slope processes in weathered volcaniclastic deposits within the city of naples the camaldoli hill case we describe the slope processes acting on camaldoli hill the main volcanic feature of the neapolitan area whose geological evolution and setting have been reconstructed the backbone of the hill includes the remnants of two partially superposed tuff cones lying between the campanian ignimbrite ci and the neapolitan yellow tuff nyt this sequence is mantled by pyroclastic anthropogenic and epiclastic deposits with abrupt thickness and facies variations the structural setting of the hill mainly results from several phases of reactivation of the ci caldera faults which were active until about 9 5 ka deformation younger than 15 ka is evidenced by landslide deposits caused by slope instability from volcano tectonism and by a high angle erosional unconformity formed in response to a base level lowering a stratigraphic analysis of the reworked deposits at the foot of the slopes allowed us to define both depositional mechanisms and sedimentation rates the results of combined volcanological geomorphological and engineering geological studies permitted us to constrain and quantify past geological processes and hypothesis about the future evolution of the hill present day slope processes on camaldoli hill are largely controlled by the presence of weathered and reworked deposits whose nature and thickness have been analysed and mapped in detail four main kinds of slope processes have been recognized falls and toppling failures from nyt small scale slides in the weathered and pedogenized loose cover mixed events represented by slides evolving to hyperconcentrated flows mud flows and debris flows and areal and linear erosion consequently a high number of mass movements not previously documented have been mapped at the same time an insight into the sedimentation rate due to the overall slope processes covering a time span of about 5 ka was given some final considerations regarding landslide hazard are presented in the context of the most suitable remedial works

diamond bearing upper karoo fluvial sediments in ne swaziland upper karoo sediments middle to upper triassic in ne swaziland consist of an upward fining fluvially dominated sequence molteno and elliot formations followed by the mainly aeolian clarens formation diamonds and associated kimberlitic garnets are strata bound in two thin upward fining sequences 4 16 m at the base of the elliot formation these sequences comprise erosionally based clast supported conglomerate channel fill overlain by coarse feldspathic cross stratified sandstone and rippled siltstone with gritty layers diamonds and garnets occur in the matrix of the conglomerate but are five times more abundant in the coarse feldspathic sandstone they also occur in the sandy mode in the overlying siltstone but in greatly diluted concentrations the diamonds were locally derived and released to the sedimentary system following a climatic change when conditions favoured more intense erosion of nearby kimberlite pipes diamonds and garnets released to the passing sediment were concentrated locally in thin

a idealized framework for the analysis of cohesive soils undergoing desiccation the authors wish to thank the discussers for their interest in the work on crack formation in clays during drying the model presented by the authors holds for soils with a high clay content in which desiccation cracks form while the soil is still saturated this was the case for the saint alban clay for which cracks appeared for a suction of about 23 kpa 18 hours after the soil surface was exposed to evaporation only the onset of cracking was modelled and crack depth is approximated with linear elastic fracture mechanics in mode i i e opening of the crack further crack propagation occurs with the ongoing evaporation leading eventually to the formation of secondary and even tertiary cracks if the soil is restrained in all directions furthermore these cracks depending on soil type may be initiated in an unsaturated soil hence we agree that the model must then also consider unsaturated soil mechanics owing to differential behaviour between a relatively dryer surface layer and soil at a higher moisture content with increasing depth secondary crack propagation may also be controlled in some cases by shear failure at the crack tip as stated by the discussers the cracking problem is complex and the subject received little attention so far

braidplain deposition of the upper triassic molteno formation in the main karoo gondwana basin south africa abstract the upper triassic carnian molteno formation in the main karoo gondwana basin south africa forms a northerly thinning intracratonic clastic wedge comprising sandstones shales and coals occurring within thick up to 140 m laterally persistent fining upward sequences these sequences were deposited by braided streams draining an alluvial plain which may have been built on to the distal slopes of alluvial fan complexes of glacial outwash type geometric relations between sequences indicate three phases of tectonic activity the lowermost fining upward sequence in the south accumulated against a rising mountain front cessation of movement and an eastward shift in the main locus of tectonism and sedimentation was followed by renewed uplift and basinwide progradation of the second fining upward sequence adjacent to a fault block granite terrain located close to the present south east coastline of south africa this is believed to be the granite at the eastern end of the falkland island plateau an interpretation consistent with its position on most continental reconstructions and the fracture zone marking its northern scarp face faulting is attributed to the first phase of extension prior to continental breakup the sourceward recession and lack of gross fining upward trends shown by the uppermost fining upward sequences is accounted for by limited back faulting of the still active basin margin cessation of activity and further basin margin recession occurred with deposition of the overlying floodplain deposits elliot formation which were distal equivalents of the braided alluvial plain

dynamic processes from plate subduction to intracontinental deformation insights from the tectono sedimentary evolution of the zhaosu tekesi depression in the southwestern chinese tianshan abstract the zhaosu tekesi depression located to the northern margin of the south tianshan orogenic belt stb is an ideal region for investigating paleozoic orogenic processes and later modifications of the southwestern chinese tianshan in this study we employ 2d seismic and borehole data revealing the geological structure and tectonic evolution of the zhaosu tekesi basin and we report new zircon u pb geochronological and hf isotopic data and whole rock elemental and sr nd isotopic analytical results of the late silurian magmatic rocks as well as the whole rock trace element data of the early carboniferous carbonate rocks borehole data wells zc1 and zc2 and seismic imaging suggest that the zhaosu tekesi depression is composed of silurian lower carboniferous permian jurassic upper cretaceous and paleogene strata with six unconformity surfaces c 1 a s 3 p 1 c 1 a p 2 t p 1 j p 2 t k 2 d j and e k 2 d the silurian strata exhibit southward thrust structures and consist of basalt andesite dacite tuff minor muddy limestone and diorite intrusions zircon shrimp u pb dating for the well zc1 diorites yield a crystallization age of 422 ma with positive e hf t values 3 3 10 6 the diorites and andesites are calc alkaline and show enrichment in liles such as th and u and depletion in nb ta and ti relatively high initial 87 sr 86 sr ratios 0 703376 0 706737 and low e nd t values 0 5 to 1 4 and high th yb 0 29 0 35 and ta yb 1 31 1 58 ratios indicating their formation in an active continental margin setting the lower carboniferous strata mainly comprise oolitic limestone bioclastic micritic limestone and interbedded calcareous siltstone high sr ba 1 31 2 33 v sc 10 4 18 6 and low th u 0 01 0 23 ratios suggest that they were potentially deposited in the mixing near shore marine environment these results in combination with previous data indicate that the southwestern chinese tianshan is controlled by northward subduction of the south tianshan ocean during the late silurian to early carboniferous the presence of the lower permian terrigenous strata and rift basin structure constrains the timing of collision between the ctb kyb and the tarim craton as the late carboniferous thrusting reactivation and syn tectonic growth strata in the basin suggest that the southwestern chinese tianshan experienced late cretaceous compression and neogene modifications after the late permian post faulting thermal subsidence triassic peneplanation and jurassic regional subsidence intracontinental subduction due to distant effect is the main tectonic control on the miocene rapid uplift of the tianshan the zhaosu tekesi basin evolution sheds light on dynamic processes of the southwestern chinese tianshan from plate subduction to intracontinental deformation

microbes play an important role in the alteration of oceanic crust microbes have been identified within altered parts of the glass rims of pillow lavas in the upper oceanic crust 237 m below the top of the volcanic basement of odp hole 896a at the costa rica rift their presence is verified by spherical and vermicular microbodies containing dna the elemental composition of the microbially processed areas differ from the parent glass further the microbially processed parts showing different morphological forms also show different elemental composition extreme k enrichment indicate that microbes are presently active in the alteration process the carbon isotopic composition of disseminated carbonates within the basaltic section of the hole 504b 1 km distance from hole 896a also give evidence for microbial activity during rock alteration the 13c values of most of these trace carbonates are very low reflecting metabolic control of the carbon cycle in these rocks microbial alteration of basaltic glass comprising a substantial volume and surface area of the upper oceanic crust may thus play an important role in the element exchange between oceanic crust and seawater

stromatolites the challenge of a term in space and time abstract several definitions of stromatolites are discussed and kalkowsky s most important statements about stromatolites are translated from german to english a new definition for stromatolites is proposed namely stromatolites are laminated rocks the origin of which can clearly be related to the activity of microbial communities which by their morphology physiology and arrangement in space and time interact with the physical and chemical environment to produce a laminated pattern which is retained in the final rock structure unconsolidated laminated systems clearly related to the activity of microbial communities and often called recent stromatolites or living stromatolites are defined as potential stromatolites the main microbial activities which are important in the formation of potential stromatolites and stromatolites are described and examples of stromatolites of various types including iron stromatolites are also described

