breccia dikes and crater related faults in impact craters on mars erosion and exposure on the floor of a crater 75 km in diameter at the dichotomy boundary environmental conditions on mars are conducive to the modification and erosion of impact craters potentially revealing the nature of their substructure on earth postimpact erosion of complex craters in a wide range of target rocks has revealed the nature and distribution of crater related fault structures and a complex array of breccia and pseudotachylyte dikes which range up to tens of meters in width and tens of kilometers in length we review the characteristics of fault structures breccia dikes and pseudotachylyte dikes on earth showing that they occur in complex network like patterns and are often offset along late stage crater related faults individual faults and dikes can undulate in width and can branch and bifurcate along strike detailed geological analyses of terrestrial craters show that faults and breccia dikes form during each of the major stages of the impact cratering process compression excavation and modification we report here on the discovery of prominent lattice like ridge networks occurring on the floor of a highly modified impact crater 75 km in diameter near the dichotomy boundary of the northern lowland and southern upland interior fill and crater floor units have been exhumed by fluvial and eolian processes to reveal a unit below the crater floor containing a distinctive set of linear ridges of broadly similar width and forming a lattice like pattern ridge exposures range from 1 4 km in length and 65 120 m in width are broadly parallel straight to slightly curving and are cross cut by near orthogonal ridges forming a box or lattice like pattern ridges are exposed on the exhumed crater floor extending from the base of the wall toward the center on the basis of the strong similarities of these features to terrestrial crater related fault structures and breccia dikes we interpret these ridges to be faults and breccia dikes formed below the floor of the crater during the excavation and modification stages of the impact event and subsequently exhumed by erosion the recognition of such features on mars will help in documenting the nature of impact cratering processes and aid in assessment of crustal structure faults and breccia dikes can also be used as data for the assessment of post cratering depths and degrees of landform exhumation

composition morphology and stratigraphy of noachian crust around the isidis basin definitive exposures of pristine ancient crust on mars are rare and the finding that much of the ancient noachian terrain on mars exhibits evidence of phyllosilicate alteration adds further complexity we have analyzed high resolution data from the mars reconnaissance orbiter in the well exposed noachian crust surrounding the isidis basin we focus on data from the compact reconnaissance imaging spectrometer for mars as well as imaging data sets from high resolution imagine science experiment and context imager these data show the lowermost unit of noachian crust in this region is a complex brecciated unit of diverse compositions breccia blocks consisting of unaltered mafic rocks together with rocks showing signatures of fe mg phyllosilicates are commonly observed in regions of good exposure layered or banded phyllosilicate bearing breccia rocks are observed suggestive of pre isidis sedimentary deposits in places the phyllosilicate bearing material appears as a matrix surrounding mafic blocks and the mafic rocks show evidence of complex folded relationships possibly formed in the turbulent flow during emplacement of basin scale ejecta these materials likely include both pre isidis basement rocks as well as the brecciated products of the isidis basin forming event at 3 9 ga a banded olivine unit capped by a mafic unit covers a large topographic and geographic range from northwest of nili fossae to the southern edge of the isidis basin this olivine mafic cap combination superimposes the phyllosilicate bearing basement rocks and distinctly conforms to the underlying basement topography this may be due to draping of the topography by a fluid or tectonic deformation of a previously flatter lying morphology we interpret the draping superposed olivine mafic cap combination to be impact melt from the isidis basin forming event while some distinct post isidis alteration is evident carbonate kaolinite and serpentine the persistence of olivine from the time of isidis basin suggests that large scale aqueous alteration processes had ceased by the time this unit was emplaced

crustal history of north central arabia terra mars detailed geological studies across the highland lowland boundary in north central arabia terra mars provide constraints on the age and nature of processes responsible for a complex array of structures and landforms the area studied is bounded by 27 5 and 47 5 n 330 and 335 w highland basement was formed in early noachian and was extensively resurfaced in late noachian materials underlying highland plateaus were emplaced in middle noachian erosion responsible for fretted terrane and fretted channels most likely occurred in early hesperian but definitely prior to the middle of the hesperian and during or after middle noachian there is good evidence for the involvement of liquid water in fretted channel formation within the area mapped the presence of fretted terrane correlates with evidence for extensive development of large grabens all lowland materials were emplaced after the erosion that formed fretted terrane and channels the earliest of these materials were emplaced in mid hesperian and the youngest were emplaced in late amazonian morphological evidence supports debris flow or rock glacier processes for the formation of the youngest materials debris flow materials on the floors of fretted channels are much too young to be related to channel formation the entire region exhibits a northerly tilt of 0 1 0 15 that formed after deposition of the oldest lowland materials but before emplacement of upper hesperian and younger debris flow rock glacier materials the nature and ages of structures and landforms in this region are not consistent with the presence of a late noachian to early hesperian convergent plate boundary in northern arabia terra

modification of the dichotomy boundary on mars by amazonian mid latitude regional glaciation 1 restoration of the dichotomy boundary to its original position to assess its origin requires a thorough knowledge of processes responsible for its degradation and retreat the unique fretted terrain located along the deuteronilus protonilus mensae northern mid latitude portion of the boundary has been long held to provide clues to dichotomy degradation processes we use new spacecraft data to show that fretted valleys display a multitude of characteristics typical of integrated valley glacial systems on earth multiple theater headed alcove like accumulation areas sharp arete like ridges typical of glacial erosion converging patterns of downslope valley flow valley lineation patterns typical of folding and shear wrap around features indicative of flow around obstacles and broad piedmont like lobes as the valley fill extends out into the northern lowlands the single integrated system containing these features covers about 30 000 km2 and is one of dozens of fretted valleys along the dichotomy boundary in this region these relationships suggest that the boundary area was subjected to very large scale regional glaciation during the amazonian recognition and documentation of this important late stage process provides information critical to reconstructing the original dichotomy boundary

ice and debris in the fretted terrain mars viking moderate and high resolution images along the northern highland margin were studied monoscopically and stereoscopically to contribute to an understanding of the development of fretted terrain results suport the hypothesis that the fretting process involved flow facilitated by interstitial ice the process apparently continued for a long period of time and debris apron formation shaped the fretted terrain in the past as well as the present interstitial ice in debris aprons is most likely derived from ground ice obtained by sapping or scarp collapse debris aprons could have been removed by sublimation if they consisted mostly of ice or by deflation if they consisted mostly of debris to remove the debris wind erosion was either very intense early in martian history or was intermittent perhaps owing to climatic cycles the location of debris aprons in two midlatitudinal belts can be attributed to atmospheric conditions that permit ice to remain in the ground at moderate depth in those latitudes and climatic factors that permit temperatures to be warm enough for ice to flow channels have minimum gradients of 5 8 m km which permits flowing ice to deform by dislocation creep at exceedingly low strain rates alternately ice charged debris may have been buoyed by brines by analogy with the fretted channels ice could have flowed through the outflow channels if the atmospheric pressure was somewhat higher in the past and permitted ice to exist in the equatorial area

the geological history of northeast syrtis major mars abstract as inferred from orbital spectroscopic data northeast syrtis major bears considerable mineral diversity that spans the noachian hesperian boundary despite its small geographic area in this study we use observations from the high resolution imaging science experiment supplemented with context camera imagery to characterize and map the lateral extent of geomorphic units in northeast syrtis major and constrain the geomorphic context of the orbital identified mineral signatures using recent observations we confirm previous mineralogy identified with the compact reconnaissance imaging spectrometer for mars and greatly extend the lateral extent of visible to near infrared investigation utilizing the greater coverage analysis of thermal emission imaging system observations reveals further physical properties and distribution of the geomorphic units the stratigraphy which spans the noachian hesperian boundary displays significant morphological heterogeneity at the decameter scale but it is unifiable under five distinct geomorphic units our paired morphological and mineralogical analysis allows us to construct a detailed geological history of northeast syrtis major several geological events that occurred in northeast syrtis major including the formation of the post isidis crust the emplacement of an olivine rich unit the formation of sulfate minerals and the emplacement of the syrtis major volcanics can be related to regional and global processes constraining the local chronology other mineralogical indicators particularly the formation of al phyllosilicates are difficult to place in the temporal sequence they are observed in isolated patches on the post isidis crust not as a distinct stratigraphic unit as observed elsewhere in nili fossae suggesting their formation via isolated leaching or through alteration of initial compositional heterogeneities within the crust exposures of an olivine rich unit are intermittently observed to form quasi circular landforms suggestive of emplacement in circular depressions which may indicate a period of cratering between the formation of the isidis basin and the deposition of the olivine rich unit we identify and discuss intriguing large linear features of the olivine rich unit reminiscent of dyke fed volcanism that have raised bounding ridges suggestive of contact metamorphism with the crust we compile review and discuss many of the outstanding questions and running hypotheses relevant to our mapping area a synthesis of our geomorphic mapping with recent literature reveals a well defined geological history with extensive aqueous activity at northeast syrtis major that is amassed in a stratigraphic sequence spanning a time likely greater than 250 million years of geological history our geomorphic and spectral analyses confirm that northeast syrtis major exhibits considerable geomorphic and mineralogic diversity within a relatively small geographic area that is representative of the geologic processes occurring throughout the broader nili fossae region during the noachian and hesperian northeast syrtis major adds to this sequence by exposing the diverse environmental history of this region as observed through the presence of alteration minerals not present in this fidelity or proximity elsewhere in nili fossae

