an experimental study of the relationship between joint spacing and layer thickness abstract two methods for measuring joint spacing are described and compared the area method is a constant for a given outcrop area and is not affected by joint distribution within that area in contrast the line method depends on the location of the linear traverse two kinds of joint sets are distinguished on bedding surfaces 1 a poorly developed set represents the early stages of development when typical joint lengths are less than typical spacing 2 a well developed set represents later stages when lengths are much greater than spacing the area method is applicable to both poorly and well developed sets whereas the line method produces inconsistent results for poorly developed joint sets surface textures on many joints indicate point fracture origins and propagation parallel to bedding whereas most numerical model studies of spacing assume linear origins and propagation perpendicular to bedding the laboratory experiments described in this paper do not suffer from these restrictions an important concept confirmed during these experiments is fracture saturation when the applied strain reaches a certain value fracture spacing stops evolving and remains nearly constant these fracture sets are well developed spacing at saturation is a function of layer thickness but is independent of strain whereas spacing before saturation varies strongly with applied strain thus plotting spacing vs thickness and comparing the slopes of lines fit to such data for poorly developed joint sets in different layers is unlikely to be a diagnostic test for differences in material properties on the other hand spacing may be a sensitive indicator of strain for layers with poorly developed joint sets assessing fracture saturation is a lirst order consideration when gathering spacing data

a new approach for characterization and prediction of natural fracture occurrence in tight oil sandstones with intense anisotropy natural fractures provide the main path for fluid flow in tight oil reservoirs and can control the flow direction in the subsurface tight sandstones commonly have intense mechanical anisotropy which means that fracture development in such tight formations may vary widely with respect to fracture orientations however the prediction of the degree of fracture development for each orientation is challenging focusing on the tight sandstones of the chang 4 and 5 member in the jiyuan oil field ordos basin china a new approach was presented for better prediction of the tectonic fracture occurrence in different directions based on fracture characterization controlling factor and formation mechanism analysis first fracture types characteristics formation time and controlling factors were determined using data from outcrops cores and image logs then triaxial tests were conducted to measure the mechanical parameters of rock samples in different directions that assessed the mechanical anisotropy of the formation and its impact on the development of fracture networks in the basin next finite element numerical simulations of the paleotectonic stress field during fracturing were performed based on the fracture formation mechanism and controlling factors lithology bed thickness sedimentary microfacies and rock anisotropy finally according to the failure criteria established using the measured mechanical parameters the failure ratio and strain energy were calculated these criteria could be employed to predict the fracture occurrence and the degree of development of each fracture network the simulation predictions in this work are in good agreement with observed data from outcrops cores and image logs

deformation pattern in a massive ponded lava flow at odp iodp site 1256 a core and log approach 1 a complete in situ section of upper oceanic crust from extrusive lavas through dikes into gabbros has been recently drilled for the first time in 15 ma old crust created at the superfast spreading east pacific rise epr the upper igneous basement cored at ocean drilling program odp integrated ocean drilling program iodp holes 1256c and 1256d consists of sheet flows and massive flows including a ponded lava flow unit lava pond interpreted as a thick lava flow delivered off axis and accumulated in a topographic depression fine scale structural analysis shows that the lava pond is much less fractured than the deeper lavas probably as the effect of structure spacing that is larger than in the underlying lava flows in hole 1256d structures are mainly subhorizontal in the lava pond and become steeper downhole toward the sheeted dike complex this reflects the distance of the lava pond from the stress field produced by the intrusion of dikes at depth inducing mostly subvertical eruptive fractures and fracture network and the distance of the site from active rifting at the epr contrasting structural patterns in the lava pond and in the deeper parts of the drilled section are reflected in contrasting physical properties patterns as obtained by shipboard and downhole logging data electrical resistivity compressional velocity and bulk density are generally higher whereas natural radioactivity and neutron porosity are lower in the lava pond than in the deeper hole the occurrence of a thick massive lava flow at the top part of the volcanic section affects the deformation pattern of the entire drilled crust and consequently its permeability regime which in turn controls the geometry and distribution of hydrothermal circulation and basalt alteration

age and sr isotopic signature of the catoctin volcanic province implications for subcrustal mantle evolution variations in chemical and sr isotopic signatures of a series of basaltic flows have been correlated with newly mapped stratigraphic sections in the upper precambrian catoctin flood basalts in the central appalachians element mobility through fluid interaction is commonly observed in flows with a well developed fabric but some samples with more massive textures commonly in the cores of thicker flows still retain relict clinopyroxene and igneous textures and show only minor element redistribution five whole rock samples with relict igneous textures and a clinopyroxene mineral separate yield an isochron with an age of 570 36 ma 2sigma and a strontium initial ratio of 0 7035 1 this age is consistent with recent paleontological evidence for a latest precambrian time for catoctin volcanism and along with isotopic age determinations of 680 710 ma for other rift related magmatism suggests either a two stage or a very long protracted episode of continental rifting paleogeographic reconstruction indicates that the original site of eruption for the lavas must have been at least 56 km to the east in the same geographic location from which the mesozoic appalachian tholeiite terrane evolved variations in sr isotopic signatures and differences in major element abundances between these two sequences suggest mantle heterogeneitiesmore that must have developed beneath the eastern north american craton during the interim 400 m y less

the role of radiogenic heat production in the thermal evolution of a proterozoic granulite facies orogenic belt eastern ghats indian shield abstract the eastern ghats belt egb is a deeply eroded proterozoic orogenic belt juxtaposed against the archaean dharwar bastar singhbhum cratons in the indian shield the tectono metamorphic history of this belt is broadly similar to the grenvillian orogenic belts the egb exposes dominantly the granulite facies rocks such as charnockites khondalites and migmatitic gneisses with less abundant intermediate to mafic granulites the granulites are intruded by small plutons of granites syenites and anorthosites the northern and southern parts of the belt negb and segb have distinct crustal histories but 1600 1000 550 ma old tectono metamorphic events are common to both the belts in this research we attempt to understand the present day and past thermal state of this orogen on the basis of our radioelement k u and th measurements at 562 sites using in situ gamma ray spectrometry with other geological and geophysical constraints the dominant rock types of the negb and segb have similar radioelement abundances 3 k 3 ppm u and 30 ppm th and heat production heat production of charnockites gneisses khondalites and intermediate granulites of the negb is 2 9 2 8 2 9 and 1 1 w m 3 respectively in the segb they are 2 7 2 5 2 8 and 0 6 w m 3 respectively and the mafic granulites are the lowest in heat production 0 3 w m 3 on the basis of the heat production data and crustal petrologic models we show that the crustal contribution of the negb and segb is 54 and 45 mw m 2 respectively which are broadly in good agreement with the available surface heat flow data thermal models of the present day egb crust indicate that the temperature at the moho is 550 c radioelements and heat production of the eastern ghats granulites are higher than the other granulite belts in the indian shield the crustal radiogenic heat contribution and moho temperatures of the egb are higher than the adjoining archaean cratons this could also be responsible for the development of inverted metamorphic isograds in the tectonic boundary between the egb and the adjoining cratons during a collisional orogenic event at about 550 ma ago

