spectral mixture modeling a new analysis of rock and soil types at the viking lander 1 site a viking lander 1 image was modeled as mixtures of reflectance spectra of palagonite dust gray andesitelike rock and a coarse rocklike soil the rocks are covered to varying degrees by dust but otherwise appear unweathered rocklike soil occurs as lag deposits in deflation zones around stones and on top of a drift and as a layer in a trench dug by the lander this soil probably is derived from the rocks by wind abrasion and or spallation dust is the major component of the soil and covers most of the surface the dust is unrelated spectrally to the rock but is equivalent to the global scale dust observed telescopically a new method was developed to model a multispectral image as mixtures of end member spectra and to compare image spectra directly with laboratory reference spectra the method for the first time uses shade and secondary illumination effects as spectral end members thus the effects of topography and illumination on all scales can be isolated or removed the image was calibrated absolutely from the laboratory spectra in close agreement with direct calibrations the method has broad applications to interpreting multispectral images including satellite images

origin and implications of early diagenetic quartz in the mississippian bowland shale formation craven basin uk abstract silica cementation exerts a key control on the compaction and geotechnical properties of mudstones and by extension the style of hydrocarbon and or mineral systems present in a given sedimentary basin integrated microscopic and bulk geochemical observations demonstrate that siliceous mudstones in the bowland shale formation a target for uk shale gas extraction exhibit abundant dispersed discrete m scale quartz cements and exhibit silica enrichment excess above a local detrital si al threshold of 2 5 dissolution of siliceous radiolarian tests during early diagenesis is identified as the main source of silica opal a required for quartz precipitation either via opal ct or directly to quartz and potentially generated as a product of anoxic marine weathering dissolution of reactive silicates during early diagenesis excess silica correlates with free hydrocarbons s1 normalised to total organic carbon oil saturation index osi we propose early diagenetic quartz precipitation suppressed pore collapse buttress effect retaining the pore space capacity to host oil quartz precipitation was likely catalysed for example via low porewater ph elevated al and or fe oxide content and or abundant labile organic matter juxtaposition of siliceous mudstones and mudstones lacking quartz cement indicates silica was immobile beyond the bed scale thus metre scale siliceous packages likely represent more prospective units within the bowland shale in terms of unconventional hydrocarbons on the basis of early diagenetic biogenic derived quartz cementation leading to improved hydrocarbon storage capacity coupled to enhanced brittleness these findings are relevant for shale oil and shale gas systems specifically where oil retained in pores subsequently cracks to generate gas these findings also suggest the bowland shale is a sub class of black shale defined by the potential to host a relatively large volume of early diagenetic fluids derived from anoxic bottom waters which were potentially s and or metal bearing this is potentially relevant for understanding the genesis of adjacent and related pb zn mineral deposits

a simple model of pressure build up caused by porosity reduction during burial summary the fluid pressure build up due to porosity reduction in sedimentary basins during burial is studied the model assumes that the void ratio decreases exponentially with depth and that the permeability is proportional to the void ratio to an arbitrary exponent simple analytical solutions are obtained for the darcy velocity and the fluid excess pressure the pressure build up during burial is studied with these solutions and it is found to be inversely proportional to the gravity number the importance of the permeability exponents on the fluid pressure is also studied gravity numbers much less than 1 are shown to yield high excess pressures during burial a reasonable approximation for the maximum darcy velocity is found to be the product of the surface void ratio and the burial rate hydrofracturing is discussed in relation to the pressure build up and cases characterized by gravity numbers much less than 1 are found to yield hydrofracturing over large depth ranges it is suggested that the average permeability of hydrofractured sediments during burial corresponds to a gravity number equal to 1

