diagenesis and hydrocarbon accumulation brent sandstone jurassic bergen high area north sea diagenesis of brent group sandstones in the bergen high area was studied and related to the history of hydrocarbon maturation and pore fluid environment calcite and kaolinite are low temperature diagenetic precipitates that formed as meteoric pore water entered the brent at the end of brent deposition and later during late cimmerian erosion a major period of feldspar dissolution associated with quartz and illite cementation occurred at huldra field precipitation of diagenetic illite in brent sandstones was closely linked with hydrocarbon maturation in the adjacent viking graben and accumulation of generated fluids in traps at structural highs at huldra field illite k ar ages suggest onset of illite diagenesis in the latest cretaceous initial reservoiring of hydroca bons during the latest paleocene to early eocene 58 ma and filling of the reservoir by early oligocene 38 ma pore water character during illite diagenesis interpreted from oxygen isotope and quartz overgrowth fluid inclusion data was warmer than predicted by burial history analysis and one and a half to two times seawater salinity the incursion of saline isotopically evolved fluids at huldra suggests replacement of paleometeoric water by basin compaction fluid illite k ar ages from veslefrikk field indicate a minimum age of hydrocarbon accumulation at about 30 ma modern pore water chemistry in the brent at veslefrikk is unlike the suggested water chemistry at huldra and indicates that migrating compaction water did not completely displace older fresh connate water this sugge ts that hydrocarbons migrated into veslefrikk as a discrete phase without great volumes of associated compaction water

quaternary blind thrusting in the southwestern sacramento valley california patterns of microearthquakes and quaternary surface deformation suggest that the tectonic setting of the sw sacramento valley is similar to areas of the western san joaquin valley known to be underlain by seismogenic blind thrust faults on the basis of previous work and analysis of geologic and seismic reflection data the following late cenozoic tectonic features and processes are identified 1 uplift of the northern coast ranges beginning approximately 3 4 ma and eastward propagation of uplift into the southwestern sacramento valley by 1 0 ma 2 uplift and homoclinal flexure of plio pleistocene strata at the eastern coast ranges mountain front 3 uplift and folding above blind thrusts approximately 15 km east of the mountain front in the southwestern sacramento valley similar associations of structures and processes have been observed in thrust belts in pakistan the peruvian andes and the canadian cordillera and are commonly attributed to thrusting within an intercutaneous wedge or triangle zone by using other thrust belts as analogs the propagation of an eastward tapering triangle zone is interpreted to be the principal mechanism for uplift and homoclinal flexure at the eastern coast ranges mountain front seismic reflection profiles reveal that 1 the triangle zone consists primarily of east vergent blind thrusts and 2 west vergent backthrusts exposed in the the eastern coast ranges and southwestern sacramento valley are rooted in the east vergent thrusts transfer of slip from the east vergent blind thrusts to the west vergent backthrusts occurs locally beneath the southwestern sacramento valley fault bend folding in the hanging walls of the backthrusts has created a north northwest striking chain of low hills approximately 15 km east of the mountain front the folds deform 3 4 1 0 ma fluvial sediments and thus are middle pleistocene in age or younger local variations in strike suggest that the fold chain is segmented like the new idria coalinga kettleman hills segmented fold chain in the southwestern san joaquin valley stein and ekstrom 1989 these data have implications for seismic hazard assessment anecdotal accounts indicate that two m 6 0 events of the 1892 winters vacaville earthquake sequence probably occurred beneath the eastern coast ranges dale 1977 toppozada et al 1981 ground cracking was observed following the main shocks along the mountain front in the southwestern sacramento valley we propose that the earthquakes were generated by slip on a blind thrust beneath the coast ranges and that the ground cracking in the valley represents propagation of the eastward tapering triangle zone the 1892 earthquake sequence suggests that blind thrusts beneath the southwestern sacramento valley are active and capable of generating moderate to large magnitude earthquakes

pre miocene palaeogeography of the los cabos block baja california sur geochronological and palaeomagnetic constraints the los cabos block lcb is located at the southern end of the baja california peninsula and is composed mainly of intrusive rocks that were emplaced into pre cretaceous heterogeneous metasediments and are partly covered by miocene volcanic and volcaniclastic rocks field observations provide evidence that the undeformed homogeneous gabbronorite of the sierra el novillo in the ne part of the lcb is the oldest crystalline unit which is intruded by undeformed to slightly deformed tonalitic quartz dioritic granitoids for these rocks an rb sr intrusion age of 129 15 ma and a biotite whole rock cooling age of 116 2 ma were obtained low initial 87sr 86sr and high e nd values of about 0 7036 and 5t o 6 respectively characterise the intrusives as primitive mantle derived granites further to the southeast and south granitoids are more silicic with abundant deformed sequences orthogneisses diatexites and migmatites among others from the southern part of the lcb an rb sr intrusion age for the undeformed granites of 115 4 ma and a biotite cooling age of 90 2 ma were determined initial 87sr 86sr was determined to be 0 7054 and e nd values fall between 2 and 0 the isotopic data do not clearly allow us to characterise the deformed granitoids of the lcb as being intruded by the undeformed unit deformation could have been partly due to syn intrusive tectonics palaeomagnetic data suggest minor if any northward displacement of the lcb with respect to continental mexico corresponding to the rifting in the gulf of california since the late miocene and significant 35 45 clockwise rotations possible tilting evects which may have occurred given the dip of tertiary volcanics on top of the intrusives would modify these values for rotation and to a lesser degree of northward displacement geochemical isotopic and palaeomagnetic results and the cooling history of the lcb are similar to those from the puerto vallarta batholith whereas other mexican continental margin plutons further to the se display lower intrusion ages more rapid cooling rates and generally more primitive magma compositions in the light of the data presented a common magmatic evolution of the lcb and the puerto vallarta batholith is strongly suggested 2000 elsevier science b v all rights reserved

insolation values for the climate of the last 10 million years new values for the astronomical parameters of the earth s orbit and rotation eccentricity obliquity and precession are proposed for paleoclimatic research related to the late miocene the pliocene and the quaternary they have been obtained from a numerical solution of the lagrangian system of the planetary point masses and from an analytical solution of the poisson equations of the earth moon system the analytical expansion allows the direct determination of the main frequencies with their phase and amplitude numerical and analytical comparisons with the former astronomical solution ber78 are performed so that the accuracy and the interval of time over which the new solution is valid can be estimated the corresponding insolation values have also been computed this analysis leads to the conclusion that the new values are expected to be reliable over the last 5 ma in the time domain and at least over the last 10 ma in the frequency domain

feldspar diagenesis in the frio formation brazoria county texas gulf coast tremendous quantities of detrital feldspar have been dissolved or albitized below about 14 000 ft 4 267 m in the frio formation oligocene chocolate bayou field brazoria county texas some sandstones no longer contain any unmodified detrital feldspar grains material transfer involved in these reactions is immense affecting at least 15 of the rock volume thus albitization has important implications for several other diagenetic processes that involve feldspars or their components these processes include formation of secondary porosity precipitation of quartz and carbonate cements and the evolution of na ca cl formation water

palaeomagnetism of the puerto vallarta intrusive complex and the accretion of the guerrero terrain mexico abstract the batholith exposed around puerto vallarta is part of the guerrero tectonostratigraphic terrain presumably accreted to mexico during the laramide orogenic phase fourteen sites on this batholith were studied palaeomagnetically and geochemically nine sites define a paleopole at 76 6 n 181 8 e with 95 6 6 coinciding with the reference pole for stable north america it is proposed that no important movements of the region relative to north america have occurred since emplacement of the intrusive rocks 80 ma ago this would mean that the guerrero terrain was accreted to the mexican continental margin before that time geochemical data support this interpretation characterizing the rocks as volcanic arc granites intruded at an active continental margin

electrically conducting ca rich brines rather than water expected in the martian subsurface 1 if mars ever possessed a salty liquid hydrosphere which later partly evaporated and froze down then any aqueous fluids left near the surface could have evolved to become dense eutectic brines eutectic brines by definition are the last to freeze and the first to melt if cacl2 rich such brines can remain liquid until temperatures below 220 k close to the average surface temperature of mars in the martian subsurface in intimate contact with the ca rich basaltic regolith nacl rich early brines should have reacted to become ca rich fractional crystallization freezing and partial melting would also drive brines toward cacl2 rich compositions in other words eutectic brine compositions could be present in the shallow subsurface of mars for the same reasons that eutectic magma compositions are common on earth don juan pond antarctica a cacl2 rich eutectic brine provides a possible terrestrial analog particularly because it is fed from a basaltic aquifer owing to their relative density and fluid nature brines in the martian regolith should eventually become sandwiched between ice above and salts beneath a thawing brine sandwich provides one explanation among many for the young gullies recently attributed to seepage of liquid water on mars whether or not brine seepage explains the gullies phenomenon dense cacl2 rich brines are to be expected in the deep subsurface of mars although they might be somewhat diluted temperatures permitting and of variable salt composition in any case they should be good conductors of electricity

