isotopic and elemental evidence for meteoric alteration of a pennsylvanian phylloid algal mound holder formation new mexico u s a the wrong figure 5 was printed with this article the correct figure follows fig 5 thin section photomicrographs of the characteristic lithologies in yucca mound a and b leopard

controls on reservoir quality of an upper jurassic reef mound in the palmers wood field area weald basin southern england an upper jurassic oxfordian reef mound in the palmers wood field area of the weald basin southern england clearly shows the relationship of facies diagenesis and porosity development to relative sea level changes the coral microbial reef mound was initiated over a drowned oolite shoal during a third order marine transgression and exhibits changes in coral morphology from base to top as the reef mound caught up with sea level during the subsequent highstand during the highstand extensive encrustation of the reef mound took place by microbial stromatolitic cements with subsequent porosity loss during the following lowstand of relative sea level extensive leaching of the aragonitic corals took place and a lowstand wedge accumulated down ramp porosity was lost during burial related cementation and compaction coral microbial reef mounds of this type have moderate potential for porosity formation unlike tighter deeper ramp reefs in the jurassic which are less prone to subaerial leaching and have less potential for secondary porosity development because they are less rich in aragonitic components although the upper jurassic reef mound is only a minor contributor to reservoirs in the palmers wood field it may provide a model for other upper jurassic reef plays

petrology of the mafic rocks of the xigaze ophiolite tibet the xigaze ophiolite yarlung zangbo suture zone southern tibet china shows an unusual crustal sequence characterized by a lack of large masses of cumulate gabbros by dolerites intrusive throughout the whole ophiolite sequence and by the injection of dolerites in already serpentinized peridotites the abyssal tholeiitic nature of all the mafic rocks indicates that they have been generated at an oceanic ridge all the geological arguments and petrological and textural data on the mafic rocks point to very low heat production and large heat losses through widespread intensive sea water circulation for the spreading centre in which they have been formed in good agreement with a slow spreading ridge origin

rheological evolution of the ocean crust a microstructural view rheological studies of the oceanic lithosphere have used direct observations of rocks and inferences based on thermal mechanical and experimental models modeling studies help to constrain an average crust and mantle rheology and the deep seated processes that control regional scale strength and stress field variations the smoothing of fine scale variations in such models can obscure many of the geological processes that influence strain localization and deformation partitioning in the ocean crust examination of deformation mechanisms and histories in ocean crust rocks provides a complementary approach to modeling examples of structures and deformation histories in diabases from deep sea drilling project ocean drilling program site 504b the hayes and atlantis fracture zones mid atlantic ridge and the troodos ophiolite cyprus are presented in conjunction with a synthesis of microstructural studies of the ocean crust over the last 25 years a survey of brittle quasi plastic and synmagmatic viscous deformation is used to demonstrate the influence of primary compositional and textural characteristics and variable magmatic and hydrothermal histories on deformation mechanisms and strain localization in the ocean crust geological evidence indicates that hydrothermal fluids strongly influence the nature of deformation and that effective stresses may be low due to fluid overpressures melt distribution will strongly influence strain localization at the base of the crust and synkinematic hydration during crystal plastic deformation plays a key role in the relative strengths of polyphase oceanic lithologies a schematic distribution of failure mechanisms in the ocean crust is used to discuss the controls on variations in lateral and vertical strength profiles and their possible relation to spreading rates although microstructural studies of ocean crust are still in their infancy they provide valuable constraints for rheological models and further insights to explain the distribution of seismicity at spreading centers and acoustic signatures

ionospheric signatures of dayside magnetopause transients a case study using satellite and ground measurements a case study is presented of coordinated ground and space measurements featuring a set of transient auroral fragments located on the poleward side of a stable cusp cleft arc optical ground data from ny alesund svalbard and heiss island franz josef land were combined with dmsp f9 satellite measurements to examine the characteristics of these auroral features a stable red arc stretched across most of the dayside auroral zone in a region dominated by westward convection in accordance with the orientation of the imf poleward of the red arc were several westward moving auroral jets having characteristics similar to midday auroral breakup events such events may be ground signatures of transitory magnetic merging at the dayside magnetopause if so the driven convective motion of these structures should contribute to the polar cap potential within this limited data set we find that although the transitory structures have an inherent potential associated with the motion of the optical signatures the structures on the whole contribute a small fraction of the total polar cap potential

geotectonic significance of gneissic amphibolites from the vema fracture zone equatorial mid atlantic ridge a large collection of gneissic amphibolites was recovered by two close dredge hauls from the deepest part of the north facing slope of the transverse ridge forming the south wall of the vema fracture zone serpentinites and various types of gabbroic rocks ranging from undeformed slightly uralitized gabbros and norites to flaser and mylonitic gabbros were associated with the gneissic amphibolites in the shallowest of the dredge hauls petrological studies indicate that the amphibolites were derived from similar gabbroic rocks that reequilibrated under stress in the conditions of the amphibolite facies on the other hand the associated meta gabbros display sequences of secondary minerals indicating complex cooling and cataclastic histories without reaching metamorphic equilibrium we tentatively suggest that the gneissic amphibolites and associated metagabbros formed in a vertical shear zone generated in oceanic layer 3 by tectonism associated with the vema fracture zone hydrothermal circulation of seawater along the highly permeable shear zone was activated by magmatic intrusions k ar dating suggests within relatively large analytical uncertainties that the amphibolite metamorphism took place 10 m y ago i e at a time when the dredging sites were located in the vicinity of the spreading center

dynamics and electric currents of morningside sun aligned auroral arcs 1 on 13 december 2004 morningside sun aligned auroral arcs were observed at about 8 mlt over svalbard around 75 mlat during about 1 5 hours after the imf bz component had turned to strongly northward 10 nt the arcs appeared periodically 6 7 min and moved poleward at a velocity of about 700 m s the arcs occurred during substorm recovery and appeared to develop from the area of enhanced luminosity seen in the western nightside horizon toward east the electric fields and currents associated with the arcs have been calculated from the eiscat svalbard radar observations the arcs were associated with periodic spatial structures of 270 km in latitudinal width each of the 270 km wide structure consists of four specific fac regions the upward fac region 75 km wide containing the optical arc the return downward fac region 100 km wide poleward of the arc and a secondary weaker arc equatorward of the main arc with a pair of facs similar to the main arc but narrower in width the interchange instability with the ionospheric feedback is suggested as a suitable generation mechanism for such kind arcs

development of phylloid algal carbonate mounds during regression expanding the build and fill model abstract the build and fill model for mixed carbonate clastic systems was originally developed to explain thin widespread icehouse depositional sequences that maintain more or less equal thickness over wide areas despite complex internal architecture that includes both topography building and topography filling depositional phases lithologic heterogeneity in each small scale sequence is driven largely by carbonate deposition providing localized relief above surrounding sediment and the eventual infilling of that relief by siliciclastics or a combination of carbonates and siliciclastics phylloid algal carbonate mounds are a critical component of these small scale sequences cyclothems during the late paleozoic ice age pennsylvanian permian and have been ascribed to both relief building and relief filling phases of build and fill which hinders the use of this model as a predictor of lithofacies distribution and geometry and limits its broader application to other carbonate mound settings this study includes detailed remapping of upper pennsylvanian outcrop and subsurface strata over roughly 3 000 km 2 of northeastern kansas which includes the type strata for the paleozoic example of the original build and fill model the dominant depositional motif for the phylloid algal lithofacies is that of relief builder with only moderate relief filling as carbonate mounds switched from aggradation to progradation during continued regression phylloid algal lithofacies in the iola wyandotte farley and plattsburg cyclothems upper missourian stage were predominantly aggradational and developed mound geometries relief building during regression as the base of the photic zone fell to a position low enough to encourage red and green algae dominated typically micritic bafflestone lithologies to develop on pre existing topographic highs as accommodation space continued to decrease mound development switched from aggradational to progradational before further regression brought nearshore shales into the basin which filled in bathymetric lows produced between adjacent mounds the revisions to build and fill presented here serve to expand the model and highlight the influence that the dominant relief building mechanism frame building binding vs baffling sequence stratigraphic position transgression vs falling stage and interplay between antecedent topography and rate of sea level rise and fall can have on interpretation of these mixed carbonate clastic depositional systems relief building can occur during both transgression and regression relief filling is largely limited to regression the timing of relief building within each sequence tst vs fsst is a result of the interplay between the dominant relief building organisms basin and or slope geometry pre existing accommodation space i e water depth vs antecedent topography and the rates of relative sea level rise and fall

early diagenetic partial oxidation of organic matter and sulfides in the middle pennsylvanian desmoinesian excello shale member of the fort scott limestone and equivalents northern midcontinent region usa a process of early diagenetic partial oxidation of organic matter and sulfides has altered the chemical composition of the middle pennsylvanian excello shale member of the fort scott limestone and equivalents in the northern midcontinent region this process was identified by comparison of organic carbon contents rock eval hydrogen indices organic carbon 13c and element compositions of core and surface mine samples of the excello shale member with analyses of three other underlying and overlying organic matter rich marine shales offshore shale lithofacies from southern iowa northern missouri eastern kansas and northeastern oklahoma the end product of the partial oxidation process is shale with relatively low contents of hydrogen poor c13 enriched organic matter lower contents of sulfur and sulfide forming elements and relatively unchanged contents of phosphorus and many trace elements e g cr ni and v however because of lower organic carbon contents element organic carbon ratios are greatly increased the partial oxidation process apparently took place during subaerial exposure of the overlying marine carbonate member blackjack creek member of the fort scott limestone following a marine regression when meteoric waters percolated down to the level of the excello muds allowing oxidation of organic matter and sulfides this hypothesis is supported by earlier workers who have identified meteoric carbonate cements within and soil horizons at the top of the blackjack creek member the period of oxidation is constrained in that organic matter and sulfides in the little osage shale member of the fort scott limestone and equivalents immediately overlying the blackjack creek member appear unaltered similar alteration of other shales in the middle and upper pennsylvanian sections may be local to regional in extent and would depend on the extent and duration of the marine regression and be influenced by local variations in permeability and topography the partial oxidation process has likely led to a redistribution of sulfur and sulfide forming elements into other organic rich lithologies in the section the altered oxidized shales are nongenerative with respect to hydrocarbon generation

