microstructure of high temperature 73 c siliceous sinter deposited around hot springs and geysers yellowstone national park the role of biological and abiological processes in sedimentation slightly alkaline hot springs and geysers in yellowstone national park exhibit distinctive assemblages of high temperature 73 c siliceous sinter reflecting local hydrodynamic conditions the main depositional zones include subaqueous pool and channel bottoms and intermittently wetted subaerial splash surge and overflow areas subaqueous deposits include particulate siliceous sediment and dendritic and microbial silica framework silica framework forms thin porous microbe rich films coating subaqueous surces spicules with intervening narrow crevices dominate in splash zones surge and overflow deposits include pool and channel rims columns and knobs in thin section subaerial sinter is composed of i dark brown nearly opaque laminated sinter deposited on surfaces that evaporate to dryness ii clear translucent silica deposited subaqueously through precipitation driven by supersaturation iii heterogeneous silica representing silica encrusted microbial filaments and detritus and iv sinter debris brownish laminations form the framework of most sinter deposited in surge and overflow zones pits and cavities are common architectural features of subaerial sinter and show concave upward pseudo cross laminations and micro unconformities developed through migration marked birefringence of silica deposited on surfaces that evaporate to dryness is probably a strain effect repeated wetting and evaporation often to dryness and capillary effects control the deposition morphology and microstructure of most high temperature sinter outside of the fully subaqueous zone microbial filaments are abundant on and within high temperature sinter but do not provide the main controls on morphology or structuring except in biofilms developed on subaqueous surfaces millimetre scale lamination cyclicity in much high temperature sinter represents annual layering and regular seasonal fluctuations in silica sedimentation

ironstone bodies of the barberton greenstone belt south africa products of a cenozoic hydrological system not archean hydrothermal vents irregular bodies of goethite and hematite termed ironstone pods in the barberton greenstone belt south africa have been previously interpreted as the earth9s most ancient submarine hydrothermal vent deposits and have yielded putative evidence about archean hydrothermal systems ocean composition and temperature and early life this report summarizes geologic sedimentological and petrographic evidence from three widely separated areas showing that the ironstone was deposited on and directly below the modern ground surface by active groundwater and spring systems probably during periods of higher rainfall in the pleistocene the deposits include a recently active spring terrace and outflow system containing iron oxide encrusted microbial filaments shallow subsurface groundwater areas showing ironstone deposited between chert blocks and replacing chert degraded spring and outflow deposits and goethite cemented pleistocene landslide breccias and regolith the predominance of goethite a thermally unstable iron oxide widely developed slope parallel stratification abundant primary open cavities some with vertical goethite dripstone and the lack of structural deformation collectively indicate that these bodies formed during relatively recent time veins and displaced masses of coarsely crystalline quartz show corrosion and represent relict archean vein and cavity fill quartz after iron oxide replacement of surrounding chert iron was sourced by archean sideritic sedimentary units that hold up high ridges on which the ironstone bodies occur siderite was dissolved by circulating groundwater and the iron deposited as oxides within the shallow subsurface groundwater system and around springs where the fe rich water flowed onto the surface these deposits represent a remarkable iron oxide depositing quaternary hydrologic system but provide no information about conditions or life on the early earth

ophiolite emplacement by collision between the sula platform and the sulawesi island arc indonesia much of the tectonic complexity displayed in eastern indonesia results from a series of neogene collision events between island arcs continental fragments and the australian continent here we examine the emplacement of a large ophiolite belt resulting from the miocene collision between the sulawesi island arc and a continental fragment the sula platform we present the results of several marine geophysical expeditions to the sw molucca sea and the nw banda sea plus gravity and geology on the east arms of sulawesi the batui thrust separates the ophiolite from sedimentary rocks deformed along the leading edge of the sula platform we mapped this thrust eastward from sulawesi along the southern margin of the gorontalo basin the latter is floored by oceanic crust and its south edge is uplifted against the thrust thus the sulawesi ophiolite can be traced offshore to its origin as basement of the gorontalo basin the ophiolite is composed of harzburgite in the southeast arm and passes upward through a complex of gabbros and diabase dikes in the east arm ophiolite melange underlies the harzburgites on the southeast arm beneath low angle thrust contacts where seen our local observations show the melange to be composed of thrust packets of both serpentine and red shale matrix varieties the packets are several hundred meters thick and the melange where studied has a moderate north to northeast dipping foliation this orientation if regionally representative of the melange fabric is consistent with a significant northward component of movement of the lower plate probably the sula platform or its margin where the ophiolite is in contact with rocks of the central schist belt it dips under the schist but where it encounters melange or mesozoic or paleogene sedimentary rocks the ophiolite is thrust over them the tectonic overlap sequence from west to east is schist over ophiolite over older sediments or melange the ophiolite appears to have been emplaced by oblique convergence of the sula platform along the southern edge of the gorontalo basin we suggest that the gorontalo basin represents a forearc basin and the ophiolite is its basement analogous to a number of other forearc settings deformation of the ophiolite may have occurred in part on the seafloor prior to emplacement but we feel that much of the deformation occurred during emplacement the sulawesi ophiolite is only one of a number of ophiolites in the indonesian region each of which has a very different origin and tectonic history

petrography and genesis of spicular and columnar geyserite from the whakarewarewa and orakeikorako geothermal areas north island new zealand domal masses of geyserite which surround many geyser vents in the whakarewarewa geothermal area are formed largely of spicules spicule columns and shrub columns the non branching spicules and the branches of branching spicules individually up to 3 cm high and 1 mm diameter have a laminated core encased by siliceous cortex silicified microbes are rare in the core but common in the cortex silicified microbial mats and pseudodendrites are found in the crevices between neighbouring spicules shrub columns up to 5 cm high and 1 5 cm in diameter are formed of opal a that was precipitated around a three dimensional branching shrub like microbial structure the shrub branches which are hollow with scalloped walls do not contain any evidence of the original microbes or minerals that formed them silicified microbial mats are present between the columns microbial boring and etching by acidic steam led to local diagenetic degradation of these columns in the geyserite mounds at whakarewarewa spicules and spicule columns are common whereas shrub columns are rare interbedding and intercalation of spicular geyserite with shrub columnar geyserite indicate however that these different morphologic entities probably formed under similar environmental conditions on the mounds around the geyser vents petrographic evidence shows that the spicules spicule columns and shrub columns grew through a combination of biotic and abiotic processes

microstructural changes accompanying the opal a to opal ct transition new evidence from the siliceous sinters of geysir haukadalur iceland opaline silica opal a has formed in marine lacustrine and geothermal environments throughout geological time during diagenesis opal a normally changes to opal ct then opal c and finally to quartz such changes commonly destroy the original fabrics and any fossils that opal a contained the physical changes that accompany the opal a to opal ct transition however are known poorly x ray diffraction analyses electron microprobe analyses and high resolution high magnification scanning electron microscope imagery of siliceous sinters from the geysir geothermal area in iceland show that opal a is formed of heterometric arrays of randomly packed microspheres up to 5 m diameter with neighbouring spheres commonly being joined by small connection pads in contrast enlarged spheres lepispheres inverse opal two types and spindle frameworks with hexagonal motifs characterize opal ct the textures in opal ct which vary on a microscale reflect the complex interplay between dissolution e g inverse opal and precipitation e g enlarged spheres spindle frameworks that probably was mediated by groundwater in a near surface environment the processes deciphered from these young rocks should however be applicable to sedimentary opal a and opal ct of all ages irrespective of their origin

the growth of siliceous sinter deposits around high temperature eruptive hot springs abstract siliceous hot spring deposits sinter are of interest as they are indicative of hydrothermal resources at depth and may provide evidence for early life on earth and possibly mars numeric models of concurrent evaporation and opal a precipitation around high temperature 73 c eruptive hot spring vents such as geysers show that silica is most efficiently precipitated by complete evaporation owing to very sluggish growth kinetics for silica precipitation from supersaturated hydrothermal water where evaporation is complete between geyser events areas of initially deeper water precipitate more silica that over time fill in topographic lows to produce a smooth surface in contrast incomplete evaporation in which water is left in low areas prior to being washed away by the next geyser event or equivalently where there is continuous surface flow in topographic low areas tends to enhance the growth of minor topographic highs and leads to an increase in surface roughness such as seen in the development of knobby geyserite sinter texture around vents

empirical ground motion relations for subduction zone earthquakes and their application to cascadia and other regions ground motion relations for earthquakes that occur in subduction zones are an important input to seismic hazard analyses in many parts of the world in the cascadia region washington oregon northern california and british columbia for example there is a significant hazard from megathrust earthquakes along the subduction interface and from large events within the subducting slab these hazards are in addition to the hazard from shallow earthquakes in the overlying crust we have compiled a response spectra database from thousands of strong motion record ings from events of moment magnitude m 5 8 3 occurring in subduction zones around the world including both interface and in slab events the 2001 m 6 8 nis qually and 1999 m 5 9 satsop earthquakes are included in the database as are many records from subduction zones in japan kyoshin net data mexico guerrero data and central america the size of the database is four times larger than that available for previous empirical regressions to determine ground motion relations for subduc tion zone earthquakes the large dataset enables improved determination of attenu ation parameters and magnitude scaling for both interface and in slab events soil response parameters are also better determined by the data we use the database to develop global ground motion relations for interface and in slab earthquakes using a maximum likelihood regression method we analyze regional variability of ground motion amplitudes across the global database and find that there are significant regional differences in particular amplitudes in cascadia differ by more than a factor of 2 from those in japan for the same magnitude distance event type and national earthquake hazards reduction program nehrp soil class this is believed to be due to regional differences in the depth of the soil profile which are not captured by the nehrp site classification scheme regional correction factors to account for these differences are proposed for cascadia and japan the results of this study differ significantly from previous analyses based on more limited data and have important implications for seismic hazard analysis the ground motion relations predict that a great megathrust earthquake m 8 at a fault distance of about 100 km would produce pseudoacceleration psa 5 damped horizontal component on soil sites of about 110 cm sec 2 at 0 5 hz 660 cm sec 2 at 2 5 hz and 410 cm sec 2 at 5 hz with a peak ground acceleration of about 180 cm sec 2 these damaging levels of motion would be experienced over a very large area corresponding to a rectangular area about 300 km wide by 500 km long large in slab events m 7 5 would produce even higher psa values within 100 km of the fault but the in slab motions attenuate much more rapidly with distance thus the hazard posed by moderate to large in slab events such as the 2001 nisqually earth quake is modest compared to that of a cascadia megathrust earthquake of m 8 in terms of the area that would experience damaging levels of ground motion

