late ordovician glacial record of the anti atlas morocco abstract late ordovician glaciogenic deposits are exposed intermittently along an 800 km long outcrop belt in the anti atlas mountains of southern morocco these deposits are of economic significance as potential oil bearing sandstones in the tindouf and boudenib basins and thus are here re examined as analogues to subsurface hydrocarbon reservoirs glaciogenic deposits of the upper second bani formation rest unconformably upon underlying shallow marine clastic deposits the unconformity is characterised by a series of palaeovalleys some 0 5 1 0 km wide and up to 100 m deep which may have been cut under elevated hydrostatic pressures as tunnel valleys beneath a late ordovician ice sheet the valleys and intervalley areas are filled with glaciogene sediments categorized into five facies associations namely 1 a tabular sandstone association shallow marine shoreface deposits 2 a massive sandstone and conglomerate ice contact debrites 3 meandriform sandstone deposits ice proximal sandur 4 stratified diamictites ice rafted debris and 5 sigmoidally bedded sandstones intertidal sandstones deformation in these sediments is ubiquitous and includes soft sediment striated pavements metre scale duplex systems thrust and fold belts of deformation affecting some tens of metres of sediment and pervasive lineations these features are interpreted to record the complex nature of deformation processes operating beneath a late ordovician ice sheet including sliding at the ice bed interface folding and deformation within the sediment column and a series of complex ramps detachments and shear zones within an unconsolidated pile of sediment beneath the ice sheet

carbon isotopes and event stratigraphy near the ordovician silurian boundary yichang south china abstract the organic carbon isotope data through the unweathered ordovician silurian boundary strata at the wangjiawan riverside section which is c 180 m southeast of the wangjiawan north section the gssp for the base of the hirnantian stage upper ordovician show that a positive 13 c org excursion begins just below the base of the hirnantian stage and peaks in the lower part of the n extraordinarius biozone this is followed by an interval of slightly reduced 13 c values and a second peak of 2 above pre hirnantian values which occurs in the lower part of the n persculptus biozone upper hirnantian the peaks in 13 c org values can be correlated well with episodes of glacial expansion described from africa and peri gondwanan europe evidence from sedimentological faunal and geochemical data from south china as well as the evidence of glacial sediments in north africa all suggests a short lived glaciation of 1 ma in the ordovician south pole region consisting of at least two major pulses the first phase of gradual glacial expansion began just before the hirnantian epoch and the second phase ended with rapid melting in the late hirnantian early n persculptus biozone

long lived deformation in the southern mississippi embayment revealed by high resolution seismic reflection and sub bottom profiler data three high resolution seismic reflection profiles and two sub bottom profiler sections acquired along the mississippi river in southern central u s image deformation in post paleozoic sediments the northernmost profile images two faults offsetting cretaceous through at least eocene cane river reflectors interpreted to strike northwest and to be part of the arkansas river fault zone the central profile shows a down to the north fault displacing cretaceous and paleocene midway group reflectors by 210 m and 160 m respectively the fault is interpreted as the northern edge fault of the monroe uplift a late cretaceous uplift associated with igneous intrusions the southernmost profile displays a down to the south fault offsetting cretaceous and paleocene eocene wilcox group reflectors by 125 m and 32 m respectively tilted reflectors in the first 80 m indicate eocene oligocene activity of the fault although quaternary activity cannot be ruled out quaternary tectonic activity is proposed for a series of faults that offset shallow 40 m depth eocene sequences and the base of the quaternary alluvium as imaged on two sub bottom profiler sections these shallow faults are imaged in the vicinity of holocene earthquake induced liquefaction fields corroborating the evidence for recent tectonic activity in the area the spatial coincidence of the imaged faults with the inferred location of the alabama oklahoma transform strongly argues toward a long lived influence of this precambrian continental margin in focusing tectonic activity in the southern u s by controlling the reactivation of triassic jurassic syn rift basement structures and guiding the emplacement of late cretaceous igneous intrusions and the location of cenozoic deformation

magnetotelluric observations in the western ouachita mountains southeastern oklahoma the first magnetotelluric mt analysis of the ouachita mountains region is presented magnetotelluric data acquired at 19 sites along a 60 km profile in southeastern oklahoma were used to image the western extension of the ouachita mountains and to determine the poorly known subsurface interaction between the pennsylvanian tishomingo belton uplift and the subsurface extension of the exposed western ouachita mountains drill hole data geologic mapping seismic reflection profiles and 1 d and 2 d mt derived models indicate that lying beneath the low resistivity gulf coastal plain sediments are 2 3 km of deep water lower pennsylvanian jackfork group sediments and 6 8 km of ouachita facies lithologies mainly consisting of the stanley group beneath the profile s northern section are 2 4 km of atoka formation sediments probably deposited within the arkoma basin that underlie thrusted zones of the stanley group the most unique feature is a high resistivity zone beneath stations 7 9 interpreted to be p

a far field record of the end ordovician glaciation the ellis bay formation anticosti island eastern canada abstract the end ordovician was a critical period in earth history it was a time of global climatic change with large scale continental glaciations a wealth of recent work has documented the stratigraphical records from the near field areas i e from various gondwanan continental relics where glaciation was more severely felt there is an urgent need to re examine the stratigraphic record from the far field areas at greater levels of resolution than it has been hitherto possible in order to understand the timing duration and character at the glaciation centred around the ordovician silurian o s boundary one of the best exposed and most complete stratigraphic records from paleotropical areas near the boundary is on anticosti island in eastern canada the east west trending 200 km long ellis bay hirnantian outcrop belt with superb coastal exposures at both ends of anticosti island is slightly oblique to the paleoshoreline with near shore siliciclastic dominated facies restricted to the eastern part of the island and more offshore carbonate dominated facies present along the central and western parts these mixed carbonate siliciclastic facies accumulated along a storm dominated beach to offshore profile they are organized into a stacked set of shallowing upward offshore to shoreline facies successions typically coarsening upward separated by deepening upward facies successions typically fining upward formed during intervening transgressions high frequency glacio eustatic fluctuations were the dominant control on the development of tr sequences observed in the ellis bay succession of anticosti island although tectonic subsidence had a substantial secondary effect on thickness five tr sequences are recognized allowing for precise correlation along the ellis bay outcrop belt in the eastern sections relatively thin tr sequences are locally bounded by unconformities a typical sequence begins with thin transgressive limestone overlain by a maximum flooding calcareous shale grading up into regressive storm influenced proximal to transitional zone fine sandstone grainstone and shale which are sharply overlain by shoreface to foreshore sandstone grainstone in the central and western sections the thicker tr sequences have more symmetrical transgressive regressive components reflecting greater subsidence compensated by a higher carbonate sediment supply two discrete glacial periods are recognized during the deposition of the hirnantian ellis bay succession with the younger one recording large sea level changes 50 m in magnitude caused by the development of very large ice sheets over western gondwana abundant ice conditions on the globe during the end ordovician and the resulting high amplitude eustasy appear to have left distinctive stratigraphical imprints on mixed paleotropical siliciclastic carbonate ramps