evolution of polygonal patterns in stratified mud during desiccation the role of flaw distribution and layer boundaries detailed analysis of diagnostic surface morphologies of fractures in natural mud indicates that mud cracks systematically nucleate at the bottom of the mud and propagate vertically upward toward the free surface and laterally outward toward adjacent cracks earlier generations of mud cracks rupture the set of desiccated layers altogether forming polygonal patterns that are similar throughout the mud sequence later generations of mud cracks subdivide each layer separately forming markedly different polygonal patterns within individual mud layers a simple mechanical model draws an analog between cooling of granular materials and drying of mud it shows that subfact defects at grain boundaries near the base may become critical before defects associated with fine particles at the top even though the stress profile due to drying is usually more tensile at the top thin section analysis and sieving method indicate that grain boundaries at the bottom are several times longer than at the top of the mud due to the natural sorting of grains illustrating the validity of the model this study suggests that stress variations with depth are less important for pattern evolution of mud cracks than previously theorized however nonuniform flaw distribution and surface discontinuities play a fundamental role during mud crack nucleation and growth these results have significance for several subjects including experimental studies of cracking induced by desiccation the previously suggested analogy between natural mud fracturing and basalt fracturing and the use of mud cracks in stratigraphic interpretation

mapping of elastic properties of gas hydrates in the carolina trough by waveform inversion gas hydrates are frozen methane gas that forms at appropriate pressure and temperature conditions they are found in the marine sediments along continental margins worldwide they have the economic potential of being tapped as a fuel source and also have the potential as a greenhouse agent after being freed into the atmosphere in seismic sections the occurrence of the base of gas hydrates in some areas is often marked by a bright amplitude reflection such reflections follow the sea floor topography and are called bottom simulating reflectors bsr the bsrs have negative polarity with respect to the sea floor reflection and in a common shot or a cdp gather the amplitude increases with offset the negative impedance contrast causing bsrs may be due to negative velocity contrast between hydrated sediments and normal sediment below or due to the presence of free gas at the base of the hydrates in this paper we carry out a prestack seismic waveform inversion of multichannel seismic data collected in

rock cities and ruiniform relief forms processes terminology abstract spectacular rock cut landscapes consisting of closely spaced residual rock blocks separated by narrow intersecting corridors are variously termed as rock cities and ruiniform relief the former term has apparently never been formally defined except in the local czech literature whereas the latter was applied in various contexts making the terminology confusing this paper provides a global overview of forms processes and controls behind such rock city ruiniform landscapes and develops geomorphological terminology for such rock cut landscapes including the proposal that rock city becomes a legitimate scientific term rock cities and ruiniform relief are two different types of rock erosional morphology typified by the presence of a street pattern in the former and highly weathered surfaces of rock residuals in the latter although there is some overlap between the two examples of rock cities or ruiniform relief may be found in different lithologies and in a wide range of climates from hyper arid to humid classic localities include northern czechia and adjacent parts of poland and germany australia central sahara and the gran sabana in venezuela lithologies particularly suitable to the development of this type of erosional morphology include mainly sandstones followed by limestones dolomites conglomerates tuffs but igneous rocks such as granites or rhyolites are also known to support this type of relief the combination of sufficient rock strength to support steep rock faces and the presence of discontinuities which preferentially eroded turn to streets and squares in the rock cities appears as the basic prerequisite a wide range of processes are involved in their formation including surface and subsurface weathering mass movements salt tectonics karst processes in both carbonate and silicate rocks piping fluvial erosion and aeolian abrasion despite being spectacular landscapes very few models of long term evolution of rock city and ruiniform relief have been presented in the literature and it is unlikely that any one scenario would apply to all situations rather rock cities are examples of geomorphological convergence

origin and evolution of polygonal cracks in hydrous sulphate sands white sands national monument new mexico the nucleation and propagation of polygonal cracks in hydrous sulphate dunes at white sands national monument new mexico are affected by water availability transport through the sand and exchange with the atmosphere these gypsum sands are cohesive enough to crack due to capillary forces and gypsum cements formed during evaporation surface cracks form five sided polygons with a variety of triple junction angles cracks extend down to variable depths and polygons increase in size with depth showing a maturation similar to experimentally produced polygonal columns in drying corn starch results from two years of monitoring crack geometries temperature and humidity demonstrate that cracks form when water is lost to the atmosphere through the transport of water vapour subsurface relative humidity below 5 to 10 cm is almost always maintained at 100 by the evaporation and condensation of water in thin films on grains the amplitude of daily temperature and thus absolute humidity changes decreases with depth consistent with lower evaporation and condensation rates with increasing depth changes in absolute humidity and the contrast between humidity in pore spaces versus the overlying atmosphere result in significant water loss from the dunes except during times of precipitation and frost dew condensation this water loss allows cracks to nucleate and propagate into the dunes this study hypothesizes that crack tips propagate into sand to the depth at which thin films of water on grains are drying and that this depth varies from the surface of the dune during precipitation events to depths greater than 45 cm when dunes are drier

cracks and fins in sulfate sand evidence for recent mineral atmospheric water cycling in meridiani planum outcrops gypsum dunes at white sands national monument new mexico provide an excellent analog to sulfate rich eolian outcrops on meridiani planum mars as characterized by the rover opportunity numerous outcrops imaged by opportunity contain polygonal cracks that crosscut bedding and extend across most surfaces of boulders some of these cracks are associated with millimeter thick platy fins that protrude as much as a few centimeters above outcrops similar cracks crosscut bedding at white sands on erosional stoss slopes and in pedestals of cemented dune sand the cracks at white sands form in response to cementation of damp gypsum sand followed by contraction due to dehydration during warm seasons cracked sand is dry and cracks rarely grow although some may form after rainstorms two types of fins form along cracks and differentially eroded laminae white fins represent subsurface differential cementation along cracks followed by differential erosion tan fins form due to cementation of crack edges and laminae on exposed dune surfaces wind blown sediment adheres to damp tan fins similar processes may be important for crack and fin formation in meridiani planum outcrops implying recent water cycling between sulfate outcrops and the martian atmosphere

desiccation cracks in siliciclastic deposits microbial mat related compared to abiotic sedimentary origin abstract siliciclastic sediment colonized by microbial mats yield a set of distinct sedimentary fabrics that are collectively called mat related structures mrs in the rock record versatile cracks are observed in biostabilized strata but the mechanisms responsible for their formation remain debated microbially stabilized sediments produce desiccation cracks that serve as modern analogs for fossil microbial cracks however since both microbial mat shrinkage and clay shrinkage may contribute to the formation of these desiccation cracks it is difficult to isolate the influence of the microbial mat on the resulting crack formation distribution and morphology to address this issue we conducted a series of desiccation experiments that determine differences between microbially influenced desiccation cracks i e biotic and those formed in identical but sterilized i e abiotic siliciclastic sediment three sediment mixtures were used 1 very fine sized sand 2 mixed ungraded silt clay and 3 normally graded silt clay in all of the experiments the water rich microbial mat contracted substantially while drying producing isolated pockets of shallow but wide cracks the distribution of which was controlled by heterogeneities in the mat structure and thickness variations of the mat in the clay poor substratum the microbial mat was the only crack forming mechanism while in the clay rich substrata experiments 2 and 3 desiccation cracks were more strongly influenced by clay shrinkage the abiotic clay rich sediment produced a polygonal network of deep cracks intersecting at 90 120o junctions in the biotic clay rich experiments the microbial mat modified these desiccation features by withstanding crack propagation or by producing curled up crack polygon margins even though a microbial mat shrinks substantially with desiccation its cohesive nature and heterogeneous distribution prevents the formation of a regular crack network but its shallow penetration into the sediment limits its influence on cracking the biotic crack formation is best characterized by a heterogeneous distribution of wide but shallow pockets of radiating cracks that do not connect to one another while abiotic cracking is best characterized by an orthogonal network of deep intersecting cracks by comparing biotic and abiotic shrinkage cracks formed in identical sediment we improve our understanding of modern desiccation features and thus strengthen our interpretation of the rock record

