calcite textures microstructures and rheological properties of marble mylonites in the bancroft shear zone ontario canada calcite textures in several microstructural domains from the bancroft shear zone reveal a sequence of transitions which indicate contributions from a variety of deformation mechanisms at greenschist facies conditions the coarse marble protolith has undergone low strains accommodated by twinning and grain boundary migration porphyroclastic mylonite has a well developed crystallographic preferred orientation and microstructures indicative of dislocation creep and rotation recrystallization mechanisms the textures are consistent with high temperature deformation experiments where r f and c slip systems were active complete dynamic recrystallization produced equant medium sized grains 50 m with a texture that is nearly random secondary calcite growth observed under cathodoluminescence and variation in 18o value with microstructurc indicate large fluid fluxes during mylonitization fluid assisted grain boundary sliding and a minor component of dislocation creep are the inferred deformation mechanisms finer grained s c mylonite has a well developed shape fabric and crystallographic preferred orientation with a point maximum and great circle girdle attributed to attainment of an easy slip orientation for most grains ultramylonites have homogeneous textures with finc elongate grains 20 30 m mantled by disseminated secondary phases textures in ultramylonites are characterized by single point maxima and great circle girdles the most evolved ultramylonite has a point maximum oblique to the shear plane with a symmetry indicating rotation of the maximum toward the extension direction texture microstructure relations in these mylonites indicate that the competition between deformation mechanisms was highly sensitive to grain size strain and secondary phases in addition to temperature strain rate and differential stress the varied deformation mechanisms and application of experimentally derived constitutive equations indicate significant differences in differential stress and strain rate among microstructural domains these differences reflect strain softening associated with dynamic recrystallization and work hardening associated with dissemination of secondary phases and the transition from grain boundary sliding to dislocation creep

cretaceous tertiary structures and kinematics of the serbomacedonian metamorphic rocks and their relation to the exhumation of the hellenic hinterland macedonia greece the kinematic pattern and associated metamorphism of the predominant ductile deformation and the subsequent deformational stages of the serbomacedonian metamorphic rocks and granitoids are presented in terms of peri tethyan tectonics a systematic record of structural and metamorphic data gives evidence of a main top to ene to ese ductile flow of cretaceous age 120 90 ma associated with a crustal stretching and unroofing a subordinate wsw to wnw antithetic sense of movement of the tectonic top is observed in places the associated metamorphic conditions are estimated at 4 5 7 5 kbar and 510 580 c during eocene to miocene times these fabrics were successively deformed by low angle extensional de ductile shear zones with top to ne and sw sense of movement and brittle shear zones of similar kinematic pattern suggesting a transition from ductile to brittle deformation de deformation was accompanied during its later stages by nw se directed shortening we also discuss the relation of this cretaceous tertiary deformation of the serbomacedonian metamorphic rocks with the eocene to miocene ductile top to southwestward crustal shear of the adjacent rhodope crystalline rocks we regard the serbomacedonian and the rhodope metamorphic rocks to represent related metamorphic provinces the most recent exhumation and cooling history of which is bracketed between the eocene and neogene

pure shear and simple shear calcite textures comparison of experimental theoretical and natural data abstract the pattern of lattice preferred orientation texture in deformed rocks is an expression of the strain path and the acting deformation mechanisms a first indication about the strain path is given by the symmetry of pole figures coaxial deformation produces orthorhombic pole figures while non coaxial deformation yields monoclinic or triclinic pole figures more quantitative information about the strain history can be obtained by comparing natural textures with experimental ones and with theoretical models for this comparison a representation in the sensitive three dimensional orientation distribution space is extremely important and efforts are made to explain this concept we have been investigating differences between pure shear and simple shear deformation incarbonate rocks and have found considerable agreement between textures produced in plane strain experiments and predictions based on the taylor model we were able to simulate the observed changes with strain history coaxial vs non coaxial and the profound texture transition which occurs with increasing temperature two natural calcite textures were then selected which we interpreted by comparing them with the experimental and theoretical results a marble from the santa rosa mylonite zone in southern california displays orthorhombic pole figures with patterns consistent with low temperature deformation in pure shear a limestone from the tanque verde detachment fault in arizona has a monoclinic fabric from which we can interpret that 60 of the deformation occurred by simple shear

exhumation and doming of the thasos metamorphic core complex s rhodope greece structural and geochronological constraints abstract the thasos island metamorphic core complex s rhodope n aegean sea was generated between 26 and 13 ma progressive extensional tectonics was associated with upwarping of the deeper parts of this core complex to a narrow gneiss dome and with formation of a steep topographic high we present a combined structural microstructural and geochronological study that constrains the deformation time path of progressive extension and exhumation thasos island consists of three superimposed more or less flat lying metamorphic units with downward decreasing geochronologic ages which are separated by two systems of low angle normal detachment surfaces trending nw se the upper unit stayed in supracrustal level since 50 ma representing the upper plate during an early phase of extension not later than 26 23 ma upper levels of the core complex intermediate unit underwent pervasive mylonitization with top to the sw shear from about 21 ma onward the intermediate unit was rigid and top to the sw slip continued at least until 13 8 ma on the discrete detachment surface that separates the core complex from the upper plate yet in the 21 15 ma period deeper levels lower unit still deformed plastically and deformation successively concentrated in zones of high strain and in discrete detachment surfaces that caused decoupling of the lower from the intermediate unit these indicate downdip movements in opposite sense associated with doming of the lower unit at that stage simultaneously the intermediate unit was fragmented on high angle normal faults that sole into the dome bounding detachment surface and roof into the core complex bounding detachment surface these faults confine tilt blocks that are arranged around the lower unit gneiss dome doming of the core complex occurred simultaneously with subsidence of basins located on the upper plate

texture development of calcite by deformation and dynamic recrystallization at 1000 k during torsion experiments of marble to large strains abstract torsion deformation experiments were performed on solid cylinders of carrara marble at high temperature 1000 k and constant twist rate about 4 10 4 rad s 1 to large twist angles between 80 and 840 these conditions correspond to simple shear deformation at constant shear strain rate 3 10 4 s 1 and variable amounts of shear strain of 1 2 5 and 11 at the outer mantle of the sample cylinders lattice preferred orientations lpo were measured on polished thin sections using automated electron backscatter diffraction ebsd to analyze the microtextures in orientation imaging micrographs oim shear deformation produced first a microstructure of elongated grains with shape and orientation close to the finite strain ellipse of the imposed shear strain 1 and 2 a deformation texture developed with monoclinic sample symmetry and an oblique c axis distribution relative to the shear plane with increasing shear the marble recrystallized by subgrain rotation and nucleation of small grains starting along grain boundaries which ultimately replaced the whole deformation microstructure a steady state recrystallization microstructure evolved with nearly equant grains of about 10 m in size 5 and 11 the lpo changed completely from the monoclinic deformation texture into a distribution with a sharp single orientation component with r 10 1 4 parallel to the shear plane and a 1 2 1 0 parallel to the shear direction this recrystallization texture has orthorhombic symmetry with respect to shear plane and shear direction during the transition the applied flow stress decreased simple shear deformation of calcite was modeled with the self consistent polycrystal plasticity theory it included a model for dynamic recrystallization based on a balance between growth and nucleation model results agree with experimental data if slip on r 10 1 4 a 1 2 1 0 is introduced as a potential slip system for calcite in that case the steady state recrystallization texture has an easy slip preferred orientation the hypothesis remains speculative until this slip system is actually confirmed by direct observation