mineral chemistry of ultramafic and mafic plutonic rocks of the appalachian ophiolites quebec canada abstract this paper presents the first detailed mineralogical record of ophiolitic plutonic rocks in the thetford mines asbestos area quebec the plutonic rocks form the lower 5 5 km section of the 8 km thick thetford mines ophiolite the ophiolitic cumulate sequence is in tectonic contact with underlying thick plastically deformed harzburgitic peridotite now largely serpentinized the mineralogy of this peridotite is a typical example of a very depleted ultramafic rock since the minerals are poor in hygromagmaphile elements and enriched in the high partition coefficient elements ni cr and mg the cumulates form a rock suite varying from the bottom to the top of the sequence as follows dunite chromitite wehrlite clinopyroxenite hornblende gabbro olivine from the very base of the sequence is highly magnesian fo91 94 and ni rich nio 0 30 0 41 wt chromite grains are characterized by high cr 0 60 0 87 and varying mg 0 30 0 62 related to host rock composition and higher fe3 than harzburgite spinels the tio2 content is usually very low calculations using mineral chemistry data suggest that the primitive liquid which was in equilibrium with early fractionated minerals was highly magnesian ni and cr rich and conversely tio2 and alkali poor this composition is comparable to that of the boninitic lavas of the lower volcanic group overlying the gabbroic rocks by taking into account subsolidus re equilibration it is possible to recalculate an average primitive magma composition the genesis of this liquid results from a 20 30 partial melting of a source depleted in hygromagmaphile elements it is concluded that both magmatic processes and source composition are responsible for the genesis of low tio2 magnesian liquids and that these conditions may not be restricted to a single geodynamic environment paleogeographic reconstructions of appalachian terranes in cambrian ordovician times the occurrence of boninitic lavas and low ti cumulate sequence constrain the formation of the thetford mines ophiolite in a marginal basin or the frontal arc region of an island arc system it is stressed that geodynamic significance of such liquids must be considered in relation to a geological context in order to ascertain their real implication

experimental syntectonic hydration of basalt abstract this paper outlines in general terms the role of metamorphic transformations during rock deformation long duration triaxial stress relaxation tests at 600 c 160 mpa confining pressure on wet crushed basalt sheared in sawcuts made in intact basalt cylinders are reported mechanical data are interpreted in terms of a material which shows a small strain rate sensitivity to large variations in applied stress but which work hardens dramatically as a result of marked microstructural changes which occur during straining permanent distortions of c 20 were produced in the samples tests at 100 mpa pore water pressure resulted in the formation of oriented overgrowths of amphibole preferentially developed on less stressed interfaces between original grains and in pore spaces substantial redistribution of feldspar also occurred at low pore pressures no amphibole formed and only feldspar was recrystallized much of the original porosity of the rock was eliminated in all cases the mechanical and microstructural data are interpreted in terms of flow by some combination of grain sliding and diffusional accommodation with the reprecipitated diffused material being the product of a chemical reaction involving all original solid phases plus water from a consideration of the effects of the reaction on the chemical potential gradients driving diffusion it is concluded that although the syntectonic hydration exerts a profound effect on the microstructural and mineralogical development of the rock during deformation there is no substantial enhancement of the rate of strain similar microstructural developments also occur in natural rocks deformed during retrogressive metamorphic events and examples are described for comparison

characteristics of the postnoon auroras inferred from eiscat radar measurements svalbard european incoherent scatter eiscat radar and polar satellite ultraviolet imager data have been used to investigate structure and dynamics of the postnoon auroral oval in the so called 14 mlt region southward turnings of the interplanetary magnetic field were associated with bright afternoon and evening arcs which drifted equatorward increase in the equatorward arc velocity occurred about 27 7 min after the imf bz turned southward at the subsolar point the observed equatorward motion may occur independently of the convection in one of the cases the velocity of the drifting arc was observed to be about 100 200 m s southward whereas the ionospheric plasma drifted northward at a velocity of 160 m s the arcs were located in the region of convection shear that was shown by the ground magnetometers and the eiscat data on ion velocity and temperature in the cases when electric field data were available the meridional component of the electric field varied from about 60 mv m southward equatorward of the arc to 30 mv m northward poleward of the arc ionospheric plasma parameters were measured during the passes of the arcs through the radar beam downward field aligned ion flow at the 300 550 km altitude was observed poleward of the arc whereas upward flows were observed equatorward of the arc this variation is explained by the difference in the vertical gradient of plasma density convection flows associated with arc and differences in electron precipitation and sunlight conditions create the vertical gradients of plasma density that determine the ambipolar diffusion and observed ion motion

thin widespread pennsylvanian black shales of midcontinent north america a record of a cyclic succession of widespread pycnoclines in a fluctuating epeiric sea

early proterozoic oceanic crust in the northern colorado front range implications for crustal growth and initiation of basement faults 1 early proterozoic oceanic crust exposed in a shear zone in the northern colorado front range provides insight into the tectonic evolution of the colorado province tectonically intermingled mylonitic amphibolite locally pillowed metagabbro clinopyroxenite metachert and melange are preserved within the buckhorn creek shear zone west of fort collins whole rock geochemical data support formation of the igneous protoliths in an oceanic environment major and trace element data overlap samples from modern oceanic ridges and are distinct from modern arc trends metagabbros and clinopyroxenites preserve a subhorizontal lineation defined by igneous pyroxenes core and mantle microstructures indicate initial deformation at temperatures 900 c secondary amphiboles define both subhorizontal and subvertical lineations kinematic analysis indicates that the rocks experienced e w strike slip deformation prior to and during amphibolite facies metamorphism followed by n s shortening 40ar 39ar data from secondary hornblende yield age steps as old as 1 9 ga based on the chemical data and kinematic evidence for strike slip deformation we propose that the buckhorn creek rocks originated at an oceanic ridge transform intersection the high t pyroxene fabrics formed during ocean floor deformation prior to 1 7 ga regional deformation of the colorado province the buckhorn creek fragment likely represents original oceanic basement for 1 8 1 7 ga arc magmatism and sedimentation that ultimately formed the colorado province continental crust reactivation of original oceanic fracture zones may be responsible for the network of regularly spaced northeast trending shear zones in colorado once formed subvertical transform faults are likely to survive the transition from oceanic to continental crust and persist as long lived crustal discontinuities

cufenis mineral assemblages in upper mantle peridotites from the table mountain and blow me down mountain ophiolite massifs bay of islands area newfoundland their relationships with fluids and silicate melts abstract plastically deformed ultramafic rocks in the table mountain and blow me down mountain ophiolites comprise a basal unit of slightly depleted iherzolites an intermediate sequence of strongly depleted harzburgites and an upper zone of dunites also referred to as a transition zone intensively percolated by basaltic melts or magmatic fluids thirty five samples including all of the above rock types have been investigated on 100 polished thin sections for cu fe ni s mineral assemblages most of them contain traces of cu fe ni sulfides native metals and locally ni arsenide compositional features of opaque assemblages as well as their textural sites in the rocks indicate that the present cu fe ni s minerals derive from an upper mantle sulfide component through extensive subsolidus re equilibration down to 100 c the primitive component predominant pentlandite minor pyrrhotite and chalcopyrite is preserved as sulfide inclusions in chromites of the transition zone due to a subsolidus re equilibration in a closed system on the contrary sulfide assemblages interstitial to silicates and spinel have extensively reacted with reducing serpentinizing fluids to produce sulfur deficient sulfides such as heazlewoodite and mackinawite and native metals native copper and awaruite microscale variations of redox conditions and the removal of fe from the silicate during serpentinization may account for the peculiar grain by grain equilibrium state of intergranular assemblages in spite of low temperature alteration a gradual depletion in sulfide component has been recognized from the basal lherzolites to the intermediate harzburgites while the sulfide content gradually increases in the transition zone up to 0 2 by volume the first pattern is consistent with the low melting nature of the sulfide component in mantle melting processes microstructural criteria such as the absence of sulfide inclusions in olivine neoblasts demonstrate that the sulfide component postdates plastic deformation of the transition zone the sulfide enrichment pattern is thus ascribed to the percolation of a sulfur saturated basaltic magma into residual dunites

effect of sudden solar wind dynamic pressure changes at subauroral latitudes change in magnetic field the observations obtained during the international magnetospheric study ims from the magnetometers of the igs network extending from cambridge england to tromso norway are used to study the response of subauroral current systems to sudden changes in solar wind dynamic pressure observations show that the response is very strong at subauroral latitudes the preliminary response in the h component is a brief small increase in the dayside morning sector and a decrease in the afternoon and night sectors the main response in the horizontal field the h and d components is toward the pole except in the dayside morning sector the inferred ionospheric current is mainly a circulatory system flowing counterclockwise when viewed from the north pole everywhere at subauroral latitudes except the dayside morning sector

reservoir characterization of the pennsylvanian caddo limestone in stephens county texas a case study of komia dominated algal mounds abstract the caddo limestone forms economic carbonate reservoirs in stephens county northern texas this study demonstrates that in the caddo limestone of the eliasville and breckenridge fields porosity and permeability are best developed in phylloid algal wackestones and packstones as well as komia wackestones and packstones prevalent within the uppermost interval i e cycle a of the formation in the study area the main reservoirs formed in the upper and middle intervals of the caddo algal mounds because of meteoric dissolution related to subaerial exposure which created a large volume of secondary pores and early cementation that prevented mechanical compaction a great portion of the secondary pores remain open providing the principal pore spaces of the reservoir interval vugs including moldic voids are abundant and dissolution enhanced intragranular pores are very common within widespread komia intercrystalline pores are prevalent in dolomitized and neomorphised lithofacies where micrite was converted to microsparitic and sparitic calcite micropores are abundant in the matrix and within grains especially komia fragments the lower or basal interval of cycle a is commonly much less porous owing to the substantial loss of primary pores by physical compaction and lack or rare presence of secondary pores laterally wells in the areas with thicker cycle a interpreted as algal mounds have higher porosity and thicker net reservoir than those in intermound areas this work provides a case study of carbonate reservoirs in which komia wackestones and packstones are the major reservoir rocks

