analysis of spatial relationships among geomorphic petrographic and structural characteristics of the dartmoor tors the granite tors of dartmoor are characterized by using the regional spatial patterns of 21 geomorphic petrographic and structural variables the data were derived from topographic maps field measurement and laboratory analysis of rock samples collected in the field identification of spatial patterns and comparison of patterns between variables were made on contour maps generated from the data using arc info three tor types summit tors valleyside tors and spur tors are semiquantitatively characterized using these data and procedures the three types of tors are distinguished with respect to variations in relative relief joint spacing and joint type rock texture grain size and composition summit tors have the highest relative relief mean 125 7 m contain the most potassium feldspar 30 per cent and have the most widely spaced primary vertical 300 cm and secondary horizontal joints 10 cm the rocks are the most megacrystic 15 per cent and the coarsest grained plagioclase is also abundant in summit tors valleyside tors have the lowest relative relief mean 72 9 m and the most widely spaced horizontal joints primary 60 200 cm secondary 10 cm grain size vertical joint spacing and quartz and potassium feldspar abundances are intermediate the shapes of valleyside tors are controlled by horizontal joints and the rocks are either very feebly magcrystic or equigranular in texture spur tors have intermediate relative relief mean 115 4 m the narrowest joint spacing vertical joints 200 cm 50 75 cm horizontal joints 60 cm 10 cm and the finest grain in addition the rocks are feebly magacrystic

controls of tor formation cairngorm mountains scotland tors occur in many granitic landscapes and provide opportunities to better understand differential weathering we assess tor formation in the cairngorm mountains scotland by examining correlation

granite geomorphology and its geological controls serra da estrela portugal abstract serra da estrela is an elevated granite massif in central portugal characterized by extensive plateau surfaces incised by deep valleys affected by quaternary glaciation bounded by steep fault generated escarpments the presence of seven major textural variants of granite provides an opportunity to study the relationships between lithology and relief whereas dem analysis helped to show the relationships between lithology and topography objectively the higher ground is associated with fine to medium grained granites and is typified by planar surfaces of low gradient with occasional angular tors and rock pedestals block fields built by angular material are common in the parts that were not previously glaciated less elevated parts of the plateau are supported by medium to coarse grained granites and show more varied topography with an abundance of tors boulder piles and depressions lithological boundaries locally coincide with slope breaks but this is not the rule in the northern part of the massif a deep topographic basin has evolved in biotite granite whereas deeply incised valleys follow major fault lines geological controls show a hierarchy in that gross relief reflects the pattern of tectonic uplift and subsidence whereas lithology and then fracture patterns become more and more important if one focuses on smaller and smaller landforms

pervasive dolomitization with subsequent hydrothermal alteration in the clarke lake gas field middle devonian slave point formation british columbia canada the clarke lake gas field in british columbia canada is hosted in pervasively dolomitized middle devonian carbonates of the slave point formation the clarke lake field consists mostly of pervasive matrix dolomite and some saddle dolomite the latter varying in volume from about zero in limestones to normally 20 40 locally up to 80 in dolostones over any given 10 m 33 ft core interval some of the saddle dolomite is replacive some is cement and both varieties are associated with dissolution porosity and recrystallized matrix dolomite the major objective of this study is to identify the causes and timing of matrix and saddle dolomite formation specifically whether these dolomites are hydrothermal a comprehensive petrographic and geochemical examination indicates that pervasive matrix dolomitization was accomplished by long distance migration of halite saturated brines during the late devonian to mississippian fluid inclusion homogenization temperatures suggest about 150 uncorrected to 190c corrected at the time of matrix dolomitization these temperatures differ markedly from most published work on the dolomitized devonian reefs in the alberta basin south of the peace river arch where pervasive matrix dolomitization was accomplished by advection of slightly modified seawater at temperatures of about 60 80c and where no hydrothermal influence was ever present the saddle dolomites at clarke lake are not cogenetic with matrix dolomite and are not the product of hydrothermal dolomitization sensu stricto instead they formed through the hydrothermal alteration of matrix dolomite by way of invasion of a gypsum saturated brine during periods of extremely high heat flow and regional plate margin tectonics in the late devonian to mississippian fluid inclusion homogenization temperatures suggest that hydrothermal alteration occurred between 230 uncorrected and 267c corrected which is significantly higher than the maximum temperature of about 190c attained by the slave point formation during burial the sources of the halite and gypsum saturated brines are middle devonian evaporite depositional environments roughly 200 km 124 mi south and or east of clarke lake near the peace river arch

isotope geochemistry of fluid inclusions in permian halite with implications for the isotopic history of ocean water and the origin of saline formation waters abstract d and 18o values have been determined for fluid inclusions in 45 samples of permian halite the inclusions are enriched in 18o relative to the meteoric water line but are depleted in d relative to ocean water inclusions with the more positive values coincide with the isotopic composition expected for evaporating sea water which follows a hooked trajectory on a d 18o diagram inclusions with more negative values may represent more highly evaporated sea water but probably reflect synsedimentary or diagenetic mixing of meteoric water with evaporite brines the isotope systematics in these inclusions are sufficiently similar to those of a modern evaporite pan to indicate that permian sea water was isotopically similar to modern sea water connate evaporite brines can have negative values because of the probable hooked isotope trajectory of evaporating sea water and or synsedimentary mixing of evaporite brines with meteoric waters subsurface formation waters composed of mixtures of remnant primary evaporite brines and later meteoric waters may be more common than previous isotopic evidence has suggested

hydrocarbon evolution for a north south section of the south caspian basin abstract a 150 km length 6 second deep seismic line across the west central and north parts of the south caspian basin was used to construct quantitative dynamical thermal and hydrocarbon evolution patterns the depth of the west part of the 2 d section of the south caspian basin is almost 30 km the computer program geopetii was used to provide quantitative evolution models the procedure provided an opportunity to investigate the development dynamics of excess fluid pressure porosity retention rock fracturing compaction heat transfer maturity generation pressure kinetic hydrocarbon generation migration and accumulation together with solubility effects on hydrocarbon transport the results suggest that i temperature is 350 400 c in the deepest part of the section at a depth of 26 29 km ii the highest values of excess pressure nearly twice hydrostatic fluid pressure are in jurassic and cretaceous formations in the west part of the section which has now subsided to a depth of about 20 27 km iii major oil and gas generation began in the last 10 5 mybp the migration in free phase and in water solution occurring dominantly in the last few million years iv trapping of hydrocarbons took place mainly but not exclusively in the 3 9 km depth interval in the sands of the productive series of the pilocene embedded in a shale sequence v oil and gas filling of the shallow reservoirs by oil and gas is on going today indicating an extremely high productivity for any reservoir found in the offshore area vi there is overlap with depth of oil and gas reservoirs and the total amount of hydrocarbons estimated to be trapped is considerable vii the high overpressure expected makes for a drilling hazard but one which it is worthwhile to overcome if the anticipated oil and gas accumulations are encountered

multiple scattering of seismic waves in fractured media crosscorrelation as a probe of fracture intensity this paper presents the theory which describes how the lateral correlation of vertical displacement is influenced by multiple scattering of seismic waves caused by the presence of fluid filled fractures along with an observational methodology to show how to extract fracture related quantities e g fracture porosity degree of anisotropy of the fractures change in orientation of the fractures density of fluid filling the fractures from seismic data using the theoretical development as an underpinning framework a simple physical picture is given of the basic process as well as some rough order of magnitude estimates to show that the parameters quantifying the size of the multiple scattering fall squarely in the seismic regime

textural elemental and isotopic variations among constituents in middle devonian limestones north america abstract biogenic components brachiopods crinoids and corals in middle devonian limestones were manually separated from their enclosing matrix and analyzed by textural elemental and isotopic methods in order to assess the preservational state of these components in general brachiopods in these rocks are better preserved texturally than other constituents as determined by sem and cathodoluminescence observations elemental variations among the major components are compatible with a differential response to diagenesis governed by the relative stability of the original mineralogy of the component however among the most stable low mg calcite brachiopods the relationship between textural preservation and trace element content is complex this apparent complexity may be due at least par ially to our method of textural characterization of whole brachipod samples the nonluminescent portions of a set of well preserved brachipods have textural characteristics and elemental contents which are virtually indistinguishable from modern representatives consequently their 18o values 3 7 0 2 are assumed to represent primary biogenic calcite these 18o values are in the heavy end of the range of values from texturally well preserved whole brachiopods 6 6 to 2 5 texturally altered whole brachiopods 6 7 to 3 7 and texturally altered crinoids and cora s 6 7 to 2 7 this comparison suggests that marine derived pore waters were a predominant component during the principal phase of diagenesis the relatively low l8o values of well preserved brachiopods compared with modern representatives implies that middle devonian seawater was either warmer by about 10 c or depleted in 18o by about 2 3 relative to modern seawater

