origin of minor limestone shale cycles climatically induced or diagenetic the opinion is widely held that each limestone bed in minor limestone shale cyclic sequences however modified subsequently by diagenesis reflects an environmental event on the assumption that this is true attempts have recently been made to relate limestone shale cycles to climatic variations affected by milankovitch orbital cycles a test is proposed and illustrated mainly with reference to the blue lias formation of dorset england that appears to indicate that many limestones can be produced solely by rhythmic unmixing of caco 3 during diagenesis unless such limestones can be clearly distinguished from those that record genuine environmental signals orbital cycle analysis based on such sequences will give meaningless results

carbon isotope excursions and sea level change implications for the frasnian famennian biotic crisis new carbon and oxygen isotope data from carbonates spanning the frasnian famennian f f boundary in the guilin area south china show a broad positive 13 c rise and fall with sharp short lived negative 13 c events this pattern is comparable to that in europe and north america the integration of the isotope stratigraphy with high resolution sequence stratigraphy corroborates the onset of the positive 13 c excursion during a third order sea level fall in the latest frasnian this can best be explained through increased burial of c org during the sea level fall brought about by increased organic productivity caused by increased continent derived nutrient flux to the ocean due to enhanced weathering through the proliferation of land plants in the devonian this scenario would have resulted in anoxic and eutrophic conditions over epicontinental seas and blooms of cyanobacteria creating a highly stressful and fragile ecosystem for oligotrophic normal marine benthic organisms and leading to their massive decline the global third order sea level fall near the end of the frasnian may have led to gas hydrate dissociation giving the negative 13 c events and caused wild climatic fluctuations the subsequent short term events of sea level rise anoxia and eutrophication in the latest part of the f f transition would have placed additional environmental stresses on the already weakened biota leading to their further demise

diagenetically enhanced bedding in argillaceous platform limestones stratified cementation and selective compaction a field and petrographic study has been made of 34 massive beds in argillaceous limestones of open marine platform facies in the u k lithologies include grainstones packstones wackestones and lime mudstones the rocks are of silurian carboniferous and jurassic ages additional information was obtained from other limestones in the u k the u s a canada and continental europe the beds are parts of sequences composed of couplets of strata fissile limestones alternating with hard limestones in the fissile limestones the effects of mechanical compaction and pressure dissolution have been concentrated whereas in the hard limestones they are minimal or absent bedding planes visible in outcrop are diagenetic in origin and lie in the middle parts of the fissile limestones where compaction has been most severe the features produced by pressure dissolution are dissolution seams and fitted fabric there are no stylolites the original carbonate sediments were bioturbated and any structures produced by flowing water were destroyed the vertical distribution of the bedding planes bears no relation to primary depositional bedding planes which are rare or absent it is inferred that the strata which were to become the hard limestones were selectively cemented before mechanical compaction had been completed thenceforth mechanical compaction and then pressure dissolution were concentrated in the less cemented strata these became the fissile limestones pressure dissolution acted late in the diagenetic history and provided only an insignificant part if any of the carbonate for cementation it is concluded that the orientation of beds couplets is parallel to successive sea floors and that the sediments that eventually became single beds accumulated synchronously similar couplets in platform limestones of the mississippian and pennsylvanian of the u s a extend over thousands of square kilometres the signal that controlled the initial selective cementation must have been widespread and synchronous and also syndepositional in its timing but otherwise cannot be further defined on the basis of the data so far collected the presumed order of events was 1 accumulation of carbonate sediment terrigenous clay and organic matter 2 hydrodynamic reworking and bioturbation the latter finally overprinting the former 3 selective cementation of the more carbonate rich strata yielding couplets each consisting of a relatively well cemented stratum and a poorly cemented stratum 4 mechanical compaction concentrated in the less cemented strata 5 pressure dissolution concentrated in the same strata

lead isotopic signatures of porphyry copper deposits in oceanic and continental settings colombian andes abstract three discrete sub belts of porphyry copper type mineralization are recognized in the colombian andes a western eocene sub belt an eastern jurassic to early cretaceous sub belt and between them a central miocene sub belt the western sub belt is part of an oceanic domain constituted by oceanic crust and overlying immature island arc rocks the eastern sub belt is within a continental domain underlain by the leading edge of the guayana shield and the central sub belt spans the faulted boundary between them the thicker continental crust includes important granulitic rocks which crop out locally as in the vicinity of the mocoa porphyry copper deposit pb isotopic ratios were determined for pyrite samples collected from 6 porphyry copper centers 3 in the western sub belt 2 in the eastern sub belt and one in the central sub belt ratios fall into 3 discrete populations the most radiogenic values represent the western sub belt the least radiogenic represent the eastern sub belt and an intermediate value corresponds to the central sub belt ratios therefore become progressively less radiogenic from the western oceanic domain to the eastern cratonic domain comparison of the pb isotopic ratios with those given in the literature for possible source materials for colombian porphyry copper leads enables the subcontinental mantle wedge subducted oceanic crust and subducted metalliferous sediments to be discounted as principal sources the relatively radiogenic signatures of 5 of the porphyry copper centers appear to be broadly compatible with either a subducted pelagic sediment source or an upper continental crust source whereas the sixth center mocoa is characterized by a distinctly less radiogenic 206 pb 204 pb ratio an admixture of a relatively small percentage of non radiogenic pb from granulitic material in the upper crust with the more radiogenic pb typical of the western sub belt centers could account for the mocoa data because much of the upper crustal section in the region of mocoa is thought to be dominated by granulitic material the radiogenic pb component could be best derived from subducted pelagic sediments if this is the case then a subducted pelagic sediment source could dominate the pb isotopic signatures of all colombian porphyry copper mineralization with some of the pb isotopic differences being produced by selective contamination with upper crustal leads of variable character if this conclusion is correct then the fundamental metal budgets of colombian porphyry copper systems are probably largely independent of upper crustal influences

deep water facies processes and models a review and classification scheme for modern and ancient sediments abstract a review of previous work on modern and ancient deep water facies processes and models is presented with a new classification scheme involving 40 distinct facies related to 15 conceptually distinct facies groups these facies are fixed points in a spectrum of facies generated in a process continuum from resedimentation processes through semi permanent bottom currents to pelagic settling in essence the scheme is descriptive of the sedimentary attributes of sediments although it is designed to aid interpretation of possible sediment transport deposition processes the classification scheme is three tier with facies classes groups and constituent facies and is hierarchical to allow flexibility in its use there are seven facies classes with classes a e defined largely on the basis of grain size differences class f on the basis of internal organization and class g on composition the facies classes are class a gravels muddy gravels gravelly muds and pebbly sands with 5 gravel grade class b sands with 80 sand grade and 80 mud 40 silt and 0 20 sand class e muds and clays with 95 mud grade for the purpose of large scale mapping or reconnaissance fieldwork either the level of facies classes or groups may be appropriate whereas for more detailed sedimentology the more detailed facies level will be necessary

carbon oxygen and sulphur isotope variations in concretions from the upper lias of n e england carbon oxygen and sulphur isotope data for transects across two pyrite bearmg carbonate concretions and their host sediments from the upper lias of n e england show symmetrical zonation 13cpdb values of the calcite cement 12 9 to 15 4 indicate that most of it originated from organic matter by bacterial reduction of sulphate augmented with marine and to a lesser extent fermentation derived carbonate organic carbon 13cpdb 26 1 to 37 0 reflects the admixture of allochtho nous terrestrial organic matter with marine material and the selective preservation of isotopically light organic material through microbiological degradation two phases of pyrite are present in each concretion the earlier framboidal pyrite formed throughout the sediment prior to concretionary growth and has 34scd values of 22 to 26 indicating formation by open system sulphate reduction the later euhedral phase is more abundant and reaches values of 2 5 to 5 5 at concretion margins this phase of sulphate reduction provided the carbonate source for concretionary growth and occurred in a partially closed system the 13c and 34s data are consistent with mineralogical and chemical evidence which suggest that both concretions formed close to the sediment surface the 18o values of the calcite in one concretion 18opdb 2 3 to 4 8 indicate precipitation in pore waters whose temperature and isotopic composition was close to that of overlying seawater the other concretion is isotopically much lighter 18opdb 8 9 to 9 9 and large 18o differences between concretions in closely spaced horizons imply that local factors control the isotopic composition of pore waters

isotopic composition of displacive fibrous calcite veins reversals in pore water composition trends during burial diagenesis abstract stable isotope and petrographic analyses of diagenetic calcite cone in cone and beef veins from british jurassic and lower cretaceous shales have been used to determine the environment of precipitation of fibrous calcites successive growth took place by antitaxial displacive addition at the vein margins away from primary sedimentary laminations or early diagenetic concretions carbon and oxygen isotopic ratios 13c 0 and 18 4 to 11 indicate a relatively late dingenetic origin for the veins after tens or probably hundreds of meters of burial and after cessation of bacterial activity nd considerable modification of the oxygen isotopic composition of the pore water vein growth was discontinuous and took place in waters of changing isotopic and trace element composition changes are not unidirectional and are unlikely to result from the simple evolution of a single conhate pore water reversals in isotopic trends indicate that precipitation took place during periods of renewed lateral groundwater flow tapping different sources of bicarbonate bearing solution

