order to highlight limiting environmental factors and ecological requirements lithioperna occurs throughout the whole formation it is the most eurytopic genus well adapted to dysaerobic habitats and with higher ecomorphic plasticity than the other two genera it frequently occurs in the life position giving rise to densely packed fan like or book like aggregates lithiotis required aerobic and euhaline marine conditions and is common in the middle part of the formation where mesotrophic conditions prevailed this genus is rarely preserved in the theoretical vertical life position probably due to lower shell stability and a less effective attachment modality cochlearites is the most common genus of the rotzo formation this genus inhabited euhaline to slightly hypersaline marine environments occurring in the upper part of the formation prevailing in oligotrophic conditions lithiotis s l fauna gave rise to biogenic accumulations with an elevation of a few decimeters to several meters above the sea bottom and with a lateral extension ranging from ten to several hundred meters therefore these accumulations represent biological build ups which document four developmental phases colonization aggradation progradation and demise the colonization phase is represented by pioneering individuals which were cemented or byssally attached to a bioclastic coarse grained storm layer which constituted the hard substrate for larval attachment the aggradation phase produced the core of the build up and its elevation above the sea bottom the core is mostly represented by individuals which acted as bafflers commonly with an apparently chaotic fabric the lithiotis s s and cochlearites cores are generally mud supported and contain loosely packed to dispersed specimens occasionally preserved in life position the lithiotis s s and cochlearites build ups can be defined as bivalve mounds the cores of the lithioperna build ups are skeletal supported with shells mostly preserved in life position which characterize the bivalve reefs the prograding phase produced lateral additions to the build up flanks of oblique and imbricated articulated individuals the flanks grew centrifugally from the core and represent the most conspicuous part of the build ups ranging from 10 to 20 times the size of the core and occasionally extending over several hundred meters

distribution of clay minerals in early jurassic peritethyan seas palaeoclimatic significance inferred from multiproxy comparisons article i nfo a set of published unpublished and new clay mineral data from 60 european and mediterranean localities allows us to test the reliability of clay minerals as palaeoclimatic proxies for the pliensbachian toarcian period early jurassic by reconstructing spatial and temporal variations of detrital fluxes at the ammonite biochronozone resolution in order to discuss their palaeoclimatic meaning a compilation of low latitude belemnite 18 o 13 c mg ca and 87 sr 86 sr values is presented for the first time for the whole pliensbachian toarcian period once diagenetic and authigenic biases have been identified and ruled out kaolinite content variation is considered as a reliable palaeoclimatic proxy for the early jurassic major kaolinite enrichments occur during times of low 18 o high mg ca and increasing 87 sr 86 sr implying warm climates and efficient runoffs during the davoei falciferum and bifrons zones conversely cooler and drier times such as the late pliensbachian or the late toarcian are characterized by low hydrolysis of landmasses and correspond to kaolinite depleted intervals secondary factors as modifications of sources or hydrothermalism may sporadically disturb the palaeoclimatic signal e g in the bakony area during the late pliensbachian in addition a spatial comparison of clay assemblages displays significant kaolinite enrichments towards northern parts of the peritethyan realm probably related to the latitudinal zonation of hydrolyzing conditions this implies enhanced runoffs on northern continental landmasses that reworked kaolinite rich

glacier fjord and seismic response to recent large calving events jakobshavn isbrae greenland 1 the recent loss of jakobshavn isbrae s extensive floating ice tongue has been accompanied by a change in near terminus behavior calving currently occurs primarily in summer from a grounded terminus involves the detachment and overturning of several icebergs within 30 60 min and produces long lasting and far reaching ocean waves and seismic signals including glacial earthquakes calving also increases near terminus glacier velocities by 3 but does not cause episodic rapid glacier slip thereby contradicting the originally proposed glacial earthquake mechanism we propose that the earthquakes are instead caused by icebergs scraping the fjord bottom during calving

the 1998 aiquile bolivia earthquake a seismically active fault revealed with insar abstract using sar interferometry insar the deformation field of the mw 6 6 1998 aiquile bolivia earthquake is mapped and the epicentre accurately located for the first time elastic dislocation modelling is used to demonstrate that the measured displacements are best explained with a n s oriented fault with a strike that is oblique to the principal topographic features in the region and a location that agrees with a modified mercalli intensity map constructed from observations of damage in the surrounding area a variable slip solution for a fault in this orientation is obtained which predicts peak slip of 1 42 m at depths of 4 5 km on the fault plane and has an estimated seismic moment m0 8 44 1018 n m which agrees with estimates from long period seismology this is the first time that a fault has been demonstrated unambiguously to be active in the central andes and since there was no previous knowledge of an active fault with this location or orientation a necessary conclusion is that our understanding of seismic hazard in this region is limited