Anhydrite Nodules Research Articles

Assessment of anhydrite dissolution as the rate-limiting step during thermochemical sulfate reduction

Thermochemical sulfate reduction (TSR) is a process observed in some gas and oil reservoirs above a temperature about 120–145°C. During this process, petroleum compounds such as methane react with anhydrite to form calcite, H 2S and other minor components. This reaction can lead to the complete destruction of petroleum accompanied by the generation of a large amount of H 2S. TSR consists of a variety of more elementary steps, including dissolution of reactants, diffusion to the point of reaction, geochemical interaction in the aqueous phase and mineral precipitation. The rate-controlling step for TSR is as yet unknown, although thermodynamic data suggest that the assemblage of hydrocarbons and anhydrite is very reactive. Experimental work revealed relatively high dissolution rates for anhydrite. Modeling TSR as complex multiphase, multistep reaction confirms that if the intrinsic dissolution of anhydrite were the rate-limiting step, the reaction could be complete within a year and would be effective even at low temperatures (e.g., 80°C). Other controls on the transformation must therefore be invoked. At the reservoir scale, a limited reactive volume at the gas–water transition zone may localize and thus limit the rate of TSR. Textural evidence indicates, however, that the controls on the reaction rate at the grain scale are more important. In model simulations, calcite precipitation on the outer edge of anhydrite nodules leads to progressive isolation of the remaining anhydrite from the petroleum phase and a dramatic decrease in the anhydrite dissolution rate. With the reactants forced to diffuse through the calcite armoring, the simulations predict completion of TSR in 200,000–300,000 years. The simulation results match closely the observed trend of reaction extent as a function of depth for temperature higher than 140°C, but overestimates this extent at lower temperature. Thus, the modeling suggests that the rate-controlling step of TSR evolves initially from (1) the rate of aqueous redox interaction between sulfate and petroleum to (2) the rate of diffusion-controlled dissolution of anhydrite. This could explain the high minimum temperature of reaction as well as the coexistence of anhydrite and petroleum even in reservoirs TSR is at an advanced stage.

The Influence of Rock Fabric and Mineralogy on Thermochemical Sulfate Reduction: Khuff Formation, Abu Dhabi

ABSTRACT Thermochemical sulfate reduction (TSR) is the reaction between anhydrite and petroleum fluids at elevated temperatures to produce H2S and calcite. In this study of the dolomite-hosted hydrocarbon gas reservoirs in the Permo-Triassic Khuff Formation, Abu Dhabi, a geochemically well constrained rock-gas system, we demonstrate for the first time a clear influence of rock texture and mineralogy on the rate and extent of TSR reactions and thus on H2S concentration in the gas phase. The controls on the rate on H2S accumulation were: TSR reaction kinetics. TSR became significant as temperature exceeded 140°C, a critical threshold temperature for chemical reaction between aqueous sulfate and aqueous methane. Anhydrite dissolution rate. Once initiated, the subsequent reaction rate was controlled by the rate of supply of aqueous sulfate to the reaction site. Sulfate limitation is indicated by the lack of fractionation of sulfur isotopes between sulfate and sulfide (i.e., suggesting total reaction for each unit of sulfate that dissolves). Also, finely crystalline anhydrite began reacting at a lower temperature than coarse crystalline anhydrite (likely a result of relative surface area), suggesting that anhydrite dissolution was the rate-limiting step. Transport rates are unlikely to have been rate-limiting at the earliest stage of reaction because anhydrite was replaced in situ by calcite on the very edges of anhydrite nodules and crystals. Transport rate. Later, as TSR proceeded, it became transport controlled, as calcite, growing on the surface of anhydrite crystals, began to isolate them from dissolved methane. TSR ceased once calcite had effectively armor-plated (totally isolated) the remaining anhydrite. Finely crystalline anhydrite underwent more extensive and more rapid TSR than coarser anhydrite crystals because these had a greater ratio of surface area to volume, allowing more and faster dissolution and requiring more calcite to isolate them from methane. Localized loss of H2S occurred in the reservoir by reaction with indigenous Fe-bearing clays. Consequently, reservoirs with a relatively high siliciclastic content have less H2S than would be expected from the advanced state of anhydrite replacement by calcite. In order to predict TSR-related H2S concentration in hydrocarbon gases it is thus important to understand the diagenetic and textural characteristics of the reservoir as well as the thermal and petroleum-emplacement history.

