thermal anomalies in the central indian ocean evidence for dewatering of the bengal fan seafloor heat flow measurements in the central indian ocean are locally higher than predicted by lithospheric cooling models these anomalously high heat flow values occur in a region affected by folding and faulting of oceanic crust and overlying distal bengal fan sediments three closely spaced sites drilled during ocean drilling program leg 116 dated the onset of deformation in this area at about 7 8 ma here apatite fission track thermochronometric data from sites 717 and 718 and numerical simulations are used to 1 quantify the effects of bengal fan sedimentation on seafloor heat flow 2 evaluate the thermal history of sediments penetrated at these two sites and 3 constrain mechanisms responsible for anomalous seafloor heat flow in this area at the rates sediments were deposited along the distal fan 350 m m y numerical simulations predict a 6 to 9 reduction in present day heat flow at the seafloor relative to heat flow through the base of the lithosphere at site 717 thermal history simulations assuming heat transfer dominated by conduction are consistent with available down hole temperature measurements and apatite fission track data site 718 is located 8 km south of site 717 and 2 km south of a high angle fault that offsets oceanic crust by about 300 m a previously published detailed heat flow survey conducted around site 718 documented seafloor heat flow ranging from 44 to 166 mw m2 over kilometer scale distances in addition a monotonic 10 to 20 reduction in the mean length of fission tracks in apatite from 560 to 960 m below the seafloor at site 718 implies bottom hole temperatures of the order of 50 60 c for a time period greater than a few million years both the seafloor heat flow and apatite fission track data argue for localized possibly intermittent advection of heat by lateral updip migration of pore fluids in the vicinity of site 718 taken together the heat flow data and temperature range inferred from the apatite fission track data at site 718 imply a fluid source in the upper crust upward movement of pore fluids at velocities of less than 1 cm yr through a basement sediment boundary maintained at temperatures ranging from 65 to 80 c is capable of explaining both sets of observations as a hypothesis to reconcile existing observations i suggest that the anomalous thermal structure in the central indian ocean represents a joint response to compaction driven dewatering of the bengal fan and local enhancement of the vertical permeability in the upper crust and sediment pile due to intraplate deformation

diagenetic history of the proterozoic carbonates and its role in the oil field development in the baikit anteclise southwestern siberia abstract the meso to neoproterozoic kamo group and ediacaran carbonate rocks of the baikit anteclise southwestern siberian platform russia were studied to reconstruct the history of development of the reservoir rocks and oil field the kamo group is composed of 3 5 to 4 km thick dolostone and rarely limestone succession that was deformed before the ediacaran time post depositional processes played the key role in forming the reservoir properties of the kamo group during the stage of deposition and subsidence 1500 720 ma silicification cementation and recrystallization caused development of a dense and low permeability matrix pronounced recrystallization led to changes in isotopic and geochemical characteristics of the primary sediments including increase in mn up to 0 093 and fe up to 3 11 contents and 87sr 86sr ratios up to 0 7239 decrease in sr concentrations down to 11 ppm and the development of flat ree normalized patterns folding and uplift in the baikit basin began about 720 ma ago and caused fracturing and subsequent healing with vein dolomite and calcite precipitated from hot 70 130 c and highly saline up to 28 7 wt fluids geochemical e g paas normalized patterns of ree and isotopic 87sr 86sr 18o and 13c signatures of host dolostones and crosscutting dolomite veins are very similar suggesting that dolomites within the veins formed through dissolution of the host rocks uplift caused subaerial erosion intense dissolution and karstification which penetrated down to 200 300 m below the surface folding uplift and peneplenization were important for porosity and permeability development in the dolostones of the kamo group the cap rocks were subsequently formed during the ediacaran and paleozoic sedimentation cycle the oil field developed in the paleozoic oil produced from the source rocks of the madra vedreshev and iremeken formations of the kamo group was dissipated during the uplift and erosion before the ediacaran sedimentation after deposition of the ediacaran and cambrian strata source rocks of the iremeken formation might have released oil when they passed the oil window but most of oil in the kuyumba field was derived from younger ediacaran source rocks and migrated through fault network from the flanks the kureika and cis sayan yenisey syneclises into the reservoir of the baikit anteclise that was a topographic high at that time

isotopic evidence for carbonate cementation and recrystallization and for tectonic expulsion of fluids into the western canada sedimentary basin carbon oxygen and strontium isotope data for calcites and dolomites from the devonian obed platform in alberta canada demonstrate that 1 both limestones and dolostones of the obed platform underwent significant deep burial cementation and recrystallization 2 calcites experienced more extensive geochemical alteration than did dolomites under deep burial conditions and 3 the fluids that facilitated deep burial carbonate diagenesis probably were partially derived from the rocky mountain fold and thrust belt the more extensive degree of recrystallization of calcite is shown especially by its higher 87 sr 86 sr ratios a lesser degree of 13 c depletion in dolomites indicates that dolomite recrystallization partially coincided with hydrocarbon oxidation evidence supporting interpretation 3 above includes fractures and vugs bearing late diagenetic calcite cements that have extremely high 87 sr 86 sr ratios including the highest ratios reported thus far for any diagenetic carbonates from western canada 0 7252 in carbonates values this high are found only in tectonic veins in proterozoic clastic rocks in the rocky mountains and in the obed platform about 100 km into the foreland basin the late diagenetic calcite cements also have highly depleted 13 c values minimum 27 1 relative to pdb peedee belemnite indicating incorporation of oxidized carbon from thermochemical sulfate reduction the process of carbonate cementation and recrystallization in strata of the obed platform probably occurred during deep burial maximum 5 7 km and was effected by a hot 100 c mixture of connate brines and hydrothermal or metamorphic fluids that were expelled from the rocky mountain fold and thrust belt during the laramide orogeny the data also suggest that 1 the common practice of using limestones to establish marine or original geochemical baselines for stable and radiogenic isotope interpretations must be conducted with caution and 2 replacement burial dolostones are quite resistant to burial recrystallization finally the geochemical trace of tectonically expelled fluids may be limited to about 100 km into the foreland basin implying that the volumes and or fluxes of fluids produced by tectonic expulsion are rather low

a model for attenuation of backscatter due to sediment accumulations and its application to determine sediment thicknesses with gloria sidescan sonar where the seafloor consists of a highly backscattering surface covered with a drape of fine grained sediment and where the sediment does not contribute significantly to the backscattered signal the sediment thickness can potentially be inferred from the amount of signal attenuation in the sediment layer a first order acoustical model is proposed in which the signal strength is reduced by an amount proportional to the sediment thickness and attenuation rate in the sediments data collected with geological long range inclined azdic gloria are used to develop and test this model at an acoustic frequency of 6 5 khz the range of sediment thickness that the sonar can potentially recover is 0 20 m a range that is often below the resolution of vertical profilers in the first example the signal variation with grazing angle over a uniformly buried surface is analyzed for attenuation which is expected to increase toward shallow grazing angles and longer attenuating paths through the sediment layer the surface is a lava flow north of hawaii covered by 1 2 m of fine grained sediments clague et al 1990 and gloria data analyzed with the model suggest an attenuation coefficient of 0 2 0 4 db m a value consistent with results from laboratory and field measurements of sediment attenuation in the second example a rocky surface is buried by varying amounts of sediment which is analyzed at almost constant grazing angle the surface is the volcanic basement of the southeast indian ridge and gloria data reveal the thickening of the sediment cover with distance away from the spreading center this is used to derive an average sedimentation rate of 6 mm ka which is generally consistent with results from other studies and suggests that this technique may be used to study differences in sedimentation rates between different regions furthermore the model is used to calculate from image amplitude distributions the sediment thickness distribution which represents the accumulation of sediments in ponds and the exposure of abyssal hills these sedimentary processes are also reflected in the standard deviation of sediment thickness which increases with seafloor age i explore the various sources of error in estimating sediment thickness using this technique and propose a model for nonuniform sediment drapes the first order model assumes a uniform thickness at the subfootprint scale

diagenesis of sandstones in the back arc basins of the western pacific ocean sandstones occur in back arc basins of the western pacific at dsdp sites 299 sea of japan 297 northern shikoku basin 445 and 446 daito ridge and basin province 453 mariana trough 286 new hebrides basin and 285 south fiji basin these sandstones are dominantly volcaniclastic arenites derived from andesitic island arcs the degree of sandstone diagenesis is dependent on original composition burial rate heat flow history of the basin and timing of sandstone deposition with respect to rifting processes and associated high heat flow sandstones containing a larger proportion of volcaniclastic components showed more diagenetic effects than sandstones containing a significant volume of other rock fragments and mineral components sandstones deposited during early stages of rifting sites 445 446 with a slow burial rate and high crustal heat flow showed the greatest degree of downhole diagenetic change these diagenetic changes include early pore space reduction and rim cementation by clay minerals followed later by calcite and subsequent pore fill cementation by clinoptilolite heulandite analcite and later calcite replacement of recognizable volcanic rock fragments by chert calcite and zeolites was observed in the deepest part of the hole sandstones deposited after rifting under conditions of associated lower heat flow showed considerably less diagenetic changes particularly if burial was rapid the high heat flow associated with earliest rifting associated fluid circulation driven by thermal convection and slow burial rate controlled the diagenetic history of these sandstones thus timing of sandstone deposition with rifting stage and associated burial rates were key factors in controlling sandstone diagenesis in back arc basins

paleomagnetism and 40ar 39ar chronology of lavas from meseta del lago buenos aires patagonia 1 as part of a larger project investigating paleosecular variation in south america 36 paleomagnetic and geochronology sites were sampled on meseta del lago buenos aires 47 s 289 e 200 km east of the active plate boundary in southern patagonia basaltic lavas sampled range from late miocene to late pleistocene yet most of the individual lavas are younger than 3 3 ma thirty eight isochron ages determined via 40ar 39ar incremental heating experiments frame a detailed stratigraphy the isochron ages range from 66 9 4 1 ka for scoria cones and a youthful lava flow in the rio pinturas valley to 10 12 1 35 ma for thick plateau forming lavas exposed along the se edge of the meseta the last 3 3 myr is characterized by seven episodes of volcanism at ca 3 2 3 0 ma 2 4 ma 1 7 ma 1 35 ma 1 0 ma 750 ka 430 330 ka and 110 ka the bulk of lavas forming the surface of the meseta erupted in the last 1 7 my all sites have stable magnetization and after step demagnetization using either thermal or alternating field techniques yield characteristic directions held by magnetite and or titanomagnetite ten sites have distinct transitional directions defined by pole latitudes 55 and associated ages indicate possible connections to known reversals within the matuyama chron including the onset of the jaramillo subchron 1 016 0 01 ma the cobb mountain subchron 1 25 0 03 ma the ontong java 1 event 1 37 0 03 ma and the termination of the olduvai subchron 1 72 0 02 ma remaining sites are divided into normal 14 sites and reversed polarity 12 sites at 95 confidence mean normal and reversed directions overlap the mean direction for 26 sites is i 63 0 d 3 4 95 5 4 which is indistinguishable from the expected geocentric axial dipole paleosecular variation measured by the dispersion of virtual geomagnetic poles about the rotation axis is higher than expected at 20 0 model g using data for the past 5 myr predicts a dispersion of 17 this discrepancy may be due to true dispersion of the field in southern south america or it may be an artifact of inadequate sampling

classification of dolomite rock textures abstract dolomite rock textures can be classified according to crystal size distribution and crystal boundary shape the classification scheme presented here is largely descriptive but carries genetic implications because size distribution is controlled by both nucleation and growth kinetics and crystal boundary shape is controlled by growth kinetics size distributions are classified as unimodal or polymodal crystal boundary shapes are classified as planar or nonplanar if the evidence permits a complete classification includes a description of recognizable allochems matrix and void filling allochems and preexisting cements may be unreplaced partially replaced replaced mimically or replaced nonmimically allochems may be dissolved leaving molds matrix can be unreplace partially replaced or replaced by a unimodal or polymodal size dolomite unimodal size distributions generally indicated a single nucleation event on a unimodal substrate polymodal sizes can be formed by multiple nucleation events on a unimodal or polymodal substrate or differential nucleation on an originally polymodal substrate planar crystal boundaries develop when crystals undergo faceted growth and nonplanar boundaries develop when crystals undergo nonfaceted growth nonplanar boundaries are characteristic of growth at elevated temperature 50 c and or high supersaturation both planar and nonplanar dolomite can form as a cement replacement of caco3 or neomorphism of a precursor dolomite

