use of fracture filling mineral assemblages for characterizing water rock interactions during exhumation of an accretionary complex an example from the shimanto belt southern kyushu japan abstract various fracture filling minerals and secondary minerals in fracture walls were formed by fluid rock interaction during the exhumation of the palaeogene shimanto belt of kyushu japan which is located in an accretionary complex each mineral formed under favourable geological conditions and can be used to estimate the conditions of accretion and formation of the related rock sequences petrographic observations mineralogical and geochemical analyses were made on fracture filling minerals and secondary minerals from boreholes of ca 140 m depth drilled in the shimanto belt results reveal that the secondary minerals were formed in three major stages distinguished by the sequential textural relationships of the minerals and the interpreted environment of mineral formation filling mineral assemblages show that the studied rock formation has been subducted to a depth of several km and the temperature reached was ca 200 300 c after the subduction the rock formation was uplifted and surface acidic water penetrated up to 80 m beneath the present ground surface the acid water dissolved calcite fracture filling minerals to form the present groundwater flow paths which allowed recent wall rock alteration to occur the results shown here imply that filling mineral assemblages can be an effective tool to evaluate the environmental changes during exhumation of an accretionary complex

dolomitization a critical view of some current views the principal models in vogue today for dolomitization are the mixing zone and the sabkha models despite the wide acceptance of these models there has been little critical assessment of their validity such an assessment is the objective of the present paper a close look at the dorag mixing zone model badiozamani 1973 reveals several serious weaknesses 1 the range of freshwater seawater mixtures that meet the dorag requirement for dolomitization shrinks to a small window if the geologically more realistic disordered dolomite is used in the calculations instead of the ordered dolomite on which the model is based 2 in none of the known modern coastal mixing zones in limestone or lime sediments has replacement by dolomite been observed 3 in a number of dolomites interpreted to be of mixing zone origin dolomite has precipitated without dissolution of the calcite substrate evidence that negates the fundamental premise of the dorag model in addition to these weaknesses isotope and trace element data used to identify mixed water dolomite are inadmissible because 1 isotopic fractionation factors for dolomites remain unresolved 2 isotope values uncorrected for temperature for a host of dolomites interpreted to be of different origins overlap 3 nonisomorphous trace elements such as na in dolomite cannot on theoretical grounds be relied on to identify dolomitizing fluids similar overall objections can be brought against folk and land9s 1975 schizohaline version of mixing zone dolomitization in summary mixing zone models have such weak underpinnings that they should be questioned as viable explanations for massive dolomitization contemporaneous dolomite formation in modern sabkhas is well documented but the important question of whether the mechanism of dolomite formation is replacement or direct precipitation remains to be resolved in the well studied sabkha at abu dhabi brine chemistry changes have been used as evidence of a replacement origin for the dolomite but it is shown here that this evidence is far from unequivocal an alternative origin of direct precipitation of dolomite is offered an origin in keeping with dolomite precipitation known from other modern sabkhas and saline lakes and in line with our laboratory experience with dolomite synthesis moreover it is suggested here that in general contemporaneous dolomite will form at low temperatures only by direct precipitation a mechanism that requires special conditions of highly super saturated waters of high mg ca ratio and elevated co 3 hco 3 concentrations this may explain why modern dolomite is mainly restricted to evaporitic environments a bias not shared by ancient dolomites in contrast replacement dolomite appears to require at low temperatures long reaction times or 10 4 yr a requirement that is mainly to be met in large regional groundwater flow systems both marine and nonmarine a third dolomitization model considered here is that of baker and kastner 1981 based on the experimental finding that sulfate ions inhibit or retard dolomitization a number of modern sedimentary dolomites are not in accord with this model in that they are forming from brines with large sulfate concentrations 2 to 70 times that of seawater thus the baker kastner model should be held in abeyance until these serious contradictions are resolved the current emphasis on mixing zone and sabkha dolomitization has diverted attention from other promising avenues of approach to the dolomite problem four of these avenues each of which deemphasizes the special water approach are briefly addressed and are as follows 1 influence of temperature and time 2 mass transfer processes 3 burial diagenesis of epigenetic dolomites 4 fluid inclusion studies at elevated temperatures the dolomite problem essentially disappears ordered dolomite can be made in the laboratory in days at 100 degrees c what is more at temperatures above 60 degrees c ca rich waters become dolomitizing fluids which makes most natural subsurface waters capable of dolomitization at low temperatures time may be the key element so that seawater will become a major dolomitizing fluid only where stable circulation systems such as kohout convention can drive seawater through carbonate platforms for many thousands to millions of years this paper has tried to show that currently favored models of dolomitization carry serious uncertainties enough to warn us to look more critically at the validity of these models and also it is hoped enough to spur new efforts to find new solutions to the problems of dolomitization