Sedimentology of the Cretaceous Maha Sarakham evaporites in the Khorat Plateau of northeastern Thailand

Evaporites of the Cretaceous to early Tertiary Maha Sarakham Formation on the Khorat Plateau of southeast Asia (Thailand and Laos) are composed of three depositional members that each include evaporitic successions, each overlain by non-marine clastic red beds, and are present in both the Khorat and the Sakon Nakhon sub-basins. These two basins are presently separated by the northwest-trending Phu Phan anticline. The thickness of the formation averages 250 m but is up to 1.1 km thick in some areas. In both basins it thickens towards the basin centre suggesting differential basin subsidence preceding or during sedimentation. The stratigraphy, lithological character and mineralogy of the evaporites and clastics are identical in both basins suggesting that they were probably connected during deposition. Evaporites include thick successions of halite, anhydrite and a considerable accumulation of potassic minerals (sylvite and carnallite) but contain some tachyhydrite, and minor amounts of borates. During the deposition of halite the basin was subjected to repeated inflow of fresher marine water that resulted in the formation of anhydrite marker beds. Sedimentary facies and textures of both halite and anhydrite suggest deposition in a shallow saline-pan environment. Many halite beds, however, contain a curious `sieve-like' fabric marked by skeletal anhydrite outlines of gypsum precursor crystals and are the product of early diagenetic replacement by halite of primary shallow-water gypsum. The δ 34 S isotopic values obtained from different types of anhydrite interbedded with halite range from 14.3‰ to 17.0‰ (CDT), suggesting a marine origin for this sulphate. Bromine concentration in the halite of the Lower Member begins around 70 ppm and systematically increases upward to 400 ppm below the potash-rich zone, also suggesting evaporation of largely marine waters. In the Middle Member the initial concentration of bromine in halite is 200 ppm, rising to 450 ppm in the upper part of this member. The bromine concentration in the Upper Member exhibits uniform upward increase and ranges from 200 to 300 ppm. The presence of tachyhydrite in association with the potassic salts was probably the result of: (1) the large volumes of halite replacement of gypsum, on a bed by bed basis, releasing calcium back into the restricted waters of the basin; and (2) early hydrothermal input of calcium chloride-rich waters. The borates associated with potash-rich beds likely resulted from erosion and influx of water from surrounding granitic terrains; however, hydrothermal influx is also possible. Interbedded with the evaporites are non-marine red beds that are also evaporative, with displacive anhydrite nodules and beds and considerable amounts of displacive halite. The δ 34 S isotopic values of this anhydrite have non-marine values, ranging from 6.4‰ to 10.9‰ (CDT). These data indicate that the Khorat and Sakhon Nakhon basins underwent periods of marine influx due to relative world sea-level rise but were sporadically isolated from the world ocean.

The alteration of glauberite in lacustrine deposits of the Taoudenni-Agorgott basin, northern Mali

A profile studied within Holocene salt lake deposits of the Taoudenni-Agorgott basin, northern Mali, comprises a macroscopically unstratified surface layer containing calcium sulfate minerals. This interval has previously been interpreted as a depositional unit that formed under different conditions than the underlying glauberite-bearing layered deposits. The present micromorphological study of this layer demonstrates that it formed by an alteration of the glauberite-bearing primary deposits. The first stage in the alteration of the syndepositional salts was the incongruent dissolution of glauberite (Na 2Ca(SO 4) 2). This resulted in the formation of gypsum (CaSO 4·2H 2O), both as crystals that are part of microcrystalline aggregates with a xenotopic fabric and as larger anhedral and elongated prismatic crystals. The gypsum crystals were subsequently subjected to a dehydration, resulting in a topotactic replacement of this mineral by bassanite (2CaSO 4·H 2O) and anhydrite (CaSO 4). This replacement was characterized by a parallelism between the fibre axes of the dehydrated mineral phases and the [001] axis of the gypsum crystals. During later periods, the alteration products repeatedly underwent hydration and dehydration reactions and interactions with both more concentrated and dilute solutions, producing aggregates with gypsum, bassanite, anhydrite and secondary glauberite as constituents. Aggregates of acicular crystals with a possible eugsterite (Na 4Ca(SO 4( 3·2H 2O) composition also formed during this stage. In the part of the surface layer in which the alteration is most advanced, the evaporite minerals mainly occur in the form of large anhydrite nodules. The constituents of these aggregates are not exclusively the direct product of a dehydration of the large gypsum crystals from which the nodules were derived. The products of these different stages in the alteration of the primary deposits occur in separate depth intervals of the surface layer, which therefore displays a general upward increase in degree of diagenesis.

