Euxinic Basins Research Articles

Filamentous microfossils in the early proterozoic transvaal supergroup: their morphology, significance, and paleoenvironmental setting

Well preserved filamentous microfossils (Siphonophycus transvaalensis n. sp.) are described here from the carbonate (Campbellrand Subgroup) to iron-formation (Kuruman Iron Formation) transition of the Transvaal Supergroup, South Africa, estimated to be 2.5-2.3 Ga years old. The microfossils occur in petrographic thin-sections of a core sample of carbonate-chert. They are preserved by permineralization in both chert and in sparry ferroan dolomite. Stratigraphically the fossiliferous core sample occurs as part of an upward transition from a stromatolitic dolomite and limestone sequence (Campbellrand) to the overlying iron-rich sediments of the Kuruman-Griquatown Iron Formations. The average δ13C value of the kerogen in the sample is about - 36.9%. The microfossils are filamentous, unbranched, tubular to somewhat flattened, 15–25, μm in diameter and a few to many hundred microns in length. They exhibit a coriaceous, finely granular external surface texture resulting from the presence of adhering, randomly distributed, fine mineralic (carbonate) needles. In salient morphological characteristics they are comparable to the tubular, originally polysaccharide, encompassing sheaths of extant oscillatoriacean cyanobacteria. In comparison with previously described Precambrian microfossils, these fossil filaments are unusual because of their preservation in sparry carbonate (in addition to chert), their relatively large diameter, and their coating by adhering, precipitated, carbonate needles; they appear to be among the oldest assured microfossils now known from Proterozoic-age sediments. The microfossils are interwoven, occurring in subparallel aggregates that form a stromatolitic mat-like fabric; they are considered to be of an endogenetic in situ benthic origin occurring at the proximal margin of a ‘deep shelf’ environment at the front (distal margin) of the Campbellrand carbonate platform. The water depth for this environment, at the break in slope between deep shelf and euxinic basin, is estimated to have been of the order of 40–45 m. Paleomagnetic data support our interpretation that the micro-organisms inhabited a warm water marine environment, probably at low latitudes.

Sulfur and organic carbon contents in sediment cores from the Tyro and Orca basins

The Tyro and Orca basins are two small, deep-water, euxinic depressions in the Eastern Mediterranean and the northern Gulf of Mexico, respectively. Anoxic bottom waters are known to be nearly saturated with NaCl. Sediment accumulation in the basins has been complicated by episodic slumping and/or redeposition. Analyses of sediment total reduced sulfur and organic carbon and resulting S C ratios are used to test the feasibility of using these parameters as indicators for the mode of sediment deposition in the basins. Sediments originally deposited in the basin are, as expected, enriched in sulfur and carbon content as compared to sediments that originated from areas beyond the limit of the anoxic brine pool and were redeposited later inside the basin. However, values of the S C ratios of these primarily deposited sediments are less than those of sediments of redeposited origin, and indicate a low rate of sulfide conversion during bacterial sulfate reduction. This inconsistency is interpreted to be due to the high-salinity basin waters inhibiting the overall activity of sulfate reducers and their associated heterotrophic microorganism communities and/or the reactivity of initially deposited organic matter. This study also demonstrates that variations of sulfur and carbon contents and their S C ratios together can be used as a geochemical tool to delineate the origin of sediments accumulated in hypersaline euxinic basins. However, the feasibility of the S C method requires a further check as to whether the rate of sulfate reduction could have been constrained, even in environments of normal salinity.

Hydrochemistry, origin and evolution of sedimentary subsurface fluids: II. Early diagenetic pore-water evolution in high sedimentation-rate offshore basins

High sedimentation-rate basins are characteristic of the continental margins, where sedimentation rates up to 500m/Ma and more are no exception. The sediments are rich in organic matter and undergo a well-established sequence of bacterial oxidation reactions, starting at the sediment/water interface with: (1) the oxidation zone (absent in euxinic basins), which is followed downwards by (2) the nitrate reduction zone, (3) the sulfate reduction zone, (4) the carbonate reduction zone, (5) the fermentation zone and (6) the decarboxylation zone, which marks the onset of thermocatalytic reactions characteristic of the intermediate stage (and deeper stages) of diagenesis.

