lithoherms on the florida hatteras slope sections of the florida hatteras slope were investigated during dives of the johnson sea link i submersible and us navy s submarine nr 1 numerous calcareous coral capped lithoherms often exceeding 40 m in elevation were encountered in the subphotic depths below 440 m one huge lithoherm complex was explored that forms a long slope parallel ridge perched on the seaward edge of mid slope terrace this lithoherm belt is 400 m wide more than 4 4 km long and 150 m high on its down slope flank direct observations and side scan imaging indicate that the long axes of individual ridge crest mounds trend perpendicular to the regional current flow away from the ridge individual lithoherms trend more parallel with regional current flow and occur without obvious bedrock control all of these structures have very steeply sloping flanks frequently 30 60 where cemented crusts are exposed by erosion thickets of living ahermatypic coral coat the crest and up stream flanks of these topographic features living deep water coral have modern 14c ages 700 80 year bp and trap older 3250 100 year bp sediment rich in fine grained aragonite elsewhere older deep water coral rubble 20 230 230 year bp traps younger sediment 13 760 140 year bp the trapped sediments contain 50 aragonite fines 62 5 m the friable crusts exposed on these flanks are generally similar in composition to the sediments that accumulate on the crests of these structures they are older 14c ages of 17 770 330 and 32 710 570 year bp and contain less aragonite 20 and more high magnesium calcite apparently lithoherms grow upwards by trapping fine sediments within ahermatypic coral thickets on their crests and upstream flanks cemented crusts may be a consequence of earlier paleoceanographic conditions or a result of gradual and ongoing cementation and diagenesis the large size platform edge position and linear trend of this deep water lithoherm complex are strikingly similar to a shallow marginal reef system

light element distributions n b li in baltic basin bentonites record organic sources abstract distributions of nitrogen boron and lithium were studied in illite smectite from ordovician silurian k bentonites in the baltic basin these trace elements are linked to thermal maturation of organic matter creating fluid signatures that are distinct from those in regionally buried sediments the fluid chemistry is ideally recorded in authigenic illite within the k bentonite because b substitutes in tetrahedral sites of illite li in octahedral sites and n is fixed in the interlayers as nh 4 three nanometric size fractions of illite 10 among these size fractions relate mainly to changes in fluid composition during illite growth bentonite volcanic ash layers mark stratigraphic timelines while illitization of the bentonite k bentonite varies as a function of burial and tectonic temperature gradients in the central baltic basin bentonites were deposited across a carbonate platform near estonia grading to deep basin shales near denmark and poland compositional variations in illite reflect fluids from different sources high n illite 2000 ppm in the southwest basin transitions to high b 250 ppm and high li 100 ppm illite to the northeast these trends follow a thermal gradient decreasing from southwest 200 c to northeast 90 c isotope ratios of b and li show opposing trends in samples across this thermal gradient the bisotopes decrease from southwest to northeast 18 to 4 while li isotopes increase from 5 to 32 the cambrian alum shale a regional hydrocarbon source rock was studied as a potential source of b and li as more than 50 of its b and li was associated with organic matter with increasing thermal maturity illite 11b decreased from 0 to 13 while 7li increased from 20 to 5 these opposing trends are similar to those observed in the k bentonites this suggests that organic sources of n b and li dominate fluids in bentonites from the southwestern baltic basin whereas basement derived fluids dominate in the northeastern baltic basin these trace elements record paleofluid changes during illite crystal growth and can identify pathways of chemically distinct oil and gas related fluids

