cover deformation above a blind duplex an example from west virginia u s a abstract pretectonic cover above blind thrust systems responds to underlying thrusting by combinations of forethrusting backthrusting and coupling the role of these responses was examined in the hanging rock cacapon mountain anticlinorium of the central appalachians this cover fold consists of middle ordovician and younger sedimentary rocks above a blind horse of cambro ordovician carbonates the cover has abundant folding and subsidiary faulting cover cleavage in carbonate and fine grained clastic rocks formed before regional folding in response to layer parallel shortening lps this 15 lps also produced grain suturing and fluid inclusion trails in quartzarenites the lps formed during cover forethrusting accommodating older blind horses in the hinterland with the formation of the underlying horse cover coupling occurred with bending folding additional secondary buckling and cleavage rotation by flexural flow with perhaps minor flattening this cover coupling was insufficient to accommodate horse formation no evidence exists for backthrusting but forelandwards abundant subsurface evidence exists for cover forethrusting with imbricate thrusting this thrusting accommodates horse formation and differs in style from the lps of the older forethrusting event so for this blind system the cover responded with a combination of coupling and forethrusting

variation in quartz arenite deformation mechanisms between a roof sequence and duplexes abstract microstructural abundances and histories in quartz arenite of the lower silurian tuscarora sandstone were used to determine the nature and role of microscale deformation in a cover sequence and underlying thrust system and to assess the degree to which the cover sequence accommodated emplacement of the thrust system sandstone samples are located across the transition from the central to southern appalachian foreland thrust system where the thrusts change from blind to emergent southwards and where southern deformation of early alleghanian age has been previously shown to be overprinted by central deformation of alleghanian age microstructures observed with transmitted light and cathodoluminescence microscopy indicate that grain scale deformation occurred by dislocation flow pressure solution and microfracturing with the last being generally the most important the sequence of deformation mechanisms is the same for the cover sequence and the thrust system pressure solution during sedimentary compaction dislocation flow during layer parallel shortening and localized microfracturing with limited pressure solution near major thrust ramps and in steep fold limbs a greater abundance of dislocation flow microstructures in the cover sequence from layer parallel shortening indicates grain scale accommodation in the tuscarora sandstone of some shortening associated with emplacement of the thrust system the transition zone between the central and southern appalachians contains the greatest occurrence of every microstructure which is consistent with the area having been affected by diachronous central and southern alleghanian deformations

coalification patterns of the pennsylvanian coal measures in the appalachian foreland basin western and south central pennsylvania detailed vitrinite reflectance ro profiles have been constructed for nine boreholes penetrating the pennsylvanian coal measures of the appalachian foreland basin in western and south central pennsylvania a revised regional coal rank map was compiled with new and pre existing data by comparing the calculated ro profiles with those of measured ro maximum burial depth coalification temperature heat flow and geothermal gradient were estimated for the coal measures using a one dimensional transient heat conductive model combined with the lopatin waples diagram the results indicate that increase in coal rank 0 7 1 9 ro and in ro gradient 0 38 1 45 ro km across the study area coincides regionally with variation in calculated maximum burial depth 3 5 km paleotemperature 90 155 c paleoheat flow 53 to 75 mw m2 and geothermal gradient 26 to 33 c km and that the region represents different burial and thermal regimes as well as different structural provinces at the western margin of the intraplateau structural front coalification continued after the alleghanian orogeny regional geothermal heating due to depositional burial is thought to have been the dominant heat source responsible for the coalification level in the area closer to the eastern edge of the intraplateau front both regional geothermal heating due to burial and advective heating due to the fluid flow accompanying the alleghanian orogeny were important in creating the observed coalification patterns a higher coal rank and reflectance gradient together with a concentric isorank pattern centred in northern somerset county appear to represent an area where advective heating due to fluid flow was the dominant heat source

