ABSTRACT Petrographic, geochemical, and fluid inclusion analysis of dolomite and calcite cements has been conducted on Mississippian carbonates collected from the surface and subsurface of the southern midcontinent of the United States (Oklahoma, Missouri, Kansas, and Arkansas). Fracture and vug, intergrain, and intragrain porosity are filled with calcite, authigenic quartz, and dolomite cements. Primary limestone porosity is filled partially by early marine and meteoric calcite cements. Equant (blocky) calcite cements were precipitated under seawater or mixed meteoric-seawater conditions in the phreatic zone and in the deep phreatic zone under late (burial) diagenetic conditions. Fracture- and breccia-filling saddle dolomite cements that were observed are late diagenetic and are likely related to the nearby Tri-State Mississippi Valley-type (MVT) mineral district. Carbon and oxygen isotope values of dolomite cements range from δ 18 O (VPDB) = −9.5 to −2.7‰ and from δ 13 C (VPDB) = −4.0 to −0.4‰. Values for calcite cements range from δ 18 O (VPDB) = −11.6 to −1.9‰ and from δ 13 C (VPDB) = −12.2 to +4.6‰. These values are consistent with three types of diagenetic fluids: seawater, seawater modified by meteoric water, and evolved basinal water. Analysis of fluid inclusions in late calcite, dolomite, and quartz cements indicates the presence of both dilute and high salinity end-member fluids. Homogenization temperatures (T h ) of fluid inclusions range from 57°C to 175°C and salinities range from 0 to 25 equivalent weight % NaCl. Fluid inclusion T h values and salinities are consistent with a saline basinal fluid variably diluted by fluids of meteoric or mixed seawater and meteoric origin. Petroleum inclusions were observed in late diagenetic calcite and dolomite cements.The late diagenetic cements filled porosity retained after early diagenetic cementation indicating that some original porosity in the Mississippian carbonate rocks remained open during petroleum migration. Elevated fluid inclusion T h values over a broad region, not just in the Tri-State Mineral District, imply that the regional thermal maturity of rocks may be higher than believed previously. This study indicates that the Mississippian carbonate resource play on the southern midcontinent has a very complex diagenetic history, continuing long after early diagenetic cementation. Possibly the most important diagenetic events affecting these rocks occurred during burial and basinal fluid migration through these strata.

Abstract The Middle Cambrian through Lower Ordovician carbonate rocks of North America host some of the largest economic Mississippi Valley-type (MVT) base-metal sulfide deposits in the world. These rocks also host numerous subeconomic MVT deposits, minor and trace occurrences of mineralization, and hydrocarbon fields. Mississippi Valley-type deposits commonly contain bitumen, pyrobitumen, and/or liquid petroleum, suggesting that MVT mineralization is associated with the generation and migration of hydrocarbons and thus is a normal part of basin evolution. In addition to sulfide and sulfate mineralization, common characteristics of MVT deposits are large-scale dissolution and brecciation of carbonate rocks, precipitation of large volumes of dolomite and calcite cements, epigenetic (hydrothermal) dolomitization, and recrys-tallization of preexisting dolomite. Mineralizing fluids have the effects both of increasing the original porosity by dissolution and brecciation and of occluding porosity because of precipitation of cements. Mississippi Valley-type fluids are not localized but affect sedimentary rocks across large regions. It is likely that most, if not all, Cambrian–Ordovician carbonate rocks in North America have undergone at least some diagenetic alteration because of exposure to these fluids. This conclusion is supported by the observation that subeconomic MVT mineralization has been observed in Cambrian and Lower Ordovician carbonates throughout much of North America. These fluids commonly have affected carbonate petroleum reservoir rocks in regions distal from known ore deposits. Mississippi Valley-type mineralization is believed to result from a complex mixing and/or cooling of saline fluids expelled from sedimentary basins. These fluids have temperatures ranging from 60 to 250°C. Most of the fluids originate from evaporated seawater or water that has dissolved halite and that has interacted with sedimentary rocks and, possibly, basement rocks. Several geochemical and hydrogeological mechanisms have been proposed for MVT deposits. However, the precise mechanisms driving fluid flow and deposition are not yet completely understood. Major tectonic events associated with MVT mineralization of the great American carbonate bank strata include the Acadian orogeny (Late Devonian–Early Mississippian) for early mineralization in the Appalachian Mountain region, the Alleghanian-Ouachita orogeny (Pennsylvanian–Permian) for mineralization in the Appalachian and midcontinent regions, and the Laramide orogeny (Late Cretaceous–early Tertiary) for the Cordilleran region.

