the motion of rigid ellipsoidal particles in slow flows abstract preferred orientations of fabric elements in tectonites are usually interpreted with reference to two continuum models namely those of march 1932 and jeffrey 1922 it is argued that the first of these is usually inappropriate if the fabric elements are rigid inhomogeneities embedded in a deformable matrix jeffrey s model is more applicable but until recently its use has been limited to problems involving axisymmetric particles and simple flow geometries in this paper jeffrey s equations have been solved numerically in order to investigate the behaviour of more general systems the motions of two particle shapes one a prolate spheroid the other a triaxial ellipsoid are considered in parallel for four specified flow geometries which involve pure or simple shear and combinations of both any departures from particle axisymmetry or from simple flow geometries have a significant effect on particle behaviour compared with that predicted by spheroid simple flow solutions

novel paleoecology of a postextinction reef famennian late devonian of the canning basin northwestern australia reefs are widely supposed to be particularly susceptible to mass extinction events and to survive only as low diversity remnant communities dominated by holdover or disaster taxa famennian late devonian reefs exposed in the windjana limestone of the canning basin northwestern australia demonstrate however that a novel reef ecology was established in the immediate aftermath of the frasnian famennian mass extinction here diverse calcimicrobes including rothpletzella spp shuguria spp ortonella and girvanella together with bryozoans brachiopods and stromatoporoid sphinctozoan and lithistid sponges grew as complex framework intergrowths in previously undocumented morphological forms forming spectacular elevated laminar to platy structures to 3 m in diameter and 0 35 m thick at least 15 morphospecies of lithistids are now identified where only two were previously documented these communities show no substantial reduction in biodiversity compared to frasnian counterparts nor any change in tiering or loss of complex ecological interactions these observations suggest that where stable carbonate platforms persisted after mass extinction events reef building could continue more important they demonstrate that no protracted interval of time was necessarily required either for recovery to ecological stability or for completely new reef ecologies to assemble such studies highlight the need to document ecosystem recovery after mass extinction events using detailed paleoecological analyses in addition to simple compilations of global biodiversity changes

resolution of the earliest metazoan record differential taphonomy of ediacaran and paleozoic fossil molds and casts abstract much of our knowledge of early metazoan evolution is derived from unmineralized death mask or endorelief mold and cast fossils in ediacaran clastic sedimentary rocks this record is often regarded as a unique ediacaran taphonomic window however the prevalence of soft bodied molds and casts in paleozoic clastic rocks has been increasing begging an extension or modification to our understanding of this preservational motif chief amongst such fossils are eldonids a non biomineralized group of stem deuterostomes because eldonids are also preserved as compressed or flattened fossils from deposits like the burgess shale chengjiang and kaili they offer a comparative case study for evaluating the taphonomic fidelity of mold cast style preservation during this interval eds and raman microspectroscopic analysis of ordovician and devonian eldonid molds and casts and comparison with burgess shale eldonids suggests the mold cast taphonomic style produces significantly lower fidelity of fossil preservation we propose that eldonid mold cast fossils are preserved by the adsorption of reduced iron ions onto tissues composed primarily of high molecular weight hmw biopolymers which require enzymatic degradation prior to decay nucleation and growth of aluminosilicates and or sulfides around these adsorbed ions forms a fossilizable surface veneer preserving a death mask mold more labile tissues could not be fossilized in this mold and cast style ediacaran mold and cast fossils from south australia the white sea region of russia namibia and newfoundland exhibit preservational characteristics consistent with this new proposed model analysis of their preservational mode suggests that the first metazoans which would have lacked hmw biopolymeric tissues could not have been fossilized in this particular style thus understanding the origin and earliest evolution of the metazoa requires a focus on alternative modes of fossilization

reef development at the frasnian famennian mass extinction boundary a newly compiled global reef database indicates that the 5 6 myr long frasnian late devonian metazoan reef episode had relatively lowdiversity compared to middle devonian highs w ith over 200 genera of calcitic rugose and tabulate corals following an initial early rise after late givetian coral and stromatoporoid extinctions reefs expanded for the last time during mid frasnian sealevel highstands but declined markedly in the late frasnian rhenana linguiformis conodont zones belowthe frasnian famennian f f boundary globally metazoan reefs were wiped out by the end frasnian some famennian reefs while partly retaining the structure of the underlying carbonate platform were built by cyanobacterial consortia such as renalcis rothpletzella girvanella and epiphyton during the famennian foraminiferans with calcite walls became abundant for the first time in the phanerozoic adding a newdimension to carbonate platforms colonial rugose corals phaceloid cerioid and thamnasterioid modules were absent in the early post extinction phases up into the mid famennian and very rare and non reefbuilding later but solitary deep water lazarus corals survived locally coral sponge reefs are unknown from the 21 myr long famennian also a time of very lowplatform carbonate production rare small isolated stromatoporoid sponge and lithistid sponge patch reefs returned episodically during the famennian in north america western europe australia and china the aragonitic stromatoporoids became extinct at the end of the famennian during a late devonian tectonically very active collisional caledonian mountain building phase oceanic and atmospheric cooling accompanied by sealevel lowstand systems exposed most carbonate platforms accelerating coastal erosion and karsting this increased the amount of clastics in the shelf slope setting in the last 1 3 myr prior to the f f boundary often burying reefs immediately following there were protracted losses in nearly all major tropical shelf benthic marine invertebrates exceeded only by the end permian extinctions in severity there is no apparent link between black organic rich horizons and reef demise at or close to the f f boundary the f f boundary not also marks the largest change from widespread flooded early and mid paleozoic continental cratons to narrow distal shelves but also spikes the largest known global phanerozoic shift in atmospheric o2 enrichment and co2 drawdown this threshold matched the rise of the first tropical rainforests and expansion of terrestrial biomes on the tropical coastal lowlands formerly occupied by carbonate platforms 6 2002 elsevier science b v all rights reserved

effect of the frasnian famennian extinction event on the stromatoporoids although paleozoic stromatoporoids were drastically reduced in abundance and diversity by the frasnian famennian f f mass extinction event at least 20 genera mostly labechiids and clathrodictyids are known from latest devonian rocks some typical frasnian genera survived the f f crisis to die out in early famennian time and others took part in the strunian resurgence of the stromatoporoids small stromatoporoid reefs are known from famennian and strunian rocks the history of the group suggests a large rapid but progressive environmental change at the f f boundary and the survival of genera better adapted to cool water

quantitative analysis and paleoecology of earliest mississippian microbial reefs gudman formation queensland australia not just post disaster phenomena small 30 m diameter 9 m thick reefs in the gudman formation of queensland australia are the oldest known mississippian reefs occurring very near the devonian carboniferous boundary following late devonian extinction events they occur in high energy shallow oolitic grainstones and consist of 70 microbialite and bound detritus on the basis of point count analysis skeletal organisms including potential frame building rugose and tabulate corals ramose bryozoans crinoids and algae occur in growth position but account for only 4 4 of sampled framework the microbial framework was syndepositionally rigid on the basis of 1 vertical and overhanging relief in a high energy setting 2 export of framework intraclasts 3 hard substrate encrusting organisms 4 abundant hard rock borings and 5 neptunian dikes unusually for mississippian reefs stromatolites make up a large part of the framework 32 but more typical thrombolites are equally abundant interfingering of skeletal organisms and microbialites suggests that they grew in welloxygenated normal marine waters and that microbial biofilms competed effectively with skeletal organisms for available substrate abundant and diverse co occurring skeletal flora and fauna are inconsistent with interpretations of gudman stromatolites as post extinction disaster taxa hence gudman reefs are stromatolite rich examples of a larger class of microbialite dominated phanerozoic reefs that occurred with abundant skeletal metazoans in normal marine settings

microbial influence on erosion grain transport and bedform genesis in sandy substrates under unidirectional flow interpreting the physical dynamics of ancient environments requires an understanding of how current generated sedimentary structures such as ripples and dunes are created traditional interpretations of these structures are based on experimental flume studies of unconsolidated quartz sand in which stepwise increases in flow velocity yield a suite of sedimentary structures analogous to those found in the rock record yet cyanobacteria which were excluded from these studies are pervasive in wet sandy environments and secrete sufficient extracellular polysaccharides to inhibit grain movement and markedly change the conditions under which sedimentary structures form here the results of flume experiments using cyanobacteria inoculated quartz sand are reported which demonstrate that microbes strongly influence the behaviour of unconsolidated sand in medium sand thin ca 0ae 1t o 0 ae5 mm thick microbial communities growing at the sediment water interface can nearly double the flow velocity required to produce the traditional sequence of ripple fi dune fi plane bed lamination bedforms in some cases these thin film like microbial communities can inhibit the growth of ripples or dunes entirely and instead bed shear stresses result in flip over and rip up structures thicker ca 1 mm thick microbial mats mediate terracing of erosional edges they also foster transport of multigrain aggregates and yield a bedform progression consisting of flipovers fi roll ups fi rip ups of bound sand

martin brasier s contribution to the palaeobiology of the ediacaran cambrian transition martin brasier s work spanned almost the entire geological column but the origin of animals and the nature of the cambrian explosion were areas of particular interest to him martin adopted a holistic approach to the study of these topics that considered the interplay between multiple geological and biological phenomena and he sought to interpret the fossil record within the broad context of geological biogeochemical and ecological changes in the earth system here we summarize martin s main contributions to this area of research and assess the impact of his findings on the development of this field karl popper would have said that palaeontology is not real science because you can t go out and sample it i think absolutely the opposite i think this is actually where science is it s trying to guess what lies over the hill and map terra incognita when people come in and colonize that s just technology martin brasier 2013 excerpt from a phone interview in moor 2016

modular construction of early ediacaran complex life forms newly discovered exceptionally preserved soft bodied fossils near spaniard9s bay in eastern newfoundland exhibit features not previously described from ediacaran terminal neoproterozoic fossils all of the spaniard9s bay taxa were composed of similar architectural elements centimeter scale frondlets exhibiting three orders of fracticality in branching frondlets were combined as modules atop semi rigid organic skeletons to form a wide array of larger constructions including frondose and plumose structures this architecture and construction define the rangeomorphs a biological clade that dominated the mistaken point assemblage 575 to 560 million years ago but does not appear to be ancestral to any phanerozoic or modern organisms