bikpela a large siliceous chimney from the pacmanus hydrothermal field manus basin papua new guinea the 2 7 m bikpela chimney differs from sulfide rich chimneys at pacmanus eastern manus basin by virtue of high silica content a stromatolite like layered structure and the presence of a large internal collapse cavity its interior is dominated by 5 to 10 mm thick primary laminae of opal a dominated fronds clouded by submicroscopic sphalerite inclusions set between thin subhorizontal growth borders larger sphalerites with sulfosalt galena intergrowths at frond ends project into discoid cavities under the growth borders a steeper second order lamination reflects arrested expansion of the 50 cm high internal collapse cavity underlying the cavity floor an axial breccia pipe contains distorted fragments of laminated interior modified by dissolution and then redeposition of opal and further growth of pyrite and sphalerite beneath a thin mn oxide crust a hard outer sphalerite barite carapace 5 cm thick and devoid of opal is banded parallel to the chimney exterior at least three stages of chimney growth are evident beginning with repetitive deposition of opaline frond layers probably developed from microbial mat precursors by overgrowth of hydrothermal opal and fine sphalerite next with rising fluid temperatures hydrothermal fe poor sphalerite with pb sulfosalts and galena deposited at frond ends and under growth borders and via lateral diffusion and mixing with seawater formed abundantly in the outer carapace of the chimney passage of hot more acid fluids into the chimney axial zone then caused dissolution of primary opal and consequent formation of the collapse cavity and underlying breccia pipe together with deposition of fe rich sphalerite and pyrite in second order fractures and cavities related to the collapse finally conductive cooling during or after this latter stage produced films and spherules of clear opal coating pores and grain margins throughout the structure apart from in the carapace bikpela contains significant gold and silver avg 8 7 ppm au 250 ppm ag n 9 but it is low overall in cu up to 0 5 wt in the pipe and pb up to 0 7 wt in the carapace its carapace is relatively rich in zn up to 54 wt ag as and sb with one sample containing 0 12 wt silver

factors governing subaqueous siliceous sinter precipitation in hot springs examples from yellowstone national park usa siliceous sinter precipitation within hot spring systems has been attributed to a variety of mechanisms evaporative concentration cooling changes in ph and cation effects repetitive in situ t ph alkalinity etc and laboratory major minor and trace elemental stable isotopic analyses of the waters plus observations of silica precipitation on natural e g twigs pine cones as well as artificial substrates glass slides and copper plates in the waters substantiate that subaqueous precipitation is occurring throughout the vent to distal end of flow in both cistern spring norris geyser basin and deerbone spring lower geyser basin yellowstone national park wyoming usa quartz and sodium potassium geothermometers indicate that cistern spring is fed by a subsurface reservoir that is between 232 and 272 c calculated reservoir temperatures are significantly lower at deerbone spring 182 197 c based on a suite of measured and theoretical saturation indices downflow changes in the system resulting from evaporative concentration e g cl increases 10 changes in ph e g 5 6 7 1 and cation effects al and fe are of negligible importance in the subaqueous precipitation of hot spring opal a similarly at the macroenvironmental scale potential biotic effects on opal a precipitation appear to be minimal modelling of the two active siliceous sinter precipitating systems indicates that cooling e g 80 17 c is the predominant process governing subaqueous mineral precipitation

physico chemical and environmental controls on siliceous sinter formation at the high altitude el tatio geothermal field chile abstract el tatio geothermal field is located 4270 m above sea level in the altiplano northern chile siliceous sinter deposits from el tatio were studied to understand the influence of water chemistry and the extreme climatic conditions on their textures and mineralogy the results of this study show that the mineralogy of el tatio sinters include of opal a and accessory minerals such as halite gypsum and cahnite ca 4 b 2 as 2 o 12 4h 2 o which precipitate by full evaporation of high arsenic boron and calcium thermal waters el tatio sinters show a high degree of structural disorder probably linked to cation incorporation into the silica structure and or the occurrence of micro to nano scale accessory minerals the high content of cations in the thermal waters is strongly tied to relatively high silica precipitation rates considering silica concentration in water 147 285 mg l sio 2 precipitation rate reach 2 5 kg m 2 per year based on in situ precipitation experiments the particular environmental conditions of this high altitude geothermal area that produce high water cooling rate and high evaporation rate may also be responsible for the fast silica precipitation low environmental temperatures create freezing related sinter textures i e silica platelets and micro columns ridges silicified microbial filaments are also characteristic of el tatio sinters and they are indicative of water temperature and hydrodynamic conditions at the moment of sinter formation however sinter textural interpretation in a high altitude andean context must be done carefully as specific relationships between microbial and hydrodynamic textures are found

reconnaissance microearthquake survey of sulawesi indonesia several hundred earthquakes were recorded during the operation of a two week five station portable seismograph network on sulawesi indonesia forty one of these events were locatable and half of these occurred beneath the eastern gorontalo basin in a north dipping zone which extends from the batui thrust on the east arm of sulawesi to about 100 km depth beneath the gorontalo basin the batui thrust is the site of thrusting of the banggai islands continental fragment beneath the east arm ophiolite and oceanic crust of the gorontalo basin the observation of a zone of earthquakes dipping to the north from this thrust zone suggests that the leading edge of the banggai island block was subducted to at least 100 km depth the eastern gorontalo basin earthquake zone may connect with a deep seismic zone beneath the celebes basin beneath the western gorontalo basin a very narrow zone of earthquakes dips to the south probably within lithosphere of the celebes basin subducted at the north sulawesi trench three shallow earthquakes occurred near lake matano in central sulawesi possibly on the matano fault and their composite focal plane solution suggests east west extension the occurrence of only one earthquake on the palu fault and the lack of short s p times at a station operated on the fault indicate that this feature was very quiet during the survey period

evolution of a dynamic paleo hydrothermal system at mangatete taupo volcanic zone new zealand abstract recent quarrying and active faulting at mangatete taupo volcanic zone tvz new zealand illuminate a rare spatial and temporal window on a dynamic late quaternary geothermal system detailed geological mapping stratigraphic logging ams 14 c dating and textural and mineralogical analyses were used to construct a complex history of hydrothermal volcanological and tectonic activity from 36 to 2 ka extinct surface hydrothermal manifestations occur over a 2 km 2 area and include in situ siliceous sinters distributed on normal fault terraces an inferred hydrothermal eruption breccia heb containing acid etched sinter blocks another probable heb that was bathed in silicifying thermal fluids and sinter clasts that were entrained in a debris flow associated with a volcanic ash event preserved sinter textures typical of near neutral ph alkali chloride spring discharge channels aprons terraces and affiliated marshes comprise plant rich palisade tufted bubble mat and domal stromatolitic fabrics in addition a packed fragmental sinter facies is shown herein to constitute silicified microbial mats that were broken transported and deposited as point bar deposits in thermal spring fed streams moreover four unusual siliceous sinter fabrics vuggy globular spongy scalloped and arcuate wavy layered are interpreted to have formed from local acid sulfate chloride thermal springs possibly associated with paleo fumaroles the reconstructed history of paleo hydrothermal activity indicates that the oldest sinters 36 ka at mangatete developed in alkali chloride hot springs but then underwent post depositional alterion overprinting by acid sulfate steam condensate and were dismembered possibly by a hydrothermal eruption low ph hot spring discharges forming the unusual inferred acid sinter fabrics were localized in the same area a shift in paleo hydrology is evidenced by unaltered alkali chloride sinters dated between 22 and 3 ka a cluster of sinter dates between 22 7 and 20 7 ka denote thermal fluid emissions around the time of the okareka eruption 22 1 ka an 11 kyr gap in sinter ages followed which overlaps with a time of relative quiescence in local fault movement in between the rotorua and rotoma eruptions as determined from previous paleo seismicity studies on the whirinaki fault in the mangatete area at 9 4 ka hot spring activity in thermally fed streams slightly post dated the rotoma eruption 9 5 ka the locations and ages of the youngest post 9 4 ka alkali chloride sinters and silicified paleosols in relation to whirinaki fault movements indicate an overall westward migration of siliceous spring discharge dated charcoal 1 8 kyr in ash at the base of a locally extensive volcaniclastic debris flow containing sinter blocks of a range of ages and mineralogical maturity suggests its affiliation with the taupo eruption mangatete sinters occur along sw ne striking faults parallel to the regional trend of the tvz which dissect the topography into stepped terraces inferred basement cross faults in the study area i e structural lineaments orthogonal to this trend may have served as fluid pathways at depth initiating and maintaining hydrothermal activity at mangatete for more than 30 kyr within the larger ngakuru graben 150 km 2 the ages and distributions of sinters and hydrothermal eruption breccias including those at mangatete show that significant hydrothermal activity occurred for more than 60 kyr in an area lacking such activity today thus mangatete retains a rare stratigraphic record within the tvz sinter archive of changing geothermal fluids resulting from shifts in paleo phreatic water levels moreover although limited the presented age data imply a relationship among seismic activity volcanic ash fall events and the occurrence of permeable pathways for upward migration of thermal fluids finally whereas currently active tvz geothermal fields appear to be stable over tens of thousands of years or more the fossil hydrothermal systems in the wider mangatete area demonstrate the possibility of much greater temporal variability in the lateral expression of local surface manifestations of thermal fluid discharge through time

new insight into the origin of manganese oxide ore deposits in the appalachian valley and ridge of northeastern tennessee and northern virginia usa manganese oxide deposits have long been observed in association with carbonates within the appalachian mountains but their origin has remained enigmatic for well over a century ore deposits of mn oxides from several productive sites located in eastern tennessee and northern virginia display morphologies that include botryoidal and branching forms massive nodules breccia matrix cements and fracture fills the primary ore minerals include hollandite cryptomelane and romanechite samples of mn oxides from multiple localities in these regions were analyzed using electron microscopy x ray analysis fourier transform infrared spectroscopy and trace rare earth element geochemistry the samples from eastern tennessee have biological morphologies contain residual biopolymers and exhibit ree signatures that suggest the ore formation was due to supergene enrichment likely coupled with microbial activity in contrast several northern virginia ores hosted within quartz sandstone breccias exhibit petrographic relations mineral morphologies and ree signatures indicating inorganic precipitation and a likely hydrothermal origin with supergene overprinting nodular accumulations of mn oxides within weathered alluvial deposits that occur near to breccia hosted mn deposits in virginia show geochemical signatures that are distinct from the breccia matrices and appear to reflect remobilization of earlier emplaced mn and concentration within supergene traps based on the proximity of all of the productive ore deposits to mapped faults or other zones of deformation we suggest that the primary source of all of the mn may have been deep seated and that mn oxides with supergene and or biological characteristics result from the local remobilization and concentration of this primary mn

