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

stable isotopic signature of a palaeoaquifer mississippian alamogordo member limestones sacramento mountains new mexico usa nine stratigraphic sections each 5 m thick were sampled from the alamogordo member limestones of the lake valley formation sacramento mountains new mexico usa four stratigraphic sections consist entirely of lime mudstone and wackestone whereas the other five sections have a prominent layer of crinoidal packstone about 1 m thick at their base stable isotopic analyses reveal that the lime muds in the sections with basal packstone layers show a downward decrease in 1 8 o and constant 1 3 c values whereas those in the sections solely composed of lime mudstone and wackestone have in general relatively uniform 1 8 o and 1 3 c values the diagenesis of the alamogordo member limestones was previously believed to have been governed by the downward percolation of meteoric water from a regional pre pennsylvanian exposure surface 100 m above this unit however the uniform 1 3 c and downward decrease in 1 8 o values in the lime muds in the sections with basal packstones indicate that the meteoric water ascended within the alamogordo member rather than descended from the overlying exposure surface this indicates that the basal packstones were probably a conduit for meteoric water this is further supported indirectly by the relatively uniform 1 8 o and 1 3 c values of the lime mud in the sections without basal packstones the implications are that the oxygen isotopic gradients may be used to identify palaeoaquifers flow directions within these aquifers and that meteoric diagenesis below an exposure surface could be governed by flow through a palaeoaquifer

paleoaquifer and deep burial related cements defined by regional cathodoluminescent patterns middle ordovician carbonates virginia middle ordovician ramp carbonates virginia were deposited in a subsiding foreland basin bordered on the southeast by tectonic highlands ramp carbonates were lithified in part by turbid marine cements but major cementation was by nonferroan clear rim and equant cements zoned defined by cathodoluminescence clear cements consist of nonluminescent oldest bright and dull youngest cements the zonation relates to increasingly reducing conditions of pore waters zoned cements in peritidal beds best developed in southeastern belts have complex zonations pendant to pore rimming fabrics and are associated with crystal silt which abuts all cement zones solutional cavities and erosional surfaces which locally truncate dull cement these cements are meteori vadose to shallow phreatic cements in northwestern exposures of peritidal beds are dominated by non zoned dull cement which lacks abundant evidence of early near surface precipitation major cementation of subtidal facies occurred under burial conditions burial cements best developed in southeastern belts have a simple zonation reflecting progressive burial up to 7 5 km 4 5 mi of the carbonate ramp shallow burial nonluminescent cement formed from oxidizing meteoric waters which expelled anoxic connate marine waters these meteoric waters were carried by aquifers from tectonic upland recharge areas on the southeastern basin margin bright cement formed under more reducing conditions following stagnation of the paleoaquifer with burial and possibly was precipitated at depths of 2 to 3 km 1 to 2 mi burial cements in northwestern exposures of subtidal beds are dominated by dull cement initial generations of which precipitated from downdip portions of aquif rs deeper burial dull calcite and ferroan dolomite cements largely formed at burial depths of 2 to 3 km 1 to 2 mi and temperatures of 75 to 135 c 165 to 275 f or more associated with hydrocarbon formation emplacement during the late devonian to mississippian latest clear dull cement fills tectonic fractures and was emplaced during late paleozoic alleghenian deformation probably at temperatures of 200 to 300 c 390 to 575 f and depths of 5 to 7 km 3 to 4 5 mi deeper burial diagenesis appears to be genetically linked to late paleozoic mississippi valley type mineralization in the southern appalachians zoned peritidal and burial cements are confined mainly to southeastern portions of the ramp where cementation was influenced by meteoric waters shed from tectonic uplands on the southeast and carried northwest by paleoaquifers northwestern portions of the ramp were influenced very little by upland sourced meteoric waters and nonzoned dull cements precipitated from relatively reducing waters the distribution of nonluminescent cement in middle ordovician subtidal facies defines the regional distribution of oxidizing portions of the paleoaquifer system such incursion of upland sourced oxidizing meteoric waters into ramp carbonates should be a common feature of foreland basin carbonates deposited adjacent to tectonic uplands furthermore foreland basin carbonates that undergo progressive burial should show a simple nonluminescent to bright to dull sequence of cement zones the close association of zoned cements and regional uplands in the middle ordovician sequence indicates the importance of assessing regional geologic relationships environmental parameters geologic history and tectonics in understanding regional cementation patterns and cementation processes of anci nt carbonate platforms

cementation in oligo miocene non tropical shelf limestones otway basin australia abstract the oligocene to miocene heytesbury group of the otway basin southeastern australia is a non tropical carbonate shelf succession this subsurface stratigraphic succession consists of bioclastic grainstones to mudstones containing bryozoa benthonic and planktonic foraminifera echinoderms brachiopods and molluscs the basal bryozoa dominated clifton formation was deposited in relatively shallow waters the overlying gellibrand marl rich in planktonic foraminifera and bryozoa formed under deep waters the uppermost unit benthonic foraminifera dominated port campbell limestone was deposited in a moderately deep middle shelf environment cementation is limited and includes 1 scalenohedral calcite non luminescent to bright mn and fe poor in inter and intraparticle porosity 2 blocky equant calcite dull mn and fe rich in interparticle porosity 3 syntaxial calcite overgrowths on echinoderm particles non bright dull zoned 4 euhedral dolomite in the interparticle pore space of the port campbell limestone 5 bladed prismatic calcite ferroan in planktonic foraminifera 6 fibrous calcite non luminescent in intraparticle porosity and 7 glauconite and iron oxides only in intraparticle porosity all caco3 cements are low mg calcites the cements observed in the heytesbury group appear to have formed in three successive diagenetic environments 1 early shallow burial early fe and mn poor non luminescent syntaxial overgrowths and non luminescent scalenohedral calcite under oxidising conditions 2 late shallow burial bright more mn rich syntaxial overgrowth zone i e more than the initial non luminescent one and bright scalenohedral calcite under moderately reducing pore water conditions and 3 moderate burial ferroan dull blocky calcite and dull syntaxial overgrowth zone under reducing conditions pressure solution was the main cement producing mechanism although seawater and aragonite dissolution could account for the precipitation of the early non luminescent cements scarcity of early interparticle cements may have been caused by the calcite rich character of the original sediments the ferroan dully luminescent blocky equant calcite and syntaxial overgrowths typically interpreted to represent products of deep burial in ancient carbonate successions are observed in the shallow buried port campbell limestone

the sequence and mechanism of low temperature dolomite formation calcian dolomites in a pennsylvanian echinoderm abstract middle pennsylvanian fossils from the buckhorn asphalt quarry of southern oklahoma are unusual in that early sealing by tar has preserved many of their original textural and mineralogic features and those of the earliest stages of diagenesis echinoderm fragments collected from this locality axe well preserved with original stereom morphology clearly visible optically one to five micrometer anhedral calcian dolomite grains occupy as much as 30 percent of the original stererom of the fragments as displayed by differential etching and mgk x my maps the composition of skeletal fragments is 75 percent ca 98mg 02co3 25 percent ca 54mg 46co3 single crystal x ray di fraction xrd empa tem and stem microanalysis were used to characterize the calcian dolomite phase tem micrographs revealed an approximate 100 a tweed structure identical to that reported by reeder and wenk 1979 our data imply that magnesium in the calcian dolomites was derived largely from the original magnesian calcite of the echinoderm stereom original biogenic high magnesium calcite is altered via local microdissolution reprecipitation to anhedral disordered calcian dolomite grains within low magnesium calcite the product phases low magnesium calcite and calcian dolomite retain the original crystallographic orientation of the stereom these products of early diagenesis are apparent precursors to well defined ordered euhedral microdolomites the relationships between the low magnesium calcite and calcian dolomite and the chemistry and complex ultrastructure of the calcian dolomites support the kind of low temperature mechanism of dolomite formation postulated by gains 1977 goldsm th and graf 1958 and others on the basis of hydrothermal experiments

diagenetic evolution of selected parasequences across a carbonate platform late paleozoic tengiz reservoir kazakhstan abstract predicting the diagenetic modification of reservoir quality in carbonate fields is poorly understood but of paramount importance to reservoir characterization the diagenetic processes and current pore system are reported for two parasequences from the platform center to the margin of the tengiz reservoir 16 km the late visean lvis13 cbs 900 and bashkirian bash2 5 msf 350 parasequences are separated by 8 million years syndepositional fibrous cement 10 rock volume and aragonite skeletal molds developed in both parasequences on subaerial exposure brown soil related calcite and meniscus and pendant cements were emplaced in both parasequences the application of cl petrography to these rocks for the first time has revealed the depositional components and early marine components are altered developing a mottled pattern implying that their original chemistry is not preserved the principal calcite cement shows a complex growth pattern by cl that is unreported from any other calcite cement cl is also used to divide the main cement into stage 1 nonluminescent and stage 2 orange to brown luminescent calcite stage 1 calcite shows vadose meniscus and pendant fabrics followed by uniform phreatic euhedral terminations stage 1 cement is patchily distributed at a millimeter scale 28 to 40 in the visean and 0 5 to 22 in bashkirian grainstones stage 1 cement is more abundant at the center of the platform in the late visean than at its margin abundant stage 1 calcite creates a framework that prevents later compaction preserving a high porosity at the center of the platform this sweet spot is reinforced by the deposition of late stage bitumen cement that occupies 8 rock volume at the margin of the platform but is almost absent at its center stage 1 13c values 2 4 to 1 7 are derived by dissolution of host rock and their 18o values 6 3 to 3 7 are compatible with precipitation from meteoric or mixed marine meteoric water depositional micrite and micritic grains are transformed by dissolution precipitation processes into porous microrhombic calcite not by ostwald ripening microrhombic calcite is ubiquitous in the tengiz icehouse dispelling the myth that this fabric is restricted to greenhouse epochs at 300 million years before significant burial the study grainstones were patchily cemented and dissolved to reach peak porosity their transformed texture set up flow paths that controlled the distribution of later diagenetic products reservoir compartmentalization did not occur through extensive hardground formation at this time as occurs in some greenhouse sequences but ash bands at parasequence boundaries were turned into tonsteins and wackestones were compacted perhaps sufficiently to influence vertical flow cl examination of stage 2 calcite that adds 0 5 to 16 to rock volume reveals that its precipitation was episodic and switched sites as the pore network became blocked through precipitation and compaction and was opened by new fractures a change from acute to equant habit occurred during stage 2 precipitation causing the change from fibrous to equant crystal fabric the 18opdb values of stage 2 calcite 3 to 9 are linked to precipitation from an 18o enriched fluid that evolved to 6 smow at temperatures of 60 c to 120 c using the equation of kim and o neil 1997 many open macropores are lined with small cement crystals that cl shows have all stages of growth present and are terminated by euhedral surfaces these macropores are only partly filled because of cement inhibition not because they were dissolved during burial as was previously claimed late stage dissolution did create some macropores and rounded micrite crystals but its effect on overall platform porosity is small the cl examination of tengiz platform calcite cement has recognized many diagenetic events that cannot be recognized without cl the new tengiz diagenetic textures recognized here are probably present in other carbonate platforms especially those from icehouse epochs