palynomorph optical analyses for thermal maturity assessment of upper ordovician katian hirnantian rocks from southern estonia abstract application of integrated multi proxy optical analytical techniques for the assessment of thermal maturity of organic matter was tested in katian to hirnantian upper ordovician sediments from a subsurface succession in southern estonia the investigated valga 10 drill core section consists of a biostratigraphically well constrained succession deposited in the outer part of a carbonate shelf livonian basin the diverse and well preserved palynological assemblages permitted the analysis of thermally induced modifications of optical properties of small and large acanthomorph acritarchs qualitative methods such as uv fluorescence and acritarch alteration index aai were calibrated against the quantitative palynomorph darkness index pdi the occurrence of vitrinite like organoclasts allowed reflectance measurements in some of the investigated samples the strong fluorescence and pale to yellow colour of acritarchs combined with low pdi and organoclast reflectance values suggest thermally immature kerogen and a limited burial diagenesis in the study area moreover pdi showed a good correlation with uv fluorescence allowing a calibration of these two techniques and proving the reliability of pdi for thermal maturity estimation in sedimentary rocks unsuitable for rock eval pyrolysis and vitrinite reflectance furthermore the applied methodologies permitted the differentiation of reworked from in situ organic matter and highlighted the potential of the former for sequence stratigraphic investigations

marine redox variability from baltica during extinction events in the latest ordovician early silurian abstract it is well documented that upper ordovician and silurian successions record multiple marine turnover events including the second largest mass extinction in the phanerozoic widespread glaciation and multiple global carbon cycle perturbations whereas causal mechanisms for the late ordovician major mass extinction event involving climate paleoceanographic variation and 13c records have been published similar records remain poorly constrained for subsequent extinction events in the early silurian here we present new organic matter carbon isotope 13corg chemostratigraphy and corresponding paleoredox proxies fe speciation mn v mo and pyrite sulfur isotopes from two organic rich drill core sections in sweden and latvia that span the upper katian through lower wenlock stages 446 431 ma pyritized fe and bulk sedimentary mn concentrations from upper ordovician strata in southern sweden suggest a local redox shift to more reducing conditions in the late hirnantian possibly including euxinic anoxic and sulfidic water column conditions that coincide with the second mass extinction pulse the new high resolution 13corg and 34spyr datasets from the late aeronian early silurian interval within both drill cores show positive excursions that are broadly coincident with the associated sandvika and sedgwickii extinction events independently fe speciation and bulk sedimentary trace metal data from this late aeronian interval record locally euxinic conditions in both the deep basinal sweden and distal shelf latvia settings before and during the late aeronian positive 13c excursion this multiproxy paleoredox dataset provides the first direct evidence for local to regional expansion of reducing marine conditions coincident with this early silurian late aeronian biotic event and positive 13c excursion additionally new 34spyr values spanning the llandovery wenlock boundary interval in the latvia core show a positive excursion coincident with fe speciation and trace metal enrichments that imply a local redox perturbation with intermittently euxinic bottom waters during the rising limb of the ireviken positive 13c excursion the combination of these geochemical data for local to regional scale more data required for global interpretations changes in marine redox conditions with paleobiological records and evidence for eustatic sea level rise indicate that environmental stresses related to an expansion of anoxic to euxinic conditions were a probable driver for several extinction events during the latest ordovician early silurian

evolution of gravity anomalies across collisional mountain belts clues to the amount of continental convergence and underthrusting a series of density models illustrates the gross form of free air and bouguer gravity anomalies anticipated during ocean basin closure and consequent development of collisional orogens when compared to gravity anomalies observed across some mountain belts the hypothetical anomalies provide a clue to the degree of under thrusting of crust associated with one lithospheric plate beneath crust of the opposing plate margin the results of the study suggest very early stage collision in the ouachita mountains of arkansas and the sulaiman range of pakistan with thin transitional or oceanic crust still intact on the lower plate in contrast the himalaya of pakistan represent a much more advanced stage of collision where continental crust may have underthrust the mountains for 600 km

gravity constraints on lithosphere flexure and the structure of the late paleozoic ouachita orogen in arkansas and oklahoma south central north america spectral analysis of bouguer gravity anomalies in western central arkansas and eastern oklahoma indicates that the thickness of the crust in the ouachita fold and thrust belt increases from 38 km in the western ouachitas to 44 km in the eastern ouachitas the change in crustal thickness occurs near the western end of the broken bow uplift and coincides with an abrupt decrease in the flexural rigidity of the lithosphere from 1 8 1024 n m in the western ouachitas to 5 0 1023 n m in the eastern ouachitas the flexural rigidity in the western ouachitas is similar to values determined in the appalachian fold and thrust belt and coincides with the depth of the 450 c isotherm predicted by conductive cooling models for the thermal evolution of the early paleozoic southern laurentian rifted continental margin the thick crust in the eastern ouachitas results in lithosphere that is anomalously weak for rifted continental crust of this age the thicker crust is attributed to an eastward transition from a rift segment to a transform segment of the paleozoic continental margin a layered density model derived from the gravity data shows that strata interpreted to be deformed ouachita facies rocks are thickest in the eastern ouachitas and are consistent with a greater amount of shortening in the central thrust belt in arkansas as compared to oklahoma the opposite relationship is observed in the frontal ouachita province where shortening appears greater in oklahoma the cross strike changes in the locus of shortening crustal thickness flexural rigidity and the inferred transition from rift to transform segments of the early paleozoic continental margin all coincide with the location of a previously hypothesized zone of diffuse rightlateral shear located at the western end of the benton uplift flexural modeling indicates that the load required to produce the observed bouguer gravity low in the arkoma foreland basin trends parallel to the benton and broken bow uplifts but is located 114 to 276 km farther south in the western ouachitas the position of the load coincides with the northern edge of the sabine uplift and is interpreted to mark the southern extent of ouachita facies rocks that were emplaced on the laurentian continental margin and or attached remnant oceanic crust