holocene sedimentary sequence foraminifera and ostracoda in west coastal lowland of bohai sea china abstract the holocene sedimentary sequences in the west coastal lowland of the bohai sea consists in ascending order of pre transgression river and lake sediments transgressive tidal flat and estuarine sediments shelf sediments and prograding delta and river deposits the delta deposits are the thickest and are typically composed of two imbricating yellow river delta superlobes that acted at different periods there are three marker horizons in the sequence at 1 the bottom of marine sediments 2 the bottom of shelf deposits and 3 the boundary between two imbricating delta superlobes environmental changes were mainly controlled by a rising sea level prior to 6000 bp and by delta progradation after 6000 bp stratigraphic distribution of foraminifera and ostracoda vary according to variations in salinity and sedimentation rate there is no evidence that the yellow river delta developed during the holocene transgression but rather during a later prograding phase an understanding of the shelly ridges in the coastal lowland indicates that the ridges were formed at a similar level to the present and there is no evidence for holocene sea levels 1 2 m higher than the present sea level

closely spaced fractures in layered rocks initiation mechanism and propagation kinematics spacings of opening mode fractures joints and veins in layered sedimentary rocks often scale with the layer thickness field observations reveal that the ratio of fracture spacing to the thickness of the fractured layer s tf ranges from less than 0 1 to greater than 10 there is a critical spacing to layer thickness ratio that defines the condition of fracture saturation and explains the observed spacing ratios between 0 8 and 1 2 values of s tf 1 2 are explained as the results of the fracturing process having not reached the saturation level to explain ratios of s tf 0 8 we study the possibility for further fracture infilling by considering flaw distributions between adjacent fractures loaded by extension of the layer results show that infilling fractures grow more easily from flaws located near the interface than from those in the middle of the fractured layer the propagation of a flaw located in the middle of the fractured layer is unstable but for the flaw to propagate toward the interfaces its height has to be greater than a critical size this critical size decreases with increasing s tf the propagation behavior of a flaw with one of its tips at the interface depends on s tf the propagation is unstable when s tf is greater than a critical value when s tf is less than this critical value the propagation is first unstable then stable and then unstable again an infilling fracture can cut through the fractured layer only if s tf is greater than another critical value otherwise the infilling fracture can only partially cut the fractured layer for models with the same elastic constants for the fractured layer and the neighboring layers this critical value is 0 546 and the minimum spacing to layer thickness ratio of fractures formed by the in filling process under extension is 0 273

geochemistry of the late mesoproterozoic mount eve granite suite implications for late to post ottawan tectonics in the new jersey hudson highlands

700 ma rift event in the blue ridge province of virginia a unique time constraint on pre iapetan rifting of laurentia discovery of compositionally distinctive rhyolites produced from a peralkaline volcanic center within the late neoproterozoic robertson river batholith and which are intercalated with terrestrial deposits of the previously undated mechum river formation indicates that rift related sedimentation and magmatism were ongoing at 702 705 ma about 130 m y before the breakup of laurentia recognition of late neoproterozoic age rhyolites in central virginia indicates that rift related magmatism was more widespread along the incipient eastern present coordinates margin of laurentia than previously thought neoproterozoic rifting in the blue ridge province of virginia appears to have been part of a broad scale process that affected most if not all of the preserved margins of laurentia the association of similar extension related terrestrial deposits and compositionally distinctive felsic rocks elsewhere may provide a unique marker for testing proposed predrift configurations of the late neoproterozoic supercontinent rodinia

delta progradation and chenier formation in the huanghe yellow river delta china abstract to clarify holocene development of the huanghe yellow river delta and the relationship between delta progradation and chenier formation detailed sediment analyses and high resolution radiocarbon dating were done on borehole samples taken from two sites on the present huanghe h9601 latitude 37 40 5 n and longitude 118 28 7 e with an altitude of 5 5 m and h9602 lat 37 47 8 n and long 118 54 3 e 4 8 m downcore changes of sediment facies and accumulation rate show that delta progradation occurred at least twice during ca 2 6 1 2 c 14 yrbp and 1855 present at the two borehole sites these phases of progradation correlate with superlobes 6 and 7 and superlobe 10 respectively of ten superlobes composing the holocene huanghe delta shown by c xue historical changes in the yellow river delta china marine geology 113 321 329 1993 the boundary of these sediments at h9602 is sharp and erosional and correlates with cheniers located between borehole sites on the delta plain the diastem period between these phases of progradation almost coincides with the periods when the river mouth of the huanghe was located in the northwestern part of the bohai sea during 1048 1128 ad superlobe 8 and the jiangsu region facing the yellow sea during 1128 1855 superlobe 9 this suggests that chenier formation and delta progradation are linked and controlled by sediment supply and river course shifts of the lower reaches of the huanghe

geochronology of the mesoproterozoic state farm gneiss and associated neoproterozoic granitoids goochland terrane virginia the goochland terrane is an isolated block of mesoproterozoic crust in the piedmont province of central virginia we report u pb zircon dates and whole rock major and trace element data for the state farm gneiss one of the main units in the terrane and additional results for a newly recognized suite of neoproterozoic granitoids that intrude the gneiss the state farm gneiss ranges in bulk composition from quartz monzodiorite to granite and three samples yield u pb zircon dates of 1046 7 6 ma 1039 18 11 ma and 1023 10 ma we interpret these dates as igneous crystallization ages which indicate a maximum emplacement interval of ca 1057 1013 ma for these samples neoproterozoic granitoids including the fine creek mills and flat rock granites are more alkaline than the state farm gneiss and display all compositional characteristics of a type granites e g high fe mg ga ga al nb zn y zr u pb zircon analyses from five separate bodies indicate crystallization ages from ca 654 to 588 ma but all results are complicated by mesoproterozoic ca 1 ga inheritance coupled in some cases with secondary pb loss we interpret the ages and compositions of the younger rocks to reflect neoproterozoic rifting of the goochland terrane but its location during rifting is uncertain the terrane may have been separated from laurentia during the neoproterozoic breakup of rodinia and later reattached

three dimensional characterization of a fractured granite and transport properties a three dimensional fracture network in a granite block is reconstructed from a series of experimental serial sections it is visualized and its most important geometrical characteristics are studied namely the orientation of the fractures the connectivity of the fractures the number of cycles per unit volume the distributions of surface areas and of the intersection lengths and the number of finite solid blocks isolated in the solid matrix by the network though the network mostly consists of two families of fractures it is interesting to note that a simple model of randomly oriented monodisperse hexagons often yields a good order of magnitude for the various geometrical properties which have been measured on the real block the flow properties are then studied examples of velocity field are provided as well as histograms of velocities the permeability tensor is determined and is found to be in good agreement with snow s formula finally dispersion is analyzed by means of a random walk method histograms of arrival times are provided and interpreted in terms of dispersion tensor