classification of dolomite rock textures abstract dolomite rock textures can be classified according to crystal size distribution and crystal boundary shape the classification scheme presented here is largely descriptive but carries genetic implications because size distribution is controlled by both nucleation and growth kinetics and crystal boundary shape is controlled by growth kinetics size distributions are classified as unimodal or polymodal crystal boundary shapes are classified as planar or nonplanar if the evidence permits a complete classification includes a description of recognizable allochems matrix and void filling allochems and preexisting cements may be unreplaced partially replaced replaced mimically or replaced nonmimically allochems may be dissolved leaving molds matrix can be unreplace partially replaced or replaced by a unimodal or polymodal size dolomite unimodal size distributions generally indicated a single nucleation event on a unimodal substrate polymodal sizes can be formed by multiple nucleation events on a unimodal or polymodal substrate or differential nucleation on an originally polymodal substrate planar crystal boundaries develop when crystals undergo faceted growth and nonplanar boundaries develop when crystals undergo nonfaceted growth nonplanar boundaries are characteristic of growth at elevated temperature 50 c and or high supersaturation both planar and nonplanar dolomite can form as a cement replacement of caco3 or neomorphism of a precursor dolomite

diagenesis of devonian reefal carbonates in the oscar range canning basin western australia abstract the oscar range part of the extensive devonian reef complex of the northern canning basin is a large atoll 80 10 km in which frasnian and famennian late devonian reefs grew around an emergent island of precambrian metasedimentary rocks many sediments in the oscar range had high depositional porosities but this porosity was largely occluded by diagenetic alteration diagenesis included marine cementation dolomitization pressure solution burial cementation and karstification field mapping petrography cathodoluminescence and carbon and oxygen isotope analyses have been used to document six major diagenetic episodes that affected oscar range sediments earliest cementation occurred in the marine environment where reef margin and associated subfacies developed abundant non luminescing radiaxial fibrous and microcrystalline calcite cements based on convergent covariant isotopic trends marine cements had a characteristic late devonian oxygen signature of 4 5 0 5 pdb and a carbon signature of 2 0 0 5 pdb in well cemented parts of the reef complex synsedimentary neptunian fissures formed and were filled with cement and sediment cements in these fissures have isotopic ratios that are essentially identical to the values determined for marine cements thereby suggesting that the fissures acted as conduits f r seawater flow and cementation early in the history of the reef complexes marine diagenesis was followed by marine burial and possible meteoric phreatic cementation and dolomitization marine cements are overgrown by non luminescing scalenohedral cements that grade into banded bright and non luminescing blocky calcite cements that are common on the platform but are missing in many reef margin and marginal slope deposits banded luminescence may represent fluctuating reducing and oxidizing conditions in pore waters across the intermittently emergent platform the reef complexes however were generally drowned reefs with little chance for prolonged meteoric exposure this sedimentologic evidence suggests that banded luminescing cements may have formed in the marine burial environment the next cement generation consists of non luminescing non ferroan bloc y calcites that occur throughout the reef complex stratigraphic relationships combined with isotopic results 18o 6 0 to 9 0 13c 2 0 suggest that this spar is a burial cement resulting from late devonian through early carboniferous burial the reef complex was exhumed and eroded in the late carboniferous dedolomite dolo moldic porosity pisolitic calcrete crusts iron oxide stained blocky cements and deep 200 m sinkholes formed at the unconformity surface non ferroan irregularly banded moderate to bright luminescing blocky calcite precipitated throughout the reef complex unconformity related cements calcrete crusts and dedolomites have distinctive isotopic signatures that show little variation in oxygen 7 0 to 9 0 but have a broad range of carbon values 1 5 to 8 0 highly negative carbon values are probably related to the interaction of meteoric fluids with isotopically light soil gas at the er sion surface in post carboniferous time most of the remaining porosity was filled with non ferroan bright luminescing blocky calcite that formed by pressure solution reprecipitation during burial isotopic analyses 18o 8 0 to 12 0 13c 1 5 support a burial origin for this generation of cements these cements fill primary porosity tension gashes along stylolites and hairline fractures along which hematite is leached a final generation of ferroan homogeneous dull to non luminescing blocky calcite cement formed in a late burial event that according to isotopic evidence 18o 10 5 to 15 0 13c 2 0 to 9 0 was probably related to movement of basinal brines through the reef complexes

the gas bearing devonian presqu ile dolomite of the cordova embayment region of british columbia canada dolomitization and the stratigraphic template white dolospar of the presqu ile dolomite forms the reservoir for many gas fields of northeastern british columbia within the keg river sulphur point and slave point formations along the shelf margin of the cordova embayment the embayment fill succession of the organic rich klua shale the otter park shale and marlstones equivalent to the sulphur point and slave point formations are aquicludes that blocked the upward and lateral flow of dolomitizing brines presqu ile dolomitization along the slave point shelf edge is interrupted in places where the klua shale extends shelfward underneath the slave point formation the watt mountain shale was also a regional aquiclude that prevented upward circulation of dolomitizing fluids in areas behind the slave point shelf edge flow of dolomitizing nacl cacl2 mgcl2 h2o brine solutions began in the shallow subsurface and precipitated dolomite cements type 1 at temperatures less than 50 degreesc possibly even during slave point deposition as northwestward directed shallow subsurface brine reflux from the elk point basin began this was followed by upward directed subsurface convective flow of nacl cacl2 mgcl2 h2o and nacl mgcl2 h2o brines and precipitation of dolomite cements at temperatures of about 130 degreesc during the late devonian carboniferous lower salinity brines that precipitated calcite cements and some dolomite type 2 may have interacted with connate fluids of marine origin or less likely with meteoric fluids modeling of organic maturation indicates that the muskwa shales above the slave point formation passed through the oil generation window in the permian and the onset of dry gas generation in the middle mesozoic postdolomitization calcite cements precipitated during these times contain abundant liquid hydrocarbon filled and methane filled inclusions high temperatures of close to 200 degreesc experienced by slave point strata in the mesozoic caused stretching but no leakage or refilling of fluid inclusions in all dolomite cements

construction of the silurian and devonian seawater 87sr 86sr curve the estimated path of seawater 87sr 86sr during the silurian and devonian is based on 86 analyses 84 whole rocks presented here and in other published results the seawater 87sr 86sr rose steadily throughout the silurian from a poorly constrained value near 0 70793 sw 114 at the ordovician silurian boundary the 87sr 86sr value at the silurian devonian boundary is near 0 70871 sw 36 based on an earliest devonian limestone suite from new york the 87sr 86sr falls through the early devonian to reach a low near 0 70772 sw 135 in the eifelian and givetian and rises to near 0 70803 sw 104 in famennian time the seawater 87sr 86sr at the devonian mississippian boundary is estimated to be near 0 70805 sw 102 the middle paleozoic sample suite is judged less suitable for defining seawater 87sr 86sr than those from other paleozoic periods the middle paleozoic seawater 87sr 86sr variation indicates domination of continental sr contributions during the silurian and later devonian the high ratio continental sr contribution is controlled by the climate and the area age and composition of exposed crystalline rocks the dominance of the oceanic low ratio sr during the early and part of middle devonian is coincident with a reported increase in continental volcanism the estimated rise and fall of sealevel leading to flooding or exposure of the continents does not appear to control seawater 87sr 86sr during the middle paleozoic

a test of petrological control in the development of bornhardts and koppies on the matopos batholith zimbabwe this paper investigates the reasons for the presence of both domed inselbergs bornhardts and boulder inselbergs koppies on the matopos batholith zimbabwe xrf and microprobe analyses of rock samples from 16 bornhardts and 24 koppies have shown no significant chemical or mineralogical differences which might indicate greater weathering resistance of the bornhardts many bornhardts are located on massive bodies of slowly cooled porphyritic granite which were emplaced at a relatively late stage in the evolution of the batholith the surrounding rocks are less porphyritic and appear to have been foliated during deformation associated with emplacement of the later bodies the entire batholith has been affected by post emplacement tectonism which has created three major fracture systems the morphological differences between bornhardts and koppies in this area are therefore considered to reflect differences in jointing which arise both from the primary igneous emplacement process and from tectonic deformation

in situ geochemistry of lower paleozoic dolomites in the northwestern tarim basin implications for the nature origin and evolution of diagenetic fluids lower paleozoic sedimentary rocks in the northwestern tarim basin were strongly altered by complicated geofluids which resulted in the occurrence of various diagenetic minerals e g dolomite here in situmajor trace and rare earth element geochemistry of lower ordovician diagenetic dolomite grains as well as petrography were performed to unravel the geochemical features the nature and origin of the diagenetic fluids the results indicate that different geochemical information can be detected within a single sample even within a single dolomite grain five generations of diagenetic dolomite have been identified based on geochemical signatures resulting from four distinct types of diagenetic fluids 1 hree enrichment paas normalized low rree no eu anomaly low mn ba moderate fe and high sr contents are probably due to early burial dolomitizing fluids 2 mree enrichment high rree high mn fe and low sr content are likely to be associated with devonian deep circulating crustal hydrothermal fluids 3 flat or lree enrichment pattern with obviously positive eu anomaly is inferred to be linked to permian magmatic hydrothermal fluids and 4 flat ree pattern moderate rree no eu anomaly low mn ba moderate fe and high sr contents are probably due to late burial dolomitizing fluids the significances of in situmethod demonstrated in this study compared with the whole rock analysis include not only contamination free analysis but also unraveling the internal geochemical variation within a single sample or a mineral grain thus for the geochemical study of complicated diagenetic process in situmethod should be preferentially considered

