an integrated stratinomic model for the genesis and concentration of small shelly fossil style phosphatic microsteinkerns in not so exceptional conditions abstract carbonate fluorapatite filled millimeter scale microsteinkerns are a key element of the small shelly fossil ssf fauna generally associated with the late proterozoic cambrian the purported disappearance of these fossils and this style of phosphatic preservation during the cambrian stage 2 botomian extinction has been linked to changes in ocean chemistry and or changes to the physical environment due to the evolution of metazoans as well to the changing role of metazoans in the marine environment through a qualitative meta analysis of existing literature we document a broader temporal distribution for ssf style preservation than previously recognized we examine these occurrences for common patterns of depositional conditions and we offer a conceptual model for both the origin of the phosphatic microsteinkerns and their eventual concentration in certain sediments based on known biogeochemical diagenetic processes of authigenic calcium fluorapatite precipitation phosphogenesis and subsequent concentration of phosphatic sedimentary particles this polycyclic phosphogenic condensation ppc model makes predictions as to how and where this form of preservation might occur in cambrian and non cambrian marine strata alike

convergence rates across a displacement transfer zone in the western transverse ranges ventura basin california three cross sections are balanced and retrodeformed to the top of the pleistocene saugus formation and to a horizon close to the base of the jaramillo subchron to yield crustal shortening and shortening rates for the western transverse ranges of california the cross sections compare the shortening that occurs along a transfer zone in which displacement is transferred eastward from a surface reverse fault red mountain fault to a blind thrust and from that to a combination of both a surface reverse fault san cayetano fault and the blind thrust using an age of 250 50 ka for the top of the saugus formation based on amino acid racemization of fossil mollusks crustal shortening rates appear to have abruptly accelerated through time from 3 4 3 to 5 4 mm yr between 250 and 975 ka to 20 6 to 28 1 mm yr since 250 50 ka using an age of 500 ka for the top of the saugus formation based on paleomagnetic stratigraphy there is little increase in deformation rates through time ranging from 5 5 to 8 6 mm yr between 975 500 ka and from 10 3 to 14 1 mm yr since 500 ka more crustal convergence rates determined by global positioning system surveys taken over 4 6 years indicate a shortening rate of 7 10 mm yr across the basin this is consistent with the slower rates of deformation calculated using an age of 500 ka for the top of the saugus formation less

magnesian calcite and chamositic ooids forming shoals peripheral to late ordovician ashgill muddy siliciclastic shores southern ontario in southern ontario ooids are associated with two distinct facies associations in the queenston formation the final stage of late ordovician ashgill taconic basin fill one facies consists of thin ooid and bioclastic grainstones interbedded with mudrock and lies near the base of the formation and in southwestern ontario also forms a local nw thickening wedge near the middle of the formation ooids have radial fibrous and radial concentric fabrics type a with chamosite illite and fe oxide laths at intercrystalline sites vertical lithologic and ooid abundance patterns indicate that thresholds to carbonate production were sensitive to changes in terrigenous sediment supply sea level circulation accommodation space and tectonism ooids in the second facies association are admixed with abraded fragments of open marine biota or occur burrow fills within a 30 cm thick interval of mudrock near the top of the preserved queenston succession a few metres below the ordovician silurian unconformity ooids have radial concentric and crosscutting patchy microcrystalline fabrics type b this unit may represent a transgressive or stillstand deposit modified by bioturbation the extent of preserved fabric suggests that both ooid types were originally magnesian calcite but type a ooids underwent greater burial alteration this is shown by crystalline mosaics that cross cut relict primary fabrics 13c values 1 82 to 0 67 and 18o values 4 46 to 10 57 more negative than marine calcite of similar age mn and fe concentrations more elevated above expected marine values and a luminescence similar to that of intergranular cements burial meteoric diagenesis was likely promoted by excellent permeability of the host sand we interpret authigenic chamosite and fe oxide to reflect diagenesis of iron bearing and clay detritus trapped during ooid growth type b ooids suffered less alteration 13c 1 1 to 6 64 and 18o 3 04 to 4 81 values overlap the expected marine range including 13c enrichment that occurs within the hirnantian latest ordovician excursion although mn and fe values are still higher than those of modern calcitic ooids negligible luminescence suggests that recrystallization occurred in the presence of marine derived pore fluids further burial alteration was inhibited due to low permeability of the host mud type a ooid facies in the queenston formation forms an ancient analogue for lesser known quaternary ooid shoals peripheral to tropical deltaic systems the facies of type b ooids while more enigmatic may preserve a geochemical herald of latest ordovician climate change the presence of minor chamosite in type a ooids defines a possible distal facies of the well known oolitic ironstones of similar age in the mid continental usa

