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Blood, D.R., and Lash, G.G., 2015, Dynamic redox conditions in the Marcellus Shale as recorded by pyrite framboid size distributions, in Larsen, D., Egenhoff, S.O., and Fishman, N.S., eds., Paying Attention to Mudrocks: Priceless!: Geological Society of America Special Paper 515, p. 153–168.
Abstract
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.
... It is important to note that while approximately 60% of all framboids measured are ≤ 5 µm, large framboids (> 10 µm) are present in nearly all analyzed samples, exclusive of two samples from the Levanna Shale in which the MFD is 8 µm (Fig. 21,Table 2). Framboids of the Union Springs and Oatka Creek members suggest accumulation within a dominantly anoxic to occasionally euxinic water column (Fig. 22), an interpretation largely consistent with studies from elsewhere in the basin, including southwestern Pennsylvania(Lash andBlood, 2014, Blood andLash, 2015) and eastern West Virginia (Blood and Lash, 2015). The greater occurrence of smaller framboids (~65% of measured framboids are ≤ 5 µm) and diminished abundance of larger framboids (< 4% are > 10 µm) in the Oatka Creek (Table 2) of the J. Lesson core are consistent with a greater degree of oxygen deprivation (perhaps occasional euxinia) compared with, the underlying Union Springs. ...
... It is important to note that while approximately 60% of all framboids measured are ≤ 5 µm, large framboids (> 10 µm) are present in nearly all analyzed samples, exclusive of two samples from the Levanna Shale in which the MFD is 8 µm (Fig. 21,Table 2). Framboids of the Union Springs and Oatka Creek members suggest accumulation within a dominantly anoxic to occasionally euxinic water column (Fig. 22), an interpretation largely consistent with studies from elsewhere in the basin, including southwestern Pennsylvania(Lash andBlood, 2014, Blood andLash, 2015) and eastern West Virginia (Blood and Lash, 2015). The greater occurrence of smaller framboids (~65% of measured framboids are ≤ 5 µm) and diminished abundance of larger framboids (< 4% are > 10 µm) in the Oatka Creek (Table 2) of the J. Lesson core are consistent with a greater degree of oxygen deprivation (perhaps occasional euxinia) compared with, the underlying Union Springs. ...
... For example,Wignall et al., (2010)maintain that half-inch sediment samples collected from the Santa Barbara Basin record on average hundreds of years or more of deposition. Further,Blood and Lash (2015), based on their study of pyrite morphology of Marcellus samples from Greene County, Pennsylvania, suggest that a one-inch-thick thin section could represent 1,100 years of deposition. The Union Springs and Oatka Creek members of the J. Leeson well inDoddridge County, WV, comprise 30 ft of compacted organic-rich mudstones. ...
Pyrite (FeS2) framboids, spheroidal groups of discrete equant pyrite microcrysts, are found in sediments of all geological ages. The size of a pyrite framboid is established during early diagenesis and preserved through time. Framboid size distributions are hence useful for the evaluation of depositional conditions. In this work, we present machine learning approaches to characterize the size distributions of pyrite framboids to understand the intensity and duration of anoxia and euxinia during the Middle Devonian of the Appalachian foreland basin by analyzing framboid size distributions of the Marcellus Shale from Lycoming County, Pennsylvania. Importantly, we overcome the time-consuming and laborious nature of current manual tracing methods to enable the processing of high volumes of micrograph data. Specifically, we implement convolutional neural networks (CNNs) to characterize framboids from 14 samples across depths in the Marcellus Shale. We show that CNNs enable the precise and fast measurement of framboid size distributions from scanning electron micrographs. CNN architectures including Inception, ResNet, Inception-Resnet, and Mask RCNN were trained and tested on a total of ∼ 6,800 framboids from 128 grayscale and 32 colored scanning electron micrographs. Kolmogorov-Smirnov tests on the framboidal equivalent diameter distributions measured from CNNs and manual tracing show that the CNN algorithms detected framboids with up to 99% precision. Importantly, once trained, the CNNs were ∼ 100 times faster than current manual tracing. A straightforward extension of this work includes the application of CNNs to characterize pores, fractures, organic matter, and/or mineral grains in geological materials.
