formation and age of sphalerite mineralization in carbonate rocks of bajocian age in the swiss jura mountains evidence of mesozoic hydrothermal activity a combination of petrographic and geochemical techniques was applied to better constrain the origin and evolution of the fluid systems responsible for the formation of disseminated cd rich up to 0 6 wt sphalerite zns mineralization in the northeastern part of the jura mountains switzerland the rb sr ages of sphalerite samples indicate that a main phase of sphalerite formation occurred near the boundary between the late middle and early late jurassic at around 162 ma the negative 34s values 22 3 to 5 3 suggest that biogenic sulfide sulfur was involved in zns precipitation the strontium isotope composition is more radiogenic than that of contemporaneous seawater reflecting the interaction of mineralizing fluids with silicate rocks lead isotope signatures are very uniform 206pb 204pb 18 63 18 67 207pb 204pb 15 63 15 64 208pb 204pb 38 51 38 63 indicating an isotopically well homogenized fluid system the basement rocks underlying the jurassic strata are considered to be the main source of metals for the sphalerite mineralization the migration of deep sourced hydrothermal saline metal bearing fluids into the bajocian host carbonates containing sedimentary reduced sulfur resulted in the precipitation of sulfides the period of sphalerite formation near the middle late jurassic boundary is characterized by enhanced tectonic and hydrothermal activity in europe related to the opening of the central atlantic and tectonic thermal subsidence during spreading of the alpine tethys our study provides evidence that the bajocian carbonate rocks in the jura mountains area were affected by the circulation of deep sourced metal bearing hydrothermal fluids in response to these continent wide tectonothermal events the presence of sphalerite mineralization and associated geochemical anomalies in zn and cd contents in carbonate rocks may also be used to trace basement features

response of shallow marine carbonate facies to third order and high frequency sea level fluctuations hauptrogenstein formation northern switzerland abstract facies patterns in shallow marine carbonates of the hauptrogenstein formation of northern switzerland middle bajocian to early bathonian reflect small changes in sea level third order sea level fluctuations calculated from large scale facies patterns alternations of marl and carbonate dominated sedimentary units on the level of members and subsidence show two phases of sea level rise and fall in the late bajocian and a relative sea level rise in the early bathonian most of the sedimentary rocks were produced during phases of relative rise and highstand of sea level no periods of emergence during lowstands are preserved sea level changes of a higher order have been analyzed by facies changes at the outcrop scale thin shallowing up parasequences 0 5 6 m thick have been correlated across platform ramp and offshore sedimentary environments in each of these environments carbonates react differently to sea level fluctuations relative rises in sea level are represented by non deposition and biogenic sediments periods of stable or slightly falling sea level are dominated by oolitic sediments 1 in the offshoal environment marl layers at the base of thin shallowing up units are covered by increasingly proximal tempestites 2 in the ramp environment oblique stratified oolitic and bioclastic grainstones are bounded by marine flooding surfaces 3 on the platform marine flooding surfaces are overlain by beds deposited under moderate energy conditions coral beds oncoid beds platform tempestites which are in turn covered by high energy oblique stratified oolitic grainstones such parasequences deposited in response to small scale high frequency sea level changes can be correlated over distances of more than 100 km a model for evolution and response of carbonate facies to sea level changes is presented

mechanical and chemical compaction in fine grained shallow water limestones abstract artificial compaction of in situ cores of modern sediments from differing sedimentary environments resulted in 1 reduction of sediment thickness by 50 percent and more 2 reduction of initial porosities of 65 to 75 percent to 35 to 45 percent 3 creation of megascopic textures almost identical to many ancient lime mud and wackestones 4 creation of organic wispy stylolite like layers that in nature probably would initiate dissolution and stylolite development given burial and time 5 chemical compaction evidenced by thin sections showing quartz grains piercing mollusc shells without causing fractures and sem evidence of solutional interfitting of 1 to 4 m size aragonitic carbonate grains 6 obliteration of pellets and birdseye or fenestral voids in those sediments here early cementation was lacking pellets were preserved in slightly cemented bahama bank muds compacted to half the original thickness 7 obliteration of identifiable marine grasses and vertical root tube voids 8 mashing of sediment filled circular burrows to produce ellipsoidal structures significant mechanical compaction resulted from pressures simulating less than 1 000 ft 305 m of burial increasing loads to an equivalent of more than 10 000 ft 3 400 m did not significantly increase compaction or reduce sediment core length chemical compaction pressure dissolution was detected only in sediment cores compacted to pressures greater than 10 000 ft 3 400 m of burial these short term experiments suggest that chemical compaction would begin at much shallower depths given geologic time compaction experiments that caused chemical compaction lend support to the well established hypothesis that cement required to produce a low porosity low permeability fine grained limestone is derived internally dissolution ion diffusion and reprecipitation are considered the m st likely processes for creating significant thicknesses of dense limestones in the geologic record continuation of chemical compaction after significant porosity reduction necessitates expulsion of connate fluids possibly including hydrocarbons

the relation between regionally consistent stretching lineations and plate motions abstract regional stretching lineations in the himalayas and southern tibet coincide with the known relative motions of the converging plates this relation is used to infer plate motions from the stretching lineations in the variscan and caledonian belts of europe and in the late proterozoic belts of northeast and east africa in each region high angle convergence of plates produced transverse stretching lineations and high angle collision was followed by or combined with large displacements first by ductile shear and later by brittle fractures parallel to the plate boundaries apparent stretching as a result of volume loss ramsay wood 1973 is common but recognisable as such reorientation by later deformations must be allowed for ramsay 1967 in ductile shear zones simple shearing leads to the progressive approach towards the shear plane of the xy plane of the strain ellipsoid and progressive approach of x the extension axis to the shear line ramsay graham 1970 thus large extensions approximate to the shear line large extensions tend to produce sheath folds whose acutely curved axes approach the stretching direction quinquis et al 1978 also the pre existence of a strong linear fabric predisposes the development of later folds with axes parallel to the earlier linear fabric for all of these reasons the extension axis x rather than the fold axis is of prime significance