the structural geometry and evolution of foreland thrust systems northern virginia seismic reflection data reveal that the structural geometry of the central appalachians of northern virginia consists of three distinct thrust systems each thrust system is characterized by a unique internal geometry the blue ridge thrust sheet is a composite thrust sheet composed primarily of imbricated precambrian crystalline rocks it over rode cambrian ordovician carbonates and formed a sheared basement cored fault bend fold thrusts within the sheet may be taconic and earliest alleghanian whereas final thrusting and emplacement of the sheet were probably slightly younger but still early alleghanian the north mountain thrust sheet is characterized by imbricated cambrian ordovician carbonates that are deformed into large amplitude mode ii fault bend folds and fault propagation folds rocks of this sheet were transported more than 60 km across a similar section of carbonates the leading edge of the north mountain thrust sheet was deformed into a fold with a mode ii fault bend fold geometry and was juxtaposed against middle paleozoic rocks the middle paleozoic rocks occur in a ramp across which displacement along the north mountain thrust was transferred to a higher detachment more than 60 km of cover rocks displaced during the emplacement of the north mountain thrust sheet either were transported across this ramp and thrust over a similar section in the western valley and ridge province or were backthrust above the sheet the timing for imbrication and emplacement of the north mountain thrust sheet is probably main phase alleghanian the lower carbonate duplex extends from beneath the blue ridge and north mountain thrust sheets westward across the western valley and ridge province the lower carbonate duplex is characterized by imbricated cambrian ordovician carbonates that form low amplitude fault bend folds this thrust system also probably formed during main phase alleghanian deformation

estimating average fracture spacing in subsurface rock knowledge of the spacing of fractures in reservoir rocks i e the distance between parallel fractures in a subsurface joint set can lead to a better understanding of the production characteristics of a reservoir and serve to quantify the relative degree of deformation in subsurface rocks in this paper i present a new method for estimating the spacing of subsurface fractures this new method is easy to use from the standpoint of both data collection and data analysis the average fracture spacing method can be applied with boreholes of any orientation relative to a fracture set the method is especially powerful when it is used for the relatively common case of a borehole nearly parallel to a fracture set e g vertical borehole intersecting vertical fractures average fracture spacing is estimated from an analytical solution based on observed borehole fracture intersections and observed fracture porosity the only data required are the dimensions of the core or imaged borehole and the total height of all sampled fractures because the likelihood of intersecting fractures increases when a well is deviated perpendicular to the fractures of a set fracture reservoirs commonly are candidates for deviated boreholes an informed decision on borehole deviation requires predicting the fracture intersection frequency as a function of both deviation magnitude and direction a new method based on probabilities of borehole fracture intersections uses spacing and height data from subsurface joint like fractures and the borehole diameter to predict fracture intersection frequencies for all possible well deviations fracture intersection frequency solutions are presented with respect to a conventional geographic reference frame thus simplifying even the most complex three dimentional situations

world wide delivery of river sediment to the oceans new data and new estimates from old data show that rivers with large sediment loads annual discharges greater than about 15 times 10 6 tons contribute about 7 times 10 9 tons of suspended sediment to the ocean yearly extrapolating available data for all drainage basins the total suspended sediment delivered by all rivers to the oceans is about 13 5 times 10 9 tons annually bedload and flood discharges may account for an additional 1 2 times 10 9 tons about 70 of this total is derived from southern asia and the larger islands in the pacific and indian oceans where sediment yields are much greater than for other drainage basins

cheniers in china an overview abstract in various regions along the east and southeast coast of china and on the outer shelf of the east china sea series of cheniers have been found and studied their geomorphological and sedimentological features have been described and the main conditions for their formation established the development of cheniers in china can be divided into three periods 24 000 15 000 yrs b p 7000 5000 yrs b p and 5000 yrs b p onwards these periods have been based on 14c datings of 80 shell samples the chenier groups of the three periods are distributed throughout various geomorphological locations using the geomorphological locations of the cheniers in combination with their age of formation the palaeopositions of the chinese coastline in the various periods since 24 000 yrs b p are described the history of the position of the east coast of china including the features attesting to sealevel changes during this era is briefly discussed

origin of mg metatholeiites of the schirmacher region east antarctica constraints from trace elements and nd sr isotopic systematics abstract the mafic granulites of schirmacher region east antarctica the rocks under study occur more or less as concordant sills or as lenses or as boudinage structures within the felsic rocks charnockites or metapelites of the region they show variation from garnet bearing two pyroxene granulites and garnet free pyroxene granulites to transitional amphibolite pyroxene granulites their major trace ree and isotopic chemistry are not distinct from each other and they represent mg basalts with mgo 7 and al 2 o 3 the majority of the analyzed samples plot in the tholeiitic field or show tholeiitic trends suggesting their metatholeiitic nature as well as general preservation of original composition the rocks are characterized by enriched large ion lithophile elemental concentrations than that of mid oceanic ridge basalts their high field strength elements and heavy rare earth elemental concentrations however are as that of mid oceanic ridge basalts a feature which is also reflected in the ratios of their large ion lithophile elements against high field strength elements and heavy rare earth elements wherein we find these ratios are higher than n type morb further the rocks show negative nb anomaly high th ta ratio and low la nb ratio which are also characteristics of subduction related magmatism the isotopic studies carried out on these samples show that the sm nd and rb sr dating did not yield much spread but suggested a sm nd metamorphic age of 960 ma rb sr dating gave ages 886 ma suggesting the reworking of the rb and sr elements during subsequent tectonothermal overprinting the nd model ages t dm nd of these rocks show a relatively restricted range of 1120 to 1357 ma suggesting mafic magmatism 1200 ma the positive end values 4 22 to 6 07 shown by these rocks represent a juvenile crustal fragment derived from melting of mantle precursors without significant reworking of older crustal material it is proposed that these rocks were produced by partial melting of a mantle source characterized by lile enrichment related principally to dehydration of subducted oceanic crust

