ABSTRACT Detailed diagenetic studies of the late Cambrian Alum Shale in southern Sweden were undertaken across an interval that includes the peak Steptoean Positive Carbon Isotope Excursion (SPICE) event to evaluate the pyrite mineralization history in the formation. Samples were collected from the Andrarum-3 core (Scania, Sweden); here the Alum was deposited in the distal, siliciclastic mudstone-rich end of a shelf system. Abundant cryptobioturbation is observed in the Alum, which points to oxic–dysoxic conditions prevailing during deposition. Petrographic examination of polished thin sections ( n = 65) reveals the presence of numerous texturally distinct types of pyrite, including matrix framboids, two different types of framboid concretions (those with rims of iron-dolomite and those lacking rims), disseminated euhedral pyrite crystals, concretions of euhedral pyrite crystals, overgrowths of pyrite on these different pyrite generations, anhedral pyrite intergrown with bedding parallel mineralized fractures (i.e., “beef”), and massive vertical/subvertical accumulations of pyrite. Paragenetic relationships outline the relative timing of formation of the texturally distinct pyrite. Framboids and framboid concretions formed prior to precipitation of any euhedral pyrite crystals, and these pyrite generations precipitated prior to the pyrite overgrowths on them. As Alum Shale sediments are all distorted by these texturally different pyrite generations, they are likely to have formed early in the postdepositional history of the formation. In contrast, pyrite associated with “beef” is likely temporally related to the onset of hydrocarbon generation, which in this part of Sweden is thought to have been many tens of millions of years after deposition. Because vertical/subvertical massive pyrite features distort “beef,” they clearly postdate it. Of all these pyrite textures, only framboid concretions appear to be restricted to the SPICE interval. The texturally distinct nature of the pyrite generations, along with evidence of their formation at different times in the postdepositional history of the Alum Shale, is the key outcome of this petrographic study. Because the petrographic data presented herein point to a postdeposition origin for all generations of pyrite, diagenetic processes—not those processes associated with deposition—were responsible for the complex pyritization history observed in the Alum, in the Andrarum-3 core.

Laguna Mar Chiquita (central Argentina; ~latitude 31°S, longitude 63°W) provides an outstanding opportunity to examine organic facies development and petroleum source-rock potential in a modern thick-skinned foreland basin lake. In this case study, we define profundal, paleodelta, and lake-margin depositional environments based on trends in bathymetry and lake-floor sediment particle size. Sedimentary geochemical analyses indicate that organic carbon–rich muds accumulate in profundal environments during the extant lake-level highstand. The lateral variability of organic facies is minimal. The quality of organic facies is controlled by lake level and depositional environment, both of which dictate patterns of algal productivity, siliciclastic dilution, and early diagenesis. We present conceptual models of lacustrine source rocks in both thick-skinned and thin-skinned foreland basins based on modern analog data from both Laguna Mar Chiquita and other lakes in the central Andean foreland. Over relatively short time intervals (10 2 –10 4 yr), climatically driven water-level fluctuations influence the source-rock potential of these basins. Over time intervals >10 5 yr, contraction and lateral migration of the basin flexural profile control stratal stacking patterns and the potential for hydrocarbon play development.

