the mechanics of continental extension in western north america implications for the magmatic and structural evolution of the great basin abstract a finite element model of continental extension within a rheologically stratified lithosphere is used to examine the structural and magmatic consequences of extensional collapse of thickened crust within the early tertiary north american cordillera crustal thickening creates a weakness in the upper mantle in the model which focuses strain within the great basin during extension marginal highlands corresponding to the sierra nevada and colorado plateau develop near the edges of the weakened region where abrupt changes in the strength of the lithosphere create large gradients in the amount of strain the great thickness of the crust results in widespread ductile flow in the lower crust within the interior of the great basin favoring the formation of metamorphic core complexes during the early stages of extension ductile flow becomes inhibited as the crust thins and cools possibly contributing to the change in the style of extension from low angle detachment faulting to high angle normal faulting during the middle miocene as the lithosphere cools the strength of the uppermost mantle increases after about 10 m y the upper mantle becomes stronger beneath the great basin than beneath adjacent unextended regions strain then begins to migrate outward from the interior of the great basin onto the marginal highlands and their interior edges begin to collapse and are incorporated into the widening great basin after 40 m y of extension the highest rates of strain are localized at the edges of the great basin in positions corresponding to the walker lane tectonic belt of western nevada and the colorado plateau transition zone the weight of the marginal highlands in the model is partially supported by flexure of the strong layer in the uppermost mantle portions of the uppermost mantle which have been flexed upward cool more rapidly than deeper portions creating periodical variations in the strength of the lithosphere which eventually develop into crustal and lithospheric boudinage with a wavelength similar to that observed in regional gravity studies rapid thinning of the model lithosphere during the first 10 m y of extension results in nearly adiabatic decompression of the lithospheric mantle partial melting of the lower lithospheric mantle is likely during this period if volatile phases or mafic lithologies were present this mechanism may account for widespread silicic magmatism in the great basin during the middle tertiary as the region undergoing extension widens the rate of lithospheric thinning decreases and the mantle beneath the great basin begins to cool this leads to a cessation of melting within the lithosphere and waning of silicic volcanism 15 20 m y after the onset of extension the appearance of predominantly basaltic and bimodal volcanic suites in the great basin during the last 5 10 ma correlates well with the onset of decompression melting in the asthenosphere if the mantle potential temperature is greater than about 1350 c approximately 70 c warmer than typical mantle potential temperature

carbonate sedimentology of the early precambrian hamersley group of western australia abstract the early precambrian hamersley group of western australia contains both major and minor occurrences of well preserved carbonate sedimentary rock the carawine dolomite and the middle or paraburdoo member of the wittenoom formation are the two most extensive occurrences the carawine dolomite is restricted to the eastern part of the hamersley basin and contains abundant stromatolites oncolites and wave ripples as well as local occurrences of evaporite crystal pseudomorphs and oolitic to pisolitic textures these carbonate strata were deposited in a shallow water paleoenvironment herein referred to as the carawine platform in contrast the wittenoom formation is restricted to the centraland western parts of the hamersley basin and such shallow water features are entirely absent from its carbonate strata these strata consist largely of thinly laminated lutite but also include some thin carbonate turbidites the carbonate sedimentary rocks of the wittenoom formation as well as some in the carawine dolomite which display similar characteristics are therefore interpreted as sediments that were deposited off platform in deeper water paleoenvironments paleocurrent thickness and grain size trends of the carbonate turbidites indicate they were deposited by paleoflows moving south and west even though the carawine dolomite lies to the northeast of the wittenoom formation stratigraphic correlations using a newly recognized marker bed of probable impact origin suggest that the carbonate strata of the carawine dolomite are younger than the paraburdoo member of the wittenoom formation if true the carawine platform could not have been a source of sediment for most of the carbonate sedimentary rocks in the wittenoom formation but it may havebeen the sediment source for similar carbonate turbidites in younger formations such as the mt mcrae shale and the dales gorge member of the brockman iron formation this in turn suggests that carbonate sediments were accumulating in the shallower parts of the hamersley basin while some of the bifs were being deposited simultaneously in the deeper parts

the burgess shale and associated cambrian formations west of the fossil gully fault zone on mount stephen british columbia west of the fossil gully fault zone on mount stephen the three lowest members only of the burgess shale formation are preserved the kicking horse shale the yoho river limestone and the campsite cliff shale the formation rests unconformably upon the takakkaw tongue formation whose dark basinal limestones conformably overlie paler shelf like limestones of the mount whyte formation mapping has resolved a long standing problem and shown that the stratigraphical position of the famous mount stephen trilobite beds lies within the campsite cliff shale it has also revealed some of the complexities of the fossil gully fault zone among which different periods and directions of component fault movements are indicated faunal evidence shows that the plagiura kochaspis to albertella and albertella to glossopleura zonal boundaries lie within the takakkaw tongue sequence within the burgess shale three distinct soft bodied communities occur at different stratigraphical levels on this mountain slope the oldest

grain boundary graphite in rocks and implications for high electrical conductivity in the lower crust the origin of zones of high electrical conductivity in the lower continental crust is a long standing mystery possible explanations include the presence of brines1 2 partial melt3 serpentine4 and graphite5 when discussing the occurrence of graphite in the crust many petrologists have considered phase relations as they would have existed at the peak of metamorphism or during igneous emplacement6 10 here we show that a fine film of graphite is present on the grain boundaries in three rocks from the laramie anorthosite complex in two of these rocks graphite was stable during igneous crystallization but in the other it was not we maintain that in all of the rocks the grain boundary graphite precipitated from a co2 rich fluid during cooling the chemical processes that produced the grain boundary graphite in these rocks are likely to operate in many lower crustal rocks we therefore contend that because the films we observe are capable of producing the high conductivity that is seen in the lower crust grain boundary graphite should be considered as a possible cause for some conductivity anomalies

origin of graphite in the pickering gneiss and the franklin marble honey brook upland pennsylvania piedmont the graphite bearing pickering gneiss and intercalated franklin marble are part of an aggregation of crystalline rocks in the honey brook upland of the southeastern pennsylvania piedmont uplands that consists of a charnockitic basement overlain by a suite of volcanic rocks and intruded by the honey brook anorthosite massif carbon isotopic analysis indicates that graphite formation in the pickering gneiss and franklin marble is related to metamorphism of organic rich muds and carbonates accompanied by localized fluid flow associated with the development of anatectic pegmatites graphite in fine grained pickering gneisses of upper amphibolite and lower granulite facies has an average 13 c 23 8 per mil relative to pdb and ranges in value from 17 4 to 29 6 indicating an organic source calcite from the franklin marble and associated calcareous gneisses has an average 13 c 0 4 as would be expected with a sedimentary carbonate protolith coarse grained pickering gneisses termed anatectic pegmatites in this study of amphibolite facies have an average 13 c 18 7 and range in value from 14 0 to 26 9 whereas those of granulite facies have an average 13 c 10 9 and range in value from 8 9 to 12 4 at the time of anatexis metamorphic fluids may have facilitated exchange of high 13 c carbon from the carbonates with low 13 c carbon of the partially melted fine grained gneisses thus producing the intermediate values of 13 c of graphite in the anatectic pegmatites calcite graphite isotope thermometry yields estimates of 680 40 c for upper amphibolite facies and 748 40 c for granulite facies metamorphism in the upland

electrical conductivity and carbon in metamorphic rocks of the yukon tanana terrane alaska electrical conductivity of a water saturated quartz mica garnet schist collected from a surface outcrop near the denali fault zone in the yukon tanana terrane of east central alaska increases slightly with pressure to about 200 mpa this behavior is unlike that exhibited by other yukon tanana samples or by most rocks from other locations detailed petrographic examination of the sample revealed the presence of a stringer of carbonaceous material generally less than 10 m thick enclosed in and intergrown with one of the muscovite layers and extending for about 2 cm along the foliation the stringer is probably responsible for the anomalous conductivity change with pressure making the sample the first for which anomalous electrical conductivity behavior can be attributed to carbon associated with a specific feature the carbonaceous stringer together with its host muscovite layer are deformed and broken around a rotated garnet porphyroclast the deformation was accommodated by plastic deformation of quartz and therefore occurred in the ductile regime under conditions at least equivalent to greenschist facies metamorphism we interpret the textural relations to indicate that the carbonaceous material was formed by fluid deposition in a fracture formed within the muscovite layer possibly during the main phase of metamorphism and deformation and that the mica and carbon stringer were then deformed by the noncoaxial deformation responsible for rotation of the garnet porphyroclasts together the facts that the deformation resulting in garnet rotation was ductile and that the garnet rotation disrupted the stringer demonstrate that the carbonaceous stringer was present at depth i e 10 km brittle deformation on the microscopic scale is observed to have broken the connectivity of the carbon stringer explaining in part why the rock does not exhibit anomalously high conductivity at 0 1 mpa 1 atm pressure the brittle deformation is interpreted to have been caused by unloading due to uplift the observations indicate that carbonaceous material may exert a primary control on crustal electrical conductivity because it may be present as interconnected arrays in grain boundaries or microfractures or in megascopic throughgoing fractures

intracrustal complexity in the united states midcontinent preliminary results from cocorp surveys in northeastern kansas unusually clear indications of complex structure in the mid to lower crust is revealed by seismic reflection surveys in northeastern kansas this complexity contrasts markedly with the layer cake simplicity of both the overlying sedimentary cover and most previous crustal models for the central united states seismic sections collected by cocorp consortium for continental reflection profiling as part of a major east west traverse across the neniaha ridge and midcontinent geophysical anomaly indicate that below a thin relatively flat layered paleozoic sedimentary section the deep crust is characterized by numerous dipping and arcuate reflections and diffractions in many places layered and crosscutting these reflections suggest convoluted three dimensional folded faulted and intruded structures specific identification of these deep features may be possible if future surveys can trace them to accessible depths the basement above these reflection complexes contains significantly fewer reflections consistent with but not necessarily diagnostic of the granitic terrane that dominates basement drill hole samples in the region among the events at these shallower basement depths are several east dipping reflections some of which may be major faults travel times corresponding to expected moho depths about 36 km are characterized less by specific reflections than by an apparent decrease in the density and number of reflections while evidence of crustal heterogeneity is common among deep reflection studies the kansas seismic results outlined in this brief report stand out as being unusually clear representations of such

syngenetic thin graphite rich horizons in lower crustal rocks from the serre san bruno calabria italy and implications for the nature of high conducting deep crustal layers abstract in order to better understand the nature of deep crustal high electrical conductivity we studied the electrical properties of a tilted section of a former lower continental crust exposed in the calabrian arc of the alpine apennine mountain system geoelectric field measurements and impedance measurements on rock samples showed that these high grade metamorphic rocks are generally highly resistive as expected for crystalline electrolytically conducting rocks of low porosity this holds for graphite free metabasites as well as for metapelites which generally contain accessory up to 3 biogenic graphite in the form of isolated grains clearly as an exception a group of thin stratiform black horizons with thicknesses of 1 15 cm within the metapelitic series was detected by means of self potential sp measurements rock samples from these horizons exhibit high quasi metallic bulk conductivities of up to 50 s m 0 02 m in agreeement with up to 20 syngenetic graphite forming a network of interconnected streaks or crack fillings the high amount of carbon most probably originates from organic matter of c org rich black shales relative enrichment of the low mobility graphitic matter compared to the carbon content of the assumed protoliths may have been due to pressure solution and partial melting during prograde metamorphism without major contribution of a fluid phase resulting in isolated graphite flakes although enriched graphite in this form has little effect on electrical conductivity for the calabrian black horizons microscopic analyses make conceivable that in a further decisive step isolated graphite grains were mechanically smeared to continuous pathways during uplift by shearing producing hereby the observed graphitic network which is needed to generate high conductivity as c org rich black shales are common members of sedimentary sequences throughout the earth s history good conductors of this type may be expected in the continental crust at any depth depending on tectonic and metamorphic history with the exception of magmatic protoliths regarding the extremely high conductivity of the meta black shale samples containing syngenetic sheared graphite a total thickness of a few meters of such rocks is sufficient to explain magnetotelluric high conductivity anomalies in the deep crust

