impact cratering in sandstone the memin pilot study on the effect of pore water abstract planetary surfaces are subjected to meteorite bombardment and crater formation rocks forming these surfaces are often porous and contain fluids to understand the role of both parameters on impact cratering we conducted laboratory experiments with dry and wet sandstone blocks impacted by centimeter sized steel spheres we utilized a 40 m two stage light gas gun to achieve impact velocities of up to 5 4 km s 1 cratering efficiency ejection velocities and spall volume are enhanced if the pore space of the sandstone is filled with water in addition the crater morphologies differ substantially from wet to dry targets i e craters in wet targets are larger but shallower we report on the effects of pore water on the excavation flow field and the degree of target damage we suggest that vaporization of water upon pressure release significantly contributes to the impact process

sandstone sample analysis and additional structural data from jabal rayah a possible impact structure in saudi arabia the 5 5 km sized jabal rayah ring structure located at 28 390 n 37 120 e in saudi arabia has been classified as a possible complex impact structure located in flat lying paleozoic clastic sediments previous detailed mapping showed that erosional processes led to a relief inversion with displaced folded and faulted blocks of silurian to early devonian strata interpreted to form a ring syncline now forming a topographically outstanding 150 m high ring crest the drainage toward the center of the structure seems controlled by a set of radial faults this central part is eroded to the level of the surrounding plateau and partially covered with gravel analysis of 28 qusaiba formation sandstones showed that at the present outcrop level the sediments seem devoid of shock features measurement of fold axes in the central part of the structure shows radially outward plunging fold axes becoming steeper toward the center and also fold axes of other orientation and folded folds this fold axis pattern is interpreted as an upward pointing kilometer sized sheath fold assuming an impact scenario and using the present size of the structure the minimum central structural uplift is estimated at 500 m which is consistent with qusaiba formation occupying the center of the ring structure

saqqar a 34 km diameter impact structure in saudi arabia here we present the first proof of an impact origin for the saqqar circular structure in northwestern saudi arabia neville et al 2014 with an apparent diameter of 34 km centered at 29 35 n 38 42 e the structure is formed in cambrian devonian siliciclastics and is unconformably overlain by undeformed cretaceous and paleogene sediments the age of impact is not well constrained and lies somewhere between 410 and 70 ma the subsurface structure is constrained by 2 d reflection seismic profiles and six drilled wells first order structural features are a central uplift that rises approximately 2 km above regional datums surrounded by a ring syncline the crater rim is defined by circumferential normal faults the central uplift and ring syncline correspond to a bouguer gravity high and an annular ring like low respectively the wells were drilled within the central uplift the deepest among them exceed 2 km depth sandstone core samples from these wells show abundant indicators of a shock metamorphic overprint planar deformation features pdfs were measured with orientations along 0001 103 and less frequently along 101 and 104 planar fractures pfs predominantly occur along 0001 and 101 and are locally associated with feather features ffs in addition some shocked feldspar grains and strongly deformed mica flakes were found the recorded shock pressure ranges between 5 and 15 gpa the preserved level of shock and the absence of an allochthonous crater fill suggest that saqqar was eroded by 1 2 km between the devonian and maastrichtian the documentation of unequivocal shock features proves the formation of the saqqar structure by a hypervelocity impact event

sedimentary processes across the continental rise of the southern antarctic peninsula abstract a series of large sediment mounds have been identified along the pacific portion of the antarctic peninsula continental rise these mounds are composed of sediment delivered to the continental rise during the advance and retreat of grounded ice across the shelf the stratigraphic development of one of these sediment deposits the tula sediment mound is examined to investigate how the onset of glaciation influenced the deep sea depositional environment along this portion of the margin the stratal relationships associated facies distribution and the physiography observed along the southern antarctic margin reflect the waxing and waning of the antarctic ice sheets various processes erode transport and deposit sediment along the outer shelf slope and rise throughout a glacial cycle a deep sea erosional unconformity is apparent at the base of the tula deposit this surface may reflect the first onset of intensified bottom water circulation along the margin perhaps induced by the tectonic opening of the drake passage the tula sediment mound is comprised predominantly of canyon overbank systems evidence for the onset of canyon cutting and the development of the thick overbank deposits 2 km is found immediately above the deep sea erosional unconformity relating increased canyon cutting across the continental rise to the fluctuation of ice across the shelf implies that the onset of predominantly glacial conditions commenced soon after the onset of intensified bottom water circulation along this margin the volume of sediment associated with the tula deposit and the apparent point source distribution of the channel systems suggests that much sediment was transported to the slope and rise perhaps by meltwater processes through these canyons

meteorite reconnaissance in saudi arabia meteorite searches in saudi arabia between 2008 and 2014 yielded 46 meteorites from the yabrin area 23 n 49 e 35 meteorites from the rub al khali sand desert 19 20 n 48 51 e and 1 meteorite from al haddar no meteorites were found near hafar al batin 29 n 45 e the 82 new meteorites represent 57 falls comprising 43 ordinary chondrites 4 carbonaceous chondrites 2 enstatite chondrites 3 ureilites 3 eucrites 1 acapulcoite and 1 lunar meteorite the median of 31 14c terrestrial ages is 6 2 ka significantly younger than the oman population 19 5 ka n 128 a further assessment of terrestrial 14c contamination is advised by a 11 15 ka 14c terrestrial age of heavily weathered meteorite khawr al fazra 014 geology indicating a terrestrial age 100 ka find densities of 0 4 2 8 km 2 for yabrin and the western rub al khali are similar to 0 5 km 2 observed in oman higher find densities of 135 km 2 29 km 2 for masses 10 g exist on small pleistocene outcrops in blowouts in the south central rub al khali 21 unpaired meteorites four 10 g were found in 11 blowouts with a combined area of 0 14 km2 the rub al khali meteorites show a relatively high degree of weathering median w 3 6 2 5 for yabrin low median mass 4 3 138 g and a high h l ratio 2 3 1 1 the high density of small meteorites is explained by prolonged sand protection and recent deflation the high meteorite density and relatively high proportion of rare meteorite types render the rub al khali blowouts an interesting target for future exploration

quaternary sediment patterns in the weddell sea relations and environmental conditions sediment patterns such as texture composition and facies from three selected areas of the antarctic continental margin of the weddell sea are discussed in relation to environmental variations of the quaternary hydrosphere and cryosphere advance and retreat of ice shelves as well as oscillations in sea ice coverage are reflected by particular sediment facies the distribution of ice rafted detritus tracks the antarctic coastal current and the weddell sea bottom water contour current can be recognized by its distinctive winnowing and erosion pattern distribution and abundance of biogenic sediment components are mainly controlled by the duration of sea ice coverage reflecting the long term climatic evolution