provenance sea level and monsoon climate controls on silicate weathering of yellow river sediment in the northern okinawa trough during late last glaciation abstract high resolution multi proxy records including geochemical and grain size data from detrital sediments of iodp site u1429 in the northern okinawa trough provide reliable evidence for constraining sediment source and transport processes they also allow silicate weathering and erosion controlled by sea level change and east asian summer monsoon evolution since 34 ka to be reconstructed provenance proxies indicated that sediments in the northern okinawa trough were mainly supplied by the yellow river middle reach at 34 8 ka the low sea level and proximity of paleo yellow river mouth to the northern okinawa trough were the causes of the dominant yellow river input after 8 ka a retreated yellow river mouth coupled with the blocking effect of the kuroshio current and its branches together with strong east asian summer monsoon precipitation resulted in relatively strong sediment input from kyushu to the northern okinawa trough meanwhile yellow river upper reach supplied more sediments to the study site than that during the 34 8 ka on glacial interglacial scale silicate weathering proxies indicate that the core sediment was more weathered during the last glacial and deglacial than that in the modern yellow river we attribute this to the increased upper reach sediment input which supplied more weak weathered sediment to the study site during the holocene besides composite effect of weathering during glacial and deglacial sediment production and additional weathering upon subaerial exposure of shelf deposits during low sea level stage as well as older weathered sediments reworking during sea level rise could also account for this weathering regime on multi millennial scale from 34 to 18 5 ka silicate weathering was mainly controlled by the east asian summer monsoon with a cooling and drying climate associated with weakened summer monsoon activity this is consistent with the reduced alteration of sediments eroded from the yellow river basin from 18 5 to 8 ka strong reworking of older weathered sediments overwhelmed the east asian summer monsoon in controlling silicate weathering which induced a continuous increase in chemical alteration of the sediments during this period

origin of granulite terranes and the formation of the lowermost continental crust differences in composition and pressures of equilibration between exposed regional granulite terranes and suites of granulite xenoliths of crustal origin indicate that granulite terranes do not represent exhumed lowermost crust as had been thought but rather middle and lower middle crustal levels application of well calibrated barometers indicate that exposed granulites record equilibration pressures of 0 6 to 0 8 gigapascal 20 to 30 kilometers depth of burial whereas granulite xenoliths which also tend to be more mafic record pressures of at least 1 0 to 1 5 gigapascals 35 to 50 kilometers depth of burial thickening of the crust by the crystallization of mafic magmas at the crust mantle boundary may account for both the formation of regional granulite terranes at shallower depths and the formation of deep seated mafic crust represented by many xenolith suites

partial melting of apatite bearing charnockite granulite and diorite melt compositions restite mineralogy and petrologic implications melting experiments p 6 9 kbar t 850 950 c nno 02 hm were done on mafic to felsic charnockites a dioritic gneiss and a felsic garnet granulite all common rock types in the grenville basement of eastern north america a graphite bearing granulite gneiss did not melt water h2o 0 60 to 2 0 wt is bound in low grade retrograde metamorphic minerals and is consumed during the earliest stages of melting most melts are water undersaturated melt compositions range from metaluminous silicic granodiorite diorite starting composition to peraluminous or weakly metaluminous granites all others in general liquids become more feldspathic less silicic and less peraluminous and are enriched in feo mgo and tio2 with increasing temperature residual feldspar mineralogy controls the cao k2o and na2o contents of the partial melts and the behavior of these elements can be used particularly if the degree of source melting can be ascertained to infer some aspects of the feldspar mineralogy of the source k feldspar a common restite phase in the charnockite and granulite but not the diorite should control the behavior of ba and possibly eu in these systems and yield signatures of these elements that can distinguish source regions and in some cases bulk versus melt assimilation apatite a common restite phase is enriched in rare earth elements ree especially middle ree retention of apatite in the restite will result in steep light ree enriched patterns for melts derived from the diorite and charnockites

transformation induced jointing as a gauge for interfacial slip and rock strength various theoretical and numerical models have been proposed in order to explain joint formation and spacing in layered rock series however most of these models assume that the interfaces between the rock layers are perfectly welded i e no slip occurs and that all the layers are subjected to the same remote strain due to various processes e g tectonic processes other factors may also induce extensional strain in rocks e g phase transformations however such processes may induce different amounts of strain on the layers in a rock series leading to a strain mismatch between these layers in this paper we present a 1 d finite difference linear elastic model which allows joint formation within the middle layer in a three layer rock series and is induced by a strain mismatch between the fractured central layer and the surrounding matrix furthermore the central layer in our model is not necessarily welded to the matrix layers and is allowed to slip along the interfaces between these layers if the shear strength of the material at the interface is reached we find that the final fracture spacing to layer thickness ratio s tf in such layered systems is directly proportional to the ratio of the tensile and shear strength of the material changes in the material properties such as the shear modulus or young s modulus do not affect these results a natural analog of joint formation driven by phase transformations is found in the orthopyroxenite dykes of the leka ophiolite complex loc norway joint formation in orthopyroxenite dykes results from serpentinization driven expansion of the surrounding dunite matrix detailed field studies and measurements 583 sample points yield s tf ratios between 0 1 and 1 0 with a mean value of 0 45 0 20 we demonstrate that the strain mismatch driven joint formation associated with interfacial slip explains the low s tf ratios obtained from field measurements and may also help us constrain rock strength

geomorphic tectonic control of sediment discharge to the ocean the importance of small mountainous rivers analysis of data from 280 rivers discharging to the ocean indicates that sediment loads yields are a log linear function of basin area and maximum elevation of the river basin other factors controlling sediment discharge e g climate runoff appear to have secondary importance a notable exception is the influence of human activity climate and geology on the rivers draining southern asia and oceania sediment fluxes from small mountainous rivers many of which discharge directly onto active margins e g western south and north america and most high standing oceanic islands have been greatly underestimated in previous global sediment budgets perhaps by as much as a factor of three in contrast sediment fluxes to the ocean from large rivers nearly all of which discharge onto passive margins or marginal seas have been overestimated as some of the sediment load is subaerially sequestered in subsiding deltas before the proliferation of dam construction in the latter half of this century rivers

quantifying opening mode fracture spatial organization in horizontal wellbore image logs core and outcrop application to upper cretaceous frontier formation tight gas sandstones usa abstract the upper cretaceous frontier formation is a naturally fractured gas producing sandstone in wyoming regionally random and statistically more clustered than random patterns exist in the same upper to lower shoreface depositional facies east west and north south striking regional fractures sampled using image logs and cores from three horizontal wells exhibit clustered patterns whereas data collected from east west striking fractures in outcrop have patterns that are indistinguishable from random image log data analyzed with the correlation count method shows clusters 35 m wide and spaced 50 to 90 m apart as well as clusters up to 12 m wide with periodic inter cluster spacings a hierarchy of cluster sizes exists organization within clusters is likely fractal these rocks have markedly different structural and burial histories so regional differences in degree of clustering are unsurprising clustered patterns correspond to fractures having core quartz deposition contemporaneous with fracture opening circumstances that some models suggest might affect spacing patterns by interfering with fracture growth our results show that quantifying and identifying patterns as statistically more or less clustered than random delineates differences in fracture patterns that are not otherwise apparent but that may influence gas and water production and therefore may be economically important

partition coefficients of hf zr and ree between zircon apatite and liquid concentration ratios of hf zr and ree between zircon apatite and liquid were determined for three igneous compositions two andesites and a diorite the concentration ratios of these elements between zircon and corresponding liquid can approximate the partition coefficient although the concentration ratios between apatite and andesite groundmass can be considered as partition coefficients those for the apatite in the diorite may deviate from the partition coefficients the hree partition coefficients between zircon and liquid are very large 100 for er to 500 for lu and the hf partition coefficient is even larger the ree partition coefficients between apatite and liquid are convex upward and large d 10 100 whereas the hf and zr partition coefficients are less than 1 the large differences between partition coefficients of lu and hf for zircon liquid and for apatite liquid are confirmed these partition coefficients are useful for petrogenetic models involving zircon and apatite