acoustic properties of neogene carbonates and siliciclastics from the subsurface of the florida keys implications for seismic reflectivity abstract results of measurements of compressional wave velocity and shear wave velocity have been compared with other rock properties xrd and thin section analyses for over fifty minicores from two cores drilled through sedimentary rocks on stock island and long key florida keys the comparisons indicate that sonic velocity in siliciclastics and carbonates is mainly controlled by three factors 1 porosity and primary pore type 2 quartz content and 3 dolomite content in the case of two pliocene prograding formations the combination of depositional fabric and diagenetic alterations produced patterns and values of almost identical impedance for fine grained carbonates and siliciclastics consequently seismic sections can not be used to distinguish between the prograding quartz sands and the prograding fine grained carbonates compressional and shear wave velocities in the limestones are strongly controlled by total porosity and the primary pore type generally high porosity rocks have lower velocities than low porosity rocks but pore types can significantly influence velocities overall an increasing percentage of dolomite in carbonate rocks results in an increase in velocity some dolomitic fabrics deviate from this trend fabric preserving mimetic dolomites yield the highest velocities whereas samples that are of purely sucrosic dolomites produce lower velocities which however are still higher in average than non or only partly dolomitized samples quartz sandstones and quartz bearing limestones have generally low and uniform velocities the low velocities of the sandstones containing 80 quartz is a result of their primary interparticle porosity due to the low diagenetic potential of quartz grains a high quartz content inhibits major diagenetic alterations which would result in cementation and higher velocities synthetic seismograms and nearby seismic sections reveal two major seismic facies as a result of the observed velocity distribution 1 carbonates of the stock island formation and siliciclastics of the long key formation are both characterized by low seismic reflectivity or even seismic transparency this seismic facies is caused by a very similar uniform vertical distribution of low velocities with almost identical impedance values in the two formations thus the transition or an interfingering between the time equivalent siliciclastic long key formation and the carbonate stock island formation might not be detectable on a seismic section 2 the over and underlying shallow marine carbonates in contrast have higher velocities with a wider range of velocities resulting in a succession of high amplitude reflections

geodynamic control on carbonate diagenesis petrographic and isotopic investigation of the upper jurassic formations of the paris basin france abstract the oxfordian and kimmeridgian carbonates of the eastern edge of the paris basin display poor reservoir properties stable isotope investigation of these non recrystallized cements through both microdrilling sampling and in situ sims secondary ion mass spectrometry analysis shows that they originated from slightly buffered meteoric fluids 3 smow to 8 smow flowing through the malm carbonate aquifers of the eastern edge of the paris basin after a review of the post upper jurassic history of the paris basin it appears that meteoric water supplies leading to such cementations were associated with the evolution of the basin s geodynamic setting and notably with the lcu late cimmerian unconformity major event which corresponds to the uplift of the london brabant massif denudation of the oxfordian and kimmeridgian carbonates on the southern flank of the london brabant massif to the north of the study area then lead to lateral recharging of the associated aquifers with meteoric fluids this scenario is similar to the one proposed to explain cementations of the jurassic carbonates in the wessex weald basins which emphasizes the main role played by the cretaceous unconformities over carbonate diagenesis at the scale of the northwestern tethys margin

controls on sonic velocity in carbonates compressional and shear wave velocities v p and v s of 210 minicores of carbonates from different areas and ages were measured under variable confining and pore fluid pressures the lithologies of the samples range from unconsolidated carbonate mud to completely lithified limestones the velocity measurements enable us to relate velocity variations in carbonates to factors such as mineralogy porosity pore types and density and to quantify the velocity effects of compaction and other diagenetic alterations

velocity distribution and bed roughness in high gradient streams a model for the velocity field over a poorly sorted bed is derived herein and applied to flow in steep streams with coarse gravel beds in such streams large clasts often are comparable in size to stream depth and act as obstacles to the flow the effect of these obstacles on a flow is estimated by partitioning total stress proportional to the depth slope product into a purely fluid component and a form drag component associated with flow around the obstacles an eddy viscosity closure is used with a length scale dependent on both distance from the boundary and wake dimension resulting velocity profiles agree well with profiles measured by marchand et al 1984 in nine mountain streams in colorado model runs for a range of conditions reveal that the velocity distribution depends primarily on d84z and flow depth h d84z is the length of the vertically oriented axis of clasts at the 84th percentile of the grain size distribution calculated values of mean velocity normalized by shear velocity are well represented by a simple log linear relationship in terms of relative roughness d84 h

a reach averaged model of diurnal discharge wave propagation down the colorado river through the grand canyon as part of the glen canyon environmental studies we have developed a discharge model that routes daily discharge waves released from glen canyon dam to diamond creek 386 km downstream owing to the length of the diurnal discharge wave and the sparseness of the available topographic data the latter were averaged over the entire length of the system terms too small to be significant in the momentum equation were identified by scaling arguments based on data from past dam releases and on channel hydraulic geometry channel friction results primarily from form drag on large topographic elements and from variations in cross sectional area and flow depth rather than bed roughness producing a stage dependent friction that is not well represented by a constant value of standard channel roughness parameters such as manning s n channel friction as a function of stage was determined from field data available at high discharge 792 m3 s and intermediate discharge 425 m3 s and by using simple kinematic wave theory together with wave speed measurements to determine channel friction at low discharge about 142 m3 s model predictions of wave speed and shape agree well with data from five streamflow gaging stations and 42 stage gaging stations located along this segment of the colorado river

assessing the mineralogy of the watershed and fan deposits of the jezero crater paleolake system mars we present results from geomorphic mapping and visible to near infrared spectral analyses of the jezero crater paleolake basin and its associated watershed the goal of this study is to understand the provenance of the sedimentary deposits within this open basin lake using a source to sink approach two fan deposits located within the basin have distinct visible to near infrared mineralogic signatures measured by the compact reconnaissance imaging spectrometer for mars crism the northern fan is spectrally characterized by a mixture of mg rich carbonate and olivine while the western fan is characterized by fe mg smectite e g saponite or nontronite with variable amounts of mg rich carbonate and olivine in isolated exposures the watersheds of these deposits contain a variety of geomorphic units that are likely to have supplied sediment to the jezero crater paleolake as the fluvial valleys that fed the basin incise these units the geomorphic units include exposures of fe mg smectite olivine and mg rich carbonate bearing terrain we show that the difference in fan deposit mineralogy is a function of the areal exposure of the major geomorphic units within their watersheds this indicates that the spectrally dominant aqueous alteration minerals in the fan deposits are primarily detrital or transported in nature and did not form in situ we conclude that the aqueous alteration of the units in the watershed occurred prior to the fluvial activity that carved the valleys of the jezero crater paleolake system and that the two periods of aqueous activity are not genetically related

acoustic behavior of sedimentary rocks geologic properties versus poisson s ratios velocity transforms are essential for the extraction of geologic information from seismic reflection data sets and or predicting porosity from wireline logging data most commonly used transforms like the wyllie time average equation the raymer modified time average equation and the gardner equation are all based on empirical relationships extracted from relatively small clean mono and bimineral and mostly siliciclastic data sets and generally fail to explain the acoustic relationships within multimineral data sets to extract globally significant relationships between geologic and acoustic parameters more than 2800 plug measurements were collected more than two thirds of the data were our own measurements the others were from the literature this data set covers nearly all sedimentary environments and mixtures of those as well as the entire range in petrophysical properties