a kinetically controlled fluid pressure and migration model new concepts of silicate diagenesis and their effects on rock properties have major implications for predictive models of geopressure and migration scale efficiency at temperatures greater than approximately 70 degrees c precipitation rate controlled internally sourced quartz cementation and neoformation of diagenetic clay become important porosity and permeability reduction factors respectively in shales clay diagenesis rapidly reduces permeability rendering them prone to overpressure the shale source rock capillary pressure is increased dramatically and later onset of hydrocarbon expulsion is facilitated by microfractures which allows for greatly increased expulsion and lateral primary migration efficiency quartz cementational porosity loss in sandstones and millstones proceeds mainly independent of pressure and hydrocarbon saturation which can result in hydrofracturing of the overlying seals shales the affect of hydrofracturing results in a coupling of thermally controlled porosity reduction processes fluid overpressure and episodic expulsion in areas of thick shales the probability of long range km scale vertical migration increases such that hydrocarbon migration to shallower sequences increases correspondingly the preservation of economically attractive hydrocarbon accumulations at depth is reduced except in reservoirs with mitigating geologic factors the underlying thermal controls of silicate diagenetic and organic maturation processes result in open quote self organized close quote basin scale fluid migration systems which optimizes the occurrencemore of economic hydrocarbon accumulations to certain stratigraphic structural levels within any given basin less

use of airborne imaging spectrometer data to map minerals associated with hydrothermally altered rocks in the northern grapevine mountains nevada and california abstract three flightlines of airborne imaging spectrometer ais data acquired over the northern grapevine mountains nevada and california were used to map minerals associated with hydrothermally altered rocks the data were processed to remove vertical striping normalized using an equal area normalization and reduced to reflectance relative to an average spectrum derived from the data an algorithm was developed to automatically calculate the absorption band parameters band position band depth and band width for the strongest absorption feature in each pixel these parameters were mapped into an intensity hue saturation ihs color system to produce a single color image that summarized the absorption band information this image was used to map areas of potential alteration based upon the predicted relationships between the color image and mineral absorption band individual ais spectra for these areas were then examined to identify specific minerals two types of alteration were mapped with the ais data areas of quartz sericite pyrite alteration were identified based upon a strong absorption feature near 2 21 m a weak shoulder near 2 25 m and a weak absorption band near 2 35 m caused by sericite fine grained muscovite areas of argillic alteration were defined based on the presence of montmorillonite identified by a weak to moderate absorption feature near 2 21 m and the absence of the 2 35 m band montmorillonite could not be identified in mineral mixtures calcite and dolomite were identified based on sharp absorption features near 2 34 and 2 32 m respectively areas of alteration identified using the ais data corresponded well with areas mapped using field mapping field reflectance spectra and laboratory spectral measurements

pressure solution and viscous compaction in sedimentary basins mathematical models of compaction in sedimentary basins typically assume a relationship between effective pressure pand porosity which is of a non linear type that is p p d however at depths greater than a kilometer pressure solution becomes important and this relationship approaches a viscous one we derive a mathematical model for viscous compaction in sedimentary basins and show how the model suggests different styles of behavior in the limits of slow nd fst compaction the initial pore pressure will be lithostatic over time the sediments compact and the resultant expulsion of pore water allows the pore pressure to approach a hy drostatic limit and in so doing the porosity is reduced at depths up to about a kilometer this compaction is elastic and due to the rearrangement of sediment particles at greater depths cementation and pressure solution occur and the latter causes further compac tion we call compaction by pressure solution viscous compaction and it forms the subject of the present pa per one of the interesting features of overpressuring is that it typically occurs suddenly at depths in excess of 3000 meters hunt 1990 and is often associated

contourite porosity grain size and reservoir characteristics abstract contourites are now recognised as having a significant potential as hydrocarbon reservoirs in the subsurface and several fields have been interpreted as comprising bottom current reworked turbidite sands however very little has been published on the porosity characteristics of contourites this study documents porosity data from iodp expedition 339 sites in the gulf of cadiz we use grain size analyses porosity depth plots and exponential models to yield a better understanding of grain size characteristics and facies porosity characteristics and the reservoir potential of contourites in the subsurface new grain size data for over 350 samples from the cadiz contourites is presented building on earlier work these data confirm the distinctive trends in textural properties linked to depositional processes under the action of bottom currents the finest muddy contourites porosity depth relationships from four cadiz sites show a moderately high initial porosity for both sand and mud facies 50 60 and a systematic decrease with depth to around 35 40 near 500 m burial depth according to the exponential models of porosity with depth contourite porosity should exceed 10 at 2500 m burial depth we compare the data from the gulf of cadiz contourite depositional system with those of the eirik drift newfoundland drift gardar drift and canterbury slope drifts similar depth trends are observed and all show anomalies linked to interbedded sandy and muddy facies composition carbonate vs siliciclastic and the presence of hiatuses in the sediment record these data provide good insight into the likely reservoir characteristics of contourites for both conventional and unconventional reservoirs they are comparable with those of existing contourite fields most of which are mixed turbidite contourite systems

