sedimentary pyrite formation an update sedimentary pyrite formation during early diagenesis is a major process for controlling the oxygen level of the atmosphere and the sulfate concentration in seawater over geologic time the amount of pyrite that may form in a sediment is limited by the rates of supply of decomposable organic matter dissolved sulfate and reactive detrital iron minerals organic matter appears to be the major control on pyrite formation in normal non euxinic terrigenous marine sediments where dissolved sulfate and iron minerals are abundant by contrast pyrite formation in non marine freshwater sediments is severely limited by low concentrations of sulfate and this characteristic can be used to distinguish ancient organic rich fresh water shales from marine shales under marine euxinic conditions sufficient h2s is produced that the dominant control on pyrite formation is the availability of reactive iron minerals calculations based on a sulfur isotope model indicate that over phanerozoic time the worldwide average organic carbon to pyrite sulfur ratio of sedimentary rocks has varied considerably high cs ratios during permo carboniferous time can be explained by a shift of major organic deposition from the oceans to the land which resulted in the formation of vast coal swamps at that time low cs ratios compared to today during the early paleozoic can be explained in terms of a greater abundance of euxinic basins combined with deposition of a more reactive type of organic matter in the remaining oxygenated portions of the ocean the latter could have been due to lower oceanic oxygen levels and or a lack of transportation of refractory terrestrial organic matter to the marine environment due to the absence of vascular land plants at that time

moderate temperature zeolitic alteration in a cooling pyroclastic deposit abstract the locally zeolitized topopah spring member of the paintbrush tuff 13 myr yucca mountain nevada u s a is part of a thick sequence of zeolitized pyroclastic units most of the zeolitized units are nonwelded tuffs that were altered during low temperature diagenesis but the distribution and textural setting of zeolite heulandite clinoptilolite and smectite in the densely welded topopah spring tuff suggest that these hydrous minerals formed while the tuff was still cooling after pyroclastic emplacement and welding the hydrous minerals are concentrated within a transition zone between devitrified tuff in the central part of the unit and underlying vitrophyre movement of liquid and convected heat along fractures from the devitrified tuff to the ritrophyre caused local devitrification and hydrous mineral crystallization oxygen isotope geothermometry of cogenetic quartz confirms the nondiagenetic moderate temperature origin of the hydrous minerals at temperatures of 40 100 c assuming a meteoric water source the topopah spring tuff is under consideration for emplacement of a high level nuclear waste repository the natural rock alteration of the cooling pyroclastic deposit may be a good natural analog for repository induced hydrothermal alteration as a result of repository thermal loading temperatures in the topopah spring vitrophyre may rise sufficiently to duplicate the inferred temperatures of natural zeolitic alteration heated water moving downward from the repository into the vitrophyre may contribute to new zeolitic alteration

zeolitic diagenesis of late quaternary fluviolacustrine sediments and associated calcrete formation in the lake bogoria basin kenya rift valley late quaternary fluviolacustrine siltstones mudstones and claystones loboi silts on the northern margins of the saline alkaline lake bogoria in the kenya rift valley contain up to c 40 authigenic analcime and minor natrolite the zeolitic sediments are reddish brown and up to 1 m thick the amount of analcime increases upward in the profile but decreases with distance from the lake the altered sediments show many pedogenic features including zeolitic root mats rootmarks concretions and carbonate rhizoliths residual patches of calcrete locally cap the zeolitic rocks the profile is interpreted as an exhumed palaeosol and land surface on the former margins of the lake the analcime occurs as submicroscopic 0 5 2 5 m subhedral and euhedral crystals which have an average si a 1 ratio of 2 33 as determined by x ray microanalysis or 2 18 d value of 639 analcime peak the analcime formed in lake marginal sediments soils by reaction of silicate detritus with na2co3 rich pore waters concentrated close to the land surface by evaporative pumping and evapotranspiration poorly ordered clay minerals were probably the main reactants authigenic illite may have been a by product of the reactions chemical analyses suggest that pore waters supplied some of na and possibly k and sio2 the associated calcrete and rhizoliths were formed during or shortly after the main period of zeolitic alteration the ca2 may have originated from infiltrating dilute runoff and groundwater authigenic smectite was precipitated in open porosity following analcime formation the zeolitic alteration at lake bogoria provides a relatively recent analogue for lake marginal zeolites found in many ancient saline alkaline lake sediments

water table effects on the measurement of earth strain abstract forced changes in the water head within a granite penetrating borehole were found to induce anomalously large free surface strains and tilts in the vicinity of the hole this deformation is shown to be due to fluid pressure induced changes in the aperture of a compliant hydraulically conductive fracture at a depth of 100 m and a quantitative analysis of the deformation data is performed to recover fracture characteristics the importance of the effect for secular earth strain measurements is discussed in the light of the ubiquitous nature of both water table fluctuations and fractures in the shallow crust