diachronous tectonic collapse of the ordovician continental margin eastern canada comparison between the quebec reentrant and st lawrence promontory the upper ordovician trenton group of southern quebec represents the last taconian foreland basin carbonate unit in the quebec reentrant prior to final collapse of laurentia s continental margin and its burial under synorogenic flysch the trenton group either conformably or unconformably overlies the black river group and is in turn conformably overlain by the utica shales the tripartite trenton carbonate unit records progressive deepening 1 very shallow to shallow subtidal 2 shallow to deep carbonate ramp and 3 shallow to deep outer shelf regional facies distribution lithotectonic elements and thickness variations indicate that the trenton shelf was dissected by extensional faults delineating blocks subsiding at various rates this scenario compares favourably with taconian foreland basin development in the middle ordovician table head group at the st lawrence promontory newfoundland a similar stratigraphic succession and tectono sedimentary history occurring 10 15 ma earlier at the s

sequence stratigraphy of a carbonate clastic ramp the cincinnatian series upper ordovician in its type area the cincinnatian series consists of five depositional sequences each is composed of a thin transgressive systems tract and a much thicker shallowing upward highstand systems tract whereas the highstand systems tracts are relatively poor in micrite but rich in terrigenous shale the transgressive systems tracts are composed of cleaner carbonate facies ranging from coarse grained calcarenites to micrite rich wackestones and micstones this alternation of carbonate and terrigenousfacies types resulted from the trapping of clastic sediment near its presumed source in the appalachians during a relative sea level rise and it may be a feature common to sequences in other mixed carbonate clastic settings these depositional sequences were produced by eustatic fluctuations as shown by the limited extent to which the sequences can be traced into more proximal portions of the appalachian basin the recognition of depositional sequences clarifies the regional paleogeography and simplifies regional lithostratigraphic nomenclature within current biostratigraphic resolution sequences in the cincinnatian series appear to have chronostratigraphic significance and thereby offer an additional means of intrabasinal correlation in the upper ordovician

the mesoscale dynamics of freezing rain storms over eastern canada abstract a severe ice storm affected the east coast of canada during the canadian atlantic storms project ii a hierarchy of cloud resolving model simulations of this storm was performed with the objective of enhancing understanding of the cloud and mesoscale processes that affected the development of freezing rain events the observed features of the system were reasonably well replicated in the high resolution simulation diagnosis of the model results suggests that the change of surface characteristics from ocean to land when the surface warm front approaches newfoundland disturbs the quasi thermal wind balance near the frontal region the cross frontal circulation intensifies in response to the thermal wind imbalance which in turn leads to the development of an extensive above freezing inversion layer in the model storm depending on the depth of the subfreezing layer below the inversion the melted snow may refreeze within the subfreezing layer to form ice pellets or they may refreeze at the surf

the late ordovician dawson point formation timiskaming outlier ontario key to a new regional synthesis of richmondian hirnantian carbonate and siliciclastic magnafacies across the central canadian craton current paleogeographic reconstructions extend late ordovician taconic derived siliciclastics across the cen tral canadian craton prior to the terminal ordovician glacioeustatic lowstand revision of the late ordovician dawson point formation of the timiskaming outlier greatly reduces the distribution of these siliciclastics and documents a greater spread of shallow water carbonate of richmondian age as revised the dawson point formation contains two informal members a deep water graptolitic shale that grades upward into shallow water siliciclastic redbeds and an up per member of shallow water muddy crinoidal limestone with interbedded shale likely representing low energy shoals on a muddy shelf deep water shale accumulation began in the upper manitoulinensis graptolite zone following foun dering of the regional foreland carbonate platform basin development documents a northward younging 1 million years from southern ontario foreland basins in keeping with regional tectonic driven transgression along eastern north america the shale to carbonate succession of the dawson point formation correlates with the georgian bay forma tion on manitoulin island wherein the upper carbonate dominated divisions of both formations are equivalent to the siliciclastic queenston formation of southern ontario in absence of additional biostratigraphic information the upper member of the dawson point formation is likely richmondian or late ashgillian in age the revised late ordovician history of the timiskaming outlier may identify a once significant volume of shallow water carbonate across the central canadian craton with related sequestration of carbon dioxide possibly aiding global cooling erosion of the carbonate driven by developing glacioeustatic lowstand conditions was likely contemporaneous with early hirnantian peritidal de position of the uppermost queenston formation in southern ontario