constraints on surface temperature 3 4 billion years ago based on triple oxygen isotopes of cherts from the barberton greenstone belt south africa and the problem of sample selection studies of earth9s surface temperature before 3 0 ga have focused heavily on the oxygen isotopic composition of silica rich sedimentary rocks called cherts interpretation of the results have suggested early surface temperatures ranging from as high as 70 15 c down to those that differ little from modern values a major controversy centers on whether differences in the oxygen isotopic compositions of cherts over time reflect changing surface temperatures changing ocean isotopic composition or post depositional diagenetic and metamorphic effects we here present results of triple oxygen measurements of 3 472 ga to 3 239 ga cherts from the barberton greenstone belt south africa the best preserved samples based on geological evidence have 917o and 918o values that plot generally on or near the equilibrium fractionation line for silica precipitated out of modern ice free sea water geologic considerations allow many potentially useful samples to be eliminated for paleotemperature analysis because of proximity to younger mafic intrusions or interactions with meteoric waters during deposition both of which tend to lower preserved isotopic values our results of triple o isotopic analyses of a suite of samples representing deposition under open marine shallow shelf conditions suggest that archean surface temperatures were well above those of the present day perhaps as high as 66 to 76 c they demonstrate that geologic context including depositional setting and post depositional history requires careful assessment before the significance of oxygen isotopic results can be evaluated

the origin of carbonaceous matter in pre 3 0 ga greenstone terrains a review and new evidence from the 3 42 ga buck reef chert abstract the geological record of carbonaceous matter from at least 3 5 ga to the end of the precambrian is fundamentally continuous in terms of carbonaceous matter structure composition environments of deposition preservation and abundance in host rocks no abiotic processes are currently known to be capable of producing continuity in all four of these properties although this broad view of the geological record does not prove that life had arisen by 3 5 ga the end of the early archean it suggests a working hypothesis most if not all carbonaceous matter present in rocks older than 3 0 ga was produced by living organisms this hypothesis must be tested by studies of specific early geological units designed to explore the form distribution and origin of enclosed carbonaceous matter the carbonaceous environmentally diverse 3416 ma buck reef chert brc of the barberton greenstone belt south africa provides an opportunity for such a study upward facies progressions in the brc reflect deposition in environments ranging from shallow marine evaporitic brine ponds to a storm and wave active shelf to a deep low energy basinal setting below storm wave base abundances and ratios of al2o3 zr tio2 and cr track inputs of various types of volcaniclastic and terrigenous clastic materials in particular zr al2o3 and zr serve as proxies for concentration of windblown dust and indirectly as proxies for sedimentation rate cu zn ni and feo were concentrated in the most slowly deposited transitional and basinal sediments inconsistent with a hydrothermal setting but consistent with a normal marine setting the distribution of microfacies defined by associations and layering of clastic ferruginous and carbonaceous grains correlates with facies transitions fine carbonaceous laminations which occur only in shallow platform settings represent photosynthetic microbial mats these were ripped up and the debris widely redistributed in shallow and deep water by waves and storms the isotopic composition of carbonaceous matter ranges from 35 to 30 in shallow water settings and to 20 in deep water units the heavier 13c in deep water carbonaceous matter is thought to reflect microbial processing possibly by fermentation and methanogenesis of organic matter originally produced in shallow water hydrothermal origins for brc carbonaceous matter are clearly excluded by the inferred depositional setting of the rocks as a whole an inference supported by field petrographic and geochemical analysis we suggest that the biological model proposed here for brc carbonaceous matter is the best currently available the hypothesis that at least some carbonaceous matter present in rocks older than 3 0 ga was produced by living organisms should be regarded as likely until extraordinary contradictory evidence is presented

topographic slope as a proxy for seismic site conditions and amplification we describe a technique to derive first order site condition maps directly from topographic data for calibration we use global 30 arc sec topographic data and v s 30 measurements here v s 30 refers to the average shear velocity down to 30 m aggregated from several studies in the u s as well as in taiwan italy and australia v s 30 values are correlated against topographic slope to develop two sets of parameters for deriving v s 30 one for active tectonic regions where topographic relief is high and one for stable shields where topography is more subdued by taking the gradient of the topography and choosing ranges of slope that maximize the correlation with shallow shear velocity observations we can recover to first order many of the spatially varying features of sitecondition maps developed for california our site condition map for the low relief mississippi embayment also predicts the bulk of the v s 30 observations in that region despite rather low slope ranges we find that maps derived from the slope of the topography is often well correlated with other independently derived regional scale site condition maps but the latter maps vary in quality and continuity and subsequently also in their ability to match observed v s 30 measurements contained therein alternatively the slope based method provides a simple approach to uniform site condition mapping

sedimentology and geochemistry of archean silica granules the production of biogenic silica has dominated the marine silica cycle since early paleozoic time drawing down the concentration of dissolved silica in modern seawater to a few parts per million ppm prior to the biological innovation of the first silica biomineralizing organisms in late proterozoic time inputs of silica into precambrian seawater were balanced by strictly chemical silica and silicate precipitation processes although the mechanics of this abiotic marine silica cycle remain poorly understood cherty sedimentary rocks are abundant in archean sequences and many previous authors have suggested that primary precipitation of amorphous silica could have occurred in archean seawater the recent discovery that many pure chert layers in early archean rocks formed as sedimentary beds of sand sized subspherical silica granules has provided direct evidence for primary silica deposition here we provide further sedimentological and geochemical analyses of early archean silica granules in order to gain a better understanding of the mechanisms of granule formation silica granules are common components of sedimentary cherts from a variety of depositional settings and water depths the abundance and widespread distribution of silica granules in archean rocks suggest that they represented a significant primary silica depositional mode and that most formed by precipitation in the upper part of the water column the regular occurrence of silica granules as centimeter scale layers within banded chert alternating with layers of black or ferruginous chert containing few granules indicates episodic granule sedimentation contrasting silicon isotopic compositions of granules from different depositional environments indicate that isotopic signatures were modified during early diagenesis looking to modern siliceous sinters for insight into silica precipitation we suggest that silica granules may have formed via multiple stages of aggregation of silica nanospheres and microspheres consistent with this hypothesis archean ocean chemistry would have favored particle aggregation over gelling granule formation would have been most favorable under conditions promoting rapid silica polymerization including high salinity and or high concentrations of dissolved silica our observations suggest that granule sedimentation was often episodic suggesting that granule formation may have also been episodic perhaps linked to variations in these key parameters

geologic record of partial ocean evaporation triggered by giant asteroid impacts 3 29 3 23 billion years ago although lunar studies suggest that large asteroid impact rates in the inner solar system declined to their present low levels at 3 8 3 7 ga recent studies in greenstone belts indicate that asteroids 20 km to 70 km in diameter were still striking the earth as late as 3 2 ga at rates significantly greater than the values estimated from lunar studies we here present geologic evidence that two of these terrestrial impacts at 3 29 ga and 3 23 ga caused heating of earth s atmosphere ocean surface boiling and evaporation of tens of meters to perhaps 100 m of seawater rapid ocean evaporation resulted in abrupt sea level drops erosion of the exposed sea floor and precipitation of distinctive layers of laminated silica representing marine siliceous sinter such events would have severely affected microbial communities especially among shallow water and photosynthetic organisms these large impacts profoundly affected archean crustal development surface environment and biological evolution until 3 2 ga or even later

silica diagenesis i solubility controls abstract the diagenetic sequence of silica is examined in terms of several thermodynamic and kinetic concepts the relationship between solubility and surface area or particle size is sufficient to explain simple opal a opal ct quartz transformations suggesting that the role of temperature and other physical parameters may not be as critical as previously thought complexation and adsorption in an impure system when added to the model allow for possibilities of opal a transformation directly to quartz of clay and zeolite formation from an opaline silica source and of variations in silica transformation rates due to presence of impurities in the system

unearthing the buried palu koro fault and the pattern of damage caused by the 2018 sulawesi earthquake using hvsr inversion

a holocene tsunami deposit in eastern scotland a thin regionally extensive laterally persistent sand layer identified within the holocene coastal sequences of eastern scotland dated to 7000 years bp is suggested to be a tsunami deposit the likely source of the tsunami wave is the earthquake induced second storegga slide on the norwegian continental slope at least 750 km northeast of the deposit

morphologic and mineralogic transitions from opal a to opal ct in low temperature siliceous sinter diagenesis taupo volcanic zone new zealand abstract the opal a to opal ct silica phase transformation in new zealand s siliceous hot spring deposits sinter is accompanied by recurring textural changes at the micron scale recognition of these changes is essential to extract paleohydrological paleoenvironmental and paleobiological signatures from ancient hydrothermal systems and to recognize diagenetic overprints upon primary depositional signals we examined 39 samples of siliceous sinter from the low temperature 35 c palisade microfacies at orakei korako and te kopia geothermal fields taupo volcanic zone new zealand in three different postdepositional environments least disturbed heat affected weathered friable to indurated samples were collected from modern presently silicifying microbial mats in the outflow of nearly neutral alkali chloride springs to sinter 3 500 years old the progressive mineralogical change from noncrystalline opal a to paracrystalline opal ct was traced by x ray powder diffraction and scanning electron microscopy one diagenetic sequence with two morphological pathways spherical and smooth polymeric and monomeric deposition respectively was identified at the micron scale both pathways ultimately lead to formation of typical opal ct bladed lepispheres along the spherical pathway initial morphological restructuring includes formation of circular holes 0 1 to 1 0 m in diameter in opal a spheres 3 0 m in diameter these changes correlate with a shift in the maximum intensity of the opal a x ray scattering broadband from 4 0 a to 4 09 a the characteristic position for opal ct an early transitional diagenetic stage is indicated in both sphere and smooth silica pathways by development of hexagonal platelets and a composite opal a ct scattering broadband that is sharp peaked in comparison and centered at 4 09 a a still sharper peaked composite opal ct a x ray diffraction band follows which has developed an incipient tridymite shoulder it is found in samples that display both hexagonal platelets and incipient fuzzy opal ct lepispheres at this late transitional stage patchy replacement textures are visible at the macroscale only with the appearance of numerous well developed opal ct bladed lepispheres is a typical sharp peaked 4 09 a opal ct x ray trace evident with a well defined tridymite shoulder this diagenetic process can produce either porous or vitreous fabrics depending on the degree to which infill of silica has reduced porosity and increased density of the deposit this study provides a textural mineralogical context for understanding silicification of microbial communities in geothermal settings and their subsequent diagenetic modifications these modifications continue long after geothermal activity has ceased with little or no burial and occur in horizons or localized patches of a given deposit moreover postdepositional conditions such as heat overprinting or weathering influence the diagenesis of siliceous sinter by increasing its maturation rate