oceanic anoxia at the precambrian cambrian boundary the precambrian cambrian pc c boundary separates fossils representing two discrete evolutionary phases the neoproterozoic soft bodied ediacarian biotas and cambrian small shelly faunas the biological discontinuity is suspected to have been a result of mass extinction however recent discoveries of the ediacarian biotas in cambrian sediments have led to an understanding that the faunal change was gradual through the pc c transition th u ratios which are high in oxidizing conditions and low in reducing conditions show a considerable positive correlation with 13c values at all studied sites of the pc c boundary this correlation indicates that reported 13c variation across the pc c boundary from numerous localities corresponds to redox variation in the depositional environment the negative 13c anomaly that occurs worldwide at the pc c boundary therefore corresponds to the widespread development of an oxygen deficient shallow marine environment this finding suggests that widespread oceanic oxygen deficiency which has been interpreted to reflect phanerozoic mass extinction events also occurred immediately before the cambrian explosion

crustal structure of the colorado plateau arizona application of new long offset seismic data analysis techniques the colorado plateau is a large crustal block in the southwestern united states that has been raised intact nearly 2 km above sea level since cretaceous marine sediments were deposited on its surface controversy exists concerning the thickness of the plateau crust and the source of its buoyancy interpretations of seismic data collected on the plateau vary as to whether the crust is closer to 40 or 50 km thick a thick crust could support the observed topography of the colorado plateau isostatically while a thinner crust would indicate the presence of an underlying low density mantle this paper reports results on long offset seismic data collected during the 1989 segment of the u s geological survey pacific to arizona crustal experiment that extended from the transition zone into the colorado plateau in northwest arizona we apply two new methods to analyze long offset data that employ finite difference travel time calculations 1 a first arrival time inverter to find upper crustal velocity structure and 2 a forward modeling technique that allows the direct use of the inverted upper crustal solution in modeling secondary reflected arrivals we find that the crustal thickness increases from 30 km beneath the metamorphic core complexes in the southern basin and range province to about 42 km beneath the northern transition zone and southern colorado plateau margin we observe some crustal thinning to 37 km thick and slightly higher lower crustal velocities farther inboard beneath the kaibab uplift on the north rim of the grand canyon the crust thickens to a maximum of 48 km we observe a nonuniform crustal thickness beneath the colorado plateau that varies by 15 and corresponds approximately to variations in topography with the thickest crust underlying the highest elevations crustal compositions as inferred from seismic velocities appear to be the same beneath the colorado plateau as those in the basin and range province to the southwest implying that the plateau crust represents an unextended version of the basin and range some of the variability in crustal structure appears to correspond to preserved lithospheric discontinuities that date back to the proterozoic era

the northeast kingdom batholith vermont magmatic evolution and geochemical constraints on the origin of acadian granitic rocks five devonian plutons west charleston echo pond nulhegan derby and willoughby that constitute the northeast kingdom batholith in vermont show wide ranges in elemental abundances and ratios consistent with major crustal contributions during their evolution the batholith consists of metaluminous quartz gabbro diorite and quartz monzodiorite peraluminous granodiorite and granite and strongly peraluminous leucogranite contents of major elements vary systematically with increasingsio 40 and have small negative eu anomalies the strongly peraluminous willoughby leucogranite has unique trace element abundances and ratios relative to the rest of the batholith including low contents of hf zr sr and ba low values of k rb 80 164 th ta 9 rb cs 7 40 k cs 0 1 0 5 ce pb 0 5 4 high values of rb sr 1 18 low to moderate ree contents and light ree enriched patterns with small negative eu anomalies flat ree patterns with large negative eu anomalies are found in a small hydrothermally altered area characterized by high abundances of sn up to 26 ppm rb up to 670 ppm li up to 310 ppm ta up to 13 1 ppm and u up to 10 ppm there is no single mixing trend fractional crystallization assemblage or assimilationscheme that accounts for all trace elementvariations from quartz gabbro to granite in the northeast kingdom batholith the plutons originated by mixing mantle derived components and crustal melts generated at different levels in the heterogeneous lithosphere in a continental collisional environment hybrid rocks in the batholith evolved by fractional crystallization and assimilation of country rocks 50 by mass and some of the leucogranitic rocks were subsequently disturbed by a mild hydrothermal event that resulted in the deposition of small amounts of sulfide minerals

cocorp deep reflections moho at 50 km 16 s beneath the colorado plateau cocorp deep reflection data on the colorado plateau reveal complex reflections to about 16 s two way travel time 50 km beyond which there is an abrupt decrease of reflectivity this boundary is interpreted to represent the moho an inference consistent with the existing but limited refraction data sets when they are reconciled with the recent independent determination of an 8 10 8 15 km s pn velocity beneath the colorado plateau this 50 km depth to moho and 8 10 8 15 km s pn velocity for the colorado plateau closely resembles that of the high plains just east of the rocky mountains the absence of continuous strong reflections at the moho suggests that the moho beneath the colorado plateau is a transition not a velocity step function on the scale of the reflection experiment the reflection boundary may represent the base of complex and discontinuous reflectors of the crust and crust mantle transition below which the mantle may be largely peridotite with a velocity of 8 1 km s these results strongly reinforce the suggestion that the moho on deep reflection data in relatively stable continental regions is represented by an abrupt decrease in reflectivity below which the mantle is relatively seismically transparent

exceptional fossil record distribution of soft tissue preservation through the phanerozoic preservation of soft bodied fossil biotas konservat lagerstaten that preserve traces of volatile nonmineralized tissues readily degraded by bacteria are not evenly spaced through geologic time when compared to outcrop area exceptional faunas appear to be over represented in the cambrian and jurassic these concentrations in time correspond to particular environments indicating that controls on the distribution of exceptional faunas may have operated on a global scale the reduction in the number of exceptional faunas after the cambrian may reflect the evolution and diversification of deep bio turbators specific conditions favoring stagnation and episodic burial were required to ensure preservation in younger rocks

post cambrian closure of the deep water slope basin taphonomic window exceptional faunas konservat lagerstatten that preserve traces of volatile nonmineralized tissues are statistically overabundant in the cambrian period almost all examples preserved in continental slope and shelf basin environments are of this age the hypothesis that an increase in the amount and complexity of bioturbation was an important agent in the elimination of this deep water slope basin taphonomic window is supported post cambrian ichnofaunal assemblages contain a higher proportion of pascichnia and agrichnia ethologies produced by a mobile infauna they also illustrate the lateral partitioning of organisms into different environmental niches both opportunistic and equilibrium infaunas occur in low oxygen environments in which the preservation of nonbiomineralized tissues was favored direct consumption of carcasses by bioturbating organisms was less important than changes to sediment properties as a result of bioturbation notably enhanced microbial degradation of reactive organic matter increased permeability and the disruption of geochemical gradients necessary for mineral authigenesis

crustal structure and evolution beneath the colorado plateau and the southern basin and range province results from receiver function and gravity studies over the past several decades contrasting models have been proposed for the physical and chemical processes responsible for the uplift and long term stability of the colorado plateau cp and crustal thinning beneath the basin and range province brp in the southwestern united states here we provide new constraints on the models by modeling gravity anomalies and by systematically analyzing over 15 500 p to s receiver functions recorded at 72 usarray and other broadband seismic stations on the southwestern cp and the southern brp our results reveal that the brp is characterized by a thin crust 28 2 0 5 km a mean vp vs of 1 761 0 014 and a mean amplitude r of p to s converted wave relative to that of the direct p wave of 0 181 0 014 that are similar to a typical continental crust consistent with the model that the thin crust was the consequence of lithospheric stretching during the cenozoic the cp is characterized by the thickest crust 42 3 0 8 km largest vp vs 1 825 0 009 and smallest r 0 105 0 007 values in the study area in addition many stations on the cp exhibit a clear arrival before the p to s converted phase from the moho corresponding to a lower crustal layer of about 12 km thick with a mafic composition we hypothesize that the lower crustal layer which has an anomalously large density as revealed by gravity modeling and high velocities in seismic refraction lines contributed to the long term stability and preuplift low elevation of the colorado plateau

carbon enhanced electrical conductivity during fracture of rocks changes in electrical resistance during rock fracture in the presence of a carbonaceous atmosphere have been investigated using nugget sandstone and westerly granite the experiments were performed in an internally heated gas pressure vessel with a load train that produced strain rates between 10 6 and 10 5 s 1 samples were deformed at temperatures of 354 to 502 c and pressures of 100 to 170 mpa in atmospheres of ar or mixtures of 95 co2 with 5 co or 5 ch4 compositions that are well within the field of graphite stability at the run conditions in experiments using nugget sandstone resistance reached a minimum value when the maximum temperature was achieved and good electrode contact was made the resistance then increased as the experiment continued probably due to dry out of the sample a change in the oxidation state of the fe oxide associated with the cement or destruction of current bearing pathways at approximately 200 mpa end load the rock sample failed plots of load and resistance versus time show several interesting features in one experiment for example as the end load reached about 175 mpa resistance stopped increasing and remained fairly constant for a period of approximately 0 5 hour during loading the end load displayed small decreases that were simultaneous with small decreases in resistance when the end load and the displacement indicated rock failure resistance decreased dramatically from 150 m to 100 m in a single experiment the westerly granite also showed a decrease in resistance during dilatancy the nature and distribution of carbon in the run products were studied by electron microprobe and time of flight secondary ion mass spectroscopy top sims carbon observed by mapping with the former is clearly observed on micro cracks that based on the microtexture are interpreted to have formed during the deformation the tof sims data confirm the electron probe observations that carbon is present on fracture surfaces these observations and experimental results lead to the hypothesis that as microfractures open in the time leading up to failure along a fracture carbon is deposited as a continuous film on the new reactive mineral surfaces and this produces a decrease in resistance subsequent changes in resistance occur as connectivity of the initial fracture network is altered by continued deformation such a process may explain some electromagnetic effects associated with earthquakes

