paleoenvironnements et dynamique de faunes de trilobites dans l ordovicien llanvirn superieur caradoc basal du massif armoricain france abstract during the ordovician the armorican massif is generally considered as a platform domain a shallow marine environment characterized by endemic invertebrate faunas however approximately from the late llanvirn to the end of the llandeilo different trilobite associations develop their geographic distribution seems to be mainly related to bathymetric variations the interpretations proposed in this paper are supported by lithological and sedimentological data and comparisons with living marine crustaceans the possible mode of life of some trilobites has also been taken into consideration thus at the eastern end of the armorican median synclinorium and south of rennes martigne ferchaud synclinorium the presence of parabarrandia a mesopelagic trilobite from open shelf indicates amongst other things a marked increase of depth from west to east and from north to south in the southeastern part of the armorican massif ancenis region the development of a cyclopygid biofacies indicates the beginning of a slope opening onto a deep marine environment south armorican ocean in some cases uralichas ectillaenus panderia the exceptionally large dimensions of the dorsal exoskeletons would result from the low water temperature at high latitudes which agrees well with the existence of a gondwanan glaciation at the end of the ordovician

do critical stresses for incipient motion and erosion really exist the concept of critical stress for the initial motion of noncohesive sediment beds under turbulent flow conditions is reviewed observational definitions of incipient motion are many and not entirely compatible some laboratory flume observations of sediment movement suggest that no true threshold exists current understanding of turbulent fluid motion at a sediment bed suggests that some particle movement must occur at all nonzero time mean velocities a combined model for flume flow and sediment transport having no threshold is seen to explain features of data that have been previously used to support the threshold concept

palaeogeographical controls on the cambrian trilobite immigration and evolutionary patterns reported in the western gondwana margin abstract southward drifting of the western gondwanan margin during the cambrian has been demonstrated by means of both palaeomagnetic methods and lithological indicators of climate such as carbonates and evaporites recent improvements in biostratigraphical correlations permit an enhanced understanding of the climatic and palaeobiogeographical constraints that controlled the distribution of cambrian benthic communities palaeogeographical and biogeographical reconstructions based on trilobites are reported in this paper in order to test interaction between migration speciation and extinction rates the variability of the documented biogeographical patterns is directly related to species diversity in which wider distribution coincides with transgressive trends and subsequent connection of neighbouring platforms early cambrian trilobite faunas show a high degree of both substrate control and endemicity although transgressions led to parallel shifts in faunal compositions by contrast mid cambrian trilobite faunas are relatively uniform across western gondwana and latest mid and late cambrian associations document influence of an increased similarity with asian trilobite faunas

ordovician and silurian global geography presidential address delivered 3 may 2000 the main palaeocontinents during the early ordovician were gondwana laurentia baltica and siberia and a brief survey is made of their limits in the ordovician and silurian in particular gondwana by far the largest continent is analysed as including a core of south america antarctica africa australia and peninsular india and also the marginal terranes all of which show faunal links with but which may not have been attached to the core of avalonia ibero armorica and other european fragments turkey arabia and terranes from the far east including south china sibumasu and parts of australasia changes within the benthic faunas reveal that baltica whilst isolated in the early ordovician became united with avalonia by the end ordovician and the two with laurentia by the mid silurian to form the new supercontinent of laurussia island arcs had distinctive faunas during the ordovician in particular those in the iapetus between laurentia baltica and gondwana and the huge kipchak arc which ran from baltica to siberia as the period progressed the iapetus arcs became subducted beneath or accreted to their neighbouring cratons and the kipchak arc gradually collapsed to form the core of the new kazakhstania terrane gondwana drifted over the south pole and this movement is reflected in the cratonic benthic faunas particularly brachiopods and trilobites which in the early ordovician had formed a cline between the high latitude faunas of north africa and the equatorial faunas of the far east and australia but by the devonian lived in much warmer seas in southern europe and cooler waters in antarctica the faunas also reflect the global palaeoclimates which were warm in the early to mid ordovician and mid to late silurian but which were much colder in the half million year period of the late ordovician and early silurian particularly in the latest ordovician hirnantian ice age with its attendant widespread hirnantia brachiopod fauna the relative closeness of the chief palaeocontinents by the early silurian enabled brachiopod and trilobite larvae to cross the narrower oceans enabling a relatively cosmopolitan benthic fauna to be established over much of the globe apart from the cooler water higher latitude clarkeia fauna to the south and the tuvaella fauna to the north

late ordovician global warming the boda event there is substantial evidence for mid ashgillian global warming before the latest ordovician hirnantian glaciation as shown by the movement of previously lower latitude benthic faunas such as trilobites and brachiopods to progressively higher latitudes and by an increase in endemic faunas at low latitudes this is shown by faunal dynamics on several paleocontinents and endorsed by published evidence of warmer water sediments from gondwana and elsewhere we term this global warming the boda event