geomagnetic paleosecular variation in easter island the southeast pacific abstract we collected more than 250 samples from 45 sites in lava flows of the three volcanoes on easter island the k ar whole rock dating was attempted on 13 sites all of the ages obtained are younger than 0 8 ma reliable paleomagnetic directions were obtained from 34 sites the mean vgp position is 89 2 n and 239 1 e with a 95 confidence value of 3 8 the angular dispersion value of easter island vgps was calculated to be 11 8 with an upper limit of 14 1 and a lower limit of 9 8 the angular dispersion value is significantly lower than the values predicted by the theoretical secular variation models the pacific dipole window appears to exist also in the southeast pacific

an interpretation of magnetic signatures of subduction zones detected by magsat abstract the magnetic anomalies detected by magsat over subduction zones are related to the thermal state and the location with respect to the geomagnetic poles of the subducting oceanic lithosphere it is shown that an old about 100 m y subducting lithosphere produces about twice as strong a magnetic anomaly as that associated with a young about 40 m y one also a given subducting oceanic lithosphere produces a stronger magnetic anomaly at high geomagnetic latitudes than near the geomagnetic equator based on the thermal evolution models of the oceanic lithosphere and assuming a mean magnetic susceptibility of 13 10 3 si the pacific plate subducting along the kuril trench produces a magnetic anomaly similar to the observed one the nazca plate subducting along the peru chile trench produces a magnetic anomaly of less than 0 5 nt no distinct anomaly observed by magsat over this region

influence of climate and dolomite composition on dedolomitization insights from a multi proxy study in the central oman mountains abstract dedolomitization is an important diagenetic process that can yield information on the history of paleo fluids in a given aquifer or outcrop and by extension it offers a window into past hydrologic conditions dedolomitization is also relevant economically as it can alter the porosity and permeability of carbonates thus affecting reservoir quality despite considerable research the process of dedolomitization is still not entirely understood here new findings from the central oman mountains highlight the importance of dolomite chemistry and fluctuating climate on dedolomitization petrographic mineralogical and geochemical data from both altered and pristine dolomite hosted in jurassic carbonate rocks from the sahtan group and outcropping at wadi mistal in the jebel akhdar tectonic window reveal two dolomite bodies with different characteristics stratabound and fault related dolostone the ferroan dolomite crystals are larger in the fault related dolostone whereas the non ferroan dolomite crystals are smaller in the stratabound dolostone petrography reveals a complex suite of alteration textures including pristine dolomite recrystallized dolomite and calcitized dolomite dedolomite iron oxides are present pervasively in the altered rocks and different alteration textures co occur in the same sample relative to unaltered dolomite in this outcrop the recrystallized dolomite is characterized by similar positive carbon isotope values but less negative oxygen isotope values and the calcitized dolomite is characterized by even less negative oxygen isotope values and more negative carbon isotope values based on field data and petrography two phases of alteration are evident an earlier dedolomitization event affecting the stratabound dolomite only and a more pervasive pleistocene holocene alteration phase associated with surface weathering that affected both the fault related dolomite and to a lesser extent the stratabound dolomite stable isotope results further suggest that the more recent subaerial weathering phase happened under two alternating climate regimes dedolomitization was triggered by meteoric fluids that interacted with soil related carbon during humid climatic period s whereas recrystallization of ferroan fault related dolomite happened during more arid climatic period s with less developed soils and lower fluid rock interaction these results suggest that weathering of ferroan fault related dolomite can lead to formation of goethite and recrystallized non stoichiometric dolomite with a reset stable oxygen isotope signature and sometimes to dedolomitization

paleosecular variation from easter island revisited modern demagnetization of a 1970s data set abstract sixty two brunhes age lava flows from easter island in the eastern pacific ocean collected and processed for paleomagnetic and secular variation studies in the 1970s have been reprocessed to meet current standards of demagnetization alternating field demagnetization in 7 10 steps was performed on all samples previously undemagnetized and characteristic directions determined from principal component analysis the final data set gives a mean direction of i 46 1 and d 357 4 with 95 3 9 eliminating sites with 95 confidence circles greater than 10 0 gave an angular dispersion of virtual geomagnetic poles vgps of 14 1 with respect to the rotation axis this value is slightly larger than that determined prior to detailed demagnetization and agrees with values predicted for this latitude by model g there is no compelling evidence from this study supporting the existence of a secular variation low in the southeastern pacific region

nature and distribution of diagenetic phases and petrophysical properties of carbonates the mississippian madison formation bighorn basin wyoming usa abstract the purpose of this study is to characterize factors controlling the nature and distribution of the diagenetic phases that gave rise to carbonate reservoirs to do so a pluridisciplinary approach was carried out integrating sedimentologic and diagenetic studies on the madison formation lower carboniferous 357 340 my which is a carbonate reservoir in subsurface the madison formation outcropping in the bighorn basin wyoming usa is a 340 m thick carbonate series composed of four to seven 3rd order depositional sequences s1 to s7 depending of the palaeogeographic location the first three sequences s1 to s3 were deposited under arid and warm conditions during tournaisian times which favoured high accumulation of carbonates leading to a morphological change from a quite flat ramp s1 to s2 to a wide platform s3 it also probably favoured the early calcite cementation isopachous and syntaxial calcite cements of the subtidal deposits and the early dolomitization d1 of the supratidal to intertidal sabkha ones in addition the very flat profile occurring during s3 was also responsible for the postponed dolomitization d2 of s1 s2 due to reflux of brines at various degrees depending on the palaeogeographic location the deposition of s4 to s7 under humid conditions during visean times were associated with 1 a decrease of the carbonate accumulation and of the dolomitization 2 dissolution processes at micro pores network and macro scales karst collapse breccia and calcite cementation c1 the spatial distribution of all these first diagenetic phases acts as a controlling factor on the distribution of the later burial diagenetic phases thus the mesogenetic calcite cements are mostly observed in the secondary porosity created by dissolution karsts collapse breccias or dolomitization the burial diagenesis of the madison formation was characterized by 1 a dolomitization phase d3 in proximal parts of the platform leading to an increase of porosity and permeability and 2 calcite cementations c2 c3 in the distal parts of the platform leading to a decrease of these properties the madison formation provides a good spatial representation of sedimentary and diagenetic heterogeneities that may occur in the carbonate reservoirs due to various palaeogeographic locations climates burial history and structures

current aspects of basin analysis abstract current research in basin analysis focuses on interdisciplinary integration of geodynamics mathematical and physical modelling and sedimentary geology these efforts are organized into a research framework emphasizing basin formation nature of the basin fill maturation diagenesis of the basin fill and timing of events research in geodynamics permits development of a proposed classification of sedimentary basins in which basin types are discriminated according to nature of the plate margin basin position on or within a tectonic plate nature of the crust and geodynamic models or known process of formation a summary of current research developments on sedimentary basin fills focuses on the interdisciplinary solutions developed to understanding carbonate distribution through time origin of black shales sea level history and global sedimentary cycles maturation of basin fills requires an understanding of both thermodynamic aspects of mineral phase changes and changing character of the chemistry of flow of water through porous sediments during basin evolution summary case histories from the illinois basin and the appalachian basin illustrate these approaches

early dolomitization and partial burial recrystallization a case study of middle triassic peritidal dolomites in the villany hills sw hungary using petrography carbon oxygen strontium and clumped isotope data

palaeosecular variation over the past 5 myr based on a new generalized database summary a generalized database of global palaeomagnetic data from 3719 lava flows and thin dykes of age 0 5 ma has been constructed for use with a relational database management system the database includes all data whose virtual geomagnetic poles vgp lie within 45 of the spin axis and can be used for studies of palaeosecular variation and for geomagnetic field modelling because many of these data were collected and processed more than 15 20 years ago each result has been characterized according to the demagnetization procedures carried out analysis of these data in terms of the latitude variation of the angular dispersion of vgps palaeosecular variation from lavas strongly suggests that careful data selection is required and that many of the older studies may need to be redone using more modern methods differences between the angular dispersions for separate normal and reverse polarity data sets confirm that many older studies have not been adequately cleaned magnetically therefore the use of the database for geomagnetic field modelling should be carried out with some caution using a vgp cut off angle that varies with latitude the best data set consists of 2636 results that show a smooth increase of vgp angular dispersion with latitude model g for palaeosecular variation which is based on modelling of the antisymmetric dipole and symmetric quadrupole dynamo families provides a good fit to these results

the paleoenvironmental evolution of the cambrian longwangmiao formation stage 4 toyonian on the yangtze platform south china petrographic and geochemical constrains abstract during the cambrian period stage 4 biodiversity and biomass declined rapidly and globally in an event known as the botomian toyonian crisis to gain a better understanding of the paleoenvironment in south china during this stage we present and interpret complex multi parameter carbonate analyses petrography carbon and oxygen isotopic composition and major trace and rare earth elements from the toyonian longwangmiao formation in the bandenggou section the longwangmiao formation comprises primarily carbonate deposits with massive detrital mineral occurrences in its lower middle sequence principal component analysis of the minerals and oxides leads to the definition of two intervals i and ii interval i is composed mainly of dolostones interbedded with sandstones and contains exceedingly high amounts of terrestrial original minerals and oxides as well as high ree y th and sc concentrations interval ii which is composed primarily of limestones has trace amounts of terrestrial original minerals and oxides and low ree y th and sc concentrations the 13c and 18o values show a significant positive shift from interval i to interval ii after eliminating the effects of diagenesis contributions through the analyses of diagenesis sensitive indexes 18o values mgo cao mn sr and fe sr ratios and the correlations of 13c 18o 18o eu eu eu eu ce ce and eu eu feo the sr and na concentrations of calcites in interval i are extremely high exceeding even the corresponding ranges of marine aragonites while calcites from interval ii have low sr and na concentrations that are within the range of marine low mg calcite however both show strong positive correlations between ca sr ca na and sr na the different calcite elemental behaviors in intervals i and ii suggest different carbonate precursors associated with seawater temperature and climate furthermore the sr ca ratios are negatively correlated with 13c and 18o values and the 13c values also negatively correlated with sio2 content these features indicate a significant change in the depositional environment during the cambrian toyonian stage from an early restricted marine environment under a warm and arid climate with massive terrestrial influxes to a later relatively open marine environment under a cool and humid climate with negligible terrestrial influxes this shift likely resulted in the observed changes in biomass and biodiversity

origin of dolomite in the third member of feixianguan formation lower triassic in the jiannan area sichuan basin china abstract the third member of feixianguan formation jiannan area consists of marine carbonates parts of which were dolomitized to form high quality reservoirs based on petrographical features three types of dolomite phases are distinguished including very finely finely crystalline dolomite type d1 medium crystalline dolomite type d2 and coarse crystalline saddle dolomite type d3 type d1 dolomite has characteristics of micritic to fine crystals 18 o values imply partly diagenetic stabilitization during burial type d2 dolomite has characteristics of mainly medium 100 250 m subhedral crystals with zoned to uniform dull red cl the estimates from 18 o values suggest precipitation at depths of 760 1100 m values of 13 c and 87 sr 86 sr of type d2 are similar to values of host limestone suggesting that dolomitization was mediated by t 1 f 3 seawater like fluids type d3 is strictly associated with type d2 high homogenization temperatures and limited distributions indicate that type d3 formed in a deep closed environment dolostone made of type d1 dolomite is volumetrically minor and is characterized by micropores and low permeability while type d3 as a void filling cement slightly reduces the reservoir porosity high quality reservoirs are only intimately associated with dolostone made of type d2 a burial compaction model is proposed for type d2 mg 2 was derived from seawater derived compaction fluid that was expelled from the trough strata the potential high quality reservoirs are likely to occur in opposing margin shoals