modeling porosity evolution and cementation of sandstones abstract recent models of quartz cementation of sandstones attribute the source of silica to dissolution of quartz at stylolites bjorkum 1993 walderhaug 1994 aase et al 1996 oelkers et al 1996 quartz dissolution is believed to be catalyzed by the presence of mica illitic clayswhich are not dissolved or destroyed during the quartz dissolution process bjorkum1993 a supersaturation of silica is created at the stylolites which leads to a diffusive transport of silica into the interstylolite regionwhere the silica precipitates as cement interstylolite cementation is observed to be smooth which has led to the conclusion that the precipitation step is the slowest step in the entire quartz cementation process this is because the silica supersaturationcreated by dissolution of quartz at the stylolites is almost uniform throughout the interstylolite region the supersaturation is estimated to be 1 above the silica equilibrium concentration aase et al 1996 oelkers et al 1996 this estimate is based on the increase in the silica equilibrium concentration needed to grow microcrystalline quartz grains of a size 1 m petrographic studies of deeply buried jurassic north sea sandstones show that sandstones with microcrystalline quartz coatings have well preserved porosities microcrystalline quartz coatings are able to inhibit quartz cementation because the supersaturation created by dissolution of quartz at the stylolites is insufficient to grow the microcrystalline quarts aase et al 1996 the idea that stylolites are the source for silica cement was already suggested by heald et al 1955

analysis of simulated advanced spaceborne thermal emission and reflection aster radiometer data of the iron hill colorado study area for mapping lithologies the advanced spaceborne thermal emission and reflection aster radiometer was designed to record reflected energy in nine channels with 15 or 30 m resolution including stereoscopic images and emitted energy in five channels with 90 m resolution from the nasa earth observing system ami platform a simulated aster data set was produced for the iron hill colorado study area by resampling calibrated registered airborne visible infrared imaging spectrometer aviris data and thermal infrared multispectral scanner tims data to the appropriate spatial and spectral parameters a digital elevation model was obtained to simulate aster derived topographic data the main lithologic units in the area are granitic rocks and felsite into which a carbonatite stock and associated alkalic igneous rocks were intruded these rocks are locally covered by jurassic sandstone tertiary rhyolitic tuff and colluvial deposits several methods were evaluated for mapping the main lithologic units including the unsupervised classification and spectral curve matching techniques in the five thermalinfrared tir channels comparison of the results of linear spectral unmixing and unsupervised classification with published geologic maps showed that the main lithologic units were mapped but large areas with moderate to dense tree cover were not mapped in the tir data compared to tims data simulated aster data permitted slightly less discrimination in the mafic alkalic rock series and carbonatite was not mapped in the tims nor in the simulated aster tir data in the nine visible and near infrared channels unsupervised classification did not yield useful results but both the spectral linear unmixing and the matched filter techniques produced useful results including mapping calcitic and dolomitic carbonatite exposures travertine in hot spring deposits kaolinite in argillized sandstone and tuff and muscovite in sericitized granite and felsite as well as commonly occurring illite muscovite however the distinction made in aviris data between calcite and dolomite was not consistently feasible in the simulated aster data comparison of the lithologie information produced by spectral analysis of the simulated aster data to a photogeologic interpretation of a simulated aster color image illustrates the high potential of spectral analysis of aster data to geologic interpretation

pressure and temperature evolution in sedimentary basins summary this article is about what influence continuous sedimentation has on the excess pressure compaction and temperature in a sedimentary basin we give an answer in terms of two dimensionless numbers 4 and a1 which characterize the pressure and the temperature solution respectively 1 is shown to define a transitional zone between excess pressured basins with low compaction and hydrostatic basins with high compaction the 4 parameter generalizes earlier results of gibson the most important physical parameter in a is the ratio between the permeability and the sedimentation rate by means of we can quantify when high sedimentation rates and low permeability yield high excess pressures this analysis is based on the porosity given by formulae of the form 4 aps where ps is the effective stress and a is some scaling factor in an analogous manner ai 1 defines a transitional zone when moving boundary effects become marked on the temperature the most important parameter in ai is the ratio between a and the sedimentation rate simulation results are obtained by solving dimensionless pressure and temperature equations the numerical pressure solution is compared with the exact solution given by gibson and is shown to be in excellent accordance