remagnetization and orogenic fluids testing the hypothesis in the central appalachians summary palaeomagnetic and geochemical studies on devonian sedimentary rocks in the valley and ridge province in west virginia indicate that the hypothesized relationship between orogenic fluids and remagnetization is not straightforward the helderberg group an aquitard and the oriskany formation a palaeoaquifer both contain similar synfolding late palaeozoic chemical remanent magnetizations crms residing in magnetite the results from the oriskany formation are consistent with remagnetization by orogenic fluids because the unit contains geochemical evidence for the migration of such fluids in contrast there is no geochemical evidence for the widespread migration of orogenic fluids into the helderberg group and thus the crm cannot be related to such fluids another remagnetization mechanism which remains elusive must be responsible for the crm in the helderberg group

multi carbonate component reconstruction of mid carboniferous chesterian seawater δ13c abstract this study presents the first stratigraphically continuous 13ccarb record for the chesterian interval developed from antler foreland basin platform successions the bulk carbonate derived 13c curve was constructed using 700 biostratigraphically constrained samples providing a 105 yr resolution a multi component study of 13c and 18o variability in petrographically and geochemically screened abiotic and biogenic calcite components delineates the degree of preservation and 13c heterogeneity in these ancient epicontinental carbonates furthermore it constrains the influence of kinetic and vital effects and local to regional scale c cycling on their 13ccarb values the 13c and 18o values of micrites and bulk carbonates overlap regardless of lithofacies and show minimal intrasample variability overall biogenic calcite 13c and 18o values are higher than the average isotope compositions of bulk carbonates and micrites although 13c values in contemporaneous samples are only minimally 0 3 2 std err of 0 3 different relative to much larger differences in 18o values although brachiopod 13c and 18o values are higher than average bulk carbonate and micrite values their 13c values exhibit considerable isotopic variability by taxa and ultrastructure notably the 13c values of brachiopod secondary shell layers overlap with bulk carbonate micrite and marine cement 13c values conversely 13c values of brachiopod tertiary prismatic layers including composita are on average 2 to 5 higher than the 13c values of contemporaneous bulk carbonates micrites and brachiopod secondary shell layers an observed average 2 decrease in the 13c values of secondary cross bladed laminar layers in productids relative to contemporaneous bulk carbonate micrite and marine cement values suggests a previously unrecognized kinetic and or vital effect on carboniferous productid 13c values given that they have the highest 18o values of all analyzed components the observed higher 13c values in these tertiary prismatic layers are comparable to greater than the 13c enrichment 1 previously documented in pennsylvanian composita the overlap in 13c values of bulk carbonates with diagenetically screened microdrilled matrix marine cements and brachiopod secondary shell layers indicates that the chesterian bulk carbonate derived 13c values may reliably record to within 1 the evolution of ambient seawater 13c in the outer carbonate ramp of the antler basin comparison of the chesterian 13c record to the bulk carbonate c isotope curve derived from the arrow canyon nv carbonate succession further suggests that these bulk carbonate derived 13c records are reliable proxies of the composition and trends in seawater 13c regionally along the equatorial eastern panthalassan margin the newly resolved chesterian stratigraphy documents multiple 13c excursions including previously unrecognized significant c isotope shifts a shift to higher frequency and amplitude shorter term 13c fluctuations in the upper third of the chesterian stage occurred simultaneously with a change in stratigraphic cyclicity from dominantly 3rd order to 4th order carbonate cycles suggesting a mechanistic linkage between ambient seawater 13c and sea level fluctuations in the antler basin this change in the character of the shorter term 13c fluctuations coupled with a subsequent significant 4 positive c isotope excursion across the mid carboniferous boundary previously recognized in all carboniferous curves is interpreted to record the onset of late paleozoic glaciation and glacio eustasy

burial dominated cementation in non tropical carbonates of the oligocene te kuiti group new zealand abstract cementation of bryozoan echinoid benthic foraminiferal temperate shelf carbonates of the oligocene te kuiti group north island new zealand occurred mainly during subsurface burial the calcite cements in the limestones are dominated by equant and syntaxial rim spar which typically becomes ferroan given an iron supply and compared to the skeletal material with normal marine 18o values from 2 to 1 more depleted in 18o with depth of burial the 18o composition of bulk cement samples ranging from 1 to 7 these trends reflect the establishment in pore waters during sediment burial of reducing conditions and gradually increasing temperatures 20 50 c respectively the 13c values 0 to 3 of the cements remain the same as the host marine shells suggesting the source of carbon in the cements was simply redistributed marine carbonate derived from shell dissolution two gradational burial diagenetic environments influenced by marine derived porewaters are arbitrarily distinguished shallow burial phase and moderate burial phase during the shallow burial phase down to 500 600 m sub bottom depth the carbonates lost at least 25 of their original porosity by mechanical compaction and were selectively cemented by non ferroan or usually ferroan variably luminescent slightly 18o depleted sparry calcite cement 18o 2 to 4 mainly as syntaxial rims about echinoid grains these shallow burial cements form less than about 10 of total cement in the majority of the limestones and their source was probably mainly mild intergranular dissolution of calcitic skeletal fragments accompanying the onset of chemical compaction during the moderate burial phase between about 600 and 1100 m sub bottom depth porosity loss continued typically to about 70 of its original value as a result of pressure solution of calcitic bioclasts associated with more advanced stages of chemical compaction this involved development of a wide variety of non sutured and microstylolitic solution seams including both single and composite wispy or continuous bedding parallel types and non parallel reticulate forms the released carbonate was precipitated as ferroan or non ferroan where iron supply was negligible dull luminescent strongly 18o depleted 18o 4 to 7 mainly equant calcite spar cement occluding available pore space in the limestones

regional trends in the timing of alleghanian remagnetization in the appalachians pole positions related to remagnetized components isolated in appalachian limestone and redbed rock units range over about 60 m y of the permian carboniferous apparent polar wander path for north america apparent ages of remagnetization are older in the southern appalachians and younger to the north if the remagnetizations are associated with fluids expelled during the alleghany orogeny then the apparent remagnetization age trend could describe the timing of thrust sheet emplacement

synfolding remagnetization and deformation results from palaeozoic sedimentary rocks in west virginia summary palaeomagnetic analysis was conducted at 27 sites from two stratigraphically adjacent palaeozoic units along two transects meldey and rig of the patterson creek anticline in the appalachian orogen of west virginia to better understand the relationship between deformation and remagnetization while no primary directions were observed we isolated identical well defined secondary late palaeozoic magnetizations in carbonates from the late silurian tonoloway formation and the early devonian helderberg group standard application of conventional palaeomagnetic fold tests suggests that the magnetizations are synfolding in the helderberg group and pre folding to early synfolding in the tonoloway formation a fold test on all data combined indicates a synfolding magnetization best grouping at 60 65 per cent unfolding but the resulting distributions are not circular an alternative interpretation based on optimal differential unfolding at the site mean level yields one group with a nearly pre folding or early synfolding magnetization 70 per cent unfolding that is both circular and concordant with north america s apparent polar wander path the second group has a variable synfolding magnetization that is elliptical at the optimum unfolding level and falls off the path differences in either the age of remagnetization or viscous partial thermoremanent magnetization contamination cannot account for this result the sites falling into the pre folding group are biased towards the tonoloway formation the medley transect the northwest limb away from small folds and lower dip angle relative to the sites in the synfolding group the two groups defined by the fold test results also have a relationship with both total strain and strain partitioning the rocks containing the early synfolding remanence exhibit less total strain and in particular less pressure solution strain relative to grain boundary sliding and calcite twinning it is likely that the remagnetization in the entire area was coincident and occurred prior to folding even though some sites reveal synfolding directions the variation in the behaviour of the remagnetization relative to folding is unclear at present but it indicates that pressure solution and stylolite formation are important processes in remagnetization in any event the relationship between deformation and magnetization is not simple apparently identical magnetizations from adjacent sites may have very different histories and or responses to deformation while the exact relationship between remagnetization and deformation remains elusive this study demonstrates that significant variation in behaviour can exist at a very local scale

cathodoluminescence and composition of calcite cement in the taum sauk limestone upper cambrian southeast missouri abstract study of calcite cements in the taum sauk limestone upper cambrian southeast missouri using cathodoluminescence petrography and microprobe analysis has shown that 1 distinct luminescent zones correspond to distinct compositional zones 2 the degree of luminescent intensity of a particular calcite zone appears to be controlled by the fe mn ratio of the zone not by the absolute concentrations of either cation 3 manganese may induce luminescence in calcite at concentrations as low as 300 ppm and 4 there may be no critical concentration of ferrous iron in calcite above which there can be no luminescene

migration of orogenic fluids through the siluro devonian helderberg group during late paleozoic deformation constraints on fluid sources and implications for thermal histories of sedimentary basins abstract fluid inclusion data and fracture filling cement fabrics in deformed but non overthrusted carbonate and siliciclastic rocks of the siluro devonian helderberg group central appalachians indicate post cementation late paleozoic migration of high pressure high temperature fluids void filling quartz and calcite cements contain secondary two phase fluid inclusions liquid vapor which occur in annealed microfractures most melting temperatures last occurrence of ice are 20 to 25 c which correspond to salinities of greater than 22 equivalent wt nacl pressure corrected liquid vapor homogenization temperatures are 200 to 300 c and greatly exceed maximum paleotemperatures 150 c given by conodont cai values vitrinite reflectance illite crystallinity or temperatures calculated from known sedimentary overburden completely cemented sandstone and limestone also have multiple crosscutting trains of secondary hydrocarbon inclusions hydrocarbon inclusions occur along deformation microstructures in cements some hydrocarbon inclusion trains crosscut cement filled fractures the nature of occurrence of the secondary inclusions suggests that fluids moved along intracrystalline deformation microstructures either during or after late paleozoic deformation rare fractures contain transported skeletal grains exotic clasts lithologies different from wall rock recemented clasts of fracture filling cement and mud cement clasts contain solid inclusions of mud and skeletal fragments and indicate several episodes of particle transport cementation and refracturing prior to final fracture filling and cementation these fracture fills locally crosscut stylolites coarse grained poorly sorted clastic fracture fills suggest migration of rapidly moving fluids which were capable of transporting clasts through fracture conduits under deep burial conditions conduction calculations place upper limits on size and duration of thermal anomalies which were caused by extrabasinal fluids these calculations indicate that hot fluid migration events most likely were short lived heat transfer by extraformational fluids moving along faults and fractures may have been the mechanism by which thermal anomalies developed in the helderberg group stratigraphic constraints suggest that likely sources for the hot brines are overthrusted terranes to the east of the study area possible stratigraphic horizons which were sources for the overpressured hot fluids may have been thick cambrian shales which are important detachment horizons for major late paleozoic thrust sheets in the eastern valley and ridge province east of the study area fluids may have migrated during thrusting along thrust faults which ramp upwards from east to west into stratigraphically higher units thrust faults ultimately die out in devonian units some of the dissolved ions in the secondary two phase inclusions may have been incorporated when shale derived fluids moved through silurian evaporites or cambro ordovician carbonates which contain evidence of cryptic evaporites potential source rocks for secondary hydrocarbon inclusions may have been lower silurian through lower devonian strata based on lopatin modeling and assuming that migration occurred during late paleozoic deformation this study may help to explain many apparently anomalous geologic phenomena in deformed foreland basin sediments it also has implications for tracking migration pathways for fluids which facilitate regional scale deformation in fold and thrust belts and how tectonic fluids may affect the rocks through which they move