early palaeozoic palaeobiogeography and palaeoecology of stylophoran echinoderms abstract stylophorans cornutes mitrates represent one of the most diverse classes of cambro ordovician echinoderms they were free living benthic non radiate forms closely related to asterozoans and crinoids taphonomic sedimentological and palaeosynecological data provide useful information on key aspects of stylophoran palaeoecology such a combined approach suggests that the rarity of stylophorans in proximal environments above storm wave base was probably original and does not exclusively result from the possession of a loosely articulated polyplated calcitic test conversely stylophorans were relatively abundant in deeper settings below storm wave base where they frequently occur in low diversity dense beds several tens to hundreds of individuals per square metre such assemblages were either largely dominated by stylophorans only middle cambrian darriwilian or by the association of both stylophorans and ophiuroids latest darriwilian silurian comparison with ecological requirements of similar modern and fossil dense populations of ophiuroids suggests that stylophoran beds occurred in well oxygenated oligotrophic waters with low predatory pressure and that sea temperature played a key role in the palaeobiogeographic distribution of stylophorans seven main stylophoran biofacies are identified and described the associated environments and tropical distribution of primitive heavily plated forms suggest that the initial radiation of the class probably took place in warm to temperate shallow settings earliest middle cambrian late cambrian however from this primitive stock stylophorans radiated several times towards deeper and or colder environments possibly because of the lower predation pressure in such settings cothurnocystid cornutes in the middle cambrian mitrates in the late cambrian hanusiids in the tremadocian as a consequence the fact that most cambro ordovician cornutes and mitrates were psychrospheric i e cold water adapted probably explains their relatively wide palaeobiogeographic distribution in high palaeolatitudes cambro ordovician stylophorans were abundant and diverse in shallow to deep environments but they were generally restricted to deep settings in low palaeolatitudes the great caradocian transgression favoured the extension of stylophorans from circum polar regions e g western margin of gondwana towards external settings at lower palaeolatitudes avalonia baltica and eastern margin of laurentia where one group of mitrates underwent a major radiation anomalocystitids and extended towards shallower and warmer palaeoenvironments in contrast all silurian stylophoran assemblages known so far are rare poorly diversified and restricted to shallow warm to temperate environments in tropical regions this distribution partly results from 1 the probable eradication of psychrospheric forms e g cornutes because of the prolonged anoxia of deep environments in the late hirnantian early silurian of west gondwana 2 the radiation of some mitrates anomalocystitids in shallow tropical settings in the late ordovician and 3 of strong taphonomic and or sampling biases numerous ghost lineages

size sorting in marine muds processes pitfalls and prospects for paleoflow speed proxies the basis for and use of fine grain size parameters for inference of paleoflow speeds is reviewed here the basis resides in data on deposited sediment taken in conjunction with flow speed measurements in the field experimental data on suspended sediment transport and deposition and theoretical treatments of the generation of size distributions of deposits from suspension controlled by particle settling velocity and flow speed in the deep sea sorting events occur under resuspension deposition events in benthic storms at flow speeds below 10 15 cm s 1 size in the noncohesive sortable silt 10 63 m range is controlled by selective deposition whereas above that range removal of finer material by winnowing also plays a role the best particle size instruments to measure a flow speed related grain size employ the settling velocity method while laser diffraction sizers can yield misleading results because of particle shape effects potential problems including source effects downslope supply on continental margins spatial variability of flow over bedforms and influence of ice rafted detritus are examined a number of studies using the sortable silt flow speed proxy are reviewed and inverse modeling of grain size distributions is examined outstanding problems are that corroboration is sparse because almost no studies have yet used the full range of proxies for flow rate and water mass identification and that the sortable silt mean size is not yet properly calibrated in terms of flow speed

the armorica microplate fact or fiction critical review of the concept and contradictory palaeobiogeographical data abstract the problem of an armorica microplate detached from gondwana and having had during part of the palaeozoic an independent latitudinal evolution is reconsidered in terms of a critical review of the palaeomagnetic data that were the very roots of this concept and through an alternative approach based on palaeoclimatic and palaeobiogeographical data palaeomagnetic data for the silurian and the devonian of the south european regions supposedly constituting the armorica microplate remain rare and ambiguous those from gondwana are more numerous but contradictory enough to give rise to diverging models regarding the latitudinal evolution of this continent consequently the reality of an armorica microplate cannot be considered as established on the contrary lithological indicators of palaeoclimate and palaeobiogeographical data are in total harmony and indicate that in actual fact the southern european regions remained permanently closely connected with gondwana of which they composed the northern margin although repeatedly maintained for more than 20 years the concept of an armorica microplate can thus be considered a fiction this conclusion should lead to the dismissal of geodynamical models proposed for the variscan belt to which this concept was integrated and are contradicted by inescapable palaeobiogeographical constraints

upper cambrian trilobites and brachiopods from severnaya zemlya arctic russia and their implications for correlation and biogeography a new late cambrian trilobite brachiopod fauna from the kurchavinskaya formation severnaya zemlya northern siberia allows correlation of the ketyi horizon of the nw siberian succession with the praecursor zone of the baltic olenid zonation the presence on severnaya zemlya of the typically siberian trilobite kujandaspis ketiensis indicates that even if severnaya zemlya lay on a separate plate whether kara or arctida as postulated by other authors then it was still probably not far from siberia however the associated brachiopods are partly endemic to severnaya zemlya thus giving some support to the independent palaeomagnetic evidence for their origin on a plate separate from siberia

evolutionary paleoecology of the earliest echinoderms helicoplacoids and the cambrian substrate revolution bioturbation in neritic siliciclastic settings during the proterozoic phanerozoic transition increased in depth and intensity causing a change in substrates from the matgrounds characteristic of the proterozoic to the mixgrounds characteristic of the phanerozoic this change in bioturbation increased the water content of surficial layers of sediment and blurred the sediment water interface leading to the first appearance of a mixed layer development of a mixed layer throughout neritic environments would have had a strong impact on any benthic metazoans particularly sessile suspension feeders that were well adapted for survival on relatively unbioturbated proterozoic substrates the impact of this substrate transition on benthic metazoans has been termed the cambrian substrate revolution the unusual early cambrian helicoplacoid echinoderms were well adapted for survival on typical proterozoic style substrates the examination of new helicoplacoid specimens collected during this study combined with extensive study of the rocks in which they are preserved indicate that helicoplacoids lived as sediment stickers on a muddy substrate that underwent only low to moderate levels of strictly horizontal bioturbation and did not have a mixed layer the significant increase of bioturbation through the cambrian in neritic siliciclastic settings is likely to have led to the extinction of the helicoplacoids other similarly adapted sessile suspension feeding echinoderms may have also been driven to extinction by the effects of the cambrian substrate revolution the co existence during the cambrian of organisms adapted to the variety of substrates characteristic of this transitional period may also have contributed to the high degree of perceived morphological disparity during the cambrian explosion