the kohuamuri siliceous sinter as a vector for epithermal mineralisation coromandel volcanic zone new zealand the kohuamuri siliceous sinter is the largest known fossil hot spring system in the hauraki goldfield a 200 40 km volcanic terrain with at least 50 adularia illite epithermal deposits formed 16 3 5 6 ma within the coromandel volcanic zone new zealand the sinter is associated with rhyolite and ignimbrite of the whitianga caldera miocene pliocene and consists of two deposits the kohuamuri deposit itself a large in situ outcrop 47 000 m2 and its associated sinter boulder field 4500 m2 and the kaitoke deposit 900 m to the southwest comprising boulders in a landslide situated on a normal fault the well preserved macroscopic and microscopic textures at kohuamuri are similar to actively forming and ancient hot spring deposits elsewhere derived from deep circulating magmatically heated near neutral ph alkali chloride fluids oversaturated in amorphous silica and that discharge at the earth s surface at 100 c lithofacies petrography mineralogy as well as trace element concentrations of the kohuamuri kaitoke deposits were used to locate likely palaeo thermal conduits from the deep reservoir and to reconstruct the palaeoenvironmental setting of the siliceous sinter as an aid to assessing the economic potential of the ancient geothermal system both deposits contain the high temperature 75 c geyserite lithofacies with the kohuamuri deposit also exhibiting textures affiliated with cooler middle and distal sinter apron areas as well as geothermally influenced marsh facies trace element analysis of sinter lithofacies revealed concentrations and zonations of au ag base metals pb cu zn and pathfinder elements as sb associated with epithermal deposits elevated in the proximal vent area and providing evidence of possible au and ag ore mineralisation at depth the methodology used in this study could be utilised globally to identify and assess as yet unidentified epithermal deposits

influence of thermophilic bacteria on calcite and silica precipitation in hot springs with water temperatures above 90 c evidence from kenya and new zealand hot and boiling springs in kenya and new zealand that are emitting water with temperatures more than 90 c are commonly characterized by a complex array of caco3 and sio2 precipitates that have been formed through abiogenic and biogenic processes thermophilic bacteria are the only microbes that can survive in the boiling water that is discharged into pools around the spring orifice analysis of modern substrates from various springs in the kenya rift valley and the taupo volcanic zone in new zealand shows that they are inhabited by a diverse array of coccoid and filamentous bacteria in some areas these bacteria produce copious amounts of mucus that coat the substrates although the coccoid and filamentous bacteria provide substrates for caco3 and sio2 precipitation the microbes do not seem to have any direct influence on the morphology of the precipitates that are produced conversely the mucus found in these hot spring pools selectively takes up elements such as si mg al and fe but is not calcifi

impact of seasonal changes on the formation and accumulation of soft siliceous sediments on the discharge apron of geysir iceland abstract unconsolidated highly porous permeable siliceous sediments found in shallow water pools on the middle and distal parts of geysir s discharge apron are formed of cryogenic opal a coa opal a spheres opal a cement silicified microbes and detrital lithic grains silicified leaves and silica coated grass stems and twigs are locally present with these pool sediments and microbial mats commonly cover the sediment surface partially consolidated layers 0 5 cm thick form when the constituent grains are bound together by filamentous microbes exopolysaccharides and or opal a cement most components of the sediment can be related to processes that are controlled largely by seasonal variations low temperatures in winter promote freezing of the medial to distal discharge waters and lead to formation of coa in ice when the ice melts the released coa grains become part of the pool sediments microbial mats trap and bind sediment and provide templates for silica precipitation during spring and summer but become semi dormant during the constant darkness that iceland experiences during winter when the waters are sufficiently supersaturated opal a precipitates as spheres in the water column and cement in pool deposits in some pools seasonal changes are evident from the alternation of soft and hard partly cemented laminae elsewhere however the distinction between the different time dependent components is lost because they become superimposed on each other or as aperiodic changes in flow patterns affect different pools

patterns of isostatic land uplift during the holocene evidence from mainland scotland preliminary results of a lengthy and detailed investigation into middle and late holocene raised shorelines in mainland scotland indicate that three shorelines are sufficiently widely distributed to permit an examination of their extent and of the patterns of uplift which they indicate the shorelines are the storegga tsunami shoreline reached at c 7100 14c years bp at the culmination of the main postglacial transgression and the blairdrummond shoreline reached after the main postglacial shoreline at c 2000 4200 14c years bp at the time of a tsunami believed to have been caused by the second storegga submarine slide the main postglacial shoreline reached at c 5800 6850 14c years bp the main postglacial shoreline previously believed to be the highest holocene raised shoreline in scotland is now believed to be overlapped around the periphery of the uplifted area by the later blairdrummond shoreline isobase models based upon quadratic trend surface analysis of available comparable height dat

depositional evolution of an extinct sinter mound from source to outflow el tatio chile siliceous sinter deposits from el tatio chile preserve a wide variety of depositional environments and biosignatures from high temperature 85 c vent proximal facies to distal deposits dominated by silicified microbial mats four cores were drilled into an el tatio sinter mound and associated distal apron to investigate changes in hydrothermal environments over geologic timescales sedimentary and geochemical analysis of multiple sinter cores records the initiation and accretion of diverse depositional features still observed today in el tatio facies adjacent to hydrothermal vents are dominated by laminated sinter crusts on the steep margins of a high temperature pool with sparse microbial preservation outer margins of the same pool contain extensive sinter columns up to ten centimeters in length precipitated during repeated cycles of pool overflow and subsequent evaporation low relief hydrothermal pools also form minor deposits within distal debris aprons and analogous pools are still active close to sampling locations debris aprons are dominated by palisade tufted and arborescent microbial fabrics with distinct mat textures revealing well preserved microfossils surficial deposits in all cores feature detrital rich and microbially influenced sinters overlying higher temperature facies indicating a relative decrease in hydrothermal activity over time geochemical proxies for hydrothermal fluids and de trital input match depositional interpretations based on sedimentary structures 14 c ages from core deposits extend the mound s history by 11 000 years recording at least three thousand years of sinter deposition on top of glacial sandstones 13 337 10 232 y cal bp importantly this work provides a detailed depositional model unavailable through surficial sedimentology alone

collision rotation and the initiation of subduction in the evolution of sulawesi indonesia the island of sulawesi indonesia has been shaped and deformed as a result of collision with the sula platform a sliver of continental material from the northern margin of australia new guinea the collision has resulted in rotation of the north volcanic arm of sulawesi and the development of the accretionary wedge of the north sulawesi trench the north sulawesi trench changes laterally from a zone of no active deformation in the eastern part to a wide accretionary wedge in the west early stages of thrusting produce a steep frontal slope 8 16 indicative of relatively high basal shear stress whereas the more advanced western zone of thrusting produces a gentle 2 slope consistent with low basal shear stress reported paleomagnetic data suggest post late eocene counter clockwise rotation of the north arm and the offshore geophysics are explained by a pivot of the north arm with respect to the celebes basin about the eastern end of the arc convergence between the north banda basin and southeast sulawesi is documented by the presence of the tolo thrust its outcrop is strongly arcuate and its accretionary wedge varies in width from a minimum of a few kilometers at each end to a maximum of 30 40 km in the central part the northern end transforms to the leftlateral matano fault with a reported offset of 20 km the southern end of the thrust projects toward the deformed rocks of buton but the structural relations there are not clear the matano fault zone appears to connect westward with the palu fault which forms the western transform of the north sulawesi trench the palu matano fault system acts as a trench trench transform between the north sulawesi trench and the tolo thrust and this system is described by the same rotation pole as that for the sulawesi north arm

holocene spit development on a regressive shoreline dornoch firth scotland the series of raised and modern spits and beaches which characterise the dornoch firth coastline are described the beaches and spits in the inner firth are relatively small having developed during the mid holocene with little subsequent modification in the central firth large spit complexes developed on the northern and southern shorelines extending westwards during the mid holocene however during the later holocene spit growth was towards the east on the southern shore and towards the southwest on the northern shore in the outer firth spit and beach development mainly occurred during the later holocene the development of spits at the mouth of the firth changed the wave energy environment in the central section and resulted in the marked changes in spit alignment in this area the features also indicate that during the rise and culmination of the main postglacial transgression coarse clastic sediments derived from cliff erosion dominated in contrast during the later holocene when relative sea level fell sand sized material derived from the nearshore and offshore zone predominated

accretionary prisms in subduction earthquake cycles the theory of dynamic coulomb wedge 1 we expand the theory of critically tapered coulomb wedge for accretionary prisms by considering stress changes in subduction earthquake cycles building on the coulomb plasticity of the classical theory we assume an elastic perfectly coulomb plastic rheology and derive exact stress solutions for stable and critical wedges the new theory postulates that the actively deforming most seaward part of an accretionary prism the outer wedge overlies the updip velocity strengthening part of the subduction fault and the less deformed inner wedge overlies the velocity weakening part the seismogenic zone during great earthquakes the outer wedge is pushed into a compressively critical state with an increase in basal and internal stresses and pore fluid pressure after the earthquake the outer wedge returns to a stable state the outer wedge geometry is controlled by the peak stress of the updip velocity strengthening part of the subduction fault achieved in largest earthquakes the inner wedge generally stays in the stable regime throughout earthquake cycles acting as an apparent backstop and providing a stable environment for the formation of forearc basins the new theory has important implications for the studies of the updip limit of the seismogenic zone the evolution of accretionary prisms and forearc basins activation of splay faults and tsunami generation evolution of the fluid regime and mechanics of frontal prisms at margins dominated by tectonic erosion

chalcedonic quartz and occurrence of quartzine length slow chalcedony in pelagic sediments abstract chalcedony is the most abundant form of quartz in silicified pelagic sediments from the north pacific varieties of chalcedonic quartz present in chert of deep sea origin include chalcedony length fast and zebraic quartzine length slow and lutecite these occurrences of quartzine in known pelagic sediments emphasize the dangers of using quartzine as an indicator of former evaporitic environments quartzine is a diagenetic mineral and does indicate pore fluids rich in sulphate and magnesium in pelagic sediments it is always associated with authigenic barite and in many cases with authigenic dolomite quartzine should not be used by itself as an indicator of any particular environment of deposition