the active southwest margin of the colorado plateau uplift of mantle origin during cenozoic time the colorado plateau was raised about 2 km above sea level the most recent and best documented uplift of the plateau 1 km has been concentratedatitssouthwestmarginbetween6 and 1 ma whereas the eastern colorado plateau may have been at high elevations sinceeocenetime tobetterunderstandthe recent tectonic activity at the southwest marginofthecoloradoplateau wecompile detailed crustal thickness and density informationfromseismicandgravitydatafor a region that includes northwest arizona and the southern tip of nevada this information is used to isolate the mantle contribution to uplift wefind that there is relatively low density mantle underlying the southernmarginoftheplateauinnorthwest arizona which could result from about 60 80 km of thinning of the dense mantle lithosphere combined with about 100 cof heating through a 100 km thick mantle layer the available estimates from earthquake source seismology in or near the studyareaarecompatiblewiththisestimate of lithospheric thinning we speculate that uplift may result from subduction related thinning of the continental lithosphere

the middle cambrian burgess shale and its relationship to the stephen formation in the southern canadian rocky mountains the burgess shale has been an anomalous geologic unit ever since walcott named it in 1911 as the geographic equivalent of the ogygopsis shale in the middle cambrian stephen formation of southeastern british columbia but it has never been recognized outside of its type locality so its status relative to the stephen formation remained uncertain the geologic setting of the burgess shale was determined by aitken and fritz in 1968 when they recognized the cathedral escarpment and divided the stephen formation into a thin platformal succession on top of the escarpment and a thick basinal succession which included walcott s burgess shale in front fieldwork by royal ontario museum parties between 1982 and 1997 has now demonstrated that the thin and thick stephen successions lie within different facies belts and should be regarded as separate formations the stephen shale formation is part of the middle carbonate belt succession whereas the name burgess shale formation is applied to the thick basinal

carbonaceous material in pelitic schists of the sanbagawa metamorphic belt in central shikoku japan abstract carbonaceous material in pelitic schists of the sanbagawa metamorphic belt in central shikoku japan was separated from the host rocks and its x ray diffraction and chemical composition were studied its crystal structure and chemistry change continously with increasing metamorphic grade and approach those of well ordered graphite near the biotite isograd as graphitization is a rate process the temperature of complete graphitization differs from one metamorphic terrain to another as a function of the duration of metamorphism in an individual metamorphic terraan however the degree of graphitization is a useful indicator of relative metamorphic temperature in lower grade rocks

moho beneath the north sea compared on normal incidence and wide angle seismic records the mohorovicic discontinuity the moho is properly defined as the step in the velocity of horizontally travelling p waves from about 7 km s 1 to about 8 1 km s 1 its detailed structure varies from place to place but it is universally interpreted as the transition from crust to mantle normal incidence reflections originating at about moho depth have been reported sporadi cally during the past 20 yr during the past 10 yr continuous seismic reflection profiles have been obtained to this depth both on the continents and in the deep sea and at the same time striking improvements have been made in the methods available for interpretation of refraction experiments here we report a comparison between the moho determined by a long range refraction experiment across the north sea and the results of a short normal incidence reflection profile shot along the same line fig 1 we believe this to be the first time that such a direct comparison has been made

herringbone calcite petrography and environmental significance abstract herringbone calcite is a previously undescribed carbonate cement and sea floor precipitate that is common in archean carbonates but rare in proterozoic and phanerozoic rocks it is abundant in the 2520 ma campbellrand malmani platform south africa where field relationships such as erosional truncation of layers of herringbone calcite and interbedding of herringbone calcite with grainstones demonstrate that it precipitated from ambient marine water this interpretation is supported by depositional relationships in the 2 6 ga huntsman limestone of the bulawayo greenstone belt zimbabwe the 2 6 ga carawine dolomite australia the 1 90 ga rocknest formation and the 1 8 1 2 ga dismal lakes group canada the ordovician porterfi ld carbonate buildup virginia and various silurian carbonate buildups in the midcontinent united states each of these occurrences is associated with anaerobic depositional environments or organic rich sediments herringbone calcite consists of alternating light and dark crenulated bands each light dark pair is 0 5 1 0 mm thick microscopically each pair of bands consists of a row of elongate crystals with their long axes aligned perpendicular to banding and along the growth direction of the cement the bases of the crystals are optically unoriented but upwards in each crystal the optical c axis rotates until it is perpendicular to crystal elongation the tops of the elongate crystal are thus optically aligned and length slow the light bands of herringbone calcite correspond to the optically oriented parts of the elongate crystals whereas the dark bands correspond to the optically unoriented lower parts of the elongate crystals microspar crystals are also present in some dark ba ds a mg calcite precursor for herringbone calcite now preserved as low mg calcite or dolomite is supported by the presence of micro dolomite inclusions and textural differences between herringbone calcite and textures interpreted as neomorphosed former aragonite or low mg calcite precipitation of herringbone calcite may be consistent with a diffusionally controlled growth model involving branching growth of fibrous crystals and diffusion of a precipitation inhibitor away from the crystallization surface since herringbone calcite is associated with anaerobic depositional environments the inhibitor promoting precipitation of herringbone calcite may be present only in poorly oxygenated sea water thus the stratigraphic distribution of herringbone calcite may be an important indicator of the abundance of oxygen in carbonate depositional environments through time

poisson s ratios of the upper and lower crust and the sub moho mantle beneath central honshu japan abstract poisson s ratios of the upper and lower crust and the sub moho mantle beneath central honshu japan are investigated using three independent methods that are based on s to p ratios of apparent velocities the wadati diagrams and an inversion of p and s arrivals shallow earthquakes at distances of 200 500 km from the nagoya university telemeter network are used for the apparent velocity ratio method crustal and subcrustal earth quakes under the network are used for the other two methods the network consists of wide band seismometers with three components which are particularly suitable for detecting s waves the three different methods give a consistent result for poisson s ratio that is 1 0 23 0 01 in the upper crust 2 0 26 0 28 in both the lower crust and in the sub moho mantle the result indicates a sharp contrast in between the upper and the lower crust rather than at the moho the low in the upper crust can only be explained by the presence of a substantial amount of free quartz indicating granitic rocks a higher in the lower crust suggests that this portion is presumably less saturated in silica and may be even undersaturated pointing to intermediate to mafic rocks the sub moho is almost equal to the averaged over the entire upper mantle that has been estimated from the wadati diagrams of deep shocks beneath japan but is higher than those calculated from pn and sn velocities in oceanic and stable continental regions

deep structure of the scottish caledonides revealed by the moist reflection profile the moine and outer isles seismic traverse moist is a deep crustal reflection profile shot at sea off the north coast of scotland in 1981 spectacular reflections are observed from the moho and from thrust zones within the caledonian fold belt and foreland deep reflection profiling of the continental crust of the united states under the direction of the cocorp group has amply demonstrated the value of this technique when applied to suitable geological problems see for example refs 1 2 a similar group the british institutes reflection profiling syndicate birps has now been set up in the uk to organize such projects funds are provided by the natural environment research council nerc through the core group based at cambridge university however a preliminary experiment of which moist is the result was organized through the institute of geological sciences igs

tepees in peritidal carbonates origin via earthquake induced deformation with example from the middle cambrian of western canada abstract tepee structures are common in many ancient carbonate tidal flat successions and because analogues are known from tropical coastal areas they are well established as evidence for synsedimentary lithification of subaerially exposed carbonate sediment existing explanations involving desiccation thermal expansion and contraction fluctuating groundwater pore pressure or diagenetic force of crystallization do not adequately account for the style and degree of deformation seen in some examples such as those in the middle cambrian cathedral formation of the southern rocky mountains western canada these tepees provide evidence for essentially instantaneous events causing brittle failure of decimetre thick crusts of ooidal and pisoidal grainstone followed by their dilation rupturing separation tilting and shingling they can be explained as breccias generated by sporadic episodes of strong ground motion from earthquakes that emanated from a syndepositionally active fault system nearby variably directed tensile and compressive stresses broke crusts shifted them upwards and shuffled them laterally subsequent microbial encrustation and vigorous flushing by seawater in the resulting shelter cavities led to rapid stabilization by cementation recognition of the role of seismically induced deformation in explaining some tepee structures furnishes a proxy for synsedimentary tectonism and adds another member to the family of seismites which reflect a range of rheological properties of unlithified and lithifying sediments

evidence from borehole samples for the role of accessory minerals in lower crustal conductivity the high electrical conductivity of the lower continental crust as inferred from geophysical measurements1 has defied a simple explanation both pore saturating brines2 and interconnected carbon films3 5 have been proposed as conducting pathways but their relative importance remains uncertain and may vary from place to place6 so far attempts to address this question in the laboratory have had to rely on samples of metamorphic rock collected from surface exposures7 10 although these rocks almost certainly originated in the lower crust chemical alteration during their transport to and residence at the surface is likely to have affected their conducting properties here we report conductivity data for pristine crustal rocks recovered from depths of up to 4 6 km in the ktb boreholes in southern germany our results show that the electrical conductivity of accessory phases such as fe ti oxides and sulphides can enhance or even surpass the high conductivity produced by saline fluids

a new hypothesis for organic preservation of burgess shale taxa in the middle cambrian wheeler formation house range utah cambrian konservat lagerstatten are the most significant fossil deposits for our understanding of the initiation of phanerozoic life although many modes of preservation may occur these deposits most frequently contain nonmineralized fossils preserved in the form of kerogenized carbon films a rare yet important taphonomic pathway that has not previously been explained for any unit by a comprehensive model the middle cambrian wheeler formation of utah one of these lagerstatten contains abundant kerogenized preservation of nonmineralized tissues which occurs within a distinctive taphofacies that accumulated under the following conditions 1 domination of the siliciclastic fraction by clay sized particles 2 close proximity to a carbonate platform which resulted in mixed carbonate clay sediments 3 a well developed oxygen minimum precluding benthic colonization and burrowing and 4 relative proximity to oxic bottom waters facilitating transport of organisms from a habitable environment to one that favored their preservation we propose that preservation of nonmineralized tissues in the wheeler formation may have resulted from a combination of influences that reduced permeability and thus lowered oxidant flux which in turn may have restricted microbial decomposition of some nonmineralized tissues those influences include near bottom anoxia preventing sediment irrigation by restriction of bioturbation reducing conditions near the sediment water interface that may have acted to deflocculate aggregations of clay minerals resulting in low permeability face to face contacts early diagenetic pore occluding carbonate cements and an absence of coarse grains such as silt skeletonized microfossils fecal pellets or bioclasts this model may be applicable to kerogenized preservation of macrofossils in other fossil lagerstatten

calcite graphite isotope thermometry a test for polymetamorphism in marble tudor gabbro aureole ontario canada graphitization and coarsening of organic material in carbonate bearing metasedimentary rocks is accompanied by carbon isotope exchange which is the basis of a refractory pressure independent geothermometer comparison of observed isotopic fractionations between calcite and graphite a13ccal gr with independent petrological thermometers provides the following empirical calibration over the range 400 800 c a13ccal gr 5 81 x lo6 x t k 2 61 this system has its greatest potential in marbles where calcite graphite is a common assemblage and other geothermometers are often unavailable the temperature dependency of this empirical calibration differs from theoretical calibrations reasons for this are unclear but the new empirical calibration yields temperature estimates in better agreement with independent thermometry from several terranes and is preferred for geological applications both calcite graphite isotopic thermometry and calcite dolomite solvus thermometry are applied to marble adjacent to the tudor gabbro in the grenville province of ontario canada the marble has undergone two metamorphic episodes early contact metamorphism and later regional metamorphism values of a13cca gr decrease regularly from c 8 0 in samples over 2 km from the pluton to values of 3 4 within 200m of the contact these samples appear to preserve fractionations from the early thermal aureole with the empirical geothermometer and indicate temperatures of 450 500 c away from the intrusion and 700 750 c near the gabbro this thermal profile around the gabbro is consistent with conductive heat flow models in contrast the distribution of mg between calcite and dolomite has been completely reset during later regional metamorphism and yields uniform temperatures of c 500 c even at the contact graphite textures are important for interpreting the results of the calcite graphite thermometer coarsening of graphite approaching the tudor gabbro correlates with the decrease in isotopic fractionations and provides textural evidence that graphite crystallization took place at the time of intrusion in contrast to isotopic exchange during prograde metamorphism which is facilitated by graphitization retrogressive carbon isotopic exchange appears to require recrystallization of graphite which is sluggish and easily recognized texturally resistance of the calcite graphite system to resetting permits thermometry in polymetamorphic settings to see through later events that have disturbed other systems