glacio eustasy depositional environments diagenesis and reservoir character of goen limestone cyclothem desmoinesian concho platform central texas the goen limestone desmoinesian of concho and runnels counties central texas consists of three stacked transgressive regressive sequences each cycle consists of a thin basal transgressive unit clayey facies deposited in a deeper water lower ramp setting and thick regressive limestones algal foraminiferal and bryozoan facies representing middle to upper ramp mound mound associated and back mound environments glacio eustatic sea level fluctuations were the dominant control on the observed cyclicity and facies distribution the major porosity forming process in the goen limestone was early leaching by freshwater lenses that migrated in response to glacio eustatic sea level changes the dominant porosity reducing processes included early cementation in a meteoric phreatic environment and late stage saddle dolomitization in a deeper burial connate environment the hydrocarbon producing regressive middle and upper ramp units foraminiferal and bryozoan facies underwent the most complex diagenetic histories and contain extensive secondary pores the presence or absence of penetrating freshwater lenses and the distribution of green phylloid algae and secondary pore filling cements were the major factors controlling goen reservoirs numerous cyclic carbonate reservoirs with depositional and diagenetic histories similar to the goen are present in the desmoinesian section of central texas e g caddo odom gardner capps and dog bend limestones late paleozoic mound associated and back mound carbonate reservoirs may be of greater importance in petroleum exploration than algal mounds one of the most frequently exploited late paleozoic carbonates of north america

meteoric burial diagenesis of pennsylvanian arkosic sandstones southwestern anadarko basin texas pennsylvanian arkosic sandstones at mobeetie field were deposited in fan deltas that prograded onto a shallow shelf in the southern anadarko basin the detrital minerals of the sandstones reflect the composition of the precambrian granites and granodiorites that were exposed in the nearby amarillo uplift distal margins of some fan delta lobes were reworked by marine processes and carbonate fossil fragments and oolites were mixed with terrigenous clastics the diagenetic history and hence reservoir quality of the distal marine reworked sandstones differ from those of the more proximal nonreworked sandstones the earliest diagenetic events formation of chlorite ooids and precipitation of fibrous submarine mg calcite cement took place in the depositional environment of the marine reworked sandstones on the basis of isotopic data the remainder of the diagenesis evidently occurred in fluids of meteoric origin shallow early meteoric diagenesis induced precipitation of iron poor calcite spar in reworked sandstones nonreworked sandstones lack calcite spar but contain early pore lining chlorite cement dissolution of aragonite by fresh meteoric ground water created moldic porosity within reworked sandstones as burial increased porosity in both the reworked and nonreworked fan delta sandstones was reduced by precipitation of authigenic quartz feldspar kaolinite fe calcite and ankerite oxygen isotope data suggest that these cements also precipitated from fluids of meteoric origin that became increasingly hot but remained relatively constant in isotopic composition the last diagenetic events were influenced by the overlying permian evaporites sodium rich fluids caused partial albitization of detrital plagioclase and sulfate derived from the evaporites precipitated as anhydrite and celestite

observations of auroral substorms occurring together with preexisting quiet time auroral patterns although horse collar configurations are commonly associated with relatively quiescent magnetospheric conditions observations made with the viking uv imager are presented which clearly demonstrate that classical auroral substorms can coexist with such patterns thus the magnetospheric conditions that lead to the production of these quiet time auroral patterns are not necessarily stable to the growth of the instability that leads to substorm onset as well the horse collar patterns provide useful natural indicators of the underlying magnetospheric topology and can be used to determine in a relative sense the source location of the substorm activity we conclude that the horse collar patterns have a distant tail source location and that the substorm onsets have a source location on closed field lines with much lower l values the observations are therefore inconsistent with the onset mechanisms proposed in either the thermal catastrophe model or the boundary layer dynamics model in addition we have shown that substorm related plasma sheet dropouts are not always produced by a neutral line induced thinning of the plasma sheet but could also be caused by the large scale motion of the open field line region plasma data from dmsp f6 dmsp f7 isee 1 and viking are consistent with the interpretation that the regions between the horse collar arcs are threaded by open lobe field lines and that open field lines can also partially intrude into the space between the transpolar arcs and the lower latitude main oval the significance of these results for substorm activity which occurs together with quiet time auroral patterns is discussed in terms of the flux rope plasmoid version of the near earth neutral line model depending upon the location and extent of the open field line regions relative to the developing plasmoid it is suggested that several possibilities exist for the subsequent evolution of the substorm activity in the tail and that these different scenarios can be related to specific types of auroral morphologies in the ionosphere

petrologic evolution of large failed rifts in the eastern pacific petrology of volcanic and plutonic rocks from the mathematician ridge area and the guadalupe trough

lithofacies character and architecture across a pennsylvanian inner platform transect sierra de cuera asturias spain abstract seismic scale outcrops of lower pennsylvanian steep margined carbonate platforms in northern spain sierra de cuera cantabrian mountains provide the opportunity to link depositional facies to platform interior architecture lithofacies character vertical stacking patterns and spatial distribution of depositional facies and shallowing upward cycles were investigated to provide a semiquantitative depositional model for inner platform strata a 70 m thick succession of eight shallowing upward cycles 2 5 15 m thick was studied across a 2 km wide transect cycles consist of algal bioherms and skeletal packstone lithofacies association b deposited in an open marine and sub wave base environment which are locally lateral to crinoid dominated packstone lithofacies association d lithofacies association b developed on a transgressive one meter thick high energy coated grain grainstone lithofacies association a and is overlain by open marine to restricted lagoonal deposits lithofacies association c indicative of decreased paleo water depth cycle boundaries are marine flooding surfaces occurring at the base of lithofacies a and the top of lithofacies c petrographic and outcrop evidence of subaerial exposure is conspicuously lacking cycle and lithofacies thickness varies laterally and all the lithofacies can be either continuous for the 2 km width of investigation or discontinuous terminating within tens to hundreds of meters in most of the cycles the lithofacies succession changes laterally because of lateral facies transitions between lithofacies b and d and pinching out of lithofacies a and c meter scale variations in depositional topography are related to the stacked lens shaped bioherms assigned to lithofacies b and appear to control the thickness and lateral continuity of the overlying facies sierra de cuera strata do not resemble other pennsylvanian cycles driven by high frequency high amplitude glacio eustasy which are commonly characterized by subaerial exposure surfaces developed on subtidal deposits except for the absence of peritidal facies and the presence of low relief algal bioherms on the platform top the necessary accommodation to enhance the growth of bioherms on the platform top was probably due to high frequency 100 240 ky moderate amplitude nearly 40 m sea level fluctuations combined with subsidence rates of 70 140 m my tectonic subsidence might have been responsible for the lack of evidence of subaerial exposure surfaces in the examined moscovian strata the studied inner platform lithofacies character and architecture were controlled by the interplay of high frequency changes in accommodation and the presence of meter scale depositional topography attributed to lateral variations of carbonate accumulation

flux transfer events on the magnetopause spatial distribution and controlling factors magnetic field profiles across the dayside magnetopause obtained during the first five years 1977 1981 of the isee mission have been examined in boundary normal coordinates to determine the spatial distribution and controlling factors of flux transfer events fte s owing to the natural evolution of the orbital plane of the spacecraft the dayside magnetopause region is eventually sampled both above and below the magnetic equator nevertheless at the end of the period surveyed the satellites still had not yet reached latitudes as far south of the equator as they were initially north of the equator two types of magnetic signature in the component normal to the magnetopause are regularly observed when an fte passes the spacecraft in the northern dawn quadrant the perturbation along the normal direction is usually first outward and then inward whereas in the southern dusk quadrant the reverse behavior occurs the dividing line between the two signatures is tilted somewhat with respect to the equator along a south dawn north dusk direction however deviations from this simple pattern are observed in both hemispheres examination of simultaneous imp 8 and isee 3 interplanetary data reveals that fte s occur almost exclusively during southward interplanetary magnetic field imf conditions fte s occur about 45 of the time when the imf has a southward component with the highest rates of occurrence for due southward imf the sign of the east west component of the imf does not exert control on the rate of fte occurrence

lithofacies stable isotopic composition and stratigraphic evolution of microbial and associated carbonates green river formation eocene piceance basin colorado lacustrine carbonates of the eocene green river formation crop out on the western margin of the piceance basin and the eastern margin of the uinta basin in western colorado this area allows tracing of vertical and horizontal facies variation over hundreds of meters limestone beds consist of littoral to sublittoral lithofacies bioclastic and oolitic grainstones oolitic wackestone intraclastic rudstone stromatolites and thrombolites facies form upward deepening cycles that start with sharp based grainstones and packstones followed by stromatolites or thrombolites and capped by fine grained stromatolites and or oil shale deposits the vertical succession of carbonate deposits correlates with evolutionary lake stages the succession starts with grainstone deposits rich in ostracods and gastropods that correspond to an initial freshwater lake thrombolites capped by laminated stromatolites or coarse agglutinated stromatolites correlate with a higher salinity transitional lake deepening upward cycles as much as 5 m 16 ft thick of thrombolites agglutinated stromatolites and fine grained stromatolites occur in the highly fluctuating lake the upper section is dominated by laminated stromatolites that correspond to a rising lake stable isotope 18o and 13c values covary and range from 8 to 0 8 and 3 to 5 respectively the 18o values indicate carbonate precipitating water evolved from fresh to saline and became less saline in the upper green river negative excursions of 13c values correspond to lake level rises and positive excursions of 13c values occur during lake level falls syndepositional to burial diagenesis modified carbonate porosity early dissolution is followed by burial compaction and fracturing compaction and late calcite cements occluded primary and secondary porosity

a statistical study of transient event motion at geosynchronous orbit the geosynchronous goes 5 and goes 6 satellites frequently observe transient events marked by magnetic field strength increases and bipolar magnetic field signatures lasting several minutes in this study we report a survey of 87 events observed simultaneously by both goes spacecraft for a total of 174 individual observations from august to december 1984 events detected in the prenoon sector outnumbered those in the postnoon sector by about a 3 to 1 ratio the distribution of the events versus local time exhibited a significant prenoon peak like the distribution of magnetic impulse events observed in high latitude ground magnetometers a cross correlation analysis of the two goes data sets indicated lags that range from 0 to over 2 min with the majority of the events moving antisunward the short lags correspond to azimuthal speeds of hundreds of kilometers per second greater than flow speeds in the magnetosheath but less than fast mode waves the short lags may indicate that the events move primarily latitudinally and or that transient events are seldom localized but rather occur over extended if not global regions investigations of event occurrence versus interplanetary magnetic field imf bz event motion versus imf by and correspondence between upstream plasma data and the events all indicate that pressure pulses are the likely source of many of the events about 27 of the events with simultaneous solar wind data were preceded by sharp reversals in one or more imf components and nearly all of this particular group of events occurred in the dawn sector this suggests that the pressure pulses may be commonly generated in the foreshock bow shock region since the prenoon magnetopause lies generally behind the quasi parallel bow shock where such pulses are thought to be triggered by imf discontinuities finally several events in the data set were also observed by the ampte cce these are presented as case studies