mafic magma intraplating anatexis and hybridization in arc crust bindal batholith norway distinct with nd and o in the migmatites from 7 6 to 9 6 the dioritic velfjord plutons 448 ma were emplaced into and from 10 9 to 13 5 and in the contact granites regional migmatitic metapelitic and metacarbonate rocks at midfrom 5 2 to 7 5 and from 9 6 to 12 3 respectively crustal levels corresponding to pressures of 700 mpa exthus the contact granites could have a source that is isotopically humation to 400 mpa began while the migmatites were in a distinct from but mineralogically similar to the diatexites or they partly molten state with increasing proximity to the plutons could result from mixing of magma similar to the high k leucosomes regional stromatic migmatites change to diatexite and diatexitic with dioritic magmas mass balance calculations are consistent dikes are common within 500 m of the contacts we interpret these with the latter interpretation with proportions of granitic to dioritic relationships to indicate that heat from the plutons resulted in contact magmas from 7 1 to 7 3 emplacement and solidification of the migmatization in a zone up to 1 km wide typical residual dioritic plutons provided zones of structural anisotropy along which mineralogy in the diatexites is plagioclase quartz biotite high k leucosome magmas and contact granite magmas collected garnet sillimanite k feldspar consistent with biotite these magmas were injected by additional dioritic magma and dehydration melting podand dike like leucosomes consist of two further hybridized because the solidi of the plutons were several types earlier high k granitic ones with mineral assemblages hundred degrees higher than that of the granitic magmas the pluton identical to the migmatites and later low k tonalitic ones in which walls acted as long lived hot rigid surfaces along which magmas sillimanite is sparse and garnet absent the high k leucosome collected and migrated magmas can be explained by biotite dehydration melting at 700 mpa within the aureole mafic magmas were locally injected into and hybridized with the diatexites and the high k leucosome

chloritization of late ordovician k bentonites from the northern baltic palaeobasin influence from source material or diagenetic environment abstract the baltic ordovician silurian sedimentary succession embodies numerous altered volcanic ash beds that are illite smectite dominated only a few beds rich in chlorite smectite are known from the upper ordovician pirgu stage of estonia mixed layer chlorite smectite occurs commonly in low grade metamorphic and hydrothermal environments however chloritic k bentonites of the pirgu stage have never been buried deeply and lack signs of metamorphic overprinting in order to understand the origin of chloritization three distinct beds were sampled in 14 drillcores from estonia and latvia and analysed by means of xrd and sem the principal authigenic assemblage of the bulk samples consists of chlorite smectite corrensite together with illite smectite and k feldspar the actual mineral composition of k bentonites though varies from sample to sample the clay mineral assemblages range from virtually pure chlorite smectite to illite smectite dominated assemblages with minor or no chlorite smectite the proportion of chlorite smectite in k bentonites shows systematic lateral variations the share of chloritic phases is highest in the shallower water part of the palaeobasin and decreases towards the deeper part of the basin such regular lateral variations suggest a possible link between chloritization and the configuration of ancient palaeobasin the present study suggests that the chloritization of primary felsic ashes occurred during early diagenesis and that it was caused by an influx of mg rich water probably from a marine sabkha type environment

high resolution carbon isotope stratigraphy of the basal silurian stratotype dob s linn scotland and its global correlation since its designation as the global stratotype section and point gssp for the base of the silurian system the choice of dob9s linn southern scotland has received criticism due to the difficulties of relating its well constrained graptolite biostratigraphy to shallow water sequences elsewhere kerogen samples from across the ordovician silurian boundary interval at dob9s linn have yielded carbon stable isotope signatures consistent with those recorded elsewhere in particular showing a clear positive 13 c excursion in the terminal ordovician the architecture of the 13 c curve from dob9s linn enables very high resolution stratigraphic subdivision and direct correlation between the deep water dob9s linn section and time equivalent carbonate shelf deposits an integrated stratigraphic scheme using isotope stratigraphy and biostratigraphy of graptolites conodonts and shelly faunas has been constructed this direct correlation shows that the shallow water successions including the former stratotype candidate at anticosti island are generally incomplete with hiatuses related to the rapid sea level changes during the hirnantian stage this confirms and greatly increases the global utility of dob9s linn as a boundary stratotype

distribution and tectonic setting of ordovician k bentonites in the united kingdom twenty five ordovician k bentonite samples ranging in age from llanvirn to ashgill and representing both sides of the lapetus suture were analysed by inaa and xrf and compared on magmatic and tectonic discrimination diagrams the proportional distribution of nb y ti zr hf ta th and ree indicates that source volcanics were erupted in a setting that changed from subduction related in the llanvirn llandeilo to marginal basin extensional volcanism in the caradoc ashgill welsh k bentonites appear to be derived from volcanoes within the welsh basin but k bentonites from the southern uplands to not have characteristics consistent with a fore arc setting further suggesting a source related to ensialic marginal basin extensional volcanism

