flexural modelling of continental lithosphere deformation a comparison of 2d and 3d techniques abstract the flexural isostatic response of the lithosphere in response to loading caused by continental tectonics has been modelled in 3d the modelling approach used has been to determine hanging wall deformation following movement over a pre defined fault surface in addition the lower crust is assumed to deform by a pure shear mechanism the changes in crustal thickness resulting from these structural processes impose loads upon the lithosphere which responds by isostatic adjustment algorithms have been developed to quantify the flexural isostatic response to these loads in 3d these deflections are then superimposed upon the results from the structural modelling to generate isostatically compensated hanging wall footwall and fault surfaces schematic models are presented for extensional compressional and strike slip deformation model results are dependent upon the interaction between fault geometry displacement along the fault which can be varied along strike and the methodology used to quantify the flexural response of the lithosphere to loading emphasis has been placed upon contrasting models which include a structural component only with those that incorporate both structural and flexural isostatic processes following extension structural processes generate a relatively deep half graben with no deformation at the basin edges isostatic compensation modifies this structure to produce a relatively shallow but variable basin depth with uplift typically between 1 and 2 km experienced at the basin edges compressional models have been generated which show the formation of large uplift structures which are modified by isostatic compensation so that they are considerably reduced in magnitude a regional depression i e foreland basin is also generated adjacent to the remaining uplift both 2d and 3d implementations of flexural isostasy have been investigated to provide insights into the validity of results provided by commonly applied 2d methods a major advantage arising from 3d tectonic modelling is the ability to investigate the effects of oblique or entirely strike slip components of fault movement strike slip deformation has been modelled in the context of a single fault surface which varies along strike to show the development of pressure ridge and pull apart basin structures the isostatic compensation of these structures shows complex patterns of uplift and subsidence due to the interference of negative and positive loading and associated flexural deflections

mobility and fractionation of rare earth elements in argillaceous sediments implications for dating diagenesis and low grade metamorphism abstract we report sm nd and rb sr data for the fine fractions of lower paleozoic argillaceous rocks from wales uk and new york usa spanning the range of low grade metamorphic conditions from the diagenetic zone zeolite facies to the epizone greenschist facies in all cases leaching of the fine fractions results in a high 147 sm 144 nd 0 09 0 29 acid soluble component and a complementary low 147 sm 144 nd 0 05 0 14 residual component the observed fractionation is an ancient feature related to diagenesis burial and metamorphism the magnitude of sm nd fractionation between leachates and residues as well as the resulting sm nd ages vary as a function of grain size and metamorphic grade uncleaved welsh mudrocks of the diagenetic zone yield sm nd leachate residue ages of 453 484 ma in agreement with their llanviian to caradocian biostratigraphic ages whereas higher grade rocks of the anchizone and epizone yield sm nd ages as young as 413 ma these ages are transitional between the time of deposition and the time of regional deformation related to the acadian orogeny at 390 ma distinct convex upward rare earth element ree patterns of the leachates suggest that the precipitation of early diagenetic apatite controls the trace element budget of the rock forcing a depletion of middle rees on the subsequently formed diagenetic phyllosilicates the amount of organic matter present and the extent of later prograde reactions are probable modifiers of this fractionation process ordovician and devonian clastic rocks associated with the trenton and onondaga limestones of new york yield single sample and multi sample sm nd isochron ages that agree well with their biostratigraphic ages of 454 ma and 390 ma respectively the ree fractionation observed in shale leachates of the ordovician utica formation is related to ca mg of the bulk rock and hence to the composition of the diagenetic carbonate cement in all cases the sm nd system remained closed subsequent to the peak of diagenesis or metamorphism including the north american rocks that show no evidence of being isotopically reset during widespread remagnetization of the subjacent limestone units in the late paleozoic