seismic stratigraphy of the central bransfield basin nw antarctic peninsula interpretation of deposits and sedimentary processes in a glacio marine environment abstract the central bransfield basin is a deep narrow trough between the northern tip of the antarctic peninsula and the south shetland islands analyses of single channel high resolution seismic reflection data are used to characterise the seismic stratigraphy of the central bransfield basin the tectonised acoustic basement is overlain by a 1 s thick sedimentary cover composed of two main sedimentary sequences the lower sequence which shows synsedimentary deformation has only been identified on the antarctic peninsula margin the upper sequence is a complex sedimentary package composed of eight seismic units whose distribution geometry and seismic facies allow two types of seismic units to be distinguished slope and basinal units the slope units constituted by progradational stratified seismic facies form a sedimentary wedge extending from the shelf edge the basinal units fill the basin floor showing chaotic and undulated seismic facies that change basinward into stratified seismic facies both types of seismic units display an interfingering pattern at the base of the slope suggesting an alternating shift of the sedimentary depocentre from the slope to the basinfloor and vice versa this alternate pattern indicates that the sedimentary processes responsible for the infilling of the central bransfield basin followed a cyclic pattern which has likely been associated with the advance and retreat of the ice sheets over the margins during glacial and interglacial episodes during glacial periods the ice sheets advanced eroded the shallower sea floor areas and deposited diamicton and debris flow deposits along the moving grounding line resulting in a progradational sedimentary wedge on the slope at the end of glacial periods coinciding with the retreat of the ice sheets extensive sediment failures affected the continental margin during interglacial periods the ice sheets remained restricted to coastal locations and glacial troughs where processes of meltwater formation might have been significant sediment laden underflows are generated within these troughs from where they flow and spread over the shelf and down the slope to the basinfloor as sediment gravity flow deposits the combined effect of these processes is a progradational build up of the shelf and an aggradational infilling of the basin floor together with the development of the interfingering pattern at the base of the slope

mid late pleistocene glacimarine sedimentary processes of a high latitude deep sea sediment drift antarctic peninsula pacific margin the effects of glaciation on sediment drifts is recognised from marked sedimentary facies variation in deep sea cores taken from the continental rise of the antarctic peninsula pacific margin nineteen sediment cores were visually described logged for magnetic susceptibility and x radiographed about 1000 analyses were performed for grain size clay minerals and biostratigraphy foraminifera nannofossils and diatoms four sediment types associated with distinct sedimentary processes are recognised based on textural compositional analysis 1 hemipelagic mud forms the bulk of the interglacial sediment and accumulated from the pelagic settling of bioclasts and ice rafted wind transported detritus 2 terrigenous mud forms the bulk of the glacial sediment and accumulated from a combination of sedimentary processes including turbidity currents turbid plumes and bottom current reworking of nepheloid layers 3 silty deposits occurring as laminated layers and lenses represent the lateral spillout of low density turbidity currents 4 lastly glacial interglacial gravelly mud layers derive from settling of ice rafted detritus five depositional settings are interpreted within sediment drift 7 each characterised by the dominance interaction of one or several depositional processes the repetitive succession of typical sedimentary facies is inferred to reflect a sequence of four climatic stages glaciation glacial deglaciation and interglacial each one characterised by a distinctive clay mineral assemblage and bioclastic content variations in clay mineral assemblage within interglacial stage 5 core sed 06 suggest minor colder climatic fluctuations possibly correlatable with substages 5a to 5e

glacially influenced deep marine sedimentation of the late precambrian gaskiers formation newfoundland canada the late proterozoic conception group exposed on the avalon peninsula in newfoundland canada is a 4 km thick turbidite succession containing a conformable 300 m thick sequence of diamictites the gaskiers formation near the base massive and crudely stratified diamictites form beds up to 25 m thick which have a tabular geometry with slightly erosive basal contacts and are interbedded with mudstones and fine grained thin bedded turbidites these diamictites are interpreted as submarine debris flow deposits disrupted diamictites form strongly deformed units that contain large complexly folded rafts of mudstone and turbidite facies these diamictite units are interpreted as submarine slumps diamictites contain glacially striated and faceted clasts clasts and matrix are predominantly of volcanic provenance one outcrop shows interbedded volcanic agglomerate and diamictite and volcanic bombs can also be identified the interbedding of diamictites with turbidites and the stratigraphic context provided by the thick sequences of turbidites below mall bay formation and above drook formation indicate a deep marine slope setting of diamictite deposition diamictite facies record remobilization and downslope transfer of large volumes of unstable volcanic and glacial debris initially deposited in a shallower water marginal marine zone the regional tectonic framework suggests the conception group accumulated in a deep southward opening ensialic rift basin with active but waning volcanic centres to the north the gaskiers formation may be representative of other late precambrian glacially influenced diamictite sequences that were deposited around the north atlantic region and in europe these deep marine diamictite sequences characterized by debris flows turbidites and slump deposits can be contrasted with more extensive shallow marine shelf diamictite sequences found in association with dolomites and tidally influenced shallow water facies in other basinal settings

weddell sea abyssal sediments a record of antarctic bottom water flow abstract deep circulation in the weddell sea is a clockwise gyre with bottom water formed by mixing along the southern and western margins most antarctic bottom water originates here leaving the weddell sea to the east depths 4500 m or to the north depths 16 m ma sediments from the deep weddell basin are hemipelagic muds with ash laminae and mainly in the centre of the gyre distal turbidites derived from the southwest thin debris flows occur near seamounts the hemipelagic muds become coarser from the centre to the edge of the gyre winnowing by strong bottom currents is localised along the northern margin of the basin the absence of diatoms from the weddell basin sediments may result from dissolution as well as low productivity sediments from jane basin consist of alternating diatomaceous and barren hemipelagic muds considered to represent interglacial and glacial conditions respectively the diatomaceous sediments contain more silt suggesting that bottom water flow increases during interglacial periods

luminescence dating of the wabar meteorite craters saudi arabia 1 luminescence dating has been used to find the age of meteorite impact craters at wabar al hadida in saudi arabia the luminescence characteristics of the shocked material were determined using a variety of luminescence dating techniques applied to impactite formed by the meteorite and to the underlying sand the age is found to be 290 38 years a comparison is made with two possible historically recorded ages an impact as young as this has implications for the assessment of hazards from the impact on earth of small meteorites

feather features a low shock pressure indicator in quartz 1 feather features are a recently discovered type of microstructure in shocked quartz they occur as short parallel to subparallel lamellae with spacing similar to that of planar deformation features pdfs these lamellae are always found in combination with a planar fracture pf feather features are crystallographically controlled 0001 101 and 112 orientations are common while 103 and 102 orientations are lacking samples from the nordlinger ries and the matt wilson impact structures are compared with plane wave shock recovery experiments 16 gpa microscopic analysis of quartz grains with feather features show that their formation is linked to shearing along the associated pfs during shock deformation the generation of shear fractures with feather features in plane wave shock recovery experiments suggests a pressure range of 7 10 gpa although further constraints are needed for an upper and lower pressure limit the orientation of feather features in rock samples is suggested to be controlled by the direction of the principal axis of stress with sheared pfs at 45 angles to the axis and most feather feature lamellae aligned parallel to the axis 20 the majority of feather feature lamellae emanate from the pfs in the same direction and in experiments point in the direction from which the shock wave came on the basis of their appearance in shocked quartz grains from 26 impact craters and the current lack of reports of these features in endogenically deformed crustal rocks their uniqueness as a new type of impact induced planar microstructure alongside pdfs and pfs is proposed