immobile and mobile elements during the transition of volcanic ash to bentonite an example from the early palaeozoic sedimentary section of the baltic basin abstract in order to check the immobility and mobility of elements during conversion of acidic volcanic glass to bentonites in normal marine environments we studied the composition of three altered volcanic ash layers from the palaeozoic of the baltoscandian region correlated through different facies regular changes in element concentrations in accordance with loss and gain of material during the transformation of volcanic ash indicate that al nb ti zr sn pt ta hf and th were generally immobile and can be used for the interpretation of source magma and correlation of ash layers cd behaves similarly with immobile elements and this can be explained with preservation only of the immobile portion of cd that is fixed in phenocrysts in bentonites in shales during the formation of kaolinite the data indicate small scale mobility of al and cd in lime muds where k feldspar forms from volcanic ash ta hf and th reveal some small scale mobility these slightly mobile elements must be used with caution for interpretation of thin ash layers with thicknesses of

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

metabentonites in the moffat shale group southern uplands of scotland geochemical evidence of ensialic marginal basin volcanism metabentonites occur extensively in the moffat shale group of the southern uplands of scotland at dob s linn 135 metabentonite beds 1 50 cm thick occur in ashgill to llandovery strata representing an aggregate thickness of 6 m of compacted ash accumulated over approximately 25 ma the metabentonites are characterized by relatively high concentrations of trace elements including ba cs hf nb rb ta th u y zr and rees which were inherited from evolved vitric ash immobile trace element data indicate that a spectrum of silicic ash compositions accumulated ranging from subalkaline to mildly peralkaline in the late ordovician n gracilis to g persculptus biozones subalkaline ash falls predominated whereas peralkaline ash falls predominated in the llandovery silurian p acuminatus to m convolutes biozones giving way to predominantly subalkaline ash falls during accumulation of the m sedgwickii to r maximus biozones changeovers in the dominant ash types are marked by increased proportions of ash the magmas from which the ash types evolved were generated in an ensialic arc transitional to a back arc setting and involved attenuated sialic crust and mantle characterized by variable depletion in hfs elements lithological petrological and ree characteristics suggest that the moffat shale group is not exclusively pelagic in origin and probably accumulated in a back arc basin bordering an ensialic arc terrane

new isotopic data solving an old biostratigraphic problem the age of the upper ordovician brachiopod holorhynchus giganteus recent work has shown that there is a pronounced positive late ordovician excursion in both 13 c and 18 o which appears to be global and identifies a clear chemostratigraphic interval that is confined to part of the hirnantian stage pre hirnantian rawtheyan brachiopod carbonate values of 13 c are typically in the range 1 to 1 pdb and lower hirnantian values are typically in the range of 4 to 7 o pdb this contrast in isotopic values has been used to assess the age of the distinctive late ordovician brachiopod holorhynchus giganteus the age of the holorhynchus association is important in terms of its relationship to the major late ordovician extinction that was initiated at the start of the hirnantian the species which traditionally was regarded as uppermost ordovician hirnantian in age has been shown on stratigraphic evidence to be pre hirnantian at many locations if this is so it predates the first phase of extinction and is not associated with the hirnantia fauna which characterizes the interval between the two main episodes of late ordovician extinction however at its type locality in the asker district of norway the stratigraphic evidence has pointed to it being hirnantian in age to resolve this ambiguity analyses have been made on holorhynchus at three locations 1 in the east baltic region where holorhynchus occurs below demonstrable hirnantian rocks 2 in the boda limestone of central sweden where holorhynchus occurs high on a carbonate mud mound where its age is unclear and 3 in the type area of the species in norway at all three locations the 13 c values are low indicating a pre hirnantian pre extinction age for holorhynchus the re assessment of the age of the asker sequence in norway implies the presence of a cryptic unconformity there and suggests uplift rather than subsidence during the latest ordovician this study emphasizes the value of chemostratigraphy in high resolution stratigraphy

geochemistry of telichian silurian k bentonites in estonia and latvia in the telychian section of estonia and latvia k bentonites from 45 volcanic eruptions were discovered the thickness of k bentonite interbeds varies from a few millimetres to 20 cm the sodium component concentration in sanidine phenocrysts measured by xrd ranges from 20 to 48 mol and was used for establishing correlations the ti zr nb th and sr concentrations and ratios show temporal trends indicating fractional crystallization in magma chambers the analysis of biotite phenocrysts revealed magnesium and iron rich biotites in bentonites synthesis of these geochemical data enabled a classification of bentonites into seven geochemical types which probably originate from seven different volcanic sources isopach schemes indicate ash transport from the west and north west directions