high energy carbonate sand accumulation the quicksands southwest florida keys abstract high resolution seismic reflection profiles of the quicksands located along a broad ridge on the platform shelf west of key west florida indicate a significant deposit of non oolitic carbonate sand occurs in a belt 47 km long by 28 km wide the surface of the belt is ornamented by large 5 m migrating tidal bars oriented in a north south direction on which sand waves oriented in an east west direction are superimposed some of the sand waves are awash at low tide the sand waves are formed by strong reversing tidal currents flowing between the gulf of mexico and the straits of florida the waves migrate directly over pleistocene bedrock to the east but the deposit thickens to the west and sand waves there overlie non oolitic holocene accumulations as thick as 12 m westward ipping accretionary bedding indicates that net migration of the sands is to the west despite north south movement of tidal currents the westward edge of the accumulation has accreted over deeper muddier deposits although tidal currents and resultant bedforms appear identical to those of active ooid deposits in the bahamas and elsewhere no oolitically coated grains were found in this study thin section analyses show the principal component average 48 of the sands is fragmented plates of species of the green alga halimeda followed by particulate coral average 17 which increases off the flanks of the main sand body short vibracores confirm the presence of cross bedding

sea water circulation on an oolite dominated carbonate system in an epeiric sea middle jurassic switzerland the middle jurassic burgundy carbonate platform occupied a central part of the central european epeiric sea during the middle jurassic the facies architecture of the oolitic calcarenite bodies was affected by tidal currents on the platform and relative sea level changes the 13c values of inorganic carbonates sampled in biostratigraphic defined intervals do not match very well between closely spaced sections and hence are of restricted use for stratigraphic purposes it appears that the platform interior might have been decoupled from the global carbon pool although deposited in a rapidly accumulating setting the recorded isotope signatures might be affected by some local stratigraphic gaps nonetheless the carbon isotope data imply lateral changes of the platform waters these appear to be related to the position on the platform and to the sediment dispersal pattern as evidenced by clay minerals adjacent to the eastern margin of the platform detrital chlorite and illite occur in considerable proportions both ascribed to a boreal source to the east and the north east in contrast smectite rich mixed layer clay mineral content increases significantly towards the platform interior pointing to a delivery from the north west all these data are suggestive of an overall clockwise current pattern in the central european epeiric sea during the middle jurassic

cyclicity and paleoenvironmental dynamics rocknest platform northwest canada the superbly exposed rocknest formation northwest territories canada contains 140 to 160 shallowing upward cycles 1 to 15 m thick that contain distinctive marker beds and can be correlated for 200 km parallel to strike and 120 km perpendicular to strike cycles are grouped according to cycle base lithology which reflects paleogeographic position on the platform shale based cycles of the distal inner shelf pass westward into dolosiltite based cycles then into tufa based cycles of the shoal complex and then into grainstone based cycles of the outer shelf cycles are bounded by erosional surfaces that become less pronounced away from the shoal complex cycles formed in response to small asymmetric rapid rise slow fall eustatic oscillations in sea level 10 m during sea level rise rapid transgression and submergence of tidal flats occurred along the eastern margin of the shoal complex followed by eastward expansion of the shoal complex and slow progradation of tidal flats over lagoonal facies during sea level fall a glacio eustatic model for cyclicity accounts for the data favoring asymmetric eustatic sea level oscillations subaerial exposure and vadose diagenesis of platform during sea level falls arrested progradation and incomplete shoaling of many cycles and cycle periods of 20 000 to 100 000 yr 18 000 to 30 000 yr for rocknest inner shelf slopes ranged from 8 cm km average at high sea level stand to 2 cm km average at low sea level stand tidal flat progradation rates were 7 to 9 km 1 000 yr interaction of inner shelf slope sea level fall rate and sedimentation rate generates complete condensed or expanded cycle profiles one dimensional computer modeling reveals that sedimentation rates of shoal complex facies were 250 cm 1 000 yr that lag times in sedimentation following transgression may have ranged from zero on the shoal complex to 2 000 yr on the distal inner shelf and that subtidal sedimentation rates may have ranged from 80 cm 1 000 yr for the proximal inner shelf to 20 cm 1 000 yr for the distal inner shelf

ultradeep metamorphic rocks the retrospective viewpoint ultradeep or ultra high pressure uhp metamorphic rocks formed from crustal protoliths within the stability field of coesite at pressures 2 5 3 0 gpa corresponding to depths 80 120 km occur locally though regionally distributed in at least five continental areas their recognition is solely based on characteristic minerals and mineral assemblages calibrated by experimental high pressure studies detailed petrographic and microprobe work especially on mineral inclusions in favorable cases allows the derivation of prograde pt paths during subduction and of retrograde ones during exhumation commonly the gneisses adjacent to the uhp rocks do not exhibit signs of ultradeep metamorphism apparently because the kinetics of their mineral reactions are sufficiently fast to allow complete reequilibration to shallower pt conditions during the retrograde path it is also possible however that uhp equilibria were not attained throughout the rock volumes subducted but only along zones of shearing and fluid introduction if it is true that not all uhp metamorphic rocks return to the crustal orogenic belts but some continue to be subducted to greater mantle depths the classical geochemical pattern of a one way mass transfer from mantle to crust throughout the earth s history is at stake the assumed gradual growth of continents may have had a counterpart of continent destruction during collision events most recent experimental studies at high pressures and relatively low temperatures show that at least three new hydrous mg a1 silicates exist that were not found in nature thus far but may be characteristic minerals in the cold portions of old subduction zones thus extending the water retentivity of subducting slabs to greater and hitherto unexpected depths