paleogene continental margin truncation in southwestern mexico geochronological evidence the reasons for and mechanisms of continental margin truncation in sw mexico where mesozoic cenozoic plutons are situated directly on the pacific coast are not yet well understood large scale dextral and or sinistral displacements of the continental margin terranes now forming parts of baja california or the chortis block have been proposed the well defined along coast nw se decreasing granitoid intrusion age trend 1 2 cm yr in the 100 ma 40 ma time interval between puerto vallarta and zihuatanejo is interpreted by us to be a geometric artifact of oblique continental margin truncation rather than the consequence of a sinistral offset of the chortis block from those latitudes toward the se changes in the dip and velocity of the nnw sse trending cretaceous tertiary subduction zone resulted in a landward migration of the magmatic arc taking into account certain stratigraphic affinities of chortis and the oaxaca and mixteca terranes together with the known displacement rates along the north america caribbean plate boundary the northwesternmost paleoposition of the chortis block with respect to sw mexico was near zihuatanejo in contrast between zihuatanejo and the isthmus of tehuantepec the cessation of the tertiary magmatism decreased more rapidly 7 7 cm yr although the trend is not so obvious starting in the late eocene chortis moved about 1100 km to the se along a transform boundary associated with the opening of the cayman trough based on our geochronological data and structural relationships between mylonite zones and plutons in the acapulco tehuantepec area we propose an approximately 650 km se movement of chortis from about 40 25 ma with a velocity of 6 5 4 3 cm yr since this is considerably slower than the decreasing age trend obtained by us using the geochronological data we consider batholith formation in this segment to predate and postdate the offshore passage of the north america farallon caribbean triple junction geological observations and paleomagnetic data do not give strong support for large scale right lateral displacements of crustal blocks like the baja california given the isotopic data presented the continental margin truncation in sw mexico seems to be the consequence of an interaction of mechanisms of these we regard tectonic erosion associated with the subduction process to be the most important in the northwestern segment on the other hand the lateral removal of material associated with the displacement of chortis is more important in the southeastern segment

sandstone diagenesis in the pattani basin gulf of thailand history of water rock interaction and comparison with the gulf of mexico abstract in the pattani basin a failed rift basin extensive water rock interaction has occurred between subquartzose alluvial sandstones of miocene age and their pore fluids diagenetic rates and pathways have been strongly influenced by high geothermal gradients high co 2 fugacities and low pore water salinities depositional pore water was fresh to brackish depending on the depositional environment of the sediments chloride concentrations in modern formation water are believed primarily to reflect the proportions of river and sea water in the depositional environment however the concentration of other important solutes and the isotopic composition of the formation waters can not be explained by roportional mixing of these two end member waters dissolution of detrital plagioclase an 3 and k feldspar are reactions of major significance that are reflected chemically in the na cl and k cl ratios of the formation water despite the high temperature of the sandstones 120 200 c diagenetic albite does not occur geochemical calculations indicate the formation water is undersaturated with respect to both orthoclase and albite this style of feldspar diagenesis differs significantly from that of sandstones of similar composition in other basins and has probably influenced other aspects of silicate diagenesis important authigenic minerals are 1 locally abundant calcite cement 13 c 12 8 18 o 17 3 pdb an early diagenetic phase that formed at about 60 c 2 pore filling kaolinite 18 o 9 9 d 83 5smow that was closely associated with feldspar dissolution and formed over a range of temperatures and 3 fibrous pore lining and pore bridging illite 18 o 9 8 d 86 7 smow the last significant cement formed at temperatures of 120 to 150 c potassium argon dates on illite indicate that sandstone diagenesis took place during a period of rapid sedimentation in the first two thirds of the burial history comparison of pattani basin diagenesis with diagenesis of sandstones of similar age in other sedimentary basins demonstrates that chemical diagenesis relative to mechanical compaction has been especially rapid in the pattani basin this reflects the effect of high temperatures on reaction rates the net effect is a high average rate of porosity loss with burial 11 km

aeromagnetic patterns of half graben and basin inversion implications for sediment hosted massive sulfide pb zn ag exploration the inverted extensional basins of the palaeoproterozoic north australian craton are amongst the most metallogenically endowed on earth for sediment hosted massive sulphide shms pb zn ag deposits faults that evolved during the evolution of these basins are an important exploration vector as they have behaved as conduits for mineralizing fluids controlled facies architecture and impacted on hydrothermal convection cells we attempt to map out ancient normal faults using integrated structural and aeromagnetic analysis to identify faults that are either buried beneath younger cover or have been inverted during later orogenic events this technique can be used for targeting potential conduits for mineralising fluids during shms pb zn ag exploration as well as a tool to map the architecture of extensional basins we have constructed simplified cross sections of half graben and inverted half graben and have forward modelled their magnetic response the aeromagnetic signature of a pre inverted half graben shows that the tilt block is characterised by a shallow magnetic gradient related to the shallowing of the magnetic marker beneath the half graben normal offset of the marker unit across a normal fault is characterised by a steeper magnetic gradient as the depth to the marker horizon increases simplified cross sections of isolated growth anticlines and the hanging wall buttressing show that the general shape of the magnetic profiles are maintained until the half graben has been completely inverted and the magnetic marker horizon displays reverse offset along the reactivated normal fault

origin and evolution of formation water at the jujo tecominoacan oil reservoir gulf of mexico part 1 chemical evolution and water rock interaction abstract the origin and evolution of formation water from upper jurassic to upper cretaceous mudstone packstone dolomite host rocks at the jujo tecominoacan oil reservoir located onshore in se mexico at a depth from 5200 to 6200 m b s l have been investigated using detailed water geochemistry from 12 producer wells and six closed wells and related host rock mineralogy saline waters of cl na type with total dissolved solids from 10 to 23 g l are chemically distinct from hypersaline cl ca na and cl na ca type waters with tds between 181 and 385 g l bromine cl and br na ratios suggest the subaerial evaporation of seawater beyond halite precipitation to explain the extreme hypersaline components while less saline samples were formed by mixing of high salinity end members with surface derived low salinity water components the dissolution of evaporites from adjacent salt domes has little impact on present formation water composition geochemical simulations with harvie m ller weare and phrqpitz thermodynamic data sets suggest secondary fluid enrichment in ca hco3 and sr by water rock interaction the volumetric mass balance between ca enrichment and mg depletion confirms dolomitization as the major alteration process potassium cl ratios below evaporation trajectory are attributed to minor precipitation of k feldspar and illitization without evidence for albitization at the jujo tecominoacan reservoir the abundance of secondary dolomite illite and pyrite in drilling cores from reservoir host rock reconfirms the observed water rock exchange processes sulfate concentrations are controlled by anhydrite solubility as indicated by positive si values although anhydrite deposition is limited throughout the lithological reservoir column the chemical variety of produced water at the jujo tecominoacan oil field is related to a sequence of primary and secondary processes including infiltration of evaporated seawater and original meteoric fluids the subsequent mixing of different water types and the formation of secondary minerals by water rock interaction a best fit between measured and calculated reservoir temperatures was obtained with the mg li geothermometer for high salinity formation water tds 180 g l whereas na k na ka ca and quartz geothermometers are partially applicable for less salinite water tds

composition of plagioclase feldspar in modern deep sea sands relationship to tectonic setting microprobe analysis of plagioclase in deep sea sands from various tectonic settings gives averages ranging from 19 an for trailing edge continental margins to 56 an for basins on the ocean side of an island arc for each setting except island arcs anomalously high amounts of pure albite are found these albitic grains may be recycled from pre existing sedimentary rocks potassium rich plagioclase thought to be indicative of a volcanic provenance is less abundant than would be expected from the proportion of volcanic rock fragments perhaps such high k plagioclases are less resistant to weathering than low k plagioclase

modeling the marine magnetic field of bahia de banderas mexico confirms the half graben structure of the bay abstract an existing aeromagnetic survey flown on the central western portion of mexico did not include an important tectonic structure bahia de banderas the bay has an extension of approximately 1400 km2 and is located within the puerto vallarta batholith a granitic structure of cretaceous origin we report here the additional gathering of 5523 magnetic values on the bay in order to complement the existing land aeromagnetic information this allowed modeling the structure of the bay from the magnetic viewpoint a late miocene age has been proposed for the bay making it roughly contemporaneous with the first stages of separation of baja california from mainland mexico initially proposed as a graben it was subsequently shown that its structure actually corresponds to a half graben of the fault growth type with reverse drag geometry it appears to have been developed in response to an extensional process in the n s direction valle de banderas neighbors the bay constituting its eastern land continuation it has also been proposed as a graben and it is also likely the result of an extensional process however it seems to be a structure more recently formed probably around 5 ma the different time origin of the bay and of the valley is strengthened by the different alignment of the valley axis where ameca river flows and discharges into the bay of around 30 from the trace of banderas fault the magnetic responses of the valley aeromagnetic and terrestrial support the existence of an extensional process upward and downward continuations of the magnetic fields show that sierra de vallejo and sierra de zapotan to the nw of the valley are deeply rooted structures and their magnetic responses are similar to those obtained in the puerto vallarta batholith these characteristics support a common origin for them three magnetic profiles trending nnw are modeled across bahia de banderas the models identify the structure as a half graben with a listric main fault and reverse drag geometry just as it was previously obtained elsewhere by an independent modeling process

geothermometry from brine analyses lessons from the gulf coast u s a abstract geothermometers based on mineral water equilibria are of limited use at temperatures characteristic of hydrocarbon production from sedimentary basins metastable detrital phases such as alkali feldspars and plagioclase persist to temperatures characteristic of the deepest wells sasmpled approaching 200 c and equilibria involving stable phases such as albite illite and quartz are not commonly achieved empirical relations such as that between mg and li are more successful in predicting subsurface formation water temperatures than are equilibrium relations involving quartz carbonates or feldspars the concentrations of most components in saline formation waters seem to be governed by kinetic rather than by equilibrium considerations