composition and correlation of volcanic ash beds of silurian age from the eastern baltic sanidine composition and bulk geochemistry of volcanic ash beds from the east baltic indicate the subalkaline nature of the volcanism near the margins of the baltica plate during the silurian several bentonites in the wenlock include a previously unknown sanidine with 48 to 58 mol of the na ca component in contrast to the earlier telychian volcanism sodium rich sanidine occurs in ash beds which originate from relatively moderately evolved dacitic magma the studied material from two drill cores integrated with previous research enables production of a more complete list of 49 volcanic eruption layers for the lower to middle wenlock in the east baltic this updated list of bentonites characterized by their sanidine compositions forms a good basis for future integrated bio and chemostratigraphic correlations in northern europe

high resolution stable isotope stratigraphy of upper ordovician sequences constraints on the timing of bioevents and environmental changes associated with mass extinction and glaciation the two phases of the late ordovician mass extinction are approximately coeval with the periods of rapid climate change associated with the onset and demise of the gondwanan glaciation in this paper we argue that the distinctive late ordovician carbon isotope profile provides a chronostratigraphic ruler against which a sequence of environmental and biotic events may be located the ruler also allows regional and global high resolution correlation of successions representing very different environments cores from the upper ordovician succession of estonia and latvia record a large 1 3 c c a r b o n a t e excursion up to 6 with a similar profile shape the consistent relationship between the chemostratigraphy and biostratigraphy in the baltic region suggests that the isotope profile has a regional chronostratigraphic value the presence of similar profiles in nevada united states suggests that the excursion is a global chronostratigraphic signal this interpretation enables a detailed correlation to be made between upper ordovician shallow marine and basinal sequences that have wholly different faunas successions in the baltic area and in canada that do not display the model profile are interpreted as incomplete reinter pretation of these important successions significantly modifies the global database used to assess the pattern of diversity change during the mass extinction key levels of environmental change have been located against the carbon isotope profile new oxygen isotope data from brachiopod and ostracode calcite set tight limits on the start of the glacial events cooling and sea level fall started at the same stratigraphic level as the start of the carbon isotope excursion the later rise in sea level and fall in oxygen isotope values record the end of the glaciation these restrict the duration of the main glaciation to only 1 5 graptolite zones we propose that models of the carbon cycle should be adapted to be consistent with the temporal relationships between carbon cycling sea level fall and temperature change documented here the chronostratigraphic ruler provided by the carbon isotope profiles is used as a scale to determine the sequence of biotic changes and to allow high resolution correlation of biotic events at different locations this approach identifies regional similarities and differences in the patterns of extinction the main phase of graptolite extinction in the monitor range nevada for example is synchronous with the chitinozoan extinction in the baltic region but chitinozoan taxa survive to higher levels in nevada the benthic faunas in the baltic region demonstrate that the main extinction event corresponded with the beginning of the isotope excursion at the start of the hirnantian the level at which the marine environment started to change rapidly but that there were further extinctions of species within the early hirnantian the cold adapted hirnantia fauna did not appear immediately after the extinction in this area the relationship between the second phase of extinction and the carbon isotope excursion is less clear but the available data suggest that the extinction coincides with a time of rapid environmental change but not at the inception of environmental change as happened in the first phase

upper ordovician δ13corg chemostratigraphy k bentonite stratigraphy and biostratigraphy in southern scandinavia a reappraisal a pioneer 13corg study through the upper sandbian and katian upper ordovician succession in the rostanga 1 drill core in the classical geological outcrop area at rostanga in southernmost sweden produced a wealth of new carbon isotope data which are useful for local and regional correlations among the upper ordovician positive 13c excursions the guttenberg gice waynesville saunja whitewater moe paroveja and hirnantian hice isotopic carbon excursions are recognized but the kope rakvere 13c excursion is missing suggesting a stratigraphic gap all these isotopic excursions are tied closely to biostratigraphy especially graptolite biostratigraphy and in the case of the waynesville saunja and whitewater moe excursions for the first time anywhere in the world the rostanga gice 13corg curve from the upper sularp shale shows a striking similarity to that of the katian gssp in oklahoma suggesting the potential of trans atlantic correlation based on a projection from the katian gssp the previously poorly constrained position of the base of the katian in southern sweden appears to be in the uppermost sularp shale in strata of the upper diplograptus foliaceus zone previous interpretations of the relations between k bentonite successions in southern scandinavia are somewhat revised and the kinnekulle k bentonite is recognized for the first time in scania based on new radiometric dates this very prominent and widespread ash bed appears to be slightly older than the deicke and millbrig k bentonites in eastern north america