reactivated basement structures affecting the sedimentary facies in a tectonically quiescent epicontinental basin an example from nw switzerland abstract the jurassic deposits in the southern part of central europe accumulated in a shallow epicontinental sea and their deposition has usually been believed to have corresponded to a phase of tectonic quiescence neither on conventional seismic records nor in outcrops obvious indications of synsedimentary tectonics were found nonetheless subtle variations in lithofacies and thickness occur above faults in the crystalline basement in fact preexisting structures became reactivated in the jurassic during major extensional phases when the tethys and the north atlantic opened this reactivation led to differential subsidence and or to rotation of fault bounded blocks but the sediments were deformed mainly flexurally as vertical movements in the basement were dissipated by triassic salt thus the depositional area was morphologically differentiated into swells and depressions in siliciclastic muddy environments swells were characterized by hardbottoms and hiatus beds depressions were filled by distal tempestites and gravity deposits in carbonate settings reefs nucleated on swells marls and muds were deposited in depressions many of them gravity deposits reactivation of faults occurred only during short episodes and was not synchronous throughout the study area and reactivation of individual faults was episodic and probably controlled by the palaeostress field and the faults orientation reactivation of deep rooted faults is also documented by hydrothermal activity which led to vein mineralization and alteration of minerals today exposed in nearby basement units the chronostratigraphic ages of the hydrothermal processes coincide with phases of enhanced subsidence during the sinemurian aalenian bajocian bathonian and oxfordian in turn facies changes of the sedimentary cover should be useful to predict basement structures when no seismic records are available

depositional facies of high energy beach to offshore sequence comparison with low energy sequence a study of the nearshore zone and continental shelf of a high energy coast was made in the ventura port hueneme area of california during the summer of 1974 to investigate depositional facies characteristics and compare them with similar facies from a low energy coastal setting analysis of box cores can cores and vibracores permits recognition of three principal zones 1 nearshore facies backshore to 9 m water depth made up primarily of parallel ripple and cross bedded clean sand bioturbation is only locally significant 2 transition facies 9 to 18 m water depth a zone of fine sand and silty sand characterized by an increase in biogenic over physical sedimentary structures that are commonly preserved as laminated to burrowed beds wave ripple bedding and parallel lamination are important structures in this facies 3 offshore facies 18 m water depth sandy silt is the primary texture and bioturbation is the dominant sedimentary structure remnant parallel lamination is the only physical sedimentary structure present comparison of the results of this study with a previous description of a low energy beach to offshore sequence at sapelo island georgia indicates that the two areas do not differ greatly in their facies or overall vertical sequence of sedimentary structures the principal difference is in the thickness of the three facies the california facies are significantly thicker than their georgia counterparts this difference is in direct response to the role of higher wave energy on the california coast

the dislocation substructures of naturally deformed omphacites abstract a transmission electron microscopy study of naturally deformed omphacite has revealed twins antiphase boundaries and a variety of dislocation substructures consisting of curved free dislocations loops isolated nodal points and networks both free dislocations and dislocation boundaries contain nodes that involve dislocations with the following burgers vectors 001 1 2 110 and 1 2 112 this implies a reaction of the type 001 1 2 110 1 2 112 by analogy with quartz and olivine the dislocation substructures in the omphacites are considered to be indicative of dislocation creep mechanisms and responsible for most of the optical defect structures using microstructural and geochemical data the metamorphic conditions during the creep deformation are estimated to be minimum 8 kbar 550 c maximum 15 kbar 625 c

the paradox of drowned reefs and carbonate platforms shallow water carbonate platforms and reefs are drowned when tectonic subsidence or rising sea level outpaces carbonate accumulation and benthonic carbonate production ceases drowned platforms are common in the geologic record but they present a paradox if one considers rates of processes involved during the early holocene when sea level rose at rates of 6 000 to 10 000 m yr mm 1 000 yr most reefs and platforms were outpaced by the rising sea during the late holocene with sea level rising 500 to 3 000 m yr in the atlantic caribbean area reefs and platforms started to recover built to sea level and prograded seaward 1 000 m yr is thus a conservative estimate of the average growth potential of modern reefs and platforms independently accumulation rates of prograding platforms in the geologic record suggest growth potential in excess of several hundred microns per year the growth potential of 1 000 m yr exceeds any relative rise of sea level caused by long term processes in the geologic record newly formed ocean crust subsides at a maximum of 250 m yr basin subsidence averages 10 to 100 m yr and sea level rises due to increased sea floor spreading amount to less than 10 m yr rapid pulses of relative rise of sea level or reduction of benthic growth by deterioration of the environment remain the only plausible explanations of drowning the geologic record shows examples of both of these processes global mass extinctions of reefs and platforms occurred in the middle cretaceous eustatic rise due to submarine volcanism or desiccation of a small ocean basin and the late devonian global crisis of ocean environment extraterrestrial cause drowning controlled by regional tectonics prevailed in the jurassic and early cretaceous of the tethyan realm and the drowning of mesozoic platforms in the western north atlantic seems to have been dictated by plate tectonic drift to higher latitudes

sea level changes an integrated approach sea level changes an integrated approach in october 1985 sepm sponsored a four day conference entitled sea level changes an integrated approach the purpose of the conference was to provide a forum for an interdisciplinary exchange of ideas on sea level changes and to provide an opportunity for integrating various types of evidence in approaching unresolved issues the conference was successful in bringing together scientists from industry academia and government representing all of the major geosciences disciplines presentations of many new papers plus significant releases of data that were previously held proprietary provided fertile ground for discussion this much cited volume represents the best of the material presented at the conference includes the early vail chart