microfractures and their role in deformation of a quartz arenite from the central appalachian foreland abstract microfractures are pervasive in the massanutten sandstone a quartz arenite exposed in a doubly plunging synclinorium in the central appalachian foreland they are present as fluid inclusion planes fips and microveins parallel the fips fips occur in three orthogonal sets horizontal vertical and normal to the fold axis and vertical and parallel to the fold axis the geometry distribution and age of fips relative to other microstructures shows that microfractures developed during folding microfracturing followed layer parallel shortening by dislocation flow and was coeval with the peak of pressure solution that marked the final stages of folding microfracture densities are extremely high 160 mm 1 in many samples rapid closure of microfractures suppressed brecciation and prevented microfractures from propagating unstably to form joints microfracture opening accounts for significant finite strain with extensional strains of up to 11 1 and associated volume increases of over 16 0 the general lack of shortening microstructures indicates that a net volume increase is likely and that the massanutten sandstone may have served as a sink for dissolved phases from adjacent units undergoing volume loss deformation

dynamic analysis of deformed quartz grains from the folded middle miocene momonoki subgroup of central japan origin of healed microcracks abstract dynamic analyses have been performed on quartz detrital grains from the sandstones of folded middle miocene strata momonoki subgroup found along the itoigawa shizuoka tectonic line istl central japan these quartz grains contain numerous deformation lamellae kink bands and healed microcracks in this study in addition to the dynamic analysis using deformation lamellae the arrow method of carter and friedman the c axis preferred orientations of kinked and unkinked grains and the preferred orientations of healed microcracks in individual grains are analyzed in order to test if these fabrics can be used to determine the palaeostress field in the case of basal a slip being dominant which probably holds for the analyzed quartz the principal stress axes can be inferred from the c axis preferred orientations of kinked and unkinked grains referred to as the kink method here because the preferred orientations indicate the favourable and unfavourable orientations respectively under stress for basal a slip results of the dynamic analyses of deformed quartz by the arrow method and by the kink method are consistent with each other and presumably indicate the palaeostress field associated with the macroscopic folding of the momonoki subgroup although the origin and timing of healed microcracking in the deformed quartz are not completely resolved the preferred orientation of poles to healed microcracks seems to consist of two or three sets of orthogonally disposed maxima whose directions coincide with the principal stress axes responsible for the crystal plasticity i e the lamellae and kinks in the quartz it is suggested that the successive release of residual tectonic stresses in the principal stress directions led to this orthogonal orientation pattern of healed microcracks in the present study then combined dynamic analyses of deformed quartz using deformation lamellae kink bands and healed microcracks provide a more complete quantitative picture of the bulk stress field during deformation particularly the combination of the arrow method with the kink method the latter applied to naturally deformed quartz for the first time in this study provides an indication of the relative magnitude of principal stresses finally in the context of the influence of fluids on the deformation processes in rocks i would like to stress that microcracking in crystalline or consolidated rocks is essential for the incorporation of enough fluid into the constituent crystals to activate fluid assisted deformation processes such as solution precipitation creep and that the regional development of microcracks may be controlled by the stress field associated with the fold development in layered consolidated rocks

primary fabric ellipsoids in sandstones implications for depositional processes and strain analysis abstract when measuring strains in deformed clastic rocks geologists usually assume one or more of the following 1 objects were initially circular 2 objects were initially non circular but had uniform orientations 3 object populations initially had a fabric but that this fabric had a symmetrical relationship to bedding or that 4 initial fabrics are recognizable even after straining to evaluate these assumptions we measured 43 fabric ellipsoids in non strained poorly sorted sandstones from four depositional settings dsdp core 174 fan and abyssal plain deposits off the coast of oregon the cretaceous great valley sequence california fore arc deposits the cretaceous pigeon point formation california accreted and slumped turbidites and crossbedded sandstones from intracontinental basins in california and australia our results indicate the following 1 in two dimensional cuts individual grains have variable but usually small axial ratios these data and microstructures in grains and matrix cements also argue against significant compaction during burial in these clast supported sand rich units and instead suggest that the primary fabrics largely reflect grain packing processes during deposition and or slumping and packing in the case of the pigeon point samples

