turbulence measurements over a sand wave in lower chesapeake bay virginia u s a abstract measurements of turbulent shear stress 5 cm above a sediment bed are described for five stations distributed along a transverse profile of an estuarine sand wave in lower chesapeake bay virginia u s a time series analyses of the velocity data reveal three kinds of constituents 1 components of long period 2 wind wave components and 3 turbulence the energy in the turbulent part of the records is highly attenuated in comparison with that of the low frequency components furthermore at all except the lowermost frequencies the energy contained in the horizontal component of the flow is less than that contained in the vertical component the excess energy at the low frequency part of the spectrum as compared to the high frequency part is explained by 1 the input of energy from wind waves 2 damping of turbulence or 3 both causes the first evidently is the main cause suspended sediments and stable density stratification reduce turbulence but they act mainly on the vertical component a schematic model presented here indicates that oscillating flow over a rugose bed induces additional fluctuations in the vertical component estimates of reynolds shear stress are small and most are negative time series plots of the reynolds stress show 1 small spikes in all records in which the instantaneous shear stresses are very close to zero and 2 intermittent large spikes in most instances within a spike the fluctuating horizontal component of the turbulent velocity u and the vertical counterpart w are of the same sign and w u these results are in agreement with the proposed model of flow over rippled surfaces the findings and concepts developed in the study indicate that even though a flow over the bed actually produces a significant boundary shear stress it cannot be properly measured very close to the bottom based on the reynolds stress idea when making nearbed measurements in turbulent flows in open waters with tidal currents surface waves stable density structures a rugose bed and with sediment in suspension one may encounter a reduction in turbulence and a degradation of the usual correlation between u and w under these conditions the profile method for determining the shear stress using mean flow values may be more reliable

characterisation of ottnangian middle burdigalian palaeoenvironments in the north alpine foreland basin using benthic foraminifera a review of the upper marine molasse of southern germany abstract considerable facies changes and varying foraminiferal assemblages have been described from the lower miocene sediments of the upper marine molasse in the north alpine foreland basin of southern germany and have resulted in the distinction of a western molasse and eastern molasse however the causes and processes leading to these local differences have largely remained unexplored to date the aim of this study is to explore possible explanations in order to better understand the palaeogeographic stratigraphic and environmental setting the study focuses on the ottnangian middle burdigalian segment of the upper marine molasse in southern germany and includes a review of previously published data together with new data from the borehole stockhausen which is located between the areas of the eastern and western molasse methods include quantitative analysis of benthic foraminiferal assemblages qualitative studies of planktonic foraminifera and calcareous nannoplankton as well as palaeoenvironmental considerations based on the actualistic approach the results of these studies demonstrate that the lower and middle ottnangian upper marine molasse include a single transgressive regressive sedimentary cycle with gradual changes of foraminiferal assemblages in the eastern molasse on the other hand two sedimentation cycles are recognizable in the western molasse and in the borehole stockhausen where the lower and middle ottnangian sediments are separated from one another by an unconformity and display an abrupt change in lithofacies and foraminiferal content the benthic foraminiferal assemblages from the area of the western molasse are indicative of a rather shallow sea but indicative of a much deeper facies in the basin area of the eastern molasse it can therefore be concluded that the shallow sea to the west was more sensitive to minor changes in sea level than the area of the eastern molasse either because of global sea level variation or as a result of regional tectonics this means that the occurrence of two sedimentary cycles in the western molasse and absence of the second cycle in the eastern molasse may be the result of shallower water depths and a lower subsidence rate in the area of the western molasse another interesting result from our studies relates to the sea level highstand and maximum extension of the upper marine molasse sea that based on our studies has occurred during the early ottnangian rather than during the middle ottnangian as suggested previously during this highstand the area of the eastern molasse displays a differentiation into a deep neritic basin facies characterised by high organic flux and moderate oxygen depletion at times and a marginal marine facies with well oxygenated environments the latter facies remained poorly understood until recently the entire region of the south german molasse basin was affected by a regression during the middle ottnangian this phase is characterised by unstable environmental conditions in the eastern molasse i e salinity fluctuations and temporarily strong oxygen depletion of bottom waters in the western molasse life conditions became even worse and as a result a drastic decline in marine life is observable

palinspastic reconstruction and topographic evolution of the eastern alps during late tertiary tectonic extrusion this paper presents a new palinspastic restoration of the eastern alps for neogene time and an attempt to reconstruct the neogene palaeogeology palaeotopography and palaeohydrography in connection with the structural evolution the eastern alps underwent radical horizontal displacement during the neogene due to large strike slip systems and formation of structural windows our palinspastic reconstruction considers a the rearrangement of tectonic units dismembered during tectonic extrusion b the tectonic denudation driven by displacement of the crystalline blocks c geochronological arguments and d the sedimentary record of the syn extrusion basins the rearrangement of tectonic blocks results in a remarkably good fit of highly dismembered zones both in crystalline and sedimentary areas and shows the pre miocene unstretched pattern of the eastern alps reduced to 65 of its present day e w elongation using this structural frame and considering the sedimentary record a set of palaeogeologic and palaeotopographic sketch maps with the palaeo river systems is presented for three time slices pre syn and post extension situation in late oligocene and early miocene times the western eastern alps were already mountainous whereas the eastern part of the orogen formed lowlands or hilly areas enhanced block movement in the course of the extrusion process around the early middle miocene boundary led to the formation of intramontane sedimentary basins and a fault induced reorientation of the drainage pattern which forms the basis of the modern river system in the area east of the tauern window this region where pre miocene land surfaces are preserved probably became a mountainous area not before late miocene time and never reached the elevations of the areas further west

vims sea carousel a field instrument for studying sediment transport preliminary results from field experiments using a newly developed annular sea bed flume vims sea carousel are presented to illustrate a method of in situ determination of critical threshold shear stress necessary for sediment entrainment this flume has been deployed in the lower chesapeake bay and on the inner shelf of the middle atlantic bight we applied a series of controlled bed shear stresses by changing the stress sequentially to examine the responses of nearly undisturbed natural sediment beds an optical backscatter sensor mounted on the flume indicated the occurrences of resuspension of bed material we identified clear threshold values of bed shear stress for resuspension generally the sandy inner shelf sediment had a higher critical bed shear stress 0 22 pa for resuspension than that for silty sediments 0 1 0 19 pa data from partially cohesive sediments in the lower chesapeake bay indicate significant seasonal differences in critical bed shear stress 0 19 pa at the beginning of summer and 0 11 pa at the end of summer these differences may be attributed to bioturbation the seasonal difference in the microflora living on the grains or spatial heterogeneity of bed