ORIGIN OF THE MID‐YPRESIAN JIRANI DOLOMITE — A MAJOR RESERVOIR ROCK IN THE NW LIBYA OFFSHORE

The Jirani Dolomite was formed during a major relative fall in sea‐level in middle Ypresian (early Eocene) times, in shallow, hypersaline lagoons on a restricted shallow platform, where supretidal and intertidal and semi‐arid to arid conditions prdominated. Localisation of the shallow logoonal facies to the region of the Jifarah Trough suggests that restriction was caused by downwarp relative to mild uplift of the Jifarah Platform to the north.Two main facies are recognised: and anhydritic dolomite facies, composed of dolomite and dolomitic limestone with anhydrite nodules; and a non‐anhydritic dolomite faices, composed of dolomite and dolomitic limestone only. Lithologic and petrographic evidence leads to the conclusion that anhydrite was also present in the non‐anhydritic dolomite facies initially, but was removed by dissolution processes. Petrographic studies show that dolomitisation proceeded in three stages. Stage I involved penecontemporaneous/early8 diagenetic dolomitisation of the precurosr limestones with gypsum/anhydrite under hypersaline seepage reflux conditions. Stage II dolomitisation, mainly confined to the non‐anhydritic dolomite facies, was probably formed in the mixing zone between meteoric and sea water, probably at shallow depths of burial. Stage III dolomitisation occured at depth in the late stage of basin evolution, causing some infilling of mouldic and vuggy porosity by coarsely crystalline, saddle dolomite. Evidence of mixing zone dolomitisation indicates that the Jirani Dolomite was exosed subaerially. Studies of the Metlaoui Group show that this event can be related to a major relative fall in sea level in early Lutetian times following deposition of the Jdeir Formation. Exposure of the deposits to flushing by meteoric waters explains the dissolution of the anhydrite from the non‐anhydritic dolomite faices, and the accentuation of the well‐development porosity in this facies, which make it one of the most important reservoir rocks in the offshore region.

Dolomitization and H2S generation in the Permian Khuff Formation, Offshore Dubai, U.A.E.

Dolomite in the Permian Khuff Formation from the T-1 well (Fateh field, offshore Dubai, United Arab Emirates) formed during the Miocene from a dense brine at a depth of approximately 15,500 ft (4,724 m), as indicated by fluid inclusion data. As the majority of the porosity is intercrystalline, most of the porosity formed at the same time. Fluid inclusion data, together with petrography, luminescence, and oxygen and carbon stable isotopes, indicate the diagenetic history was extremely complex with multiple episodes of precipitation of cements (calcite, saddle dolomite, anhydrite, fluorite), but fluid inclusion geothermometry data indicate that the majority of cementation occurred from the Eocene to the Miocene at depths of at least 12,300 ft (3,749 m). The matrix dolomite, anhydrite nodules, and calcite and fluorite cements have higher87Sr/86Sr ratios (0.70780 to 0.71084) than expected for late Permian seawater, which supports a non marine (burial) origin. Anhydrite with higher than expected δ34S (16.5 to 20.2o/oo CDT) for late Permian seawater gives supporting evidence for this interpretation.

Nonmarine Evaporitic Sedimentation and Associated Diagenetic Processes of the Southwestern Margin of the Ebro Basin (Lower Miocene), Spain

ABSTRACT In the southwestern margin of the Tertiary Ebro Basin (northeastern Spain) several evaporitic units are found among distal alluvial-fan deposits. They are made up of bioturbated microlenticular primary gypsum, secondary nodular and meganodular gypsum after anhydrite, chert nodules, and charophyte limestones. These evaporites formed in shallow saline lakes along the basin margin during the early Early Miocene (Autol gypsum unit) and the late Early Miocene (Ablitas, Gravalos, and Ribafrecha gypsum units). The lakes were fed principally by deeply circulated groundwaters loaded with dissolved salts from the old evaporite formations (mainly Triassic rocks) that make up the surrounding mountain chains. Shallow meteoric waters of low concentration also reached the lakes episodically. Concentration of these waters led to deposition of microlenticular gypsum and charophytic carbonates, which were then intensely bioturbated. Early-diagenetic chert nodules and nodular anhydrite also formed. In the early stages of burial large anhydrite nodules (meganodules) overprinted these sedimentary and early-di genetic deposits. With more advanced burial the primary gypsum was profoundly anhydritized in some areas. With subsequent exhumation the anhydritized zones were transformed to secondary gypsum. Marginal lakes acted as preconcentrators of drainage waters coming from the surrounding mountain chains, causing early precipitation of low-solubility salts as Ca-carbonates, gypsum, and silica. The remaining dissolved salts then drained toward the basin center, where gypsum, anhydrite, glauberite, polyhalite, and halite precipitated in contemporaneous high-salinity lakes.