upper ordovician facies in the lac saint jean outlier quebec eastern canada palaeoenvironmental significance for late ordovician oceanography upper ordovician caradocian carbonates of eastern north america were deposited along the iapetus continental margin and record a transition from warm to cool water settings despite this margin having been within the southern hemisphere tropical belt this event has been documented from virginia usa to southern quebec canada although not previously from areas close to the palaeoequator field petrographic and major element geochemistry data have been gathered from the poorly known upper ordovician carbonate succession outcropping in the lac saint jean outlier in central quebec the succession consists of a lower siliciclastic formation tremblay overlain by three limestone formations simard shipshaw and galets and capped by shales pointe bleue shale from macro and microfaunal evidence carbonate sedimentation occurred during the late caradoc and is younger than the early to mid caradoc carbonate succession present farther south relative sea level fluctuations recorded in the sediments suggest an overall sea level rise briefly halted by a minor end caradocian sea level fall the lower limestone formation simard consists of muddy sediments with algal coral stromatoporoid boundstones green algae are abundant this unit reflects low energy sedimentation on a shallow warm water carbonate ramp colonized by a diverse chlorozoan fauna the upper limestone formation galets is typified by coarse grained bioclastic sediments punctuated by numerous phosphate rich hardgrounds with evidence for high energy shallow marine conditions faunas were dominated by crinoids and bryozoans this unit represents high energy sedimentation on a cool shallow water carbonate ramp colonized by a brynoderm faunal association between both units a deeper marine outer shelf limestone formation shipshaw was developed in the lac saint jean area a transition from warm to cool water carbonate ramps occurred in latest caradoc times and is litho and biofacies wise similar to what is documented for lower caradocian limestones present farther south upwelling of nutrient rich cool bottom oceanic waters was a probable cause for this transition

late ordovician chatfieldian catastrophic volcanism and abrupt carbonate platform interior subsidence a tectonic link across a taconian foreland basin quebec embayment due to inherited crustal weakness the late ordovician trenton group of new york state and equivalent strata in south central canada peripheral to the quebec embayment form a regional carbonate platform succession within the taconian foreland basin the upper and lower group boundaries record geodynamically controlled changes in regional subsidence whereas a higher order allocyclic stratigraphy traced south along the foreland interior of the eastern united states may identify shorter term eustatic controls a tectonomagmatic event extending across the foreland basin is recorded a few million years prior to the regional collapse of the foreland platform in the quebec embayment within the lower c spiniferus graptolite zone platform margin retreat 50 km in southern quebec is contemporaneous with a shale rich limestone succession that thins 30 10 m westward over 300 km into the ottawa embayment eastern ontario west of the appalachian structural front this stratigraphic record coincides with catastrophic volcanism within the once distant volcanic arc terranes as recorded by a bentonite traced geophysically from the platform margin to the michigan basin core from the ottawa embayment demonstrates western directed transport of lithic bearing ash the lithic material representing reworked deep water sedimentary and or volcaniclastic sediment was entrained by eruption of calc alkaline felsic to basaltic andesitic and trachyandesitic magmatic sources the above geological history documents a short period of diminishing influence of subsidence moving into the platform interior an event that enhanced briefly cross platform oceanographic circulation as recorded by elevated shale abundance and change in macrofaunal assemblages short lived differential subsidence of the continental crust in this region in response to distal tectonism may have been predicated by inherited weakness of a shallowly buried intracratonic 400km precambrian fault system manifest today by the ottawa bonnechere graben the resulting thin 10 to 30 m stratigraphic record of the tectonomagmatic event is of similar thickness to high order allostratigraphic patterns within the taconic foreland interior of the eastern united states because there is a heterogeneous distribution of precambrian fault systems underlying the taconian foreland interior of eastern laurentia differential structural response of the foreland interior can be anticipated the apparent regional allostratigraphy may contain subtle facies differences along the strike of the foreland basin that reflect this tectonic component