metamorphism in oceanic layer 3 gorringe bank eastern atlantic gorringe bank is an anomalously high structure of the eastern part of the north atlantic which was known to be composed of mantle derived peridotites layer 4 and gabbros layer 3 during the submersible cruise cyagor ii in 1981 the contact between layer 4 and layer 3 was observed on mount gettysburg and interpreted as tectonic the overlying series of gabbro was extensively sampled on both mounts composing the bank gettysburg and ormonde coarse grained to pegmatoid clinopyroxene gabbros predominate and are associated with differentiated rocks ferrogabbros and diorites cumulate gabbros are missing the gabbroic section sampled is therefore interpreted as the upper part of the plutonic section most samples were strongly recrystallized during two distinct events metamorphism occurred close to the ridge axis from interaction of a seawater derived fluid with still hot gabbros high temperature shear zones favoured fluid circulation but the water rock ratio estimated from the sodium input was very small in undeformed rocks 1 the low w r ratio explains the strong evolution of the fluid phase and therefore some particular compositions of secondary minerals low temperature alteration occurred when the gabbros were tectonically emplaced close to the sea bottom

directly dating geologic events u pb dating of carbonates 1 carbonates are ubiquitous forming in a variety of settings including caves soils the oceans during burial from hydrothermal fluids and along faults u pb dating of carbonates is applicable to almost the entire geologic time scale with results from pleistocene speleothems showing great promise for dating records 1 ma old here we review studies of u pb dating of carbonates from a range of depositional and diagenetic environments and consider published results with regard to stratigraphic associations and the behavior of u and pb in various fluids we detail the steps required to select suitable samples summarize current laboratory techniques and provide directions for improvement of analytical protocols the u pb dating of carbonates has tremendous potential for dating events of geologic interest and is virtually untapped

flux transfer event footprint patterns and implications for convection the spatial relation between an open polar cap and the footprints of open magnetopause patches is calculated for a variety of imf orientations patches mapped from an otherwise closed magnetopause elongate and fan out from the cusp point located at noon on the polar cap boundary the fan extends into the closed field region equatorward of the polar cap the footprints of patches well down the flanks elongate the most and align with the polar cap boundary as the patches convect with the magnetosheath flow their footprints pivot antisunward about the cusp until they coalesce with the polar cap in the course of pivoting the footprints act like paddles and push adjacent closed flux antisunward forming a low latitude boundary layer on open and closed field lines if the patches are regions of enhanced flux imbedded in an open magnetopause the footprints shorten detach from the cusp and extrude from positions increasingly displaced from noon as the field component normal to the magnetopause strengthens only if the normal component is large will the footprints degenerate to bulges on the polar cap boundary and move directly poleward as previously supposed

multistation observations of auroras polar cap substorms double wavelength 630 0 and 427 8 nm all sky imagers were operated at the automatic geophysical observatory ago sites p1 and p4 magnetic latitude 80 south pole station magnetic latitude 74 and p3 magnetic latitude 72 during the austral winter of 1994 and 1995 a new data format was developed to present the 24 hour auroral displays from the station arrays which provided 24 hour optical data during the antarctic winter night for several months auroras at these stations exhibit the typical magnetic local time mlt features of high latitude precipitation on the nightside mlt substorm poleward expansions were observed to reach very high latitudes a typical event june 3 1994 0400 0500 ut started equatorward from the south pole and propagated to the high latitude limit of the field of view at p1 and p4 the morphological appearance of these high latitude auroral substorm features was similar to that of substorm auroras observed at lower latitudes the intense 427 8 nm emission accompanied by a weak 630 nm emission showed that the particles were energetic the interplanetary magnetic field imf bz component was slightly negative before and during this event emphasizing that these were substorm auroras and not polar cap auroras which tend to occur during times of prolonged positive imf bz statistical analysis of the 1995 ago optical data shows that over 20 of substorms are seen to penetrate to regions greater than 80 of latitude

dolomite reservoirs porosity evolution and reservoir characteristics systematic analyses of the published record of dolomite reservoirs worldwide reveal that the majority of hydrocarbon producing dolomite reservoirs occurs in 1 peritidal dominated carbonate 2 subtidal carbonate associated with evaporitic tidal flat lagoon 3 subtidal carbonate associated with basinal evaporite and 4 nonevaporitic carbonate sequence associated with topographic high unconformity platform margin buildup or fault fracture reservoir characteristics vary greatly from one dolomite type to another depending upon the original sediment fabric the mechanism by which dolomite was formed and the extent to which early formed dolomite was modified by postdolomitization diagenetic processes e g karstification fracturing and burial corrosion this paper discusses the origin of dolomite porosity and demonstrates the porosity evolution and reservoir characteristics of different dolomite types

the influence of nacl h2o fluids on reactions between olivine and plagioclase an experimental study at 0 8 gpa and 800 900 c abstract the lower to middle continental and oceanic crust is dominated by mafic gabbroic rocks which are variably metamorphosed at amphibolite granulite facies conditions forming reaction rims or veins containing secondary pyroxenes amphibole and phlogopite metamorphic fluids in the mid crust are typically nacl rich brines in this study olivine and plagioclase were reacted with varying amounts of fluid 0 2 wt added that had compositions ranging from pure h2o to nacl solutions 0 2 mol in a series of experiments using the piston cylinder apparatus at 0 8 gpa and temperatures from 800 to 900 c coronas of clinopyroxene clinopyroxene amphibole and clinopyroxene phlogopite were formed the thicknesses of reaction rims i e the relative amount of reaction rim products increase with the relative amount of fluid present in the system and are therefore linked to the level of fluid saturation along grain boundaries and interphase boundaries samples with no added h2o show little to no signs of reaction indicating the necessity for fluids in initiating a reaction between olivine and plagioclase the concentration of nacl in the fluid also increases the rates of rim growth at 900 c and 2 wt added fluid cl rich hydrous dacite trachyte melt compositions formed and are similar to low degree partial melts of oceanic crust the cl contents of secondary amphiboles are positively correlated with the feo and k2o contents of the amphibole consumption of fluid during the formation of hydrous rim phases may lead to high cl within transient fluid filled pores along grain boundaries which may lead to locally cl rich amphiboles

complexity of the crustal sequence in the northern oman ophiolite fizh and southern aswad blocks the effect of early slicing abstract on the basis of detailed mapping of the fizh and southern aswad blocks in northern oman the complexity of the crustal sequence is shown lateral facies variations are at least as important as vertical ones and no section can be reconstructed as a stratigraphic section sharp and transitional moho boundaries alternate over a distance of 10 km the important residual mantle crust transition zones are root zones for intrusive wehrlites and cause doming in the moho fractures facilitating the penetration of seawater in the upper crustal parts dikes and lavas show intense hydrothermal alteration and can be related to shear zones in the lower parts of the crustal sequence and occasionally be followed into the peridotites intrusive wehrlites reach as high into the crust as the base of the dike complex and occasionally even the lavas they follow fault zones or n s to nw se directions recrystallization of isotropic gabbro occurs in the vicinity of the wehrlites the geometry observed is not compatible with an oceanic transform zone it may indicate the location of an overlapping spreading centre although too little is known of the deeper structures to confirm this there was certainly an important reaction to the early compression at the oceanic spreading ridge prior to final consolidation confirming an earlier model of the setting

events of enhanced convection and related dayside auroral activity in this paper we study the high latitude plasma flow variations associated with a periodic 8 min sequence of auroral forms moving along the polar cap boundary which appear to be the most regularly occuring dayside auroral phenomenon under conditions of southward directed interplanetary magnetic field satellite data on auroral particle precipitation and ionospheric plasma drifts from dmsp f10 and f11 are combined with ground based optical and ion flow measurements for january 7 1992 ionospheric flow measurements of 10 s resolution over the range of invariant latitudes from 71 to 76 were obtained by operating both the european incoherent scatter eiscat uhf and vhf radars simultaneously the optical site ny alesund svalbard and the eiscat radar field of view were located in the postnoon sector during the actual observations the west greenland magnetometers provided information about temporal variations of high latitude convection in the prenoon sector satellite observations of polar cap convection in the northern and southern hemispheres show a standard two cell pattern consistent with a prevailing negative by component of the interplanetary magnetic field the 630 0 nm auroral forms located poleward of the persistent cleft aurora and the flow reversal boundary in the 1440 1540 mlt sector were observed to coincide with magnetosheath like particle precipitation and a secondary population of higher energy ions and they propagated eastward tailward at speeds comparable with the convection velocity it is shown that these optical events were accompanied by bursts of sunward return flow at lower latitudes in both the morning and the afternoon sectors consistent with a modulation of dungey cell convection the background level of convection was low in this case kp 2 the variability of the high latitude convection may be explained as resulting from time varying reconnection at the magnetopause in that case this study indicates that time variations of the reconnection rate effectively modulates ionospheric convection

conjugacy of isolated auroral arcs and nonconjugate auroral breakups fine examples of both conjugate and nonconjugate isolated auroral arcs were observed at two geomagnetically conjugate stations near l 6 syowa station in antarctica and husafell in iceland on september 12 1988 these events exhibited some interesting characteristics an auroral loop structure that appeared in both hemispheres was 2 0 times larger in the north south direction at syowa than at husafell this scale difference is greater than expected from the difference in geographic and geomagnetic igrf coordinates between the two points of observation however temporal and spatial variations in the loop structures were almost identical in both hemispheres after the disappearance of the loop structure closely conjugate auroras were formed nonconjugate auroral features appeared again at syowa on the poleward side while the equatorward auroras maintained conjugacy the nonconjugate aurora at syowa then began to break up showing fast moving vortex like structures auroral spirals at this time all auroral features at husafell seemed to have their conjugate counterparts in equatorward auroras at syowa and none exhibited rapid motions these conjugate auroras at husafell were gradually extending poleward while the corresponding features at syowa were compressed toward the equator and shrinking in size the onset of auroral breakup was about one minute earlier at syowa than at husafell the nonconjugate auroral features were reflected in corresponding magnetic field variations on the ground the events summarized above give interesting clues to the development and decay of auroral conjugacy and the question why the beginning of auroral breakup is not simultaneous at conjugate stations the time lag and nonconjugacy of auroral breakup in conjugate areas suggests that the triggering source of auroral breakup was not located near the equatorial plane in the magnetosphere but most likely in a localized region near the ionosphere in the southern hemisphere the nonconjugate auroral spirals also suggest the existence of asymmetrical field aligned currents