Chert appearance in the Cueva-Bedón carbonate platform (upper Cretaceous, northern Spain)

AbstractNodular chert from the Cueva-Bedón carbonate platform (late Turonian-earliest Coniacian) occurs above the type 1 sequence boundary and in association with a transgressive systems tract deposit. It formed by selective replacement of grainstones-packstones, and Thalassinoides burrow fillings. Other minor types of selective silicification include the replacement of anhydrite nodules, forming quartz geodes, and the partial or total replacement of oysters and inoceramids. The chert nodules consist predominantly of microquartz with volumetrically minor fibrous quartz (length-fast chalcedony) and megaquartz. The silica has a mainly biogenic source (siliceous sponge spicules and radiolarians). Lepisphere textures that are indicative of an intermediate opal-CT precursor are evident in chert nodules, which indicates the chert formed by a maturation process from opalA to opal-CT to quartz. The δ 18O values of microquartz suggest a precipitation from marine fluids. The fabric of quartz geodes consists of irregular bands of heterometric megaquartz, quartzine and lutecite spherulites, and a well developed band of petaloidal megaquartz that contains small anhydrite inclusions. Bivalve shells are replaced mainly by fibrousquartz (quartzine-lutecite), which forms extensive beekite rings. The silica source of the replaced fossils is also biogenic, but silicification couldhave occurred by direct quartz replacement without an opal-CT precursor. Field and petrographic evidence indicate that silicification was an early diagenetic process that affected both uncompacted and compacted sediments, andwas inhibited when the dolomitization began. There seems to exist a syncrony in age (late Turonian-earliest Coniacian) between the nodular chert in the Cueva-Bedón carbonate platform and the bedded and nodular chert ofthe Plentzia carbonate turbidites in the deeper Basque Arc.

Anhydrite Nodules Formed During Deep Burial

ABSTRACT Anhydrite nodules are known from, or have been interpreted to have formed in, geologic environments that range from near surface to several hundred meters of burial depth. Anhydrite nodules from the Upper Devonian Leduc Formation in the subsurface of Alberta, Canada, have feeder channels of anhydrite and contain inclusions of bitumen-stained wall rock, indicating that these nodules formed after oil migration and hence at depths of probably more than 2500-3000 m. These new findings indicate that anhydrite nodules can form at burial depths ranging from nearly zero to several thousand meters, and that the mere presence of anhydrite nodules has no particular significance genetically (displacive or replacive) or paleoenvironmentally (sabkha, subaqueous, etc.). The origin of anhydrite nodul s in any particular area can often be determined by means of petrographic and/or geochemical criteria.

Continental origin of the Maha Sarakham evaporites, northeastern Thailand

Recent sedimentologic analysis of the potash-bearing Maha Sarakham Formation (Upper Cretaceous) shows that it is not of marine origin as previously thought. The main reason is the lack of evidence suggesting transgression onto the underlying redbeds of the Khok Kruat Formation, which are desert stream deposits. Similar lack of evidence also holds for the salt/red clastic contacts within the evaporites. Sedimentary features preserved in cores also point towatd the nonmarine origin. Algal lamination, palmate and swallow-tail fabrics in anhydrite (formerly gypsum) suggest shallow gypsum pan environment, while truncation and dissolution of the primary halite crystals suggest ephemeral salt pan conditions. The clastic units alternating in the formation contain features which characterize saline and dry mudflat environments, for instance, desiccation cracks, caliche, and displacive anhydrite nodules. In the potash zone, neither carnallite nor sylvite shows structures that may indicate sedimentation in a standing brine body or growing upward from the pan floor. Moreover, it is discernible at places that carnallite and sylvite were formed interstitially in the salt pan halite host, which implies their diagenetic origin. Considering the information gained so far, it is apparent that there was no restricted sea in northeastern Thailand during Late Cretaceous time. The Maha Sarakham Formation was in fact deposited in arid desert, probably deep in the continent. The new interpretation raises several quest0ons to the consensus on the formation, i.e. basin history, stratigraphic subdivision of the Khorat Group, and relationship among the formations within. It may also affect the economic aspects of the formation, i.e. continuity of the potash horizons and occurrence and migration of subsurface brines.