origin and evolution of a cold based tropical mountain glacier on mars the pavonis mons fan shaped deposit 1 each of the large tharsis montes volcanoes in the equatorial region of mars has an unusual amazonian aged fan shaped deposit on its west northwestern flank on the basis of viking orbiter data the origin of these deposits has been variously ascribed to volcanic mass wasting tectonic and glacial processes using new mgs and odyssey data combined with recent developments in the study of cold based glaciers we reassess the geology and mode of origin for these deposits with particular emphasis on pavonis mons the deposits share three characteristic facies including 1 a ridged facies consisting of tens to 100 parallel concentric ridges around the margins of the deposits 2 a knobby facies composed of irregular hills and hummocks and 3 a smooth facies of broad lobate plains that superposes all other units within the deposits on the basis of morphology topography superposition relationships and close terrestrial analogs in the dry valleys of antarctica we interpret the pavonis fan shaped deposit as the depositional remains of a cold based glacier that formed on the northwestern flank in recent martian history we interpret the ridged facies as drop moraines formed around the margins of a retreating cold based glacier the knobby facies as a sublimation till derived from in situ down wasting of cold based glacial ice and the smooth facies as extant debris covered glacial ice in addition to the three main facies the fan shaped deposit at pavonis mons contains several unique features including arcuate scarps high relief flow like features and radial ridges which suggest that volcanism played a role during its formation using recent results from mars general circulation model simulations we outline a model of glacier formation involving atmospheric deposition of water ice on the northwestern flanks of the tharsis montes during periods of high mean obliquity reconstructed ice sheet profiles for each of the tharsis montes glaciers suggest that the ice sheets attained average thicknesses of 1 6 2 4 km values that are consistent with a cold based glacial origin analysis of crater size frequency distributions using new data indicates that the age of the glaciation lies within the late amazonian 10 200 ma thus our results suggest that multiple phases of tropical mountain glaciation occurred on mars within the past few hundred myr and that significant amounts of near surface equatorial ice may remain within the deposit today

geology mineralization and emplacement of the whistle parkin offset dike sudbury the whistle parkin dike is a 12 km long radial offset dike located in the northeast sector of the 1 85 ga sudbury impact structure the dike is connected to the sudbury igneous complex via a 0 5 km long 250 m wide embayment the whistle segment of the dike narrows to a width of approximately 30 m and extends for about 1 5 km from the complex where it is cut by the post creek fault zone this fault displaces the dike 2 km to the northwest beyond which it continues as the parkin portion of the dike the parkin segment extends a further 10 km north northeast the dike and genetically associated embayment are comprised of numerous rock types these include the sublayer radial breccia mafic sulfide bearing breccia inclusion rich quartz diorite and inclusion poor quartz diorite economic ni cu pge mineralization is found in association with the sublayer mafic sulfide bearing breccia and inclusion bearing quartz diorite phases rare earth element ree data reveal that the dike lithologies are representative of bulk sudbury igneous complex melt we propose a multistage emplacement mechanism for the dike the breccia units including the inclusion bearing quartz diorite phase were forcefully injected laterally into an impact generated radial crack during the excavation stage of crater formation subsequent early modification processes facilitated the intrusion of inclusion poor quartz diorite particularly along the dike margins inclusion poor quartz diorite emplacement may have been gravitationally driven from the overlying impact melt sheet later stage modification i e final transient cavity collapse caused decoupling of the whistle and parkin dike segments via faulting this occurred concurrently with the settling of the sublayer unit and associated economic sulfides into the embayment and main dike segments

burial dissolution of micrite in middle east carbonate reservoirs jurassic cretaceous keys for recognition and timing abstract this paper discusses burial diagenesis and especially a burial dissolution phase and its effects on micrite microtexture in reservoirs in the middle east three microporous reservoirs were selected the cenomanian mishrif formation in iraq the kimmeridgian tithonian arab d and the barremian kharaib formations both in abu dhabi staining cathodoluminescence and scanning electron microscopy were used for petrographic observations in the selected reservoirs three typical micrite microtextures are distinguished micro rhombic compact anhedral and rounded the chronology of diagenesis based on petrographic observations shows that mineralogical stabilisation through high magnesian calcite replacement by low magnesian calcite led to the formation of micro rhombic micrites non reservoir facies such as seals present compact anhedral micrites correlation between these micrites and high aluminium content suggests alumino silicate minerals clays as a catalyst for pressure dissolution and thus for reservoir degradation through cementation micrites from the upper part of oil reservoirs are dissolved during burial diagenesis dissolution is responsible for rounded shape and for reduction of crystal size it also implies pore diameter and pore throat size enlargement comparison of porosity and permeability with scanning electron microscope data confirms that rounded micrites are better reservoirs than micro rhombic micrites

mars global surveyor observations of martian fretted terrain the martian fretted terrain between latitudes 30 and 50 n and between 315 and 360 w has been reexamined in light of new mars orbiter camera moc and mars orbiter laser altimeter mola data from mars global surveyor much of the terrain in the 30 50 latitude belt in both hemispheres has a characteristic stippled or pitted texture at moc 1 5 m scale the texture appears to result from partial removal of a formerly smooth thin deposit as a result of sublimation and deflation a complex history of deposition and exhumation is indicated by remnants of a former thicker cover of layered deposits in some hollows and on some slopes particularly those facing the pole are smooth textured deposits outlined by an outward facing escarpment throughout the study area are numerous escarpments with debris flows at their base the escarpments typically have slopes in the 20 30 range at the base of the escarpment is commonly a deposit with striae oriented at right angles to the escarpment outside this deposit is the main debris apron with a surface that typically slopes 2 3 and complex surface textures suggestive of compression sublimation and deflation the presence of undeformed impact craters indicates that the debris flows are no longer forming fretted valleys contain lineated fill and are poorly graded they likely form from fluvial valleys that were initially like those elsewhere on the planet but were subsequently widened and filled by the same mass wasting processes that formed the debris aprons slope reversals indicate that downvalley flow of the lineated fill is minor the ubiquitous presence of breaks in slope formed by mass wasting and the complex surface textures that result from mass wasting deflation and sublimation decreases the recognizability of the shorelines formerly proposed for this area

geomorphic analysis of lobate debris aprons on mars at mars orbiter camera scale evidence for ice sublimation initiated by fractures 1 lobate debris aprons known to be geomorphic landform indicators of the presence of ground ice are of special interest for future missions devoted to the research of water on mars lobate debris aprons in fretted terrains of deuteronilus and protonilus mensae 35 50 n show typical convex shapes interpreted to be the result of viscous deformation at the scale of mars orbiter camera moc high resolution images the surface of these debris aprons shows complex patterns with small pits and buttes these patterns can be explained by the mantling of dust the accumulation of interstitial ice and the subsequent removal of ice by sublimation the sublimation of the ground ice is especially initiated and accelerated by subsurface heterogeneities like fractures theoretical quantification of sublimation rates therefore minimizes sublimation which is not a homogeneous process at least over the landforms studied crater counts show that the sublimation occurred in the last tens of millions of years up to the recent past in the point of view of future searching of subsurface ice only surface layers are submitted to sublimation favoring the conservation of ground ice in deeper layers since the formation of the landform the geophysical survey of lobate debris aprons would give interesting insights into the subsurface distribution of ice and its seasonal variations especially in order to measure current sublimation of ground ice

flow measurement in streams using video imagery 1 a video imagery technique for making flow measurements in streams and waterways is presented and used to estimate discharge for clear creek near oxford iowa a video camera was used to visualize the flow seeded with tracers measurements of free surface flow velocities were then made using particle image velocimetry piv techniques the velocity measurements were used as input to a hydraulic model which uses kinematic principles conservation of mass to derive three dimensional flow fields for discharge estimation a survey of five channel cross sections over a 7 15 m length of stream were made to define the channel bathymetry for the hydraulic model the average channel width was 5 7 m and the average depth was 0 2 m the discharge estimate using the video technique 0 183 m3 s compares well with the current meter discharge measurement 0 192 m3 s with an estimated standard error of 6 4 the root mean square difference of the depth averaged velocity at the locations of the current meter measurements was 0 032 m s potential applications of the video imagery technique include measurements of flood flows at ungaged sites river monitoring from a remote site and estimation of two and three dimensional flow components