geochemistry and diagenesis of miocene lacustrine siliceous sedimentary and pyroclastic rocks mytilinii basin samos island greece abstract a late miocene non marine stratigraphic sequence composed of limestone opal ct bearing limestone porcelanite marlstone diatomaceous marlstone dolomite and tuffite crops out on eastern samos island this lacustrine sequence is subdivided into the hora beds and the underlying pythagorion formation the hora beds is overlain by the clastic mytilinii series which contains turolian late miocene mammalian fossils the lacustrine sequence contains volcanic glass and the silica polymorphs opal a opal ct and quartz volcanic glass predominantly occurs in tuffaceous rocks from the lower and upper parts of the lacustrine sequence opal a diatom frustules is confined to layers in the upper part of the hora beds beds rich in opal ct underlie those containing opal a the occurrence of opal ct is extensive encompassing the lower hora beds and the sedimentary rocks and tuffs of the pythagorion formation a transition zone between the opal a and opal ct zones is identified by x ray diffraction patterns that are intermediate between those of opal ct and opal a perhaps due to a mixture of the two polymorphs diagenesis was not advanced enough for opal ct to transform to quartz or for volcanic glass to transform to opal c based on geochemical and mineralogical data we suggest that the rate of diagenetic transformation of opal a to opal ct was mainly controlled by the chemistry of pore fluids pore fluids were characterized by high salinity moderately high alkalinity and high magnesium ion activity these pore fluid characteristics are indicated by the presence of evaporitic salts halite sylvite niter high boron content in biogenic silica and by dolomite in both the opal a and opal ct bearing beds the absence of authigenic k feldspar borosilicates and zeolites also support these pore fluid characteristics additional factors that influenced the rate of silica diagenesis were host rock lithology and the relatively high heat flow in the aegean region from miocene to holocene

modern sands of south america composition provenance and global significance standard petrographic methods were used to study 604 modern sands from south america of which 351 came from rivers and 253 from beaches in spite of the wide geomorphic contrasts these sands belong to only three great families 1 an immature andean family of lithic arenites rich in volcanic and metamorphic grains that covers about 30 of south america 2 a cratonic association rich in quartz that covers about 62 of the continent and 3 a transitional molasse association which occurs between these two and covers only about 8 of the continent the grand arithmetic average q f rf ratio for the entire continent is 60 11 29 and the area weighted carbonate free average qa fa rfa is 68 10 22 a superior continental estimate and the first ever for an entire continent

subsurface dissolution of heavy minerals frio formation sandstones of the ancestral rio grande province south texas abstract oligocene sandstones frio formation of south texas deposited by the ancestral rio grande contain a heavy mineral assemblage that is markedly altered even in the shallowest sample examined approximately 1 km depth the weight fraction of detrital heavy minerals in the modern rio grande is approximately 1 whereas subsurface frio sandstones contain as little as 0 1 or less heavy minerals modern rio grande sand contains a diverse and unstable heavy mineral assemblage dominated by pyroxenes amphiboles and an assortment of iron titanium oxides minor components include apatite garnet sphene epidote and zircon heavy minerals in subsurface oligocene sandstones include abundant apatite zircon and titanium oxide among detrital components minor iron rich garnet also survives in the subsurface to depths of about 3 km other very stable but also very minor heavy minerals include tourmaline chromite spinel and monazite locally the heavy mineral fraction in the frio is dominated by authigenic components such as siderite pyrite or barite authigenic sphalerite is also observed among the heavy minerals feldspars and rock fragments suggest that sediment deposited by the ancestral rio grande was more strongly dominated by volcanic detritus than is the modern river it is reasonable to infer that the initial heavy mineral assemblage of the frio formation was likewise more unstable than that which appears today in the rio grande thus heavy mineral loss determined by comparing subsurface frio sandstones with the sand in the modern rio grande is conservative comparison of sr and nd concentrations in modern rio grande sands and subsurface frio sandstones suggests that heavy mineral destruction in the subsurface does not cause major perturbations of whole rock isotopic systems involving these elements

ammonium enrichment associated with diagenetic alteration in tertiary pyroclastic rocks from greece the ammonium ion has been measured in fresh volcanic rocks in vitric tuffs and in tuffs which have undergone secondary alteration the ammonium content of fresh lavas of a wide range of composition is shown to be negligible in contrast high concentrations of ammonium are found in pyroclastic rocks that have undergone diagenetic alteration data are given for three types of diagenetic alteration namely 1 hydration 2 zeolitization and 3 alteration to secondary k feldspar different levels of ammonium enrichment are associated with each type hydrated vitric tuffs from kozani in northern greece and heraklion in crete contain from 158 to 390 ppm nh 4 zeolitized ash and lapilli tuffs from santorini and from lefkimi ne greece are much lower in their ammonium content ranging from 12 to 56 ppm tuffs containing secondary k feldspar in addition to zeolites from metaxades n e greece have ammonium contents from 51 to 512 ppm measurements of fixed and exchangeable ammonium were used to distinguish between the ammonium held by zeolites and that held by k feldspars in the clinoptilolite and mordenite bearing tuffs from santorini and lefkimi most of the ammonium is in cation exchangeable form and must be held by the zeolites by contrast in the feldspar bearing tuffs from the metaxades area almost all the ammonium is fixed and it is inferred to be held mainly by secondary k feldspar