middle cambrian echinoderms from north spain show echinoderms diversified earlier in gondwana new fossil discoveries in the middle cambrian of spain have considerably expanded our knowledge of the temporal and spatial distribution of some major clades of echinoderms including sucocystid cinctans isorophid edrioasteroids cothurnocystid stylophorans ctenocystoids and a new group of blastozoans eocystitids because many of these taxa appear close to the beginning of the middle cambrian it seems likely that their origins must be placed in the early cambrian these results based on articulated specimens provided from echinoderm lagerstatten agree with the hidden diversity provided from isolated ossicles from other gondwanan areas

a fault controlled trough through a carbonate platform the middle cambrian house range embayment during the cambrian part of western north america was a rapidly subsiding passive continental margin a belt of carbonate deposition dominated the central shelf during middle cambrian time the belt was bounded by fine grained terrigenous sediments that accumulated in deeper water to the west and in a coastal setting to the east rapid and possibly repeated movement along a northeast southwest oriented fault produced a conspicuous eastward re entrant into the carbonate belt the house range embayment in nevada and utah during the middle middle cambrian the embayment was an asymmetrical trough that deepened and widened as it extended 400 km westward across the shelf toward the edge of the continent the trough axis lay near its abrupt southern margin which was a normal fault separating an area of continuous shallow water accumulation to the south from deep water deposition the northern flank of the embayment was initially a drowned platform that formed a southward sloping ramp facies within the embayment represent deposition in an anoxic basin on a deep ramp that was locally steepened and on a shallow low energy ramp the facies rapidly prograded basinward across the northern flank and through vertical accretion a broad carbonate platform was re established there in contrast to the northern flank a continuous sequence of deep water lithofacies accumulated throughout the late middle cambrian in the trough axis of the embayment this region seemingly subsided more rapidly than did that of the northern flank and it may have functioned as a sediment bypass zone although a source of periplatformal carbonates was nearby sedimentation rates in the trough were thus inadequate to re establish a shallow water depositional setting

palaeogeographical and palaeoecological aspects of the cambro ordovician radiation of echinoderms in gondwanan africa and peri gondwanan europe abstract ecology and tempo of the lower palaeozoic radiation of echinoderms are discussed in this paper based on comparison of the diversity patterns observed in cambro ordovician faunas from laurentia and the northern gondwana margin the cambrian agronomic revolution triggered a global radiation of echinoderms with the progressive disappearance of biomat related lifestyles and the colonisation of new environments both in laurentia and on the northern gondwana margin soft substrate echinoderm assemblages related to cold and or deep environments were dominated by blastozoans and stylophorans these assemblages show a pattern of continuous diversification from the middle cambrian to the middle ordovician the major radiation of crinoids in the lower to middle ordovician of laurentia is only a local diversification correlated with the development of hardgrounds in warm shallow environments no comparable major ordovician radiation event occurred in the cooler environments of the northern gondwana margin

bathymetric and isotopic evidence for a short lived late ordovician glaciation in a greenhouse period the end ordovician glaciation is distinct among phanerozoic glaciations in that co 2 levels were generally high yet major continental ice sheets accumulated on the gondwana supercontinent new oxygen isotopic data indicate substantial changes in sea water temperatures and ice volume coinciding with glacio eustatic changes in sea level reflecting the growth and decay of the gondwana ice cap major glaciation was apparently confined to the hirnantian and was 0 5 1 m y long rather than the 35 m y of earlier estimates carbon isotope values indicate significant changes in carbon cycling as the oceans changed from a state with warm saline bottom waters to a state with cold deep water circulation and then back again we believe that the changes in the carbon cycle effected a reduction in p co 2 levels in the oceans and atmosphere and thus promoted glaciation but were unable to sustain icehouse conditions in a greenhouse world

the fezouata shale lower ordovician anti atlas morocco a historical review abstract exceptionally preserved fossils yield crucial information about the evolution of life on earth the fezouata biota from the lower ordovician of morocco is a konservat lagerstatte of major importance and it is today considered as an ordovician burgess shale this biota was discovered only some 15 years ago but geological studies of the area date back to the beginning of the 20th century pioneering geological investigations lead to the discovery of ordovician strata in the anti atlas 1929 and ultimately resulted in their formal subdivision into four main stratigraphic units 1942 in the agdz area the presence of fossiliferous tremadocian lower ordovician strata was suspected as early as 1939 but only definitively confirmed in 1955 in the 1960s 1990s jacques destombes provided the first detailed biostratigraphic framework for the lower ordovician of the anti atlas and collected thousands of fossils that were subsequently described in a series of monographs in the early 2000s exceptionally preserved fossils were discovered in the fezouata shale tremadocian late floian in the central anti atlas by mohamed ou said ben moula this biota now known as the fezouata biota is of utmost importance for it demonstrates the extent in the fossil record of non biomineralising animals typical of the cambrian explosion into the ordovician during the great ordovician biodiversification although most components are still in need of formal descriptions a fairly good picture of the composition and organisation of this biota and how it contributes to our understanding of the early evolution of metazoan communities can now be depicted moreover recent studies have substantially clarified the biostratigraphical and palaeoenvironmental context of the fezouata shale and are now being followed up by detailed investigations of the taphonomy geochemistry and micropalaeontology of this unique konservat lagerstatte these efforts will soon greatly benefit from the recovery of fresh unweathered samples from drill cores