rotation of ductile fabrics across the alpine fault and cenozoic bending of the new zealand orocline this study examines the role and kinematic mechanism of vertical axis rotations and folding during early development phases of a transform plate boundary in continental crust near the alpine fault in new zealand this involved an initial period of widely distributed shear including wrench folding together with vertical axis rotations and orocline development followed by a focusing of slip onto a single fault in the south island the haast schist has been displaced horizontally in the neogene by c 480 km of dextral slip on the alpine fault ls tectonites on either side of the fault lie on different limbs of a profound z shaped orocline rotation of these limbs took place about a vertical axis resulting in 25 30 of clockwise rotation of fabrics to the nw of the alpine fault relative to those to the south foliation anisotropy caused bending to take place locally by folding about an inclined hinge in the limbs of a pre existing synform which tightened during the deformation on both sides of the fault involvement of post oligocene age structures in wrench folding and bending suggests that the bending took place entirely in the cenozoic before or during miocene development of the alpine fault as a through going structure during this early phase of plate boundary deformation shear was distributed across a zone 100 km wide vertical axis rotation affected fault bounded basement slivers that shortened perpendicular to their strike a style of deformation like that in the transverse ranges in california today

an evaluation of criteria to deduce the sense of movement in sheared rocks some of the most useful criteria for the deduction of the sense of shear are summarized for use in areas where unequivocal field evidence is lacking apparently conflicting evidence from rotated pressure shadow regions around porphyroclasts and porphyroblasts is clarified the use of quartz crystallographic fabric asymmetry to deduce the shear sense in the bulk rock should be treated with caution and used only together with detailed microstructural observations

strain path partitioning within thrust sheets microstructural and petrofabric evidence from the moine thrust zone at loch eriboll northwest scotland abstract quartz c axis fabrics and microstructures have been investigated within a suite of quartzites collected from the loch eriboll area of the moine thrust zone and are used to interpret the detailed processes involved in fabric evolution the intensity of quartz c axis fabrics is directly proportional to the calculated strain magnitude a correlation is also established between the pattern of c axis fabrics and the calculated strain symmetry two kinematic domains are recognized within one of the studied thrust sheets which outcrops immediately beneath the moine thrust within the upper and central levels of the thrust sheet coaxial deformation is indicated by conjugate mutually interfering shear bands globular low strain detrital quartz grains whose c axes are aligned sub parallel to the principal finite shortening direction z and quartz c axis fabrics which are symmetric both in terms of skeletal outline and intensity distribution with respect to mylonitic foliation and lineation non coaxial deformation is indicated within the more intensely deformed and recrystallized quartzites located near the base of the thrust sheet by single sets of shear bands and c axis fabrics which are asymmetric with respect to foliation and lineation tectonic models offering possible explanations for the presence of kinematic strain path domains within thrust sheets are considered

silica secreting biota and mass extinctions survival patterns and processes abstract high survival and at least regional blooms of siliceous marine groups with coeval retreat of calcareous organisms is known from the frasnian famennian radiolarians silicisponges and end cretaceous diatoms radiolarians mass extinctions a strictly actualistic approach to the palaeoecology of silica secreting biota is of limited significance especially for silica enriched and oligotrophic palaeozoic epeiric seas and in particular during biotic turnovers adaptation among silica secreting marine plankton has led toward more efficient utilization of shrinking dissolved silica resources within surface waters of cenozoic oceans as shown by both radiolarians and diatoms this biosiliceous signal during the major biotic crises is mostly explained by a large scale increase in volcano hydrothermal activity during major plate boundary re arrangements triggering global ecosystem perturbations the cumulative stimulus favoured siliceous versus calcareous biota growth deposition and preservation because of 1 a higher rate of input of silica and other nutrients promoting eutrophication pulses 2 a punctuated or reversed greenhouse climatic effect episodically coupled with 3 vigorous oceanic circulation due to a variety of volcanogenic upwellings and turnovers siliceous communities adapted to more eutrophic conditions have thrived in the stressed niches on the other hand expanding anoxia and nutrification may lead to a stepwise loss of deep water niches and consequent selective decline of more specialized oligotrophic radiolarians and other pelagic biota by contrast siliceous faunas experienced severe non selective losses during the end permian mass extinction recorded as a chert gap thus the greatest environmental disaster of the phanerozoic was driven by a unique set of killing factors extremely effective for radiolarian productivity e g due to the combination of a drastic volcanic winter with expanding superanoxia

resolving miss conceptions and misconceptions a geological approach to sedimentary surface textures generated by microbial and abiotic processes the rock record contains a rich variety of sedimentary surface textures on siliciclastic sandstone siltstone and mudstone bedding planes in recent years an increasing number of these textures have been attributed to surficial microbial mats at the time of deposition resulting in their classification as microbially induced sedimentary structures or miss research into miss has developed at a rapid rate resulting in a number of misconceptions in the literature here we attempt to rectify these miss misunderstandings the first part of this paper surveys the stratigraphic and environmental range of reported miss revealing that contrary to popular belief there are more reported miss bearing rock units of phanerozoic than precambrian age furthermore miss exhibit a pan environmental and almost continuous record since the archean claims for the stratigraphic restriction of miss to intervals prior to the evolution of grazing organisms or after mass extinction events as well as claims for the environmental restriction of miss appear to result from sampling bias in the second part of the paper we suggest that raised awareness of miss has come at the cost of a decreasing appreciation of abiotic processes that may create morphologically similar features by introducing the umbrella term sedimentary surface textures of which miss are one subset we suggest a practical methodology for classifying such structures in the geological record we illustrate how elucidating the formative mechanisms of ancient sedimentary surface textures usually requires consideration of a suite of sedimentological evidence from surrounding strata resultant interpretations microbial or non microbial should be couched within a reasonable degree of uncertainty this approach recognizes that morphological similarity alone does not constitute scientific proof of a common origin and reinstates a passive descriptive terminology for sedimentary surface textures that cannot be achieved with the current miss lexicon it is hoped that this new terminology will reduce the number of overly sensational and misleading claims of miss occurrence and permit the means to practically separate initial observation from interpretation furthermore this methodology offers a scientific approach that appreciates the low likelihood of conclusively identifying microbial structures from visual appearance alone informing the search for true miss in earth s geological record and potentially on other planetary bodies such as mars

flow in natural shear zones the consequences of spinning flow regimes abstract the ancient flow regime in natural shear zones is often considered to have followed a deformation path comparable to that in theoretical shear zones i e progressive simple shear between rigid wall rocks with a persistent flow plane orientation parallel to the edges of the zone this is often based on the presence of monoclinic fabric elements in the zones which indicate a dominantly non coaxial flow regime though not necessarily persistent simple shear the deformation fabric of a shear zone from the pyrenees illustrates that even at a kinematic vorticity number of w 1 simple shear along the entire deformation path obliqueness of the flow planes with the edges of the zone is possible for some time if incremental stretching axes were rotating with respect to zone edges this implies that ductile deformation must have taken place in the wall rock of the zone such a flow regime may be difficult to recognize with the fabric criteria used at present but leads to an unusual kinematic significance of the shear zone involved in extreme cases the zone may have acted as a passive marker in a ductilely deforming medium it also means that not only w must be known to reconstruct the ancient flow regime in natural shear zones but at least also the time dependence of the orientation of the incremental stretching axes

calcification of cyanobacterial filaments girvanella and the origin of lower paleozoic lime mud the origin of lime mud in ancient shallow marine carbonate platforms is one of the most vexing mysteries in sedimentary geology possible sources previously suggested include the physical and biological disintegration of animal and algal bioclasts mineralizing pelagic organisms and spontaneous whitings from either abiotic precipitation or calcification of suspended picoplankton and organic matter the styles of occurrence of micrite walled tubes referred to girvanella in lower paleozoic limestones argue that occasional permineralization of mucilaginous sheaths of the benthic filamentous cyanobacteria precursor could have produced lime mud when the tubes fell apart estimated volumes are comparable to those calculated for the modern bahama platform the copious growth of cyanobacteria typical in shallow marine settings indicates that this mechanism could account for a considerable amount of lime mud especially in the early paleozoic

evolutionary relationships within the avalonian ediacara biota new insights from laser analysis abstract we report new high resolution laser scanning of the type material for the earliest complex ediacaran genera charnia bradgatia charniodiscus and ivesheadia from charnwood uk and compare these with beothukis mistakensis gen et sp nov and the recently described taxa charnia wardi charnia antecedens and fractofusus spp from broadly coeval strata in newfoundland we use the laser and other techniques to map the similarities and differences in morphology between these ediacaran rangeomorphs key features are suggested to include the number of growth axes the number and placement of growth tips the presence of radiating or subparallel axes for the first and higher order branches the extent of displayed or undisplayed leaf like rangeomorph9 architecture and the extent of furling of the margins of these leaf like elements these features are then used to propose suggested homologies between these taxa leading to a preliminary phylogenetic hypothesis for the evolution of the avalonian ediacara biota

the eigenvalues of steady flow in mohr space abstract the eigenvalues of the velocity gradients tensor and of the position gradients tensor describing instantaneous flow or incremental deformations are easily determined in mohr space as the points of intersection between a mohr circle and the horizontal mohr axis intersections occur only if the tensor has real eigenvalues eigenvectors associated with the eigenvalues are directions of no rotation a line of material particles under an eigenvector does not rotate during steady flow although the line is instantaneously stretching at a rate equal to its corresponding eigenvalue for non spinning steady sub simple shearing there will always be two eigenvectors the angle between the eigenvectors is dependent on the ratio of pure to simple shearing rates this angle decreases as the pure shearing component decreases relative to the simple shearing component superimposed increments of pure shear and simple shear produce an equivalent set of non rotated material lines but the deformation histories of these lines are rotational the influence of eigendirections on geological structures is constrained by the rheology of a rock during deformation in homogenous steady flow no influence may be obvious spatially periodic material softening along eigenvectors during bulk hardening may result in a complex fabric of intersecting shear zones the geometry of the fabric elements provides important information on the bulk deformation of the rock particularly on the bulk shear sense

wrinkle structures microbially mediated sedimentary structures common in subtidal siliciclastic settings at the proterozoic phanerozoic transition sedimentary structures produced by microbial activity are well known from ancient carbonate facies but little is known about equivalent microbial structures in ancient siliciclastic facies wrinkle marks and kinneyia ripples are closely related sedimentary structures that are commonly found in ancient siliciclastics and they may represent microbial activity to evaluate this possibility these structures were studied in vendian cambrian strata of north america and compared to structures formed by modern microbial mat communities in redfish bay texas striking similarities in sedimentologic petrographic and morphologic characteristics of modern and ancient occurrences suggest that the enigmatic but widespread ancient structures could have been formed by microbial processes the relative abundance of these structures in siliciclastic facies of this interval further suggests that prior to the onset of relatively intense bioturbation in the ordovician microbial mats could have played a significant role in the evolution and diversification of early life in a broad variety of sea floor environments

estimation of vorticity from fibrous calcite veins central maine usa an estimate for the vorticity of a mid paleozoic deformation was made using deformed and or rotated calcite veins as natural vorticity and shear strain gauges in a northern part of the u s appalachians the spatial distribution of extended shortened and shortened then extended elongate fibrous calcite veins in limestone layers were used to construct a mohr circle for the position gradient tensor h from which vorticity and flow parameters were calculated the amount of rotation of large axial ratio fibrous calcite veins in quartz phyllite layer boudin necks and linear veins in limestone layers were used to calculate shear strain as a check on flow parameters calculated from the distribution of deformed veins these analyses demonstrate that the d3 phase of deformation was characterized by a mean kinematic vorticity number of 0 67 the existence of a non coaxial fabric defined by reactivated foliation and dextral asymmetric folds is also established age constraints suggest that this non coaxial deformation in eastern central maine began early in the devonian and continued at least through regional peak metamorphism this study represents the first quantitative assessment of the vorticity of devonian deformation in maine and the results suggest that a transpressional tectonic model is appropriate for the area at that time