basin evolution and unconformity related uranium mineralization the camie river u prospect paleoproterozoic otish basin quebec the paleoproterozoic otish basin quebec hosts several uranium prospects that until recently remained underexplored and poorly understood in this study the camie river u prospect which shows similar charac teristics to high grade unconformity related u deposits is the focus of an integrated basin analysis in the western otish basin conglomerate and sandstone of the indicator formation which were deposited in at least six depositional sequences were affected by insignificant early diagenetic compaction and cementation this allowed the for mation of regional peak diagenetic aquifers which became muscovite altered due to interaction with fluids having 18 o and 2h values similar to those of seawater influenced basinal brines at 250 c u mineralization at camie river occurred at 1721 20 ma based on a 207 pb 206 pb date obtained by laser ablation of uraninite which coincides with a phase of the otish gabbro intrusion that has been dated at ca 1730 ma the intrusive event promoted circulation of u bearing basinal brines triggering u mineralization at several locations in the western otish basin interaction of basinal brines with the otish gabbro produced coarse grained chlorite tourmaline and phengitic muscovite which decreased the fluid conducting capabilities of diagenetic aquifers and resulted in fault zone and fracture dominated fluid flow subsequent fluid alteration events produced limited u remobilization sulfides sudoite and siderite between ca 1670 and 1410 ma based on mineral paragenesis and 40 ar 39 ar dates of muscovite metamorphic fluids having high 18 o values and temperatures around 300 c accompanied 1 2 to 1 0 ga grenville orogene sis and subgreenschist grade metamorphism in the otish basin but were present at low water rock ratios at camie river and therefore produced little alteration post grenville uplift of the otish basin likely produced late low temperature alteration minerals that have been influenced by recent meteoric water suggesting that the fault zones and fractures the minerals occupy remain as preferential fluid flow pathways to the present day radiogenic pb and the characteristic trace elements mo w nb have also preferentially dispersed from the mineralization along fault zones fractures and depositional sequence boundaries and can be used to explore for camie river style u mineralization

astronomical theory of the pleistocene ice ages a brief historical review abstract this paper traces the evolution of the astronomical theory of the pleistocene ice ages from its origin in the work of adhemar and croll through its quantitative transformation in the hands of milankovitch to its current renaissance emphasis is given to the interplay between climatic theory and geologic observation from 1941 when milankovitch published his final version of the theory until the early 1970s most attempts to test the theory empirically were carried out in the time domain by comparing the geologic record of climate with predicted climate curves these attempts ran into serious difficulties particularly the inaccuracy of the geologic time scale and the ambiguity of climate predictions early attempts to evaluate the theory with simple radiation balance models appeared to indicate that the climatic response to secular changes in the orbit would be too small to be observed geologically but by the mid 1970s the advent of an improved time scale for pleistocene marine deposits and the availability of long closely sampled records made it possible to arrange generalized but unambiguous tests of the theory in the frequency domain by comparing orbital and climatic spectra these results decisively confirmed the idea that orbital variations significantly influenced the earth s climate during the pleistocene but they did not tell us how this influence was exercised the emphasis of current work has therefore shifted to the identification and quantitative modeling of physical mechanisms by which the climate system responds so dramatically to the predicted rather subtle changes in radiative boundary conditions

neotectonics in central mexico regional neotectonic activity in central mexico was mapped through analysis of eleven landsat thematic mapper images the study area stretches from mexico city on the east to the gulf of california on the west between 17 and 21 n latitude fault scarps of plio quaternary age define several large scale tectonic features within the mexican volcanic belt dominated by rifting transtension and shear with probable sinistral components three large crustal blocks south of the mexican volcanic belt are separated from the north american plate and from each other by zones of neotectonic activity that express the relative motions between the adjacent blocks a simple vector diagram for plates and crustal blocks in central mexico has been constructed using information from the literature combined with the orientations of fault zones as mapped in this study the relative motion between the blocks varies from about 2 5 to 8 mm year 1 which is comparable to motions of crustal blocks in areas of active rifting in other parts of the world in the mexican volcanic belt there is a clear relationship between volcanism and neotectonic activity fault zones that traverse nearly the entire region have the same location and orientation as the volcanic belt this may indicate that deformation within the crust of the overriding slab plays a significant role in determining the unusual orientation of the mexican volcanic belt relative to the trench the ultimate factor controlling the location of plio quaternary volcanism and tectonism in central mexico may be large zones of crustal weakness formed during major mesozoic and cenozoic tectonic events

geology of the northern colima and zacoalco grabens southwest mexico late cenozoic rifting in the mexican volcanic belt the south trending northern colima graben intersects two other grabens the northwest trending zacoalco graben and the east trending chapala graben to form a rift rift rift triple junction 50 km south southwest of guadalajara in southwest mexico the northern colima graben represents the northernmost extension of the 190 km long 20 to 65 km wide colima rift which overlies the projection of the subducting boundary of the rivera and cocos plates and represents a rift of probable pliocene pleistocene age of the east trending mexican volcanic belt mvb associated with the northernmost 90 km of the rift there are 4 6 m y old to holocene alkaline volcanic rocks rarely found in continental volcanic arc environments and which are more typically associated with either continental intraplate rifting or oceanic islands these alkaline lavas have erupted side by side with 10 m y old to holocene calc alkaline lavas that represent typical arc volcanism it is proposed that the grabens are associated with the initiation of a rifting of a portion of southwest mexico away from the mexican mainland similar to the rifting of baja california away from the mexican mainland the calc alkaline k ar dates are consistent with post mid miocene development of east trending mvb volcanism

apatite fission track analysis of oligocene strata in south texas u s a testing annealing models analysis of apatite fission track data from forty seven surface and subsurface samples of fine grained sandstone to pebbly conglomerate from the oligocene catahoula gueydan frio and vicksburg formations documents the response of fission track ages and track length distributions to simple post depositional thermal histories characterized by monotonic heating data from outcrop samples document that the apatite component was derived from source areas characterized by rapid cooling through the temperature range from 120 to 50 c during the early tertiary the apatite population is characterized by a log normal chlorine distribution mean 0 3 wt 0 3 wt consisting predominantly of low cl apatites 80 is 0 5 wt mean track lengths from core samples decrease systematically with increasing depth and temperature the decrease in apatite fission track ages with increasing depth and temperature is less systematic due to the scatter in inherited ages at the time of deposition no tracks were observed in apatite from samples at temperatures above 130 c mean track lengths for the south texas data agree well with those for a similar data set from the otway basin southeastern australia for present day temperatures of 70 c at higher temperatures mean lengths are 1 to 9 m longer than those observed from the otway basin this discrepancy is interpreted to reflect up to 40 ma longer residence times at maximum burial temperatures for the otway basin samples standard deviations about mean lengths are also lower than observed in the otway basin study consistent with the wider variation of cl contents mean 0 8 wt 0 7 wt that characterize apatite from the otway basin recently published apatite fission track annealing models predict significantly different mean track length vs present day temperature profiles for thermal histories believed to be representative of those experienced by sampled south texas strata because of the large uncertainties attributed to present day temperatures 10 c the predicted mean length vs temperature trends for these models essentially fall within or on the uncertainty envelope for the observed trend residuals of predicted minus observed mean lengths suggest that the previously published carlson composite parameter model using an initial mean length of 15 8 m instead of 16 5 m or the crowley et al model for track length reduction down to 11 m provide the best estimates of mean track length reduction over geologic time scales for near end member f apatite populations

sensitivity of paleoclimate simulation results to season definitions according to the milankovitch theory slow variations of the earth s orbital parameters change the amplitude of the seasonal cycle of insolation and are considered to be the main forcing mechanism of glacial interglacial cycles because of the precession and changes in eccentricity the length of seasons also varies no absolute phasing is then possible between the insolation curves of two different periods various solutions to compare different periods have been given either for astronomical computations e g berger and loutre 1991 laskar et al 1993 or for model simulations e g kutzbach and otto bliesner 1982 mitchell et al 1988 but the sensitivity of model results to the different possible solutions has never been quantified our results based on simulations of the last interglacial climate 126 kyr b p where changes in the length of the seasons are large clearly show that phase leads or lags between the various solutions used introduce biases in the analysis of insolation and climate change of the same order of magnitude as the milankovitch forcing our main conclusions are that 1 when comparing various model simulations the date of the vernal equinox i e the phasing of the seasonal cycle of insolation as well as the definition of seasons must be the same for all models in order to avoid artificial differences 2 seasons based on astronomical positions are preferred to seasons defined with the same lengths as today since they better account for the phasing of insolation curves however insolation is not the only forcing in most atmospheric general circulation model simulations we also discuss the impact of the calendar hidden behind the definition of the seasonal cycle of the other boundary conditions such as sea ice or sea surface temperatures

strontium isotopic compositions and potassium and rubidium contents of formation waters in sedimentary basins clues to the origin of the solutes abstract strontium isotopic compositions of formation waters in sedimentary basins relate primarily to sr from dissolution replacement of alkali feldspar minerals dissolution recrystallization precipitation of marine carbonate and sulfate minerals and illitization formation waters in sedimentary basins are often depleted in k but enriched in rb relative to evaporated seawater and have k rb ratios commonly between 200 and 2000 the k and rb characteristics of formation waters suggest that deep burial conversion of smectite to illite has a major influence on the evolutionary history of these solutes

mechanism of plagioclase albitization abstract observations of closely spaced porous and cemented sandstones from the san joaquin basin the los angeles basin and the gulf coast reveal that albitization of plagioclase occurs during fracturing of the detrital plagioclase grains albitization is found to be a chemically selective hydrous reaction in which an rich plagioclase is preferentially albitized first relative to an poor plagioclase the observed non positive volume change during albitization indicates that aluminum mobility takes place on a scale at least as large as a thin section these findings are supported by thermodynamic stability calculations for plagioclase with different compositions and degrees of aluminum silicon ordering at 25 c in a representative sea water and at 100 c in an oil field water the calculations predict that 1 an rich plagioclase is less stable than an poor plagioclase and 2 plagioclase with a random al si distribution is less stable than plagioclase with the same composition but an ordered al si distribution any composition and structural state of plagioclase including albite is unstable in sea water at 25 c the width of the albitization zone may be controlled by the degree of aluminum silicon ordering this agrees well with the observed smaller temperature interval of albitization in sandstones containing only volcanically derived plagioclase

banderas rift zone a plausible nw limit of the jalisco block 1 echo soundings recently made in bahia de banderas show that this region is a graben with steeply dipping walls and several basins it is the offshore continuation of the valle de banderas graben and of a branching rift rio ameca rift originating in the tepic zacoalco rift zone the general trend of the three structures is ene with some ne trending offsets and they have a total length of 150 km this banderas rift zone is proposed as the nw limit of the jalisco block the existence of this limit suggests that there is another platelet or block between the jalisco block and a portion of the rivera plate probably bounded by the tres marias escarpment the jalisco block and the north america plate