origin of reservoir fractures in little knife field north dakota thin vertical planar fractures observed in the mission canyon formation at the little knife field are naturally occurring and appear to be extension fractures the predominant east west trend of the fractures measured in oriented core from six wells parallels the contemporary maximum horizontal compressive stress in the williston basin the fractures occur only in carbonate units but within the carbonates their occurrence is not lithology dependent fracture density measured in cores of the reservoir carbonates averages 1 ft 3 m of fracture per 2 3 ft 7 m of core the formation and mineralization of reservoir fractures were the most recent diagenetic events in the mission canyon formation at little knife study of aqueous and hydrocarbon fluid inclusions associated with the fractures reveals 1 fractures formed after the strata were buried to at least their present depth of 9 800 ft 2 987 m which indicates their age is post mesozoic 2 the pore fluid pressure gradient was normal hydrostatic immediately after if not during fracture system development 3 formation water salinity has remained fairly constant since fracture initiation 4 migration of hydrocarbons into the reservoir probably preceded fracture genesis and 5 methane concentration may have decreased since fracture initiation

chronology of holocene cheniers and oyster reefs on the coast of bohai bay china cheniers and oyster reefs are two essential components of holocene strata on the coast of bohai bay china the existing nonconventional14c dates often with unsuitable sample positions less tested samples and unreasonable data comparisons limit the refined analysis of the local chronostratigraphy on the basis of a number of pretreatment routines including geological investigations x ray diffraction analysis xrd 13c measurement of shells selection of appropriate shell species umboniumsp and terebridae for14c dating and determination of the local mean 13c value 2 68 pdb for the common shells a set of samples was radiocarbon dated by accelerator mass spectrometry ams these new ages obtained from the lower part of cheniers enable us to estimate the initiation of the cheniers and confirm that the existing nonconventional dates are often questionable due to unsuitable sample positions another two ams ages dated for two different microgrowth layers precipitated in a varying water body of the samecrassostrea gigasshell are statistically identical within 2 error this implies that the different water masses in the coastal environment would be rapidly in balance with the contemporaneous atmospheric co2 both marine93 and intercal93 were used for calibration of radiocarbon dates these amended the time frame of the local holocene history

a method for estimating subsurface fracture density in core quantitative information on fracture density may be used to delineate areas structures or lithologies that have a relatively high density of subsurface fractures a quantitative estimate of subsurface fracture density can be obtained from core data previously no unequivocal technique existed for estimating the degree of subsurface fracturing from oil well core data the spacing of joints at outcrops is commonly observed to be a direct linear function of the thickness of the jointed bed our method for estimating subsurface fracture density is a statistical estimate based on this empirical relation the probability of a given number of core fracture intersections is determined for a range of possible fracture densities after accounting for core diameter variations bed thickness geometric corrections necessitated by bedding dip and variations in fracture pitch the true fracture density is then estimated by finding the fracture density value that yields a calculated number of fractured beds equal to the observed number of fractured beds the technique was tested by measuring fracture density in the field at 2 outcrops hypothetical cores were constructed on photographs of the outcrops and estimates of fracture density from the core data yielded values in agreement with those obtained by field measurement the core data required for estimation of fracture density are core diameter bed thickness number of beds fractured and unfractured and the angles between core axis bedding and fractures the mathematics should be programmed on a computer for ease of use

combining outcrop data and three dimensional structural models to characterize fractured reservoirs an example from wyoming combining a detailed outcrop characterization of fracture and fault occurrence with attributes from a three dimensional model of an anticlinally folded clastic reservoir body we determine which characteristics of the structural form and evolution are most closely related to the development of important reservoir scale structures our example reservoir body studied is the frontier formation 1 sandstone in oil mountain an asymmetric anticline on the western flank of casper arch in central wyoming the three dimensional model of the structure was constructed using an iterative scheme designed to maximize interpretation accuracy and precision the model was analyzed to determine the spatial variance in morphologic and kinematic attributes using a quantitative testing approach we found that the intensity of tectonically produced fractures is closely related spatially to rate of dip change and total curvature with the former having the strongest correlation this folding is a low strain process compared to tear faulting which has the strongest spatial correlation to larger strains the location and magnitude of these higher strain areas can be adequately predicted by three dimensional restoration and forward modeling of the upper bounding surface of the reservoir body we use these results to build a predictive model for fault and fracture distribution at oil mountain and to discuss how this approach can aid in the exploitation of analogous producing reservoirs

accessory phase controls on the geochemistry of crustal melts and restites produced during water undersaturated partial melting the profound geochemical conseqences of accessory phase behaviour during partial melting of highgrade metapelites are demonstrated with reference to two geochemically distinct crustal melts produced by biotite dehydration melting reactions under granulite facies kbar 860 cc conditions these two leucogneiss suites from the brattstrand bluffs coastline eastern antarctica have similar field relations transport distances 10 100 s of metres and major element chemistry type 1 leucogneisses have low zr th and lree positive eu anomalies and zr zr and lreet lreet values less than 1 0 i e less than required to saturate the melt mass balance constraints suggest that these melts have been extracted before equilibration with host melanosomes the dry peraluminous nature of vapour undersaturated melts inhibits monazite and zircon solubility and results in concentration of these phases in the residue melts are consequently depleted in lree and hree melanosomes show complementary enrichment in lree while hree patterns are dominated by residual garnet type 2 leucogneisses in contrast have strongly enriched zr th and lree abundances negative eu anomalies and zr zr and lreet lreet 1 resulting from accessory phase entrainment vapour absent partial melting under moderate 6 8 kbar pressure granulite facies conditions of a pelitic source containing monazite is likely to give disequilibrium melts depleted in lree and hree as monazite and garnet are concentrated in the residue if temperatures are high enough 850 870 c to permit relatively large degrees of partial melting then the feldspar component of the source will be removed almost completely giving melts with large positive eu anomalies melts formed under vapour present conditions are unlikely to show such extreme lree and hree depletion or positive eu anomalies even at high degrees of partial melting disequilibrium melting coupled with source entrainment could fortuitously produce ree and trace element signatures similar to those typical of s type granites and usually ascribed to equilibrium melting conditions