the sedimentary legacy of a palaeo ice stream on the shelf of the southern bellingshausen sea clues to west antarctic glacial history during the late quaternary a major trough belgica trough eroded by a palaeo ice stream crosses the continental shelf of the southern bellingshausen sea west antarctica and is associated with a trough mouth fan belgica tmf on the adjacent continental slope previous marine geophysical and geological studies investigated the bathymetry and geomorphology of belgica trough and belgica tmf erosional and depositional processes associated with bedform formation and the temporal and spatial changes in clay mineral provenance of subglacial and glaciomarine sediments here we present multi proxy data from sediment cores recovered from the shelf and uppermost slope in the southern bellingshausen sea and reconstruct the ice sheet history since the last glacial maximum lgm in this poorly studied area of west antarctica we combined new data physical properties sedimentary structures geochemical and grain size data with published data shear strength clay mineral assemblages to refine a previous facies classification for the sediments the multi proxy approach allowed us to distinguish four main facies types and to assign them to the following depositional settings 1 subglacial 2 proximal grounding line 3 distal sub ice shelf sub sea ice and 4 seasonal open marine in the seasonal open marine fades we found evidence for episodic current induced winnowing of near seabed sediments on the middle to outer shelf and at the uppermost slope during the late holocene in addition we obtained data on excess pb 210 activity at three core sites and 44 ams c 14 dates from the acid insoluble fraction of organic matter aio and calcareous micro fossils respectively at 12 sites these chronological data enabled us to reconstruct for the first time the timing of the last advance and retreat of the west antarctic ice sheet wais and the antarctic peninsula ice sheet apis in the southern bellingshausen sea we used the down core variability in sediment provenance inferred from clay mineral changes to identify the most reliable aio c 14 ages for ice sheet retreat the palaeo ice stream advanced through belgica trough after similar to 36 0 corrected c 14 ka before present b p it retreated from the outer shelf at similar to 25 5 ka b p the middle shelf at similar to 19 8 ka b p the inner shelf in eltanin bay at similar to 12 3 ka b p and the inner shelf in ronne entrance at similar to 6 3 ka b p the retreat of the wais and apis occurred slowly and stepwise and may still be in progress this dynamical ice sheet behaviour has to be taken into account for the interpretation of recent and the prediction of future mass balance changes in the study area the glacial history of the southern bellingshausen sea is unique when compared to other regions in west antarctica but some open questions regarding its chronology need to be addressed by future work c 2010 elsevier ltd all rights reserved

the circular uneged uul structure east gobi basin mongolia geomorphic and structural evidence for meteorite impact into an unconsolidated coarse clastic target abstract the uneged uul structure is a 10 km circular complex multi ridged domal feature in the unegt subbasin of the east gobi basin southeastern mongolia as revealed by remote sensing and recent field reconnaissance the central part of the uneged uul structure comprises a complex central peak of outward radiating curved ridges composed of stratigraphically uplifted greenschist facies basement schists surrounded by an annular moat the most prominent feature of the structure is a central annular ridge 3 km in diameter composed of pebble boulder conglomerates and gravels of the upper jurassic sharilyn formation surrounded by three outer domal ridges composed of lower cretaceous conglomeratic sandstones and gypsum clays jurassic conglomerates forming the main part of the central annular ridge show effects of severe internal deformation the original population of pebbles cobbles and boulders appears moderately displaced and mostly broken but nowhere aligned along shear planes or foliated primary sedimentary features such as cross lamination or imbrication have been obliterated we explain this penetrative brecciation as a result of dissipative shearing caused by a strong and rapid singular event that in magnitude was beyond the range of the common crustal tectonics recorded elsewhere in this region disrupted and chaotically distributed conglomeratic sandstone beds in the central annular ridge dip in highly variable directions on a local scale but show an apparent se nw trend of bedding plane alignment further outside the tilted and uplifted upper jurassic to lower cretaceous strata of the domal area are overlain by the flat lying upper cretaceous which stratigraphically constrains the timing of deformation at the uneged uul structure to most likely the early cretaceous endogenic formation models such as magmatism and salt gypsum or mud diapirism fail to explain the nature of the uneged uul structure the uneged uul structure bears a set of geomorphic and structural features resembling those at some eroded complex impact structures on earth morphologically similar central peaks are observed at the spider and matt wilson impact structures in australia the central annular ridge reminds of that at gosses bluff in australia the outer domal ridges might correspond to ring like features as known from tin bider in algeria we therefore cautiously propose that an impact may have produced the uneged uul feature causing structural uplift 1000 m of basement rocks at its center so far no convincing evidence for shock metamorphism could be proven by field work and petrographic analyses however it is likely that at the time of the deformation event the unconsolidated conglomerates were highly porous and possibly immersed in groundwater buffering the propagation of sudden stress reducing deformation further studies will be in order to unravel the nature of the uneged uul structure which should be considered a promising possible impact structure

tertiary palaeogeography and tectonostratigraphic evolution of the northern and southern peri tethys platforms and the intermediate domains of the african eurasian convergent plate boundary zone abstract the increasing effects of african eurasian convergence during the tertiary resulted in the uplift and emergence of the northern and southern peri tethys platforms palaeogeographic maps covering six selected time slices including the middle eocene late early oligocene late early miocene early middle miocene early late miocene and middle to late pliocene illustrate that environmental and depositional differentiations on the northern platform and along the bordering domains of the convergence zone were more pronounced than on the southern platform and its adjacent areas the tectonic evolution of the northern platform included an overall eastward directed trend in the onset of basin uplift and emergence which started at about the eocene oligocene transition at 34 ma tectonostratigraphic analyses indicate a striking contemporaneity of the events which defined the temporal and spatial development of both the domains of the convergent plate boundary zone and the bordering platforms five episodes of major regional change in palaeogeographic and tectonic setting are distinguished they occurred in the late eocene 37 34 ma early late oligocene 30 27 ma latest early to earliest middle miocene 17 15 ma early late miocene 9 8 ma and late early to early middle pliocene 4 3 ma these episodes encompassed changes which were most probably induced by geodynamic events primarily related to the relative motions of the african arabian and eurasian plates in turn the plate motions are assumed to have triggered discrete steps in the regional kinematics and geodynamics that governed the palaeogeographic evolution of the peri tethys platforms and the intermediate domains of the african eurasian plate boundary zone

