hydraulic fracture reopening pressure and the estimation of maximum horizontal stress abstract in hydrofracture stress measurements the magnitude of the maximum horizontal stress s h is commonly estimated from the borehole pressure required to reopen an induced axial crack examination of the processes which govern the borehole pressure history recorded during the reopening cycle of such tests indicates two sources of error in the estimates of s h derived using the conventional method proposed by bredehoeft et al bredehoeft jd wolff rg keys ws and shutter e 1976 colorado geol soc amer bull 87 250 8 the first arises from the failure to include a term arising from pressure penetration into the crack prior to reopening in the force balance acting across the mouth of the induced axial cracks the problem can be remedied by using a modified reopening equation which includes pressure penetration of the crack the second source of error is more problematic and concerns the correct identification of the true reopening pressure from the borehole pressure records analysis of the process of reopening aided by numerical simulations shows that the true reopening pressure is generally less than the apparent i e that detected reopening pressure the discrepancy between true and apparent reopening pressures increases with larger hydraulic compliance of the test equipment the compliance in question refers to that of the fluid volume between the flow meter and the crack mouth s simulation of a pair of 1 m high axial cracks with 2 m residual hydraulic aperture in a 100 mm borehole indicates that the system compliance must be reduced to 5 10 7 m 3 mpa to enable the true reopening pressure to be estimated to better than 10 at flow rate is less than 10 4 m 3 s this is several orders of magnitude less than applies to conventional hydrofracture systems but is attainable for tests in small holes at shallow depth by making relatively minor system modifications tests at greater depth however would seem to require downhole measurement of flow at the packers we validate our assertions with a field test in which reopening pressure was determined mechanically and hydraulically

ammonite and inoceramid bivalve extinction patterns in cretaceous tertiary boundary sections of the biscay region southwestern france northern spain cretaceous tertiary boundary sections located in the biscay region of southwestern france and northern spain are among the most complete of all known land based sequences across this boundary new collections of ammonites and inoceramid bivalves from five localities show quite distinct extinction patterns for these two groups of characteristic cretaceous molluscs from eight to ten ammonite species extend into the latest cretaceous and possibly to the boundary in contrast inoceramid bivalve diversity gradually diminished from a minimum of six species in the early maastrichtian to zero at the beginning of the late maastrichtian when they were replaced by the cryptic pteriomorph tenuipteria extinction of typical inoceramids well before the end of the maastrichtian is supported by data from other regions indicating that it was a global rather than local phenomenon

investigation of pre extinction dwarfing in cenozoic planktonic foraminifera abstract we propose a phenomenon termed pre extinction dwarfing in cenozoic planktonic foraminifera high resolution morphometric analysis shows a reduction in maximum or mean test size in the 2 20 kyr interval preceding the extinction of pliocene globorotalia exilis and globorotalia miocenica from the caribbean sea and middle eocene morozovelloides crassatus from the western north atlantic through a series of case studies and review of qualitative observations from the literature we suggest that dwarfing occurs in a number of species of planktonic foraminifera foreshadowing their demise and that such size changes are not just a phenomenon associated with mass extinction events we propose that this is a common stress induced pattern that results in diminutive size in the pre extinction interval

can the lilliput effect be detected in the brachiopod faunas of south china following the terminal ordovician mass extinction abstract in the immediate aftermath of global extinctions organisms were normally much smaller than those prior to these events this lilliput effect can be subdivided into two types 1 a specific type following the original definition of the effect which targets species level taxa associated with inhospitable environments and 2 a more general type related to the reactions of higher rank taxa above the species level the body sizes of brachiopods from south china through the ordovician and silurian transition late katian hirnantian and earliest rhuddanian are compared at generic superfamilial ordinal and class levels the results indicate that the body sizes of the taxa of lower rank e g genus level are highly variable within these different intervals the type of evidence for the lilliput effect through the end ordovician mass extinction is thus quite different from that of the end permian mass extinction probably reflecting differences in the intensity of these two major bioevents however the relationships between the contrasting trends in body size change of some taxa of higher rank e g at the ordinal level and the relative dominance of these taxa in the latest ordovician and earliest silurian suggest that the brachiopods of the two major ordovician groups the strophomenoids and orthoids adopted different survival strategies during and immediately after the crisis from those of the pentamerides and rhynchonellides that were common in silurian assemblages