the source of lead in the osen sandstone lead deposit on the baltic shield norway the sandstone hosted lead deposits on the baltic shield occur in marine late precambrian to early cambrian sedimentary rocks a ground water model in which the basement is postulated as the metal source has been proposed for these deposits to test this model the osen deposit was selected because it lies directly on a homogeneous granite basement on the basis of u th pb whole rock isotopic analyses for three samples of unweathered basement granite trysil granite a basement age of 1 598 m y was established the lead isotopic compositions of four out of five specimens of galena from the osen deposit plot between isochrons calculated for the granite at 500 m y and 540 m y this is strong support for the hypothesis that the lead source was basement granite and suggests that mineralization took place soon after sedimentation a basement source for the lead is further substantiated by the regional pattern of lead isotopic compositions of sandstone lead deposits which reflects the character of adjacent basement terranes

formation of complex fibrous calcite veins in upper triassic strata of wrangellia terrain british columbia canada abstract fibrous calcite veins are ubiquitous throughout the thinly bedded organic rich upper triassic marine mdrocks of the queen charlotte islands and their lateral equivalents on vancouver island these veins show variable and complex morphologies and can be grouped into several types a simple b anastomosing or composite c boxwork and 4 polygonal network oriented normal to bedding field petrographic and geochemical evidence suggest that vein opening resulting from hydraulic fracturing due to elevated pore fluid pressures was an early phenomenon and occurred prior to significant compaction of the host sediments calcite fibers in the veins are up to 30 mm long and commonly oriented perpendicular to the wall but locally display conical structures fibrous calcites with the exception of those in boxwork veins are generally non ferroan and dull to very weakly orange luminescent the boxwork calcites are ferroan zoned and show dull luminescence with some bright rims 18 o values range from 8 2 to 21 6 pdb and 13 c values range from 2 0 to 4 4 pdb although some variations are present among the different morphological types of calcite veins oxygen and carbon isotopic values display important variations when compared geographically the most depleted oxygen and carbon isotopic values are those of boxwork calcite and they are associated with areas where the effects of early mesozoic plutonism were most severe precipitation of boxwork fibrous calcites is interpreted to have been related to hydrothermal discharge into unconsolidated host sediment rather than to later burial although the hydrothermal influence on the formation of vein calcite is related to geological events specific to the wrangellia terrain this study provides an alternative mechanism for the generation of fibrous calcite veins and demonstrates the local importance of hydrothermal input in the evolution of pore water chemistry

origins of aircraft damaging clear air turbulence during the 9 december 1992 colorado downslope windstorm numerical simulations and comparison with observations results from numerical simulations of the colorado front range downslope windstorm of 9 december 1992 are presented although this case was not characterized by severe surface winds the event caused extreme clearair turbulence cat aloft as indicated by the severe structural damage experienced by a dc 8 cargo jet at 9 7 km above mean sea level over the mountains detailed measurements from the national oceanic and atmospheric administration environmental research laboratories environmental technology laboratory doppler lidar and wind profilers operating on that day and from the defense meteorological satellite program satellite allow for a uniquely rich comparison between the simulations and observations four levels of grid refinement were used in the model the outer domain used national centers for environmental prediction data for initial and boundary conditions the finest grid used 200 m in all three dimensions over a 48 km by 48 km section the range of resolution and domain coverage were sufficient to resolve the abundant variety of dynamics associated with a time evolving windstorm forced during a frontal passage this full range of resolution and model complexity was essential in this case many aspects of this windstorm are inherently three dimensional and are not represented in idealized models using either 2d or so called 2d 3d dynamics both the timing and location of wave breaking compared well with observations the model also reproduced cross stream wavelike perturbations in the jet stream that compared well with the orientation and spacing of cloud bands observed by satellite and lidar model results also show that the observed cat derives from interactions between these wavelike jet stream disturbances and mountain forced internal gravity waves due to the nearly east west orientation of the jet stream these two interacting wave modes were orthogonal to each other thermal gradients associated with the intense jet stream undulations generated horizontal vortex tubes hvts aligned with the mean flow these hvts remained aloft while they propagated downstream at about the elevation of the aircraft incident and evidence for such a vortex was seen by the lidar the model and observations suggest that one of these intense vortices may have caused the aircraft incident reports of strong surface gusts were intermittent along the front range during the period of this study the model showed that interactions between the gravity waves and flow aligned jet stream undulations result in isolated occurrences of strong surface gusts in line with observations the simulations show that strong shears on the upper and bottom surfaces of the jet stream combine to provide an episodic downburst of turbulence in the present case the perturbations of the jet stream provide a funnel shaped shear zone aligned with the mean flow that acts as a guide for the downward transport of turbulence resulting from breaking gravity waves the physical picture for the upper levels is similar to the surface gusts described by clark and farley resulting from vortex tilting the cat feeding into this funnel came from all surfaces of the jet stream with more than half originating from the vertically inclined shear zones on the bottom side of the jet stream visually the downburst of turbulence looks similar to a rain shaft plummeting to the surface and propagating out over the plains leaving relatively quiescent conditions behind

transformation of aragonite dominated lime muds to microcrystalline limestones scanning electron microscopy of textures in several well lithified pleistocene micrites mixed micrite and microspar from south florida and the bahamas shows significant diagenetic alteration of the lime mud precursors the dominance of aragonite in those precursor muds is supported by the surprising presence of abundant aragonite relics in the neomorphic calcite crystals confirmed by feigl stain x ray and electron diffraction polished very lightly etched surfaces of neomorphic calcite crystals even in the microspar show aragonite relics and pits both absent in coexisting void fill cements similarities between micrite and microspar crystals indicate their common origin through a one step neomorphic process of calcitization that is the mud did not first calcitize to micrite and then alter to microspar by aggrading neomorphism this conclusion has more general implications for the interpretation of the origins of other older microsparitic carbonates both early calcitization and concurrent early meteoric cementation were significant in producing the florida bahamas micrites studied which have low porosities similar to those found in most ancient fine grained limestones these low porosities despite the absence of significant overburden indicate that major porosity reduction without compaction can occur in calcitized aragonite dominated muds

dolomitization of deep water sediments cow head group cambro ordovician western newfoundland abstract dolomite in the cow head group occurs as 1 dolomitic siltstone interbedded with dolomitized or undolomitized shale 2 prominent tan weathering argillaceous partings between bedded limestones 3 disseminated crystals in limestones and 4 as a component of the interparticle matrix in conglomerates dolomite crystals are present as detrital cores cement or replacement of limestone and shale crystals are commonly characterized by inclusion rich cores inclusion poor rims and late iron enrichment many inclusion rich cores in disseminated dolomite crystals and dolomitic siltstones are detrital most inclusion rich cores however probably originated from replacement of peloids and possibly other allochems these detrital and authigenic cores are overgrown by inclusion poor authi enic rims other authigenic crystals lack an obvious core region and consist entirely of inclusion poor dolomite the authigenic dolomite originated in uncompacted or partially compacted sediments during shallow burial below the sediment seawater interface as indicated by 1 the occurrence of dolomitic siltstone and shale clasts in polymictic limestone conglomerates 2 sharp erosive contacts between dolomitic siltstone and shale and overlying undolomitized shale or conglomerate and 3 spatial association with columnar calcite cement interpreted to be a shallow burial precipitate in clast supported conglomerates the dolomite crystals continued to enlarge rather than recrystallize during deeper burial as indicated by variable and complex cathodoluminescence zoning an organic signature is not apparent in their carbon isotopic compositions the oxygen isotopic composition of these crystal reflects the deeper burial conditions of higher temperatures more evolved pore waters or both the cow head group also contains minor late diagenetic dolomites which occur as cement and a replacement of limestone shale and earlier dolomite this dolomite postdates tectonic stylolites and is spatially associated with joints and faults

cathodoluminescence and composition of calcite cement in the taum sauk limestone upper cambrian southeast missouri abstract study of calcite cements in the taum sauk limestone upper cambrian southeast missouri using cathodoluminescence petrography and microprobe analysis has shown that 1 distinct luminescent zones correspond to distinct compositional zones 2 the degree of luminescent intensity of a particular calcite zone appears to be controlled by the fe mn ratio of the zone not by the absolute concentrations of either cation 3 manganese may induce luminescence in calcite at concentrations as low as 300 ppm and 4 there may be no critical concentration of ferrous iron in calcite above which there can be no luminescene

windstorms along the western side of the washington cascade mountains part i a high resolution observational and modeling study of the 12 february 1995 event abstract this paper describes the leeside wind event of 12 february 1995 in which a swath of strong low level easterly flow with gusts to 35 40 m s 1 extended downwind of a major gap in the cascade mountains over the puget sound lowlands low level easterly flow approaching the olympics was decelerated to less than 5 m s 1 resulting in enhanced over 30 cm snowfall along the windward side of the barrier in addition to conventional synoptic observations high resolution data sources were utilized profilers on both sides of the cascades a wsr 88d doppler radar located in northern puget sound and nonhydrostatic model simulations down to 1 km resolution with the exception of underpredicting heavy snowfall to the east of the olympics many of the mesoscale features of this event were realistically simulated including cold air damming over eastern washington strong winds downwind of the gaps a mountain wake downstream of mount rainier and flow splitting around the olympics a major objective of th