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

deformation partitioning and porphyroblast rotation in meta morphic rocks a radical reinterpretation abstract most porphyroblasts never rotate during ductile deformation provided they do not intern ally deform during subsequent events with the exception of relatively uncommon but spectacular examples of spiralling garnets instead the sur rounding foliation rotates and reactivates due to partitioning of the deformation around the porphyroblast consequently porphyroblasts commonly preserve the orientation of early foliations and stretching lineations within strain shadows or inclusion trails even where these structures have been rotated or obliterated in the matrix due to subsequent deformation these relationships can be readily used to help develop an understanding of the processes of foliation development and they demonstrate the prominent role of reactivation of old foliations during subse quent deformation they can also be used to determine the deformation history as porphyro blasts only rotate when the deformation cannot partition and involves progressive shearing with no combined bulk shortening component

anoxic events in the late frasnian causes of the frasnian famennian faunal crisis carbon isotope data collected from five frasnian famennian boundary sections in central europe show two positive 13 c excursions in the late frasnian 367 ma both anomalies coincided with the deposition of the bituminous lower and upper kellwasser horizons the carbon isotope patterns indicate two phases of enhanced burial and subsequent recycling of organic carbon a maximum formation of warm saline waters on the subtropical to tropical epicontinental shelves during late frasnian transgressive episodes may have induced oceanic oxygen deficits variations in the c org burial rates may have resulted in changes in the co 2 concentrations in the oceans and in the atmosphere culminating in global climatic changes we propose that the repeated co occurrences of sea level fluctuations anoxic conditions and global climatic changes during the late frasnian would affect especially the subtropical to tropical shallow water communities and reef ecosystems which were severely affected during the frasnian famennian faunal crisis

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

explaining the exceptional preservation of ediacaran rangeomorphs from spaniard s bay newfoundland a hydraulic model exceptional 3 d preservation of ediacaran rangeomorph fossils is found on a single bedding plane at upper island cove spaniard s bay newfoundland this high quality preservation has previously been explained by entrainment of organisms within the td e mudstone division of a distal turbidite followed by encasement within concretions our sedimentological and taphonomic analysis reveals a clear association between these fossils and evidence for erosive unidirectional flows including scours marks tool marks ridge and groove marks parting lineations and current crescents we suggest an alternative sequence of events that runs broadly as follows i rangeomorph discs were anchored to the seafloor during deposition of planar laminated silts our unit 2 10 mm thick td now bearing pyrite framboids and pyritized organic matter ii rangeomorph fronds were then felled and entrained by high velocity unidirectional currents to lie within their own erosional scours at the top of unit 2 or to form tool marks iii this topography was then draped and cast by soft weathering sand unit 3 tc associated with the growth of early diagenetic pyrite around sand grains pyrite grains also appear to have replaced clumps of organic matter fossil impressions have since been exposed by differential weathering of the ferruginous sands with respect to the silts this new context now provides a parsimonious explanation for a range of hitherto paradoxical structures features previously regarded as microbial mats bubble trains that formed in the lee of sinuous ripples on the top of unit 2 may be explained as load casts or by localised gas escape within areas of lowered hydraulic pressure rangeomorph fronds remarkably preserved in positive rather than the more usual negative epirelief are explained by means of sediment casting of branches that became ruptured in the high velocity current paradoxical structures previously thought to be enclosing biological sheaths around rangeomorph fronds are reinterpreted as scour marks whereas imbricate overlaps of first order branches in beothukis trepassia and avalofractus are explained by hydraulic shear driven by overlying currents across ruptured and deflated fronds we find that rangeomorph bodies could be deflated imbricated folded over inverted and infilled with fine sediment our hydraulic model provides a null hypothesis against which future observations of rangeomorph fronds can now be tested it removes some significant anomalies in our understanding of rangeomorph architecture and provides a better understanding of the physical properties of their body tissues permitting the possibility of a reasoned consideration of their puzzling biological affinities

bedding parallel shear thrusting and quartz vein formation in witwatersrand quartzites abstract evidence is presented which clearly demonstrates the presence of thrust faults in the gold bearing sedimentary rocks of the witwatersrand supergroup a package of these rocks subjected to shear strain allowed overlying beds on the northern margin of the witwatersrand basin to move outwards in a n nw direction movement took place on ductile shear zones which were parallel to the bedding and often reveal excellent examples of ramp and duplex structures it has also been established that syntectonic foliation planes were utilized as planes of shear in certain circumstances quantitative estimates of strain for the pile which are of necessity minimum values give 1 fluids from which vein quartz formed played a critical role in the shear plane movement and may have relieved a portion of the overburden load during deformation

the role of stress in ductile deformation abstract the concept of progressive ductile deformation has previously been discussed mainly in kinematic terms little attention has been paid to the principal stresses that control the formation of ductile deformation patterns many deformation patterns of rocks may have been created in a stress field of stable orientation an equation derived here directly links the orientation of the major axis of deviatoric stress to the stretch and rotation components of plane isochoric deformation in isotropic rocks estimations of rotation and stretch of finite strain ellipsoids may therefore aid the recovery of palaeostress axes using a nomogram introduced here the inclination of the major principal deviatoric stress axis with respect to a reference plane controls both the particle movement paths and mode of progressive deformation the deformation tensor obtained by integrating the rate of displacement or velocity gradient equations is first expressed in time dependent terms comprising only the normal and shear components of the strain rate tensor mohr s equations of stress can then be used to link strain rates to the major principal stress responsible for them the rate of progressive deformation is determined by the rheology of the deforming rocks and the magnitude of the deviatoric stress this derivation yields a time dependent deformation tensor which is expressed in terms of the dynamic viscosity and the magnitude and orientation of the major deviatoric stress with respect to a reference plane the significance of this deformation tensor is illustrated by forward modelling using computer graphics these allow the progressive deformation of a unit volume of rock in response to deviatoric stresses of various stable orientations to be visualized

famennian upper devonian calcimicrobial renalcis reef at miaomen guilin guangxi south china famennian upper devonian reefs represent a calcimicrobial and stromatolitic reef framework with a few skeleton dominated stromatoporoid examples after the skeletal metazoans in reef ecosystems were impacted on a global scale by the frasnian famennian extinction event calcimicrobes thrombolites stromatolites and biologically induced cement formed the major part of reef framework volume and contributed to rigidity of reefs in this study one example of famennian non skeletal carbonate buildups at miaomen guilin south china has been documented in detail thick and massive limestones in the platform margin facies show a lateral transition to well bedded fenestral and laminated limestone in the back reef facies southeastward and to well bedded intraclastic grainstone mudstone and shale with lithoclasts and breccias in fore reef slope facies northwestward the miaomen reef is almost exclusively constructed by calcimicrobes and cement major reef builders are renalcis izhella paraepiphyton garwoodia and a keega like in microbe other reef builders are wetheredella rivularia rothpletzella ortonella and girvanella some less common algae and calcimicrobes also occur in the miaomen reef including parachaetetes solenopora tharama like objects and unidentifiable microbes cavities are well developed in the reef limestone the miaomen renalcis reefs developed along leeward platform margin settings adjoined by intraplatform depressions and rimmed famennian carbonate platforms along with stromatolite reefs ooid shoals brachiopod shell shoals nautiloid shoals and previously described renalcis epiphyton and renalcis cement reefs miaomen reefs demonstrate the significant roles of calcimicrobes and microbial carbonates in the development of famennian carbonate systems in south china famennian microbial reefs in guilin represent an interval of profound biotic change in the style and extent of carbonate buildups and in the composition of buildup communities and indicate important environmental and ecological changes within the carbonate system c 2003 elsevier b v all rights reserved

was phanerozoic reef history controlled by the distribution of non enzymatically secreted reef carbonates microbial carbonate and biologically induced cement throughout most of the phanerozoic reef rigidity resulted as much or more from early lithification by microbial carbonates and biologically induced cements non enzymatic carbonates than from biological encrustation of or by large enzymatically secreted metazoan skeletons reef framework is divided into four categories 1 skeletal metazoan 2 non skeletal microbialite stromatolite and thrombolite 3 calcimicrobe and 4 biocementstone in which small or delicate organisms serve as scaffolds for rigid cement crusts the last three categories are dominated by non enzymatic carbonates skeletal framework and non skeletal microbialite framework were the most abundant framework types through the phanerozoic the composition and abundance of skeletal framework was controlled largely by mass extinction events but most reefs consisted of both microbialite and skeletal organisms in a mutually beneficial relationship microbialite framework was abundant throughout the palaeozoic and early mesozoic but declined after the jurassic calcimicrobe framework was important during the cambrian early ordovician and devonian and biocementstone framework was important from the late mississippian to the late triassic the phanerozoic history of reefs does not correlate well with the stratigraphic distribution of large skeletal reef builders or with a variety of physicochemical parameters including sea level history wilson cycle or global climate cycles because non enzymatic carbonates result from induction by non obligate calcifiers and not enzymatic precipitation by obligate calcifiers the distribution of these carbonates was controlled to a larger extent by temporal changes in physicochemical parameters affecting the saturation state of sea water with respect to carbonate minerals changes in pco ca mg ratios cation concentrations and temperature may have affected the abundance of non enzymatic carbonates and hence reefs independently from the effects of these same parameters and mass extinction events on skeletal reef biota the decline in abundance of reefal microbialite and absence of calcimicrobe and biocementstone reef framework after the jurassic may be a result of relatively low saturation states of sea water owing to increased removal and sequestration of finite marine carbonate resources by calcareous plankton since the jurassic reef history is difficult to correlate with temporal changes in specific global parameters because these parameters affect skeletal biota and biologically induced carbonate precipitation independently hence reef history was regulated not just by skeletal reef biota but by parameters governing non enzymatic carbonates