zebra dolomitization as a result of focused fluid flow in the rocky mountains fold and thrust belt canada discordant zebra dolomite bodies occur locally in the middle cambrian cathedral and eldon formations of the main ranges of the canadian rocky mountains fold and thrust belt they are characterized by alternating dark grey a and white b bands forming an abba diagenetic cyclicity these bands developed parallel to both bedding and cleavage dark grey a bands consist of fine 300 m non planar crystalline impure dolomite the white b bands are composed of coarse up to several millimetres milky white pure saddle dolomites b1 which are often covered by pore lining zoned dolomite b2 the b phases often possess a saddle shaped morphology in contrast to the replacement origin of the a dolomite the zoned b2 dolomite rims are interpreted as a cement formed in open cavities the b1 dolomite is interpreted as the result of recrystallization with diagenetic leaching of non carbonate components all the zebra dolomites studied are nearly stoichiometric and are characterized by enriched na and depleted sr concentrations fe and mn concentrations in these dolomites differ depending on the sample locality fluid inclusion data indicate that the dolomites formed from relatively hot th 130 200 c saline 20 23 wt cacl2 eq fluids a diagenetic high temperature origin is also supported by depleted 18o values 20 to 14 vpdb a contribution of 87sr enriched fluids is reflected in the 87sr 86sr values 0 7091 0 7123 zebra dolomite development is explained by focused fluid flow which exploited areas of structural weaknesses e g basin platform rim areas faults etc expulsion of hot basinal brines in a tectonically active regime generated overpressures which explains the development of secondary porosity during zebra dolomitization as well as the intra zebra fracturing at decimetre to micrometre scale

time space mapping of easter chain volcanism abstract new 40ar 39ar and published k ar ages show that the locus of volcanism along the easter volcanic chain evc has shifted systematically from the nazca ridge at about 26 m y to the recently active sala y gomez island easter island region this indicates a plume rather than a hotline i e mantle roll origin for the evc the time space distribution of ages combined with published ages for the galapagos and juan fernandez volcanic chains is used to reconstruct nazca plate velocities over the past 26 m y a plume now located in the region of sala y gomez island is most compatible with these data west of the plume the evc records neither nazca nor pacific plate motions this section of the evc may be related to westward channeling of plume material to the pacific nazca spreading boundary region

constraints upon water advection in sediments of the mariana trough thermal gradient measurements consolidation tests and pore water compositions from the mariana trough imply that water is moving through the sediments in areas with less than about 100 m of sediment cover the maximum advection rates implied by the thermal measurements and consolidation tests may be as high as 10 5 cm s 1 but are most commonly in the range of 1 to 5 10 6 cm s 1 theoretical calculations of the effect of the highest advection rates upon carbonate dissolution indicate that dissolution may be impeded or enhanced depending upon the direction of flow by a factor of 2 to 5 times the rate for diffusion alone the average percentage of carbonate is consistently higher in two cores from the area with no advection or upward advection than the average percentage of carbonate in three cores from the area with downward advection this increase in average amount of carbonate in cores with upward moving water or no movement cannot be attributed solely to differences in water depth or in amount of terrigenous dilution if the sediment column acts as a passive boundary layer then the water velocities necessary to affect chemical gradients of silica are in the range 10 9 to 10 10 cm s l however if dissolution of silica occurs within the sediment column then the advection velocities needed to affect chemical gradients are at least 3 10 8 cm s l and may be as high as 3 10 6 cm s l this order of magnitude increase in advection velocities when chemical reactions occur within the sediments is probably applicable to other cations in addition to silica if so then the advection velocities needed to affect heat flow 10 8 cm s 1 and pore water chemical gradients are much nearer in magnitude than previously assumed

from tabular to rhombohedral dolomite crystals in zechstein 2 dolostones from scharzfeld sw harz germany a case study with combined cl and ebsd investigations abstract tabular dolomite crystals found within dolomite rhombs have been investigated by cathodoluminescence cl microscopy and spectroscopy combined with electron backscatter diffraction ebsd for the first time the dolomites formed in the upper permian stassfurt carbonate ca2 at the southern margin of the german polish zechstein basin cathodoluminescence petrography of the dolostone succession revealed that the dolomites developed in four phases electron backscatter diffraction analysis reveals tabular crystal growth during the two first generations while the last two generations are characterized by rhombohedral crystal shapes the tabular dolomite cement crystals and their microcrystalline equivalents in matrix and components have a stoichiometric composition with good to very good lattice ordering manganese and iron contents of the tabular crystals are low and their carbon and oxygen isotope composition confirms an early diagenetic dolomite formation under marine evaporitic conditions from precursor carbonates of upper permian age cl spectroscopy reveals that the tabular dolomite generation 1 has a very high percentage of mn2 on the ca lattice position which results in a visually yellowish green cl emission although relatively increased mn2 contents at the ca lattice position appear to be rather common in evaporitic dolomites the combination of a tabular crystal shape and a preferred input of mn2 at the ca lattice position is a remarkable phenomenon as tabular dolomite crystals so far are exclusively reported from evaporitic diagenetic settings they could be the result of a high mg ca ratio which blocks c axis orientated growth of dolomite crystal the occurrence of well ordered dolomite of which the geochemical zoning can be studied in such detail is rare for the earliest synsedimentary stages of dolomite formation in marine environments because these early stages commonly consist of not or badly ordered ca dolomites a primary geochemical zoning of such dolomite usually gets lost during stabilisation and transformation to better ordering and stoichiometry

paleomagnetism of miocene volcanics on sao tome paleosecular variation at the equator and a comparison to its latitudinal dependence over the last 5 myr a collection was made in january 2009 of 10 oriented samples from each of 54 sites in lavas on sao tome island nominal location 0 3o n 6 5o e some sites were affected by lightning leaving a total of 42 sites for analysis of paleosecular variation overall magnetic properties were excellent highly stable magnetizations carried by pseudo single domain magnetite after principal component analysis of progressive alternating field demagnetization data for the samples twenty two sites had normal polarity magnetizations d 0 6o i 8 3o 95 4 3o 53 1 and 20 had reverse magnetizations d 176 0o i 4 2o 95 7 3o 20 8 the directions are within 5 of antiparallel yielding a positive reversal test the combined data set of 42 site mean virtual geomagnetic poles converted to common normal polarity yields a pole position at 86 0 n 211 5oe a95 3 1o ar ar and k ar dating reveals that these rocks are miocene in age 5 11 ma old enough to allow northerly plate motion to help explain the slightly far sided pole position the between site dispersion in virtual geomagnetic poles was estimated as the angular standard deviation sb and equaled 11 4 with 95 confidence interval between 9 9 and 13 4 this article is protected by copyright all rights reserved

paleomagnetic directions and k ar dating of 0 to 1 ma lava flows from la guadeloupe island french west indies implications for time averaged field models twenty six lava flows spanning the last million years were sampled in la guadeloupe french west indies because of the lack of continuous volcano stratigraphic sections in la guadeloupe dating is necessary in order to describe the temporal evolution of the geomagnetic field in this time interval new k ar ages ranging from 50 ka to 1 ma have been obtained on andesites using the cassignol gillot technique at the universite paris sud institut de physique du globe de paris orsay laboratory additional flows with k ar ages obtained using the same dating technique were also sampled for paleomagnetic investigations more than 200 samples were analyzed using both alternating field af and thermal stepwise demagnetization techniques duplicate samplings of two flows at three different sites demonstrate that within flow dispersion is negligible for the andesitic lava flows sampled in this study direct comparison with an earlier paleomagnetic study performed on the island indicates that for the three investigated flows modern demagnetization techniques yield much better defined paleomagnetic directions the matuyama brunhes transition was recorded in a three flow section and is dated at 781 18 ka in good agreement with other recent radiometric age determinations the mean paleomagnetic pole calculated from the 23 normal polarity flows is indistinguishable from geographic north which implies that no significant persistent axial quadrupole term can be identified at this site for the last million years this result contradicts earlier results and has important implications for models of the time averaged field taf an occam algorithm was used to construct a taf model from the global volcanic database from the last 5 ma substitution of the mean direction calculated from the earlier study for the lesser antilles by our new mean value reduces the quadrupole term by more than 30 this effect which was produced by changing data from a single site demonstrates that older paleomagnetic sites may need to be reinvestigated furthermore it also highlights the limitations of taf models that can be inferred from paleomagnetic databases in their present state

the myth of the pacific dipole window the angular dispersion of virtual geomagnetic poles vgp from lava flows is often cited as having been anomalously low in the pacific during the brunhes epoch because the dispersion from hawaiian lavas is said to be much lower than measured elsewhere this led to the concept of the pacific dipole window or pacific non dipole low because lavas tend to be erupted in bursts of activity many of the hawaiian data are serially correlated and thus cannot necessarily be treated as independent observations geochronological controls as are available for hawaii have been used in a previous analysis to thin the data to try to avoid repeated sampling of the same geomagnetic field unfortunately in that analysis the angular dispersion of vgps was calculated about the mean vgp instead of about the spin axis and thus underestimated the dispersion we have therefore reanalyzed the relevant data using the following criteria 1 only those lavas with 95 10 have been considered 2 a latitude dependent cut off angle is used to eliminate those vectors that are not part of the normal secular variation 3 we have developed a statistical method to rationalise those flows that have repeatedly sampled the same geomagnetic field vector because suitable geochronological controls are rarely available application of this new method to the hawaiian data shows excellent agreement with the results from using geochronological controls where possible we have therefore included this method in our overall analysis as expected the resulting global data are compatible with a fisher distribution about the spin axis brunhes age data from hawaii n 96 independent measurements give a vgp angular dispersion about the spin axis of sf 12 4 and for the pacific region as a whole sf 12 5 n 190 between latitudes 15 and 30 north or south these values are the same as sf 12 4 n 160 calculated for the rest of the world in the same latitude range this clearly demonstrates that the hypothesis of the pacific dipole window may confidently be rejected

quantitative analysis of north sea subsidence stratigraphic and geophysical maps of the northern and southern north sea basin are used to study mechanisms of intracratonic basin formation these maps include 1 tectonic subsidence for ten stratigraphic units 2 predicted and observed sediment overpressure 3 geographic variation of compaction constants 4 net basement fault extension 5 free air gravity 6 flexural rigidity 7 heat flow and 8 predicted hydrocarbon maturation in northern areas of the north sea observed basement fault extension crustal thinning present day flexural rigidity and tectonic subsidence are predicted reasonably well by a simple stretching model of basin formation with rifting phases during the triassic and mid jurassic early cretaceous in southern areas of the north sea however a similar analysis suggests that other non extensional rift related processes dominated basin evolution the effects of anomalously active aesthenospheric convection coupled with basalt eclogite and deep crustal dewatering reactions are considered as possible additional mechanisms the troughlike form of quaternary tectonic subsidence suggests a new phase of rifting may be starting in the north sea the importance of such thermal rejuvenation is borne out by the association of areas of differential subsidence with high heat flow and low flexural rigidity one component of tectonic reactivation during this period may be due to thermally induced isostatic reequilibration of older tectonic loads

microbial communities and their primary to early diagenetic mineral phases the record from neoproterozoic microbialites of qarn alam oman

stabilization of early formed dolomite a tale of divergence from two mississippian dolomites abstract several large hydrocarbon accumulations in alberta canada are hosted in dolomitized successions of stacked thin sabkha capped cycles of visean age porosity is micro intercrystalline and occurs in dolomitized restricted subtidal and intertidal muds that have their fine primary fabric preserved two such fields are here considered that illustrate divergent and contrasting modes of dolomite stabilization despite initial similarities in facies and textures the dolomite in the upper debolt formation of the dunvegan field nw alberta forms planar e or microsucrosic fabrics with crystals in the 1 20 m range the dolomite is non ferroan ca rich average of 58 mol caco 3 and poorly ordered its stable isotopic signatures range from 0 12 to 3 4 vpdb for 18 o mean 1 3 and 0 9 to 4 3 vpdb for 13 c mean 2 6 the average radiogenic 87 sr 86 sr ratio for this dolomite is 0 7077 both sets of values are consistent with dolomite precipitation from mississippian marine or modified marine evaporated seawater these parameters are strongly reminiscent of holocene protodolomites and hence suggestive of a sabkha dolomitization process shallow seepage reflux or evaporitive pumping this dolomite with its high associated porosity average of 15 and up to 38 relatively unaltered mineralogical and chemical signatures both preserved despite 4 km of burial depth suggests a very unique set of relatively non reactive physico chemical conditions during burial likely a closed system in contrast the dolomite from the mount head formation of the shell waterton field sw alberta has undergone measurable neomorphic alteration in several stages in deeper burial environments open system such alteration has affected its crystal size range 18 o ranges between 1 5 in the least altered dolomite and 13 2 vpdb 13 c ranges from 3 9 to 1 5 vpdb and 87 sr 86 sr ranges from 0 7078 to 0 7090 the dolomite has retained a degree of non stoichiometry with an average ca content of 55 mol caco 3 the mount head dolomites clearly indicate ongoing reactivity between the rocks and basinal fluids during burial and are in this respect representative of the norm for dolostones in the western canada sedimentary basin