The recent increase in unconventional oil and gas exploration and production has prompted a large amount of research on hydraulic fracturing, but the majority of chemical reactions between shale minerals and organic matter with fracturing fluids are not well understood. Organic matter, primarily in the form of kerogen, dominates the transport pathways for oil and gas; thus any alteration of kerogen (both physical and chemical properties) upon exposure to fracturing fluid may impact hydrocarbon extraction. In addition, kerogen is enriched in metals, making it a potential source of heavy metal contaminants to produced waters. In this study, we reacted two different kerogen isolates of contrasting type and maturity (derived from Green River and Marcellus shales) with a synthetic hydraulic fracturing fluid for two weeks in order to determine the effect of fracturing fluids on both shale organic matter and closely associated minerals. ATR-FTIR results show that the functional group compositions of the kerogen isolates were in fact altered, although by apparently different mechanisms. In particular, hydrophobic functional groups decreased in the Marcellus kerogen, which suggests the wettability of shale organic matter may be susceptible to alteration during hydraulic fracturing operations. About 1% of organic carbon in the more immature and Type I Green River kerogen isolate was solubilized when it was exposed to fracturing fluid, and the released organic compounds significantly impacted Fe oxidation. Based on the alteration observed in both kerogen isolates, it should not be assumed that kerogenic pores are chemically inert over the timeframe of hydraulic fracturing operations. Shifts in functional group composition and loss of hydrophobicity have the potential to degrade transport and storage parameters such as wettability, which could alter hydrocarbon and fracturing fluid transport through shale. Additionally, reaction of Green River and Marcellus kerogen isolates with low pH solutions (full fracturing fluid, which contains hydrochloric acid, or pH 2 water) mobilized potential trace metal(loid) contaminants, primarily S, Fe, Co, Ni, Zn, and Pb. The source of trace metal(loid)s varied between the two kerogen isolates, with metals in the Marcellus shale largely sourced from pyrite impurities, whereas metals in the Green River shale were sourced from a combination of accessory minerals and kerogen.
High resolution event and cyclic stratigraphic study of Pragian-, Emsian- and Eifelian-age strata across the Appalachian Balsin (New York to southwestern Virginia and Ohio) indicates a need for some changes in stratigraphic nomenclature. Following procedures of the International and North American Stratigraphic codes, new units are proposed, older units are revised, refined, or reinstated, and some units are abandoned (owing to their synonymy). The changes include: 1) abandonment of the Ridgeley Formation as a synonym of the Oriskany Formation; 2) three proposed formal members of the Esopus Formation in New York and eastern Pennsylvania (Spawn Hollow, Quarry Hill and Wiltwyck members); 3) one new member (Gumaer Island Member) in the lower part of the Schoharie Formation, and assignment of the term Carlisle Center as a member-level unit within the Schoharie Formation; 4) assignment of the term "Marcellus" to subgroup status within the classic Hamilton Group of New York, and recognition to two formation-level successions within the Marcellus (Union Springs and Oatka Creek-Mount Marion formations, low to high). Upper Marcellus strata are assigned to two laterally equivalent formations, the Oatka Creek (dominated by basinal black shale facies) and the Mount Marion (proximal basin to shoreface shales to sandstones). Two members occur within the Union Springs Formation (Bakoven and Stony Hollow members) and three members occur in the lower part of the overlying Oatka Creek and Mount Marion formations (Hurley, Cherry Valley and East Berne members). Some units are revised or restricted, based on changes in unit boundaries, recognition of multiple lithosomes, or other reasons. Other units are abandoned (e.g., Buttermilk Falls Formation and associated members, eastern Pennsylvania).
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.