fractures in sandstone reservoirs with ultra low permeability a case study of the upper triassic yanchang formation in the ordos basin china the upper triassic yanchang formation in the ordos basin central china is a typical sandstone reservoir with an ultra low permeability high angle tectonic fractures and diagenetic fractures such as near horizontal bedding fractures intragranular fractures and grain boundary fractures are abundant fractures are major pathways and enhance fluid flow in sandstone reservoirs with ultra low permeability because of their weak lateral continuity and their small apertures under lithostatic pressure bedding fractures make a relatively small contribution to the overall permeability of reservoirs as they are both of small size and low permeability intragranular fractures and grain boundary fractures mainly improve the connectivity of reservoirs by connecting the matrix pores high angle tectonic fractures control the fluid movement in the ultra low permeability reservoirs under the effect of different successive tectonic stress fields four assemblages of high angle tectonic fractures developed in the sandstone reservoirs under the present day stress differently oriented fractures have different connectivities apertures and permeabilities the northeast southwest fractures parallel to the maximum principal stress have a good connectivity large apertures and a high permeability forming the dominant flow paths knowledge of these paths can be used for optimizing well placement because of their sensitivity to pressure fractures in different directions will show varying apertures as the formation pressure decreases therefore the permeability of the fractures of different orientations and its impact on reservoirs vary at different developmental stages

extensional and contractional deformation in the blue ridge province virginia basement rocks of the blue ridge anticlinorium in central and northern virginia were involved in two temporally distinct deformation episodes the older event is characterized by a locally pervasive low strain fabric and high strain shear zones a preferred grain shape orientation in recovered quartz feldspar and biotite combined with a poorly developed symmetric quartz c axis pattern defines the low strain fabric kinematic indicators are typically symmetric but where asymmetric they consistently indicate top down to the southeast extensional motion discrete shear zones are within and close to late proterozoic plutons kinematic evidence fron these zones records large noncoaxial strains also with a down to the southeast extensional shear sense feldspar microstructures range from cataclastic fractures to complete recovery suggesting that deformation occurred at greenschist to amphibolite grade both low strain foliations and high strain shear zones are cut by paleozoic thrust related tectonites on the basis of kinematic evidence and spatial association with late proterozoic plutons cross cutting relations with thrust related structures and regional geologic relations we suggest that extensional deformation is related to late proterozoic rifting of the laurentian margin the kinematic histories of the rockfish valley fault zone and related shear zones appear to be relatively simple and consistent with paleozoic west to northwest directed thrusting the rockfish valley fault zone does not separate distinctly different basement terranes and thus its displacement is thought to be small mylonitic rocks from this fault zone display crystal plastic deformation in quartz and cataclastic deformation in feldspars under lower to middle greenschist facies conditions solution transfer processes were an important deformation mechanism in these tectonites thrust related mylonitic rocks are overprinted by related cataclasite and semibrittle shear bands suggestive of a transition toward more brittle deformation as the thrust system cut higher in the crust the rockfish valley fault zone appears to predate the formation of the south mountain cleavage in the valley and ridge province but it may be part of a lengthy progressive thrust related deformation

chemical diversity and origin of precambrian charnockitic rocks of the central pedlar massif grenvillian blue ridge terrane virginia abstract middle proterozoic rocks of the pedlar massif are comprised of varied charnockitic and granulitic lithologies representing 1150 1000 ma grenvillian metamorphic and igneous events in the central appalachians of virginia chemically and mineralogically diverse units include metaluminous to peraluminous charnockites enderbites jotunites and granulite gneisses ranging in sio2 from 47 to 79 wt relative to bulk continental crust typical charnockites are enriched by 3 6 in rb k ba lree zr and hf less enriched in ta sr p and hree and depleted in cs th ti and sc although low sio2 members have relatively high ti and p and some high sio2 units are depleted in k rb and ba the pedlar river charnockite suite prcs vesuvius megacrystic charnockite vmc nellysford granulite gneiss ngg and the lady slipper granulite gneiss lsgg exhibit common chemical signatures and trends notably evident in uniform lree enriched patterns with variable eu anomalies which reflect their derivation from a mixed protolith of volcanics and reworked volcaniclastic sediments dated at 1130 1150 ma pedlar river charnockites represent portions of deep seated granulite gneisses that were mobilized by granulite facies dehydration melting and emplaced en masse as plutons into overlying and surrounding gneisses during the grenville episode divergent ree patterns in the prcs are attributed to crystallization of quartz feldspar garnet or hree compatible mafic and accessory phases leading to complementary chemical signatures in mafic and felsic layers depletion of cs throughout the pedlar massif is attributed to its incompatibility with major mineral phases during either protolith evolution or granulite facies dehydration while dispersions in k rb sr ba and eu are due to the relative proportions of the major phases quartz k feldspar plagioclase and biotite mobility of insoluble trace elements zr hf ta and th require partial melting and crystallization of accessory mineral phases whereas sc cr and ree mobility depends on the proportions of pyroxene and garnet several leucocharnockites and other felsic rocks of the prcs show markedly elevated th values ranging from 10 to 83 ppm which complement depleted th in typical prcs units these charnockitic rocks represent low degree partial melts 1 5 from normal prcs or ngg protoliths to produce high silica magmas which separated from the main charnockitic body

petrology and isotopic composition of a grenvillian basement fragment in the northern appalachian orogen blair river inlier nova scotia canada mesoproterozoic metaplutonic rocks in northern cape breton island introduction nova scotia occur in a tectonic inlier within the appalachian in eastern north america mesoproterozoic laurentian orogen although they have been multiply metamorphosed and basement rocks and their neoproterozoic to mid paleovariably deformed the petrology and geochemistry of these rocks zoic volcanic and sedimentary cover preserve a record provide insight into the tectonomagmatic evolution of easternmost of the proterozoic to mid paleozoic geologic history of laurentia anorthosite syenite and granitoid plutons eastern laurentia the record includes mesoproterozoic 1100 980 ma intruded the sailor brook and polletts cove river orogeny on the ancient north american craton breakgneisses new nd isotopic data are presented from a biotite rich up of a neoproterozoic supercontinent development of part of the sailor brook gneiss ndi 0 7 two anorthosite a passive continental margin and mid paleozoic orogeny samples ndi 2 1 and 2 8 and a charnockite unit exposures of mesoproterozoic laurentian basement in ndi 0 4 new u pb zircon data from the anorthosite yield easternmost canada and the usa occur in a dispb pb ages between 975 and 1095 ma disrupted u pb continuous band of inliers in the appalachian orogen systematics preclude a unique age interpretation but the 1095 ma from newfoundland to alabama fig 1 they consist single grain date is a minimum age for the anorthosite field relations mostly of rocks formed or deformed during major and trace element geochemistry and isotopic characteristics 1190 980 ma grenvillian tectonic and magmatic indicate that the anorthosite and charnockite were probably coexisting events and are distinct from neoproterozoic and melts but not differentiates of a single parent magma the lithological paleozoic units of accreted appalachian terranes in that and chemical characteristics of the lowland brook syenite are they include granulite facies metamorphic rocks and similar to those of a contemporaneous belt of ultrapotassic plutons in many cases anorthosite charnockite mangerite suites in the southern grenville province and both have the chemical bartholomew 1984 and papers therein pettingill et al characteristics of certain modern continental margin arc magmas 1984 rankin et al 1989 1993 owen erdmer 1990 these data indicate that the mesoproterozoic units in the blair hughes et al 1997 river inlier were juvenile crustal additions to eastern laurentia the sinuous trend of the band of basement inliers during grenville related orogenic events broadly reflects promontories and re entrants that formed