structural development and petrofabrics of eclogite facies shear zones bergen arcs western norway implications for deep crustal deformational processes caledonian eclogite facies shear zones developed from grenvillian garnet granulite facies anorthosites and gabbros in the bergen arcs of western norway allow direct investigation of the relations between macroscopic structures and crystallographic preferred orientation cpo in lower continental crust field relations on the island of holsnoy show that the eclogites formed locally from granulite facies rocks by progressive development of 1 eclogite adjacent to fractures 2 eclogite in discrete shear zones 2 m thick 3 eclogite breccia consisting of 80 well foliated eclogite that wraps around rotated granulite blocks and 4 anastomosing subparallel eclogite facies shear zones 30 100 m thick continuous over distances 1 km within the granulite terrane these shear zones deformed under eclogite facies conditions at an estimated temperature of 670 50 c and a minimum pressure of 1460 mpa which corresponds to depths of 55 km in the continental crust detailed investigation of the major shear zones shows the development of a strong foliation defined by the shape preferred orientation of omphacite and by alternating segregations of omphacite garnet rich and kyanite zoisite rich layers a consistent lineation throughout the shear zones is defined by elongate aggregates of garnet and omphacite the cpo of omphacite determined from five axis universal stage measurements shows a strong b axis maximum normal to foliation and a c axis girdle within the foliation plane with weak maxima parallel to the lineation direction these patterns are consistent with deformation of omphacite by slip parallel to 001 and suggest glide along 010 the lineation and cpo data reveal a consistent sense of shear zone movement although the displacement was small localized faulting of high grade rocks accompanied by fluid infiltration can be an important mode of failure in the lower continental crust field relations show that granulite facies rocks can exist in a metastable state under eclogite facies conditions and imply that the lower crust can host differing metamorphic facies at the same depth deformation of granulite and partial conversion to eclogite such as is exposed on holsnoy island may be an orogenic scale process in the lowermost crust of collisional orogens

late oxfordian to late kimmeridgian carbonate deposits of nw switzerland swiss jura stratigraphical and palaeogeographical implications in the transition area between the paris basin and the tethys abstract geological sections of the shallow water carbonate dominated sedimentary system of the late jurassic reuchenette formation in northwestern switzerland have been studied between the southern jura mountains and the tabular jura the largest sections show a characteristic cyclic stacking pattern up to now the age of these sediments including the type section linking the boreal and tethyan realms was biostratigraphically poorly constrained in the tabular jura five 3rd order sequences can be assigned to the planula to eudoxus zone late oxfordian to late kimmeridgian using index fossils ammonites and ostracodes jank m 2004 new insights into the development of the late jurassic reuchenette formation of nw switzerland late oxfordian to late kimmeridgian jura mountains dissertationen aus dem geologisch palaontologischen institut der universitat basel 32 121 pp this time control and several new outcrops in combination with mineralostratigraphical and lithological marker beds allow the correlation and dating of the thickest sections of the reuchenette formation and thus serve to improve the previously estimated ages of their sequence boundaries the variability of stacking pattern and facies between sections also reveals distinctive changes in facies evolution related to late palaeozoic basement structures and synsedimentary subsidence these structures acted as important controlling factors for the sediment distribution of the reuchenette formation besides the sea level fluctuations the interplay of sea level changes and synsedimentary subsidence is outlined by lateral thickness variations and shifting depositional environments a close examination of these changes also sheds much light on the nature of platform topography in the transition area between the paris basin in the north and the tethys in the south or more generally between the boreal and tethyan realms during the planula to divisum time intervals the study area was a flat platform with a more or less uniform facies distribution which connected the above mentioned realms during the divisum to acanthicum time intervals this platform changed into a pronounced basin and swell morpoholgy with specific depositional environments and separated the paris basin from the tethys

facies and climate environmental changes recorded on a carbonate ramp a sedimentological and geochemical approach on middle jurassic carbonates paris basin france a detailed sedimentological geochemical and mineralogical study is carried out on the early bajocian to early callovian middle jurassic limestones of the paris basin objectives are to document and explain the facies changes in the context of the climate environmental evolution at the nw european scale deposits include 18 lithofacies which are stacked into 10 third order depositional sequences at a greater time scale 4 biosedimentary packages are distinguished by their allochems associations 1 an intracratonic carbonate environment with coral reefs and crinoid rich facies is typical of the early bajocian 2 a major facies change occurred at the early late bajocian transition with a shift from crinoid and coral rich facies to ooid rich facies during the late bajocian a southward dipping ooid ramp with successive progradational trends was emplaced 3 a large lagoon protected by ooid shoals developed during the bathonian in a muddy ramp setting 4 at the bathonian callovian boundary a second major change occurred with the lagoonal facies being superseded by an ooid bioclastic crinoids corals ramp associated with a waning of carbonate productivity retrogradational trend our geochemical study including a compilation of bibliographic data allows us to refine the existing sea surface palaeotemperature pattern a cooling from the latest early bajocian to the late bajocian and a subsequent warming from the earliest callovian to the early middle callovian transition are described using a proxy of sea surface temperatures together with 13c and mineralogical data our new 18o values suggest a palaeo climatic palaeo environmental control of facies in this shallow carbonate ramp environment