depositional models for the glaciomarine sequences of andean late paleozoic basins of argentina abstract diamictite bearing late paleozoic sequences of probably glacial origin crop out in three basins of western argentina 1 the tarija basin 2 the calingasta uspallata basin and 3 the central patagonian basin the tarija section shows clear evidence of soft sediment deformation in a shallow marine environment slump overfolds and hook like and pull apart structures are common in lenses of well sorted sandstones within a sequence mostly composed of massive matrix supported pebbly mudstones the overall shallowing upward trend of the section suggests alluvial progradation underlying fine grained rocks represent quiet water prodelta deposits the diamictites suggest a delta slope position and the overlying red mudstones with cross bedded sandstones and conglomerates correspond to delta plain deposits in the calingasta uspallata basin the hoyada verde formation represents the best example of an ice contact glaciomarine depositional system with minor gravity flow deposits massive matrix supported sandy diamictites associated with a boulder pavement together with sandstone and conglomerate channels represent grounded ice deposits grading to subglacial flow tills thin bedded matrix supported muddy diamictites with dropstones are the product of a process of gravity resedimentation debris flows and ice rafting pebbly shales represent shelf sedimentation with local deposition from floating ice the ansilta formation shows characteristic features of low and high turbidity current dominated environments basal diamictites resemble debris flow deposits and are associated with thickening upward sequences of bouma tb c turbidites and massive sandstones with fluid scape structures these two sections described above represent the end members of a continuum from glaciomarine rocks to gravity induced deposits formed by subaqueous reworking of glacial debris fine grained rocks associated with diamictites contain marine fossils belonging to the livipustula zone of westphalian age middle carboniferous the central patagonian diamictites appear to be clearly associated with classical turbidites with bouma sequences resedimented conglomerates and high density turbidity current sandstones the conglomerates and sandstones form composite channels with common amalgamation and internal scour surfaces the depositional models proposed for the diamictite bearing sequences of the three basins can be placed in a broader model which embraces several types of complex interactions among glaciomarine nearshore and gravity flow dominated environments

steinheim suevite a first report of melt bearing impactites from the steinheim basin sw germany the 3 8 km steinheim basin in sw germany is a complex impact crater with central uplift hosted by a sequence of triassic to jurassic sedimentary rocks it exhibits a well preserved crater morphology intensely brecciated limestone blocks that form the crater rim as well as distinct shatter cones in limestones in addition an impact breccia mainly composed of middle to upper jurassic limestones marls mudstones and sandstones is known from drilling into the impact crater no impact melt lithologies however have so far been reported from the steinheim basin in samples of the breccia that were taken from the b 26 drill core we discovered small particles up to millimeters in size that are rich in sio2 50 wt and al2o3 28 wt and contain particles of fe ni co sulfides as well as target rock clasts shocked and unshocked quartz feldspar limestone and droplet shaped particles of calcite the particles exhibit distinct flow structures and relicts of schlieren and vesicles from the geochemical composition and the textural properties we interpret these particles as mixed silicate melt fragments widely recrystallized altered and or transformed into hydrous phyllosilicates furthermore we detected schlieren of lechatelierite and recrystallized carbonate melt on the basis of impactite nomenclature the melt bearing impact breccia in the steinheim basin can be denominated as steinheim suevite the geochemical character of the mixed melt particles points to middle jurassic sandstones eisensandstein formation that crop out at the center of the central uplift as the source for the melt fragments

micropaleontological and taphonomic characteristics of mass transport deposits in the northern gulf of eilat aqaba red sea abstract submarine mass transport deposits mtds are a well known phenomenon in tectonically active regions evidence for such deposits is commonly found in the continental slope sedimentary records as distinct units with coarser grain size compared to the usual and continuous pelagic sedimentation the gulf of eilat aqaba is located between the southernmost end of the dead sea transform and the spreading center of the red sea and is considered as an active tectonic region in this study an innovative approach using symbiont bearing larger benthic foraminifera lbf to identify mtds in the gulf of eilat aqaba gea sedimentary record is presented the abundance size and preservation state of lbf shells were analyzed in two radiocarbon dated sediment cores collected at different deposition environments at water depth of 532 m and 316 m the microfaunal and taphonomic results show that the coarse units are characterized by a generally higher numerical abundance of lbf dominated by operculina ammonoides amphistegina papillosa and amphistegina bicirculata these benthic assemblages are found in deeper depths than their original habitat ranging between 50 and 120 m in accordance with their symbionts light requirements in the coarse units lbf 1 mm appear in high frequency up to 161 specimens per g sediment and poorly preserved shells are also abundant containing up to 247 specimens per g sediment in addition these units also contain high numbers of yellowish and blackish colored lbf shells as opposed to null in the non disturbed units and unlike their natural white color the large shell size indicates that high energy is involved in the displacement of the sediments the poor state of preservation also suggests a turbulent flow during transportation which requires a high energy triggering mechanism the color alteration is probably associated with a diagenetic process related to increasing burial time depth also supported by the stratigraphic older ages of the mtds suggesting a long burial before the sediments were displaced in addition according to the dating of the record some units correlate with historical and pre historical earthquakes reinforcing lbf species as a reliable proxy for mass transport events

impact into unconsolidated water rich sediments at the marquez dome texas the marquez dome leon county texas represents a 13 km diameter paleocene eocene impact structure formed in largely unconsolidated sediments in a near shore environment the present study is an analysis of samples from cores taken from boreholes drilled separately on the edge of the central uplift and in the surrounding annular basin the borehole drilled in the annular basin of the structure penetrated a sequence of interbedded sands silts and shales that is typical of the stratigraphy of the surrounding area in contrast the borehole drilled on the edge of the central uplift penetrated material that is relatively homogeneous in chemical composition and texture and may represent a mixture of sand silt clay and minor carbonate derived from deeper levels in the preimpact stratigraphy veins containing pseudotachylitic breccias are not found and are not expected in this environment because low strength target materials are not conducive to frictional melting similarly the low strength and unconsolidated nature of these target materials are not conducive to the formation of other types of typical impact breccias e g melt rocks or suevites the absence of such lithologies results either from explosive ejection of these materials caused by the water rich character of the target sediments or more probably from removal of these materials by deeper postimpact erosion than has been suggested previously planar deformation features pdfs were not found in quartz grains from any of these samples the scarcity of quartz grains with pdfs which have only been reported in rare impact breccias from the central uplift and the large amount of vertical displacement indicated for the central uplift of this structure may also be a consequence of the low strength of target materials