relationships among in situ stress fractures and faults and fluid flow monterey formation santa maria basin california we used borehole televiewer bhtv data from four wells within the onshore and offshore santa maria basin california to investigate the relationships among fracture distribution orientation and variation with depth and in situ stress our analysis of stress induced well bore breakouts shows a uniform northeast maximum horizontal stress sh max orientation in each well this direction is consistent with the sh max direction determined from well bore breakouts in other wells in this region the northwest trend of active fold axes and kinematic inversion of nearby earthquake focal plane mechanisms in contrast to the uniformity of the stress field fracture orientation dip and frequency vary considerably from well to well and within each well with depth fractures can be divided into distinct subsets on the basis of fracture frequency and orientation which correlate with changes of lithology and physical properties although factors such as tectonic history diagenesis and structural variations obviously have influenced fracture distribution integration of the in situ stress and fracture data sets indicates that many of the fractures faults and bedding planes are active small scale strike slip and reverse faults in the current northeast trending transpressive stress field in fact we observed local breakout rotations in the wells providing kinematic evidence for recent shear motion along fracture copyright 1997 the american association of petroleum geologists all rights reserved 1manuscript received april 15 1996 revised manuscript received december 27 1996 final acceptance july 15 1997 2department of geophysics stanford university stanford california 94305 2215 we wish to thank tom zalan of the chevron u s a production company for providing the offshore well data unocal corporation for providing the data on the onshore well and marcia mclaren from pacific gas and electric company for providing the earthquake focal mechanisms used in the stress inversion analysis the data used as background seismicity in figure 1 were extracted from the world wide web of the southern california seismic network scsn catalog operated jointly by the seismological laboratory at caltech and the u s geological survey both in pasadena california we appreciate the comments and helpful discussions from daniel moos steve graham and lev vernik

alternate fault model for the santa barbara california earthquake of 13 august 1978 published reports attribute the 1978 santa barbara earthquake to a north dipping reverse fault supported in part by first motion studies surface geology augmented by oil well and trench logs shows a predominance of late quaternary reverse faults which dip steeply south suggesting that the alternate nodal plane determined from first motion studies is preferred a steep fault would better explain the narrow rectangular map pattern of aftershocks the santa barbara region has a high historical seismicity compared to the onshore ventura basin to the east where late quaternary displacements on seismically active north dipping faults are much higher a paradox if the 1978 event were also produced by a north dipping fault on the other hand the tectonic subprovince characterized by steeply south dipping faults is much better developed at santa barbara where the historical seismicity is higher than in the onshore ventura basin

petrology and potential tectonic significance of a k bentonite in a taconian shale basin eastern ontario canada northern appalachians a 6 cm thick k bentonite herein defined as the russell bed occurs in an upper ordovician deep basin shale succession in eastern ontario canada forming part of the distal taconic foreland in eastern north america the bed lies within the pygmaeus graptolite biozone which is about 451 to 452 ma in age although some bentonites are reported from this interval in eastern north america we are reporting the first set of compositional data for a bentonite of this age gamma log correlation identifies a potential minimum distribution area of 5 km 2 for the k bentonite covering part of southern quebec new york state and eastern ontario the deposit coincides with the first influx of distal turbdites into this shale basin associated with taconic flysch and simultaneous abrupt ventilation of the once anoxic deep water basin which had formed initially after foundering of the upper ordovician carbonate platform concurrent intrabasinal extinction of several graptolite species suggests that change in sedimentation palaeoceanography and volcanism were linked to a regional external process compositionally the bentonite is distinct from the older ordovician platform deposits in eastern north america the deposit contains abundant titaniferous phlogopite with 1 6 bao fluoroapatite with 2 5 f and dynamically shaped glass spherules now altered to clay the spherules and clay matrix constitute 45 of the bed and compositionally define an illite 90 smectite i s structure with about 7 5 k 2 o age dating by ar ar analysis of the phlogopite crystals yielded a younger than expected 440 445 ma age this difference along with evidence of localized chloritization of phlogopite likely reflects known post ordovician hydrothermal activity within the basin on the basis of several geochemical proxies the magmatic source of the russell k bentonite falls within the trachyandesite field and was ba enriched comparison of geochemistry and mineralogy with older middle to late ordovician and younger early silurian k bentonites within the taconic orogen along eastern laurentia and baltica reveals that the potential source magma for the russell bed was more mafic more alkaline and less fractionated than sources typical of older platform bentonites instead it is more similar to the younger llandovery bentonites of scandinavia and scotland it remains uncertain if it signals local or regional compositional change in volcanic source in the northern appalachians

the mesoscale and microscale structure and organization of clouds and precipitation in midlatitude cyclones xi comparisons between observational and theoretical aspects of rainbands abstract comparisons are made between the characteristics of several types of rainbands observed in an extratropical cyclone and dynamical mechanisms relevant on the mesoscale the warm sector flow ahead of the cold front and above the cold frontal zone aloft was unstable to conditional symmetric instability and theoretical predictions for this mechanism am consistent with several aspects of the warm sector and wide cold frontal rainbands in the case of the warm sector rainbands other mechanisms e g wave cisk and mixed dynamic convective instabilities may have also played a role the core structure of the narrow cold frontal rainbands appeared to be affected by an instability that derived its energy from the horizontal shear across the surface front also many aspects of the narrow cold frontal rainband were similar to a density current shear induced gravity waves appeared to be responsible for the wavelike rainbands observed in the vicinity of the cold frontal zone aloft the orientation of the