burial diagenesis in the upper devonian reef complexes of the geikie gorge region canning basin western australia the devonian carbonates of the geikie gorge region canning basin have undergone a long and complex diagenetic history that began in devonian seawater with extensive marine cementation of platform margin lithologies devonian lower carboniferous burial diagenesis was the most important porosity occluding episode because almost all primary porosity was destroyed by equant calcite cements nonluminescent to brightly luminescent to dully luminescent during this interval dolomitization and consequent secondary porosity development also occurred during early burial diagenesis the distribution and geochemistry of the major calcite cements and dolomite types are consistent with these phases having been precipitated from connate marine or basinal brines karstification and mi or calcite cementation took place during late carboniferous subaerial exposure minor calcite cementation occurred during permian cenozoic burial predominantly in secondary porosity within pervasively dolomitized lithologies karstification dedolomitization and calcite recrystallization took place in association with cenozoic meteoric diagenesis secondary moldic and intercrystalline porosity within the completely dolomitized lithologies were the longest lived porosity types in the carbonates some secondary porosity escaped both devonian carboniferous and permian cenozoic burial cementation probably due to a lack of nucleation sites for calcite cements within completely dolomitized lithologies

petrography geochemistry and origin of burial diagenetic facies siluro devonian helderberg group carbonate rocks central appalachians petrographic and geochemical studies of pore filling cements and replacement products in the helderberg group upper silurian lower devonian central appalachians document the diagenetic history of these rocks shallow ramp skeletal limestones and buildups were partially lithified by marine cements however most pore filling cement formed under shallow 300 m to deep burial 300 4 000 m conditions regional cathodoluminescent zonation patterns in early calcite cements indicate meteoric ground waters were involved in shallow burial cementation early zoned calcite cements consist of updip nonluminescent cements with thin luminescent laminae in limestone or sandstone that pass downdip into subzoned dull cements interlayered bright and dull laminae and then into nonzoned dull cement in basinward limestone the regional distribution stable isotopic compositions and timing of early zoned cements relative to other diagenetic events suggest that all of the early cements formed synchronously from meteoric ground waters that became progressively more reducing as they flowed downdip at least 150 km helderberg sandstone tongues probably were preferential conduits for meteoric gro nd waters because early calcite cements formed only on scattered skeletal grains in sandstones late void filling dull cement formed from deep burial pore fluids at burial depths of 300 to 4 000 m deep burial pore fluids were dilute to saline na ca cl brines from primary fluid inclusion data with stable isotopic compositions that probably were similar to isotopic compositions of formation waters from modern oil fields void filling dull calcite cements occluded nearly all remaining porosity before late paleozoic fracturing and deformation latest diagenetic phases include dolomite fluorite silica and fracture filling dull calcite this study shows that integrated petrographic and geochemical approaches to diagenetic studies are useful for relating various diagenetic phases and fabrics to burial history and paleoaquifer distribution

trenching exposures of the surface rupture of 2008 mw 7 9 wenchuan earthquake china implications for coseismic deformation and paleoseismology along the central longmen shan thrust fault abstract the may 12 2008 mw 7 9 wenchuan earthquake was induced by failure of two of the major faults of the longmen shan thrust fault zone along the eastern margin of tibet plateau our study focused on trenches across the yingxiu bichuan fault the central fault in the longmen shan belt that has a coseismic surface break of more than 200 km long trenching excavation across the 2008 earthquake rupture on three representative sites reveals the styles and amounts of the deformation and paleoseismicity along the longmen shan fault styles of coseismic deformation along the 2008 earthquake rupture at these three sites represent three models of deformation along a thrust fault two of the three trench exposures reveal one pre 2008 earthquake event which is coincident with the pre existing scarps based on the observation of exposed stratigraphy and structures in the trenches and the geomorphic expressions on ground surface we interpret the 2008 earthquake as a characteristic earthquake along this fault the interval of reoccurrence of large earthquake events on the central longmen shan fault the yingxiu beichuan fault can be inferred to be about 11 000 years according to 14c and osl dating the amounts of the vertical displacement and shortening across the surface rupture during the 2008 earthquake are determined to be 1 0 2 8 m and 0 15 1 32 m respectively the shortening rate and uplift rate are then estimated to be 0 09 0 12 mm yr and 0 18 0 2 mm yr respectively it is indicated that the deformation is absorbed mainly not by shortening but by uplift along the rupture during the 2008 earthquake

regional distribution of microdolomite inclusions in mississippian echinoderms from southwestern new mexico abstract crinoid grains from mississippian skeletal packstones and grainstones from the lake valley and kelley formations new mexico have retained most of their original mg which now resides in 1 to 10 m microdolomite inclusions the mean microdolomite content of these crinoids is about 9 3 1 1 95 confidence limit by volumes as determined by petrography the microdolomite contents show regional variations being lowest in the northern part of the study area about 6 and highest about 11 170 km to the south there are no consistent stratigraphic gradients in microdolomite contents the average apparent mg content of the mississippian crinoids is estimated from the 9 3 microdolomite by assuming 50 original intraskeletal porosity this estimate of 10 4 mole mgco3 is lower than the average 14 5 mole of modern low latitude crinoids in spite of this the higher values of mg contents of mississippian crinoids fall within the range of mg contents of modern low latitude crinoids we interpret these data to mean that original mg contents of mississippian crinoids were comparable to those of modern crinoids and that the relatively low mean mg content of mississippian crinoids is due to diagenetic loss of mg we rule out lower temperatures or mg contents of mississippian sea waters and biological differences of modern and mississippian crinoids as causin the low mg contents of mississippian crinoids mg loss from mississippian crinoids is supported by the decrease in microdolomite content toward rims of grains and by the fact that intragranular variation is lower than intergranular variations of microdolomite content on the basis of the above evidence we suggest that on average mississippian crinoids have retained about 75 of their mg the southward increase in mg content of mississippian crinoids we interpret to reflect a greater diagenetic mg loss in the north than in the south rather than a southward increase in sea water temperature or a southward change in taxa this interpretation is supported by the southward decrease in intensity of chemical compaction and by the covariance of mg and 18o contents of crinoids our diagenetic model based on these data and on previous work invokes regional fresh phreatic groundwater systems that formed in response to late mississippian early pennsylvanian emergence of the shelf the groundwaters were recharged in the north and flowed generally southward northern areas contained diagenetically more aggress ve groundwaters which leached more mg from crinoid grains than did groundwaters farther south

problem of secondary porosity frio formation oligocene texas gulf coast secondary porosity formed by the dissolution of both carbonate and silicate minerals especially k feldspars is widely developed in sandstones of the frio formation oligocene in the texas gulf coast co 2 produced by decarboxylation of organic matter is commonly suggested as the acid required for dissolution material balance calculations indicate that co 2 produced by decarboxylation of organic matter in frio formation shales can account for a regional average of only 1 or 2 secondary porosity in frio formation sandstones yet point count data indicate an average of 10 secondary porosity long distance fluid transport many kilometres and or other mechanisms of acid generation should therefore be considered carbon isotopic data on dissolved inorganic carbon in formation water co 2 in produced natural gas and carbonate cements indicate that co 2 produced by decarboxylation had a minor impact on these carbon reservoirs the reaction kerogen water methane carbon dioxide can explain the isotopic data but alone it is insufficient to account for all the secondary porosity unless long distance material transport is involved

sedimentology and basin evolution of the siluro devonian helderberg group central appalachians abstract the late silurian early devonian helderberg group central appalachians is a sequence of mixed siliciclastic carbonate sediments that was deposited during relative tectonic quiescence on a ramp that was built out from low relief tectonic highlands bordering the eastern side of the appalachian basin the helderberg group is a third order vail cycle that lasted about 8 10 m y three transgressive regressive sequences are also recognized each sequence was deposited over 2 3 m y subsidence rates during deposition were about 1 2 cm 1 000 yr lithofacies patterns resulted from eustatic sea level changes differential basin subsidence and varying sedimentation rates skeletal grainstone rudstone formed fringing skeletal banks that developed during regression and prograded away from the eastern side of the basin at rates of 35 50 m 1 000 yr quartz sandstone tongues in the helderberg group probably were reworked from underlying silurian sandstone and were introduced to the basin by rivers that nearly coincided with ancestral silurian drainage systems siliciclastic influx was never great enough to shut carbonate production off completely thick middle devonian siliciclastic sediments buried the helderberg group and updip subaerial exposures accompanying the onset of the acadian orogeny

regional cementation from unconformity recharged aquifer and burial fluids mississippian newman limestone kentucky abstract the mississippian newman limestone in kentucky was studied to assess the effects of unconformities within and above the sequence on calcite cementation early cements are low in fe and show a nonluminescent to dull to nonluminescent zonation early nonluminescent cement mainly formed in updip areas from oxidizing meteoric waters in a regional paleoaquifer that formed during regional regression in the late mississippian during late mississippian transgression waters became reducing to form early dull cement up to 600 ppm mn up to 2 700 ppm fe but averaging 570 ppm the aquifer underwent later recharge during mississippian pennsylvanian regression to form a second nonluminescent cement this event continued into the pennsylvanian when the sequence underwent increasing burial earl cements have 13o values that become lighter downdip perhaps reflecting a downdip temperature increase due to burial 13c values are lightest in the north postulated recharge area and reflect input of organically derived co2 in waters there most calcite cementation appears to have been associated with two post newman unconformities which generated regional aquifers and reflects a shift to a wetter climate in late mississippian pennsylvanian time early cement abundance decreases away from postulated recharge areas later iron rich burial cements up to 2 200 ppm mn up to 11 000 ppm fe fill remaining pore spaces compaction induced fractures spalled cement rims and tectonic fractures lined with saddle dolomite and pyrite they formed above 60 c at depths of 1 to 3 km 13o values are compatible with derivation by pressure solution of preexisting calcites in isotopically heavy waters highly modified by burial diagenesis light 13c values reflect isotopically light organic carbon in the deltaic waters entering the subsurface or being expelled from shales or buffering of waters by light c early calcite cements the 13c calcite cements become heavier downdip by equilibration of waters with host limestone newman sediments may have been subjected to late postcementation migration of warm fluids based on secondary fluid inclusions these late stage fluids most likely were chemically complex warm brines 7 17 equiv wt naci 50 to 160 c that resided in the formation for sufficiently short times that conodont cai values were not affected