late cenozoic extension in northeastern greece strymon valley detachment system and rhodope metamorphic core complex the strymon valley detachment system accommodated at least 25 km of extension at the northern margin of the aegean extensional province from middle miocene through early pliocene time the primary detachment surface is exposed as a gently southwest dipping low angle normal fault similar to those widely known in core complex settings of the u s basin and range province for more than 150 km along strike from the aegean sea coast in northeastern greece northward into the struma valley of southern bulgaria hanging wall rocks in the strymon valley detachment system moved in a s53 w direction relative to the footwall resulting ultimately in the tectonic unroofing of the rhodope metamorphic core complex several segments of the strymon valley detachment have previously been interpreted as thrust faults formed during a miocene compressional event our results negate the principal evidence for this event and imply instead that extensional deformation has characterized the northern aegean region since at least middle miocene time displacement ceased on the strymon valley detachment with the late pliocene development of the strymon and drama basins we propose that those basins together with the other principal basins of the northern aegean region are subsiding above an active northeast dipping extensional detachment zone that forms a unified kinematic system with the western offshore continuation of the dextral north anatolian strike slip fault the north aegean trough if this hypothesis is correct then the north aegean trough the western strands of the north anatolian fault and the principal modern depocenters of the northern aegean region may all have originated no earlier than late pliocene time

a holocene history of dune mediated landscape change along the southeastern shore of lake superior abstract causal links that connect holocene high stands of lake superior with dune building stream damming and diversion and reservoir impoundment and infilling are inferred from a multidisciplinary investigation of a small watershed along the se shore of lake superior radiocarbon ages of wood fragments from in place stumps and soil o horizons recovered from the bottom of 300 ha grand sable lake suggest that the near shore inland lake was formed during multiple episodes of late holocene dune damming of ancestral sable creek forest drownings at 3000 1530 and 300 cal years bp are highly correlated with local soil burial events that occurred during high stands of lake superior during these and earlier events sable creek was diverted onto eastward graded late pleistocene meltwater terraces ground penetrating radar gpr reveals the early holocene valley of sable creek now filled and its constituent sedimentary structures near planar paleosols identified with gpr suggest two repeating modes of landscape evolution mediated by levels of lake superior high lake stands drove stream damming reservoir impoundment and eolian infilling of impoundments falling lake superior levels brought decreased sand supply to dune dams and lowered stream base level these latter factors promoted stream piracy breaching of dune dams and aerial exposure and forestation of infilled lakebeds the bathymetry of grand sable lake suggests that its shoreline configuration and depth varied in response to events of dune damming and subsequent dam breaching the interrelated late holocene events apparent in this study area suggest that variations in lake level have imposed complex hydrologic and geomorphic signatures on upper great lakes coasts

calcite textures examples from nappes with strain path partitioning abstract strain path partitioning partitioning of deformation into components of pure and simple shear in two alpine nappes is qualitatively indicated by a variation in orientation of foliation microkinematic criteria quartz textures and a comparison with strain path partitioning in theoretical nappe models the degree of non coaxial deformation relative to the total deformation is quantified by using the asymmetry of textures in calcite tectonites the texture interpretation is based on both polycrystal plasticity models and experimental data calcite tectonites in the graz paleozoic thrust sheet austria and the morcles nappe switzerland were deformed in a strain path with both pure and simple shear components texture asymmetry supports the strain path derived from geological observations as an exception a high strain ultramylonite deformed nearly exclusively in pure shear it is suggested that this represents a softening instability since deformation in pure shear is energetically more favorable than in simple shear the incompatibilities caused by heterogeneous pure shear are accommodated by detachments and heterogeneous simple shear adjacent to the pure shear zone

simple shear experiments on calcite rocks rheology and microfabric ideal simple shearing of carrara marble and solnhofen limestone was achieved by inserting calcite rock slices into sandstone cylinders precut at 35 the experiments covered a wide range in temperatures 25 900 c at confining pressures of 200 and 250 mpa and at strain rates between 10 3 and 10 5 s 1 shear strains of up 2 7 were achieved the stress strain data for simple shear favourably compare with predictions on the basis of existing flow laws established in coaxial testing this comparison demonstrates that the simple shear experiments covered most of the previously established flow regimes for calcite rocks at the highest temperatures carrara marble exhibited substantial work softening after high shear strains a complete texture analysis was carried out with the u stage and a texture goniometer the following four distinct microfabric regimes were found 1 twinning regime an oblique c axis maximum deflected from the pole to the flattening plane towards 1 and against the imposed sense of shear results directly from e twinning extensive twinning some grains are completely twinned rapidly rotates the c axes into near parallelism with 1 this regime was observed for carrara marble deformed at temperatures below 700 c and for solnhofen limestone at room temperature 2 intracrystalline slip regime three c axis maxima were observed in both rock types at more elevated temperatures and in the absence of twinning these c axis orientations result from an alignment of slip planes and slip directions into parallelism with the shear zone boundary and the shear direction respectively this finding supports the idea of an orientation for easy slip resulting from simple shearing in addition to r and f glide basal glide is inferred to have been operative 3 grain boundary sliding regime a great circle distribution of c axes approximately perpendicular to the direction of finite shortening is observed this texture is weaker compared to the other regimes it is interpreted to result from minor contributions of f slip towards total strain this regime is restricted to fine grained solnhofen limestone deformed at the highest temperatures 4 grain boundary migration regime only in carrara marble deformed at the highest temperatures is there substantial grain boundary migration recrystallization the texture consists of a dominant c axis maximum oriented perpendicular to the shear zone boundary this single maximum corresponds to an orientation for easy slip for basal slip grain boundary migration assists rapid lattice reorientation and leads to geometrical work softening quantitative analysis of grain boundary surface orientations gave the following results within the twinning and the intracrystalline slip regimes the characteristic grain shape is elliptical for both rock types the strain ellipses derived from this analysis closely correspond to the known finite strain at the lowest temperatures at intermediate temperatures and especially in carrara marble there is a consistent trend for the aspect ratios to indicate too little flattening and for the angles between the long axis of the strain ellipse and the shear zone boundary to be too small in the absence of grain boundary migration recrystallization this finding indicates minor translations of grains with respect to one another grain boundary sliding or translations accommodated in the mantle regions of grains at the highest temperatures where extensive grain boundary migration occurs in carrara marble grain boundary migration regime two preferred orientations of grain boundaries are observed 1 parallel to the shear zone boundary and 2 parallel to an oblique direction deflected 20 away from the normal to the shear zone boundary in the sense of the imposed shear the same pattern of grain boundary alignment is observed in solnhofen limestone deformed in the grain boundary sliding regime the geological applications of this experimental work are discussed in terms of calcite tectonites in particular and sheared rocks in general