fluid deposited graphitic inclusions in quartz comparison between ktb german continental deep drilling core samples and artificially reequilibrated natural inclusions we used raman microsampling spectroscopy rms to determine the degree of crystallinity of minute 2 15 m graphite inclusions in quartz in two sets of samples experimentally reequilibrated fluid inclusions in a natural quartz grain and biotite bearing paragneisses from the ktb deep drillhole in se germany our sequential reequilibration experiments at 725 c on initially pure co2 inclusions in a quartz wafer and the j krautheim 1993experiments at 900 1100 c on organic compounds heated in gold or platinum capsules suggest that at a given temperature 1 fluid deposited graphite will have a lower crystallinity than metamorphosed organic matter and 2 that the crystallinity of fluid deposited graphite is affected by the composition of the fluid from which it was deposited we determined that the precipitation of more crystalline graphite is favored by lower f h2 higher f o2 and that the crystallinity of graphite is established by the conditions including gas fugacities that pertain as the fluid first reaches graphite saturation graphite inclusions within quartz grains in the ktb rocks show a wide range in crystallinity index reflecting three episodes of carbon entrapment under different metamorphic conditions isolated graphite inclusions have the spectral properties of totally ordered completely crystalline graphite such crystallinity suggests that the graphite was incorporated from the surrounding metasedimentary rocks which underwent metamorphism at upper amphibolite facies conditions much of the fluid deposited graphite in fluid inclusions however shows some spectral disorder the properties of that graphite resemble those of experimental precipitates at temperatures in excess of 700 c and at elevated pressures suggesting that the inclusions represent precipitates from c o h fluids trapped under conditions near those of peak metamorphism at the ktb site in contrast graphite that is intimately associated with chlorite and other presumably low temperature silicates in inclusions is highly disordered and spectrally resembles kerogens this graphite probably was deposited during later greenschist facies retrograde metamorphism at about 400 500 c the degree of crystallinity of fluid deposited graphite is shown to be a much more complex function of temperature than is the crystallinity of metamorphic graphite to some extent experiments can provide temperature calibration of the crystallinity index however the difference in time scales between experimental runs and geologic processes makes it difficult to infer specific temperatures for naturally precipitated graphite

paleoproterozoic stark formation athapuscow basin northwest canada record of cratonic scale salinity crisis abstract the paleoproterozoic stark formation a thick breccia unit formed during the transition from marine to non marine foreland basin fill in the athapuscow basin northwest territories canada four types of breccia occur in the stark formation 1 bedded breccias at the base of the unit overlying the basinal facies of the underlying pethei group are interpreted as debris flows 2 massive chaotic breccias occur in the lower part of the stark formation throughout the basin and these are interpreted to be evaporite collapse breccias 3 siltstone and mudstone breccia occurs in the upper part of the stark formation and is also interpreted as evaporite collapse breccia though much less evaporite was involved in its formation than the underlying chaotic breccia and 4 laminar accretionary breccia consisting of laminar carbonate clasts commonly with flat bottoms and convex up upper surface geometries these clasts occur in lenticular pods and are interpreted to form in caves following dissolution of evaporites or carbonates the widespread presence and abundance of evaporite pseudomorphs especially halite in all breccia types the stratigraphic restriction of the stark formation and irregular filling above this unit indicates that all but the bedded breccia formed by the dissolution and collapse of evaporites chaotic breccia directly above underlying pethei group shallow water and deep water carbonates indicates the evaporates formed across the platform and adjacent deep basin foundered megaclasts up to 1 5 km long and 40 m thick in basinal settings suggest cumulative evaporite thicknesses on the order of a few tens to a few hundreds of meters carbonate clasts in all the breccias contain abundant wave ripples planar laminations stromatolites and ooids these structures along with a paucity of subaerial exposure features indicates nearly all carbonate deposition in the stark was subaqueous thus by inference the evaporites in the stark formation were also partially subaqueous displacive halite pseudomorphs are common throughout all facies in the stark formation silicified and dolomite filled halite pseudomorphs are common along the contact between the evaporite collapse breccia and underlying carbonate platform pethei group and surrounding large clasts in basinal settings silicified hopper crystal casts within deep water rhythmites of the uppermost pethei group indicate precipitation of halite over deep waters gypsum pseudomorphs are exceedingly rare and anhydrite pseudomorphs were not observed the dominance of halite features in this basin and two penecontemporaneous basins kilihigok basin and wopmay orogen surrounding the slave craton suggests that relative to the present day seawater at 1 9 to 1 8 ga was oversaturated with respect to calcium carbonate had higher hco3 concentration and possibly was depleted in so42 the dearth of extensive thick bedded marine gypsum in the archean and early paleoproterozoic can be explained by low atmospheric oxygen content prior to 2 ga in conjunction with increased carbonate saturation before 1 6 ga

seismic reflectors conductivity water and stress in the continental crust the uppermost 10 15 km of the earth s continental crust differs in several geophysical properties from the lower crust the upper crust is electrically resistive seismically transparent contains nearly all intracontinental earthquake hypocentres and responds to stress elastically with brittle fracture the lower crust is electrically conductive contains many seismic reflectors is aseismic and shows ductile response to stress i show here that the characteristics of both regions can be explained if the entire crust contains saline water in separated cavities in the compressively stressed rocks of the upper crust but in the lower crust forming an interconnected film on the crystal surfaces the model may apply principally to technically active parts of the continental crust beneath shields the lower crust may be dry

metamorphism of carbonaceous material in the tono contact aureole kitakami mountains japan abstract optical and x ray studies of carbonaceous material in the tono contact metamorphic aureole kitakami mountains northeast japan have revealed that metamorphic graphitization proceeded through two discontinuous changes first optically anisotropic domains develop within the coaly phytoclast forming transitional material and then ordered graphite crystallizes by the decomposition of pre existing carbonaceous materials coaly material disappears in the uppermost chlorite zone transitional material appears in the middle of the lower chlorite zone graphite appears in the upper chlorite zone and its modal abundance increases across the andalusite iso grad to the cordierite isograd where all the carbonaceous materials have converted to graphite the apparently continuous variation in the crystallographic parameters of the bulk carbonaceous material during graphitization is largely due to variation in the modal proportions of three types of carbonaceous materials the temperature of graphitization in the present area is at least 100 c higher than the temperature in the sanbagawa and new caledonia high pressure metamorphic terrains probably due to the slow reaction rate of metamorphic graphitization and to the short duration of contact metamorphism

cocorp arizona transect strong crustal reflections and offset moho beneath the transition zone the consortium for continental reflection profiling cocorp transect across the southwest margin of the colorado plateau in arizona reveals prominent subhorizontal zones of reflections here termed the bagdad reflection sequence brs which dominate the upper and middle crust in the transition zone tz between the colorado plateau and the basin and range province a cenozoic origin for the brs is considered most likely it may represent fragmented or sheared zones related to detachment faulting or a series of intrusions possibly emplaced along preexisting zones of weakness moho reflections observed beneath the tz and the basin and range province contrast with the nonreflective moho beneath the colorado plateau suggesting that extension and its associated igneous processes probably played a key role in the development of reflectors at the moho the moho and a prominent midcrustal reflector are offset by about 3 km in a normal sense in at least one place this is the best example of offset moho yet found by cocorp and contrasts sharply with cocorp observations of a smooth moho elsewhere this offset is probably a late tectonic expression of crustal extension and thinning across the tz

comparison of geochemical and distinctive mineralogical features associated with the kinzers and burgess shale formations and their associated units article i nfo article history accepted 12 august 2007 this paper compares two north american occurrences of fossils that exhibit burgess shale type preservation bst the middle cambrian burgess shale formation of southeastern british columbia and the lower to middle cambrian kinzers fm of southeastern pennsylvania redox sensitive trace elements mo ni co v sc indicate that palaeo oxygenation conditions varied with respect to faunal assemblage and location ogygopsis bearing fauna in both the burgess shale and kinzers formations are associated with a dysoxic metal signature burgess shale type preservation in the kinzers fm was deposited in a fluctuating oxycline whereas the bst fossil beds of fossil ridge and mt stephen burgess shale fm yield a geochemical signature that consistently indicates oxic conditions thus sustained low oxygen conditions do not correlate with higher quality of fossil preservation at these sites ogygopsis is associated with dysoxic conditions in both localities and may therefore be a faunal indicator of such environments in middle cambrian strata carbonate units associated with both the burgess shale and kinzers formations contain large generally concordant near surface cavities that contain herringbone calcite cements that show no significant enrichment in the potential calcite inhibitor ions such as reduced fe sr s pb and zn the abundance of micro inclusions of dolomite suggests that mg may have been the calcite inhibitor ion responsible for herringbone cement in these cambrian occurrences another distinctive mineralogical feature is the presence of doubly terminated euhedral zoned authigenic quartz as a conspicuous part of a carbonate host rock replacement assemblage the association of both herringbone calcite and zoned quartz crystals in units associated with the burgess shale and kinzers formations suggest a common feature in the depositional and diagenetic environment of these two regions in which burgess shale type fossil preservation occur syn depositional brine flow is the likely cause of these distinct mineralogical features brine seeps in the depositional environment may have provided nutrients for stable high abundance faunal communities in a slope setting thereby accounting for the abundance of fossil material furthermore high salinity brine in the pore waters of carcass bearing cambrian muds may have further limited shallow burrowing scavenging and decomposition rates accounting for the exceptional preservation in these two localities

coincident seismic reflection refraction studies of the continental lithosphere a global review summary vertical incidence reflection profiling has identified several characteristic features of the continental iithosphere including a generally transparent upper crust a reflective lower crust reflections from the crust mantle boundary and a commonly transparent upper mantle the underlying physical causes of these characteristic features remain poorly understood this review summarizes additional information brought to bear on the physical properties of these characteristic crustal structures through the use of coincident wide angle refraction profiling