hydrogeologic control on zeolitic diagenesis of the white river sequence smectite opal ct and clinoptilolite are common alteration products in the white river sequence a mid tertiary unit in the great plains and northern rocky mountains the white river sequence is largely made up of fine grained tuffaceous material that is rich in unaltered rhyolitic glass smectite is ubiquitous and it or a precursor phase was present during white river soil formation but also formed after burial smectite fe from 5 to 7 fe 2 o 3 and ti from 0 4 to 0 8 tio 2 concentrations in mud rocks suggest formation from dacitic to latitic glass rhyotitic glass however was the primary source for opal ct and clinoptilolite opal ct and clinoptilolite have a highly variable distribution over macroscopic to regional scales of observation stable isotopic compositions of smectite 18 o smow 11 8 to 13 3 opal ct 18 o smow 19 9 to 24 3 and late diagenetic calcite 18 o smow 10 3 2 0 suggest that burial diagenetic reactions took place at temperatures ranging from 27 to 55 c clinoptilolite apparently formed in areas with high contact times between paleoground water and vitroclastic material in white river rocks of wyoming clinoptilolite occurs only in localities that are interpreted as lying within paleo ground water discharge zones in the great plains clinoptilolite is rare in eastern colorado and southern nebraska but progressively increases in abundance to the north reflecting a northward decrease in permeability zeolitic alteration near douglas wyoming changed the hydrologic role of tuffs from aquifers to aquitards tuffs were originally aquifers because of their high permeability 30 md relative to interbedded mud rocks 10 4 to 1 md signigicant thicknesses 0 5 to 2 m presentday and lateral continuity tuff alteration however decreased permeability by 4 orders of magnitude relative to glass rich portions similar diagenetic effects on mud rock permeabilities are likely mass flux into altered tuffs at douglas is indicated by an increase in bulk rock density from 1 3 g cm 3 for glass rich tuff to 1 7 and 2 0 g cm 3 respectively for zeolitic and siliceous counterparts local silica redistribution accouts for the bulk of the density increases a systematic increase in ba content of clinoptilolite over a 150 m vertical section from 0 1 to 6 6 mol of the exchangeable cations suggests that it was derived from underlying units

formation of zeolites in open hydrologic systems in contrast to those deposits formed by the alteration of volcanic ash in saline alkaline lakes discussed in the chapter by hay and sheppard this volume large volumes of tuffaceous sediments around the world have been transformed to zeolites and other authigenic silicate minerals by the action of percolating water in open hydrologic systems these include systems that have experienced significant chemical and hydrologic exchanges with the surrounding environment tephra sequences exposed to open system alteration commonly show a vertical zonation of zeolites and other authigenic minerals that reflects the chemical changes in meteoric water moving through the system flow in an open hydrologic system can either be downward or have a downward component where meteoric water enters the system resulting in a vertical or nearly vertical zonation of water composition and authigenic minerals the original pyroclastic materials of tuffaceous sediments may have been laid down in the sea close to the volcanic sources air laid onto the land surface or reworked into fluviatile and freshwater lacustrine environments open system zeolite deposits are most common in nonmarine rocks although many are known in sediments that were deposited in shallow marine environments zeolite deposits of the open system type are commonly several hundred meters thick and can be traced laterally for several tens of kilometers many examples of zeolite deposits that formed from land laid tephra have been recognized in the western united states but most of the large marine deposits are in japan and in southern and southeastern europe because of their relatively large and commonly discontinuous areal extent open system deposits have not been studied in as much detail as some other types of zeolite deposits in sedimentary rocks however studies of several key areas have provided a sound basis for the current understanding of this type of zeolite body zeolitic alteration can take place

authigenic silicates and silica polymorphs in the miocene saline alkaline deposits of the karlovassi basin samos greece the lower neogene unit of the karlovassi basin samos island greece was deposited in a continental basin during miocene times and consists of four successions overlying a basal conglomerate the four successions are from the base upward carbonates tuffaceous rocks claystones and porcelanites volcanic glass was originally abundant in the tuffaceous succession but is now represented by its diagenetic products including boron bearing k feldspar the zeolites clinoptilolite analcite and mordenite smectite opal disordered tridymite cristobalite cristobalite and minor amounts of the zeolites erionite phillipsite and chabasite and tridymite evaporite minerals such as gypsum celestite and thenardite are present along with authigenic silicates in tuffaceous intervals into the claystone succession authigenic silica polymorphs represented by opal and chalcedony were identified in the uppermost succession of porcelaneous limestones to chert five irregular concentric zones related to the presence and kind of zeolites and k feldspar are recognized in the lower neogene unit 1 a marginal zone of carbonate and detrital silicate minerals nonauthigenic silicates 2 a clinoptilolite zone 3 a mixed clinoptilolite analcite zone 4 an analcite zone and 5 a central boron bearing k feldspar zone the general pattern of zonation and distribution of authigenic silicates as well as their chemical characteristics indicates the presence of a saline alkaline lake during miocene times this lake is analogous to those of the same age which existed in southern yugoslavia and western turkey where borate sulfate and double carbonate deposits were formed along with authigenic silicates it is possible that a large continental area in the eastern mediterranean underwent large scale dessication which took place during the miocene messinian salinity crisis resulting in the development of highly saline alkaline lacustrine environments