the influence of crystallographic orientation and grain boundary migration on microstructural and textural evolution in an s c mylonite abstract the analysis of the subgrain and dislocation sub structure present in grains in different crystallographic orientations within a domainal s c mylonite is reported the study reveals a range in subgrain size and development which is related to the crystal orientation of the host grain in the mylonite subgrains in grains with easy slip orientations contain large subgrains while grains in more complex slip orientations contain small subgrains the dislocation densities preserved in the mylonite are extremely variable even within individual subgrains and may be the result of a late stress pulse the analysis indicates that grain boundary migration was an important process in the microstructural evolution initiating the selective removal of some orientations and thus influencing the final crystallographic preferred orientation present in different domains the data collected is also used to assess the potential stability and preservation of microstructures during the non steady state deformation expected at the end of a deformation event

porphyroclast systems as kinematic indicators porphyroclasts of relatively strong minerals in mylonites commonly have an internal monoclinic shape symmetry defined by tails of dynamically recrystallized material the geometry of a porphyroclast and its tails called a porhyroclast system can serve as a valuable indicator of the sense of vorticity porphyroclast systems have been divided into and types on the basis of the geometry of the tails types have wedge shaped recrystallized tails whose median lines lie on opposite sides of a reference plane parallel to the tails and containing the symmetry axis for the system types are further subdivided into a a types in which the porphyroclast is isolated in a relatively homogeneous matrix and b types in which the porphyroclast system is associated with a shear band foliation in the matrix types typically have narrow recrystallized tails whose median lines cross the reference plane adjacent to the porphyroclast consequently embayments of matrix material occur adjacent to the porphyroclasts and the tails display characteristic bends a porphyroclast system in a mylonite develops when the relatively weak dynamically recrystallized grain aggregate in the porphyroclast mantle changes its shape due to non coaxial flow in the adjacent matrix this behaviour has been modelled in shear box experiments passive marker lines around rigid cylinders embedded in silicone putty were subjected to simple shear the experiments were modified to simulate a change in recrystallization rate r with respect to rate of deformation by decreasing the diameter of the rigid cylinder during deformation at variable rates the ratio r appears to be one of the most important factors in determining which porphyroclast system will develop at high r values flow of recrystallized material away from the porphyroclast is continuously appended by the production of new grains and wedge shaped a type tails develop at low r values relatively few new grains are added to the tails which become thinned and deflected by drag due to the spinning motion of the porphyroclast in addition most porphyroclast systems at low shear strains are of a type or lack monoclinic symmetry whereas types are only developed at high shear strain values complex porphyroclast systems characterized by two generations of tails are observed in many of the natural and model shear zones studied and may form due to fluctuating r conditions that allow isolated a and type porphyroclast systems to be used as sense of vorticity indicators are the systems should have a monoclinic shape symmetry matrix grain size should be small with respect to porphyroclast size matrix fabric should be homogeneous deformation history should be simple and observations should be made on sections normal to the inferred bulk vorticity vector for the mylonite

an experimental steady state foliation the history of the intensity and orientation of a grain shape foliation was investigated in octachloropropane deformed in simple shear at 80 of its absolute melting temperature and a shear strain rate of 4 10 5s 1 foliation orientation developed from the beginning of the deformation remains steady throughout the deformation foliation intensity becomes steady after a bulk shear strain of about 0 9 has been accumulated the steadiness of foliation orientation and intensity is achieved by some balance between foliation strengthening and foliation weakening processes the main foliation strengthening process is intragranular plastic deformation foliation weakening processes include dynamic recrystallization by migration of straight or or slightly wavy grain boundaries grain dissection rotational recrystallization grain amalgamation relative rigidity of hard grains and grain boundary sliding a small but definite angle although not constant is observed between the foliation and the long axis of the bulk total strain ellipse the average aspect ratio of grains is lower than that of the bulk total strain ellipse by a factor of about 0 4 at total shear strain of 1 3 the average grain size also stabilizes becoming steady from a shear strain of 0 5 onwards a steady grain shape foliation may be a possible paleostress or a paleostrain rate indicator but cannot be a paleostrain indicator

rapid changes in the nature of the h chondrites falling to earth we have previously identified a subgroup of antarctic h chondrites that are significantly different from h chondrites among the modern falls in terms of induced thermoluminescence tl metallographic cooling rate and cosmogenic inert gas contents here we examine their terrestrial and thermal history as apparent in their natural tl and radioactive cosmogenic isotope abundances these meteorites have a tendency towards high 26al activities and fairly short 14c and 36cl terrestrial ages generally 1 2 au to 1 au within the last 105 years we suggest that the nature of the meteorites falling to earth is not independent of time but depends on stochastic events such as the breakup of parent bodies and recent variations in orbit

comments on the interpretation of deformation textures in rocks abstract in rocks that undergo ductile deformation preferred orientation develops as a result of intracrystalline slip and mechanical twinning the orientation distribution is a consequence of the microscopic mechanisms and of the strain path it can be used to get some insight into the deformation history however it is never unique the interpretation relies largely on polycrystal plasticity theory the concepts of stress equilibrium and strain compatibility which are two extreme assumptions made to model deformation are discussed new approaches such as the viscoplastic self consistent theory are a compromise and may be applicable to mineral systems which display a high degree of plastic anisotropy important extensions allow for heterogeneous deformation in the polycrystal from grain to grain and even within grains in correspondence with microstructural observations all these theories defy the popular notion which is becoming entrenched in the geological literature that the microscopic slip plane normal aligns with the axis of maximum principal compressive stress and that in simple shear the crystallographic slip plane rotates into the macroscopic shear plane and the slip direction into the macroscopic shear direction an orientation referred to by geologists as easy glide it is emphasized that future work on texture development of rocks should be based on rigorous physics rather than ingenious intuition in accordance with an old recommendation of walter schmidt

petrology of marble and peridotite in the seiad ultramafic complex northern california usa peridotite and infolded marble of the seiad ultramafic complex were recrystallized in the upper amphibolite facies as part of the regional progressive metamorphism of the rattlesnake creek terrane field relations including the occurrence of metarodingites and metasomatic zones between dissimilar rock types demonstrate that the metasediments and serpentinized ultramafic rocks were juxtaposed prior to regional barrovian metamorphism temperatures are estimated to have reached 760 800 c at pressures of 7 8 kbar during the peak of metamorphism four low variance parageneses have been identified within a small 3 km2 area of the complex which may reasonably be assumed to have formed under the same p and t conditions isobaric t xco2 diagrams of appropriate equilibria are presented for three different internally consistent sets of thermodynamic data despite the seemingly small numerical differences between the standard state thermodynamic properties of the data sets only one diagram allows the four observed assemblages to coexist within a reasonable temperature range all three phase diagrams require differences in fluid composition on the scale of a thin section strong evidence for effective control of pore fluid composition by local mineral reactions during metamorphism