evoluton of polygonal fracture patterns in lava flows cooling induced fractures also known as columnar joints divide basaltic lava flows into prismatic columns with polygonal cross sections the regularity and symmetry of the fracture patterns have long fascinated naturalists in view of the recent selection of two candidate nuclear waste sites in areas where polygonally fractured volcanic rocks are located a better understanding of the fracture patterns is required field data indicate that the tetragonal networks at flow surfaces evolve systematically to hexagonal networks as the joints grow inward during solidification of lava this evolution occurs by the gradual change of most orthogonal intersections to nonorthogonal intersections of about 120 degrees the surface features and intersection geometries of columnar joints show that joint segments at any given level form sequentially yet harmoniously

the holocene history of the white sands dune field and influences on eolian deflation and playa lakes abstract white sands national monument is the world s largest gypsum dune field the dunes are downwind of a 20 m deep 19 km wide deflation basin containing large playa lakes today the gypsum sand is derived from the edge of the deflation basin next to the dune field rather than the alkali flat and playa lakes where gypsum crystals are forming three erosional shorelines mark wetter episodes when playa lakes formed in the deflation basin successively younger shorelines formed within older ones as the basin deepened but was not widened by deflation the modern shoreline is forming around lake lucero playa the oldest shoreline termed l1 is degraded and formed near the pleistocene holocene transition deflation from the l1 to the l2 shoreline cut through pleistocene bedded evaporites and probably marks initiation of the dune field this event was before 5840 yr bp based on radiocarbon dates this reinforces an evolving consensus that episodes of deflation have characterized desert basins in the southwestern united states regional deflation events have been dated at 7000 and 4000 yr bp the shorelines imply the white sands dune field was created in short episodes and the modern dune field may not represent conditions during expansion of the dune sea

desiccation of a sensitive clay field experimental observations an excavation with three different levels was exposed to continuous evaporation during 35 days in order to observe and document the formation of shrinkage cracks in an intact and weathered sensitive marine clay desiccation of the intact saint alban clay under restrained conditions at an average evaporation rate of 0 018 cm h and an initial moisture content of 103 produced visible primary cracks after 17 h with an average spacing of 20 to 24 cm secondary cracks within each original polygon appeared after about 70 h of evaporation subhorizontal cracks at a depth of 6 to 8 cm were also created owing to differential strains induced by differential shrinkage of the soil polygons further evaporation resulted in the formation of a new set of vertical cracks after about 150 h of evaporation in the soil below the subhorizontal shear plane the shrinkage induced volume change associated with these different sets of cracks produced polygons with a protuberance at their bottom part indicating that the depth of crack propagation below the subhorizontal shear plane was about 3 to 4 cm in weathered clay at an initial moisture content of about 50 the average spacing was 10 to 12 cm indicating that spacing between primary cracks is related to soil type in the cohesionless topsoil no cracks were observed

structural characteristics of columnar jointed basalt in drainage tunnel of baihetan hydropower station and its influence on the behavior of p wave anisotropy abstract researches about the structural characteristics of rock mass are the basis for understanding their failure modes mechanism properties as well as evaluation of structural stability during the process of excavation natural columnar jointed basalt has complex structures and exhibits remarkable anisotropy and size effect in this study the observation window method was firstly applied to describe the natural columnar joints and pillar shapes at engineering scale and then the structural characteristics of columnar joints were analyzed the roughness of columnar joints scanned from large size rock core obtained at drainage tunnel 1 1 was secondly investigated based on the 3d scanning method some techniques such as field statistical analysis and ct scanning were used to analyse the characteristics of internal implicit joints in columnar jointed basalt thirdly the large scale columnar jointed basalt cores were tested by p wave which assisted to investigating the anisotropy of columnar jointed basalt the experimental and analytical results indicate that at the engineering scale natural columnar jointed basalt is heterogeneous exhibiting the features of mosaic structure and mainly quadrilateral in shape at the large size rock core scale the roughness of columnar joints is low with the value of 5 558 at the basalt column scale the internal implicit joints are not randomly distributed instead showing obvious tendency the internal implicit joints and columnar joints are important factors contributing to the special failure mode of columnar jointed basalt at the rock core scale with increasing angle between the p wave propagation direction and column the anisotropy of columnar jointed basalt core gradually increases and the degree of anisotropy is proportional to the number of columnar joints the internal implicit joints could also affect the p wave anisotropy of basalt column and the results in this study can provide some references for projects related to columnar jointed rock mass

eastern mississippi delta late wisconsin unconformity overlying transgressive facies sea level and subsidence abstract analysis of 92 engineering core logs located in the balize sector of the eastern mississippi delta focuses on the late wisconsin unconformity and lithofacies of strata lying immediately below and above this stratigraphic horizon this major sequence boundary is a key feature used to correlate strata across shelf and slope to the basin observations emphasize lithofacies distributions of the latest pleistocene sediments underlying the unconformity the late pleistocene to early holocene transgressive facies and the immediately overlying deltaic deposits maps and a cross section compiled with this information highlight the critical relation between lithofacies distributions and late wisconsin sea level oscillations core analysis reveals that the transgressive facies comprises distinct environments of deposition offshore to onshore findings identify criteria to determine the approximate position of the shoreline at the late wisconsin maximum sea level lowstand and at the extent of early holocene maximum marine inundation in the balize complex mapping of lithologies along the late wisconsin unconformity serves to improve correlation of sediment facies with changes in acoustic response along high resolution seismic profiles an estimate of long term averaged land subsidence suggests lowering of at least 1 mm year near the shelfedge during the past 18 000 years this mapping also refines sea level response models for the mississippi delta including criteria to locate key paleogeographic features such as shelfedges and maximum flooding surfaces in other recent marine deltas and in older stacked deltaic sequences

origin of polygonal fractures in sand uppermost navajo and page sandstones page arizona abstract polygonal fractures in eolian sandstones occur along the upper surface of the navajo sandstone and along four additional surfaces within the page sandstone near page arizona the polygons are largely rectangular and range from 1 to 6 m across most of the fractures are less than 1 5 m deep and fracture widths range from hairlines to tens of centimeters or more fracture fill most commonly shows vertical layering and in some cases a series of layers is evident symmetrical about the midline of the fracture other fractures are filled with contorted or structureless deposits subhorizontal laminae and displaced or brecciated host strata in some cases surfaces marked by polygonal fractures can be shown to have been topographically low at the time of deposition and the fracture sandstone below these low areas contains calcite or siliceous nodules and siliceous cement interpreted as contractional in origin the polygonal fractures formed on an exposed evaporite encrusted surface in a manner similar to ice wedges irregularities in these fractures are attributed to erosion by wind or water evaporites evidenced by anhydrite inclusions pore shapes megaquartz replacement and pseudomorphs of evaporitic minerals provided the cohesion and contraction that caused the tensional stresses necessary for polygonal fracturing stresses were the result of thermal contraction such surfaces are thought to represent hiatuses in eolian sand sea development and probably represent deflation to near the water table during net deflationary conditions

assessment of crustal velocity models using seismic refraction and reflection tomography two tomographic methods for assessing velocity models obtained from wide angle seismic traveltime data are presented through four case studies the modelling inversion of wide angle traveltimes usually involves some aspects that are quite subjective for example 1 identifying and including later phases that are often difficult to pick within the seismic coda 2 assigning specific layers to arrivals 3 incorporating pre conceived structure not specifically required by the data and 4 selecting a model parametrization these steps are applied to maximize model constraint and minimize model non uniqueness however these steps may cause the overall approach to appear ad hoc and thereby diminish the credibility of the final model the effect of these subjective choices can largely be addressed by estimating the minimum model structure required by the least subjective portion of the wide angle data set the first arrival times for data sets with moho reflections the tomographic velocity model can be used to invert the pmp times for a minimum structure moho in this way crustal velocity and moho models can be obtained that require the least amount of subjective input and the model structure that is required by the wide angle data with a high degree of certainty can be differentiated from structure that is merely consistent with the data the tomographic models are not intended to supersede the preferred models since the latter model is typically better resolved and more interpretable this form of tomographic assessment is intended to lend credibility to model features common to the tomographic and preferred models four case studies are presented in which a preferred model was derived using one or more of the subjective steps described above this was followed by conventional first arrival and reflection traveltime tomography using a finely gridded model parametrization to derive smooth minimum structure models the case studies are from the se canadian cordillera across the rocky mountain trench central india across the narmada son lineament the iberia margin across the galicia bank and the central chilean margin across the valparaiso basin and a subducting seamount these case studies span the range of modern wide angle experiments and data sets in terms of shot receiver spacing marine and land acquisition lateral heterogeneity of the study area and availability of wide angle reflections and coincident near vertical reflection data the results are surprising given the amount of structure in the smooth tomographically derived models that is consistent with the more subjectively derived models the results show that exploiting the complementary nature of the subjective and tomographic approaches is an effective strategy for the analysis of wide angle traveltime data