did cooling oceans trigger ordovician biodiversification evidence from conodont thermometry the ordovician period long considered a supergreenhouse state saw one of the greatest radiations of life in earth9s history previous temperature estimates of up to 70 c have spawned controversial speculation that the oxygen isotopic composition of seawater must have evolved over geological time we present a very different global climate record determined by ion microprobe oxygen isotope analyses of early ordovician silurian conodonts this record shows a steady cooling trend through the early ordovician reaching modern equatorial temperatures that were sustained throughout the middle and late ordovician this favorable climate regime implies not only that the oxygen isotopic composition of ordovician seawater was similar to that of today but also that climate played an overarching role in promoting the unprecedented increases in biodiversity that characterized this period

paleozoic continent ocean transition in the ouachita mountains imaged from passcal wide angle seismic reflection refraction data a wide angle reflection reflection experiment sponsored by the program for array seismic studies of the continental lithosphere passcal was conducted in the ouachita mountains area of southwestern arkansas and northwestern louisiana this experimental employed 400 state of the art seismic recorders and overlapped the southern one third of the cocorp consortium for continental reflection profiling deep seismic reflection profile in the area a wide variety of data processing and interpretation techniques was employed to derive an earth model from these data the model depicts a preserved early paleozoic continental margin buried beneath allochthonous paleozoic strata and younger sedimentary rocks the southern part of the model indicates the presence of oceanic or highly extended continental crust overlain by about 15 km of mostly paleozoic sedimentary rock these results are consistent with little if any shortening of crystalline continental crust during the ouachita orogeny

bioproductivity rise in the east baltic epicontinental sea in the aeronian early silurian abstract the occurrence of black shale in deep shelf environments chertification in shoreward carbonate facies barite occurrences positive 13 c excursions in carbonate sections and enhanced micritic limestone accumulation give evidence of a primary bioproductivity rise suggesting upwelling in the aeronian of the east baltic the end rhuddanian onset of the rise in primary bioproductivity corresponds to the beginning of black shale formation on the deep shelf and micritic limestone formation on the shallower shelf the bioproductivity maximum was in the mid aeronian judging from the highest values of 13 c upwelling ceased and primary productivity decreased at the end of aeronian

early paleozoic transform margin structure beneath the mississippi coastal plain southeast united states a geophysical transect across the central gulf of mexico coastal plain shows that the early paleozoic continental margin of southern laurentia is preserved in a nearly pristine state beneath younger strata that were emplaced during the late paleozoic ouachita orogeny and formation of the modern gulf of mexico coastal plain the thickness of the crystalline crust decreases abruptly across the margin over a distance of 50 km from 35 km beneath the black warrior foreland basin to 10 km beneath the ouachita fold and thrust belt this abrupt decrease in crustal thickness is similar to modern transform margins but very different from most rifted margins which display much more gradual transitions in crustal thickness the geophysical data indicate an absence of synrift intrusive and volcanic rocks underplated mafic rocks at the base of the crust and abnormally thick oceanic crust adjacent to the margin the lack of these features is also characteristic of modern transform margins combined with transects across the margin farther west the data confirm previous suggestions that the central gulf of mexico coastal plain overlies an 800 km long transform segment of the late proterozoic early paleozoic southern laurentian continental margin that extends continuously from western arkansas to southeast alabama

the end ordovician glaciation and the hirnantian stage a global review and questions about late ordovician event stratigraphy abstract this paper proposes a global review of hirnantian event stratigraphy the hirnantian gssp in south china is tentatively correlated with latest ordovician strata from the peri gondwanan glacial regions problems of biostratigraphical correlation are highlighted at a worldwide scale the major biostratigraphically useful fossil groups graptolites chitinozoans brachiopods conodonts acritarchs are analysed and their limits for global correlation of the uppermost ordovician are discussed palaeobiogeographical disparities are invoked as the primary cause of the difficulty in establishing an effective late ordovician global biostratigraphical scheme as an alternative correlative tool the hice hirnantian isotopic curve excursion event is often put forward in the literature however carbon isotope chemostratigraphy shows like biostratigraphy some limits to the present state of knowledge no good independent biostratigraphical control of the hice exists in both shallow carbonate deposits and deeper shaly ones recent studies have also demonstrated inconsistencies between carbon isotopic signals obtained from organic 13corg and inorganic 13ccarb carbon species further complicating the use of the hice as an isochronous benchmark all of these difficulties for hirnantian event stratigraphy are discussed in detail in order to enable them to be overcome in the future precise late ordovician and early silurian event stratigraphies are essential for the understanding of the mechanisms linked to the first of the big five extinctions