orientation analysis of localized shear deformation in quartz fibres at the brittle ductile transition abstract the crystallographic preferred orientation of fibrous quartz veins deformed under subgreenschist facies conditions along a temperature gradient from 270 to 370 c has been investigated to explore the active mechanism during deformation of the quartz fibres the complete crystallographic orientation of individual recrystallized grains and subgrains was determined by using a combination of cip computer integrated polarization microscopy ebsd electron back scatter diffraction and additional specially developed image analysis techniques it was found that the quartz fibres deformed along crystallographically controlled shear bands from inverse pole figures of normals to the shear band boundaries it is evident that these bands occur along the positive and negative rhomb planes of quartz it is suggested that they are initiated as fractures along these planes inside the shear bands small grains broken off from the host grain show passive rotation of the lattice consistent with the shear sense of the shear bands continued deformation and increasing temperature lead to different recrystallization microstructures inside and outside the shear bands and to a partial obliteration of the sharp orientation relationships

upstream control of river anastomosis by sediment overloading upper columbia river british columbia canada anastomosing rivers systems of multiple interconnected channels that enclose floodbasins constitute a major category of rivers for which various sedimentary facies models have been developed while the sedimentary products of anastomosing rivers are relatively well known their genesis is still debated a rapidly growing number of ancient alluvial successions being interpreted as of anastomosing river origin including important hydrocarbon reservoirs urge the development of robust models for the genesis of anastomosis to facilitate better interpretation of ancient depositional settings and controls the upper columbia river british columbia canada is the most studied anastomosing river and has played a key role in the development of an anastomosing river facies model two hypotheses for the origin of upper columbia river anastomosis include the following i downstream control by aggrading cross valley alluvial fans and ii upstream control by excessive bedload input from tributaries both upstream and downstream control may force aggradation and avulsions in the upper columbia river in order to test both hypotheses long term millennia scale floodplain sedimentation rates and avulsion frequencies are calculated using 14c dated deeply buried organic floodplain material from cross valley borehole transects the results indicate a downstream decrease in floodplain sedimentation rate and avulsion frequency along the anastomosed reach which is consistent with dominant upstream control by sediment overloading the data here link recent avulsion activity to increased sediment supply during the little ice age ca 1100 to 1950 ad this link is supported by data showing that sediment supply to the upper columbia study reach fluctuated in response to holocene glacial advances and retreats in the hinterland upstream control of anastomosis has considerable implications for the reconstruction of the setting of interpreted ancient anastomosing systems the present research underscores that anastomosing systems typically occur in relatively proximal settings with abundant sediment supplied to low gradient floodplains a situation commonly found in intermontane and foreland basins

natural calcite c axis fabrics an alternate interpretation microstructures and crystallographic fabrics in natural calcite mylonites from newfoundland canada are discussed and compared with other natural and experimental examples in the newfoundland rocks porphyroclasts deformed mainly by twinning and intracrystalline slip and the recrystallized grains deformed by dislocation creep assisted by dynamic recrystallization by grain boundary migration t 350 c porphyroclast c axis fabrics consist of a broad point maximum oriented normal to the shear plane recrystallized grains in the ultramylonites have strong c axis fabrics consisting of a single point maximum oriented either parallel or oblique to the normal to the shear plane experimental simple shear calcite c axis fabrics have been divided by previous workers into low lt and high temperature ht fabrics twinning and intracrystalline r slip have been identified as the dominant deformation mechanisms in low temperature experiments whereas intracrystalline r and f slip dominate in high temperature experiments lt fabrics have a single point maximum which is oriented approximately parallel to the principal stress axis and is oblique to the normal to the shear plane ht fabrics have three point maxima which are asymmetrical both in orientation and intensity with respect to the normal to the shear plane the transition between lt and ht fabrics occurs at temperatures lower than 400 c in fine grained limestones and lower than 700 c in marbles at laboratory strain rates natural calcite c axis fabrics in agreement with study almost invariably consist of a point maximum oriented either normal or slightly oblique to the shear plane because their fabrics are similar to experimental lt fabrics they have been interpreted as lt fabrics by comparison with experimental and computer simulated lt fabrics the obliquity of their c axis point maximum with respect to the shear plane normal has been used as a shear sense indicator and as a measure of the degree of non coaxiality of the deformation two lines of evidence mitigate against calcite c axis fabrics as shear sense and strain path indicators in mylonite zones first the obliquity of the c axis point maximum in natural examples is not consistent for a given sense of shear secondly mylonite microstructures reported here and in the literature supports the contention that dynamic recrystallization and intracrystalline slip are responsible for the strong c axis fabrics of most calcite mylonites therefore natural calcite mylonite c axis fabrics do not generally correlate with either lt or ht experimental fabrics instead natural calcite mylonites have similar microstructures and fabrics to marbles that were deformed experimentally at very high temperatures 800 850 c and underwent pervasive dynamic recrystallization by grain boundary migration consequently c axis fabrics of calcite mylonites cannot be used as a measure of the degree of non coaxiality of the deformation nor can they be used as shear sense indicators

preferred crystallographic orientation development during the plastic and superplastic flow of calcite rocks abstract grain size sensitive flow of fine grained rocks such as are often found in mylonitic shear zones is commonly inferred to be accomplished by grain boundary sliding so that no preferred crystallographic orientation is expected to develop pre existing fabrics are supposed to weaken and grain shapes to remain equant however many fine grained mylonites commonly exhibit marked preferred crystallographic orientation whilst displaying a stable but weak grain shape fabric we have examined preferred crystallographic orientation and shape fabric development during the flow of synthetic ultrafine grained 700 c the strong crystal plastic fabric develops more slowly but is not eliminated through the broad transition into true superplastic flow this is interpreted to reflect an important role of intracrystalline plasticity in the accommodation of grain boundary sliding from experiments designed to investigate the extent of survival or modification of an initial twinning fabric through recrystallization and subsequent hot working it was shown that although the microstructure could be totally annealed the early formed fabric survived even subsequent high temperature crystal plastic or superplastic flow this may explain the frequent occurrence of strong low temperature fabrics in calcite ultramylonites