origin of bottom simulating reflectors geophysical evidence from the cascadia accretionary prism vertical seismic profile vsp data from two drill sites on the cascadia margin show low velocity zones indicative of free gas be neath a bottom simulating reflector bsr offshore oregon at ocean drilling program odp site 892 velocities drop from an average of 1750 m s above the bsr to less than 1250 m s below it sonic logs confirm that seismic velocity in the sediments adjacent to the borehole is less than that of water for at least 50 m beneath the depth of the bsr at this site similarly at odp site 889 offshore vancouver velocities range from 1700 to 1900 m s in the 100 m above the bsr and drop abruptly to 1520 m s in the 15 m just beneath it the low velocities observed beneath the bsr are strong evidence for the presence of 1 5 free gas by volume the bsr at these two sites results from the contact between gas free sediments containing a small quantity of hydrate above the bsr and a low velocity free gas zone beneath it although the bsr is associated with the base of the hydrate stability field hydrate appears to account for relatively little of the velocity contrast that produces the bsr velocity above the bsr at site 889 is only about 100 m s greater than that expected for sediments of similar porosity sediments above the bsr at site 892 appear to have normal velocity for their porosity and may contain little hydrate

assessment of gas hydrate and free gas distribution on the south shetland margin antarctica based on multichannel seismic reflection data summary processing and interpretation of a grid of intermediate resolution multichannel seismic reflection profiles collected on the ne sector of the south shetland continental margin allowed us to map the lateral extent of a bottom simulating reflector bsr the margin an accretionary wedge located off the northern tip of the antarctic peninsula consists of two distinct and superimposed tectonic regimes an older regime is related to mesozoic middle cenozoic subduction related tectonism a younger one is associated with a mainly extensional tectonic phase and related to the oligocene development of the western scotia sea the occurrence of the bsr appears to be controlled by the geological structure of the margin the bsr lacks continuity near basement structures main geological discontinuities and faults on the other hand the amplitude and continuity of the bsr are not affected by the presence of folded structures and undeformed sedimentary layering we found that the bsr is mostly confined to the ne sector of the south shetland margin where propagation of faulting associated with the shackleton fracture zone may have driven migration of natural gas towards the surface and created the conditions for a bsr to appear the application of reflection tomography techniques allowed us to reconstruct the averaged seismic velocity field between the seafloor and bsr in order to map the depth of bsr by averaging the observed velocity structure above and below the bsr and applying a theoretical model of elastic wave propagation in porous media we attempted as rigorous a quantitative assessment as possible of the natural gas present as gas hydrate above the bsr and as free gas between the bsr and the base of gas reflector bgr

sedimentary textures formed by aqueous processes erebus crater meridiani planum mars new observations at erebus crater olympia outcrop by the mars exploration rover opportunity between sols 671 and 735 a sol is a martian day indicate that a diverse suite of primary and penecontemporaneous sedimentary structures is preserved in sulfate rich bedrock centimeter scale trough festoon cross lamination is abundant and is better expressed and thicker than previously described examples postdepositional shrinkage cracks in the same outcrop are interpreted to have formed in response to desiccation considered collectively this suite of sedimentary structures provides strong support for the involvement of liquid water during accumulation of sedimentary rocks at meridiani planum

early diagenesis of eolian dune and interdune sands at white sands new mexico abstract the degree of early diagenesis in eolian dune and interdune sands at white sands new mexico is largely a function of the relationship between sand location and the water table most active and vegetation stabilized dune sands are in the vadose zone whereas interdune sands are in the capillary fringe and phreatic zones crystallographically controlled dissolution of the framework gypsum grains results in elongate prismatic etch pits on sand grains from the capillary fringe and phreatic zones whereas dissolution of sand grains in the vadose zone is slight causing minute irregularities on grain surfaces vadose water percolating through the sand is manifest as meniscus layers consequently dune sands in the vadose zone are cemented mainly by meniscus shaped gypsum at grain contacts pendant cements formed on the lower margins of some sand grains cementation in the capillary fringe and the phreatic zone is more extensive than the vadose regardless of strata type typically well developed gypsum overgrowths form along the entire edge of a grain or may encompass the entire grain complex diagenetic histories are suggested by multiple overgrowths and several episodes of dissolution on single grains attesting to changing saturation levels with respect to gypsum in the shallow ground water these changes in saturation are possibly due to periods of dilution by meteoric recharge alternating with periods of concentration of ions and the formation of cement due to evaporation through the capillary fringe

estimating the thickness of the free gas zone beneath hydrate ridge oregon continental margin from seismic velocities and attenuation recent ocean drilling program odp results in the oregon accretionary prism and on the blake ridge indicate that the zone containing free gas beneath the hydrate bearing near surface sediments is considerably thicker than previously thought in this paper we present results from travel time inversion of refracted seismic waves that show very low 100 in the normal accretionary complex sediments west of hydrate ridge these results suggest that refraction seismic techniques are a powerful way to constrain the depth to which free gas is present in sediments beneath the hydrate stability zone defining the extent of the free gas zone is an important factor for estimating the total volume of gas present and for evaluating its impact on slope stability and potential contribution to global climate change

characterizing the gas hydrate stability zone using a vertical hydrophone array this paper describes a set of unconventional seismic experiments designed to characterize the subsurface hydrate distribution at mississippi canyon block 798 in the northern gulf of mexico the sea floor at the survey site lies within the hydrate stability zone and heat flow data indicate that the zone may be 400 m thick sediments near the sea floor are characterized by relatively weak seismic reflections it is possible that the weak reflectivity partially is due to dispersed distribution of hydrates within the fine grained sediments the data were acquired using a vertical hydrophone array deployed at the sea bottom two different approaches were used at the site one used a conventional seismic source and the other used the noise generated by the ship the results from the experiments indicate that the vertical array performs well for high resolution characterization of the gas hydrate stability zone

seismic velocity structure at a gas hydrate reflector offshore western colombia from full waveform inversion seismic reflection profiles across many continental margins have imaged bottom simulating reflectors bsrs which have been interpreted as being formed at the base of a methane hydrate stability field such reflectors might arise either from an impedance contrast between high velocity partially hydrated sediments and water saturated sediments or from a contrast with partially gas saturated sediments these alternatives may be hard to distinguish by conventional amplitude versus offset or waveform modeling approaches here we investigate the origin of a high amplitude bsr in the accretionary wedge offshore of western colombia by seismic waveform inversion the inversion procedure consists of three steps firstly determination of root mean square velocities and hence estimates of the interval velocities between major reflectors by a global grid search for maximum normalized energy along elliptical trajectories in the intercept time slowness domain secondly determination of accurate interval velocities between these reflectors by a monte carlo search for maximum energy and thirdly a waveform fit in the frequency slowness domain using differential reflectivity seismograms and a conjugate gradient optimization algorithm to minimize the sample by sample waveform misfit between data and synthetic at two locations near a structural high we find a 30 m thick low velocity zone beneath the bsr with the properties of a partially gas saturated zone while at a third location where the bsr amplitude is lower we find no evidence for anomalously low velocities the preferential development of the bsr in structures that would tend to intercept fluid flow or migrating gas and the presence of free gas beneath the bsr indicate a mechanism of bsr formation in which free methane gas migrates upward into the hydrate stability field or is carried there in advecting pore water

columnar joint morphology and cooling rate a starch water mixture experiment 1 an analogue experiment using a starch water mixture has been carried out in order to understand the effect of cooling rate on the morphological characteristics of a basalt columnar joint if the contraction of material is essential for the formation of columnar joint structure the water loss rate by desiccation hereafter referred to as desiccation rate in the experiment is analogous to the cooling rate in solidifying basalt in the experiment the desiccation rate is controlled by varying the distance between the starch water mixture and a lamp used as the heat source we find that there are three regimes in the relation between joint formation and desiccation rate 1 at desiccation rates higher than 1 4 10 2 g cm 2 h 1 normal columnar joint regime the average cross sectional area s of a column is inversely proportional to the average desiccation rate i e s with 1 2 between that desiccation rate and a critical desiccation rate 0 8 10 2 g cm2h s approaches infinity as decreases close to a critical desiccation rate i e exponent monotonically increases from unity to infinity critical regime 3 below the critical desiccation rate no columnar structure forms no columnar joint regime forms applying the present experimental result to the formation of basalt column the basalt columnar cross sectional area is inversely proportional to the cooling rate with factors including elasticity crack growth coefficient thermal expansion glass transition temperature and crack density ratio at stress maximum also it can be predicted that there exists a critical cooling rate below which the columnar joint does not form the presence of a critical regime between the normal columnar jointing and no columnar jointing during a certain cooling rate range can also be predicted we find that at higher cooling rate the preferred column shape is a pentagon whereas at lower cooling rate it is a hexagon