the margins of avalonia during cambrian and earliest ordovician times avalonia was an area forming an integral part of the huge gondwanan continent probably along the northern margin of amazonia until in early ordovician late arenig or llanvirn time it split off from gondwana leaving a widening rheic ocean to its south today its southern margin with gondwana extends northeast from east of cape cod massachusetts through nova scotia north of the meguma terrane and thence below sea level to the south of newfoundland on the eastern side of the present atlantic the southern margin may separate southwest portugal from the rest of the iberian peninsula it can be traced eastwards with more certainty from the south cornwall nappes to a line separating the northern phyllite belt on the southern margin of the rhenohercynian terrane and the mid german crystalline high there is no certain evidence of avalonian crust to the northeast of the elbe line the northern margin of avalonia extends westwards from south of denmark to the british isles where it merges with the iapetus ocean suture between scotland and england traced westwards it crosses ireland and reappears in northern newfoundland to the east of new world island where it may follow the trace of the dog bay line and the cape ray fault recent work suggests that the northern margin of avalonia may clip the northern tip of cape breton island in nova scotia and then enter the north american mainland at the bay of chaleur it may then be traced from north and west of the popelogan and bronson hill arcs to long island sound near newhaven connecticut the cambrian to devonian faunas reflect the history of avalonia initially they were purely gondwanan but as ordovician time proceeded more genera crossed firstly the tornquist ocean as it narrowed between avalonia and baltica to close in latest ordovician and early silurian times and secondly the iapetus ocean so that by the early silurian most of the benthic shelly faunas apart from the ostracods were the same round the adjacent margins of all three palaeocontinents

a trilobite fauna from the highest shineton shales in shropshire and the correlation of the latest tremadoc a rich fauna dominated by trilobites and calcified chordates has been collected in shropshire from the arenaceous beds the highest member of the tremadoc shineton shale formation and hitherto regarded as poorly fossiliferous this fauna shows that shelf conditions persisted longer in shropshire than has been supposed it is likely that even younger tremadoc is cut out at the unconformity below the caradoc in the shineton inlier the correlation of the later tremadoc is reviewed and the nomenclature of british tremadoc biozones is revised the trilobites described here are a mixture of previously known and new forms the name shumardia conophrys salopiensis callaway is revived for british material traditionally assigned to the scandinavian species shumardia pusilla sars from which it is distinct the new taxa litagnostus meniscus sp nov apatokephalus sarculum sp nov and skljarella cracens sp nov are described the type species of asaphellus a homfrayi and leptoplastides l salteri are redescribed and geragnostus callavei pseudokainella impar and parapilekia sp are recorded new information on the ontogeny of s c salopiensis and a homfrayi is given litagnostus and skljarella are recorded from the british isles for the first time

two dimensional model of mud transport a two dimensional in plan finite difference model is presented which models the transport of well mixed cohesive sediments in estuaries and coastal waters a description of each of the physical processes incorporated in the model is given an erosion mechanism is not included explicitly because of the difficulties involved in a realistic formulation instead areas of net deposition throughout a tidal cycle are identified and rates of mud deposition within these areas are predicted two applications of the model to siltation studies are presented involving different modes of operation of the model in each case field data in the form of bed core samples were available so that the models could be both verified and calibrated under these circumstances the model can be used to make useful engineering predictions such as projected dredging requirements in estuaries and ports

cohesive material erosion by unidirectional current the initiation of motion of consolidated cohesive sediments under a unidirectional flow of clear water was studied experiments were performed in a flume tunnel capable of providing a bed shear stress up to 26 pa and a velocity of 3 5 m s 3 mm above the bed samples were prepared in a specially designed press using a carefully controlled consolidation procedure critical shear stress and velocity were found to increase with compressive strength vane shear strength plasticity index clay content and consolidation pressure

fission track thermochronology applied to phanerozoic thermotectonic events in central and southern sweden abstract central and southern sweden is characterized by a precambrian basement and a few lower palaeozoic sedimentary remnants with a probable initial thickness of less than 500 m upper palaeozoic deposits have not been documented in the swedish part of the fennoscandian shield regional deposition of upper palaeozoic sediments on the platform appears possible though since vast amounts of material must have been eroded from the caledonian mountain range during this period fission track thermochronology has been used in this study in order to validate the former existence of a post caledonian sedimentary load trace the variation in thickness and extent of the deposits and get an indication of the cooling history in the area samples from an east west trending profile from the caledonian border zone in the west to the aland archipelago in the east together with a few samples from surrounding areas have been analysed the obtained pooled apparent apatite fission track ages along the profile vary from 250 ma in the west to 500 ma in the east while the mean track lengths decrease from 14 m in the west to 11 m in the east in southern sweden ages between c 170 and 320 ma and a mean track length of 13 4 m were obtained the dating results show that all samples have experienced raised temperatures since the pre cambrian peneplanation temperatures leading to total annealing of fission tracks during the palaeozoic were reached in the western part of central sweden and in southern sweden while partial annealing is traced further east to the aland archipelago the obtained fission track results are interpreted by the former existence of a devonian foreland basin covering most of central and southern sweden an estimation of the upper palaeozoic sedimentary thickness yields 2 5 km of deposits in the ludvika area and at least 1 km in the aland archipelago inverse modelling of the fission track data resulted in several alternative modelled thermal histories one of these suggests that central sweden was covered by palaeozoic and mesozoic sediments until tertiary time in that case the surface relief of western central sweden has been chiselled out during the tertiary another possible thermal history includes denudation due to surface level conditions during the mesozoic in the west in that case reheating to 40 80 c during the late mesozoic cenozoic is required according to the present apatite annealing models none of the proposed modelled thermal histories can be rejected so far