natural nakmgcl solutions and solid derivatives trapped in euhedral quartz from danish zechstein salt abstract highly saline mgcl2 kcl metamorphic solutions trapped in euhedral quartz from the top of the zechstein 2 salt the deck halite in the mors dome denmark have been studied by means of microthermometry the inclusions are classified after their content of solution and daughter minerals or solid derivatives type i contains much solution and little carnallite kmgcl3 6h2o which dissolves under vapour pressure from 80 to 320 mm hg in the temperature range 75 140 c the salinity is between 35 and 50 eq wt nacl type ii contains more carnallite than solution and the carnallite dissolves in the temperature range 121 169 c under pressures up to 77 mpa the salinity ranges from 38 to 52 eq wt nacl based on the measurements of type ii inclusions a pressure dependent melting curve of carnallite melting incongruently is proposed the curve is expressed by the equation t m car c 167 5 c 0 1p m car where dt dp 0 1 c mpa 1 the types iiia b and c contain the parageneses carnallite sylvite halite carnallite mgcl2 tetrahydrate sylvite halite and tetrahydrate sylvite halite respectively the salinity ranges from fully 50 to 60 eq wt nacl in several of these inclusions grains of carnallite sylvite or halite were trapped in suspension in the solutions the incipient melting temperatures of tetrahydrate in negative crystals 175 8 180 6 c are minimum crystallization temperatures of the host quartz type iv inclusions consist of trapped dry grains of carnallite the melting temperatures of which 172 4 179 6 c are maximum crystallization temperatures of the host quartz these crystals probably crystallized during early cretaceous times at a depth of more than 5000 m four quartz crystals with inclusions containing concentrated seawater probably formed in the span of time from the compaction of the bedded rock salt in triassic times to the initial diapirism in late jurassic times

middle cambrian carbonate platform collapse southeastern canadian rocky mountains regional erosion features termed megatruncation surfaces at the seaward margin of a middle cambrian carbonate platform in southeastern british columbia are inferred to be the proximal parts of gravity slide scars comparable in scale and morphology to modern examples in the caribbean region the surfaces truncate up to 200 m of outer platform strata and have low angle to near vertical headwalls they can be traced a minimum of 56 km along depositional strike and appear to extend at least 8 km basinward of the platform edge they therefore profoundly affect regional stratigraphy

origin of meter scale submarine cavities and herringbone calcite cement in a cambrian microbial reef ledger formation u s a abstract meter scale submarine cavities in middle cambrian shelf margin microbial reef strata indicate large scale dewatering processes in conjunction with substrate instability related to interreef channeling and shelf edge downslope creep and slip syndepositional cement precipitation within the cavities preserved delicate microbial fabrics and stabilized the reef system radiaxial fibrous calcite and herringbone calcite cements line the cavity interiors isopachously the two phases cannot be discriminated on the basis of fe mn or sr contents but do have different isotopic signatures slightly more negative 13c values in herringbone calcite suggest that abrupt transitions between radiaxial fibrous and herringbone calcite cement are the result of rapid and repeated changes in pore fluid oxygen levels storm driven pore water circulation renewed oxygenated seawater flow into the cavities resulting in precipitation of radiaxial fibrous calcite a threshold level of oxygen reduction resulted in the change to herringbone calcite precipitation the pore fluids associated with herringbone calcite did not have elevated mn or fe concentrations as suggested in previous studies herringbone calcite appears to be more susceptible to diagenetic alteration than radiaxial fibrous cement however as indicated by greater resetting of oxygen isotope values

source of silica and silicification of the lowermost cambrian yanjiahe formation in the three gorges area south china abstract the rise of siliceous organisms is generally considered to have driven the abrupt decrease of silica in the ocean during the ediacaran cambrian e c interval even if the exact timing and biological triggers of this event are still debated although ediacaran cambrian chert depositions are widely distributed multiple evidences converge to support their hydrothermal origin in this study we provide petrographic study supplemented by elemental concentration sem and raman spectral data on the siliceous rocks from the yanjiahe formation the three gorges area south china in order to ascertain the origin of included silica and discuss the involved processes of silica deposition contrary to most previous studies our analyses suggest that studied cherts mainly resulted from biogenic silica and subsidiary from abiotic precipitation influenced by surrounding hydrothermal activity and from detrital input the deposition model of silica includes direct deposition of siliceous bioclasts secondary replacement of carbonate by dissolved biosilica and microbial induced silicification from seawater the sink of silica recorded in the yanjiahe formation was characterized of a transition from typical precambrian to phanerozoic process in addition micropalaeontological and taphonomical analyses of the yanjiahe formation revealed two pulses of microfossil abundance and diversity in the fortunian and stage 2 respectively they can be correlated with a regression and an enrichment in dissolved phosphate that probably derived from enhanced detrital flux

a summary of electromagnetic studies on the abitibi grenville transect electromagnetic surveys on the abitibi grenville lithoprobe transect have elucidated a number of conductivity signatures that can be genetically linked to precambrian tectonic processes some major fault zones are moderately conductive possibly signalling graphite deposition from a mantle co2 flux along crust penetrating fault systems however conductive graphitic metasedimentary rocks characteristic of foreland basins are apparently absent from the transect area a weak inverse correlation between metamorphic grade and electrical conductivity was observed by following rock units across the grenville front into high grade equivalents within the parautochthonous belt a uniformly conductive mid crustal layer extends across the grenville front apparently without change in character the existence of this ubiquitous mid crustal conductor has been interpreted to mean that electrical conductivity is controlled by the present day pressure temperature and fluid saturation of the lower crust independent o

carbonaceous matter in peridotites and basalts studied by xps sali and leed abstract carbonaceous matter in peridotite xenoliths and basalt from the hualalai volcano in a basalt glass collected directly from an active lava lake on the east rift of kilauea in garnet and diopside megacrysts from the jagersfontein kimberlite and in gabbros from the stillwater and bushveld complexes has been studied by x ray photoelectron spectroscopy xps thermal desorption surface analysis by laser ionization sali and low energy electron diffraction leed the basalt and two of the four xenoliths from hualalai and both jagersfontein megacrysts yielded trace quantities 10 nanomoles of organic compounds on heating to 700 c organics were not detected in the rocks from the layered intrusions and neither carbonaceous matter nor organics were detected in the glass from the lava lake where detected organics appear to be associated with carbonaceous films on microcrack surfaces carbonaceous matter exists as films less than a few nm thick and particles up to 20 m across both of which contain elements expected to be present in significant quantities in magmatic vapors namely si alkalis halogens n and transition metals leed studies suggest that the carbonaceous films are amorphous the data suggest two possible mechanisms for the formation of the organics one is that they are a product of abiotic heterogeneous catalysis of volcanic gas on new chemically active mineral surfaces formed by fracturing during cooling alternatively organics may have been assimilated into the volcanic gases prior to eruption and then deposited on cracks formed during eruption and cooling in any case there is no evidence to suggest that the organics represent laboratory or field biogenic contamination

electrical conductivity temperatures and fluids in the lower crust we have compiled both laboratory and worldwide field data on electrical conductivity to help understand the physical implications of deep crustal electrical profiles regional heat flow was used to assign temperatures to each layer in regional electrical conductivity models we avoided those data where purely conductive heat flow suggested temperatures more than about 1000 sup 0 c substantially higher than solidus temperatures and outside the range of validity of heat flow models the resulting plots of log conductivity sigma versus 1 t demonstrate that even low conductivity layers lcl have conductivities several orders of magnitude higher than dry laboratory samples and that the data can be represented by straight line fits in addition tectonically active regions show systematically higher conductivities than do shield areas because volatiles are usually lost in laboratory measurements and their absence is a principal difference between laboratory and field conditions these materials probably account for the relatively higher conductivities of rocks in situ in the crust free water in amounts of 0 01 0 1 in fracture porosity could explain crustal conductivities other possibilities are graphite hydrated minerals in rare instances and sulfur in combination with other volatiles as most of the temperatures are less than 700 sup 0 c partial melting seems likely onlymore in regions of highest heat flow where the conductive temperature profiles are inappropriate another result is that at a given temperature crustal high conductivity layes hcl are more conductive by another order of magnitude and show more scatter than do lcl s less

carbonate ramp to rimmed shelf evolution lower to middle cambrian continental margin virginia appalachians drill cores through the lower to middle cambrian shady dolomite carbonate platform 600 to 1 200 m thick in the austinville virginia region have provided critical platform margin facies relations not exposed in outcrop the control provided by the closely spaced drill cores has allowed detailed cross sections to be constructed through the margin which allow the evolution of the carbonate platform from a gently sloping carbonate ramp to a high relief reef rimmed shelf to be interpreted the shady dolomite records the inception of carbonate sedimentation on the lower cambrian passive margin and forms the initial carbonate foundation for the over lying cambrian ordovician carbonate shelf sequence carbonate sedimentation was initiated on a submerged lower cambrian clastic shelf resulting in deposition of a thick sequence 300 m of deep ramp nodular limestones this facies shallows upward into a carbonate ramp fringed by low relief stromatactoid mud mound buildups as much as 30 m thick and 3 5 km wide downslope these buildups pass into slope conglomerate and deep ramp carbonate facies landward of the buildups cyclic peritidal sequences developed by latest early cambrian time as the ramp increased in relief skeletal algal bioherms and lime sands were established on the high energy margin to form a high relief as much as 500 to 700 m rimmed shelf with thick 600 m off platform deposits immediately seaward of the shelf edge in the shelf interior a major regression during early to middle cambrian time resulted in development of a major unconformity and in deposition of red beds on the more rapidly subsiding outer platform subsidence rates exceeded sea level fall rates allowing continued upbuilding of the reefal rimmed shelf this rimmed shelf persisted for about 30 m y and influenced carbonate sedimentation throughout the cambrian until it was destroyed by incipient collision during the early ordovician period read in press

the moho in the northern basin and range province nevada along the cocorp 40 n seismic reflection transect cocorp seismic reflection profiles across nevada at about 40 n image a prominent essentially continuous band of reflectors at a two way traveltime of 9 to 11 s the approximate correspondence of this reflection time with estimates of the two way traveltime to the moho in this area provided by seismic refraction data suggests that the prominent reflections are from the moho the relief on these reflectors the reflection moho beneath nevada across a latitudinal transect of

birps deep seismic reflection studies of the british caledonides the western isles north channel winch traverse fig 1 an extension of the successful moist profile1 was recorded in 1982 at sea along the west coast of britain for birps british institutions reflection profiling syndicate by the geophysical company of norway geco the purpose of the winch traverse was to study crustal structure of the caledonian foreland to cross the caledonian orogen and to establish the three dimensional geometry of mantle reflectors originally seen on moist we describe here the data first emphasizing british caledonian structures and then discussing features of the deep crust of wider significance the data are of very good quality and contain clear moho and upper mantle reflections the lower crust is surprisingly reflective deeply penetrative thrusts are observed and there is firm evidence that several of the mesozoic basins round the shores of the united kingdom were formed by reactivation of older features