Widespread porous and permeable sandstones of the Heiberg Formation/Group (uppermost Triassic-Lower Jurassic) are host to oil and/or gas in sixteen fields in the western Sverdrup Basin of the Canadian Arctic Islands. The sandstones are part of three, third-order, T–R (transgressive–regressive) sequences which are of Rhaetian–Hettangian, Sinemurian and Pliensbachian–Toarcian age. The general depositional/tectonic setting for the three T–R sequences was a marine shelf undergoing relatively low subsidence. Sediment influx to the shelf was generally low and increased eastward toward a deltaic input centre located in the south-central portion of the basin. Each T–R sequence is bounded by subaerial unconformities (usually modified by shoreface erosion) on the basin margin and by correlative transgressive surfaces farther seaward. A maximum flooding surface within each sequence separates a lower transgressive systems tract from an overlying regressive systems tract. The transgressive systems tract thickens eastward in each sequence and consists of a variety of facies including massive to cross-bedded sandstone (inner shelf), rippled to burrowed sandstone (mid-shelf), glauconitic sandstone, siltstone and shale (outer shelf) and oolitic ironstone (starved inner shelf). The predominant facies recognized in the regressive systems tracts are burrowed, grey shale and siltstone (offshore shelf), rippled to burrowed siltstone and sandstone (lower shoreface-mid shelf), cross-bedded to massive sandstone (upper shore face-inner shelf) and carbonaceous shale and siltstone with a highly variable sandstone content (lagoon, coastal plain). In the Rhaetian–Hettangian sequence a distinctive restricted marine facies association (foreshore-offshore shelf) is widespread and is characterized by an almost total lack of burrowing and by red and green coloration of shale–siltstone units. Oil and gas are structurally trapped almost exclusively in the uppermost porous sandstones of the succession beneath thick argillaceous deposits of the Jameson Bay Formation. The stratigraphic position of such sandstones varies across the study area. Regressive shoreface–foreshore sandstones of the Rhaetian–Hettangian sequence form the main reservoirs along the southwest basin margin. Farther northeast the regressive inner shelf to coastal plain sandstones of the Sinemurian sequence and the transgressive inner shelf sandstones of the Pliensbachian–Toarcian sequence are the main reservoirs. Comparison of the sequence bounding events with the Haq et al. (1988) curve reveals both matches and mismatches. This result, plus the occurrence of major changes in the amount and location of sediment input and in subsidence pattern from one sequence to the next, suggest that regional tectonics was the main factor controlling sequence development.
The size distribution of pyrite framboids from selected British Jurassic black shales is calibrated against a paleoecologically-determined, oxygen-restricted biofacies (ORB) scheme, revealing a close correspondence between the two data sets. Thus, euxinic conditions (H2S-bearing, O2-free bottom waters) are characterized by tiny framboids (<5 μm) with a narrow size range and an absence of benthic and nektobenthic taxa. Fissile strata with discrete, benthos-bearing bedding planes, variously termed lower dysaerobic, exaerobic, and poikiloaerobic biofacies in the literature, also have a euxinic framboid signature but with the addition of rare large diameter framboids. Upper dysaerobic biofacies contain substantially larger framboids with a broader size distribution. The utility of framboid size distributions can be modified by sedimentary reworking events (for example, during storms) which tend to winnow/remove the smallest size fraction. Therefore, care must be taken in the selection and analysis of samples. In addition to framboids, a large proportion of the sedimentary pyrite in the Jurassic shales consists of microcrystals and amorphous lumps. The varying proportions of these morphologies show only weak correspondence with ORB fluctuations, and their size variations are unrelated, making them unsuitable for determining ancient redox levels. The presence of large amounts of non-framboidal pyrite in black shales will complicate the assessment of whole rock S isotope fluctuations, because each population is likely to have formed at different locations within the sedimentary environment.
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Field relations and regional correlations show that organic-rich mudrocks in the Devonian Marcellus to Dunkirk Shales and the Ordovician Utica Shale of New York, which have historically been interpreted to have been deposited in deep, still, permanently anoxic basins greater than 150 meters deep, may in fact have been deposited in relatively shallow, moving water that was at least periodically not anoxic and no more than a few tens of meters deep. These black shales were mainly deposited on the present-day western or cratonward side of the basin, not in the deepest part or at the toe of a prograding clastic wedge as is commonly interpreted. The deepest part of the basin in both the Ordovician and the Devonian was the site of organic-poor turbidite deposition. The organic-rich black shales commonly overlie, onlap and pinch out on unconformities, some of which are demonstrably subaerial in origin. Both the Ordovician and Devonian black shales were deposited during periods of high tectonic subsidence driven by thrust-loading to the east. This black shale deposition occurred in the Appalachian Basin while areas to the west were exposed land suggesting relatively low eustatic sea level during deposition. Scour surfaces and cross-lamination are common throughout both of these shales suggesting storm influence and possibly other types of currents. Benthic fossils and burrows occur in the shales indicating periods when the sea floor had sufficient oxygen levels to sustain life.