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

diagenetic modification of sm nd model ages in tertiary sandstones and shales texas gulf coast sm nd model ages of sandstones and shales from the oligocene frio formation of south texas and the paleocene eocene wilcox group of south central texas increase with increasing depth of burial depleted mantle model ages tdm of frio shales increase from ca 800 ma at 1 km of burial to ca 1000 ma at 4km in frio sandstones tdm increases from ca 750 ma at 1 km to over 1200 ma at 4 km of burial in wilcox shales tdm increases from ca 1300 ma to ca 1500 ma over a burial depth of 1 5 to 4 5 km tdm of wilcox sandstones increases from ca 1450 ma at 2 5 km of burial to as much as ca 2000 ma at 4 0 km the sampling scheme employed minimizes but does not eliminate the possibility that these trends in tdm are due to detrital variability close correlation of these trends with diagenetic changes in the rock and evidence of rare earth element mobility preserved in the diagenetic products suggests that diagenesis has modified tdm diagenetic modification of tdm in frio sandstone and wilcox shale is due primarily to loss of radiogenic nd whereas increased 147sm 144nd produces elevated tdm in deeper wilcox sandstones the tdm of frio shales does not change appreciably with depth owing to the opposite effects on tdm of decreased 147sm 144nd and decreased nd 0 loss of radiogenic nd is due primarily to reactions involving unstable volcanic detritus which is more abundant in the frio than in the wilcox decreased 147sm 144nd in authigenic illite and consequent increased 147sm 144nd in pore fluids results in elevated tdm in wilcox and frio sandstones containing late diagenetic cement and in decreased 147sm 144nd in deeper frio shale the similar tdm trends observed in both units which have different detrital characteristics suggest that modification of sm nd model ages in siliciclastic rocks may be commonplace during burial diagenesis

burial diagenesis of argillaceous sediment south texas gulf of mexico sedimentary basin a reexamination cuttings from a well through a thick section of miocene oligocene mudrocks from kenedy county texas spanning a depth range of 2130 to 5490 m 7000 to 18 000 ft have been studied petrographically and geochemically on the basis of whole rock chemical analyses the deepest samples have lost 18 wt and approximately vol mostly as caco3 mineral bound h2o and sio2 but including additional ca as well as sr light rare earth elements ree la ce nd sm fe and li k2o and rb have been added to the deeper rocks the large chemical changes are accompanied mineralogically by loss of detrital calcite kaolinite k feldspar ca plagioclase and muscovite gain of chlorite and albite and continued reaction of smectitic illite smectite i s to more illitic and k rich compositions throughout the entire depth interval of the well the large chemical changes in this thick mud rich interval almost certainly require advection of water free convection to accomplish the mass transfer initial variation in sediment composition is ruled out as a cause of the observed compositional changes with increasing depth because 1 a variety of immobile elements al2o3 tio2 zr hf heavy ree er yb th and sc remain constant relative to each other despite their uneven distribution across various particle size fractions in the sediments 2 deep frio shales are unlike quaternary gulf of mexico sediments or average shales and 3 unreasonable primary mineralogic compositions would be necessary to explain the chemical composition of the deep samples these results indicate that burial diagenesis of argillaceous sediment can be a considerably more open chemical process than is conventionally assumed that it can account for the two major chemical cements calcite and quartz in associated sandstones and that it mirrors secular changes in shales throughout geologic time

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

redistribution of rare earth elements during diagenesis of turbidite hemipelagite mudrock sequences of llandovery age from central wales

application of the indophenol blue method to the determination of ammonium in silicate rocks and minerals abstract a colorimetric method is given for determining ammonium in silicate rocks and minerals at concentrations down to 1 ppm the sample is digested in cold hf and ammonia is then separated by distillation and measured colorimetrically as the indophenol blue complex using the reagents and procedures described here the problems of colour instability and poor reproducibility encountered by previous workers have been overcome