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

tectonics of complex crater formation as revealed by the haughton impact structure devon island canadian high arctic the results of a systematic field mapping campaign at the haughton impact structure have revealed new information about the tectonic evolution of mid size complex impact structures these studies reveal that several structures are generated during the initial compressive outward directed growth of the transient cavity during the excavation stage of crater formation 1 sub vertical radial faults and fractures 2 sub horizontal bedding parallel detachment faults and 3 minor concentric faults and fractures uplift of the transient cavity floor toward the end of the excavation stage produces a central uplift compressional inward directed deformation results in the duplication of strata along thrust faults and folds it is notable that haughton lacks a central topographic peak or peak ring the gravitational collapse of transient cavity walls involves the complex interaction of a series of interconnected radial and concentric faults while the outermost concentric faults dip in toward the crater center the majority of the innermost faults at haughton dip away from the center complex interactions between an outward directed collapsing central uplift and inward collapsing crater walls during the final stages of crater modification resulted in a structural ring of uplifted intensely faulted sub vertical and or overturned strata at a radial distance from the crater center of 5 0 6 5 km converging flow during the collapse of transient cavity walls was accommodated by the formation of several structures 1 sub vertical radial faults and folds 2 positive flower structures and chaotically brecciated ridges 3 rollover anticlines in the hanging walls of major listric faults and 4 antithetic faults and crestal collapse grabens oblique strike slip i e centripetal movement along concentric faults also accommodated strain during the final stages of readjustment during the crater modification stage it is clear that deformation during collapse of the transient cavity walls at haughton was brittle and localized along discrete fault planes separating kilometer size blocks

a case study of impact induced hydrothermal activity the haughton impact structure devon island canadian high arctic the well preserved state and excellent exposure at the 39 ma haughton impact structure 23 km in diameter allows a clearer picture to be made of the nature and distribution of hydrothermal deposits within mid size complex impact craters a moderate to low temperature hydrothermal system was generated at haughton by the interaction of groundwaters with the hot impact melt breccias that filled the interior of the crater four distinct settings and styles of hydrothermal mineralization are recognized at haughton a vugs and veins within the impact melt breccias with an increase in intensity of alteration towards the base b cementation of brecciated lithologies in the interior of the central uplift c intense veining around the heavily faulted and fractured outer margin of the central uplift and d hydrothermal pipe structures or gossans and mineralization along fault surfaces around the faulted crater rim each setting is associated with a different suite of hydrothermal minerals that were deposited at different stages in the development of the hydrothermal system minor early quartz precipitation in the impact melt breccias was followed by the deposition of calcite and marcasite within cavities and fractures plus minor celestite barite and fluorite this occurred at temperatures of at least 200 c and down to 100 120 c hydrothermal circulation through the faulted crater rim with the deposition of calcite quartz marcasite and pyrite occurred at similar temperatures quartz mineralization within breccias of the interior of the central uplift occurred in two distinct episodes 250 down to 90 c and 80 c appreciation of the structural features of impact craters has proven to be key to understanding the distribution of hydrothermal deposits at haughton

the relationship between acoustic properties and the petrographic character of carbonate rocks laboratory studies of the detailed relationships between acoustic properties and the petrographic character of brine and air saturated carbonate rocks with a wide range of facies porosities lithologies and rock fabrics indicate that porosity is the major factor influencing both p and s wave impedance and velocity primary lithology and secondary mineralogy have only a small influence on impedance and velocity combined use of p and s wave velocity data discriminates porosity changes from lithologic changes all other variables including pore fluid type and petrographic fabric have no significant influence on velocities laboratory measurements of p wave velocity under simulated in situ conditions reproduce well log velocity values reliably laboratory porosity velocity trends agree with the time average equation when the correct matrix velocities are used rock property results were used to interpret porosity lithology variations for an inverted seismic section from the williston basin where well

the formation of irregular polygonal ridge networks nili fossae mars implications for extensive subsurface channelized fluid flow in the noachian abstract roughly 16 000 20 50 m wide morphologically diverse erosionally resistant ridges have been exhumed and mapped in noachian aged terrain across a large region in nili fossae and the nilosyrtis highlands however the formation of these landforms has been widely debated their morphology and geologic context suggest surface processes such as aeolian fluvial and glacial sedimentary deposition are unlikely candidates for their formation possible subsurface ridge formation mechanisms include 1 volcanic dike intrusions along pre existing fractures 2 breccia dikes from impact cratering 3 clastic dikes or deformation bands and 4 chemical alteration or precipitation of minerals in or along pre existing fractures or faults the ridges are being exhumed from a phyllosilicate bearing host rock but whether the ridges themselves have an aqueous origin remains unknown although previous studies used comparative morphology to assess their formation each hypothesized formation mechanism implies different mineralogical suites possibly detectable in remotely sensed spectral data here we combine spectral and morphological analyses to assess the origin of a subset of ridges n 797 that have irregular polygonal ridge network morphology we use observations from the compact reconnaissance imaging spectrometer for mars crism and high resolution imaging science experiment hirise supplemented with context camera ctx imagery to analyze the spectral signatures and geometry in order to 1 measure ridge morphology orientation and intersection angles and 2 isolate the spectral signatures of the ridges from their host unit to evaluate hypothesized ridge formation mechanisms the irregular polygonal nature near orthogonal intersections and lack of dominant orientation are characteristics consistent with fracture propagation under horizontal near isotropic extensional stress in a physically heterogeneous host rock inconsistent with impact or volcanic related features alternatively very shallow clastic intrusion could produce similar ridge geometry our key findings are 1 ridge orientation frequency and distribution suggest mineralization or cementation of subsurface fractures or shallow clastic intrusions as favored formation mechanisms and 2 ridges share all the diagnostic spectral features of mg smectite saponite and or mixed layered talc saponite clays of the host materials however ridges express weaker absorptions if the crism spatial and or spectral resolution has not limited our ability to detect a spectrally unique cementing agent then both grain size or texture and visible to near infrared inactive mineralogy could be responsible for the ridge s weaker spectral absorptions such as quartz and silica some oxide group minerals and amorphous materials our results imply that shallow subsurface groundwater and hydrothermal activity was likely extensive prior to and during the opening of the nili fossae and played an integral role in the formation of the ridges observed throughout the region

origin of periodically spaced wrinkle ridges on the tharsis plateau of mars ridged plains are a major geologic unit on mars and are a predominant unit on the tharsis plateau a region that has undergone uplift extension and extensive shield and flood volcanism these units probably flood volcanic in origin are characterized by landforms classed as wrinkle ridges wrinkle ridges are interpreted to be folds resulting from buckling followed by reverse to thrust faulting flexure fracture a prominent characteristic of many of the wrinkle ridges on mars particularly those in the tharsis ridge system is the periodic nature of their spacing the periodic spacing has been evaluated in six major provinces on the tharsis plateau with regions further divided into domains based largely on variation in ridge orientation the average spacing of the tharsis ridges based on 2934 measurements is 30 km in an effort to account for the periodic nature of the wrinkle ridges the ridged plains material is modeled as both a single member and a multilayer with frictionless contacts that has buckled at a critical wavelength of folding free slip between layers is assumed based on the possible existence of mechanically weak interbeds in the ridged plains sequence separating groups of flows the near surface mechanical structure in the area of ridged plains is approximated by a strong layer or layers overlying a weak megaregolith of finite thickness overlying a strong basement buckling of the ridged plains is assumed to be decoupled from the basement by the weak megaregolith decoupling is enhanced if the megaregolith were water or ice rich at the time of deformation the rheologic behavior of the ridged plains and megaregolith is approximated by a linear elastic and linear viscous material the models are examined for a range in 1 the strength contrast between the ridged plains material and the underlying megaregolith 2 thickness of the ridged plains material 3 thickness of the megaregolith and 4 number of layers the elastic model can explain the observed ridge spacing only if the ridged plains are relatively thick 2400 m with multiple layers n 1 and relatively high contrasts in young s modulus e e0 100 the high e e0 required is possible only if the megaregolith was water rich at the time of deformation over the same range in values of the parameters viscous buckling is much less restricted the viscous model can explain the observed ridge spacing over a wide range in ridged plains thickness 250 m to several kilometers megaregolith thickness number of layers n 1 to 8 and contrasts in viscosity 0 10 in addition viscous buckling is viable if the megaregolith were dry water rich or ice rich at the time of deformation the absence of folds with a cross sectional geometry in the shape of a sinusoid anticline syncline pairs may be the result of initial deformation of the ridged plains material into low amplitude folds in infinitesimal strain followed by plastic yielding in the cores of the anticlines in finite strain initial elastic or viscous buckling coupled with plastic yielding confined to the hinge area followed by the development of reverse to thrust faulting could account for the asymmetric fold geometry of many of the wrinkle ridges