multistage dolomitization in rainbow buildups middle devonian keg river formation alberta canada abstract more than 80 partially to completely dolomitized buildups of middle devonian keg river formation occur in the rainbow sub basin of northwestern alberta in six buildups a b e f g and tehze four petrographic types of dolomites are identified gray fine crystalline dolomite floating dolomite rhombs patches matrix dolomite and saddle dolomite the petrographic and geochemical data indicate that these dolomites probably formed in three principle stages of dolomitization during progressive burial gray fine crystalline dolomite less than 1 of total dolomite by volume is interpreted as penecontemporaneously dolomitized lime muds by either normal marine waters or evaporitic brines during early exposure the following evidence suggests that this dolomite probably formed early 1 it usually occurs in fractures and breccias presumably related to the early exposure of the buildups 2 it is absent in rugs and molds that formed in the subsurface environment and 3 locally clasts of gray fine crystalline dolomite are embedded in marine limestones rhomb patch dolomite is the major type of dolomite in dolomitic limestones and is widespread throughout the limestone buildups matrix dolomite is fabric destructive and is most abundant in the dolostone buildups both these dolomites probably formed during shallow to intermediate burial because they 1 postdate stylolites 2 are widespread throughout the buildups and crosscut various facies and 3 have light oxygen isotope compositions and low sr but high fe and mn concentrations dolomitizing fluids for dolomite rhombs patches and matrix dolomites were possibly derived from 1 mechanical compaction 2 dehydration of the adjacent muskeg gypsum at relatively shallow burial depths and 3 intermediate depth basinal fluids conducted updip along porous and permeable conduits during d vonian and carly carboniferous times compaction fluids were probably responsible for the early dolomite rhombs and patches however the main phase of matrix dolomitization took place later probably during late mississippian to jurassic time when the east side of the basin was intermittently tilted and uplifted causing updip flow of intermediate depth basinal brines saddle dolomite occurs as late stage vug and fracture fillings it is most depleted in 18o but most enriched in mn and fe suggesting precipitation during deeper burial from warm basinal fluids these fluids were expelled during upper cretaceous sedimentary and tectonic loading of the western side of the basin and then moved up the east side of the basin along existing platform fault and fracture conduits the source of the mg is uncertain some or most of the mg may have resulted from chemical compaction of the earlier matrix dolomite

petrology and u pb geochronology of footwall porphyritic rhyolites from the wolverine volcanogenic massive sulfide deposit yukon canada implications for the genesis of massive sulfide deposits in continental margin environments porphyritic rhyolite sills form an important component of the footwall of the wolverine volcanogenic massive sulfide vms deposit yukon canada and occur proximal to mineralization in the immediate deposit area wolverine lynx zone and at similar stratigraphic levels along strike fisher puck and sable zones porphyritic rhyolites are of two types an older quartz feldspar porphyritic qfp rhyolite suite and a younger feldspar porphyritic fp suite both the qfp and fp suites of intrusions are semiconcordant suggesting a silllike morphology and are altered and crosscut by veinlet mineralization suggesting that they are pre to synmineralization the margins of qfp suite of intrusions contain minor xenoliths of surrounding shales and poorly developed chilled margins suggesting emplacement into partially consolidated sedimentary rocks whereas the fp suite of intrusions shows well developed chilled margins indicative of emplacement into fully solidified sedimentary rock these features suggest that the qfp suite of intrusions represents an older phase of rhyolitic magmatism whereas the fp suite represents a younger event this is supported by u pb zircon ages which indicate a 352 4 1 5 ma emplacement age for the fp suite and a 347 to 346 ma emplacement for the fp suite two ages at 347 8 1 3 and 346 0 2 2 ma both suites of porphyries have inherited proterozoic zircon and have ratios of la smucn 1 and nb thucn 1 ucn upper continental crust normalized indicating derivation from and or extensive interaction with ancient upper continental crustal materials the fp suite however has elevated high field strength element hfse and rare earth element ree contents high zircon saturation temperatures and higher nb ta ratios and lower ti sc ratios than the qfp suite these features are interpreted to reflect that the fp suite of magmas was hotter 900 c melts with a greater mantle component in their genesis both suites however are interpreted to have formed due to basaltic upwelling crustal melting and crust mantle mixing during ensialic back arc basin activity the presence of mantle heat within the wolverine basin from 352 to 347 to 346 ma a minimum of 5 m y suggests that sustained mantle heat flow was critical to the genesis of the wolverine porphyries it is also suggested that this sustained mantle heat was responsible for the wolverine hydrothermal system and that upwelling mantle may be essential in providing the heat to drive hydrothermal systems even in continental margin type vms environments e g bathurst iberian pyrite belt the wolverine porphyries are among the most hfse and ree enriched felsic rocks associated with vms mineralization globally the high hfse and ree concentrations in these rocks are interpreted to be due to

δ18o values 87sr86sr and sr mg ratios of late devonian abiotic marine calcite implications for the composition of ancient seawater abstract late devonian frasnian abiotic marine calcite has been microsampled and analyzed for 87 sr 86 sr ratios 18o and 13c values and minor element concentrations portions of marine cement crystals from the alberta and canning basins have escaped diagenetic alteration and preserve original marine 18o values 4 8 0 5 pdb 13c values 2 0 to 3 0 pdb 87 sr 86 sr ratios 0 70805 3 and sr mg weight ratios 0 04 to 0 05 marine 87 sr 86 sr ratios are globally consistent and can be correlated within the alberta basin and among the alberta canning and williston basins correlation of isotopic and chemical data strengthen the conclusion that marine cements from the leduc formation preserve original marine 18o values which are 3 to 4 lower than those of modern marine cements these low 18o values are best explained by precipitation from 18o depleted seawater and not by elevated seawater temperature or diagenetic alteration for comparison with devonian data analogous data were collected from holocene mg calcite and aragonite marine cements from enewetak atoll marshall islands mg calcite and aragonite marine cements are in isotopic equilibrium with ambient seawater and mg calcite cements are homogeneous with respect to sr and mg contents empirically derived homogeneous distribution coefficients for mg and sr in modern abiotic mg calcite from enewetak atoll are 0 034 and 0 15 respectively an equation describing the dependence of dsr on mg content was based on a compilation of sr and mg data from holocene abiotic marine calcite dsr 3 52 10 6 ppm mg 6 20 10 3 unlike that derived from experimental data this sr mg relation is consistent over a range of 4 to 20 mol mgco3 and may represent precipitation phenomena which are minimally controlled by kinetic effects comparison of sr and mg contents of analogous devonian and holocene marine cements suggests that the mg ca ratio of late devonian seawater was significantly lower and that the sr ca ratio was significantly higher than that of modern seawater

pine point orebodies and their relationship to the stratigraphy structure dolomitization and karstification of the middle devonian barrier complex mississippi valley type ore field on the south side of great slave lake pine point has produced 58 2 million tons of 3 0 percent pb and 6 7 percent zn with present reserves of 25 7 million tons of 2 7 percent pb and 6 3 percent zn most pine point orebodies were deposited in interconnected paleokarst networks that developed at the lower limits of coarse dolomite alteration of upper barrier limestones karstic structures which exhibit the largest cavity development and consequently the greatest amounts of infilling internal sediments host the best grades of mineralization the following conclusions are suggested the barrier developed due to subtle epeirogenic crustal movements and or eustatic sea level changes affecting normal sedimentary processes deformation of the barrier is possibly due to differential sediment compaction or warping of underlying evaporites coarse dolomitization and paleokarstification metals introduced by chloride rich brines brines expelled by overburden pressures migrated eastward along the barrier and precipitated metals in paleokarst structures upon encountering reduced sulfur species modified journal abstract

scattering and attenuation abstract an extensive existence of strong small scale heterogeneity in the earth is suggested by the great variability of direct waves and remarkable stability of scattered waves both in the case of teleseismic p coda and in the case of local earthquake s coda quantitative studies of small scoale heterogeneities have been made by the following three methods a lower bound of heterogeneity in the lithosphere has been estimated by the deterministic or stochastic modeling of travel time and amplitude fluctuation of teleseismic p waves across a large aperture seismic array an upper bound of heterogeneity has been estimated by modeling the attenuation of s waves as a scattering loss due to heterogeneity these two approaches suggest the existence of heterogeneity with rms velocity fluctuations from 3 to 10 per cent the third method is based on the estimation of heterogeneity from the amplitude of coda waves we found a significant discrepancy between the result obtained by the third method and those from the first two and attribute the discrepancy to the assumption of a single scattering model adopted in the coda generation theory the need for including multiple scattering as well as for considering vector elastic waves rather than scalar waves is emphasized