petrology and geochemistry of diagenetically altered tuffaceous rocks from the middle triassic of central spain abstract petrographical and geochemical analysis of altered tuffaceous rocks of the middle triassic from the iberian range in central spain shows that pedogenic processes and subsequent diagenesis have strongly modified the original mineralogy and textures these rocks are related to a magmatic episode that occurred at the buntsandstein muschelkalk unconformity in this region during the evolution of the iberian trough the tuffaceous rocks accumulated in saline ponds in an open and terminal flood plain setting their alteration is typified by neoformation of k feldspar and illite precipitation of chert and carbonate cements and replacements were also important processes resulting in intercalated chert beds and nodules much of the silica appears to be derived from extensive devitrification and weathering of glassy volcanic ash x ray diffraction analysis microscopic observations and edx microanalysis reveal that the chert consists of various types of chalcedony tridymite micro and megaquartz illite k feldspar and small amounts of albite in some of the nodules dolomite ankerite is also common enclosing anhydrite and barite relicts which are partially silicified different levels of ammonium enrichment have been found in each petrofacies type tuffs and tuffaceous rocks contain from 27 to 326 ppm mean 139 of nh4 underlying arkosic sandstones with authigenic k feldspar contain nh4 ranging from 48 ppm to 191 ppm mean 124 the nh4 was probably derived from decay of organic matter in the evaporitic environment of deposition and its incorporation into feldspar and illite as they formed from unstable volcanic glass sanidine plagioclase and vitric volcanic fragments this transformation could have occurred via the intermediate phases of smectite and zeolites the tuffaceous rocks also have a high content of b ranging from 61 ppm to 170 ppm mean 101 however high levels of b are not positively correlated with high levels of nh4 in the samples both b and nh4 can be contained in either k feldspar or illite but it is not possible to know how much is present in each mineral without separating the minerals comparison of spidergrams of the tuffaceous rocks with those in the underlying arkosic sandstones indicates similar patterns with positive anomalies in la sm and y and negative in nb p ti and yb although the tuffaceous rocks are slightly richer in total ree these data suggest that the distribution of ree must also be controlled by weathering and diagenetic reactions and cannot be used for discrimination between volcanic and nonvolcanic sandstones

present day stress analysis of the st lawrence lowlands sedimentary basin canada and implications for caprock integrity during co2 injection operations abstract a geomechanical analysis of the st lawrence lowlands sedimentary basin is important to reliably estimate the maximum sustainable fluid pressures for co 2 injection that will not reactivate pre existing faults in the caprock thereby inducing a breeched co 2 reservoir this requires the determination of prevailing stresses orientations and magnitudes fault and fractures geometries and rock strengths the average maximum horizontal stress orientation s hmax is estimated n59 e 20 in the st lawrence lowlands the stress orientations were obtained from stress induced wellbore breakouts inferred from four arm dipmeter caliper data in 17 wells these wellbore failure features are confined to paleozoic lithological units of the st lawrence platform succession and frontal thrusts of the quebec appalachians at depths from 250 m to 4 km our results are consistent with the regional ne sw s hmax stress orientation trend that is generally observed in eastern canada and the u s the stresses pressure gradients estimated for the st lawrence lowlands depths hmin 20 5 3 kpa m s v 25 6 kpa m s hmax 40 7 5 kpa m pore pressure p p 9 8 kpa m indicating a strike slip stress regime s hmin v hmax the high angle ne sw regional faults and fractures in the paleozoic sedimentary succession and the grenvillian basement are oblique to the s hmax stress orientations 10 to 36 and could be reactivated slip tendency 0 34 to 0 58 under the present day stress field if fluid pressures exceeded the critical threshold further refinement of regional geomechanical model is required to estimate the maximum sustainable injection pressure necessary for shear reactivation along the regional faults the regional pore pressure stress coupling ratio under assumed parameters is about 0 5 0 65 and may contribute to reduce the risk of shear reactivation of faults and fractures the maximum sustainable p p that would not cause opening of vertical tensile fractures during co 2 operations is about 18 5 20 mpa for the depth of 1 km

recognition and implications of eocene tufas and travertines in the chadron formation white river group badlands of south dakota there are few descriptions of pre quaternary tufa or travertine yet these deposits have important climatic hydrological and tectonic implications in the badlands of south dakota the late eocene chadron formation contains a variety of tufa fluvial lacustrine paludal and perched springline types travertine and calcrete pedogenic and non pedogenic types in fluvial channels tufas and tufa barrages oncolites tufa lithoclast conglomerates and phytoherms are transitional to travertines pseudostromatolitic boundstone gravels and small pinnacle resurgences with rimstone edges and pools lacustrine tufas formed in small ponds adjacent to other types of tufas and are similar to the low energy bench margin facies types observed in modern groundwater fed temperate zone hardwater lakes paludal tufas are laterally discontinuous sheets of pisoid intraclast wackestone that fill palaeotopography distal perched springline tufas are mounds of numerous sloping tufa sheets concretions and rhizoliths associated with palaeosols are recognized as pedogenic calcretes whereas non pedogenic calcretes are variably cemented vertically oriented conduits for palaeogroundwater discharge that are located in fault zones these deposits resulted from palaeogroundwater discharge along faults during the late eocene coeval unroofing of the black hills uplift created a regional groundwater flow system through karsted palaeozoic limestones exposed in the black hills the chadron formation is an avulsion dominated siliciclastic unit yet important bodies of carbonate accumulated in it as a result of groundwater processes