estimate of bottom and surface stress during a spring neap tide cycle by dynamical assimilation of tide gauge observations in the chesapeake bay dynamical assimilation of surface elevation from tide gauges is investigated to estimate the bottom drag coefficient and surface stress as a first step in improving modeled tidal and wind driven circulation in the chesapeake bay a two dimensional shallow water model and an adjoint variational method with a limited memory quasi newton optimization algorithm are used to achieve this goal assimilation of tide gauge observations from 10 permanent stations in the bay and use of a two dimensional model adequately estimate the bottom drag coefficient wind stress and surface elevation at the bay mouth subsequent use of these estimates in the circulation model considerably improves the modeled surface elevation in the entire bay assimilation of predicted tidal elevations yields a drag coefficient defined in the hydraulic way varying between 2 5 x 10 4 and 3 1 x 10 3 the bottom drag coefficient displays a periodicity corresponding to the spring neap tide cycle with a maximum value during neap tide and a minimum value during spring tide from assimilation of actual tide gauge observations it is found that the fortnightly modulation is altered during frontal passage furthermore the response of the sea surface to the wind forcing is found to be more important in the lower bay than in the upper bay where the barometric pressure effect seems to be more important

the northern coast of the ottnangian middle burdigalian early miocene molasse sea in germany sediments foraminiferal assemblages and biostratigraphy this study provides new data on the litho and biostratigraphy of the middle burdigalian ottnangian marine sediments at or close to the former northern coastline of the german molasse basin based on cores from three boreholes burgau hamlar 2 and lutzingen and two outcrops haunsheim and dattenhausen methods include quantitative analysis of benthic foraminiferal assemblages 90 samples studies of planktonic foraminifers and ostracods and investigations of lithofacies the data indicate that the transgressive regressive marine sediments of the upper marine molasse omm at burgau and hamlar 2 can be subdivided into the well known lower and middle ottnangian sedimentation cycles with the first cycle being represented by the omm basisschichten and kalkofen formation and the second by the baltringen and steinhofe formations we show for the first time that also the northernmost marginal marine omm facies lutzingen haunsheim and dattenhausen can be correlated with the lower and middle ottnangian sedimentation cycles consequently our results do not support the presence of a previously suggested third sedimentation cycle within the omm sediments our micropaleontological data reveal regionally diverse depositional environments reflecting different water depths and co varying environmental variables significant differences in abundance diversity and species composition of the benthic foraminiferal assemblages demonstrate marginal marine facies for lutzingen haunsheim and dattenhausen nearshore facies for hamlar 2 and shallow marine basin facies for burgau the characteristic lower ottnangian benthic foraminiferal species exhibit restricted ecological tolerances hence the absence or scarcity of these species in nearshore to marginal marine deposits has no stratigraphic significance the presence of two sedimentation cycles in the western german molasse basin alone may have resulted from the interplay of regional tectonics and basin development that have led to different rates of subsidence and sedimentation in the western and eastern sectors of the german molasse basin

seasonal oxygen depletion in continental shelf waters of louisiana historical record of benthic foraminifers a strong spring and summer oxygen depletion is induced in nearshore bottom waters of the louisiana continental shelf by density stratification and by the carbon flux from phytoplankton production which in turn is related to the nutrient load of the mississippi and atchafalaya rivers in an attempt to read the historical record of this shelf hypoxia during the past two centuries we compared the stratigraphic signals of benthic foraminifera as reflected in a relative dominance index for two common species of ammonia and elphidium in 210 pb dated cores and we found evidence of an overall rise in oxygen stress in intensity or duration especially in the past 100 yr this implies a progressive increase in the influence of river borne nutrients particularly anthropogenically influenced nitrates judging by our results foraminiferal indices based on appropriate species ratios should prove useful in testing hypotheses about long term environmental stresses including eutrophication and paleohypoxia on other marine shelves

spatial variability of bottom types in the lower chesapeake bay and adjoining estuaries and inner shelf abstract the spatial distributions of the bed textural and morphologic properties that influence boundary layer roughness characteristics in the lower chesapeake bay the lower portions of the york james and elizabeth rivers and the adjacent inner continental shelf were systematically mapped a high resolution fully corrected side scan sonar mapping system 100 khz was used for remote acoustic detection of bottom roughness supported by ground truthing by direct in situ observations by divers these complementary methods proved to be especially effective in detecting a wide range of roughness controlling bed surface properties at various scales fine scale variations in sediment size and associated bottom texture are considered to be the main source of heterogeneity in nikuradse skin friction roughness a wide variety of small and intermediate scale morphologic elements provide meso scale and small scale distributed form drag roughness depending upon location the distributed roughness may be either biogenic or hydrodynamically induced by currents and waves although anthropogenic roughness prevails in certain instances e g port areas in terms of particular combinations of roughness scales and types combined sonar and diver observation data allow the beds to be systematically but qualitatively classified into 10 bottom types each of which is associated with a particular type of subenvironment

evidence for the influence of form drag on bottom boundary layer flow posed of two intersecting segments smith 1977 and smith and mclean 1977 demonstrate that multiple roughness scales can generate velocity profiles with more than two segments thus when form drag is important the constantstress assumption will not be valid in an earlier paper caldwell and chriss 1979 we demonstrated the existence of the viscous sublayer in the bottom boundary layer and found a logarithmic layer above it in examining additional data from the experiment segmented profiles were found in the logarithmic region figures 1 and 2 as expected if form drag influences the flow when the original data set was reanalyzed using thinner averaging intervals in the upper portion of the profile and also incorporating both upward and downward traverses it too shows two distinct logarithmic slopes in the original study only downward traverses were used although deviation from a single logarithmic form is not necessarily large the slope of the logarithmic regression is significantly different in the two regions implying that the turbulent stress above is significantly larger than it is nearer the bed

sequence stratigraphy and depositional history in the south eastern german molasse basin abstract a study of seismic lines and well data from the region between the isar and inn rivers se germany indicates the presence of three main depositional cycles in the molasse basin upper eocene chattian sands upper chattian marls aquitanian sandstone shale series and aquitanian fish shales upper freshwater molasse the flexural subsidence caused by thrust loading has the greatest influence in increasing reducing the accommodation space for the deposition which controlled the displacement of the depositional axis in a n s direction however eustatic sea level changes which caused the shoreline to shift in a w e direction have a strong impact on the stratigraphic development of this typical foreland basin