hydrothermal dolomitization of the bekhme formation upper cretaceous zagros basin kurdistan region of iraq record of oil migration and degradation abstract the common presence of oil seepages in dolostones is widespread in cretaceous carbonate successions of the kurdistan region of iraq this integrated field petrographic chemical stable c o and sr isotopes and fluid inclusion study aims to link dolomitization to the origin and geochemical evolution of fluids and oil migration in the upper cretaceous bekhme carbonates flux of hot basinal hydrothermal brines which is suggested to have occurred during the zagros orogeny resulted in dolomitization and cementation of vugs and fractures by coarse crystalline saddle dolomite equant calcite and anhydrite the saddle dolomite and host dolostones have similar stable isotopic composition and formed prior to oil migration from hot 81 115 c basinal nacl mgcl2 h2o brines with salinities of 18 22 wt nacl eq the equant calcite cement which surrounds and hence postdates saddle dolomite has precipitated during oil migration from cooler 60 110 c nacl cacl2 h2o brines 14 18 wt nacl eq the yellowish fluorescence color of oil inclusions in the equant calcite indicates that the oil had api gravity of 15 25 composition which is lighter than present day oil in the reservoirs api of 10 17 this difference in oil composition is attributed to oil degradation by the flux of meteoric water which is evidenced by the low 13c values 8 5 to 3 9 vpdb as well as by nil salinity and low temperature in fluid inclusions of late columnar calcite cement this study demonstrates that linking fluid flux history and related diagenesis to the tectonic evolution of the basin provides important clues to the timing of oil migration degradation and reservoir evolution

large scale fluid flow in the middle devonian presqu ile barrier western canada sedimentary basin saddle dolomite cements are regionally extensive in the middle devonian presqu9ile or keg river barrier and are spatially associated with mississippi valley type mineralization at pine point northwest territories from northeastern british columbia to pine point over a lateral distance of 400 km these dolomite cements show general trends of decreasing 87 sr 86 sr ratios 0 7106 to 0 7081 and homogenization temperatures 178 to 92 c with some increase in 18 o values 16 to 7 pdb these regional trends suggest that hotter and more radiogenic basinal fluids moved eastward updip along the presqu9ile barrier and mixed with cooler ambient formation waters these movements of basinal fluids were probably related to tectonic thrusting and compression sedimentary loading and tectonic uplift on the western margin of the western canada sedimentary basin which began to form during late jurassic to early cretaceous time and climaxed during the late cretaceous to paleocene the middle devonian presqu9ile barrier appears to have acted as a deeply buried regional conduit system that played an important role in focusing and channeling these basinal fluids these regional fluid flows appear to have been responsible for extensive burial dolomitization secondary migration of hydrocarbons and local mississippi valley type mineralization in the western canada sedimentary basin

spatial and temporal correlation between coda q 1 and seismicity and its physical mechanism from the analysis of local earthquake coda waves recorded by the benioff short period vertical seismograph at riverside california we found a surprisingly large and systematic temporal variation in coda q 1 in the southern california during the 55 year period from 1933 to 1987 comparison of the coda q 1 variation with the b value obtained for earthquakes with m 3 occurred in the area within 180 km from riverside corresponding to the area for which q 1 is estimated by the coda method led to an extraordinary result that coda q 1 correlates positively with b value with the correlation coefficient of 0 79 the significance of correlation cannot be rejected at the confidence level of 0 974 since the data used for measuring q 1 namely the coda part of seismograms and the data used for measuring b namely the seismicity catalog are totally independent the observed correlation supports strongly the physical reality of temporal change in both coda q 1 and b value the correlation between coda q 1 and b value has been reported for several other cases but the correlation was positive in some cases and negative in others our current preferred model for explaining the observed correlation is the following aseismic creep is going on from time to time over various sizes of cracks in the ductile part and in some cases in the brittle part of lithosphere at and near the plate boundary creep activities tend to increase the crack density and coda q 1 observed in the region if the creek occurs over distributed cracks with a certain predominant scale length spatial stress concentration may enhance seismicity for earthquakes with a similar size if this characteristic size is in the lower part of the magnitude range for which the b value is estimated the enhanced seismicity will increase b on the other hand if it is in the upper part of the magnitude range it will decrease b thus we can explain the observed strong correlation between coda q 1 and b value for both positive and negative cases the existence of such a characteristic crack size in a seismic region has been supported by various observations on earthquake scaling

formation of coarsely crystalline hydrothermal dolomite reservoirs in the presqu ile barrier western canada sedimentary basin late stage coarsely crystalline replacement dolomite and associated saddle dolomite cement form a widespread diagenetic facies in the middle devonian presqu ile barrier that extends southwestward from pine point to the subsurface of the foothills in northeastern british columbia these dolomites create hydrocarbon reservoirs in otherwise tight limestones in the subsurface of the northwest territories and northeastern british columbia and host mississippi valley type deposits at pine point the coarsely crystalline replacement dolomite and saddle dolomite cement are interpreted to have formed during burial because they replace blocky sparry calcite cements occur continuously across the sub watt mountain unconformity postdate stylolites and earlier replacement dolomites and overlap sulfide mineralization the dgr18o values of both replacement and saddle dolomites increase eastward updip along the presqu ile barrier from 16 pmil pdb in the deeper subsurface of northeastern british columbia to 7 pmil pdb at pine point whereas the corresponding homogenization temperatures of saddle dolomite fluid inclusions decrease from 178 to 92 c the 87sr 86sr ratios of coarsely crystalline and saddle dolomites decrease eastward along the pres u ile barrier from about 0 7106 in the subsurface of northeastern british columbia to 0 7081 at pine point these geochemical trends suggest a possible basin scale migration of hot radiogenic dolomitizing fluids updip eastward along the presqu ile barrier such large scale fluid movements probably were related to western canada sedimentary basin tectonic compression and sedimentary loading which occurred at least twice during early burial between the late devonian and early carboniferous and during deep burial between the late jurassic and early tertiary

quantifying the origin and geometry of circular sag structures in northern fort worth basin texas paleocave collapse pull apart fault systems or hydrothermal alteration three dimensional seismic data reveal numerous subcircular sag structures in the northern fort worth basin the structures are defined by concentric faults that extend vertically upward 760 1060 m 2500 3500 ft from the lower ordovician ellenburger group the largest structures remained active into the lower desmoinesian strawn group criteria are outlined for defining seismically resolvable sag structures and a detailed quantitative analysis of the geometries of these circular features was undertaken results are compared and contrasted with reviews of subsurface collapse mechanisms and strike slip processes that are known to produce subsurface circular to subcircular sag geometries in plan view in this manner we develop several constraints for differentiating collapse related sag structures from strike slip related sag structures qualitative analyses indicate that the geometries observed are strongly analogous to subsurface collapse features where material is removed at depth to create a void into which the overburden subsequently sags and collapses quantitative analyses support the formation of these features by incremental collapse and suprastratal deformation above a linked system of coalesced collapsed paleocaves within the ellenburger group observations and criteria presented herein provide valuable information in defining seismically resolvable collapse features worldwide and help distinguish sag features of collapse affinity from those of other origins

petrological influence on differential weathering and inselberg development in the kora area of central kenya in the kora area of central kenya domed inselbergs are well developed on outcrops of granitoid migmatite while positive relief features are rare on the surrounding gneiss block strewn vegetated hills occur on restricted areas of granoblastite gabbro and metagabbro schmidt hammer measurements have shown that the apparent differences in resistance to weathering and erosion are not due to variations in rock hardness since all the rock types have similar r values the results of geochemical analyses have shown that the migmatites are significantly more potassic than the surrounding gneiss samples of migmatite from the inselbergs were also found to be slightly richer in potassium than migmatite samples from the inter inselberg areas the variations in potassium content probably reflect differences in protolith composition chemical fractionation during partial melting and the effects of metasomatism these findings support earlier suggestions that other things being equal potassium rich granitoid rocks weather more slowly than less potassic rocks

origin of dissolution vugs caverns and breccias in the middle devonian presqu ile barrier host of pine point mississippi valley type deposits mississippi valley type lead zinc ore deposits at pine point occur in dissolution vugs caverns and breccias which previously were interpreted to be related to meteoric karstification during middle devonian sub watt mountain exposure studies of open pits at pine point and core samples from 34 wells from the subsurface presqu9ile barrier revealed that dissolution related to the sub watt mountain exposure was relatively minor and differed from the large scale hydrothermal dissolution that occurred prior to or was contemporaneous with the emplacement of mississippi valley type deposits dissolution vugs caverns and breccias related to the sub watt mountain exposure are 1 commonly filled in with green shale and locally with pendant calcite cements 2 preserved both in limestones and dolomites 3 restricted to the strata immediately beneath the unconformity and 4 formed prior to stylolitization dolomitization and mineralization diagnostic meteoric karst features such as rundkarren dolina calcrete terra rossa and speleothems are not observed in the presqu9ile barrier the limited evidence of meteoric water during sub watt mountain exposure is possibly due to an arid climate and relatively short periods of exposure in contrast hydrothermal dissolution vugs caverns and breccias that host mississippi valley type ore deposits at pine point are filled with laminated dolomites saddle dolomites sulfide minerals late stage calcite and pyrobitumen that are associated with hydrothermal fluids occur both above and below the watt mountain unconformity follow the regional conduit system that parallels the barrier postdate stylolites suggesting that dissolution must have occurred during burial and occur only in dolomites interpreted to have been formed from burial fluids fluid inclusion measurements indicate that the saddle dolomites precipitated from fluids between 92 degrees and 178 degrees c having salinities between 11 and 29 wt percent nacl equiv thus the majority of dissolution vugs caverns and breccias that host mississippi valley type ore deposits at pine point appear to be related to hydrothermal brines not meteoric fluids and formed before and or during late stage dolomitization and pb zn mineralization several mechanisms have been proposed to account for the hydrothermal dissolution including acid basinal fluids generated by thermochemical sulfate reduction diagenesis of organic matter and or cooling of co 2 rich brines during their updip movement all of which may have been active at pine point