delineating fracture zones using surface tunnel surface seismic data p s and s p mode conversions a surface tunnel surface seismic experiment was conducted at the aspo hard rock laboratoryto study the seismic response of major fracture systems intersecting the tunnel a newly developedthree com

experimental tumbling of echinoderms taphonomic patterns and implications abstract despite a wide array of published actualistic studies on echinoderm taphonomy the detailed pattern of decay and disarticulation of their skeletons is still not well understood here we provide results of tumbling experiments using a rotating barrel filled with artificial seawater and medium sized quartz sand to mimic physical forces experienced by echinoderms during transportation in high energy conditions in particular we determined semi quantitatively transportation induced rates and patterns of damage and disintegration of freshly killed ophiuroid asteroid and crinoid skeletons that were not allowed to decay initially our experiments showed that echinoderm specimens disintegrated in a characteristic sequence toward an increase of the degree of disarticulation abrasion and roundness or thinness of echinoderm ossicles the sequence of disintegration in crinoids began with the partial disintegration of distal arms after 2 h a time equivalent to 1 km of transport the initial split of ophiuroid and asteroid arms and crinoid cirri occurred after 24 h 12 km and complete destruction of the asteroid mouth and ophiuroid disk area occurred after 72 h 36 km the duration of transport necessary to promote initial fragmentation in asteroid and ophiuroid arms and crinoid cirri into isolated ossicles was 120 h 60 km the complete disarticulation of crinoid ophiuroid and asteroid arms and crinoid cirri occurred after 312 h 156 km and 408 h 204 km respectively although it has been argued that the quality of preservation can be a poor index of post mortem transport echinoderms allowed limited initial decay in the presence of rapid and relatively constant physical disturbance an approximation of the distance of transport can be made our data demonstrate that articulated ossicles can remain for several days sufficient time for long even a few hundred km transportation this finding illustrates that articulated echinoderm remains do not necessarily imply low energy and highlights the importance of a reliable discrimination of autochthonous and allochthonous components of fossil echinoderm assemblages application of in particular isolated fossil echinoderm ossicles in e g paleoenvironmental and paleoecological reconstructions may lead to serious misinterpretations and should be supplemented by observations of abrasion traces

measurement related uncertainties in overcoring data at the aspo hrl sweden part 2 biaxial tests of csiro hi overcore samples this paper is the second dealing with measurement related uncertainties of overcoring data undertaken at the aspo hard rock laboratory and focuses on the biaxial test data from csiro hi overcore sa

evaluation of measurement related uncertainties in the analysis of overcoring rock stress data from aspo hrl sweden a case study a re evaluation of overcoring results from the borre probe and the csiro hi cell was carried out at the aspo hrl the re analysis revealed a number of measurement related uncertainties for the two cells e g bonding temperature effects and identification of elastic parameters these uncertainties were corrected and the re analyzed strains were then used to determine the in situ stress field using standard least squares technique the results indicate lower stress magnitudes compared to the original interpretations and the results are in good agreement with existing hydraulic fracturing data as well as with theoretical vertical stress overall the re analysis indicates that the stress field at aspo hrl is well constrained and relatively consistent with depth