fluid inclusion technique for determining maximum temperature in calcite and its comparison to the vitrinite reflectance geothermometer theory laboratory experiments and empirical observation suggest that many aqueous fluid inclusions in calcite reequilibrate during overheating and therefore some homogenization temperatures t h record a temperature close to the maximum reached by the rock this characteristic suggests that aqueous fluid inclusions in calcite can be used to establish maximum temperature t peak to test this hypothesis we have compiled fluid inclusion t peak mean random vitrinite reflectance r m and present day t peak from 46 diverse geologic systems that have been at t peak from 10 4 to 10 6 yr present t peak ranged from 65 to 345 c t h modes and means ranged from 59 to 350 c and r m data ranged from 0 4 to 4 6 spanning the temperature and thermal maturity range associated with burial diagenesis hydrothermal alteration and low grade metamorphism plots of t h and t peak data for systems thought to be currently at maximum temperature demonstrate close agreement between t h and present t peak in sedimentary basins although caution should be applied the relation suggests that t h of aqueous fluid inclusions in calcite may be a useful measure of maximum temperature this study also compares t h to mean random vitrinite reflectance r m to offer further support for the use of t h as a measure of t peak and to provide a better understanding of r m t h correlates well with r m and results in a curve similar to r m vs t peak calibrations determined by other workers the strong correlation correlation coefficient r 0 93 between t peak and r m in these systems suggests that maximum temperature is the major control on thermal maturation

fault whispers transmission distortions on prestack seismic reflection data transmission distortions are observed on prestack seismic data at two locations in the gulf of mexico these distortions produce anomalous amplitude versus offset avo signatures the locations of the distortion zones are determined using acquisition geometry and ray tracing no obvious reflection events such as shallow gas zones are observed at the predicted locations of the distortion zones instead the distortion zones correlate with buried faults and unconformities it is postulated that the distortions are produced by velocity changes across buried faults and unconformities the distortions result from an interference pattern resulting from seismic waves arriving from different sides of the faults a simple model is developed to explain many of the characteristics of the distortion pattern

structurally controlled hydrothermal alteration of carbonate reservoirs introduction langhorne taury smith heads the reservoir characterization group at the new york state museum he holds a b s degree from temple university a ph d from virginia tech and did postdoctoral work at the university of miami he also worked for chevron as a development geologist and current research interests are focused on carbonate reservoir characterization and hydrothermal alteration of carbonate reservoirs graham davies received his b sc honors degree and ph d from the university of western australia his doctoral thesis was on modern carbonates in shark bay western australia published in aapg memoir 13 after a postdoctoral fellowship with james lee wilson at rice university he joined the geological survey of canada gsc in calgary after 7 years with the gsc graham co founded and became principal owner of agat applied geoscience and technology consultants laboratories since 1983 he has operated through gdgc ltd he has published about 70 papers on the geology of australia and canada and has authored or coauthored more than 600 consulting reports graham received the cspg douglas medal in 2002 for his work on arctic paleozoic carbonates and evaporites and on the triassic and other aspects of western canadian geology his principal current interest is in hydrothermal dolomites this issue of the aapg bulletin is dedicated to structurally controlled hydrothermal alteration of carbonate rocks and reservoirs this type of alteration covers the spectrum of carbonate hosted sulfide ore deposits to hydrothermal dolomite to leached limestones and in many cases has a major impact on reservoir type geometry quality and distribution hydrothermal dolomite htd reservoirs have a long history of exploration and development in north america extending from the earliest discoveries in the ordovician lima indiana and other trends of the northeastern united states hurley and budros 1990 wickstrom et al 1992 and continuing today with the trenton black river play