isotopic and elemental evidence for meteoric alteration of a pennsylvanian phylloid algal mound holder formation new mexico u s ameteoric diagenesis of a pennsylvanian phylloid algal mound new mexico

properties of 6300 a auroral emissions at south pole optical 6300 a observations of the polar cap region were recorded during the april to august austral winters of 1983 through 1986 with an image intensified all sky camera located at south pole station auroral structures seen in these all sky images are correlated with the signatures registered by a meridian slit keogram imager the keograms provide a convenient survey of auroral activity but the all sky images are required in order to determine the precise auroral morphology associated with the commonly observed keogram traces typical images of auroral activity around the diurnal period are presented during morning and afternoon magnetic local time the auroral oval is usually seen crossing zenith and frequent meridional movements appear on the keograms during the midday interval discrete poleward moving arcs occasionally appear likely associated with flux transfer events or similar changes in the plasma flow pattern nightside keograms show the characteristic signatures of sun aligned arcs which are well defined in the associated all sky images two thirds of the sun aligned arcs are observed on the dawnside of local midnight the arcs drift overhead at velocities of typically less than 250 m s with reference to a sun oriented noncorotating frame no significant preference is shown for easterly dawnward or westerly duskward drift directions these sun aligned arcs occur with greater frequency than previously thought we observe a total of 114 such events during 219 diurnal periods of image collection the available interplanetary magnetic field measurements though limited indicate bz 0 for two thirds of the sun aligned arcs while no correlation is found between by and their local time distribution there is a suggestion that dawnward motions appear preferentially with by 0 and duskward motions with by 0

diagenetic history and reservoir characteristics of a middle carboniferous dolomite reservoir eastern sichuan china abstract reservoirs of commercial gasfields in eastern sichuan china occur in middle carboniferous dolomite porous reservoir rocks in this dolomite are bounded by an upper disconformity and were deposited in tidal flats two types of diagenetic histories are suggested the first involves dolomite breccia and relict algal pellet dolomite and occurs in three stages 1 1 penecontemporaneous desiccation evaporation and replacement which formed polygonal mudcracks and dolomitic crusts and gypsum in a supratidal environment gypsum is laminated nodular and preserved as anhydrite pseudomorphs 2 2 solution developed porosity vadose silts and cements indicating early leaching and cementation by meteoric water subhedral crystalline dolomite formed from mixing of interstitial sea water with meteoric water causing intercrystalline porosity and pinpoint vuggy porosity 3 3 cavernous and fracture porosity formed in association with a regional erosional hiatus latest carboniferous the second type involves cryptalgal dolomite grainstones and packstones occurring as interbedded laminae and are characterized by dolomicritic cryptalgal grains with dolomite rim cement dissolution porosity and rhombic dolomitic fillings they were deposited in marine phreatic environments in the lower intertidal zone subsequent hypersaline seepage reflux caused dolomitization solution porosity including integranular porosity moldic porosity and intergranular porosity filled partially with limpid rhombic dolomite indicated leaching in meteoric vadose environments during early diagenetic stages subsequently precipitation of dolomite rhombs indicates a mixing zone

record of late pleistocene oxygen isotopic stage 5 climate changes during episodes of karst development on the northern coast of crete sequence stratigraphic implications abstract in late pleistocene times during oxygen isotopic stage ois 5 aeolian calcareous sand dunes were deposited along the northern coast of crete where they now discontinuously crop out wedged between thin shallow marine tyrrhenian deposits late pleistocene marine sediments are scarce and amalgamated because the combination of slow uplift movement and low elevation of ois 5 highstand shorelines close to present mean sea level promoted their erosion in the shallow marine and subaerial environments the studied succession is organized in transgressive regressive t r sequences bounded below and above by transgressive surfaces of erosion superimposed on subaerial unconformities the vertical stacking of sedimentary facies between consecutive key surfaces demonstrates that the preservation potential of marine transgressive deposits tst was higher than that of regressive sediments similarly aeolian strata and palaeosols located landward were well preserved detailed sedimentological and petrographic studies of the aeolianites and their framing units from several sites revealed a complex microstratigraphy of diagenetic features that relate to 1 the succession of highstand shorelines during ois 5 and 2 episodes of subsurface karstification interpreted to result from soil cover development on limestone substrate during relative sea level falls a complex sequence of climate changes during karst formation is suggested by solution pipes that cross cut or contain calcified root systems and whose inner walls are covered with carbonate crusts interpreted as calcretes the repetition during ois 5 of this transition from rhizocretion formation to karstification to calcretization in the studied deposits is argued to occur in response to contrasting climate conditions at times of sea level lowstand it is proposed that palaeosols and rhizocretions formed during the falling limb of the relative sea level curve under semi arid conditions karstification developed at the turn between falling and rising sea level when climate was at its wettest and calcretization occurred during the rising limb of the relative sea level curve under arid conditions identification of wave cut surfaces of erosion on top of transgressive marine sediments and below aeolian deposits suggest that the latter are contemporaneous with falling sea level marine units deposited basinward below the lowstand shoreline namely forced regressive systems tracts frst otherwise not recognized in the field the complex suite of alteration processes recorded within the palaeosols above aeolian deposits are interpreted to represent periods extending from the latest stage of sea level fall to the earliest stage of sea level rise including lowstand systems tracts lst this proves a satisfactory complement to reconstruct palaeosea level fluctuations in settings where the shallow marine nature of sedimentary environments means that most regressive and lowstand deposits are not recognized in the stratigraphic record

relationship of uranium to petrography of caliche paleosols with application to precisely dating the time of sedimentation abstract we use petrography combined with fission track mapping to evaluate qualitatively the phases that concentrate uranium in caliche paleosols from late paleozoic cyclothems of texas and new mexico we also discuss geochemical analyses of u and pb concentrations and pb isotopic compositions uranium concentrations in brown caliche calcite range from 1 to 9 ppm lead concentrations in the same calcites range from 0 5 to 1 ppm lead isotope compositions range from common no apparent radiogenic component to lead with a substantial radiogenic component caliches with brown calcite most likely developed near the vadose phreatic interface often a redox boundary where organic material and uranium is expected to be concentrated in contrast uranium concentrations in light colored caliche nodules from the vadose zone have uranium concentrations that are 1 ppm or less fifteen of forty five samples from this study have a range in u pb that would permit precise dating of the rocks but only the data from three give precise ages some of the samples have hematite which has high concentrations of u and pb for one of the precisely dated samples the two sigma uncertainty was improved from 10 to 2 6 ma by avoiding hematite when sampling

diagenetic model for carbonate rocks in midcontinent pennsylvanian eustatic cyclothems abstract diagenetic patterns in cyclic midcontinent pennsylvanian carbonates are readily explained in terms of a predictive diagenetic model derived logically from the eustatic depositional model for widespread pennsylvanian cyclothems developed by heckel 1977 1980 transgressive shoal water calcarenites are characterized by pervasive overpacking of grains neomorphism of originally aragonite grains ooids green algae molluscs and ferroan calcite and dolomite cement which indicate movement from the marinephreatic environment of deposition and diagenesis into the low oxygen deeper burial connate zone with substantial compaction before any cementation offshore invertebrate calcarenites associated with offshore core shales also are characterized by pervasive overpacking of grains an ferroan carbonate cements which indicate a similar diagenetic history their lack of discernible neomorphism relates to original absence of shallow water aragonite grains ooids green algae and probable removal of thin shelled molluscs by dissolution during long periods of sediment starvation in cool water which also corroded the edges of many calcite grains regressive shoal water calcarenites show a much greater variety of diagenetic features including early marine cement rims and large scale leaching of originally aragonite grains often with subsequent collapse of micrite envelopes grain fragments and overlying material in samples insufficiently stabilized by early cement rims this was followed by pervasive cementation by blocky calcite before much further compaction then by ferroan calcite and finally ferroan dolomite in remaining voids this pattern indicates replacement of depositional marine phreatic water by meteoric water which dissolved unstable carbonate grains and then deposited stable carbonate cements in environments that eventually became increasingly oxygen depleted and otherwise chemically changed probably as mixing zone and deeper connate water moved back into the rock and replaced the meteoric water during and after the succeeding transgression trends in calcilutites are essentially similar to those in calcarenites of equivalent phase of deposition with evidence of subaerial exposure and meteoric vadose soil formation in strata at the top of many regressive limestones it is apparent that with the regression of the sea and emergence that terminated deposition of a cyclothem meteoric water penetrated the permeable pans of the regressive carbonate and left its distinctive diagenetic patterns of early leaching and cementation before much compaction took place but rarely did meteoric water penetrate the impermeable offshore shale which acted as a seal this allowed associated offshore and underlying transgressive carbonates to become more deeply buried and substantially compacted before cementation took place with unstable grains undergoing slow neomorphism in the absence of meteoric leaching

stratigraphic architecture of lower permian cyclic carbonate reservoirs chase group in the mid continent usa based on outcrop studies seven depositional sequences each of which consists of two intermediate order cyclothems compose outcrops of the chase group permian wolfcampian in the mid continent principal reservoir analog facies are restricted mainly to the highstand systems tracts of cyclothems these facies include subtidal lime sands deposited mainly in downdip distal ramp settings and peritidal dolomudstones deposited farther updip on the ramp the stratigraphic and depositional architecture of the section was controlled by the interplay among paleobathymetry glacio eustasy and periodic syndepositional tectonism porous lime sands dominate in the lower part of the chase group the forced regressive unconformity bounded sequences within which they occur were deposited during relatively high magnitude eustatic fluctuations that resulted in marine accommodation increases great enough to preclude peritidal deposition porous peritidal facies instead dominate in the upper part of the chase group the sequences within which these normal regressive deposits are present were deposited during lower magnitude eustatic fluctuations and decreased marine accommodation which allowed for their eventual progradation across the ramp complex porosity heterogeneity is evident within individual depositional sequences and component cyclothems as well as vertically within the chase group