chemical evolution of a gas capped deep aquifer southwest of iran the kangan aquifer ka is located below a gas reservoir in the crest of the kangan anticline southwest of iran this aquifer is composed of permo triassic limestone dolomite sandstone anhydrite and shale it is characterized by a total dissolved solid of about 332 000 mg l and na ca cl type water a previous study showed that the source of the ka waters is evaporated seawater chemical evolution of the ka is the main objective of this study the major minor and trace element concentrations of the ka waters were measured the chemical evolution of ka waters occurred by three different processes evaporation of seawater water rock and water gas interactions due to the seawater evaporation process the concentration of all ions in the ka waters increased up to saturation levels in comparison to the evaporated seawater the higher concentrations of ca li sr i mn and b and lower concentrations of mg so4 and na and no changes in concentrations of cl and k ions are observed in the ka waters based on the chemical evolution after seawater evaporation the ka waters are classified into four groups 1 no evolution cl k ions 2 water rock interaction na ca mg li and sr ions 3 water gas interaction so4 and i ions and 4 both water rock and water gas interactions mn and b ions the chemical evolution processes of the ka waters include dolomitization precipitation ion exchange and recrystallization in water rock interaction bacterial reduction and diagenesis of organic material in water gas interaction also occur a new type of chart caexcess versus mgdeficit is proposed to evaluate the dolomitization process

feldspar albitization and feldspar cementation in the albert formation reservoir sandstones new brunswick canada abstract feldspar albitization and feldspar cementation especially albite are common in the carboniferous fluvio lacustrine albert formation reservoir sandstones of the moncton subbasin new brunswick both albitization and albite cements occur in early and late diagenesis burial diagenetic origin of albite is supported by 1 the chemical purity of albite 2 the growth of euhedral albite on numerous sites of the host feldspar 3 the continued growth of the replacement albite into adjacent pores 4 the delicate skeletal texture in partially dissolved areas of albitized feldspar and 5 the occurrence of ankerite cement inclusions in authigenic albite backscattered electron imaging suggests that feldspar albitization occurs in irregular vein like and perthite elbite forms feldspar albitization is commonly associated with dissolution pores and twins backscattered electron imaging suggests that up to 50 of the mean total 4 detrital k feldspars have been albitized the volume percentage of feldspar albitization shows no trend with increasing burial depth or temperature most of the sandstones have 2 3 albite cement except in a high minus cement porosity framework where it may exceed 30 of the sandstone

astronomical frequencies for climate research at the decadal to century time scale short term variations of the elements representing the earth s motion around the sun and its rotation have been analyzed over the last 6000 years using 1 year steps their low frequency part is compared first to the values obtained from a secular theory of the planetary long term motion showing that they can be considered reliable enough to represent adequately the motion of the earth over the last 5000 years spectral analysis of these values shows that the main periodicities are 2 67 3 98 5 26 5 93 7 9 9 8 11 9 14 7 15 8 29 42 61 122 165 and 250 years for the eccentricity as well as for the climatic precession with an additional component at around 930 years for the eccentricity and around 840 years for the climatic precession periodicities at 2 67 3 8 5 9 8 0 9 3 11 9 14 7 18 6 29 135 250 and 840 vr are also shown for the obliquity spectral analyses of the daily july mid month insolation at 65 degrees n show essentially the same periodicities as the climatic precession and the obliquity i e 2 67 3 98 5 92 8 1 11 9 15 7 18 6 29 40 61 and around 900 years finally a wider analysis of the insolation pattern was performed related to the large periodicity band of the insolation time series for the solstices and the equinoxes for 7 different latitudes in equatorial latitudes the insolation variance is largely explained by precession but precession dominates everywhere with the obliquity signal being stronger at polar latitudes at the solstices the amplitudes of the insolation change at these frequencies is of the order of 0 2 wm 2 at the maximum

geochemistry of tectonically expelled fluids from the northern coast ranges rumsey hills california usa tectonic compression has created abnormally high pressure on deep basinal fluids causing their expulsion from areally exposed upper cretaceous rock along the eastern margin of the california coast ranges the fluids emerge as near neutral perennial sodium chloride springs at high elevations with flow rates as high as 10 l per min higher spring discharges are more common around the exposure of a west vergent fault propagation fold axis spring waters range from 1000 to 27 000 mg l tds the least saline water 18o 7 5 closely represents local meteoric water that mixes with saline fluid 18o 5 3 and forms a slope of 3 5 on a d vs 18o plot a na 125 to 8000 mg l vs cl 150 to 17 000 mg l plot shows a linear dilution trend that extends close to but below the values for modern seawater calcium 75 3000 mg l is considerably enriched relative to seawater and forms a nonlinear trend with chloride in detail the na deficit defined by the difference between the measured na content and the na concentration on a hypothetical seawater dilution line is approximately balanced by the ca excess similarly defined by the seawater dilution line this relationship strongly suggests that the fluid is diluted seawater that is being modified by active albitization of plagioclase at different depths simultaneous b and 18o enrichment of the fluids accompanied by deuterium depletion further suggest that the seawater modification is influenced by clay diagenesis bicarbonate and sio2 concentrations show an inverse correlation with cl with most waters being saturated or slightly oversaturated with calcite and quartz at the discharge temperatures some freshwater springs with near meteoric stable isotope values may represent mixing of young groundwater from perched aquifers but in many cases the freshwater springs emerge along the same structures and have the same perennial nature as the saline fluids and expulsion of an older fresh groundwater component that is under abnormal fluid pressures cannot be ruled out basinal fluids elsewhere commonly show dilution trends with local meteoric water and in the case of the rumsey hills some of the dilute saline waters may indicate deep penetration of meteoric water 1 km in the pleistocene before the latest tectonic uplift geothermometry of the spring waters maximum 90 c suggest an origin from as deep as 4 0 km this depth is consistent with the depth of the core of a fault propagation anticline below the surface of the rumsey hills developed by active internal deformation of an east tapering wedge beneath the southwestern sacramento valley active tectonic compression causes near lithostatic fluid pressures in the shallow subsurface below the rumsey hills and volume strain within the core of the anticline that results in upward expulsion of the saline fluids from the indicated depths

eutectic brines on mars origin and possible relation to young seepage features abstract if the recently observed young gullies on mars represent seeps or outflows of aqueous fluid that fluid must be highly concentrated brine to be even moderately stable under current martian surface conditions such brines are to be expected in the megaregolith if an initial saline hydrosphere underwent evapoconcentration via water escape from the atmosphere evolved into subsurface cacl 2 enriched brines by chemical interaction with mafic rocks and then froze until eutectic compositions were reached although most such brines would tend to sink deep into the megaregolith they could be preserved locally at relatively high elevations as aquifers or frozen paleoaquifers perched between ground ice and crystalline salts

active rifting in southwestern mexico manifestations of an incipient eastward spreading ridge jump a major ongoing episode of continental rifting has formed a system of three intersecting grabens in the western mexican volcanic belt these structures may mark the initial stage of an eastward spreading ridge jump beneath continental crust eastward ridge segment jumps have been propagating northward along this part of the east pacific rise for the past 12 m y but this is the first plate boundary reorganization event to involve continental crust since the separation of baja california in the miocene since pliocene time small volumes of unusual alkaline magmas have erupted in the graben areas in close association with the more abundant subduction related calc alkaline magmas erupted from andesitic stratovolcanoes the alkaline magmas are varieties found in zones of active rifting elsewhere in the world

diagenesis of the purington shale in the illinois basin and implications for the diagenetic state of sedimentary rocks of shallow paleozoic basins abstract the clay minerals micas and feldspars of the pennsylvanian age purington shale have been more diagenetically active than generally recognized they have undergone diagenetic changes comparable to those of cenozoic and mesozoic sedimentary rocks buried three to four times as deeply and heated to approximately twice the temperature the purington shale on the western shelf of the illinois basin has experienced maximum burial of 1 0 km and was never heated to more than 60 c except for a very brief time at about 80 c the illite smectite i s of the purington shale has 90 illite the chemical compositions of the feldspar assemblage in the 0 09 0 063 mm fraction differ from most modern detrital suites of feldspars but are similar to diagenetic sequences described by others from much greater burial depths and are extensively but not completely albitized the simplest interpretation of the apparently advanced stage of diagenesis is that the diagenetic processes have been operating at less press

correlation of carbon dioxide abundance with temperature in clastic hydrocarbon reservoirs relationship to inorganic chemical equilibrium the mol co2 of gas reservoired in sandstones tends to increase with increasing temperature along both the us gulf coast and the norwegian continental shelf each of these provinces displays identical linear correlations between temperature and the log of the partial pressure of co2 i e pco2 in the reservoir fluid that is the product of the mol co2 the reservoir pressure this correlation is the same both in gulf coast reservoirs younger than 17 ma and in norwegian reservoirs of jurassic to triassic age 144 245 ma indicating that it is not dependent on reaction kinetics norwegian shelf data indicate that the same correlation also applies for gas condensate and oil reservoirs we suggest that the log pco2versus temperature correlation is the result of inorganic chemical equilibria between feldspar clay and carbonate minerals a trend of decreasing ph with increasing temperature is fixed by equilibria between aluminosilicate minerals feldspar kaolinite at lower temperatures followed by various reactions between kaolinite illite chlorite and feldspar as temperatures exceed 110 140 c responding to the ph set by the aluminosilicate system co2 activity in the formation water and thereby co2 partial pressure in any gas phase present is controlled by equilibrium with carbonate minerals this model predicts that co2 released by organic maturation will in general result in precipitation of carbonate minerals below 100 120 c and that there should be widespread carbonate dissolution at higher temperatures