contributions of compaction and aquathermal pressuring to geopressure and the influence of environmental conditions reply luo and vasseur 1992 presented a useful methodology of simplifying the complex equations that describe basin compaction and thereby directly assessing the contribution of aquathermal effects in the development of abnormal pressures their methodology leads to the conclusion that aquathermal pressuring plays only a small role in the development of overpressures in sedimentary basins although this conclusion follows from the cases studied it is possible to conclude otherwise using 1 a more general model of sediment compaction and 2 material properties that are more representative of conditions at depth the primary factor behind objection to the conclusion in luo and vasseur 1992 is that is fundamental equations which formed the basis for the simplifying methodology do not account fully for conditions in which a sediment s effective stresses are reduced we emphasize here that we do not claim that thermal effects are the dominant cause of overpressures rather our calculations suggest that they are not negligible in all cases there is a complex interplay of the various mechanisms that cause excess pore water pressures no one mechanism of overpressuring dominates all others for all situations because thermal effects not only cause higher excess pore pressures but also reduce effective stresses thermal effectsmore will be minimized by default in calculations that do not account for effective stress reduction the resulting reduced effective stresses also have important implications for geological questions such as acoustic velocities and fracture potential less

the effect of grain coating microquartz on preservation of reservoir porosity clay coatings have been widely accepted by many workers as an explanation for preserving high porosity in deeply buried sandstones but few workers have realized that similar effects can be produced by microcrystalline quartz coatings this phenomenon can be expected only under special circumstances but in such cases it can have profound consequences for exploration in the central graben area of the southern north sea unusually high porosity 20 27 and permeability 100 1000 md are found in certain zones in upper jurassic sandstones at depths of 3 4 4 4 km the porosity in these zones is 5 15 higher than expected based on average porosity depth trends from brent and haltenbanken sandstones we propose that the high porosity is due to continuous grain coatings of euhedral microcrystalline quartz crystals that are 0 1 2 m thick the distribution of microcrystalline quartz coatings is controlled by the presence of siliceous sponge spicules rhaxella which implies a sedimentological control on the reservoir quality we present a thermodynamic model showing how continuous microcrystalline quartz coatings inhibit development of no mal macrocrystalline quartz overgrowths sourced mainly from stylolites high porosities in parts of various upper jurassic oil fields ula and gyda have previously been explained by inhibition of quartz cementation by early hydrocarbon charge we suggest that the microcrystalline quartz coatings provide a more plausible explanation

mechanical and chemical compaction model for sedimentary basin simulators abstract this article presents a sediment compaction model for sedimentary basin simulators the concepts previously used in sedimentary basin models are generalized and described in our model based on the formalism specific to rock and soil mechanics sediment compaction is described on a geological time scale by an elastoplastic model in which the elastic modulus and the strain hardening modulus increase when deformation increases the plastic limit is the maximum vertical effective stress reached by the sediment the rheology of the sediment is defined by a relationship that couples the porosity or volume of the sediment with the vertical effective stress assuming uniaxial deformation the model also incorporates a viscoplastic term in the compaction equation this component macroscopically considers viscous compaction phenomena such as pressure solution the viscosity coefficient is considered to be a function of the temperature some theoretical considerations allow us to conclude that the thermal dependency of the viscosity is given with an arrhenius law in which the activation energy ranges from 20 kj mole to 50 kj mole using viscosity coefficients extrapolated from previous laboratory experiments a sensitivity study shows significant effects of viscous deformation on the compaction of basins older than 1 ma in another study the viscosity coefficient is determined by matching the results of numerical simulations with laboratory and borehole data obtained from literature for chalk a constant viscosity coefficient of 2 5 gpa ma 8 1022 pa s has been determined assuming viscosity as a function of temperature with an activation energy of 40 kj mole chalk viscosity at 15 c is calibrated around 25 gpa ma simulations with different thermal gradients show that porosity is a function of the temperature furthermore simulations covering different lengths of time show that porosity is also a function of time