cathodoluminescence of carbonate cemented holocene beachrock from the togo coastline west africa an approach to early diagenesis carbonate cemented holocene beachrock outcrops sporadically along more than 100 km of the togo and benin coasts the sands are siliciclastic with subsidiary carbonate skeletal fragments and intraclasts 14c radiometric dating indicates the sand deposits have formed within the last 5000 years beachrock has been indurated in phreatic environments undergoing afterwards several stages of burial and subaerial exposure cathodoluminescence cl proved a useful tool in unravelling the intricate cementation of this beachrock which can be subdivided into three stages firstly magnesian calcite was precipitated in a marine phreatic active zone very dull orange and very dull blue colours emitted in isopachous rims of needle like crystals and peloidal micrite cl1 fabric reveal different eh and ph precipitating conditions in this environment secondly bright yellow orange luminescing acicular and equant spar cement cl2 fabric is interpreted to have formed in a mixed water phreatic zone the third stage of cementation occurred dominantly in the landward part of the beachrock and accompanied neomorphism of the earlier marine cements the resultant cl3 fabric dull blue medium orange luminescing equant spar is typical of continental fresh water phreatic zones the progression from marine to fresh water pore fillings accompanied sand bar progradation during a stable sea level stage ultimately there was erosion stage of the sand bar deposits which led to subaerial exposure and partial erosion of the beachrock recognition of the effects of repeated shoreline progradation during the early diagenesis of this holocene beachrock indicates the potential value of diagenetic studies in aiding recognition of marine regressions and transgressions in the geological record

dolomite occurrence evolution and economically important associations abstract dolomite is not a simple mineral it can form as a primary precipitate a diagenetic replacement or as a hydrothermal metamorphic phase all that it requires is permeability a mechanism that facilitates fluid flow and a sufficient supply of magnesium dolomite can form in lakes on or beneath the shallow seafloor in zones of brine reflux and in early to late burial settings it may form from seawater from continental waters from the mixing of basinal brines the mixing of hypersaline brine with seawater or the mixing of seawater with meteoric water or via the cooling of basinal brines bacterial metabolism may aid the process of precipitation in settings where sulfate reducing species flourish and microbial action may control primary precipitation in some hypersaline anoxic lake settings dolomite is a metastable mineral early formed crystals can be replaced by later more stable phases with such replacements repeated a number of times during burial and metamorphism each new phase is formed by the partial or complete dissolution of an earlier dolomite this continual re equilibration during burial detracts from the ability of trace elements to indicate depositional conditions and resets the oxygen isotope signature of the dolomite at progressively higher temperatures because subsurface dolomite evolves via dissolution and reprecipitation a bed of dolomite can retain or create porosity and permeability to much greater burial depths and into higher temperature realms than a limestone counterpart dolomitization also creates new crystals with new rhomb growth following the dissolution of less stable precursors repetition of this process without complete pore cementation can generate intercrystalline porosity a number of times in the rock s burial history intercrystalline porosity is a highly interconnected style of porosity that gives dolomite reservoirs their good fluid storage capacity and efficient drainage the fact that many dolomite reservoirs formed via brine reflux means that they sit beneath an evaporite seal in both platform and basinwide evaporite settings the same association of evaporites sulfate source and entrained hydrocarbons means that burial conditions are also suitable for thermochemical sulfate reduction and the precipitation of base metals this tends to occur at higher temperatures 60 c 80 c and so the resulting dolomites tend to be ferroan and consist of saddle shaped crystals

the common origin and alteration history of the hypabyssal and volcanic phases of the wadi tarr albitite complex southern sinai egypt new data and interpretations are presented for the igneous albitites of the wadi tarr area southern sinai egypt the albitite masses are isolated in outcrop from any granitic intrusions and have intrusive contacts against the country rocks without any structural control they have marginal zones of breccias with jigsaw fit angular clasts suggesting explosive in situ formation the albitites are of two types the western medium grained hypabyssal albitite and the eastern fine grained porphyritic albitite the field relations suggest emplacement at different levels in a magmatic cupola the hypabyssal texture and steeply dipping slope of the upper contact of the western albitite imply deeper emplacement whereas the gently dipping contacts and porphyritic texture of the eastern albitite masses indicate that they define the probable location of the cupola apex both types of albitites consist of albite 92 97 with minor amounts of quartz k feldspar and biotite the accessory minerals include fe oxides augite sulphides zircon rutile xenotime titanite allanite and monazite the whole rock compositions of the hypabyssal and porphyritic albitites are closely related but the porphyritic type has lower abundances of sr ba y nb th and zr we show that the hypabyssal and porphyritic albitites have a common petrogenetic origin most likely as late stage cumulates from a fractionating strongly alkaline a type magma consistent with the compositions of the mafic minerals the source magma was probably a tephritic liquid we use melts models to show that only a sufficiently alkaline magma follows a differentiation path that both avoids quartz saturation and encounters the alkali feldspar solvus reaching a residual liquid in equilibrium with highly sodic feldspar although the melts results show a chemically consistent means of forming igneous albitite they are incomplete in that physical segregation mechanisms are still required to isolate the albite from mafic minerals and or a low temperature aqueous alteration stage is needed to leach k from the feldspar alteration surrounding the wadi tarr albitites is extensive and dominated by alkali metasomatism similar to fenitization alteration in the marginal breccia zone of the albitite is dominated by precipitation of amphibole and carbonate in veins and in the breccia matrix whereas the volcanic country rocks show replacement of feldspars by sericite carbonate and epidote as well as vein carbonate the altered volcanic country rocks show lower concentrations of fe 2o 3 sr cu pb ba and ce accompanied by higher concentrations of na2o and mgo compared to unaltered equivalent samples

amphibolitized sheared gabbros from ophiolites as indicators of the evolution of the oceanic crust bay of islands newfoundland abstract in the bay of islands ophiolite complex both the basal part of the cumulate gabbro sequence and the underlying peridotites have been plastically deformed higher in the series the cumulate gabbros are undeformed except in localized shear zones slightly to strongly oblique to the layering the resulting shear bands are narrow and composed either of mylonitic amphibole bearing gabbros or of highly foliated amphibolites the amphibole composition and nature and the chemistry of coexisting minerals combined with structural evidence imply that a first episode of shearing subhorizontal in the crust occurred at decreasing temperatures ranging from 750 to 450 c this shearing probably represent the response of the lithosphere to stresses also responsible of asthenospheric flow in the upper mantle a second episode of shearing oblique to the first one and probably related to cooling of the crust away from the ridge occurred at lower temperatures ranging from 500 to 300 c the surrounding gabbros have undergone static hydrothermal metamorphism at temperatures decreasing from 650 to 450 c all metamorphic events occurred in hydrous conditions in the vicinity of the ridge axis

trace metal records of regional paleoenvironmental variability in pennsylvanian upper carboniferous black shales abstract regional geochemical differences within a laterally continuous cyclic pennsylvanian upper carboniferous shale in midcontinent north america are interpreted in light of models of glacioeustatic forcing and new views on water column paleoredox stability and trace metal behavior in black shale environments specifically we characterize differences in transition metal fe mn mo v ni zn pb and u concentrations in black shales of the hushpuckney shale member of the swope limestone in iowa and equivalent black shale beds of the coffeyville formation in oklahoma although c s fe systematics and uniform 34 s depleted isotope ratios of pyrite indicate pervasive euxinic deposition anoxic and sulfidic bottom waters for these shales regional variations can be inferred for the efficiency of mo scavenging and for the rates of siliciclastic sedimentation as expressed in spatially varying fe al ratios black shales in iowa show mo enrichment roughly five times greater than that observed in coeval euxinic shales in oklahoma by contrast fe al ratios in oklahoma shales are as much as five times greater than the continental ratio of 0 5 observed in the over and underlying oxic facies and in the coeval black shales in iowa recent work in modern marine settings has shown that enrichments in fe commonly result from scavenging in a euxinic water column during syngenetic pyrite formation in contrast to fe the concentrations of other transition metals mo v ni pb zn u are typically more enriched in the black shales in iowa relative to oklahoma the transition metal trends in these paleozoic shales are reasonably interpreted in terms of early fixation in organic rich sediments due to euxinic water column conditions however regional variations in 1 rates of siliciclastic input 2 organic reservoirs including relative inputs of terrestrial versus marine organic matter and 3 additional inputs of metals to bottom waters from contemporaneous hydrothermal vents are additional key controls that lead to geographic variation in the extent of metal enrichments preserved in ancient organic rich sediments

ocean floor hydrothermal metamorphism in the limousin ophiolites western french massif central evidence of a rare preserved variscan oceanic marker the limousin ophiolite is located at the suture zone between two major thrust sheets in the western french massif central this ophiolitic section comprises mantle harzburgite mantle dunite wehrlites troctolites and layered gabbros it has recorded a static metamorphic event transforming the gabbros into undeformed amphibolites and the magmatic ultramafites into serpentinites and or pargasite bearing chloritites with various thermobarometric methods it is possible to show that the different varieties of amphibole have registered low p c 0 2 gpa conditions with temperature ranging from high t late magmatic conditions to greenschist zeolite metamorphic facies the abundance of undeformed metamorphic rocks which is typical of the lower oceanic crust the occurrence of ca al mg metasomatism illustrated by the growth of ca al silicates in veins or replacing the primary magmatic minerals the p t conditions of the metamorphism and the numerous similarities with oceanic crustal rocks from ocean drilling program and worldwide ophiolites are the main arguments for an ocean floor hydrothermal metamorphism in the vicinity of a palaeo ridge among the west european variscan ophiolites the limousin ophiolites constitute an extremely rare occurrence that has not been involved in any hp subduction related or mp orogenic metamorphism as observed in other ophiolite occurrences i e france spain and germany

polar uvi observations of dayside auroral transient events we analyze polar ultraviolet imager uvi observations of auroral transient events ates in the dayside northern hemisphere during 5 winter months in 1996 and 1997 we found 31 prenoon ates but only 13 afternoon events prenoon and afternoon event characteristics differ prenoon ates generally appear as bright spots of auroral luminosity in the area from 0800 to 1000 magnetic local time mlt and 74 5 and 76 5 corrected geomagnetic latitude cgl bright aurorae then quickly expand westward and poleward accompanied by high latitude magnetic impulsive events mie and traveling convection vortices tcv afternoon ates usually appear as a sudden intensification of aurorae in the area from 1400 to 1600 mlt and 75 5 to 78 5 cgl within 15 20 min the bright band of luminosity extends eastward to reach 2000 2100 mlt at 70 72 cgl although midlatitude and low latitude ground magnetograms in the evening sector record increases in the horizontal component of the magnetic field no corresponding features occur at stations in the morning sector afternoon ates correspond to abrupt changes in the interplanetary magnetic field imf orientation but not to significant variations of the solar wind dynamic pressure indicating that the auroral transient events occur as part of the magnetospheric response to abrupt changes in the foreshock geometry