dating silica sinter geyserite a cautionary tale abstract we describe a new effort to date hydrothermal silica sinter deposits geyserite from the upper geyser basin of yellowstone national park using 14c of co deposited organic matter u series and cosmogenic 10be methods a majority of the samples were collected from stratigraphic sections mainly at riverside giant and castle geysers ages obtained from 41 14c analyses range from modern to 12 1 cal ka bp nearly all the 14c ages show inconsistencies with their stratigraphic positions and several replicate 14c analyses from the same sample result in significantly different ages the 13c values of the organic material in the sinter range from 26 6 to 12 7 the more enriched values are attributed to microbial fixation of dissolved inorganic carbon dic which has heavier 13c values and is 14c depleted relative to atmospheric co2 leading to an apparent older age u series analyses on 4 samples yielded ages between 2 2 and 7 4 ka large 230th u age uncertainties in the sinter due to low uranium concentrations along with elevated 232th and associated initial 230th make these ages imprecise for use on holocene deposits a single cosmogenic 10be exposure age of 596 18 ka is considerably older than the age of underlying rhyolite and is thus unreliable this apparent old age results from contamination by meteoric 10be trapped in the opal that overprints the very small amount of cosmogenic 10be by presenting the problems we encountered and discussing their probable cause this paper highlights the difficulty in obtaining reliable high precision geochronological data necessary to use sinter deposits as paleoenvironmental and paleo hydrothermal archives

a geothermally influenced wetland containing unconsolidated geochemical sediments hot spring waters flowing from porkchop geyser norris geyser basin yellowstone national park usa enter a shallow wetland basin and precipitate opal a silica particulate particulate formation by chemical rather than biochemical and colloidal mechanisms is suggested by floc and shard like particle morphologies comprising opal a silica nanospheres and microspheres of colloidal dimensions and precipitation from waters with opaque milky blue colouration indicative of aqueous silica sol conditions sediment accumulates in the wetland at a rate of ca 20 25 mm year is unconsolidated and massive to diffusely bedded to laminated post depositional features include soft sediment deformation and scouring and in drying conditions relatively deep desiccation establishment of geochemically dominated wetland sedimentation is favoured where alkali chloride hot spring fluids of circum neutral to basic ph and high silica concentration discharge to and cool to ca 35 c within topographic depressions that receive only small volumes of non hot spring water local wetland vegetation which is composed of hydrophytes halophytes and alkali tolerant species more typical of coastal wetlands colonizes the soft wetland substrate and may be relatively quickly buried by rapid sediment accumulation prior to the evolution of the diatom silica sink geothermal wetlands containing geochemically precipitated silica sediments may have been much more common and widespread rhizoliths chert nodules with organic cores scour fabrics soft sediment deformation desiccation cracks and massive to diffuse bedding preserved in palaeozoic geothermal environments may all be evidence of ancient unconsolidated geochemical sediments and geothermal wetland conditions

the holocene storegga slide tsunami in the united kingdom all currently known sites in the united kingdom with evidence for the holocene storegga slide tsunami are described information on the altitude distribution stratigraphical context age particle size profile and microfossil characteristics of the deposits is presented the tsunami involved a greater area than previously described reaching a coastline over 600 km long the ubiquitous sand layer which forms the main deposit associated with the event is shown to exhibit a consistent morphology and a particle size profile marked by fining upwards sequences an analysis of new and previously published radiocarbon dates indicates that from evidence in the united kingdom the event took place sometime around 7100 radiocarbon years bp 7900 calibrated years bp a new isobase model for mainland scotland and adjacent areas providing a preliminary estimate of land uplift since the tsunami is presented the model estimates contemporary sea surface level offshore at 14 m below the present day mean high water spring tides tsunami sediment run up is greatest in inlets where it reaches at least 25 m on shetland and at least 5 m along the mainland coastline to the south and run up of the tsunami would have exceeded these values the tsunami sediments identified here are considered particularly valuable as a synchronous marker horizon

a unified source model for the 2011 tohoku earthquake abstract the devastating 2011 tohoku earthquake was observed by dense networks of geophysical instruments a unified source model was constructed through joint inversion of teleseismic strong motion and geodetic datasets the result indicates that the earthquake consists of three main ruptures after small rupture in the initial 50 s the first rupture expanded at a slow speed of 1 8 km s to the northeast and east the second rupture began 20 s later at the slowest speed of 1 5 km s and became dominant with the largest slip of 36 m the third rupture then played the leading role propagating southward at a speed of 2 5 km s only the tsunami inversion requires an extension of the source fault to a shallow part between the first rupture area and the japan trench recovering tsunamigenic slips larger than 40 m the slow rupture speed and tsunamigenic slips can explain the features of the disaster by the earthquake

implications of fault slip rates and earthquake recurrence models to probabilistic seismic hazard estimates abstract increasingly fault slip rates are being used to constrain earthquake recurrence relationships for site specific probabilistic seismic hazard ground motion assessments this paper shows the sensitivity of seismic hazard assessments to variations in recurrence models and parameters that incorporate fault slip rates two models are considered to describe the partitioning of the slip rate or seismic moment rate into various magnitude earthquakes an exponential magnitude distribution and a characteristic earthquake distribution assuming an exponential distribution the activity rate n m 0 is constrained by the upper bound magnitude m u the b value for the region and the fault slip rate s for a given s variations in m u and b value have significant effects on recurrence and computed hazard depending on whether the assumption is made that the seismicity rate is constant or the moment rate is constant there is increasing evidence that the characteristic earthquake model is more appropriate for individual faults than the exponential magnitude distribution based on seismicity data from areas having repeated large earthquakes a generalized recurrence density function is developed and the resulting recurrence relationship requires only m u b value and s a comparison of the recurrence relationships from this model with the historical seismicity and paleoseismicity data on the wasatch and san andreas faults shows a good match the computed hazard based on the characteristic earthquake model differs from that obtained for the exponential model as a function of the fault to site distance and the acceleration level one check on the recurrence models using slip rate is to compare over large regions activity rates based on seismicity data and slip rate data such a comparison in the western transverse ranges shows a reasonable match for both the exponential and characteristic earthquake models in the moderate to large magnitude range

cyclic sedimentation in the eocene allenby formation of south central british columbia and the origin of the princeton chert fossil beds extensive deposits of eocene conglomerate sandstone shale and sub bituminous coal occur within a half graben at princeton british columbia these strata comprise 2000 m of fluvial sediment interbedded with tephra and volcanic flows near the southeast margin of the basin 200 m of rhythmically bedded strata are exposed along the similkameen river cyclic deposition resulted from repeated episodes of hydrologic and topographic change related to seismic activity along the boundary fault on the east flank of the princeton basin where allenby formation sediments are juxtaposed against metamorphic and metavolcanic rocks of the upper triassic nicola group interbedded carbonaceous shale and tuffaceous sandstone record influxes of distal lahar deposits into a lowland swamp coal breccia transitions originated when debris flows along the scarp carried volcanic debris into adjacent wetlands coal chert rhythmites are inferred to be evidence of hydrologic changes associated with floodplain tilt events that c

formation of silica oncoids around geysers and hot springs at el tatio northern chile siliceous oncoids up to 4 cm in diameter are common on the laterally extensive sinter aprons that surround the spectacular geysers and hot springs at el tatio in northern chile many of these complex oncoids developed close to geyser and spring vents that discharge boiling water internally the oncoids which are composed of precipitated amorphous silica are formed of complex arrays of spicules and concentric laminae as well as detrital volcanic grains spicular growth is dominant in most examples the formation and growth of the spicules and concentric laminae were mediated by a microbial community which included filamentous microbes mucus and possibly bacteria the microbes and mucus were silicified by replacement and encrustation in some laminae the filamentous microbes lay parallel to the growth surface in other laminae most filaments forming the thin mats were suberect amorphous silica precipitated between the filaments occluded porosity and commonly disguised the microbial fabric the oncoids grew on the proximal sinter aprons around the geyser vents and hot spring pools most growth took place subaerially with the silica delivered to the precipitation sites by splashing water from the geysers and or periodic shallow flooding of the discharge aprons unlike silica oncoids at other geothermal sites vertical growth of oncoids that formed in some rimstone pools was not limited by water depth

tectonic controls on atmospheric climatic and biological evolution 3 5 2 4ga abstract this paper presents a speculative scenario for the evolution of the earth s surface environment and biological community 3 5 2 4 ga based on the geologic record and its interpretation available geologic evidence suggests that the earth s climate before 3 2 ga was hot probably 60 73 c as a result of a co 2 and ch 4 greenhouse the early biological community was probably dominated by anoxygenic photosynthetic thermophiles cyanobacteria if they evolved before 3 1 ga would have struggled to survive at such high temperatures as a result the atmosphere contained extremely low levels of o 2 and 33 s shows wide departures from 0 indicating mass independent fractionation of s isotopes in the upper atmosphere this hot early climate collapsed 3 1 3 0 ga due to growth of large new blocks of continental crust cratons the weathering of which resulted in the depletion of atmospheric co 2 and eventual drawdown of ch 4 due to formation and rainout of methane aerosols cooling may have culminated in glaciation about 3 0 2 9 ga this climatic catastrophe and attendant changes in atmospheric composition drove a major biological revolution 3 0 2 7 ga characterized by the emergence of new low temperature taxa including cyanobacteria and their spread throughout surface environments at the expense of extreme thermophiles including methanogens during a period of general tectonic stability 3 0 2 7 ga the earth s surface was characterized by overall clement conditions with temperatures 2 after 2 75 ga and by 2 7 ga surface temperatures had returned to 60 c cyanobacteria were again suppressed o 2 production waned and atmospheric mass independent fractionation is again indicated by extreme variations in 33 s these conditions persisted perhaps intermittently until about 2 5 2 4 ga when weathering and erosion of vast new blocks of continental crust formed about 2 65 2 6 ga caused the second and probably last collapse of a 60 c surface climate broad tectonic climatic and biological events 3 5 2 9 ga are remarkably parallel to those of the late archean and paleoproterozoic 2 75 2 2 ga and neoproterozoic 1 0 0 50 ga these 500 myr long cycles of greenstone volcanism and crustal generation climatic and atmospheric instability and biological innovation reflect the long term interaction of the tectonic atmospheric climatic and biological components of the earth s surface system they suggest that the evolution of the earth s interior expressed through its control on the formation of large blocks of continental crust has influenced atmospheric composition and climate which in turn have provided a fundamental control on the timing and directions of biological evolution throughout earth history