deformation structures and an alteration zone linked to deposition of volcanogenic sulphate in an ancient playa oligocene of nebraska usa historic sulphur rich volcanic eruptions have altered global climate for as much as five years and much larger events are known from the geologic record at scotts bluff nebraska early oligocene strata of the lower arikaree group contain a tephra bed with abundant calcite pseudomorphs after gypsum previous work has shown sulphate from the pseudomorphs in this tephra bears a high 17 o anomaly indicative of oxidation of sulphur gases by ozone or hydrogen peroxide in the atmosphere possible sources of the tephra were caldera eruptions at about 28 ma in the san juan volcanic field of southwestern colorado 500 km sw of the study site and the eastern great basin 1000 km wsw the present sedimentological study shows that tephra and volcanogenic sulphate were deposited and preserved within a small surfacedischarging playa that developed on the irregular upper surface of aeolian siltstones of the subjacent white river group sulphate solutions including perhaps sulphuric acid percolated downward within the vadose zone dissolving early formed smectite cement within underlying volcaniclastic sandstones reddening these rocks along an irregular alteration front preserved fine scale stratification within the sandstones precludes the possibility that reddening took place during pedogenesis displacive growth of gypsum at the playa centre folded tephra beds and forced tephra into underlying sandstones forming elongate cones the large mass fraction of gypsum now replaced by calcite in the playa sediments suggests a huge long distance delivery of sulphate aerosols some of the sulphate and tephra may have come from the same eruption or the fine grained tephra may simply have aided preservation of dry fog sulphate derived from an unrelated effusive eruption of lava

characteristics of manganese ore deposits in china abstract manganese ore deposits are widely distributed in china based on features of regional tectonics and sedimentary environments as well as genetic characteristics seven important metallogenic provinces can be classified the commercial manganese ore deposits are distributed mainly on platforms and their marginal areas especially those of relatively mobile platforms the main metallogenic epochs of manganese ore deposits of china are sinian and devonian in contrast with the famous proterozoic cretaceous and oligocene manganese deposits elsewhere in the world based on their modes of origin manganese ore deposits in china can be divided into six types among them sedimentary and supergene deposits are the main types accounting for commercial manganese deposits according to the lithological association of the ore bearing series sedimentary manganese ore deposits can be classified into 1 mud rock type 2 black shale type 3 carbonate rock type in addition eight element association types can also be identified among which b mn s mn ca mg mn and p mn types are quite special for manganese ore deposits in china

alteration processes of the roque nublo ignimbrites gran canaria canary islands abstract the non welded lithic rich ignimbrites of roque nublo are the most characteristic deposits of the second magmatic cycle pliocene in age on gran canaria they are very heterogeneous with 35 55 by volume of lithic clasts 15 30 of moderately vesiculated pumice 5 7 of crystals and 20 30 of ash matrix the juvenile components pumice fragments and ash matrix are largely altered the mineral assemblage that originated by the alteration of vitric components consists mainly of zeolites chabazite phillipsite and analcime and subordinate smectites the alteration does not show any lateral or vertical gradation in the deposits and does not change in intensity with respect to the distance from the emission center these alteration processes affect only the roque nublo ignimbrites their associated fall and surge deposits remain unaltered a geo autoclave mechanism is proposed to explain the alteration processes which devitrified the juvenile components of the roque nublo ignimbrites and gave the rocks their characteristic strong induration this implies that the eruptions which led to the formation of the roque nublo ignimbrites had an important phreatomagmatic component and that the resulting pyroclastic flows were too dense and too poorly expanded to permit the water vapour to be separated from the vitric fragments during transport the alteration took place in situ and immediately after the deposition of the deposits when they were still hot by the reaction between the vitric components and the condensed vapour and cannot be attributed to late diagenesis or weathering

great salt lake and precursors utah the last 30 000 years sediment cores up to 6 5 m in length from the south arm of great salt lake utah have been correlated radiocarbon ages and volcanic tephra layers indicate a record of greater than 30 000 years a variety of approaches have been employed to collect data used in stratigraphic correlation and lake elevation interpretation these include acoustic stratigraphy sedimentologic analyses mineralogy geochemistry major element c o and s isotopes and organics paleontology and pollen the results indicate that prior to 32 000 year b p an ephemeral saline lake playa system was present in the basin the perennial lake which has occupied the basin since this time rose in a series of three major steps the freshest water conditions and presumably highest altitude was reached at about 17 000 year b p the lake remained fresh for a brief period followed by a rapid increase in salinity and sharp lowering in elevation to levels below that of the present great salt lake the lake remained at low elevations and divided at times into a north and south basin until about 8 000 year b p since that time with the exception of two short rises to about 1290 m the lake level has remained near the present elevation of 1280 m

consolidation settlements above deep tunnels in fractured crystalline rock part 1 investigations above the gotthard highway tunnel surface subsidence associated with tunnelling in fractured crystalline rock masses is rarely considered to be large enough to be a cause for concern recent high precision levelling measurements along the gotthard pass road in central switzerland however have revealed up to 12 cm of subsidence along sections that pass several hundred metres above the gotthard highway tunnel large scale consolidation associated with pore pressure reduction in the rock mass arising from tunnel drainage is believed to be the contributing mechanism although these settlements may appear to be small compared to those associated with groundwater or oil and gas withdrawal from more compliant porous media they are large enough to adversely affect the structural integrity of sensitive concrete structures on the surface e g thin arch concrete dams this is a concern for the 57 km long gotthard base tunnel which is currently under construction as it will pass through similar geological conditions as the gotthard highway tunnel and underneath several important dams the prediction of the expected settlements requires a more complete understanding of the processes underlying the settlements than exist at present this paper part 1 and a companion paper part 2 are a contribution towards this end in part 1 we examine the question of consolidation in fractured crystalline rock introduce the geodetic data and the geological setting of the subsidence associated with the gotthard highway tunnel possible mechanisms for the consolidation are described and estimates obtained for the key hydro mechanical parameters that govern their action such as the normal stiffness of fault zones these data are used to condition analytical scoping calculations of expected subsidence based upon poro elastic theory and to provide input for a series of numerical models used to obtain a more detailed understanding of the data which are presented in part 2 r 2008 elsevier ltd all rights reserved