strains and vorticity beneath a tabular batholith in the zambesi belt northeast zimbabwe structures on scales of cm to dm in formerly universally orientated quartz veins are used to analyse deformation in part of the pan african e w zambezi orogen by integrating their orientations over length scales of six orders of magnitude 2 cm to 20 km sixty two strain ellipsoids on locality scales 20 200 m and six on district scales 2 20 km demonstrate that each of the constituent lithologies together with the metasedimentary cover sequence as a whole strained as homogeneous materials after the quartz veins developed related to orogenic structures these homogeneous strains demonstrate that major folds in the cover initiated as buckles on e w axes before the quartz veins were inserted but tightened passively as the metamorphic grade rose from greenschist to upper amphibolite facies by contrast post vein f2 flow folds originally on ne axes amplified actively at the same time but without noticeable shortening such f2 folds are interpreted to have been driven by the gravitational stabilization of the relatively dense proterozoic cover and its less dense archaean basement the instability due to this primary density inversion was aided significantly by a secondary density inversion induced by the orogenic thermal gradient while the metamorphic gradient was below 30 c km the basement rose in conformable upright ridges on the same ne trends preserved in the archaean rocks in the kalahari craton to the south the early e w trending folds and later ne trending folds of the zambezi orogeny interfered to generate class ic patterns of elongate domes and basins throughout the cover sequence when the regional thermal gradient rose above 30 c km the basement rose diapirically into or through the cover as domes of migma migmatite or magma on centres 5 10 km apart melts of the deep basement eventually drained up the stems of earlier migma diapirs and presumably fed a tabular batholith in the epizone that has since been removed by erosion diapirs of basement or remobilized basement displaced the earlier class ic fold interference pattern in the cover in all directions this led to uniaxial radial flattening and vorticity down to scales of 100 m in the cover within about 2 km of the basement contact meanwhile away from the immediate influence of the diapirs the cover sequence continued to undergo vorticity about a nnw shortening axis on district scales but only spun on locality scales the mean post vein n s orogenic shortening of the cover sequence is found to be 60 on the locality scale linear strain fields for data from appropriate districts are interpreted in terms of plane strains with about 50 60 of the total shortening being due to uniaxial nnw volume loss of sio2 and water on both locality and district scales all the post vein shortening measured in the orogen can be attributed to lateral displacement of the cover by diapirs of a sialic basement which may not have shortened on the regional scale at any stage pre vein shortening of the cover by thrusts and ductile flow has not been quantified but n s shortening due to early buckles is calculated to have been about 17 the upward loss of sio2 in solution and either solid or molten basement from levels still exposed to others since eroded appears to be a far more characteristic aspect of the eastern part of the zambezi orogen than lateral shortening on the regional scale

strain ellipsoids from incompetent dykes application to volume loss during mylonitization in the singo gneiss zone central sweden we describe how to constrain bulk strain ellipsoids by distinguishing the attitudes of mullioned from planar contacts in incompetent dykes our method is demonstrated by using metabasaltic dykes to

ediacaran fossil preservation taphonomy and diagenesis of a discoid biota from the amadeus basin central australia abstract aspects of the preservation of the enigmatic soft bodied ediacara biota in relatively coarse grained siliciclastic sediments remain unresolved the taphonomic model proposed by gehling in 1999 is here validated through examination of previously unpublished discoid fossils from the shallow marine deltaic arumbera sandstone in the amadeus basin within the flinders style preservation the model predicts stalked frondose organisms attached by a discoid holdfast to a microbial mat encrusted sea bed with sedimentation as periodic storm event beds during a terminal neoproterozoic taphonomic window a pyrite sole veneer provided critical fossil rigidity during burial the sequence of sedimentation events presented here advances a slightly modified gehling model no direct evidence of pyrite is detected instead a thin haematite clay coating covers fossil hyporelief surfaces nevertheless an original pyrite sole veneer is indirectly validated by tracing its diagenetic history through to today s haematite detailed petrography demonstrates reducing conditions during eodiagenesis that remained anoxic mildly alkaline through mesodiagenesis thus the pyrite sole veneer survived deepest burial meteoric water penetrated the deep regolith during prolonged telodiagenesis creating oxic mildly acidic pore water so pyrite oxidised and hydrolised to haematite the applicability of gehling s model is higher where marine shelf sediments subsided continuously in shallower or eroded areas that experienced intermittent meteoric eodiagenesis diagenetic history was different and ediacaran fossil preservation unlikely diagenetic history therefore has crucial implications for ediacaran fossil preservation

dickinsonia from ediacara a new look at morphology and body construction abstract during groundbreaking fieldwork on the taphonomy and sedimentology of the famous fossil deposits at the ediacara sheep station goldring r currnow c n 1967 the stratigraphy and facies of the late precambrian at ediacara south australia journal of the geological society of australia 14 195 214 roland goldring formed one of the few collections of original ediacara fossil materials that can still be accessed outside of australia this material collected from the type locality at ediacara is now housed in the natural history museum at oxford and includes excellent specimens of the enigmatic fossil dickinsonia costata sprigg r c 1949 early cambrian jellyfishes of ediacara south australia and mount john kimberly district western australia transactions of the royal society of south australia 73 72 99 this first detailed description of the oxford dickinsonia material and its associated wrinkle structures is discussed with relevance to some wider problems facing ediacaran taxonomy serial photographic reconstruction techniques are used to produce high resolution analyses of these fossils attention is drawn to features such as relief inversion of the upper surface in which a spatulate b segment is typically preserved in positive hyporelief while the rest of the fossil is mainly preserved in negative hyporelief segments can also appear to bifurcate and merge a feature which together with the glide plane of symmetry is inconsistent with the bilaterian model often argued for this taxon controversial and conflicting claims for the presence of circular muscles a rigid outer cuticle an axial gut and even radial tentacles can all be explained by a single hypothesis that involves a simple possibly coelenterate grade organism which had a distinctive hydraulic body structure that underwent stretching strain collapse and contraction as the internal turgor pressure in the segments changed especially during death and burial

calcimicrobes and microbialites in lagoonal sediments from mississippian midale beds williston basin southeastern saskatchewan canada carbonates of mississippian age visean in the midale beds charles formation of southeastern saskatchewan canada produce significant amounts of hydrocarbons the midale beds represent deposition in a shallow water periodically restricted epeiric setting the sedimentation is characterized by a variety of shallow water carbonate lithologies ranging from wackestone packstone grainstone to microbial boundstone algae calcimicrobes and related microbial fabrics are common features in these limestones cores and thin sections through the midale beds in the glen ewen and midale pools of southeastern saskatchewan were examined in order to study the contribution of calcimicrobes and microbial fabrics to the sedimentation of mississippian carbonate rocks in southeastern saskatchewan calcimicrobes and microbial fabrics are the important components in the grainstones and microbial boundstone the calcimicrobes are commonly found as porostromate forms including garwoodia sp and ortonella sp and other forms such as archaeolithoporella like girvanella like wetheredella like and problematic microbes also occur but are not common microbial fabrics are characterized by microstromatolites microbial laminations thrombolite clotted peloids and fenestrate forms calcimicrobes stabilized grains and modified and created sediments and the related syndepositional microbial fabrics affected the development of porosity permeability of midale carbonates calcimicrobes and microbial fabrics in midale beds highlighted a significant account of microbial facies associated with the mississippian carbonates worldwide

metazoan microbial framework fabrics in a mississippian carboniferous coral sponge microbial reef monto queensland australia microbial fabrics stromatolites thrombolites and calcimicrobes occur in many paleozoic carbonate buildups and commonly dominated reefs after mass extinction events e g middle cambrian famennian late devonian and early mississippian by visean middle mississippian time eastern australian reefs were mostly small microbialite dominated structures but they contained diverse reef building metazoans e g rugose and tabulate corals bryozoans that came to dominate limited reef facies in some cases reefs in the cannindah limestone at old cannindah homestead monto region queensland are exceptional in being the largest such reefs and in having the most complex and differentiated reef facies they occurred on an oolitic crinoidal bank characterized by long tenn continuous carbonate deposition in a shallow high energy setting cannindah reef framework contained lithistid sponges and diverse corals but was dominated by microbialite the microbialites contain diverse thrombolites microdigitate stromatolites and calcimicrobes abundant syndepositional cavities in the microbial framework supported a diverse cryptic fauna including numerous calcimicrobes e g renalcis palaeomicrocodium girvanella ortonella aphralysia and problematica crinoids and ostracodes cavities indicate that the framework was suprastratal both where microbialite dominated and where skeletal organisms played a role in framework construction although these reefs grew following late devonian extinction events that affected skeletal reef builders the dominance of microbialites is difficult to attribute to the absence of appropriate skeletal reef builders the reefs occurred similar to 20 million years after the devonian mississippian transition and diverse potentially reef building corals and algae occur throughout the reefs but never rose to dominate framework construction high siliciclastic flux turbidity abnormal salinity low oxygen levels low light penetration and climatic deterioration can be eliminated as limiting factors for skeletal organisms in the reefs suggesting either that 1 subtle nutrient or ecological factors governed the community membership or 2 that the microbial biofilms competed well for reef substrate c 2005 elsevier b v all rights reserved