high resolution seismic reflection profiling of neotectonic faults in lake timiskaming timiskaming graben ontario quebec canada the timiskaming graben is a 400 km long 50 km wide north west trending morphotectonic depression within the canadian shield of eastern north america and experiences frequent intraplate earthquakes the graben extends along the border of ontario and quebec connecting southward with the nipissing and ottawa bonnechere grabens and the st lawrence rift system which includes a similar structure underlying the hudson valley of the eastern usa together they form a complex failed rift system related to regional extension of north american crust during the breakup of rodinia and later pangea the timiskaming graben lies within a belt of heightened seismic activity western quebec seismic zone with frequent moderate magnitude greater than magnitude 5 earthquakes including a magnitude 6 2 in 1935 these events threaten aging urban infrastructure built on soft glacial sediments post glacial landslides along the ottawa valley suggest earthquakes as large as magnitude 7 the inner part of the timiskaming graben is filled by lake timiskaming a large 110 km long post glacial successor to glacial lake barlow that was ponded by the laurentide ice sheet 9500 years ago the effects of frequent ground shaking on lake floor sediments was assessed by collecting more than 1000 line kilometres of high resolution chirp seismic profiles late glacial lake barlow glaciolacustrine and overlying post glacial sediments are extensively deformed by extensional faults that define prominent horsts and grabens multibeam bathymetry data suggest that faults influence the morphology of the modern lake floor despite high sedimentation rates and indicate recent neotectonic deformation the lake timiskaming area provides evidence of post glacial intracratonic faulting related to recurring earthquake activity along a weak spot within the north american plate

mesoscale circulations forced by melting snow part i basic simulations and dynamics abstract the melting of snow extracts latent heat of fusion from the environment the basic response of the atmosphere to this cooling by melting mechanism is investigated by using a nonlinear two dimensional numerical model it is found that the resultant melting induced circulations consist of a forced downdraft which spreads out laterally like a gravity current and transients which are gravity waves the characteristics of these mesoscale thermally driven circulations are studied under idealistic atmospheric conditions model results show that the melting associated with realistic precipitation rates up to 10 mm h 1 can induce horizontal wind perturbations of several meters per second and vertical motions of tens of centimeters per second since the gravity waves and the cold outflow current propagate away from the source they can have significant dynamic effects on the environment remote from the precipitation region moreover the melting induced near o c isothermal layer in the atmosphere alters

paleobiogeography of the early late ordovician trentonian latest sandbian to middle katian brachiopod fauna during a major marine transgression and colonization of the epicontinental seas in laurentia abstract during the late sandbian and early to middle katian a major marine transgression flooded the intracratonic basins and platforms of eastern north america allowing brachiopods to disperse into the newly formed epicontinental seas two main brachiopod faunas can be differentiated using multivariate analysis of faunal data from localities across laurentia during this time a relatively diverse scoto appalachian fauna confined to the eastern margin of north america and peri laurentian terranes and a less diverse but more widely distributed epicontinental fauna that expanded into the remainder of the flooded continent the epicontinental fauna can be divided into four subfaunas with subtle compositional differences that correspond to different paleoenvironmental settings of eastern north america influenced by such paleoecological factors as nutrient rich cool water upwelling onto the continent through the sebree trough a greater range of habitable environments on the continental margin and in the adjacent deep taconic foreland basin and the interactions between pericratonic and intracratonic water masses of different physical and chemical properties the epicontinental brachiopod faunas dominated basins and platforms of continental interior until the hirnantian extinction event

late ordovician platform foundering its paleoceanography and burial as preserved in separate eastern michigan basin ottawa embayment basins southern ontario comparison of litho bio and chemostratigraphy in two cores from the northeastern margin of the michigan basin manitoulin island and from within the ottawa embayment eastern ontario identifies interbasinal differences of late ordovician platform foundering linked to taconic orogenesis graptolite biostratigraphy defines an east to west younging late edenian to early maysvillian of platform burial a regional unconformity likely caps the platform succession in both basins an increased supply of mafic material appears during the final stages of platform collapse with the accumulation of organic rich 8 petroliferous shales collingwood member michigan basin eastview member ottawa embayment both units preserve evidence for deposition coincident with increased dysoxic to possible anoxic bottom water conditions but the collingwood member accumulated under a relatively stable paleoceanographic environment rhythmic interbedding with platform limestone in eastern ontario combined with ev