40ar 39ar ages and paleomagnetism of sao miguel lavas azores we present new 40 ar 39 ar ages and paleomagnetic data for sao miguel island azores paleomagnetic samples were obtained for 34 flows and one dike successful mean paleomagnetic directions were obtained for 28 of these 35 sites 40 ar 39 ar age determinations on 12 flows from the nordeste complex were attempted successfully ages obtained are between 0 78 ma and 0 88 ma in contrast to published k ar ages of 1 ma to 4 ma our radiometric ages are consistent with the reverse polarity paleomagnetic field directions and indicate that the entire exposed part of the nordeste complex is of a late matuyama age the duration of volcanism across sao miguel is significantly less than previously believed which has important implications for regional melt generation processes and temporal sampling of the geomagnetic field observed stable isotope and trace element trends across the island can be explained at least in part by communication between different magma source regions at depth the 40 ar 39 ar ages indicate that our normal polarity paleomagnetic data sample at least 0 1 myr 0 0 1 ma and up to 0 78 myr 0 0 78 ma of paleosecular variation and our reverse polarity data sample approximately 0 1 myr 0 78 0 88 ma of paleosecular variation our results demonstrate that precise radiometric dating of numerous flows sampled is essential to accurate inferences of long term geomagnetic field behavior negative inclination anomalies are observed for both the normal and reverse polarity time averaged field within the data uncertainties normal and reverse polarity field directions are antipodal but the reverse polarity field shows a significant deviation from a geocentric axial dipole direction 1998 elsevier science b v all rights reserved

influence of sedimentation local and regional hydrothermal circulation and thermal rebound on measurements of seafloor heat flux 1 we quantify the influence of environmental processes on measurements of seafloor heat flux with a new one dimensional thermal model that includes time varying sedimentation and boundary conditions represents several scales of hydrothermal and transient conductive processes within basement and allows fluid seepage through accumulating sediments variations in basement thermal conductivity the extent of hydrothermal mixing in upper basement and fluid seepage through sediments each influence seafloor heat flux by 2 8 conductive thermal rebound following the cessation of advective heat loss from the crust may lower seafloor heat flux values by 5 10 even after several to several tens of million years the new models indicate that thermal rebound takes much longer than suggested by earlier analytical calculations mainly because earlier models did not account for the heat capacitance of the conductive lithosphere application of the new model to 3 5 3 6 ma seafloor on the eastern flank of the juan de fuca ridge suggests that anomalously low heat flux in this area is best explained by incomplete conductive thermal rebound in the last 100 200 ka following the burial of numerous basement outcrops application of the new model to the eastern flank of the east pacific rise and extrapolation out to the age of some of the oldest remaining seafloor indicate that sedimentation corrections may be important even where accumulation rates are typical of global values model results also suggest that conductive thermal rebound through thick sediments may bias measurements made on moderate to old seafloor even where there is little evidence at present for ridge flank hydrothermal circulation

the influence of seawater carbonate chemistry mineralogy and diagenesis on calcium isotope variations in lower middle triassic carbonate rocks abstract the geological calcium cycle is linked to the geological carbon cycle through the weathering and burial of carbonate rocks as a result calcium ca isotope ratios 44ca 40ca expressed as 44 40ca can help to constrain ancient carbon cycle dynamics if ca cycle behavior can be reconstructed however the 44 40ca of carbonate rocks is influenced not only by the 44 40ca of seawater but also by diagenetic processes and fractionation associated with carbonate precipitation in this study we investigate the dominant controls on carbonate 44 40ca in upper permian to middle triassic limestones ca 253 to 244 ma from south china and turkey this time interval is ideal for assessing controls on ca isotope ratios in carbonate rocks because fluctuations in seawater 44 40ca may be expected based on several large carbon isotope 13c excursions ranging from 2 to 8 parallel negative 13c and 44 40ca excursions were previously identified across the end permian extinction horizon here we find a second negative excursion in 44 40ca of 0 2 within lower triassic strata in both south china and turkey however this excursion is not synchronous between regions and thus cannot be interpreted to reflect secular change in the 44 40ca of global seawater additionally 44 40ca values from turkey are consistently 0 3 lower than contemporaneous samples from south china providing further support for local or regional influences by measuring 44 40ca and sr concentrations sr in two stratigraphic sections located at opposite margins of the paleo tethys ocean we can determine whether the data represent global conditions e g secular variations in the 44 40ca of seawater versus local controls e g original mineralogy or diagenetic alteration the sr and 44 40ca data from this study are best described statistically by a log linear correlation that also exists in many previously published datasets of various geological ages using a model of early marine diagenetic water rock interaction we illustrate that this general correlation can be explained by the chemical evolution of bulk carbonate sediment samples with different initial mineralogical compositions that subsequently underwent recrystallization although early diagenetic resetting and carbonate mineralogy strongly influence the carbonate 44 40ca values the relationship between sr and 44 40ca holds potential for reconstructing first order secular changes in seawater 44 40ca composition

multiple episodes of dolomitization and dolomite recrystallization during shallow burial in upper jurassic shelf carbonates eastern swabian alb southern germany abstract the upper jurassic of the eastern swabian alb is composed of oolitic platform sands with associated microbe siliceous sponge mounds at the platform margins they are surrounded by argillaceous or calcareous mudstones and marl limestone alternations deposited in adjacent marl basins partial to complete dolomitization is predominantly confined to the mound facies six types of dolomite as well as one type of ankerite document a complex diagenetic history during shallow burial with multiple episodes of dolomite formation and recrystallization the earliest massive matrix dolomitization is ca rich has slightly depleted oxygen isotope values relative to late jurassic seawater and carbon isotopic values in equilibrium with late jurassic seawater this initial massive matrix dolomitization occurred during latest jurassic to earliest cretaceous and is related to pressure dissolution during very shallow burial at temperatures of at least 50 c hydrologic conditions and mass balance calculations indicate that burial compaction provided sufficient fluids for dolomitization mg is derived from negligibly modified seawater that was expelled from the adjacent off reef strata into the mound facies position of the mounds along the platform margins controlled the distribution of the shallow burial dolomite covariant trends between textural modification increasing stoichiometry partial changes in trace element content mn fe sr and depletion in stable isotopes as well as distinctive cl pattern illustrate two recrystallization phases of the precursor matrix dolomite during further burial at elevated temperatures strong sr enrichment of the second phase of recrystallized dolomite is ascribed to sr rich meteoric waters descending from overlying aragonite bearing reef limestones or evaporite bearing peritidal carbonates late stage coarsely crystalline dolomite cements occur as vug and fracture fillings and formed during burial ankerite associated with sulphide and sulphate minerals and saddle dolomite are assumed to have formed from hydrothermal waters that moved to higher stratigraphic levels along fracture conduit systems that developed during late cretaceous to tertiary alpine orogenesis

a map of the pacific geomagnetic anomaly during the brunhes chron abstract the persistence of an anomaly of the geomagnetic field in the central pacific ocean has been suggested by many authors however the amplitude and the extent of this feature remain poorly constrained due to insufficient spatial and temporal coverage of the volcanic database records from marine sediments are appropriate to document the long term behavior of the geomagnetic field we studied 11 piston cores from the central and southwestern pacific in order to document the anomaly during the past 800 kyr ten records were retained after applying selection criteria dealing with physical disturbances deformations and any possible sources of inclination errors the results have been integrated into a new data set for the pacific ocean and the eastern indian ocean which combines sedimentary and volcanic records a strong negative inclination anomaly 10 in the southwestern pacific is the main characteristic of the new map in contrast the sedimentary records from the hawaiian area do not indicate any stable feature but rather alternating positive and negative anomalies the new inclination map of the brunhes chron bears striking similarities with the configuration of the historical field particularly the presence of a negative anomaly centered in the eastern indian ocean and southwestern pacific ocean detailed records constraining the archeological period in the southwestern pacific and documenting the spatial extension of the anomaly are now needed to establish a direct link with the present field

permeability underpressures and convection in the oceanic crust near the costa rica rift eastern equatorial pacific in situ permeability and pore pressures were measured 200 m deep beneath the top of the oceanic crust at dsdp site 504b these measurements have relevance for the transition from convective to conductive heat flow on the south flank of the costa rica rift conventional slug and constant rate injection tests were made below a hydraulic packer set at various depths in the hole the packer was first set in a massive flow unit 37 m below the sediment basement interface the bulk permeability of the 172 5 m of pillow basalts and basaltic flows below the packer was found to be about 40 millidarcys 4 10 10 cm2 measurements over 3 and 15 m intervals at the bottom of the hole in an altered pillow zone indicated a bulk permeability of 2 4 millidarcys these values are thought to be accurate to 30 formation pore pressures were found to be approximately 8 bars 2 below hydrostatic interpretation of the data with respect to simple numerical convection models suggests that the transition from convective to conductive heat flow is controlled by the cessation of convective heat transport through the sedimentary layer rather than the cessation of convection in the sediment furthermore the agreement between observed and modeled underpressures implies that hole 504b penetrated an active ocean crustal convection system the thick sedimentary layer layers of basal chert and massive flow basalts above the layer 2a pillow flows apparently form an impermeable lid effectively isolating the convection system from the ocean

large rotation of the easter microplate as evidenced by oriented paleomagnetic samples from the ocean floor one of the goals of the pito93 cruise along the perimeter of the easter microplate was to obtain oriented rock samples from the ocean floor for paleomagnetic study using a new orienting device the geocompass 14 oriented blocks were obtained during 6 dives conducted using the ifremer dsrv nautile paleomagnetic analysis shows that 13 of these 14 blocks give consistent results in thermal and af demagnetizations and reveals a mean characteristic remanent magnetization chrm direction within each block with both normal and reverse polarities after elimination of data from 5 blocks situated outside the microplate and one block which bears an anomalous direction we have computed a mean chrm direction at dm 49 0 im 39 0 k 10 2 95 19 8 n 7 although the mean inclination is in agreement with the inclination of the recent dipole magnetic field d 0 i 43 the average declination is significantly different from this direction which we interpret as due to a bulk clockwise rotation of 48 5 11 after the bivariate average of the easter microplate over the past 2 3 ma this value is in excellent agreement with previous estimates based on remote geophysical measurements and models

microanalysis of carbonate cement δ18o in a co2 storage system seal insights into the diagenetic history of the eau claire formation upper cambrian illinois basin oxygen isotope 18o zonation in carbonate mineral cements is often employed as a proxy record typically with millimeter scale resolution of changing temperature regimes during different stages of sediment diagenesis recent advances in secondary ion mass spectrometry allow for highly precise and accurate determinations of cement 18o values to be made in situ on a micrometer scale thus significantly increasing the spatial resolution available to studies of diagenesis in sandstone shale and carbonate systems chemo isotopically zoned dolomite ankerite cements within shaly sandstone beds of the predominantly silty shaly eau claire formation cambrian illinois basin were investigated revealing the following with increasing depth of burial from 0 5 to 2 km 1500 to 6500 ft cement 18o values decrease from a high of approximately 24 down to approximately 14 on the vienna standard mean ocean water vsmow scale equivalent to 6 5 to 16 5 on the vienna peedee belemnite vpdb scale the observed cross basin trend is largely consistent with cements having formed in response to progressive sediment burial and heating within the context of independent burial and thermal history models for the illinois basin cementation began soon after deposition and continued intermittently into the mid permian however temperatures in excess of burial model predictions are inferred at the time of latest ankerite cement precipitation which we propose overlapped in time with conductive heating of the eau claire formation a closed system from under and overlying sandstone aquifers that channeled the flow of hot mississippi valley type mineralizing brines during the mid permian ca 270 ma