genetic and temporal relations between formation waters and biogenic methane upper devonian antrim shale michigan basin usa abstract controversy remains regarding how well geochemical criteria can distinguish microbial from thermogenic methane natural gas in most conventional deposits has migrated from a source rock to a reservoir rarely remaining associated with the original or cogenetic formation waters we investigated an unusual gas reservoir the late devonian antrim shale in which large volumes of variably saline water are coproduced with gas the antrim shale is organic rich of relatively low thermal maturity extensively fractured and is both source and reservoir for methane that is generated dominantly by microbial activity this hydrogeologic setting permits integration of chemical and isotopic compositions of coproduced water and gas providing a unique opportunity to characterize methane generating mechanisms the well developed fracture network provides a conduit for gas and water mass transport within the antrim shale and allows invasion of meteoric water from overlying aquifers in the glacial drift steep regional concentration gradients in chemical and isotopic data are observed for formation waters and gases dilute waters grade into dense brines 300 000 ppm over lateral distances of less than 30 km radiogenic 14c and 3h and stable isotope 18o and d analyses of shallow antrim shale formation waters and glacial drift groundwaters indicate recharge times from modern to 20 000 yr bp carbon isotope compositions of methane from antrim shale wells are typical of the established range for thermogenic or mixed gas 13c 47 to 56 however the unusually high 13c values of co2 coproduced with methane 22 and dissolved inorganic carbon dic in formation waters 28 require bacterial mediation the d values of methane and coproduced formation water provide the strongest evidence of bacterial methanogenesis methane ethane propane ratios and 13c values for ethane indicate 1 the presence of a thermogenic gas component that increases basinward and 2 progressive bacterial oxidation of ethane as the antrim shale subcrop is approached multiple episodes of pleistocene glaciation over northern michigan appear critical to the development of these gas deposits loading of thick ice sheets may have provided hydraulic head that enhanced dilation of preexisting fractures and influx of meteoric water the physical erosion cycle of repeated glacial advances and retreats exhumed the antrim shale around the northern margin of the michigan basin subjecting it to near surface physiochemical and biochemical processes the chemical and hydrologic relations demonstrated in the antrim shale reservoir suggest a dynamic connection between pleistocene glacial history of the midcontinent region and development of recoverable microbially generated natural gas reserves

fractured hydrothermal dolomite reservoirs in the devonian dundee formation of the central michigan basin the middle devonian dundee formation is the most prolific oil producing unit in the michigan basin with more than 375 million bbl of oil produced to date reservoir types in the dundee formation can be fracture controlled or facies controlled and each type may have been diagenetically modified although fracture controlled reservoirs produce more oil than facies controlled reservoirs little is known about the process by which they were formed and diagenetically modified in parts of the dundee preexisting sedimentary fabrics have been strongly overprinted by medium to coarse grained dolomite dolomitized intervals contain planar and saddle dolomite with minor calcite anhydrite pyrite and uncommon fluorite fluid inclusion analyses of two phase aqueous inclusions in dolomite and calcite suggest that some water rock interaction in these rocks occurred at temperatures as high as 120 150c in the presence of dense na ca mg cl brines these data in conjunction with published organic maturity data and burial reconstructions are not easily explained by a long term burial model and have important implications for the thermal history of the michigan basin the data are best explained by a model involving short duration transport of fluids and heat from deeper parts of the basin along major fault and fracture zones connected to structures in the precambrian basement these data give new insight into the hydrothermal processes responsible for the formation of these reservoirs

hydrothermal dolomite reservoir in the precambrian dengying formation of central sichuan basin southwestern china abstract the precambrian dengying formation is maximum buried carbonate reservoir in the sichuan basin reservoir types are thought to be dominated by sedimentary facies or karst controlled precursory sedimentary fabrics have been intensively superimposed by medium to coarse grained dolomite in most areas dolomitized intervals contain planar and saddle dolomite quartz and few hydrothermal replacive minerals fluid inclusion analyses of dolomite suggests that rocks are formed at temperatures ranging from 132 6 c to 218 7 c in the presence of dense brines while the dolomite phases are demonstrated by negative 18 o and 13 c vpdb values strontium isotopes enriched in 87 sr and the fluid source could support the conclusion the dolomites of the dengying formation in central sichuan basin that formed around basement rooted wrench faults in turn mainly oriented towards the north south and east west strike slip faults are detectable lastly the grabens take the form of negative flower structures the result of an intra cratonic rift that took place during the sinian and early cambrian period through tensional faulting our primary contention is that basement fault which resulted in the magmatic or deep clastic fluids migration was key for the formation of the obvious high temperature coarse dolomite and saddle dolomite replacement subsequently hot fluids that circulated within the matrix dolomite were aided by fractures or vugs and 1 leached into the dolomite producing vugs and pores 2 precipitated saddle dolomite and 3 led to hydrofracturing fractures enlargement and further brecciation the dolomite eventually formed porous hydrocarbon reservoirs through diagenesis this model better illustrates how fluids that originated from deep basin migration along strike slip transfer faults and fractures flowed out to structures in precambrian basement rooted faults inheriting the rift in the cambrian the data involved offers a fresh perspective pertinent to deep hydrocarbon exploration of dolomite reservoirs in southwestern china