These observations have led to the development of a new depositional model for black shales in actively subsiding foreland basins. In this model, eustatic sea level is interpreted to have been relatively low and there was subaerial exposure on peripheral bulges to the west of the basin. The black shale is most organic rich where it was deposited in the shallowest water far to the west and becomes progressively less organic rich approaching what was the deepest part of the basin. This is at least in part due to progressively more dilution from clay and silt that are sourced from the highlands to the east but it may be that the longest duration of anoxic conditions occurred in the shallowest water. Shallow water would be warmer than deep water and the warmer water is the less oxygen it can hold in solution. Seasonally anoxia or dysoxia similar that what occurs on many shallow continental shelves today may have helped to preserve organic matter but could also explain the presence of benthic fossils and bioturbation. Because it was relatively shallow, storms and other currents influenced deposition throughout. The deepest part of the basin is the site of turbidite and organic-poor shale deposition, not black shale. A similar model may apply to other black shales worldwide.
Variations in the concentration of redox sensitive elements combined with pyrite framboid size data documented from a Marcellus Formation (Middle Devonian) core recovered from southwestern Pennsylvania elucidate the redox, organic matter accumulation, and diagenetic history of these deposits in this region of the basin. Uranium and Mo enrichment and Fe/Al display sharp increases coincident with diminishing Th/U upward through the initial 3rd order trangressive systems tract (lower Union Springs Member). These data as well as abundant small (<6 μm) pyrite framboids record establishment of strongly reducing benthic conditions, perhaps related to the expansion of an oxygen minimum zone induced by increased surface productivity. Strongly anoxic, even euxinic, conditions were interrupted by episodes of dysoxia, perhaps seasonal or longer term. Overlying regressive systems tract (RST) deposits record modestly improved redox conditions, likely a reflection of a receding oxygen minimum zone as base level dropped. A subsequent 3rd order base level rise and renewed expansion of the oxygen minimum zone triggered by increased surface productivity resulted in the accumulation of the organic-rich lower Oatka Creek Member. Still, the mix of abundant small and subordinate large (>10 μm) framboids preserves the record of oxygen deficient to sulfidic bottom conditions frequently interrupted by episodes of (dys)oxia. Improving redox conditions through the overlying RST were accompanied by a two-fold increase in Al and consequent dilution of the organic matter flux and authigenic trace metal uptake at the sediment–water interface. The upper half of the Oatka Creek comprises a depositional sequence not obvious from core inspection or gamma-ray signature but revealed by Mo enrichment and Al concentration profiles. Diagenetic modification of the Marcellus includes several horizons of authigenic calcium carbonate concretions and marked Ba enrichment. Both features reflect the effects of non-steady state microbial diagenesis within a methane-rich sedimentary column.
Devonian-Mississippian black shales apparently were deposited in foreland basins formed in response to deformational loading in the Acadian orogen. These black shales were deposited in a succession of partially overlapping basins separated from each other by coarser clastic units, which migrated episodically southwestward in time parallel to the orogen. If foreland basins are results of an isostatic response to deformational loading in the orogen, migration of successive basins along the orogen must reflect concomitant shift of orogeny, and hence relative plate motion in the same direction. These black-shale deposits, then, not only demonstrate the diachronous nature of the Acadian orogeny, a fact previously indicated by other lines of evidence, but also enable approximation of rates of relative plate motion to be made. Using three different dating schemes, the black-shale deposits indicate Late Devonian plate motion along an 800 km segment of the orogen during approximately 11 Ma, for an average rate of relative plate motion slightly greater than 7 cm/yr. The facts that the black-shale basins do apparently track plate movement and resulting orogeny and that the along-strike component of basin migration exceeds the perpendicular-to-strike component suggest that at least Late Devonian parts of the Acadian orogeny are related to episodes of oblique convergence involving major strike-slip movement with substantial amounts of transpression. 72 references.
Size analysis of pyrite framboids has been undertaken on epicontinental Permian- Triassic boundary sections throughout the world in order to evaluate the intensity and duration of anoxia. Mid-paleolatitude sections from the margins of the Boreal (Spitsbergen, Greenland) and Neotethyan oceans (Western Australia) reveal intense anoxia throughout the Permian-Triassic boundary interval with euxinic conditions frequently developing, and dysoxia encountered even in relatively shallow-water settings above storm wave base. At equatorial paleolatitudes, weakly oxygenated (dysoxic) conditions are widely developed in a broad range of water depths including those shallow enough to produce oolite deposition, although euxinia was rare. Western and eastern Tethyan locations reveal a complex and unstable redox history: anoxia in the Hindeodus praeparvus Zone was replaced by oxygenated facies in the Permian- Triassic boundary interval (H. changxingensis to H. parvus zones). Oxygen-poor deposition returned during the succeeding Isarcicella isarcica Zone. The more persistent and intense development of oxygen restriction in cooler water, mid-paleolatitude sections argues against warming and dissolved oxygen decline as the key cause of Permian-Triassic boundary anoxia. In higher paleolatitudes the benthic invertebrate extinctions occurred during a prolonged phase of oxygen-poor deposition, while in equatorial Tethyan locations benthic losses occurred at the end of the first anoxic phase (in the late H. praeparvus Zone).