the hot spring and geyser sinters of el tatio northern chile abstract the siliceous sinter deposits of el tatio geothermal field in northern chile have been examined petrographically and mineralogically these sinters consist of amorphous silica opal a deposited around hot springs and geysers from nearly neutral silica saturated sodium chloride waters water cooling and evaporation to dryness are the main processes that control the opal a deposition in both subaqueous and subaerial settings in close spatial relation to microbial communities all fingerprints of organisms observed in the studied sinter samples represent microbes and suggest that the microbial community is moderately diverse cyanobacteria green bacteria and diatoms the most important ecological parameter is the temperature gradient which is closely related to the observed depositional settings 1 geyser setting water temperature 70 86 c boiling point at el tatio 4200 m a s l coarse laminated sinter macrostructure with rapid local variations biota comprises non photosynthetic hyperthermophilic bacteria 2 splash areas around geysers water temperature 60 75 c laminated spicule and column macrostructure locally forming cupolas

relationship between spring and geyser activity and the deposition and morphology of high temperature 73 c siliceous sinter yellowstone national park wyoming u s a abstract six classes of alkaline hot springs and geysers are here distinguished in yellowstone national park based on activity and eruptive style 1 non surging non boiling springs 2 gently surging non boiling springs 3 non surging to gently surging boiling springs 4 strongly surging springs 5 fountain geysers and 6 cone geysers each displays morphologically distinctive siliceous sinter deposits in the high temperature 73 c parts of the vent and outflow systems non surging and gently surging non boiling springs that do not overflow their rims develop thin digitate to lilypad like sinter rims that grow horizontally over the pool surfaces similar springs with rim overflow have rims that accrete vertically boiling springs often have nearly circular steep sided pools and rims dominated by splash and rim overflow and composed mainly of spicular and columnar sinter fountain geysers characterized by broad fountain like eruption bursts have a central vent surrounded by a zone of spicular sinter beyond which is a broad flat bottomed moat but lack steep sinter cones cone geysers have narrow vents jet like discharges that cool rapidly and fall close to the vent and steep sided spicule dominated sinter cones sinter deposition takes place within three main zones 1 fully subaqueous sides and bottoms of vent pools outflow basins and outflow channels 2 periodically wetted splash zones surrounding hot springs and geysers and 3 largely subaerial parts of the rim and outflow areas wetted mainly by overflow and surging and that dry quickly fully subaqueous surfaces are covered by fine earthy particulate siliceous sediment pss composed largely of amorphous silica spherules and spherule aggregates that appear to have precipitated around heterogeneous nuclei including microbes splash zones of hot springs and geysers are characterized by spicular sinter largely subaerial zones wetted mainly by rim overflow and surging with little or no splash show complex varieties of terraced columnar and knobby sinter the highest silica deposition rates occur where surfaces are wetted frequently and evaporate to dryness such as the tops of spicules columns and knobs associated lower areas and crevices that remain wet or moist show slow accumulation of pss most continuously wet surfaces in high temperature 73 c hot spring and geyser vent and outflow systems are probably covered by microbial biofilms although microbes may locally influence rates of silica deposition and sinter microstructure the results of this study suggest that the depositional sites morphology and macrostructuring of high temperature sinter are controlled largely by the processes of surface wetting evaporation and drying

textural preservation in siliceous hot spring deposits during early diagenesis examples from yellowstone national park and nevada u s a abstract a petrographic mineralogical and geochemical investigation reveals evidence for selective preservation of microfacies in siliceous sinters plio pleistocene sinters from steamboat springs nevada contain identical but fewer petrographic textures than pleistocene sinters from artist point yellowstone national park wyoming the latter are preserved in altered sediments of unknown origin a sequence of pore filling and mineralogical changes explains the excellent preservation of nine microfacies and two petrographic textures plio pleistocene steamboat springs sinters are preserved in successions of andesitic basalt flows and were likely subjected to a more extreme thermal and a different hydrological environment than those from yellowstone national park most microfacies were obliterated during postdepositional heating of steamboat spring sinters differences in diagenetic histories related to depositional environment water chemistry and or subsequent burial must account for the loss of textural evidence between the diagenetic stage represented by artist point sinters and that of steamboat springs sinters hence early postdepositional history affects both the likelihood and quality of microfossil preservation

radiocarbon dating of silica sinter deposits in shallow drill cores from the upper geyser basin yellowstone national park abstract to explore the timing of hydrothermal activity at the upper geyser basin ugb in yellowstone national park we obtained seven new accelerator mass spectrometry ams radiocarbon 14 c ages of carbonaceous material trapped within siliceous sinter five samples came from depths of 15 152 cm within the y 1 well and two samples were from well y 7 depths of 24 cm and 122 cm these two wells at black sand and biscuit basins respectively were drilled in 1967 as part of a scientific drilling program by the u s geological survey white et al 1975 even with samples as small as 15 g we obtained sufficient carbonaceous material a mixture of thermophilic mats pollen and charcoal for the 14 c analyses apparent time of deposition ranged from 3775 25 and 2910 30 14 c years bp at the top of the cores to about 8000 years bp at the bottom the dates are consistent with variable rates of sinter formation at individual sites within the ugb over the holocene on a basin wide scale though these and other existing 14 c dates hint that hydrothermal activity at the ugb may have been continuous throughout the holocene

the miocene atastra creek sinter bodie hills volcanic field california and nevada 4d evolution of a geomorphically intact siliceous hot spring deposit abstract the atastra creek siliceous hot spring deposit or sinter occurs in the paramount bald peak alteration zone due north of the bodie precious metals mining district in the miocene bodie hills volcanic field california and nevada u s a distinctive features include its geomorphically intact geyser vent mounds the presence of growth fault stepped sinter terraces that developed westward along a nnw trending fault and atypical hg sb and as concentrations high temperature 75 c subaerial proximal apron sinter occurs in two settings 1 radiating botryoidal geyserite in vent mounds and 2 bedded geyserite columnar nodular size sorted geyser eggs intercalated with mid to low temperature

geothermal mineralization i the mechanism of formation of the beowawe nevada siliceous sinter deposit the siliceous sinter deposit at beowawe nev has a volume of 7 71 x 10 sup 7 m sup 3 and contains 1 28 x 10 sup 11 kg silica this silica precipitated from geothermal solutions that welled up along the malpais fault a graben bounding fault sinter formation involved three steps 1 geothermal solutions saturated with quartz in the reservoir moved to the surface where they cooled and became supersaturated with amorphous silica 2 amorphous silica particles nucleated heterogeneously to produce a colloidal suspension 3 the amorphous silica particles agglomerated and were cemented together by amorphous silica precipitating in the embayments between the particles at least 5 58 x 10 sup 14 kg of geothermal solution must have been produced over time to account for the mass of silica in the beowawe deposit this fluid would have carried at least 4 95 x 10 sup 17 kj of heat to the surface 21 references 6 figures 1 table

attenuation relations of strong ground motion in japan using site classification based on predominant period a spectral acceleration attenuation model for japan is presented in the present study the data set includes a very large number of strong ground motion records up to the 2003 off tokach main and aftershocks site class terms instead of individual site correction terms are used the site classes of recording stations are from a recent study on site classification for strong motion recording stations in japan according to a classification scheme that has been used in japanese engineering design the use of site class terms enables tectonic source type effects to be identified and accounted for in the present model the effects of a faulting mechanism for crustal earthquakes also are accounted for for crustal and interface earthquakes a simple form of an attenuation model with respect to distance is able to capture the main strong motion characteristics and achieves unbiased estimates for subduction slab events a simple distance modification factor is employed to achieve plausible and unbiased predictions the effects of source depth tectonic source type and faulting mechanism of crustal earthquakes are significant the need for magnitude squared terms is evaluated and the use of magnitude squared terms reduces the interevent error further

geyserite in hot spring siliceous sinter window on earth s hottest terrestrial paleo environment and its extreme life siliceous hot spring deposits or sinters typically form in active terrestrial on land volcanic terrains where magmatically heated waters circulating through the shallow crust emerge at the earth s surface as silica charged geothermal fluids geyserites are sinters affiliated with the highest temperature 75 100 c natural geothermal fluid emissions comprising localized lithologically distinctive hydrothermal silica precipitates that develop around geysers spouters and spring vents they demarcate the position of hot fluid upflow zones useful for geothermal energy and epithermal mineral prospecting near vent areas also are extreme environment settings for the growth of microbial biofilms at near boiling temperatures microbial biosignatures e g characteristic silicified microbial textures carbon isotopes genetic material lipid biomarkers may be extracted from modern geyserite however because of strong taphonomic filtering and subsequent diagenesis fossils in geyserite are very rare in the pre quaternary sinter record which in and of itself is patchy in time and space back to about 400 ma only a few old examples are known such as geyserite reported from the devonian drummond basin australia devonian rhynie cherts scotland and a new example described herein from the spectacularly well preserved late jurassic 150 ma yellowstone style geothermal landscapes of patagonia argentina there geyserite is associated with fossil vent mounds and silicified hydrothermal breccias of the claudia sinter which is geologically related to the world class cerro vanguardia gold silver deposit of the deseado massif a part of the chon aike siliceous large igneous province tubular filament like micro inclusions from claudia were studied using integrated petrographic and laser micro raman analysis the results of which suggest a biological origin the putative fossils are enclosed within nodular geyserite a texture typical of subaerial near vent conditions overall this worldwide review of geyserite confirms its significance as a mineralizing geological archive reflecting the nature of earth s highest temperature habitable terrestrial sedimentary environment hot spring depositional settings also may serve as analogs for early earth paleoenvironments because of their elevated temperature of formation rapid mineralization by silica and morphologically comparable carbonaceous material sourced from prokaryotes adapted to life at high temperatures

towards improved empirical isobase models of holocene land uplift for mainland scotland uk a new approach to modelling patterns of glacio isostatic land uplift during the holocene in mainland scotland uk is described the approach is based upon altitude measurements at the inner margin or locally highest point of raised estuarine surfaces dated by radiocarbon assay supported by microfossil analyses 2241 altitudes have been analysed by a technique new to studies of former sea levels gaussian trend surface analysis and isobase models for four holocene shorelines the holocene storegga slide tsunami shoreline abandoned rapidly circa 7900 sidereal years bp the main postglacial shoreline abandoned during circa 6400 7700 sidereal years bp the blairdrummond shoreline abandoned during circa 4500 5800 sidereal years bp and a speculative fourth shoreline the wigtown shoreline abandoned during circa 1520 3700 sidereal years bp are shown in a series of maps the implications of the shoreline patterns for glaicio isostasy in the area are discussed it is maintained that the statistical technique used enables broad estimates to be made of nearshore sea surface change