stable isotope evidence for the petrogenesis and fluid evolution in the proterozoic harney peak leucogranite black hills south dakota abstract oxygen and hydrogen isotope systematics of the proterozoic harney peak granite black hills south dakota were examined in order to constrain its petrogenesis and to examine the role of fluids in a peraluminous granite pegmatite magmatic system the leucogranite and its satellite intrusions were emplaced as hundreds of sills and dikes which are texturally heterogeneous ranging from aplitic to pegmatitic the dominant ferromagnesian mineral in the core of the granite is biotite while along its perimeter and in satellite intrusions it is tourmaline biotite and tourmaline are for the most part mutually exclusive oxygen isotopes among minerals in non pegmatitic rocks from throughout the pluton equilibrated for the most part at magmatic temperatures between 800 to 650 c in the pegmatitic samples quartzfeldspar oxygen isotope fractionations point to disequilibrium probably a result of the sequential crystallization of these minerals the isotopic composition of most pegmatites suggests local derivation by differentiation of emplaced batches of magma the whole rock 18 o values of the granites are heterogeneous ranging from 10 4 to 14 3 however there is a pronounced difference in the isotopic composition of the biotite containing granites from the core of the pluton 11 5 0 6 and the tourmaline rich granites from its perimeter and the satellite intrusions 13 2 0 8 the average oxygen isotopic composition of the surrounding schists is identical to that of the latter granites biotite muscovite and tourmaline muscovite d values vary from 20 to 10 and from 0 to 10 respectively although these values differ from theoretical values the narrow ranges suggest that the minerals are in isotopic equilibrium and that their isotopic compositions record the composition of the magmatic fluid the calculated d value of the magmatic fluid is 67 to 58 well within the magmatic water range however somewhat elevated mineral d values in the southern part of the harney peak granite where the metasediments reached the second sillimanite isograd suggest that limited interaction with fluids derived from the dehydrating metasediments may have occurred alternatively the elevated values could reflect a hydrogen isotope heterogeneity in the source or interaction with another magmatic system to the south of the harney peak granite it is shown that fractional crystallization or subsolidus interaction of the harney peak granite with the magmatic fluid or a fluid derived from the schist cannot explain the difference between the 18 o values of the core and perimeter granites although some oxygen isotope heterogeneity in the granite could be explained by assimilation of the country rocks assimilation cannot explain all of the difference between the two granite types instead it is proposed that intrusion of the magma which led to the biotite granites in the core of the pluton at the culmination of regional metamorphism initiated melting of the schists at a depth somewhat greater than the present level of erosion the melts were emplaced into the overlying schist and differentiated into the many tourmaline rich granite pegmatite sills and dikes comprising much of the perimeter of the harney peak granite and its satellite plutons alternatively the different melts may have resulted from melting along an isotopically heterogeneous vertical section of the crust in response to the ascent of a thermal pulse

carbon isotopic thermometry calibrated by dolomite calcite solvus temperatures abstract the temperature dependence of carbon isotopic fractionations between calcite and graphite and between dolomite and graphite are calibrated by the calcite dolomite solvus geothermometry using marbles collected from the contact metamorphic aureole in the kasuga area central japan the carbon isotopic fractionations 13ccc gr and 13cdo gr systematically decrease with increasing metamorphic temperature the concordant relationships between the fractionations and solvus temperatures are approximately linear with t 2 over the temperature range 400 to 680 c 13ccc gr 5 6 106 t 2 k 2 4 13cdo gr 5 9 106 t 2 k 1 9 these systematic relationships between fractionation and temperature suggest that carbon isotopic equilibria between carbonates and graphite were attained in many cases the equation for the calcite graphite system has a slope steeper than bottinga s 1969 results it is however in good agreement with that of valley and o neil 1981 in the temperature range from 600 to 800 c because of the relatively high sensitivity to temperature these isotopic geothermometers are useful for determining the temperatures in moderate to high grade metamorphosed carbonate rocks

free carbon electrical conductivity in the earth s mantle laboratory measurements indicate that solid carbon either amorphous or as graphite enhances the electrical conductivity of rocks carbon in quantities of only a few ppm providing it forms an interconnected phase could give rise to electrical conductivities of the order seen in the high conductivity layers in the earth s mantle this possibility is consistent with carbon petrology in mantle rocks and puts the formation and migration of abiogenic carbon compounds into a broader context

geochemical evidence for oxygenated bottom waters during deposition of fossiliferous strata of the burgess shale formation abstract deciphering the oxygenation potential of bottom waters during deposition of the fossiliferous strata of the burgess shale is key to understanding the paleoecology of the organisms preserved as fossils and the processes involved in their preservation methods of paleo redox determination that are based upon trace fossils organic carbon content or size analysis of pyrite framboids are problematic when applied to the burgess shale fortunately patterns in redox sensitive trace elements hold great potential for determining paleo redox conditions for these greenschist facies rocks because they are independent of both age and metamorphic grade four burgess shale type bst fossil bearing sections were studied three lie within the burgess shale formation whereas one lies within the slightly younger duchesnay unit of the middle chancellor formation the geochemical proxies of seawater redox conditions that were used are mo u th v cr v v ni ni co and v sc each of these redox sensitive indices suggests that oxygenated conditions prevailed in the overlying seawater during deposition of beds within which bst preservation developed non fossiliferous beds at the base of the mt stephen trilobite beds yield three geochemical indices v v ni v sc ni co that indicate deposition under dysoxic to anoxic conditions thus the only beds to have been deposited under anoxic conditions are barren of fossils of soft bodied taxa further contradicting the notion that bottom water anoxia is a factor in bst preservation the laminated nature of the burgess shale and evidence for subsurface anoxia coupled with evidence for oxygenated bottom waters presence of bacterial mats and surficial trace fossils suggest that the burgess shale was deposited in the exaerobic zone oxic anoxic boundary at the water sediment interface implications of this reinterpretation of the depositional environment of the burgess shale include 1 taphonomic models for bst preservation must account for the presence of oxygen in bottom waters 2 paleoecological interpretations of the burgess shale need to account for a probable autochthonous benthic component in what is a complex assemblage of fossils that includes elements derived from the platform and 3 the possibility of burgess shale communities based upon chemosynthesis and or bacteria grazing must be considered

a new type of shelf margin deposit rigid microbial sheets and unconsolidated grainstones riddled with meter scale cavities middle cambrian microbial limestone contains a network of unusual predominantly horizontal cavities up to 2 m in length and 0 5 m in height the microbialite experienced rapid syndepositional lithification but adjacent grainstone sediments remained unlithified during deposition this juxtaposition contributed to sediment instability resulting in fracturing and brecciation of the lithified microbialite while unconsolidated grainstones underwent slumping and injection into some cavities remaining space within the cavities was colonized by a series of encrustations thin crusts 2 8 mm of laminated algal mats followed by several generations of calcified renalcis like cyanobacteria up to 45 mm thick remaining void space was partially filled by internal sediment and then sequentially occluded by banded radiaxial fibrous calcite herringbone calcite and finally saddle dolomite cements the radiaxial and herringbone calcite cements precipitated from porewaters derived from seawater that became anoxic through the breakdown of organic matter in the microbialite noteworthy is the presence of herringbone calcite cement not as a seafloor precipitate but as an early cavity fill we propose that the unusual bedding parallel fractures were caused by gravity collapse along a shallow platform margin coeval foreslope sediments show syndepositional slumping faulting and mass flow deposits these redeposited sediments contain boulders of microbialite and grainstone of platform margin provenance 1999 elsevier science b v all rights reserved

13c 12c exchange between calcite and graphite a possible thermometer in grenville marbles the fractionation of 13c between calcite and graphite cc gr is consistently small 2 6 4 8 permil in 34 assemblages from upper amphibolite and granulite facies marbles of the grenville province in 25 samples from the adirondack mountains new york it decreases regularly with increasing metamorphic temperature the fractionations are independent of absolute 13c values of calcite 2 9 to 5 0 for t 600 800 c the adirondack data are described by cc gr 0 00748t c 8 68 this good correlation between and t suggests that carbon isotope equilibrium was attained in these high grade marbles and that the theoretical calculations of this fractionation by bottinga are approximately 2 permil too large in this temperature range because of the relatively high temperature sensitivity suggested by these results and by bottinga s calculations and the pressure independence of isotope fractionation cc gr may provide a very good thermometer for high grade marbles comparison of this field calibration for cc gr vs temperature with results from other terranes supports the utility of cc gr for geothermometry and suggests that graphite is much more sluggish to exchange than calcite that exchange between calcite and graphite occurs at temperatures as low as 300 c and that equilibrium may normally be attained only when peak metamorphic temperatures are greater than 500 600 c because 13c exchange is an unavoidable metamorphic process at temperatures above 300 c high values of 13c gr in moderate to high grade carbonate bearing rocks do not provide a sufficient criterion to infer an abiogenic origin for the graphite

seismic profiling the continental lithosphere deep seismic reflection profiling is successfully being used worldwide to study the structure of the continental crust1 7 the moist8 profile recorded by birps british institutions reflection profiling syndicate was the first to show that the structure of the upper mantle could also be imaged given a powerful source and long recording times birps has recently acquired an ultra deep seismic reflection line drum deep reflections from the upper mantle the drum line was designed specifically to investigate the reflectivity of the lower continental lithosphere and was therefore recorded to a two way time twt of 30 s giving a depth of penetration into the lithosphere of 110 km this profile has now revealed strong reflections which are unequivocally from the lower lithosphere well below the base of the crust and which are as far as we are aware the deepest and most continuous structures imaged in the upper mantle by multi channel seismic reflection profiling

grain boundary graphite in kapuskasing gneisses and implications for lower crustal conductivity surface electromagnetic measurements commonly reveal that the intermediate and lower continental crust has an appreciable electrical conductivity much higher than that found for laboratory analogues of deep crustal rocks or for upper crustal rocks either formed at shallow depths or uplifted from deeper levels conducting films at grain boundaries are thought to play an important part in enhancing deep crustal conductivity with brines and graphite being the main candidates1 at present however there is a lack of direct evidence for such films frost et al 2 have reported an enhancement in conductivity of igneous lower crustal rocks from the la ramie complex apparently owing to the presence of graphite films but the possible role of brines was not clear from this work here we present a study of the grain surface composition of metamorphic and one igneous rocks exhumed from 20 km depth by the kapuskasing uplift of the canadian shield3 we expect these samples to be representative of metamorphic and igneous rocks of the deep archaean crust auger spectroscopy provides evidence for graphite films at grain boundaries which can account for the present electrical signature of the kapuskasing crust4 6 we also detect traces of chlorine sulphur and iron which suggest that brines and solid conductors other than graphite may have helped to enhance the conductivity but their abundance suggests at best a minor role relative to graphite

trace element behavior and redox facies in core shales of upper pennsylvanian kansas type cyclothems the black shale submember of core shales of pennsylvanian kansas type cyclothems is highly enriched in redox sensitive trace elements tes relative to the overlying gray shale submember as well as to many other modern and ancient organic rich deposits controls on trace element behavior in the black shale can be inferred from trace element enrichment factors efs relative to gray shale and the relationship of tes to total organic carbon toc most tes conformed to one of two patterns 1 mo u v zn and pb exhibited moderate efs and strong covariation with toc in samples having 10 wt toc and 2 cu ni cr and co exhibited low efs and moderate to strong covariation with toc in all samples these patterns are inferred to represent different responses to benthic redox conditions the first pattern is characteristic of tes of strong euxinic affinity i e those taken up in solid solution by fe sulfide or involved in other reactions catalyzed by free h2s and resident mainly in authigenic phases whereas the second pattern is characteristic of tes of weak euxinic affinity i e those not strongly influenced by the presence of free h2s and resident mainly in the organic carbon or detrital fractions of the sediment these inferences allowed development of a multiproxy technique for assessing redox facies in black shale samples euxinic conditions were considered to have existed if at least two of four redox indicator trace elements mo u v zn showed euxinic levels of enrichment and nonsulfidic anoxic conditions were inferred otherwise the validity of the procedure is indicated by 1 agreement among all four redox indicators for a large majority of samples 69 of 185 2 among samples yielding a mixed redox signal a systematic sequence of te enrichment v zn mo u and 3 for tes of weak euxinic affinity reduced variance among samples representing each redox facies sequential enrichment may be a response to differential redox thresholds for accumulation e g v or postdepositional remobilization of trace elements e g u and may provide a basis for finer assessment of redox conditions in low oxygen paleoenvironments than permitted by simple classification schemes the multiproxy procedure for redox facies analysis developed in this study is likely to be more reliable than widely used single proxy indicators based on trace elements e g mo authigenic u or v v ni or c s fe systematics e g s toc or dop d 2004 elsevier b v all rights reserved