The Devonian System of Euramerica contains at least 14 transgressive-regressive (T-R) cycles of eustatic origin. These are separated into three groups (or depophases) and from Carboniferous cycles by three prominent regressions. Twelve post-Lochkovian T-R cycles are recognized, and they commonly appear to result from abrupt deepening events followed by prolonged upward shallowing. Deepening events in the western United States (especailly Nevada), western Canada, New York, Belgium, and Germany have been dated in the standard conodont zonation and are demonstrably simultaneous in several or all five regions. This synchroneity indicates control by eustatic sea-level fluctuations rather than by local or regional epeirogeny. Facies shifts in shelf sedimentary successions are more reliable indicators of the timing of sea-level fluctuations than are strandline shifts in the cratonic interior, because the latter are more influenced by local epeirogeny. Strandline shifts are most useful in estimating the relative magnitude for sea-level fluctuations. Devonian facies progressions and the three prominent regressions are of a duration and an order of magnitude that could have been caused by episodes of growth and decay of Devonian oceanic ridge systems. The described T-R cycles could have formed in response to mid-plate thermal uplift and submarine volcanism. The latter process may have been a control on small-scale (1-5 m thick) upward-shallowing cycles within the major T-R cycles. Continental glaciation could have been a factor in sea-level fluctuations only in the Famennian and could not have been responsible for the Devonian facies progressions or the numerous T-R cycles. The Frasnian extinctions were apparently cumulative rather than due to a single calamity. Two rapid sea-level rises occurred just before, and one at, the Frasnian-Famennian boundary. It is probable that this series of deepening events reduced the size of shallow-shelf habitats, caused repeated anoxic conditions in basinal areas, and drowned the reef ecosystems that had sustained the immensely diverse Devonian benthos. -Authors
Integrated geochemical data suggest that black shale deposition in the
Devonian Geneseo Formation of western New York was initiated by the
coincidence of siliciclastic starvation and the intensification of
seasonal water column stratification and mixing. Once established,
however, black shale deposition was maintained through efficient
recycling of biolimiting nutrients which enhanced primary productivity.
Recycling efficiency was achieved through a positive feedback loop of
oscillating benthic redox conditions that enhanced N and P regeneration
from sediments, sustained high primary productivity by returning
nutrients to the photic zone during mixing, and ensured a downward flux
of organic matter that drove or enhanced the episodic development of
benthic anoxia during stratification. This feedback was ultimately
disrupted by rising siliciclastic influx, which diluted organic matter
and restored benthic redox stability. The abrupt overturn of diverse,
long-standing Appalachian basin marine communities may have been the
result of trophic resource destabilization during Geneseo deposition.
The Khunamuh Formation of the Guryul Ravine section in Kashmir provides one of the most detailed deep-water records of the end-Permian mass extinction in Perigondwanan palaeolatitudes. Examination of pyrite framboid size distributions and sediment microfabric reveals a distinct change in the uppermost Permian strata from nonlaminated, pyrite-free silty mudstones to finely laminated, silty shales rich in framboidal pyrite around 1 m above the base of the formation. The size frequency distribution of the framboids is identical to populations from severely oxygen-restricted modern depositional environments. This change to dysoxic benthic conditions coincides with the rapid demise of diverse Permian brachiopod and bryozoan assemblages. Thus, the link between extinction and anoxia is clearly seen at Guryul Ravine and it provides an interesting contrast to other Perigondwanan sections developed in shallower water where both the onset of anoxia and the extinction occurred somewhat later in the earliest Triassic.
A new synthesis of the Lower to Middle Devonian (upper Pragian, Emsian and Eifelian) succession across the Appalachian Basin has been developed by high resolution event and sequence stratigraphic analysis. The correlation of numerous marker beds and a hierarchy of cycles in the interval of the Oriskany Sandstone to lower Hamilton Group provide a refined picture of the depositional patterns, faunal changes, formation to member-level (and finer) relationships, and sea-level trends. The succession begins above the Wallbridge Unconformity, or its correlative conformity, which lies beneath the Oriskany Sandstone, not above it as previously thought. The new sequence-stratigraphic framework of Oriskany to lower Hamilton strata comprises nine 'third order' stratigraphic sequences (cycles), though an interval of some 25 million years. At a coarse scale, the eustatic Pragian to Eifelian sea-level curve for Euramerica of Johnson et al. (1985) shows broad variance with the Appalachian curve, reflecting the regional influence of the Acadian orogeny. However at the finer sequence-scale, the Euramerican sea-level trends are recognizable in the Appalachian Basin succession.