Depositional Environments and Reservoir Properties of the Upper Queen Formation, Concho Bluff and Concho Bluff North Fields, Midland Basin, Texas

The upper Queen Formation (Upper Permian) in Concho Bluff and Concho Bluff North fields (Crane, Upton, and Ector counties, Texas) consists of four thick (6-11 m) clastic members separated by evaporite members, and contains several major sandstone reservoirs. A study of cores and logs from the upper Queen was done to determine its depositional environments and the properties of its sandstone reservoirs. Four facies are present in the upper Queen. Facies 1 consists of planar- and wavy-laminated fine and very fine sandstones and silty sandstones deposited in fluvial sandflats and delta-plains. Facies 2 consists of wavy-laminated siltstones and haloturbated mudstones with anhydrite nodules deposited in subaqueous prodelta environments and subaerial saline mudflats. Facies 3 consists of well-sorted fine to very fine sandstones with horizontal and inclined planar laminae deposited in eolian sand sheets. Facies 4 consists of massive to laminated halite and anhydrite deposited in hypersaline playas. The vertical sequence of these facies indicates that the upper Queen was deposited during four cycles of fan-delta progradation into and retreat from coastal-plain playas during a sea level lowstand in the Midland basin. The sandstones of the fluvial sandflat and eolian sand-sheet facies constitute the reservoirs of the upper Queen in themore » fields. The average cumulative thickness of these facies is 8 m, and the fields average 16% porosity and 40-50 md of permeability. The remaining facies are all nonproductive with averagge porosities of less than 10% and average permeabilities of less than 1 md.« less

Facies and depositional environments of the Shattuck Sandstone on the northwestern shelf of the Permian Basin

ABSTRACT The Artesia Group is a sequence of carbonates, evaporites, sandstones, siltstones, and shales that was deposited on the back-reef shelves of the Permian Basin of Texas and New Mexico during late Permian (Guadalupian) time. The facies and depositional environments of the shelf carbonates and evaporites in this group are fairly well established, but there is still debate about the origin of the shelf clastics and the role of sea-level fluctuations in their formation. This problem is not unique to the Artesia Group but is common to shelf clastics in stratigraphic sections around the world. This study examined cores and logs of one of the more prominent shelf clastics in the Artesia Group, the Shattuck Sandstone Member, from two oil fields on the Northwest Shelf of the Permian Basin in or er to address this problem. The Shattuck Sandstone is underlain and overlain by carbonates, evaporites, siltstones, and sandstones that were deposited in a shelf-interior lagoon and adjacent coastal sabkhas during relative highstands of sea level. However, the textures, colors, sedimentary structures, fabrics, and fossil content of the Shattuck, and the nature of its contacts with these bounding lagoonal and coastal sabkha deposits, indicate that it was deposited in semi-arid and poorly-vegetated desert coastal plain environments that prograded across the Northwest Shelf after exposure of the shelf by a sea-level fall. Three facies can be distinguished in the Shattuck Sandstone. Facies 1 consists largely of reddish-brown and non-fossiliferous cross-bedded and bedded sandstones, silty sandstones, and siltstones with silty mudstone drapes and infiltrated detrital clay; it is interpreted to be the deposits of a fluvial sandflat system. Facies 2 consists of non-fossiliferous well-sorted fine to very fine sandstone with dune cross-beds and wind-ripple, grainfall, and deflation-lag laminae; it is interpreted to be the deposits of an eolian sand sheet. Facies 3 is largely composed of reddish-brown and non-fossiliferous sandstones, silty sandstones and siltstones with haloturbation and dewatering structures, displacive anhydrite nodules, and thin nodular-mosaic anhydrite beds; it is interpreted to be the de osits of a clastic-dominated sabkha.