This study examines the relationship between depositional environment and sedimentary organic geochemistry in Lake Malawi, East Africa, and evaluates the relative significance of the various processes that control sedimentary organic matter (OM) in lacustrine systems. Total organic carbon (TOC) concentrations in recent sediments from Lake Malawi range from 0.01 to 8.80 wt% and average 2.83 wt% for surface sediments and 2.35 wt% for shallow core sediments. Hydrogen index (HI) values as determined by Rock-Eval pyrolysis range from 0 to 756 mg HC g −1 TOC and average 205 mg HC g −1 TOC for surface sediments and 228 mg HC g −1 TOC for shallow core samples. On average, variations in primary productivity throughout the lake may account for ~33% of the TOC content in Lake Malawi sediments (as much as 1 wt% TOC), and have little or no impact on sedimentary HI values. Similarly, ~33% to 66% of the variation in TOC content in Lake Malawi sediments appears to be controlled by anoxic preservation of OM (~1–2 wt% TOC), although some component of the water depth–TOC relationship may be due to physical sediment transport processes. Furthermore, anoxic preservation has a minimal effect on HI values in Lake Malawi sediments. Dilution of OM by inorganic sediment may account for ~16% of variability in TOC content in Lake Malawi sediments (~0.5 wt% TOC). The effect of inputs of terrestrial sediment on the organic character of surface sediments in these lakes is highly variable, and appears to be more closely related to the local depositional environment than the regional flux of terrestrial OM. Total nitrogen and TOC content in surface sediments collected throughout the lake are found to be highly correlated (r 2 = 0.95), indicating a well-homogenized source of OM to the lake bottom. The recurring suspension and deposition of terrestrial sediment may account for significant amounts of OM deposited in offshore regions of the lake. This process effectively separates denser inorganic sediment from less dense OM and allows terrestrial OM to preferentially be transported farther offshore. The conclusion is that for the organic carbon content in these regions to be elevated a mixed terrestrial-lacustrine origin is required. The hydrodynamic separation of mineral and organic constituents is most pronounced in regions with shallow bathymetric gradients, consistent with previous findings from Lake Tanganyika.

Substantial amounts of authigenic clay minerals can accumulate in terrestrial mudstones where the following conditions are met: surface or pore waters are alkaline, aqueous silica activity is high, at least some dissolved magnesium (Mg) is present, and detrital input is relatively low. Availability of Al- or Fe-rich detrital clays likely leads to Mg-rich smectite formation, whereas sepiolite or kerolite is favored in environments with no detrital substrates. Surface waters of the Ngorongoro Crater, Tanzania, provide a good example of alteration of incoming detrital clay minerals into authigenic sediments with strong partitioning of Mg into silicate rather than carbonate phases. Quaternary deposits of Olduvai Gorge (Tanzania) and the Olorgesailie Basin (Kenya) provide end members for comparing processes in highly saline and alkaline settings (Olduvai) versus those in diatomaceous fresher water environments (Olorgesailie). Authigenic clays from around the world suggest that illitization and octahedral alterations are decoupled, and therefore indicative of different processes, emphasizing the need to supplement basal layer X-ray diffraction analyses with analyses of hkl reflections and geochemistry of purified phases. In general, authigenic clay minerals are more common in underfilled lake basins, usually associated with evaporitic basins with siliceous input from volcaniclastics or hydrothermal discharge.

Short-term sediment accumulation directly on basalt flows without the damming of drainage is not well studied. Can lakes form on basalt surfaces during times of volcanic quiescence between outpourings? Large faulted downthrown blocks of the lower Jurassic Kalkrand basalt within the Hardap Recreational Resort in central Namibia provided depressions for sediment accumulation during two periods of volcanic quiescence. The sedimentary infill was preserved as two sedimentary interlayers from 6 to 8 m in thickness between lava flow units. These sedimentary rocks, formerly interpreted as dominantly lacustrine deposits, now can be recognized as alluvial fill because of the presence of pedogenic mud aggregates. Sedimentation processes filling these fault-bound depressions included sheetflooding and grain flow, archived in successions containing sandstone, siltstone, and mudrock. Large cracks and fractures on the basalt surfaces, filled with quartz sandstone, showed that the regional water table was below the basalt flow surface, making the formation of lakes impossible, especially because of the permeability and porosity of such flows in general. No pattern or cyclicity during sediment deposition was found and paleocurrent measurements showed random sedimentary influx directions, probably related to the lava field surface topography. Autochthonous weathering of basalts added to the allochthonous drainage input of quartz-rich sand and silt that produced sandstone and siltstone containing smectitic clays. Most of these clay-rich siltstones and sandstones contained mud laminae within traction-load sedimentary structures, providing evidence for the presence of pedogenic mud aggregates within these sheetflood deposits. Geothermal groundwaters circulating through the basalt and the sediments, mixing with meteoric waters, produced Ca-rich diagenetic waters with an elevated temperature to precipitate calcite cement and dendritic crystals. This work demonstrates that characteristics of mud deposition can provide important clues in determining the true depositional paleoenvironment of these Jurassic sedimentary interlayers between basalt flows.