A sequence, as originally defined by Sloss and colleagues, was a stratigraphic unit bounded by subaerial unconformities. Such a stratigraphic unit proved to be of limited value because, in most instances, sequences could be recognized only on the margins of a basin where subaerial unconformities were present. Vail and colleagues greatly expanded the utility of sequences for basin analysis when they redefined the term as a unit bounded by unconfor- mities or correlative conformities. The addition of correlative conformities allowed a sequence to potentially be recognized over an entire basin. This revised definition has led to the formulation of four different types of sequences, each having a different set of bounding surfaces. Vail and colleagues have defined two types: a type 1 depositional sequence and a type 2 depositional sequence. A type 1 depositional sequence utilizes a subaerial unconformity as the unconformable portion of the boundary and a time line equivalent to the start of base level fall for the correlative conformity. Because the subaerial unconformity migrates basinward during base level fall, much of it is therefore included within such a sequence rather than being on the boundary. Also it is impossible to objectively recognize a time line that corresponds to the start of base level fall. For these rea- sons a type 1 depositional sequence has little practical value. A type 2 depositional sequence also uses the subaerial unconformity as the unconform- able portion of the boundary but uses a time line equivalent to the end, rather than the start, of base level fall for the correlative conformity. This resolves the problem of including a portion of the unconformity inside the sequence. However, it is essentially impossible to objectively recognize a time line that corresponds with the end of base level fall (start of base level rise) and thus this type of sequence also has no practical value. Galloway proposed the use of maxi- mum flooding surfaces as sequence boundaries and named such a unit a genetic stratigraphic sequence. This alleviated the problem of major subjectivity in boundary recognition because maximum flooding surfaces can be determined by objective scientific analysis. However, this sequence type founders on the problem that the subaerial unconformity occurs within the sequence and thus it lacks genetic coherency on the basin margins. To overcome these major deficiencies in sequence definition, Embry and Johannessen have defined a fourth type of sequence that they term a T-R sequence. This sequence uses the subaerial unconformity as the unconformable portion of the boundary and the maximum regres- sive surface as the correlative conformity. This methodology keeps the subaerial unconformity on the boundary and also provides for a correlative conformity that can be objectively deter- mined. It thus avoids the fatal flaws of previously defined types. A T-R sequence can be divided into a transgressive systems tract below and a regressive systems tract above by using the maxi- mum flooding surface as a mutual boundary. T-R sequence stratigraphy, unlike the other proposed methodologies, has maximum practical utility with a minimum of stultifying jargon. Papers
Transgressive–regressive (T–R) sequence analysis has been applied to the Jurassic succession of the Sverdrup Basin with sequence boundaries drawn at subaerial unconformities or the correlative transgressive surfaces. A hierarchal system of sequence order that reflects the different nature of the boundaries has been formulated on the basis of boundary characteristics. Second- through fifth-order sequences have been recognized in the Jurassic succession, which itself is part of a first-order sequence of mid-Permian – Early Cretaceous age.The Jurassic strata occur within four second-order sequences. The boundaries of these sequences are characterized by widespread subaerial unconformities across which major changes in depositional and subsidence regimes occur. These boundaries are earliest Rhaetian, earliest Pliensbachian, earliest Bajocian, earliest Oxfordian, and Hauterivian in age.Each second-order sequence is divisible into a number of third-order sequences bounded mainly by basin-wide transgressive surfaces with subaerial unconformities present on the basin margins. The ages of the 10 Jurassic third-order sequences are Rhaetian – Hettangian, Sinemurian, Pliensbachian – Toarcian, late Toarcian – Aalenian, Bajocian, Bathonian, Callovian, Oxfordian – early Kimmeridgian, late Kimmeridgian – early Tithonian, and late Tithonian. The third-order sequences commonly contain three to six fourth-order sequences. These sequences are bound entirely by transgressive surfaces that can be correlated only over a portion of the basin.A good correlation between the second- and third-order transgressive events of the Sverdrup Basin and proposed global events is observed. This worldwide occurrence suggests that the events in part reflect eustatic sea-level changes. The characteristics of the second- and third-order boundaries also indicate that each had a tectonic influence that resulted in a rapid relative sea-level fall (uplift) followed by a rapid rise (subsidence). Given the apparent combination of tectonic and eustatic influence on the generation of the second- and third-order sequence boundaries, they are interpreted to reflect significant plate-tectonic reorganizations that affected the intraplate stress regimes of the oceanic (eustatic) and continental (tectonic) portions of each lithospheric plate.