unconformities and porosity development in carbonate strata ideas from a hedberg conference the purpose of this paper is to highlight ideas from a hedberg conference concerning detection of unconformities and associated porosity in carbonate strata many types of information including cores and outcrops seismic data eustatic sea level curves wireline logs biostratigraphic data stable isotope trends cycle stacking patterns and tectonic and basin evolution models can be used to predict and or detect subaerial unconformities all methods of detecting subaerial exposure with these types of information have limits and pitfalls detailed analysis of the stratigraphy sedimentology and diagenetic history of a rock sequence from outcrop or core data is considered the most reliable means of detecting subaerial exposure though an integrated approach using all a ailable information is recommended the relationship between subaerial exposure and subsurface porosity in carbonates is not simple or easily predicted subsurface porosity is the product of many factors operating during deposition subaerial exposure and burial several concepts should be recognized when evaluating porosity associated with subaerial unconformities 1 carbonate diagenesis during subaerial exposure rearranges pore networks but usually does not increase the total porosity near surface dissolution can simply lower a topographic surface without increasing porosity within the host limestone in many cases total porosity in carbonates under unconformities is actually reduced during subaerial exposure 2 permeability is likely to change more than porosity during subaerial exposure increasing in some c ses but decreasing in others 3 pore systems evolve with time during subaerial exposure short periods of subaerial exposure 10 000 400 000 yr are often associated with greater porosity than long intervals of subaerial exposure 1 20 million yr prolonged subaerial exposure may change permeability less than porosity because high permeability karst related conduits can form quickly and persist for millions of years 4 many carbonates subjected to subaerial exposure have little or no porosity in the deep subsurface because compaction cementation and stratal collapse reduce porosity during burial 5 unconformity related diagenesis may enhance reservoir potential by creating pore systems that are resistant to compaction during deeper burial 6 vugs caverns and breccias can fo m in the deep subsurface because of dissolution by basinal fluids independent of subaerial exposure 7 lithologic changes at unconformities like shales overlying carbonates can influence the flow of subsurface fluids during end page 857 deeper burial resulting in deep burial dissolution localized along unconformities if subaerial exposure and exposure surfaces are integrated into a sequence stratigraphic paleogeographic and climatic framework systematic and hopefully predictable patterns will emerge however we do not fully understand the complex interplay between the factors critical to creating and preserving porosity in carbonates additional research is needed to quantify critical processes and products so we can reliably predict porosity associated with subaerial exposure during exploration in frontier basins and on a smaller scale during reservoir analysis

the saltwater freshwater interface in the tertiary limestone aquifer southeast atlantic outer continental shelf of the u s a abstract hydrologic testing in an offshore oil well abandoned by tenneco inc determined the position of the saltwater freshwater interface in tertiary limestones underlying the florida georgia continental shelf of the u s a previous drilling joides and u s g s amcor projects established the existence of freshwater far offshore in this area at the tenneco well 55 mi 88 km east of fernandina beach florida drill stem tests made in the interval 1050 1070 ft 320 326 m below sea level in the ocala limestone recovered a sample with a chloride concentration of 7000 mg l 1 formation water probably is slightly fresher pressure head measurements indicated equivalent freshwater heads of 24 29 ft 7 3 8 8 m above sea level at the coast fernandina beach a relatively thin transition zone separating freshwater and saltwater occurs at a depth of 2100 ft 640 m below sea level fifty five miles 88 km offshore at the tenneco well the base of freshwater is 1100 ft 335 m below sea level the difference in approximate depth to the freshwater saltwater transition at these two locations suggests an interface with a very slight landward slope assuming the hubbert interface equation applies here because the interface and therefore freshwater flow lines are nearly horizontal the equilibrium depth to the interface should be 40 times the freshwater head above sea level using present day freshwater heads along the coast in the hubbert equation results in depths to the interface of less than the observed 2100 ft 640 m substituting predevelopment heads in the equation yields depths greater than 2100 ft 640 m thus the interface appears to be in a transient position between the position that would be compatible with present day heads and the position that would be compatible with predevelopment heads this implies that some movement of the interface from the predevelopment position has occurred during the past hundred years the implied movement is incompatible with the hypothesis that the freshwater occurring far offshore in this area is trapped water remaining since the pleistocene epoch

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

marine diagenesis of lower ordovician carbonate sediments dumugol formation korea cementation in a calcite sea abstract the lower ordovician dumugol formation exhibits many features that indicate early lithification such as calcite nodules hardgrounds mud mounds and intraclasts detailed observations of these early lithified features reveal that rapid marine cementation was instrumental in their formation marine lithification took place in a low energy subtidal environmental that was influenced by intermittent storms marine cements include syntaxial overgrowth bladed calcite fibrous calcite and fine crystalline equant calcite cements syntaxial overgrowths precipitated on echinoderm grains and contributed to rapid marine lithification of echinoderm bearing sediments bladed fibrous and fine crystalline equant calcite cements precipitated in locally suitable sites but their occurrence is limited and thus played a minor role in marine lithification microcrystalline calcites also precipitated in lime mud rich fine grained sediments and participated in rapid marine lithification of the dumugol sediments the absence of aragonite allochems and cement and the predominance of calcite cement suggest that the dumugol sea was undersaturated with respect to aragonite but supersaturated with respect to calcite which is indicative of a calcite sea

patterns of groundwater salinity changes in a deep continental oceanic transect off the southeastern atlantic coast of the u s a abstract manheim f t and paull c k 1981 patterns of groundwater salinity changes in a deep continental oceanic transect off the southeastern atlantic coast of the u s a in w back and r letolle guest editors symposium on geochemistry of ground water 26th international geological congress j hydrol 54 95ndash 105 investigations of formation fluid salinities in a transect from western georgia to the edge of the blake plateau off the coast of georgia show surprisingly similar hydro chemical features offshore and onshore a fresh brackish wedge of groundwater 25g kg total dissolved solids lies beneath the shelf to a depth of 900 m on land brackish waters extend to a maximum depth of 1 2 km below sea level in lowndes county georgia in deeper horizons hypersaline brines 100g kg occur in lower cretaceous strata these strata have a pronounced evaporitic anhydritic character in the offshore segment strong salinity gradients in interstitial waters signify buried evaporite deposits at drill sites beneath the blake plateau

fluid control of localized mineral domains in limestone pressure solution structures grain to grain and stylolitic solution structures in two central appalachian siluro devonian limestone macroscale folds contain one of four distinct mineral assemblages that are characterized by the dominant iron phase mineral present 1 chloritegillitegpyritegcalciteg quartzgtio2ggoethite 2 chloritegillitegpyrite altered to iron oxide hydroxidegcalcite gquartzgtio2 3 chloritegilliteg magnetitegcalcitegquartz and 4 chloritegilliteggoethitegcalcitegquartzgtio2 optical reflectance microscopy and sem eds was used to characterize the mineralogy and mineral morphology of these structures geochemical modeling was used to constrain the conditions of formation and preservation the primary control on solution structure mineral assemblage was the redox conditions present in the solution structures during burial and deformation the redox conditions on the microscale may have been controlled by the local fluid chemistry and the presence absence of hydrocarbons and organic acids within the formation fluids and the influx of externally derived fluids by fracture formation during the folding process the wide variation in mineralogy of the solution structures shows that they were chemical factories where a variety of chemical reactions took place during rock dissolution in particular the formation of authigenic magnetite in solution structures has significant implications for paleomagnetic applications and use of anisotropy of anhysteretic remanent magnetization and anisotropy of magnetic susceptibility fabrics q 2005 elsevier ltd all rights reserved

chronostratigraphic significance of cathodoluminescence zoning in syntaxial cement mississippian lake valley formation new mexico abstract echinoderm syntax ial cement crystals have been collected at several stratigraphic levels within the lake valley formation which is overlain by a major pre pennsylvanian subaerial exposure surface the crystals were microsampled along growth bands and yielded high resolution elemental and isotopic information that record fluid evolution during their growth although cement crystals show little variation in cathodoluminescence character and bulk chemistry throughout the regional extent of the formation intracrystalline patterns in minor element and stable isotope chemistry allow for the identification of several settings of cementation including the marine phreatic marine meteoric mixing and meteoric phreatic zones when placed in a regional stratigraphic context crystal growth records enable reconstruction of the temporal and spatial extent of these diagenetic environments isotopic petrographic and stratigraphic constraints indicate that cementation was related to two temporally distinct meteoric systems most lake valley formation syntaxial cement formed in the marine phreatic and marine meteoric mixing zones during the earliest phase of cementation of this cement in excess of 60 formed in the marine phreatic zone and lowermost marine meteoric mixing zone smaller volumes precipitated from progressively fresher mixtures of marine and meteoric fluid during the gradual expansion of a freshwater lens that developed in response to pre pennsylvanian lowering of sea level meteoric phreatic fluids were relatively unimportant during this episode of cementation in contrast cement of entirely meteoric phreatic origin is associated with a later meteoric system but is found only in the northern part of the study area and at stratigraphic levels immediately adjacent to the pre pennsylvanian unconforrnity our results indicate that cements in the upper and lower parts of the lake valley formation are genetically and temporally unrelated thus the cement cathodoluminescence stratigraphy which has been previously applied to the lake valley can not be valid over the full extent of the formation this has wider implications our demonstration that diverse and temporally distinct diagenetic settings can produce identical cl characteristics shows that caution should be employed when using cement cl stratigraphy alone to extend diagenetic interpretations over large geographic regions or through substantial stratigraphic successions