geology of the devonian dundee reservoir west branch field michigan 1 west branch field located near the center of the michigan basin is a low relief asymmetric northwest southeast trending anticline since 1934 the dundee reservoir middle devonian dundee and rogers city limestones has produced more than 12 million bbl 2 million m 3 of oil from this field dundee sediments were deposited on a carbonate ramp that dipped gently to the northwest core studies and log correlations show that the reservoir is dominated by normal marine somewhat lenticular beds of skeletal peloidal grainstones and packstones crinoids are very abundant comprising nearly 100 of the grains in some intervals the uppermost dundee consists of micritic carbonates with a restricted faunal assemblage the dundee rogers city contact is a disconformity that ppears to be a pyritized extensively bioeroded hardground the overlying rogers city limestone is a dark nodular wackestone with rare packstone interbeds trace amounts of dolomite are common throughout the field however pervasive dolomite is common only in the top 10 15 ft 3 4 5 m of the dundee this dolomite generally has high porosity 5 15 but very low permeability 0 1 0 md primary interparticle porosity in limestones accounts for most of the reservoir porosity although previous workers have concluded that fracture controlled dolomites are responsible for porosity and permeability in this field we conclude that such dolomites are not important we also conclude that at west branch field there is no evidence for reefal growth over a pre existing fold as has been reported in other dundee rogers city reservoirs

thermal subsidence and generation of hydrocarbons in michigan basin temperature histories for selected stratigraphic horizons in the michigan basin are computed from three dimensional continually filled subsidence models mechanical evolution of the michigan basin is modeled as flexure of the lithosphere caused by thermal contraction results are compatible with the subsidence record of the sediments and free air gravity anomalies paleotemperature is determined from excess temperature due to the thermal anomaly plus burial temperature predicted from subsidence curves for an equilibrium temperature gradient of 22 c km 1 2 f 100 ft surface temperature of 20 c 68 f and an equilibrium surface heat flow of 1 1 hfu excess temperature paleotemperature and surface heat flow do not exceed 15 c 27 f 110 76 c 230 f and 2 5 hfu respectively these estimates are consistent with upper limits set by paleomagnetic studies the low value for excess temperature is caused by concentration of the thermal anomaly below 15 km 9 mi in agreement with gravity results the great depth of the thermal anomaly can explain the lack of evidence for an initial heating event prior to subsidence once the thermal history of the sediments is specified the oil potential of the basin can be determined from laboratory derived kinetic equations for degradation of kerogen to petroleum for the michigan basin predicted temperature conditions are sufficient for source rock 25 conversion of type i and type ii kerogen only in ordovician and older rocks in the southern peninsula of michigan by this model petroleum found in rocks younger than ordovician would have had to migrate upward from the older rocks geochemical studies of dundee devonian and trenton ordovician crude oils in michigan are compatible with this interpretation niagaran silurian crude oils appear to come from a different source than the dundee and trenton oils if their source is silurian rocks on the flanks of the basin either the scheme used to calculate maturity is not applicable to these oils or the temperature of the source rocks was significantly higher in the past than today more likely the model underestimates the depth of burial of silurian source rocks near the center of the basin especially in local regions of downfaulting

thermal history of michigan basin and southern canadian shield from apatite fission track analysis apatite fission track ages and confined length distributions were collected from 38 basement outcrop and 5 basement drillcore samples in order to reconstruct the phanerozoic thermal history of the michigan basin and southern canadian shield the apatite data indicate two periods of thermal activity in the region triassic heating cooling that affected the basin and adjacent shield and cretaceous or post cretaceous heating cooling that primarily affected the basin the magnitude timing and cause of cretaceous thermal activity cannot be identified with the present data model calculations suggest that some of the shield samples and probably all of the basin samples were heated to temperatures of at least 90c just prior to relatively rapid cooling in the triassic available stratigraphic and geochemical constraints suggest that these elevated temperatures were the result of burial by an additional 2 5 km of late paleozoic probably pennsylvanian and permian sediments it is likely that the basin was buried during the alleghenian orogeny as observed for the adjacent appalachian basin