Foraminiferal proxies of hypoxia indicate apparent low oxygen to hypoxic conditions in several hotspots in the Mississippi Bight. The foraminiferal hypoxia proxies, the Ammonia to Elphidium (A/E) index and the Pseudononion-Epistominella-Buliminella (PEB) index, were tabulated from three sets of core tops collected in 1951-1956. Additionally, the oxygenation history of a site near the Balize delta was evaluated over the past one hundred years based on A/E and PEB indices and size distributions of pyrite framboids in a gravity core dated by Pb-210 geochronology. The results from the 1950's core-top collections show apparent, recurrent low-oxygen to hypoxic bottom water on the inner shelf at hotspot locales seaward of the Mississippi-Alabama barrier islands and the eastern distributaries of the Balize delta. Specifically, the A/E index exceeds 90% on the inner shelf seaward of Horn and Dauphin islands, both of the Mississippi-Alabama barrier islands, and a center between Pass a Loutre and Main Pass of the Balize delta. In partial support of these results are reports of present-day low oxygen to hypoxic concentrations in bottom waters associated with seasonally high surface chlorophyll a and seasonal strengthening of a brackish-water cap at these locales. In contrast, the PEB index in core tops suggests good oxygenation at midshelf depths > 30 m. The PEB index, size distributions of framboidal pyrite, and other indicators in a gravity core 44 km northeast of Pass A Loutre indicate no clear change in conditions over the past century, constraining the seaward extent of the hotspot near the Balize delta.
THE Black Sea is the largest anoxic marine basin in the world today1. Below the layer of oxygenated surface water, hydrogen sulphide builds up to concentrations as high as 425 μM in the deep water down to a maximum depth of 2,200 m (ref. 2). The hydrographic regime is characterized by low-salinity surface water of river origin overlying high-salinity deep water of Mediterranean origin1,3. A steep pycnocline, centred at about 50 m is the primary physical barrier to mixing and is the origin of the stability of the anoxic (oxygen/hydrogen sulphide) interface. Here we report new observations, however, that indicate dramatic changes in the oceanographic characteristics of the anoxic interface of the Black Sea over decadal or shorter timescales. The anoxic, sulphide-containing interface has moved up in the water column since the last US cruises in 1969 and 1975. In addition, a suboxic zone overlays the sulphide-containing deep water. The expected overlap of oxygen and sulphide was not present. We believe that these observations result from horizontal mixing or flushing events that inject denser, saltier water into the relevant part of the water column. It is possible that man-made reduction in freshwater inflow into the Black Sea could cause these changes, although natural variability cannot be discounted.
1990.04 We review the highlights of the 1988 symposium on Palaeozoic Biogeography and Palaeogeography, and present a revised set of 20 Palaeozoic base maps,that incorporate much,of the new data presented at the symposium. The maps include 5 major innovations: (1) A preliminary attempt has been made,to describe the motion of the Cathaysian terranes during the Palaeozoic; (2) a more detailed description of the events surrounding the Iapetus Ocean is presented; (3) an alternative apparent polar wandering,path for Gondwana,has been constructed using the changing distributions of palaeoclimatically restricted lithofacies; (4) new,palaeomagnetic,data have been incorporated that places Laurentia and Baltica at more southerly latitudes, and adjacent to Gondwana, during the Early Devonian; Siberia is also placed further south in the light of biogeographic data presented at the symposium; (5) Kazakhstan is treated as a westward extension of Siberia, rather than as a separate palaeocontinent. The relationships between climatic changes, sea level changes, evolutionary radiations and intercontinental migrations are discussed. A symposium on Palaeozoic Biogeography and Palaeogeography,
Pyrite as a proxy for the redox states of bottom waters was evaluated in the context of modern salt marsh sediments. Two sites, 40 m apart, with very different physicochemical properties were sampled seasonally. The sites differed from each other in (1) the amount of tidal flushing, (2) vegetation, (3) redox condition below the sediment–water interface, (4) carbon content and (5) solid phase iron oxyhydroxide and iron sulfide content of the sediment. The calculated degree of pyritization (DOP) varies between 0.44 and 1.0, depending on the site and the season. Different DOP values were obtained for the same samples when different techniques were applied for extraction of reactive iron. Framboidal pyrite was dominant; however, other textural forms of pyrite were also observed. The size distribution of the individual pyrite crystals and the framboids was fairly narrow; with the majority of them having diameters <10 μm. Despite deposition taking place in aerobic waters (fully oxygenated waters) at the sampled sites, both the DOP and pyrite textures indicated dysoxic (waters with low oxygen concentrations) to euxinic (waters with no oxygen present, and H2S may be continually or intermittently present) bottom waters. These results indicate that caution must be applied when using DOP and pyrite morphologies as proxies for interpreting the redox conditions of different depositional environments.