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

microstratigraphy of calcite cements in pliocene cool water limestones new zealand relationship to sea level burial and exhumation events

dolomite calcitization and cement zonation related to uplift of the raisby formation zechstein carbonate northeast england abstract the raisby formation is the basal zechstein carbonate of northeast england it has undergone a complex diagenetic history during burial including multiphase dolomitization and replacement by sulphates upon uplift meteoric derived fluids have dissolved this replacive sulphate and precipitated calcite cements in the resultant porosity study of these cements by cathodoluminescence and trace element geochemistry shows that they are zoned and different cathodoluminescence zones correlate well with iron and manganese geochemical changes demonstrating a gradual increase in oxygenation of the precipitating fluids through time major diagenetic processes during cement growth include precipitation of anhydrite fluorite and barite calcitization of dolomite and internal sedimentation relating these processes to cement zonation allows this complex diagenesis to be explained in terms of uplift under the influence of meteoric derived groundwaters of constantly changing geochemical and electrochemical characteristics

disequilibrium trace element partitioning in jurassic sparry calcite cements implications for crystal growth mechanisms during diagenesis sparry calcite cements from middle jurassic limestones in central england contain complex non concentric zoning patterns produced by a combination of sector zonation and a sector specific lamellar or polygonal zonation electron microprobe analyses of individual zones within crystals document 1 a sympathetic covariation of fe mg and sr between two non equivalent sectors and 2 episodic further increase of sr contents coupled with exclusion of fe mg and mn within the more enriched sectors comparison of results from three different localities demonstrates the regional consistency of this pattern although the magnitude of the differential trace element partitioning varies from combined petrographic and geochemical data the trace element variation in the crystals can be shown to bear no simple relationship with either 1 the chemical evolution of the 9bulk9 precipitating porefluid or 2 the relative overall growth rates of the different crystal faces previous sector zoning models based on crystal surface configurations and internal structure are unable to account directly for the geochemical patterns observed in this study differences between growth mechanisms on the crystal faces of the studied spars are likely to have exerted a significant influence on trace element partitioning in particular they offer a plausible explanation for the geometric and compositional characteristics of the sector specific episodic zoning on the most enriched crystal faces in general crystal morphology and composition will be intimately linked by the growth mechanisms operating on different faces and by their response to changes in the diagenetic environment the validity of diagenetic interpretations from bulk geochemical analysis of sparry calcite cements thus rests upon a detailed analysis of any zonation that they contain

diagenetic history of pipe creek jr reef silurian north central indiana u s a abstract calcite cements in the silurian ludlovian pipe creek jr reef north central indiana are compositionally zoned with characteristic minor element concentrations and stable isotope signatures and were precipitated in different diagenetic environments superposition and crosscutting relationships allow us to group cement zones and to relate them to the sequence stratigraphic evolution of the reef pipe creek jr reef grew in normal marine waters with the reef top high greater than 50 m above the platform floor flank facies are volumetrically important and are preserved largely as limestone in contrast to most dolomitized silurian reefs in the midcontinent syndepositional marine cements fill primary porosity and synsedimentary fractures and are interlayered with marine internal sediment now low magnesium calcite their isotopic compositions are similar to those of depositional grains and cements estimated to have precipitated from ludlovian sea waters depositional porosity was reduced by 75 by the precipitation of these syndepositional cements which stabilized the steeply dipping flank slope postdepositional clear calcite cements are interpreted as shallow phreatic and burial cements on the basis of their relationship to periods of karstification and fracturing shallow phreatic cements with concentric cathodoluminescent cl zonation precipitated in primary pores and are postdated by fractures and caves filled with middle devonian sandstone cl zonal boundaries are sharp and some near a major stratigraphic unconformity show evidence of dissolution the volumetric abundance of the individual cl zones varies in the reef indicating a complex superposition of waters of varying chemistry and rock water interaction that are probably related to relative sea level changes this important aspect of the reef stratigraphy is recorded only by the diagenetic succession because evidence of earlier sea level changes is removed by a major later regional unconformity burial cements are the youngest diagenetic feature recognized and they rest conformably or unconformably over older cements they exhibit both concentric cl zonation and sectoral zoning they are ferroan to nonferroan and they contain thin sulfide zones along growth band boundaries their isotopic compositions do not overlap with shallow phreatic or marine cement values degraded oil postdates burial cements and is composed of the same sterane class as the devonian age antrim shale the probable source rock this source contrasts with that of reef reservoirs in the michigan basin where silurian strata are commonly the hydrocarbon source

diagenesis thermal maturation and burial history of the upper cambrian bonneterre dolomite southeastern missouri an interpretation of thermal history from petrographic and fluid inclusion evidence abstract in this report fluid inclusion thermal alteration index tai and petrographic data are used to interpret thermal maturation and burial history of the bonneterre formation of southeastern missouri tai visual estimates based on indigenous upper cambrian acritarchs indicate maximum thermal exposure into the mid upper peak gas window this interpretation is in agreement with maximum fluid inclusion homogenization temperatures which average 235 c 455 f or an approximate vitrinite equivalent of 1 7 r0 thin section petrography and fluid inclusion evidence further indicate that three separate episodes of burial have occurred initial burial of unknown depth or thermal exposure culminated in a period of uplift and karstic subaerial exposure possibly during late ordovician time maximum burial and thermal exposure to the mid upper peak gas window followed and is represented in the rock by a high temperature 235 c randomly oriented two phase fluid inclusion population in matrix dolomite subsequent uplift possibly coincident with ouachita overthrusting resulted in a variety of diagenetic fabrics including late burial cements and fractures secondary two phase fluid inclusions within these late calcite cements and those occurring in linear trains that cut across matrix dolomite record lower average homogenization temperatures than their unoriented precursors average th 170 c or an approximate equivalent of 1 3 r0 further indicating that a significant amount uplift has occurred since maximum burial present day bottom hole temperatures of only 74 c an approximate equivalent of 0 7 r0 indicate that uplift has occurred after this second burial episode this event probably coincides with the development of the new madrid fault system during the tertiary which still active today these results demonstrate the utility of fluid inclusion microthermometry complex burial thermal histories

burial diagenesis and porosity evolution upper jurassic smackover arkansas and louisiana the jurassic upper smackover ooid lime grainstone of the central gulf coast has been a major hydrocarbon exploration target for nearly half a century study of the geologic and sedimentologic framework petrography two phase fluid inclusions and trace element and stable isotope chemistry indicates the following diagenetic history the upper smackover is a blanket ooid grainstone ranging in thickness from 300 to 400 ft 91 to 122 m and covering at least 4 000 sq mi 10 400 sq km in the northern part of the arkansas shelf a regional meteoric water system was established in the grainstone shortly after deposition and prior to significant burial this meteoric water system gave way to conditions which were increasingly more marine toward the south near the louisiana arkansas state line the fresh to marine hydrologic gradient is responsible for the strong precompaction regional diagenetic overprint that eventually controlled available diagenetic pathways for the unit during burial three diagenetic zones are present 1 a northern zone dominated by oomoldic porosity and preburial spar cementation 2 a sou hern zone characterized by compaction effects late subsurface cement primary porosity preservation and late secondary subsurface porosity generation and 3 a transitional zone having characteristics common to both the northern and southern zones during the shallow burial phase compaction became increasingly more important to the south in zones dominated by marine connate fluid whereas rock integrity and a secondary oomoldic porosity system had already been established in the northern zone as burial increased brine originating in the underlying louann salt reached the shelf grainstone and began to displace the original connate fluid the brine triggered the inversion to calcite of remaining grains and cement composed of unstable mineralogy when the louann brine had displaced the original connate fluid caso4 replacement as gypsum and anhydrite was common across all three diagenetic zones also precompaction fine cement associated with oomoldic porosity was recrystallized and reequilibrated with the concentrate brine localized replacement dolomitization may well have developed throughout the burial history of the sequence but was most commonly postcompactional postcompaction poikilitic calcite cement and baroque dolomite were precipitated from the louann brine under conditions of elevated temperature and pressure as subsidence continued organic material from the basinward equivalents of the upper smackover lime grainstone arrived on the shelf and catalyzed so4 reduction resulting in pyrite and galena replacement in the southern and transitional zones as hydrocarbon maturation continued in the fine grained equivalents of the upper smackover h2s and co2 by products of these processes began to increase in the brine of the southern and transitional zones ultimately causing regional dissolution of existing carbonate and significantly enhancing preserved porosity porosity in the northern zone was established early and preserved through moderate burial although porosity is well developed permeability has been largely destroyed by recrystallization dolomitization is a prerequisite for economic reservoir development in the northern zone porosity in the southern zone is solution enhanced preserved primary intergranular porosity compaction is the most important porosity destruction process compaction in the absence of an early diagenetic overprint is affected primarily by grain type and secondarily by size and sorting pellets and oncolites compact easily yet destroy porosity during burial ooids and rhodolites resist compaction during burial therefore are related to porosity preservation grain type distribution as well as size and sorting are controlled by depositional environment processes with the most favorable areas for porosity development concentrations of ooids and rhodolites associated with early salt related structural movement porosity distribution in the southern zone therefore can be outli ed with the aid of geophysical end page 597 techniques porosity in the transitional zone is early secondary or preserved primary intergranular or a combination porosity in this zone therefore is highly unpredictable and a strong potential exists for diagenetic traps unrelated to structure or original depositional environment documentation of significant deep subsurface secondary porosity possibly associated with hydrocarbon maturation opens the potential for hydrocarbon exploration in deeply buried carbonate dominated basins this would include carbonate rocks heretofore thought to be well below the depth of carbonate porosity maintenance because of lithostatic load

dedolomitization in the taum sauk limestone upper cambrian southeast missouri abstract coarse dolomite crystals in the taum sauk limestone upper cambrian southeast missouri contain both calcite and iron oxides in rhombohedral zones and along fractures and cleavage traces the dolomite crystals are demonstrably nonstoichiometric averaging 57 mole percent caco3 and are compositionally zoned ranging from 1 40 to 4 56 mole percent feco3 cathodoluminescene study and microprobe analyses of the dolomite coupled with scanning electron microscope sem examination of etched fracture surfaces reveal that 1 scattered dolomite within the calcite iron oxide zones is the most iron rich 2 calcite within dolomite rhombs is brightly luminescent nonferroan and is intergrown with iron oxides 3 the iron oxides display two crystal habits thin red hematite circular plates typically 0 7 to 1 0 microns across grouped in rosettes and prismatic bundles and prismatic radiating aggregates of brown crystals goethite typically 1 to 3 microns in length 4 narrow veinlets of dolomite associated with calcite and iron oxides along cleavage traces form a three dimensional network this feature which aided in interpreting the iagenetic history of the dolomite was not observed during petrographic examination but was readily observed with the sem the occurrence of nonferroan calcite and ferric oxides in rhombohedral zones in taum sauk dolomite suggests that dedolomitization occurred by the oxidation and alteration of ferroan dolomite zones and probably reflects alteration related to recent weathering

implications of thermal events on thrust emplacement sequence in the appalachian fold and thrust belt some new vitrinite reflectance data interpretation of existing geothermometry data combined with new vitrinite reflectance data within the framework of a detailed composite tectonic setting elucidates the evolution of structural sequencing of thrust sheets during the alleghanian event in the valley and ridge province in virginia that the pulaski thrust sheet preceded the saltville thrust sheet in the emplacement sequence and that both reached thermal maxima prior to or during respective emplacement may be inferred from vitrinite and other geothermometry data in contrast the narrows and st clair thrust sheets probably each attained their thermal maximum after emplacement new vitrinite reflectance data are consistent with cai and other temperature sensitive information heretofore ascertained in the valley and ridge province and support previously established maximum temperatures of ca 200 c for strata of the saltville thrust sheet as young as mississippian r max values from mississippian coals in the price formation of the sal

cathodoluminescence petrography of microstructural fabrics in deformed limestones abstract cathodoluminescence petrography is a valuable tool for the analysis of strain microfabrics in deformed limestones finely banded growth zones in calcite cements visible only in the cathodoluminescent mode act as very sensitive displacement markers they reveal the varied and heterogeneous nature of microstructures within the deformed calcite