factors controlling the thermo mechanical deformation of oil shales implications for compaction of mudstones and exploitation abstract six permian to miocene organic matter rich mudstones were subjected to unconfined compression tests under three thermo mechanical regimes the aims of this study were to assess the role of factors that control deformation and highlight implications for mudstone compaction and oil shale exploitation axial strain ranged from 1 9 to 23 at 25 c 5 3 70 mpa 12 79 at 25 310 c 31 42 mpa and 1 38 40 at 25 350 c 0 8 mpa the axial strain data showed that the principal factor controlling mudstone compaction is temperature with a secondary role for effective stress during high temperature tests dehydration of smectite in the 94 150 c interval resulted in a distinct axial strain effect decomposition of organic matter and other minerals caused visible strain only above 280 c tests up to 350 c resulted in mass loss of 4 54 43 8 wt with 1 1 8 wt due to organic matter a strong positive correlation was found between the petroleum generation indices 4 6 55 8 and expulsion efficiencies 38 6 96 2 while compaction 0 68 51 4 correlated with axial strain 1 38 40 solid volume loss 2 55 12 15 due to petroleum generation from kerogen resulted in an increase in porosity 1 56 6 36 low rate heating and low retorting temperature are necessary to optimise yield from oil shale exploitation

generation of overpressure by cementation of pore space in sedimentary rocks summary overpressure build up is studied when the main cause for porosity reduction is cementation of the pore space sourced locally the average porosity reduction for siliclastic sediments is modelled with nth order kinetics it is shown that the overpressure in one layer at a constant depth will decrease exponentially with time in the case of first order kinetics for the porosity reduction the overpressure is studied in one layer subjected to cementation during constant burial along a thermal gradient a small overpressure build up is shown above the window for cementation with a steep rise in overpressure in the upper part of the window the overpressure build up is then seen to decrease rapidly towards the end of the window for cementation overpressure build up is also studied when cementation is the main cause for porosity reduction in the entire column of sediments during deposition and burial the overpressure regime characterized by gravity numbers larger than one is studied this regime corresponds to low or moderate overpressures in the case of mechanical compaction cementation is shown to imply a steep pressure build up in the window of cementation which will easily exceed the lithostatic pressure the porosity loss due to cementation is seen to have a strong impact on the permeability which leads to the formation of a pressure seal although most of the potential for fluid expulsion is exhausted below the seal because most of the porosity is cemented up the permeability of the seal is sufficiently low for hydrofracturing to take place this scenario is consistent with overpressure observations in many wells

overpressure development in sedimentary basins induced by chemo mechanical compaction of sandstones abstract mechanisms of overpressure build up resulting from chemically induced compaction are investigated by considering that intergranular pressure solution controlled by temperature and effective stresses is the dominant process of chemical deformation in sandstones the numerical simulations are performed using a thermo poro mechanical tool based on the finite element method specifically devised to deal with sedimentary basin modeling at the material level purely mechanical and chemo mechanical deformations are respectively addressed by means of plastic and viscoplastic constitutive components porosity data of the middle jurassic garn formation from the haltenbanken area of the mid norwegian continental shelf are taken as reference for calibration of the sandstone model to be used on synthetic cases of a siliciclastic basin in oedometric conditions two situations are proposed involving different depositional sequences of sandy and shaly sediments with the aim to assess the permeability effect in the numerical model the results have shown that early overpressure development in low permeability formations preserves sandstone porosities by reducing effective stresses and thus retarding pressure solution compaction whereas higher effective stresses associated with permeable depositional environments may lead to important chemo mechanical deformation resulting in low porosity sandstones and significant overpressure generation at later stages of basin history an additional case is finally analyzed to investigate the consequences of pressure solution inhibition due to diagenetic grain coating in a sandstone reservoir the simulation resulted in porosity preservation and lower overpressure values showing that a sedimentary basin submitted to an important level of chemo mechanical compaction can present substantially higher overpressure distribution than basins where this phenomenon did not take place the paper highlights the importance of integrating stresses fluid pressure and temperature to represent the porous material evolution in order to describe the geological state of sedimentary basins