birth and demise of a middle jurassic isolated shallow marine carbonate platform on a tilted fault block example from the southern iberian continental palaeomargin abstract subbetic middle jurassic oolitic limestones of the jabalcuz formation crop out in san cristobal hill near jaen city andalucia spain between hemipelagic limestone and marl successions the jabalcuz limestones range in facies from calcareous breccias and micritic limestones to white cross bedded oolitic limestones recent erosion has exhumed a jurassic isolated shallow water carbonate platform on the san cristobal hill this shallow platform developed on a tilted fault block an almost continuous laterally extensive outcrop reveals tectono sedimentary features distinctive of block tilting in the different margins of the fault block the studied sections represent various palaeogeographic positions in the ancient shallow water carbonate platform and basin transition this exceptional outcrop allows to decipher the triggering mechanisms of the birth evolution and drowning of this jurassic isolated shallow water carbonate platform two shallowing upward depositional sequences separated by flooding surfaces can be distinguished on two different sides of the fault block in the southeastern part of the outcrop proximal sections grade vertically from distal talus fault breccias with bivalve and serpulid buildup intercalations to white cross bedded oolitic limestones defining the lowermost depositional sequence upwards overlying a flooding surface the second sequence with oolitic limestones prograding over micritic deposits is recorded in the southwest oolitic peloidal and more distal micritic facies alternate with notable southeastern progradation of oolitic facies in the upper part of the section which represents the upper depositional sequence the top of this second depositional sequence is another flooding surface recorded by the sedimentation of marls with radiolarians from the overlying formation in the northwestern outcrops the two depositional sequences are also almost completely preserved and can be differentiated a 100 m thick succession composed of a basal slumped unit of micritic and peloidal limestones overlain by cross bedded oolitic limestones represents the lowermost sequence the superposition of a newer slumped unit of micritic and peloidal limestones on the cross bedded oolitic limestones defines the lower part of the uppermost depositional sequence finally the central sections are less thick and comprise a facies succession consisting almost entirely of oolitic limestones locally oolitic limestones often show cross bedding and prograding clinoforms the cross bedding has centrifugal directions around the perimeter outcrops of san cristobal hill whereas in the centre palaeocurrents are highly variable the sense of the palaeoslopes as deduced from slumps also fits with a centrifugal pattern finally in the southwestern outcrops the demise of the carbonate platform is marked by backstepping oolitic facies the presence of lumachelle facies and onlapping hemipelagic facies all the data point to the birth evolution and drowning of an isolated shallow water carbonate platform that developed on a tilted fault block the fault block tilting controlled the development of talus breccias in the southeastern part and of slumps in the northwest dipping hangingwall ramp in the shallowest environments produced by the tilting uppermost part of the fault block a shallow marine carbonate platform developed carbonate production mainly oolitic kept up with relative sea level fluctuations and extended the platform at least to the northwest southeast and southwest based on observable data finally tectonics and sea level fluctuations drowned this carbonate platform

upper mantle anisotropy a preliminary model seismic anisotropy in the upper mantle can be explained by crystallographic mineral alignment achieved through dislocation motion the physical mechanism of mineral alignment requires upper mantle shear flow which reorients and aligns minerals by dislocation glide and climb governed by the dominant glide system of each mineral the dominant glide systems are assumed to be 100 0l0 for olivine and 001 100 for the pyroxenes these yield a predominantly orthorhombic fabric with the olivine 100 and the pyroxene 001 axes aligned in the upper mantle flow direction and the olivine 010 and the pyroxene 100 axes aligned normal to the upper mantle flow plane these glide systems have a threshold temperature of enhanced mobility of 1100 1200 k which yields a solid state thermally defined lithosphere asthenosphere boundary in an olivine pyroxene mantle consistent with recent seismic determinations of the thickness of the lithosphere mantle anisotropy due to mineral alignment is then actively maintained below this boundary asthenosphere and mesosphere and is a fossil state above this boundary lithosphere we use the dominant glide systems to establish the crystallographic orientation of olivine and pyroxene in calculating the maximum seismic anisotropy of two petrologic models pyrolite and piclogite for the upper mantle by an extrapolation of single crystal anisotropic mineral elastic properties to a depth of 400 km the real earth seismic anisotropy will be bound by the limits of maximum anisotropy from perfect mineral alignment and minimum anisotropy isotropy from random mineral alignment the seismic anisotropy of the upper 220 km is best represented by the pyrolite model which reduces to quasi hexagonal symmetry with the unique axis in the direction of mantle flow the lehmann discontinuity is conjectured to be due to a change in composition from pyrolite to piclogite and therefore may represent a change in anisotropy the piclogite model has orthorhombic symmetry distinctly different from that of the pyrolite the piclogite anisotropy model can appear quasi isotropic when measured by transverse isotropy parameterization if the mantle flow is mainly horizontal with a horizontal shear plane that is the shear flow gradient duh dr dominates

omphacite fabric variation in the cabo ortegal eclogite nw spain relationships with strain symmetry during high pressure deformation petrostructural study of cabo ortegal eclogite tectonites provides an insight into the relationships between omphacite lattice preferred orientation and the fundamental deformation parameters in particular strain symmetry microstructural evidence is consistent with a tectonic origin of eclogite fabrics by coeval pyroxene plastic deformation and recrystallization under eclogite facies metamorphic conditions omphacite shape fabrics and garnet spatial distributions are anisotropic and representative of the fields of either apparent flattening or constriction omphacite lattice preferred orientations are dominated by 010 axis maxima normal to the foliation s type or flattening fabrics by 001 axis maxima parallel to the lineation l type or constriction or correspond to other fabric patterns described in the literature omphacite crystallographic fabrics are consistently asymmetric with respect to the structural framework defined by foliation and lineation this suggests that non coaxial deformation components either simple or general shear accompanied eclogite fabric development in addition to any flattening or constriction strain paths and permit shear sense determination these petrostructural features were acquired in the course of eo hercynian ductile deformation probably during deep subduction under 1 5 1 7 gpa confining pressure temperatures above 600 700 c low differential stress 15 20 mpa and slow strain rate 10 12 10 15s 1