Starved Euxinic Basin Concept for Oil and Gas Genesis in Oklahoma: ABSTRACT

Starved Euxinic Basin Concept for Oil and Gas Genesis in Oklahoma: ABSTRACT

Geologic and geochemical evidence of possible bedded barite deposits in Devonian rocks of the Valley and Ridge Province, Appalachian Mountains

Middle Devonian rocks in the Appalachian basin valley and ridge province offer new ground for exploration of economic bedded barite deposits. The depositional strike length of documented barite exposures extends from central Pennsylvania through eastern West Virginia and southern Virginia. Appalachian barite has features similar to other sediment-hosted barite deposits: (1) deposition in restricted marine conditions, (2) deposition in a trough or geosynclinal setting, (3) insignificant amounts of volcanic rocks in the stratigraphic succession, (4) subjection to structural deformation, and (5) delta 34 S values similar to nodular barite from western Canada and Nevada.Barite is restricted to the Needmore Shale Formation, the Marcellus-Millboro Shale Formations, and the Tully Limestone. The latter two stratigraphic horizons offer the best potential for hosting economic barite deposits. The Purcell Limestone Member of the Marcellus-Millboro Formations hosts many exposures of barite and should be the primary target of an exploration program.Syngenetic to early diagenetic barite includes barite nodules and incipient barite beds. Diagenetic barite includes baritized ammonoids, barite-filled syneresis cracks in barite nodules and limestone concretions, and small barite nodules which fringe zones of abundant barite nodules. Epigenetic barite includes vein barite in rock fractures, weathering-produced barite rinds, and acicular crystals in limestone which appear to be replacement structures.Minor and trace element contents show no apparent enrichment in metals relative to similar rock types from other stratigraphic settings. An exception is that barite-bearing rock units appear enriched in barium which becomes more concentrated toward zones of abundant barite nodules.The delta 34 S values (28.0-47.4ppm) and delta 18 O values (19.3-30.4ppm) show that barite nodules are considerably enriched in these constituents relative to coeval seawater. Systematic decreases of delta 34 S values updip from zones of abundant nodules combined with a slight decrease of 34 S content downdip suggest two styles of deposition: deposition of smaller nodules updip because of increasing distances from the critical redox boundary or by remobilization during diagenesis, and deposition from a warm Ba-rich spring system subject to biogenic alteration.Sulfur isotope values of vein- and vug-filling barite from shrinkage cracks in barite nodules are generally heavier than those of the host nodule. This fact coupled with elevated temperature (72 degrees -142 degrees C) fluid inclusions, some of which are organic bearing, suggest that barite enriched in 34 S was remobilized during diagenesis.We speculate that these sediment-hosted barite occurrences represent the margins of larger, yet undiscovered, bedded barite deposits. We feel that barium-bearing fluids were derived from submarine springs developed during the initial stages of the Acadian orogeny and deposited barite in minor submarine euxinic basins.

Sulphate reduction, organic matter decomposition and pyrite formation

Laboratory, field, and theoretical studies have shown that the rate of bacterial sulphate reduction during early diagenesis depends primarily on the reactivity of sedimentary organic matter whose decomposition follows first-order kinetics, with rate constants varying over six orders of magnitude. Decay rates decrease with decreasing sediment burial rate and, for a given sediment, with depth, because o f the successive utilization by bacteria of less and less reactive organic compounds. High burial (and bioturbation) rates enable reactive compounds to become available for sulphate reduction, at depth, which otherwise would be destroyed by molecular oxygen at or above the sediment-water interface. An important consequence of bacterial sulphate reduction is the formation of sedimentary pyrite, FeS 2 . In normal marine sediments (those deposited in oxygenated bottom waters) pyrite formation is limited by the concentration and reactivity of organic matter, whereas in euxinic (sulphidic) basins pyrite is limited by the abundance and reactivity of detrital iron minerals, and in non-saline swamp and lake sediments by the low levels of dissolved sulphate found in fresh water. Because of these differences in limiting factors, the three environments can be distinguished in both modern sediments and ancient rocks by plots of organic carbon, C against pyrite sulphur, S. Values of the C:S ratio based on theoretical calculations indicate that worldwide the bulk of organic matter burial has shifted considerably between these environments over Phanerozoic time.