age and origin of anorthosites charnockites and granulites in the central virginia blue ridge nd and sr isotopic evidence rb sr isotopic data for anorthosites charnockites ferrodioritic to quartz monzonitic plutons and high grade gneisses of the blue ridge of central virginia show evidence of post emplacement metamorphism but in some cases retain grenville ages the pedlar river charnockite suite yields an isochron age of 1021 36 ma initial 87sr 86sr ratio of 0 7047 6 which agrees with published u pb zircon ages five samples of that unit which contain paleozoic mylonitic fabrics define a regression line of 683 ma interpreted as a mixing line with no age significance samples of the roseland anorthosite complex show excessive scatter on a rb sr evolution diagram probably due to paleozoic 475 m y metamorphism data from the ferrodioritic to quartz monzonitic plutons of the area yield an age of 1009 26 ma inital ratio 0 7058 4 which is in the range of the u pb zircon ages of 1000 1100 ma the stage road layered gneiss yields an age of 1147 34 ma initial ratio of 0 7047 5 sm nd data for the pedlar river charnockite suite reflect a pre grenville age of 1489 118 ma nd 6 7 1 2 data for the roseland anorthosite complex and the ferrodioritic to quartz monzonitic plutons yield grenville isochron ages of 1045 44 ma nd 1 0 0 3 and 1027 101 ma nd 1 4 1 0 respectively two roseland anorthosite samples plot far above the isochron demonstrating the effects of post emplacement disturbance of sm nd systematics while mylonitized pedlar river charnockite suite samples show no evidence of sm nd redistribution the disparity of the sm nd age and other isotopic ages for the pedlar river charnockite suite probably reflects a sm nd source age suggesting the presence of an older crust within this portion of the ca 1 ga old basement

fractures generated during folding of the palmerton sandstone eastern pennsylvania a syncline of devonian palmerton sandstone exposed in a quarry in the fold thrust belt of eastern pennsylvania contains a set of fractures that are perpendicular to bedding and strike parallel to the fold axis fracture spacing is between 0 25 and 30 cm these fractures are zones of cataclasis that have striated shear surfaces displacement on fractures where measurable is less than 1 cm sparse kinematic evidence suggests that slip on fractures could have contributed to growth of the fold petrographic evidence shows that slip is in part localized along pre existing mineralized fractures which probably formed during burial lithification and diagenesis of the palmerton sandstone in an extensional structural setting the unusual shear fracture pattern illustrated by the palmerton sandstone may result from slip on these early fractures during thnist sheet displacement along a thrust ramp this exposure also illustrates the operation of contrasting deformation mechanisms in sandstone and in vertically ad

spatial arrangement of faults and opening mode fractures spatial arrangement is a fundamental characteristic of fracture arrays the pattern of fault and opening mode fracture positions in space defines structural heterogeneity and anisotropy in a rock volume governs how faults and fractures affect fluid flow and impacts our understanding of the initiation propagation and interactions during the formation of fracture patterns this special issue highlights recent progress with respect to characterizing and understanding the spatial arrangements of fault and fracture patterns providing examples over a wide range of scales and structural settings five papers describe new methods and improvements of existing techniques to quantify spatial arrangement one study unravels the time evolution of opening mode fracture spatial arrangement which are data needed to compare natural patterns with progressive fracture growth in kinematic and mechanical models three papers investigate the role of evolving diagenesis in localizing fractures by mechanical stratigraphy and nine discuss opening mode fracture spatial arrangement two papers show the relevance of complex cluster patterns to unconventional reservoirs through examples of fractures in tight gas sandstone horizontal wells and a study of fracture arrangement in shale four papers demonstrate the roles of folds in fracture localization and the development spatial patterns one paper models along fault friction and fluid pressure and their effects on fault related fracture arrangement contributions address deformation band patterns in carbonate rocks and fault size and arrangement above a detachment fault three papers describe fault and fracture arrangements in basement terrains and three document fracture patterns in shale this collection of papers points toward improvement in field methods continuing improvements in computer based data analysis and creation of synthetic fracture patterns and opportunities for further understanding fault and fracture attributes in the subsurface through coupled spatial size and pattern analysis

determination of subsurface distance between vertical parallel natural fractures based on core data i present a simplified method to estimate subsurface distance between vertical parallel fractures based on core analysis and the binomial theorem the geologic model used in this paper is identical to the one presented by narr and lerche in 1984 required input data include core diameter d thickness of each bed intercepted by the core t angle between core axis and fractures s and angle between core axis and bedding thgr the method is corroborated with the use of outcrop data previously published by narr and lerche in 1984 the approach is illustrated with a step by step example which easily can be reproduced with a hand held calculator by the practicing geologist

fracture density in the deep subsurface techniques with application to point arguello oil field because monterey formation reservoirs rely on fractures joints for permeability quantitative information on fracture spacing is important to exploration strategies and for understanding reservoir behavior density of joints in cores of four wells from the point arguello reservoir has been determined with a new probability based method and these subsurface joint densities are compared with joint densities measured at nearby outcrops my measure of joint density is the fracture spacing index which is the slope of the trend of layer thickness to joint spacing a single set of extension joints that strike parallel to bedding dip predominates at outcrops likewise the cores contain a single set of extension joints perpendicular to the anticline axis and parallel to present day maximum horizontal compressive stress in core and at outcrop the only lithologic control on joint density is between nonjointed mudstone and harder more brittle jointed rocks within each well the fracture spacing index is the same for all hard rocks though it varies between wells in the reservoir joint density relates to structural position the most densely jointed strata fracture spacing index 0 45 are from the hinge of a minor anticline where its plunge steepens a less steeply plunging anticlinal nose has lower joint density fracture spacing index 0 30 the lowest joint density fracture spacing index 0 095 and 0 12 is in cores from gently dipping areas at outcrops in various structural settings the fracture spacing index is the same approximately 1 29 in chert dolostone and porcelanite and siliceous shale these rocks may be saturated with joints so that differences in brittle strain due to local structural variations have been overwhelmed as joints continued to form during unroofing of these strata chert looks more fractured than other lithologies because of thin bedding fracture spacing index is used to compute such parameters as fracture porosity and volume of fractures that directly contact the well bore these parameters may be important when trying to model the behavior of a petroleum reservoir or when trying to assess the feasibility of strongly deviating wells to improve the performance of a fracture reservoir