the fluvial architecture of the upper buntsandstein in the iberian basin central spain the cercadillo sandstone and siltstone css formation forms the upper part of the buntsandstein fluvial facies in the western margin of the iberian basin sedimentation was controlled by two major pre triassic tectonic systems the iberian ranges system nw se and the central system ne sw fluvial sedimentation during this period took place in coexisting braided and meandering channels together with associated ephemeral episodes this unusual association was due to a combination of structural control by the main normal faults coupled with large discharge fluctuations the css formation consists of two major fluvial sequences with very similar vertical patterns comprising low sinuosity stream deposits that pass upwards into high sinuosity deposits associated with extensive floodplain sediments a widespread calcrete horizon is developed at the top of the lower sequence and a surface with pedogenic features is present at the top of the upper sequence these horizons are interpreted to be the result of subaerial exposure during a period of no subsidence and equilibrium in the erosion sedimentation processes in the area probably due to quiescent periods on the master faults that delineate the basin minor changes within the sequences are probably due to discharge fluctuations of a climatic origin whereas the general vertical trend of the sequences is suggested to be tectonically controlled palaeocurrent data show a general flux of sediments to the south east parallel to the axis of the iberian basin but the final part of the upper sequence palaeocurrents to the north east are interpreted to indicate a local reactivation of secondary transverse faults in the south east part of the study area

diagenetic k feldspar pseudomorphs in the triassic buntsandstein sandstones of the iberian range spain early diagenetic k feldspar in the triassic buntsandstein of the iberian range spain occurs as pseudomorphs after detrital k feldspar or 99 untwinned commonly heavily clouded by vacuoles and tiny inclusions dark luminescing and are composed of numerous fine euhedral crystals of k feldspar the latter property suggests that the pseudomorphs form via dissolution of detrital k feldspar and plagioclase and precipitation of authigenic k feldspar x ray diffraction analysis shows that the authigenic k feldspar is intermediate microcline

geochemistry of mississippian tuffs from the ouachita mountains and implications for the tectonics of the ouachita orogen oklahoma and arkansas the ouachita orogeny was the result of plate convergence at the southern margin of the north american continent although the nature of the converging southern plate and the direction of subduction remain uncertain the presence of areally extensive tuff layers interbedded with shale in the mississippian stanley group of the ouachita mountains oklahoma and arkansas provides the potential to define the tectonic environment of volcanism from the geochemistry of the tuffs and thereby delimit the subduction configuration the tuffs contain relic primary magmatic quartz plagioclase and alkali feldspar and range from crystal to vitric rich mineralogical sorting and diagenetic effects have caused chemical variability within individual tuff units but overall the tuffs have retained their primary igneous geochemical characteristics the beavers bend and hatton tuffs are geochemically very similar and are more evolved higher sio2 rb th ree lower sr ba than the mud creek tuff the stratigraphically equivalent sabine rhyolite is geochemically distinct from the tuffs having less fractionated rare earth element ree patterns and different trace element ratios both the ouachita tuffs and sabine rhyolite have the geochemical characteristics of subduction related magmas for example strong depletion of nb and ta relative to other incompatible trace elements consideration of trace element systematics in the tuffs compared to those of modern high silica volcanic rocks from different subduction related tectonic settings suggests a continental arc origin and implies southward subduction beneath a southern continent during carboniferous ocean basin closure

authigenic k nh4 feldspar in sandstones a fingerprint of the diagenesis of organic matter abstract in arkosic sandstones of the san joaquin and los angeles basins presently at temperatures between 35 c and 174 c trace amounts of authigenic k nh4 feldspar are present as microfracture fillings and overgrowths on detrital k feldspar microchemical analyses of this authigenic phase reveal up to 80 mole buddingtonite the largest ammonium concentrations are observed in a sandy interval of the antelope shale 80 mole and in the stevens sands 50 mole of the san joaquin basin this latter unit was deposited as a turbidite in the organic rich fruitvale shale an equivalent of the antelope shale the lowest ammonium contents 0 16 mole are present in the shallow marine vedder sands and the marginal marin san joaquin formation in the same basin petrographic 18o 13c and 87sr 86sr analyses of a dolomite cement that postdates authigenic k nh4 feldspar indicate that the feldspar precipitated below 28 c in the zone of methanogenesis from pore waters with the same sr signature as sea water at the time of sedimentation authigenic k nh4 feldspar is thus an early diagenetic phase that crystallized prior to oil migration under anoxic conditions when organic matter releases ammonium the source of ammonium is bacterial decay of organic matter in the sandstones themselves and or in contemporaneous shales