deep burial hydrothermal alteration of the pre salt carbonate reservoirs from northern campos basin offshore brazil evidence from petrography fluid inclusions sr c and o isotopes abstract petrographic mineralogical elemental isotopic and fluid inclusion analyses were integrated to unravel the diagenetic evolution of brazilian pre salt lacustrine carbonate reservoirs of northern campos basin southeast brazilian margin detailed thin section and cathodoluminescence petrography scanning electron microscopy and electron microprobe analyses established a paragenetic evolution of diagenetic processes and products comprising extensive dolomitization silicification and dissolution a paragenesis including saddle dolomite macrocrystalline calcite mega quartz sr barite celestine fluorite dickite sphalerite galena and other metallic sulfides filling fractures and dissolution porosity and aqueous fluid inclusions with homogenization temperatures of 92 152 c and salinities between 13 and 26 wt eq nacl characterized a hydrothermal system with some analogy to carbonate hosted pb zn mississippi valley mvt and irish type deposits petroleum inclusions and solid bitumen testify atypical oil generation and migration associated with the hydrothermal flow the host pre salt spherulitic and fascicular carbonates present highly radiogenic 87sr 86sr ratios indicating strong interaction with continental crust materials hydrothermal phases show 18o values more negative than syngenetic and diagenetic carbonates the 13c values are interpreted as result of interaction between the hydrothermal fluids and the host rocks the combined data set provides clear evidence of intense hydrothermal alteration of northern campos basin pre salt reservoirs at deep burial conditions 2 km possibly related to late cretaceous or more probably paleogene magmatic activity mixed sourced fluids bearing a basinal signature fed the hydrothermal system and promoted dissolution of the host rocks the hydrothermal alterations had strong impact on the porosity permeability and heterogeneity contributing together with the associated fracturing to the excellent production performance of the pre salt reservoirs

discrete fracture network model of the vapor zone leakages at the copahue geothermal field abstract the volcano hosted geothermal system of copahue is one of the most promising resources of renewable energy in argentina it is driven by a layered reservoir composed of a steam cap above a deep liquid dominated reservoir we study the surficial leakages of the shallow steam zone by performing a discrete fracture network model and a 3d forward strain stress model of the upper caprock unit the hydrothermal zones linked to steam cap leakages are subordinated to deep rooted n60 striking faults associated damage zones exhibit higher values of fracture intensity vertical and anisotropy of permeability than the host rocks joints and cross joints likely control leakage zones where the formers are subject to high dilation tendency the anfiteatro monocline might constitute the northwestern boundary of the shallow steam zone variations of h2o co2 ratio helium isotopic signature and co2 between hydrothermal zones are explained in terms of petrophysical properties of the upper ignimbrites and changes in their structural control our results suggest that the upper caprock ignimbrite avoids the decompression of the reservoir because it provides the low fractured and low permeability layer that seals the geothermal system conversely ignimbrites affected by the extensional faults system enhance the shallow vapor zone given its high fracture density and permeability

devonian dundee formation crystal field michigan basin recovery of bypassed oil through horizontal drilling carbonate reservoirs of the devonian dundee formation represent the most productive interval in the michigan basin yet have suffered widespread abandonment due to improper early reservoir management and completion practices dundee carbonate reservoirs deposited in marginal marine and shallow marine environments have produced over 351 mmbbl oil from 137 fields most of which were discovered in the 1930s and 1940s initial potentials for many dundee wells ranged from 2000 to 9000 bbl day presumably due to high permeability fractures and solution enhanced porosity in some areas and excellent primary interparticle porosity in other areas crystal field discovered in 1935 at a depth of less than 3500 ft 1060 m was aggressively developed and produced at high rates causing widespread water coning such that 95 of the field was drilled and abandoned in less than 5 yr leaving considerable interwell reserves in the ground no cores or wireline logs existed for the field prior to 1995 when the field became the subject of a multiyear study aimed at determining the feasibility of employing horizontal drilling to recover bypassed reserves a vertical test portion of the tow 1 3 horizontal well located in a structurally elevated portion of the field cored and logged the entire reservoir interval yielding crucial new information on the basis of relevant data a horizontal leg was drilled resulting in an excellent producer with estimated recoverable reserves of 200 000 bbl two subsequent horizontal tests with poor results were drilled in downdip locations and oriented perpendicular to the tow well suggesting the importance of both structural position and azimuth selective use of horizontal drilling is considered an appropriate cost effective means for recovering bypassed reserves in dundee fields where sufficient geologic data exist to adequately characterize the reservoir and oil occurrence for some fields this may require new coring and logging operations