acoustic properties of ancient shallow marine carbonates effects of depositional environments and diagenetic processes middle jurassic paris basin france abstract examination of petrophysical properties acoustic velocity porosity permeability and density and petrographical characteristics texture facies composition and diagenesis of more than 250 core plugs from the middle jurassic carbonates of the eastern paris basin provides insights into the parameters controlling acoustic velocities in relatively low porosity carbonate rocks 20 the pore type observations reveal distinct acoustic velocities in samples with intergranular macropores and samples with micropores in subhedral micrite such that velocities in microporous mudstone wackestone lagoonal deposits are greater than in macroporous grainstone shoal samples at a given porosity range 15 20 the standard wyllie and raymer transforms fit very well with the linear regression between acoustic velocity and porosity from mudstone or lagoonal facies marls and fine grained deposits interpreted as lagoonal facies include statistically significant correlation r 0 9 between velocity and porosity however the data suggest that the wide scatter in velocity porosity relationship from grainstones are not the result of different sorting grain size pore type dolomite content or clay content instead early cementation greatly influences acoustic properties during diagenesis and are interpreted to account for the high variability of velocities over a given porosity range specifically at a given porosity acoustic velocities in compacted grainstone that did not undergo early cementation are higher than in early cemented grainstone petrographic observations suggest that early cementation limits mechanical compaction creating a heterogeneous medium from the earliest stages of diagenesis non touching grains preservation of intergranular macropores that are partially to totally filled by later blocky calcite cement the abundant interfaces between micritized ooids early cement fringes and blocky calcites in grainstones may induce significant wave attenuation as a result the standard time average equations fail to predict the effect of diagenetic features such as early cementation on sonic velocity conversely an absence of early cementation favors mechanical compaction grain to grain contact and suturing the result is a homogeneous micritized grain supported network that may facilitate wave propagation through demonstration of the key role of early cementation in the explanation of variability in acoustic properties the results of this study illustrate the complicated factors influencing velocity transforms in carbonates wyllie and raymer i e classical tools for predicting reservoir properties these insights on the interpretation of vp and the refinement of velocity porosity transforms in grainstone units may be broadly applicable to enhancing seismic based exploration in carbonate successions

velocities of compressional and shear waves in limestones carbonate rocks are important hydrocarbon reservoir rocks with complex textures and petrophysical properties porosity and permeability mainly resulting from various diagenetic processes compaction dissolution precipitation cementation etc these complexities make prediction of reservoir characteristics e g porosity and permeability from their seismic properties very difficult to explore the relationship between the seismic petrophysical and geological properties ultrasonic compressional and shear wave velocity measurements were made under a simulated in situ condition of pressure 50 mpa hydrostatic effective pressure at frequencies of approximately 0 85 mhz and 0 7 mhz respectively using a pulse echo method the measurements were made both in vacuum dry and fully saturated conditions in oolitic limestones of the great oolite formation of southern england some of the rocks were fully saturated with oil the acoustic measurements were supplemented by porosity and permeability measurements petrological and pore geometry studies of resin impregnated polished thin sections x ray diffraction analyses and scanning electron microscope studies to investigate submicroscopic textures and micropores it is shown that the compressional and shear wave velocities vp and vs respectively decrease with increasing porosity and that vp decreases approximately twice as fast as vs the systematic differences in pore structures e g the aspect ratio of the limestones produce large residuals in the velocity versus porosity relationship it is demonstrated that the velocity versus porosity relationship can be improved by removing the pore structure dependent variations from the residuals the introduction of water into the pore space decreases the shear moduli of the rocks by about 2 gpa suggesting that there exists a fluid matrix interaction at grain contacts which reduces the rigidity the predicted biot gassmann velocity values are greater than the measured velocity values due to the rock fluid interaction this is not accounted for in the biot gassmann velocity models and velocity dispersion due to a local flow mechanism the velocities predicted by the raymer and time average relationships overestimated the measured velocities even more than the biot model

calculation of stage discharge relations for gravel bedded channels 1 a method for calculating stage discharge relations rating curves in gravel bedded streams is presented and applied to five reaches at u s geological survey usgs gaging stations the approach which builds on the work of kean and smith 2005 uses a fluid mechanically based model to convert measurements of stage into flow and boundary shear stress fields as appropriate for determining water discharge the model does not use site specific empirical roughness coefficients such as the manning coefficient but rather determines channel roughness from field measurements of the channel geometry and the dominant physical and biological roughness elements in the modeled reach the approach is fully compatible with current usgs style river gaging procedures and can be used in conjunction with or instead of the standard empirical gaging methods when used in parallel the theoretical rating curves produced by the model are in good agreement with direct measurements of discharge made by the usgs the results of our analyses indicate that the theoretical rating curve approach has the potential to substantially reduce the number of measurement visits and therefore costs required to develop rating curves for gaging stations owing to this lower cost it is well suited for regional hydrologic studies in which rainfall distributions are measured in space and time and are compared with measured discharges on the links of the river network in addition our method is ideally suited for sites such as remote locations where it is difficult or impossible to define a complete rating curve using conventional methods alone

generation and verification of theoretical rating curves in the whitewater river basin kansas 1 a new method for generating stage discharge relations rating curves for geomorphically stable channels is presented and applied to two streams in the whitewater river basin kansas the approach converts measurements of stage into discharge using a fluid mechanically based model the model does not use empirical roughness coefficients such as manning coefficients but rather determines channel roughness from field measurements of the 1 channel geometry 2 the physical roughness of the bed banks and floodplain and 3 the vegetation density on the banks and floodplain these measurements are used to calculate explicitly the drag on the small scale topographic features on the boundary the drag on the stems and branches of woody vegetation and the friction on the bed banks and floodplain the theoretical rating curves produced by the model for two study reaches which are near u s geological survey usgs streamflow gauging stations are in good agreement with direct measurements of discharge made by the usgs our method has the potential of providing accurate estimates of stream flows less expensively than conventional gauging methods in addition the method can be used to obtain more accurate discharge estimates than conventional indirect methods for determining discharge which are based on estimates of manning s coefficient

impactites of the haughton impact structure devon island canadian high arctic contrary to the previous interpretation of a single allochthonous impactite lithology combined field optical and analytical scanning electron microscopy sem studies have revealed the presence of a series of impactites at the haughton impact structure in the crater interior there is a consistent upward sequence from parautochthonous target rocks overlain by parautochthonous lithic monomict breccias through allochthonous lithic polymict breccia into pale grey allochthonous impact melt breccias the groundmass of the pale grey impact melt breccias consists of microcrystalline calcite silicate impact melt glass and anhydrite analytical data and microtextures indicate that these phases represent a series of impact generated melts that were molten at the time of and following deposition impact melt glass clasts are present in approximately half of the samples studied consideration of the groundmass phases and impact glass clasts reveal that impactites of the crater interior contain shock melted sedimentary material from depths of 920 to 200 to 760 m and are interpreted as remnants of the continuous ejecta blanket the pale grey impact melt breccias although similar to the impact melt breccias of the crater interior are more carbonate rich and do not appear to have incorporated clasts from the crystalline basement thus the spatial distribution of the crater fill impactites at haughton the stratigraphic succession from target rocks to allochthonous impactites the recognition of large volumes of impact melt breccias and their probable original volume are all analogous to characteristics of coherent impact melt layers in comparatively sized structures formed in crystalline targets