suspended sediment transport and benthic boundary layer dynamics abstract the mellor yamada level ii turbulence closure model is used to examine the dynamics of a benthic boundary layer bbl carrying sediment in suspension using flow and sediment properties from observations in the florida straits implications for sediment transport in this and similar environments are discussed model results indicate that three size classes provide an adequate representation of the frequency distribution of a moderately well sorted sand size sediment the benthic boundary layer responds to a stabilizing suspended sediment concentration gradient by a reduction of about 45 in the bottom stress damping of turbulence and by a decrease in the slope of the velocity profile the velocity field in the lowermost portion of a neutrally stratified bottom boundary layer is governed by the relationship u u k ln z z 0 to recover the predicted velocity distribution in the sediment stratified bbl from this relationship von karman s constant k must be reduced by about 15 the model however treats k as internally invariant applying similarity theory to the model output yields u u k ln z z 0 where k k 1 ar f a is an empirically determined constant and rf is the flux richardson number in this case k may be interpreted as a modified von karman s constant increases in the angle of turning of the current vector 5 and enhancement of the ekman like nature of the current spiral are predicted these model predictions are consistent with findings from studies of stably stratified atmospheric boundary layers and sediment laden channel flows

sedimentary sequences seismic facies subsidence analysis and evolution of the burdigalian upper marine molasse group central switzerland the sedimentological interpretation of seismic and borehole data derived from the burdigalian upper marine molasse group of the central north alpine foreland basin enables a detailed reconstruction of the shallow marine architecture the different seismic facies are assigned to the shoreface foreshore nearshore and offshore depositional systems mapping of the seismic facies on the seismic line reveals the presence of two prograding sequences luzern and st gallen formations that are separated by an unconformity the subsidence analysis suggests that the evolution of the upper marine molasse was primarily controlled by the interaction between tectonic subsidence and sediment supply the distal transgressions of the luzern and st gallen formations and the establishment of shallow marine environments were caused by enhanced subsidence rates in the distal reaches associated with increased sediment supply rates the development of the sequence boundaries separating the luzern and st gallen formations and the regression at the end of the st gallen formation however were controlled by uplift in the distal part of the basin the subsidence and sediment flux analyses of the upper marine molasse allow an improved understanding of the tectonic and denudation history of the northern alps out of sequence thickening in the rear of the northern alps by underplating of the aar massif and forward thrusting of the helvetic nappes is interpreted to have controlled uplift in the distal part of the basin and the formation of the sequence boundaries in sequence crustal thickening at the tip of the orogenic wedge by forward underthrusting of lower freshwater molasse thrust sheets beneath molasse deposits associated with enhanced erosion of the northern alps caused a northward shift of the location of the major orogenic load and an increase of the sediment supply to the molasse basin these processes are interpreted to have controlled the distal shifts of the basin depocenter and the distal transgressions of the luzern and st gallen formations

effects of biological activity on the entrainment of marine sediments nowell a r m jumars p a and eckman j e 1981 effects of biological activity on the entrainment of marine sediments mar geol 42 133 153 the effects of animal tracks and fecal pellet production on the critical entrainment velocity of marine sediments were examined experimentally laboratory measurements in a free surface seawater flume were made using three sediment sizes effects of three species of polychaetes and two species of bivalves were tested boundary shear velocity was calculated from the mean velocity profile in the logarithmic region of the boundary layer measurements were made with a hot film anemometer tracking doubled the boundary roughness and decreased the critical entrainment velocity by 20 ambient or free sediments were more easily entrained than fecal mounds which were restrained from movement by mucous adhesion between the fecal coils isolated pellets such as those egested by amphicteis scaphobranchiata transported readily as bedload over a cohesive sediment surface

ries and chicxulub impact craters on earth provide insights for martian ejecta blankets terrestrial impact structures provide field evidence for cratering processes on planetary bodies that have an atmosphere and volatiles in the target rocks here we discuss two examples that may yield implications for martian craters 1 recent field analysis of the ries crater has revealed the existence of subhorizontal shear planes detachments in the periphery of the crater beneath the ejecta blanket at 0 9 1 8 crater radii distance their formation and associated radial outward shearing was caused by weak spallation and subsequent dragging during deposition of the ejecta curtain both processes are enhanced in rheologically layered targets and in the presence of fluids detachment faulting may also occur in the periphery of martian impacts and could be responsible for the formation of lobe parallel ridges and furrows in the inner layer of double layer and multiple layer ejecta craters 2 the ejecta blanket of the chicxulub crater was identified on the southeastern yucatn peninsula at distances of 3 0 5 0 crater radii from the impact center abundance of glide planes within the ejecta and particle abrasion both rise with crater distance which implies a ground hugging erosive and cohesive secondary ejecta flow systematic measurement of motion indicators revealed that the flow was deviated by a preexisting karst relief in analogy with martian fluidized ejecta blankets it is suggested that the large runout was related to subsurface volatiles and the presence of basal glide planes and was influenced by eroded bedrock lithologies it is proposed that ramparts may result from enhanced shear localization and a stacking of ejecta material along internal glide planes at decreasing flow rates when the flow begins to freeze below a certain yield stress

paleobathymetry in the backstripping procedure correction for oxygenation effects on depth estimates abstract this paper aims to provide a straightforward and easily applicable method for estimating the depositional depth evolution of marine basins vertical movements of the basin floor can be reconstructed from the sedimentary record and more accurately constrained when information from the sedimentary history is combined with palaeodepth estimates derived from fauna to this end we propose to extend an existing method based on the percentage of planktonic foraminifera with respect to the total planktonic and benthic foraminiferal association which is expressed as the percentage planktonics p the ratio between planktonic and benthic foraminifera is related to water depth and the p generally increases with increasing distance to shore however next to water depth the oxygen level of bottom waters has a profound effect on the abundance of benthic foraminifera and as such influences the p depending on basin configuration the oxygen level at the sea floor can vary on milankovitch time scales and is reflected by the fraction of benthic foraminiferal species that indicate an effect of oxygen stress on the biotic system these species can be used as stress markers and their percentage with respect to the total benthic population is here expressed as s to assess whether the effect of sea floor oxygenation impairs depth reconstructions we studied the percentage of planktonic foraminifera p in five well dated sedimentary successions from the lower pliocene of crete corfu and milos in greece additionally we assessed whether different foraminiferal size fractions and counting methods affect the determination of the percentage of planktonic foraminifera the palaeobathymetric evolution calculated for each basin was confirmed for all successions by an independent check on depth related occurrences of benthic foraminifera after correction for bathymetry changes of the basin due to sedimentation compaction and eustatic sea level variations the vertical movement history of the basin floor was inferred we propose a standard methodology for reconstructions of palaeobathymetry of marine sedimentary successions from foraminiferal associations