field and modelling studies of cambrian carbonate cycles virginia appalachians reply abstract field studies coupled with computer modelling of carbonate cycles in the elbrook conococheague formations middle to upper cambrian in the appalachians have helped define the mechanisms of formation the sequence is up to 1 6 km thick and composed of cyclic peritidal carbonates these formed on an aggraded rimmed shelf on a mature passive continental margin the cycles 1 7 m thick have lower parts of subtidal intertidal carbonate and caps of dolomitic laminite containing minor quartz arenite shale and breccia sedimentation rates for these peritidal carbonate environments exceeded long term subsidence of the platform 0 01 to 0 1 m 1 000 yr thus the shelf stayed near to sea level i e was aggraded milankovitch 20 000 to 100 000 yr low amplitude sea level fluctuations few meters superimposed on 3rd order sea level fluctuation of 1 to 3 m y duration and less than 20 m amplitude caused periodic submergence of the gently sloping platform less than 0 02 m km slopes resulting in deposition of over 400 peritidal cycles in about 26 m y cycles that formed during 3rd order sea level rise are thick and limestone rich whereas those formed during 3rd order fall are thin quartzose and highly dolomitized the modelling suggests that fewer and thinner cycles formed during the long term falls compared to the rises reflecting decreased accommodation cycle frequency decreases into areas of lower subsidence rate open marine cycles typical of the outer shelf and cycles formed during 3rd order sea level rise contain digitate bioherms grainstone and ribbon carbonates in lower parts they reflect high subsidence rate and an open marine setting inner shelf cycles and cycles that formed during 3rd order sea level fall commonly are thrombolitic or have thin subtidal parts grainstone conglomerate or stromatolite and reflec the low subsidence rates interior areas of quaternary carbonate shelves are characterized by incipiently drowned facies and pinnacle reefs and are punctuated by soil caliche horizons and karst surface that reflect 100 m glacio eustatic sea level fluctuations in contrast many ancient shelves like the cambro ordovician shelf of the appalachians were flat topped and dominated by cyclic peritidal sequences lacking evidence of high amplitude sea level fluctuations such aggraded shelves may represent the typical state of mature carbonate shelves that formed in response to low amplitude milankovitch sea level fluctuations as opposed to the large scale fluctuations that typified the quaternary

pore networks in holocene carbonate sediments abstract permeabilities of holocene carbonate sediments from florida and the bahamas determined for 74 samples by water flow rates in a variable head permeameter range from 0 6 to 57 000 millidarcies porosities range from 40 to 78 percent these parameters are related to depositional texture as follows table the muddiest sediments i e the finest grained have the highest porosities but lowest permeabilities this negative correlation between porosity and permeability is the reverse of the situation found in carbonate rocks even as young as pleistocene high porosity and low permeability in sediments show a strong correlation with percentage of fines 62 m from capillary pressure curves it is inferred that many of the pore throats of muddy carbonate sediments are less than 1 m in diameter at least after drying measured permeabilities are used to calculate cementation rates for simple models of upper phreatic zone cementation the calculated rates would require excessive time to produce the degree of cementation seen in late pleistocene rocks of florida and the bahamas the amounts of rainfall and evaporation are the most important factors in the degree of cementation

high temperature deformation of diopside single crystal 1 mechanical data laboratory deformation experiments were carried out on diopside single crystals creep tests were made in a dead load apparatus at temperatures t 1020 to 1320 c axial compressive stresses 50 to 170 mpa with strain rates e ranging from 2 10 9 s 1 to 4 10 7 s 1 the specimens were oriented such that mechanical twinning was not possible the experiments were designed to activate 110 1 2 a b slip systems our results demonstrate that they are the major operative systems for t 1000 c and appear to be predominant over the 100 c slip system the mechanical data are fitted to a power law e a n exp e rt an inversion method was used to determine the parameter in a the activation energy e and the stress exponent n below a critical temperature tc 1130 1140 c e is found to be 440 30 kj mol associated with the 110 1 2 a b slip systems activated symmetrically without any contribution from the 100 c system orientation 2 e is found equal to 740 30 kj mol if all three systems have the same resolved shear stresses orientation 1 for t tc values of e drop to 50 15 and 85 30 kj mol for orientations 2 and 1 respectively so that the creep law of diopside becomes nearly temperature independent the law derived from the orientation 2 crystals could be consistent with natural deformations for this orientation and within the range of temperatures covered n is 6 5 0 4 the strong decrease in the activation energy at tc may be ascribed unambiguously to the occurrence of partial melting leading to microdroplets which pin the mobile dislocations ingrin et al this issue the high value found for the stress exponent n in the creep law suggests that clinopyroxene is softer than olivine at high stresses at t 900 c stress 20 mpa and e 10 15 s 1 while olivine is softer at lower stresses this competence inversion is predicted to occur within the ranges of stresses and strain rates expected in the lower crust and upper mantle

a comparison of mineral grain fabrics and finite strain in amphibolites from eastern finland fabrics in amphibolite rocks from the kuopio area eastern finland are described and compared with results of finite strain analysis the area consists of mantled gneiss domes which are deep seated diapiric structures results concern the cover rocks of the domes strain ellipsoids range in shape from prolate to oblate 0 67 v 0 52 strain intensities vary strongly preferred orientations of hornblende have been measured by x ray texture goniometry and corresponding fabric ellipsoids calculated using an eigenvalue method the same has been done for biotite where present fabric ellipsoids obtained for biotite are all oblate 0 04 vf 0 76 those obtained for hornblende range from prolate to oblate 0 60 vf 0 73 intensities of hornblende fabric are weaker than those of biotite fabric local variations of results are induced by grain scale factors such as grain size or original fabrics no simple correlation is found between intensities of fabric and strain in general increasing stra

modern shallow water graded sand layers from storm surges bering shelf a mimic of bouma sequences and turbidite systems abstract a sequence of graded sand layers interbedded with mud extends offshore over 100 km from the yukon delta shoreline across the flat shallow 100 kin such shallow water graded layers off lobate deltas may be distinguished from similar deep water turbidites by 1 the predominance of trough cross lamination perhaps resulting from wave oscillation effects in the proximal part of the system and 2 gradation to common shallow marine fossils bioturbation and storm lag layers n distal areas

quaternary aragonite cycles and oxygen isotope record in bahamian carbonate ooze carbonate ooze in bahamian troughs displays cyclic variation in the content of aragonite a 10 m core devoid of turbidites shows 3 5 cycles each defined by a sharp increase followed by a gradual decrease of aragonite the aragonite curve matches closely the oxygen isotope curve of planktic foraminifera from the same core aragonite highs correspond to light oxygen isotope values and thus to interglacial stages content of terrigenous material is low during interglacials and higher during glacial periods intermittent flooding and exposure of the bahama banks is most probably not the cause of the cycles because the latest increase in aragonite precedes bank flooding by 8 000 yr and because the cycles are distinctly asymmetric with rapid increase and gradual decrease of aragonitic content carbonate dissolution cycles tied to the glacial rhythm of earth9s climate are a more likely explanation of the variations in aragonite content