strain variations and three dimensional strain factorization at the transition from the southern to the central appalachians abstract the transition between the southern and the central appalachian foreland thrust system is a region subjected to at least two non coaxial deformations where typical two dimensional analyses such as cross section balancing or plane strain retro deformation cannot be applied three dimensional factorization can simulate the non coaxial strain history simulation results for compaction and layer parallel shortening lps were compared to finite strains of 16 quartz arenite samples from the tuscarora sandstone successful simulations indicate in the southern appalachians 30 35 total compaction by volume loss including a 4 5 9 e w horizontal shortening during lithification and 5 lps along 150 330 by plane strain or axial symmetric flattening in the central appalachians 30 35 total compaction by volume loss including a 3 7 n s horizontal shortening during lithification and 10 lps along 120 300 by plane strain or axial symmetric flattening in the transition zone 30 compaction by volume loss including a 3 e w horizontal shortening during lithification 5 southern lps along 150 330 by axial symmetric flattening and 5 10 central shortening along 130 or 140 by plane strain or axial symmetric flattening at 10 to the normal central trend from the simulations horizontal shortening in the southern appalachians during lithification in the mississippian was greater than the later alleghanian deformation this shortening probably represents a very early lps developed in unlithified sand that required less deviatoric stress to deform than did the cemented quartz arenites during later tectonic deformation also the oblique and weaker central tectonic lps in the transition zone probably represents behavior at the limits of central appalachian deformation finally the lps simulations indicate that axial symmetric flattening oblate strain is more successful than plane strain as the representative behavior so caution is recommended in assuming plane strain as the dominant strain behavior in orogenic forelands

joints microfractures and the formation of solution cleavage in limestone the geometry of solution cleavage surfaces in limestones indicates a close connection between cleavage formation and joint and microfracture surfaces these relations suggest that networks of microfractures and joints act both as conduits for transport of dissolved phases and as surfaces on which the cleavage nucleates material transport is by darcyian type bulk flow and is thus not a rate limiting process this contrasts with metamorphic terranes where the transport mechanism diffusion has been identified as a rate limiting step cleavage surfaces of weak to moderate intensities or spacing may form parallel to nonprincipal planes of finite strain

rate laws for water assisted compaction and stress induced water rock interaction in sandstones mineral water interactions under conditions of nonhydrostatic stress play a role in subjects as diverse as ductile creep in fault zones phase relations in metamorphic rocks mass redistribution and replacement reactions during diagenesis and loss of porosity in deep sedimentary basins as a step toward understanding the fundamental geochemical processes involved using naturally rounded st peter sand we have investigated the kinetics of pore volume loss and quartz water reactions under nonhydrostatic hydrothermal conditions in flow through reactors rate laws for creep and mineral water reaction are derived from the time rate of change of pore volume sand water dissolution kinetics and flow rate independent steady state silica concentrations and reveal functional dependencies of rates on grain size volume strain temperature effective pressure confining minus pore pressure and specific surface areas together the mechanical and chemical rate laws form a self consistent model for coupled deformation and water rock interaction of porous sands under nonhydrostatic conditions microstructural evidence shows a progressive widening of nominally circular and nominally flat grain grain contacts with increasing strain or equivalently porosity loss and small quartz overgrowths occurring at grain contact peripheries the mechanical and chemical data suggest that the dominant creep mechanism is due to removal of mass from grain contacts termed pressure solution or solution transfer with a lesser component of time dependent crack growth and healing the magnitude of a stress dependent concentration increase is too large to be accounted for by elastic or dislocation strain energy induced supersaturations favoring instead the normal stress dependence of molar gibbs free energy associated with grain grain interfaces