modeling of porosity loss during compaction and cementation of sandstones irreversible inelastic processes are responsible for mechanical and chemical compaction of sedimentary rocks at the time of burying our purpose is to describe the inelastic response of the rock at large time scales in order to do this we build a model that describes how porosity progressively decreases at depth we use a previous geometrical model for the compaction process of a sandstone by grain interpenetration that is restricted to the case of mass conservation in addition we introduce a compaction equilibrium concept solid grains can support stresses up to a critical effective stress c before plastic flow occurs this critical stress depends on temperature and is derived from the pressure solution deformation law pressure solution is the plastic deformation mechanism implemented during compaction our model predicts a porosity destruction at a depth of about 3 km this model has the property to define a range of compaction curves we investigate the sensitivity of the model to the main input parameters liquid film thickness grain size temperature gradient and activation energy

a unified approach to geopressuring low permeability zone formation and secondary porosity generation in sedimentary basins geopressuring secondary porosity and low permeability zones occur in many of the world s sedimentary basins and are often associated with major petroleum reservoirs we present a mechanical model which simultaneously reproduces these phenomena assuming pressure solution to be the dominant rock deformation mechanism a solid state rock skeleton viscosity is introduced which relates the deformation of the rock skeleton to the effective stress numerical solutions for compaction indicate that a soft sedimentary layer preferentially compacts to form a low permeability zone there is a large liquid pressure gradient through the low permeability zone in the more slowly compacting region beneath the low permeability zone the pressure gradient remains hydrostatic but the absolute pressure can exceed the lithostatic pressure in the absence of hydraulic fracturing this results in the formation of secondary porosity

simulated aster data for geologic studies the advanced spaceborne thermal emission and reflectance radiometer aster is a high spatial resolution imaging instrument scheduled to be launched on nasa s earth observing system am 1 satellite platform in 1988 aster acquires data in 14 bands spanning the visible near infrared short wavelength infrared and thermal infrared spectral regions with spatial resolution varying from 15 90 m depending on wavelength in order to evaluate the authors ability to use aster data for geological mapping they created a simulated 14 band aster data set for cuprite nevada the study site has sparse vegetation and exposes a wide range of unaltered and hydrothermally altered volcanic rocks the wide range of wavelengths covered by aster allowed them to distinguish iron oxide minerals clay bearing minerals sulfate minerals ammonia minerals siliceous rocks and carbonates based on interpretation of the aster data and in conjunction with laboratory and field spectral measurements they produced an alteration map showing the distribution of argillized rocks opalized rocks with alunite silicified rocks and areas dominated by kaolinite and buddingtonite the map was as accurate as published maps made by traditional field methods aster should be an improvement over existing satellite systems for geologic mapping

pressures and hydrogeology in petroleum basins abstract in most deep sedimentary basins in the world there is a layered arrangement of at least two superimposed hydrogeological systems the shallowest hydrogeological system can extend to great depths however in many basins it extends from the surface down to only about 10 000 ft greatest historical depth of burial the shallow hydrogeological systems generally are basin wide in extent and exhibit normal pressures the pore water apparently is free to migrate approximately in accordance with the theory of fluid migration proposed by munn 1909 and quantified by hubbert 1940 and 1953 the deeper hydrogeological systems usually are not basin wide in extent and exhibit abnormal pressures they generally consist of a layer of individual fluid compartments which are sealed off from each other and from the overlying system in some basins mainly in the onshore u s there is an even deeper near normally pressured noncompartmented section the compartmented layer in those basins generally is in the sequence of rocks which were deposited during the period of most rapid deposition the underlying noncompartmented layer where present usually is in prebasin shelf deposits and basement rock the uppermost noncompartmented layer usually is in rocks which were deposited during the last 10 000 ft in basin filling correlations of petroleum in reservoirs with the rocks from which the petroleum was generated has imposed a system of mainly upwards paths for the first several thousand feet of migration of petroleum from source rocks largely within the compartmented layer to pools generally in the shallow hydrogeological system the migration paths of petroleum and water may not be coincident