petrology of cordierite gedrite bearing sodic granulite from the grenvillian long range inlier newfoundland sodium rich potassium poor granulite in the long range inlier contains ab qtz crd bt opx ilm and either gedrite or garnet but rarely both the distribution of garnet in the sodic gneiss was influenced by bulk compositional controls e g higher al2o3 sio2 textural evidence indicates that gedrite was metastable with respect to crd opx during granulite grade metamorphism peak metamorphic conditions are estimated to have been 700 c at 600 mpa the metamorphic fluid was co2 rich the preservation of pelitic sil crd bt grt qtz mc and layers in which k2o na2o suggests a sedimentary precursor for the sodic gneiss but the granulites are more sodic and less magnesian than primary sediments recently identified as possible precursors to cordierite orthoamphibole rocks e g ultramafic derived greywackes evaporitic mudstones leucosomes tend to be more sodic mean na2o k2o 23 5 than mesosomes mean na2o k2o 17 8 m garnet free gneiss and 1 4 m garnetiferous gneiss so unl

pore fluid chemistry from the second gas hydrate drilling expedition in the ulleung basin ubgh2 source mechanisms and consequences of fluid freshening in the central part of the ulleung basin east sea during the second gas hydrate drilling expedition in the ulleung basin ubgh2 pore fluids were collected from sites that sampled sediments from the continental slope to the basin center and included locations within and outside seismic identified chimney like features at five non chimney sites established during the ubgh2 discrete excursions to low cl concentrations within the gas hydrate occurrence zone ghoz are associated with water enriched in d and 18o as expected from gas hydrate dissociation during core recovery estimates of gas hydrate saturation values sh based on cl and d values are in good agreement and show that when gas hydrate occurs in discrete horizons or within strata bound fractures throughout the ghoz it occupies 5 65 of the pore space fluids recovered from three sites characterized by seismic inferred chimneys show massive hydrate accumulation near the seafloor accompanied by the formation of shallow cl rich brines with d and 18o values lower than seawater chemical and isotopic data from fluids sampled at all drilled sites reveal significant fluid freshening below the gas hydrate stability zone ghsz the distribution of cl k and d indicate that the observed freshening is predominantly caused by clay mineral dehydration illitization previously reported heat flow values in the center of the basin indicate that the sediments deeper than 800 mbsf meter below seafloor are warm enough to expect clay diagenesis with the onset of smectite to illite transformation source fluid temperatures 70 c are confirmed through the use of na li and mg2 li geothermometry based on these observations we propose the following scenario for the evolution of formation fluids in the ulleung basin where high formation temperatures and rapid sedimentation at the basin center led to sediment compaction and clay dehydration reactions that result in fluid overpressures and associated fracturing of the formation sediment fractures and fluid possible gas migration in the chimney features imaged in seismic records are consistent with our postulated upward transport of fluids from 800 m depth though the ghsz and to the seafloor

comment on geochemistry of tectonically expelled fluids from the northern coast ranges rumsey hills california usa by m l davisson t s presser and r e criss

experimental diagenesis exploratory results we conducted experiments in which a granular mixture of quartz and labradorite was deformed in compression at elevated temperatures 200 350 c and pressures 100 mpa in the presence of deionized water following initial consolidation of the material by application of the loads at room temperature a second hydrothermal phase of consolidation was observed when the temperature was increased with the loads fixed the strain rate during this hydrothermal consolidation decayed approximately exponentially with time and was accompanied by only minor porosity reduction the sample permeability decreased rapidly during the hydrothermal consolidation and continued to decrease following cessation of the deformation the rate of this self sealing process was a complex function of temperature scanning electron microscopy and microprobe analysis of the indurated post experimental material showed albitization of the feldspar especially in highly deformed regions analysis of post experimental fluids show that si in solution increased with increasing flow and temperature fluid rock equilibrium was not achieved within the short durations of the experiments 50 60 hours our interpretation is that deformation occured by grain boundary sliding aided by solution and reaction at high stress contacts and sealing occurred by precipitation in low stress regions in the throats of pores

plate interactions control middle late miocene proto gulf and basin and range extension in the southern basin and range middle late miocene proto gulf 12 6 ma extension around the gulf of california gulf extensional province is commonly interpreted as resulting from partitioning of oblique pacific north american plate motion into strike slip displacement along the margin and east northeast extension perpendicular to the margin within the north american plate we propose that this mechanism also applies to kinematically similar predominantly east northeast extension that occurred at the same time throughout the southern basin and range province from southern arizona and new mexico to the trans mexican volcanic belt new field and 40ar 39ar data in sinaloa and durango confirm that this episode of extension occurred on the mainland side of the gulf and in the basin and range east of the sierra madre occidental which is generally considered the eastern margin of the gulf extensional province published data indicate the middle late miocene episode also occurred across the northern and southern ends of the sierra madre where the gulf extensional province connects with the basin and range 1 from central sonora into southern arizona and new mexico and 2 from nayarit into central mexico north of the trans mexican volcanic belt this episode appears to have avected an area that continues to the eastern edge of the basin and range province in texas and san luis potosi recognition that this episode of extension avected the entire southern basin and range resolves the discrepancy between the amount of extension calculated based on plate reconstructions and that based on field data within the gulf extensional province alone published plate reconstructions require 160 to 110 km of east northeast extension between 12 and 6 ma if taken up solely within the gulf extensional province this would have generated 66 to 78 extension which is much greater than observed spread across the entire southern basin and range it requires only 20 total extension which is more consistent with observations of cumulative extension between 12 and 6 ma extension was partitioned into the gulf extensional province because 1 it lies between two stable batholith belts mesozoic peninsular ranges on the west and mid tertiary sierra madre occidental on the east that resisted extension and 2 the gulf was thermally weakened by immediately preceding arc magmatism extension in the main basin and range province in part probably avoided the relatively strong batholithic crust of the sierra madre occidental 2000 elsevier science b v all rights reserved

ion microprobe study of intragrain micropermeability in alkali feldspars alkali feldspar cleavage fragments from the klokken layered syenite south greenland were heated to 700 c at 0 1 gpa in 99 h218o for 75 h these samples were then polished and imaged by ion microprobe for 18o the feldspars were known to contain areas of pristine braid micro perthite which were not turbid and areas of deuteric patch perthite which were turbid turbidity is related to the presence of micropores in the feldspars on imaging the grain it was found that the 18o had penetrated into the parts of the grain which were microporous and not into the pristine areas micropores are therefore responsible for rendering the feldspars permeable as well as porous the implications of micropermeable feldspars in several areas of geology are discussed

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

short term orbital forcing a quasi review and a reappraisal of realistic boundary conditions for climate modeling abstract the aim of this paper is to provide geoscientists with the most accurate set of the earth s astro climatic parameters and daily insolation quantities able to describe the short term orbital forcing stof as represented by the ever changing incoming solar radiation we provide an updated review and a pragmatic tool database using the latest astronomical models and orbital ephemeris for the entire holocene and 1 kyr into the future our results are compared with the most important database produced for studying long term orbital forcing showing no systematic discrepancies over the full thirteen thousand years period studied our detailed analysis of the periods present in stof as perturbed by solar system bodies yields a very rich dynamical modulation on annual to decadal timescales when compared to previous results in addition we addressed for the first time the error committed considering daily insolation as a continuous function of orbital longitudes with respect to the nominal values i e calculating the corresponding daily insolation with orbital longitudes tabulated at noon we found important relative differences up to 5 which correspond to errors of 2 5 w m 2 in the daily mean insolation for exactly the same calendar day and set of astro climatic parameters this previously unrecognized error could have a significant impact in both the initial and boundary conditions for any climate modeling experiment

drag along faults abstract the bending of lines in the proximity of faults known as fault drag is examined here by analytical and numerical finite element models frequently the bent lines are convex toward the direction of the fault motion and this convexity is known as normal drag whereas an inverted sense of convexity is known as reverse drag we first analyze the slip along a short fault embedded in a large elastic or elastic plastic plate the analysis indicates that reverse drag is the expected drag along the short fault models with faults of high friction coefficient display smaller drag than frictionless faults this suggests that the drag intensity is not simply related to the frictional resistance we also model the drag along a normal fault with curved anti listric surface embedded in an elastic plastic medium this model also indicates that the reverse drag is the prevailing one the predictions of the present models agree well with previous experimental results of slip along short faults in wax and plasticene samples the normal drag observed in association with long faults reflects prefaulting deformation which is concentrated within a narrow shear zone

cretaceous albitization and dequartzification of hercynian peraluminous granite in the salvezines massif french pyrenees abstract the salvezines massif in northern french pyrenees has undergone extensive albitization the massif consists of gneisses and a leucogranite hosted in paleozoic sediments schists and carbonates the leucogranite was emplaced at the end of the hercynian orogeny the granite underwent monazite fractionation during magmatic stages which induced lowering of ree and th contents in the most evolved rocks hydrothermal alteration during late magmatic stages is also identified through the development of a tetrad effect in the ree patterns of the granite and through the fractionation of ratios like zr hf out of the charac charge radius controlled range the granite underwent partial to complete sub solidus albitization progressive metasomatic overprint induced metasomatic replacement of feldspars into albite and dissolution of quartz episyenitization together with new albite formation hydrothermal muscovite with a sheaf like structure sometimes crystallised in albitites inherited muscovites from magmatic stage and newly formed muscovites from albitites have distinct chemical compositions with the latter being much more phengitic the initial peraluminous chemistry of the leucogranite was lost during albitization and composition evolved towards a pure albite one a cnk 1 albitization induced gains in na and al and losses in si and k leaching of ree and u is also identified during albitization as well as the non charac behaviour of some elements 18o values of quartz and muscovite in albitized granites are in the magmatic range about 12 and 9 5 respectively feldspars acquired high 18o values during albitization up to 15 muscovite grains from albitites have higher 18o values than inherited ones and tend towards isotopic equilibrium with albite the albitizing fluids have interacted with high 18o rocks probably local carbonates prior to alter granite muscovites inherited from magmatic stages display very characteristic disturbed 39ar 40ar age spectra with saddle shapes when a muscovite associated with albitization provided a plateau age at 117 5 ma for this alteration event albitization in the salvezines massif just preceded the regional talc chlorite hydrothermal mineralization both events might then represent two outward signs of a huge hydrothermal system at the time of the rotation of the iberian plate around europe the north pyrenean metamorphism is identified by 39ar 40ar analyses at ca or younger than 100 ma