the importance of sediment gravity flow to sediment transport and sorting in a glacial marine environment eastern weddell sea antarctica offshore sediment transport on the eastern weddell sea continental margin cannot be modeled after examples from nonglaciated margins transport of terrigenous material to the marine environment is accomplished by floating or grounded ice instead of fluvial processes there is no coastal zone where sediments are washed as they enter the marine environment and because the average shelf depth is 400 m wind or wave generated currents have little if any influence on sediments texturally immature glacial sediment on the continental shelf and slope remains unsorted except by processes of sediment gravity flow piston core coverage demonstrates that downslope transitions between slumps debris flows and turbidity currents are active on the unstable intercanyon slope areas and on the glacially scoured continental shelf sediment textural analyses indicate that size sorting and mineralogic maturity are achieved within sediment gravity flow transitions and that lithic 40 lithics glacial sediment is transformed into sorted arkosic sand 3 lithics over distances of less than 10 km on the upper continental slope and shelf arkosic or quartz sand thus generated is often interbedded with diamictons which are either glacial or nonglacial slumps and debris flows in origin therefore such associations in ancient sequences do not rule out a glacial origin for diamictites or mixtites nor do they mandate deep water sedimentation canyon systems are at present isolated from major sediment input however an extensive 662 000 km2 abyssal submarine fan complex containing glaciogenic sands implies canyon activity since initial glaciation of the region sediment may have been supplied to canyons in the past by subglacial meltwater discharge at times of maximum ice grounding sand may also have been generated directly at canyon heads by strong contour currents impinging on the shelf break as typically occurs elsewhere around the antarctic margin today this focus of current energy at the shelf break does not exist in the eastern weddell sea today thus given the great depth of the continental shelf eustasy is far less important than glacial and oceanographic conditions that is the position of the ice sheet grounding line relative to the shelf break and the level and intensity of contour currents in regulating sediment supply to submarine canyons in the absence of major canyon activity intercanyon sediment transport appears to be the most significant source of slope and abyssal sands

upper oligocene to lowermost miocene strata of king george island south shetland islands antarctica stratigraphy facies analysis and implications for the glacial history of the antarctic peninsula the cape melville formation cmf exposed on southeastern king george island south shetland islands provides rare evidence of extensive earliest miocene glaciation in the antarctic peninsula region the formation records the presence of regional marine based grounded ice on the continental shelf it overlies disconformably the upper oligocene destruction bay formation which consists of sandstones recording nonglacial shallow marine conditions four units have been identified within the approximately 150 m thickness of the cmf the basal unit a consists of coarse glacigenic debris flow facies interbedded with glaciomarine mudstone and sandstone the overlying unit b is mainly fine grained this succession may represent relatively ice proximal deposition followed by glacial retreat and or relative sea level rise the upper cmf units c and d was deposited in an ice distal marine environment with intermittent input of coarse glacigenic debris mainly from ice rafting thin beds of pelagic carbonate ooze within unit c indicate periods of low terrigenous sediment input and high productivity lithologically diverse glacigenic gravel clasts mainly ice rafted debris in the cmf had a wide regional source area suggesting that ice cover was widespread regionally and included calving ice margins for a small proportion of clasts the nearest known source is the mountains fringing the southern weddell sea such clasts were presumably transported north in debris laden icebergs by a strong cold weddell sea surface current a temperate glacial setting is tentatively inferred from the cmf palynological results confirm and enhance the paleoenvironmental interpretation from the sedimentology and include the first early miocene dinoflagellate cyst assemblages recorded on the antarctic peninsula this reappraisal of the glacial record from the cmf provides valuable constraints on the antarctic cryosphere and regional paleoenvironments in the mid cenozoic

quaternary mud turbidites from the south shetland trench west antarctica recognition and implications for turbidite facies modelling a piston core from the basinal part depth of 5188 m of the south shetland trench west antarctica yielded a terrigenous mud section 11 m long which can be subdivided with great precision into turbidite and hemipelagite layers mud turbidites mean bed thickness 44 cm alternate regularly with and are best distinguishable from their hemipelagite host mean bed thickness 17 cm by the following features i sharp basal contacts ii terrigenous sand free textures except basal well sorted silt laminae and the absence of outsized ice rafted components iii a laminated little to non bioturbated internal structure iv distinct textural and compositional grading and v marked steps on water content and sediment density logs mud turbidites recovered from the south shetland trench differ from an earlier model mud turbidite sequence by their i excessive about six times larger bed thickness ii complex internal organization manifested in multiple repetitions up to four of the same structural interval s in sequential or nonsequential order iii distinctive very fine grained cap of highly porous clay rich in fragments of siliceous biogenics iv widespread zones of penesyndepositional deformation and v evidence of flow reversals these features are interpreted to record deposition from large muddy turbidity currents subjected to flow transformations including soliton and or seiche related reversals induced by ponding and interactions of the flow with the topographical confinements of the trench it is concluded that contained muddy turbidites cannot be adequately modelled using published sequences differentiation of single model and contained mud turbidites offers obvious advantages in basin analysis and in understanding the plethora of turbidity current related depositional mechanisms of deep sea mud

shock metamorphism of quartz in nature and experiment ii significance in geoscience the occurrence of shock metamorphosed quartz is the most common petrographic criterion for the identification of terrestrial impact structures and lithologies its utility is due to its almost ubiquitous occurrence in terrestrial rocks its overall stability and the fact that a variety of shock metamorphic effects occurring over a range of shock pressures have been well documented these shock effects have been generally duplicated in shock recovery experiments and thus serve as shock pressure barometers after reviewing the general character of shock effects in quartz the differences between experimental and natural shock events and their potential effects on the shock metamorphism of quartz are explored the short pulse lengths in experiments may account for the difficulty in synthesizing the high pressure polymorphs coesite and stishovite compared to natural occurrences in addition post shock thermal effects are possible in natural events which can affect shock altered physical properties such as refractive index and cause annealing of shock damage and recrystallization the orientations of planar microstructures however are unaffected by post impact thermal events except if quartz is recrystallized and provide the best natural shock barometer in terms of utility and occurrence the nature of planar microstructures particularly planar deformation features pdfs is discussed in some detail and a scheme of variations in orientations with shock pressure is provided the effect of post impact events on pdfs is generally limited to annealing of the original glass lamellae to produce decorated pdfs resulting from the exsolution of dissolved water during recrystallization basal 0001 pdfs differ from other pdf orientations in that they are multiple mechanical brazil twins which are difficult to detect if not partially annealed and decorated the occurrence and significance of shock metamorphosed quartz and its other phases namely coesite stishovite diaplectic glass and lechatelierite are discussed for terrestrial impact structures in both crystalline non porous and sedimentary porous targets the bulk of past studies have dealt with crystalline targets where variations in recorded shock pressure in quartz have been used to constrain aspects of the cratering process and to estimate crater dimensions at eroded structures in sedimentary targets the effect of pore space results in an inhomogeneous distribution in recorded shock pressure and temperature which requires a different classification scheme for the variation of recorded shock compared to that in crystalline targets this is discussed along with examples of variations in the relative abundances of planar microstructures and their orientations which are attributed to textural variations in sedimentary target rocks examples of the shock metamorphism of quartz in distal ejecta such as at the k t boundary and from nuclear explosions are illustrated and are equivalent to that of known impact structures except with respect to characteristics that are due to long term post shock thermal effects finally the differences between the deformation and phase transformation of quartz by shock and by endogenic tectonic and volcanic processes are discussed we confirm previous conclusions that they are completely dissimilar in character due to the vastly different physical conditions and time scales typical for shock events compared to tectonic and volcanic events well characterized and documented shock effects in quartz are unequivocal indicators of impact in the natural environment