Sedimentary pyrite formation: An update

Sedimentary pyrite formation during early diagenesis is a major process for controlling the oxygen level of the atmosphere and the sulfate concentration in seawater over geologic time. The amount of pyrite that may form in a sediment is limited by the rates of supply of decomposable organic matter, dissolved sulfate, and reactive detrital iron minerals. Organic matter appears to be the major control on pyrite formation in normal (non-euxinic) terrigenous marine sediments where dissolved sulfate and iron minerals are abundant. By contrast, pyrite formation in non-marine, freshwater sediments is severely limited by low concentrations of sulfate and this characteristic can be used to distinguish ancient organic-rich fresh water shales from marine shales. Under marine euxinic conditions sufficient H 2S is produced that the dominant control on pyrite formation is the availability of reactive iron minerals. Calculations, based on a sulfur isotope model, indicate that over Phanerozoic time the worldwide average organic carbon-to-pyrite sulfur ratio of sedimentary rocks has varied considerably. High C S ratios during Permo-Carboniferous time can be explained by a shift of major organic deposition from the oceans to the land which resulted in the formation of vast coal swamps at that time. Low C S ratios, compared to today, during the early Paleozoic can be explained in terms of a greater abundance of euxinic basins combined with deposition of a more reactive type of organic matter in the remaining oxygenated portions of the ocean. The latter could have been due to lower oceanic oxygen levels and/or a lack of transportation of refractory terrestrial organic matter to the marine environment due to the absence of vascular land plants at that time.

Pyritized ooids, stringed ooids and reticulate tubular structures from within the black shale at Amjhor, Bihar, India

Carbonate ooids, notionally incompatible with an euxinic environment, and some curious reticulate tubular structures with fibrous carbonate walls are described from intraclasts occurring in Proterozoic black shales containing the pyrite deposit at Amjhor. While some of the tubes are visibly due to coalescence of ooids, others are interpreted as worm burrows on the basis of their strata-transgressive orientation, branching nature and larger diameter than those of the ooids. All stages of pyritization of ooids and tube-cores, displaying truncation of the growth-fabric of calcite grains by growing pyrite crystals, point to a late- or post-diagenetic age of pyritization. The clasts containing ooids and tubes are believed to have been transported from a shallow-bank and/or a nearshore environment into a lagoonal, euxinic basin. An environmental model, consistent with the observed data, is proposed to account for the ooid—black shale association.