deformation of single crystal clinopyroxenes 1 mechanical twinning in diopside and hedenbergite laboratory deformation experiments were carried out on two single crystal clinopyroxenes chrome diopside and hedenbergite tests were made in a griggs solid medium triaxial hot creep tester a confining pressure of 1000 mpa was applied in all experiments crystals were deformed at strain rates from 10 4 to 10 8 s 1 and at temperatures from 400 c to 1200 c two orientations of the crystals with respect to the maximum principal compressive stress were tested the first orientation subjects the 100 001 and 001 100 mechanical twinning systems in clinopyroxene to a high resolved shear stress in the sense appropriate for mechanical twinning to occur the second orientation subjects these systems to an equal resolved shear stress but in the opposite sense so that mechanical twinning is not possible effects of temperature and strain rate on the flow stress are observed for the two clinopyroxene compositions mechanical twinning on the system 100 001 is observed to be the primary deformation mechanism in crystals oriented favorably for twinning the resolved shear stress required for mechanical twinning on this system is 140 mpa for hedenbergite and 100 mpa for chrome diopside the smaller twinning stresses for diopside are considered to be due to abundant inclusions in the crystals the mechanical twinning stress is nearly independent of temperature from 400 c to 850 c and strain rate from 10 4 to 10 8 s 1 above 850 1000 c for diopside and 1000 c for hedenbergite deformation occurs by dislocation glide and is strongly temperature and strain rate dependent crystals tested in the orientation for which mechanical twinning is not possible deform by two different dislocation glide mechanisms depending on temperature details of these experiments are reported in part 2 results for mechanical twinning predict that this is not an important deformation mechanism in the earth unless resolved shear stress on the twinning system exceeds 140 mpa lack of dependence of twinning stress on strain rate implies that large rapid strains will occur in this mineral if the twinning stress is reached independence of twinning stress in clinopyroxenes from temperature strain rate or composition allows use of this mineral as a geopiezometer at plate margins where stresses are high presence of mechanically twinned clinopyroxene grains allows determination of the orientation and magnitude of stresses responsible for the deformation

working with natural cohesive sediments summary of stabilizing and destabilizing biotic in uenceson natural cohesive sediment stabilizing destabilizingeps secretion extracellularpolymeric substances bybacteria and microphytobenthosenhances cohesion promotes occulation and hence deposition blistering trapping ofoxygen bubbles in bio lmsincreases buoyancy of thebio lm to such an extentthat it pulls away fromthe sediment sediment compaction burrowingmacrofauna increasesediment densityand hence stability pelletization theformationof faeces and psuedo faeces can enhanceerodibility increased drainage burrowand channel formationpromotes dewatering grazing organismsfeedingon intertidal ats causesphysical disturbance andresuspensionof the sediment andreduces the stabilizingin uence ofmicrophytobenthos network effects filamentous biotaramify through the sediment matrixbinding sediment particles together burrow cleaning somebenthic fauna cleantubes they inhabit inthe sediment giving riseto a localizedbenthic ux flow effects

sea carousel a benthic annular flume a benthic annular flume sea carousel has been developed and tested to measure in situ the erodibility of cohesive sediments the flume is equipped with three optical backscatter sensors a lid rotation switch and an electromagnetic em flow meter capable of detecting azimuthal and vertical components of flow data are logged at rates up to 10 66 hz erodibility is inferred from the rate of change in suspended sediment concentration detected in the annulus the energy density wave number spectrum of azimuthal flow showed peaks in the energy spectrum at paddle rotation wave numbers k of 14 and 7 m 1 macroturbulent time scales but were not significant friction velocity u measured 1 at 1 hz using a flush mounted hot film sensor and 2 derived from measured velocity profiles in the inner part of the logarithmic layer gave comparable results for 1 at higher values of u method 2 underpredicted by up to 20 method 1 showed radial increases in in the annulus for y 0 32 m s 1 radial velocity gradients were proportional to y 0 32 m s 1 maximum radial differences in u were 10 for y 0 5 ms 1 suspended sediment mass concentration s in the annulus resulted in a significant decrease 10 5 in derived by method 1 over the range 0 s 1 these decreases were not evident in method 2 method 1 may therefore be subject to changes in stress sensor calibration with changes in s subaerial deployments of sea carousel caused severe substrate disturbance water losses and aeration of the annulus submarine deployments produced stable results though dispersion of turbid flume water took place results clearly demonstrated the existence of type i and type ii erosion documented from laboratory studies

development of shape and lattice preferred orientations application to the seismic anisotropy of the lower crust abstract we review the physical basis of the development of fabrics in plastic and viscous flow and illustrate the typical fabrics formed by these processes in the main rock forming silicates of the lower crust feldspar quartz pyroxene and amphibole the orientation process in plastic deformation where a single slip is dominant is recalled and the role of the constraint of neighbouring grains is emphasized the fabric development of anisometric crystals in viscous flow is discussed as a function of the main controlling parameters shear strain aspect ratio and interference between crystals the same sense of fabric asymmetry is introduced by plastic and viscous flow between the flow plane and the shape preferred orientation and hence coherent kinematic analysis can be undertaken in both modes of flow in order to assess the role of such fabrics in the seismic laminations of the lower continental crust we have calculated the seismic p wave properties of typical fabrics for hypothetical monomineralic and polymineralic rocks the calculations show that the strongest anisotropies develop in monomineralic rock with values between 5 and 16 compared with 5 and 8 for typical rock compositions the strongest anisotropies for layered monomineralic rocks generated by fabrics is only 6 compared to the 14 suggested by model studies of the observed seismic laminations we suggest that other effects such as compositional layering and or constructive interference of seismic waves are responsible for augmenting the apparent anisotropy