ash shutbah a possible impact structure in saudi arabia we have investigated the ash shutbah circular structure in central saudi arabia 21 degrees 37n 45 degrees 39e using satellite imagery field mapping thin section petrography and x ray diffraction of collected samples the approximately 2 1km sized structure located in flat lying jurassic tuwaiq mountain limestone has been nearly peneplained by erosional processes satellite and structural data show a central area consisting of dhruma formation sandstones with steep bedding and tight folds plunging radially outward open folding occurs in displaced younger tuwaiq mountain limestone formation blocks surrounding the central area but is absent outside the circular structure an approximately 60cm thick unique folded and disrupted orthoquartzitic sandstone marker bed occurring in the central area of the structure is found 140m deeper in undisturbed escarpment outcrops located a few hundred meters west of the structure with exception of a possible concave shatter cone found in the orthoquartzite of the central area other diagnostic shock features are lacking some quartz rich sandstones from the central area show pervasive fracturing of quartz grains with common concussion fractures this deformation was followed by an event of quartz dissolution and calcite precipitation consistent with local sea or groundwater heating the combination of central stratigraphic uplift of 140m concussion features in discolored sandstone outward dipping concentric folds in the central area deformation restricted to the rocks of the ring structure a complex circular structure of 2 1km diameter that appears broadly consistent with what one would expect from an impact structure in sedimentary targets and a possible shatter cone all point to an impact origin of the ash shutbah structure in fact the ash shutbah structure appears to be a textbook example of an eroded complex impact crater located in flat lying sedimentary rocks where the undisturbed stratigraphic section can be studied in escarpment outcrops in the vicinity of the structure less

nature and classification of waterlain glaciogenic sediments exemplified by pleistocene late paleozoic and late precambrian deposits this study of waterlain glaciogenic sediments is designed to present both a review and new information on glaciogenic subaquatic deposits of differing age in a number of localities in north and south america and south africa the late paleozoic glaciogenic deposits of the parana basin in brazil and the karoo basin of south africa are singled out for special attention as they show a reasonably complete lateral sequence of terrestrial to off shore glaciogenic sedimentation although the environment of subaquatic glaciogenic sedimentation varies from one area to the next certain common elements are found which can be used to develop a generalized model for both glaciomarine and glaciolacustrine sedimentation for descriptive purposes the model is divided into two broad categories a shelf facies and a basinal facies the shelf facies is marked by massive diamicton ite which may be 200 m or more in thickness and which is frequently overlain by a complex of clastic sediments consisting primarily of gravity and fluid flows the basinal facies is marked by products of subaquatic slumps and more distal turbidites and glaciomarine sediments new terminology is introduced the massive diamicton ite which is diagnostic of the shelf facies probably represents deposition from the base of active ice in a subaquatic environment and is termed undermelt diamicton ite the gravity and fluid flows which are usually found overlying undermelt diamicton ite and in the basinal facies are subdivided into six categories glaciogenic subaquatic outwash glaciogenic suspension flow glaciogenic chaotic debris flow glaciogenic subaquatic debris flow glaciogenic slurry flow and glaciogenic turbidity flow the relative abundance of undermelt diamicton ite and the various types of gravity and fluid flows can be used to define inner shelf outer shelf inner basin and outer basin facies of glaciomarine sedimentation

glacimarine processes and inductive lithofacies modelling of ice shelf and tidewater glacier sediments based on quaternary examples abstract three variables control the type of glacial debris source and where and when debris is introduced to an ocean 1 type of glacial ice source ice sheet or valley glacier 2 condition of a grounded glacier melting freezing or frozen base and cold or warm internal ice and 3 type of glacier front ice shelf or tidewater using these variables in different combinations eight glacimarine regimes are postulated to exist by integrating information from holocene sediment and pleistocene glacimarine sequences with the eight inductive regimes lithofacies relationships are predicted and modelled the models show a complexity of glacimarine lithofacies the importance of recognizing this complexity lies in realistic interpretations of ancient sequences ice shelf sediment may be limited in the geologic record because of tectonic and climatic constraints required for ice shelf formation alternatively in some tidewater glacier regimes larger volumes of glacimarine sediment are produced than in ice shelf regimes ice proximal sediment is distinctive especially near subglacial meltwater stream effluxes ice shelves and tidewater glaciers may produce similar ice proximal lithofacies e g morainal bank associations and laminites in certain regimes however details of lithofacies may aid in distinguishing ice shelf from tidewater regimes ice shelf and tidewater iceberg zone sediment can be distinguished from each other if basal and high level englacial debris can be distinguished ice shelf zone sediment out from the grounding line has a basal debris source high level englacial debris is deposited in more distal ice shelf zones resulting in sediment by pass in corresponding positions of a tidewater glacier regime iceberg rafted debris comprises mixed basal and high level englacial debris deeper water zones receive large volumes of sediment when a melting freezing base grounding line is at the edge of a continental shelf when the grounding line retreats sediment supply may be drastically reduced because of a back sloping continental shelf and trapping by morainal banks cool water bioclastic carbonates may accumulate once a grounding line has retreated although ice shelf iceberg zones are more extensive than those in tidewater regimes ice shelf icebergs have low debris content and resulting ice rafted debris may be scarce the smallest volume of sediment is produced by a frozen base ice sheet ending as a tidewater front and the largest volume by a melting freezing base valley tidewater glacier glacimarine sediment has a relatively high preservation potential near continental interiors or margins however at continental margins mass flow olistostrome and tectonic melange deposits may be preserved with glacimarine diamictites these deposits may be difficult to distinguish from each other especially if metamorphosed such as at a convergent margin the models presented do not fully represent the glacimarine environment because many other variables are omitted however the principal concepts of generating glacimarine lithofacies associations are considered continued research will verify if each regime exists and will provide data on those regimes that exist today to compare and modify these models

structure and evolution of upheaval dome a pinched off salt diapir upheaval dome canyonlands national park utah is an enigmatic structure previously attributed to underlying salt doming cryptovolcanic explosion fluid escape or meteoritic impact we propose that an overhanging diapir of partly extrusive salt was pinched off from its stem and subsequently eroded many features support this inference especially synsedimentary structures that indicate jurassic growth of the dome over at least 20 m y conversely evidence favoring other hypotheses seems sparse and equivocal in the rim syncline strata were thinned by circumferentially striking low angle extensional faults verging both inward toward the center of the dome and outward near the dome s core radial shortening produced constrictional bulk strain forming an inward verging thrust duplex and tight to isoclinal circumferentially trending folds farther inward circumferential shortening predominated radially trending growth folds and imbricate thrusts pass inward into steep clastic dikes in the dome s core we infer that abortive salt glaciers spread from a passive salt stock during late triassic and early jurassic time during middle jurassic time the allochthonous salt spread into a pancake shaped glacier inferred to be 3 km in diameter diapiric pinch off may have involved inward gravitational collapse of the country rocks which intensely constricted the center of the dome sediments in the axial shear zone beneath the glacier steepened to near vertical the central uplift is inferred to be the toe of the convergent gravity spreading system