experimental dissolution of a fossil foraminiferal assemblage paleocene eocene thermal maximum dababiya egypt implications for paleoenvironmental reconstructions abstract dissolution experiments were carried out on a foraminiferal assemblage from the paleocene eocene thermal maximum petm at dababiya egypt in order to 1 reveal the effects of differential dissolution on the composition of the foraminiferal assemblage and 2 develop objective criteria for the evaluation of dissolution in foraminiferal assemblages used in early paleogene paleoenvironmental reconstructions particularly with respect to neritic midway type assemblages from the paleocene eocene transition our results confirm two general observations on modern foraminifera 1 planktic foraminifera are much more vulnerable to dissolution than benthic foraminifera leading to depressed p b ratios and 2 dissolution susceptibility differs between size fractions with the smaller specimens dissolving more rapidly than the bigger ones leading to a larger average size of the remaining assemblage within a size fraction wall structure and thickness are considered to be the main factors controlling differential dissolution susceptibility we propose a ranking scheme for taxa with respect to dissolution resistance among the benthic taxa lenticulina is most resistant followed by the agglutinated gaudryina cf ellisorae and alabamina midwayensis biserial and triserial hyaline taxa and the porcelaneous spiroloculina sp are most susceptible to dissolution whereas rotaliines such as cibicidoides and anomalinoides have an intermediate susceptibility this implies that mild dissolution of a midway type benthic assemblage leads to a relative enrichment in lenticulina gaudryina and rotaliines amongst planktic foraminifera the muricate taxa acarinina and morozovella are most resistant followed by the cancellate subbotina the smooth and generally small globanomalina and zeauvigerina are least resistant to dissolution our data enable to objectively evaluate various degrees of dissolution in benthic and planktic foraminiferal assemblages retrieved from the lower paleogene tethyan outcrops in this way taphonomic artifacts can be readily distinguished from paleoenvironmental signals affecting the primary composition of the assemblages more generally we propose that the combined use of foraminiferal numbers p b ratio and relative abundances of non calcareous agglutinated taxa and lenticulina may provide a powerful proxy for assessing dissolution in hemipelagic assemblages from cenozoic and upper cretaceous continental margins in order to achieve more robust pre quaternary paleoenvironmental reconstructions based on quantitative foraminiferal data application of dissolution proxies like proposed here or in slightly modified form should become a more widely used micropaleontologic procedure particularly continental margin studies dealing with major biotic events e g petm or employing p b ratios for sea level reconstructions should benefit from such an approach

an efficient horizontal two dimensional hydrodynamic model abstract an efficient numerical model is applied to estimate the detailed tidal current fields in the middle reach of the james river near hogpoint virginia this model uses an economic full implicit finite difference scheme and variable grid size to solve the depth averaged equations of motion and the continuity equation terms of atmospheric pressure gradient wind stresses tidal force bottom and lateral friction coriolis force and convective acceleration are all included thus this model can be used to simulate tidal circulation storm surge and shear induced flow a variable grid system was used to give better resolution at specified subareas the governing equations and numerical scheme are relatively simple and it can be easily maintained and adopted for other applications the computing time of this model is approximately linear proportional to the number of fluid cells the model can be run on a personal computer with sufficient efficiency

post collisional orogen parallel large scale extension in the eastern alps abstract large scale extension affected the eastern alps during post collisional lateral tectonic extrusion in early and middle miocene times the tauern window was mainly tectonically exhumed by the 160 km pullapart of the rigidly behaving austroalpine basement blocks forming the tectonic lid of the penninic contents of the window an evaluation of the syn extrusion fault pattern reveals displacements of several tens of kilometers along the low angle extensional shear zones at the western and eastern margins of the window and along the important strike slip fault zones north and south of the window large scale shear is reflected by discontinuities in mineral cooling patterns across important shear zones the cooling histories of the tectonic blocks show that the window boundaries during exhumation coincide only partly with the classical boundaries of the window defined by thrust units reconstruction of the exhumation history shows an asymmetric evolution of the tauern window we argue that the eastern and western low angle extensional shear zones formed in sequence which is reflected by different evolutions of the fragmentation and uplift history of the austraolpine tectonic blocks to the east and west of the window in the central and eastern alps three structural domes the lepontin dome in the central alps and the tauern and rechnitz domes in the eastern alps were formed by large scale extension in miocene times total e w stretch amounts to more than 300 km and results in 70 extension we propose that the central and eastern alps are part of an extensional province that also includes the pannonian basin and has a counterpart separated by the moesian indenter in the rhodope mountains the aegean sea and the menderes massif in asia minor in these regions a number of mainly tectonically exhumed domes or deep sedimentary basins formed by miocene large scale extension

modes of cross shore sediment transport on the shoreface of the middle atlantic bight the mechanisms responsible for onshore and offshore sediment fluxes across the shoreface zone seaward of the surf zone were examined in a 3 year field study the study was conducted in the southern part of the middle atlantic bight in the depth region 7 17 m using instrumented tripods supporting electromagnetic current meters pressure sensors suspended sediment concentration sensors and sonar altimeters the observations embraced fairweather moderate energy swell dominated and storm conditions cross shore mean flows ranged from near zero during fairweather to 20 cm s 1 during the storm oscillatory flows were on the order of 10 cm s 1 during fairweather and 100 cm s 1 during the storm suspended sediment concentrations at about 10 cm above the bed were 5 kg m 3 during the storm three methods were applied to evaluate the relative importance of incident waves long period oscillations mean flows and gravity in effecting shoreward or seaward sediment flux 1 an energetics transport model was applied to instantaneous near bottom velocity data 2 higher moments of near bottom flows were estimated and compared and 3 suspended sediment fluxes were estimated directly from the instantaneous products of cross shore velocity and suspended sediment concentration the results show that measurable contributions were made by all four of the processes most significantly mean flows were seen to dominate and cause offshore fluxes during the storm and to contribute significantly to onshore and offshore flux during fairweather and moderate energy incident waves were in all cases the major source of bed shear stress but also caused shoreward as well as seaward net sediment advection low frequency effects involving wave groups and long period waves made secondary contributions to cross shore sediment flux contrary to expectations low frequency fluxes were just as often shoreward as seaward whereas cross correlations between suspended sediment concentration and the instantaneous near bottom current speed were high and in phase under storm conditions they were weak and out of phase during fairweather conditions this suggests that simple energetics models are probably inadequate for predicting fairweather transport of suspended sediment