meteoric calcite cementation diagenetic response to relative fall in sea level and effect on porosity and permeability las negras area southeastern spain abstract a dolomitized upper miocene carbonate system in southeast spain contains extensive upper and lower zones of calcite cementation that cut across the stratigraphy cement textures including isopachous and circumgranular which are consistent with phreatic zone cementation cements in the upper cemented zone are non luminescent whereas those in the lower cemented zone exhibit multiple bands of luminescent and non luminescent cements in the upper cemented zone isotopic data show two meteoric calcite lines mcl with mean 18 o at 5 1 and 5 8 vpdb whereas no clear mcl is defined in the lower cemented zone where mean 18 o for calcite cement is at 6 7 vpdb 13 c values in both cement zones are predominantly negative ranging from 10 to 2 vpdb suggestive of carbon from soil gas or decayed organics measurements of tm ice in primary fluid inclusions yield a mode of 0 0 c in both zones indicating calcite cementation from fresh water these two zones define the positions of two different paleo water tables that formed during a relative sea level fall and erosional downcutting during the plio pleistocene the upper cemented zone pre dated the lower cemented zone on the basis of known relative sea level history meteoric calcite cementation reduced porosity and permeability but measured values are inconsistent with simple filling of open pore space each texture boundstone grainstone packstone wackestone produces a different relationship between percent calcite cement and porosity permeability distribution of cements may be predictable on the basis of known sea level history and the effect of the cementation can be incorporated into subsurface geomodels by defining surfaces of rock boundaries that separate cemented zones from uncemented zones and applying texture specific relationships among cementation porosity and permeability

sedimentary and botanical factors influencing peat accumulation in the mississippi delta the holocene mississippi delta system consists of six cyclic deltaic packages associated peat deposits are planar and eutrophic and accumulate in areas abandoned by deltaic sedimentation peats that formed in large scale interlobe basins are the subject of this paper true peats 75 organic matter average 81 7 organic matter on a dry weight basis controls on quality and lateral distribution of peat deposits are botanical parent material the balance between subsidence and accretion rates detrital clastic influx and marine inundation botanical parent material inferred from 13 c signatures and microtome sections is always freshwater originating either in forested swamps or under herbaceous floating mats sphagnum spp is essentially absent in order for peat to accumulate rates of accretion and subsidence must balance long term subsidence rates in the study area increase from about 0 10 to 0 30 cm a 1 over a distance of 40 km from upper to lower delta plain subsurface accretion rates for true peats at a depth of 2 3 m however average about 0 05 cm a 1 renewed coarse detrital clastic influx terminates peat accumulation the largest quantities of true peat are found together with relatively large amounts of organic poor clastic sediments mostly clays delta lobe abandonment and consequent coastal erosion causes salt water intrusion into freshwater environments resulting saline marsh sediments have maximum organic matter contents of 35 in a transgressive setting these may overlie older high quality low ash freshwater peats leaching experiments reveal the presence of water soluble salts in both transgressed and fresh peats leaching reduces ash values by 1 5 to 1 3 suggesting that such losses may occur during early diagenesis it is possible that some commercial coal seams originated in analogous interlobe environments where leaching and possibly silica mobilization reduced the originally high ash contents to acceptable levels

origin of shallow burial cements in the late visean of the campine basin belgium abstract the diagenesis of late visean buildup and shelf carbonates from three boreholes in the campine basin northern belgium has been studied to determine the early diagenetic pore fluid evolution in the basin the earliest cementation occurred by isopachous fibrous and radiaxial fibrous calcites which precipitated in a marine environment under oxidizing conditions neomorphism during subsequent diagenesis altered these cements extensively precipitation of these first stage cements was followed by an episode of dissolution of carbonate components gastropods pelecypods and echinoderms after this episode non bright and dull luminescent stages successively developed in the shelf carbonates in response to the pore waters becoming progressively more anaerobic fluid inclusions and oxygen stable isotopes in the dull luminescent stage indicate that this cement formed in a meteoric fluid at relatively low temperatures 30 c and thus under burial conditions of a few hundred metres buildup carbonates from the shelf margin show a similar diagenetic evolution but dull luminescent cements appear to have precipitated at higher temperatures 45 c than in the shelf carbonates the meteoric waters responsible for the cementation are interpreted to have been sourced from near the brabant massif where dissolution of visean limestones could have taken place when the limestones to the north were already buried underneath namurian strata lateral gravity driven groundwater flow may be a very important type of fluid flow causing the cementation of shelf carbonates fracturing of the late visean strata started before the precipitation of the meteoric cements but was most intense just before and during the precipitation of the dull luminescent calcites this fracturing probably took place during the sudetic phase at the end of the visean and early namurian times

nature and origin of authigenic rhodochrosite and siderite from the paleozoic aquifer northeast mississippi u s a abstract botryoidal rhodochrosite siderite and pyrite are the principal authigenic phases in fractured chert comprising the paleozoic aquifer of northeast mississippi locally alternating bands of near end member composition rhodochrosite and siderite occur in veinlets up to 2 mm wide in the fractured chert carbon isotope data for the carbonates 13c values ranging from 9 6 to 11 3 pdb indicate that both inorganic and organic c were incorporated into the authigenic phases the paleozoic aquifer is hydraulically connected with the overlying upper cretaceous coastal plain strata groundwater in the coastal plain units is undersaturated with respect to calcite and has relatively high fe and mn contents the latter apparently caused by the dissolution of fe mn oxyhydroxides the natural downward recharge of these waters into the paleozoic aquifer causes calcite dissolution in limestone at the base of the chert which in turn leads to the precipitation of rhodochrosite and siderite additionally pyrite is precipitated as so4 reduction occurs in the same zone rhodochrosite has not been previously documented as an authigenic phase in a fresh water aquifer system suggesting that the geochemical environment necessary for its precipitation is not common the alternating bands of rhodochrosite and siderite observed in this study coupled with available thermodynamic data indicate that the groundwater a mn2 afe2 ratio must have fluctuated over time variations in the ratio are best explained by variable eh conditions at the site where fe mn oxyhydroxide dissolution occurs

remagnetization of the tonoloway formation and the helderberg group in the central appalachians testing the origin of syntilting magnetizations summary palaeomagnetic rock magnetic geochemical and strain studies of the helderberg group and the tonoloway formation in the valley and ridge province of west virginia were performed to test the origin of syntilting magnetizations and to test the mechanisms of chemical remanent magnetization crm acquisition the test for a connection between strain and remagnetization was performed by comparing the types and levels of strain with the magnetic properties between the generally coarse grained thickly bedded helderberg and the thinly bedded and finer grained tonoloway standard tilt tests as well as optimal differential untilting from two transects on the wills mountain anticline indicate that the helderberg contains a pervasive late syntilting 30 per cent untilting characteristic remanent magnetization chrm whereas the tonoloway contains a well defined early syntilting chrm 1 76 per cent untilting as well as a chrm 2 which is similar to the chrm in the helderberg the chrms reside in magnetite the rock magnetic results from the two units are similar although the helderberg samples have a finer apparent grain size perhaps as a result of magnetic hardening strain data reveal an apparent correlation between the syntilting magnetizations and types degrees of pressure solution strain pressure solution strain is higher in the helderberg and lower in the tonoloway strain may have caused rotation of magnetic minerals but it not clear that the differences in strain between the units were high enough to cause the rotations that are needed to explain the tilt test results several other viable hypotheses were also investigated one hypothesis is a strain enhanced chemical process that caused dissolution and precipitation of new magnetite in solution structures during folding a preliminary test of this hypothesis produced inconclusive results another hypothesis consistent with some of the characteristics of the magnetizations in the helderberg and tonoloway units is that the rocks were partially to completely remagnetized by a piezoremanent remagnetization process geochemical results show that origin of the remanence carrying grains cannot be attributed to orogenic type fluids but were probably caused by a burial diagenetic process

strain partitioning of deformation mechanisms in limestones examining the relationship of strain and anisotropy of magnetic susceptibility ams abstract in order to investigate the relationship between rock strain and anisotropy of magnetic susceptibility ams strain partitioning and ams analysis was conducted at 35 sites from two stratigraphically adjacent paleozoic limestone units in the patterson creek and wills mountain anticlines in the central appalachian orogen of west virginia in addition anisotropy of anhysteretic remanent magnetization aarm was conducted on selected samples to examine the role of preferentially oriented magnetite on the ams fabric strain is partitioned into bed normal shortening due to compaction solution strain 35 0 shortening bed parallel shortening due to tectonic solution strain 13 3 shortening calcite twinning strain 5 8 shortening and grain boundary sliding 26 7 shortening the ams fabrics in the rocks were found to be a result of a complex interaction between rock lithology deformation mechanisms and strain magnitude although all the rocks have experienced the same deformation conditions six different ams fabrics are exhibited each of the different ams fabrics is a composite fabric resulting from the overprinting of three components 1 an inherent primary depositional ams fabric that is attributed to preferentially oriented phyllosilicates in the rock matrix 2 a diagenetic and or compaction ams fabric formed during burial that is due to preferentially oriented phyllosilicates in solution structures and in the rock matrix and 3 a tectonic ams fabric that was imparted on the rocks by layer parallel shortening deformation prior to folding and is also attributed to preferentially oriented phyllosilicates in solution structures and in the rock matrix as well as twinning of ferroan calcite

systematic sequence scale controls on carbonate cementation in a siliciclastic sedimentary basin examples from upper cretaceous shallow marine deposits of utah and coloradao usa whilst the relationship between stratigraphic development and carbonate cementation within siliciclastic succession has been documented through a number case studies these studies have been generally restricted to observations upon individual sequences and or limited sub surface data in this paper long term 5 million years large scale 200 km stratigraphic controls on carbonate cementation patterns are documented from the upper cretaceous panther tongue member blackhawk formation and castlegate sandstone exposed in the book cliffs in utah and colorado usa together these comprise eight progradational wedges of sandstones which interfinger with the mancos shale deposited within the western interior seaway foreland basin petrographic analyses of ferroan dolomite cement bodies within these sandstone wedges show that the ferroan dolomite cements are all early relative to burial diagenesis within the host sandstones stable isotope analyses indicates that a significant meteoric component was present in precipitating fluids and this is consistent with the observation that cements are always present down dip of sequence boundaries and or leached whitecaps beneath coals in addition the lateral distribution of cement bodies increases consistently up succession from less than 5 km in extent in the older sequences to 30 km in extent in the youngest sequences these changes in distribution are in response to the increased progradation and increased and more aerially extensive sequence boundary development in younger sequences the implications of these data are that whilst localized spatial patterns of diagenesis and in particular carbonate cementation are predictable and controlled by the nature and presence of individual stratal surfaces systematic diagenetic alteration patterns are also present at the sedimentary basin scale and controlled by the nature of larger scale stratigraphic development and basin evolution this evolution may be driven by eustatic shifts or through tectonic or climatic driven base level shifts these observations allow an improved insight into the basin scale processes that control the macroscopic diagenetic properties of sedimentary successions and sub surface hydrocarbon reservoirs