the diagenetic history of some septarian concretions from the kimmeridge clay england abstract large septarian concretions from the kimmeridge clay up to 1 2 m in diameter have centres comprising anhedral calcite microspar passing into margins of radiating fibrous calcite microspar with a pyrite rich zone at the transition septarian veins formed and were lined with brown calcite synchronously with fibrous matrix growth with white calcite precipitated in septarian cavities after concretion growth ceased septarian veins filled only with white calcite formed later at the same time as the outermost calcite microspar crystals were enlarged the concretions were buried in the late jurassic to about 130 m and in the late cretaceous to about 550 m with uplift between oxygen isotopes show that the concretion grew throughout the first burial with septarian veins forming from about 30 m depth onwards later septarian veins formed between about 200 and 500 m during the second burial carbon isotopes show that the compact inner matrix grew in the sulphate reduction zone the end of which is marked by the pyrite enriched zone dissolving shells and possibly minor methanogenic carbonate slowly diluted sulphate reduction zone carbonate during deeper burial during early concretion growth mg and sr were depleted in the pore water during later stages of the first burial mg sr mn and fe all increased especially after concretion growth ceased during the second burial fe mn and mg decreased as calcite precipitated implying relatively closed systems for these elements synchronous formation of septarian fractures and fibrous calcite matrix shows that the kimmeridge clay became overpressured during the later stages of both burials

carbon isotope geochemistry of the frasnian famennian transition abstract carbon isotope records for inorganic and total organic carbon toc as well as for individual organic biomarkers show two positive excursions in the late frasnian with comparable shape and amplitude in 13c the positive shifts in 13c can be correlated with the deposition of the kellwasser horizons the inorganic carbon isotope excursions are explained by an enhanced organic carbon burial that is expected to lower the concentration of oceanic dissolved co2 toc 13c values do not exhibit a larger shift than that recorded by inorganic carbon 13c values this is unexpected since a decrease in the dissolved co2 concentration should result in lower carbon isotope fractionation during photosynthesis ep 13c values of presumed algal biomarkers low molecular weight n alkanes pristane phytane although offset by 2 5 exhibit essentially identical records as toc and confirm this unexpected result it is proposed that high atmospheric and oceanic co2 concentrations during the devonian resulted in maximum photosynthetic fractionation any change in the co2 concentration would thus not affect ep as such 13c of primary organic carbon and 13ccarb will exhibit parallel excursions the data imply that carbon isotopes and the relationship between ep and co2 will not be effective to study changes in pco2 levels during the devonian since co2 concentrations were too high

observations simulations and analysis of nonstationary trapped lee waves abstract although considerable understanding of mountain waves has been gained with the aid of the steady state assumption it is clear that mountain waves evolve over time group velocity arguments indicate that this evolution can occur in less than 1 h this study uses observations of trapped lee waves to measure the rate at which their horizontal wavelengths change including a detailed analysis of two events in which such changes are clearly documented in one case doppler lidar observations show a steady increase in horizontal wavelength of 6 h 1 over 4 h and clearly illustrates the relationship between the wave clouds and wave motions in a second case visible satellite imagery reveals an increase in wavelength of 14 h 1 which is related to temporal changes in vertical air motions measured by wind profilers within the wave field hourly vertical profiles of wind and virtual temperature measured by radio acoustic sounding systems rass and wind profilers reveal important changes in the wave env

galena lead isotopic variations in a mesozoic cenozoic andean arc antarctic peninsula the antarctic peninsula magmatic arc was the site of andean type subduction from at least silurian times to the late cenozoic hydrothermal galenas from eight localities in accretionary complex arc volcanic sequences and plutonic rocks along a strike length of 1100 km have restricted 206 pb 204 pb 207 pb 204 pb and 208 pb 204 pb compositions 18 628 18 734 15 599 15 673 and 38 441 38 689 respectively plotting on crustal average growth curves the local geological setting and comparison with andean leads suggest that the major source reservoir was isotopically homogeneous sedimentary rocks forming the late palaeozoic triassic accretionary complex the lead data suggest that old continental basement is absent at least as far south as 65 s galenas from a large 13 2 km epithermal vein system on hurd peninsula fall in three populations a main isotopically homogenous array 18 643 15 647 and 38 503 2 0 020 0 020 and 0 050 n 33 interpreted as a large hydrothermal system that extracted homogenous leads from early triassic turbidites two smaller sub populations with more radiogenic 206 pb 204 pb are associated with mid to late cretaceous intrusive rocks and may represent either local contributions of magmatic lead or younger hydrothermal episodes in western graham land two localities associated with mantle derived mafic intrusions of cenozoic age have low 207 pb 204 pb and 208 pb 204 pb ratios indicating addition of some mantle type lead

water column anoxia enhanced productivity and concomitant changes in δ13c and δ34s across the frasnian famennian boundary kowala holy cross mountains poland the investigation of the trace element and organic geochemistry of the frasnian famennian boundary section at kowala holy cross mountains poland shows that the lower water column was oxygen deficient during late frasnian and early famennian times the abundance and carbon isotopic composition of diaryl isoprenoids biomarkers indicative for green sulfur bacteria prove that euxinic waters reached into the photic zone at least episodically total organic carbon toc contents show two maxima that are time equivalent to the kellwasser horizons deposited in shallower water settings enhanced toc concentrations are explained by a higher primary productivity presumably as a consequence of an enhanced nutrient supply from the continent the increase in the abundance of hopanes and bituminite suggests that the bacterial contribution to toc increased at the frasnian famennian transition the sulfur isotopic composition of pyritic and organically bound sulfur shows a 27 excursion across the boundary the observation that the 34s values of organic bound sulfur closely resemble that of pyrite sulfur indicates a common sulfur source likely early diagenetic sulfide a change in the 13c of total dissolved inorganic carbon as a consequence of an enhanced burial of 12c enriched organic carbon is indicated by a 3 excursion measured for toc as well as for individual n alkanes and isoprenoids the burial of large amounts of organic carbon is expected to result in a decrease in pco2 and should affect the photosynthetic carbon isotope fractionation ep the fact that we observe no change in ep can be explained by the circumstance that ep was most probably at maximum values as a consequence of high atmospheric and oceanic dissolved co2 concentrations during the devonian

paleotemperature analysis by authigenic minerals and its application to petroleum exploration paleotemperature analysis in sedimentary basins is very important to petroleum exploration because generation of petroleum from kerogen is controlled mainly by temperature vitrinite reflectance generally is used for this analysis a new technique of paleotemperature analysis depends on the relationship between transformation temperatures of authigenic minerals in diagenetically altered argillaceous sediments and their depth of first appearance in any specific well study of both the paleogeothermal gradient and porosity depth relationships in a basin make possible predictions of hydrocarbon pools in the exploration area for example primary migration of oil in neogene rocks of the niigata basin in central honshu japan has occurred at burial depths between 2 260 and 3 50 m 7 410 and 12 960 ft this corresponds to the analyzed paleogeothermal gradients of exploited oil and gas fields in the basin 3 0 to 4 0 c 100 m 1 6 to 2 2 f 100 ft

rapid burial diagenesis of deep water carbonates exuma sound bahamas well lithified deep water periplatform limestones of late miocene age occur at subsurface depths from 113 m to total depth of 283 m at ocean drilling program odp site 632 in exuma sound bahamas this occurrence marked by a dramatic increase in compressional wave velocities up to 5 0 km s represents the most rapid transformation of ooze to limestone yet discovered in either periplatform or open ocean settings the burial diagenetic pathway is similar to those for meteoric and deep burial diagenetic environments aragonite dissolves magnesian calcites exsolve or dissolve low mg calcite precipitates as cement and secondary moldic vuggy porosity develops stable isotope ratios suggest precipitation of low mg calcite from cold marine derived pore waters accelerated diagenesis is a response to the influx of large quantities up to 80 of bank derived metastable carbonates and rapid burial via high accumulation rates up to 210 m m y our findings 1 document carbonate mineral stabilization and lithification in a seawater mediated shallow burial diagenetic environment 2 highlight the very high diagenetic potential of periplatform carbonates and 3 call into question the use of seismic velocities as support for the bahama megabank hypothesis

the frasnian famennian boundary interval in the south polish moravian shelf basins integrated event stratigraphical approach abstract the late devonian mass extinction event near the frasnian famennian f f boundary has been analysed using conodont biostratigraphy and biofacies sedimentology magnetic susceptibility and geochemistry in reference sections of the south polish moravian shelf holy cross mountains cracow and brno areas high resolution biostratigraphic study revealed difficulties in the precise recognition of this natural stage boundary but confirmed the occurrence of a major third order sequence boundary in the f f transition in an active synsedimentary tectonic setting marked by erosional discontinuities hardgrounds and brecciation or omission surfaces conclusive evidence of an extraterrestrial impact has not been found among earth bound factors the main devastating role in the shelf habitats is ascribed to fluctuating anoxia and or nutrient dynamics in a disturbed greenhouse climatic setting the long term facies changes were determined by a conspicuous break in carbonate production accompanied by replacement of mature stromatoporoid coral reefs by pioneer shelly crinoid banks microbial mounds and localised oolitic bars the key f f passage interval was marked by intermittent but generally accelerated periplatform ooze debris input and severe storm events as well as by probably highly fluctuating oxygenation and biological overproduction best manifested in radiolarian silicisponge and cephalopod acmes eutrophication phenomena at least partly stimulated by various hydrothermal and volcanic processes were one of the major biogeochemical processes during this climax of the late devonian biotic crisis however significant oceanographic perturbations were also linked with the onset of the transgressive hypoxic upper kellwasser event still within the latest frasnian linguiformis zone i e well before the f f conodont collapse implied variations in the redox state of seawater through the kellwasser timespan probably in regionally highly variable temporal scales are in agreement with recent palaeoecological and biogeochemical inferences in particular suggesting recovering oxygenation just prior to the f f boundary in the other laurussian intrashelf basins all the data support a long time multicausal earth bound crisis instead a worldwide cosmic catastrophe