Abstract The Brooks Range is a north-directed fold and thrust belt that forms the southern boundary of the North Slope petroleum province in northern Alaska. Field-based studies have long recognized that large-magnitude, thin-skinned folding and thrusting in the Brooks Range occurred during arc-continent collision in the Middle Jurassic to Early Cretaceous (Neocomian). Folds and thrusts, however, also deform middle and Upper Cretaceous strata of the Colville foreland basin and thus record a younger phase of deformation that apatite fission-track data have shown to occur primarily during the early Tertiary (~60 and ~45 Ma). A structural and kinematic model that reconciles these observations is critical to understanding the petroleum system of the Brooks Range fold and thrust belt. New interpretations of outcrop and regional seismic reflection data indicate that from the modern mountain front northward to near the deformation front under the coastal plain, the basal thrust detachment for the orogen is located in the Jurassic and Lower Cretaceous Kingak Shale in the upper part of the regionally extensive, gently south-dipping, north-derived Mississippian to Early Cretaceous Ellesmerian sequence. The frontal part of the orogen lies in middle Cretaceous foreland basin strata and consists of a thin-skinned fold belt at the deformation front and a fully developed passive-roof duplex to the south. Near the mountain front, the orogen is composed of a stacked series of allochthons and thrust duplexes and associated Neocomian syntectonic deposits that are unconformably overlain by proximal foreland basin strata. The foreland basin strata and underlying deformed rocks are truncated by a younger generation of folds and thrusts. Vitrinite reflectance and stable isotope compositions of veins provide evidence of two fluid events in these rocks, including an earlier higher temperature (~250–300°C) event that was buffered by limestone and a younger, lower temperature (~150°C) event that had distinctly lower δ 13 C values as a result of oxidation of organic matter and/or methane. Zircon fission-track data from the host rocks of the veins show that the higher temperature fluid event occurred at 160–120 Ma, whereas the lower temperature event probably occurred at about 60–45 Ma. It is proposed that the Brooks Range consists of two superposed contractional orogens that used many of the same mechanically incompetent stratigraphic units (e.g., Kayak Shale, Kingak Shale) as sites of thrust detachment. The older orogen formed in a north-directed arc-continent collisional zone that was active from 160 to 120 Ma. This deformation produced a thin-skinned deformational wedge that is characterized by fartraveled allochthons with relatively low structural relief, because it involved a thin (1–4-km [0.6–2.5-mi]-thick) stratigraphic section. Deeper parts of the deformational wedge are envisioned to have contained relatively high-temperature fluids that presumably migrated from or through limestone-rich source areas in the underlying autochthon or from deeper parts of the orogen. The younger orogen, which formed initially at about 60 Ma and reactivated at 45 Ma, produced a thrust belt and frontal triangle zone with low amounts of shortening and relatively high structural relief, because it involved a structural section 5–10 km (3–6 mi) thick. Fluids associated with this deformation were relatively of lower temperature and suggest that hydrocarbon migration occurred at this time. We conclude that hydrocarbon generation from Triassic and Jurassic source strata and migration into stratigraphic traps occurred primarily by sedimentary burial principally at 100–90 Ma, between the times of the two major episodes of deformation. Subsequent sedimentary burial caused deep stratigraphic traps to become overmature, cracking oil to gas, and initiated some new hydrocarbon generation progressively higher in the section. Structural disruption of the traps in the early Tertiary released sequestered hydrocarbons. The hydrocarbons remigrated into newly formed structural traps, which formed at higher structural levels or were lost to the surface. Because of the generally high maturation of the Colville basin at the time of the deformation and remigration, most of the hydrocarbons available to fill traps were gas.

Abstract The bulk of the replacive dolomite in the Lower Carboniferous (Dinantian) of Ireland is characterized by planar texture and (C, O) isotope signatures that are consistent with a modified Dinantian seawater origin at near-surface temperatures. Only a volumetrically minor (~ 5%) part of the dolomite is characterized by nonplanar texture. Petrographic evidence for fabric-preserving neomorphism of mimetic replacement planar dolomite is the preservation of concentric ooid textures and planar dolomite zonation (in plane-polarized light) retained within mosaics of mainly planar/nonplanar and nonplanar crystals. Neomorphosed dolomite commonly forms an interlocking mosaic of planar/nonplanar crystals and appears to have slightly increased crystal size relative to fine crystalline planar dolomite. In cathodoluminescence, neomorphosed dolomite tends to possess a uniform medium-intensity red color that lacks zonation. The oxygen isotope data from the planar and neomorphosed dolomites (planar and planar/nonplanar morphologies) further supports the petrographically defined division of the two dolomite types.