friction melt distribution in a multi ring impact basin it is generally accepted that multi ring basins are the consequence of very large impacts but the mechanism by which they form is still a matter of contention1 2 most of what is currently known about multi ring basins is based on remote studies of the moon3 5 and to a lesser extent mars and mercury4 6 but at least two multi ring impact basins have been recognized on earth7 the sudbury8 canada and vredefort9 south africa impact structures providing an opportunity to study their properties directly here we describe the distribution of friction melt pseudotachylyte in the floor of the sudbury impact basin although the veins and dykes of pseudotachylyte decrease in both thickness and frequency of occurrence towards the basin periphery the greatest volumes of friction melt appear to define four rings around the central impact melt sheet field evidence indicates that the rings originated as zones of large displacement which facilitated localized frictional melting of the basin floor during the modification collapse stage of the cratering process by analogy we argue that the rings of other multi ring impact basins are also likely to be the remnants of such large displacement fault zones

the terrestrial impact cratering record approximately 130 terrestrial hypervelocity impact craters are currently known due to variations in preservation and in geologic knowledge this sample is biased towards young x 20 km craters on the cratons of australia europe including the former u s s r and north america the rate of discovery of new craters is 3 5 craters per year although modified by erosion terrestrial impact craters exhibit the range of morphologies observed for craters on other terrestrial planetary bodies such as the moon terrestrial craters provide essential ground truth data on the geologic effects of impact and the subsurface structure of impact craters which can be used to constrain interpretations of lunar samples and models of crater formation due to erosion and its effects on form terrestrial craters are recognized primarily by the occurrence of shock metamorphic effects these include shatter cones microscopic planar deformation features solid state and fusion glasses high pressure polymorphs and whole rock melting and vaporization shock recovery experiments indicate that these features occur over shock pressures of 5 gpa to x 100 gpa terrestrial craters have a set of geophysical characteristics which are largely the result of the passage of a shock wave and impact induced fracturing they include gravity and magnetic lows and reductions in seismic velocity the gravity anomalies are seldom greater than 30 mgal due to the limiting effects of lithostatic pressure on fracturing at large complex craters the gravity signature may include a central relative gravity high due to uplift and short wavelength central magnetic anomalies due to a variety of processes much current work is focused on the effects of impact on earth evolution previous work on shock metamorphism and the contamination of impact melt rocks by meteoritic siderophile elements provides a basis for the interpretation of the physical and chemical evidence from cretaceous tertiary boundary sites as resulting from a major impact suggestions that other biological boundaries in the stratigraphie record are due to periodic impacts are not supported by time series analysis of the terrestrial cratering record by analogy with the lunar record and modelling of the effects of very large impacts it has been proposed that biological and atmospheric evolution of the earth could not stabilize before the end of the late heavy bombardment 3 8 ga ago the present terrestrial cratering rate is 5 4 2 7 10 15 km 2a 1 for a diameter 20 km this represents a local threat on historic time scales on a global scale a major impact sufficient to cripple human civilization severely will occur on time scales of 106 a

geology mineralization and emplacement of the foy offset dike sudbury impact structure the foy offset dike is the longest of the known radial dikes associated with the 1 85 ga sudbury igneous complex the foy dike extends for at least 30 km from the complex into the north range archean and proterozoic footwall rocks of the sudbury impact structure the dike ranges in width from 400 m where it connects to the sudbury igneous complex to 50 m at its most distal known exposure we divide the dike into a proximal segment and a distal segment based on where it intersects the concentric hess offset dike three main dike lithologies are distinguished all referred to as varieties of quartz diorite inclusion rich pyroxene quartz diorite occurs in the embayment adjacent to the base of the main mass of the sudbury igneous complex this constitutes the sublayer northward the sublayer gives way to amphibole biotite inclusion bearing quartz diorite and amphibole biotite inclusion poor quartz diorite the dike is mineralized and deposits in the proximal segment have been mined mineralization is associated with the inclusion rich dike phase at the nickel offset mine the dike contains inclusion bearing massive ni cu sulfides that also host platinum group minerals the orebodies occurred as strike parallel sheets within the central parts of the dike in association with inclusion bearing quartz diorite the geochemistry of the dike lithologies indicates that both the inclusion poor and inclusion bearing quartz diorite varieties are close in composition to the postulated bulk composition of the sudbury igneous complex this implies that the dike was emplaced prior to any significant fractionation of the main melt body and hence very early in the evolution of the sudbury complex we envisage dike emplacement rapidly following rebound and central uplift formation as part of the impact process early dike emplacement implies that the ore was also introduced early in the evolution of the melt sheet when the inclusion poor quartz diorite phases were still unconsolidated uplift would have caused extension in the target rocks and so dilated the footwall fractures and faults that then became hosts primarily via gravitationally driven intrusion to the primitive polymict impact melt breccias and sulfide ores the chilled margin compositions of the foy inclusion poor quartz diorite are considered good indicators of the original bulk composition of the sudbury igneous complex this idea is supported by comparison with calculated estimations of the bulk composition a shift in geochemical trends a grain size increase and a decrease in inclusion content occurs 22 km along the length of the dike which we believe corresponds to a fault system that downfaults to the north given that the dense ore would have preferentially settled toward the base of the foy dike fracture system postimpact downfaulting of the distal segment of the dike may have displaced any mineralized zones present to deeper levels relative to the proximal segment

palaeomagnetic dating of the borne de fer ferricrete ne france lower cretaceous continental weathering abstract a palaeomagnetic study was carried out on the borne de fer ferricrete in north eastern france close to the luxembourg border this ferricrete is overlain by kaolinitic red clay and caps a silto argilaceous saprolite 15 to 40 m thick showing weathering features evolution of the clay minerals from the base to the top and karstic solution pipes beneath this weathering profile lies above jurassic marl and limestone from around 425 to 450 m a s l and may correspond to a scarce remnant of an old weathering palaeosurface palaeomagnetic rockmagnetic and petrographic analyses reveal presence of a dominant hydroxide contribution with a magnetic remanence carried by goethite a small contribution of maghemite or hematite carries a more scattered but similar direction the virtual geomagnetic pole derived from the goethite component lies at lat 73 4 n lon 205 9 e dp 4 3 dm 6 2 this location close to the 120 140 ma reference poles and presence of normal and reversed polarities are consistent with a lower cretaceous age close to the long normal cretaceous superchron our results are consistent with the ones obtained recently in germany belgium and france and prepare the ground for reconstructing the lower cretaceous continental palaeosurface geometry and palaeogeography

attenuation of p and s waves in limestones ultrasonic compressional and shear wave attenuation measurements have been made on 40 centimetre sized samples of water and oil saturated oolitic limestones at 50 mpa effective hydrostatic pressure confining pressure minus pore fluid pressure at frequencies of about 0 85 mhz and 0 7 mhz respectively using the pulse echo method the mineralogy porosity permeability and the distribution of the pore types of each sample were determined using a combination of optical and scanning electron microscopy a helium porosimeter and a nitrogen permeameter the limestones contain a complex porosity system consisting of interparticle macropores dimensions up to 300 microns and micropores dimensions 5 10 microns within the ooids the calcite cement and the mud matrix ultrasonic attenuation reaches a maximum value in those limestones in which the dual porosity system is most fully developed indicating that the squirt flow mechanism which has previously been shown to occur in shaley sandstones also operates in the limestones it is argued that the larger scale dual porosity systems present in limestones in situ could similarly cause seismic attenuation at the frequencies of field seismic surveys through the operation of the squirt flow mechanism

extensive linear ridge networks in nili fossae and nilosyrtis mars implications for fluid flow in the ancient crust 1 we have undertaken high resolution mapping of the distribution physical characteristics orientation and stratigraphic occurrence of 4000 linear ridge segments in two study areas the nilosyrtis highlands and the nili fossae region covering 1 105 km2 ridges are typically hundreds of meters in length meters in width and up to tens of meters in height ridges form intersecting networks of anastomosing and bifurcating segments and consequently have variable orientations ridges are expressed in altered and brecciated basement materials on the floors and rims of impact craters deeply eroded by fluvial and other processes the ridge bearing basement is the lowest exposed stratigraphic unit and is overlain by a relatively unaltered mafic cap rock ridges are observed to terminate at the contact of these two units we interpret the ridges to represent a complex of fractures faults shear zones clastic and melt bearing dikes and pseudotachylytes that have been variably indurated by fluid percolation and mineralization the fluid conduits were hardened relative to the host rock and are expressed as ridges due to differential erosion the orientations of the exhumed ridges record the state of stress in the crust at the time of formation in nili fossae a significant population of ridges is aligned with the orientations of the nili fossae graben indicating that their emplacement may be related to crustal loading of the isidis basin after the impact event the association of ridges with the hydrated mineral bearing basement suggests that in the postimpact environment the fractures served as conduits of preferential fluid flow which were cemented by mineral precipitation