remarkable preservation of microbial mats in neoproterozoic siliciclastic settings implications for ediacaran taphonomic models abstract it is beyond doubt that the appearance of infaunal bioturbation and metazoan biomineralization across the ediacaran cambrian transition irreversibly affected the nature of marine sediment architecture and biogeochemistry here we review those changes in relation to their likely effect upon the processes of fossil preservation especially within siliciclastic sediments processes of soft tissue preservation in siliciclastic settings from the ediacaran period including microbes and microbial mats as well as ediacaran macrofossils are here reviewed within this context highlighted examples include the exceptional preservation of microbes found in association with wrinkle structures and ediacaran macrofossils in england and newfoundland replicated by silicate minerals and in the white sea region of russia replicated by iron sulphide these occurrences show that soft tissue preservation in siliciclastic settings went well beyond that typical for ediacaran macrofossils alone and also extended to similar modes of preservation in associated microbes using these new observations it can be argued that several existing explanations for ediacaran fossil preservation can be united within a biogeochemical model that involves evolution of the sediment mixed layer across this transition

biosedimentological features of major microbe metazoan transitions mmts from precambrian to cenozoic abstract biotic activities are involved in almost all sedimentation processes throughout the evolutionary history of life on our planet however deep time organism induced sedimentation and biosedimentary records remain unclear in terms of lithologic types strata stacking patterns and possible controlling factors we document biosedimentary features of major transitions from microbe dominated switching to metazoan dominated biosedimentary systems based on the global distributions of both microbial and metazoan carbonates through precambrian to phanerozoic times with emphasis on sedimentary records from china the compilation of 150 and 180 well documented metazoan and microbial reefs respectively from china reveals that metazoan reefs proliferated during the middle ordovician middle devonian and middle permian whereas microbial reefs were well developed during the cambrian late devonian and early middle triassic plus a moderate development during the early silurian these stratigraphic abundances of metazoan and microbial carbonates of china generally match the global patterns the updated variation trends of microbial and metazoan carbonates throughout the late precambrian and phanerozoic reveal that there were five major microbe metazoan transitions mmts the late ediacaran the cambrian and the aftermaths of the mass extinctions of the end ordovician late devonian and end permian the late ediacaran mmt began with microbe dominated oceans with occasional occurrences of metazoans the presence of cloudina dominated reefs in the latest ediacaran marks the completion of the switching of this microbe dominated depositional system into a metazoan dominated system the cambrian saw the expansion of skeletal microbes i e epiphyton renalcis in the oceans and the stratigraphic successions yield the most diverse biosedimentary deposits and or structures of the entire phanerozoic the cambrian mmt was the longest microbial metazoan alternation period and is marked by two metazoan occurrence peaks marked by dominance of abundant archaeocyath buildups during its epoch 2 and by maceriate and lithistid sponge reefs during the late furongian epoch the early silurian in china saw the deposition of a thick suite of organic rich black shales followed by alternations of microbe rich sediments oil shales and metazoan bearing deposits which are replaced by microbial and metazoan reefs during the late early silurian the late devonian mmt started during the late frasnian and persisted into the early mississippian and thus extended slightly longer than the aftermath of the frasnian famennian extinction interval alternating occurrences of microbial and metazoan reefs characterize this late devonian mmt almost all microbe mediated sediments structures observed in the cambrian mmt reoccurred in the aftermath of the end permian mass extinction during the early middle triassic mmt suggesting high similarities between those two mmts cambrian and early middle triassic mmts also share comparable carbon and sulfur isotopic perturbations warming regimes and generally oxygen deficient seawaters some of these environmental and climatic extremes may also occur during other mmts but they usually did not occur synchronously most mmts seem to have undergone four developmental stages they initiated as microbe dominated successions stage a and then were characterized by alternations of microbe dominated and of metazoan bearing or bioturbated successions stage b both microbial and metazoan reefs co occurred during stage c and a dominance of metazoan reefs marks the development of stage d ediacaran and cambrian mmts seem to have undergone the first three development stages whereas the three post extinction mmts experienced the full set of stages a d corresponding to metazoan survival initial recovery and full recovery the majority of volatile rich large igneous provinces lips coupled with intensive acidification events anoxia and global warming regimes took place during the mesozoic cenozoic however microbe dominated sediments were only widely deposited during the early triassic and greatly declined after that time therefore it seems that microbial abundance in mmts may not be directly related to these extreme lip events this is probably because a primary source of food for the metazoans might have shifted to phytoplankton e g coccoliths dinoflagellates and radiolarians in the marine waters since the triassic certainly the pre mesozoic oceans were not dominated by phytoplankton perturbations in the carbon isotope record characterize all mmts and thus may be reliable proxies indicating mmt biosedimentary systems

fossils and matgrounds from the neoproterozoic longmyndian supergroup shropshire uk body and trace fossils of ediacaran affinity are described from strata of the late neoproterozoic longmyndian supergroup exposed near church stretton shropshire uk the almost spherical soft bodied ediacaran fossil beltanelliformis brunsae menner occurs rarely in the burway formation but much smaller simpler discoidal structures are common in both the burway and synalds formations and are referred to beltanelliformis minutae sp nov similar discoidal structures but with a distinct central depression are included in intrites punctatus fedonkin and are common at several horizons two blocks with numerous examples of medusinites aff asteroides sprigg glaessner wade were recovered from the burway formation the purported ediacaran body fossil arumberia glaessner walter is common at several horizons but its biogenicity is not accepted herein arumberia is thus treated along with evidence for microbially bound sediment surfaces or matgrounds that have been suggested by several authors to be necessary for some types of ediacaran preservation the assemblage of simple trace and body fossils along with matgrounds is typical of latest neoproterozoic time though some elements range into the phanerozoic

a tectonic model for the emplacement of the main donegal granite nw ireland the main donegal granite is shown to lie in a major late stage ne sw trending transcurrent sinistral shear zone in the caledonides of nw ireland the zone which is bent both laterally and upwards around the pluton has displacements which diminish sw along its length from a maximum of approximately 20 km at the ne end of the pluton the granite has also been deformed by this sinistral shear but it has a strain component of its own the nature and development of which indicate that the pluton was accommodated rather than forcefully emplaced in the shear zone strain variations between granite and wall rock and an early plagioclase fabric associated with the petrographic banding of the granite suggest that intrusion and deformation were more or less synchronous at around 400 ma it is suggested that unconsolidated granite in the shear zone locally created high strain rates and instability which caused the zone to bend split lengthwise and progressively create an internal low pressure zone into which the main granite was emplaced the wide variety of emplacement styles of the other radiometrically synchronous plutons of the batholith can be attributed to the progressive development of the shear zone thorr and fanad in tension between the zone and a complimentary structure to the n ardara and toories in local compression at the shear zone termination rosses in tension on the outer arc of the bending zone and trawenagh bay as displaced main granite magma in a similar tensional setting during the final stages of deformation

a re examination of the nama type vendian organism rangea schneiderhoehni the need to re examine rangea has been motivated by two factors first by the recent progress in the understanding of three dimensional mouldic preservation of vendian fossils and second by discoveries of this taxon outside gondwana albeit in the same sedimentary environment as seen in namibia several important features are revealed including the in situ posture in the sediment the double layered quilted structure the tripartite stemless body and the mucous supported sheath in the sediment it is suggested that rangea represents an infaunal organism and that the similarity with other members of the nama type biota reflects convergence in functional and fabricational constraints in relation to infaunal life habit

a critique of vorticity analysis using rigid clasts abstract estimating the kinematic vorticity numbers from rock fabrics presents many problems in this paper we use numerical modeling to investigate the reliability of those widely used vorticity analysis methods using rigid clasts we use very simple flows steady state homogeneous and having monoclinic symmetry to represent plane straining zones and transpressional zones and assume that the rotation of rigid clasts perfectly obeys the theory of jeffery 1922 proceedings of the royal society of london a102 161 179 these are assumptions made in current vorticity analysis using rigid clasts even with these simple assumptions our modeling shows that the current methods have intrinsic uncertainties so large that it is pointless to use the estimated vorticity numbers to constrain shear zone boundary conditions and kinematics it is perfectly consistent with numerical modeling results if the currently reported vorticity numbers estimated from rigid clasts in the range of 0 50 0 85 are all interpreted as being from natural shear zones with close to simple shearing flows the large uncertainties arise because the motion of rigid clasts is intrinsically a three dimensional problem

stable positions of rigid objects in non coaxial flow a study in vorticity analysis abstract distortions of a homogeneous fabric which occur around rigid objects in rocks deformed by non coaxial ductile flow are useful to determine the sense of shear but also contain information on other aspects of the flow rigid objects in a non coaxial flow fall into two categories those which are permanently rotating and those which can reach stable positions at high finite strain the orientation of such immobilized objects with respect to eigenvectors of instantaneous flow is a function of the vorticity number of the flow the ratio of instantaneous stretches and the axial ratio of the object in naturally deformed rocks immobilized rigid objects may be recognized from the geometry of the surrounding fabric tails of recrystallized material around porphyroclasts in a mylonite rotate towards parallellism with the extensional eigenvector of the flow and the shape of tails reflects the rotational behaviour of the porphyroclast the axial ratio of immobilized porphyroclasts and their orientation with respect to the tails can theoretically be used to determine the vorticity number of the flow and deviations from isochoric plane strain in practice such an analysis is as yet difficult but a tentative example is given of the way in which the vorticity number can be calculated from a population of feldspar porphyroclasts in a quartzite mylonite from the french pyrenees

novel reef fabrics from the devonian canning basin western australia abstract large cement filled cavities 0 2 to 1 5 m wide are well developed in slope margin sediments of the spectacular upper devonian frasnian reefs of the canning basin western australia where they account for up to 50 of the primary porosity these are here interpreted as primary reef framework cavities that formed beneath a variety of domal tabular or laminar stromatoporoid sponges of particular note are those created by unusual very thin 2 to 8 mm laminar stromatoporoids mainly stachyodes australe that formed arching hollow domes up to 0 3 m in height and 1 5 m in diameter over the sediment surface to enclose flat based cavities the free undersurface of these stromatoporoids often supported a hitherto unrecognised cryptic community dominated by pendent growth of the putative calcified cyanobacterium renalcis with rare intergrown lithistid sponges and spiny atrypid brachiopods the uneven growth surface of the cryptos imparts an irregular stromatactis like texture to the upper surface of the remaining cavity which is filled by early marine finely banded fibrous cements mainly radiaxial calcite interbedded with often multiple generations of geopetal sediment containing peloids and ostracod debris this ecology yields the tabular stromatoporoid renalcis fabric described ubiquitously from the canning basin reef complex such unusual reef fabrics are a consequence of the ecology of shallow marine mid palaeozoic reefs which were quite unlike that of modern coral reefs the frequent preservation of relatively delicate in situ communities was due to 1 rapid and pervasive early cementation 2 growth under non energetic conditions and 3 the relative insignificance of bioeroders associated with reefs at this time