mineralogy nucleation and growth of dolomite in the laboratory and sedimentary environment a review dolomite camg co3 2 forms in numerous geological settings usually as a diagenetic replacement of limestone and is an important component of petroleum reservoir rocks rocks hosting base metal deposits and fresh water aquifers dolomite is a rhombohedral carbonate with a structure consisting of an ordered arrangement of alternating layers of ca2 and mg2 cations interspersed with co32 anion layers normal to the c axis dolomite has r3 symmetry lower than the caco3 r3 c symmetry of calcite primarily due to ca mg ordering high magnesium calcite also has r3 c symmetry and differs from dolomite in that ca2 and mg2 ions are not ordered high magnesium calcite with near dolomite stoichiometry 50 mol mgco3 has been observed both in nature and in laboratory products and is referred to in the literature as protodolomite or very high magnesium calcite many dolomites display some degree of cation disorder ca2 on mg2 sites and vice versa which is detectable using transmission electron microscopy and x ray diffractometry laboratory syntheses at high temperature and pressure as well as studies of natural dolomites show that factors affecting dolomite ordering stoichiometry nucleation and growth include temperature alkalinity ph concentration of mg and ca mg to ca ratio fluid to rock ratio mineralogy of the carbonate being replaced and surface area available for nucleation in spite of numerous attempts dolomite has not been synthesized in the laboratory under near surface conditions examination of published x ray diffraction data demonstrates that assertions of dolomite synthesis in the laboratory under near ambient conditions by microbial mediation are unsubstantiated these laboratory products show no evidence of cation ordering and appear to be very high magnesium calcite elevated temperature and elevated pressure experiments demonstrate that dolomite nucleation and growth always are preceded by very high magnesium calcite formation it remains to be demonstrated whether microbial mediated growth of very high magnesium calcite in nature provides a precursor to dolomite nucleation and growth analogous to reaction paths in high temperature experiments

selective dolomitization by syntaxial overgrowth around detrital dolomite nuclei a case from the jurassic of the ligurian brianconnais ligurian alps abstract dolomite crystals occurring in middle jurassic dolostones and dolomitic limestones of the ligurian brianconnais domain french italian ligurian alps show irregular complex zoning evidenced by cathodoluminescence cl and backscattered electron imaging comparable zoning patterns from other study areas have been attributed alternatively to recrystallization or dissolution precipitation processes however none of these mechanisms can account for the range of observations from the ligurian brianconnais domain which include 1 the presence of multiple core types each with different cl characteristics 2 marked compositional differences between cores and surrounding rims ferroan vs nonferroan mg ca ratio 3 a marked shape difference between cores with irregular outlines and rims which progressively approach a well developed rhombohedral habit these features are most reasonably interpreted as dolomitic rims that formed as syntaxial overgrowths around detrital dolomite nuclei which in turn originated from erosion of underlying triassic rocks and triggered selective replacement of fine grained calcareous sediment failure to recognize the exact nature of this type of dolomite crystal may lead to overestimating the degree of dolomitization and to overlooking possible detrital inputs in a basin otherwise supplied with pure allochemical sediments with consequent loss of information on the tectonosedimentary evolution

early dolomitization and recrystallization in shallow marine carbonates mississippian alida beds williston basin canada evidence from petrography and isotope geochemistry abstract the mississippian lower carboniferous alida beds of the northern williston basin canada are carbonate rocks that contain a number of significant hydrocarbon reservoirs these shallow marine carbonate platform deposits consist of abundant very finely to finely crystalline dolomite that occurs mainly in mud dominated sediments dolomite crystals are 4 to 30 m in length subhedral to anhedral in shape and typically replace the matrix but also occur as thin lamellae within ooid cortices textural relations indicate that the dolomite predates all other diagenetic phases including coarse calcite cement anhydrite and bitumen dolomite 18o values 9 3 to 4 5 pdb are depleted relative to that expected for precipitation from osagean late tournaisian seawater 0 5 to 1 5 pdb while 13c values 2 3 to 3 4 pdb are within the range of osagean seawater values 1 8 to 4 8 pdb dolomite 87sr 86sr values 0 70816 to 0 70921 are somewhat elevated with respect to osagean seawater 0 70768 to 0 70817 a trend of increasing average dolomite crystal size with decreasing 18o is observed in matrix replacive dolomite based on petrographic relations 13c compositions and the pervasive occurrence of finely crystalline dolomite initial dolomitization is interpreted to have taken place in the presence of slightly evaporated seawater the absence of depositional evaporites in dolomitized intervals suggests that dolomitization was driven by reflux of slightly evaporated seawater that had not reached gypsum saturation depleted 18o values elevated 87sr 86sr ratios and correlation of dolomite crystal sizes and 18o values suggest that the early formed dolomite was variably recrystallized during burial

early diagenetic recrystallization of holocene 3000 years old peritidal dolomites ambergris cay belize three peritidal carbonate crusts and associated intercrust sediments total thickness of 30cm aged 3000 years bp on ambergris cay belize contain 32 100 calcian dolomite x 72 5 dolomite ranging in composition from 40 to 46 mol mgco3 x 43 3 dolomite replaced high mg calcite foraminiferal muds penecontemporaneously with sedimentation forming partially dolomitized sediments and lithified crusts dolomitization probably occurred in normal to moderately evaporated seawater and is apparently continuing at the present detailed scanning electron microscope analysis shows a linear increase in mean dolomite crystal size with depth 0 4 m near the top of the section to 1 0 m near the base of the dolomitized section this size increase is not accompanied by any significant decrease in porosity crystal size distributions appear to be log normal and become increasingly broad and flat with depth rietveld x ray pattern fitting structure refinements indicate increasing ca and mg concentrations on their respective sites cation ordering as a function of increasing depth most of the ordering occurs within the first 15 cm of the surface stoichiometry does not increase with depth indicating no relationship between the ca mg ratio and cation ordering strong geochemical trends were observed down section in the dolomite including 1 increasing mn content 44 to 274 ppm and 2 decreasing 13c values 0 9 to 5 5 pdb oxygen isotope values range from 18o 1 3 pdb in the upper part of the section to 2 6 pdb in the lower part of the section and are interpreted to represent two distinct groups of values rather than a continuous trend down section dolomite crystal size increase and shapes of crystal size distributions are consistent with recrystallization via a surface energy driven dissolution reprecipitation process ostwald ripening the observed trends in carbon isotopes and mn content probably result from geochemical re equilibration during recrystallization and reflect reducing conditions and an isotopically light organically derived carbon source oxygen isotope compositions probably reflect relict original dolomite values and are a result of decreasing evaporation due to rising sea level

multi phase dolomitization and recrystallization of middle triassic shallow marine peritidal carbonates from the mecsek mts sw hungary as inferred from petrography carbon oxygen strontium and clumped isotope data abstract shallow marine to peritidal carbonates of the triassic csukma formation in the mecsek mts of sw hungary are made up of dolomites limestones and dolomitic limestones that show evidence of a complex diagenetic history integration of petrographic conventional stable oxygen and carbon isotope clumped isotope and strontium isotope data with the paleogeography paleoclimate and burial history of the region revealed four major diagenetic stages stage 1 the peritidal carbonates were dolomitized penecontemporaneously during the middle triassic by refluxing evaporatively concentrated brines stage 2 increasing burial during the late triassic jurassic resulted in recrystallization of the kan dolomite member in an intermediate burial setting stage 3 during the early cretaceous seawater was drawn down and circulated through rift related faults causing renewed recrystallization of the kan dolomite member as well as dolomitization of the kozar limestone member and the underlying limestones in a deep burial setting but only in the vicinity of the faults stage 4 during the late cretaceous and cenozoic thrusting resulted in tectonic expulsion of basinal fluids and precipitation of multiple saddle dolomite cement phases near the faults the results of this study imply that the clumped isotope method integrated with other geochemical data can successfully be applied to identify the nature and potential sources of extra formational diagenetic fluids responsible for dolomitization and recrystallization this study provides conclusive evidence for multi phase dolomitization and dolomite recrystallization over several millions of years middle triassic through early cretaceous and several thousands of meters of burial in the csukma formation in sw hungary furthermore this study is the first to identify fault controlled dolomitization by circulating cretaceous seawater within triassic carbonates of central europe further supporting the viability of the interpretation of dolomitization by seawater initially drawn down and then geothermally circulated through faults in extensional basins

persistently anomalous pacific geomagnetic fields a new average geomagnetic field model for the past 3kyr als3k helps bridge a large temporal sampling gap between historical models and more traditional paleo magnetic studies spanning the last 5 myr a quasi static feature seen historically in the central pacific has the oppo site sign in als3k its structure is similar to but of larger amplitude than that in the time averaged geomagnetic field for the last 5 myr anomalous geomagnetic fields exist be neath the pacific over timescales rangingfrom 10 2 10 6 years it is unlikely that bias over such long time scales arises from electromagnetic screening but conceivable that the lorentz force is influenced by long wavelength thermal variations and or localized regions of increased electrical conductiv ity associated with compositional anomalies and possibly partial melt this is consistent with recent seismic obser vations of the lower mantle

vertical open burrows in deep sea sediments 2 m in length previous studies of the mixed layer in deep sea sediments suggest a maximum burrowing depth of about 10 30 cm ref 1 however we now report that two recently obtained cores from the north east atlantic s126 2 and s126 4 display open burrows to a depth of 214 cm within a distal pelagic turbidite an organic lining in one of the burrows indicates a modern origin many of the burrows are less than 0 5 mm across and are only visible on fractured surfaces their effects on sediment mixing are probably minimal but they provide a network of open vertical channels which considerably increases the permeability this is of crucial importance to studies involving pore water movement such as those associated with the radioactive waste disposal project the effect of the burrows changes the calculated permeability from the equivalent of a clay to that of a coarse sand the delicate structure of the burrows may result in their closure during handling thus requiring their effect on pore water fluxes to be studied in situ

the occurrence and origin of late paleozoic remagnetization in the sedimentary rocks of north america although it has been known for 25 years that some paleozoic sedimentary rock units in europe and north america were remagnetized during pennsylvanian or permian time it is only very recently that the complex and widespread nature of the late paleozoic remagnetization phenomenon has been generally acknowledged it is now recognized that many paleozoic paleomagnetic poles for north america that were once considered reliable are in fact the result of remagnetization and as a consequence the paleomagnetic data base for the paleozoic is undergoing rapid and drastic revision the causes of late paleozoic remagnetization in north america are currently the focus of much interest and active research the remagnetization can reside in either hematite or magnetite and different remagnetization mechanisms have been important in different settings chemical remagnetization processes some related to specific diagenetic events are dominant in hematite bearing sandstones and carbonates in magnetite bearing carbonates both chemical and thermoviscous remagnetization processes appear to have been important but it is difficult to determine which process is the dominant one in some settings some of the observed remagnetizations can be linked to the migration of chemically active and perhaps hot fluids during the mountain building events that affected much of the continent during the late paleozoic paleomagnetic studies promise to be important in assessing the role of orogeny in driving fluid migrations within sedimentary basins and in constraining the age of the migrations and the nature of the fluids

chemistry and origin of zeolites in sandstones at dsdp sites 445 and 446 daito ridge and basin province northwest pacific abstract andesitic volcaniclastic sandstones of middle eocene age recovered from dsdp sites 445 and 446 a back arc basin contain clinoptilolite heulandite and analcite as a pore filling cement clinoptilolite and heulandite at sites 445 and 446 contain different chemical composition from other deep sea clinoptilolites and heulandites the dominant cation of both clinoptilolite and heulandite is na ion activity in pore water was greater at the time of their formation volcanic glass has acted as a precursor for both clinoptilolite and heulandite at sites 445 and 446 biogenic silica is also suggested as a precursor high na ion activity in pore water also helped to transform clinoptilolite and heulandite to analcite downhole the necessary na ions for this clinoptilolite heulandite and analcite at sites 445 and 446 might have been provided by hydrothermal circulation of seawater through more permeable sandstones deposited during early stages of back arc basin rifting