eocene pliocene time scale and stratigraphy of the upper rhine graben urg and the swiss molasse basin smb we present a general stratigraphic synthesis for the upper rhine graben urg and the swiss molasse basin smb from eocene to pliocene times the stratigraphic data were compiled both from literature and from research carried out by the authors during the past 6 years an index of the stratigraphically most important localitites is provided we distinguish 14 geographical areas from the helvetic domain in the south to the hanau basin in the north for each geographical area we give a synthesis of the biostratigraphy lithofacies and chronostratigraphic ranges the relationships between this stratigraphic record and the global sea level changes are generally disturbed by the geodynamic e g subsidence evolution of the basins however global sea level changes probably affected the dynamic of transgression regression in the urg e g middle pechelbronn beds and serie grise corresponding with sea level rise between ru1 ru2 and ru2 ru3 sequences respectively as well as in the molasse basin regression of the umm corresponding with the sea level drop at the ch1 sequence the urgent project upper rhine graben evolution and neotectonics provided an unique opportunity to carry out and present this synthesis discussions with scientists addressing sedimentology tectonics geophysics and geochemistry permitted the comparison of the sedimentary history and stratigraphy of the basin with processes controlling its geodynamic evolution data presented here back up the palaeogeographic reconstructions presented in a companion paper by the same authors see berger et al in int j earth sci 2005

impact of interrelated and interdependent ecological controls on benthic foraminifera an example from the gulf of guinea living and dead benthic foraminiferal assemblages from two size fractions 63 250 m and 250 m were studied at 128 stations from the shelves down to abyssal plains of the gulf of guinea in the eastern equatorial atlantic the sample size ranged between 300 and 400 cm2 of the sediment water interface with a penetration depth of 1 cm environmental parameters were quantified for sediment composition food webs bottom water oxygen concentrations and benthic oxygen respiration seven major station groupings named g1 g7 are subdivided by factor analysis the three most abundant species therein are discussed with respect to their ecological requirements for both size classes and for living and dead counts for 49 living species the range of environmental gradients is established and for five species live observations are reported five station groupings can be attributed toward stepwise ranges scaled by bathymetry and flux rates of organic carbon g4 27 75 m g5 68 269 m g6 250 740 m g2 674 2007 m g1 1475 4970 m two geographically restricted station groupings locally interfinger with other groupings in the upper bathyal and abyssal water depths g7 82 451 m g3 1002 4658 m g7 shows lowered oxygen concentrations and benthic oxygen respirations whereas hemipelagic sediments are typical of g3 the interfingering of g7 and g3 however is not purely a result of the increasing number of species most perfectly adapted to these environments but rather that these groupings are structured by the decrease of species with lower tolerance towards environmental perturbation in fact the more common species have an environmental range much broader than the environmental frame of the groupings they dominate this observation holds for all size classes and for live and dead counts we conclude that species specific preferences and thresholds provide a more valuable and more consistent tool in environmental research than the environmental ranges observed for assemblages

the bottom boundary layer of the bay stem plains environment of lower chesapeake bay field observations of bottom boundary layer processes in the bay stem plains environment of lower chesapeake bay depth 11 12 m were made using a bottom mounted tripod supporting electromagnetic current meters and a pressure sensor the prandtl von karman equation the inertial dissipation method and the grant and madsen wave current interaction model were applied to the field data to estimate time varying bed stress and to evaluate the likely contributions of waves and currents to total shear stress and to skin friction biogenic bed micromorphology dominated bed roughness a background signal of long period low amplitude swell was present for most of the time although these oscillations were too weak to agitate the bottom sediments interactions of the swell with tidal currents in the presence of a biogenically roughened bottom caused the hydraulic roughness height z 0 and drag coefficient c d sensed by the mean current to be significantly increased in addition internal waves with frequencies in the neighbourhood of the pycnoclinal brunt vaisala frequency period range approximately 60 100 s appeared intermittently and caused near bottom velocity fluctuations with amplitudes of over 10 cm s 1

facies development along the tide influenced shelf of the burdigalian seaway an example from the ottnangian stratotype early miocene middle burdigalian abstract herein we report quantitative micropaleontological benthic foraminifers dinoflagellate cysts calcareous nannoplankton sedimentological grain size analysis and geophysical background gamma radiation analyses from ottnang schanze the stratotype for the regional ottnangian stage central paratethys lower miocene middle burdigalian the revealed trends in bathymetry primary productivity bottom water oxygenation and water energy allow exemplary insights into the paleoenvironment of the terminal burdigalian seaway several facies of a eutrophic environment are distinguished that reflect a transition from a suboxic outer neritic to upper bathyal towards a better oxygenated middle neritic setting under the influence of storm events and currents a comparison with available data from upper austria and bavaria consistently shows the regressive trend during the late early ottnangian in upper austria the deep water facies from the lower part of the stratotype represents the most distal sediments the upper part together with localities closer to the northern coast records inner to middle neritic environments that are heavily affected by tidal currents the facies distribution results from the progradation of a tide influenced environment along the northern shelf of the north alpine foreland basin heralding the closure of the burdigalian seaway the available age estimate for the stratotype constrains the onset of the regressive phase to 18 ma

kinematics of rock flow and the interpretation of geological structures with particular reference to shear zones abstract current interpretations of structures are generally based on homogeneous and steady deformation models despite the fact that both the heterogeneity of rocks materially rheologically and geometrically and the time dependence of imposed geological conditions give rise to significant heterogeneous and non steady flow in concert with field observations we emphasize that the expectation of heterogeneity and non steadiness is the key to understanding natural deformation and that in order to carry out successful structural analysis and tectonic interpretation it is necessary to recognize the first order distinction between imposed boundary conditions typically used to define the tectonic regime e g transcurrent transpression and the response recorded by rocks within the zone structures and fabrics using s c fabric as an example it is demonstrated how flow with a non zero spinning component resulting from the rheological contrasts and or geologically realistic time dependent boundary displacement can drastically change the ideal geometric and kinematic relations between the fabric and the host zone in agreement with both theoretical analysis and field observation it is shown that natural flow regimes range from pure shear to pure rotation including super simple shear in consideration of the heterogeneity and non steadiness of natural deformation kinematic analysis is justifiable only within a homogeneous domain and steady period flow kinematics and mechanisms are interrelated in that firstly mechanisms provide internal constraints on kinematics ensuring that only certain flows are possible and secondly flow kinematics will favour development of certain mechanisms

symmetry of pebble deformation involving solution pits and slip lineations in the northern alpine molasse basin abstract deformation features on pebbles of the alpine molasse basin are most clearly developed in carbonate components ductile distortion is small most of the pebbles moved against each other to produce solution pits and slip lineations on the pebble surfaces the complete lineation field has a triaxial geometry from a compressional axis of divergence with maximum solution fields of diverging lineations extend to meet at a plane of convergence their ends bend away from an intermediate axis towards an extensional axis of convergence the strain symmetry is pure shear for orthogonal lineation field axes uniaxial compression and extension representing special cases the angle between the divergent and the convergent axes decreases from 90 to 0 with the transition from pure to simple shear for molasse pebbles angles between 90 and 60 were usually observed regional compression developed perpendicular to the alpine structures and parallel to bedding with increasing deviations near the alpine border zones are recorded of vertical and horizontal extension and of compression perpendicular to bedding and horizontal extension perpendicular to the rhinegraben existing at the northern border of the molasse basin