a study of mountain lee waves using clear air radar the tropospheric and lower stratospheric response to flow over complex terrain is investigated using observations of vertical and horizontal motions made by a clear air doppler radar stationed at toulon in southern france analysis of the data has disclosed that episodic enhancements in the variability of vertical and horizontal velocities are due to lee waves excited by obstacles located within 5 km of the radar three separate cases of lee wave activity are studied two dimensional linear perturbation theory and existing three dimensional simulation results for appropriate parameter values are used as guides to check against the vertical structure observed by radar the magnitudes of vertical velocities observed during lee wave incidence are well within the range reported in previous observational and numerical simulation studies from a detailed examination of one of the cases of trapped waves the time variation observed by the radar of timescales of less than an hour has emerged as the likely result of small directional changes in the upstream winds causing a bodily rotation of the downstream three dimensional wave pattern centred on the wave generating obstacle the observed tendency of the vertical velocity perturbation to change sign with time above the radar location follows as a consequence of the presence of large horizontal gradients in the motion field which are inherent in the structure of three dimensional lee waves the temporal variability of the lee wave pattern has allowed the radar to sweep out a partial horizontal cross section revealing vertical circulation features that are broadly consistent with gravity wave behaviour however analysis of the effects of finite antenna beam spacing on the sampled data indicates that a spurious phase shift might have been introduced between the vertical and horizontal velocity perturbations observed in the cross section in contrast the measured vertical velocities are not seriously affected by finite beam width effects the lee waves examined in this study are quite moderate nevertheless this circumstance has provided a favourable opportunity to conduct a critical evaluation of the capabilities and limitations of this radar technique for observing lee wave phenomena in general we have been able to show clearly how the incidence and vertical structure of the waves respond to the synoptic scale situation in particular we have been able to distinguish between trapped and vertically propagating waves and to relate a transition between these wave types to synoptic scale changes in the static stability structure

doppler lidar observations of a downslope windstorm abstract during january and february 1987 the noaa wpl pulsed doppler lidar was deployed in the foothills west of boulder colorado to study orographically induced flows over the continental divide on 29 january 1987 the lidar with its unique spatial and temporal data gathering capabilities documented a downslope windstorm affecting the boulder area and the rest of the front range the lidar recorded in detail 1 a low level leeside wind maximum 2 propagating wind gusts exhibiting two distinct periodicities and 3 the eastern edge of a mountain wave feature where a jumplike flow reversal occurred such structures have not previously been observed with comparable detail by conventional in situ and remote sensing instruments the observed phenomena were similar to results obtained from mountain wave numerical models the most notable of the structural similarities was between the observed and modeled wind gusts

synsedimentary submarine slope failure and tectonic deformation in deep water carbonates cow head group western newfoundland the cow head group interpreted as a southeast dipping base of slope carbonate apron contains intraformational truncation surfaces and slide masses synsedimentary shear zones are formed 1 below intraformational truncation surfaces 2 in the basal parts of slide masses and 3 in the shallow subsurface because of downslope creep shear zones are characterized by a variety of synsedimentary deformation structures limestones are subject to folding brecciation rotation of fragmented beds and the development of fitted lenticular bedding in the interbedded shales there is both disruption of fine laminations and small scale isoclinal folding and faulting outcrops characterized by these features and the lack of truncation surfaces or slide masses may reflect minor downslope creep the presence of truncation surfaces slide masses and shear zones indicates deposition on an unstable sloping surface the recognition of intraformational truncation surfaces and slide masses usually requires extensive str

chemical model for the origin of minor limestone shale cycles by anaerobic methane oxidation chemical and isotopic studies of jurassic carbonate concretions indicate an origin by anaerobic methane oxidation concretion growth occurred within the top 1 m of sediment in a thin zone where methane was consumed to stimulate a late rejuvenated phase of sulfate reduction similar chemical and isotopic characteristics are shown by diagenetic carbonate in several limestone shale sequences and an origin by anaerobic methane oxidation is proposed for these sequences also the evolution of isolated concretions into diagenetic limestones depends on the extent of and depth variations in carbonate supersaturation arising from anaerobic methane oxidation and the persistence of a reduced sedimentation rate or a depositional hiatus diagenetic limestones formed in this way can often be recognized by their association with a later pyrite phase that is more abundant and has more positive sup 34 s values than the adjacent shales

origin of fine grained carbonate and siliciclastic sediments in an early palaeozoic slope sequence cow head group western newfoundland the cow head group is an early palaeozoic base of slope sediment apron composed of carbonate and shale whereas coarse grained conglomerate and calcarenite are readily interpreted as debris flow and turbidite deposits calcilutite lime mudstone calcisiltite and shale combine to form three distinct lithofacies whose present attributes are a function of both sedimentation and early diagenesis shale is the most common lithology black green and red shale colour variations reflect the abundance of organic matter in the source area and oxygenation conditions of the sea bottom in black and green shale millimetre to centimetre thick alternating dark and light laminations represent terrigenous mud turbidites and hemipelagites respectively the calcisiltite shale facies is uncommon and is composed of numerous graded carbonate shale sequences gcss deposited from waning carbonate turbidites and fall out of terrigenous muds some of the characteristics of ribbon and parted lime mudstones in the calcilutite shale facies can be explained by deposition of carbonate mud from dilute turbidity currents or hemipelagic settling other features are diagenetic in origin the lack of micrite in gcss and in the interbedded shales of the calcilutite shale facies is interpreted to reflect early dissolution of the finer carbonate from these sediments this remobilized carbonate was precipitated locally to lithify lime mudstone turbidites or hemipelagites form diagenetic lime mudstone beds and nodules cement calcisiltites and form dolomite many of the calcisiltites and calcilutites were therefore carbonate enriched at the expense of adjacent argillaceous sediments these attributes characterize not only fine grained sediments of the cow head group but many other early palaeozoic slope carbonates as well suggesting that the model proposed here for depositionl diagenesis has wider application

cenozoic plate kinematics subduction and magmatism south american andes revised plate reconstructions and enlarged isotopic age data sets strongly support three mechanisms for controlling the configuration and associated volcanism and tectonism of the andean subduction zone these include age of subducted plate convergence rate and interaction with hotspot traces from 50 to 25 ma in the andes the age of oceanic plate subducted progressively decreased apparently resulting in a9gradual decrease in subduction angle and consequent eastward shift in the loci of magmatic activity a major reorganization of plate motions at 25 ma resulted in an increased convergence rate normal to the chilean andes and was accompanied by subsequent broadening of the locus of magmatic activity into adjacent portions of bolivia and argentina convergence rates changed only slightly across the peruvian margin and no clear effect on subsequent magmatism is indicated interaction of the andean subduction zone with the juan fernandez and easter nazca hotspot traces apparently resulted in lower angle subduction and consequent gaps in magmatic activity as predicted continuations of the traces correspond with regions lacking dated volcanic rocks the current gaps in igneous activity correspond with contemporary subduction of the two traces and this association apparently occurred in the past as well

recristallisation des calcaires micritiques en fonction de la maturation thermique dans les basses terres du saint laurent du quebec typical sections of the ordovician carbonate platform sequence in the st lawrence lowlands beekmantown chazy black river trenton and utica groups were sampled at neuville joliette and montreal in the province of quebec the purpose of this work was to determine the level of thermal maturation of the carbonates using organic matter and clay studies and to correlate it with the crystal characteristics of the microcrystalline calcite the studied formations are supermature in the joliette and montreal areas whereas they are only mature at neuville the numerical analysis of the petrographic data shows that some microstructures observed in the rock samples and the size and morphology of the microcrystalline calcite crystals are partly related to the thermal maturation as determined by other methods

sedimentological and marine eogenetic control on porosity distribution in upper cretaceous carbonate turbidites central albania results of a detailed petrographic and stable isotope study illustrate that sedimentological differences and eogenetic dissolution precipitation processes controlled porosity distribution within carbonate turbidites of the ionian basin central albania based on lithology characteristics and porosity distribution observed in outcrop individual turbidite beds can be subdivided into four distinct intervals i e from base to top a a non porous wackestone floatstone or packstone followed by b porous packstone grainstone that grades into c wackestone and d non porous mudstone with pelagic foraminifera wackestone interval c is characterized by an alternation of porous and non porous laminae changes in turbidity current flow regime controlled the initial presence of matrix micrite giving rise to both matrix and grain supported lithologies within turbidite sequences these are non porous and porous respectively four eogenetic diagenetic processes dissolution cementation neomorphism and compaction acted shortly after deposition and modified primary porosity characteristics and distribution alteration by meteoric water is excluded based on the continuous burial until the oligocene of the studied deep marine carbonates moreover the stable isotope data with values between 2ae1 and 0ae7 for d 18 ov pdb and between 1 and 3 for d 13 cv pdb favour alteration by marine derived pore waters compaction and aggrading neomorphism occurred dominantly in intervals characterized by higher matrix micrite content i e the floatstone base and the wackestone mudstone upper turbidite part framework stabilizing cementation occurred dominantly in the packstone grainstone middle part of the turbidite beds in the latter porous lithologies matrix micrite was not compacted because of the grain fabric and the framework stabilizing cements here neomorphism of micrite into microrhombic euhedral calcite occurred and microporosity was preserved