alleghanian episode of k bentonite illitization in the southern appalachian basin mixed layer illite smectite i s from middle ordovician potassium k bentonites was found to be uniformly illitic throughout the southern appalachian basin regardless of variable depths of burial of the k bentonites the k ar ages of illitization are narrowly confined between 272 and 303 ma late pennsylvanian to early permian this suggests that illitization was a short lived episode coincident with and prompted by the alleghanian orogeny the illitization is explainable by the fluid expulsion hypothesis recently proposed by others e g oliver hot saline fluids were flushed to the basin edges from the deeply buried part of the foreland basin during the orogeny these fluids are thought to be effective agents of illitization some of our k bentonite samples are stratigraphically and geographically close to mississippi valley type lead zinc deposits suggesting a similar mode of origin rapid illitization of shales in a fashion similar to that observed for the bentonites should have led to high pore water pressures that enabled them to act as ideal decollements during alleghanian thin skinned deformation

origin of fracture porosity example from altamont field utah the occurrence of natural fracture systems in subsurface rock can be predicted if careful evaluation is made of the geologic processes that affect sedimentary strata during their cycle of burial diagenesis uplift and erosional unloading variations in the state of stress within rock arise for example from changes in temperature pore pressure weight of overburden or tectonic loading hence geologic processes acting on a sedimentary unit should be analyzed for their several contributions to the state of stress and this information used to compute a stress history from this stress history predictions may be made as to when in the burial cycle to expect fracture joint formation what type of fractures extension or shear may occur and which geologic factors are most favorable to development of fractures a stress history is computed for strata of the naturally fractured altamont oil field in utah s uinta basin calculations suggest that fractures formed in extension that the well cemented rocks are those most likely to be fractured that fractures began to develop only after strata were buried to great depth and that the fracture system continued to develop as strata were uplifted and denuded of overburden geologic evidence on fracture genesis and development is in accord with the stress history prediction stress history can be useful in evaluating a sedimentary basin for naturally fractured reservoir exploration plays

historical changes in the yellow river delta china abstract the lower yellow river delta has experienced numerous avulsions during the last 2000 years and the floodplain can be divided into four provinces alluvial fan western plain middle plain and eastern plain coastal deltas the yellow river delta complex is composed of several delta superlobes resulting from shifts in the lower river channel and each delta superlobe consists of several smaller lobes formed by shifts in the distributary channels these two kinds of channel shifts have resulted in a complicated imbrication pattern in the delta complex two imbricated delta superlobes are divided by a reworked bed and buried shelf deposits shelly ridges form the surface boundaries between the various delta superlobes the delta superlobes passed through an active stage a reworking stage and were then finally overlain by later superlobes

tectonic evolution of the grenville orogen in the central appalachians abstract remnants of the grenville orogeny a globally pervasive tectonothermal event resulting from the amalgamation of the supercontinent rodinia between ca 1 3 and 1 0 ga are exposed throughout eastern north america for ca 3000 km from labrador to alabama although tectonic models for the development of the grenville have historically been based on observations from the large and well exposed canadian grenville province recent investigations suggest that initial continental collision occurred proximal to the appalachian blue ridge mesoproterozoic rocks within this portion of the orogen are exposed within a series of allochthonous terranes that were extensively reworked during paleozoic orogenesis to better understand the tectonic history of this critical region we utilized garnet and monazite geochronology to reconstruct the metamorphic history of eleven samples of ortho and paragneiss from the appalachian blue ridge province the resultant dates together with new results from trace element thermobarometry define a prolonged record of high temperature metamorphism coincident with the timing of continent continent collision as recognized in the northern portion of the orogen initial garnet and monazite growth commenced between 1100 and 1065 ma with peak granulite facies conditions 750 900 c 0 55 0 85 gpa occurring from 1050 to 1010 ma burial and heating of the paragneisses progressed rapidly with peak metamorphism occurring 20 30 myr after deposition of the sedimentary protoliths local hydrothermal alteration followed peak metamorphism and occurred at 1000 990 ma with later cooling through 660 c between 980 and 935 ma 3 2 c myr the metamorphic history outlined here together with insights from previous studies is consistent with formation of the blue ridge rocks within a continental suture zone that had an orogenic architecture comparable to that of the himalayan tibetan orogen

toward a quantitative definition of mechanical units new techniques and results from an outcropping deep water turbidite succession tanqua karoo basin south africa the physical properties of reservoirs are strongly influenced by distributed fracture fields outcrop studies are commonly used to determine them but have provided unsatisfactory results because the definition of mechanical units i e groups of layers displaying homogeneous fracture patterns is difficult and typically conducted in a qualitative manner we have developed a systematic methodology to acquire and process fracture patterns in outcrops and to define their relation with stratigraphy we dedicate particular attention to the vertical distribution of fractures in a sedimentary succession commonly composed of layers of different thicknesses and compositions the method makes full use of geographic information system technologies and allows for direct digital acquisition in the field leading to time efficient acquisition data are processed with a newly developed routine that permits an objective description of the changes of fracture characteristics along the stratigraphy of the outcrop the operator is then able to define the most suitable fracture stratigraphy the integration of results from different outcrops is thought to provide a tool for predicting fracture distributions in subsurface target areas

using borehole images for target zone evaluation in horizontal wells horizontal wells are rarely horizontal instead operators commonly try to drill such wells into particular rock layers or target zones which may or may not be truly horizontal thicknesses of target zones commonly range from a few feet to a few tens of feet 1 10 m target zone evaluation concerns whether a horizontal well was successfully located and drilled in a given rock layer borehole imaging logs provide a powerful tool for stratigraphic interpretation and target zone evaluation in the austin chalk niobrara formation san andres formation and other units this study uses borehole images generated by schlumberger s formation microscanner fms a microconductivity logging device open fractures and clay rich interbeds appear as dark high conductivity traces on the fms log these traces can be fit with sinusoidal curves and oriented on a computer workstation the shape of the sinusoidal curve that fits a particular bedding plane tells the interpreter whether the borehole was moving upward or downward through the strata stratlog trademark of sierra geophysics inc a halliburton company software has been used to display borehole profiles by combining fms data on fracture intensities and bedding plane intersections with gamma ray logs mud logs and borehole deviation surveys to aid in planning future wells multiple penetrations of he same horizon can be detected and used to calculate highly accurate bedding plane dips fault interpretation including the detection of rollover beds is also possible finally stratigraphic interpretation can be combined with observed fractures to determine which rock layers are most highly fractured and therefore should be target zones

u pb geochronology of the grenville orogen of ontario and new york constraints on ancient crustal tectonics based on lithological structural and geophysi cal characteristics the proterozoic grenville orogen of southern ontario and new york has been divided into domains that are separated from each other by ductile shear zones in order to constrain the timing of meta morphism u pb ages were determined on metamorphic and igneous sphenes from marbles calc silicate gneisses amphibolites granitoids skarns and pegmatites in addi tion u pb ages were obtained for monazites from meta pelites and for a rutile from an amphibolite these mineral ages constrain the timing of mineral growth the duration of metamorphism and the cooling history of the different domains that make up the southern part of the exposed grenville orogen based on the ages from metamorphic minerals regional and contact metamorphism occurred in the following intervals central granulite terrane