mechanisms of pore pressure stress coupling which can adversely affect stress measurements conducted in deep tunnels rock stress measurements performed in boreholes drilled from a tunnel at a depth of 800 m indicate that the minimum stress has been reduced from pre excavation levels by drawdown in pore pressure the disturbance to the ambient stresses extends well beyond two tunnel diameters a model is developed to explain the inferred strong coupling between the pore pressure and minimum stress in the rock mass around the tunnel the model includes the effects of poro elasticity of the intact rock and the stiffness of the fractures for the field situation under study where the fractures are unusually complaint both poro elasticity and fracture compliance could be important contributors to the coupling changes in pore pressure are shown to produce reductions in minimum stress that are at least 50 of the drop in pore pressure and probably higher in more general situations the poro elastic component is likely to be the more significant of the two mechanisms and alone accounts for a coupling factor of 50 in this context the importance of fractures in crystalline conditions probably lies more in their promotion of penetrative drainage than their contribution to coupling coefficient the results highlight the importance of establishing the extent and distribution of pore pressure drawdown about galleries from which stress measurements are to be conducted

clay mineral assemblages and analcime formation in a palaeogene fluvial lacustrine sequence maiz gordo formation palaeogen from northwestern argentina abstract the palaeogene maiz gordo formation is one of the main lacustrine events recorded in northwestern argentina it consists of sandstone mudstone and limestone beds 200 m thick deposited in a brackish alkaline lake and braided alluvial systems the maiz gordo lake evolved mainly as a closed system with brief periods as an open one x ray diffraction xrd and scanning electron microscopy sem were used to study samples from seven sites corresponding respectively to proximal intermediate and transitional positions of the fluvial environment and marginal and inner lake environment focusing on the clay mineralogy and analcime formation the basinward zonation of diagenetic minerals identified in the maiz gordo lake was mordenite analcime k feldspar although not a typical zonation of saline alkaline lakes it does indicate an increase in salinity and alkalinity towards the centre in proximal fluvial settings smectite predominates at the base of the sequence with scarce kaolinite towards the top a striking increase in kaolinite content suggests a change from a relatively arid climate with alternating humid and dry seasons towards a warm and humid climate kaolinite content clearly decreases in a basinward direction such a variation is attributable to changes in hydro geochemistry denoting the progressive influence of the brackish and alkaline lake water on interstitial pores sem images of intermediate fluvial samples reveal authigenesis of illite at the expense of kaolinite booklets in littoral and inner lake settings the clay fraction is composed of muscovite sometimes with subordinate smectite analcime occurs in variable amounts in all sedimentary facies in rock pores or filling veins it forms subhedral square to hexagonal or anhedral rounded crystals denoting that they coarsened at low to moderate degrees of supersaturation although the mordenite identified in a fluvial level would have been the precursor of analcime in the maiz gordo basin no textural evidence of analcime formation through replacement of mordenite or other precursor zeolite was found hence it is more probable that analcime formation took place by direct authigenic precipitation or through the reaction between interstitial brines and clay minerals or plagioclase

thermogenetic degradation of early zeolite cement an important process for generating anomalously high porosity and permeability in deeply buried sandstone reservoirs abstract diagenetic explanations to anomalously high porosities and permeabilities in deeply buried sandstones have been presented for many reservoirs for example grain coatings of microquartz or chlorite can preserve porosity by preventing succeeding quartz overgrowths and dissolution of grains or cement can generate secondary porosity anomalously high porosity and permeability up to 25 and 500 md respectively characterize the deeply buried 5 km upper jurassic shoreface sandstones of the gert member heno formation in the hejre field in the danish central graben optical cathodoluminescence and scanning electron microscopy were used to characterize the petrography and diagenetic changes that resulted in this high pressure and high temperature hpht reservoir this was supplemented by chemical analyses icp oes and icp ms mineralogical composition xrd and oxygen isotope composition of quartz and calcite cement sims the petrographic investigation shows that none of the known explanations for anomalously high porosity and permeability can be applied hence a new hypothesis for deeply buried sandstones in contact with lavas and volcaniclastic lithologies is proposed the new hypothesis includes reducing mechanical compaction due to early patchy zeolite cement which was sourced from adjacent lavas and volcaniclastic rocks locally stylolites developed where zeolite cement was less effective hence the present overpressure formed at a later stage increased temperatures during burial resulted in thermal instability of the zeolite cement which was dissolved and authigenic k feldspar and quartz precipitated instead this resulted in a reduction of the cement volume of up to 70 if the zeolite was k heulandite and a corresponding relative porosity increase thus a reduction in cement volume was generated by replacing zeolite of an open crystal structure with mineral phases k feldspar and quartz of denser crystal structures the thermogenetic breakdown of zeolite was forced by increased temperatures during burial this process may explain previously enigmatic highly porous and permeable deeply buried sandstone reservoirs