deformation induced clinopyroxene fabrics from eclogites abstract clinopyroxene omphacite microfabric studies in eclogites are useful in characterizing the deformation that has affected deeper parts of subduction zones we present the microfabrics and tem structures of four samples representative of the various deformation types known in eclogites the 001 axis tends to be parallel to the lineation while the 010 plane is close to the foliation but these general features undergo strong variations in relation to the shape fabric in strongly lineated samples 010 tends to be spread out in a great circle normal to the lineation whereas 001 is dispersed in the foliation in the case of strongly foliated samples the slip systems identified by tem are 001 100 001 110 and 1 2 110 110 operation of dislocation creep is testified by the presence of dislocations and dislocation walls but this process alone cannot explain all the observed features particularly some aspects of the crystallographic preferred orientation hence other deformation mechanisms must be involved these may be dynamic migration recrystallization by grain boundary migration as well as mass transfer processes this study illustrates that not every crystallographic mineral preferred orientation can be explained by dislocation glide and or twinning processes alone in addition it also demonstrates that eclogite microfabrics can be used to unravel deformation parameters in subduction and or collision zones

low differential stress during high pressure metamorphism the microstructural record of a metapelite from the eclogite zone tauern window eastern alps constraints on the state of stress at deep levels of convergent plate boundaries can be obtained from the microstructural record of high pressure metamorphic rocks provided that this record has remained unchanged during exhumation the inclusion patterns in poikiloblastic garnet of a metapelite from the eclogite zone of the tauern window eastern alps reveal a foam microstructure of quartz controlled by grain boundary free energy the garnet blasts show concentric chemical zonation thermobarometry based on correlation between compositional domains in garnet chloritoid and phengite coexisting with kyanite rutile and quartz shows that garnet growth commenced before peak p t conditions close to 25 kbar and 600 c had been reached and continued during the early stage of decompression this implies that the microstructure of quartz controlled by grain boundary free energy persisted continuously during the latest stage of burial to ca 80 km depth and the early stage of exhumation involving detachment from the downgoing slab and release into the subduction channel the foam structure of the overgrown quartz indicates that the differential stress was too low to drive dislocation creep during that period when the rock crossed the interface between the plates with a high differential stress resulting in tectonic overpressure or significant shear heating being ruled out outside the protective garnet the interfacial free energy controlled quartz microstructure has been destroyed completely due to dynamic recrystallization during deformation by dislocation creep this happened while the crustal slice was exhumed to a depth of about 45 km the preservation of this synkinematic quartz microstructure indicates a tectonic scenario where the final cooling was rapid and no marked stress peak was associated with the brittle plastic transition

deformation processes in eclogitic rocks evidence for the rheological delamination of the oceanic crust in deeper levels of subduction zones the deformation mechanisms and associated growth processes operating in omphacite during foliation development and veining have been studied in a km scale eclogitic ductile shear zone from the monviso meta ophiolitic complex italian western alps dislocation creep due to multiple slip and or climb is the dominant deformation mechanism whereas 100 and 001 twinning fracturing and intergranular fluid phase diffusional creep are subordinate active slip systems include 110 001 12 110 110 100 001 and 12 010 100 fracturing is interpreted in terms of brittle instabilities arising from strain and pore fluid pressure partitioning between adjacent omphacite domains the transition from plastic to brittle behaviour of rocks is cyclic in nature as the internally generated fluids can be released and trapped repetitively during progressive deformation the significance of the eclogitic shear zone is discussed in terms of a mechanically weak zone that developed during burial or at least prior to the peak of temperature at a minimum depth of 40 km in a subduction zone and which subsequently acted as a zone of detachment during a late stage of the alpine orogeny a rheological model for oceanic crust delamination in deeper levels of subduction zones is presented

punctuated aggradational cycles a general hypothesis of episodic stratigraphic accumulation according to the hypothesis of punctuated aggradational cycles presented here the stratigraphic record consists of small scale 1 5 m thick shallowing upward cycles pacs separated by surfaces marked by abrupt change to deeper facies this motif pervasive in time and environment is produced by relatively long periods tens of thousands of years of base level stability punctuated by geologically instantaneous relative base level rises of at least basin wide extent as basin wide lithologic time stratigraphic units pacs are fundamental to all aspects of stratigraphic analysis including correlation paleoenvironmental interpretation and paleogeographic reconstruction of several possible allogenic mechanisms for a pervasively cyclic stratigraphic record glacial eustasy driven by orbital perturbations is preferred the pac hypothesis is presented as a comprehensive model to be tested and modified by examination of the stratigraphic record from an episodic perspective

models for generation of carbonate cycles computer modeling provides a quantitative approach to a better understanding of actual carbonate cyclic sequences to model carbonate cycles the authors can use water depth dependent sedimentation rate for each facies an initial lag time linear subsidence tidal range and period and amplitude of sea level oscillation about a horizontal datum tidal flat capped cycles up to a few meters thick result from low amplitude sea level oscillation of a few meters and short lag times nonerosive caps reflect sea level lowering being balanced by subsidence and basinward migration of the shoreline not exceeding tidal flat progradation rate when higher amplitude sea level oscillations occur the tidal flats are abandoned on the inner shelf during sea level fall because seaward movement of the strandline outpaces progradation rate of flats increased amplitude also results in sea level falling faster than flats can subside so that disconformities with thick vadose profiles develop high amplitude 100 m or more oscillations result in incipient drowning of platforms and juxtaposition of deep water facies against shallow water facies within cycles sea level falls before the platform can build to the sea level highstand and the shoreline migrates much more rapidly than tidal flats can prograde thus cycles are disconformity bounded and lack tidal flat caps 10 references

fabric asymmetry and shear sense in movement zones asymmetry between lattice preferred orientation and shape fabrics is commonly observed in microstructural studies of many deformed mineral aggregates e g ice carbonate rocks peridotites and quartzites theoretical predictions are limited by the nature of the models available and more can be gained by discussion of the mechanisms which give rise to such asymmetry in specific natural and experimental situations