The Cambrian Alum Shale Formation in the Andrarum-3 core from Scania, southern Sweden, consists of black siliciclastic mudstone with minor carbonate intercalations. Four facies comprise three siliciclastic mudstones and one fine-grained carbonate. The facies reflect deposition along a transect from deep ramp to basin on a Cambrian shelf. The three mudstone facies contain abundant clay clasts and laterally variable siltstone laminae. Bed-load transport processes seem to have dominated deposition on this deep shelf. These sedimentary rocks record mainly event deposition, and only relatively few, thin laminae probably resulted from suspension settling. The Alum Shale Formation deep shelf did not show a bioturbation gradient, but fecal strings are common and Planolites burrows are rare in all mudstone facies. Evidence for biotic colonization indicates that this mudstone environment was not persistently anoxic, but rather was most likely intermittently dysoxic. The Alum Shale Formation in the Andrarum-3 core shows an overall decrease of grain size, preserved energy indicators, and carbonate content upsection interpreted to reflect a deepening upward. The succession can also be divided into four small-scale fining-upward cycles that represent deepening, and four overlying coarsening-upward cycles that represent upward shallowing.

The origin of clays and clay minerals in the Paleocene Clayton (CF) and Porters Creek (PCF) Formations within the Mississippi embayment of central North America has been debated for more than 50 years. X-ray diffraction and petrographic analysis of samples of the CF and PCF from a mine in southeastern Missouri are used to evaluate contributions from Cretaceous-Paleogene (K-Pg) impact debris and the role of sediment diagenesis in the fine-grained sediment. Expandable clay minerals increase in abundance relative to illite and kaolinite above the K-Pg unconformity in the CF and PCF, and include dioctahedral smectite, vermiculite, and minor mixed-layered clay components, along with trioctahedral smectitic clays in the CF and lowermost PCF. Additional diagenetic phases include clinoptilolite (in the CF and lower PCF), pyrite, siderite, and opal CT (mainly in the PCF). The results of the petrographic analysis show no evidence for volcanic ash contributing directly to the sediment in the PCF. The detrital silicate minerals are mainly quartz, muscovite, biotite, and metamorphic minerals, consistent with an ancestral Appalachian Mountains source rather than volcanic ash or a Cretaceous western interior sediment source. The illite, kaolinite, dioctahedral smectite, and ordered illite-smectite mixed-layered clays are present in varying quantities in Cretaceous through Paleogene marine and nonmarine mudstones from the Mississippi embayment, and appear to be detrital in origin. Trioctahedral smectite and clinopti-lolite in the CF and lowermost PCF are argued to derive from alteration of glassy impact debris; clasts in the basal CF contain microtektites replaced by trioctahedral smectitic mixed-layered clay with randomly interstratified illite. The X-ray diffraction characteristics of the vermiculite in the PCF indicate a hydroxyl-interlayered aluminous variety that is argued to have a diagenetic origin, formed by clay mineral reactions under variably anoxic conditions in the PCF sediments during early diagenesis. Throughout the CF and PCF, pyrite precipitated under reducing conditions during diagenesis, locally along with siderite replacement of micritic carbonate. In addition, opal-CT precipitated as a result of silicate reactions and dissolution of diatoms under alkaline conditions and replaced matrix and calcitic microfossils throughout much of the upper PCF.