influence of a major exposure surface on the development of microporous micritic limestones example of the upper mishrif formation cenomanian of the middle east microporous platform top limestones of the cenomanian mishrif formation offshore qatar were studied in order to investigate the diagenetic processes associated with the top mishrif subaerial unconformity and its influence on the development of microporosity in underlying carbonates petrographical and stable isotope results indicate that complex diagenetic changes occurred during subaerial exposure of the mishrif formation including pervasive dissolution and meteoric cementation as well as neomorphism of the micritic matrix micrites at the top of the mishrif formation are coarse i e 2 m sub rounded and very dull luminescent under cathodoluminescence in this uppermost part of the studied interval the limestone matrix first underwent dissolution of unstable grains in the vadose zone with subsequent precipitation of low magnesium calcite lmc overgrowths within an oxidising phreatic setting this process explains the poor luminescence of the micrite crystals and their relatively coarse crystallometry which results in the present day in relatively good reservoir properties 13c ratios within the microporous limestones are negative up to 4 v pdb due to the incorporation of isotopically light carbon derived from palaeosols which developed during exposure by contrast fine i e 2 m polyhedral and bright luminescent micritic matrix with higher 13c values up to 4 v pdb and lower permeability are dominant within the underlying carbonate interval these fine micrites are likely to be the product of early neomorphic recrystallisation in the lowermost part of the palaeo aquifer associated with the top mishrif unconformity and were precipitated from meteoric or mixed dysoxic waters which were slightly supersaturated with respect to calcite

the depositional and diagenetic evolution of the coastal ridges of northwestern egypt since the lower pliocene successive generations of coastal ridges made up of mixed oolitic and biogenic grainstones formed along the mediterranean coastline of egypt west of alexandria and have undergone progressive stages of diagenesis the ridges are complex structures resulting from several phases of deposition in which shallow marine components are partially and progressively reworked as subaerial and aeolian deposits diagenetic alteration resulted from increasing freshening of the pore waters in the ridges diagenesis began with marine phreatic cementation followed by freshwater phreatic cements aggrading recrystallization of microcrystalline calcite to microspar and minor dolomitization non marine hypersaline diagenesis soil profiles developed on the upper surfaces of the oldest dunes

cathodoluminescence in quaternary carbonate deposits abstract the cathodoluminescent oscillatory and sectoral growth zones common in crystals formed in ancient limestone successions in a variety of putative environments appear to be rare or absent from recent and pleistocene marine carbonate sequences the factors controlling cathodoluminescence and reasons for this disparity are examined the cathodoluminescent zones in the cements of ancient rocks have been interpreted as responses to variations in the redox potential of formative pore waters during crystal growth although similar cathodoluminescent behaviour is recorded from some deposits including travertines and quaternary speleothems formed in what are thought to have been strongly oxidizing environments the apparent absence of cathodoluminescence in the most recent and pleistocene marine deposits that presumably reflect deposition and diagenesis in environments that are also characteristically oxidized therefore seems anomalous the controlling influences on cathodoluminescence are reviewed together with evidence relating to observations of pleistocene marine deposits and likely conditions of formation but where it is present the mechanism s for its development remain elusive

fabric retentive non luminescent brachiopods as indicators of original delta 13 c and delta 18 o composition a test abstract fabric retentive non luminescent helderbergian brachiopods from open marine deposits analogous to brachiopods used in other studies to determine paleo oceanic stable isotopic composition exhibit depleted 13c values immediately below the erosional contact and more enriched values of 13c at greater stratigraphic distances below this surface in addition 13c values of these brachiopods display a marked shift across the erosional surface and values above it are essentially the same as those 30 m or more beneath the surface complementing these trends whole rock analyses metastable high agnesian calcite constituents echinoderms as well as early equant calcite cement also exhibit an increased depletion trend in 13c values as the helderberg oriskany contact is approached from below fabric retentive luminescent as well as obviously recrystallized brachiopods display stable isotopic values similar to those of fabric retentive non luminescent brachiopods thus best preserved fabric retentive non luminescent brachiopods exhibit values similar to obviously recrystallized brachiopods diagenetically altered metastable constituents echinoderms stable early meteoric cements and whole rock samples helderberg strata of new york state were affected by early meteoric diagenesis during a siegenian exposure event prior to deposition of the overlying marine oriskany sandstone meteoric diagenesis during this exposure gave rise to a characteristic carbon isotopic depletion trend approaching the helderberg oriskany contact all of the skeletal constituents underwent isotopic exchange during early diagenetic exposure to meteoric waters consequently the practice of using fabric retentive non luminescent brachiopods as indicators of their original chemistry and by extension paleo oceanic chemistry is questioned

subaqueous shrinkage cracks in the sheepbed mudstone implications for early fluid diagenesis gale crater mars the sheepbed mudstone yellowknife bay formation gale crater represents an ancient lakebed now exhumed and exposed on the martian surface the mudstone has four diagenetic textures including a suite of early diagenetic nodules hollow nodules and raised ridges and later diagenetic light toned veins that crosscut those features in this study we describe the distribution and characteristics of the raised ridges a network of short spindle shaped cracks that crosscut bedding do not form polygonal networks and contain two to four layers of isopachous erosion resistant cement the cracks have a clustered distribution within the sheepbed member and transition laterally into concentrations of nodules and hollow nodules suggesting that these features formed penecontemporaneously because of the erosion resistant nature of the crack fills their three dimensional structure can be observed cracks that transition from subvertical to subhorizontal orientations suggest that the cracks formed within the sediment rather than at the surface this observation and comparison to terrestrial analogs indicate that these are syneresis cracks cracks that formed subaqueously syneresis cracks form by salinity changes that cause sediment contraction mechanical shaking of sediment or gas production within the sediment examination of diagenetic features within the sheepbed mudstone favors a gas production mechanism which has been shown to create a variety of diagenetic morphologies comparable to the raised ridges and hollow nodules the crack morphology and the isopachous layered cement fill show that the cracks were filled in the phreatic zone and that the sheepbed mudstone remained fluid saturated after deposition and through early burial and lithification

isotope signatures associated with early meteoric diagenesis the environments in which carbonate diagenesis proceeds have been documented in previous studies of holocene and late pleistocene sediments and limestones on barbados west indies variations in the carbon and oxygen isotopic composition of limestones produced during early freshwater diagenesis have been observed in this study to occur in specific patterns six potentially useful patterns emerge when one views stable isotope data within a stratigraphic framework 1 the subaerial exposure surface is characterized by strongly 12c enriched limestones 13c compositions of underlying limestones grow progressively heavier with increasing depth 2 the subaerial exposure surface may also be marked by slight 18o enrichment 3 an abrupt shift in 18o values may differentiate sediments above the exposure surface from those below 4 sediments altered in the marine meteoric mixing zone may be characterized by positive covariance between their 18o and 13c compositions 5 the vadose phreatic boundary may be marked by a sharp increase in 13c values in the seaward portions of a fresh groundwater system and 6 samples contemporaneously altered in a single fresh groundwater system within an areally restricted region should display a narrow range of 18o and a wide range of 13c compositions analysis of samples from five palaeozoic and mesozoic formations which contained petrographic evidence of early freshwater diagenesis showed that isotope patterns similar to those observed in barbados limestones have been preserved in rocks as old as mississippian these isotope patterns could prove to be useful for identifying diagenetically induced porosity trends in carbonate rocks they might be used to identify limestones diagenetically altered in meteoric environments to identify mixing zone cements and dolomites and to trace the regional and vertical distributions of early meteoric groundwater systems in ancient carbonate formations

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

the ore forming process of the maoping pb zn deposit northeastern yunnan china constraints from cathodoluminescence cl petrography of hydrothermal dolomite abstract most mississippi valley type mvt pb zn deposits have a common host rock lithofacies sequence known as the hydrothermal dolomite htd facies however the spatial and temporal relationships between dolomitization and pb zn mineralization remain unclear in this paper we report a detailed cathodoluminescence cl study on the htd facies of the no ii orebody in the maoping pb zn deposit in northeastern yunnan province southwestern china the results indicate that the dolomite in the carboniferous limestone is part of a spectrum of hydrothermal mineral deposits and the diagenesis and mineralization processes in which the acidity of fluids played an important role can be divided into six steps a dolomite prototype formed during the first and second steps in the third step acid fluids produced by sulfide precipitation interacted with the dolomite prototype resulting in dissolution and alteration thus forming cavities the fluid that formed during the first three steps flowed along open tensional faults dolomitization in the first step covered most of the region of later mineralization and gradually extended toward the center of the orebody in the fourth and fifth steps vugs and fractures were the main pathways for fluids with only a small volume of fluid in the fifth step dedolomitization occurred in the sixth step and vugs were filled mostly by calcite however the fact that voids are present in only a small number of vugs is indicative of fluid exhaustion dolomite with growth zoning is characteristic of precipitation by ore forming fluids accompanied by pulsed filling there is no significant correlation between vugs and the locus of mineralization rather the vugs indicate that the ore forming fluids had passed through them and the locus of massive ore formation was the center of the migrating fluid body the saddle dolomite was deposited during pauses in sulfide precipitation cathodoluminescent zonation of htd can be used as an aid in understanding the ore forming process and for mineral prospecting in the maoping deposit and other mvt deposits

diagenesis and crystal caskets echinoderm mg calcite transformation dry canyon new mexico u s a abstract the stereom of living echinus esculentus skeletons is composed of mg calcite interambulacral plates 6 mole mgco3 spines 4 mole mgco3 and is compared with the stereom of pennsylvanian echinoderms from the holder formation dry canyon some dry canyon echinoderms had their pore system 55 volume filled by magnesian ferroan calcite cement before the stereom transformed type 1 into small 20 m diameter in ossicles that underwent type 2 transformation indicates ionic transfer over tens of microns the margins of some type 2 ossicles set in a clay matrix are fe rich and contain barite crystals the fe2 and ba2 were transferred from the surrounding clay hundreds of microns into these ossicles type 1 transformation was catalyzed by internal water from the stereom the presence of a ferroan calcite crystal casket confined alteration to the skeletal mg calcite type 2 transformation involved both stereom and pore filling mg calcite cement the unstable dolomite crystals formed by both transformation processes were protected from further change by the crystal caskets in which they are encased the transformation of dry canyon echinoderms was probably triggered or accelerated by rising temperature caused by burial the bulk composition of stereom that has undergone type 1 transformation can be used as a proxy for the original major element chemistry of echinoderm skeletons minor amounts of mg calcite cement that occur in dry canon type 1 ossicles indicate the chemistry of porewater close to the sediment water interface fossil echinoderms are preserved in many ways and it is suggested that their textural variations can be used to map the distribution of diagenetic reactions the stereom is destroyed in most fossil echinoderms and even in those where the stereom shape is preserved its internal structure is changed echinoderm mg calcite does not transform with perfect textural preservation