Burial-Diagenetic Sabkha-Like Gypsum and Anhydrite Nodules

ABSTRACT Anhydrite nodules in the Late Devonian Dismal Creek and Blue Ridge Members in the subsurface of Alberta, Canada, recovered from depths in excess of 3000 m, resemble sabkha nodules. However, several features are inconsistent with a sabkha origin or are consistent with both a shallow and a deep burial origin. These nodules occur in facies that represent deposition on a marine carbonate platform or ramp close to or below fair weather wave base. Almost all nodules are associated with dark, bituminous, argillaceous seams and pockets, carbonaceous halos, anastomosing bituminous veinlets, and stylolitic contacts. Petrographic, UV-fluorescence, x-ray diffraction, and pyrolysis data indicate that the dark material consists of diagenetic residues from pressure solution and concretionary cementa ion, and that the organic matter is mature kerogen, solidified crude oil, or a mixture of the two. Sulfur and oxygen isotope ratios of the anhydrite suggest bacterial sulfate reduction before or during nodule growth. All features taken together indicate a burial-diagenetic origin for the Dismal Creek and Blue Ridge nodules. It is suggested that gypsum or anhydrite nodules resembling those commonly formed in sabkhas can be formed during burial at depths of at least several tens to several hundreds of meters. The main criteria for recognizing a burial origin for sulfate nodules are: 1) nodules occur within, or juxtaposed to, residues of pressure solution seams, stylolites, and diagenetic argillaceous and carbonaceous pockets between diagenetic carbonate nodules; 2) nodules replace host rock, and they may crosscut the boundaries between host rock and pressure solution accumulates; 3) nodules contain inclusions that had formed during burial, e.g., authigenic pyrite, dolomite, etc.; 4) there is an absence of features indicative of breakage during mechanical compaction, e.g., broken pile-of-brick texture withi the nodules; 5) anhydrite crystals in nodule margins have circum-nodular, tangential alignment; 6) compared to sabkha nodules, burial-diagenetic nodules may occur in facies that are not lagoonal to peritidal and supratidal, and that are unlikely to have been subaerially exposed in evaporitic climates.

Bedded Cherts in the Early Ordovician Arbuckle Group of Southwestern Oklahoma

Bedded cherts are a rarely occurring but environmentally significant facies in the Early Ordovician Arbuckle Group. Two such units have been identified: one in the Cool Creek Formation and one in the Kindblade Formation. In each, microcrystalline calcite and dolomite alternate in thin laminae with microcrystalline silica in units up to 25 cm in thickness. The areal extent of the bedded cherts encompasses more than 130 km{sup 2}. It is hypothesized that the interlaminated cherts and micrites were deposited in very shallow marine conditions. A relatively small part of the Oklahoma Aulacogen was cut off from open ocean circulation, resulting in an isolated lagoon. The chert/micrite couplets may represent seasonal deposits in the lagoon. During seasonal flooding, slightly deeper water resulted in algal growth with accompanying precipitation of carbonates and solution of detrital quartz sand. During the dry season, the dissolved silica precipitated, resulting in thin layers of microcrystalline chart. Some carbonate layers contain small pseudomorphs after what may have been gypsum and anhydrite crystals and nodules, indicating that salinities increased enough to permit formation of evaporites. At least ten couplets occur in the thickest units, suggesting that the lagoon existed for ten wet-dry seasons.

Oceanographic Restriction and Deposition of the Permian Park City and Phosphoria Formations, Northeastern Utah and Western Wyoming

Detailed lithofacies analyses of the Permian Park City Formation, in northeastern Utah and western Wyoming, reveal that it was deposited in both open and restricted continental shelf and slope environments bordering the Oquirrh and Sublett basins. The Park City and the intercalated Phosphoria Formation document the interplay between carbonate, clastic, evaporite, and organic-rich sedimentation, fluctuating sea-level and bottom water oxygenated, and oceanic upwelling. New data from the Park City and Phosphoria formations imply that paleoceanographic models for the deposition of these units must be revised. Both physical and chemical restriction, resulting from paleogeographic constraints, regressive conditions, and the decay of organic matter produced in nutrient-rich upwelled waters, were important to the development of lithofacies patterns. Evidence of restriction includes massive and bedded anhydrite deposits and calcite replaced anhydrite nodules, carbonate facies with low levels of bioturbation and significant quantities of authigenic pyrite, and laminated black, organic-rich shales indicating low oxygen conditions. Park City and Phosphoria lithofacies imply that upwelling began during regression that resulted from a glacio-eustatic drop in sea level. This was accompanied by a greater pole-to-equator temperature gradient and intensified atmospheric circulation that induced eastern ocean basin upwelling. Physical and chemical restriction of marginal Permian basins was importantmore » in the development of dysaerobic to anaerobic conditions that facilitated the preservation of organic matter.« less