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

evidence of organic structures in ediacara type fossils and associated microbial mats ediacara type fossils represent a group of soft bodied organisms mainly known from imprints in proterozoic coarse grained siliciclastic sedimentary rocks circular compressions of beltanelliformis brunsae and remains related to ediacara type fossils such as cucullus fraudulentus and mucuplagum primitivum are reported here in an organic mode of preservation from the neoproterozoic doushantuo and liulaobei formations of china they can be interpreted as prokaryotic colonies a charnid fossil with circular attachment disc and stalk but torn off frond is documented in a three dimensional and partly organic mode of preservation from the neoproterozoic ust pinega formation white sea coast russia according to their morphology and structure the charniidae are not regarded by us as pennatulaceans modern myxobacteria illustrate that macroscopic size complexity and even compartmentalization can also be developed by prokaryotic colonies part of the ediacara type fossils may therefore represent prokaryotic colonies or symbiotic organisms involving prokaryotes finally direct evidence indicates that biofilms with associated prokaryotic sheaths preserved in both organic and pyritic fashion form the wrinkled surfaces elephant skin that were preferentially colonized by ediacara type fossils this finding supports previous interpretations based on comparative morphological and sedimentological approaches that ancient wrinkle structures were microbial mats

microbial carbonates as indicators of environmental change and biotic crises in carbonate systems examples from the late devonian alberta basin canada microbial precipitation of calcium carbonate has played a vital role in the development of carbonate platforms since their initiation in the proterozoic we report here the varied roles that microbial carbonates played in late devonian carbonate platforms in the alberta basin canada we recognize microbial carbonates as important contributors within the carbonate system during times of major environmental change including transgressive events in platform environments and the recovery interval following the frasnian famennian mass extinction detailed sequence stratigraphic analysis of two isolated platforms in the canadian rockies was used to document their evolution from a regional ramp to isolated platforms with phases of progradation aggradation and backstepping and renewed progradation related to rates of second order and third order sea level change and basin infill the carbonate system was reorganized following annihilation of many carbonate producing biota during the frasnian famennian mass extinction microbial carbonates figure prominently in both frasnian platform development and the famennian recovery of the carbonate system following the frasnian famennian mass extinction during transgressive platform phases deeper water facies with a crinoid rugose coral and microbe dominated biota abruptly overlie prograding stromatoporoid framestones microbial carbonates consist of abundant girvinellid oncoids that nucleated around other fossil clasts the temporary replacement of stromatoporoid communities during transgression implies a shift from oligotrophic to mesotrophic conditions and the microbes likely capitalized on the change in nutrient supply microbes played a significant role in highstand reef margin facies where renalcis was a binder and cementer in stromatoporoid framestones and downslope rugose coral stromatoporoid mounds microbially laminated carbonates also form stromatolitic mats in peritidal shallowing upward cycles and repetitive stromatolitic intervals commonly indicate sea level lowstands associated with the development of sequence boundaries stromatoporoid reefs and their associated reef interior facies indicate an oligotrophic ecosystem where microbial carbonates were relegated mainly to cryptic or stressed marine habitats an end frasnian sea level lowstand exposed the isolated carbonate platforms transgression and mass extinction characterize the frasnian famennian boundary event and microbial carbonates occur at this horizon in western canada stromatolites oncoids and large scale microbial thrombolites appear to be

mountain building enhanced continental weathering and organic carbon burial as major causes for climatic cooling at the frasnian famennian boundary c 376 ma the late devonian was a period of drastic environmental changes as exemplified by a major biotic crisis at the frasnian famennian boundary ffb and the onset in famennian times of glaciations across southern gondwana worldwide evidence for the coeval development of the major acadian eovariscan belt led us to propose a model relating the late frasnian famennian environmental perturbations to extensive continental uplift through two atmospheric co2 depleting mechanisms 1 the intensification of silicate weathering on the continental areas as attested by a major rise in the 87 sr 86 sr composition of sea water at the ffb and 2 the massive burial of organic carbon kellwasser events in partially confined basins due to the collisional induced reduction of equatorial oceanic communications between the palaeotethysian and panthalassic oceans this process is also suggested to have been controlled by an important primary productivity connected to an increased nutrient availability triggered by the enhanced continental run off

superposed deformations by mohr construction abstract the mohr diagram can provide a simple constructional method for superposing various kinds of deformation examples are given for superposed pure shears and various combinations of pure shear simple shear and area change construction methods are developed on a mohr diagram for reciprocal stretch vs rotation which is actually a representation of the backwards deformation tensor d the superposition of apure shear or a simple shear on an earlier deformation follows elementary principles in mohr space enabling the total deformation to be determined simply more general deformations can be considered as a combination of dilation pure shear and simple shear if body rotations arc omitted which are all amenable to the mohr construction method although confined to problems of two dimensional superposed deformation this new method provides a useful alternative to mathematical or computer based methods to determine the effects of oblique superposition of several deformations the visual appeal of such graphical constructions should aid in the teaching and understanding of deformation processes

earliest known carboniferous shallow water reefs gudman formation tn1b queensland australia implications for late devonian reef collapse and recovery the phanerozoic history of reefs extensively has been considered a direct reflection of the history of skeletal reef building organisms however such a relationship does not characterize global mid paleozoic reef history the extinction of most reef building stromatoporoids and corals at the frasnian famennian boundary correlates with the collapse of north american and european stromatoporoid dominated reefs but western australian russian and chinese reefs were much less severely affected until the late famennian when algae calcimicrobes and nonskeletal microbialites i e stromatolites thrombolites declined globally additionally reef recovery was more rapid than previously thought small early tournaisian tn1b shallow water reefs in the gudman formation of eastern central queensland substantially reduce the duration of the reefless lag time following late devonian reef decline essentially confining it to the strunian gudman reefs are dominated by microbialite but contain a diverse although volumetrically insignificant skeletal fauna and flora including large colonial corals bryozoans crinoids brachiopods and calcareous algae hence mid paleozoic reef collapse and recovery reflect an amalgam of more or less independent histories of skeletal organisms calcimicrobes and nonskeletal microbialites in response to regional and global environmental parameters a better understanding of mid paleozoic reef history will require detailed local and regional scale studies to isolate global from nonglobal signals

microbial mounds prior to the frasnian famennian mass extinctions hantang guilin south china late frasnian mounds of the yunghsien formation guilin south china developed as part of the guilin platform mostly in reef flat and platform margin settings microbial mounds in platform margin settings at hantang about 10 km west of guilin contain frasnian biota such as stachyodes and kuangxiastraea and thus occur below the frasnian famennian mass extinction boundary platform margin facies were dominated by microbes algae and receptaculitids massive corals and stromatoporoids are not common and rarely show reef building functions as they did in givetian time the margin mounds are composed of brachiopod receptaculitid cementstone and a variety of boundstones that contain rothpletzella renalcis thrombolite and stromatolite other microbial communities include girvanella izhella ortonella and wetheredella solenoporoid algae are abundant locally zebra structures and neptunian dykes are well developed at some intervals pervasive early cementation played an important role in lithification of the microbial boundstones and rudstones frasnian reefs of many regions of the world were constructed by stromatoporoids and corals although a shift to calcimicrobe dominated frameworks occurred before the famennian however the exact ages of many frasnian margin outcrops are poorly constrained owing to difficulties dating shallow carbonate facies the hantang mounds represent a microbe dominated reef building community with rare skeletal reef builders consistent with major late devonian changes in reef composition diversity and guild structure occurring before the end of the frasnian a similar transition occurred in the canning basin of western australia but coeval successions in north america western europe and the northern urals are either less well known or represent different bathymetric settings the transition in reef building style below the frasnian famennian boundary is documented here in the two best exposed successions on two continents which may have been global set in the larger context of late devonian and mississippian microbial reef building the hantang mounds help to demonstrate that controls on microbial reef communities differed from those on larger skeletal reef biota calcimicrobes replaced stromatoporoids as major reef builders before the frasnian famennian extinction event and increasing stromatoporoid diversity towards the end of the famennian did not result in a resurgence of skeletal reef frameworks calcimicrobes dominated margin facies through the famennian but declined near the devonian carboniferous boundary stromatolite and thrombolite facies which occurred behind the mound margin at hantang rose to dominate mississippian shallow water reef frameworks with only a minor resurgence of the important frasnian calcimicrobe renalcis in the visean when well skeletonized organisms corals also became volumetrically significant frame builders again

upper devonian reefs and microbialite at maoying south china implications for paleoenvironmental changes abstract reefal carbonate deposits in south china preserve a record of community structure and environmental change across the frasnian famennian boundary and through the late devonian detailing a regional response to one of the largest mass extinction events in earth history the upper devonian frasnian to famennian limestone outcrops in the maoying area of northern ziyun county south china record a carbonate platform depositional setting with some patch reef development the upper devonian carbonate deposits in maoying area are distinctive from those in correlative sections in dushan county with rare argillaceous limestone and dolomitic limestone as well as less dolostone exposures to the south in the guilin area of northeastern guangxi province are similar to those seen in the study area upper devonian carbonates in the maoying area are divided into two lithologic groups reef limestones or dolostone beds composed of five lithologically distinctive intervals the upper devonian strata have a cumulative thickness of 530 m the upper devonian reefs in the maoying area were developed on a shell pavement of brachiopod and bivalve shell shoal packstones and evolved into patch reefs in the interior of the carbonate platform patch reef development was limited to early frasnian time reef building organisms are dominated by massive and tabular stromatoporoids amphipora sp stachyodes sp and coral thamnopora sp while brachiopods and algae accumulated between stromatoporoid skeletal frameworks three evolutionary stages of the frasnian patch reefs including an initial development stage maturity stage and demise stage can be recognized according to sedimentary structures and fossil assemblage characteristics the frasnian strata overlying the patch reefs represent a well bedded back reef depositional setting and are mainly composed of amphipora floatstone and packstone containing a few bulbous stromatoporoids and small fasciculate corals famennian strata are characterized by laminated limestone and fenestrate limestone that are interbedded with infrequent mudstone and argillaceous limestone horizons a prominent microbialite bed developed in the laminated limestone of the famennian carbonate platform consists of stromatolite and thrombolite framestone this bed can be correlated in horizon and age with microbial reefs and mounds in the famennian offshore carbonate platform in the huanan epeiric sea such as famennian microbial reefs and mounds in guilin this represents a biogenic carbonate formation characterized by microbial fabrics that accumulated in a nearshore carbonate platform setting after the global frasnian famennian biotic extinction event which promoted the growth of microbialites during the latest devonian in south china