vein cements and the geochemical evolution of subsurface fluids in the visean of the campine basin poederlee borehole belgium upper visean reef mound carbonates from the campine basin northern belgium have a complex diagenetic history which reflects the evolution of subsurface fluids in the basin primary pores became cemented by marine fibrous stage a and shallow burial stage b c and d scalenohedral and blocky calcites tectonic deformation then created successive networks of fractures which acted as conduits for aggressive fluids and which were subsequently cemented by six stages of carbonate cement fluid inclusion evidence suggests that the first cloudy columnar calcite cements in fractures stage e formed at or below 50 c these calcites have a 18 o of 13 3 pdb and they are interpreted to have grown in meteoric waters 18 o 6 7 to 7 4 smow after a further period of fracturing and dissolution white blocky calcites stage f 18 o 9 1 pdb precipitated around 60 c from fluids with a 18 o 0 9 smow later fractures contain brown columnar and blocky calcites stage g with a 18 o of 12 1 pdb and a minimum homogenization temperature t h of c 90 c giving a pressure corrected precipitation temperature of 95 c an oxygen isotopic composi tion of 0 6 smow is indicated for the fluid granular to bladed ferroan calcites stage h blocky ferroan calcites stage i 18 o 9 8 pdb and t h 128 c and saddle dolomites stage j 18 o 10 0 pdb and t h 147 c are the last diagenetic carbonate phases methane has been detected in the vapour phase of the fluid inclusions in the blocky ferroan calcites the ferroan calcites and the saddle dolomites are interpreted to have formed respectively at 150 c and 200 c in 18 o rich fluids 7 9 smow and 10 9 smow the stable isotope values of the fluids indicate an abrupt change from a meteoric composition towards marine derived compactional fluids and basinal brines enrichment in 18 o is mainly attributed to the dewatering dehydration of clay minerals in the adjacent upper carboniferous strata the paragenetic sequence of cements described here is comparable to that encountered in other lower carboniferous limestones where there is not such an extensive data base of fluid inclusion and stable isotopic data it is suggested that the large change in the pore fluid composition documented here might be a common subsurface process and that the interpretation of diagenetic temperatures from isotopic data alone can be very misleading

sequential vein growth with fault displacement an example from the austin chalk formation texas to determine the opening and precipitation history and characteristics of vein forming fluids analyses of oxygen and carbon isotopes and trace elements were carried out on multilayered crack seal calcite veins in the austin chalk formation near san antonio texas the veins developed within the normal fault zones possessing unique chemical and textural characteristics which indicate sequential vein development they are composed of alternating millimeter to submillimeter thick calcite veinlets and host lithons occasionally crosscut by coarse equant grained secondary calcite veins regular changes in 18o e g 2 6 to 5 6 pee dee belemnite pdb of the calcite veinlets along the length of veins suggest that the individual calcite veinlets were sequentially developed a systematic 18o decrease in the vein opening direction primarily resulted from a continuous increase in temperature of the ascending fluids delivered to the austin chalk relatively constant 13c approximately 1 4 0 4 pdb for the multilayered veins and most secondary veins indicates that the composition of fluids from which the calcite veins precipitated was initially buffered by the bulk chalk there is no spatial variation in trace element composition of the calcite veinlets along the length of veins low sr concentrations in both calcite veinlets and secondary veins relative to those of the host chalk reflect a low partition coefficient of sr in calcite during vein formation normal faults in the cretaceous austin chalk were conduits to upwardly mobile vein forming fluids

tectonic setting of outer trench slope volcanism pillow basalt and limestone in the taconian orogen of eastern new york the only pillow basalt in synorogenic sedimentary rocks at the exterior margin of the taconic orogen in eastern north america is at stark s knob in eastern new york earlier reported as extrusive into allochthonous ordovician slope and rise facies this small lens ca 125 m long 39 m thick is a fault bounded block in upper ordovician melange under the taconian frontal thrust its n morb normal mid ocean ridge basalt basalt geochemistry and spinel composition are characteristic of oceanic ridge settings at a water depth of 2 km or more abundant limestone lenses on pillows and lava shelves within pillows yielded a middle late ordovician gastropod the limestones are reconciled with this extrusion depth and with limited early paleozoic pelagic carbonate production by lime mud transport from the laurentian platform or abiotic carbonate precipitation with sea water heating during basalt extrusion a genetic relationship between the parautochthonous stark s knob basalts and the allochthonous jonestown volcanics in slope and rise facies of the hamburg klippe eastern pennsylvania is likely both are ordovician morb basalts that reflect volcanism on the subducting outer trench slope prior to the taconic arc laurentia collision taconic orogenesis may have led to basalt production on the subducting plate by 1 the setting up of orogen parallel predominantly strike slip motion on the subducting slab with morb basalt generated at offsets in a setting analogous to the gulf of california or 2 development of faults in a flexure induced extensional regime by either process mafic volcanism appears to be a rare but tectonically significant process on outer trench slopes as continental margins or oceanic plates enter subduction zones resume l unique basalte a coussinet dans les roches sedimentaires syntectoniques a la bordure exterieure de

cathodoluminescence in calcite cements new insights on pb and zn sensitizing mn activation and fe quenching at low trace element concentrations abstract the role of pb and zn as sensitizers of cathodolumines cl in sedimentary carbonates is still speculative as is the relationship between mn fe and cl intensity at low concentrations of mn and fe we address these questions using calcite cements from the lower oligocene suwannee formation of florida which have mean mn pb zn and fe concentrations less than 45 ppm only a small fraction of these cements exhibit visible cl but when present the luminescence is orange to reddish orange trace element contents were measured using a synchrotron x ray fluorescence microprobe and cl intensity was approximated by spot metered photomicrograph exposure times based on mn and fe contents four cement populations can be defined group i is luminescent calcite with mn 25 ppm and molar mn fe 1 8 group ii contains both luminescent and nonluminescent cements all with mn 25 ppm molar mn fe 25 ppm molar mn fe 100 ppm group iv is nonluminescent calcite with mn 25 ppm and variable fe these groupings and statistical analyses of the data mann whitney tests and factor analyses confirm that 25 ppm mn is necessary to initiate visible cl and that mn is the sole control on cl intensity when mn and fe contents are very low 100 ppm the data also establish 100 ppm fe as a new minimum for the quenching of cl when mn is below 200 ppm there is no evidence in our data that pb or zn concentrations in tens of parts per million either activate or sensitize cl the mann whitney test shows that there is no statistical evidence at the 95 confidence level to indicate that luminescent and nonluminescent cements reflect different populations of pb and zn except for group ii cements that have more pb in nonluminescent cements than in otherwise chemically equivalent luminescent cements factor analysis shows that cl intensity in all luminescent cements is unrelated to pb or zn concentrations given that pb or zn sensitizing is lacking in low mn calcites like the suwannee cements it is hard to imagine how these elements could play a significant role in calcites containing even more mn as is typical of most sedimentary carbonates

new data on relative stability of carbonate minerals implications for diagenesis and cementation abstract redetermination of magnesian calcite solubilities indicates that the constants currently in use overestimate their solubility by 300 the relative chemical stability of aragonite and magnesian calcite plays a fundamental role in controlling the behavior of these metastable minerals during the diagenetic transformation to stable low magnesian calcite limestone via dissolution and reprecipitation reactions temporal evolution of rock and groundwater geochemistry as well as development of limestone fabrics depends on the relative chemical reactivity of carbonate grains during dissolution as well as their receptiveness to cementation in both marine and meteoric diagenetic systems following the laboratory determination of magnesian calcite stabilities by l n plummer and f t mackenzie in 1974 it has been accepted that magnesian calcites containing greater than 7 5 mole mgco3 are more unstable than aragonite and become progressively more unstable with increasing mole mgco3 for example 18 mole magnesian calcite was thought to be seven times more soluble than aragonite the solubilities of several biogenic magnesian calcites echinoid 12 mole mgco3 red algae 18 mole mgco3 have been redetermined the new data for the stability of the magnesian calcites indicate that previously determined solubilities for magnesian calcites were significant overestimates using the new stability constants from the present study 12 5 mole magnesian calcite is equivalent in stability to aragonite and an 18 mole magnesian calcite is only twice as soluble as aragonite thus the solubility of magnesian calcites must be reduced by 300 from existing values in the literature it is believed that the discrepancy with previous work on magnesian calcite stability is due to sample preparation procedure in the present study care was taken both to eliminate submicron fine particles by ultrasonic cleaning and to reduce the crystal strain induced by crushing experiments show samples not so treated yield larger magnesian calcite solubilities because of enhanced reactivity and solubility of adhering submicron particles and strained crystal surfaces the new stability relations have significant implications for the sequence and pathways of diagenetic reactions mineralogic stability and available surface area are the main chemical controls on the rate of diagenetic alteration given the reduced solubility offset between aragonite and magnesian calcite aragonites having more complex microstructures and hence more reactive surface area dissolve more rapidly than coexisting high magnesian calcites the lower solubilities for the magnesian calcites sharply reduce the potential chemical region in ground waters where the incongruent dissolution of magnesian calcite is possible since the specific nature of dissolution is closely related to the rate and style of cementation these findings prompt the reevaluation of possible pathways which may be taken during early diagenesis more work remains to be done to quantify the precise effects of mineralogy and grain microstructure into a complete predictive model for patterns of cementation but it is clear that the reduced mineralogic stability offset between aragonite and magnesian calcite makes microstructural detail and ground water geochemistry prime controls over the reaction paths followed during the early diagenetic stabilization of shallow water carbonate deposits end of article last page 566

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

chemostratigraphic and u pb geochronologic constraints on carbon cycling across the silurian devonian boundary abstract the devonian period hosts extraordinary changes to earth s biosphere land plants began their rise to prominence with early vascular vegetation beginning its colonization of near shore environments in the latest silurian across the silurian devonian pridoli lochkovian transition carbon isotope analyses of bulk marine carbonates c carb 13 from laurentian and baltic successions reveal a positive c carb 13 shift known as the klonk event values reach 5 8 making it one of the largest carbon isotope excursions in the phanerozoic assigning rates and durations to these significant events requires a robust precise devonian time scale here we present 675 micritic matrix and 357 fossil specific c carb 13 analyses from the lower devonian helderberg group of new york and west virginia that exhibit the very positive c carb 13 values observed in other silurian devonian basins this chemostratigraphic dataset is coupled with 66 id tims u pb dates on single zircons from six ash falls intercalated within helderberg sediments including dates on the stratigraphically lowest devonian ashes yet developed in this work we a demonstrate that matrix and fossil specific c carb 13 values track one another closely in the helderberg group b estimate the silurian devonian boundary age in new york to be 421 3 1 2 ma 2 including decay constant uncertainties and c calculate the time required to evolve from baseline to peak c carb 13 values at the onset of the klonk event to be 1 00 0 25 myr under these constraints a steady state perturbation to the global carbon cycle can explain the observed excursion with modern fluxes as long as dic concentration in the devonian ocean remained below 2 the modern value therefore potential drivers such as enhanced burial of organic carbon need not rely on anomalously high total fluxes of carbon to explain the klonk event