mid carboniferous deglaciation of the protoprecordillera argentina recorded in the agua de jaguel palaeovalley abstract the agua de jaguel formation near uspallata mendoza province argentina was deposited within a palaeovalley along the southeastern margin of the calingasta uspallata basin adjacent to the protoprecordillera the basal glacigenic sequence of the formation was deposited in the serpukhovian early bashkirian near the beginning of the late palaeozoic ice age the sequence is composed of diamictite conglomerate sandstone and mudrock and records four depositional stages within a palaeofjord 1 morainal bank deposition by a wet based tidewater glacier 2 glacial retreat succession where ice retreated up the fjord out of the immediate area allowing iceberg deposition of dropstones and dump deposits 3 continued glacial retreat with ice receding onto land allowing a shoreface to develop within the palaeovalley and 4 transgression across the shoreface and resumption of iceberg deposition the thickness and facies of this succession are similar to deposits within modern alaskan fjords housing temperate tidewater glaciers comparison of the agua de jaguel formation with alaskan fjords indicates that sequence stratigraphy used for low latitude deposits must be applied with caution as glacimarine ice retreat within a fjord produces strata surfaces that are easily misidentified as flooding surfaces resulting from changes in water depth likewise the transgression during stage 4 is significant as it indicates a rise in sea level that occurred during glaciation of the protoprecordillera thus suggesting that the transgression between stages 3 and 4 was not driven by input of glacial meltwater additionally the sediment geochemistry suggests that the bottom waters of the palaeovalley were anoxic which may help explain the absence of bioturbation in the sequence as well as in other ancient glacimarine palaeovalleys the deglaciation succession and transgression are also recorded in the nearby hoyada verde and tramojo formations so it is proposed that all three formations record one glacial event in the protoprecordillera overlying fluvial and shallow marine strata in the agua de jaguel hoyada verde and tramojo formations show no indication of continued glaciation in the protoprecordillera following stage 4 and equivalent strata the deglaciation succession of the agua de jaguel formation affirms the emerging concept that the late palaeozoic ice age was characterized by alpine glaciers ice caps and small ice sheets that were not massive enough to have driven eustatic fluctuations of 100 m as previously understood and that ice never covered westernmost gondwana during later lpia events

stable isotope evidence for regional scale fluid migration in a barrovian metamorphic terrane vermont usa we measured the c and o isotopic composition of carbonate minerals in the waits river formation eastern vermont to determine the extent of fluid infiltration during regional metamorphism over an approx 2000 km2 area in a deep seated 25 km barrovian terrane from a petrologic study of this terrane ferry proposed the existence of a large regional metamorphic hydrothermal system with two first order features 1 on the scale of the entire terrane fluid flow was focused into the axes of two major antiforms of regional extent 2 on a smaller scale about 100 km2 flow was further focused around synmetamorphic granitic plutons that intruded along the axes of the antiforms we find isotopic evidence for both the regional hydrothermal activity along the antiforms and the more intense fluid flow around synmetamorphic plutons the evidence for hydrothermal activity around the plutons is large heavy isotope depletions up to 6 9 in 18ocarb and 13ccarb in diopside zone rocks adjacent to the plutons these isotopic shifts are greater than can be explained solely by prograde metamorphic reactions we find two lines of evidence for the more diffuse regional flow that was focused into axes of the antiforms first 18ocarb and 13ccarb within individual outcrops become increasingly homogeneous with increasing grade towards antiform axes indicating that the rocks equilibrated with a permeating fluid second there are depletions in 18o near the margins of the waits river formation which can be interpreted as a dispersed advective infiltration front displaced toward the antiform axes these fronts were modelled using eq 13 of bickle and baker and imply time integrated fluid of 105 106 cm3 cm2 which are consistent with values derived by ferry from measured progress of prograde devolatilization reactions

ree and trace element patterns from organic rich rocks of the ediacaran cambrian transitional interval abstract it has been established that important changes in the marine environment and the biosphere occurred during the cambrian however the relationships between the so called cambrian explosion and the concomitant environmental changes are not yet fully understood this study presents new geochemical data from the black shale successions from different facies belts of the yangtze platform in south china variations in the concentrations of ree and trace elements varying ce ce th u v sc and v cr ratios in kerogen as well as in bulk rocks from different depositional environments along a transect from platform to basin indicate two oxidation events which led to the oxygenation of the water column in shallow marine environments and euxinic conditions weak correlation between toc v u and mo in the deeper sea during the first oxidation event in the late terreneuvian anoxic conditions in bottom waters rapidly changed to euxinic conditions subsequently the second oxidation event during the early epoch 2 of the cambrian led to oxic suboxic conditions in deeper seawater

mapping high pn velocity beneath the colorado plateau constrains uplift models the massive international seismological centre data set of the past 20 years and the two station method are used to determine pn velocities in the mantle lid beneath the colorado plateau in this method the event is located at distances where pn is the first arrival 2 16 and the path is in or very near the azimuth of the two station pair and crosses the plateau this technique to a large extent minimizes the hypocenter mislocation effect and possible errors due to variations in the crustal structure near the source since only the difference in travel times at the two stations is used however this technique has a few underlying assumptions and possible sources of errors such as the quality of the pn data base and station delays caused by varying crustal structure that require an extremely careful application of the method a detailed study of the source of errors and a methodology of selection of the data are presented application of this method to the colorado plateau using all possible two station pairs from 53 stations located within or along the margin of the plateau yields an average high pn velocity of 8 12 0 09 km s this value is considerably larger than the average value of 7 83 km s based on available but very limited seismic refraction profiles but is remarkably similar to the average value of 8 1 km s for the relatively stable midcontinent region our new pn velocity for the colorado plateau eliminates the paradox in the literature that emphasizes the rather close similarity between average pn velocities beneath the colorado plateau and the basin and range province while their tectonic and magmatic cenozoic history is dramatically different previous models for the structure and evolution of the plateau have used the low pn velocity as an important constraint on density and thermal state of the lithosphere hence such models should be reexamined on the basis of this new uppermost mantle pn velocity determination there are two main models that have been proposed to explain the 2 km uplift of the colorado plateau one is based on a combination of thermal thinning of the lithosphere and crustal thickening and the other involves a combination of the delamination of the subducted subhorizontal farallon oceanic plate from the overriding north american plate and crustal thickening we show that the delamination model is more readily consistent not only with our velocity determination and the elevation of the plateau but also with varied geological observations reported in the literature that concern the cenozoic evolution of western north america

high resolution geochemical evidence for oxic bottom waters in three cambrian burgess shale type deposits abstract the cambrian radiation of complex animals is a fundamental event in the history of life on earth much of our understanding of this event is made possible through the study of exceptionally preserved fossils in burgess shale type bst deposits based on bioturbation levels in bst deposits they are often interpreted as representing restricted oxygen settings this study tests the low oxygen interpretation through analysis of geochemical paleoredox proxies and bioturbation levels in three bst deposits the early cambrian maotianshan shale china the middle cambrian wheeler shale usa and the middle cambrian spence shale usa results from 96 samples show fine scale geochemical evidence for oxic bottom water conditions during the deposition of these three bst deposits trace element paleoredox indices pi give consistent oxic signals in all three shales in addition total organic carbon toc and total sulfur ts levels in the spence shale are extremely low the average c s ratio is 3 1 comparable to values for modern oxic marine water bioturbation levels in all three shales are low with rare development of minimal to moderate horizontal bioturbation while short term oxygen fluctuations or poikiloaerobic conditions could still have sporadically existed in these settings this study shows that these bst deposits were deposited in a dominantly oxic paleoredox setting evidence for low bioturbation levels coupled with paleoredox geochemistry indicative of oxic bottom waters suggests that pervasive exaerobic conditions with an oxic water column positioned above oxygen depleted sediment pore waters may have existed in many cambrian bst deposit paleoenvironments

taphonomy and depositional circumstances of exceptionally preserved fossils from the kinzers formation cambrian southeastern pennsylvania abstract the emigsville member of the kinzers formation cambrian southeastern pennsylvania is a deposit of exceptional preservation containing three main lithofacies that were part of a mixed siliciclastic carbonate debris fan developed seaward of a carbonate shelf along the appalachian margin of laurentia fossils were exceptionally preserved in a shelf environment subject to tempestite deposition most remains are fragmentary which emphasizes the importance of predation as a taphonomic process where emigsville sediments were deposited remains are preserved mostly as organic carbon films pyrite crusts and aluminosilicate films these preservation styles apparently depended upon the development of microbial consortia in isolated microenvironments and some of the microbes were autolithified sedimentary and biological evidence suggests that anoxia salinity fluctuations and sedimentary obrution played relatively minor roles in the emigsville member of the kinzers formation the abundance of exceptionally preserved remains suggests that exceptional sedimentary conditions were not necessary in the cambrian in order for exceptional preservation to occur the abundance of predation evidence in a deposit of exceptional preservation reinforces the concept that predation was the primary taphonomic filter during cambrian time

deep seismic reflection characteristics of the continental crust four types of seismic reflection fabrics appear to characterize the continental crust 1 a cratonic fabric dominated by diffractions and dipping reflections but lacking a pronounced reflection moho 2 regions of thin skinned foreland deformation with abundant shallow but sparse deep reflections 3 a unidirectional fabric that dips toward the hinterland and may represent a crustal scale ramp zone and 4 zones of highly layered lower crust and high amplitude reflection moho the first three types are commonly observed in transects across single mountain belts but the fourth appears to correspond to regions of elevated lower crustal temperatures such as those encountered in rift provinces and perhaps elsewhere

laramide crustal thickening event in the rocky mountain foreland and great plains the rocky mountain foreland and great plains of the western united states were formerly part of a continental platform adjusted by erosion and deposition in cambrian through jurassic time to near mean sea level and therefore to a near uniform crustal thickness of approximately 33 km today the region stands at regional elevations up to 2 km isostatically supported by a crust exceeding 50 km in thickness reasonable estimates of tertiary sedimentation and laramide strain do not account for more than 15 of the implied thickening however from approximately 70 40 m y b p this region was underlain by a horizontally subducting slab of farallon plate lithosphere moving northeast this slab could have been the cause of the thickening finite difference thermal models with specified kinematics show only a temporary cooling of the base of the north american lithosphere by this slab the excess weight of the slab would have depressed the region plate bending calculations show a quantitative agreement of predicted depression with upper cretaceous isopachs since depression by this slab lasted until the eocene at least the latest cretaceous regression was probably caused by a laramide crustal thickening event the farallon slab might have caused crustal thickening in two ways its excess weight would have drawn in ductile lower crust from surrounding regions however calculations show that this effect is too slow too local and too reversible to explain most of the crustal thickening therefore it seems likely that ductile lower crust was transported from sw to ne by shear stresses which the farallon plate exerted on the base of the north american lithosphere a preliminary finite element calculation based on this hypothesis shows the correct general pattern of crustal thickening an unexpected but encouraging result is that predicted principal compression directions are orthogonal to many laramide basement uplifts