cambrian palaeomagnetic data from baltica implications for true polar wander and cambrian palaeogeography a reliable early cambrian c 535 ma and a preliminary late cambrian c 500 ma palaeomagnetic pole from baltica sweden overlap within uncertainty and they are also broadly compatible with vendian c 583 ma palaeomagnetic data apparent polar wander for baltica amounts to less than 25 between 583 and 500 ma and therefore negates recent speculations that the earth tipped 90 during the early cambrian true polar wander throughout vendian and cambrian times baltica lay at southerly latitudes c 30 60 s baltica was geographically inverted and present day northern baltica faced the nw margin of gondwana which covered the south pole laurentia eastern baltica and laurentia west gondwana were separated by the iapetus ocean while the aegir sea separated western baltica from the taimyr region of siberia during the cambrian baltica probably moved eastward along the gondwana margin and by c 515 520 ma subduction in the aegir sea was initiated a major event is recognized in late cambrian or early ordovician times c 500 478 ma when baltica must have undergone a 55 counter clockwise rotation in c 22 million years 3 ma we relate this to the early caledonian finnmarkian orogeny which involved arc continent collision following subduction

review of trilobite biofacies distributions at the cambrian ordovician boundary detailed analysis of trilobite assemblages at the cambrian tremadoc transition reveals the presence of four possibly five biogeographic provinces baltic north china southeast china north american and possibly siberian one or more discrete biofacies characterize the transition in each province when plotted palaeogeographically these appear to be largely temperature controlled interdigitation of provincialized trilobite biofacies is recognized by the representation of dominant types of trilobite olenaceans ceratopygaceans remopleuridaceans dikelocephalaceans and leio stegiaceans in particular on key boundary sections these sections at nochixtlan mexico batyrbay southern kazakhstan broken skull river district of mackenzie canada and jiangshan chang shan western zhejiang china permit the correlation of biogeographic provinces globally as a result of the application of trilobite biostratigraphy relatively few problems prevent the maintenance of a cambrian ordovician boundary stratotype section in its traditional baltic biogeographic setting

fine particle deposition to initially starved stationary planar beds reported here are results from new flume experiments examining deposition and entrainment of inert silt sized particles with spherical diameters in the range from 20 to 60 m to and from planar impermeable and initially starved beds underlying channel flows bed surfaces comprised smooth or fixed sand size granular roughness and provided hydraulically smooth to transitionally rough boundaries results of these experiments were analysed with a simple model that describes the evolution of vertically averaged concentration of suspended sediment and accommodates the simultaneous delivery to and entrainment of grains from the bed the rate of particle arrival to a bed diminishes linearly and the rate of particle entrainment increases by the 5 2 power as the value of the dimensionless saffman parameter s u 3 g approaches a threshold value of order unity where u is the conventional friction velocity of the turbulent channel flow g is the acceleration due to gravity adjusted for the submerged buoyancy of individual particles and is the kinematic viscosity of the transporting fluid this transport behaviour is consistent with the notion that non cohesive silt sized particles can neither reach nor remain on an impermeable bed under flow conditions where mean lift imposed on stationary particles in the viscous sublayer equals or exceeds the submerged weight of individual particles within the size range of particles used in these experiments particle size and the characteristic size of granular roughness up to that of medium sand did not affect rates of dimensionless arrival or entrainment to a significant degree instead a new but consistent picture of fine particle transport is emerging silt sized material at least is subject to potentially significant interaction with the bed during intermittent suspension transport at intermediate flow speeds greater than the value required for initiation of transport ca 20 cm sec 1 but less than the value ca 50 cm sec 1 required by the saffman criterion ensuring transport in fully passive suspension or equivalently wash load

predicting erosion resistance of muds abstract when measured or apparent yield stress of cohesive muds is assumed to reflect average particle particle bond strength in network structures critical bed shear stress required for grain by grain entrainment can be predicted from an analysis of the forces acting on component grains of geometrically flat mud beds erosion resistance is then defined in terms of cohesive yield stress as well as nominal particle size shape relative density and packing geometry predicted values of critical bed shear stress compare favorably with existing data from selected studies in which appropriate yield stress and hydrodynamic parameters have been reported

a heuristic examination of cohesive sediment bed exchange in turbulent flows abstract prediction of the concentration of suspended cohesive sediment in the marine environment is constrained by difficulties in interpreting experimental evidence on bed exchange i e erosion and deposition of particles which remains sparse in mechanistic details in this paper conditions under which bed exchange in turbulent flows collectively determines the concentration of suspended matter have been examined in the heuristic sense based on selective experimental data it is argued that interpretation of such data can be significantly facilitated when multi class representation of particle size collisional interaction between suspended particles and probabilistic representations of the bed shear stress along with variables describing particle behavior critical shear stress for deposition bed floc shear strength are taken into account aggregation floc growth and breakup kinetics brings about shifts in the suspended particle size distribution bed exchange is accordingly modulated and this in turn determines concentration dynamics probabilistic representation of the governing variables broadens the suspended sediment size spectrum by increasing the possibilities of inter particle interactions relative to the mean value representation simple models of bed exchange which essentially rely on single size assumption and mean value representation of variables overlook the mechanistic basis underpinning particle dynamics