origin and emplacement of offset dykes in the sudbury impact structure constraints from hess the hess offset is a steeply dipping dyke located 12 15 km north of the 1 85 ga sudbury igneous complex sic within the 200 250 km diameter sudbury impact structure it is up to 60 m wide and strikes subconcentrically to the sic for at least 23 km the main phase of the dyke is granodioritic but it conforms with what is locally referred to as quartz diorite a term used for all the offset dykes of the sudbury impact structure rare earth element data shows that the hess offset is genetically related to the sic hess is most closely affiliated with an evolved felsic norite component of sic and not bulk impact melt this indicates that hess was emplaced during fractionation of the impact melt sheet rather than immediately following impact the main quartz diorite phase of the dyke comprises a quartz plagioclase hornblende biotite clinopyroxene orthopyroxene assemblage critically the hess offset occupies a concentric fault system that marks the northern limit of a pseudotachylyte rich shatter cone bearing annulus about the sic this fault system was active during the modification stage of the impact process

atmospheric effects on ejecta emplacement and crater formation on venus from magellan the venus cratering record provides a unique environment for assessing the effects of both gravity and an atmosphere on impact crater formation this contribution uses surface signatures of energy partitioning as a framework for testing extrapolations from laboratory experiments and other planetary settings seven general conclusions can be drawn first the dense lower atmosphere of venus takes on the role of a low density target for bodies smaller than about 4 km in diameter air blasts created by cratering in the atmosphere create distinctive surface signatures that allow the derivation of an independent assessment of impactor energy at the limit of break up second dynamic pressures during entry of larger bodies will exceed their strength limit but may not prevent penetration of the atmosphere due to aerodynamic reshaping that minimizes the drag coefficient such a process may account for the formation of unusually small craters 1 3 km third the dense atmosphere of venus preserves signatures of early time cratering processes on the surface that are typically lost on atmosphere free surfaces such signatures not only provide another estimate of impactor energy but also include a distinctive record of the impactor i e comet versus asteroid in distinctive run out flows created before the crater has finished formation strong winds and turbulence associated with the atmospheric disturbance at later times create wind streaks behind topographic barriers fourth ballistic ejection of excavated debris occurs from craters on venus just as it does on planets without an atmosphere thereby underscoring the fundamental mechanical transfer of energy from impactor to target fifth ejecta emplacement is nonballistic due to the large dynamic forces acting on the advancing curtain and its constituent ejecta the outward moving ejecta curtain induces strong response winds that entrain ejecta and drive a ground hugging debris flow outward without returning to the cavity flow separation creates an overriding run out ejecta flow further sustained by atmospheric turbulence and identified as radar dark lobate lobes sixth radar dark parabolas are proposed to be late time fallout deposits created as the downrange fireball evolves aloft perhaps analogous to terrestrial tektite strewn fields and seventh the response of crater formation to the atmosphere is conversely expressed by a reduction in cratering efficiency as revealed by the unusually large central peak complexes and the unexpected diameter depth relations of craters hence surface ages may be significantly underestimated

dike indicators in the hadriaca patera promethei terra region mars abstract the eastern hellas basin rim has been highly modified by a multitude of both volcanic and fluvial processes it is covered by a large part of the circum hellas volcanic province consisting of the volcanic plains of hesperia planum and western promethei terra as well as the two central volcanoes hadriaca and tyrrhena patera and their associated lava flows large outflow channels of dao niger harmakhis reull and teviot valles dissect the wrinkle ridged plains postdating the major volcanic events the origin of these outflow channels is still ambiguous as the mobilization mechanism for the underground volatile reservoirs has yet to be revealed due to the proximity of the volcanic structures many previous studies have suggested via models that the released ice or ground water was mobilized due to endogenic heating from magmatic activity in the region to find evidence of this we conducted a systematic search for any magmatic intrusions at 50 m pixel resolution this paper provides detailed mapping of several areas where we found features which we interpret to be dikes and shows the overall distribution of such structures in the eastern hellas rim region the hadriaca patera volcano flanks exhibit a concentric pattern of linear ridges and fractures indicative of a vast ring dike system the dikes occupy mostly the area near the summit and the hellas facing flank extending there for over 400 km away from the caldera a hadriaca radial dike swarm pattern is observed extending southwards from the volcano composed of isolated parallel linear ridge group outcrops and cut by the valles canyons an additional isolated dike ridge group exists south of lower reull valles possibly controlled by the tyrrhena patera volcano some 1300 km away all dikes within the study region are roughly late hesperian in age either predating the valles formation or being roughly concurrent with them in total 500 dike segments were identified with a combined length of 2500 km since 90 of the dikes are ridges they suggest that the entire region has undergone large scale erosion of at least a few tens of meters to expose the dikes the dike orientations were most probably controlled by the volcanic centers and possibly partly by a hellas concentric fracture pattern

evidence for persistent flow and aqueous sedimentation on early mars landforms representative of sedimentary processes and environments that occurred early in martian history have been recognized in mars global surveyor mars orbiter camera and mars odyssey thermal emission imaging system images evidence of distributary channelized flow in particular flow that lasted long enough to foster meandering and the resulting deposition of a fan shaped apron of debris indicate persistent flow conditions and formation of at least some large intracrater layered sedimentary sequences within fluvial and potentially lacustrine environments

the south range breccia belt of the sudbury impact structure a possible terrace collapse feature the south range breccia belt srbb is an arcuate 45 km long zone of sudbury breccia in the south range of the 1 85 ga sudbury impact structure the belt varies in thickness between tens of meters to hundreds of meters and is composed of a polymict assemblage of huronian supergroup 2 49 2 20 ga nipissing diabase 2 2 ga and proterozoic granitoid breccia fragments ranging in size from a few millimeters to tens of meters the srbb matrix is composed of a fine grained 100 m assemblage of biotite quartz and ilmenite with trace amounts of plagioclase zircon titanite epidote pyrite chalcopyrite pyrrhotite and occasionally chlorite the srbb hosts the frood stobie vermilion and kirkwood quartz diorite offset dykes the former being associated with one of the largest ni cu pge sulphide deposits in the world optical petrography and whole rock geochemistry concur with previous studies that have suggested that the matrix of the srbb is derived from comminution and at least partial frictional melting of the wall rock huronian supergroup lithologies rare earth element ree data from all sampled lithologies associated with the srbb exhibit crustal signatures when normalized to c1 chondrite values additionally ree data from the quartz diorites disseminated sulphides in sudbury breccia and a sample of an aphanitic biotite hornblende tonalite dyke exhibit flat slopes when compared to the mafic and felsic norites quartz gabbro and granophyre units of the sudbury igneous complex sic which suggests that these lithologies are representative of bulk sic melt we suggest that the srbb was formed by high strain rate 1 m s gravity driven seismogenic slip of the inner ring of the sudbury impact structure during postimpact crustal readjustment crater modification stage failure of the hanging wall may have facilitated the injection of bulk sic melt into the srbb along with the ni cu pge sulphides of the frood stobie deposit postimpact penokean 1 9 1 7 ga tectonism particularly northwest directed shearing along the south range shear zone and associated thrust faulting could account for the present subvertical orientation of the srbb and the apparent lack of a connection at depth with the sic