formation of saddle dolomites in upper cambrian carbonates western tarim basin northwest china implications for fault related fluid flow abstract the saddle dolomites occur more intensely in cores closely to fault than that in cores far away from the fault in upper cambrian carbonate of western tarim basin suggesting that formation of the saddle dolomites is likely related to fault controlled fluid flow they partially fill in fractures and vugs of replacement dolomite the saddle dolomites exhibit complex internal textures commonly consisting of core and cortex in comparison with the matrix dolomites the saddle dolomites show lower sr content and 87 sr 86 sr ratios higher fe and mn content and more negative 18 o values combined with high th 100 130 c of primary fluid inclusions it is suggested that the saddle dolomites precipitated from hydrothermal fluid derived from the deep evaporite bearing middle cambrian strata and the magnesium source may be due to dissolution of host dolomite during hydrothermal fluid migration fault activity resulted in petrographic and geochemical difference of the core and cortex of the saddle dolomites the cores precipitated from the formation water mixed by deep brines at the early stage of fault activity and the cortexes precipitated from the deep fluid with higher temperatures through the middle cambrian later in summary the formation of the saddle dolomites implies a hydrothermal fluid event related to fault activity which also resulted in high porosity in upper cambrian carbonate in western tarim basin

thermal history of michigan basin the elevated organic maturity observed in shallowly buried units from the michigan basin implies that higher temperatures and thicker overburdens once existed in the basin evidence from sediment accumulation rates regional dips and maturity of pennsylvanian age coals suggests that up to 1 000 m 3 280 ft of sediment were removed by erosion prior to the late jurassic when the basin became stable geothermal gradients during paleozoic basin subsidence probably ranged from 35 to 45 c km 19 to 25 f 1 000 ft in contrast to the average present value of 25 c km 14 f 1 000 ft depths to the top of the oil window ranged from 1 900 to 2 300 m 6 230 to 7 540 ft during the paleozoic post pennsylvanian erosional uplift and further thermal m turation of the basin have combined to raise the top of the oil window to its present level of 500 m 1 640 ft

evidence against the dorag mixing zone model for dolomitization along the wisconsin arch a case for hydrothermal diagenesis ordovician carbonates near the wisconsin arch represent the type locality in ancient rocks for the dorag or mixing zone model for dolomitization field petrographic and geochemical evidence suggests a genetic link between the pervasive dolomite trace mississippi valley type mvt minerals and potassium k silicate minerals in these rocks which preserve a regional hydrothermal signature constraints were placed on the conditions of water rock interaction using fluid inclusion methods cathodoluminescence and plane light petrography stable isotopic analyses and organic maturity data homogenization temperatures of two phase aqueous fluid inclusions in dolomite sphalerite and quartz range between 65 and 120c freezing data suggest a na ca mg cl h2o fluid with salinities between 13 and 28 wt nacl equivalent the pervasive dolomitization of paleozoic rocks on and adjacent to the wisconsin arch was the result of water rock interaction with dense brines at elevated temperatures and it was coeval with regional trace mvt mineralization and k silicate diagenesis a reevaluation of the dorag mixing zone model for dolomitization in conjunction with convincing new petrographic and geochemical evidence has ruled out the dorag model as the process responsible for pervasive dolomitization along the wisconsin arch and adds to the abundant body of literature that casts serious doubt about the viability of the dorag model in general

early history of the michigan basin subsidence and appalachian tectonics geometries of cambrian to silurian stratigraphc sequences in the michigan basin record discrete episodes of basin centered subsidence separated by periods of regional tilting backstripping reveals irregular subsidence rates that argue against a simple thermal contraction model depositional facies architecture also reflects episodic subsidence patterns basin centered facies tracts dominating during subsidence reactivations these three lines of evidence indicate that subsidence cessations and reactivations characterize the early history of the michigan basin periods of episodic subsidence correlate temporally with orogenic events in the appalachians suggesting that reactivation of basin subsidence is related to tectonic activity we propose that appalachian orogenic activity caused the episodic subsidence of the michigan basin possibly through weakening of the lower crust and reactivation of a preexisting upper crustal isostatic imbalance