The organic-rich upper shale member of the upper Devonian–lower Mississippian Bakken Formation (Williston Basin, North Dakota, USA) has undergone significant diagenetic alteration, irrespective of catagenesis related to hydrocarbon generation. Alteration includes precipitation of numerous cements, replacement of both detrital and authigenic minerals, multiple episodes of fracturing, and compaction. Quartz authigenesis occurred throughout much of the member, and is represented by multiple generations of microcrystalline quartz. Chalcedonic quartz fills radiolarian microfossils and is present in the matrix. Sulfide minerals include pyrite and sphalerite. Carbonate diagenesis is volumetrically minor and includes thin dolomite overgrowths and calcite cement. At least two generations of fractures are observed. Based on the authigenic minerals and their relative timing of formation, the evolution of pore waters can be postulated. Dolomite and calcite resulted from early postdepositional aerobic oxidation of some of the abundant organic material in the formation. Following aerobic oxidation, conditions became anoxic and sulfide minerals precipitated. Transformation of the originally opaline tests of radiolaria resulted in precipitation of quartz, and quartz authigenesis is most common in more distal parts of the depositional basin where radiolaria were abundant. Because quartz authigenesis is related to the distribution of radiolaria, there is a link between diagenesis and depositional environment. Furthermore, much of the diagenesis in the upper shale member preceded hydrocarbon generation, so early postdepositional processes were responsible for occlusion of significant original porosity in the member. Thus, diagenetic mineral precipitation was at least partly responsible for the limited ability of these mudstones to provide porosity for storage of hydrocarbons.

Pyrite framboid diameters were examined in 31 samples taken from 2 Marcellus Shale cores recovered from Greene County, Pennsylvania, and Upshur County, West Virginia (USA). Analysis of framboid diameters in those samples from the more proximally located Upshur County core suggests that anoxic to anoxic-euxinic conditions persisted during accumulation of the transgressive-regressive cycle (MSS1) that comprises the Union Springs Member of the Marcellus Shale, with intermittent episodes of dysoxia. An increased abundance of large framboids documented from the overlying transgressive-regressive cycle (MSS2), which comprises the bulk of the Oatka Creek Member of the Marcellus Shale, indicates improved bottom-water conditions. Redox conditions recorded by framboid diameters of the MSS1 cycle of the Greene County core are generally similar to those of the Upshur County core; however, conditions in that region of the basin from which the Greene County core was recovered appear to have remained dominantly anoxic to anoxic-euxinic. Furthermore, the presence of small syngenetic framboids and large diagenetic framboids in the same thin section samples suggests that redox conditions fluctuated on a temporal scale beyond that observed at the scale of a centimeter-scale thin section. Framboid diameter trends established for both cores enhance our understanding of how much redox conditions varied both spatially and stratigraphically during accumulation of the Marcellus Shale.

Abstract A study of the Upper Devonian-Lower Mississippian Woodford Shale was undertaken on samples at low thermal maturity from the Arbuckle Mountains, southern Oklahoma, to dientify possible mechanisms by which natural gas might be stored in Woodford reservoirs in the adjacent Anadarko Basin. The two main lighologies in the Woodford, chert and mudstone, display different inorganic and organic characteristics. Cherts have (1) variable porosity from 0.59% to 4.90%, (2) low calculated permeabilities, and (3) small mean pore apertures. Intercrystalline pores dominate in cherts. In contrast, mudstones generally have (1) porosities ranging from 1.97% to 6.31%, (2) low calculated permeabilities, and (3) small mean pore apertures. Interparticle, intraparticle, and moldic pores all are present in mudstones. Because of their high quartz content, cherts are brittle and commonly demonstrate microfracturing that is lithologically controlled and bedding perpendicular, whereas much less microfracturing exists in mudstones. The early diagenetic intercrystalline porosity in cherts has likely been preserved since it formed because of the rigid, internal framework provided by the abundant authigenic quartz. Coupled with their relatively high TOC contents, cherts then may be important intervals of gas generation and storage in the Woodford. Where abundant, cherts may then play a significant role as source and reservoir intervals within the formation in the Anadarko Basin.