microfractures in chalks of albuskjell field norwegian sector north sea possible origin and distribution production data from many north sea chalk fields have indicated moderate to considerable contribution to production from natural fractures this paper illustrates a detailed study of natural fracturing in the albuskjell field where gas condensate hydrocarbons are contained in upper cretaceous maastrichtian and lower tertiary danian chalks the field is a large halokinetically induced dome located on the northern limits of the productive chalk region known as the greater ekofisk area examination of core material from wells 1 6 3 2 4 f10 and 1 6a 10 albuskjell has revealed the presence of two types of fractures the first appear to be early are commonly compacted predominantly healed and resemble conjugate shear fractures the second type is related mainly to the tips of stylolites these are vertical preferentially open and are interpreted as tension fractures tension fractures form when the minimum effective stress is reduced to the tensile strength of the chalk as a result of increased pore fluid pressure and or decreased total confining stress due to relative extension construction of effective stress versus progressive burial depth profiles using three models of overpressure generation suggests that high pore fluid pressures alone could not have formed the tension fractures in albuskjell measurements and calculations from depth converted seismic sections have shown that halokinesis occurred throughout chalk deposition and continued in a series of pulses until early miocene time incremental stress values associated with halokinesis predict optimum conditions for tension fracture formation at 4 500 ft 1 370 m burial owing to a combination of halokinetic doming and overpressuring with an important contribution from source rock maturation these fractures possibly formed therefore post hydrocarbon emplacement 3 000 ft 1 000 m burial identity mid oligocene and have been preserved by hydrocarbon invasion the healed shear fractures formed prior to significant pressure solution and hydrocarbon emplacement and are thought to be associated with extra dome processes possibly involving graben tectonics or halokinetic reactivation of earlier northwest southeast trending major faults their distribution is expected to be fairly uniform over the field with broadly northwest southeast orientations shear fracture density is moderately constant between the danian and maastrichtian chalk sequences but is significantly reduced to absent in the argillaceous base danian model studies predict that tension fractures should be concentrated in crestal areas of the field where near random orientations may occur preferentially radial with some concentric orientations relative to the structural dome may occur off the crest with radial fractures persisting farther downflank predicted tension fracture orientations agree well with those measured from the oriented cores of well 1 6 a10 open tension fractures are preferentially absent from tight early cemented or argillaceous chalks they are also poorly developed in soft lithologies and predominantly occur in chalks with moderate to high porosities with stylolites the spacing of open tension fractures is assumed to be constant over the field whereas fracture width decreases from crest to flank this is in line with the concept of lower horizontal stresses in the structural crest a plot of optimum fracture spacing versus fracture width has been constructed using strain values calculated from the halokinetic models analysis of production data supports the proposed model and agrees well with a strong halokinetic influence on fracture development although significant halokinesis ceased during the early miocene residual strain may still exist in albuskjell with larger horizontal effective stresses occurring in the flanks of the structure calculations suggest that tension fractures are probably open under initial field conditions to a potentia depth of 11 500 ft 3 500 m

correction the timing of the last deglaciation in north atlantic climate records nature 412 724 727 2001 we directly used the observed leads of sea surface temperature with respect to air temperature dated in calendar years whereas the air temperature calendar ages should have been converted into 14c ages with reservoir ages computed as the difference between marine and atmospheric 14c ages

intergranular pressure solution in four quartzose sandstones abstract cathodoluminescence petrography and scanning electron microscopy reveal that several geologic variables significantly influenced intergranular pressure solution and quartz cementation in the bromide ordovician oklahoma upper minnelusa permian wyoming nugget jurassic utah and tuscarora silurian pennsylvania sandstones a linear relationship exists between mean grain size and intergranular pressure solution with finer grained samples having experienced more intergranular pressure solution than coarser grained ones samples that display pervasive illite grain coatings have also experienced more intergranular pressure solution than those with no or poorly developed grain coatings sorting and limited framework grain compositional variability have exerted little influence o intergranular pressure solution silica budgets indicate that in the bromide upper minnelusa and nugget sandstones more quartz has been dissolved by intergranular pressure solution than is present as cement these sandstones have exported silica during burial diagenesis in the tuscarora the amount of quartz dissolved by intergranular pressure solution is about equal to the amount of quartz cement present implying approximate mass balance however petrographic evidence suggests that most of the quartz cement in at least three of these sandstones predates most intergranular pressure solution implying that they acted as silica importers during shallow diagenesis and silica exporters during deeper diagenesis intergranular pressure solution has played varying roles in determining the ultimate porosity of these four sandstones in the nugget intergranular pressure solution was the single most important diagenetic process that influenced porosity evolution in the bromide intergranular pressure solution was slightly more important than mechanical compaction and considerably more important than cementation in the tuscarora intergranular pressure solution was slightly more important than mechanical compaction and of about equal importance to quartz cementation in the upper minnelusa intergranular pressure solution was less important than both mechanical compaction and gypsum anhydrite cementation