reconstruction of deformation and flow parameters from deformed vein sets abstract in any homogeneously deformed material sets of material lines with a similar stretch history occupy sectors with a scale and orientation that depend on finite strain area change rate and vorticity of the parent flow if the size and shape of these material line sectors are known they can be used to determine the sense of shear and to calculate deformation and flow parameters even if actual stretches along material lines are unknown in rocks which have undergone near homogeneous deformation on a large scale the orientation of sets of boudinaged folded and folded boudinaged or boudinaged folded veins can be used for such calculations a general theoretical background to the problem is presented and a mohr circle construction to carry out the deformation and flow analysis is introduced among other statements the theory predicts that shortened or folded boudins must develop in non coaxial monophase deformation with considerable area change in the plane of observation or in polyphase deformation

paleoenvironmental and basin analysis of the late neoproterozoic ediacaran upper conception and st john s groups west conception bay newfoundland the conception and st john s groups consist of deep marine fine grained siliciclastic volcaniclastic rocks with abundant impressions of ediacaran body fossils based on previous tectonic reconstructions these strata are interpreted to have accumulated during the transition from a fore arc phase 1 ca 630 560 ma to a strike slip pull apart basin phase 2 ca 560 540 ma the drook and mistaken point formations accumulated during phase 1 and are characterized by tabular thick to medium bedded turbidites east southeast paleoflow directions contourites southwest paleoflow directions and volcanic ash beds the high proportion of each bed consisting of te bouma turbidite division mudstone and the lack of correlation between bed thickness and basal grain size suggest ponding of the turbidity currents the differences in paleoflow directions and texture between conception bay and equivalent units at mistaken point imply that the two areas may have been geographically isolated from each other p

environmental controls on marine ecosystem recovery following mass extinctions with an example from the early triassic abstract the recovery of marine ecosystems following a mass extinction event involves an extended interval of increasing biotic diversity and ecosystem complexity the pace of recovery may be controlled by intrinsic ecosystem or extrinsic environmental factors here we present an analysis of changes in marine conditions following the end permian mass extinction with the objective of evaluating the role of environmental factors in the protracted 5 myr long recovery of marine ecosystems during the early triassic specifically our study examines changes in weathering productivity and redox proxies in three sections in south china chaohu daxiakou and zuodeng and one in northern india mud our results reveal 1 recurrent environmental perturbations during the early triassic 2 a general pattern of high terrestrial weathering rates and more intensely reducing marine redox conditions during the early griesbachian late griesbachian mid smithian and more weakly the mid spathian 3 increases in marine productivity during the aforementioned intervals except for the early griesbachian and 4 stronger and more temporally discrete intervals of environmental change in deepwater sections chaohu and daxiakou relative to shallow and intermediate sections zuodeng and mud our analysis reveals a close relationship between episodes of marine environmental deterioration and a slowing or reversal of ecosystem recovery based on metrics of biodiversity within community alpha diversity infaunal burrowing and ecosystem tiering we infer that the pattern and pace of marine ecosystem recovery was strongly modulated by recurrent environmental perturbations during the early triassic these perturbations were associated with elevated weathering and productivity fluxes implying that nutrient and energy flows were key influences on recovery more regular secular variation in deepwater relative to shallow water environmental conditions implies that perturbations originated at depth i e within the oceanic thermocline and influenced the ocean surface layer irregularly finally we compared patterns of environmental disturbance and ecosystem recovery following the other four big five phanerozoic mass extinctions to evaluate whether commonalities exist in general the pace of ecosystem recovery depends on the degree of stability of the post crisis marine environment

vorticity analysis in a metachert from the sanbagawa belt sw japan abstract common tectonic features of deformed metachert include both sets of extended and shortened veins and quartz c axis preferred orientation patterns in such rocks information is also commonly available on the orientation and magnitude of the finite strain these features can be used to derive two independent estimates of the degree of non coaxiality during deformation in an example from the sanbagawa belt sw japan the two methods give consistent results and show that deformation was intermediate between simple and pure shear such types of progressive deformation distributed over a broad region could cause substantial subvertical thinning and be a significant factor in the exhumation of high p t rocks in the region

platform margins reef facies and microbial carbonates a comparison of devonian reef complexes in the canning basin western australia and the guilin region south china devonian reef complexes were well developed in western australia and south china but no detailed direct comparison has been made between reef building in the two regions the regions differ in several respects including tectonic stratigraphic and palaeoceanographic palaeogeographic settings and the reef building styles reflect minor differences in reef builders and reef facies similarities and differences between the two reef complexes provide insights into the characteristics of platform margins reef facies and microbial carbonates of both regions here we present a comparison of platform margin types from different stratigraphic positions in the late devonian reef complex of the canning basin western australia and middle and late devonian margin to marginal slope successions in guilin south china comparisons are integrated into a review of the reefal stratigraphy of both regions reef facies reef complex architecture temporal reef builder associations 2nd order stratigraphy and platform cyclicity in the two regions were generally similar where the successions overlap temporally however carbonate deposition began earlier in south china carbonate complexes were also more widespread in south china and represent a thicker succession overall platforms in the canning basin grew directly on precambrian crystalline basement or early palaeozoic sedimentary rocks but in south china carbonate complexes developed conformably on older devonian siliciclastic strata pre frasnian reef facies in south china had more abundant skeletal frameworks than in canning basin reefs of equivalent age and famennian shoaling margins containing various microbial reefs may have been more common and probably more diverse in south china however late devonian platform margin types have been documented more completely in the canning basin deep intra platform troughs deep depressions containing non carbonate pelagic sediments nandan type successions that developed along syndepositional faults characterize devonian carbonate platforms in south china but have no equivalent on the lennard shelf canning basin where inter reef areas were more shallow the south china platform to depression pattern was generally continuous from the lower to upper devonian indicating that many pre devonian tectonic features continued to exercise considerable effect through deposition localized fault controlled subsidence was an important factor in both regions but similarities in 2nd order aggradation progradation cycles suggest that eustasy was also an important control on the larger scale stratigraphic development of both regions

efficient use of the velocity gradients tensor in flow modelling passchier c w 1987 efficient use of the velocity gradients tensor in flow modelling tectonophysm 136 159 163 for models of fabric development in rocks with vorticity as a variable parameter the choice of an unsuitable reference frame for instantaneous flow can hamper clear presentation of results the orientation of most fabric elements which develop in deforming rocks is attached to some principal direction of instantaneous flow syntaxial fibres crystallographic preferred orientation patterns steady state foliations and immobilised rigid objects seem to orient themselves in a fixed position relative to instantaneous stretching axes di while shape fabric elements and mica preferred orientation approach parallelism with the extensional apophysis i of the flow models for development of such fabric elements benefit from a choice of reference frame attached to either d or i

stress and strain evolution during single layer folding under pure and simple shear abstract folds in rocks are commonly used in geology for strain analysis they contain information about rock rheology and the kinematics and mechanics of deformation the systematic analysis of the evolution of stress and strain during rock folding can provide key information on the mechanical behaviour of rocks and the efficiency of the folding process in order to investigate the evolution of rock rheology during fold development a series of two dimensional simulations of single layer folding are presented here they are run using the finite element method basil integrated within the software platform elle to simulate linear and non linear viscous deformation the kinematics of deformation the competence ratio between the folding layer and surrounding matrix as well as the stress exponent of the power law viscous material are systematically varied the results allow comparing the stages of folding under different deformation kinematic conditions for all simulations the folding amplification process starts when the second invariant of the strain rate tensor in the competent layer deviates from the theoretical strain rate curve for a homogeneous material which corresponds to a viscosity ratio between layer and matrix of m 1 the relative time when the fold amplification starts is determined by the viscosity ratio between the competent layer and its surrounding matrix the initial layer orientation with respect to the shear plane the kinematics of deformation and the stress exponent the folding process is more effective in cases with high viscosity ratio non linear rheology and layers initially oriented at a low angle with respect to the shear plane because the second invariant of the strain rate tensor at the layer deviates earlier from the theoretical curve the results also show differences depending on the boundary conditions where 1 folding a competent layer requires less work in simple shear than in pure shear and 2 the geometrical softening experienced by the competent layer due to fold development is followed by a hardening stage in pure shear and by a major softening phase in simple shear the simulation results suggest that the decrease of stress of a competent layer without decreasing the mechanical strength has a direct influence on the behaviour of a lithospheric layer around the crust mantle boundary which may experience geometrical softening depending on the tectonic settings rather than material softening due metamorphic reactions or grain size reduction

famennian mud mounds in the proximal fore reef slope canning basin western australia abstract famennian late devonian carbonate buildups and in particular mud mounds are poorly known in general and few have been documented in detail relatively small famennian mud mounds occur in proximal fore reef slope settings in the canning basin western australia the famennian platform margin facies passes from typical shoaling carbonate facies in the back reef through massive calcimicrobial cement rich reef margin facies to relatively steeply dipping 20 30 well bedded fore reef slope facies containing shelf derived winnowed grainy sediments and extremely coarse reef block debris isolated or coalescing mounds occur in the proximal slope immediately adjacent to and in some cases possibly grading into the margin facies mounds are elongate perpendicular to the margin and some had synoptic relief greater than 2 m mounds are lithologically variable and consist of varying proportions of micrite multiple generations of marine cement abundant rothpletzella renalcis poorly preserved sparry microbial crusts and sporadically distributed laminar stromatoporoids surrounding grainy slope facies abut and slope off of mound flanks mound facies are very similar to nearby reef margin facies with the exceptions that stromatoporoids have not been observed in margin facies and solenoporoid algae which occur in the margin have not been observed in the mounds stromatolites are conspicuously absent from both facies mound facies appear to be more closely related to frasnian and famennian calcimicrobe cement dominated reef margin facies than to famennian deep water stromatolite sponge mound facies such as those that occur elsewhere in the canning basin the observed canning famennian reef and mound frameworks were constructed by communities that appear to be very similar to earlier frasnian communities despite the frasnian famennian extinction event and provide good examples of microbial reef framework construction in a high energy setting