hydrothermal dolomite a product of poor definition and imagination abstract the latest dolomite bandwagon is the hydrothermal dolomite model in its present form this bandwagon is doomed or at least very much overstated for at least two reasons 1 there are several definitions of hydrothermal and hardly any author specifies which one s he is using 2 very few of the dolomites hitherto called hydrothermal have been demonstrated to be hydrothermal according to any definition except the worst as presently applied the term hydrothermal dolomite is confusing and or meaningless we suggest to use white s geol soc amer bull 68 1957 1637 time honored definition of hydrothermal as aqueous solutions that are warm or hot relative to its surrounding environment with no genetic implications regarding the fluid source hence a dolomite should be called hydrothermal only if it can be demonstrated to have formed at a higher than ambient temperature regardless of fluid source or drive furthermore this definition does not carry a lower or upper temperature limit even a dolomite formed at 40 c could be hydrothermal by extension dolomites formed at temperatures lower than ambient are not hydrothermal even if they formed at a rather high temperature for example groundwater may penetrate a rock sequence through a highly permeable pathway such that it is heated to 150 c at a depth where the surrounding rock has a temperature of 250 c we suggest to call dolomite formed from this water hydrofrigid dolomite formed in or near thermal equilibrium with the surrounding rocks may be called geothermal furthermore not all saddle dolomite formation requires advection fluid flow to transport mg saddle dolomite can be formed in at least three ways i e from advection local redistribution of older dolomite during stylolitization and as a by product of thermochemical sulfate reduction in a closed or semi closed system only the first and the last of these three possibilities have a chance of being hydrothermal almost all dolomites and dolostones in the western canada sedimentary basin have recently been re interpreted as hydrothermal applying the rationale outlined above reveals however that this basin contains very little hydrothermal dolomite rather most dolomites in this basin and almost all dolostones south of the peace river arch are geothermal and or the proof of a hydrothermal origin has not been made this has important implications beyond the various case studies at hand as attempts to tie dolomitization to orogenic events become moot at least in the southern part of the basin

source of diagenetic fluids from fluid inclusion gas ratios abstract alteration of marine carbonates such as those from the mesoproterozoic and lower ordovician during burial history is carried out by diagenetic fluids that could potentially have different sources during precipitation diagenetic minerals may trap tiny amounts of those solutions in fluid inclusions thus providing fossil archives and aliquots that offer valuable record about the origin of fluids the analysis of various fluid inclusion gases e g ch 4 co 2 n 2 he and ar may provide reliable information which allows the discrimination among different fluid sources generally speaking the meteoric cements should have n 2 ar ratio around 38 the value for air saturated water however the gases in inclusions of the current study trapped by diagenetic carbonates cements from independent depositional basins of different ages mesoproterozoic early ordovician and modern and locations canada usa brazil and java sea have n 2 ar ratios 92 which match non magmatic modified marine or meteoric fluids also the ar he ratios 3 in the studied fluid inclusions are significantly lower than those of calc alkaline magmatic fluids 20 thus excluding magmatic fluids as a potential source on the other hand some n 2 ar and ar he ratios of the studied samples approach values reported for basinal fluids from the hansonburg mvt deposit in new mexico 68 and 1 respectively suggesting that both meteoric mixed marine meteoric evolved meteoric and basinal fluids may be involved during carbonate diagenesis

mid paleozoic initiation of the northern cordilleran marginal backarc basin geologic geochemical and neodymium isotope evidence from the oldest mafic magmatic rocks in the yukon tanana terrane finlayson lake district southeast yukon canada the fire lake formation of the yukon tanana terrane in the finlayson lake region yukon canada consists primarily of late devonian ca 365 360 ma mafic metavolcanic rocks and smaller volumes of mafic and ultramafic subvolcanic metamorphosed intrusions in this paper field geochemical and nd isotope attributes of these rocks are presented in an attempt to understand their tectonic setting the magmatic processes involved in their formation and their roles in cordil leran crustal growth the mafic rocks of the fire lake formation exhibit a wide diversity of geochemical signatures and are classified into seven chemically defined suites 1 back arc basin basalt 2 enriched mid oceanic ridge basalt e morb 3 oceanic island basalt oib 4 th rich oib 5 boninite 6 island arc tholeiite and 7 light rare earth element lree enriched island arc tholeiite the diversity of geochemical signatures is interpreted to represent variable mixtures of asthenospheric morb type mantle subarc mantle wedge and lithospheric oib type mantle with or without elemental contributions from the subducted slab and or continental crust these suites of rocks are also associated with fine grained basinal sedimentary facies variations in metavolcanic and metasedimentary unit thickness extensional synvolcanic faults and apparent extensional fault controlled emplacement of mafic intrusive rocks and hydrothermal volcanic hosted massive sulfide mineralization the suites also exhibit a broad spatial distribution those with arc signatures nb thmn 1 mn normalized to primitive mantle values are located primarily in the western parts of the formation and suites with nonarc signatures nb thmn 1 are located primarily within the eastern parts of the formation collectively these geologic and geochemical attributes are interpreted to stem from the transition from arc magmatism to the initiation of an extensional backarc basinal environment associated with an east dipping subduction zone the initiation of backarc basin magmatism recorded in the fire lake formation was part of a much larger late devonian backarc basinal system forming along the western edge of the margin of north america the fire lake formation is interpreted to represent 1 the commencement of yukon tanana arc rifting and separation from the north american cratonic margin and 2 the initiation of a marginal backarc basin now the slide mountain terrane inboard of the yukon tanana arc system this tectonic evolution likely occurred either as a result of slab rollback toward the west within the convergent margin responsible for yukon tanana arc activity or as a result of the propagation of the slide mountain backarc basin spreading ridges into the yukon tanana arc system this yukon tanana arc rifting episode was also broadly coincident with rifting and hydrothermal activity within rocks of the north american craton the geochemical and isotopic signatures of magmatic rocks in the fire lake formation have some features similar to intraoceanic arc rocks e g boninites island arc tholei ites and many have juvenile nd isotope signatures i e nd t 0 most have nd t 5 suggesting that the pericratonic terranes of the northern cordillera have a significant juvenile component if this is the case throughout the yukon tanana terrane then the pericratonic terranes may have contributed much more juvenile material to cordil leran crustal growth in the phanerozoic than has previously been considered

u pb zircon and geochemical evidence for bimodal mid paleozoic magmatism and syngenetic base metal mineralization in the yukon tanana terrane alaska new shrimp sensitive high resolution ion microprobe u pb zircon ages and trace element geochemical data for mafic and felsic metaigneous rocks of the pericratonic yukon tanana terrane in east central alaska help define the tectonic setting of mid paleo zoic magmatism and syngenetic hydrothermal zn pb ag mineralization along the ancient pacific margin of north america we compare data from similar greenschist facies sequences of bimodal volcanic and subvolcanic rocks associated with carbonaceous and siliciclastic marine sedimentary rocks in the wood river area of the alaska range and the salcha river area of the yukon tanana upland and from amphibolite facies augen gneiss and mafic gneiss amphibolite in the goodpaster river area of the upland allowing for analytical uncertainties igneous crystallization age ranges of 376 353 ma 378 346 ma and 374 358 ma are indicated by 13 new shrimp u pb dates for the wood river salcha river and goodpaster river areas respectively bimodal magmatism is indicated by late devonian crystallization ages for both augen gneiss 371 3 and 362 4 ma and associated orthoamphibolite 369 3 ma in the upland and by stratigraphic interleaving of mafic and felsic rocks in the alaska range metabasites in all three study areas have elevated hfse high field strength element and ree rare earth element contents indicative of generation in a within plate extensional tectonic setting within plate trace element signatures also are indicated for peralkaline metarhyolites that host the largest volcanogenic massive sulfide deposits of the bonnifield district in the wood river area and for metarhyolite tuff interlayered with the carbonaceous nasina assemblage which hosts sedimentary exhalative sulfide occurrences in the salcha river area most of the other felsic metaigneous samples from the alaska range and the yukon tanana upland have geochemical signatures that are similar to those of both average upper continental crust and continental margin arc rocks generated in thick continental crust given the absence in our study areas of intermediate composition magmatic products generally found in most arcs and the presence of bimodal magmatism the alkalic within plate chemistry of the mafic rocks and some of the felsic rocks and the widespread occurrence of interlayered carbonaceous sedimentary rocks indicative of deposition within a restricted marine basin or submerged continental margin we consider it most likely that this prolonged late devonian to early mississippian magmatic episode resulted from attenuation of the ancient continental margin of western north america rather than development of an arc as proposed by many others