cs relationships in shales hosting manganese ores from mexico china and newfoundland implications for depositional environment and mineralization abstract organic carbon and sulfide sulfur contents of host rocks and ore bodies selected from three manganese deposits were tested and plots of carbon against sulfur of the type proposed by berner were used to distinguish depositional environments molango in east central mexico is the largest of the three deposits examined the ore bed of the molango manganese deposit east central mexico is the basal unit of the chipoco facies taman fm kimmeridgian and is underlain by laminated black shales of the santiago formation oxfordian black shales of the santigo formation are high in organic carbon 0 88 2 43 and sulfide sulfur 2 45 9 27 s c plots suggest an euxinic marine environment the manganese ore bed consists of very finely laminated mn carbonate shows a poor correlation between organic carbon and sulfide sulfur and has a low content of sulfide sulfur the upper part of the chipoco facies consists of organic rich limestones and interbedded black shales both lithologies have a positive correlation between organic carbon and sulfide sulfur and may have been euxinic although less so than the santiago taojiang a manganese deposit of south china occurs in the middle ordovician modaoxi formation and is smaller than the molango deposit but is still minable the ore body consists of well crystallized rhodochrosite and mn calcite and is hosted by interbedded black and gray shales below the ore body and gray mudstone above the ore body the direct hanging wall and foot wall are mn limestones black shales contain high organic carbon 0 58 4 3 and sulfide sulfur 0 13 3 96 gray shale and gray mudstone contain low organic carbon and sulfide sulfur rhodochrosite ores and mn limestone have low organic carbon and sulfide sulfur contents the plots of carbon against sulfur show that the host rocks below the ore body are euxinic marine products whereas the gray mudstone above the ore body is normal marine the manual s river prospect is a thin but widespread bed of manganese in eastern newfoundland canada a manganese carbonate bed is the basal unit of the middle cambrian chamberlain s brook formation and consists of very fine grained rhodochrosite host rocks consist of sideritic gray shale gray shale and black shale above the ore body and green shale and gray shale below the ore body both the ore body and host rocks contain very low organic carbon 0 21 2 19 and sulfide sulfur 0 02 1 84 which indicates nomal marine conditions comparison of the three manganese occurrences suggests that a thicker development of euxinic strata before the onset of manganese deposition leads to a thicker manganese ore body if no euxinic conditions were present only sub economic manganese enrichments occur

jurassic lake t oo dichi a large alkaline saline lake morrison formation eastern colorado plateau a large alkaline saline lake lake t9oo9dichi9 occupied the entire eastern part of the colorado plateau region during deposition of the brushy basin member of the upper jurassic morrison formation the lake extended from near the site of albuquerque new mexico to near the site of grand junction colorado and occupied a region that encompassed the san juan and ancestral paradox basins making it the largest ancient alkaline saline lake known the lake was typically shallow and frequently evaporated to dryness although the water table was probably never far beneath the surface intermittent streams carried detritus from source areas to the west and southwest far out into the lake basin a reflection of a low depositional gradient and the ephemeral nature of the lake prevailing westerly winds carried silicic volcanic ash to the lake basin from an arc region to the west and southwest a lateral hydrogeochemical gradient characterized by increasing salinity and alkalinity from the margin of the lake to the center developed in pore waters of the lacustrine sediments and resulted in alteration of the ash to a variety of authigenic minerals that define concentric zones within the lake basin the basinward progression of diagenetic mineral zones is smectite clinoptilolite analcime potassium feldspar albite a concomitant lateral zonation of mixed layer illite smectite developed from highly smectitic in the outermost zones to highly illitic in the central zone authigenic albite and illitic mixed layer clay minerals previously thought to require elevated temperatures for formation crystallized during early diagenesis at syndepositional temperatures in the lacus trine sediments of lake t9oo9dichi9 this study presents evidence for a syndepositional origin for both of these minerals these findings have important implications in terms of limiting the use of these minerals as geothermometers recognition of alkaline saline lake deposits in the morrison formation significantly alters interpretations of depositional environments of this formation and it also has important implications for paleoclimatic interpretation late jurassic climate was apparently much more and than had previously been thought in fact sedimentologic evidence suggests that the lake basin was typically dry for extended periods and enjoyed only brief wet intervals this conclusion has important consequences for environmental interpretation of the habitat that was favorable for large herbivorous dinosaurs which thrived in the late jurassic

superheavy s isotopes from glacier associated sediments of the neoproterozoic of south china oceanic anoxia or sulfate limitation black shales and mn carbonates interbedded with glacial deposits from the neo proterozoic of southern china exhibit extremely heavy values of pyrite s isotopes that may reflect the peculiar environment of earth at this time 34 s averages 30 at tanganshan and 44 at xiangtan compared with typical values of 0 to 5 found in younger deposits furthermore there is no distinction between the shales and the mn carbonate ores in the neoproterozoic unlike the younger deposits which show much lighter 34 s in the shales than in the mn ores the spread is 25 most other chemical parameters are very similar to both the younger mn deposits and those from the paleoproterozoic the exception is the rare earth elements ree all neoproterozoic fe ores and most neoproterozoic mn ores lack the positive eu anomaly that characterizes archean and paleoproterozoic fe mn accumulations on the other hand neoproterozoic mn deposits have positive ce anomalies on north american shale composite nasc normalized plots in contrast to other mnco 3 ores the ree is also higher than in other mn deposits but lower than in modern deep sea crusts sulfide s values in all neoproterozoic shales tend to be exceptionally variable and to often show much heavier values than can be found in marine strata from the phanerozoic therefore the anomalous 34 s values we observed reflect peculiar conditions in the world oceans at this time rather than purely local effects times of enrichment of seawater sulfate in 34 s do not correspond to periods of glaciation so the likely cause of the s isotope patterns is not worldwide glaciation but a generally low level of dissolved sulfate s in the neoproterozoic oceans that allowed modest increases in the amounts of s removed as pyrite to drive down the oceanic s reservoir enough to produce strong rayleigh reservoir effects the abundance worldwide of sturtian age mn and fe deposits indicates an increase in fe flux to the oceans that would have been sufficient to depress so 4 2 levels severely and to result in residu al dissolved s extremely enriched in 34 s ree evidence indicates that most of this enhanced fe and mn flux came from diagenetic remobilization of detrital oxides