mid cretaceous albian santonian sea surface temperature record of the tropical atlantic ocean paleoclimate records of geologic time periods characterized by extreme global warmth such as the mid cretaceous are important for a better understanding of the earth9s climate system operating in an exceptionally warm mode here we applied an organic geochemical proxy tex 86 on organic matter rich albian santonian sediments recovered from ocean drilling program leg 207 sites 1258 and 1259 on demerara rise to reconstruct sea surface temperatures ssts in the western equatorial atlantic preceded by a stepwise cenomanian warming trend 31 35 c the onset of the cretaceous thermal maximum coincided here with the cenomanian turonian boundary event once established this extreme warm climate regime characterized by averaged tropical ssts close to 35 c lasted up to the turonian coniacian transition two pronounced cooler intervals 2 3 c interrupt this otherwise remarkably stable record providing the first 18 o independent evidence for middle turonian cooling that previously has been attributed to glacioeustatic sea level lowering coniacian ssts decline stepwise reaching a minimum in the santonian 32 33 c where cooling is most pronounced presumably concomitant with the first progressive opening of a deep water passage through the equatorial atlantic gateway

dwarfism of irregular echinoids echinocorys from poland during the campanian maastrichtian boundary event abstract the echinoid genus echinocorys is one of the dominant irregular echinoid in the cretaceous of poland our analysis of the body size trends of these echinoids shows that they underwent dwarfism in the early maastrichtian the reduction in body size in these echinoids was likely an ecological response to the major climate perturbations connected with the campanian maastrichtian boundary event cmbe that was characterized by a long lasting negative carbon isotope excursion and cooling this is consistent with the fact that food limitations and cold temperatures are among the major factors decreasing growth rates in modern echinoids so far in the case of echinoids the so called lilliput effect has been only recognized in the early danian early paleogene heart urchins atelostomata thus our study shows that the reduction in the body size in echinoids was a more common biotic response to environmental stress than previously thought

recovery from the end permian extinction event evidence from lilliput listracanthus abstract we report the reduction of dermal denticle size in the chondrichthyan listracanthus in the context of other in faunal changes observed in a lower triassic section near wapiti lake british columbia listracanthus is the first chondrichthyan to show this lilliput effect following the great end permian mass extinction in our attempt to collate diverse information from associated faunal assemblages and large macrofossil collections we interpret the change in denticle size in listracanthus as occurring at around the induan olenekian boundary the relatively small change in denticle size can be interpreted in terms of the relative lack of effect of the biotic crisis on this chondrichthyan and maybe others or alternatively as the relatively much larger body size of listracanthus in comparison with other permo triassic boundary lilliput taxa it is speculated the lack of denticle enameloid and well developed vascularisation along with the moderate minimum in size below the induan olenekian boundary may indicate that listracanthus denticles are not necessarily external skeletal elements but possibly modified dermal denticles gill rakers of a large basking shark it is concluded that discovery of the early triassic species listracanthus pectenatus creates a ghost lineage of 30 ma and highlights the significance of discovering perishable remains of chondrichthyans in order to understand faunal changes across the permo triassic boundary

the evolutionary history of size variation of planktic foraminiferal assemblages in the cenozoic abstract the iterative evolutionary radiation of planktic foraminifers is a well documented macroevolutionary process here we document the accompanying size changes in entire planktic foraminiferal assemblages for the past 70 my and their relationship to paleoenvironmental changes after the size decrease at the cretaceous paleogene k p boundary high latitude assemblages remained consistently small size evolution in low latitudes can be divided into three major phases the first is characterized by dwarfs 65 42 ma the second shows moderate size fluctuations 42 14 ma and in the third phase planktic foraminifers have grown to the unprecedented sizes observed today our analyses of size variability with paleoproxy records indicate that periods of size increase coincided with phases of global cooling eocene and neogene these periods were characterized by enhanced latitudinal and vertical temperature gradients in the oceans and high diversity polytaxy in the paleocene and during the oligocene the observed minor size changes of the largely low diversity oligotaxic assemblages seem to correlate with productivity changes however polytaxy per se was not responsible for larger test sizes

new developments in the integrated stress determination method and their application to rock stress data at the aspo hrl sweden the integrated stress determination method isdm is a powerful tool for estimating the regional stress tensor from in situ measurements of local stress tensors using a wide variety of stress measu