geologic analysis of the upper jurassic haynesville shale in east texas and west louisiana the upper jurassic haynesville shale is currently regarded as one of the most prolific emerging shale gas plays in the continental united states it has estimated play resources of several hundred trillion cubic feet and per well reserves estimated as much as 7 5 bcf the reservoir spans more than 16 counties along the boundary of eastern texas and western louisiana although this basin has a long history of exploration and analysis of its mesozoic section a comprehensive subsurface study characterizing the haynesville shale has not been conducted this article is the first to address the structural setting stratigraphy depositional environment and facies fracturing and production challenges of the haynesville shale gas play basement structures and salt movement influenced carbonate and siliciclastic sedimentation associated with the opening of the gulf of mexico the haynesville shale is an organic and carbonate rich mudrock that was deposited in a deep partly euxinic and anoxic basin during the kimmeridgian to the early tithonian related to a second order transgression that deposited organic rich black shales worldwide the haynesville basin was surrounded by carbonate shelves of the smackover and haynesville lime louark sequence in the north and west several rivers supplied sand and mud from the northwest north and northeast into the basin haynesville mudrocks contain a spectrum of facies ranging from bioturbated calcareous mudstone laminated calcareous mudstone and silty peloidal siliceous mudstone to unlaminated siliceous organic rich mudstone framboidal to colloidal pyrite is variably present in the form of concretions laminae and individual framboids and replaces calcite cement and mollusk shells haynesville reservoirs are characterized by overpressuring porosity averaging 8 to 12 sw of 20 to 30 nanodarcy permeabilities reservoir thickness of 200 to 300 ft 70 to100 m and initial production of as much as 30 mmcf day reservoir depth ranges from 9000 to 14 000 ft 3000 to 4700 m and lateral drilling distances are 3000 to 5000 ft 1000 to 1700 m typical haynesville wells exhibit a steeper decline curve 80 in the first year than other shale gas plays which is attributed to a very high overpressure

evidence for a widespread carbon isotopic event associated with late middle ordovician sedimentological and faunal changes in estonia an episode of remarkable biotic climatic sea level and facies changes took place during the late viruan caradoc epoch in the baltoscandian area we studied the carbon isotopic composition of carbonate sediments from this period data on the stable carbon isotopic composition of whole rock carbonates from three south estonian core sections together with those on ostracode distribution are presented in two core sections a positive 13 c shift of 2 was revealed in the upper part of the keila stage mid caradoc the comparison of isotope and ostracode data in the sections suggested the occurrence of a gap of late keilan age in the marginal area of the basin equivalent to the north estonian confacies belt the beginning of the excursion approximately coincided with climatic and sea level changes in the shallow shelf area the peak of the late keilan excursion preceded the biotic crises and maximum black shale accumulation in the baltoscandian palaeobasin an approximately synchronous carbon isotopic event has been reported from north america referring to a possible global oceanographic event in the caradoc epoch although the positive carbon isotopic excursion and related environmental events of the late keilan age have some unique features they show more similarities to the end ordovician and silurian events characterized by oceanic change from stratified state to thermohaline circulation state than to warm anoxic events related to eustatic sea level rise

some comparisons of the structure and evolution of the southern appalachian ouachita orogen and portions of the trans european suture zone region abstract many aspects of the tectonic history of the appalachian ouachita orogen and the trans european suture region are similar in a tectonic sense they are geographically linked because at the end of the paleozoic one could travel from texas to poland following a continuous orogenic belt much of each of these orogenic belts is however buried by younger strata and thus geophysical studies are a key element of efforts to better understand them many more geophysical data are needed to provide a comprehensive picture of the deep structure but in several areas new models of the lithospheric structure are emerging in this paper we review some of these models and introduce two new ones for the southern appalachians in addition we explore the implications of these models for the evolution of these orogenic belts drawing attention to similarities and differences in their structure and evolution the appalachian and caledonian orogens represent temporally parallel development along irregular late proterozoic continental margins but the crustal structure is different because of different accretionary elements and complications in the geometry of the original margins lateral transport via thrusting can confuse efforts to tie geologic structures identified on the surface with deep structures for example in the appalachian ouachita orogen the thrust related transport distance for major features such as the blue ridge piedmont sheet and the benton uplift is at least 200 km on a crustal scale deformation in the appalachian ouachita orogen varies greatly in style and intensity in the ouachita orogen the late proterozoic continental margin seems to be preserved while in the northern appalachians crustal scale deformation is intense along the us appalachians the major crustal structure variation is the thin dense crust of avalonia abutting the relatively thick crust of laurentia along the ouachita orogen several large crustal blocks can be identified that are unlike anything seen along the appalachians

inversion for crustal structure using reflections from the passcal ouachita experiment an interface inversion has been tested and applied to wide angle reflection data from the 1986 passcal ouachita experiment an interface corrugation with a relief of 10 km and a width of 30 km was successfully imaged in a test of the interface inversion velocity depth curves derived for shot points 14 19 using the tail sum method are similar to an average one dimensional velocity model from forward modeling using travel time correlations from southern profiles 14 19 the similarity of these velocity depth curves positioned over the southern half of the passcal experiment suggests that crustal structure in this region is approximately laterally homogeneous lower crustal velocities are not well resolved due to the recording on most profiles of only small segments of the travel time triplications a two dimensional velocity structure was derived by using the average one dimensional velocities for the deeper crustal layers and formally inverting for depth to interfaces the final inversion model is found to be consistent with previous refraction interpretations south of the ouachita orogenic trend and concurrent interpretations of the passcal data set inversion results for the central and southern portion of the passcal profile indicate a depth of 10 12 km for a midcrustal layer which thins southward from approximately 10 km to about 4 km a lower crustal layer with an average thickness of 12 km and a moho depth of approximately 29 5 km are also determined interface depths are in agreement with a normal moveout stack of the passcal data set in particular the shallowing of the moho to a depth of 30 km over the northern 50 km of the profile matches previous interpretations of the data set and has been interpreted here and in previous studies as the location of the paleozoic continental margin geophysical studies of the modern atlantic continental passive margin provide the simplest comparison to the crustal structure derived here the lower crustal layer found south of the shallowing of the moho to 30 km beneath the passcal profile is analogous in thickness and position to rift stage lower crust however other tectonic interpretations are possible given the uncertainties in velocity and lithology relationships

carbon isotope stratigraphy in the latest ordovician of estonia on the basis of bulk carbonate carbon isotope data from six drill cores and the type section of the porkuni stage of estonia 13c curves were compiled which altogether cover nearly the whole succession of the latest ordovician hirnantian well known as a time of short lived glaciation and mass extinction in pre porkuni and silurian rocks 13c values were low from 0 2 to 1 7 for the porkuni stage three intervals of the curve were established 1 lower a positive shift up to 6 7 2 middle part with variable values from 3 3 to 5 4 and 3 upper phase with slightly decreased values from 3 2 to 3 6 the correlation with the dob s linn scotland kerogen 13c curve shows that the first two intervals coincide with the normalograptus extraordinarius graptolite biozone the third one with the n persculptus biozone