neomorphism of the middle devonian ratner limestone related to dehydration of gypsum in south central saskatchewan the middle devonian ratner formation in south central saskatchewan consists of laminated carbonate and interlaminated to interbedded carbonate and anhydrite and the ratner carbonate is preserved as limestone in many cored intervals medium to very coarsely crystalline mvc limestone up t o4mi n thickness is dominantly present in the uppermost part or the upper cycle of the ratner formation directly overlain by the whitkow anhydrite gen erally both abundance and size of the mvc calcite crystals decrease downward away from contact between the ratner carbonate and the whitkow anhydrite the average 87 sr 86 sr ratio of the mvc calcite is 0 7078 comparable to that of middle devonian seawater the ratner mvc limestone is interpreted to result from neomorphism of the precursor microcrystalline lime mudstone in burial environments and neomorphic fluids might have consisted of gypsum dehydration water mixing with minor various ratios of formation waters based on petrographic observation o and sr isotopic data and stratigraphic evidence this research provides a case study in which the neomorphism of limestone is related to gypsum dehydration water

meteorological controls on wind erosion during foehn wind events in the eastern southern alps new zealand foehn winds in the lee of mountain barriers create highly favourable meteorological conditions for wind erosion in alpine areas and over adjacent lowlands this paper presents meteorological observations made during winter and summer foehn wind erosion events as part of a 3 year investigation into the physical nature and incidence of eolian processes in the eastern southern alps new zealand foehn winds were observed to initiate eolian activity including dust storm genesis when mean near surface wind speeds exceeded 7 8 m s 1 over a 20 min period in the absence of precipitation wind speeds of 25 30 m s 1 were frequently recorded during typical foehn events and wind speed maxima of 40 50 m s 1 were monitored during severe foehn windstorms saltation clouds sampled at 0 5 and 1 m above glaciofluvial deposits during foehn wind erosion events were found to display a mean grain size of between 300 and 435 m the entrainment of fine grained surface sediments by the airstream appeared to be enhanced in late autumn may and early spring september by freeze thaw cycles that were observed to result in needle ice growth in exposed glaciofluvial and lacustrine deposits the needle ice fragmented such deposits producing surfaces that were aerodynamically rough and highly susceptible to deflation by the airstream following a thaw observations made by this study are thought to be similar to foehn wind erosion events reported in the lee of other substantial mountain ranges such as the rocky mountains andes hindu kush and karakoram

two phase diagenetic alteration of carbonate matrix middle ordovician limestones taiyuan city area china both pseudospar and micrite are present in carbonate matrix of middle ordovician wackestones in the taiyuan city area china under the scanning electron microscope pseudospar crystals display unpitted surfaces and an absence of aragonite relics in polished and slightly etched sections micritic crystals show a similar feature with slightly pitted surfaces and absence of aragonite relics this possibly indicates that both pseudospar and micrite have a similar original mineralogy i e as calcites this is further supported by their low sr contents 417 ppm the geochemistry of pseudospar and micrite suggests that they were the products of two distinct episodes of diagenesis affecting the matrix micrite was possibly formed in near surface meteoric environments but pseudospar which is characterized by a population of more depleted 18o values 9 3 to 10 2 pdb than that of micrite 7 4 to 8 7 pdb is explained to have formed during shallow burial diagenesis because its 18o composition is too light to be accounted for by near surface meteoric origin the lower sr contents 417 ppm of both pseudospar and micrite in conjunction with their more depleted 18o values 7 4 to 10 2 pdb are interpreted to be the results of meteoric diagenesis when compared with the geochemistry of marine diagenetic calcites the more negative and wider range of 13c values of pseudospar and micrite 0 3 to 4 7 in comparison with those of other coeval meteoric calcites was attributed to the influence of the decay of organic matter in carbonate matrix as well as to meteoric diagenesis although the contents of fe and mn between pseudospar and micrine overlap widely an inverse relationship exists between the concentrations of fe and mn and the 13c values in both pseudospar and micrite this can be accounted for by variable redox by variable redox conditions during diagenesis the 18o values of void fill sparry calcite cements in micrite 8 2 to 8 6 pdb fall into the category of near surface meteoric diagenesis possibly indicating that the majority of micrites were cemented earlier and consequently were resistant to any further alterations so that only minor amount of pseudospars were formed during shallow burial diagenesis by recrystallization

carbon isotope δ13ccarb heterogeneity in deep water cambro ordovician carbonates western newfoundland abstract carbonates of western newfoundland span the cambro ordovician interval and preserve a record of slope basinal deposition in the cow head group near cow head this unit consists of conglomerates and ribbon and laminated limestone interbedded with shale that is well exposed in sea cliffs at cow head peninsula these conglomerates although prevalent throughout the section vary in thickness and abundance stratigraphically and record both local disruption and large scale episodic sedimentation events microfacies drilled for carbon isotope 13 c carb analysis of conglomerates reveal isotopic heterogeneity within individual samples in some cases more than 1 while this might be an expected outcome of drilling multiple areas of a heterogeneous conglomerate hand sample permil level variability was observed both between individual clasts in a sample between different parts of the same matrix and between a clast and its surrounding matrix no associated variation in 18 o carb or trace element distributions exists to suggest that this 13 c carb variability is the result of later stage meteoric diagenesis the 13 c carb variability suggests multiple sources of dissolved inorganic carbon dic associated with carbonate precipitation for phases within these individual samples these data indicate that processes such as local organic matter remineralization and early authigenic carbonate precipitation during lithification at the sediment water interface swi are either contributing to or controlling 13 c carb values in cambrian carbonates perhaps more so than at other intervals in earth history

remobilization and mineralization of selenium tellurium in metamorphosed red beds evidence from the munster basin ireland abstract selenium and tellurium are vital elements for the ongoing development of carbon free energy generation but are only recovered in small volumes as by products of base and precious metal processing security of supply of these commodities is of high importance as their demand is forecast to increase in the near future thus more research is required on the varying mineralization systems where their occurrence and behavior are poorly understood such as sedimentary basins and metasedimentary terranes the munster basin of southern ireland is a good study area for sedimentary se te in such environments as it has a regionally high soil selenium anomaly and is characterized by devonian terrestrial red beds which were metamorphosed during the hercynian variscan orogeny the area also contains ore grade biogenic sediment hosted copper and remobilized vein hosted copper sulfides which are prospective for chalcophile selenium and tellurium quartz vein hosted se te mineralization was identified across the munster basin accompanied by trace gold silver mercury accessory phases new sulfur isotope data reinforces the model of sulfur and selenium remobilized from a biogenic red bed source which reflects a wider association of selenium deposits with oxidizing environments the co occurrence of tellurium mineralization suggests that it too may be concentrated in oxidizing environments associated traces of gold add to a growing body of evidence that gold is also transported and precipitated in oxidizing conditions in continental basins

coastal lithofacies and biofacies associated with syndepositional dolomitization and silicification draken formation upper riphean svalbard the draken formation 120 250 m of northeast spitsbergen svalbard forms part of a thick upper proterozoic carbonate platform succession it consists predominantly of intraformational dolomitic conglomerates with excellent textural preservation six main lithofacies were recognized in the field quartz sandstones stromatolitic mats conglomerates with silicified intraclasts dolostone conglomerates with desiccated mudrocks oolitic pisolitic grainstones and fenestral dolostones a series of five main gradational biofacies were recognized from silicified and rare calcified microfossils biofacies 1 represents low energy subtidal benthos erect filaments and plankton acritarchs and vase shaped microfossils whereas biofacies 2 to 5 are microbial mat assemblages with filamentous mat builders and associated dwellers and washed in plankton ranging from basal intertidal to high intertidal supratidal colour values a measure of the lightness of the colour shade of sawn rock samples were quantified using a munsell chart and exhibit a pronounced variation means of major groups varying from 4 0 to 5 95 across the spectrum of subtidal to supratidal sediments as inferred from other criteria the lightening in progressively more exposed sediments is related to lowering of organic carbon contents probably mainly by oxidation six types of early cement have been recognized calcite microspar type 1 is common as a subtidal cement in many proterozoic formations whereas types 2 subtidal isopachous fringes 3 subtidal hardground dolomicrite and 4 intertidal meniscus dolomicrite are very similar to phanerozoic examples except for their dolomitic mineralogy types 5 and 6 are complex and variable dolomite growths associated with expansion and replacive phenomena they characterize the fenestral lithofacies and compare with modern supratidal cements consideration of diagenetic fabrics and truncation textures of intraclasts indicates that leaching dolomitization silicification were all significant syndepositional processes altering the original metastable carbonates the data set provides evidence for a spectrum of peritidal environments including ooid shoals protected subtidal tidal sandflats and protected carbonate mudflats different sections show a preponderance of particular facies the coastal lithofacies continuum was completely dolomitized unlike offshore to ooid shoal facies of adjacent formations dolomitization thus bears a relationship to depositional bathymetry although hydrodynamics clearly have a role the potential importance of whiting precipitation in raising mg ca in marginal marine environments is also stressed