Middle East--Stratigraphic Evolution and Oil Habitat: ABSTRACT

The post-Hercynian sequence of the Middle East is dominated by carbonate sedimentation on a stable platform flanked on the northeast by the Tethys ocean. Two principal types of depositional systems alternated in time: (1) ramp-type mixed carbonate-clastic units and (2) differentiated carbonate shelves. The first type was deposited during regressive conditions, when clastics were brought into the basin and resulted in layer-cake formations. The second type was formed during transgressive periods and is dominated by carbonate cycles separated by lithoclines, time-transgressive submarine lithified surfaces. Differentiation is marked, with starved euxinic basins separated by high-energy margins from carbonate-evaporite platforms. The tectonic development of the Middle East can be divided into several stages. The first stage, which ended with the Turonian, was characterized by very stable platform conditions. Three types of positive elements were dominant: (1) broad regional paleohighs; (2) horsts and tilted fault blocks trending NNE-SSW; and (3) salt domes. All three influenced deposition through synsedimentary growth. The subsequent stage, from Turonian to Maestrichtian, was one of orogenic activity, with the formation of a foredeep along the Tethys margin and subsequent ophiolite-radiolarite nappe emplacement. From the Late Cretaceous to the Miocene, the platform regained its stability, only to lose it again at the close of the Tertiary, when the last Alpine orogenic phase affected the region, creating the Z gros anticlinal traps. Source rocks were formed in the starved basins during the transgressive periods. Marginal mounds, rudist banks, oolite bars and sheets, and regressive sandstones form the main reservoirs. Supratidal evaporites and regressive shales are the regional seals. The spatial arrangement of these elements and the development of source maturity through time explain the observed distribution of the oil and gas fields. The Middle Jurassic to Albian sequence in the central part of the Gulf, around the Qatar Peninsula, provides a well-studied example of the control on oil distribution by the distribution in space and time of mature source beds, effective regional seals, and reservoirs. Oil, tar, and extract typing as well as maturation studies show that the Upper Jurassic Hanifa bituminous limestone is the source for the oil and tar in the Jurassic as well as the Lower Cretaceous reservoirs, the latter reservoirs being only charged where the intervening regional Hith seal is either absent through nondeposition or is breached through faulting. Growth structures draining mature Hanifa kitchens contain sizable accumulations, whereas the Jurassic and Cretaceous reservoirs of the large Qatar dome contain o ly minor amounts of oil, which can be ascribed to insufficient source maturity and too late closure. Geochemical and geologic evidence indicates that the tar mats present at the base of many oil accumulations are not the result of biodegradation or early, immature expulsion from the source, but probably the product of gas deasphalting of reservoired oil. End_of_Article - Last_Page 1358------------

Middle East: Stratigraphic Evolution and Oil Habitat

The post-Hercynian sequence of the Middle East is dominated by carbonate sedimentation on a stable platform flanked on the northeast by the Tethys ocean. Two principal types of depositional systems alternated in time: (1) ramp-type mixed carbonate-clastic units and (2) differentiated carbonate shelves. The first type was deposited during regressive conditions, when clastics were brought into the basin and resulted in layer-cake formations. The second type was formed during transgressive periods and is dominated by carbonate cycles separated by lithoclines, time-transgressive submarine lithified surfaces. Differentiation is marked, with starved euxinic basins separated by high-energy margins from carbonate-evaporite platforms. The tectonic development of the Middle East can be divided into several stages. The first stage, which ended with the Turonian, was characterized by very stable platform conditions. Three types of positive elements were dominant: (1) broad regional paleohighs, (2) horsts and tilted fault blocks trending NNE-SSW, and (3) salt domes. All three influenced deposition through synsedimentary growth. The subsequent stage, from Turonian to Maestrichtian, was one of orogenic activity, with the formation of a foredeep along the Tethys margin and subsequent ophiolite-radiolarite nappe emplacement. From the Late Cretaceous to the Miocene the platform regained its stability, only to lose it again at the close of the Tertiary, when the last Alpine orogenic phase affected the region, creating the Za ros anticlinal traps. Source rocks were formed in the starved basins during the transgressive periods. Marginal mounds, rudist banks, oolite bars and sheets, and regressive sandstones form the main reservoirs. Supratidal evaporites and regressive shales are the regional seals. The spatial arrangement of these elements and the development of source maturity through time explain the observed distribution of the oil and gas fields.