high resolution sequence stratigraphy of the upper ordovician montoya group southern new mexico and western texas outcrop analog of an unconventional chert and carbonate reservoir the upper ordovician montoya group crops out in southern new mexico and westernmost texas and records predominantly subtidal deposition on a gently dipping carbonate ramp that was subsequently almost entirely dolomitized the montoya group is a third order composite sequence composed of six regionally correlative shallowing upward third order depositional sequences m0 m5 sequence m0 has sandstone at its base that is overlain by skeletal packstone grainstone sequence m0 occurs only locally and was likely deposited in a topographic low formed during regional development of the unconformity following el paso group deposition sequence m1 marking the initial widespread transgression over the ellenburger unconformity consists of sandstone updip that passes downramp into skeletal packstone the highstand systems tract hst of m1 consists of a prograding skeletal grainstone that was subaerially exposed upramp sequence m2 which contains the second order maximum flooding surface has abundant subtidal cherty carbonate at its base which shallows upward into a widespread prograding coral packstone grainstone in the hst sequence m3 also contains abundant downramp chert that passes upramp into an aggrading crinoidal shoal and farther upramp into peritidal mudstone sequence m4 records an extensive basinward shift in facies as peritidal burrowed and cryptalgalaminated mudstone prograded over subtidal carbonate sequence m5 is only locally developed downramp and consists of crinoidal grainstone with abundant evidence of subaerial exposure a regional unconformity separates the montoya group from the silurian fusselman dolostone or younger units parasequences meter scale cycles recording low to moderate amplitude relative sea level fluctuations are ubiquituous features at individual outcrops but are difficult to correlate regionally the abundance of syn or early depositional chert in the subtidal facies indicates that the montoya group was deposited within a region of strong regional upwelling along southern laurentia this early formed chert was the reservoir facies in a successful upper ordovician gas play in ward and reeves counties texas

anatomy of a phylloid algal buildup raytown limestone iola formation pennsylvanian southeast kansas u s a abstract quantitative petrography and temporal spatial relationships of microfacies were used to interpret the depositional history of iola carbonates the paola limestone consisting of four microfacies was deposited on a carbonate ramp and appears to record the maximum transgression within the iola cycle of deposition the overlying raytown limestone was composed of nineteen microfacies arranged in three vertically successive depositional models of similar general configuration these models revealed an initial bioclastic bar evolving upward into a complex bioaccumulated bioconstructed phylloid algal buildup the latter was replaced landward by a siliciclastic distal deltaic environment and graded basinward into upper ramp and lower ramp carbonates locally restricted circulation caused the development of hypersaline conditions the shallowing upward depositional evolution of the phylloid algal buildup consisted of four distinct stages bioclastic hydrodynamic buildup bioaccumulation bioconstruction and bioclastic destruction in storm dominated conditions and intermittent emergence diagenesis of iola microfacies was complex and included marine phreatic 1 bioturbation syneresis micritization micrite lithification and hardground genesis freshwater vadose phreatic undersaturated 1 dissolution of evaporites moldic dissolution collapse brecciation and vadose silt infiltration marine phreatic 2 isopachous rim and botryoidal cementation freshwater phreatic saturated 2 sparite cementation and extensive neomorphism deep burial anhydritization dolomitization stylolitization and silicification and late uplift fracturation and cementation most of the diagenesis was microfacies specific

evolution of oceanic gabbros from dsdp leg 82 influence of the fluid phase on metamorphic crystallizations abstract gabbro breccias were recovered from an anomalously shallow level of the ocean crust during dsdp leg 82 the rocks display evidence of metamorphic crystallization related either to localized deformation or to hydrothermal circulation of a seawater derived fluid under static conditions secondary phases consist of plagioclase amphibole and minor clinopyroxene ilmenite sphene and chlorite petrological study indicates that deformation took place at high temperature under anhydrous conditions and was followed by hydrothermal circulation the compositions of secondary minerals i e strong zonations presence of chlorine in amphiboles varying compositions of secondary plagioclase indicate that reactions of the gabbros with the fluids occurred at a low water rock ratio relations between cl na and k in amphiboles suggest penetration of at least two distinct fluids of different compositions metamorphic crystallization stopped when greenschist facies conditions were reached 350 c probably because hydrothermal circulation faded out

auroral morphology of the midday oval auroral displays in the noon sector were examined by using hundreds of defense meteorological satellite program auroral images taken over the southern polar region in austral winters in order to determine the morphology the auroral displays in the midday part of the auroral oval can be grouped into five characteristic types depending on the geomagnetic activity and the bz component of the interplanetary magnetic field an important characteristic is the clear disconnection in appearance between the noonside and the nightside discrete auroras also there is the lack of correlation between the concurrent nightside substorm activity and midday discrete auroral activity thus the occurrence of bright discrete auroras in the midday oval may be caused by the local injection of the magnetosheath plasma into the dayside boundary layer the observed discontinuity of discrete auroras between the dayside and the nightside ovals is consistent with the existence of two separated major injection regions along the auroral oval the dayside cusp and the nightside plasma sheet

glacioeustatic cyclicity of a pennsylvanian carbonate platform in a foreland basin setting an example from the bachende formation of the cantabrian zone nw spain abstract by analysis of outcrop data sequence stratigraphy was used to interpret the stratal architecture and evolution of a moscovian pennsylvanian carbonate platform bachende formation located in the variscan foreland basin of the cantabrian zone nw spain the bachende formation represents a delta top carbonate platform developed in highly subsiding average subsidence rate of 254 m my distal shelf areas of the basin it reached a thickness of 650 m and covered an area of 170 km 2 after conservative palinspastic restorations the platform interior succession is mostly composed of low relief mound shaped micritic boundstones alternating with bioclastic mudstones wackestones peloidal foraminiferal packstones grainstones calcareous algal bafflestones and skeletal and ooidal grainstones landwards to the w carbonate platform beds alternate and interfinger with deltaic sandstones and shallow marine shales the platform interior consists of three sequences s1 s2 and s3 ranging from 180 to 230 m in thickness and from 1 3 to 0 5 my in age these sequences are in turn subdivided into several meter scale 10 6 m average thickness high frequency 110 to 15 ky subtidal transgressive regressive cycles these cycles are bounded by subaerial exposure or marine flooding surfaces and are inferred to be laterally continuous across the platform top they vary from mixed carbonate siliciclastic to carbonate dominated cycles cyclicity on the bachende platform was strongly influenced by glacioeustatic sea level variations and changes in subsidence and sedimentation rates subaerial exposure surfaces capping high frequency cycles are better developed in the sequence 1 late kashirian early podolskian when subsidence was relatively low and glacioeustatic sea level fluctuations could have exposed subtidal deposits during base level falls on the basis of the conservative estimated duration of each cycle and spectral analysis of cycle thickness the precessional 20 ky and obliquity 40 ky cycles were detected in all the sequences the short eccentricity cycle 100 ky in the sequence 1 and the long eccentricity cycle 400 ky in the sequences 2 and 3 podolskian nevertheless precessional and obliquity frequencies seem to dominate during sequences 2 and 3 which were deposited under significant higher subsidence and sedimentation rates additionally spectral peaks at 60 80 ky are randomly present across the whole succession and they cannot be explained as a linear response of the climate system to the milankovitch forcing

podogenic columnar calcite from the oolite group lower carboniferous south wales abstract calcrete palaeosols in the oolite group have microspar hardpan layers overlying clay rich nodular layers which in turn overlie the degraded host oolite within and immediately below hardpan layers there are extensive sheet like and fan developments of columnar calcite with individual crystals up to 30 cm long crystals range from those with straight extinction and normal growth zonation through feathery crystals to those showing sweeping extinction the calcretes accumulated within a seasonal climate with hardpan accumulation probably occurring through evaporation during the dry season the columnar calcite precipitated from more dilute solutions with maximum growth probably occurring during the wet season precipitation may have been partly or wholly bacterially mediated or induced by the reduction in p co 2 of fluids as they migrated out of the extremely restricted microporosity of the hardpan columnar fabrics reflect low supersaturations relative to those at which the hardpan accreted inhibition of crystal nucleation and of growth on x axis parallel faces and an excess of co 2 2 availability over ca 2 growth and nucleation may have been inhibited by the presence of particular organic compounds or clays the range in crystal form is related to the degree of poisoning of crystal growth faces by organic compounds

influence of water column anoxia on the burial and preservation of carbon and phosphorus in marine shales abstract organic p and organic c concentrations were measured in several well characterized phanerozoic marine shale sequences with the primary focus being the camp run member of the devonian mississippian new albany shale sequences were selected with close spatial association of bioturbated and laminated sediments which reflect deposition from oxic and anoxic waters respectively average organic c p mole ratios calculated from the data are 150 for bioturbated shales and 3900 for the laminated shales of the new albany differences in the extent and mechanisms of early diagenesis related to the oxygenation of waters at the sediment water interface can account for the c p ratios of buried organic matter low c p ratios of bioturbated sediments are attributed to 1 1 the increased ability of bacteria to store p in well oxygenated environments which leads to the production of low c p ratio bacterial biomass and 2 2 the extensive oxidation of sedimentary organic matter resulting in the formation of residual organic phases with low c p ratios high organic c p ratios of laminated sediments are explained by a combination of 1 1 the limited ability of bacteria to store p under anoxic conditions 2 2 extensive p regeneration from sedimentary organic matter 3 3 enhanced c preservation relative to the bioturbated shales it is also shown that relative to organic c laminated shales are not as effective a p sink as are bioturbated shales this provides a mechanism in anoxic environments for burying large quantities of organic matter without simultaneously sequestering the p needed to sustain further productivity