correlated sediment thickness temperature gradient and excess pore pressure in a marine fault block basin measurements of temperature gradient and excess pore pressure in the surficial sediment of a fault block basin in the guatemala basin correlate with sediment thickness the temperature gradient is smaller and the excess pore pressure gradient is more negative in areas of thinner sediment this correlation is explained by postulating downward pore water advection within the sediments with flow velocities on the order of 10 9 to 10 8 m s in the thinnest sediments and much less flow in the thickest sediments sediment physical properties and pore water chemistry also support this interpretation since the conductive heat flow of the basin as a whole is less than one third that predicted by sea floor spreading models the oceanic basement may be the site of a vigorous hydrothermal circulation system the pore water advection in the sediments may be driven by this larger scale circulation

numerical simulations of hydrothermal circulation resulting from basalt intrusions in a buried spreading center a two dimensional one by two kilometer section through the seafloor was simulated with a numerical model to investigate coupled fluid and heat flow resulting from basalt intrusions in a buried spreading center boundary and initial conditions and physical properties of both sediments and basalt were constrained by field surveys and drilling in the guaymas basin central gulf of california parametric variations in these studies included sediment and basalt permeability anisotropy in sediment permeability and the size of heat sources faults were introduced through new intrusions both before and after cooling background heat input caused fluid convection at velocities 3 cm a 1 through shallow sediments eighty to ninety percent of the heat introduced at the base of the simulations exited through the upper horizontal surface even when the vertical boundaries were made permeable to fluid flow the simulated injection of a 25 50 m thick basalt intrusion at a depth of 250 m resulted in about 10 yr of pore fluid expulsion through the sea floor in all cases leaving the sediments above the intrusions strongly underpressured a longer period of fluid recharge followed sometimes accompanied by reductions in total seafloor heat output of 10 in comparison to pre intrusion values additional discharge recharge events were dispersed chaotically through the duration of the cooling period these cycles in heat and fluid flow resulted from the response of the simulated system to a thermodynamic shock the sudden emplacement of a large heat source and not from mechanical displacement of sediments and pore fluids which was not simulated water rock mass ratios calculated from numerical simulations are in good agreement with geochemical estimates from materials recovered from the guaymas basin assuming a bulk basalt permeability value of at least 10 17 m2 10 2 md the addition of faults through intrusions and sediments in these simulations did not facilitate continuous rapid venting increased heat input at the base of the faults resulted in temporarily greater fluid discharge but the flow could not be sustained because the modeled system could not recharge cold fluid quickly enough to remove sufficient heat through the vents

paleosecular variation and the average geomagnetic field at 20 latitude we assembled a new paleomagnetic directional data set from lava flows and thin dikes for four regions centered on 20 latitude hawaii mexico the south pacific and reunion we investigate geomagnetic field behavior over the past 5 myr and address whether geographical differences are recorded by our data set we include inclination data from other globally distributed sites with the 20 data to determine the best fitting time averaged field taf for a two parameter longitudinally symmetric zonal model values for our model parameters the axial quadrupole and octupole terms are 4 and 6 of the axial dipole respectively our estimate of the quadrupole term is compatible with most previous studies of deviations from a geocentric axial dipole gad field our estimated octupole term is larger than that from normal polarity continental and igneous rocks and oceanic sediments but consistent with that from reversed polarity continental and igneous rocks the variance reduction compared with a gad field is 12 and the remaining signal is attributed to paleosecular variation psv we examine psv at 20 using virtual geomagnetic pole vgp dispersion and comparisons of directional distributions with simulations from two statistical models regionally the hawaii and reunion data sets lack transitional magnetic directions and have similar inclination anomalies and vgp dispersion in the pacific hemisphere hawaii has a large inclination anomaly and the south pacific exhibits high psv the deviation of the taf from a gad contradicts earlier ideas of a pacific dipole window and the strong regional psv in the south pacific contrasts with the generally low secular variation found on short timescales the taf and psv at hawaii and reunion are distinct from values for the south pacific and mexico demonstrating the need for time averaged and paleosecular variation models that can describe nonzonal field structures investigations of zonal statistical psv models reveal that recent models are incompatible with the empirical 20 directional distributions and cannot fit the data by simply adjusting relative variance contributions to the psv the 20 latitude data set also suggests less psv and smaller persistent deviations from a geocentric axial dipole field during the brunhes

dolomitization of the lower ordovician watts bight formation of the st george group western newfoundland evidence of hydrothermal fluid alterationgeological survey of canada contribution 20080544 the watts bight formation in western newfoundland consists of a lower ordovician succession of shallow water carbonates and has been extensively dolomitized these dolomites occur as both replaceme

recrystallization versus neomorphism and the concept of significant recrystallization in dolomite research abstract the term neomorphism originally introduced in 1965 as a substitute for the term recrystallization in limestones is now in use also for dolostones however only very few dolomite researches use the term neomorphism even though it is well and strictly defined many do not know what this term means and using the term commonly leads to confusion among carbonate researchers as well as between them and other geologists partially or largely for these reasons most dolomite researchers use the term recrystallization rather than neomorphism in addition the term neomorphism does not encompass all the important characteristics of recrystallization known today it is recommended therefore to abandon the term neomorphism in dolomite research and apply the term recrystallization with an extended definition that includes those characteristics that commonly are involved in recrystallization and that can be measured in dolomites and dolostones textural changes size shape structural changes ordering strain compositional changes stoichiometry isotopes trace elements including zoning fluid inclusions and changes in the paleomagnetic properties for practical applications the absence presence and or degree of recrystallization is often important for a genetic interpretation of dolostones if changes via recrystallization in texture structure composition and or paleomagnetic properties results in data ranges that are larger than the original ones which must be known for reference a dolomite dolostone is said to be significantly recrystallized and the reset properties are no longer representative of the fluid and environment of dolomitization but characterize the last event of recrystallization however not all measurable properties must be reset during recrystallization in this case inherited properties represent the event of dolomitization whereas reset properties represent the fluids and process of recrystallization in recrystallization resets only one of 10 properties e g 87 sr 86 sr ratios beyond the range of the pristine samples within the analytical errors this dolomite is significantly recrystallized with respect to sr isotopes yet insignificantly recrystallized with respect to the other nine properties in either case values of those properties that are identical to the pristine reference values can be used for genetic interpretations of dolomitization the concept of significant recrystallization is of great use in genetic interpretations of dolomites dolostones in particular a reliable interpretation of the chemistry of the dolomitizing fluids no longer depends on an absolute recognition of recrystallization rather it is sufficient to recognize that a dolomite is insignificantly recrystallized application of this principle also shows that many ancient dolomites and dolostones are insignificantly recrystallized finally the concept of significant recrystallization is independent of mineralogy hence it can and should be applied also to limestones

thermochemical sulphate reduction in the upper devonian cairn formation of the fairholme carbonate complex south west alberta canadian rockies evidence from fluid inclusions and isotopic data the fairholme carbonate complex is part of the extensively dolomitized upper devonian carbonate reefs in west central alberta the studied formations contain moulds up to 10 cm in diameter which are filled partially with saddle dolomite quartz and calcite cements these cements precipitated from a mixture of brines that acquired high salinity by dissolution of halite and brines derived from evaporated sea water the fluids were warm homogenization temperature of primary fluid inclusions of 76 to 200 c and saline 20 to 25 wt nacl equivalent and testify to thermochemical sulphate reduction processes the latter is deduced from s in solid inclusions co2 and h2s in volatile rich aqueous inclusions and depleted d 13 c values down to 26 vienna pee dee belemnite high 87 sr 86 sr values 0ae7094 to 0ae7110 of the cements also indicate interaction of the fluids with siliciclastic sequences the thermochemical sulphate reduction related cements probably formed during early laramide burial another younger calcite phase characterized by depleted d 18 o values 23ae9 to 13ae9 vienna pee dee belemnite low na 27 to 37 p p m and sr 39 to 150 p p m concentrations and non saline 0 wt nacl equivalent fluid inclusions is attributed to post laramide meteoric water

the effects of sedimentation and compaction on oceanic heat flow summary the estimation of environmental effects forms an important part of the interpretation of oceanic heat flow measurements in particular the perturbations associated with sedimentation and surface temperature changes must be taken into account analytical solutions can be obtained only for individual simplified versions of these problems whereas any real example is complicated by the process of sediment compaction which changes the bulk thermal properties with depth a physical model is developed which uses sediment porosity trends to predict the thermal parameters and material advection rates for an evolving sediment basement system these values are then used in a numerical solution to the heat flow equation to give estimates of the perturbed surface heat flux through time in addition to variations in sedimentation rate sediment type radioactive heat production and surface temperature changes are considered heat flow corrections may vary by up to a factor of 2 according to sediment type while radioactive heat production can offset the effects of sedimentation by as much as 40 per cent the results also indicate that alterations determined from simple analytical models tend to over estimate the true perturbation to the flux

evaluating reflux dolomitization using a novel high resolution record of dolomite stoichiometry a case study from the cretaceous of central texas usa

origin and diagenetic evolution of gypsum and microbialitic carbonates in the late sag of the namibe basin sw angola abstract ephemeral evaporitic conditions developed within the uppermost part of the transgressive late sag sequence in the namibe basin sw angola leading to the formation of extensive centimetre to metre thick sulphate bearing deposits and correlative microbialitic carbonates rich in pseudomorphs after evaporite crystals the onshore pre salt beds examined in this study are located up to 25 m underneath the major mid aptian evaporitic succession which is typified at the outcrop by gypsiferous bambata formation and in the subsurface by the halite dominated loeme formation carbonate evaporite cycles mostly occur at the top of metre thick regressive parasequences which progressively onlap and overstep landward the former faulted rift topography or fill major pre salt palaeo valleys the sulphate beds are made up of alabastrine gypsum associated with embedded botryoidal nodules dissolution related gypsum breccia and are cross cut by thin satin spar gypsum veins nodular and fine grained fabrics are interpreted as being diagenetic gypsum deposits resulting from the dissolution and recrystallisation of former depositional subaqueous sulphates whereas gypsum veins and breccia result from telogenetic processes the carbonates display a broader diversity of facies characterised by rapid lateral variations along strike thin dolomitic and calcitic bacterial mediated filamentous microbialitic boundstones enclose a broad variety of evaporite pseudomorphs and can pass laterally over a few metres into sulphate beds dissolution related depositional breccias are also common and indicate early dissolution of former evaporite layers embedded within the microbialites sulphate and carbonate units are interpreted as being concomitantly deposited along a tide dominated coastal supra to intertidal sabkha and constitute high frequency hypersaline precursor events prior to the accumulation of the giant saline mid aptian bambata and loeme formations petrographic and geochemical analyses reveal successive dissolution recrystallisation and cementation phases that occurred during burial uplift and exhumation implying a complex diagenetic evolution of both gypsum and carbonates influenced by pore fluids of diverse composition which distinctly varied from meso to telogenetic domains

