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

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

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

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

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

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 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

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

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

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

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

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

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

an emerging technique multi ice core multi parameter correlations with antarctic sea ice extent using results stemming from the international trans antarctic scientific expedition itase ice core array plus data from ice cores from the south pole and siple dome we investigate the use of sodium na non sea salt sulfate nssso4 2 and methylsulfonate ms as proxies for antarctic sea ice extent sie maximum and mean annual chemistry concentrations for these three species correlate significantly with maximum mean and minimum annual sie offering more information and clarification than single ice core and single species approaches significant correlations greater than 90 exist between na and maximum sie nssso4 2 with minimum and mean sie and ms with mean sie correlations with sie within large geographic regions are in the same direction for all ice core sites for na and nssso4 2 but not ms all ice cores display an sie correlation with nssso4 2 and ms but not all correlate with na this multi core multi parameter study provides the initial step in determining which chemical species can be used reliably and in which regions as a building block for embedding other ice core records once established the resulting temporal and spatial matrix can be used to relate ice extents atmospheric patterns biological productivity and site conditions

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

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

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

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

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

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