Abstract A numerical model is developed to calculate the rates at which minerals precipitate or dissolve in basin strata as groundwaters migrate along temperature and pressure gradients. The calculation is based on the assumption that minerals maintain local equilibrium with migrating groundwater and their solubilities depend only on temperature or temperature and pressure. The model integrates predicted groundwater flow patterns with geochemical reaction path modeling; this approach allows us to predict the rate at which minerals dissolve and precipitate in complex geochemical systems open to groundwater flow and mass transfer. The model is formulated and solved in geologic time and basin distance scales and can therefore be applied to study basin-wide diagenesis related to long-distance fluid migration. The calculation can adjust sediment porosity from the net volume of precipitation and dissolution, therefore accounting for feedback effects of chemical diagenesis on porosity, which in turn affects permeability and fluid flow. The model is used to study the rates and nature of diagenetic alteration in several hydrologic systems, including (1) diagenesis of quartz by flow through a wavy sandstone, a sloping aquifer and a faulted aquifer, (2) cementation of amorphous silica and its feedback effect on thermal convection, (3) cementation of anhydrite in the Lyons Sandstone, Denver basin, and (4) diagenesis by migrating brines in the deep aquifers of the Illinois basin. The sample calculations shed light on the rates and patterns of chemical diagenesis that Likely accompany fluid migration in sedimentary basins. When the predicted results can be compared to diagenetic patterns observed in basin strata, the model provides an interpretation for the origin of diagenetic alteration.

Abstract Precambrian basins in Australia and Canada host massive unconformity-type uranium ore deposits, which constitute roughly 25% of the world’s known uranium resources and are the focus of international study as ancient analogs for nuclear waste repositories. Simulation of reactive mass transport and variable-density groundwater flow within these basins indicates that free convection of a uranium-bearing chloride brine at rates of about 1 ni/yr precipitated ore-grade quantities of uraninite (U0 2 ) near the unconformity between basin sandstones and graphiterich basement rocks within 100 to 1000 ky. Hydrothermal/diagenetic alteration surrounding the ore deposits resulted from mass transport through temperature gradients and across compositional boundaries. This alteration, consisting primarily of chlorite and muscovite precipitation, is strongly dependent on the pattern of groundwater flow and heat transport within the basins. The primary condition required for mineralization is the coincidence in space of graphite-rich basement rocks and upwelling limbs of free convection cells.

Abstract Models of burial diagenesis in the Gulf of Mexico sedimentary basin must explain a wide variety of phenomena, including: (1) uranium mineralization in volcanogenic Oligocene sandstones where the sandstones overlie growth fault zones in Eocene units, (2) lead-zinc mineralization in salt dome caprocks, with dissolved lead and zinc being known only from saline formation waters locally present in deeply buried Mesozoic reservoirs, (3) discharge of NaCl at the land surface, contributing to the dissolved chloride load of rivers, (4) natural seepage of oil and gas, which also leads to “vent” marine communities and IJ C-depleted CaCO,-cemented zones on the continental shelf and slope, (5) the presence of hydrocarbons above, and not uncommonly displaced laterally by ten’s or hundred’s of kilometers from mature source rocks, and (6) allochthonous, non-metalliferous saline water in Cenozoic clastic units. Fluid movement along faults is important in most, if not all of these processes. Convection within the self-fractured overpressured zone is inferred, based on the volumes of water necessary to: (1) remove Si0 2 and CaC0 3 from mudrocks and emplace authigenic quartz and calcite cements in sandstones, (2) transfer K 2 0 from feldspar dissolution in sandstones into mudrocks, where it is consumed by illitization, (3) remove sufficient volumes of hydrocarbons from “lean” Gulf Coast mudrocks as kerogen maturation proceeds and transport the hydrocarbons to permit the accumulation of significant volumes of oil and gas. Because the basement rocks beneath Gulf sediments are probably undergoing prograde metamorphism and devolatiUzation, material transfer into the sedimentary basin from the basement is inferred. The rate at which water (and C0 2 ) are added to the sedimentary basin from underlying rocks can potentally affect not only the volumes of water available for diagenesis but can maintain geopressures (and convection) within the sedimentary section long after pressure would normally decay back to hydrostatic values if compaction were the only operative process.