variation of al2o3 content in late cenozoic japanese basalts a re examination of kuno s high alumina basalt al2o3 variation in late cenozoic japanese basalts is re examined using the mg value as the differentiation index there exist no apparent differences in al2o3 among three postulated parental basalt magma types low alkali tholeiite high alumina basalt and alkali olivine basalt when the undifferentiated basalts with high mg values 55 are compared the al2o3 content of each type of basalt increases with decreasing mg value at mg values 55 and most basalts lack phenocryst plagioclase this feature is indicative of al2o3 increase by predominant mafic mineral fractionation and the absence of significant plagioclase fractionation among the differentiated basalts mg value 55 systematic differences of al2o3 content are observed the more alkalic basalt has the higher al2o3 content the origin of this variation is considered to result from a systematic difference in timing of the first appearance of plagioclase during crystallization from tholeiite through high alumina basalt to alkali basalt the progressive delay of plagioclase crystallization relative to mafic silicates may be explained by the gradual increase in h2o content in basalt magmas from tholeiite to alkali basalt

lithologic analysis from multispectral thermal infrared data of the alkalic rock complex at iron hill colorado airborne thermal infrared multispectral scanner tims data of the iron hill carbonatite alkalic igneous rock complex in south central colorado are analyzed using a new spectral emissivity ratio algorithm and confirmed by field examination using existing 1 24 000 scale geologic maps and petrographic studies color composite images show that the alkalic rocks could be clearly identified and that differences existed among alkalic rocks in several parts of the complex an unsupervised classification algorithm defines four alkalic rock classes within the complex biotitic pyroxenite uncompahgrite augitic pyroxenite and fenite nepheline syenite felsic rock classes defined in the surrounding country rock are an extensive class consisting of tuff granite and felsite a less extensive class of granite and felsite and quartzite the general composition of the classes can be determined from comparisons of the tims spectra with laboratory spectra carbonatite rocks are not classified and we attribute that to the fact that dolomite the predominant carbonate mineral in the complex has a spectral feature that falls between tims channels 5 and 6 mineralogical variability in the fenitized granite contributed to the nonuniform pattern of the fenite nepheline syenite class the biotitic pyroxenite which resulted from alteration of the pyroxenite is spatially associated and appears to be related to narrow carbonatite dikes and sills results from a linear unmixing algorithm suggest that the detected spatial extent of the two mixed felsic rock classes was sensitive to the amount of vegetation cover these results illustrate that spectral thermal infrared data can be processed to yield compositional information that can be a cost effective tool to target mineral exploration particularly in igneous terranes

analysis of potential debris flow source areas on mount shasta california by using airborne and satellite remote sensing data remote sensing data from nasa s airborne visible infrared imaging spectrometer aviris and the first spaceborne imaging spectrometer hyperion show hydrothermally altered rocks mainly composed of natroalunite kaolinite cristobalite and gypsum on both the mount shasta and shastina cones field observations indicate that much of the visible altered rock consists of talus material derived from fractured rock zones within and adjacent to dacitic domes and nearby lava flows digital elevation data were utilized to distinguish steeply sloping altered bedrock from more gently sloping talus materials volume modeling based on the imagery and digital elevation data indicate that mount shasta drainage systems contain moderate volumes of altered rock a result that is consistent with mount shasta s holocene record of mostly small to moderate debris flows similar modeling for selected areas at mount rainier and mount adams washington indicates larger altered rock volumes consistent with the occurrence of much larger holocene debris flows at those volcanoes the availability of digital elevation and spectral data from spaceborne sensors such as hyperion and the advanced spaceborne thermal emission and reflectance radiometer aster greatly expands opportunities for studying potential debris flow source characteristics at stratovolcanoes around the world

analysis of airborne visible infrared imaging spectrometer aviris data of the iron hill colorado carbonatite alkalic igneous complex the iron hill colorado carbonatite alkalic igneous rock complex consists mainly of pyroxenite uncompahgrite fenite and nepheline syenite and carbonatite the study area has been explored intermittently for several decades for thorium niobium rare earth elements titanium and vermiculite airborne visible infrared imaging spectrometer aviris data were acquired for the iron hill complex on august 28 1992 under clear sky conditions the data were calibrated using a laboratory reflectance spectrum of a tuff sagebrush mixture to represent the spectral response of a topographically flat homogeneous image area within the ash flow tuff analysis of the data was conducted using the spectral angle mapper sam and linear spectral unmixing algorithms in the spectral image processing system sips image spectra in the 2 1 to 2 45 mu m wavelength region were used in the sam method to map dolomite in the iron hill carbonatite rauhaugite stock and in several smaller bodies calcite in a small carbonatite soevite plug and in travertine spring deposits illite mainly in sericitized felsite and granite and kaolinite in hydrothermally altered ash flow tuff the general distribution of the main alkalic igneous rocks pyroxenite uncompahgrite and ijolite was mapped using a full aviris wavelength range 0 40 2 45 mu m image spectrum of pyroxenite excluding channels affected by strong atmospheric absorption however some areas consisting of felsite and fenite were not spectrally distinguished from the alkalic rocks linear spectral unmixing using full wavelength image spectra was used to improve the compositional mapping achieved using the sam algorithm the fractional abundance images obtained for dolomite and calcite were similar to the sam results but the distinction between illite and kaolinite was significantly improved especially where scattered coniferous trees are present hematite and goethite which showed widely overlapping patterns in the sam similarity images were distinguished much better in the fractional abundance images biotite was used instead of pyroxenite as a reference end member in the unmixing analysis and the results were better than those obtained in the sam analysis