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

evidence from apatite fission track analysis for regional cretaceous cooling in the ouachita mountain fold belt and arkoma basin of arkansas geologic note 1 a reconnaissance study involving apatite fission track analysis of surface paleozoic sedimentary and cretaceous intrusive rocks was undertaken to evaluate the low temperature thermal history of the ouachita mountain fold belt and arkoma basin of arkansas paleozoic rocks have apparent apatite fission track ages ranging from 184 or 12 ma to 95 or 11 ma with mean track lengths in the range of 12 95 or 0 17 micrometers to 13 51 or 0 13 micrometers indicating that these samples have undergone significant postdepositional annealing cretaceous samples have apatite fission track ages between 104 or 4 and 88 or 6 ma that are concordant with rb sr and k ar ages and mean track lengths in the range 14 69 or 0 16 micrometers to 15 36 or 0 10 microme ers indicating that these rocks cooled rapidly to paleotemperatures below 50 60 degrees c following their emplacement and have subsequently remained cool paleozoic strata of the ouachita mountain fold belt and arkoma basin in arkansas are inferred to have undergone regional cooling from paleotemperatures greater than 110 degrees c beginning in the early cretaceous to paleotemperatures generally less than 80 90 degrees c prior to the intrusion of alkaline plutons along the western margin of the mississippi embayment during the middle cretaceous whether regional cooling during the cretaceous was caused by uplift and erosion or whether a dramatic reduction in the paleogeothermal gradient was involved remains uncertain although a significant component of uplift and erosion is inferred from independent geological observations the results of this study indicate that paleozoic rocks of the ouachita mountain fold belt and arkoma basin of ar ansas were exposed to maximum paleotemperatures during late paleozoic burial prior to the emplacement of cretaceous plutons in the region

a review of the geochemistry of methane in natural gas hydrate the largest accumulations on earth of natural gas are in the form of gas hydrate found mainly offshore in outer continental margin sediment and to a lesser extent in polar regions commonly associated with permafrost measurements of hydrocarbon gas compositions and of carbon isotopic compositions of methane from natural gas hydrate samples collected in subaquatic settings from around the world suggest that methane guest molecules in the water clathrate structures are mainly derived by the microbial reduction of co2 from sedimentary organic matter typically these hydrocarbon gases are composed of 99 methane with carbon isotopic compositions 13cpdb ranging from 57 to 73 in only two regions the gulf of mexico and the caspian sea has mainly thermogenic methane been found in gas hydrate there hydrocarbon gases have methane contents ranging from 21 to 97 with 13c values ranging from 29 to 57 at a few locations where the gas hydrate contains a mixture of microbial and thermal methane microbial methane is always dominant continental gas hydrate identified in alaska and russia also has hydrocarbon gases composed of 99 methane with carbon isotopic compositions ranging from 41 to 49 these gas hydrate deposits also contain a mixture of microbial and thermal methane with thermal methane likely to be dominant published by elsevier science ltd

wave field processing of data from the 1986 program for array seismic studies of the continental lithosphere ouachita experiment in may 1986 a wide aperture seismic field experiment was conducted in southwestern arkansas and northwestern louisiana this experiment was sponsored by the incorporated research institutions for seismology as part of the program for array seismic studies of the continental lithosphere the survey line runs from the south edge of the ouachita mountains out into the gulf coastal plain the experiment involved 21 shot points and two major deployments each of 400 recorders with an effective aperture of 200 km the goal of the experiment was to attempt to provide constraints on the diverse development sequences that have been proposed to explain the structure of the area the data were processed using a previously developed prestack common source acoustic finite difference migration to produce images of reflectivity in a slice 200 km long and 40 km deep this migration algorithm has no dip restrictions and uses a two dimensional variable velocity distribution for wave extrapolation interpretation of the results of the prestack processing together with the results of complementary two dimensional ray trace modeling not shown here of the data provides an enhanced picture of the structure of the paleozoic continental margin that is consistent with the observations in most previous geophysical and geological studies the main physical and temporal features of the model are crustal thinning during precambrian rifting a continent ocean boundary sedimentary wedge and carbonate bank thrusting of deepwater sediments during the ouachita orogeny and triassic rifting

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

middle and upper ordovician carbon isotope chemostratigraphy in baltoscandia a correlation standard and clues to environmental history abstract the baltoscandian epeiric sea was characterised by relatively slow but continuous carbonate deposition from the middle ordovician up to the silurian relatively good preservation of ordovician and silurian carbonate rocks in baltoscandia makes this region unique for chemostratigraphical and palaeoenvironmental studies previous palaeozoic carbon isotope studies have revealed isotopic events in different lithologies over the baltoscandian palaeobasin and some of them on different palaeocontinents we propose a composite carbon isotope curve for the baltoscandian region a new subdivision of the middle and upper ordovician succession into seventeen chemostratigraphic zones and an improved nomenclature of isotopic excursions based on analyses of 13 c carb data from 25 drillcore and outcrop sections of estonia latvia and sweden the composite carbon isotope curve is compared with biostratigraphical data on conodont and chitinozoan distribution the katian hirnantian interval with highly fluctuating isotopic values shows climatic cyclicity of about two million years in baltoscandia no abrupt changes occur in the dapingian sandbian part of the baltoscandian isotope curve but the mid darriwilian excursion nearly coincides with the onset of cooler temperatures similar to modern like equatorial sea surface temperatures and related global biodiversification increase suggesting a climatic background for this event