ordovician organic walled microphytoplankton acritarch distribution the global scenario abstract a number of palaeobiogeographical models for ordovician organic walled microphytoplankton acritarchs prasinophytes and related groups have been published during the past 30 years a modern synthesis of ordovician acritarch palaeobiogeography based on previously published acritarch provinces and global distribution models as well as new plots on recently compiled palaeogeographical maps is presented review of the literature and new plots indicate that a number of preliminary conclusions can be drawn following minor biogeographical differentiation of acritarch assemblages during the cambrian provincialism started at the cambrian ordovician boundary in the late tremadocian a warm water assemblage containing the genera aryballomorpha athabascaella and lua but no diacrodians seems to be limited to low latitude localities such as laurentia and north china from the late tremadocian and throughout most of the arenig a peri gondwana acritarch assemblage with the easily recognisable taxa arbusculidium filamentosum coryphidium and striatotheca is present on the southern margin of gondwana and its distribution corresponds almost exactly with that of the calymenacean dalmanitacean trilobite fauna it seems reasonable to consider the acritarchs of baltica as belonging to a temperate water province which was probably not restricted to the palaeocontinent of baltica but had a wider distribution at about the same latitude as some of the elements recorded from baltica also occur in south china and argentina the maximum separation of the continents during the arenigian reflected by a pronounced biogeographical differentiation of most ordovician fossil groups led to the development of geographically distinct acritarch assemblages data from the late middle ordovician and the late ordovician remain too poor to elucidate global palaeobiogeographical patterns the biogeographical distribution of ordovician acritarchs appears similar to that of the resting cysts of modern dinoflagellates primarily controlled by latitude but also following the continental margins

rise of echinoderms in the paleozoic evolutionary fauna significance of paleoenvironmental controls echinoderms were a major enduring component of paleozoic marine faunas beginning with the rapid diversification of crinoids and other classes in the early ordovician this diversification was triggered by increased availability of habitable areas accompanying sea level rise and changed sedimentation styles newly collected early ordovician echinoderms facilitate determination in detail of life modes and important adaptations during this transition interval substrate consistency and stability were crucial spatial limiting factors that resulted in heterogeneous distribution patterns for echinoderm classes crinoids were overwhelmingly large food groove suspension feeders with relatively open filtration fans attached by small discs to hard ground surfaces in shallow water they were preadapted for this life mode rhombiferans were mobile small food groove suspension feeders they lived on soft substrates at various depths several important crinoid clades underwent a second phase of diversification during the middle ordovician by developing holdfasts adapted to soft substrates and densely pinnulate arms with small food grooves leading to dominance of this class among paleozoic echinoderms our findings are in basic agreement with the overall onshore diversification pattern of most paleozoic benthic invertebrates but we argue for an extrinsic environmental control substrate availability for the observed echinoderm distribution

sea carousel a benthic annular flume a benthic annular flume sea carousel has been developed and tested to measure in situ the erodibility of cohesive sediments the flume is equipped with three optical backscatter sensors a lid rotation switch and an electromagnetic em flow meter capable of detecting azimuthal and vertical components of flow data are logged at rates up to 10 66 hz erodibility is inferred from the rate of change in suspended sediment concentration detected in the annulus the energy density wave number spectrum of azimuthal flow showed peaks in the energy spectrum at paddle rotation wave numbers k of 14 and 7 m 1 macroturbulent time scales but were not significant friction velocity u measured 1 at 1 hz using a flush mounted hot film sensor and 2 derived from measured velocity profiles in the inner part of the logarithmic layer gave comparable results for 1 at higher values of u method 2 underpredicted by up to 20 method 1 showed radial increases in in the annulus for y 0 32 m s 1 radial velocity gradients were proportional to y 0 32 m s 1 maximum radial differences in u were 10 for y 0 5 ms 1 suspended sediment mass concentration s in the annulus resulted in a significant decrease 10 5 in derived by method 1 over the range 0 s 1 these decreases were not evident in method 2 method 1 may therefore be subject to changes in stress sensor calibration with changes in s subaerial deployments of sea carousel caused severe substrate disturbance water losses and aeration of the annulus submarine deployments produced stable results though dispersion of turbid flume water took place results clearly demonstrated the existence of type i and type ii erosion documented from laboratory studies

late ordovician molluscs of the central and eastern anti atlas morocco more than 30 species of tergomyan gastropod bivalve and cephalopod molluscs are described fromthe late ordovician of central and eastern anti atlas morocco for the cephalopods this represents t

controls of sea level fluctuations on the formation of ordovician siliceous nodules in terrigenous offshore environments abstract siliceous nodules of the ordovician period of the armorican massif and of southwest sardinia are made of a mixture of terrigenous origin material quartz and phyllosilicates and of biogenic origin components microcrystalline quartz calcium phosphate and calcite they are associated with fine grained sequences which were deposited between the distal part of the inner shelf and the outer shelf within these sequences two facies can be distinguished clayey silty facies without nodules related to episodes of high sedimentation rate and nodule bearing clayey facies linked to episodes of reduced sedimentation the genesis of the siliceous nodules can be integrated into a model taking account of the fluctuations in terrigenous flux due to eustatic variations during periods of rising sea level the terrigenous flux decreases and the sedimentation is dominated by bioclastic elements calcareous siliceous and phosphatic stability of the sediment water interface facilitates biogenic silica dissolution during early diagenesis silica precipitation leads to the formation of proto nodules the distribution of the two facies can be explained by the superposition of two eustatic signals one of very high frequency vhf and the other of high frequency hf the silty clayey facies could be accumulated during the period of falling sea level within the hf cycle and the nodule bearing clayey facies could correspond to amalgamation of several vhf cycles related to periods of low sedimentation rate in the hf cycle