geochemical trends and palaeohydrological significance of shallow burial calcite and ankerite cements in middle jurassic strata on the east midlands shelf onshore uk this paper discusses isotopic and geochemical trends in shallow burial 1500 m carbonate cements in limestones from a middle jurassic shale dominated marine to paralic succession situated on the east midlands shelf the results are used to investigate carbonate shale interactions between the near surface realm of microbially mediated diagenesis and the deep thermobaric regime of clay mineral transformations and hydrocarbon generation they are also combined with published data from cements in stratigraphically and geographically adjacent strata to better understand the late mesozoic cenozoic palaeohydrology of central eastern england east midlands shelf midland platform shallow burial cements sampled from the blisworth limestone formation consist of ferroan calcite and calcic ankerite which are associated with ferroan neomorphic calcite and beef cone in cone calcite veins calcite cements are petrographically complex and variable yet record consistent compositional trends carbon isotope values 13c 1 2 to 1 7 pdb show that organic matter breakdown did not contribute significant carbon to the shallow burial pore fluids oxygen isotope data 18o 4 5 to 11 2 pdb are consistent with precipitation from modified marine pore fluids that displaced the original brackish depositional fluids from the limestone during palaeocene burial the same fluids accompanied cementation in underlying bajocian limestones ferroan calcite cements neomorphic spar and beef record a surprising correlation in fe2 and mg2 concentration that suggests a common source for the ions in the adjacent shales this trend is not present in the bajocian limestones and indicates that ionic mass transfer was on a smaller scale than the regional plumbing system the results suggest that middle jurassic limestone palaeoaquifers on the east midlands shelf hosted a westerly directed up dip flow from compacting upper and lower jurassic marine mudrocks in the sole pit basin during the cretaceous and early tertiary however early cretaceous meteoric groundwater palaeoaquifers recorded from cements in laterally equivalent middle jurassic limestones on the midlands platform and in the weald basin appear not to have extended northward into the east midlands shelf

stable isotopic and elemental relationships of ancient shallow marine slope carbonates cambro ordovician cow head group newfoundland implications for fluid flux abstract carbon and oxygen stable isotopic and trace elemental variations in diagenetic calcite and dolomite from slope and platform margin carbonates of the cambro ordovician cow head group western newfoundland are explained by calculations that consider variations in the water rock ratio during burial history of a reconstructed cross section of the continental margin platform margin deposits are preserved as blocks of limestone with calcified algae in slope strata the blocks contain neomorphic radial 18o 9 3 to 5 8 pdb 13c 0 7 to 1 4 pdb and blocky spar 18o 8 3 to 6 7 pdb 13c 0 6 to 1 2 pdb calcite sparry dolomite x18o 5 8 pdb x13c 0 6 pdb and lutecite length slow chalcedony 18o 27 9 to 29 7 smow reaction of these almost isotopically homogeneous rocks with sea water meteoric water and upward migrated basinal water is suggested by their distinctive fabrics and isotopic compositions the beds of slope limestone contain abundant calcite microspar and pseudospar 18o 9 2 to 6 6 pdb 13c 5 6 to 0 6 pdb dolomite 18o 9 3 to 2 8 pdb 13c 3 0 to 1 0 pdb and minor chert 18o 23 4 to 29 1 smow the three dolomite fabrics are fine crystalline rhombohedra that grew in clay rich carbonate mudstone medium crystalline replacive mosaics that are nonfabric selective and coarse white sparry rhombohedra associated with stylolites and fractures the microspar and pseudospar grew by widespread neomorphism of 1 fine grained detrital calcite or aragonite in lime mudstone and 2 calcite cement in calcarenite cathode luminescent patterns of calcites and dolomites suggest a prolonged history of recrystallizations dissolution precipitation on a fine scale during burial diagenesis with increasing distance basinward from the platform margin diagenetic calcite and dolomite are systematically depleted in the heavy isotopes of oxygen and carbon and diagenetic calcite is enriched in magnesium suggesting crystal growth and recrystallizations in evolved disgenetic fluids these data along with the burial history indicate that the limestones of the cow head group were systematically altered as greater amounts of burial fluids were either focused along preferred permeability pathways during compactive expulsion or recirculated during possible convective flow through strata farther from the platform margin

biogenic chert in the cow head group cambro ordovician western newfoundland chert in the cow head group is mainly a replacement of limestone and shale and to a lesser extent an interparticle cement its field occurrences are distinct as 1 silicified margins on coarse conglomerates and thinly bedded limestones 2 nodules within limestone and shale 3 pervasively silicified beds of limestone and shale and 4 clasts or partial replacement of clasts within conglomerate radiolarians and sponge spicules are composed of microquartz or calcite and are particularly common in the ordovician part of the succession where most chert occurs in limestone spatially associated with chert the use of cathodoluminescence demonstrates that calcite replaced radiolarians and spicules are volumetrically more important than realized through transmitted light petrography petrographic relations between siliceous and rare pyritized radiolarians further indicate that these particles may be dissolved prior to compaction no trace of their former existence remains other than indirectly through the presence of silicified limestone and shale crushed grains cemented by chalcedony indicate that chert was precipitated during or after compaction the history of silicification and the replacement or dissolution of siliceous bioclasts is protracted ranging from near the sediment water interface where it is concomitant with early limestone lithification to deeper burial postdating mechanical compaction

origin of anhedral sr rich calcite in deep water mixed sediment north east australia at burial depths of 800 1000 m within the epicontinental queensland trough of north east australia odp site 823 microcrystalline inter and intraskeletal mosaics of anhedral loaf shaped rounded calcite have sr2 values ranging from below microprobe detection limits 400 500 m depths petrographic observations illustrate that the calcite microfabric formed through coalescing crystal growth resulting from one or a combination of displacive growth in clay porphyroid neomorphism of aragonite vaterite and clay replacement by calcite sr2 mean concentrations in calcite between depths of 800 and 1000 m are similar to an expected equilibrium pore water concentration using a dsr of 0 06 and may indicate active calcite precipitation however sr2 variation 2000 5000 ppm within and among crystals and concentrations that range well above predicted equilibrium values for a given depth illustrate either variable sr2 retention during recrystallization of shelf derived aragonite and authigenic local vaterite or relative uptake of sr2 during calcite precipitation with burial within the context of calcite formation during burial to 1 km diagenetic attributes that affect the latter process include increased concentrations of pore water sr2 with depth associated with aragonite recrystallization dissolution upward migration of sr rich pore water and increased dsr related to local variation in precipitation recrystallization rates differential crystal sector growth rates and or microvariation in aragonite distribution

carbonate debris flows cow head group western newfoundland abstract the cow head group is an allochthonous cambro ordovician sequence of deep water limestone conglomerates mudstones to grainstones minor quartz sandstones siltstones and shales platy clast conglomerates and boulder rich megaconglomerates as much as 100 m thick exhibit features similar to those described for subaerial debris flows that is flat sharp bases with only minor evidence for erosion poorly developed clast fabric with tilting of platy clasts at right angles to the flow direction boulders floating within flows or projecting above the tops of flows irregular flow tops indicating primary relief above the sea floor and tapered flow margins snouts snout geometry and the extent of boulder projection both allow calculation of debris static strength these results combined with measured flow thickness permit calculation of minimum and maximum paleoslope gradients original debris static strengths were in the range of 103 to 105 dynes cm2 similar to the observed range in subaerial flows calculations yield bottom gradients of l l 8 but independent field data suggest that the paleoslope dip was on the order of only a few degrees this constraint suggests that debris strength was substantially reduced during flow probably by excess pore fluid pressures thus permitting flow on very gentle slopes the paleoslope orientations used in this study are substantially different from those previously published by hubert et at 1977

sedimentary and diagenetic controls on red bed ore genesis the middle tertiary san bartolo copper deposit antofagasta province chile the middle tertiary pacencia group of antofagasta province northern chile consists of over 2 km of continental sediments these include alluvial fan playa and lacustrine facies representing deposition in an arid closed basin copper mineralization is hosted by sheet and ribbon sandstone bodies which were deposited in a high energy playa marginal sandflat environment ore minerals occur dominantly as a matrix to these lithic arkoses replacing earlier carbonate and sulfate cements the main ore minerals are native copper cuprite and atacamite although there is an along strike passage into copper sulfide cements in a structurally complex zone copper iron and copper sulfides were deposited in crosscutting fractures commonly replacing evaporite veins primary chalcopyrite and bornite exhibit progressive oxidation to anilite digenite and djurleite chalcocite is not found much of this material has oxidized further to covellite of normal and blaubleibender varieties galena and native silver are present in uneconomic amounts atacamite has a complex paragenesis representing at least locally the earliest copper bearing phase provenance studies of the host sediments and lead isotope data for the sulfides indicate that both the sandstones and the metals were derived from a calc alkaline volcano plutonic complex to the east of the basin an area now obscured by the recent volcanic andes sulfur isotope results for copper sulfides show very light delta 34 s values 33ppm indicative of biogenic sulfate reduction under conditions of unrestricted sulfate supply

brachiopods as indicators of original isotopic compositions in some paleozoic limestones the original stable isotopic composition of paleozoic marine carbonate rocks is important to establish a baseline for evaluating diagenetic alteration and providing a reliable data base for modeling exogenic geochemical cycles of the elements we have compared the 18 o and 13 c values in the nonluminescent portions of brachiopods with similar published data of whole brachiopods other whole fossils and estimates of original isotopic composition of marine cements the oxygen and carbon isotopic records for each of these paleozoic components show generally good internal consistency significant discrepancies however were noted between results on the different components isotopic and trace element data presented in this study suggest that permo carboniferous whole brachiopods are little altered chemically nonetheless a more reliable but not fool proof approach for obtaining pristine isotopic values is microsampling of least altered regions based on cathodoluminescence in conjunction with staining techniques and elemental analyses the carbon and oxygen isotopic compositions of all components decrease in 18 o and 13 c in geologically older material neither temporal trend is monotonic however oxygen isotopic results suggest an important positive shift in 18 o of about 2 during the early carboniferous whereas the carbon isotope trend shows a major positive shift 2 3 during the mid carboniferous