marble mylonites in the bancroft shear zone ontario canada microstructures and deformation mechanisms abstract mylonitization of medium grade marbles in the bancroft shear zone ontario canada is characterized by decreasing grain size of both calcite and graphite and a variety of textures calcite grain sizes vary from several millimeters in the protolith to 50 200 m in mylonite to m in ultramylonite corresponding calcite grain shapes are equant in the protolith elongate in protomylonite first developed dimensional preferred orientation equant in coarse mylonite elongate in fine mylonite second developed dimensional preferred orientation and generally equant in ultramylonite which suggests that external energy applied stress that tends to elongate grains competed with internal energy sources e g distortional strain that favor equant shapes graphite grain size changes from several millimeters to centimeters in the protolith to submicroscopic in ultramylonite in the mylonitic stages graphite is present as dark bands while in the ultramylonitic stage it is preserved as a fine coating on calcite grains based on textural evidence twinning exponential creep regime i dynamic recrystallization power law creep regime ii and possibly grain boundary sliding superplasticity regime iii are considered the dominant deformation mechanisms with increasing intensity of mylonitization their activity is largely controlled by calcite grain size calcite grain size reduction occurred predominantly by the process of rotation recrystallization during the early stages of mylonitization as indicated by the occurrence of core and mantle or mortar structures and by the grain size of subgrains and recrystallized grains grain elongation in s c structures indicates the activity of migration recrystallization these structures are not the result of flattening of originally equant grains differential stress estimates in coarse mylonites and ultramylonites based on recrystallized grain size are 2 5 and 14 38 mpa respectively initial grain size reduction of graphite occurred by progressive separation along basal planes analogous to mica fish formation in quartzo feldspathic mylonites calcite graphite thermometry on mylonitic and ultramylonitic samples shows that the metamorphic conditions during mylonitization were 475 50 c which combined with a differential stress value of 26 mpa gives a strain rate of 1 2 x 10 10 s 1 based on constitutive equations corresponding displacement rates are mm yr 1

postnappe stacking extension and exhumation of high pressure low temperature rocks in the island of crete greece the island of crete consists largely of nappes of contrasting lithologies and metamorphism that were stacked southward during an oligocene early miocene n s compression with the lower nappes undergoing a high pressure low temperature hp lt metamorphism this was followed by a n s miocene crustal extension that caused thinning of the nappes and uplift of the hp lt metamorphic rocks ductile conditions took place in the lower nappes associated with a retrograde greenschist metamorphism while semiductile to brittle conditions took place in the upper nappes a major normal detachment fault separates the lower and the upper nappes the quartz texture analyses and the symmetry of structures indicate bulk coaxial deformation while the results of strain analysis suggest both constrictional and flattening strains a younger ne sw compression affected both the thinned nappe pile and the late miocene pliocene sedimentary basins finally a pliocene ne sw extension led to further uplift and exhumation of the hp lt metamorphic rocks this cyclic tectonometamorphic process of alternate compression and extension took place during the migration of the hellenic orogenic belt toward the most external units including successive tectonic events

rotational quantities in homogeneous flow and the development of smallscale structure abstract a summary discussion is given of attempts to quantify the degree of non coaxiality of rock flow from observations of rock structure and of some principles governing the sensitivity of structure to non coaxiality this is preceded by an extended review of relevant terminology since some of the current confusion about rotational features in rocks stems from confusion about basic concepts and terminology from continuum mechanics the kinematic vorticity number has been estimated for naturally deformed rocks from three localities these yield vorticity numbers ranging from 0 35 to 0 9 corresponding to flows intermediate in character between pure and simple shearing

the origin of shape preferred orientations in mylonite inferences from in situ experiments on polycrystalline norcamphor abstract polycrystalline norcamphor c 7 h 10 o undergoing plane strain simple shear in a see through deformation rig develops intergranular microshears whose activity and orientation is closely related to shape preferred orientation spo of dynamically recrystallized grains and grain aggregates intergranular microshears nucleate at 80 90 to the sample s shear zone boundary szb and rotate synthetically toward this boundary they accommodate increasing amounts of incremental shear strain at angles ranging from 40 to 60 s b orientation with respect to the szb with progressive simple shear both the rotation rate and the amount of strain accommodated by these microshears decrease but strain accommodation increases as the microshears attain a lower angle 10 30 s a orientation to the szb the microshears gradually deactivate as they acquire inclination angles to the szb of less than 10 the deactivation of such microshears is accompanied by the nucleation of fresh high angle microshears this heterogeneous deformation is associated with two shape preferred orientations in the norcamphor mylonite a steep oblique grain spo comprising the long axes of dynamically recrystallized grains 40 60 subparallel to the s b oriented microshears and gently inclined domainal spo 20 subparallel to the s a oriented microshears the domain spo is defined by the length axes of grain aggregates with a uniform crystallographic preferred orientation the angle between domainal and grain spos is a potential measure of the bulk vorticity during mylonitization

the removable cap suction corer an inexpensive and durable device to extract unconsolidated wet sediments obtaining sediment samples in unconsolidated sediment presents many challenges the locale substrate can be soft penetration of the sediment may be limited preservation of the sedimentary fabric may be incomplete and extraction of the sample may be difficult some of these issues are overcome with vibracore or suction core techniques vibracoring requires heavy bulky and relatively expensive equipment which is costly to ship to the remote field study locations and difficult to use in muddy settings such as muddy tidal flats or coastal marshes the van der staay suction corer van de meene et al 1979 is cost effective and fixes logistical problems but sample extraction has to be done in the field the tess 1 suction corer mendez et al 2003 was designed on the basis of the van der staay suction corer and allows recovering intact sediment samples for further laboratory analysis but it requires custom machinery with more man hours and uses small diameter core tubes with a high possibility of disturbance of sedimentary structures in unconsolidated and waterlogged deposits here we present a new removable cap suction corer that is inexpensive to construct light in weight highly portable and can be designed to extract any core diameter the corer can be used for coarse and fine grained wet sediments tidal flats riverine and estuarine bars coastal marshes and shallow subaqueous environments lakes rivers shallow shoreface

s c mylonites two types of foliations are commonly developed in mylonites and mylonitic rocks a s surfaces related to the accumulation of finite strain and b c surfaces related to displacement discontinuities or zones of relatively high shear strain there are two types of s c mylonites type i s c mylonites described by berthe et al typically occur in deformed granitoids they involve narrow zones of intense shear strain which cut across mylonitic foliation type ii s c mylonites described here have widespread occurrence in quartz mica rocks involved in zones of intense non coaxial laminar flow the c surfaces are defined by trails of mica fish formed as the result of microscopic displacement discontinuities or zones of very high shear strain the s surfaces are defined by oblique foliations in the adjacent quartz aggregates formed as the result of dynamic recrystallization which periodically resets the finite strain clock these oblique foliations are characterized by grain elongations alignments of segments of the grain boundary enveloping surfaces and by trails of grains with similar c axis orientations examples of this aspect of foliation development in mylonitic rocks are so widespread that we suggest the creation of a broad class of s c tectonites and a deviation from the general tradition of purely geometric analysis of foliation and time relationships kinematic indicators such as those discussed here allow the recognition of kilometre scale zones of intense non coaxial laminar flow in crustal rocks and unambiguous determination of the sense of shear

a 3 d kinematic model of fabric development in polycrystalline aggregates comparisons with experimental and natural examples abstract a 3 d generalization of the approach previously developed by etchecopar for the simulation of fabrics in polycrystalline aggregates is presented the model is based on a geometric minimization of gaps overlaps and boundary displacements between deformed neighbouring cells each cell may deform using a small number