physical chemical and biological investigation of an unconformity between limestone and sandstone in a coastal area iriomote island case study abstract although physical and chemical weathering due to groundwater and or wind flow is frequently observed in outcrops the geological mechanism responsible for these processes is not fully understood in the northern coastal area of iriomote island japan rock collapse caused by weathering of an unconformity between pleistocene limestones and miocene sandstones has been observed the boundary is characterised by distinctive columnar structures in the sandstone possibly induced by preferential weathering along desiccation induced fractures we found that the formation processes of the columns may be closely related to the rock collapse in this study the unconformable layer between the pleistocene ryukyu group and the miocene yaeyama group was investigated to determine the geological processes involved such as the formation weathering mechanisms and alteration of chemical and physical properties consequently the macroscopic and microscopic geometric forms compressive strength and mineral composition of the columnar structures were determined carbon stable isotopic analysis of the columns indicated that microbial solidification phenomena might have been involved in column formation a formation scenario for the columns has subsequently been proposed and we found that past stratification processes based on chemical and biological reactions with desiccation cracks on sandy soil directly influenced the modern dynamic rock collapse

palaeoenvironmental significance of lacustrine stromatolite forms from the middle old red sandstone of the orcadian basin the form of microbialite accumulations is largely the product of environmental processes and microbial activity recent work has largely concentrated on the identification and classification of microbialites with little attention being paid to their environmental significance this study describes the environmental distribution of the varied stromatolite forms recorded from the middle old red sandstone sequences of the orcadian basin comparisons are made with triassic examples from east greenland and modern microbialite accumulations the middle old red sandstone of northern scotland was deposited in a predominantly lacustrine setting stromatolites are recorded from both steep basin margin coincident settings and lower gradient settings where the lake margin was distant from the basin margin in the latter case stromatolite development is largely restricted to transgressive lacustrine sequences during the deposition of which reduced rates of sedimentation resulted from the migration of sediment input points towards the basin margin stromatolite sheets domal mounds aligned mounds and associated runnels sand cored stromatolite mounds and reefal stromatolite accumulations have been identified representing the transition from more sheltered to more exposed environments in basin margin coincident settings stromatolite accumulation is restricted to areas of low sedimentation where microbialites coat boulders and pebbles a model for the palaeoenvironmental distribution of the stromatolite forms described is proposed and is shown to be applicable to similar examples from the triassic of east greenland it is suggested that this model may be more widely applicable to stromatolitic accumulations in similar lacustrine settings through large portions of the phanerozoic

experimental simulation of basalt columns abstract basalt columns can be simulated by the desiccation of mixtures of starch and water this note presents photos of starch columns from laboratory experiments and summarizes results of an extended study muller g 1998 starch columns analog model for basalt columns j geophys res 103 15239 15253 basalt cooling and starch desiccation are diffusion processes causing contraction and cracks column related crack patterns in basalt and starch are largely similar differences in their spatial and time scales are due to the difference in the diffusion constants which is about two orders of magnitude starch experiments give a few new insights into basalt column formation

starch columns analog model for basalt columns desiccation of starch water mixtures produces tensile crack patterns which appear to be interesting but largely unknown study objects for fracture mechanics structural geology and volcanology this paper concentrates on columnar jointing and on columns in starch starch columns have polygonal cross sections and are very similar to basalt columns they are produced by lamp drying starch specimens with dimensions of several centimeters and have diameters in the millimeter range the columns develop behind a crack front which propagates from the surface into the interior the experiments supported by x ray tomograms show that polygonal regularity of the crack pattern is not present at the surface but develops during penetration this transition is steered by a minimum fracture energy principle the analogy between basalt cooling and starch desiccation is far reaching water concentration in starch is analogous to temperature in basalt both quantities obey diffusion equations water loss is equivalent to heat loss the resulting contraction stresses have similar dependences on depth and time and in both cases the material strength is exceeded the starch experiments show that column diameters are controlled by the depth gradient of water concentration at the crack front high low gradients are connected with thin thick columns by analogy a similar relation with the temperature gradient exists for basalt columns the normalized starch gradients are about 3 orders of magnitude larger than the normalized gradients in basalt this explains why starch columns are much thinner than basalt columns the gradients are so different because the crack front speeds differ by a factor of about 10 after 3 days the speed is about 10 mm d in starch but about 100 mm d in basalt peck 1978 the speed difference in turn results from the difference of the diffusion constants the hydraulic diffusivity of starch is 2 orders of magnitude lower than the thermal diffusivity of basalt

building biogenic beachrock visualizing microbially mediated carbonate cement precipitation using xfm and a strontium tracer the fate of reef islands is a topic of ongoing debate in the face of climate change induced sea level rise and increased cyclone intensity increased erosion and changes to the supply of reef derived sediment may put sand reef cays at risk of dramatic morphological changes these changes may have negative implications for the existence of reef cay environments which host vital sea turtle and bird rookery habitats beachrock consolidated carbonate beach sediment in the intertidal zone forms naturally on many tropical beaches and reduces the erosion rates of these beaches when compared to unconsolidated sand in spite of the critical role beachrock plays in stabilizing some reef cay shores the method of beachrock formation is still incompletely understood in this investigation beachrock was synthesized using beach sand and beachrock samples from heron island great barrier reef australia in aquarium experiments in which natural beachrock formation conditions were simulated beachrock was produced in two aquaria wherein the water chemistry was influenced by microorganisms derived from the natural beachrock inoculum whereas no cementation occurred in an aquarium that lacked a microbial inoculum and was controlled only by physicochemical evapoconcentration the new cements in the synthesized beachrock were analyzed using synchrotron based x ray fluorescence microscopy and were identified using a sr tracer added to the experimental seawater the resulting precipitates cement sand grains together and contain abundant microfossils of the microorganisms on whose exopolymer they nucleated demonstrating the fundamental role microbes play in beachrock formation these results are of interest because beachrock could be utilized as a natural coastline stabilization strategy on sand reef cays and in turn protect the unique habitats reef islands support

seismic velocities on the nova scotian margin to estimate gas hydrate and free gas concentrations abstract this article provides new constraints on gas hydrate and free gas concentrations in the sediments at the margin off nova scotia two dimensional 2 d velocity models were constructed through simultaneous travel time inversion of ocean bottom seismometer obs data and 2 d single channel seismic scs data acquired in two surveys in 2004 and 2006 the surveys separated by 5 km were carried out in regions where the bottom simulating reflection bsr was identified in seismic reflection datasets from earlier studies and address the question of whether the bsr is a good indicator of significant gas hydrate on the scotian margin for both datasets velocity increases by 200 300 m s at a depth of approximately 220 m below seafloor mbsf but the results of the 2006 survey show a smaller velocity decrease 50 80 m s at the base of this high velocity layer 310 330 mbsf than the results of the 2004 survey 130 m s when converted to gas hydrate concentrations using effective medium theory the 2 d velocity models for both datasets show a gas hydrate layer of 100 m thickness above the identified bsr gas hydrate concentrations are estimated at approximately 2 10 for the 2006 data and 8 18 for the 2004 survey the reduction in gas hydrate concentration relative to the distance from the mohican channel structure is most likely related to the low porosity within the mud dominant sediment at the depth of the bsr free gas concentrations were calculated to be 1 2 of the sediment pore space for both datasets

resurfacing time of terrestrial surfaces by the formation and maturation of polygonal patterned ground 1 to help interpret the polygonal patterned ground on mars we present recent findings about a similar form of patterned ground in a particularly cold and arid region on earth the dry valleys of antarctica in this region distinct arrays of interconnected polygons which we refer to herein simply as patterned ground characterize many surfaces reflecting a subsurface network of interconnected subvertical wedges of sand that grow incrementally as sand progressively fills soil fractures the fractures form initially as thermoelastic stresses arise during periods of rapid cooling of frozen ground and they continue to open and close in response to thermal cycles we describe the initiation and maturation of the patterned ground using data for the growth of sand wedges and for the evolution of crack patterns and microrelief over time scales ranging up to 106 years