hirnantian sea level changes in the baltoscandian basin a review abstract correlation of hirnantian sections of the baltoscandian basin reveals four major transgressive regressive sea level cycles related to waxing and waning of ice sheets at high southern hemisphere paleolatitudes the correlation between stratigraphic members in estonia and latvia shows transgressions regressions in the first half of the hirnantian when sea level fluctuated between shallow and deep shelves during the second half of the hirnantian the sea regressed into the former deep shelf leaving north estonia as a mainland after that the sea retreated to the central baltoscandian basin and the whole baltic shelf stayed subaerial this final regression brought the coastline to the vicinity of scania in the west and of lublin basin in the east in the latest hirnantian during ice cap melting sea level began to ascend the baltic shelf submerged again in the early silurian this transgression of latest hirnantian earliest silurian indicates fast melting of ice in gondwanan ice sheets we are proposing that the destabilisation of great amount of gas hydrates due to diminishing of water column pressure during the last deepest regression acted as a trigger for rapid warming of climate the ascertainment of sea level changes presumes correlations between different parts of the baltoscandian basin comparison of published data on stratigraphy lithology faunal finds chemical elements and stable carbon isotopes was the ground for tracking hirnantian changes

a chronology of paleozoic sea level changes sea levels have been determined for most of the paleozoic era 542 to 251 million years ago but an integrated history of sea levels has remained unrealized we reconstructed a history of sea level fluctuations for the entire paleozoic by using stratigraphic sections from pericratonic and cratonic basins evaluation of the timing and amplitude of individual sea level events reveals that the magnitude of change is the most problematic to estimate accurately the long term sea level shows a gradual rise through the cambrian reaching a zenith in the late ordovician then a short lived but prominent withdrawal in response to hirnantian glaciation subsequent but decreasingly substantial eustatic highs occurred in the mid silurian near the middle late devonian boundary and in the latest carboniferous eustatic lows are recorded in the early devonian near the mississippian pennsylvanian boundary and in the late permian one hundred and seventy two eustatic events are documented for the paleozoic varying in magnitude from a few tens of meters to 125 meters

nd isotopes of atoka formation pennsylvanian turbidites displaying anomalous east flowing paleocurrents in the frontal ouachita belt of oklahoma implications for regional sediment dispersal abstract within the pennsylvanian atoka formation of oklahoma strata in the frontal ouachita belt deposited by east flowing turbidity currents yield the same end values as strata in more interior parts of the ouachita orogen deposited by west flowing turbidity currents the congruence of end values throughout the atoka formation is consistent with widespread paleozoic dispersal and recycling of detritus from the evolving appalachian orogen across the surface of the laurentian craton and along its margins sandstone in the olistostromal johns valley shale underlying the atoka formation yields a similar end value reflective of reworking appalachian derived sediment from the paleozoic oklahoma shelf succession

did changes in atmospheric co2 coincide with latest ordovician glacial interglacial cycles article i nfo the late ordovician hirnantian stage 444 million years ago was one of three time periods during the past half billion years in which large continental glaciers formed over earth s polar regions the effects of this glaciation were far reaching and coincided with one of the largest marine mass extinction events in earth history the cause of this ice age is uncertain and a paradoxical association with evidence for high atmospheric co2 levels has been debated precise linkages between sea level ice volume and carbon isotope 13 ccarb and 13 corg proxy records of pco2 have been poorly understood due in part to uncertainties in stratigraphic correlation and the interpretation of globally important sections although correlation difficulties remain recent hirnantian biostratigraphic studies now allow for improved correlations here we show that consistent trends in both 13 ccarb and 13 corg from two well dated stratigraphic sequences in estonia and anticosti island canada coincide with changes in late ordovician hirnantian climate as inferred from sea level and the extent of ice sheets the integrated datasets are consistent with increasing pco2 levels in response to ice sheet expansion that reduced silicate weathering ultimately the time period of elevated pco2 levels is followed by geologic evidence of deglaciation

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

polar front shift and atmospheric co2 during the glacial maximum of the early paleozoic icehouse our new data address the paradox of late ordovician glaciation under supposedly high pco2 8 to 22 pal preindustrial atmospheric level the paleobiogeographical distribution of chitinozoan mixed layer marine zooplankton biotopes for the hirnantian glacial maximum 440 ma are reconstructed and compared to those from the sandbian 460 ma they demonstrate a steeper latitudinal temperature gradient and an equatorwards shift of the polar front through time from 55 70 s to 40 s these changes are comparable to those during pleistocene interglacial glacial cycles in comparison with the pleistocene we hypothesize a significant decline in mean global temperature from the sandbian to hirnantian proportional with a fall in pco2 from a modeled sandbian level of 8 pal to 5 pal during the hirnantian our data suggest that a compression of midlatitudinal biotopes and ecospace in response to the developing glaciation was a likely cause of the end ordovician mass extinction

observed relations between snowfall microphysics and triple frequency radar measurements recently published studies of triple frequency radar observations of snowfall have demonstrated that naturally occurring snowflakes exhibit scattering signatures that are in some cases consistent with spheroidal particle models and in others can only be explained by complex aggregates until recently no in situ observations have been available to investigate links between microphysical snowfall properties and their scattering properties in this study we investigate for the first time relations between collocated ground based triple frequency observations with in situ measurements of snowfall at the ground the three analyzed snowfall cases obtained during a recent field campaign in finland cover light to moderate snowfall rates with transitions from heavily rimed snow to open structured low density snowflakes the observed triple frequency signatures agree well with the previously published findings from airborne radar observations a rich spatiotemporal structure of triple frequency observations throughout the cloud is observed during the three cases which often seems to be related to riming and aggregation zones within the cloud the comparison of triple frequency signatures from the lowest altitudes with the ground based in situ measurements reveals that in the presence of large 5 mm snow aggregates a bending away in the triple frequency space from the curve of classical spheroid scattering models is always observed rimed particles appear along an almost horizontal line in the triple frequency space which was not observed before overall the three case studies indicate a close connection of triple frequency signatures and snow particle structure bulk snowfall density and characteristic size of the particle size distribution