petrography and diagenesis of silcrete from the kalahari basin and cape coastal zone southern africa abstract silcrete is widely distributed in southern africa and occurs both in genetic association with deeply weathered bedrock weathering profile type and as a significant component of the cenozoic kalahari beds where it has formed through silicification of a variety of continental deposits nonweathering profile type silcrete fabrics are classified into four major types gs grain supported skeletal quartz grains in contact f floating skeletal grains floating in matrix m matrix skeletal grains 5 of volume and c conglomeratic detrital component containing pebbles f and m fabric types are subdivided on the presence absence of glaebules replacement by silica of a precursor matrix can account for the development of all four fabric types but at least some gs and c fabr c silcretes have formed by passive silica precipitation in packing voids glaebules and colloform features predominantly composed of concentrations of anatase and iron oxide are common in weathering profile silcretes but their precise origin is uncertain authigenic silica is present as megaquartz microquartz length slow and length fast chalcedony predominantly in void fills cryptocrystalline silica and opal ct depending on the silica and ionic concentrations of pore waters ph and host sediment characteristics diagnostic petrographic features including glaebules colloform structures and chalcedonic vugh fills may be used in conjunction with geochemical data and sedimentary associations to interpret silcretes in the stratigraphic record with respect to weathering profile types formed in humid low ph environments or nonweathering profile types formed in high ph semiarid to arid environments

a lower palaeozoic shallow water sequence in the eastern european variscides sw poland provenance and depositional history the gackowa formation of inferred cambrian ordovician age is part of the metamorphosed pre variscan basement of the southern kaczawa mountains of the sudetes region south west poland previously variously interpreted as lavas tuffs and sandstones it is shown to consist of about 200 m of originally well sorted siliclastic sedimentary rocks within a sequence of predominantly mafic volcanic rocks four facies have been distinguished based on relict sedimentary structures and textures and these suggest that deposition took place on a storm dominated shelf mostly above the wave base the petrography detrital zircon morphologies and geochemistry suggest affinity with and derivation in large part from acid volcanic rocks at a comparable stratigraphic position in a separate tectonostratigraphic unit to the south the gackowa formation and its acid volcanic source rocks have a clear continental geochemical signature in contrast with the mantle derived basic bimodal volcanic rocks below and above it is suggested that the parent acid magma originated by crustal melting during the ascent of these mantle derived magmas a continental crust setting is inferred for the deposition of the gackowa formation sandstones thus suggesting a continental setting for the associated within plate basalts the entire volcanic sedimentary succession resembles elements of the lower palaeozoic of germany and all may have formed in an initial rift setting during late precambrian to ordovician times

fixed ammonium in clays associated with crude oils abstract the association of ammonium nh 4 silicates with organic rich sedimentary environments has stimulated interest in the chemical cycle of n and its possible application as an indicator of in situ organic maturation reactions or crude oil migration fixed nh 4 in clay minerals was determined from three hydrocarbon occurrences of similar depositional environment but different ages depth and thermal maturity to determine whether anomalously high nh 4 substitution occurs near mature hydrocarbons results show higher fixed nh 4 concentrations in marginally mature mudstones than in immature sediments the highest fixed nh 4 concentrations were found in clays from sandstone reservoirs containing migrated crude oil fixed nh 4 in clays from holocene oil seep sediments in the gulf of mexico continental slope offshore louisiana averages 0 08 wt and increases with depth in shallow cores 420 cm reflecting an early diagenetic trend that is apparently not influenced by migrating crude oil programmed pyrolysis shows that the sediments are thermally immature av t max 419 c high hydrogen index values av 359mg g are the result of biodegraded crude oil and a high oxygen index av 182mg g reflects the presence of authigenic carbonate fixed nh 4 averages 0 16 wt in wilcox group eocene mudstones enclosing two sandstone reservoirs at fordoche field onshore louisiana in comparison to these mudstones anomalously high nh 4 fixation appears to occur in reservoir clay minerals pyrolysis shows that the sediments are marginally mature for crude oil generation av t max 432 c average hydrogen index 187 mg g and oxygen index values 75 mg g are consistent with oil prone type ii and type iii kerogen increased pyrolysis production index values and solvent extraction shows the presence of migrated crude oil this suggests that a reaction which releases nh 3 during crude oil generation or migration is recorded bynh 4 substitution in clays fixed nh 4 and total organic carbon toc at fordoche field show no statistically significant correlation suggesting that nh 4 substitution in clay minerals is not simply related to the amount of organic matter in the section but is also influenced by the presence of crude oil once nh 4 has been fixed in clays it is a more stable hydrocarbon proximity indicator than pore fluid tracers because it is less influenced by later chemical or geological changes