origin of overpressures in shales constraints from basin modeling techniques for detection evaluation and prediction of pore pressures in low permeability rocks and equations for fluid pressure computations in most integrated basin modeling software are based on relationships between porosity and effective stress in shales however recent data show that overpressured shales in the north sea do not exhibit higher porosities than the normally pressured shales of the same formation at similar depths to further evaluate the existence of porosity vs effective stress relationships in shales fluid flow simulations and porosity modeling in a typical high pressure and high temperature well in the north sea were undertaken the parameters in the permeability and porosity equations were adjusted until a satisfactory fit was achieved between the observed and modeled porosity and fluid pressure at present however the modeled porosity and pore pressure vs depth history of the sediments deviated significantly from known porosity and pore pressure vs depth relationships that have been observed in north sea shales and elsewhere today because the results from basin modeling based on porosity stress relationships were unacceptable irrespective of parameter choices and the well data from the north sea show no signs of elevated porosities in the overpressured shales it is inferred that effective stress driven compaction alone has not generated the hard overpressures observed in deeply buried north sea shales these conclusions are suggested to be generally applicable to shales with low porosities and hard overpressures worldwide both because of the physics involved and because similar results can be extracted from published modeling in the niger delta

a mathematical model for compaction in sedimentary basins summary a mathematical model for the non equilibrium compaction of clay rocks in sedimentary basins is formulated the model generalizes those of earlier authors the simplest assumptions are made concerning the rheology and diagenesis is neglected in this case we show that the model reduces to a generalized consolidation equation which for the classical darcy flow is a non linear diffusion equation for the porosity with a free boundary the model is non dimensionalized and it is shown that solutions depend on one significant dimensionless parameter a which is the ratio of the darcy flow rate and the sedimentation rate the model is solved numerically and asymptotic descriptions of the solutions are given for the cases of large and small a

mapping playa evaporite minerals with aviris data a first report from death valley california abstract efflorescent salt crusts in death valley california were mapped by using airborne visible infrared imaging spectrometer aviris data and a recently developed least squares spectral band fitting algorithm eight different saline minerals were remotely identified including three borates hydroboracite pinnoite and rivadavite that have not been previously reported from the death valley efflorescent crusts the three borates are locally important phases in the crusts and at least one of the minerals rivadavite appears to be forming directly from brine borates and other evaporite minerals provide a basis for making remote chemical measurements of desert hydrologic systems for example in the eagle borax spring area the aviris mineral maps pointed to elevated magnesium and boron levels in the ground waters and to the action of chemical divides causing subsurface fractionation of calcium many other chemical aspects of playa brines should have an expression in the associated evaporite assemblages certain anhydrous evaporites including anhydrite glauberite and thenardite lack absorption bands in the visible and near infrared wavelength range and crusts composed of these minerals could not be characterized by using aviris in these situations thermal infrared remote sensing data may complement visible and near infrared data for mapping evaporites another problem occurred in wet areas of death valley where water absorption caused low signal levels in the 2 0 2 5 m wavelength region that obscured any spectral features of evaporite minerals despite these difficulties the results of this study demonstrate the potential for using aviris and other imaging spectrometer data to study playa chemistry such data can be useful for understanding chemical linkages between evaporites and ground waters and will facilitate studies of how desert ground water regimes change through time in response to climatic and other variables