dolomitization and neomorphism of mississippian visean upper debolt formation blueberry field northeastern british columbia geologic petrologic and chemical evidence petrographic chemical and isotopic studies of the mississippian visean upper debolt formation in the blueberry field british columbia canada reveal that dolomitization was the result of several diagenetic events and that neomorphic alteration of these dolomites significantly modified their original chemical signatures these studies also demonstrate how tectonics play an important role in controlling and modifying reservoir dolomites in the area petrographic investigations have documented two early dolomite phases 1 early matrix dolomite and 2 pervasive dolomite and two later generations 3 coarse cement and 4 pseudomorphic replacement of crinoids early matrix dolomite occurs as small average 25 m subhedral to euhedral crystals that replace the matrix of carbonate mudstones wackestones and packstones petrographic evidence suggests that early matrix dolomite had a relatively early precompaction origin possibly from marine fluids however geochemical evidence indicates that later fluids have altered their original geochemical signatures copyright 1997 the american association of petroleum geologists all rights reserved 1manuscript received february 1 1996 revised manuscript received august 16 1996 final acceptance january 19 1997 2department of earth sciences university of windsor windsor ontario canada n9b 3p4 present address chauvco resources ltd 2900 255 5th avenue s w calgary alberta t2p 3g6 canada 3department of earth sciences university of windsor windsor ontario canada n9b 3p4 this research was supported by individual operating grants from canada s natural sciences and engineering research council nserc awarded to i s al aasm and additional funding from pancanadian petroleum resources limited and suncor inc s durocher acknowledges support from an ontario graduate scholarship s brame talisman energy l williams phillips petroleum resources ltd g burrowes and i mcllreath pancanadian petroleum ltd and j packard rhomb carbonate consulting ltd are thanked for the valuable information insights and ideas for tackling the complexities of the blueberry field thanks are also due to j veizer university of bochum and university of ottawa for strontium isotope analysis an earlier version of this paper benefited from critical reviews by m malone texas a m university c viau shell canada ltd s schutter j dravis dravis interest inc and an anonymous reviewer

rethinking the use of seabed sediment temperature profiles to trace submarine groundwater flow submarine groundwater fluxes across the seafloor facilitate important hydrological and biogeochemical exchanges between oceans and seabed sediment yet few studies have investigated spatially distributed groundwater fluxes in deep ocean environments such as continental slopes heat has been previously applied as a submarine groundwater tracer using an analytical solution to a heat flow equation assuming steady state conditions and homogeneous thermal conductivity these assumptions are often violated in shallow seabeds due to ocean bottom temperature changes or sediment property variations here heat tracing analysis techniques recently developed for terrestrial settings are applied in concert to examine the influences of groundwater flow ocean temperature changes and seabed thermal conductivity variations on deep ocean sediment temperature profiles temperature observations from the sediment and bottom ocean water on the scotian slope off eastern canada are used to demonstrate how simple thermal methods for tracing groundwater can be employed if more comprehensive techniques indicate that the simplifying assumptions are valid the spatial distribution of the inferred groundwater fluxes on the slope suggests a downward groundwater flow system with recharge occurring over the upper middle slope and discharge on the lower slope we speculate that the downward groundwater flow inferred on the scotian slope is due to density driven processes arising from underlying salt domes in contrast with upward slope systems driven by geothermal convection improvements in the design of future submarine hydrogeological studies are proposed for thermal data collection and groundwater flow analysis including new equations that quantify the minimum detectable flux magnitude for a given sensor accuracy and profile length

tectonic evolution of the easter microplate the plate tectonic history of the easter microplate has been reconstructed by closing the microplate in a series of steps using the pacific nazca magnetic anomalies north and south of the microplate and the nuvel 1 global plate motion model after each step the easter microplate was rotated rigidly to match the nazca and pacific anomalies gaps and overlaps formed by this kinematic treatment indicate compressional and tensional deformation respectively and show that rigid plate motions are insufficient to explain the complete tectonic evolution analysis of the magnetic anomaly data was guided by contoured seamarc ii sea beam and 3 5 khz bathymetry data and a lineament map derived from seamarc ii side scan and sea beam bathymetry data the patterns of lineaments and bathymetric structures suggest that rotational deformation of the nazca plate is the general mechanism that accommodates the space problems arising from transfer of the nazca plate to the microplate and rapid rotation of the microplate against the nazca plate similar but smaller amounts of deformation are predicted along the southern boundary of the microplate prior to the origin of the microplate the east pacific rise epr was offset in at least two places according to the older magnetic anomalies yet there is no evidence of linear fracture zones within the sparse data set except for occasional small consistent changes in regional depth across these age offsets the magnetic bathymetry and satellite altimetry data indicate that the microplate initially formed at or perhaps southeast of easter island near a left lateral offset of the epr sometime between anomaly 3 and 3 the east rift started propagating north from the present location of easter island at 4 5 ma which is 1 5 m y earlier than previously proposed however the magnetic data that support this interpretation are sparse and complicated by recent volcanic flows and associated rough bathymetry west of easter island the geometry of the microplate changes very rapidly during its evolution at the initial stages of development the microplate resembles a large propagating rift system suggesting that deformation may have been occurring throughout most of its interior up to about 2 47 ma at this time the length to width of overlap ratio of the two rifts reaches a value of 3 the northward propagation slows down the curved opening of the southwest rift becomes well established and rigid rotation of the previously deformed transferred lithosphere probably starts to predominate at this time the offset distance between the two overlapped rifts starts to increase some time after 2 47 ma and before 1 ma the east rift starts propagating northwestward probably in response to the microplate rotation and continues up until present also during this time period the east rift breaks into a series of northward propagating rifts each propagating into the microplate interior thereby transferring lithosphere from the microplate to the nazca plate and reducing the total growth rate of the microplate

epigenetic dolomitization and the origin of xenotopic dolomite texture abstract xenotopic texture which is commonly observed in pre cenozoic rocks is defined here as a mosaic of anhedral crystals with irregular or curved intercrystalline boundaries and usually undulatory extinction xenotopic dolomite texture is similar in appearance to neomorphic limestone textures idiotopic dolomite texture euhedral to subhedral crystals with straight intercrystalline boundaries contrasts with xenotopic texture and is commonly observed in both cenozoic and more ancient dolomites texture may be controlled by the temperature at which crystals grow crystal growth theory predicts that at low temperature a smooth crystal surface is energetically favored and atoms are added to crystal faces layer by layer with dislocations acting as nucleation sites this results in faceted crystals and euhedral to subhedral crystal mosaics above a critical roughening temperature crt a rough surface is energetically favored surface nucleation does not require dislocations and atoms are randomly added to the crystal surface resulting in nonfaceted growth and an anhedral crystal mosaic it is hypothesized that a critical roughening temperature exists for dolomite above 50 c xenotopic dolomites are produced by dolomitization of limestone and or neomorphic recrystallization of dolomite at elevated temperature above the crt after burial idiotopic dolomites are produced below the crt by near surface processes and are occasionally produced above crt if growth inhibiting effects of the solution or impurities stabilize crystal faces calcite has a crt below 25 c and therefore develops anhedral crystal mosaics characteristic of neomorphic texture both at near surface and at elevated temperatures synthetic xenotopic dolomite was produced in the laboratory at 250 c and 300 c by dolomitization of aragonite and calcite skeletal fragments and by recrystallization neomorphism of nonstoichiometric cenozoic dolomites xenotopic dolomite resulted from the metamorphic recrystallization of the idiotopic hueco dolomite permian texas near the marble canyon intrusion at temperatures between 350 c and 600 c hydrothermal dolomitization of periclase calcite marble near the intrusion also resulted in a xenotopic texture xenotopic dolomite in the galena group ordovician wisconsin was produced by neomorphism of a preexisting dolomite during the emplacement of lead zinc sulfides at temperatures between 80 c and 227 c in the trenton limestone ordovician michig n xenotopic dolomite replaced limestone during the migration of hot 50 c fluids along fracture systems xenotopic dolomite was not observed in cenozoic dolomites that were subjected to temperatures near surface values only

dolomitization of the late cretaceous paleocene platform carbonates golkoy ordu eastern pontides ne turkey abstract late cretaceous paleocene agillar formation in the eastern pontide zone ne turkey is composed of shallow marine platform carbonates that were buried until uplift in late paleocene time the formation which is represented by 100 m thick reefal limestones around golkoy is completely dolomitized on the basis of petrographic and geochemical features four different replacive and cement dolomite phases are present the replacive dolomites which are cut by low amplitude stylolites include 1 fine crystalline planar s dolomite rd1 and 2 coarsely crystalline planar s e dolomite rd2 two types of dolomite cements were developed in minor amounts 1 a coarse dolomite cement cd1 that mostly fills molds vugs and fractures in rd1 dolomites and 2 rim dolomite cement cd2 that commonly develops on the space facing surfaces of rd2 dolomite the replacive dolomites are nearly stoichiometric ca 49 51 mg 48 50 and have 18 o values of 3 5 to 1 6 vpdb 13 c values of 1 1 to 0 8 vpdb and 87 sr 86 sr ratios of 0 707104 to 707458 sr na mn and fe concentrations of replacive dolomites are 102 171 ppm bdl 371 ppm 244 9941 ppm and 2643 8951 ppm respectively petrographic and geochemical data indicate that the replacive dolomites were formed prior to chemical compaction at shallow to intermediate burial depths from late cretaceous paleocene seawater and or seawater slightly modified by water rock interaction process and then recrystallized by meteoric waters at increasing burial depths and temperatures compared with replacement dolomites cd1 dolomites yield lower 18 o values 4 9 to 5 3 vpdb similar 13 c values 1 2 to 0 5 vpdb and higher 87 sr 86 sr ratios 0 707660 0 707609 the cd1 dolomites overlie the low amplitude stylolites and have homogenization temperatures between 94 and 112 c indicating that they were deposited from basinal brines although replacive dolomites and dolomite cements have similar na bdl 413 ppm mn 259 6990 ppm and fe 2184 6690 ppm concentrations the cements have lower sr contents 63 114 ppm like replacive dolomites cd1 dolomite is nearly stoichiometric ca 49 50 mg 47 49 but cd2 dolomite is slightly ca rich ca 51 52 mg 47 48 the cd2 dolomite is overlain by coarsely crystalline mosaic calcite cement indicates that diagenetic fluids eventually evolved to undersaturation with respect to dolomite and oversaturation with respect to calcite

evidence for excess pore pressures in southwest indian ocean sediments brown clay cores from the madagascar and crozet basins show the following evidence of excess pore pressures large amounts of flow in increasing average sedimentation rate with age and nonlinear temperature gradients additionally many hilltops in these basins have no visible sediment cover the bare hilltops may result from periodic slumping caused by excess pore pressures calculated excess pore pressures which equal or exceed the overburden pressure were inferred from water fluxes predicted by nonlinear temperature gradients and laboratory permeability measurements by using darcy s law since pore pressures which exceed the overburden pressure are unreasonable we attribute this discrepancy to laboratory measures which underestimate the in situ permeability the widespread presence of overpressured sediments in areas of irregular topography provides a process for resuspension of clay sized particles this mechanism does not require high current velocities for the erosion of clay and therefore can be applied to many areas where no strong currents are evident carbonate rich sediments from the madagascar ridge the mozambique ridge and the agulhas plateau had almost no flow in and occurred in areas where all topography was thickly draped with sediment since the age and tectonic location of the ridges and plateaus preclude water circulation in the basement wemore attribute these differences between the brown clay and the carbonate rich material to an absence of significant excess pore pressures in the plateau and ridge sediments less

paleomagnetic and paleosecular variation study of the mt cameroon volcanics 0 0 0 25 ma cameroon west africa abstract mt cameroon 4075 m is a large volcanic horst and is presently the only active volcano on the cameroon volcanic line eight eruptions during the last century this location belongs to the famous bangui magnetic anomaly and is thus of particular interest for the investigation of the persistence and the evolution of the anomaly with time we have conducted a paleomagnetic study of 94 samples drilled from 13 lavas flow on mount cameroon 0 0 0 25 ma low field susceptibility versus temperature k t and sirm experiments indicated that magnetite 575 c was the dominant magnetic carrier and that several sites were characterized by low temperature mineral phases e g 100 200 c and 300 400 c reflecting the presence of titanomagnetite with low titanium content at least eight samples from each site were demagnetized using both thermal treatment and alternating fields a f one contemporaneous site provided inconsistent and scattered directions and was rejected successful mean paleomagnetic directions were obtained for 12 sites but two sites exhibited inclinations which were significantly away from the inclination of the geocentric axial dipole gad and vgp latitudes lower than 60 their age indicates that they were likely recorded during a period of reduced dipole intensity and they were thus discarded the time averaged direction derived from this final selection has a mean declination of 354 2 and a mean inclination of 5 1 which is 3 lower than the gad inclination the corresponding vgp position is slightly far sided but contains the north geographic pole the angular standard deviation 13 7 of the vgps is also not significantly different from the predictions of model g at this latitude thus we found no evidence for a long term anomaly in this area but additional sites are needed to provide more conclusive indications