albitisation related to the triassic unconformity in igneous rocks of the morvan massif france the carboniferous morvan massif in the northern part of the french massif central consists of granite and some rhyolite a triassic erosional unconformity has developed on the massif which is covered by mesozoic sediments of the paris basin the igneous rocks of the morvan massif show a strong alteration with pseudomorphic replacement of the primary plagioclases into albite pseudomorphic replacement of primary biotite into chlorite and minor precipitation of neogenic minerals like albite chlorite apatite haematite calcite and titanite the geometry and arrangement of these alterations give significant constraints about their development some of the altered facies develop in a pervasive manner others are restricted to centimetric to metric wide joints that imply fluid flow phenomena moreover the alteration facies are arranged in a clear succession with strongly altered facies at the top and weakly altered facies towards the depth which point to a genetic relationship with the triassic unconformity regional distribution of the alterations which affect the carboniferous igneous and volcanic formations beneath the jurassic sedimentary cover also leads to associate these alterations with the triassic unconformity dating of the alterations provides even a further constraint alterations are of triassic age that means the same age as the unconformity taking into account all these geological constraints it is proposed that albitisation of the morvan massif was developed under low temperature subsurface conditions in relation to the triassic palaeosurface

miocene shearing along the northern boundary of the jalisco block and the opening of the southern gulf of california new geologic and structural mapping in western mexico shows that the tectonic boundary between the eocene early miocene sierra madre occidental volcanic province and the cretaceous paleocene jalisco block was reactivated as a transfer zone accommodating a limited amount of lateral motion in middle and late miocene time transpressional folding left lateral strike slip faulting and minor thrusting occurred between 14 5 and 11 5 ma during initial rifting at the mouth of the gulf of california inferred right lateral transtension between 12 and 9 ma took place concurrently with widespread alkaline volcanism and east west extension at the gulf mouth and east of the jalisco block these events can be explained in light of the long extensional history that led to the separation of baja california from mainland mexico since the birth of the magdalena microplate about 14 5 ma left lateral shearing at the sierra madre occidental jalisco block boundary indicates that initial 14 12 ma rifting at the mouth of the gulf was accomplished mainly by east southeast motion of the block this was caused by differential stresses applied to the north america plate at the two sides of the subducting magdalena cocos plate boundary as a consequence of the waning of subduction of the magdalena plate subsequent right lateral transtension at the sierra madre occidental jalisco block boundary and extension to the east imply a limited west northwest motion of the block which possibly accounts for the discrepancy between slow rifting at the mouth of the gulf and fast northwest motion of baja california

open system chemical behavior in deep wilcox group mudstones texas gulf coast usa abstract wilcox group mudstones have been mechanically and geochemically transformed over a temperature range of 20 200 c our research controlled for provenance and age by sampling from five wells parallel to the paleodepositional axis all within the houston delta system across the sampled depths mudstone porosity has been reduced from 25 to

calendar effect on phase study in paleoclimate transient simulation with orbital forcing several studies have shown that the use of different calendars in paleoclimate simulations can cause artificial phase shifts on insolation forcing and climatic responses however these important calendar corrections are still often neglected in this paper the phase shifts at the precession band is quantitatively assessed by converting the model data of the transient gcm climate simulation of kutzbach et al clim dyn 30 567 579 2008 from the fixed day calendar to the fixed angular calendar with a new and efficient approach we find that insolation has a big phase shift in september october november son when the vernal equinox ve is fixed to march 21 at high latitude the phase bias is up to 60 about 3650 years the insolation phase bias in son in southern hemisphere sh is especially important because it can influence the timing of the sh summer monsoon response due to the large heat capacity of ocean the calendar correction has minor effect 2 on the phase relationships between forcing and precipitation responses of the six global summer monsoons studied in kutzbach et al 2008 after correcting the calendar effect especial on sh ocean temperature the new phase wheel results are more similar for both hemispheres the results suggest that the calendar effect should be corrected before discussing the dynamics between orbital forcing and climatic responses in phase studies of transient simulations

facies history of the upper jurassic lower cretaceous great valley sequence response to structural development of an outer arc basin abstract the facies history of the great valley sequence is closely linked to structural events in an arc trench system upper jurassic lower cretaceous marine rocks as much as 8 500 m thick along the western margin of the sacramento valley northern california were deposited on slopes and submarine fans that extended over an unstable basement floored by ophiolitic rocks paleocurrent and paleoslope analyses indicate that eastward subduction initiated as early as kimmeridgian created an outer arc ridge which bounded the west side of the basin debris was shed dominantly from the north and east but significant contributions were ridge derived sediment fabrics structures and bedding styles indicate that earliest deposition on basin plains and on eastward facing slopes was followed at le st until the valanginian by a stable sedimentological framework of prograding and avulsing submarine fans characterized by broad shallow midfan channels depositional lobes and abundant thick deposits of interchannel mud the great valley rocks are transected by northwest trending faults that produced strike separation of the east dipping strata smooth facies transitions but with abrupt thickness variations across faults suggest that faulting with some normal component of motion was contemporaneous with deposition during latest jurassic and earliest cretaceous later movement either strike slip or reverse brought coarse inner fan facies that were deposited on klamath basement into fault contact with basin plain facies that were deposited on ophiolitic basement

hydrochemical constraints on fluid mineral equilibria during compaction diagenesis of kerogen rich geopressured sediments abstract hydrochemical conditions characteristic of kerogen rich geopressured sediments contribute to the development of a predictable alteration mineralogy the following conditions result in the geopressured fluid becoming enriched and in some cases saturated with co 2 and ch 4 1 1 the upward and generally restricted flow of fluids in geopressured sediments in contrast to the generally more rapid flow of fluids in nongeopressured sediments 2 2 the coincidence of the depths of geopressuring with the geothermal temperatures necessary for co 2 and ch 4 release and 3 3 the opposing rates of sediment subsidence and co 2 and ch 4 transfer into the upward flowing fluid calculation of the phase relations in the systemcao feo mgo na 2 o al 2 o 3 co 2 h 2 s h 2 so 4 sio 2 h 2 o hcl as constrained by the above hydrochemicalcriteria allows prediction of three patterns of mineral deposition during diagenesis of kerogen rich geopressured sediments quartz deposition is favored in the upper portions and margins of the geopressured section and at the contact between geopressures and normal pressures because of increased fluid flux atshallower depths within the geopressured section carbonate deposition could occur above the zone of co 2 release from kerogen degradation as a result of the upward flux of co 2 and ch 4 enriched andpossibly saturated fluids and the subsequent decrease in fluid temperature pressure and co 2 solubility kaolinite carbonate could deposit within and above the zone of co 2 release from kerogen as a result ofsilicate dissolution by co 2 rich acid pore fluids followed by the potential for illite carbonate and albitecarbonate deposition upon co 2 depletion in contrast laumontite and anhydrite should not depositduring diagenesis of kerogen rich geopressured sediments but could deposit during diagenesis of nongeopressured or kerogen poor geopressured sediments these mineralogic relationships compare favorablywith observed relationships in the kerogen rich geopressured sediments of the frio formation from thetexas gulf coast

textures and geochemistry of authigenic albite from miocene sandstones louisiana gulf coast feldspars in louisiana miocene sandstones at depths of 2 400 to 6 000 m 80 to 150 degrees c show progresssive reaction with increasing burial plagioclase undergoes both dissolution and replacement by albite whereas k feldspar undergoes only dissolution reactions do not reach completion by 6 000 m 150 degrees c where a small amount of oligoclase and k feldspar remain within an assemblage of nearly pure authigenic albite dissolution of plagioclase gradually becomes volumetrically important near 3 600 m 100 degrees c and for the most part ends by 5 000 m 130 degrees c albite replacement of plagioclase also occurs primarily between 3 600 and 5 000 m 100 to 130 degrees c but continues to the limit of sample control at 6 000 m 150 degrees c k feldspar dissolution is negligible until 4 800 to 5 600 m 125 to 140 degrees c below 5 600 m k feldspar is nearly absent in the rocks discontinuous zones of blocky to tabular extinction characterize grains of partially albitized plagioclase backscattered electron images of these grains coupled with electron microprobe analyses indicate that these zones represent domains of unreacted calcic plagioclase an sub 10 30 interspersed with domains of nearly pure authigenic albite an sub 0 2 sem images show that some albite domains form from precipitation of small albite overgrowths on nucleation sites within secondary intragranular megascopic pores in thin section the overgrowths are optically clear other highly vacuolized albite domains may develop from direct replacement of calcic domains by dissolution reprecipitation in submicroscopic pores without the formation of megascopic pores here pore fluid may have been trapped between large numbers of subcrystals that grew together with imperfect boundaries operating on both megascopic and submicroscopic scales albite replacement of plagioclase grains can be completed