integrated stratigraphy and 40ar 39ar chronology of the early to middle miocene upper freshwater molasse in eastern bavaria germany a detailed integrated stratigraphic study biostratigraphy and magnetostratigraphy was carried out on five sections from the western part of the bavarian upper freshwater molasse of the north alpine foreland basin nafb greatly improving the chronostratigraphy of these sediments the sections belong to the lithostratigraphic units limnische untere serie ul and fluviatile untere serie uf and contain 19 mostly new small mammal bearing levels significantly refining the local biostratigraphy radiometric ages obtained from glass shards from tuff horizons are used together with the biostratigraphic information for constructing and confirming the magnetostratigraphic correlation of the studied sections to the astronomical tuned time scale ants04 lourens et al in geologic time scale 2004 cambridge university press 2004 this correlation implies that the ul lithostratigraphic unit corresponds to the latest ottnangian and the early karpatian whereas the uf corresponds to the karpatian and the early badenian this indicates that the brackish to freshwater molasse transition already occurred during the late ottnangian the pre riesian hiatus occurred in the latest karpatian and lower early badenian in eastern bavaria and bohemia and in the late karpatian and earliest badenian in western bavaria the geochemical and ar ar data of volcanic ashes suggest that highly evolved silicic magmas from a single volcano or volcanic center characterized by a uniform nd isotopic composition erupted repetitively over the course of at least 1 6 myr three phases of eruptive activity were identified at 16 1 0 2 ma zahling 2 15 6 0 4 ma krumbad and 14 5 0 2 ma heilsberg hegau the correlation of the local biostratigraphic zonation to the ants04 enables further the characterization of both the ottnangian karpatian and karpatian badenian boundaries in the nafb by small mammal biostratigraphy according to these results the ottnangian karpatian boundary is contemporaneous with the first appearance datum of megacricetodonbavaricus in the size of the type population and the first common occurrence of keramidomys thaleri whereas ligerimys florancei melissiodon dominans and prodeinotherium aff bavaricum have been already disappeared during the late ottnangian the karpatian badenian boundary is characterized by a significant size increase of the large megacricetodon lineage and possibly a re immigration of prodeinotherium bavaricum

foraminifera from the borehole altdorf se germany proxies for ottnangian early miocene palaeoenvironments of the central paratethys abstract during the ottnangian early miocene the shallow epicontinental sea of the upper marine molasse in the western and central paratethys realm experienced major sea level fluctuation leading to significant environmental changes however the effects of these changes remain largely unexplored because detailed palaeoenvironmental analyses of the respective time interval are scarce we here present a thorough analysis of the benthic foraminiferal composition 36 samples 5616 individuals and 90 taxa from a 105 25 m thick marine ottnangian succession of the borehole altdorf and from two coeval outcrops neuhofen oberschwarzenbach all located in the western central paratethys studies on the planktonic foraminifera and calcareous nannoplankton were conducted as well we show that benthic foraminiferal assemblages clearly differ with regard to species composition diversity indexes relative abundances of infauna epifauna and oxygen indicators we introduce five foraminiferal biofacies which together are suggestive of a transgressive regressive development and several distinct environmental changes from the early to middle ottnangian the planktonic foraminiferal assemblages from altdorf and the central paratethys suggest cold temperate surface water however high nutrient availability and taphonomic factors such as selective dissolution could also have affected the planktonic assemblages in most samples the relative abundance of infaunal epifaunal benthic foraminifera planktonic foraminifera and calcareous nannoplankton clearly indicates nutrient rich waters we did not find indicators for upwelling as a possible source for the nutrient flux and therefore assume that the high nutrient levels indicate large scale river influx in a humid climate based on our foraminiferal biofacies we postulate the presence of a particularly high fluviatile runoff and high organic matter supply resulting in a moderate oxygen depletion of marine bottom waters for the period of relative sea level highstand in the early ottnangian strong oxygen depletion of bottom waters occurs in the middle ottnangian and is mainly caused by salinity stratification and a reduced water exchange to the open ocean during that time moreover our data demonstrate that p b ratios for early and middle ottnangian samples only partially support the palaeobathymetric reconstructions based on the benthic foraminifera we suggest that fluviatile input and estuarine circulation strongly affected the relative abundances of planktonic foraminifera besides tidal and other marine currents

the n variscan margin of the ruhr coal district western germany structural style of a buried thrust front the ruhr coal district represents part of an external fold and thrust belt of the variscan orogen in central europe extensive mining exposed molasse type coal bearing clastic sequences of upper carboniferous age deformed by folding and thrusting

some aspects on the origin of fold type fabrics theory experiments and field applications an applicable interpretation of fabrics should be based mainly on geometrical considerations in order to cover available field data therefore a theory on the formation of foldtype fabrics including congruent and concentric flexural slip folds as well as kink bands in materials subjected to rhombic and different monocline strain types is derived by means of particle motion fields for homogeneous and isotropic bodies the analysis of experimental results and their comparison with field observations largely confirms the theory and contributes to its improvement some trends can be established with increasing monocline character of the strain type the probability of concentric and congruent flexural slip folding is reduced it is substituted by kink band formation while the portion of monocline strain is enlarged with depth rhombic fold symmetry indicates in the realm of elastic plastic behaviour the proximity of the surface of the earth or of a detachment surface with gradual increase of the rock anisotropy the development of shear faults kink folds and finally congruent and concentric flexural slip folds is favoured

the distribution of megablocks in the ries crater germany remote sensing field investigation and statistical analyses the ries crater is a well preserved complex impact crater that has been extensively used in the study of impact crater formation processes across the solar system however its geologic structure especially the megablock zone still poses questions regarding crater formation mechanics the megablock zone located between the inner crystalline ring and outer morphologic crater rim consists of allochthonous crystalline and sedimentary blocks bunte breccia deposits patches of suevite and parautochthonous sedimentary blocks that slumped into the crater during crater modification our remote sensing detection method in combination with a shallow drilling campaign and geoelectric measurements at two selected megablocks proved successful in finding new megablock structures 25 m mean diameter within the upper approximately 1 5 m of the subsurface in the megablock zone we analyzed 1777 megablocks of the megablock zone 81 of which are new discoveries in our statistical analysis we also included 2318 ejecta blocks 25 m beyond the crater rim parautochthonous megablocks show an increase in total area and size toward the final crater rim the sizes of allochthonous megablocks generally decrease with increasing radial range but inside the megablock zone the coverage with postimpact sediments obscures this trend the size frequency distribution of all megablocks obeys a power law distribution with an exponent between approximately 1 7 and 2 3 we estimated a total volume of 95 km3 of bunte breccia and 47 km3 of megablocks ejecta volume calculations and a palinspastic restoration of the extension within the megablock zone indicate that the transient cavity diameter was probably 14 15 km