carbonaceous material in metamorphosed carbonate rocks from the waits river formation ne vermont and its effect on electrical conductivity the relations among electrical conductivity and graphite content metamorphic grade and fluid rock interaction are investigated for a suite of regionally metamorphosed graphitic carbonate rocks from the waits river formation ne vermont graphitization was complete by the lowest grade of metamorphism attained in the area 450 c and 450 mpa in low grade rocks graphite occurs as inclusions within calcite and along grain boundaries and in medium grade rocks it concentrates near porphyroblasts in neither suite does graphite form interconnected networks average reduced carbon abundances are 4400 and 2800 ppm for the low and medium grade rocks respectively high grade rocks are almost void of graphite except for one specimen the 13c values were determined for graphite and carbonates and 18o values were determined for the carbonates and silicate residues there is no change in isotopic values between low and medium grade rocks in contrast high grade rocks show marked decreases in 13c and 18o values of 4 compared to low and medium grade rocks both isotopic analyses and graphite depletion in high grade rocks were caused by influx of large quantities of magmatic water during peak metamorphism electrical conductivity was measured in the laboratory on representative specimens rocks with high carbon content 7000 ppm from both the low and high grade zones display almost an order of magnitude higher electrical conductivity than expected from their fluid content alone graphite does not form an interconnected network in these rocks yet it combines with the saline fluids to significantly increase electrical conductivities

reductive deposition of graphite at lithological margins in east central vermont a sr c and o isotope study o sr and c isotopes from east central vermont are used to provide information on the timing and volume of metamorphic fluid flow the results are then used to assess the evidence for redox transformations between c species oxygen profiles are homogenised on a metre scale comparison with sr isotopes suggest that o alteration may have occurred over a significantly larger timescale than that of sr possibly because o was modified during dewatering and diagenesis in addition to the high temperature alteration recorded by strontium sr isotope distributions are consistent with cross layer fluid fluxes of 104 106 moles m 2 absolute values depend on the sr fluid rock distribution coefficient which is poorly known however reaction progress constraints suggest that fluxes were towards the lower end of this range high 13c values observed at lithological boundaries cannot be explained by volume loss or closed system processes and are taken to indicate reductive precipitation of graphite as a result of mixing between co2 and ch4 bearing fluids mass balance calculations indicate that redox reactions occurring under metamorphic conditions convert a minimum of 10 of the co2 released from limestones into graphite thus providing a potentially important control on the average residence time of c within the crust with implications for c cycling models

the initiation of the early paleozoic cordilleran miogeocline evidence from the uppermost proterozoic lower cambrian hamill group of southeastern british columbia the uppermost proterozoic lower cambrian hamill group of southeastern british columbia contains geologic evidence for a phase of extensional tectonism that led directly to the onset of thermally controlled subsidence in the cordilleran miogeocline moreover the hamill group contains the sedimentological record of the passage of the ancient passive margin from unstable tectonic conditions associated with rifting and or the earliest phases of thermal subsidence to post rift conditions characterized by stabilization of the margin and dissipation of the thermal anomaly generated during the rift phase the rift to post rift transition widespread uplift that occurred prior to and during the deposition of the lower hamill group is indicated by an unconformable relation with the underlying windermere supergroup and by stratigraphic relations between middle and upper proterozoic strata and unconformably overlying upper lower cambrian quartz arenites upper hamill group in the southern borderlands of the hami

an early cambrian rift to post rift transition in the cordillera of western north america the upper proterozoic and lower palaeozoic wedge of miogeoclinal strata in the north american cordillera is widely regarded as evidence for a proto pacific passive margin1 3 the rifting history of this margin appears to have been protracted possibly spanning 200 myr in a tectonic setting that is not well understood3 quantitative subsidence analyses of lower palaeozoic strata between british columbia and utah however provide indirect evidence that the transition from rifting to post rift cooling occurred within a relatively short interval of time although probably not synchronously between 600 and 555 myr refs 4 6 this age is significantly younger than that implied in previous studies1 2 we describe here new field data which together with published geological data provide direct evidence of a latest proterozoic or early cambrian age for the rift to post rift transition the data support recent arguments for widespread initiation of passive margins around the edge of the north american craton close to the cambrian precambrian boundary7

carbon isotopic study on coexisting calcite and graphite in the ryoke metamorphic rocks northern kiso district central japan carbon isotope fractionation between coexisting calcite and grpahite 13ccc gr has been determined in metamorphosed limestones and calc silicate rocks from the ryoke metamorphic belt in the northern kiso district in this district the ryoke metamorphic rocks ranging from the lower greenschist facies to the upper amphibolite facies are widely distributed the fractionation of 13c 12c between calcite and graphite decreases regularly with increasing metamorphic grade and is independent of absolute 13c values of calcite this evidence suggests that carbon isotopic exchange equilibrium has been attained during metamorphism even in the greenschist facies and isotopic modification possibly caused by retrogressive metamorphism is not distinguished for t 270 650 c the fractionation is expressed by the following equation 13ccc gr 8 9 106t 2 7 1 t in k

petrophysical model of deep electrical conductors graphite lining as a source and its disconnection due to uplift petrophysical information including stress release data recently made available for low porosity 0 1 0 4 crystalline rocks has been used to develop a broken conductor model that may explain the origin of enhanced electrical conductivity of the intermediate to lower continental crusts of shields and its disappearance when the rocks are uplifted average pore size and pore density of their fluid flow paths are 0 1 0 4 m and 1 400 m respectively the model assumes that these paths are filled or lined with interconnected electrical conducting minerals such as graphite resulting in the rocks having resistivities as low as 40 to 800 m in situ graphite film thicknesses required to produce these resistivities are 5 20 nm 50 200 a the model also assumes that electrical conducting paths break up during uplift due to stress release resulting in a poorly conducting rock petrophysical data of granites and observations from deep drill holes suggest a porosity increase of the order of 3 from an uplift of 10 km or more however the newly formed pore space would fill with deposits or intruded material so that surface rock porosities would not be much larger than at depth electrical resistivities of rock samples containing disconnected grain boundary graphite presently exposed by the kapuskasing uplift of the canadian shield are in the range of 3000 12000 m for the direction of foliation these values are 10 100 times larger than the estimates for similar rocks containing interconnected graphite but are 10 100 times smaller than usually observed in the canadian shield for similar rocks absent of grain boundary graphite these observations are in good agreement with the results predicted by the broken conductor model for an uplift of 10 km or more if a gross linear relationship is assumed between crack porosity and uplift for the upper 10 km of the crust

palaeontology and depositional environments of ancient brine seeps in the middle cambrian burgess shale at the monarch british columbia canada abstract the monarch a mountain in southeastern british columbia exposes the cathedral escarpment which abuts the basinal burgess shale and an underlying limestone the upper takakkaw tongue at one section a dolomitized megabreccia representing an intraformational collapsed cavern forms the cathedral escarpment wall the megabreccia is cross cut by the subsequent megatruncation event marking the collapse of the cathedral margin which supports previous hypotheses that dolomitizing brines were active at the cathedral margin prior to and during deposition of the burgess shale epiphyton rich carbonate megablocks within the basinal strata previously interpreted as olistoliths spalled off the escarpment are unlike the peritidal strata comprising the cathedral margin and are more likely deep water carbonate mud mounds that grew in the basin near the escarpment face evidence for early brines in basinal environments at this locality includes lenses of dark magnesium rich clinochlore 30 wt mgo vs 2 in background shales within the burgess shale at the escarpment wall these clinochlores are interpreted as precipitates from seafloor brine seeps analogous to ones currently found at the foot of the florida escarpment and on the distal shelf and slope of the northern gulf of mexico like the gulf examples the brines formed standing pools on the cambrian seafloor and hosted abundant organisms in a narrow fringe around their peripheries but not within the pools sponges priapulid worms hyoliths linguliform brachiopods and trilobites predominate shales only a few metres basinward from the pools are nearly barren of animal fossils increased biologic productivity localized at the seeps would account for the patchy distribution of fossils in the burgess shale in a narrow zone close to the escarpment the discovery of brine seep deposits and associated fringing communities at the monarch shows that at least some assemblages in the burgess shale were preserved within their habitat and were supported by chemosynthesis

paleolithospheric structure revealed by continental geoid anomalies abstract lithospheric geoid anomalies record changes in elevation and potential energy experienced by continental lithosphere estimates of local isostatic equilibration and potential energy in tandem with lithosphere related geoid anomalies can be used to estimate paleolithospheric thickness providing a clearer understanding of how and why continental topography is developed we employ several simplifying assumptions about the crustal and mantle lithosphere density and structure and readily acknowledge that our results are therefore first order approximations to predict the pre orogenic structure of the lithosphere at the outset we emphasize that while this approach does not provide an exhaustive evaluation of the deformation mechanism it does serve to quantify the relative role played by the variations in the crustal and upper mantle components of the lithosphere in this way we are able to use independent measurement of lithospheric geoid anomalies current post orogenic elevation and lithospheric structure and paleoelevation information to estimate topographic development and structural support over time application of this technique to the southwestern united states indicates that the uplift of the colorado plateau is the result of processes in both the crust and mantle lithosphere and that the lithosphere of the pre orogenic southern basin and range was thinned relative to the northern basin and range and colorado plateau although we use the southwestern u s as an example this method can help constrain uplift mechanisms for any region for which the structure and geoid anomaly of the modern lithosphere is well understood

authigenic euhedral megaquartz crystals in a quaternary dolomite abstract autochthonous doubly terminated euhedral megaquartz crystals as well as overgrowths on detrital quartz grains are present throughout a gypsum and anhydrite bearing pleistocene sabkha dolomite accumulation from the arabian gulf the siliceous precipitates whose source was probably recycled biogenic silica formed within meters of the sediment surface these quaternary precipitates provide the first modern analog for euhedral megaquartz crystals a relatively common constituent in ancient evaporite bearing tidal flat carbonate strata in the quaternary dolomites studied megaquartz is present as individual crystals and clusters of crystals as well as overgrowths on detrital quartz grains individual crystals commonly attain lengths of 1 mm and many show shallow dolomite mold impressions on their crystal facies and or partly or completely engulf dolomite rhombohedra the common occurrence of dolomite molds within the quartz crystals indicates that precipitation of the quartz took place either concurrently with or more likely after dolomitization of the host carbonates additionally the oft cited association of authigenic megaquartz and evaporite minerals plus the existence of void spaces into which the quartz precipitated imply a causal relationship between dissolution of the evaporites and or dolomites and precipitation of the megaquartz thus these findings demonstrate that authigenic megaquartz crystals as well as overgrowths on detrital quartz can originate under near surface temperatures and pressures and very early in the diagenetic history of evaporite associated carbonate strata