geological mineralogical and geochemical characteristics of zeolite deposits associated with borates in the bigadic emet and kirka neogene lacustrine basins western turkey the bigadic emet and kirka lacustrine basins of western turkey may be considered as tibet type graben structures that were developed during the miocene within the izmir ankara suture zone complex the volcanic sedimentary successions of these basins are made up of mudstone carbonate limestone and dolomite and detrital rocks and also of crystal or vitric tuffs about 135 to 200 m thick the degirmenli bigadic emirler bigadic kopenez emet and karaoren kirka tuffs constituting the zeolite deposits are situated beneath four borate deposits colemanite ulexite borax the most abundant diagenetic silicate minerals are k and ca clinoptilolites in the zeolite deposits and li rich trioctahedral smectites stevensite saponite and hectorite and k feldspar in the borate deposits in the degirmenli emirler kopenez and karaoren deposits the following diagenetic facies were developed from rhyolitic glasses rich in k and poor in na glass smectite k clinoptilolite opal ct caclinoptilolite k feldspar analcime quartz and k feldspar analcime quartz k feldspar which is also rarely associated with phillipsite karaoren and heulandite degirmenli and karaoren succeeds clinoptilolite and precedes analcime in these diagenetic facies where dioctahedral smectites opal ct and quartz are the latest minerals no diagenetic transformations exist between clinoptilolite k feldspar and analcime that were formed directly from glass the lateral facies distributions resulted from the differences in salinity and ph of pore water trapped during deposition of the tuffs but vertical distributions in vitric tuffs seem to have been controlled by the glass liquid ratio of the reacting system and the permeability or diffusion rate of alkali elements the bigadic emet and kirka zeolite deposits which were formed in saline basins rich in ca and mg ions show similar chemical changes i e loss of alkalis and gain in alkaline earth elements that have taken place during the diagenetic transformation of rhyolitic glasses to dioctahedral smectites or clinoptilolite the absence of sodic zeolites such as mordenite erionite chabazite and silica rich phillipsite is mainly due to the very high k na ratio of the starting materials rather than initial alkaline conditions or high na content in lake waters

adverse effects of pore pressure drainage on stress measurements performed in deep tunnels an example from the lower kihansi hydroelectric power project tanzania abstract we describe a campaign to measure stress within a gneissic ridge near the edge of the tanzanian craton the measurements were conducted to determine whether the level of minimum principal stress along the trajectory of a hydropower pressure tunnel attained the level of 10 mpa as required to leave the tunnel unlined extrapolation of early hydrofracture and overcoring measurements conducted mostly in boreholes drilled behind face suggested that the requisite level was not quite met however the minimum principal stress estimates from the instantaneous shut in pressures isips of the hydrofracture tests generally showed large scatter and were sometimes less than the estimated pre disturbance pore pressure it was suspected that the stress tests were affected by stress alteration around the tunnel due to pore pressure drainage to test this hydrofracture and hydro jacking tests were conducted in two long horizontal holes drilled ahead of the excavation face into relatively undrained rock and a third vertical hole drilled behind the face all were near the critical location where the pressure tunnel enters the powerhouse the minimum principal stress estimates in the ahead of face holes were found to be much higher than those from the behind face hole consistent with the hypothesis estimates of the drawdown in pore pressure about the tunnel and test holes suggested a coupling factor between drawdown and minimum principal stress magnitude of approximately unity this is slightly higher than predicted from an hydromechanical model that explains the coupling between the pore pressure and stress fields in terms of fracture compliance and poro elasticity of intact blocks nonetheless both results demonstrate the importance of accounting for drawdown when conducting stress measurements in boreholes drilled from tunnels we also note some instances where hydrofracture and hydro jacking tests yielded different estimates for the minimum principal stress magnitude

zeolites in closed hydrologic systems zeolites were first described from vugs and fissures in basaltic flows in the mid eighteenth century cronstedt 1756 and they are now known to be widespread in a variety of geological environments more zeolite species occur in vesicles and fractures of basaltic rocks than in any other geologic setting however zeolites from sedimentary rocks in particular represent the most important occurrences both in terms of aerial extent of deposits and in terms of the abundance of certain zeolite species the introduction of new analytical techniques in the latter part of the 20th century provided a significant stimulus to zeolite research these techniques allowed identification of large quantities of zeolites in widespread deposits of sedimentary rocks and in particular in sediments from saline alkaline lakes of the western united states sheppard and gude 1968 1969a 1973 based on geological and hydrologic environments zeolite occurrences in sedimentary rocks can be classified in the following framework a saline alkaline lakes b alkaline soils and land surfaces c deep sea sediments d open hydrologic systems e as products of hydrothermal alteration and f burial diagenetic or low metamorphic environments from a broader perspective zeolite deposits can be classified into two main groups namely closed hydrologic systems and open hydrologic systems the work of sheppard and gude 1968 1969a 1973 and later surdam 1977 discussing not only the mineralogy of the deposits but also the geology hydrology and chemistry of the depositional basins represent milestones in the study of closed hydrologic system formation of zeolites surdam 1977 considered closed hydrologic basins in two different tectonic settings a block faulted regions in arid and semiarid regions and b trough valleys associated with rifting examples of the first setting are the closed lakes of the basin and range province of the western united states whereas the