the concept of steady state foliation abstract a steady state foliation is thought of as a foliation with constant orientation and intensity in a rock undergoing steady state flow the problem discussed is whether such straininsensitive foliations exist in rocks and how they might be distinguished from the more familiar strain sensitive foliations steady state foliations would be useful indicators of stress or of strain rate yet misleading if used as indicators of finite strain one requirement for maintenance of steady state foliations is operation of processes that locally weaken grain shape preferred orientation or reduce grain aspect ratios or destroy lithological layering during flow these are the inverses of the processes normally discussed in connection with foliation development many such processes are known though the details of how and when they operate efficiently enough to stabilize a foliation are unknown there is however a tendency for some of them to operate more efficiently at high strains high temperatures and in flow regimes with high vorticity natural and experimental high temperature shear zones are accordingly one environment in which steadystate foliations may be encountered an experiment is described in which a thin specimen of para dichlorobenzene was given a large shearing deformation at 90 of its absolute melting point a steady state foliation was not produced but the technique is a promising one for further microstructural studies where in situ observation of material undergoing large deformations is desirable

fluid rock interaction and thermal evolution during thrusting of an alpine metamorphic complex tinos island greece this study examines the fluid rock interaction and thermal evolution along a thrust that juxtaposes calcite rich marbles of high p t metamorphic unit of the attic cycladic massif greece on top of a lower grade dolomite marble unit the tertiary thrust represents a major phase of tectonic movement related to the decompression of the alpine orogen in the hellenides the stable isotope signatures of the thrust plane and adjacent sections of the footwall and hanging wall rocks are characterized by significant carbon and oxygen isotope depletions the depletion is most pronounced in calcite but is almost entirely missing in coexisting dolomite the isotopic patterns in the thrust zone can be explained by the infiltration of an externally derived water rich h2o co2 ch4 fluid x c x co 2 x ch 4 0 05 at water rock ratios on the order of 0 1 to 0 5 by weight the fluid induced calcite recrystallization is viewed as an important rheological control during thrusting the temperature evolution of the footwall hanging wall and mylonitic tectonic contact was determined by calcite dolomite solvus thermometry histograms of calcite dolomite temperatures are interpreted as indicating a heating of the footwall dolomite marble during the thrusting of the hotter upper plate conversely the hanging wall marble unit was cooled during the thrusting the calcite dolomite thermometry of the thrust plane gives temperatures intermediate between the initial temperatures of the lower and upper marble units and this leads to the conclusion that conductive heat transfer rather than fluid infiltration controlled the thermal evolution during thrusting

block hydraulic conductivity of cross bedded fluvial sediments this paper concerns the three dimensional block hydraulic conductivity of blocks of limited size in cross bedded sediments of meandering rivers based on a simplified sedimentological model of a single point bar a stochastic model is proposed to simulate the spatial distribution of core scale conductivities within point bar deposits the required parameters for these simulations are obtained from actual hydraulic conductivity measurements and sedimentological field data the simulations are then used to calculate the three dimensional block conductivity tensor with a numerical upscaling procedure the stochastic simulation model is used to investigate the influence of slope and aspect angle of the cross beds and the core scale anisotropy ratio on the block hydraulic conductivity it is expected that the off diagonal elements of the block conductivity tensor generally cannot be neglected the core scale anisotropy ratio which reflects the small scale layering within the cross beds can potentially have a significant impact on the elements of the block conductivity tensor

anisotropy of diamagnetic susceptibility in thassos marble a comparison between measured and modeled data abstract a study of shear zones within the calcite marble complex of the island of thassos greece shows that the low field anisotropy of magnetic susceptibility ams technique can be successfully applied to diamagnetic rocks for characterizing rock fabrics the strain path involves both an early pure shear stage and a simple shear overprint that is documented by a transition from triaxial neutral to uniaxial prolate shapes of ams ellipsoids the maximum susceptibility is oriented perpendicular to the rock foliation reflecting the preferred orientation of calcite c axes in the protolith as well as in the mylonites for three samples that represent different types of calcite fabrics the ams was recalculated from neutron and electron backscatter diffraction textural data a comparison of the measured and modeled data shows a good coincidence for the orientation of the principal ams axes and for the recalculated anisotropy data both measured and modeled data sets reflect the change from neutral to distinct prolate ellipsoids during progressive deformation

on the relationship between the formation of shear zones and the form of the flow law for rocks undergoing dynamic recrystallization abstract the formation and maintenance of shear zones in rocks at high temperatures when viscous flow takes place at constant volume requires strain weakening several processes can lead to a strain weakening characteristic in the flow of such rocks to high strains but conventional experimental rock mechanics techniques cannot normally provide high quality mechanical data to high strains recent experimental data for calcite marble have demonstrated strain weakening accompanied by dynamic recrystallization a power law with strain exponent r can be used to describe the dependence of the flow stress on strain together with the conventional description of the dependence of strain rate on stress described by a stress exponent n the stable localization of strain into shear zones is favoured by more negative values of r e g r 0 8 and more positive values of n e g n 5 to 7 a further important factor is the critical strain e0 for the onset of recrystallization and hence strain weakening shear zone formation is favoured by smaller values for e0 shear zone formation weakens the crustal section in which they occur but the amount of weakening may not be great perhaps no more than 50 of the previous steady state flow stress certain rock types such as mafic igneous rocks may through their mechanical properties be more susceptible to shear zone formation than others such as mica schists it is emphasized that the analysis presented is based upon a strain dependent flow law that describes single phase dynamic recrystallization by grain boundary migration that leads to enhanced recovery without any change in deformation mechanism no consideration is given to possibly important effects arising from the activation of grain size sensitive flow or to potential weakening effects arising from syntectonic metamorphic reactions

the evolution of buctouche spit new brunswick canada buctouche spit is an 11 km long sandy spit on the northeast shore of new brunswick canada it is a typical flying spit having a narrow proximal section which is characterized by a single foredune that is prone to overwash and transgression and a much wider prograding distal section which is characterized by a well developed foredune backed by a series of relict foredunes an optical luminescence dating method in which infrared stimulation is used on potassium feldspars was developed for this study it is a very promising method for dating holocene deposits yielding good resolution and having simplicity of measurement the luminescence ages obtained indicate that buctouche spit extended at a rate of 4 m yr 1 between 715 45 years ago and 240 25 years ago this extension rate equates to a distal end accumulation rate of 56 000 m3 yr 1 for this period at this rate of accretion the present buctouche spit would have formed during the past 2000 years the core data indicate that the spit is 2200 years old which is consistent with the preceding estimate since 240 25 years ago the rate of sediment accumulation at buctouche spit has apparently been falling the accumulation rate between 1839 and 1945 was 23 000 m3 yr 1 less than half of the historic rate of 56 000 m3 yr 1 since 1945 the distal end of buctouche spit has not prograded extended measurably this is because buctouche spit s present vertical growth rate is at most equal to the rate of relative sea level rise at the spit and may be as little as half of the rate of relative sea level rise present sediment supply to buctouche spit is probably 16 000 m3 yr 1 it is likely that the much larger sediment supply required to sustain an historic accretion rate of 56 000 m3 yr 1 was derived from the reworking of a former sandy barrier located seaward and possibly updrift of the present spit buctouche spit is now in a limited sediment supply situation and it has become a constrained spit sediment is being eroded from the centre section of the spit transported to the distal end and then carried away by ebb tidal currents the centre section of the spit will likely breach within the next hundred years if a breach or breaches becomes permanently established in the centre section of the spit the distal end will suffer significant erosion and may be destroyed after this the cycle of rebuilding and landward migration would likely start again at a new location landward of the present spit as relative sea level continues to rise it is concluded that the key to understanding buctouche spit s long term evolution is recognizing that it is migrating landward both by continuous processes like overwash and by the more discrete and catastrophic process of overstepping