ichnological evidence for the environmental setting of the fossil lagerstatten in the devonian hunsruck slate germany analysis of the ichnology and sedimentology of the lower devonian hunsruck slate germany reveals that the distribution and preservation of the famous pyritized fauna were controlled by the deposition of fine grained turbidites that formed a firm substrate the nature of this substrate is evidenced by the preservation of laminae and the finest details of arthropod trackways the trace fossils are dominated by two ecological groups those made by epifaunal organisms and those involving burrow systems connected to the sediment water interface trace makers that moved through the sediment are poorly represented the diversity of in situ body fossils and epifaunal traces confirms that conditions within the water column remained well oxygenated even though the sediment rapidly became inhospitable the hunsruck slate konservat lagerstatten are remarkable in preserving soft tissues where unusual geochemical conditions prevailed in the environment where the animals lived rather than following transport to a different setting

the role of abiotic factors in the cambrian substrate revolution a review from the benthic community replacements of west gondwana abstract the cambrian substrate revolution refers to a substantial and rapid change to the nature of marine sedimentary substrates in the early cambrian and is widely interpreted as a biologically driven event a direct response to evolutionary innovations in metazoan burrowing and the development of new shelly faunas however abiotic factors such as tectonic and climatic evolution also had the potential to restructure cambrian substrates and are here shown to be more plausible drivers of change in the benthic faunas of western gondwana the western mediterranean region underwent a southward drift during cambrian times which drove a switch from subtropical carbonates to temperate siliciclastic substrates with short term episodes of temperate carbonate productivity as a result microbial and shelly carbonates disappeared diachronously in a stepwise manner across the lower middle cambrian boundary interval archaeocyathan microbial reefs were replaced by chancelloriid eocrinoid spiculate sponge meadows in which the stepwise immigration of new echinoderm taxa was primarily controlled by extensional tectonic events first recorded in rifting settings and later in passive margin platforms availability of new kinds of substrate was thus the primary factor that controlled where and when evolutionary innovations in benthic strategies arose examples of this include the early cambrian colonization of phosphatic hardgrounds and thrombolite crusts by chancelloriids archaeocyathan and spiculate sponges and the exploitation by benthos to the increasingly widespread availability of shelly grounds and carbonate firmgrounds by early diagenetic cementation a microbial mat epifaunal antagonistic relationship is demonstrated for echinoderm pelmatozoans based on the non overlapping palaeogeographic distributions of microbial reefs and mats versus mud sticker pelmatozoans cambrian benthic communities thus evolved in parallel with substrates in response to abiotic factors rather than being the primary drivers of substrate change

taphonomy of cystoids echinodermata diploporita from the napoleon quarry of southeastern indiana usa the lower silurian massie formation as an atypical lagerstatte abstract diploporites like most pelmatozoan echinoderms are characterized by complex multi element skeletons prone to complete disarticulation if left exposed for extended periods of time consequently preservation of abundant articulated diploporite thecae is rare and generally restricted to environments where rapid burial events could catastrophically entomb individuals one such environment is represented by the lower portion of the wenlock age massie formation at the new point stone quarry near napoleon southeastern indiana which we recognize as a konservat lagerstatte yet taphonomic evidence does not support live burial diploporites holocystites spp are dominantly preserved as intact thecae without any portions of brachioles and with damage plate shifting and plate loss plate jumbling on one side post mortem encrustation is common but epibionts are typically present on the well preserved sides of thecae geopetal sediment fill of thecae shows that the well preserved side commonly faced upward following burial this taphonomic state is paradoxical as burial of live diploporites would have resulted in preservation of complete individuals with no encrusters and only compaction induced damage rapid burial of diploporites following a short interval of exposure would have resulted in preservation similar to observed patterns but with encrusters and geopetal infills indicating that the poorly preserved side faced up and burial after extended exposure would have resulted in thorough disarticulation of thecae rather a variable and in some cases complex taphonomic history is suggested for diploporites from the napoleon quarry with at least some individuals having experienced one or more brief episodes of exhumation prior to final burial early diagenetic cementation of initial theca filling sediment is the most likely mechanism for keeping thecae intact during subsequent exposure

palaeoecological aspects of the diversification of echinoderms in the lower ordovician of central anti atlas morocco abstract echinoderms are one of the major components of benthic faunas in the lower ordovician sequence near zagora central anti atlas morocco the fezouata shale tremadocian late floian has yielded numerous exquisitely preserved echinoderm assemblages ranging through several stratigraphic levels and palaeoenvironmental conditions these associations offer a unique opportunity to document both evolutionary and palaeoecological aspects of echinoderm diversification in high latitude siliciclastic dominated western gondwana sediments where rapid in situ burials facilitated excellent faunal census conditions lower shoreface deposits of the fezouata shale provide the most complete record of successive echinoderm faunas in late tremadocian times these relatively shallow shelf deposits show the progressive replacement of low diversity opportunistic cambrian like dwarfed communities dominated by cornute stylophorans in unhospitable dysoxic environmental conditions by higher diversity benthic assemblages more typical of the great ordovician biodiversification event and dominated by blastozoans on well oxygenated sea floors the turnover of the ordovician radiation was apparently slightly delayed in more proximal settings eocrinoid meadows persisted in shallower environmental conditions up to the middle floian in the late floian they were replaced by diploporite dominated communities typical of later ordovician high latitude peri gondwanan faunas from a palaeobiogeographic point of view low diversity assemblages display relatively strong affinities with cosmopolitan late cambrian echinoderm faunas whereas high diversity communities are dominated by peri gondwanan taxa