correlation of ordovician mohawkian k bentonites in the upper mississippi valley using apatite chemistry implications for stratigraphic interpretation of the mixed carbonate siliciclastic decorah formation abstract four ordovician k bentonites have been chemically fingerprinted using trace element content of apatite phenocrysts contained within the altered ash layers trace element analysis was performed using electron microprobe on individual crystals the bentonites include the deicke millbrig elkport and dickeyville beds these beds lie within the decorah formation of the northern mississippi valley outcrop area all four k bentonite beds are believed to have a taconic source whereas volcanic ash was transported by wind and deposited in the ordovician north america epeiric sea the decorah is a marine unit shaly to the north and west and carbonate rich to the south and east thirteen k bentonite samples were collected from six localities only one locality dickeyville wi contains all four beds in succession one hundred seventy one apatite crystals were handpicked and analyzed for major minor and trace elements using an electron microprobe each crystal was analyzed at three to six separate spots to ascertain compositional variation the most discriminating and diagnostic elements were mg and mn plotting mg vs mn content produces data clusters characteristic for each k bentonite clusters show only minimal change between localities allowing individual k bentonites to be identified and used for stratigraphic correlation time slices provided by the k bentonite horizons show that the decorah formation is composed of reciprocal wedges stratigraphic sequences of shale and carbonate shale was deposited in deeper water within the hollandale embayment but pinched out southeastward onto the flank of the wisconsin dome carbonates were formed largely in shoal water on the arch prograding northwestward and shaling out downramp into the embayment

tectonics and distribution of gold deposits in china an overview gold exploration in china has expanded rapidly during the last two decades since a modern approach to economic development has become a national priority china currently produces 180 tonnes t of gold annually which is still significantly less than south africa usa and australia however china is now recognized as possessing significant gold resources in a wide range of mineral deposit types present estimates of gold resources in china exceed 4 500 t which comprise 60 in gold only deposits more than 25 in base metal rich skarn porphyry and vein deposits and more than 10 in placer accumulations the major gold provinces in china formed during the main episodes of phanerozoic tectonism such tectonism involved interaction of china s three major precambrian cratons north china tarim and yangtze or south china when combined with cathaysia block with the angara or siberian kazakhstan kyrgyzstan and indian cratons resulting collisions included deformation of accreted oceanic sequences between the cratonic blocks the most important ore forming orogenies were 1 the late paleozoic variscan 405 270 ma which led to amalgamation of the angara north china and yangtze cratons 2 the indosinian 270 208 ma which led to the collision of north china and south china cratons 3 the yanshanian 208 90 ma which was largely influenced by the subduction of the izanagi pacific plates beneath eastern china and 4 the himalayan 90 ma indentation of the indian continent into eurasia no important precambrian gold systems are recognized in china mainly because of reworking of exposed precambrian rocks by these younger orogenies but there are a few caledonian 600 405 ma gold bearing systems in northern xinjiang most of china s orogenic epithermal and carlin like gold deposits are in the reworked margins of major cratonic blocks and in metasedimentary rock dominated fold belts adjacent to these margins accordingly the major gold provinces are present along the northern southeastern and southern margins of the north china craton along the southwestern and northwestern margins of the yangtze craton in the tianshan and altayshan orogenic belts in northern xinjiang and throughout the southeastern china fold belt gold placer deposits derived from these primary deposits are concentrated in the northernmost part of northeastern china and along the northwestern margin of the yangtze craton the major provinces with significant gold in porphyry related copper systems and base metal skarns are present in the yangtze river area along the northeastern and southeastern margin of the yangtze craton in the fold belt in southwestern china and scattered through northern china three quarters of the chinese gold only deposits occur within the north china craton margins half are located in the uplifted precambrian metamorphic rocks and most of the remainder are hosted in the phanerozoic granitoids that intruded the reworked precambrian terranes the abundance of granite hosted gold contrasts the north china craton with other precambrian cratons such as those in western australia central canada and zimbabwe where gold is mainly hosted in the archean greenstone belts this difference may be explained by the multiple episodes of phanerozoic tectonism along the north china craton margins resulting from the collision of the angara north china and south china cratons and from subduction of the izanagi pacific oceanic plates underneath the eastern china continent