deep crustal extensional faulting in the ivrea zone of northern italy abstract the ivrea zone in northern italy consists of amphibolite and granulite facies metasedimentary metabasic and ultrabasic rocks and is thought to represent a section through the deep continental crust detailed mapping of the high grade part of this zone has revealed a series of high temperature shear zones which after the effects of later low temperature faulting and folding have been removed appears to accommodate extension of the sequence the shear zones formed at temperatures similar to those at which thermal equilibration occurred in the host rocks and locally show evidence for development under prograde conditions these observations fit well with thermal effects calculated from a simple heat flow model for extensional faulting in this model the effects of shear heating and displacement of cool hanging wall rocks against hotter footwall rocks are calculated these observations indicate that faulting as a mode of failure may be important during extension of the lower continental crust especially when basic rocks are the dominant components of the rock pile a restored section through the extended lower crust as observed in the ivrea zone is presented and the extent to which the features observed are able to explain seismic reflectivity of the lower crust is discussed

evolution of a calcite marble shear zone complex on thassos island greece microstructural and textural fabrics and their kinematic significance abstract the deformation history of a monophase calcite marble shear zone complex on thassos island northern greece is reconstructed by detailed geometric studies of the textural and microstructural patterns relative to a fixed reference system shear zone boundary szb strain localization within the massive marble complex is linked to decreasing p t conditions during the exhumation process of the metamorphic core complex solvus thermometry indicates that temperatures of 300 350 c prevailed during part of the shear zone deformation history the coarse grained marble protolith outside the shear zone is characterized by symmetrically oriented twin sets due to early coaxial deformation a component of heterogeneous non coaxial deformation is first recorded within the adjacent protomylonite enhanced strain weakening by dynamic recrystallization promoted strong localization of plastic deformation in the ultramylonite of the calcite shear zone where high strain was accommodated by non coaxial flow this study demonstrates that both a pure shear and a simple shear strain path can result in similar crystallographic preferred orientations single c axis maximum perpendicular to the szb by different dominant deformation mechanisms separated a axis pole figures a and a axis show different density distributions with orthorhombic texture symmetry in the protolith marble and monoclinic symmetry in the ultramylonite marble consistently with the observed grain fabric symmetry

the three metamorphic belts of the hellenides a review and a kinematic interpretation summary new results obtained in the three metamorphic belts of the hellenides suggest that each belt has a different structure geotectonic setting and evolution the external metamorphic belt of peloponnesus crete has probably resulted from nappe movement in the vicinity of an evolving island arc system the blueschists occurring within this external belt are allochthonous and have been transported with other nappes over the external carbonate platform of the pre apulian plattenkalk ionian tripolitza almyropotamos units this implies that there was only one true blueschist domain initiated in the cycladic area during early eocene the kinematic interpretation is based on the distinction of a and b structures and on the nature of the shear zones between the different domains especially that between the metamorphic hellenides below and the non metamorphic hellenides above the existence of the probably lower palaeozoic meta sedimentary sequence of kastoria in the northern pelagonian zone indicates that the overlying ophiolite bearing almopias unit has been derived from within the hercynian domain the kastoria sedimentary sequence the ophiolites of vertiskos and the carbonate platform of pangeon are considered to be most likely elements of a probable hercynian orogeny

lattice preferred orientations and microstructures of deformed cordilleran marbles correlation of shear indicators and determination of strain path marbles from nine cordilleran deformation zones the raft river and snake ranges the ruby rincon and riverside mountains the eastern peninsular ranges mylonite zone and three sites in the death valley region were sampled for microstructural analysis of thin and ultra thin sections and pole figure measurements by x ray diffraction to interpret their deformation history microstructural observations reveal various degrees of recrystallization fabrics are predominantly symmetric but some samples display consistent asymmetries of the long grain axes schistosity and the macroscopic foliation plane defined by compositional layering these asymmetric microstructures are most pronounced in non recrystallized fabrics well developed textures are found at all sites with c axes perpendicular to the foliation and a axes in a girdle in the foliation plane the sense of shear derived from slightly asymmetric 1120 pole figures is consistent with the shear sense determined in the field and by s c structures in adjacent gneisses the magnitude of the obliquity of 1120 girdles suggests that the component of simple shear is relatively small even in samples with highly asymmetric microstructures we conclude that the obliquity of the 1120 girdles is a reliable indicator of the strain path during the last substantial increment of strain and indicates that the carbonate rocks studied deformed dominantly in pure shear this implied strain path for carbonate rocks may not necessarily reflect the regional strain path because strain may be partitioned between rocks of different compositions or between zones of predominantly pure and simple shear at all scales

late holocene environments at verlorenvlei western cape province south africa abstract evidence on late quaternary palaeoenvironments of the western and southwestern cape of south africa has facilitated detailed reconstructions mainly for the mountains where organic sediment accumulation has been favoured by relatively humid climates on the lowlands particularly those of the west coast region the search for sites traditionally regarded as suitable for the accumulation of sediments containing preserved fossil pollen has commenced only more recently this area of the western cape province has however provided quaternary scientists with a rich archaeological record that has yet to be tested against independent lines of palaeoecological evidence a sequence of sediments which have accumulated in what is today a large freshwater coastal lake verlorenvlei has been sampled at grootdrift 15 km inland of the coast a series of seven sediment cores was extracted during 1991 and three of these have now been sub sampled radiocarbon dated and subjected to a range of palaeoenvironmental techniques in particular pollen analysis sedimentology and geochemistry three pollen diagrams are presented which reveal the vegetation history of both the immediate grootdrift environment and the wider catchment during several periods over the last 5500 years a mid holocene higher sea level is evident in two of the cores which were examined for pollen throughout their length the surrounding area appears at this time to have been somewhat more arid than today following a hiatus in sedimentation some time after 4300 bp the marine conditions at the site disappear and are replaced by fresh water as the dominant hydrological and ecological influence around the time of colonial occupation of the region some 300 years ago lacustrine conditions prevailed at grootdrift and greater moisture availability is indicated in the catchment during the last 300 years the picture revealed by high resolution palynology of the third core is one of progressive levels of human disturbance both in and around the verlorenvlei evidence from a textural and geochemical analysis of the sediments is consistent with the interpretation based on fossil pollen the implications of this information are examined against the archaeology and palaeoecology of several cave sites in the vicinity of the vlei the consistency apparent in the range of types of evidence suggests that some reliance can be placed on the mid late holocene palaeoenvironmental reconstruction at verlorenvlei the paper thus provides insight into the nature of the complex interactions between late holocene climate change sea level fluctuation vegetation change vlei hydrology and human activity in the western cape coastal region