geochemistry of brachiopods oxygen and carbon isotopic records of paleozoic oceans abstract combined trace element and isotope studies of 319 brachiopods covering the ordovician to permian time span show that 13c and 18o in well preserved specimens varied during the paleozoic the overall 13c secular trend is in accord with the previously published observations but its details are obscured by vital isotopic fractionation effects at generic level nonetheless the results suggest that the negative correlation between marine 13ccarbonate and 34ssulphate deteriorates at time scales of 106 years due to the long residence time and thus slow response of so42 in the ocean for oxygen isotopes all devonian and older specimens have 18o of 4 while the well preserved permian samples have near present day 18o of about 1 pdb this isotopic dichotomy is probably not due to post depositional phenomena salinity or biogenic fractionation effects this leaves open the perennial arguments for a change in 18 o 16 o of sea water versus warmer ancient oceans the present data are difficult to explain solely by the temperature alternative the coincidence of the proposed shift in 18o with the large late paleozoic changes in marine 87 sr 86 sr 13 c 12 c 34 s 32 s and sea level stands argues for a tectonic cause and for a change in 18 o 16 o of sea water although such explanation is difficult to reconcile with global balance considerations and with isotopic patterns observed in alteration products of ancient basalts and ophiolites whatever the precise cause or combination of causes the implications for tectonism and or paleoclimatology are of first order significance

diagenesis of tertiary playa sandstones of northern chile implications for andean uplift and metallogeny the oligo miocene pacencia group of antofagasta province northern chile consists of over 2 km of molassic sediments deposited in a series of alluvial fan and playa sub environments sandstones of the artolla member exposed around san bartolo represent playa marginal sandflat deposits and locally host stratiform copper mineralization a detailed study of these sediments has revealed a complex sequence of diagenetic modifications to the arkosic detrital mineral assemblage early dissolution of unstable ferro magnesian minerals and calcic feldspar resulted in the release of ions into intra stratal solution and the subsequent formation of a suite of eogenetic authigenic minerals these include early hematite coatings on framework grains pore lining zeolite and pore filling calcite gypsum and celestite cements syntaxial overgrowths of albite quartz and less common k feldspar are well developed following early oxidative intrastratal conditions the influx of acidic fluids derived from mudrocks during mesogenesis resulted in an important secondary porosity generation and the establishment of an acidic reducing intrastratal environment this facilitated the subsequent introduction of low temperature mineralizing solutions and formation of the san bartolo copper deposit a return to oxidative diagenetic conditions is recorded by late stage overgrowths of k feldspar and quartz and the oxidation of native copper chalcopyrite cements to cuprite tenorite and chalcocite covellite assemblages this final telogenetic modification to the mineral assemblage is related to flushing by meteoric waters and provides independent evidence for an important phase of andean deformation and uplift in the middle to upper miocene

oil shales evaporites and ore deposits abstract the relationships between oil shales evaporites and sedimentary ore deposits can be classified in terms of stratigraphic and geochemical coherence oil shale and black shale deposition commonly follows continental red beds and is in turn followed by evaporite deposition this transgressive regressive sequence represents an orderly succession of depositional environments in space and time and results in stratigraphic coherence the amount of organic carbon of a sediment depends on productivity and preservation both of which are enhanced by saline environments work on great salt lake utah allows us to estimate that only 5 of toc originally deposited is preserved inorganic carbonate production is similar to toc production but preservation is much higher oil shales and black shales commonly are enriched in heavy metals through scavenging by biogenic particles and complexation by organic matter ore deposits are formed from such rocks through secondary enrichment processes establishing a geochemical coherence between oil shales and ore deposits the permian kupferschiefer of n europe is used as an example to define a kupferschiefer type kst deposit here oxygenated brines in contact with red beds become acidified through mineral precipitation and acquire metals by dissolving oxide coatings oxidation of the black shale leads to further acid production and metal acquisition and eventually to sulfide deposition along a reducing front in order to form ore bodies the stratigraphic coherence of the red bed black shale evaporite succession must be joined by the geochemical coherence of the ore body evaporite black shale association the cretaceous cu zn deposits of angola the zambian copperbelt as well as the creta oklahoma deposits are other kst examples in the zambian copperbelt evaporites are indicated by the carbonate lenticles thought to be pseudomorphs after gypsum anhydrite nodules mvt deposits are also deposited by acid brines but at more elevated temperatures and with carbonates as principal host rocks the pine point deposits are cited for their close association with evaporites alkaline metal rich brines are postulated for the hyc deposit of mcarthur river australia such brines are known from the green river formation and deposits formed from such brines constitute the grt class they can be recognized by the presence of magadi type cherts and zeolite analcime k spar tuffs the cu co ore bodies of outokumpu finland might also belong to this type a new classification of sedimentary ore deposits is proposed based on their geochemical environment kst and mvt are formed from acid ore fluids while grt and ct creede type are derived from basic ore fluids ph of the fluids is best evaluated not from the ores themselves but from their effect on the host rocks

oxide mineralization at the radomiro tomic porphyry copper deposit northern chile the main copper oxide minerals in the radomiro tomic deposit are atacamite copper clays chrysocolla and copper wad their relative abundance provides the basis for dividing the oxide zone into two major geologic units the upper oxide unit and the lower oxide unit furthermore the study of mineral abundances in the oxide zones allowed the definition of an oxide mineral zonation that consists of five mineral assemblages based on the relative proportion of the three main copper oxide components atacamite copper clays and chrysocolla these classifications allow both an understanding of the distribution of copper minerals throughout the zone and an interpretation of the supergene processes that formed it the upper oxide unit is mineralogically heterogeneous the copper content is furnished on average by 40 vol percent atacamite 31 vol percent copper clays with minor chysocolla and copper wad the lower oxide unit is more homogeneous atacamite accounts for 70 vol percent of the copper ore minerals with minor amounts of copper clays and chrysocolla copper and chlorine concentrations and porosity are greater in the upper oxide unit whereas hematite chalcocite and molybdenite concentrations and specific gravity are greater in the lower oxide unit the vertical distribution of the cl cu ratio shows a chlorine deficient zone located between elevations of 2 780 and 2 870 m between these elevations the proportion of atacamite is lower and the proportion of copper clays chrysocolla and copper wad is higher than in other portions of either the upper oxide or lower oxide units the oxide zone at radomiro tomic was originated through the in situ oxidation of a preexisting secondary sulfide enrichment blanket oxidation produced an extensive zone of atacamite accompanied by weakly developed argillic alteration the upper part of the deposit was further enriched by lateral migration of copper rich solutions to the north and east where copper precipitated as chrysocolla copper wad and copper in clays in these areas strong argillic alteration characterized by the presence of montmorillonite was developed this second alteration phase impacted only the uppermost portion of the oxide zone and the base of overlying gravels

strong downslope winds and application of hydraulic like theory an experimental campaign was conducted at the lee side of a 1 km high steep mountain in order to study the development of strong downslope winds under favorable conditions using combined remote and insitu instrumentation the examination of the upstream atmospheric conditions reveals that the development of strong downslope winds is favored by a mean state critical layer or a significant decrease in static stability such as at the top of a temperature inversion at the proper height above the mountain top strong downslope winds could occur even for wind directions with a deviation of 60 off the axis perpendicular to the ridgeline as long as the cross mountain wind has a significant value at least 7 ms 1 the developed disturbances are associated with intense downdrafts of the order of 4 5 ms 1 within the first 600 m above ground and characteristic vertical turbulent structures that were observed by sodar the same phenomenon is observed not only during nighttime but also under unstable and neutral conditions of the atmospheric boundary layer abl albeit with weaker intensity fourier analysis of the vertical velocity field demonstrated that the typical time period of intense disturbances was about 5 min further evidence is also provided for the application of the hydraulic like theory under real atmospheric conditions

diagenetic processes and the mineralization in the triassic of central england continental red beds are the host rocks of a characteristic style of mineralization which suggests a genetic link between red bed formation and ore formation samples of unmineralized and mineralized triassic sediments from central england have been studied sedimentologically petrographically and geochemically with the aim of clarifying this link which may provide valuable guides for exploration on the basis of sedimentological and petrographic observations it is suggested that these red beds were formed as a result of diagenetic alteration whereby detrital silicates and oxides are progressively dissolved and the iron released is deposited as hematite a model for mineralization is proposed involving the release of trace metals from detrital minerals during diagenesis their retention in saline interstitial solutions migration to suitable sites of precipitation and deposition by reaction with trapped hydrocarbons or reduced sulphur

the frasnian famennian biotic crisis how many if any bolide impacts the prime causation of the mid late devonian mass extinction near the frasnian famennian f f boundary remains uncertain nevertheless geochemical evidence has been presented recently as decisive evidence of a giant bolide impact occurring precisely at the f f boundary which promoted the global mortality episode palaeobiological data however imply a gradual global change which is otherwise seen as a record of either multiple extraterrestrial catastrophes or of impact triggered earth bound mechanisms sedimentological mega tsunami physical craters microtektites and geochemical records remain either elusive in many aspects or incompatible with the predicted impact crisis pattern biotic succession across the f f horizon is still poorly known especially in continental domains to evidence a synchronous bedding plane killing event at the close of the crisis instead the commonly documented stepwise loss of biomass and an unproved distinctive dead zone are hard to explain simply as sampling artifacts the assumed mass mortality precisely at the f f boundary may be limited mainly to the pelagic realm the underestimated role of early variscan tectonism and associated volcanic hydrothermal processes resulting in thermal and nutrient pulses as possible prime controls of the f f crisis is suggested as well as resemblances to the superplume conditioned eventful mid cretaceous interval exemplified in the cenomanian turonian mass extinction additional shocks generated by minor cometary strikes are not excluded but may have affected some f f biotas or areas