Silcrete in the Uppermost Cambrian and Lower Ordovician of the Wisconsin Arch and Michigan Basin--Implications for Subaerial Exposure

Silcrete within uppermost Cambrian strata and the Lower Ordovician Prairie du Chien Group delineates the basinward extent of subaerial exposure during three relative sea-level lowstands that exposed portions of the Wisconsin arch and Michigan basin before, during, and after Prairie du Chien deposition. Shallow-water Prairie du Chien carbonates and Cambrian sandstones were deposited in tropical, epeiric seas during relative sea-level highstands. Fabrics identical to Cenozoic silcretes consist of cryptocrystalline microquartz (chert), microquartz ({lt}0.02 mm), megaquartz ({lt}0.02 mm), and length-slow and length-fast chalcedony. Silicified crusts of oolitic grainstones and/or anhydrite nodule typically coat uppermost Cambrian and Prairie du Chien unconformities. Silicification associated with intra- and post-Prairie du Chien unconformities is generally restricted to within 5 m of the overlying unconformity. Highly permeable lithologies, such as oolitic grainstones, boundstones, and paleokarst breccias, were preferentially silicified during intra- and post-Prairie du Chien exposure. Reworked silcrete clasts typically overlie unconformities. Silcrete is a practical exposure indicator as it is easily identified in outcrop and well cuttings. Silcrete within uppermost Cambrian strata is generally restricted to the Wisconsin arch axis, but occurs as far east as western Michigan. Silcrete below the intra-Prairie du Chien unconformity indicates subaerial exposure of the entire arch and basin. Inmore » contrast, silcrete below the post-Prairie du Chien, pre-St. Peter unconformity indicates that exposure was restricted to the Wisconsin arch and western margin of the Michigan basin in eastern Wisconsin.« less

Depositional Environments of Permian Phosphoria Formation and Related Rocks, Leach Mountains, Northeast Nevada

In the Leach Mountains, northeastern Nevada, the Phosphoria Formation is represented by phosphatic rocks of the late Leonardian Meade Peak Phosphatic Shale Tongue. The Meade Peak is overlain by dolomitic siltstones and chert, including black bedded chert, of the upper Leonardian( ) to lower Guadalupian Murdock Mountain Formation. The black bedded chert is related to the Rex Chert Member of the Phosphoria Formation. There is little consensus regarding depositional environments of the Meade Peak. Also, little work has been presented on the origin of the black bedded cherts such as in the Murdock Mountain and Rex Chert. Locally, a shallow subtidal to perhaps partly intertidal depositional model for the Meade Peak is based upon characteristics of reworked phosphatic clasts, a crowded and mixed shallow-water fauna, oolite beds, and stratigraphic position between shallow subtidal to supratidal rocks. Supratidal deposition of dolomitic siltstones and units of black bedded chert in the Murdock Mountain is based largely on abundant cauliflower-shaped blebs of chalcedony with shapes resembling modern anhydrite nodules. Also, zebraic chalcedony in the black bedded chert is interpreted as a replacement product of evaporite deposits. The setting in this southwestern part of the depositional basin of the Phosphoria Formation, in the latemore » Leonardian, during the deposition of the Meade Peak, is interpreted as a network of shallow to very shallow marine basins and intervening, perhaps periodically merged shoals. Subsequently, during the late Leonardian( ) to early Guadalupian, there was a marine regression and the supratidal environment of the Murdock Mountain Formation was established.« less

Case history of deep-burial sulfide mineralization in the northern Appalachian Basin

In the northern Appalachian Basin minor concentrations of sulfide mineralization occur throughout most of the undeformed strata, especially those composed of carbonate rock. In a hostrock of gray, dense, hard, massive, fine-grained dolostone (dolomicrite) the Lockport Formation (Middle Silurian) contains a remarkable mineral suite filling fractures, vugs, and cavities, especially a geode filling of former leached anhydrite nodules. The minerals include saddle dolomite, sphalerite, galena, marcasite, pyrite, fluorite, anhydrite and gypsum (mostly selenite), calcite, quartz, barite and celestite.