upper wenlock bentonites from wren s nest hill dudley comparisons with prominent bentonites along wenlock edge shropshire england the upper wenlock series homerian stage of the northern midland platform england contains numerous volcanic bentonite clay layers at wren9s nest hill dudley 15 bentonites have been investigated and comparisons with the type wenlock have been made by means of two key sections along wenlock edge shropshire in total 22 bentonites have been investigated and their clay and sand grade mineralogies determined rare earth element ree and yttrium concentrations of apatite grains contained within ten of the bentonites have been established allowing geochemical fingerprinting as an indication of provenance of source magmas and identification of geochemical marker beds based on the analysis of ree and yttrium concentrations it seems likely that the majority of these bentonites originated from a granodiorite magmatic source comparisons with published llandovery and lower wenlock age bentonites indicate generally more enrichment in light rees relative to heavy rees in addition close geochemical similarities between bentonites along wenlock edge and at wren9s nest hill strongly argue for their presence as precise stratigraphic equivalents within the upper much wenlock limestone formation these correlations are further supported by geophysical data from borehole wire line logs across the west midlands finally a chemically distinct mid homerian episode of volcanism is identified and represents a potentially important marker interval between the study area and other similarly aged bentonites reported from the island of gotland sweden

upper ordovician sequences of western estonia abstract the upper ordovician uppermost caradoc ashgill section of western estonia consists of a series of seven open shelf carbonate sequences depositional facies grade laterally through a series of shelf to basin facies belts grain supported facies shallow shelf mixed facies middle shelf mud supported facies deep shelf and slope and black shale facies basin locally a stromatactis mud mound occurs in a middle to deep shelf position shallow to deep shelf facies occur widely across the estonian shelf and grade laterally through a transitional slope belt into the basinal deposits of the livonian basin each sequence consists of a shallowing upward prograding facies succession sequences 1 upper nabala stage and 2 vormsi stage record step wise drowning of underlying shelf units lower nabala that culminated in the deposition of the most basinal facies fjacka shale in the livonian basin sequences 3 6 comprise the overlying pirgu stage and record the gradual expansion of shallow and middle shelf facies across the estonian shelf the porkuni stage sequence 7 is bracketed by erosional surfaces and contains the shallowest water facies of the preserved strata the uppermost part of the section normalograptus persculptus biozone is restricted to the livonian basin and includes redeposited carbonate and siliciclastic grains it is the lowstand systems tract of the lowest silurian sequence 8 sequence 7 and the overlying basinal redeposited material i e the lowstand of sequence 8 correspond to the latest ordovician hirnantian glacial interval and the bracketing unconformities are interpreted as the widely recognized early and late hirnantian glacial maximums the sequences appear correlative to upper ordovician sequences in laurentia graptolite biozones indicated that the estonian sequences are equivalent to carbonate ramp sequences in the western united states great basin and mixed carbonate siliciclastic sequences in the eastern united states appalachian basin cincinnati arch region these correlations indicate a strong eustatic control over sequence development despite the contrasting tectonic settings of these basins

multiphase silurian bentonites in the baltic palaeobasin abstract the mineral composition of altered volcanic ash beds bentonites deposited in marine environments is typically characterized by smectite or illite smectite type clay minerals silurian bentonites from an unmetamorphosed normal marine sedimentary sequence in the eastern baltic palaeobasin however show a complex and varied mineral composition with illite smectite k feldspar and kaolinite as the dominant authigenic phases the whole rock xrd study of 52 bentonite beds from two drill cores ruhnu and ohesaare southwestern estonia reveals a range of compositions varying between different beds as well as laterally within a single bed the observed variations are found to reflect the bentonite development and diversification during initial stages of volcanic ash transformation and diagenesis that was probably overprinted by a later k metasomatic event