low mg calcite marine cement in cretaceous turbidites origin spatial distribution and relationship to seawater chemistry lower cretaceous hauterivian bioclastic sandstone turbidites in the scapa member north sea basin were extensively cemented by low mg calcite spars initially as rim cements and subsequently as concretions five petrographically distinct cement stages form a consistent paragenetic sequence across the scapa field the dominant and pervasive second cement stage accounts for the majority of concretions and is the focus of this study stable isotope characterization of the cement is hampered by the presence of calcitic bioclasts and of later cements in sponge spicule moulds throughout the concretions nevertheless trends from whole rock data augmented by cement separates from synlithification fractures indicate an early calcite 18o value of 0 5 to 1 5 pdb as such the calcite probably precipitated from marine pore fluids shortly after turbidite deposition carbon isotopes 13c 0 to 2 pdb and petrographic data indicate that calcite formed as a consequence of bioclastic aragonite dissolution textural integrity of calcitic nannoplankton in the sandstones demonstrates that pore fluids remained at or above calcite saturation as expected for a mineral controlled transformation electron probe microanalyses demonstrate that early calcite cement contains 2 mol mgco3 despite its marine parentage production of this cement is ascribed to a combination of an elevated aragonite saturation depth and a lowered marine mg2 ca2 ratio in early cretaceous calcite seas relative to modern oceans scapa cement compositions concur with published models in suggesting that hauterivian ocean water had a mg2 ca2 ratio of 1 this is also supported by consideration of the spatial distribution of early calcite cement in terms of concretion growth kinetics in contrast to the dominant early cement late stage ferroan 18o depleted calcites were sourced outwith the scapa member and precipitated after 1 2 km of burial our results emphasize that bioclast dissolution and low mg calcite cementation in sandstone reservoirs should not automatically be regarded as evidence for uplift and meteoric diagenesis

observations of nonclassical frontal propagation and frontally forced gravity waves adjacent to steep topography through the integrated analysis of remote sensing and in situ data taken along the front range of colorado this study describes the interactions that occurred between a leeside arctic front and topographically modulated flows these interactions resulted in nonclassical frontal behavior and structure across northeastern colorado the shallow arctic front initially advanced southwestward toward the front range foothills before retreating eastward then a secondary surge of arctic air migrated westward into the foothills during its initial southwestward advance the front exhibited obstacle like density current characteristics its initial advance was interrupted by strong downslope northwesterly flow associated with a high amplitude mountain wave downstream of the continental divide and by a temporal decrease in the density contrast across the front due to diurnal heating in the cold air and weak cold advection in the warm air the direction and depth of flow within the arctic air also influenced the frontal propagation the shallow obstacle like front actively generated both vertically propagating and vertically trapped gravity waves as it advanced into the downslope northwesterly flow resulting in midtropospheric lenticular wave clouds aloft that tracked with the front because the front entered a region where strong downslope winds and mountain waves extended downstream over the high plains the wave field in northeastern colorado included both frontally forced and true mountain forced gravity waves a sequence of scorer parameter profiles calculated from hourly observations reveals a sharp contrast between the prefrontal and postfrontal wave environments consequently analytic resonant wave mode calculations based on the scorer parameter profiles reveal that the waves supported in the postfrontal regime differed markedly from those supported in the prefrontal environment this result is consistent with wind profiler observations that showed the amplitude of vertical motions decreasing substantially through 16 km above mean sea level msl after the shallow frontal passage

sem observations on the replacement of bahaman aragonitic mud by calcite conversion of holocene aragonitic mud to calcite microspar occurs in fresh pore waters beneath hammocks on the tidal flats of west andros island bahamas to a subsurface depth of 2 7 m the conversion process involves both dissolution of aragonite and precipitation of calcite as a cement in pre existing pores and displacive calcite crystallization neomorphism of aragonite to calcite with preservation of grain shape has not been observed resulting diagenetic products are lithified nodules composed of well sorted interlocking rhombs of microspar and of stiff unlithified calcitic aragonite mud

conglomerate hosted copper mineralization in cretaceous andean molasse the coloso formation of northern chile alluvial fan sediments of the lower cretaceous coloso formation of northern chile were deposited during the evolution of a half graben arid intra arc basin the sediments were derived from an andesitic volcanic arc and show compositional variation which can be interpreted as an unroofing sequence copper mineralization is stratigraphically restricted to the upper 150 m of the sequence in conglomerates of predominantly granodioritic composition the presence of mineralized clasts at this level indicates the erosion of bedrock mineralization lead isotopic signatures of ore sulphide support the sedimentological evidence for derivation of the sediments and metals from an island arc volcano plutonic complex to the west of the basin a genetic model is proposed in which uneconomic metal concentrations were transported into the basin as detrital phases and released by diagenetic reactions the precipitation of copper minerals followed authigenic formation of quartz feldspar analcime and calcite and also some secondary parosity generation copper minerals include the oxychloride atacamite and the suphides bornite digenite djurleite and covellite sulphur isotope values for the sulphides are relatively heavy isotope enriched 34 s 3 6 to 12 8 although these results could be interpreted in terms of closed system bacterial sulphide reduction there is no known occurrence of organic material within the coloso basin an alternative explanation involving the mixing of diagenetically derived copper rich brines and sulphur rich hydrothermal fluids is proposed to account for the coloso mineralization

hydraulic aspects of fohn winds in an alpine valley this study examines the applicability of single layer hydraulic theory to alpine fohn winds in the example of the flow in the brenner pass region austria italy the south fohn is described as the gap flow over the pass and along the associated wipp valley numerical shallow water simulations for a wide range of initial conditions including shallow and deep fohn cases are discussed and compared with selected measurements collected within the mesoscale alpine programme map the observational analyses are based on doppler and aerosol backscatter lidar data for a specific fohn case and on surface observations and radiosoundings for all fohn cases observed within the 70 day map special observing period sop radiosoundings at a location upstream south and downstream north of the brenner pass reveal that the average map sop fohn case had subcritical flow south and nearly critical flow north of the brenner pass the hydraulic model indicates flow transition to a supercritical state near the pass a hydraulic jump 2 km further north mainly subcritical flow in the upper wipp valley except near the brenner and especially for shallow fohn a tendency to becoming supercritical in the lower wipp valley the model results suggest that the vertical topographic contraction exerts stronger control for the flow at the brenner gap than the lateral contraction in accordance with doppler lidar measurements the hydraulic model captures the decrease of the wind speed in the across valley direction from east to west in the northern part of the valley this asymmetric flow pattern is a result of the complex valley geometry rather than of the influence of upper level synoptic winds as suggested in a previous study for some observed cases the strength of the temperature inversion at the top of the fohn layer decreased across the brenner pass presumably due to entrainment processes consequently the reduced gravity decreased as well there are two major reasons for the only moderate agreement between the hydraulic parameters retrieved from soundings and those provided by the model the model assumes a constant value of reduced gravity and the calculation of hydraulic parameters for fohn cases without a strong and sharp temperature inversion is ambiguous essential flow patterns in the simulated fields such as the location of the pressure minimum and wind speed maxima compare well with the surface observations along a valley transect indications for a steeply amplified or breaking gravity wave resembling a hydraulic jump to the lee of the two mountain ridges are found in lidar observations as well as in the simulations copyright 2004 royal meteorological society

neomorphism and cementation in ancient deep water limestones cow head group cambro ordovician western newfoundland canada abstract lithification of thinly bedded lime mudstones and grainstones in the cow head group resulted mainly from precipitation of calcite cement and neomorphic spar at shallow burial depths below the sea floor these crystals are predominantly non ferroan although the iron content and the size of neomorphic spar commonly increase near the margins of nodular and continuous beds of lime mudstone displacive fringes of ferroan fibrous calcite represent the last stage of carbonate precipitation in some limestones most calcite 13c values range between 0 and 3 pdb but 13c values as negative as 15 indicate that bacterial degradation of orgnaic matter variably influenced pore water composition serial analyses from the centres to the margins of nodular and continuously bedded lime mudstones suggest that many of the limestones in this sequence were lithified prior to the precipitation of calcite having the prominent negative 13c values typical of concretionary carbonate calcite 18o values range from 5 to 8 pdb indicating precipitation from 18o depleted pore waters of uncertian origin cathodoluminescence shows that aggrading neomorphism in the lime mudstones did not take place by porphyroid or coalescive consumption of a more finely crystalline mosaic to yield a more coarsely crystalline mosaic instead scattered crystals enlarged by precipitation of progressively younger and eventually ferroan calcite especially at the margins of nodular and continuous limestone beds the cathodoluminescence zoning in calcite crystals is similar to that observed in meteorically altered limestones where pore waters were initially oxidizing but became progressively more reducing with time the preservation of trace element zoning crystal growth microfabrics and 13c values in authigenic calcite suggest collectively that recrystallization did not occur during subsequent burial

high resolution doppler lidar observations of transient downslope flows and rotors abstractthe authors present observations of the temporal evolution of downslope windstorms with rotors and internal hydraulic jumps of unprecedented detail and spatiotemporal coverage the observations were carried out by means of a coherent doppler lidar in the lee of the southern sierra nevada range during the sixth intensive observational period of the terrain induced rotor experiment t rex in 2006 two representative flow regimes are analyzed and juxtaposed in this paper the first case shows pulses of high momentum air that propagate eastward through the valley with an internal hydraulic jump on the leading edge the region downstream of the transient internal hydraulic jump is characterized by turbulence but no coherent rotor circulation was observed during the second case the strongest windstorm of the field campaign t rex occurred the observed features of this event resemble the classical notion of a rotor altogether the doppler lidar observations of both downslope flow events reveal a comp