storm deposited sandstones tempestites and related ichnofossils of the late ordovician georgian bay formation southern ontario canada the late ordovician geogian bay formation of southern ontario canada comprises up to 250 m of grey to blue grey shales interbedded with highly fossiliferous calcareous sandstones these strata were deposited in equatorial paleolatitudes after 448 ma in a shallow foreland basin created by overthrusting along the eastern margin of north america the taconic orogeny the georgian bay formation comprises the middle part of an upward shallowing progradational sequence from deep water transgressive shales of the underlying whitby formation to muddy tidal flat sequences of the overlying queenston formation exposures in brickyard and river cuts near toronto and northwards along a narrow outcrop belt along the foot of the niagara escarpment show laterally extensive 100 m sharp based sheets of sandstone up to 1 m thick with gutter casts and washed out hypichnial trace fossils dominantly planolites and paleophycus on their lower bedding surfaces detailed examination of sandstone beds in outcrop and i

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

a weathering hypothesis for glaciation at high atmospheric pco2 during the late ordovician new paired carbonate and organic carbon isotope analyses from nevada usa together with a consideration of the effects of mountain building and ice sheet coverage of the continents on atmospheric pco2 lead to a new hypothesis for the cause of the late ordovician glaciation we suggest that the taconic orogeny which commenced in the late middle ordovician caused a long term decline in atmospheric pco2 through increased weatherability of silicate rocks ice sheet growth was triggered when pco2 decreased to a threshold of 10 present atmospheric level and proceeded by positive ice albedo feedback in the midst of glaciation atmospheric pco2 began to rise as continental silicate weathering rates declined in response to coverage of weathering terrains by ice sheets at first this enhanced greenhouse effect was overcompensated for by ice albedo effects ultimately however atmospheric pco2 reached a level which overwhelmed the cooling effects of ice albedo and the glaciation ended the isotope results can be interpreted to indicate that atmospheric pco2 rose during the glaciation consistent with other proxy information although alternative interpretations are possible the large positive carbonate isotope excursion observed in late ordovician rocks around the world is explained as the expected response to increased carbonate platform weathering during glacioeustatic sea level lowstand rather than as a response to increased organic carbon burial

focused fluid flow along faults in the monterey formation coastal california fluid flow in fractured siliceous mudstone of the miocene monterey formation of california is inferred to be highly focused toward map scale faults that locally contain extensive amounts of carbonate and minor silica cement the distance of cross stratigraphic flow as inferred based on the strontium isotopic composition of carbonate fault cement is close to the thickness of the monterey formation of 700 m in one of two study locations jalama beach and less than the formation thickness at another location arroyo burro beach fluid is thus derived from within the monterey formation rather than from underlying older units based on mass balance estimates of the fluid volume required for fault cementation at jalama beach the minimum distance of formation parallel flow into the fault zone is 4 km and possibly 12 km the inferred distance of flow parallel to the formation into this fault thus exceeds the distance of cross formational upward flow along the fault by at least a factor of six the mass balance estimate requires that fluid flow along the fault is channeled into a pipe shaped conduit rather than distributed along fault strike fluid flow from the surrounding formation into fault pipes is inferred to follow a radial rather than uni or bilateral flow symmetry using bedding confined sets of extension fractures and stratabound breccia bodies radial fluid flow toward fault pipes requires fairly isotropic fracture permeability for flow along bedding and a low permeability across bedding the inferred flow geometry illustrates the combined effect of fault permeability structure permeability anisotropy of the surrounding formation and hydraulic head distribution in controlling basinal fluid flow in faulted sequences

the queenston formation shale dominated mixed terrigenous carbonate deposits of upper ordovician semiarid muddy shores in ontario canada the upper ordovician queenston formation constitutes the westernmost part of the taconic clastic wedge of eastern north america in ontario the queenston formation consists of a mixed terrigenous carbonate succession composed primarily of red silty shales which are locally gypsiferous alternating in the middle and lower parts of the formation with thin grey shales calcareous siltstone and sandstone and bioclastic limestone brachiopods some ostracodes and in the lower beds bryozoans constitute the body fossil assemblage diplocraterion skolithos chondrites and occasional large vertical burrows of unknown origin form the ichnofauna plane beds ripple marks and occasional swaley and hummocky cross stratifications are present in the calcareous units mudcracks gypsum nodules occasional halite hopper casts and gilgai like structures typically occur in horizons where shales alternate with siltstone and limestone these features are indicative of prograding muddy and storm and tide affected