maximum effective moment criterion and the origin of low angle normal faults abstract the origin of the low angle normal faults of metamorphic core complexes has been debated for over two decades proponents of andersonian fault mechanics have long argued that it is mechanically unfeasible for slip to occur along shallowly dipping normal faults a new theory named the maximum effective moment criterion is proposed here for their origin using an effective moment approach formulated as m eff fh where f is the force tangentially acting on the unit shear boundaries and h represents its arm the maximum value appears at angles of 54 7 with the 1 since extensional crenulation cleavages eccs and the contractional crenulation cleavages cccs occur in conjugate pairs with 110 angle between them it is suggested that they tend to be oriented in the directions of maximum effective moment the differential stress for formation of the ecc or ccc are less than that for fracturing the orientations of conjugate eccs depending on relative magnitudes of simple shear versus coaxial strain components the synthetic ecc set is much better developed than the antithetic set due to anisotropy upward propagation of the synthetic ecc set from mid crustal domains of mylonitization through strain localization and strain softening is considered an effective mechanism for the formation of the low angle normal faults of metamorphic core complexes

early burdigalian infill of the puchkirchen trough north alpine foreland basin central paratethys facies development and sequence stratigraphy abstract to improve the prediction of gas and oil in strata along the tectonically imbricated southern margin of the puchkirchen trough north alpine foreland basin central paratethys a better understanding of facies distribution and stratigraphic control of the basin is essential the present study provides bio and chemofacies analyses and a biostratigraphic evaluation for the pelitic hall formation from the borehole hochburg 1 in the central part of the puchkirchen trough a statistical evaluation of benthic foraminiferal assemblages together with geochemical proxy records toc sulphur hydrogen index 13 c toc 15 n tn reveals a succession of early burdigalian depositional environments following a major subaqueous erosional hiatus conglomeratic sands at the base of the section which contain reworked chattian and aquitanian foraminiferal assemblages record the reactivation of a basin axial channel system during the early burdigalian eustatic sea level rise the channel was cut off from its sediment sources on the shelf and a deepening bathyal environment was established agglutinated foraminiferal assemblages with abundant bathysiphon filiformis developed that were adapted to an unstable environment with frequent deposition of turbidites the middle part of the hall formation is characterized by prograding high relief clinoforms which are fed by the wachtberg delta of the paleo inn river high sedimentation rates are interpreted and increased input of terrestrial derived organic matter is documented with a strong correlation of toc and toc s and low hi values as well as frequent occurrences of ammodiscus spp and other opportunistic agglutinating foraminifers a renewed transgression reestablished a eutrophic and suboxic bathyal environment followed by the development of an oxygenated outer middle neritic shelf environment heralding the ultimate infill of the deep water basin based on a comparison of the revealed development to existing sequence stratigraphic models for the puchkirchen trough three sequences and their corresponding systems tracts can be identified for the lower middle and upper hall formation biostratigraphic evidence from benthic foraminifers and calcareous nannoplankton indicates that they correspond to the regional substages of the middle and upper eggenburgian and lower ottnangian and to global 3rd order sequences bur 1 3 the observations suggest a primary control of eustatic sea level on the puchkirchen trough rather than alpine tectonics during the burdigalian

the benthic boundary layer and sediment transport the major problems facing scientists studying marine boundary layer fluid mechanics in 1979 concerned the specific mechanismic operating in the regions of the top and the bottom of the boundary layer at the top the mechanisms of fluid escapement from the boundary layer and mixing into the ocean interior could only be postulated approximations of diffusivity across the top of the layer were being debated the values of which were more articles of faith than physically validated estimates cf armi 1979 garrett 1979 at the bottom the intermittent generation of stress cf gordon 1975 heathershaw and simpson 1979 was held responsible for the wide variability observed in the drag coefficient and the thickness of the viscous sub layer was claimed to be greater than that traditionally found from laboratory studies caldwell and chriss 1979

the depth dependency of planktonic benthic foraminiferal ratios constraints and applications abstract study of the ratio between planktonic and benthic foraminifera in a great number of areas shows that variation of this ratio with depth can be described using organic matter flux equations essentially the share of benthic foraminifera in the total association is inversely proportional to depth as is the amount of organic matter resulting from primary productivity reaching the sea bottom variation in the p b ratio from area to area appears to be strongly dependent on the amount of inbenthic living foraminifera if the ratios are corrected for this the regressions between the proportion of planktonic foraminifera p and depth appear to be near identical in the gulf of mexico the gulf of california the west coast of the usa and the adriatic sea this regression can be described by depth e 3 58718 0 03534 p a study of the behaviour of this function in nine cores from the adriatic upper quaternary reveals that the palaeodepth reconstructions are not influenced by fluctuations in productivity and temperature but that they are significantly influenced by redeposition of fine grained deposits

sea bed noises reveal role of turbulent bursting phenomenon in sediment transport by tidal currents in many engineering and sedimentological applications it is usual to relate sediment movement to the time averaged bed shear stress however the shear stress exerted on the sea bed by tidal currents is intermittent in nature due to the turbulent bursting phenomenon1 2 sediment movement at the sea bed also occurs in bursts3 4 however attempts to link these two processes have failed because of the poor temporal resolution of existing sediment transport measurement techniques recent experimental advances now make this possible and we describe here the first detailed observations of sediment movement and fluid turbulence in tidal currents flowing over sandy gravels these results show that high stress values are a necessary but not sufficient condition for bed load sediment movement and that form drag on individual particles may have greater dynamical significance than the stress this work also suggests that it may now be possible to describe sediment movement at a level which is deterministic in terms of the turbulence and at very much shorter timescales than have been used previously

movable bed roughness in unsteady oscillatory flow a model to predict the roughness in unsteady oscillatory flows over movable noncohesive beds is presented the roughness over movable beds is shown to be a function of the boundary shear stress rather than a fixed geometrical scale as is the case for fully rough turbulent boundary shear flows over immobile beds the model partitions the roughness into two distinct contributions these two contributions are due to the form drag around individual bed forms and to the near bed sediment transport the form drag over the bed forms is treated explicitly as a function of the boundary geometry and shear stress the ripples are predicted as a function of the local skin friction and a semiempirical expression is derived using standard law of the wall arguments which gives the ripple or form roughness as a function of the boundary geometry the ripple roughness is found to be proportional to the product of the ripple steepness and height favorable comparison of the form drag model with the results of bagnold s 1946 fixed ripple study is found the value of z0 associated with intense sediment transport in oscillatory flow over a flat bed is determined from carstens et al s 1969 experiments this value is found to be 7 or 8 grain diameters an expression is derived for the roughness associated with the maximum thickness of a near bottom sediment transporting layer consistent with owen s 1964 roughness hypothesis for saltation of uniform grains in air at large values of the boundary shear stress relative to the critical value for initial sediment motion the derived expression is similar to the results of smith and mclean s 1977 unidirectional flow approach modified for oscillatory flow the total roughness model is found to compare favorably with carstens et al s 1969 data in contrast to smith and mclean s 1977 steady flow findings the results here show that when ripples are present they account for a significant portion of the boundary roughness