u pb zircon age constraints on the timing and duration of wenlock silurian paleocommunity collapse and recovery during the big crisis high precision isotope dilution u pb zircon dating was conducted on three volcanic ash fall bentonite samples from the swedish island of gotland and on a fourth bentonite from the west midlands england zircons from the ireviken grotlingbo djupvik gotland and wren s nest hill 15 west midlands bentonites yielded weighted mean pb 206 u 238 ages of 431 83 0 23 0 67 ma 428 45 035 0 73 ma 428 06 0 2110 66 ma and 427 86 032 0 71 ma respectively analytical total uncertainties these biostratigraphically well controlled age dates effectively bracket the wenlock epoch of the silurian period and provide control for the duration of one of the major paleozoic biotic events and associated perturbations to the global carbon cycle the big crisis or lundgreni event graptolites the niulde event conodonts the mulde excursion carbon isotopes these new data suggest an older and shorter duration for the recalibration of the wenlock series and demonstrate that the cascade of biological and chemical events that took place during the big crisis happened on time scales of tens to hundreds of thousands of years less

precise timing of the late ordovician sandbian super eruptions and associated environmental biological and climatological events two of the largest known eruptions in the phanerozoic produced the ordovician millbrig k bentonite of north america and the kinnekulle k bentonite of scandinavia which have been previously suggested to be coeval the millbrig k bentonite from kentucky usa and the kinnekulle k bentonite from bornholm denmark yielded chemical abrasion thermal ionization mass spectrometry u pb zircon dates of 452 86 0 29 and 454 41 0 17 ma 2 analytical uncertainty respectively thus showing significant age differences contrary to what is generally held these data and four additional newly dated k bentonites directly establish the first radioisotopic age constraints for the ordovician katian sandbian global stage boundary refine global stratigraphic correlations date associated chemostratigraphic events and suggest an alternative volcanic climate hypothesis for the late ordovician supplementary material u pb radioisotopic data table and analytical methods are available at www geolsoc org uk sup18636

upper katian ordovician bentonites in the east baltic scandinavia and scotland geochemical correlation and volcanic source interpretation altered volcanic ash interbeds bentonites in the upper katian of baltoscandia indicate significant volcanic activity in neighbouring tectonically active areas katian bentonites in the east baltic can be reliably correlated using sanidine phenocryst composition ratios of immobile trace elements tio 2 nb zr and th to al 2 o 3 enable extension of the correlations to scandinavia where late diagenetic alterations could have caused recrystallization of sanidine phenocrysts at least seven volcanic eruptions were recognized in baltoscandian sections several bentonites found in deep sea sediments are absent in shallow sea sediments indicating extensive breaks in sedimentation and erosion during late katian and hirnantian times the areal distribution pattern of katian bentonites in baltoscandia indicates a volcanic source from the north or northwest present day orientation from the margins of the iapetus palaeo ocean signatures of ultra high pressure metamorphism in the seve nappe central sweden and intrusions in the helgeland nappe complex in central norway have been proposed as potential sources of the magmas that generated the volcanic ashes deposited in the east baltic in katian times geochemical similarities between baltoscandian and dob9s linn bentonites from southern scotland suggest a common volcanic source in katian times

end ordovician extinctions a coincidence of causes the end ordovician hirnantian extinction was the first of the five big phanerozoic extinction events and the first that involved metazoan based communities it comprised two discrete pulses both linked in different ways to an intense but short lived glaciation at the south pole the first occurring at or just below the normalograptus extraordinarius graptolite biozone mainly affected nektonic and planktonic species together with those living on the shallow shelf and in deeper water whereas the second within the n persculptus graptolite biozone was less focused eradicating faunas across a range of water depths in all about 85 of marine species were removed proposed kill mechanisms for the first phase have included glacially induced cooling falling sea level and chemical recycling in the oceans but a general consensus is lacking the second phase is more clearly linked to near global anoxia associated with a marked transgression during the late hirnantian most recently however new drivers for the extinctions have been proposed including widespread euxinia together with habitat destruction caused by plate tectonic movements suggesting that the end ordovician mass extinctions were a product of the coincidence of a number of contributing factors moreover when the deteriorating climate intensified causing widespread glaciation a tipping point was reached resulting in catastrophe