flandrian relative sea level changes in the ythan valley northeast scotland relict flandrian estuarine deposits in the ythan valley are confined to an area in the lower valley around the present estuary they rest upon a discontinuous layer of peat over an irregular surface of sand and gravel and consist for the most part of a grey silty clay or carse deposit which forms terraces or small areas of carseland at the seaward end of the estuary the grey silty clay contains a layer of grey micaceous silty fine sand which tapers into the basal peat whilst in places the surface of the grey silty clay is covered by peat and or blown sand the vegetational context and age of these deposits have been determined at waterside near the mouth of the present estuary here the grey silty clay accumulated as the result of a marine transgression which began in the early flandrian and culminated between about 6189 95 and some time prior to 4000 80 radiocarbon years b p the carseland surface produced by this phase of accumulation is the highest flandrian estuarine surface in the valley at about 4 5 m o d and is correlated with the main postglacial shoreline elsewhere in eastern scotland withdrawal of the sea from this level may have been interrupted by the formation of a lower carseland surface before present levels were reached though this is far from certain the grey micaceous silty fine sand layer is dated at approximately 6850 140 radiocarbon years b p and is correlated with similar layers that may have been deposited following a storm surge the paper concludes with some considerations on the age and distribution of the main postglacial shoreline in eastern scotland

fault behavior and characteristic earthquakes examples from the wasatch and san andreas fault zones paleoseismological data for the wasatch and san andreas fault zones have led to the formulation of the characteristic earthquake model which postulates that individual faults and fault segments tend to generate essentially same size or characteristic earthquakes having a relatively narrow range of magnitudes near the maximum analysis of scarp derived colluvium in trench exposures across the wasatch fault provides estimates of the timing and displacement associated with individual surface faulting earthquakes at all of the sites studied the displacement per event has been consistently large measured values range from 1 6 to 2 6 m and the average is about 2 m on the basis of variability in the timing of individual events as well as changes in scarp morphology and fault geometry six major segments are recognized along the wasatch fault on the basis of the most likely number of surface faulting events 18 that have occurred on segments of the wasatch fault zone during the past 8000 years an average recurrence interval of 400 666 years with a preferred average of 444 years is calculated for the entire zone geologic data on the distribution of slip associated with prehistoric earthquakes and slip rates along the south central segment of the san andreas fault suggest that the m 8 1857 earthquake is a characteristic earthquake for this segment comparisons of earthquake recurrence relationships on both the wasatch and san andreas faults based on historical seismicity data and geologic data show that a linear constant b value extrapolation of the cumulative recurrence curve from the smaller magnitudes leads to gross underestimates of the frequency of occurrence of the large or characteristic earthquakes only by assuming a low b value in the moderate magnitude range can the seismicity data on small earthquakes be reconciled with geologic data on large earthquakes the characteristic earthquake appears to be a fundamental aspect of the behavior of the wasatch and san andreas faults and may apply to many other faults as well

geyser eggs from te whakarewarewatangaoteopetauaawahiao north island new zealand waikorohihi and mahanga two geysers located on geyser flat at te whakarewarewatangaoteopetauaawahiao new zealand discharge chloride waters with a ph of 8 5 9 25 at 100 c many large up to 10 8 5 cm coated grains composed of amorphous silica opal a that are informally termed geyser eggs surround the vent of waikorohihi whereas smaller 3 cm geyser eggs are present in the proximal outflow channel of mahanga the geyser eggs have nuclei of detrital sinter or rhyolitic tuff their cortices are formed of thin concentric laminae and or spicular geyserite most cortical laminae contain a low diversity biota that includes filamentous microbes three types and scattered spherical microbes silicified grains of pinus radiata pollen are common in some laminae multiple phases of silica encrustation around the filamentous microbes disguised most features that are needed to confidently identify them most filaments are probably cyanobacteria variations in the microporosity and or microbe concentrations define individual cortical laminae growth of the geyser eggs in the pool around waikorohihi vent was controlled by the eruptive behavior of the geyser the eggs are submerged by boiling water during eruptions during dormant phases the geyser eggs are subaerially exposed while exposed their surfaces may remain moist from steam or rainwater or they may desiccate the low microbial diversity in the geyser eggs is probably related to their periodic inundation by boiling water and the highly variable conditions the abundant microbial laminae in the geyser eggs confirm that microbial templates are found in siliceous coated grains from almost all environmental settings in siliceous thermal spring systems

the storegga slides evidence from eastern scotland for a possible tsunami abstract the second storegga slide on the continental slope off western norway has been dated at between 8000 and 5000 yrs b p a prominent sand layer in flandrian holocene deposits along the coast of eastern scotland and dated at approximately 7000 yrs b p may have been deposited by a tsunami generated by the slide the altitude and stratigraphy of the layer allow estimates to be made of the magnitude of the earthquake which initiated the slide

the stratigraphy of flandrian relative sea level changes at a site in tayside scotland peat deposits in a gully at the inland margin of the firth of tay carselands contain a transgressive wedge of estuarine sediments laid down at the culmination of the main postglacial transgression in previous work the sequence of deposits in the gully and their age have been examined at one location the present study both extends and evaluates that previous work employing further field work and laboratory analyses the detailed stratigraphy of the deposits is identified whilst from a borehole close to the limit of the transgressive deposit diatom analysis and geochemical analyses involving the determination of al mg c and 13cpdb are outlined eight 14c assays from peat at the transgressive and regressive contacts in four boreholes along the wedge of estuarine sediments are described whilst the results of diatom analysis are inconclusive the geochemical analyses indicate that the sediment accumulated broadly continuously without major changes in the depositional environment it is concluded that the main postglacial transgression in the gully was at first rapid then slowed and culminated between 6240 80 and 6030 80 or 6170 90 radiocarbon years b p it is subsequently maintained that the geochemical analyses described here may be of value in determining the continuity of transgressive deposits whilst the practice of obtaining multiple radiocarbon assays at such locations should enable assessment of age to be refined this may in turn assist in testing the hypothesis of shoreline diachroneity in glacioisostatically affected areas

gently dipping normal faults identified with space shuttle radar topography data in central sulawesi indonesia and some implications for fault mechanics abstract space shuttle radar topography data from central sulawesi indonesia reveal two corrugated domal landforms covering hundreds to thousands of square kilometers that are bounded to the north by an abrupt transition to typical hilly to mountainous topography these domal landforms are readily interpreted as metamorphic core complexes an interpretation consistent with a single previous field study and the abrupt northward transition in topographic style is interpreted as marking the trace of two extensional detachment faults that are active or were recently active fault dip as determined by the slope of exhumed fault footwalls ranges from 4 to 18 application of critical taper theory to fault dip and hanging wall surface slope and to similar data from several other active or recently active core complexes suggests a theoretical limit of three degrees for detachment fault dip this result appears to conflict with the dearth of seismological evidence for slip on faults dipping less than 30 the convex upward form of the gently dipping fault footwalls however allows for greater fault dip at depths of earthquake initiation and dominant energy release thus there may be no conflict between seismological and mapping studies for this class of faults

actively growing siliceous oncoids in the waiotapu geothermal area north island new zealand oncoids that are actively growing in some of the shallow water pools around champagne pool waiotapu new zealand are formed of amorphous silica opal a with minor amounts of native sulphur the growth of these oncoids is being mediated by a high diversity microbial biota that achieves optimal growth in waters that have a temperature of 35 42 c and ph of 6 6 6 9 although this biota is dominated by coccoid bacilliform and filamentous cyanobacteria sulphur oxidizing bacteria are common in the cortical laminae of some oncoids in addition diatoms and silicified pollen grains are present in some cortical laminae the silicified microbes are superbly preserved with their three dimensional form and some of their internal structures being apparent including the sites of sulphur globules in the sulphur oxidizing bacteria the microbes probably acted as templates for silica precipitation and thereby mediated the growth of the oncoids some cortical laminae are formed entirely of silicified pollen grains that came from the pinus radiata that grow in the exotic pine plantations around champagne pool by using these laminae as a record of the annual pollination event typically in september november it can be shown that many of the oncoids are

shear wave velocities and primary influencing factors of campi flegrei neapolitan deposits abstract the campi flegrei caldera italy has been the site of intense seismic activity over the past decades this area is densely populated and includes important towns such as napoli with historical sites and supporting many industries for the proper use and management of the region the evaluation of the dynamic properties of near surface rocks is necessary the volcanological pattern has been reconstructed from the lithostratigraphies of several drillings the most interesting and widespread pyroclastic products are the pozzolana deposit soil and the neapolitan yellow tuff rock both pozzolana and tuff products are covered by recent eluvial and coastal sandy deposits and younger volcanic products

petrology and geochemistry of chert on the marginal zone of yangtze platform western hunan south china during the ediacaran cambrian transition the ediacaran cambrian transition was one of the most critical intervals in earth history during this interval widespread chert was precipitated commonly as a stratal wedge in carbonates along the southern margin of the yangtze platform south china the chert wedge passes into a full chert succession further basinward to the south east four lithotypes of chert are identified across the marginal zone in western hunan mounded vein brecciated and bedded chert the mounded chert is characterized by irregular to digitiform internal fabrics generally with abundant original vesicles and or channels that mostly are lined by botryoidal chalcedony cements with minor quartz and barite crystals the host chert or matrix of these mounds is dominated by amorphous cryptocrystalline silica commonly disseminated with pyrite the vein chert with minor quartz locally generally cross cuts the overlying dolostone and chert horizons and terminates under the mounded and or bedded chert bodies the brecciated chert commonly occurs as splayed intrusions or funnel shaped wedges and cross cuts the topmost dolostones the bedded chert the most common type generally is thin to medium bedded and laminated locally it is composed of amorphous silica with minor amounts of black lumps microthermometry of fluid inclusions from vein and void lining minerals mainly quartz and barite revealed homogenization temperatures from 120 to 180 c for the trapped primary fluids compositionally these chert deposits generally are pure with sio2 92 wt and only minor fe2o3 and al2o3 contents most of which show positive europium anomalies in rare earth element patterns especially for the mounded chert all these data suggest that the marginal zone chert deposits resulted from a low temperature silica rich hydrothermal system in which the mounded chert was precipitated around the releasing vents i e as silica chimneys the vein and splayed brecciated chert however was formed along the syndepositional fault fracture conduits that linked downward while the bedded chert was precipitated in the quieter water column from the fallout of hydrothermal plumes onto the sea floor these petrological and geochemical data provide compelling evidence and a new clue to the understanding of the extensive silica precipitation rapid tectonodepositional and oceanic changes during the ediacaran cambrian transition in south china