petrographic and geochemical characteristics of dolomite types and the origin of ferroan dolomite in the trenton formation ordovician michigan basin u s a three major types of dolomite occur in the trenton formation mid ordovician of the michigan basin these are 1 regional dolomite which is confined to the extreme western edge of the basin 2 cap dolomite which occurs in the upper portion of the trenton and is confined to the basin s southern margin and 3 fracture related dolomite which occurs in association with both large and small scale faults and fractures these three dolomite types can be distinguished from one another by their major element chemistry oxygen isotope ratios and rock texture the regional dolomite is fine grained has 0 34 mol feco3 and mean 18o of 6 8 opbd the cap dolomite is texturally similar to regional dolomite but contains 3 13 0 mol feco3 and has a mean 18o of 7 7 fracture related dolomites are coarse grained low in iron and have the most depleted 18o ratios x 9 0 pdb petrographic relationships imply that the regional dolomite formed prior to the cap dolomite probably during early diagenesis the cap dolomite formed at relatively shallow depths as a result of the interaction of the overlying utica shale and the trenton limestone fracture related dolomites post date the cap dolomite and formed during deeper burial a temperature of precipitation of approximately 80 c was calculated for fracture related dolomites using oxygen isotope data the distribution of the cap dolomite was controlled by the availability of fe2 which was in turn controlled by the availability of s2 in the centre of the basin trenton utica deposition was continuous the upper trenton contained relatively high concentrations of organic matter which was used by sulphate reducing bacteria to produce h2s from seawater sulphate the precipitation of iron sulphides pyrite iron monosulphide followed and used up most of the available fe2 as a result only small amounts of ferroan dolomite formed on the periphery of the basin subaerial exposure resulted in the oxidation of most of the available organic matter sulphate reducing bacteria were therefore limited and produced limited amounts of h2s as a result only a minor amount of iron sulphide iron monosulphide formed the remaining fe2 was then available for the formation of the ferroan cap dolomite this model is supported by the following 1 in the southern margin of the basin the contact between trenton cap dolomite and the overlying utica shale is sharp and probably unconformable in the centre of the basin the contact is gradational 2 in the centre of the basin the total organic carbon content in the upper trenton is an order of magnitude higher than in the cap dolomite 3 the whole rock concentration of iron is high in both the cap dolomite and in slightly dolomitized equivalent beds in the basin centre 4 iron sulphides are abundant in the centre of the basin and mostly in the form of pyrite in the cap dolomite iron sulphide is minor and primarily in the form of iron monosulphide

hydrothermal origin of early permian saddle dolomites in the junggar basin nw china abstract hydrothermal related saddle dolomites widely exist in the lower permian fengcheng formation in the northwestern junggar basin and this formation is a set of carbonate rich deposits in an alkaline saline basin core and thin section observations cathodoluminescence cl imaging and carbon and oxygen isotope composition analyses have been used to study the origins of saddle dolomites with various occurrences hydrothermal fluids include si rich and si poor fluids and saddle dolomites occur along with siliceous bands as debris and assemblages in fragmented rocks and as cements filling pores and microfractures different types of dolomites in the fengcheng formation have similar 13c values while the 18o values differed from each other which demonstrated that the dolomites originated from the same primary carbonates and underwent multiple stages of recrystallization the 18o values were extremely negative which was caused by fractionation at high temperatures from hydrothermal fluids dolomites are directly or indirectly affected by hydrothermal diagenetic fluids according to the diagenetic stages the types of migration paths for hydrothermal fluids and the patterns of hydrothermal fluids acting on the dolomites a model was proposed to illustrate the origins and evolution of the dolomites and three paths were utilized path i occurred during the contemporaneous diagenetic stage when the carbonate particles were still free and could be captured by high density bottom hydrothermal flows and distorted path ii occurred during the long diagenetic history after the carbonate bearing sediments had consolidated saddle dolomite assemblages and saddle dolomite debris occurred with the aid of compressive tectonic forces path iii also occurred in the long diagenetic history and the host rocks were not fragmented the saddle dolomites reprecipitated as cements in the pores stylolites fractures microfractures or laminated seams