heat flow in the western abyssal plain of the gulf of mexico implications for thermal evolution of the old oceanic lithosphere the seafloor depth of an oceanic basin reflects the average temperature of the lithosphere thus the western abyssal plain of the gulf of mexico which has tectonically subsided much 1 km deeper than other basins of comparable ages late jurassic should be underlain by an anomalously cold lithosphere in order to examine this hypothesis we made suites of high accuracy heat flow measurements at 10 sites along a line connecting deep sea drilling project dsdp sites 90 and 91 in the sigsbee abyssal plain the new heat flow sites were initially surveyed by 3 5 khz echo sounding 4 channel seismic reflection seismic refraction with eight ocean bottom seismometers and nine piston cores we occupied a total of 48 heat flow stations along the seismic survey line 3 to 6 at each site including 28 where we measured in situ thermal conductivities over the practical depth interval 4 m of the new multioutrigger bow heat flow probe we determined the heat flow associated with the lithosphere by correcting the values measured at the seafloor 41 to 45 mw m2 for 1 the thermal effect of the sedimentation and 2 the additional heat from the radioactive elements within the sediments the sedimentation history required for the first was reconstructed at each heat flow site based on ages and thicknesses of the major seismic stratigraphical sequences age data from the dsdp cores 3 5 khz subbottom reflectors and correlation of turbidite units found in the piston cores radiogenic heat production was measured for 55 sediment samples from four dsdp holes in the gulf whose age ranged from present to early cretaceous 0 83 w m3 on the average this provided the correction for the second the effects of these two secondary factors approximately cancel one another the lithospheric heat flow under the abyssal plain thus estimated ranges from 40 to 47 mw m2 these heat flow values are among the lowest in the mesozoic ocean basins where highly reliable data 45 to 55 mw m2 have been reported therefore the lithosphere under the gulf seems indeed colder than that under other old ocean basins however it is not as cold as expected from the large tectonic subsidence the inconsistency between the depth and heat flow may imply an anomaly in the regional thermal isostasy

microbial geomorphology a neglected link between life and landscape abstract whilst recognition is increasing that life and landscapes are intimately related as evidenced by growing research into ecosystem engineering biogeomorphology and allied topics the microbial contribution to such interactions has been relatively neglected a revolution in environmental microbiology based on molecular techniques is now driving a reconsideration of the role of microbial processes in geomorphology at all scales recent research illustrates the hitherto unknown microbial diversity present in many extreme geomorphic environments such as hyperarid deserts subglacial lakes hot springs and much richer microbial life than previously suspected within the soils and sediments that blanket most other landscapes such microbial communities have been found to play important geomorphic roles across a wide range of environments notably in weathering precipitation of minerals and protecting surfaces from erosion these geomorphic roles can also be conceptualised as examples of ecosystem engineering and can pave the way for further plant geomorphology and zoogeomorphology processes three key aspects which emerge from a review of microbial influences on earth surface processes are a that microbes play roles on a continuum from full control to passive involvement b that complex and widespread communities of microorganisms are involved and c that microbial activity usually affects several earth surface processes at once examples of the contribution of microbial life to geomorphology over long medium and short timescales suggest that microorganisms play key geomorphological roles in two major situations on the cusp between stable states and in extreme environments where higher plant and animal life is limited and many abiotic processes are also constrained the dominant link between microbial life and geomorphology appears to take on different forms depending on the timescale under consideration with a stabilising microbial role apparent over short timescales being replaced by a denudational role over longer timescales further research involving microbiologists and geomorphologists is now needed to address three main questions i e under what conditions are microbial and geomorphological processes most closely linked what scale issues surround links between microbes and geomorphology and how do microbiological processes underpin broader biogeomorphological interactions

modelling the evolution of columnar joints abstract cooling cracks developing from the margins of solidifying lava flows or intrusive dykes or sills often propagate inward and divide the rock into prismatic columns it has been shown that 1 the initial crack pattern that forms at the surface of the body is not as regular as the pseudo hexagonal pattern which evolves later 2 columnar structures develop by repeated step wise crack advances forming cm to dm transverse bands on the column faces 3 even within well developed colonnades the polygonal outlines continue to shift slightly from one crack advance to the next we propose that each new crack should propagate parallel to the highest thermal gradient ahead of the current crack tip when adjacent columns are of unequal size the local asymmetry of the isotherms drives the new crack toward the biggest and hottest column we present a geometrical model and algorithm that follows this rule and mimics the successive crack advances as the cooling front migrates into the lava sheet or dyke the model polygonal pattern obtained consists of convex irregular polygons with a variable number of sides but in which pentagons and hexagons predominate the algorithm successfully reproduces the rapid evolution from an initial immature crack pattern to a pseudo hexagonal mature one in which the average number of sides approaches six it predicts also the predominance of y type crack junctions the rapid evolution toward columns of approximately equal sizes and the persistent changes in length and diverging orientation of growth steps observed along the sides of columns the model polygonal patterns exhibit geometrical properties which are very similar to those observed in well developed columnar jointed flows such as the giant s causeway

biogenic origin of coastal honeycomb weathering honeycomb weathering occurs in two environments in late cretaceous and eocene sandstone outcrops along the coastlines of south west oregon and north west washington usa and south west british columbia canada at these sites honeycomb weathering is found on subhorizontal rock surfaces in the intertidal zone and on steep faces in the salt spray zone above the mean high tide level in both environments cavity development is initiated by salt weathering in the intertidal zone cavity shapes and sizes are primarily controlled by wetting drying cycles and the rate of development greatly diminishes when cavities reach a critical size where the amount of seawater left by receding tides is so great that evaporation no longer produces saturated solutions encrustations of algae or barnacles may also inhibit cavity enlargement in the supratidal spray zone honeycomb weathering results from a dynamic balance between the corrosive action of salt and the protective effects of endolithic microbes subtle environmental shifts may cause honeycomb cavity patterns to continue to develop to become stable or to coalesce to produce a barren surface cavity patterns produced by complex interactions between inorganic processes and biologic activity provide a geological model of self organization surface hardening is not a factor in honeycomb formation at these study sites salt weathering in coastal environments is an intermittently active process that requires particular wind and tidal conditions to provide a supply of salt water and temperature and humidity conditions that cause evaporation under these conditions salt residues may be detectable in honeycomb weathered rock but absent at other times honeycomb weathering can form in only a few decades but erosion rates are retarded in areas of the rock that contain cavity patterns relative to adjacent non honeycombed surfaces copyright 2010 john wiley sons ltd

diamondiferous neoarchean fan delta deposits western superior province canada sedimentology and provenance abstract most fluvial diamond placers occur in non aggrading braided systems where the diamonds are preferentially trapped by very rough substrates the recognition that some of the neoarchean diamond bearing breccias near wawa ontario canada are actually fluvial conglomerates deposited by a mixture of traction current and mass flow processes provided the opportunity to add information on aggradational diamond placer systems the 454 m thick leadbetter conglomerate was studied to ascertain the depositional processes that led to the development and provenance of the placer the conglomerate is composed of a series of weakly sheared and highly viscous debris flows interbedded with boulder to pebble conglomerates representing longitudinal bars and channels pebble conglomerate and sandstone channels become more dominant upwards with a decrease in mass flow processes an abrupt marine transgression flooded this fan delta assemblage ending deposition of the coarse grained lithofacies the lowest 100 m of the conglomerate is composed of clasts derived solely from the underlying mafic volcanic assemblage above this zone the clast types become more diverse and felsic volcanic clasts become progressively more important upwards whole rock geochemistry performed on coarse grained sandstone samples indicates that major sediment contributions came from mafic rocks and nearby diamondiferous lamprophyres and related breccias though the possible existence of a localized kimberlitic source cannot be excluded geochemistry also indicates that minor enrichments of heavy minerals formed on the alluvial fan delta surface but the primary cause of the conglomerate s diamondiferous nature was a nearby diamond bearing source rock

desiccation of a sensitive clay application of the model crack field data obtained during the desiccation of an intact clay were used to assess the performance of a model enabling the prediction of depth and spacing of primary shrinkage cracks the model requires several input parameters such as the soil water characteristic curve to predict the evolution of the suction profile with time the tensile strength of the soil to define the conditions at crack initiation and the fracture toughness to calculate ultimate crack depth predictions of suctions at crack initiation time of crack initiation and depth of crack propagation were reasonable using laboratory determined values of these soil properties obtained from undisturbed samples field data suggest that neighbouring cracks form where the computed relief stresses reach 80 to 88 of the tensile strength of the soil