multiple scattering of seismic waves in fractured media velocity and effective attenuation of the coherent components of p waves and s waves we show that the multiple scattering by small fractures of seismic waves with wavelengths long compared to the fracture size and fracture spacing is indistinguishable from multiple scattering effects produced by regular porosity except for an orientation factor due to fracture alignment the fractures reduce thep wave ands wave velocities and produce an effective attenuation of the coherent component of the seismic waves the attenuation corresponds to 1000 q of about unity for a gaussian spectrum of fractures and it varies with frequencyf asf3 for a kolmogorov spectrum of fractures of spectral index the attenuation is an order of magnitude or so larger and varies with frequency asf3 v the precise degree of attenuation depends upon the matrix properties the fracture porosity the degree of fracture anisotropy the type of fluid filling the fractures and the incidence angle of the wave

geothermal gradients hydrodynamics and hydrocarbon occurrences alberta canada using coal rank data and the demonstrated genetic link between the present hydrodynamic regime and geothermal gradient pattern it is possible to reconstruct the geothermal history of alberta over the past 60 m y palinspastic adjustment for tectonic compression caused by the major laramide thrusting shows that the predeformation isoreflectance lines increased logarithmically with depth in the late paleocene the geothermal gradient was about 23 c km 1 25 f 100 ft in the eastern alberta plains compared to about 30 c km 1 65 f 100 ft in the western alberta plains a regional trend opposite to the post laramide trends reconstruction of the early eocene surface indicates western uplands with geothermal gradients as low as 21 c km 1 15 f 00 ft and eastern lowlands with geothermal gradients of 27 c km 1 5 f 100 ft compared to the present situation this represents an enhanced topographic surface and a subdued geothermal gradient pattern the genetic relations of topography water table elevation hydrodynamic regime and geothermal gradient pattern in both the early eocene and the present conform to a model developed for any compacted sedimentary basin with subaerial relief in this model on a regional scale high topographic areas have high water table elevations with correspondingly high potentiometric surfaces these areas control the regional recharge of cold meteoric water and hence have low geothermal gradients areas of medium elevation exhibit regional lateral flow and intermediate geothermal gradients regional topographic lows correspond with low potentiometric surfaces and the regional discharge of warm formation waters from deep in the basin results in high geothermal gradients local topographic featur s cause perturbations in this picture study of both the regional pattern and fine detail of the early eocene and present topographic surfaces and geothermal gradient patterns shows the validity of the model the result of the second laramide orogenic pulse was therefore to reverse the geothermal gradient pattern resulting mainly from compaction flow and to impose a topographically controlled hydrodynamic regime with flow in the same direction but with a 9 c km 0 5 f 100 ft decreased geothermal gradient in western alberta and a 4 c km 0 2 f 100 ft increased geothermal gradient in eastern alberta as erosion proceeded during the tertiary the entire alberta basin warmed up until the present temperature regime was attained study of some major hydrocarbon occurrences shows a genetic link between the hydrocarbon occurrence and the present fluid regime which itself is controlled by the present topography for at least the significant upper cretaceous shallow gas reserves in southeastern alberta however their position may already have been determined as early as the beginning of the eocene by the topographic surface at that time therefore the extent to which the present topographic surface is responsible for hydrocarbon accumulation through gravity induced cross formational flow remains unknown because this process was operative as long ago as the early eocene what does not ap ear to be in doubt is the fact that gravity induced cross formational flow does control the hydrodynamic regime which in turn influences the geothermal gradient pattern and the accumulation of hydrocarbons

mid to late paleozoic k feldspar augen granitoids of the yukon tanana terrane yukon canada implications for crustal growth and tectonic evolution of the northern cordillera potassic feldspar bearing augen granitoids are a fundamental component of the architecture of the yukon tanana terrane and the ancient pacific margin of the northern cordillera these augen granitoids form a belt that extends from alaska to southeast yukon territory vary in age and provide probes of the crustal evolution and tectonic history of the yukon tanana terrane and ancient pacific margin of north america in the paleozoic we present results of an integrated field mapping geochemical sm nd tracer isotopic and u pb zircon geochronologic study of the augen granitoids in the stewart river area in an attempt to understand their role in the crustal evolution and tectonic history of the yukon tanana terrane and ancient pacific margin of north america augen granitoids of the stewart river area are of three distinct ages late devonian early mississippian and permian u pb zircon geochronology of these augen granitoids has yielded ages of 362 1 2 7 ma stewart river augen granite 347 5 0 7 ma mount burnham augen granite and 264 8 3 7 ma wounded moose augen granite all of the augen granitoids regardless of age have negative ndt values 2 0 to 15 3 and proterozoic archean depleted mantle model ages t dm 1 37 2 56 ga these geochemical and isotopic attributes coupled with the presence of inherited zircon with precambrian ages suggestthatthesegranitoidsaretheproductof crustal melting and crust mantle mixing during three different cycles of arc magmatism in the paleozoic furthermore these granitoids represent net crustal recycling along the ancient pacific margin of north america in the paleozoic importantly however there are minor secular variations in crustal recycling and the younger permian augen granitoids exhibit higher ndt nb ta v yb and sc yb consistent with a greater juvenile component in their genesis this juvenile component is probably due to assimilation of underplated mafic material derived from older early mississippian yukon tanana terrane arc magmatism and or a greater mantle component due to enhanced infiltration of underplated mafic material into augen granitoid magma chambers through rheologically weak crust associated with permian subduction the older late devonian and early mississippian augen granitoid suites represent two pulses of yukon tanana terrane arc magmatic activity that developed in response to east dipping subduction along the western edge of the north america craton in the mid paleozoic this east dipping yukon tanana terrane arc system continued to evolve throughout the mississippian to early permian and was coincident with the development of the slide mountain backarc basin that formed between the yukon tanana terrane arc system and the north american craton this east dipping arc backarc system continued until ca 275 ma after ca 275 ma the east dipping arc and backarc magmatism ceased and was replaced by ca 270 269 ma high pressure metamorphism and the establishment of a new subduction zone that formed in response to the closure of the slide mountain backarc basin the permian augen granitoids from the stewart river are the magmatic record of this new west dipping subduction zone although there are subtle variations the petrogenetic and tectonic histories of the three suites of augen granitoids in the stewart river area are remarkably similar and attest to the constancy of magmatic and tectonic processes that occurred along the ancient pacific margin of north america in the paleozoic

multiphase partial and selective dolomitization of carnian reef limestone transdanubian range hungary partially dolomitized carbonate successions provide a good opportunity to understand the commonly multistage process of dolomitization petrographic methods fluid inclusion microthermometry and stable isotope measurements were applied to reconstruct the diagenetic evolution and dolomitization of a partially dolomitized carnian reef limestone from the transdanubian range hungary the diagenetic history began with reef diagenesis and formation of dolomite micro aggregates in microbial fabric elements this was followed by the development of euhedral porphyrotopic dolomite crystals through overgrowths around the previously formed dolomite micro aggregates during the earliest burial stage increasing burial resulted in the extension of the dolomite patches via formation of finely crystalline replacement dolomite from the late norian when the carnian reef carbonates reached the depth of 1 0 to 1 8 km the diagenetic evolution continued in an intermediate to deep burial setting contemporaneously an extensional regime was established leading to fracturing the progressive burial resulted in the recrystallization of the pre existing dolomite with increasing temperature while saddle dolomite cement was precipitated in fractures in connection with the alpine orogeny intense denudation took place during the late cretaceous accompanied by fracturing similar tectonically controlled denudation and fracturing occurred in several stages during the cenozoic as a result of these processes the studied carnian carbonates were raised to a near surface position or became subaerially exposed leading to dedolomitization of the last dolomite phase and precipitation of calcite cement in cavities and fractures this study revealed that by investigating partially and selectively dolomitized rock types it is possible to document and understand those stages of the multiple dolomitization process which can barely be detected in the completely dolomitized rock bodies recognition of the dolomitization phases could provide the basis for the analysis of their relations with the depositional diagenetic and tectonic processes and stages of basin evolution

fate of reflux brines in carbonate platforms active reflux may occur during periods of platform top brine generation but the role and fate of these brines after reflux events are uncertain we have used a numerical flow model to investigate and quantify the response of reflux brines to changes in platform top salinity simulations suggest that reflux brines originally concentrated to gypsum saturation 150 have a relatively long platform residence time on the order of 100 times the duration of the reflux event when brine generating conditions cease brines will continue to sink through the platform entraining seawater a variant of reflux circulation we term latent reflux mesosaline brines intercepted by drilling of carbonate margins by the ocean drilling program may have originated from pleistocene reflux event s on the adjacent platform top and be currently moving by latent reflux latent reflux circulation could deliver a significant quantity of dissolved reactants to platform carbonates including mg for dolomitization