ca mg of brines in miocene oligocene clastic sediments of the texas gulf coast buffering by calcite disordered dolomite equilibria the compositions of brine collected from geopressured and nongeopressured sediments of the northeast texas gulf coast were used to predict the ca mg activity ratios of water in equilibrium with calcite and dolomite to 150 c ca mg activity ratios of these water samples show an excellent linear correlation with temperature giving the equation log aca amg 0 22 7 21t c 1000 r 0 94 n 51 this line parallels and is very near to the equilibrium surface for calcite disordered dolomite suggesting that equilibrium between calcite and poorly ordered dolomite may control the ca mg activity ratios in the water samples in these sediments fe rich dolomite and calcite cements are the last cementation phases loucks et al 1984 indicating they are the most likely minerals affecting the present water chemistry in addition in these sediments calcite cements precipitated in oxygen isotopic equilibrium with present day water and calcite and dolomite cements coprecipitated in oxygen isotopic equilibrium milliken et al 1981 other possible processes such as mixing cation exchange and water rock interaction involving other carbonate and silicate minerals may act as sinks or sources for both ca and mg but they appear not to control the ca mg activity ratios at the study site the ca mg activity ratios of brines from other groundwater systems clastics and dolomite poor carbonates of the u s gulf coast agree well with the ca mg temperature trend determined in this study suggesting equilibrium with calcite and similar composition disordered dolomite in all these sediments water from dolomite rich carbonate sediments did not fit the ca mg activity ratio curve presented in this study and is more likely in equilibrium with calcite and more ordered dolomite a possible effect of dedolomitization this linear ca mg trend was used to estimate the ordering parameter s 0 4 and with that the g f 516 05 kcal mol h f 554 20 kcal mol and a solubility product constant of 10 16 9 at 25 c and 1 bar were calculated for the disordered dolomite found in these sediments this dolomite is likely typical of that found in other clastic and dolomite poor carbonate sediments

pb isotope constraints on the provenance and diagenesis of detrital feldspars from the sudbury basin canada abstract residues after hf leaching of detrital feldspar grains from the early proterozoic chelmsford formation of the sudbury basin ontario record isotopic compositions very close to the initial pb isotopic composition of their late archean superior province source these grains have been recrystallized to albite and k feldspar residues of chelmsford feldspar grains lie within the range of reported 206 pb 204 pb compositions of feldspars from plutons of the adjacent abitibi subprovince of the superior province with a steep 207 pb 206 pb slope of 0 35 0 15 3 7 0 7 ga leaches are more radiogenic than residues but leach residue pairs do not record in situ u and th decay since either the late archean time of feldspar crystallization in the superior province or the early proterozoic time of recrystallization in the chelmsford formation instead leach residue pairs lie along an average 207 pb 206 pb slope of 0 250 0 036 representing mixing trends most whole rock samples from the chelmsford formation lie close to a 2 7 ga slope collinear with detrital feldspar compositions we interpret the pb data from the leaches as recording the passing of altering fluids these fluids likely carried pb derived from rocks or minerals with relatively high u pb th pb and th u within the sedimentary basin itself the residues appear to record very close to initial values because they lie along the very steep trend defined by feldspars of the superior province

gravimetric analysis of the rifts and volcanic fields of the jalisco block mexico abstract the gravity map of the complete jalisco block jb is obtained from satellite derived measurements at a resolution of 1 km separation including the offshore region up to the middle america trench mat tepic zacoalco tz and colima delimiting rifts of the jb are identified as gravimetric minima the nw limit of the block appears also as a rifted zone with a gravity minimum connecting bahia de banderas with the tz rift defining a new triple point the compostela triple point rift rift rift that together with the guadalajara triple point determine the north boundary of the jalisco block the largest negative gravity anomalies are observed associated with the central jalisco volcanic lineament cjvl and bahia de banderas they appear to be connected since they are only separated by a small positive anomaly east of bahia de banderas the cjvl is parallel to the middle america trench exhibiting a discontinuous alignment of quaternary volcanism from the colima volcanic complex to the northwestern rifted limit of the jalisco block ameca and santiago rivers flow parallel to each other in a northwesterly direction for 100 km then santiago river changes course to continue 64 km in a north northwesterly direction up to aguamilpa dam ameca river should follow the same direction instead it abruptly changes its course to a southwesterly direction to discharge in bahia de banderas the drastic course deviation is attributed to the intersection of ameca river causeway with the bahia de banderas compostela rift proposed in this study to be the northwestern limit of the jalisco block

diagenetic history of eocene wilcox sandstones south central texas abstract sandstones and shales of the lower eocene wilcox group extend from outcrop to depths greater than 5 km where temperatures exceed 200 c eight authigenic minerals have been identified quartz kaolinite calcite ankerite and albitized plagioclase are volumetrically significant whereas chlorite illite albite overgrowths and dolomite are present only in trace amounts quartz kaolinite and ankerite cements are present over the entire depth range sampled 1528 to 4446 m calcite cement occurs only in sandstones shallower than 3300 m albitized plagioclase is the predominant feldspar in samples deeper than 2600 m whereas detrital potassium feldspar has been completely dissolved out of sandstones deeper than about 3200 m no systematic variations in the chemical composition of calcite with depth no zoning within calcite or ankerite and no compositional differences between pore filling and grain replacing calcite or ankerite were found calcite cements have 87 sr 86 sr ratios generally less than 0 708 whereas ankerite cements are significantly enriched in 87sr the oxygen isotopic composition of carbonate cement ranges between 9 and 14 pdb for calcite and between 8 and 12 for ankerite with no systematic variation with depth calculated 18o values for authigenic quartz and kaolinite range from 22 to 31 smow and from 14 to 17 respectively volumetrically important amounts of authigenic quartz and kaolinite precipitated during early burial before calcite cementation secondary porosity generation or smectite to illite transformation in encasing shales the isotopic composition of strontium in calcite cement shows most of it to be of eocene or older marine carbonate origin ankerite cementation followed the generation of secondary porosity and occurred after the products of organic maturation and shale diagenesis plagioclase albitization or potassium feldspar dissolution significantly affected the sandstone pore water system

the origin and evolution of saline formation water lower cretaceous carbonates south central texas u s a abstract land l s and prezbindowski d r 1981 the origin and evolution of saline formation water lower cretaceous carbonates south central texas u s a in w back and r letolle guest editors symposium on geochemistry of groundwater 26th international geological congress j hydrol 54 51ndash 74 systematic chemical variation exists in formation water collected from a dip section through lower cretaceous rocks of south central texas chemical variation can be explained by an interactive water rock diagenetic model the cyclic lower cretaceous shelf carbonates of the edwards group dip into the gulf of mexico coast geosyncline and can be considered to a first approximation as part of a complex aquifer contained by paleozoic basement beneath and by relatively impermeable upper cretaceous clay and chalk above the hydrodynamic character of this carbonate system is strongly controlled by major fault systems major fault systems serve as pathways for vertical movement of basinal brines into the lower cretaceous section formation water movement in this sytem has strong upfault and updip components the parent na ca cl brine originates deep in the gulf of mexico basin at temperatures between 200 and 250 c by the reaction halite detrital plagioclase quartz water albite brine other dissolved components originate by reaction of the fluid with the sedimentary phases k feldspar calcite dolomite anhydrite celestite barite and fluorite significant quantities of pb zn and fe have been mobilized as well as the brine moves updip out of the overpressured deep gulf of mexico basin and encounters limestones of the stuart city reef trend the buried platform margin small amounts of galena precipitate in late fractures continuing to rise upfault and updip the brine becomes progressively diluted on encountering significant quantities of dolomite in the backreef facies the ca rich brine causes dedolomitization although thermochemical consideration suggests that small amounts of several authigenic phases should precipitate most have yet to be found minor amounts of several kinds of calcite spar are present however as the brine evolves by dilution and by cooling no systematic changes in any cation cl ratio occur except for regular updip gain in mg as a result of progressive dedolomitization the formation water highly diluted by meteoric water eventually discharges along faults as hot mineral water

chemical constraints and origins of four groups of gulf coast reservoir fluids pitzer s method for computing activity coefficients has been applied to reported fluid compositions in four groups of gulf coast reservoirs the mississippi and arkansas smackover formation the texas edwards group the texas frio formation and the louisiana miocene sequence the activity quotients in fluids from carbonate reservoirs imply a reaction relation between calcite and disordered dolomite in which ordering increases at higher temperatures this relationship indicates that an increasing mg ca molality ratio with decreasing temperature does not necessarily imply dolomite dissolution by an updip moving fluid instead it may reflect an approach to equilibrium between calcite and metastable dolomite activity quotients in fluids from clastic reservoirs imply a rea tion relation between chlorite illite potassium feldspar and quartz at temperatures above 100 c 212 f present fluid compositions in the smackover and the edwards appear to be related to the updip movement of louann brines from the gulf coast basin accompanied by dolomitization in particular bromide plots for the edwards fluids indicate simple dilution of louann brines with interstitial fluids similar plots for fluids from the smackover show the complicating effects of illite formation sylvite dissolution and halite recrystallization albitization may have occurred during fluid movement through clastics prior to entering the smackover and edwards present fluid compositions in the frio and miocene are apparently not related to louann brines the compositional data reflect the effects of halite dissolution the subsequent albitization of plagioclase and potassium feldspar and the dissolution of carbonates during secondary porosity formation