lithofacies of the silurian keefer sandstone east central appalachian basin usa abstract fourteen lithofacies of the keefer sandstone in the area of northeastern west virginia and western maryland were identified depositional environments of the lithofacies include storm dominated marine shelf basin barrier island and backbarrier tidal flat and lagoon coastal lithofacies document a dominant basinward progradation with some sea level rise which placed backbarrier lithofacies above barrier island lithofacies shelf lithofacies as stacked coarsening upward sequences document as many as four fluctuations within the major progradation coarsening upward sequences are typically capped by transgressive sands which represent material eroded from relatively landward environments and transported basinward during coastal retreat deposition of cycles within the keefer occurred under conditions of tectonic quiescence dominant steady or falling sea level followed by rise in sea level and in a climate which was warm and humid

determination of pressure solution shortening in sandstones abstract a new method for the determination of pressure solution shortening in sandstones uses the geometry of grain to grain interpenetrations and grains truncated against solution surfaces these features are used to construct plots from which the magnitude and direction of the pressure solution shortening can be determined using simulated pressure solution deformation of artificial and natural grain populations the new method is shown to correctly assess a variety of coaxial and non coaxial shortenings although primarily intended to determine shortening the method can also quantify extension related to growth of beards or overgrowths during pressure solution application of the method to naturally deformed quartz arenite samples shows that pressure solution shortening of up to 26 occurred during compaction and 22 during layer parallel shortening

sandstone porosity loss a big picture view of the importance of compaction proper assessment of the relative importance of porosity loss by compaction and cementation is important to the prediction of porosity in sandstones studies of mass transport during diagenesis and the modeling of basin fluid flow quantitative estimation of the amounts of porosity loss by compaction and cementation can be made from standard point count data on cement and pore space abundance this method is discussed and its sources of error are evaluated error in the method arises mainly from uncertainty in initial sandstone porosities and the amount of local grain dissolution during burial data on the porosity of modern sands indicate that assumed initial porosities of 40 to 45 will produce small average errors in the calculation of compactional porosity loss for clean sandstones grain dissolution whether resulting in local porosity enhancement by export in solution of former grain mass or local precipitation of authigenic cement will cause an underestimation of the significance of compaction a large database of point count data from diverse sandstones is used to provide a broad perspective on the importance of compaction and cementation to sandstone porosity loss it is shown that the global importance of compactional porosity loss has generally been under appreciated in fact compaction mechanical and chemical is probably the dominant mechanism of porosity loss in most sandstones the extent of compactional porosity loss is related to sand grain composition and basin thermal structure

assessing brittle volume gain and pressure solution volume loss processes in quartz arenite abstract quartz arenite of the tuscarora sandstone has been deformed by dislocation flow pressure solution and microfracturing of which the last two are most important pressure solution involved shortening with no extension most samples have approximately 20 shortening normal to bedding interpreted to be compaction and a few have up to 20 layer parallel shortening of tectonic origin because there is no extension these shortenings resulted in an equivalent amount of bulk volume loss multiple sets of microfractures in the form of fluid inclusion planes and microveins account for up to 10 extension and bulk volume increases up to 17 when both microfracture volume gains and pressure solution volume losses are considered together all samples show a net bulk volume loss ranging from 14 to 35 changes in material volume are different than bulk volume because of the presence of porosity at the time of pressure solution whereas all samples show bulk volume losses most samples show net material volume gains of up to 16 with only those having experienced both compactional and tectonic pressure solution showing material volume losses pressure solution during compaction can account for most but not all cement compactional pressure solution surfaces do not extend into pore filling cement suggesting that much of the cement was externally derived after compaction therefore the flux of material may have been considerably greater than the material volume changes would indicate

evolution of solution structures in a deformed quartz arenite geometric changes related to permeability changes abstract quartz arenite of the silurian keefer sandstone at a site in west virginia u s a records an early pervasive solution related deformation by grain interpenetration volume losses measured by the pss method show shortening directions from both diagenetic compaction and early tectonic layer parallel shortening later solution related tectonic deformation during folding formed transgranular stylolites along pre existing faults or in rock volumes with abundant microfractures thus the geometry of solution related deformation changed progressively from pervasive at grain scale to focused along transgranular surfaces we propose that this geometric change resulted from intense porosity and permeability reduction during diagenetic cementation and early layer parallel shortening which required fluids to exploit tectonically created pathways along faults or networks of microfractures during later deformation