large scale quaternary detachments in ventura basin southern california the ventura basin is an east north east trending trough in the california transverse ranges which records major quaternary detachment faulting at three levels the earliest thrusting occurred along weak siltstone interbeds in a sequence dominated by competent basin plain turbidite sandstone because sedimentation continued during thrusting the age rate and direction of thrusting can be worked out faulting began 1 3 m y ago and ceased 0 65 m y ago with a maximum slip rate of 2 8 mm yr to the southeast the fault set moved up a 45 ramp and ended as a blind thrust the ramp had topographic expression on the seafloor diverting turbidites around the ramp and preserving ash beds along with other hemipelagic sediments on its crest following the end of deposition 0 2 m y ago the competent basin plain turbidites underwent flexural slip folding over an incompetent miocene sequence dominated by shale underlying competent paleogene strata were not folded the south flank of the ventura avenue anticline tilted at 3 4 rad yr the anticlinal crest rose at a rate of 15 16 mm yr decelerating to 4 3 5 2 mm yr and the anticline and an adjacent syncline shortened at a rate of 20 mm yr the high rate of folding in the ventura avenue oil field resulted in overpressured sandstone reservoirs and oil water interfaces which have not had time to reach gravity equilibrium the red mountain san cayetano and santa susana faults mark the surface expression of a seismically active midcrustal detachment which produced convergence across the ventura basin at rates as high as 23 mm yr total convergence across the eastern san cayetano fault near fillmore is 11 600 2000 m in the last million years

structural evolution of carpinteria basin western transverse ranges california the pleistocene carpinteria basin is an east trending northward verging faulted syncline containing up to 4 000 ft 1 220 m of partially intertonguing santa barbara and casitas formations deposited on previously folded pre pleistocene strata with up to 80 discordance structures subcropping against the unconformity indicate that most of the deformation in the santa ynez mountains prior to deposition of the pleistocene santa barbara formation was by folding during that time the carpinteria area was the northern margin of the offshore ventura basin subsequent uplift during middle to late pleistocene time isolated the carpinteria basin north of the red mountain fault quaternary faults in the area are either south dipping reverse faults related to bedding slip in pre pleistocene strata or north dipping reverse faults that truncate bedding and are seismogenic the rincon creek fault dips 35 to 60 south steepening to nearly vertical at depth where it apparently passes into bedding within the sespe formation the rincon creek fault offsets late pleistocene marine terraces and is itself deformed by the red mountain fault the red mountain fault dips 55 to 63 north at the surface and steepens to 75 north with depth it also steepens westward south of the summerland offshore oil field to 85 north vertical separation decreases westward from 14 750 ft 4 500 m north of the rincon field to 1 150 ft 350 m at rincon point and 330 ft 100 m south of summerland the main branch of the red mountain fault offsets a 45 000 year old marine terrace but not a 4 500 year old terrace the summerland offshore oil field is situated within a disharmonically folded anticline in which severely deformed structurally incompetent miocene mudstone overlies broadly folded competent oligocene sandstone because the anticline formed after deposition of the santa barbara formation oil could not have migrated into this field until middle to late pleistocene time

active fault hazard in southern california ground rupture versus seismic shaking there are exceptions to the common assumption that surface rupture by faulting is always accompanied by a damaging earthquake near ventura california faults of the oak view ojai area and orcutt and timber canyons displace late pleistocene and or holocene alluvial materials and soils by several tens of metres the movement is almost exclusively along bedding planes in response to flexural slip during folding therefore these faults constitute a ground rupture hazard however because they do not extend downward to rocks of high shear strength which could store large amounts of elastic strain energy they do not constitute a seismic shaking hazard in contrast the oak ridge fault near the coast does not cut strata shallower than 1 250 to 1 500 m below the surface although pressure ridges attest to recent activity similarly the newport inglewood fault between long beach and inglewood oil fields in the los angeles basin is expressed at the surface as a line of anticlinal hills rather than a throughgoing fault historical faulting at inglewood and possibly at rosecrans and dominguez oil fields may be related to oil field development rather than earthquake related displacement those portions of the oak ridge and newport inglewood faults pose seismic shaking hazard but not ground rupture potential from earthquakes alone for planning purposes it is necessary to distinguish faults with only ground rupture potential or with only seismic shaking potential from the well known faults with potential for both

upper ordovician conodont biostratigraphy and the age of the collingwood member southern ontario canada11earth science sector ess contribution 20100302 the upper ordovician stratigraphy in southern ontario represents the clastic foredeep associated with the appalachian taconic orogeny transitioning northwest into coeval carbonate platform facies ten measured and sampled sections in both the collingwood area and on manitoulin island lake huron provide two relatively complete composite sections 277 and 95 m respectively through the marine part of the sequence a total of 100 2 kg samples collected for a conodont biostratigraphic study yielded 77 215 well preserved specimens taxonomic study of the fauna illustrated herein identified 34 species representing 22 genera and three taxa in open nomenclature taxonomic revisions are made to five species of pseudobelodina and one of rhipidognathus a new species pseudobelodina microdentata is established the fauna primarily represents the midcontinent province with incursions from the north atlantic province primarily in the collingwood area four conodont zones are recognized that help refine the ages