Abstract Most sedimentary basins contain close to 20% by volume pore water; much of which is of high ionic strength and is classified as brine. Although considerable variation occurs in the composition of basinal waters, some correlation exists between major ion concentrations and total dissolved solids. It is consequently possible to construct geochemical models for basinal waters that have general utility for investigating a variety of subsurface processes. In this paper, we examine the potential role of brine composition and movement on carbonate mineral mass transport in sedimentary basins up to pressure and temperature conditions of 300 bars and 100°C. Primary emphasis is placed on the impact of vertical and lateral migration of brines and the dispersive mixing of brines with waters containing widely varying concentrations of total dissolved solids. Model results indicate that mixing of subsurface waters may produce fluids which are highly supersaturated and at times slightly undersaturated with respect to calcite. This process may represent a major mechanism for production and destruction of carbonate cements in sediments. It may also offer an explanation as to how basinal scale mass transfer of carbonates can occur in waters that are close to equilibrium with respect to major sedimentary carbonate minerals.

Abstract Burial diagenesis of platform carbonates is a very complex process occurring over tens of millions of years, encompassing several different tectonic settings, exhibiting many diagenetic products and involving waters of diverse origins and compositions. To facilitate understanding of these processes, burial diagenesis of platform carbonates is divided into three hydrotectonic realms: (1) passive margin burial diagenesis, (2) collision margin burial diagenesis, and (3) post-orogenic burial diagenesis. The passive margin burial diagenetic realm, exemplified by the northern Gulf of Mexico, is characterized by extensional tectonics, growth faulting, a relatively uniform subsidence rate, slow upward flow of compaction-driven fluids, and increases in temperature, pressure and salinity of pore waters with burial depth. The passive margin burial diagenetic realm can be divided into three subrealms: (1) pre-oil window diagenesis (<100°C), (2) oil-window diagenesis (100-140°C) and (3) gas-window diagenesis (140-200°C). Quantitative analyses of Upper Jurassic calcite ooid grainstones that have experienced all 3 subrealms indicate that, of the original 45% porosity, an average of 12% can be filled by carbonate cement, whereas 33% is lost by compaction and pressure solution. Unless a well-developed convection system exists, low HC0 3 contents of formation waters and slow fluid flow rates in passive margin settings suggest that most carbonate cements are derived locally from pressure solution of host carbonates. In passive margin settings, Mg 2 + contents of burial calcite cements should initially increase and then decrease with burial. This trend develops because initial increases in Mg 2+ distribution coefficient (DMg 2+ ) with temperature outweigh decreases in the mMg 2 V mCa 2+ ratio of pore waters, resulting in increases in Mg 2+ contents of calcite cements. But later, continued lowering of mMg 2+ /mCa 2+ ratios reverses this trend. Data suggest that Sr 2+ contents of calcite cements, initially very low, increase with temperature during deep burial. Wide variations in mFe^/mCa 2 * and mMn 2+ /mCa 2+ ratios of formation waters indicate that concentrations of Fe 2+ and Mn 2+ in carbonate cements are of limited value beyond local scales. The 8 ,8 0 compositions of calcite cements first decrease and then increase during passive margin burial. This trend develops because temperature-dependent calcite-water fractionation produces an initial decrease in 8 18 0 values of cements; but later, progressive increases in 8 ,8 0 values of formation waters overcome temperature fractionation effects, resulting in increases in 8 ,s O composition of carbonate cements. The 8 ,3 C values of carbonate cements can follow either one of two paths. In the absence of hydrocarbons, 8 I3 C compositions of pore waters are buffered by host carbonates and change little with burial. Where hydrocarbons are present, their decarboxylation at temperatures above 150°C promotes precipitation of carbonate cements with low 8 ,3 C values. Therefore, two burial 8 18 0-d l3 C trends are generated: (1) a “C trend” when organic-derived C0 2 is not available and (2) a “D trend” when hydrocarbon destruction generates significant amounts of CO,. The collision margin burial diagenetic realm, exemplified by the Ouachita collision belt, is characterized by compressional tectonics, thrust faulting, variable uplift/subsidence rates and episodic focused expulsion of tectonic fluids toward the craton. Three subrealms are recognized based on the intensity and types of diagenetic alterations: thrust belt diagenetic zone, foreland diagenetic zone, and craton margin diagenetic zone. The more important carbonate diagenetic events in collision margin settings are: (1) extensive pressure solution and fracture-fi11 carbonate cementation, (2) Mississippi Valley-type (MVT) mineralization, and (3) K-feldspar and magnetite precipitation. As much as 50% (by volume) of carbonates can dissolve by tectonic pressure solution and be available to precipitate in tectonic fractures. H 2 S produced by thermochemical sulfate reduction in deep carbonate reservoirs during passive margin settings is expelled during collision margin diagenesis to react with base metal-rich fluids in shallow depths leading to the formation of MVT ore deposits. Sulfide mineralization generates significant amounts of acids which can dissolve syndepositional dolomites and reprecipitate them as burial dolomites. K-feldspars form when K + -rich pore waters (generated by K-feldspar dissolution during passive margin diagenesis) are flushed to shallow parts of the basin. The post-orogenic burial diagenetic realm, exemplified by the Madison aquifer in the U.S. midcontinent, is characterized by a lack of tectonic activity, dominance of topographically-driven fluid flow and high fluid flow rales. Rainwater charged with soil C0 2 enters aquifers in highland recharge areas dissolving carbonates and evaporites along its flow paths. Temperature, pressure, alkalinity, pH, Ca 2 + , Mg 2+ , Cl + and Na + concentrations of groundwater increase and Eh decreases toward the discharge area. Important diagenetic processes in this realm are evaporite and carbonate dissolution, dedolomitization, calcite precipitation and bacterial sulfate reduction.