age constraints on the devonian shale hosted zn pb ba deposits gataga district northeastern british columbia canada sedimentary exhalative barite and barite sulfide deposits occur in upper devonian basinal clastic sedimentary rocks of the earn group of the gataga district northern canadian cordillera the deposits formed in a stratigraphic sequence from which conodont species and assemblages range in age from the upper triangularis zone to the lower praesulcata zone of the early to late famennian a time span of no more than 7 m y at the driftpile deposit which is the best constrained example conodonts bracket at least two distinct mineralization events during the middle famennian the older event is the lower upper marginifera zone or north trench zone and east zone and the younger event is the uppermost marginifera trachytera zone or main zone the other gataga deposits although somewhat less well defined are roughly of these ages or somewhat younger one barite pyrite unit at cirque is late famennian postera and expansa zones the youngest unit so far recognized in the gataga district exhalation of metalliferous brines into the gataga basin was of relatively short duration and episodic and the sites of active venting probably migrated along extensional faults through time this data allow temporal correlation of these sedimentary exhalative deposits with other devonian mississippian mineralization events on the north american margin such as other sedimentary exhalative sulfide deposits and volcanogenic sulfide deposits located on the outermost western margin of the miogeocline the sedimentary exhalative deposits of the kechika trough formed during a brief geologic episode within a much longer lived time span of are and back arc rifting and sea floor hydrothermal mineralization

glacial modification of granite tors in the cairngorms scotland a range of evidence indicates that many granite tors in the cairngorms have been mod ified by the flow of glacier ice during the pleistocene comparisons with sw england and the use of a space time transformation across 38 tor groups in the cairngorms allow a model to be developed for progressive glacial modification tors with deeply etched surfaces and no or limited block removal imply an absence of significant glacial modification the removal of superstructure and blocks locally forming boulder trains and the progressive reduction of tors to stumps and basal slabs repre sent the more advanced stages of modification recognition of some slabs as tor stumps from which glacial erosion has removed all superstructure allows the original distribution of tors to be recon structed for large areas of the cairngorms unmodified tors require covers of non erosive cold based ice during all of the cold stages of the middle and late pleistocene deformation beneath cold based glacier ice is capable of the removal of blocks but advanced glacial modification requires former wet based glacier ice the depth of glacial erosion at former tor sites remains limited largely to the partial or total elimination of the upstanding tor form cosmogenic nuclide exposure ages phillips et al 2006 together with data from weathering pit depths hall and phillips 2006 from the surfaces of tors and large erratic blocks require that the glacial entrainment of blocks from tors occurred in marine isotope stages mis 4 2 6 and probably at least one earlier phase the occurrence of gla cially modified tors on or close to the main summits of the cairngorms requires full ice cover over the mountains during these stages evidence from the cairngorms indicates that tor morphology can be regarded as an important indicator of former ice cover in many formerly glaciated areas particularly where other evidence of ice cover is sparse recognition of the glacial modification of tors is impor tant for debates about the former existence of nunataks and refugia copyright 2006 john wiley sons ltd

the glaciation of dartmoor the southernmost independent pleistocene ice cap in the british isles the granite uplands of dartmoor have traditionally been considered to be relict permafrost and periglacial landscapes that lay beyond the limits of quaternary glaciations but a variety of landform evidence indicates that a plateau icefield existed on the northern part of the moor constituting the southernmost independent ice cap in the british isles overdeepened or weakly u shaped valley segments fringing north dartmoor document an early extensive phase of glaciation but the most convincing landform evidence relates to more recent valley based glacier occupancy a moraine ridge on the slipper stones represents the most unequivocal palaeo glacier on north dartmoor with a palaeo ela of c 460 m above sea level asl although this relates to the youngest and most restricted phase of glaciation a longer term ela is likely to be represented by the corn ridge proto cirque at 370 410 m asl more extensive valley glaciers are recorded in each of the major drainage basins of north dartmoor by arcuate and linear bouldery ridges and hummocky valley floor drift which are interpreted as latero frontal moraines deposited by outlet lobes of a plateau icefield recession of these lobes is marked by inset sequences of such ridges and occasional meltwater channels plateau ice was predominantly thin and protective and snowblow and preferential accumulation in valley heads facilitated the modest glacial erosion and debris transport recorded in the landforms and sediments it is proposed that the highest plateaux have been occupied by ice for the longest cumulative period of time throughout the quaternary average glacial conditions explaining the distribution of different tor types on northern dartmoor this also explains the lack of tors on the most expansive of the highest plateau terrain ice dispersal centres as the product of a average glacial conditions preferentially removing tors or dampening their production rates b the survival of high relief type 1 tors during glaciation if they occupy summits too narrow to develop significant plateau icefields and or ridges that are bypassed by faster moving ice in adjacent deep valleys and c the survival of subdued type 2 tors in areas glaciated less regularly during the quaternary simple ice flow modelling indicates that a plateau icefield type glaciation is required for significant ice flow to occur and confirms thin ice cover in particular on narrow summits thereby supporting the explanation of tor class distribution the modelling allows us to spatially correlate the geomorphological evidence of margin positions into two major stages and further indicates a strong altitude mass balance feedback leading to an ice cap that is not in balance with its climate and with an extent that is limited by the length of the cold phases rather than their severity

the distribution of inselbergs and their relationship to geomorphological structural and lithological controls in southern zimbabwe abstract the gneissic granitic basement of southern zimbabwe is characterized by a wide variety of inselbergs preferentially developed on the granitoid rocks however there are significant differences in the frequency of inselbergs and most inselbergs occur in the younger granitoid rocks particularly in rocks with microcline as the dominant k feldspar but their distribution is not uniform completely isolated inselbergs are rare and most inselbergs are arranged in clusters or are associated with plateaus or they occur as the detached ends of spurs and ridges chemical and mineralogical investigations indicate significant differences in the composition of the older and younger granitoid rocks the latter are characterized by a higher content of alkali feldspars and a lower content of plagioclase and mafic minerals the influences of texture fabric and fracture characteristics on the resistance of the rocks vary in the southern part of the study area the majority of rock domes is associated with porphyritic granites and fine grained granites whilst inselbergs of the large batholith complexes in the northern part of the study area show virtually no relationships to the grain size of the rocks thin sections analysis indicate that the rocks of the terrains with a higher density of inselbergs are characterized by an intense intergrowth of minerals and a higher number of contacts between quartz and alkali feldspars particularly in the case of the porphyritic granites the influence of texture on the frequency of rock domes is linked with two factors these are a high content of microcline minerals that have filled the interstices of the surrounding mineral framework and enclosed the surrounding minerals overgrowths and replacement micro structures indicate a long lasting crystallization which appears to be responsible for a lower density of cooling fractures enclosure of the more vulnerable minerals in the more resistant microcline megacrysts retards the loosening of the rock constituents the marked differences in the lithological and structural characteristics of the various granitoid rocks influence the weathering behaviour of the rocks and illustrate that the divergent weathering characteristics of the rocks are strongly dependent on lithological and structural factors

a scattering model to explain seismic q observations in the lithosphere between 1 and 30 hz observations of lithospheric q for shear waves in the range 1 30 hz reported in the literature are examined using a scattering model consisting of randomly distributed spheres for the single scattering approximation i have assumed 1 q 1 q sub i gv omega where q sub i is the intrinsic q due to anelasticity v is the velocity of shear waves omega is angular frequency and g is the turbidity g is taken as a constant in this paper since g integral n a sigmada where n a da is the number of scattering spheres of radius a unit volume and sigma is the scattering cross section for a sphere this suggests that geometrical scattering dominates this will be true for ka 1 where k is wavenumber based on this criterion and the observation that q is large i e g small at a frequency of 0 05 hz a is of the order of magnitude 1 10 km fits of observations of shear wave q from japan and central asia indicate that q sub i 2000 500 for the lithosphere of both regions g 0 01 km for the japan observations and g 0 005 km sup 1 for central asia

rare earth element geochemistry of the woxi w sb au deposit hunan province south china the woxi w sb au deposit in hunan south china is hosted by proterozoic metasedimentary rocks a turbiditic sequence of slightly metamorphosed greenschist facies gray green and purplish red graywacke siltstone sandy slate and slate the mineralization occurs predominantly 70 as stratabound stratiform ore layers and subordinately as stringer stockworks the former consists of rhythmically interbedded banded to finely laminated stibnite scheelite quartz pyrite and silty clays whereas the latter occurs immediately beneath the stratabound ore layers and is characterized by numerous quartz pyrite gold scheelite stringer veins or veinlets that are typically either subparallel or subvertical to the overlying stratabound ore layers the deposit has been the subject of continued debate in regard to its genesis rare earth element geochemistry is used here to support a sedimentary exhalative sedex origin for the woxi deposit the ree signatures of the metasedimentary rocks and associated ores from the woxi w sb au deposit remained unchanged during post depositional processes and were mainly controlled by their provenance the original ore forming hydrothermal fluids as demonstrated by fluid inclusions in quartz from the banded ores are characterized by variable total ree concentrations 3 5 to 136 ppm marked lree enrichment lan ybn 28 248 lree hree 16 to 34 and no significant eu anomalies eu eu 0 83 to 1 18 they were most probably derived from evolved seawater that circulated in the clastic sediment pile and subsequently erupted on the seafloor the bulk banded ores are enriched in hree lan ybn 4 6 11 4 lree hree 3 to 14 and slightly depleted in eu eu eu 0 63 to 1 14 relative to their parent fluids this is interpreted as indicating the influence of seawater rather than a crystallographic control on ree content of the ores within a single ore layer the degree of hree enrichment tends to increase upward while the total ree concentrations decrease reflecting greater influence and dilution of seawater there is a broad similarity in chondrite normalized ree patterns and the amount of ree fractionation of the bandedoresinthisstudyandexhalitesfromothersedex typepolymetallicoredeposits suggestingasimilargenesisforthesedeposits