Gas-Bearing Ohio Shale along Lake Erie--Stratigraphy, Petrology, and Sedimentology: ABSTRACT

Increased interest in low-yield gas-bearing strata has led to reexamination of the Ohio Shale in the vicinity of Lake Erie where it has had a history of marginal commercial production for over a century. These strata thin depositionally westward from a maximum of 2,000 ft (600 m) at the Pennsylvania border to less than 600 ft (180 m) near Sandusky where they are truncated by erosion on the Findlay arch. Common lithotopes are black and gray shale, siltstones, and rare carbonate rocks. Black shales are most abundant in the west where gray shales and siltstones are rare. In the east, gray shales and siltstones constitute 75% of the section and the remainder is mainly black shale and minor carbonate rocks. Thin-section petrology revealed that the black shale commonly contains, by volume, less than 20% silt-size quartz and feldspar, 30 to 60% clay and mica, and 15 to 35% organic material. Gray shale commonly contains less than 30% silt-size quartz and feldspar, 50 to 90% clay and mica, and less than 10% organic material. Much of the silt is concentrated in discrete laminae a few grains thick. Production records are related to facies distribution to provide an exploration tool. Cross sections and paleocurrents indicate that the Upper Devonian prodeltaic muds and turbidite siltstones were deposited episodically in a euxinic basin. End_of_Article - Last_Page 1576------------

Cyclic anoxic events in the Early Cretaceous Tethys Ocean

Cyclic black shale and nannofossil limestone units of the Lower Cretaceous Maiolica Formation of the southern Alps have been studied. From sedimentological data, the black shale units appear to have been formed in an euxinic basin that experienced periodic overturn and basinal aeration. The carbon-13 values of the calcareous nannofossils serve as indicators of paleocirculation. In the Barremian, the palynological spectra of the black shale units reveal a drastic alteration from a warm, dry to a cooler, humid climate. As a result, thermohaline circulation in the western Tethys Ocean was replaced by density stratification, and stagnation of the deeper basins followed.

The paleoenvironment of anaerobic sediments in the Late Mesozoic South Atlantic Ocean

Laminated dark calcareous oozes/chalks/limestones as well as clayey and marly mudstones/claystones with high organic carbon contents were deposited during Jurassic, Early and Late Cretaceous times in the Atlantic, Pacific and Indian Oceans. In the South Atlantic, this sedimentary facies has been encountered only in drill sites close to the American and African continental margins. The reconstructed paleogeography of the South Atlantic, the paleodepth of deposition, and the fossil content of these sediments make clear that the Late Cretaceous anaerobic paleoenvironment developed under the influence of an oceanic mid-water oxygen minimum at moderate water depths (500–2500 m), because oxygenated sediments have been observed in the deep basin elsewhere. Whether the Early Cretaceous and Jurassic anaerobic sediments were deposited in an euxinic basin or under an oceanic mid-water oxygen minimum remains an open question because the few drill sites have not sampled the deepest part of the Early Cretaceous and (?) Jurassic South Atlantic basins.

Influence of sedimentary environments on sulfur isotope ratios in clastic rocks: a review

Strata-bound sulfide deposits associated with clastic, marine sedimentary rocks, and not associated with volcanic rocks, display distributions of δS34 values gradational between two extreme types: 1. a flat distribution ranging from δS34 of seawater sulfate to values about 25‰ lower; and 2. a narrow distribution around value δS34 (sulfide)=δS34 (seawater sulfate) −50‰, and skewed to heavier values. δS34 (seawater sulfate) is estimated from contemporaneous evaporites. There is a systematic relation between the type of δS34 distribution and the type of depositional environment. Type 1 occurs in shallow marine or brackish-water environments; type 2 occurs characteristically in deep, euxinic basins. These distributions can be accounted for by a model involving bacterial reduction of seawater sulfate in systems which range from fully-closed “batches” of sulfate (type 1) to fully open systems in which fresh sulfate is introduced as reduction proceeds (type 2). The difference in the characteristic distributions requires that the magnitude of the sulfate-sulfide kinetic isotope effect on reduction be different in the two cases. This difference has already been suggested by the conflict between δS34 data for modern marine sediments and laboratory experiments. The difference in isotope effects can be accounted for by Rees' (1973) model of steady-state sulfate reduction: low nutrient supply and undisturbed, stationary bacterial populations in the open system settings tend to generate larger fractionations.