white sands dune field new mexico age dune dynamics and recent accumulations abstract the white sands dune field situated within the tularosa basin in southern new mexico is thought to have been largely derived by a stepwise progressive deflation of pleistocene lake otero strata with the onset of regional aridity optically stimulated luminescence osl dating of samples from a core that penetrated the gypsum accumulation of the dune field confirm a time of origin at 7000 yr dune sediment is characterized as lagged influx from previously stored lake otero sediment and contemporaneous influx derived from subsequent playas sediment became available for aeolian transport and dune field construction because of a falling water table driven by the regional aridity the current dune field is primarily a wet aeolian system in which the behavior of the accumulation surface over time is a function of the water table although surface cementation by gypsum also imparts aspects of a stabilizing system dune crests are oriented borderline transverse to the annual transport resultant and a monitored dune and its cross strata show that transverse winds from the sw during the late winter and spring account for most of the crest normal migration but a significant along crest component of migration of dune sinuosity occurs with fall and winter winds from the nw and n that strike the crests obliquely trenches across three interdune areas show dune sets and interdune strata climbing at about 0 1 depth of interdune scour increases with interdune streamwise length which acts to enhance the probability of dune cross strata deflation during arid years but also increases the probability of interdune accumulation because the water table shows a net rise over time within the trenches dune sets show a bundling of foresets between reactivation surfaces interpreted as annual cycles that reveal lee face reworking by the oblique nw and n winds and slipface progradation fostered by transverse winds from the sw a progression from grainflow dominated to wind ripple dominated cross strata within a single set revealed in the longest trench is consistent with along crest migration of dune sinuosity interdune laminations show light dark couplets interpreted as annual varves reflecting dry wet portions of the year the number of laminations preserved in vertical sections across an interdune area shows that the record is incomplete and endorses the interpretation of periods of interdune deflation a calculation of the current accumulation rate as based upon the climbing strata within the trenches is significantly greater than the long term accumulation rate determined by osl dates taken from the core in addition to compaction and possible dissolution of gypsum as partial explanations the long term accumulation rate may reflect significant variation in the accumulation rate over time short term deflation of accumulations on a regular basis and or the presence of unconformities within the gypsum body

placer formation in gravel bedded rivers a review the literature on placer formation processes within fluvial systems is widespread and ranges between detailed laboratory studies of the hydrodynamic segregation processes through to the intuitive interpretation of the distribution of minerals within geological sections however there are few if any comprehensive reviews of the literature surprisingly given the economic importance of placers the theoretical framework relating to the hydrodynamics of physical grain sorting is not well developed and there are relatively few detailed laboratory hydraulic investigations to inform theory in this wide ranging review the history of the development of principles of placer formation is explored as far as possible in a non technical fashion a consideration is given to the hydrodynamics of physical grain sorting above lower stage and upper stage bedforms and the typical internal sedimentary structures associated with placer concentrations are detailed finally examples of the depositional environment of diamond tin and gold placers are considered

methane hydrate and free gas on the blake ridge from vertical seismic profiling seismic velocities measured in three drill holes through a gas hydrate deposit on the blake ridge offshore south carolina indicate that substantial free gas exists to at least 250 meters beneath the bottom simulating reflection bsr both methane hydrate and free gas exist even where a clear bsr is absent the low reflectance or blanking above the bsr is caused by lithologic homogeneity of the sediments rather than by hydrate cementation the average methane hydrate saturation above the bsr is relatively low 5 to 7 percent of porosity which suggests that earlier global estimates of methane in hydrates may be too high by as much as a factor of 3

geology and tectonics of the ryukyu islands abstract the ryukyu islands are divided morphologically and geologically into three island groups north central and south ryukyus north central ryukyu represents the geological continuation of the outer belt of southwest japan composed of mesozoic eocene sedimentary sequences whereas south ryukyu is characterized by high pressure metamorphic rocks eocene volcanics and limestone and lower miocene sediments the geological and structural contrasts between north central and south ryukyus are conspicuous before the late miocene transgression covered the whole area the ryukyu islands have been established since then the successive developments of the goto tunghai senkaku basins since the paleogene of the okinawa trough since the miocene and of the grabens near the island group since the pliocene signify a southeastward shift of the basins in relation to the activity in the granite diapir zones accompanied by volcanism

elastic wave velocity in marine sediments with gas hydrates effective medium modeling we offer a first principle based effective medium model for elastic wave velocity in unconsolidated high porosity ocean bottom sediments containing gas hydrate the dry sediment frame elastic constants depend on porosity elastic moduli of the solid phase and effective pressure elastic moduli of saturated sediment are calculated from those of the dry frame using gassmann s equation to model the effect of gas hydrate on sediment elastic moduli we use two separate assumptions a hydrate modifies the pore fluid elastic properties without affecting the frame b hydrate becomes a component of the solid phase modifying the elasticity of the frame the goal of the modeling is to predict the amount of hydrate in sediments from sonic or seismic velocity data we apply the model to sonic and vsp data from odp hole 995 and obtain hydrate concentration estimates from assumption b consistent with estimates obtained from resistivity chlorinity and evolved gas data

seismic investigation of near surface geological structure in the paducah kentucky area application to earthquake hazard evaluation abstract seismics method were used to evaluate shallow geological conditions at 33 sites in the vicinity of paducah kentucky a combined set of p and s wave seismic refraction and reflection soundings were used in addition to local borehole information to produce structure maps of 1 a shallow 160 m deep shear wave velocity contrasts across the shallow unconformity were generally 2 to 1 while the contrast at the top of the paleozoic bedrock exceeds 5 to 1 these seismic boundaries have been determined to be very important in modelling and interpreting earthquake ground motion amplification in the paducah area the quality and accuracy of the data and the cost effective nature of the methods suggest that other communities in areas at risk to damage from seismic activity with foundation conditions comparable to paducah might benefit from similar characterization in order 1 to identify seismically hazardous near surface geological conditions and 2 to develop geological models that could be used in computer simulations of site response

tectonic and climatic controls on continental depositional facies in the karoo basin of northern natal south africa abstract the eastern karoo basin south africa contains a thick sequence of terrigenous clastic sediments comprising a meanderbelt facies braided channel facies divided into coarse and fine subfacies fluviolacustrine facies and aeolian facies depositional trends and changes in fluvial style reflect a progressive increase in aridity of the climate under stable tectonic conditions interrupted by two phases of source area tectonism and the development of fine and coarse clastic wedges of the braided channel subfacies the latter signifying a short interlude of cool wet conditions the fine braided channel subfacies occurs in the upper part of the meanderbelt facies which was deposited by ephemeral meandering mixed load streams of variable discharge and sinuosity under dry semi arid climatic conditions these deposited complex internally discordant channel sands and well developed levee deposits following deposition of the coarse braided channel subfacies semi arid conditions returned and fluvial deposition was dominated by ephemeral straight to slightly sinuous mixed load streams characterised by simple channel sand bodies as the aridity of the climate increased the streams became more localised and carried an increasing proportion of fines interbedded with and overlying the fluvial deposits is a mudstone dominated lacustrine sequence grading up into aeolian sands suggesting a playa lake type situation the general absence of evaporites from these sediments is attributed to the fresh nature of the lake waters as evidenced by the freshwater aquatic organisms and clay mineral suite the lack of adequate inflow for solute accumulation and the removal of dust impregnated by salts from the surface of the dry lake bed during the dry season by superheated upward spiralling columns of air broadly similar environments to the fluvio lacustrine and aeolian facies sequence are to be found in the lake eyre basin of central australia and the okavango delta of northern botswana the okavango delta model has an important bearing on patterns of fluvial sedimentation in arid regions since it shows many characteristics of temperate well vegetated anastomosed fluvial systems despite its location in the kalahari desert

vertebrate fossil bearing paleosol units willwood formation lower eocene northwest wyoming u s a implications for taphonomy biostratigraphy and assemblage analysis abstract in the lower 300 m of the fluvial willwood formation of northwest wyoming most large concentrations of vertebrate fossils consist of disarticulated and broken skeletal remains that occur in widespread tubular thin 2 cm 1 m greenish and bluish gray mudstones that are low in percentage of free iron aluminum and manganese relatively high in percentages of organic carbon and often show evidence of clay eluviation these units lie atop sesquioxide rich sometimes clay illuviated variegated unit red yellow purple and mottled variants and are relicts of the a horizons of podzolic spodosols aquods the wide distribution of bone fragments the numerical proportions of bone elements weathering of the fragments and biogenic evidence indicate the vertebrate remains accumulated gradually as litter on the surface of the soils and became incorporated into the a horizon the bones were disarticulated broken and otherwise disturbed by the action of scavengers prior to burial occurrence of abundant vertebrate remains in discrete readily identifiable and widespread paleosol units makes these units valuable biostratigraphic markers zonules faunules evidence of geologically short term lag accumulation of the bones is important to assemblage analysis the paleosol concentrations result from attritional mortalities that more closely reflect composition of the life assemblage than do remains concentrated by fluvial transport or predators