Relationships between cyclicity and stromatolite form in the Late Proterozoic Bitter Springs Formation, Australia

ABSTRACTStromatolite biostromes and bioherms in the lower two units of the Late Proterozoic Loves Creek Member of the Bitter Springs Formation represent shallowing upward and deepening upward sequences. In the central unit stromatolite form is governed by relative position in an asymmetric shallowing upward sequence. Ooid and/or peloid‐intraclast grainstones and small, irregular bulbous and columnar stromatolites characterize the basal, transgressive portion of cycles. Domal, columnar and stratiform stromatolites comprise the bulk of the cycle. These forms accreted in a gradually shallowing epeiric sea. Domal stromatolites predominate in the deeper parts of cycles. Here synoptic relief gradually increases upwards. Columnar and stratiform stromatolites predominate in the shallower parts of cycles, where synoptic relief rapidly diminishes upwards. In thin‐bedded dolo‐mudstones at the tops of cycles the co‐occurrence of desiccation cracks, tepee structures, scalloped dissolution surfaces, gypsum moulds and anhydrite nodule pseudomorphs provides evidence for subaerial exposure. In contrast, stromatolites in a unit at the base of the Loves Creek Member accreted during a gradual rise in sealevel. Stratiform, columnar and domal stromatolitic building blocks of the shallowing upward cycle are present in this deepening sequence, but only the lower half of the shallowing upward cycle is represented. Synoptic relief of the stromatolitic laminae gradually increases upward throughout the basal stromatolitic unit. Recognition of a deepening upward stromatolite sequence at the base of the Loves Creek Member, and a disconformity surface between this sequence and the underlying Gillen Member, permits palaeoenvironmental re‐interpretation of the Loves Creek Member as a single ‘large scale’ sea‐level cycle.

Diagenesis of sabkha-related, sulphate nodules in the early Proterozoic Gordon Lake formation, Ontario, Canada

Unusually old (c. 2.4 Ga) loosely-packed anhydrite nodules and replacement nodules of gypsum, quartz and jasper are well preserved in transgressive marine siliciclastic sabkha sediment of the Huronian Gordon Lake Formation. There is no evidence of a gypsum precursor. The nodules are commonest in mud chip breccia at the base of sandstone, siltstone and mudstone upward-fining storm cycles. Anhydrite nodules are composed of a mosaic of blocky crystals. An earlier lath form is preserved in silicified outer rims. Its texture is similar to that of Recent displacive sabkha anhydrite nodules. Siliceous nodules are composed of megaquartz, some calcite-cored, or of jasper. Megaquartz is normally anhedral with strongly undulose (flamboyant) extinction. Crystals may be columnar or crudely spheroidal. Some megaquartz crystals are lath-shaped with the oblique extinction of lutecite. Rarer euhedral megaquartz in calcite nodule cores may have simple extinction but was probably not formed by cavity filling. Megaquartz crystals and anhydrite inclusions may preserve the texture of pre-existing early anhydrite laths. Smaller “jasper” nodules are composed of hematite-bearing fibrous fine-grained quartz, that is coarser than true jasper. Fibrous structure in this jasper may have oblique, length fast or length slow extinction, suggestive of recrystallization after lutecite, length fast and length slow chalcedony. True chert and chalcedony (<20μ) are absent. The jasper of the smaller nodules may contain ghost laths, after anhydrite? and scattered anhydrite and carbonate crystals. The jasper of larger nodules is granular. The jasper nodules contain rare gypsum crystals as well as anhydrite crystals which have been replaced to varied degrees by pseudomorphous quartz mosaics and carbonate rhombs. Some jasper nodules have an outer rim of megaquartz with anhydrite inclusions and associated carbonate. Some larger jasper nodules may have been formed in subaqueous sediments. Some weakly-silicified anhydrite nodules have been hydrated to gypsum nodules that are connected with subhorizontal gypsum veinlets. The hydration postdates silicification and conversion of anhydrite laths to blocky anhydrite. The veinlets, probably related to late unroofing, are a possible access route for water of hydration. They are later than host-rock lithification and calcite formation in the anhydrite nodules. Where the Gordon Lake Formation is metamorphosed megaquartz replacement nodules can be recognized by inclusions of anhydrite and interstitial calcite patches. These nodules are a useful indicator of vanished evaporites in such rocks. If the evaporite nodules of the Gordon Lake Formation, and the replaced evaporites of the Kona Dolomite of Michigan and the Peribonca Formation of Quebec are stratigraphically equivalent they indicate a cupriferous sabkha with an original extent of at least 1400 km.