deep structure of the texas gulf passive margin and its ouachita precambrian basement results of the cocorp san marcos arch survey this cocorp deep seismic survey provides a comprehensive image of the southeast texas part of the gulf passive margin and its accreted ouachita arc foundation beneath the updip limit of the cenozoic sediment wedge a prominent antiformal structure is imaged within the interior zone of the buried late paleozoic ouachita orogen the structure appears to involve precambrian grenville basement the crest of the antiform is coincident with the cretaceous tertiary luling mexia talco fault zone some of these faults dip to the northwest counter to the general regional pattern of down to the basin faulting and appear to sole into the top of the antiform suggesting that the ouachita structure has been reactivated as a hingeline to the subsiding passive margin the antiform may be tied via this fault system and the ouachita gravity gradient to the similar devils river waco and benton uplifts interpreted as precambrian basement cored massifs above the paleozoic sequence a possible rift related graben is imaged near the updip limit of jurassic salt paleoshelf edges of the major tertiary depositional sequences are marked by expanded sections disrupted by growth faults and shale diapirs within the wilcox formation the transect crosses the mouth of the 900 m deep yoakum canyon a principal pathway of sediment delivery from the laramide belt to the gulf beneath the wilcox the comanchean lower cretaceous shelf edge capped by the stuart city reef is imaged as a pronounced topographic break onlapped by several moundy sediment packages because this segment of the line parallels strike the topographic break may be interpreted as a 2000 m deep embayment in the cretaceous shelf edge and possibly a major submarine canyon older and deeper than the yoakum canyon

local and global perspectives on carbon and nitrogen cycling during the hirnantian glaciation abstract the impact of climate and ocean circulation changes on marine carbon and nitrogen cycling was investigated for a period covering the late ordovician hirnantian glaciation three sections were studied two from nevada one proximal and one distal and one from the yukon the opportunity to compare 13 c carb 13 c org and 13 c grap profiles between ocean and epeiric platform settings enabled a more thorough assessment of environmental and diagenetic factors influencing the preservation of seawater 13 c values at each stratigraphic section using the best preserved sedimentary components from the two nevada sections a lowstand sedimentary gradient of 4 was found between platform and ocean settings during the hirnantian glaciation gradients of similar magnitude have been reported from hirnantian platforms in the canadian arctic and baltic basins taking the outboard sections as the most representative of the ocean carbon pool a mean positive 13 c shift of just 2 7 0 4 is calculated which is significantly smaller than the 5 7 positive shifts recorded in epeiric platform settings the discrepancy is attributed to the effects of local carbon cycling in tropical and subtropical epeiric seas it is hypothesized that increased carbonate weathering caused 13 c values of local carbon weathering fluxes to increase which in turn caused seawater 13 c values to increase in basin proximal settings in addition nutrient dust fluxes may have contributed to increased levels of photosynthetic activity in proximal settings driving sedimentary 13 c values even higher much higher than sediment deposited in outboard settings where carbon exchanges with the surface ocean were less restrictive a 15 n profile reconstructed from deposits of the distal vinini creek section records a 1 5 positive shift through the hirnantian glacial interval interpreted as a major late ordovician upwelling zone high organic carbon 1 wt and high hydrogen indices 400 600 make the vinini creek section a suitable candidate for the preservation of original sedimentary 15 n records to explain the results a conceptual model of ocean nitrogen cycling under greenhouse climate conditions is presented that highlights the role of cyanobacteria as both primary producers and nitrogen fixers it is hypothesized that in greenhouse climates the fixed nitrogen inventory of the abyssal ocean was low due to efficient denitrification in the ocean s expansive oxygen minimum zones this caused algal productivity to become strongly dependent on continuous supplies of fixed nitrogen from cyanobacteria in the photic zone yielding sedimentary 15 n values between 0 and 2 by contrast the cooler icehouse climate of the hirnantian led to increased ocean ventilation greater partitioning of atmospheric oxygen into downwelling surface waters and oxygen minimum zone shrinkage as a result fixed nitrogen levels climbed in the ocean s abyssal depths due to the lower denitrification rates this in turn allowed nitrogen with high 15 n values to recycle back into the photic zone driving sedimentary 15 n values higher during the hirnantian glaciation

late ordovician carbon isotope trend in estonia its significance in stratigraphy and environmental analysis abstract carbon isotope changes during most of late ordovician time from the mid caradoc kinnekulle k bentonite until the beginning of the silurian were investigated as the corresponding sequence of rocks is stratigraphically nearly complete in estonia an attempt was made to use it to elaborate the general pattern of carbon isotope changes in the late ordovician complications were caused by several local or regional hiatuses in the middle and late caradoc and hirnantian a total of 385 whole rock samples were studied from eight drill cores in northern and central estonia the following positive carbon isotope events were observed 1 the mid caradoc excursion peak 13c value 2 2 in the uppermost part of the keila stage also known in sweden 2 the first late caradoc excursion 1 9 in the lower part of the rakvere stage 3 the second late caradoc excursion 2 4 in the upper part of the nabala stage 4 the early ashgill excursion 2 5 in the lowermost part of the pirgu stage 5 the widely known large hirnantian excursion in estonia the peak value reaches 6 7 in the porkuni stage the study interval comprises a long 10 ma period characterized by low magnitude carbon isotope changes and a following brief 2 ma interval with large changes no obvious lithological preference for hosting the positive shifts was recorded in principle the 13c values exceeding the background values may occur in all types of rocks present in a sedimentary basin several 13c positive excursions values 1 5 to 3 in the mohawkian of north america are evidence that the minor caradoc and early ashgill 13c positive shifts in baltoscandia may have counterparts in laurentia if correctly correlated these shifts may have global significance the hirnantian excursion is usually linked to a major glacial event even if some carbon cycling mechanisms are not completely understood the environmental causes suggested for the earlier minor shifts range from global climatic and glacial events to very local changes in basin regime and sea level our study supports the primary role of climatic or climatically triggered oceanic processes

first observations of triple frequency radar doppler spectra in snowfall interpretation and applications the potential of multifrequency doppler spectra to constrain precipitation microphysics has so far only been exploited for dual frequency spectra in rain in this study we extend the dual frequency concept to triple frequency doppler radar spectra obtained during a snowfall event which included rimed and unrimed snow aggregates a large selection of spectra obtained from low turbulence regions within the cloud reveals distinctly different signatures of the derived dual spectral ratios due to the third frequency a characteristic curve can be derived which is almost independent of the underlying particle size distribution and velocity size relation this approach provides new opportunities for validating existing and future snow scattering models and reveals how the information content of triple frequency radar data sets can be further exploited for snowfall studies