Abstract Upper Cambrian and Lower Ordovician sedimentary rocks in southeastern Missouri host the world-class Mississippi Valley-type (MVT) lead-zinc deposits of the region. Sedimentary facies of the lower part of the Upper Cambrian section are dominated by distinct clastic and carbonate facies belts associated with a high relief Cambrian topography. The upper part of the Upper Cambrian (post Davis Formation) and the Lower Ordovician section were deposited under epeiric sea conditions on a low relief topography. These latter rocks are characterized by cyclic sequences of shallow-water platform carbonates, with good lateral continuity of facies. The Davis Formation is composed of interbedded carbonates and shales and forms an effective aquiclude separating the upper and lower parts of the section into two distinct aquifers. Petrographic and cathodoluminescent studies of epigenetic dolomite cements in these Cambro-Ordovician sedimentary rocks document that: (1) dolomite cements of the Bonneterre Dolomite (lower aquifer) in the Viburnum Trend and the Old Lead Belt, which are related closely to mineralization, have a relatively complex, four zone cathodoluminescence (CL) pattern; (2) dolomite cements, in the Bonneterre and Davis Formations, which are not related spatially to mineralization commonly display less complex CL patterns; and (3) dolomite cements in the post- Davis part of the Cambrian and in the lower Ordovician section (upper aquifer) display a CL stratigraphy unrelated to that observed lower in the section. Carbon isotope compositions of host dolomite show two types of statistical variation. From the bottom of the Bonneterre Dolomite to the top of the Davis Formation, 8 I3 C values become higher (from -2.5 toward +3.0%»). Above the Davis to the Lower Ordovician, the trend reverses and 8 n C values become lower (toward — 3.0%o). A similar trend exists for 5 I8 0 values in the Bonneterre and Davis, as values become higher up section. However, above the Davis Formation, 8 1S 0 values for host dolomites display no statistical trends. The pattern of upwardly decreasing 8 I3 C values in post-Davis rocks may be the result of a secular trend in ocean carbon. The trend of upwardly increasing S' 3 C and 8 I8 0 values in the Bonneterre Dolomite and Davis Formation is likely the result of interaction with hydrothermal fluids emanating from the underlying Lamotte Sandstone, reflecting increased buffering by host dolomite as l2 C- and l6 0-enriched light fluids moved higher in the section. Distribution of sedimentary facies had a profound effect on the hydrological framework of southern Missouri during mineralization. Linear facies belts that developed on high-relief topography during deposition of the Bonneterre and Davis strata resulted in focused fluid flow and a greater degree of alteration of host dolomite. Broad, laterally continuous distribution of sedimentary facies in post-Davis rocks resulted in less focused fluid flow and less alteration of the host dolomite. The distinct C and O isotopic trend observed in the Bonneterre-Davis Formations versus that observed in post-Davis Formation rocks, coupled with differences in CL microstratigraphies of dolomite cements, indicate that the these two parts of the section acted as distinct aquifers, with relatively little fluid communication during mineralization.