the partitioning of deformation in flowing rock masses abstract most geological structures owe their development to heterogeneous and or non steady flow flow partitioning refers to the division of the instantaneous velocity field into components related to translation strain and rotation flow partitioning during the development of a geological structure usually changes with time and different histories of flow are involved at different points in the developing structure heterogeneity leads to the development of flow domains with different characteristics notably the degree of non coaxiality of deformation and different partitionings of the rotational component of flow into shear induced vorticity and spin during non steady flow there is often continual repartitioning of vorticity between the spin and shear induced components spin is an important aspect of flow leading to the development of structure this paper discusses the partitioning of deformation in a flowing rock mass coaxial flow tends to occur in isotropic materials in regions near weakly constrained boundaries non coaxial flow is favoured by suitably oriented material anisotropy and or the presence of well defined boundary constraints involving parallel relative displacements a developing anisotropy e g a crystallographic fabric in a shear zone can induce a change from coaxial to non coaxial flow other effects are related to the scale of approximation for example bulk shear induced vorticity can be locally converted to spin flow in the crust and mantle of the earth produces geological structures on all scales and involves many different types of rheological response the factors that affect flow partitioning e g spin versus non coaxial deformation are therefore of interest

deposits of the sandy braided south saskatchewan river implications for the use of modern analogs in reconstructing channel dimensions in reservoir characterization estimation of the dimensions of fluvial geobodies from core data is a notoriously difficult problem in reservoir modeling to try and improve such estimates and hence reduce uncertainty in geomodels data on dunes unit bars cross bar channels and compound bars and their associated deposits are presented herein from the sand bed braided south saskatchewan river canada these data are used to test models that relate the scale of the formative bed forms to the dimensions of the preserved deposits and therefore provide an insight as to how such deposits may be preserved over geologic time the preservation of bed form geometry is quantified by comparing the alluvial architecture above and below the maximum erosion depth of the modern channel deposits this comparison shows that there is no significant difference in the mean set thickness of dune cross strata above and below the basal erosion surface of the contemporary channel thus suggesting that dimensional relationships between dune deposits and the formative bed form dimensions are likely to be valid from both recent and older deposits the data show that estimates of mean bankfull flow depth derived from dune unit bar and cross bar channel deposits are all very similar thus the use of all these metrics together can provide a useful check that all components and scales of the alluvial architecture have been identified correctly when building reservoir models the data also highlight several practical issues with identifying and applying data relating to cross strata for example the deposits of unit bars were found to be severely truncated in length and width with only approximately 10 of the mean bar form length remaining and thus making identification in section difficult for similar reasons the deposits of compound bars were found to be especially difficult to recognize and hence estimates of channel depth based on this method may be problematic where only core data are available i e no outcrop data exist formative flow depths are suggested to be best reconstructed using cross strata formed by dunes however theoretical relationships between the distribution of set thicknesses and formative dune height are found to result in slight overestimates of the latter and hence mean bankfull flow depths derived from these measurements this article illustrates that the preservation of fluvial cross strata and thus the paleohydraulic inferences that can be drawn from them are a function of the ratio of the size and migration rate of bed forms and the time scale of aggradation and channel migration these factors must thus be considered when deciding on appropriate length thickness ratios for the purposes of object based modeling in reservoir characterization

avulsions channel evolution and floodplain sedimentation rates of the anastomosing upper columbia river british columbia canada ages of channels of the anastomosing upper columbia river south eastern british columbia canada were investigated in a cross valley transect by c 14 dating of subsurface floodplain organic material from beneath levees the avulsion history within the transect was deduced from these data and morphological stages in channel development were recognized additionally floodplain sedimentation rates were established the new data demonstrate that the upper columbia river is a long lived dynamic anastomosing system results show that anastomosis at the study location has persisted since at least 2700 cal years bp with avulsions occurring frequently at least nine channels have formed in the studied cross valley transect within the past 3000 years channel lifetimes from formation to abandonment appear to be highly variable ranging from approximately 800 to 3000 years log jams provoking avulsions and or silting up of old channels are proposed as reasons for this variability long term average floodplain sedimentation rates appear to be significantly lower than previously proposed by smith 1983 int assoc sedimentol spec publ 6 155 168 a long term 4550 years average of 1 75 mm per year after compaction was based on c 14 dates whilea short term sedimentation rate of 0 8 mm was determined for a single relatively small seasonal flood in 1994 using sediment traps however short term sedimentation rates vary considerably over the floodplain with levees aggrading up to four times faster than floodbasins channels of the upper columbia river anastomosed reach follow a consistent pattern in their development with each stage being characterized by different morphology and processes channel evolution comprises the following succession 1 avulsion stage in which a crevasse splay channel deepens by scour and levee sedimentation 2 widening and deepening stage in which bank slumping and bed scouring dominates 3 infilling stage in which either channel narrowing bank accretion or channel shallowing bed accretion takes place and 4 abandonment stage in which the residual remnant channel is filled exclusively by silt clay and organic material vertical stacking super imposition of active channels on recent channel fill sand bodies is a notable feature of the upper columbia river which suggests that reoccupation of residual channels is a common process

vibracoring a new method for coring deep lakes abstract a major limitation facing lacustrine researchers is an inability to take long 6 to 18 m continuous sediment cores in water depths greater than several metres at a low cost these problems have limited the advancement of palaeolimnology and quaternary stratigraphy working from a frozen lake surface new developments make it possible to take 18 m long vibracores in water depths of at least 55 m high frequency low amplitude vibrations are transferred down a series of 6 m long 7 6 cm diameter core tubes 3 inch aluminum irrigation pipe rigidly coupled end to end vibrations liquify sediment adjacent to the core tube walls thus allowing penetration of the sediment a core catcher prevents sediment from extruding out of the tube end during extraction this cost effective and labour efficient two persons coring system has been achieved by developing rigid core tube couplers a hand powered drum cable hoist and a set of operation procedures the hoist cable end is attached to a clamp 1 m above the core catcher and at each coupler along the pipe string at 6 m intervals lifting or lowering using a drum cable hoist allows for quick coupling and decoupling of core tubes although vibracoring is an improvement over other systems it still has portability limitations because of the bulk and weight of the equipment and large amount of pipe required for deep coring three sleds are required to transport equipment from the nearest vehicle access to the coring sites