the nature and distribution of bottom simulating reflectors at the costa rican convergent margin summary bottom simulating reflectors bsrs at the base of the hydrate stability zone are often observed in marine seismic reflection data the compressional p wave velocity structure of a bsr offshore costa rica was analysed by applying a full waveform inversion technique the resulting velocity profile indicates that the bsr is caused by a thin low velocity layer suggesting the presence of at least a small amount of free gas in the sediment pore space in undisturbed sediment sections bsrs offshore costa rica are observed over much of the continental margin they frequently occur at shallow depths beneath the seafloor usually 100 400 m at some locations bsrs appear to intersect the seafloor this was interpreted as an indication that hydrates in the study area form partly from methane which was produced beneath the hydrate stability zone and migrated upwards in the study area the vertical movement of the hydrate stability zone relative to a given point in the sediment column one of the potential factors leading to bsr formation or suppression is controlled by both sedimentation and vertical tectonism both processes may hence play a role in controlling bsr distribution bsrs are absent in areas which have been affected by slumping except where the sedimentary section above the bsr remained intact during slumping this indicates that slope failure can cause the destruction of bsrs

seismic characterization of a gas hydrate system in the gulf of mexico using wide aperture data summary gas hydrates were discovered in a mud mound in the lease block mississippi canyon 798 gulf of mexico through piston coring subsequently a seismic experiment was carried out to investigate the dynamics behind the hydrate formation during the experiment high resolution multichannel seismic reflection data using a 24 channel 240 m long streamer and wide aperture data using six ocean bottom seismometers were collected along five lines high reflectivity zones hrzs are present in the reflection data along all lines to better constrain the interpretation of the reflection data the traveltimes from the multichannel and wide aperture data sets were jointly inverted to estimate a 2 d p wave layered velocity model for each line a minimum parameter minimum structure modelling approach yielded simple models and a comparison of the models at their intersection points shows they are consistent to within 10 m s 1 in velocity and 20 m in depth in the final p wave velocity models the hrzs are associated with a lowering of velocity in the reflection data the top of the hrzs show a polarity reversal with respect to the seafloor presence of free gas in the hrzs best explains the velocity lowering and polarity reversal it is speculated that the gas has deeper sources and migrates upwards through conduits formed by salt movement in the vicinity the upward migrating gas accumulates in the axis of a channel complex and manifests itself as hrzs in the reflection data the fluids circulating along the conduits push the base of the hydrate stability zone close to the seafloor from the channel axis the free gas migrates further upwards and close to the seafloor and as it comes within the gas hydrate stability zone it forms hydrates

study of out of plane effects in the inversion of refraction wide angle reflection traveltimes abstract in the presence of three dimensional 3d inhomogeneous structure the results of 2d traveltime inversion will be in error since the effects of out of plane or 3d ray sampling are ignored we have inverted synthetic data using 2d and 3d algorithms to examine the errors caused by 3d inhomogeneities which produce significant out of plane ray bending the results of inverting data from 2d experiments are compared with vertical slices through 3d models obtained by inverting data using a number of recently employed 3d recording geometries our results show that even for strong 3d inhomogeneities out of plane effects are relatively small with crustal velocity errors of less than 0 15 km s and intra crustal boundary depth errors generally less than 2 km these errors are approximately equal to the uncertainties commonly assigned to crustal models derived from traveltime inversion the artifacts are also similar in magnitude to the lateral smearing that occurs in 3d models when using relatively coarse 3d geometries only for a dense network of profiles will a 3d inversion using off and in line data provide greater lateral resolution than a 2d independent or simultaneous inversion of in line data along each profile 2d and 2 5d inversion of crooked line data in the presence of strong velocity variations produces erroneous small scale velocity structure we conclude that most 3d crustal experiments cannot be justified on the basis that the results from a 2d experiment or a network of 2d profiles will be significantly in error due to out of plane effects 3d experiments can be justified when a dense grid of shots and receivers is used or if a volume image as opposed to a cross sectional image is required

erratum invasion of drilling mud into gas hydrate bearing sediments part i effect of drilling mud properties fulong ning 1 2 keni zhang 3 4 nengyou wu 2 ling zhang 1 gang li 2 guosheng jiang 1 yibing yu 1 li liu1 and yinghong qin1 1faculty of engineering china university of geosciences wuhan hubei 430074 china e mail nflzx cug edu cn 2guangzhou center for gas hydrate research chinese academy of sciences guangzhou 510640 china 3earth system division lawrence berkeley national laboratory berkeley ca 94720 usa 4college of water sciences beijing normal university beijing 100875 china

surface morphology of columnar joints and its significance to mechanics and direction of joint growth columnar joints in basaltic lava flows display conspicuous bands oriented normal to column axes new observations show that each band contains a single plumose structure and thus represents an individual crack or joint segment formed during a discrete growth event analysis of plumose structure and intersections of cracks leads to a new kinematic model of columnar jointing and provides the first direct proof that columnar joints grow incrementally from exterior to interior regions of solidifying magma bodies columnar joints form by nucleation and growth of new cracks on the edges of older cracks each new crack begins at a point and propagates mostly normal to column axes and along the leading edge of a developing column face where thermal stress is concentrated inward propagation of cracks toward hotter regions is limited by a decrease of thermal stress and by the brittle ductile transition of lava outward and lateral propagation is limited by mechanical interaction with previous cracks and by low thermal stress in already fractured lava cracks often diverge slightly from the planes of previous cracks probably because of spatial and temporal changes in directions of local principal stresses mechanical interaction causes a diverging crack to overlap curve toward and usually intersect the previous crack behind its edge leaving a blind tip that points in the overall growth direction of the columnar joints this and other directional criteria are applied to determine joint growth patterns in several lava flows of the western united states in two tiered and multi tiered flows downward growing columnar joints usually meet upward growing joints well below the middle of the flows which indicates very rapid cooling of upper portions relative to lower portions this supports the idea that convection of water in columnar joints connected to the surface may be an important mechanism for cooling the upper portions of these flows whereas conduction is probably the dominant cooling mechanism at the bases

carbonate stromatolites from a messinian hypersaline setting in the caltanissetta basin sicily petrographic evidence of microbial activity and related stable isotope and rare earth element signatures lower messinian stromatolites of the calcare di base formation at sutera in sicily record periods of low sea level strong evaporation and elevated salinity thought to be associated with the onset of the messinian salinity crisis overlying aragonitic limestones were precipitated in normal to slightly evaporative conditions occasionally influenced by an influx of meteoric water evidence of bacterial involvement in carbonate formation is recorded in three dolomite rich stromatolite beds in the lower portion of the section that contain low domes with irregular crinkly millimetre scale lamination and small fenestrae the dominant microfabrics are i peloidal and clotted dolomicrite with calcite filled fenestrae ii dolomicrite with bacterium like filaments and pores partially filled by calcite or black amorphous matter and iii micrite in which fenestrae alternate with dark thin wispy micrite the filaments resemble beggiatoa like sulphur bacteria under scanning electron microscopy the filaments consist of spherical aggregates of dolomite interpreted to result from calcification of bacterial microcolonies the dolomite crystals are commonly arranged as rounded grains that appear to be incorporated or absorbed into developing crystal faces biofilm like remains occur in voids between the filaments the dolomite consistently shows negative d 13 c values down to 11ae3 and very positive d 18 o mean value 7ae9 that suggest formation as primary precipitate with a substantial contribution of organic co2 very negative d 13 c values down to 31ae6 of early diagenetic calcite associated with the dolomite suggest contribution of co2 originating by anaerobic methane oxidation the shale normalized rare earth element patterns of sutera stromatolites show features similar to those in present day microbial mats with enrichment in light rare earth elements and m type tetrad effects enrichment around pr coupled to a decline around nd and a peak around sm and eu taken together the petrography and geochemistry of the sutera stromatolites provide diverse and compelling evidence for microbial influence on carbonate precipitation