advances in the estimation of ice particle fall speeds using laboratory and field measurements accurate estimates for the fall speed of natural hydrometeors are vital if their evolution in clouds is to be understood quantitatively in this study laboratory measurements of the terminal velocity yt for a variety of ice particle models settling in viscous fluids along with wind tunnel and field measurements of ice particles settling in air have been analyzed and compared to common methods of computing yt fromthe literature it is observed that while these methods work well for a number of particle types they fail for particles with open geometries specifically those particles for which the area ratio ar is small ar is defined as the area of the particleprojectednormaltotheflow dividedbytheareaofacircumscribing disc inparticular thefallspeeds of stellar and dendritic crystals needles open bullet rosettes and low density aggregates are all overestimated these particle types are important in many cloud types aggregates in particular often dominate snow precipitation at the ground and vertically pointing doppler radar measurements based on the laboratory data a simple modification to previous computational methods is proposed based on the area ratio this new method collapses the available drag data onto an approximately universal curve and the resulting errors in the computed fall speeds relative to the tank data are less than 25 in all cases comparison with the much more scattered measurements of ice particles falling in air show strong support for this new method with the area ratio bias apparently eliminated

ordovician and silurian sea water chemistry sea level and climate a synopsis following the cambrian explosion and the appearance in the fossil record of most animal phyla associated with a range of new body plans the ordovician and silurian periods witnessed three subsequent major biotic events the great ordovician biodiversification event the end ordovician extinction the first animal extinction and second largest of the five mass extinctions of the phanerozoic and the early silurian postextinction recovery there are currently no simple explanations for these three major events combined extrinsic geological and intrinsic biological factors probably drove the biodiversifications and radiations and the appearance and disappearance of marine habitats have to be analysed in the frame of changing palaeogeography palaeoclimate and sea water chemistry the present paper reviews the relationships of the three biotic events to chemical and physical processes occurring in the ocean and atmosphere during the ordovician and silurian including sea level changes geochemical proxies 13c 18o 87sr 86sr of the ocean waters and the evolution of the atmosphere oxygen and carbon dioxide content less

slant stack velocity analysis of shot point 16 from the 1986 passcal ouachita experiment a x p slant stack algorithm is applied to shot point 16 of the 1986 passcal ouachita experiment to determine the optimal me dimensional velocity structure several enhancements to a standard slant stack algorithm are used to image a one dimensional velocity fimction from the shot point gathered data because only postcritical reflections and refractions are needed to image a velocity function significant muting of precritical energy is perfonned which reduces the effect of noise in calculating the stack for data sets with long offsets scaling the data by a factor equal to or greater than offset distance compensates for the effects of geometrical spreading and attenuation this enhances imaging of the deeper regions of the velocity model and balances amplitudes across the shot point gather finally ray parameter depth images can have difficulty imaging higher velocities since for higher velocities a small change in ray parameter results in a large increment in velocity this problem is overcome by slant stacking in equal increments of velocity instead of ray parameter the enhanced slant stack algorithm is applied to shot point 16 of the 1986 passcal ouachita data set which sufficienfiy satisfies the assumption of lateral homogeneity based on previous studies of the region other velocity models are found to be consistent with the envelope of velocity fimcfion imaged by the slant stack method maximum amplitudes on the x p image are used as a constraint to delineate a more detailed velocity model a seven layer velocity model is derived for shot point 16 which is found to be similar to those of others who have studied the area the top three layers are sedhnents associated with the gulf coastal plain followed by a thick layer derived from sedimentation associated with the closing of the proto gulf of mexico and the associated ouachita orogeny the determination of a more detailed velocity function from the envelope suggests somewhat higher velocities in the lower crustal layer than previous studies however given the one dimensional assumptions of the method as well as the signal to noise ratio of the data the velocity envelope is the most important constraint on the velocity models and tectonic interpretation provided by this analysis

snow studies part iv ensemble retrieval of snow microphysics from dual wavelength vertically pointing radars abstractbased on the theory developed in part iii this paper introduces a new method to retrieve snow microphysics from ground based collocated x and w band vertically pointing doppler radars to take into account the variety of microphysical relations observed in natural precipitating snow and to quantify the uncertainty in the retrieval results caused by this variety the retrieval is formulated using the ensemble based method the ensemble is determined by the spread of uncertainties in the microphysical descriptions applied to map the same radar observables to the retrieved quantities the model descriptors use diverse assumptions concerning functional forms of particle size distribution and mass velocity relations all taken from previous observational studies the mean of each ensemble is assumed to be the best estimate of the retrieval while its spread is defined by the standard deviation that characterizes its uncertainty the main retrieved products are the characteristic size the snow mass con

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

a record of late ordovician to silurian oceanographic events on the margin of baltica based on new carbon isotope data elemental geochemistry and biostratigraphy from two boreholes in central poland abstract new stable isotope data from organic carbon 13corg and inorganic elemental geochemistry data have been generated from upper ordovician to silurian strata in two boreholes in the lublin basin of poland grabowiec 6 and zwierzyniec 1 they have been integrated here with biostratigraphical data from graptolites acritarchs and chitinozoans faunal assemblages from grabowiec 6 indicate that it spans from the katian clingani graptolite biozone to the gorstian scanicus graptolite biozone 13corg values from this section record the sheinwoodian ireviken excursion the homerian mulde excursion and a minor positive shift associated with the lower ludfordian leintwardinensis biozone the second section zwierzyniec 1 spans the sandbian through gorstian nilssoni biozone 13corg values record the hirnantian carbon isotope excursion hice and the ireviken excursion as well elemental geochemistry data is used to recognize subtle changes in provenance and lithology significant increases in the abundance of v and mo are recognized in strata deposited above the ireviken excursion the enrichment of these redox sensitive elements suggests that persistent regional anoxia and euxinia may be associated with the aftermath of these oceanographic disturbances some of these same trace elements along with fe2o3 and pb are also abundant in strata coeval with or just below the ireviken and hice excursions this may have a causal link with malformed palynomorphs observed at these intervals by some workers which are thought to reflect toxic levels of dissolved heavy metals in the world s oceans