discussion on lazarus taxa and fossil abundance at times of biotic crisis journal vol 156 1999 pp 453 456 r j twitchett writes palaeontologists grudgingly accept that the fossil record of most taxa is very patchy and incomplete a particular taxon will appear in the fossil record and then disappear for any number of millions of years only to reappear again apparently unchanged in younger strata this disappearance and reappearance is termed the lazarus effect thus in any particular interval of time a lazarus taxon is one that is not present in the fossil record but which we know must have existed by virtue of the presence of older and younger specimens wignall benton 1999 are concerned with the importance of the lazarus effect during extinction recovery intervals traditional explanations for this phenomenon are i a reduction in the quality of the fossil record and or ii a migration of taxa into as yet undiscovered refugia wignall benton make the novel suggestion that the lazarus effect is instead due to periods of low abundance implicit in this argument is the hypothesis that population size has to exceed a critical threshold before a taxon stands any chance of becoming fossilized below this threshold the population remains viable but we simply have no record of its existence they make the reasonable assumptions that this threshold will vary between environments presumably also between taxa and is essentially impossible to quantify while i agree that this is a credible explanation for the lazarus effect i do not consider that the tests constructed by wignall benton are sufficiently rigorous to disprove the traditional alternatives testing the quality of the fossil record for the permian triassic event the authors use the number of fossiliferous shallow marine formations from each stage as a measure of the quality of the fossil record as there is no change in the number of formations from the changxingian to the griesbachian they infer that the quality of

lilliput effect in late maastrichtian planktic foraminifera response to environmental stress the lilliput effect marks morphologic and intraspecies size reductions in response to environmental stresses commonly associated with the aftermath of mass extinctions this study shows that the lilliput effect is a universal biotic response associated with greenhouse warming mesotrophic or restricted basins shallow marginal settings and volcanically active regions during the late maastrichtian sedimentary sequences analyzed from tunisia egypt texas argentina the south atlantic and indian ocean reveal that the biotic stress response appears uniform regardless of the cause varying only with the degree of biotic stress overall late maastrichtian environments span a continuum from optimum conditions to the catastrophic mass extinctions with a predictable set of biotic responses relative to the degree of stress induced by oxygen salinity temperature and nutrient variations as a result of climate and sea level changes and volcanism early stages of biotic stress result in diversity reduction and the elimination of large specialized species k strategists leading to morphologic size reduction via selective extinction disappearances and intraspecies dwarfing of survivors later stages of biotic stress result in the complete disappearance of k strategists intraspecies dwarfing of r strategists and dominance by low oxygen tolerant small heterohelicids at the extreme end of the biotic response are volcanically influenced environments which cause the same detrimental biotic effects as observed in the aftermath of the k t mass extinction including the disappearance of most species and blooms of the disaster opportunist guembelitria

lilliput effect in freshwater ostracods during the permian triassic extinction abstract the lilliput effect following the permian triassic mass extinction and its aftermath has been documented in a variety of marine animal groups but it is less known in terrestrial and freshwater invertebrates here we present new investigations of the size variations of terrestrial ostracods of the genus darwinula based on fossil records from a permian triassic section on the northern limb of the dalongkou anticline section in northwest china quantitative analyses reveal that ostracod test sizes decreased sharply through the terrestrial permian triassic mass extinction interval the lilliput effect in terrestrial ostracods is characterized by the extinction of large taxa and the rise of small sized and elongate new forms coupled with the dramatic loss of conchostracans charophytes and the blooming of lycopod spores the size decrease in terrestrial ostracods following the biotic crisis through the permian triassic interval was probably triggered by several interacting events including global warming anoxia and enhanced sediment input following acid rain and wildfire

uncertainty in the maximum principal stress estimated from hydraulic fracturing measurements due to the presence of the induced fracture the classical theory for hydraulic fracturing stress measurements assumes an ideal case with a linear elastic homogenous and isotropic medium and a fracture that reopens distinctly when the minimum tangential borehole stress is exceeded the induced fracture disturbs this ideal picture in several aspects which are important for the evaluation of the maximum horizontal principal stress using the fracture reopening pressure this disturbance can be attributed to the fracture normal stiffness and the initial hydraulic fracture permeability in this paper the hydraulic fracturing reopening test is studied by coupled hydromechanical modeling that takes into account an induced fracture that is incompletely closed the result shows that with realistic equipment compliance the apparent fracture reopening evaluated from the well pressure is close to the magnitude of the minimum horizontal principal stress with little or no correlation to the maximum horizontal principal stress this observation suggests that the determination of maximum principal stress by hydraulic fracturing using the reopening pressure is very uncertain