rock matrix structures in a zone influenced by a stylolite abstract a stylolitic structure is one of the visible manifestations of the pressure solution process in a rock the stylolitic process zone as defined in previous studies on both sides of a stylolite is studied by sem the chosen samples concern tectonic stylolites within a high porosity limestone 16 without tension gashes this zone corresponds to a local increase of porosity and comprises a large number of stylolitic joints showing a decreasing size from the macroscopic structure to simple contact between two grains two hypotheses are presented for the stylolite formation 1 the macroscopic stylolite is the result of the coalescence of earlier minor pressure solution structures 2 small stylolites nucleate gradually from a major stylolite joint the mass transfer can be realized either by diffusion between particles or by seepage through previously undescribed process zone fractures the mass deposition can be localized in the porous volume and the microcracks without requiring any additional tectonic openings including tension gashes the rock deformation is therefore limited to shortening parallel to 1 without normal extension

solution transfer compaction of quartzites progress toward a rate law data on dissolution diffusion and growth in the quartz water system show that diffusion through grain boundary films is rate limiting for solution transfer and too slow to permit significant compaction in geologic time but that rates are more reasonable if the rough texture of natural grain boundaries is considered on the basis of scanning electron microscope observations a model is proposed that uses a hierarchical plumbing system in which grain boundaries consist of islands capped by grain boundary films separated by microcapillaries and connected to intergranular porosity a rate law based on such a model has an inverse grain size dependence up to a critical grain size that is large 10 cm under typical compaction conditions

revised tuning of ocean drilling program site 964 and kc01b mediterranean and implications for the δ18o tephra calcareous nannofossil and geomagnetic reversal chronologies of the past 1 1 myr 1 high resolution color reflectance records of kc01 and kc01b calabrian ridge ionian sea are presented and compared with a modified spliced high resolution color reflectance record of ocean drilling program odp site 964 this comparison revealed that kc01b is characterized by intensive deformation between 27 and 28 5 m piston depth and that some sapropels are tectonically reduced in thickness moreover the piston coring has caused considerable stretching in the top of kc01 and kc01b using a new splice of odp site 964 as guide previous astronomical tuned timescales of kc01b and odp site 964 were evaluated this evaluation resulted in a new sapropel based astronomical timescale for the last 1 1 myr the new timescale implies a much more uniform change in sedimentation rate for the ionian sea cores two prominent excursions to lighter values in the 18o record of the planktonic foraminiferal species globigerinoides ruber occur during marine isotopic stages 12 and 16 applying the new timescale these shifts correspond with maxima in obliquity and are punctuated by minima in the precession cycle they are absent in global ice volume records and are interpreted as reflecting a summer low salinity surface water lens that floats on top of extremely saline intermediate and deep waters at times of the very low sea levels during these glacial periods all biostratigraphic and magnetostratigraphic events found in kc01b and odp site 964 were re dated according to the new timescale and the ages of 33 tephra layers were reviewed the new ages for the calabrian ridge 2 and 3 magnetic events in the brunhes are concordant with minima in the global sint800 composite record derived from worldwide deep sea records of relative paleointensity and have been attributed to the big lost and la palma excursions respectively

paleostress directions from the preferred orientation of closed microfractures fluid inclusion planes in sandstone east texas basin u s a abstract closed extensional microfractures are useful indicators of paleostress directions in shallowly buried quartz cemented quartz arenites of the lower cretaceous travis peak formation a tabular sandstone and shale unit approximately 670 m thick that was deposited in the gradually subsiding northern gulf of mexico basin the closed microfractures are syndiagenetic fluid inclusion rich planes that have an ene strike that is subparallel to contemporary quartz filled extension veins and regional normal faults indicating that closed microfractures contain the maximum paleostress direction these microstructures are potentially useful for mapping paleostress trajectories in passive margin and platform settings particularly where samples are obtained from core previous experimental studies of microfracture closure suggest that microfractures evolved into fluid inclusion planes by dendritic quartz precipitation in east texas microfracture closure may have occurred at temperatures as low as 85 c and depths as shallow as 1500 m