erosion of soft cohesive sediment deposits erosion behavior of soft cohesive sediment deposits has been investigated in laboratory experiments such deposits are representative of the top active layer of estuarial beds an experimental procedure involving layer by layer erosion under a range of bed shear stresses b of successively increasing magnitude was utilized interpretation of the resulting concentrationtime data together with bed density profiles yielded a description of the variation of the bed shear strength s with depth as well as an expression for the rate of surface erosion in general s increased with depth and was also influenced by the type of sediment bed consolidation period and salinity the rate of erosion was found to vary exponentially with b s 1 2 in modeling estuarial bed erosion it is essential to take these characteristics of s and the rate of erosion into account

factors controlling critical shears for deposition and erosion of individual grains abstract we have quantified critical deposition and erosion shear stresses for individual fine 24 141 m natural quartz particles in a flow chamber with radial parallel flow in seawater 30 10 15 c critical erosion shear stresses as expected are slightly larger than estimates from previous studies which strove for cohesionless conditions while critical deposition shear stresses are three orders of magnitude larger than predicted by hjulstrom 1935 and equal to the stress on a particle settling at stokes terminal fall velocity manipulative experiments indicate that critical deposition shear stress of medium to coarse silt is little affected by physical chemical adhesion bed roughness or microbial adhesion while critical erosion shear stress is at least doubled by microbial adhesion or bed roughness critical stresses for individual grains of comparable size are normally distributed supporting the concept of well behaved thresholds for both erosion and deposition because the thresholds do not differ greatly in magnitude for particles of a given size organism produced and maintained features e g pits and tubes can be important in shifting local conditions from erosive to depositional and vice versa

which is the better proxy for paleo current strength sortable silt mean size ss or sortable silt mean grain diameter d ss a case study from the nordic seas the coarseness of the 10 63 m terrigenous silt i e sortable silt fraction tends to vary independently of sediment supply in current sorted muds in the world s oceans with coarser sediments representing relatively greater near bottom flow speeds traditionally the coarseness of this size fraction is described using an index called sortable silt mean size ss which is an arithmetic average calculated from the differential volume or mass distribution of grains within the 10 63 m terrigenous silt fraction where the relative weights of the individual size bins become increasingly disproportionate with respect to the actual number of grains within those size bins towards the coarse end of the size range this not only increases the absolute value of the apparent mean size within the 10 63 m terrigenous silt fraction but it may also affect the apparent pattern of relative changes in the coarseness of the sortable silt fraction along the core in addition it makes ss more prone to biases due to for example analytical errors here we present a detailed analysis of grain size distributions over three selected holocene time intervals from two complementary sediment cores jm97 948 2a md95 2011 extracted from the center of a high accumulation area along the flow path of the main branch of the atlantic inflow into the nordic seas and show that differential number based statistics which likely better describes variations in the actual coarseness of the sortable silt fraction may provide a more robust alternative to ss this article is protected by copyright all rights reserved

biotic response to a middle ordovician paleoceanographic event in eastern north america global paleontological events such as mass extinctions have been the focus of attention in recent years yet regional biotic events account for a greater proportion of extinction in the fossil record and should receive greater attention if we are to fully understand extinction processes over the phanerozoic we use a sequence stratigraphic framework to document a major extinction and migration episode that occurred in the middle and late ordovician of north america this event is characterized in eastern north america by coordinated changes in faunal assemblages carbonate lithologies siliciclastic influx and phosphatization we ascribe these changes to a regional paleoceanographic event brought on by thrust driven flexural subsidence and sea level fluctuations that lowered water temperature and increased turbidity and nutrient input

sortable silt and fine sediment size composition slicing parameters for palaeocurrent speed and palaeoceanography fine sediment size 63 ixm is best measured by a sedimentation technique which records the whole size distribution repeated size measurement with intermediate steps of removal of components by dissolution allows inference of the size distribution of the removed component as well as the residue in this way the size of the biogenic and lithogenic noncarbonate fractions can be determined observations of many size distributions suggest a minimum in grain size frequency curves at 8 to 10 ixm the dynamics of sediment erosion deposition and aggregate breakup suggest that fine sediment behavior is dominantly cohesive below 10 1xm grain size and noncohesive above that size thus silt coarser than 10 ixm displays size sorting in response to hydrodynamlc processes and its properties may be used to infer current speed silt that is f mer than 10 ixm behaves in the same way as clay 2 xm useful parameters of the distribution are the 10 63 ixm mean size and the percentage 10 63 ixm in the fine fraction we cannot use size distributions to distinguish the nature of the currents therefore to infer water mass advection speeds i e the mean kinetic energy of the flow ku regions of high eddy kinetic energy ke must be avoided at the present such abyssal regions lie under the high surface k e of major current systems gulf stream kuroshio agulhas antarctic circmpolar current and brazil falldand currents in the argenthe basin this is probably a satisfactory guide for the pleistocene with regard to the carbonate subfraction of the size spectrum size modes due to both cocco iths and foramlnlferal fragments can be recognized and analyzed with the boundary between them again at about 10 lm the flux of less than 10 lm carbonate at pelagic sites above the lysocline is another candidate for a productivity indicator

a mixed ophiuroid stylophoran assemblage echinodermata from the middle ordovician llandeilian of western brittany france