bioturbation inoceramid extinction and mid maastrichtian ecological change notable among the biological changes of the middle part of the maastrichtian age was a major pulse of extinction among inoceramid bivalves analysis of the distribution of inoceramid shell fragments suggests that the population of burrowing organisms increased across the same stratigraphic interval that the bivalve population decreased a global reorganization of ocean circulation that resulted in cooler better oxygenated bottom waters is proposed as an explanation for the observed changes in the deep sea other observations regarding the mid maastrichtian are consistent with this hypothesis

late cretaceous campanian carbon isotope events sea level change and correlation of the tethyan and boreal realms abstract carbon stable isotope stratigraphy provides unique insights into environmental change during the campanian the longest 83 5 71 3 ma but least well understood stage of the late cretaceous a new carbon isotope 13c profile for a 500 m thick campanian basal maastrichtian tethyan pelagic hemipelagic section near el kef northern tunisia is calibrated using data from a biostratigraphically well constrained succession at kalaat senan the general shapes of the tunisian 13c reference curve and published tethyan 13c profiles from elles tunisia and bidart sw france and a boreal curve for the trunch borehole eastern england are remarkably similar in all three areas a positive carbon isotope event of 0 2 13c in the mid campanian dated at 78 7 ma and a negative excursion of 0 4 in the upper campanian at 74 8 ma can be correlated between tunisia and england a positive excursion of 0 3 at 83 7 ma spans the santonian campanian boundary these isotope events enable precise inter regional correlations that are consistent with published nannofossil data review of campanian sea level data from north africa the middle east and northern europe indicates that major shifts in 13c profiles coincide with changes in eustatic sea level relatively stable 13c values in the lower campanian and their long term fall through the upper campanian reflect high and then falling eustatic sea levels and increased carbonate production short term 600 kyr positive excursions record greater organic productivity and or organic matter preservation and decreased carbonate fluxes during periods of rapid sea level rise and the drowning of carbonate platforms excursions were terminated by falling nutrient supply and increased carbonate deposition associated with epicontinental sea expansion and renewed carbonate platform growth during the late transgression and highstand negative excursions are linked principally to reworking of marine and terrestrial organic matter during rapid sea level fall carbon isotope stratigraphy is a powerful tool for correlation which can be used to test the validity of campanian global biostratigraphic frameworks and improve our understanding of the nature and timing of late cretaceous sea level change

deterministic chaos and historical geomorphology a review and look forward the study of nonlinear dynamics in geomorphology has been dominated by analytical techniques derived largely from other fields ultimately however confronting nonlinear complexity requires problematizing from within a geomorphological context this paper focuses on deterministic chaos the major classes of techniques for detecting chaos in geomorphic systems are reviewed while the long time series most amenable to traditional methods of chaos detection are only occasionally available to geomorphologists there are methods for detecting chaos in spatial data including qualitative and categorical data many of these are similar to or are adaptations of familiar techniques for partitioning within and between unit variation and for identifying elementary uniform landscape units the manifestations of chaos in landforms and landscapes take the form of divergent vs convergent evolution increasing vs decreasing irregularity disproportionality vs proportionality of response to perturbations or initial variations and the lack of geographical consistency or commonality of response these also serve as criteria for distinguishing between achievement of a new steady state and unstable divergence in interpreting geomorphic changes study of nonlinear dynamics in earth science need not be dominated by theoreticians and modellers as complex nonlinear behaviors can be identified and diagnosed using the types of data observations and methods typically available to geomorphologists even more important is the need to further inform the study of nonlinear dynamics on the basis of field based geomorphology