distal sedimentation in a peripheral foreland basin ordovician black shales and associated flysch of the western taconic foreland new york state and ontario black shale is characteristic of early siliciclastic fill along the distal western flank of the middle to late ordovician taconic peripheral foreland basin this facies referred to as the utica black shale magnafacies includes at least five intervals bounded by unconformities and or condensed beds each unit records a pulse of subsidence and subsequent siliciclastic sedimentation in the foreland basin progressive episodic shifts from carbonate to organic rich mud deposition may be due to steepening and subsidence of the western carbonate ramp followed by onlap of axially dispersed sediment in general silt poor organic rich mud was deposited on a subsided carbonate ramp that sloped eastward toward the nearest siliciclastic sediment source this mud may have accumulated from the pelitic tails of axial fan turbidites derived from source areas to the east and possibly the south the resultant black shale does not necessarily represent the deepest part of the basin the deepest part of the basin the basin axis contained coarser grained axial submarine fan deposits throughout the later part of the middle ordovician and the first half of the late ordovician the basin axis migrated westward over 100 km yet the orientation of the axis and the basin margins were persistent this basin orientation generally parallel to the orogenic belt along with the large scale basin shape resulted from a combination of deformational loading of the continental margin and progressive flexure in the foreland smaller scale structural elements normal fault bounded basement blocks were superposed on large scale taconic foreland basin geometry abrupt shifts from carbonate ramp sedimentation to deeper water organic rich mud deposition may reflect movement along these faults during the orogeny

sedimentology of an early silurian carbonate ramp the manitoulin formation southern ontario the manitoulin formation is a pervasively dolomitized lower silurian carbonate unit that was deposited in the michigan basin and locally in the appalachian basin the formation reaches a maximum thickness of 11 1 m in southern ontario and can be subdivided into eight facies and four regionally correlatable facies assemblages owing to the relatively continuous transition of shallow to deeper water facies from the northern to southern portions of the study area the manitoulin formation is interpreted as having formed on a carbonate ramp with a southerly component of dip our study suggests that the algonquin arch which is transected by the outcrop belt did not significantly influence deposition or separate the michigan and appalachian basins depending on its location on the ramp the manitoulin formation shows evidence of varying degrees of episodic storm events alternating with fair weather processes such as wave shoaling sediment reworking and bioturbation apart from bioturbation these fair weathe

the ireviken event in the lower silurian of gotland sweden relation to similar palaeozoic and proterozoic events abstract for a long time the silurian was thought to represent a time of stable environmental conditions in the greenhouse period that followed the late ordovician glaciation during the past decade knowledge about the silurian has increased markedly and today it is known that the conditions in the silurian were much more variable than previously assumed detailed isotopic investigations have revealed several distinct positive excursions in both carbon and oxygen isotope values in low latitudes these periods of high c and o isotope values are in many cases characterised by the growth of reefs and the formation of extended carbonate platforms the sediments deposited during these excursions contain impoverished fossil assemblages especially with respect to conodonts graptolites and trilobites a conspicuous isotope excursion coincident with facies changes and a marked mass extinction is observed near the llandovery wenlock boundary this event is called the ireviken event after its type locality on the island of gotland sweden jeppsson l 1987 in palaeobiology of conodonts ellis horwood ltd chichester pp 129 145 here isotope data from nine sections at the nw coast of gotland are presented that cover the time interval of the ireviken event the 13c mean values rise from 1 4 to 4 5 and the 18o values increase from 5 6 to 5 0 the relative timing of stable isotope development extinctions and facies development is discussed it is shown that first extinctions precede the isotope excursion this indicates that extinction events and stable isotope development are only indirectly connected but might reflect the same causes other events characterised by similar relationships between positive isotope excursions mass extinctions and facies development are found in younger parts of the silurian late wenlock and ludlow in the late ordovician the late cambrian and with some reservations in the proterozoic the similarities between these events indicate analogous controlling mechanisms for the silurian climatic changes between humid and arid conditions in low latitudes were postulated in an earlier study of the authors a palaeoceanographic climatic model was postulated which is consistent with most of the sedimentological palaeontological and geochemical data here we demonstrate that this model may be applicable also for the older events