an organodiagenetic model for marinoan age cap carbonates abstract the nuccaleena formation cap carbonate is the global stratotype for the marinoan glaciation and ediacaran gssp a designation that emphasises the importance of cap carbonates to our current understanding of global neoproterozoic biogeochemistry sedimentology and stratigraphy however to date there is no agreed depositional model for cap carbonates and there remains minimal detailed paragenetic data for developing process based depositional models here an early diagenetic organodiagenetic dolomite cementation model for marinoan cap carbonate formation is hypothesised and explored it is demonstrated how this process based model can explain the three main atypical sedimentary structures that are the basis for correlation of marinoan age cap carbonates giant tepee like structures sheet veins and tubestones these features are interpreted to be products of fluid overpressure deformation induced by organodiagenetic expansive cementation giant tepee like structures the hallmark of marinoan cap carbonates conform to a simple structural analysis that is consistent with the predicted stress field induced by expansive cementation driven fluid overpressure the undeformed to highly deformed gradient of deformation features in cap carbonates is consistent with the range of rheologies in a shallowly buried cementing carbonate modification of matrix textures and bedding surfaces also conforms to the range of expected textures within this model overall it is hypothesised that the aggradational condensed section architecture of cap carbonates was the primary control over the generation of these atypical sedimentary features the available paragenetic analysis suggests that the geochemical data particularly the c isotopic data can be reinterpreted as supportive of the organodiagenetic model and that the neoproterozoic ocean was perhaps similar to that of today the proposed model indicates that the current criteria for the placement of the ediacaran gssp are non unique and potentially non isochronous perhaps the most important aspect of this model is that it is testable and is a call for focused research on the much overlooked paragenesis of cap carbonates

paleomagnetism and 40ar 39ar ages from la palma in the canary islands 1 the structure of the time averaged geomagnetic field has been known forcenturies to be approximately dipolar significant departures of the time averagedfield from that of an axial geocentric dipole however have been reportedfor decades the data on which time averaged field models are based must beof the highest quality in order to document subtle long term features wepresent here new paleomagnetic data and 40ar 39ar ages for the island of la palma the canary islands paleomagnetic samples were obtained from 28 lava flows of these 21 met ourminimum acceptance criteria for use in time averaged field models the 40ar 39ar age determinationswere successfully carried out on samples from eight of the flows our isotopicages and paleomagnetic polarities are consistent with the currently acceptedgeomagnetic reversal timescales the reversed data in the updated databaseare antipodal to the normal data within the uncertainties and the time averageddirection is indistinguishable from that expected from a geocentric axialdipole

diagenesis of the devonian southesk cairn carbonate complex alberta canada marine cementation burial dolomitization thermochemical sulfate reduction anhydritization and squeegee fluid flow abstract one of the largest geological features in the deep part of the alberta basin in western canada is the middle to upper devonian southesk cairn carbonate complex sccc the sccc is located at depths of about 5000 m adjacent to the disturbed belt of the rocky mountains rising to depths of 3000 m in a northeastward direction over a distance of 150 km the sccc produces oil and gas with up to about 30 h2s from several stratigraphic levels the diagenesis of the sccc was investigated by means of petrography and geochemistry stable isotope compositions were determined for fibrous calcite cements matrix dolomite saddle dolomite and sparry calcite fibrous calcites have stable isotope values of 18o 7 5 to 4 5 pdb and 13c 2 0 1 0 pdb matrix dolomite samples have stable isotope values of 18o 8 1 to 4 9 pdb and 13c 0 to 1 0 pdb saddle dolomite samples range from 18o 9 8 to 7 1 pdb and 13c 1 0 to 0 pdb sparry calcite samples have 18o 13 7 to 7 2 pdb and 13c 24 0 to 0 pdb the sr isotope values of all analyzed samples span a broad range from 0 7080 to 0 7320 showing considerable scatter but also one distinctive pattern i e values higher than 0 7220 occur only at depths in excess of about 4000 m whereas lower values scatter throughout the entire depth range furthermore the sr isotope values of sparry calcites decrease northeastward similarly the fluid inclusion homogenization temperatures of these calcites display a general decrease updip whereas the salinities show no general spatial trends the uncorrected homogenization temperatures range from about 85 to 180 c and the nacl equivalent salinities range from about 17 to 24 weight the d values of brines obtained from seven wells range from 79 to 20 smow and their 18o values range from 1 5 to 8 7 smow the sulfur isotope values of the dissolved sulfate ranges from 34s 10 1 to 24 5 cdt the sr isotope values of the brine samples range from 0 7109 to 0 7255 elemental sulfur isotope values cluster tightly around 34s 18 0 to 18 4 cdt the values of secondary anhydrites range from 34s 18 4 to 26 7 cdt values for solid bitumen vary from 34s 19 5 to 20 2 cdt integration of all data reveals a complex diagenetic evolution from the middle devonian to the recent the paragenetic sequence consists of 24 distinct diagenetic phases which have been grouped into 5 diagenetic stages stage i density driven convection in near surface settings likely superimposed on and or aided by tidal pumping facilitating marine cementation stage ii cellular convection facilitating pervasive matrix dolomitization aided by compaction stage iii hydrocarbon migration stage iv thermochemical sulfate reduction forming sour gas and regional squeegee flow forming sparry calcite cements and localized flow through faults stage v fluid flow driven by erosional rebound in the overlying mesozoic strata and remnant squeegee flow this study yielded several important results 1 the prolific repertoire of diagenetic textures found in the southesk cairn carbonate complex serves as a means of comparison for petroliferous carbonate complexes elsewhere especially in sour gas provinces close to orogenic fronts 2 the diagenetic evolution contains long periods of relative tranquility that were punctuated by several episodes of intense diagenetic activity 3 both thermochemical sulfate reduction and squeegee type fluid flow are recorded in late diagenetic sparry calcite cements 4 the effect of the various burial diagenetic episodes on the hydrocarbon reservoir properties throughout the entire reef complex was highly variable specifically matrix dolomitization thermochemical sulfate reduction and the emplacement of secondary and tertiary anhydrites affected the hydrocarbon reservoir properties throughout the entire reef complex on the other hand squeegee fluid flow was insignificant in terms of affecting the reservoir properties

dolomites of the boat harbour formation in the northern peninsula western newfoundland canada implications for dolomitization history and porosity control the boat harbour formation constitutes the upper part of the lower ordovician st george group on the northern peninsula western newfoundland it ranges in thickness from 140 m 459 ft at main brook to 96 m 315 ft at daniel s harbour dolomitization of the carbonate sequence is more pervasive in the lower 30 to 40 m 98 to 131 ft at main brook whereas at daniel s harbour the section is entirely dolomitized petrography suggests that the boat harbour formation has been affected by three phases of dolomitization the earliest near surface or synsedimentary phase is d1 dolomicrite 4 55 m which exhibits dull to no luminescence it commonly occurs as laminae capping cycles and as breccias in the younger dolomite phases it has low sr 228 30 ppm and an average 18o value of 6 0 0 8 vienna peedee belemnite vpdb in the main brook section but more depleted signatures for 18o of 8 8 1 vpdb and lower sr contents 45 8 ppm in the daniel s harbour section the geochemical composition suggests that d1 was developed from fluids of a mixture of meteoric and marine waters the midburial phase d2 dolomite consists of coarse planar subeuhedral crystals 30 400 m that show concentric cathodoluminescence zoning and are also crosscut by microstylolites its 18o values range between 6 6 1 3 vpdb at main brook and 9 0 0 5 vpdb at daniel s harbour this dolomite likely precipitated from fluids that circulated through crustal rocks with progressive burial th value of 114c 11c and salinity value of 23 1 8 eq wt nacl the late stage d3 dolomite has large and coarse nonplanar crystals 125 m 7 mm that exhibit sweeping extinction under crossed polars which is characteristic of saddle dolomite and also sometimes shows thin brightly luminescent rims it was likely precipitated during deeper burial in pulses and from hot fluids th values of 148c 19c and 115c 19 6c and mean salinities of 23 2 and 22 2 eq wt nacl at main brook and daniel s harbour respectively this is also supported by their relatively more depleted 18o 11 1 1 2 and 12 3 1 4 vpdb respectively and low sr contents 88 36 and 38 5 9 ppm respectively porosity in the boat harbour formation is mainly associated with the midburial d2 dolomite intercrystalline porosity is the dominant type and it ranges in the formation from less than 1 to 8 at main brook and from 7 to 12 at daniel s harbour vugs are less common but are associated with d3 dolomite the porous zone in the formation at main brook starts approximately 10 to 15 m 33 to 49 ft below the lower boat harbour disconformity and extends down to the lower formational boundary whereas porous zones in the formation at daniel s harbour are indiscriminately distributed throughout the section

tectonically controlled origin of three unusual rock suites in the woodlark basin we propose alternative mechanisms for the origin of three unusual rock suites high mg andesites nati basalts and arclike rocks that have been dredged from the woodlark basin southwest pacific ocean we show that the high mg andesites and nati basalts are associated with an unusually cool ridge environment the cooling is due to increased hydrothermal circulation stimulated by an unusually high crustal permeability the high permeability is mainly due to cracking of the woodlark basin lithosphere as it passes over the flexural bulge in front of the subduction zone although local processes such as faulting in fracture zones may also make some contribution this increased convective cooling affects magma dynamics and chemistry at the ridge crest the high mg andesites occur where the hydrothermal circulation lowers the temperatures in the upper oceanic crust causing the magma chamber to sit in the mantle rather than in the crust and promoting the interaction of basalt magma with harzburgite the nati basalts are also the indirect result of increased crustal cooling which causes anomalously low degrees of partial melting of their depleted mantle source the arclike rocks are caused by interaction with volatiles from lithosphere that was emplaced beneath the edge of the basin less than 6 m y ago by a now inactive subduction zone to the north of the currently active one simple thermal models indicate that this formerly subducting plate is still cold enough to retain volatiles and to remain seismically active as the old plate loses volatiles they rise into the mantle convection cell which feeds the woodlark basin ridge crest because ridge subduction increases the likelihood of ophiolite obduction these observations and explanations of woodlark basin tectonics are potentially important for ophiolites

recent investigations of the 0 5 ma geomagnetic field recorded by lava flows we present a synthesis of 0 5 ma paleomagnetic directional data collected from 17 different locations under the collaborative time averaged geomagnetic field initiative tafi when combined with regional compilations from the northwest united states the southwest united states japan new zealand hawaii mexico south pacific and the indian ocean a data set of over 2000 sites with high quality stable polarity and declination and inclination measurements is obtained this is a more than sevenfold increase over similar quality data in the existing paleosecular variation of recent lavas psvrl data set and has greatly improved spatial sampling the new data set spans 78 s to 53 n and has sufficient temporal and spatial sampling to allow characterization of latitudinal variations in the time averaged field taf and paleosecular variation psv for the brunhes and matuyama chrons and for the 0 5 ma interval combined the brunhes and matuyama chrons exhibit different taf geometries notably smaller departures from a geocentric axial dipole field during the brunhes consistent with higher dipole strength observed from paleointensity data geographical variations in psv are also different for the brunhes and matuyama given the high quality of our data set polarity asymmetries in psv and the taf cannot be attributed to viscous overprints but suggest different underlying field behavior perhaps related to the influence of long lived core mantle boundary conditions on core flow psv as measured by dispersion of virtual geomagnetic poles shows less latitudinal variation than predicted by current statistical psv models or by previous data sets in particular the brunhes data reported here are compatible with a wide range of models from those that predict constant dispersion as a function of latitude to those that predict an increase in dispersion with latitude discriminating among such models could be helped by increased numbers of low latitude data and new high northern latitude sites tests with other data sets and with simulations indicate that some of the latitudinal signature previously observed in vgp dispersion can be attributed to the inclusion of low quality insufficiently cleaned data with too few samples per site our matuyama data show a stronger dependence of dispersion on latitude than the brunhes data the taf is examined using the variation of inclination anomaly with latitude best fit two parameter models have axial quadrupole contributions of 2 4 of the axial dipole term and axial octupole contributions of 1 5 approximately 2 of the octupole signature is likely the result of bias incurred by averaging unit vectors