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

calcite fabrics in a natural shear environment the helvetic nappes of western switzerland the aim of this contribution is to describe and interpret the crystallographic orientations and microstructures of calcite in the upper jurassic limestones of the helvetic nappes between the mont blanc and aar massifs over a distance along strike of about 180 km the c axis preferred orientation pattern which could be inferred is regionally very constant the c axis point maxima are everywhere subperpendicular to the regional strike of the chain there is an obliquity between the orientation of the macroscopically dominant cleavage and the microscopically visible long axes of individual grains the c axis maxima in most specimens are parallel to the axis of shortening of the microscopic grain shape fabric this fabric geometry indicates either continuous shear deformation or alternatively a two stage deformation history the obliquity of the grain shape fabric may be due to the fact that the flattening plane of the individual grains reflects only the last increment of a continuous shear process or it may represent a later finite deformation which has led to the formation of a regionally developed second cleavage there is a systematic relationship between the direction of these late shear movements and the main overthrust direction as given by the finite strain pattern this suggests that the microfabric of the rocks was acquired during a progressive rotation with time of the main overthrust direction the observed pattern of preferred orientation has been correlated by several authors with intracrystalline deformation mechanisms a combination of slip on r and twinning on e however the e twinning observed in the specimens investigated is younger than the development of the crystallographic fabric a new method for stress determination from e twinning is proposed

dynamics of cyclic carbonate deposition and biotic recovery on platforms during the famennian of late devonian in guangxi south china constraints from high resolution cycle and sequence stratigraphy to better understand the dynamics of platform carbonate deposition evolution and biotic recovery during the famennian high resolution cycle and sequence stratigraphic analyses were carried out upon three platform sections one in ramp and two in rimmed platforms along with a biostratigraphically well constrained basinal section in guangxi south china four main facies and fourteen lithofacies are identified in the three isolated platforms these are vertically arranged into decimetre to metre scale shallowing upward peritidal and subtidal cycles or parasequces in which the peritidal cycles predominate over the lower middle famennian six third order sequences s1 to s6 are identified based on the cycle stacking patterns as illustrated by fischer plots and vertical facies changes these sequences are temporally calibrated through correlation with basinal sequences constrained with conodont zonation and match well the six main famennian transgressive regressive cycles t1 to t6 in euramerica and elsewhere indicating a eustatic control however an opposite longer term trend in accommodation change s3 to s4 between platform interior and deeper open shelf to basin settings during the middle famennian suggests a differential subsidence across the platform basin section likely driven by block tilting thus both eustatic fluctuations and local tectonism had played an important role in platform development and evolution after the frasnian famennian biotic crisis main marine benthos preferentially recovered in the normal marine niches on the nearshore open shelves and radiated gradually into offshore restricted platforms brachiopods notably rhychonellides were the taxa recovering the earliest s1 in the normal or near normal marine conditions then echinoderms s3 stromatoporoids upper s3 and corals s5 stepped in episodically however these benthos generally appeared later in the restricted platform interiors only as if the ecological conditions were improved well enough for their colonization therefore the biotic recovery processes were diachronous both taxonomically and ecologically specified

famennian upper devonian stromatolite reefs at shatang guilin guangxi south china stromatolites occurred in many reef complexes throughout the phanerozoic and microbial communities played an important role in both ancient and modem stromatolite reefs famennian upper devonian limestone outcrops at shatang northwest of guilin south china contain stromatolite reefs calcimicrobes and algae are also abundant in the reef limestones highlighting the importance of microbial communities in initial reef formation after the frasnian famennian biotic crisis in some cases the shatang stromatolites are connected to form a rigid framework showing a significant ability for construction of biohermal reefs and in other cases they occur as isolated structures the cores of stromatolite reefs were formed by massive limestone containing columnar and dichotomous branching stromatolites calcimicrobes and algae e g solenopora parachaetetes garwoodia and problematic forms are also common some of the calcimicrobes formed stromatolites and other calcimicrobes played a role in sediment stabilization and were more common in biostromal thickets shatang stromatolitic reefs grew on a shoal deposit composed of sand and pebble sized bioclasts field observation and facies analysis suggest that shatang stromatolitic reefs formed platform margin facies which together with previously described renalcis epiphyton reefs renalcis reefs ooid shoals brachiopod shell shoals and nautiloid shoals rimmed the northwest part of the guilin carbonate platform the fossil microbial algal communities together with a complex of sedimentary features allow reconstruction of the depositional history of the environment based on comparison with coeval reefs in the guilin area and the previously developed regional tectonic history a model is proposed describing the sedimentary history of famennian reefs in guilin interpretation of the shatang stromatolites as disaster biota is equivocal as other factors may also have influenced the abundance and type of microbialites in shallow platform margin reef settings in the late devonian c 2004 elsevier b v all rights reserved

analysis of deformation paths in shear zones the geometry of ductile shear zones can be used to solve problems of regional tectonics if the deformation path of material in the zones is sufficiently understood flow in many shear zones may have deviated from simple shear and consequently data on the final deformation state such as finite strain and volume change are insufficient for reconstruction of the deformation path even if flow parameters were constant during progressive deformation additional information on the flow vorticity number is also needed and can be obtained from fabric elements such as sets of folded boudinaged veins rotated porphyroblasts and blocked rigid objects mohr circle constructions are presented as a tool to calculate deformation parameters from fabric data to represent the deformation path graphically and to reconstruct flow parameters from the shape of this path if the vorticity number or the volume change rate changed during progressive deformation the deformation path can be partly reconstructed using sets of fabric elements which register mean and final values of these parameters

a mohr circle construction to plot the stretch history of material lines abstract mohr circles for stretch can be used to find the spatial distribution of material lines which have been shortened extended shortened then extended and extended then shortened in a material plane which underwent area constant homogeneous deformation by steady state flow the distribution is mainly a function of the vorticity of the parent flow and can be used to determine sense of shear finite strain and even vorticity and orientation of finite strain axes the method may be useful in theoretical and experimental studies and can help to understand the complex deformation patterns in sets of cross cutting veins in deformed rocks

progressive deformation of single layers under constantly oriented boundary stresses abstract competent single layers embedded in incompetent rock display a variety of structures if deformed by shortening or stretching folds boudins or pinch and swells similarly incompetent single layers hosted by more competent matrix rock may develop mullions inverse folds or both the change in length of material planes with time is controlled by the type of bulk deformation and the initial layer orientation the length and rotation history of material planes in a unit volume of rock is studied systematically by using for the first time deviatoric stresses of arbitrary but constant orientation making use of a fundamental deformation tensor the traces of the eight planes in the unit volume of rock used in the analysis are in the plane of deformation and are treated as passive marker lines the change in the length of these material lines is computed for arbitrary orientations of the principal deviatoric stresses the history of the length change and rotation of the passive marker planes carefully scaled against time is subsequently used to predict the structures which would evolve in similarly oriented single layers of either competent or incompetent rock the theoretical range of structural patterns evolving in the model is controlled by the orientations of the principal deviatoric stress conversely the geometry of deformation patterns in nature can be used to constrain the possible orientations of the palaeostress axes

the use of mohr circles to describe non coaxial progressive deformation abstract mohr circles are useful in strain analysis but are also potentially powerful tools to translate geological fabric data into tensors describing the deformation and to reconstruct flow parameters this paper shows how mohr circles can be used to visualize the effects of variable flow parameters such as stretching rate volume change rate and vorticity number on parameters of cumulative deformation as described by the position gradient tensor a mean vorticity number is introduced as a deformation parameter besides finite cumulative strain and volume change

structures and fabrics in a crustal scale shear zone betic cordillera se spain abstract a broad zone of dominantly ductile high strain deformation lies beneath the aguilon nappe in the sierra alhamilla southern spain it forms part of a crustal scale movement zone traceable through much of the betic cordillera which separates the higher betic nappes from the underlying nevado filabride complex the zone is characterized in outcrop by a distinctive platy foliation and a strong nne trending stretching lineation microstructural characteristics include quartz ribbons mica fish augen of feldspar and other minerals in a matrix of dynamically recrystallized quartz and extensional crenulation cleavages narrow bands of ultramylonite and cataclasite occur within and on the margins of the movement zone deformation occurred under lower greenschist facies conditions and was accompanied by retrogression of earlier higher grade mineral assemblages structures in the movement zone developed in a temporal sequence beginning with isoclinal folding and transposition of older foliations this was followed by the formation of extensional crenulation cleavages and the progressive localization of strain into the ultramylonite bands mylonitic foliation in these bands is deformed by syn mylonite folds restricted to the bands all these structures were then deformed by s to se vergent small scale folds restricted to the movement zone as a whole cataclasis associated with alteration is localized along the ultramylonite bands and indicates a transition to late stage brittle deformation the lower boundary of the movement zone is gradational strain decreases recrystallized grainsize and the degree of recrystallization of quartz increases and pressure solution becomes the dominant deformation mechanism in mica schist asymmetric quartz fabrics in the movement zone indicate a nne sense of shear but variations in the degree of asymmetry suggest that flow was partitioned with the ultramylonite bands taking up much of the shear strain and the intervening rocks deforming more slowly and with a lower degree of non coaxiality diffuse fabrics in the fine grained ultramylonite bands may indicate a switch to a grainsize sensitive deformation mechanism and an overall downward increase in the opening angle of crossed girdle fabrics may reflect increased water activity at depth

taphonomy and ontogeny of a multibranched ediacaran fossil bradgatia from the avalon peninsula of newfoundland rangeomorphs an extinct group of ediacaran organisms with a fractal architecture and modular construction occur abundantly in the mistaken point assemblage 575 560 ma of the avalon peninsula of newfoundland bradgatia is represented by hundreds of specimens collectively permitting analysis of its morphology growth mode and life habits bradgatia is a petalage that consists of a radial array of up to eight petals each exhibiting up to four visible orders of rangeomorph branching the taphonomy and ontogeny of bradgatia are tightly linked by a change in preserved morphology from juvenile i to v shaped specimens to larger u shaped specimens to the largest and ontogenetically oldest o shaped specimens bradgatia probably maintained a constant number of petals that gradually spread out with age the number of frondlets per petal does not correlate with length suggesting either fractal or inflationary growth presence of a structure from which petals branch abundance of contour parallel specimens