Lower Triassic Lithofacies of Cordilleran Miogeosyncline, Western United States: ABSTRACT

The Lower Triassic marine formations of the Cordilleran miogeosyncline (Dinwoody, Thaynes, and Moenkopi) make up a wedge-shaped body that intertongues on the east with redbed shelf deposits (Chugwater and Moenkopi). The miogeosynclinal strata are limestones with minor amounts of shale, siltstone, and sandstone. The shelf deposits are redbeds with tongues of nonred siltstone, sandstone, dolomite, limestone, and evaporites. Carbonate tongues in the redbeds in eastern Utah consist of oolites and poorly sorted echinoderm-rich limestones. There are no obvious lateral facies changes and the deposits appear to have formed in a shallow, laterally extensive sea. In most areas, however, lacustrine or shallow-marine redbed environments passed westward into a sand channel--oolite bar complex or into tidal flats with intraformational conglomerate channels. Seaward, lagoons (burrowed muds) and offshore banks (echinoderm-rich wackestones and oolites) were present. Farther seaward open marine (gray bioclastic limestones and tan calcareous siltstones) and euxinic basin (black limestones and shales) conditions existed. End_Page 347------------------------------ Thin chert-rich conglomerates and sandstones and the absence of much of the Lower Triassic in eastern Nevada indicate a positive area in that region. Evidence of Lower Triassic volcanism is present in western Utah and eastern Nevada. End_of_Article - Last_Page 348------------

Bank to Basin Transition in Permian (Leonardian) Carbonates, Guadalupe Mountains, Texas: ABSTRACT

Light-colored, parallel-stratified, non-reef marine limestone and dolomite (Victorio Peak Formation) grade southeastward abruptly into dark cherty limestone (Bone Spring) along the northwestern margin of the Delaware basin (King, 1948). Study of the well-exposed transition zone in the Guadalupe Mountains suggests the presence of three major contemporaneous environments along a gentle basinward slope: bank, and bank margin (0.5-1 mile wide), both Victorio Peak; and euxinic basin, Bone Spring. Basinward regression of transition facies was 2-3 miles during accumulation of 1,500 feet of section. Middle Permian erosion truncated the transition strata creating basin-sloping unconformities on which limestone, megabreccia, and sandstone (Cutoff and Brushy Canyon Formations) were eposited. Principal lithologic facies of the transition zone are successively: carbonate grainstone and dolomite packstone (bank); dolomite wackestone and mudstone (bank margin); and carbonate mudstone (basin). Paucity of desiccation or solution features, algal laminates, oolites, and coated or composite grains suggests prevalence of sub-tidal bank environments. Allochthonous channel fills and sheet deposits of skeletal packstone and wackestone occur in bank-margin and basin-edge facies. Sand-size skeletal grains and carbonate silt are dominant constituents in bank and bank-margin rocks. Carbonate silt predominates in basin facies. Characteristics of bank relative to basin strata are lighter color, dominance of grain-supported rocks, coarser grain size, normal-marine fauna, more dolomitization, less chert (but larger, more rounded nodules), and thicker, more massive beds. Undulatory bedding and disturbed laminations characterize many basin strata adjacent to the bank margin. Two major controls on sedimentation and subsequent diagenesis were diminished turbulence with depth, and an abrupt change along the depositional slope from normal-marine to euxinic water at inferred depths of a few hundred feet. Bank proximity and finer particle size favored dolomitization. Low permeability of organic-rich basin facies apparently inhibited downdip dolomitization. Cementation obliterated most depositional porosity. End_of_Article - Last_Page 474------------

Les olistholites du Riou d'Entraix et leur signification paleostructurale

Abstract The upper Bajocian (middle Jurassic) heterogeneous polygene breccias of the Riou d'Entraix (southeast France) are interpreted as olistostromes formed by gravity sliding along a paleoscarp. The scarp was to the east, in the area presently covered by overthrusts, and separated a euxinic deep basin from a well-oxygenated shallow shelf. Similar deposits in the Barcelonette window and the semi-window at Embrun are probably of the same origin.