hydrothermal minerals and alteration rates at surtsey volcano iceland the volcanic island surtsey off the south coast of iceland was created by volcanic activity in 1963 1967 core from a 181 m deep hole extending 123 m below sea level shows the results of 12 yr of hydrothermal alteration of basaltic tephra the primary cause of heating of the tephra and of development of the hydrothermal system was the intrusion of dikes below sea level at present the hottest part of the hole at a maximum temperature of 150 c is cooling at 0 9 c per year palagonitization of sideromelane glass a dominant constituent of the tephra is an important alteration process that is strongly temperature dependent the rate doubling for every 12 c increase at 60 c 90 is palagonitized above 120 c olivine crystals are replaced on their edges by nontronite the thickness of clay doubles for each 8 c increase ten hydrothermal minerals have crystallized in the tephra at 25 to 150 c the dominant species are smectite nontronite analcite phillipsite and tobermorite the primary clay species of palagonite is probably nontronite other minerals are halite opal calcite chabazite xonotlite anhydrite and gypsum no major differences in mineral occurrence are noted above and below sea level but phillipsite and tobermorite tend to grow larger below sea level even at the same temperature analcite appears at lower temperature 55 c above sea level than below sea level 75 c anhydrite is most abundant deep in the hole where inflowing cool sea water precipitated sulfate due to reduced sulfate solubility at higher temperatures

diatom paleolimnology of late pliocene baringo basin kenya paleolakes abstract kenya s baringo tugen hills barsemoi drill site is one of six localities across kenya and ethiopia from which the hominin sites and paleolakes drilling project has obtained sediment cores in an effort to investigate the role of environmental forcing in shaping human evolution the baringo basin site features extensive exposures of the chemeron formation which contains 100 fossil vertebrate localities including five hominin sites the 228 m drill core dating from 3 29 to 2 56 ma is characterized by fluvio lacustrine sediments including multiple diatomites with evidence of variable degrees of later pedogenic modification in the lower part of the core 3 29 3 04 ma diatoms were preserved only in very low abundance consistent with predominantly fluvial or lake marginal environments in contrast five diatomites and two additional diatom rich intervals were deposited after 3 04 ma reflecting a major shift in the basin hydrology planktonic freshwater species dominated these diatom rich intervals whereas periphytic taxa were present in proportions less than 2 suggesting that these intervals represent open water deposition during lake highstands littoral or saline assemblages are largely absent throughout the core instead we observed a pattern of increasing diatom frustule dissolution at the tops and bottoms of diatomite units indicating increased alkalinity during the transgressive regressive phases a na bearing zeolite analcime indicative of saline waters precipitated in clastic dominated intervals between diatomites suggesting extreme environmental variability between lake highstands and lowstands diatom assemblages were consistently dominated by a few species belonging to the genera aulacoseira and stephanodiscus which were at times co dominant we infer that assemblages dominated by aulacoseira represent a well mixed lake with abundant supply of silica when stephanodiscus was dominant which occurred more frequently in the later freshwater phases we infer incomplete mixing and reduced silica flux to the epilimnion upper water layer

occurrence of zeolites in sedimentary rocks an overview zeolites have been known since the mid 1750s but prior to the early 1950s most reported occurrences of zeolites were in fracture fillings and amygdules in igneous rocks particularly basaltic lava flows indeed most of the large attractive zeolite specimens in museum collections were obtained from lavas in recent years zeolites have been recognized as important rock forming constituents in low grade metamorphic rocks and in a variety of sedimentary rocks most zeolites in sedimentary rocks are finely crystalline that is they occur as microscopic or submicroscopic crystals and they are therefore of little appeal to mineral collectors however deposits of this type are voluminous and have great geologic significance and economic potential zeolites are among the most common authigenic silicate minerals that occur in sedimentary rocks and they form in sedimentary rocks of diverse lithology age and depositional environment about twenty species of zeolites have been reported from sedimentary rocks but only eight zeolites commonly make up the major part of zeolitic rocks these are analcime chabazite clinoptilolite erionite heulandite laumontite mordenite and phillipsite this chapter will consider chiefly the zeolites in sedimentary rocks with emphasis on volcaniclastic deposits which contain the largest concentrations of zeolites the occurrence of zeolites in lava flows is mentioned only briefly journal articles on the occurrence and origin of natural zeolites have multiplied at a rapid rate since the mineralogy and geology of natural zeolites was first published in 1977 as volume 4 of the mineralogical society of america s reviews in mineralogy the present review will highlight areas of more recent research on the occurrence and origin of zeolites and some of the coexisting minerals a wide variety of zeolites has been identified in sedimentary deposits with the most common being clinoptilolite analcime heulandite laumontite and phillipsite less abundant zeolites include chabazite erionite mordenite natrolite