Abstract The Upper Cretaceous Milk River Formation in southeastern Alberta and southwestern Saskatchewan has produced more than 2 tcf of dry (>99% methane) microbial gas ( δ 13 C PDB –65 to –71‰) that was internally sourced. Production is from underpressured fine-grained sandstone and siltstone reservoirs, whereas the gas was generated in interbedded organic-bearing mudstones with low organic carbon contents (0.5–1.50%). The formation experienced a shallow burial history (maximum burial, <1.3 km [<0.8 mi]) and cool formation temperatures (<50°C [<122°F]). Petrologic and isotopic studies suggest that methanogenesis began shortly after deposition and continued for at least 20 to 25 m.y. Mercury injection capillary pressure data from the Milk River Formation and the overlying Upper Cretaceous Pakowki Formation, which contains numerous regionally extensive bentonitic claystones, reveal a strong lithologic control on pore apertures and calculated permeabilities. Pore apertures and calculated permeabilities in Milk River mudstones range from 0.0255 to 0.169 μm and less than 0.002 to 0.414 md, respectively, and claystones from the overlying Pakowki Formation have pore apertures from 0.011 to 0.0338 μm and calculated permeabilities of 0.0017 to 0.0065 md. The small pore apertures and low permeabilities indicate that claystones and mudstones served as seals for microbial Milk River gas, thereby permitting gas to accumulate in economic quantities and be preserved for millions of years. Based on the timing of gas generation, the gas system of the Milk River Formation can be considered an ancient microbial gas system, which is one of several ways it differs from that of the Devonian Antrim Shale, Michigan Basin, where microbial gas generation is a geologically young (Pleistocene and younger) phenomenon. The difference in timing of gas generation between the Milk River and Antrim systems implies that gases in the two formations represent end members of a spectrum of microbial gas accumulations in fine-grained rocks, with the Milk River Formation being an excellent example on which to base a paradigm for an ancient microbial gas system.

Abstract Detailed study of the Late Cambrian Mt. Simon Sandstone, in the Illinois and southern part of the Michigan Basins as well as on the intervening Kankakee and Wisconsin Arches in Illinois and Indiana, reveals that the unit has undergone a complex diagenetic history with widespread alterations. Numerous events of authigenic mineral precipitation that occur throughout the study area and at paragenetically similar times include precipitation of early grain-coating illitic illite/smectite, subsequent potassium feldspar (as overgrowths and rhombic cement) and quartz and a late void-filling illitic illite/smectite. Dolomite (both ferroan and nonferroan), pyrite, barite, anhydrite and fluorite occur locally as minor cements. Dissolution of at least some authigenic cements, such as potassium feldspar and dolomite, also occurred throughout the study area. Mechanical compaction occurred throughout the alteration history of the Mt. Simon. Homogenization temperatures for fluid inclusions in quartz overgrowths range from 100° to 130°C, and freezing point depression data indicate that the inclusions formed from solutions that were very saline (>20 wt % equivalent NaCl). Petrographic observations reveal that the quartz formed at about the same time as the potassium feldspar; previous radiometric dating of authigenic potassium feldspar indicates that it formed at about 400 Ma, during the Late Silurian to Early Devonian. Burial history reconstruction of the Mt. Simon Sandstone reveals that burial temperatures expected in the Mt. Simon Sandstone during Late Silurian to Early Devonian time were significantly less than the temperature of fluids from which quartz precipitated. Thus, it is likely that the fluids from which quartz precipitated migrated from other, probably deeper parts of the protoIllinois basin or other deep sources where fluid temperatures and salinities could be expected to be higher than within the shallower Mt. Simon Sandstone. The widespread nature of at least some of the alterations in the Mt. Simon Sandstone taken together with the similar timing of alterations throughout the region and the fluid inclusion data suggest that diagenesis in the Mt. Simon Sandstone resulted from regional migration of fluids that were al times hotter than expected temperatures due to burial alone. Thus, the Mt. Simon Sandstone appears to have been a paleofluid-flow conduit through which thermally anomalous pore waters of varying composition migrated over time. Because it has served as a regional paleoflow conduit, it seems likely that solutions responsible for formation of economic deposits of base metals in the region may also have flowed through the unit. In fact, the Mt. Simon Sandstone may well have been a principal conduit through which ore-forming fluids migrated regionally from their source to sites where mineralizing processes resulted in ore deposits.