terrestrial impact structures impact structures are the dominant landform on planets that have retained portions of their earliest crust the present surface of the earth however has comparatively few recognized impact structures this is due to its relative youthfulness and the dynamic nature of the terrestrial geosphere both of which serve to obscure and remove the impact record although not generally viewed as an important terrestrial as opposed to planetary geologic process the role of impact in earth evolution is now receiving mounting consideration for example large scale impact events may have been responsible for such phenomena as the formation of the earth s moon and certain mass extinctions in the biologic record the importance of the terrestrial impact record is greater than the relatively small number of known structures would indicate impact is a highly transient high energy event it is inherently difficult to study through experimentation because of the problem of scale in addition sophisticated finite element code calculations of impact cratering are gen erally limited to relatively early time phenomena as a result of high com putational costs terrestrial impact structures provide the only ground truth against which computational and experimental results can be com pared these structures provide information on aspects of the third dimen sion the preand postimpact distribution of target lithologies and the nature of the lithologic and mineralogic changes produced by the passage of a shock wave they also provide data for cratering rate estimates and in some cases on the nature of the impacting body in this review emphasis is placed on the nature of terrestrial impact structures the criteria for their identification and their contribution to

the wabar impact craters saudi arabia revisited the very young wabar craters formed by impact of an iron meteorite and are known to the scientific community since 1933 we describe field observations made during a visit to the wabar impact site provide analytical data on the material collected and combine these data with poorly known information discovered during the recovery of the largest meteorites during our visit in march 2008 only two craters philby b and 11m were visible philby a was completely covered by sand mapping of the ejecta field showed that the outcrops are strongly changing over time combining information from different visitors with our own and satellite images we estimate that the large seif dunes over the impact site migrate by approximately 1 0 2 0myr 1 southward shock lithification took place even at the smallest 11m crater but planar fractures pfs and undecorated planar deformation features pdfs as well as coesite and stishovite have only been found in shock lithified material from the two larger craters shock lithified dune sand material shows perfectly preserved sedimentary structures including cross bedding and animal burrows as well as postimpact structures such as open fractures perpendicular to the bedding slickensides and radiating striation resembling shatter cones the composition of all impact melt glasses can be explained as mixtures of aeolian sand and iron meteorite we observed a partial decoupling of fe and ni in the black impact glass probably due to partitioning of ni into unoxidized metal droplets the absence of a ca enriched component demonstrates that the craters did not penetrate the bedrock below the sand sheet which has an estimated thickness of 20 30m less

low angle collision with earth the elliptical impact crater matt wilson northern territory australia nearly all meteorite impact craters on earth are circular however 4 of craters should be formed by impacts at angles lower than 12 from the horizontal which should result in elongated crater structures the crater forming process that produces elliptical shapes is poorly understood we document the first elliptical crater on earth that contains a central uplift and that provides insights into the mechanisms of crater formation at a critical threshold angle of 10 15 the dimensions of the proterozoic matt wilson impact structure northern territory australia are 7 5 by 6 3 km corresponding to an aspect ratio of 1 2 with its long axis trending northeast southwest the exposed crater floor shows a preferred stacking of thrust sheets within the central uplift and in the surrounding syncline indicating northeast southwest shortening and a material transport top to the sw this is consistent with an up range to down range motion of rock caused by remnant horizontal momentum transferred from the impacting projectile to the target this preferential deformation interferes with a radially oriented convergent material flow characteristic for crater collapse the matt wilson crater provides evidence for the usefulness of structural asymmetries as a diagnostic tool to infer impact vectors the new impact crater is confirmed by the presence of planar deformation features planar fractures in quartz grains and its structural inventory

circular geological structures outcropping in the sedimentary basins of saudi arabia abstract a number of circular geological structures outcrop in the sedimentary basins of saudi arabia several are reviewed here to assess their mode of origin they are unrelated to each other are easily seen on aerial imagery and their origins are assessed here on the basis of new fieldwork and reflection seismic data the structures range in size from hundreds of meters to several kilometers in diameter jabal rayah 28 39 n 37 11 30 e is 5 km in diameter with ring faults and anomalously steep dips in siluro devonian strata reflection seismic data demonstrate that underlying strata are structureless and jabal rayah is interpreted as a probable impact structure ash shutbah 21 37 08 n 45 39 21 e is 2 3 km in diameter and consists of a zone of concentric sub horizontal folds in mesozoic carbonates surrounding a central area of disharmonic steeply plunging folds and a mass of stratigraphically anomalous sandstone there are dissolution structures in the vicinity and although this is a viable explanation the uniquely large and complex nature of ash shutbah suggests an alternative origin by impact reflection seismic data show that underlying strata in the vicinity of ash shutbah are structureless a province of at least 100 circular structures occurs in northeast saudi arabia aerial imagery and reflection seismic data shows that these structures are a mixture of bioherms and dissolution collapse features the kidan crater 22 06 53 n 53 26 38 e is a subcircular feature approximately 100 m in diameter exposed in quaternary sabkha between large sand dunes in the rub al khali field investigation reveals an unfaulted bowl with a slightly fractured floor in sabkha facies 3d reflection seismic data shows a small fault underlying the crater leading to a fluid escape interpretation for the depression

the martian dust storm of sol 1742 after nearly five earth years on mars the mutch memorial station viking lander 1 finally witnessed a local dust storm that eroded trenches conical piles and other disturbed surfaces in the sample field and near the lander the event called the dust storm of sol 1742 occurred late in the third winter of lander observations between sols 1728 and 1757 analyses of tiny new wind tails and movement of materials indicate that the eroding winds were variable but more northeasterly than those that had previously shaped the surface pebbly residues and movement of 4 5 mm clods suggest drag velocities or friction speeds of the winds were about 2 2 4 0 m s wind speeds at the height of the meteorology boom 1 6 m were probably about 40 50 m s much of the observed erosion could have occurred in a few to several tens of seconds but somewhat longer times are suggested by analogy with the erosion of terrestrial soils most of the erosion occurred where preexisting equilibrium conditions of surface configurations and surface material properties had been altered by the lander during landing and during surface sampler activities but thin layers of bright fine grained dust were also removed and redistributed surfaces where preexisting equilibrium conditions were unaltered appeared to be uneroded