sequence stratigraphic hierarchy of the upper devonian foreknobs formation central appalachian basin usa evidence for transitional greenhouse to icehouse conditions abstract the foreknobs formation upper devonian upper frasnian to basal famennian comprises the uppermost marine strata of the progradational catskill clastic wedge of the south central appalachian mountains virginia west virginia usa the foreknobs formation consists of 14 lithofacies arranged in four facies associations which record the following depositional settings 1 storm dominated distal to proximal offshore to shoreface facies association a 2 sharp based conglomeratic shoreface facies association b 3 fluvial redbed facies association c and 4 incised valley fill ivf facies association d vertical juxtaposition and stacking patterns of lithofacies and facies associations permit recognition of a hierarchy of three scales of cyclicity up to 70 short term 5th order cycles each averaging 65 kyr consist of coarsening upward parasequences of storm dominated offshore marine facies in the distal setting which correspond to high frequency unconformity bound sequences hfs of fluvial redbed strata overlain by offshore marine strata in the proximal setting these facies relationships are a consequence of 10 15 m of sea level fluctuations up to 12 intermediate term 4th order cycles each averaging 375 kyr consist of stacked 5th order cycles the 4th order cycles are bounded by regressive surfaces of marine erosion rsme at the base of sharp based conglomeratic shoreface sandstones in the distal setting that correspond with paleosols in the proximal setting in some cases the 5th order cycles within each 4th order cycle exhibit stacking patterns indicative of increasing or decreasing accommodation space these facies relationships are a consequence of 25 35 m of sea level fluctuations three complete and portions of two additional 3rd order cycles each averaging 1 12 myr consist of stacked 4th order cycles the 3rd order sea level trends reflected in the foreknobs formation are nearly identical to published eustatic sea level curves incised valley fills are present at one of the 3rd order cycle boundaries and are a consequence of a 35 45 m sea level fluctuation the amplitudes of the inferred sea level fluctuations are comparable to the expansion and contraction of ice volumes within the current greenland and antarctic ice sheets which suggests glacioeustasy was the primary control on sea level fluctuations and cyclicity within the foreknobs formation such an interpretation is consistent with knowledge of devonian climate transitioning from middle devonian greenhouse to late devonian icehouse as indicated by evidence of glaciation during parts of the late devonian in south america and the appalachians

dayside auroral activity as a possible precursor of substorm onsets a survey using polar ultraviolet imagery we have analyzed the dayside auroral oval with particular emphasis on the postnoon aurorally active region prior to the onset of isolated substorms using images acquired from the ultraviolet imager uvi on board the polar spacecraft the uvi data set used for this investigation covers a time of approximately 3 months from march 30 to july 13 in 1996 it is found that dayside auroral hot spots were active in 70 out of 95 surveyed substorm events at least 15 min before the onset while 25 cases did not involve the dayside bright spots at all of the 70 cases with dayside activity during substorms 51 cases of the dayside events were found to be spatially confined and showed little discernible changes prior to an active substorm onset while only six cases were found to be in association with apparent eastward propagation through the dusk sector to the nightside prior to a substorm onset this statistical result indicates that most of the postnoon bright spots are spatially confined in longitude and that only a few candidate cases are possibly associated with substorms triggering it also suggests that dayside auroral bright spots are distinct features from the nightside auroral substorms these results suggest that the physical processes responsible for the dayside auroral bright spots are different from those responsible for the nightside substorm activity

late ediacaran 605 580 ma post collisional alkaline magmatism in the arabian nubian shield a case study of serbal ring shaped intrusion southern sinai egypt abstract the serbal pluton is a late neoproterozoic 605 580 ma post collisional a type granites in southern sinai egypt northernmost arabian nubian shield ans it is characterized by discontinuous ring shaped outcrops dislocated by later faulting the pluton intrudes late neoproterozoic metamorphic and high k calc alkaline rocks the serbal pluton mostly comprises an outer zone of alkali feldspar granite surrounding a core of peralkaline granite gradational and sharp contacts in the serbal granites suggest that they were emplaced with a very short time interval still before complete crystallization of the earlier batch serbal granites are highly evolved 75 98 78 52 wt sio2 and display the typical geochemical characteristics of post collisional a type granites namely high sio2 na2o k2o feo mgo ga al zr nb ga and y and low cao mgo ba and sr they are rich in ree and have extreme eu negative anomalies eu eu 0 01 0 23 the chemical characteristics indicate that the peralkaline granite shares many features of granites with the tetrad ree effect the serbal pluton evolved through fractional crystallization of a parental magma derived through partial melting of a juvenile crustal protolith that had been extracted from a source having mantle geochemical and isotopic characteristics the crystallization temperatures using fe ti oxides of the serbal granites point to their formation at high temperatures up to 650 850 c at a shallow depth of emplacement

hemispheric asymmetry of the structure of dayside auroral oval a comprehensive analysis of long term and multispectral auroral observations made in the arctic and antarctica demonstrates that the dayside auroral ovals in two hemispheres are both presented in a two peak structure namely the prenoon 09 00 magnetic local time mlt and postnoon 15 00 mlt peaks the two peak structures of dayside ovals however are asymmetric in the two hemispheres i e the postnoon average auroral intensity is more than the prenoon one in the northern hemisphere but less in the southern hemisphere the hemispheric asymmetry cannot be accounted for by the effect of the interplanetary magnetic field by component and the seasonal difference of ionospheric conductivities in the two hemispheres which were used to interpret satellite observed real time auroral intensity asymmetries in the two hemispheres in previous studies we suggest that the hemispheric asymmetry is the combined effect of the prenoon postnoon variations of the magnetosheath density and local ionospheric conductivity

hydrothermal activity in a peculiar oceanic ridge oxygen and hydrogen isotope evidence in the xigaze ophiolite tibet china the xigaze ophiolite locally displays a 7 km thick continuous sequence which probably represents an oceanic crust formed at a slow spreading ridge it consists from top to bottom of 1 1 pillow lavas with 18o between 9 and 16 and d around 90 vs smow 2 2 rare trondjhemites 9 7 18o 13 4 and d around 75 3 3 dolerite sills and dikes with 18o between 7 0 and 9 5 and d between 77 and 40 4 4 isotropic gabbros occurring as screens of the sills and dikes and rare layered gabbros these gabbros have 18o between 4 8 and 11 0 and d around 65 and bear high 18o albitized plagioclases 18o 10 which coexist with clinopyroxenes of mantle values 18o 5 5 or having values being slightly depleted down to 3 0 5 5 ultramafic rocks are either fresh or serpentinized they have 18o mainly around mantle values and d s are between 250 and 100 for the mafic part of the ophiolite the isotopic compositions correspond to hydrothermal interaction with seawater derived fluids under zeolite to greenschist facies but the trondjhemites were additionally affected by fluids poorer in d in particular the gabbros reacted with seawater derived fluids under amphibolite facies conditions as seen from their secondary mineralogy and their 18o depleted pyroxenes and subsequently were pervasively reacted under greenschist zeolite facies conditions during which the high 18o albitized plagioclase formed this occurrence of amphibolite facies conditions followed by greenschist zeolite facies conditions can be interpreted either to be a result of slow cooling of the oceanic crust at the ridge or to result from a fundamentally lower temperature hydrothermal interaction if compared with other ophiolites in contrast the d s of the serpentine 100 from the ultramafics are attributed to fluids of meteoric origin at high elevation nevertheless the particular location of the serpentinized harzburgites just beneath the gabbros and the high d dolerite sills and dikes 60 d 40 enclosed in these serpentinized harzburgites suggests an oceanic origin and therefore the possibility of hydrogen isotope exchange between serpentine and meteoric fluids after the obduction the xigaze ophiolite sequence displays very little 18o depleted rocks relative to mantle oxygen composition they cannot balance the amount of 18o enriched rocks consequently the result of the hydrothermal interaction between the xigaze ophiolite sequence and the seawater is an 18o depletion of the seawater however the contribution of xigaze type ridges to the buffering of 18o in cretaceous seawater was probably small

carbon isotope excursions and crinoid dissolution at exposure surfaces in carbonates west texas u s a abstract carbonates from the horseshoe atoll reinecke field west texas u s a show four subaerial exposure events by the presence of rhizoliths alveolar septal fabric and thin calcretes that subdivide the pennsylvanian succession into five sequences vertical profiles of bulk rock 13c analyses show values that are generally 1 pdb but negative shifts of up to 2 occur beneath exposure surfaces the bulk rock 13c values 1 could be explained as a mixture of 63 marine carbonate 13c 4 and 37 pedogenic carbonate 13c 4 while a negative bulk rock excursion of 13c 1 can be explained as a mixture of 37 marine and 63 pedogenic calcite distinctive negative shifts in the 13c profiles do not occur under some exposure surfaces that contain soil generated features and one negative shift occurs that apparently is unrelated to an exposure surface bulk rock 13c values are difficult to interpret because the volume of their various components is unknown furthermore 13c profiles may miss subaerial exposure surfaces certain styles of crinoid alteration may indicate paleo subaerial exposure surfaces fossil crinoids can be preserved in a variety of ways but crinoids with dissolution holes that subsequently either become filled with calcite cement or were crushed during burial are restricted to 2 m below exposure surfaces dissolution of mg calcite crinoids is due to their relatively high mg composition 12 mole mgco3 combined with their location beneath soil capped surfaces from which meteoric waters greatly undersaturated with respect to calcite emerged late stage dissolution removed finely crystalline calcite but crinoids by this time had stabilized to coarsely crystalline calcite and dolomite and thus were unaffected evaluation of other stratigraphic systems is needed to determine whether the use of holey crinoids to identify subaerial exposure surfaces has broad application

source region of 1500 mlt auroral bright spots simultaneous polar uv images and dmsp particle data we compare auroral images from the polar ultraviolet imager uvi and simultaneous particle observations from the defense meteorological satellite program dmsp in the afternoon 1300 1600 mlt sector along the oval in the northern hemisphere to determine the magnetospheric source region of postnoon auroral bright spots auroral bright spots are determined with polar uvi while their magnetospheric source regions are determined from dmsp f13 particle data a total of 65 events of good temporal and spatial coincidence were identified after searching through over 1 year of data from april 1996 to june 1997 instances occur of auroral arcs mapping to each of several different regions including the plasma sheet the low latitude boundary layer and the plasma mantle however our results indicate that 2 3 of the time the most prominent auroral arcs are associated with plasma sheet electron precipitation and slightly less than 1 3 of the time they are found to occur near less than 1 in magnetic latitudes the boundary between the plasma sheet and other regions