linking diagenesis to sequence stratigraphy in fluvial and shallow marine sandstones evidence from the cambrian ordovician lower sandstone unit in southwestern sinai egypt abstract by integrating diagenesis and sequence stratigraphy the distribution of diagenetic alterations and their impact on reservoir quality was investigated within a sequence stratigraphic framework using the fluvial and shallow marine sandstones in the cambrian ordovician succession of southwest sinai the perographic and geochemical analysis of the studied sandstone revealed that the eogenetic alterations display fairly systematic spatial and temporal distribution patterns within the lowstand system tract and transgressive system tract as well as along the sequence stratigraphic surfaces i e sequence boundaries transgressive surfaces and parasequence boundaries during relative sea level fall percolation of meteoric waters through sandstones of the lst and below sequence and parasequence boundaries resulted in extensive dissolution of detrital grains and formation of kaolinite authigenic k feldspar and feldspar overgrowths as well as formation of mechanical infiltrated clays around the detrital grains during relative sea level rise invasion of marine water into the sandstones as a consequence of landward migration of the shoreline as well as low sedimentation rates encountered in the tst resulted in the formation of glauconite apatite and pyrite development of pseudomatrix which was formed by mechanical compaction of mud intraclasts is mostly abundant along transgressive surfaces and parasequence boundaries of the tst and is related to the abundance of mud intraclasts in the transgressive lag deposits the types and extent of eogenetic alterations have an important impact on the distribution of the mesogenetic alterations including the formation of quartz overgrowths and dickite distribution of mesogenetic quartz overgrowths in the sandstones was controlled by the distribution of mechanically infiltrated clays and the presence of eogenetic cement sandstones that remained poorly cemented during eodiagenesis and that have thin or discontinuous infiltrated clay rims around the detrital grains were cemented during mesodiagenesis by quartz the absence of extensive eogenetic cements in the sandstones suggested that the partial deterioration of porosity was mainly due to mechanical compaction partial transformation of kaolinite to dickite which indicates neomorphic change to a better ordered and more stable crystal structure at the elevated temperatures during mesodiagenesis is partially a function of distribution of kaolinite during eodiagenesis the conceptual model developed in this study shows the diagenetic evolutionary pathways in the reservoir sandstones within a sequence stratigraphic context which in turn provides some insights into the controls on reservoir heterogeneity

continental boundaries of the jalisco block and their influence in the pliocene quaternary kinematics of western mexico extensional faulting observed in southwestern mexico has been related to the incipient rifting of the jalisco block from the mexican mainland since the pliocene on the basis of new structural and geophysical data we propose that 1 the continental boundaries of the jalisco block are ancient structures reactivated since the pliocene at a low 1 mm yr rate of deformation and 2 pliocene quaternary extensional faulting at the edges of jalisco block is a basement controlled intraplate deformation related to plate boundary forces rather than to active continental rifting the jalisco block boundaries first developed in response to the uplift of the puerto vallarta batholith in pre neogene time and underwent a complex contractile deformation before the pliocene during pliocene quaternary times north northeast extension reactivated the northern boundary forming the tepic zacoalco rift whereas east southeast extension formed the northern colima rift south of the colima volcano active extension is found only west of the so called southern colima rift and partly reactivates old northeast trending basement faults the parallelism between the subducted rivera cocos plate boundary zone and the eastern neotectonic boundary of the jalisco block supports east southeastward motion of the southern mexican blocks induced by the differential motion and oblique subduction of the cocos and rivera plates on the other hand we envisage pliocene quaternary extension along the northern boundary as an upper plate response to the low convergence rate and the steep subduction angle of the rivera plate

importance of mineral surface areas in rotliegend sandstones for modeling co2 water rock interactions abstract investigations of rotliegend sandstones from the ne netherlands have been carried out to determine the mineralogical composition and spatial mineral distribution specific mineral and whole rock surface areas and the integration of these results in geochemical water rock co 2 simulations therefore two scenarios were considered 1 volumetric rock composition derived from point counting on thin sections and 2 effective mineralogy derived from 2d pore lining image analysis data which represent the percentage amount of each mineral phase exposed within open or accessible pores specific mineral surface areas were defined for each mineral phase respectively and integrated in both model scenarios based on point counting data the volumetric compositions of the sandstones are dominated by the mineralogy of detrital grains like quartz and k feldspar authigenic minerals esp clays volumetrically make up a small portion of the bulk rock volume only but are exposed due to their small crystal size nm to m scale and a large specific surface area 13 to 195 m 2 g in comparison to e g feldspar grains 0 03 to 1 79 m 2 g hence the size distribution and accessibility of detrital and authigenic minerals regulate the availability of chemical species for water rock interactions in the pore space the effective mineralogy of the sandstones is a function of the presence of grain covering and pore filling authigenic minerals in the two rotliegend sandstones studied approximately 23 and 43 of the detrital grains are covered by cements blocking the grain surface areas accessible for potential water rock interactions these minerals are fe oxide clay cutans mainly illite kaolinite quartz carbonate dolomite ankerite and siderite sulfate anhydrite and barite and galena the specific surface area ssa of each mineral was calculated using electron microscopy to determine the sample geometry e g size thickness and standard density values for the volumetric and effective mineralogy data the total specific surface area of the bulk rock was calculated by summing the specific mineral surface areas in relation to their relative content in the rock as determined using x ray diffraction these results were compared to surface area values determined using low pressure n 2 adsorption bet method of the bulk rock mean value 1 4 m 2 g it is shown that the ssa values using the effective mineralogy approach mean value of 0 46 m 2 g are closer to the whole rock bet than the ssa for volumetric mineralogy 0 05 m 2 g these different values for the volumetric and the effective mineralogy show significant differences in geochemical modeling results the initial amount of k feldspar kaolinite hematite and carbonate controls the availability of species for clay mineral precipitation e g mg smectite during co 2 storage effective mineral carbonation is calculated using the volumetric compared to the effective mineralogy within the first 100 years carbonate precipitation occurs for the volumetric mineralogy but is absent for the effective mineralogy opposing trends are observed for long term reactions 100 years similarly when using the effective mineralogy the porosity increases over the entire simulation time of 10 000 years while it initially decreases for the volumetric mineralogy followed by an increase and a net zero porosity change after 10 000 years this implies that different results for co 2 trapping can be expected depending upon the mineral assemblage and its effective mineralogy

bahia de banderas mexico morphology magnetic anomalies and shallow structure the bahia de banderas lies within a tectonically complex area at the northern end of the middle america trench the structure morphology subsurface geology and tectonic history of the bay are essential for unraveling the complex tectonic processes occurring in this area with this focus marine geophysical data multi beam bathymetry high resolution seismic reflection and total field magnetic data were collected within the bay and adjacent areas during four campaigns aboard the b o el puma conducted in 2006 and 2009 these data image the detailed morphology of and sedimentation patterns within the banderas canyon a prominent submarine canyon situated on the south side of the bay as well as the shallow subsurface structure of the northern part of the bay and the submarine marietas ridge which bounds the bay to the west we find that the marietas ridge is presently a transtensional feature the course of the banderas canyon is controlled by extensive turbidite fan sedimentation in its eastern extremity and by structural lineaments to the west the canyon floor is filled by sediments and exhibits almost no evidence for recent tectonic movements the southern canyon wall is quite steep and a few sediments are deposited as submarine fans at the base of the southern wall and extensive turbidite fans form the lower part of the northern canyon wall producing a gently sloping lower northern wall we find no evidence for a regional east west striking lineament between the bay and the middle america trench which casts doubts on the previous assertion that the banderas canyon is unequivocally related to the presence of a regional half graben finally a n71 e oriented normal fault offsets the seafloor reflector by 15 m within the central part of the bay suggesting that the bay is currently being subjected to nnw sse extension

albitized microcline grains of post depositional and probable detrital origins in brottum formation sandstones upper proterozoic sparagmite region of southern norway untwinned albite ab 99 4 an 0 4 or 0 2 grains forming during diagenesis and very low grade metamorphism are common in sandstones of the brottum formation upper proterozoic in the sparagmite region of southern norway these untwinned albite grains are usually clouded by vacuoles and tiny inclusions which are mostly made of carbonaceous material although some albite grains also contain some combination of carbonates quartz phengite chlorite pyrite halite and anatase the presence of microcline grains replaced partly by albite indicates that the albite grains are pseudomorphs after microcline chessboard albite which occurs in the brottum formation is suggested to be detrital in origin

stable isotope imaging of a dynamic groundwater system in the southwestern sacramento valley california usa abstract a dynamic image of a shallow 45 160 m below the surface aquifer in a 25km2 area in the southwestern sacramento valley california has been obtained by 18 o 16 o and d h determinations of groundwater from municipal wells the regular summertime drawdown of the water table is strongly correlated with municipal wide increases in the 18o values particularly in wells with shallow perforation levels thus demonstrating that the high 18o water resides in the upper reaches of the aquifer and penetrates more deeply with increased discharge rates these 1 5 enrichments in the 18o values are primarily attributed to infiltration of irrigation water that has penetrated to depths of at least 80 m the increased 18o values correlate with increased deviation from the meteoric water line and with increased nitrate levels indicating that the high 18o groundwater component represents local meteoric water that has been enriched by soil evaporation processes in nitrogen fertilized flood irrigated cropland isotopic gradients provide a new method for qualitatively determining subsurface permeability zones of low isotopic gradients interpreted as high 18o plumes probably follow coarse textured deposits of ancient stream channels the 18o gradients are primarily controlled by subsurface permeability although additional influences may include 1 the heterogeneous distribution of the high 18o groundwater 2 isotopic contributions from deeper groundwater 3 variable perforation depths of the wells 4 local differences in the water table level deeper sodium bicarbonate groundwaters 300 650 m below the surface have lower 18o values to 9 4 than the more shallow magnesium bicarbonate groundwaters suggesting derivation from 18o depleted meteoric groundwaters of the sierra nevada minor mixing of the deeper groundwater into the municipal discharge occurs even though previous workers considered the deeper groundwater to be confined

pre pliocene extension around the gulf of california and the transfer of baja california to the pacific plate late miocene 12 5 ma extension around the edges of the gulf of california has been alternatively attributed to basin and range extension back arc extension or development of the pacific north america plate boundary this extension was ene directed and similar in structural style to extension in the basin and range province timing constraints permit nearly synchronous onset of this deformation in a belt extending sse from northernmost baja california to the mouth of the gulf where this extensional faulting continued through pliocene time to the present synchronous with motion on the modern transform plate boundary in the gulf of california no change in direction of extension can be resolved revised constraints on pacific north america plate motion support the development of this late miocene extension as a component of pacific north america displacement that could not be accommodated by strike slip displacement along the existing plate boundary west of the baja california peninsula this scenario implies that transfer of baja california from the north america plate to the pacific plate was a gradual process beginning about 12 10 ma when motion of the pacific plate relative to north america was partitioned into separate regimes of strike slip and dip slip displacement on opposite sides of baja california