precambrian to jurassic rocks of arabian gulf and adjacent areas their facies depositional setting and hydrocarbon habitat the first sediments to onlap the metamorphosed precambrian arabian shield were infracambrian proterozoic to middle cambrian carbonates clastics and evaporites the oldest arabian reservoir rocks occur in the precambrian to lower paleozoic ara salt of the huqf group which forms the birba field of oman the middle cambrian sequence was followed by late cambrian through early permian marine sandstones and continental to littoral siltstones and variegated shales the first commercial oil discovered in the arabian gulf region occurs in fluvial sands of the ordovician to permian haima and haushi groups of the marmul field in south oman these strata are also productive in other fields and are sealed by unconformable contact with the al khlata formation or beneath shale of he albian nahr umr formation the deeply buried kerogen sediments of the huqf group to the southeast are believed to be the source rocks for these fields of south oman the late permian to triassic deposits of the arabian peninsula are mainly widespread carbonates and evaporites that were deposited during a period of relative tectonic stability their deposition on an epeiric shelf was punctuated by a series of transgressions and regressions significant gas reserves have been proven in deep wells in the arabian gulf these wells penetrate large deep structures in the permian khuff shelf carbonates these carbonates have developed secondary porosity and lie beneath interbedded shale and dolomites of the sudair or suwei formation the source of gas in the khuff is unknown but could lie in more deeply buried formations the large deep structures of the khuff are considered to be among the most attractive for gas potential in the region today the stable shelf depositional environment established during the permian and triassic continued through the mesozoic jurassic rock units of the arabian gulf formed a sedimentary megacycle of epeiric shelf carbonates these include relatively deep water mudstones and wackestones to shallow water grainstones and packstones with the greatest facies variations occurring in the post dhruma to arab d rocks some of the best and most extensive reservoirs of the area occur in the primary and secondary porosity of the upper jurassic arab formation and in the middle jurassic araej and dhruma formations of saudi arabia qatar and abu dhabi the sources for hydrocarbons in these jurassic sediments are thought to be in the dhruma and lower hanifa formation diyab equivalent of the gulf area n general the jubailah is a source in qatar and some of the offshore gulf the most common seals are the argillaceous carbonate at the top of the dhruma and the hith anhydrite the latter also marking the termination of jurassic deposition across the arabian platform the upper jurassic cycles of marine limestones and anhydrites of the arab and hith formations accumulated in response to changes of sea level caused either by regional epeirogenic movements or by eustasy

yallalie a buried structure of possible impact origin in the perth basin western australia an enigmatic buried structure located in mesozoic sedimentary rocks in the perth basin western australia was discovered in 1990 by ampol exploration the basin like yallalie structure centred on 30 26 40 3 s 115 46 16 4 e is circular in plan view and about 12 km in diameter high resolution seismic reflection profiles across the structure show a basin shaped area of chaotic reflec tions that extend to a depth of approximately 2 km below the surface the structure has sharp boundaries with surrounding faulted but otherwise relatively undisturbed rocks in the centre of the structure there is an uplifted area approximately 3 4 km across similar to those described from com plex meteorite impact structures the seismically defined structure coincides with a circular topo graphic depression and image processing of digital elevation data has allowed recognition of concentric and radial structures extending as far as 40 km from the centre of the depression gravity surveys show the structure to be associated with a positive gravity anomaly of about 30 gu aeromagnetic surveys have defined annular anomalies associated with the central uplifted section and possibly margins of the structure a search for siderophile element enrichments by neutron activation analysis in the rocks of the structure which would indicate the presence of a meteorite component proved negative quartz grains in cores that penetrate the structure show the development of prismatic cleavage fractures and irregular slightly curved planes formed by brittle fracture an allochthonous breccia of late cretaceous rocks occurs a few kilometres west of the western margin of the structure quartz grains from a thin veneer of tertiary sediments that drape the structure are essentially unde formed however multiple sets of closely spaced planar deformation features in quartz characteristic of highly shocked rocks have yet to be observed in the rocks of the yallalie structure and the allochthonous breccia the morphology of the yallalie structure determined from topographic and geophysical data suggests strongly that it is of impact origin geological and geochemical evidence is equivocal but is not inconsistent with this interpretation

the first memin shock recovery experiments at low shock pressure 5 12 5 gpa with dry porous sandstone as part of the memin research program this project is focused on shock deformation experimentally generated in dry porous seeberger sandstone in the low shock pressure range from 5 to 12 5 gpa special attention is paid to the influence of porosity on progressive shock metamorphism shock recovery experiments were carried out with a high explosive set up that generates a planar shock wave and using the shock impedance method cylinders of sandstone of average grain size of 0 17 mm and porosity of about 19 vol and containing some 96 wt sio2 were shock deformed shock effects induced with increasing shock pressure include 1 already at 5 gpa the entire pore space is closed quartz grains show undulatory extinction on average 134 fractures per mm are observed dark vesicular melt glass of the composition of the montmorillonitic phyllosilicate component of this sandstone occurs at an average amount of 1 6 vol 2 at 7 5 gpa quartz grains show weak but prominent mosaicism and the number of fractures increases to 171 per millimeter two additional kinds of melt both based on phyllosilicate precursor could be observed a light colored vesicular melt and a melt containing large iron particles the total amount of melt all types increased in this experiment to 2 4 vol raman spectroscopy confirmed the presence of shock deformed quartz grains near the surface 3 at 10 and 12 5 gpa quartz grains also show weak but prominent mosaicism the number of fractures per mm has reached a plateau value of approximately 200 and the total amount of the different melt types has increased to 4 8 vol diaplectic quartz glass could be observed locally near the impacted surface in addition local shock effects most likely caused by multiple shock wave reflections at sandstone container interfaces occur throughout the sample cylinders and include locally enhanced formation of pdf as well as shear zones associated with cataclastic microbreccia diaplectic quartz glass and sio2 melt overall findings from these first experiments have demonstrated that characteristic shock effects diagnostic for the confirmation of impact structures and suitable for shock pressure calibration are rare so far they are restricted to the limited formation of pdf and diaplectic quartz glass at shock pressures of 10 gpa and above

diaplectic quartz glass and sio2 melt experimentally generated at only 5 gpa shock pressure in porous sandstone laboratory observations and meso scale numerical modeling abstract a combination of shock recovery experiments and numerical modeling of shock deformation in the low pressure range from 2 5 to 17 5 gpa in dry porous seeberger sandstone provides new significant insights with respect to the heterogeneous nature of shock distribution in such important upper crustal material for which to date no pressure calibrated scheme for shock metamorphism exists we found that pores are already completely closed at 2 5 gpa shock pressure whole quartz grains or parts of them are transformed to diaplectic quartz glass and or sio 2 melt starting already at 5 gpa whereas these effects are not observed below shock pressures of 30 35 and 45 gpa respectively in shock experiments with quartz single crystals the appearance of diaplectic glass or melt is not restricted to the zone directly below the impacted surface but is related to the occurrence of pores in a much broader zone the combined amount of these phases increases distinctly with increasing shock pressure from 0 03 vol at 5 gpa to 80 vol at 17 5 gpa in accordance with a previous shock classification for silica phases in naturally shocked coconino sandstone from meteor crater that was based on varied slopes of the coconino sandstone hugoniot curve our observations allow us to construct a shock pressure classification for porous sandstone consistent with shock stages 1b 4 of the progressive shock metamorphism classification of kieffer 1971 numerical modeling at the meso scale provides the explanation for the discrepancy of shock deformation in porous material and single crystal quartz in keeping with our experimental results it confirms that pore space is completely collapsed at low nominal pressure and demonstrates that pore space collapse results in localized pressure amplification that can exceed 4 times the initial pressure this provides an explanation for the formation of diaplectic quartz glass and lechatelierite as observed in the low shock pressure experiments the numerical models predict an amount of sio 2 melt similar to that observed in the shock experiments this also shows that numerical models are essential to provide information beyond experimental capabilities