neodymium isotope geochemistry of felsic volcanic and intrusive rocks from the yukon tanana terrane in the finlayson lake region yukon canada devonian mississippian felsic rocks from the finlayson lake region have variable geochemical and nd isotopic characteristics that provide insights into the tectonic and metallogenic evolution of the yukon tanana terrane ytt and the northern cordillera late devonian 365 360 ma calc alkaline and tholeiitic arc felsic rocks in the mafic dominated fire lake unit yield end350 4 8 and 0 1 respectively and have 1 49 1 94 ga depleted mantle model ages tdm devonian mississippian 360 356 ma felsic volcanic kudz ze kayah unit wolverine succession and intrusive rocks grass lakes suite associated with volcanogenic massive sulphide vms deposits have end350 7 8 to 9 5 with tdm 1 59 2 25 ga a granitoid sample from the early mississippian 350 345 ma simpson range plutonic suite has end350 12 9 and tdm 2 01 ga similar to previously reported values for this suite the vms associated grass lakes suite of granitoids has higher high field strength element hfse and rare earth element ree contents and higher zr sc zr tio2 nb la and zr la values relative to the simpson range plutonic suite these geochemical features are similar to coeval vms associated felsic volcanic rocks in the kudz ze kayah unit the identification of similar hfse ree enriched felsic volcanic and subvolcanic intrusive rocks may aid in delineating prospective regions for vms mineralization in the ytt and other continental margin arc to back arc environments the geochemical and nd isotopic data for these ytt felsic rocks suggest that they reflect episodic mid paleozoic arc fire lake unit simpson range plutonic suite and back arc magmatism kudz ze kudz unit wolverine succession built upon a transitional basement with variable but significant influence from evolved proterozoic crustal materials

quantification of rock control in geomorphology abstract rock control the influence of rock properties on landform development has for long been a relatively under researched topic but numerous techniques have been developed especially by geotechnical engineers and stone conservationists that are now being applied to geomorphological issues by geomorphologists these techniques range in scale from the quantification of major discontinuities through to analysis of micro pores rock control studies that have taken place over the last few decades have contributed important insights into our understanding of such diverse phenomena as 1 river terraces bedrock channel forms and gradients pot hole development drainage density and gully badland development 2 slope forms and failures especially with respect to rock mass strength 3 the form of glacial troughs and of subglacial erosional landforms 4 weathering controls on rock durability and the association of weathering with porosity 5 classic karst features including various types of karren 6 sandstone landforms 7 inselbergs and tors and 8 cliffs and shore platforms

epigenetic dolomitization and the origin of xenotopic dolomite texture abstract xenotopic texture which is commonly observed in pre cenozoic rocks is defined here as a mosaic of anhedral crystals with irregular or curved intercrystalline boundaries and usually undulatory extinction xenotopic dolomite texture is similar in appearance to neomorphic limestone textures idiotopic dolomite texture euhedral to subhedral crystals with straight intercrystalline boundaries contrasts with xenotopic texture and is commonly observed in both cenozoic and more ancient dolomites texture may be controlled by the temperature at which crystals grow crystal growth theory predicts that at low temperature a smooth crystal surface is energetically favored and atoms are added to crystal faces layer by layer with dislocations acting as nucleation sites this results in faceted crystals and euhedral to subhedral crystal mosaics above a critical roughening temperature crt a rough surface is energetically favored surface nucleation does not require dislocations and atoms are randomly added to the crystal surface resulting in nonfaceted growth and an anhedral crystal mosaic it is hypothesized that a critical roughening temperature exists for dolomite above 50 c xenotopic dolomites are produced by dolomitization of limestone and or neomorphic recrystallization of dolomite at elevated temperature above the crt after burial idiotopic dolomites are produced below the crt by near surface processes and are occasionally produced above crt if growth inhibiting effects of the solution or impurities stabilize crystal faces calcite has a crt below 25 c and therefore develops anhedral crystal mosaics characteristic of neomorphic texture both at near surface and at elevated temperatures synthetic xenotopic dolomite was produced in the laboratory at 250 c and 300 c by dolomitization of aragonite and calcite skeletal fragments and by recrystallization neomorphism of nonstoichiometric cenozoic dolomites xenotopic dolomite resulted from the metamorphic recrystallization of the idiotopic hueco dolomite permian texas near the marble canyon intrusion at temperatures between 350 c and 600 c hydrothermal dolomitization of periclase calcite marble near the intrusion also resulted in a xenotopic texture xenotopic dolomite in the galena group ordovician wisconsin was produced by neomorphism of a preexisting dolomite during the emplacement of lead zinc sulfides at temperatures between 80 c and 227 c in the trenton limestone ordovician michig n xenotopic dolomite replaced limestone during the migration of hot 50 c fluids along fracture systems xenotopic dolomite was not observed in cenozoic dolomites that were subjected to temperatures near surface values only

the importance of siliceous radiolarian bearing mudstones in the formation of sediment hosted zn pb ba mineralization in the selwyn basin yukon canada sediment hosted massive sulfide shms mineralization occurred in three main episodes during the evolution of the selwyn basin yukon and northwest territories canada in the macmillan pass district tom and jason deposits the upper devonian strata of the earn group host the youngest least deformed examples of shms mineralization the earn group comprises gray to black homogeneous mudstones which have been deformed in the cordilleran fold and thrust belt are weathered and poorly exposed this makes stratigraphic correlations and the recognition of prospective units very challenging the host rock siliceous mudstone and mineralized intervals at the tom and jason zn pb ba deposits have been sampled from drill core major element chemistry and total organic carbon toc data are presented for the host rock samples from two drill holes tyk 5 n 18 76 17 n 19 all mudstones contain between 2 and 5 wt toc and 67 to 83 wt sio2 subtle alteration marked by potassium loss k2o al2o3 50 radiolarians by volume within the mineralized horizons and in barren time equivalent strata and therefore are a useful stratigraphic marker unit in otherwise homogeneous featureless fine grained mudstones the presence of highly siliceous carbonaceous mudstone units with preserved radiolarians suggests that in the late devonian there is a link between biological productivity in the water column the resultant sedimentation and the shms deposits these biosiliceous lithologies may be particularly favorable host rocks to shms mineralization for the following reasons 1 high porosities during early diagenesis preserve volume and enhance permeabilities in partially lithified sediments which are important parameters for subseafloor replacement mineralization 2 these units are rich in organic materials due to their deposition in productive settings and have a high capacity for the generation of sulfide via sulfate reduction therefore we suggest there is a link between times of high biological activity and productivity in oceans sediments derived from those processes and shms deposits

exploring for dolomitized slave point carbonates in northeastern british columbia in northeastern british columbia a number of significant gas fields have been discovered in dolomitized slave point carbonates located along the edge of the mid devonian barrier reef system reflection seismic techniques have been used extensively in exploring for these fields a common exploration practice has been to detect the reef edge on seismic data and then to drill on isochron thins and structural highs close to this edge the remaining reserves however are proving more difficult to find and this exploration technique does not always guarantee that the slave point formation is dolomitized at this location as with many other complex play types combining geologic concepts with geophysical exploration techniques can improve the explorationist s success ratio in this example a geologic model the dorag dolomitization model provides a possible explanation for the localization of the slave point dolomites in this model dolomitizing solutions are created by mixing fresh phreatic water with sal

fractal properties of fault systems by scattering of body seismic waves abstract one can obtain the fractal dimension of fault systems from the power frequency dependence of the scattering coefficient of x raying p or s waves this estimation may be based on the use of one of the two alternative models of media a medium with discrete random inclusions or a random continuous medium we have shown that if spectral densities of fluctuations in these models are identical the frequency dependencies of the scattering coefficients of the corresponding body waves are identical too in the weak scattering regime however fractal dimensions of a total surface of inclusions and isosurfaces in a random continuous turbulent medium are different the fractal dimension of the total surface of inclusions estimated by scattering coefficient coincides with the estimations of fractal dimensions of the fault systems in japan and the san andreas fault system we think that the fractal dimension of the total surface of faults in seismic active regions is 2 0 2 5 and in aseismic regions is about 3