biological mediation of bottom boundary layer processes and sediment suspension in the lower chesapeake bay abstract in relatively low energy estuarine environments where bottom skin friction shear stresses do not appreciably exceed the critical shear stress required for sediment transport benthic biology can profoundly affect the responsiveness of the bed to benthic flows the bottom boundary layer processes that suspend and transport fine sediments at two sites in the lower chesapeake bay eastern usa were examined in an interdisciplinary study that embraced all seasons instrumented boundary layer tetrapods were used to record time varying bed stress and suspended sediment concentrations box cores surface and profile photographs and direct observations provided data on benthic biology an annular seabed flume was used to obtain in situ estimates of the threshold shear stresses at which sediments were resuspended observed bed stresses related to semi diurnal tidal currents were in the range of 0 10 to 0 14 pa at the lower energy site but often exceeded 0 20 pa at the more energetic site these stresses were often enhanced by wind stress and waves biological effects dominated bed roughness and influenced the susceptibility of bottom sediments to physical entrainment wave or tide generated bed forms rarely were observed at either site epifaunal organisms increased the relief amplitude of the sediment water interface by more than 8 cm temporal and spatial variability in the abundance and species composition of epifauna was high at both sites making the effects of these organisms on hydraulic roughness difficult to predict application of the seabed flume showed that the critical stress at which sediment was suspended varied from 0 10 to 0 14 pa this variability is attributed to seasonal and interannual variability in biological activity the lowest critical stress values coincided with the most intense bioturbation the floor of the bay stem appears to approximate a quasi equilibrium surface with normally recurrent physical bed stresses remaining near or slightly below threshold values the bed remains active because benthic biota mix the sediment column provide bed relief and mediate the responsiveness of the bed to physical processes

paleogeography of the upper rhine graben urg and the swiss molasse basin smb from eocene to pliocene twenty paleogeographic maps are presented for middle eocene lutetian to late pliocene times according to the stratigraphical data given in the companion paper by berger et al this volume following a first lacustrine continental sedimentation during the middle eocene two and locally three rupelian transgressive events were identified with the first corresponding with the early rupelian middle pechelbronn beds and the second and third with the late rupelian serie grise fischschiefer and equivalents during the early rupelian middle pechelbronn beds a connection between north sea and urg is clearly demonstrated but a general connection between north sea urg and paratethys via the alpine sea is proposed but not proved during the late rupelian whereas in the southern urg a major hiatus spans early aquitanian to pliocene times early and middle miocene marine brackish and freshwater facies occur in the northern urg and in the molasse basin omm osm however no marine connections between these basins could be demonstrated during this time after the deposition of the molasse series a very complex drainage pattern developed during the late miocene and pliocene with a clear connection to the bresse graben during the piacenzian sundgau gravels during the late miocene pliocene and quaternary sedimentation persisted in the northern urg with hardly any interruptions the present drainage pattern of the rhine river from alpine area to the lower rhine embayment was not established before the early pleistocene

shear criteria and structural symmetry abstract during the last decade it has been shown that most relevant shear criteria within ductile rocks are asymmetric structures e g pressure shadows shear bands c s structures fabrics tension gashes folds veins the correspondence between coaxial or non coaxial deformation and symmetric or asymmetric particle velocity fields respectively accounts for the use of structural symmetry as an indicator of strain history the application of this symmetry concept to various field examples emphasizes that i the degree of symmetry of a given structural pattern reflects the bulk strain regime irrespective of the size and the mechanical behaviour of the considered system and ii the strain regime can also be inferred from the order of appearance and dominance of structures which contribute to the total deformation pattern even where the progressive deformation results in a complex pattern which cannot be directly interpreted

implications of centrifuge simulations of channel flow for opening out or destruction of folds abstract centrifuge models of modelling clay and silicone microlaminates upon silicone putty layers simulate folding and subsequent opening out and unfolding of a layered sedimentary cover sequence the superstructure upon a ductile substrate e g migmatitic gneiss in the infrastructure of large hot orogens or archaean terrains after initial layer parallel shortening a process akin to channel flow in nature is induced in models by removal of material to simulate foreland erosion isostatic readjustments due to lateral difference in gravitational loading without further shortening result in extrusion and exhumation of lower ductile layers and vertical collapse opening out and unfolding of microlaminate layers of the cover sequence many active folds developed in the superstructure during initial layer parallel shortening open out and unfold during subsequent ductile flow of the infrastructure especially where they are rounded in profile and a basal decollement to the cover sequence is present angular to chevron folds tend to be translated or passively rotated and undergo less increase in interlimb angle isolated synforms or synform antiform pairs separated by broad areas of gentle undulations or plateaux in nature may be indicative that unfolding during channel flow has taken place fold axial surfaces especially for folds of finely laminated layers may also be rotated so that folds become hinterland verging experiments suggest that where channel flow has occurred in nature if unfolding were not recognised through detailed field observations the amount of regional shortening would be underestimated field criteria to recognise unfolding in nature are suggested our models predict that veins and saddle reefs developed during folding may be potentially preserved in an area of open or sub horizontal strata following unfolding which has implications in exploration for fold related mineralisation in large hot orogens or archaean terrains

shear zone related folds folds in ductile shear zones are common structures that have a variety of origins these can be pre existing folds that become modified or folds developed during the shearing event among the syn shearing folds a second subdivision is based on the relative age of the folded surface which can be pre existing or newly formed during the shearing event in each of the three categories final fold geometry and orientation show complex relationships with the kinematic frame the final fold geometry depends on the vorticity within the shear zone the rheology and the initial orientation of the folded surface relative to the kinematic framework it follows that folds are complex structures difficult to use as kinematic indicators however in shear zones where undeformed wall rocks with pre shear structures are accessible and where kinematics can be well established folds can provide a valuable natural means to understand the initiation and evolution of structures under non coaxial regimes we point to the need of discriminating among different plausible categories based on the nature of the folded surface and on the inherent structural features of each category q 2004 elsevier ltd all rights reserved