micro rhombic calcite and microporosity in limestones a geochemical study of the lower cretaceous thamama group u a e abstract euhedral micro rhombic calcite forms a microporosity network in limestones of the lower cretaceous thamama group u a e isotopic and trace element data indicate that micro rhomb formation was a two step process associated with diagenesis in meteoric waters the micro rhombic calcite and the micropores are both products of the second event sr fe mn and mg contents of the micro rhombs are all relatively low ranging from 108 to 310 ppm 34 to 308 ppm 3 to 32 ppm and 0 42 to 0 68 mole respectively the low sr and mg contents indicate extensive flushing of these constituents from the rock the low fe and mn contents are probably inherited from the original sediments and indicate the lack of any fe or mn in the pore fluids during micro rhomb formation the 13c of the micro rhombs 3 1 to 4 5 pdm are also primary unaltered by diagenesis the 13o of the micro rhombs 4 5 to 6 0 pdb reflect alteration by mateoric waters a large number of pore volumes and an open water buffered system would be necessary to flush sr and mg yet decreasing 18o with increasing porosity indicates that 18o values in non porous intervals are partially rock buffered two diagenetic events are thus implicated the first flushing sr and mg and lowering 18o in all rocks and the second event actually forming the micro rhombs and further altering 18o as a function of porosity the first diagenetic event was most likely mineralogical stabilization in an unconfined meteoric aquifer immediately after shuaiba deposition if mineralogical stabilization was completed in the first event then calcite to calcite recrystallization is implicated for the second event the second event may have occurred in the unconfined meteoric aquifer or it may have occurred in a confined meteoric aquifer during the early late cretaceous

extensive hesperian aged south polar ice sheet on mars evidence for massive melting and retreat and lateral flow and ponding of meltwater local and regional mars orbiter laser altimeter mola topographic data support the presence of an extensive hesperian aged volatile rich south polar deposit the dorsa argentea formation hd and related units underlying the present amazonian aged polar cap api residual ice and apl layered terrain and covering a surface area that could be as large as 2 94 106 km2 about 2 of the surface of mars over twice the area of the present amazonian aged south polar deposits the deposit characteristics indicate that it contained significant quantities of water ice in amounts comparable to present day polar deposits several lines of evidence for melting indicate that the ice sheet deposits underwent melt back and liquid water drainage into surrounding lows including a large valley near the crater schmidt and the argyre basin narrow sinuous ridges lie in a broad linear depression extending from a high near the present polar cap continuously downslope to near the distal portion of hd the new topographic data support the interpretation of these ridges as eskers representing meltwater distribution networks at the base of the receding deposit extensive development of large pits and depressions cavi have previously been interpreted as eolian etching or basal melting of ice rich deposits analysis of mola topography supports the interpretation that they represent basal melting of ice rich deposits and shows that they have links to the esker systems volumetric considerations and topographic lineations suggest that some of the basal melting occurred beneath regions presently occupied by apl and that some of the liquid water formed ponds and lakes in the distal parts of hd the presence of pedestal craters is further evidence of the removal of extensive volatile rich deposits and contributes to the quantitative measure of the former deposit thickness where did the melt products go inspection of the margins of the dorsa argentea formation reveals several large channels that begin there and drain downslope for distances between 900 and 1600 km onto the floor of the argyre basin some 3 5 4 0 km below their origin these channels do not exhibit tributaries their broad lateral distribution supports other evidence that deposit melting was areally very widespread and volumetrically significant and that a large part of the meltwater entered the surface distribution system and was deposited on the floor of the argyre basin over 1000 km away estimates of the present deposit thicknesses together with amounts of the deposit removed by meltback suggest that the original volume could have been as much as 5 9 106 km3 equivalent to a global layer of water 20 m deep if the deposit consisted of 50 volatiles a portion of these volatiles migrated across the surface to pond in adjacent valleys and basins and into the groundwater system a significant portion of the volatiles remain in the deposit representing a net removal from the atmosphere and from the active hydrologic system in early to middle mars history and forming an accessible record of aqueous conditions and possible biological environments dating from that time

intra crater sedimentary deposits at the haughton impact structure devon island canadian high arctic detailed field mapping has revealed the presence of a series of intra crater sedimentary deposits within the interior of the haughton impact structure devon island canadian high arctic coarse grained well sorted pale gray lithic sandstones reworked impact melt breccias unconformably overlie pristine impact melt breccias and attest to an episode of erosion during which time significant quantities of impact melt breccias were removed the reworked impact melt breccias are in turn unconformably overlain by paleolacustrine sediments of the miocene haughton formation sediments of the haughton formation were clearly derived from pre impact lower paleozoic target rocks of the allen bay formation which form the crater rim in the northern western and southern regions of the haughton structure collectively these field relationships indicate that the haughton formation was deposited up to several million years after the formation of the haughton crater and that they do not therefore represent an immediate post impact crater lake deposit this is consistent with new isotopic dating of impactites from haughton that indicate an eocene age for the impact event sherlock et al 2005 in addition isolated deposits of post miocene intra crater glacigenic and fluvioglacial sediments were found lying unconformably over remnants of the haughton formation impact melt breccias and other pre impact target rock formations these deposits provide clear evidence for glaciation at the haughton crater the wealth and complexity of geological and climatological information preserved as intra crater deposits at haughton suggests that craters on mars with intra crater sedimentary records might present us with similar opportunities but also possibly significant challenges

cold based mountain glaciers on mars western arsia mons surface environmental conditions on mars are currently extremely cold and hyperarid most equivalent to polar deserts on earth coupling newly acquired mars data with fieldbased observations regarding the flow surface morphology and depositional history of polar glaciers in antarctica we show that the multiple facies of an extensive fan shaped deposit on the western flanks of arsia mons volcano are consistent with deposition from cold based mountain glaciers an outer ridged facies that consists of multiple laterally extensive arcuate and parallel ridges resting without disturbance on both well preserved lava flows and an impact crater is interpreted as drop moraines formed at the margin of an ablating and predominantly receding cold based glacier a knobby facies that consists of equidimensional knobs each to several kilometers in diameter is inward of the ridges this facies is interpreted as a sublimation till derived from in situ downwasting of ashrich glacier ice a third facies comprising distinctive convex outward lobes with concentric parallel ridges and aspect ratios elongated downslope likely represents rock glacier deposits some of which may still be underlain by a core of glacier ice taken together these surficial deposits show that the western flank of arsia mons was occupied by an extensive mountain glacial system accumulating on and emerging from the upper slopes of the volcano and spreading downslope to form a piedmont like fan

hillslope derived blocks retard river incision the most common detachment limited river incision models ignore the effects of sediment on fluvial erosion yet steep reaches of mountain rivers often host clusters of large 1 m blocks we argue that this distribution of blocks is a manifestation of an autogenic negative feedback in which fast vertical river incision steepens adjacent hillslopes which deliver blocks to the channel blocks inhibit incision by shielding the bed and enhancing form drag we explore this feedback with a 1 d channel reach model in which block delivery by hillslopes depends on the river incision rate results indicate that incision dependent block delivery can explain the block distribution in boulder creek colorado the proposed negative feedback may significantly slow knickpoint retreat channel adjustment and landscape response compared to rates predicted by current theory the influence of hillslope derived blocks may complicate efforts to extract base level histories from river profiles

pseudotachylyte petrogenesis constraints from the sudbury impact structure pseudotachylytes and their host rocks from the north range of the 1 85 ga sudbury impact structure have been investigated using analytical scanning electron microscopy electron microprobe analysis and xrf spectrometry the results show that the pseudotachylytes were produced in high speed slip zones by the frictional comminution and selective melting of wall rock lithologies the preferential assimilation of hydrous ferromagnesian phases during frictional melting produced relatively basic melts leaving the more mechanically resistant quartz and to a lesser extent plagioclase as included mineral clasts three distinct assemblages are identified within the pseudotachylytes a pre impact 1 85 ga rock and mineral clasts derived from host lithologies b a syn to immediately post impact 1 85 ga rapidly cooled quartz sanidine labradorite phlogopitic biotite matrix assemblage formed due to crystallization from a melt at 800 900 c and c a post impact 1 85 ga retrograde assemblage which overprints both clasts and matrices field evidence indicates that most pseudotachylyte formed in large displacement fault systems during gravitational collapse of the impact generated transient cavity the sudbury pseudotachylytes like endogenic pseudotachylytes were generated by frictional melting on fault surfaces the difference is primarily one of scale large km displacements occurring on impact induced ring faults can generate immense volumes of friction melt resulting in spectacular pseudotachylyte bodies up to 0 5 km thick and more than 10 km long