silica diagenesis origin of inorganic and replacement cherts abstract silicification of originally non siliceous sediments affects a wide variety of rock types and materials and ranges from minor to pervasive partial and minor chertification occur mostly in phanerozoic carbonates carbonate bearing sandstones evaporites and fossil wood the source of the silica is predominantly biogenic in petrified wood the silicification mechanism is a permeation or void filling process not a replacement in this example the sequence of silica phase transformation is the same as that in deep sea siliceous sediments in many silicified rocks particularly in certain carbonates the transformation sequence is different from that in radiolarites or diatomites the chemical environment and conditions of early diagenetic chert formation in shallow water carbonates are delineated by the general mixing model of knauth 1979 but remain unknown for most other types an exception are the flint nodules and bands of the english chalk a detailed geochemical study of the paramoudra flint structures by clayton 1986 provided remarkable insight into the replacement process seven different recurring silica fabrics have been recognized in chert replaced carbonates including equigranular microcrystalline quartz or microquartz and megaquartz and fibrous types chalcedony quartzine or length slow chalcedony lutecite zebraic chalcedony and microflamboyant quartz among the latter quartzine and microflamboyant quartz are common in but by no means restricted to chert replaced evaporites for which milliken 1979 recognized a sequence of seven quartz fabrics as a single criterion only anhydrite inclusions in megaquartz quartzine or microflamboyant quartz provide unequivocal evidence for an evaporite precursor the relative timing between silicification and well established diagenetic carbonate reactions shows that virtually all theoretically possible sequences occur chertification of carbonate host sediment thus may take place during early intermediate or late diagenesis and even during anchimetamorphism pervasive to complete silicification has been described in lacustrine pedogenic and hydrothermal volcanogenic rocks and occurs on the scale of individual beds members or entire formations it may affect some of the aforementioned rock types too for example carbonates the source of the silica is predominantly inorganic chert formation in these environments includes direct chemical silica precipitation from solution through a gel stage magadi type cherts result from the conversion of the hydrous sodium silicate magadiite into microcrystalline quartz which has occurred in east african rift valley lakes such as lake magadi in late pleistocene time they are closely related to directly precipitated inorganic cherts which have been observed in alkaline environments of playa lakes silcretes originate from weathering and soil forming processes under climatic and environmental conditions conducive to the formation of duricrusts and laterites in more humid climates however non weathering profile silcretes occur which have been distinguished from weathering profile silcretes four to five different silcrete fabric types have been established including the 1 grain supported or quartzitic 2 floating terrazzo 3 matrix albertinia and opaline and 4 conglomeratic types in volcanic edifices silicification related to hydrothermal activity occurs 1 along the ascent routes of the fluids vents within the volcanic complexes 2 in isolated ponds and depressions on the sea floor where the fluids discharge mostly in the median rift valley of mid ocean ridges and 3 in geothermal areas on land associated with spreading lineaments volcanic islands arcs transform faults or continental hot spots attention has focussed on the associated base metal concentrations and less on the silicification and accompanying iron enrichment processes the precambrian banded iron formations may be indirectly or directly related to hydrothermal volcanic processes the origin of these cherty iron formations certainly requires more than one genetic model and at present remains a major unresolved problem despite massive efforts to tackle it

loch lomond stadial and flandrian shorelines in the inner moray firth area scotland morphological mapping and stratigraphical investigations have identified surface and buried relict marine features in the inner moray firth area the features consist of a buried gravel layer formed during the loch lomond stadial a buried beach of early flandrian age and surface beaches and estuarine flats of mid late flandrian age analysis of the altitudes of morphological features has identified two buried and five possibly six surface glacio isostatically tilted raised shorelines the steepest shoreline is associated with the buried gravel layer and slopes down towards n20 e at a gradient of 0 20m km younger shorelines have lower gradients between 0 16 0 03m km the shoreline sequence combined with published data defines relative sea level movements in the area during the last 11000 years the inner moray firth shorelines are correlated with similar features in other areas of scotland which include the main lateglacial main buried and main postglacial shorelines

late devensian and holocene relative sea level changes in northwestern scotland new data to test existing models abstract pollen diatom lithostratigraphic and radiocarbon data from five sites in northwestern scotland provide new data from an area previously devoid of reliable and precise information on late devensian and holocene sea level changes the sites cover a range of palaeoenvironments indicative of diversity in coastal evolution since deglaciation for each site and palaeoenvironment the reference water tide level indicative range age and tendency of sea level movement of all sea level index points are quantified to enable correlation of the diverse coastal environments the data record patterns of relative sea level change and tendencies of sea level movement from 12 ka bp to 1 ka bp this is the longest and most comprehensive published record of relative sea level change from the area the information is used to test the accuracy of existing models of relative sea level change the results are only broadly consistent with a quantitative rebound model and there is significant disagreement with empirical models during the late devensian and the early holocene

the altitude and age of the main postglacial shoreline in eastern scotland abstract this paper discusses the patterns of altitude and age variation of the main postglacial shoreline in eastern scotland from the firth of forth to the dornoch firth for reasons of comparability attention is confined to the carseland areas the mean altitude of the shoreline declines eastwards from 14 8 m to 6 1 m od in the forth area from 11 2 m to 8 1 m in the earn tay area and northwards up the coast from the tay area to ca 6 4 m in the montrose basin area ca 3 9 m around the ythan estuary and ca 2 0 m near fraserburgh in the beauly firth inner moray firth area it declines northeastwards from ca 9 m to 7 1 7 9 m and declines to 5 7 m in the dornoch firth the isobase pattern for eastern scotland as represented by a quadratic trend surface conceals a significant regional variation in the tay area the age of the shoreline has been investigated at 14 sites grouped in 8 locations at varying distances from the centre of uplift the ages range from ca 6800 14c years bp in the upper forth carselands to 6095 75 5700 90 bp near fraserburgh with the exception of one site the data are consistent with the hypothesis that the shoreline is measurably diachronous having been abandoned progressively later with increasing distance from the centre of uplift the implications of these findings for understanding isostatic eustatic relationships are discussed

life inhabits rocks clues to rock erosion from electron microscopy of pisolite at a unesco heritage site in brazil rock erosion is attracting increasing attention from scientists worldwide the area encompassing the saint john baptist church saint john village xvii century ruins in rio grande do sul at the unesco world heritage site is considered a brazilian treasure however the risk of damage to this site from rock erosion has recently increased tremendously generally the rocky construction such as fence wall and tomb stone seems strong but is actually extremely sensitive to erosion caused by lichens fungi molds and bacteria because of biological erosion and massive exposure the fresh rock is dominated by clays and microorganisms water adsorbing clays and microorganisms influence the mechanisms of the rock erosion in this study the formation of bio clay minerals in porous structure of pisolite was demonstrated using electron microscopy bacterial clay mineralization can deform the rock structure and even produce organic materials biological activity could easily corrode rocky constructions around the saint john baptist church site the rocks are pisolitic laterites possibly formed in tertiary over the kretaceous parana flood basalts samples inhabited by lichens and fungi were collected from a collapsed wall in the ancient church the zonal reddish brown pisolites are 4 mm in diameter in a matrix of clays associated with porous and empty spaces elemental distribution maps from x ray fluorescence microscopy show iron rich spherules of pisolite whereas the matrix is composed of al si mn and sr thus producing goethite and kaolinite transmission electron microscopic observation showed that various types of bacteria inhabit the spherule and are associated with clay minerals and graphite stem elemental analysis confirmed the bio clay mineralization with al si s and fe around bacterial cells the results presented here will improve our understanding of nm scale bio mineralization and bio erosion in lateritic rocks

updip rupture of the 2004 sumatra earthquake extended by thick indurated sediments sediments accumulated in a subduction trench are usually unconsolidated and impede the updip propagation of fault rupture during an earthquake seismic images of the southern sumatra andaman trench reveal blocks of consolidated sediment that may have enabled fault rupture in 2004 to propagate up fault dip thus further seaward increasing the tsunami magnitude

microblock rotations and fault coupling in se asia triple junction sulawesi indonesia from gps and earthquake slip vector data the island of sulawesi eastern indonesia is located within the triple junction of the australian philippine and sunda plates and accommodates the convergence of continental fragments with the sunda margin we quantify the kinematics of sulawesi by modeling gps velocities and earthquake slip vectors as a combination of rigid block rotations and elastic deformation around faults we find that the deformation can be reasonably described by a small number of rapidly rotating crustal blocks relative to the sunda plate the southwestern part of sulawesi makassar block rotates anticlockwise at 1 4 myr the northeastern part of sulawesi the bangai sula domain comprises three blocks the central north sula block moves toward the nnw and rotates clockwise at 2 5 myr the northeastern manado block rotates clockwise at 3 myr about a nearby axis and east sulawesi is pinched between the north sula and makassar blocks along the boundary between the makassar block and the sunda plate gps measurements suggest that the trench accommodates 15 mm yr of slip within the makassar strait with current elastic strain accumulation the tectonic boundary between north sula and manado blocks is the gorontalo fault moving right laterally at about 11 mm yr and accumulating elastic strain the 42 mm yr relative motion between north sula and makassar blocks is accommodated on the palu koro left lateral strike slip fault zone the data also indicate a pull apart structure in palu area where the fault shows a transtensive motion and may have a complex geometry involving several active strands sulawesi provides a primary example of how collision can be accommodated by crustal block rotation instead of mountain building

seasonal changes in silica deposition in hot spring systems abstract monthly monitoring of water chemistry at octopus spring yellowstone national park reveals no changes in the chemical composition over this eight month study differences in silica deposition are attributed to flow dynamics and seasonal air temperatures colder air temperatures in winter promote visible accumulation of siliceous sinter probably as a result of decreased solubility periodic drying promotes accumulation and subsequent dehydration of the silica this combination of intermittent supply and periodic dehydration may account for the laminated siliceous deposits observed in the geological record the processes described herein are important in the siliceous preservation of microbial fossils but the following observations underscore the difficulty in preserving microbial fabrics in the geological record pools populated by cyanobacteria are bounded by margins built of silicified microbial mats silicification of the pool margins occurs through an evaporative process called wicking silica rich water is transported upward through the microbial mats by capillary action depositing silica at the surface of the mat as water evaporates the upper surface of such wicking deposits is extremely resistant to brecciation but the underlying support is highly friable and would be difficult to preserve intact in winter months temporary isolation of some pools from active flow causes fragile silica deposition around biological and other debris film coated gas bubbles on the surfaces of microbial mats are observed during all seasons and probably are filled with oxygen released during photosynthesis although this has not been confirmed they become coated with silica during winter months presumably because of increased cooling possibly forming thin discontinuous silica sheets as observed in the geological record