The preferred or parallel orientation of clay platelets in shale may be produced during early diagenesis by the compression of flocculated clay. During burial clay flakes reoriented around the framboids and subsequently into a parallel to subparallel position forming typical shale fabric. Also significant is the spherical rather than elliptical shape of framboids. Thus, shale fabric is a diagenetic feature, produced after framboid crystallization, and during the initial stages of sediment compaction. -from Author
The Middle Devonian Acadian orogeny affected the entire Appalachian orogen from Newfoundland to Alabama with varying intensities of deformation and metamorphism. Part of the erosional debris from this uplifted area of tectonism was shed westward into the adjacent Appalachian basin. The basin subsidence and filling varied along its length, but not in concert with the adjacent tectonism. In the Early Devonian, the basin was stable with very little subsidence. Beginning in the Middle Devonian and continuing throughout the Late Devonian, the eastern part of the basin (now Pennsylvania and central New York) underwent rapid subsidence, where the largest volume and coarsest sediments were deposited as the Catskill Delta. To the southwest, grain size and thickness progressively decrease to the thin sequence of black shales in eastern Tennessee and Kentucky. To the north (northern New York, eastern Ontario and southern Quebec), no real evidence exists that a Devonian basin developed-the sediment from the orogenic zone probably passed over the Laurentian shield to the Michigan basin and perhaps elsewhere. The Acadian orogeny did not impinge upon the basin and its contained sediments, in distinct contrast with the Taconian and Alleghanian orogenies, both of which directly affected the basin, albeit in different ways. Reported Devonian structures within the basin are insignificant (growth folds), questionable (radiometric dating of faults), or probably incorrect (folding and angular unconformity). Acadian structures that occur between the basin and the main tectonic belt include open, upright folds and steep faults; cooling of Taconian metamorphic terrane during the Devonian and Early Carboniferous is also indicated for these rocks.
The Acadian orogeny in the North Atlantic region is assessed in this chapter in the light of mid-Paleozoic tectonics; throughout, plate tectonic nomenclature is used, and cycles are avoided. In North America nine regions bearing the imprint of the Acadian orogeny are recognized. In Newfoundland, in the Maritime Provinces of Canada, and in Vermont and New Hampshire a continuous sequence of lithotectonic belts correlates along the orogen. The Bronson Hill belt, although a continuous structure in southern New England, is not recognized as such but splits into two structures northeast of the Maine-New Hampshire border: the Boundary Mountain anticlinorium and the Lobster Mountain anticlinorium. Other lithotectonic belts are partly continuous from Canada into the United States. The plate tectonic interpretation of the orogenic system is based on a model of successive blocks (terranes) approaching and colliding with North America and squeezing intervening sediments and volcanics. This took place over a fairly prolonged period of time. -from Authors
The Late Devonian Catskill Delta is made up of marine and non-marine facies built up on the flank of the tectonic Appalachian Peninsula during assembly of the Old Red (Laurasian) Continent. Much of the continent was under the influence of tropical climates showing a wide range of rainfall. Over the delta, the climate was either tropical wet and dry or desert, due in part, to a rainshadow effect caused by the mountains to the east. Streams showed great variations in discharge and an extended period of drought was an annual event over the region. Alluvial processes were dominant on land. Braided streams deposited the coarsest sediments on alluvial fans and sinuous, channelized streams deposited sand and mud on the alluvial plains. Interfluves on the alluvial plains were sufficiently long-lived to permit the formation of carbonate soils. Plants were most common near stream courses. Fine sand and mud were carried across the shoreline in distributaries to the floor of the adjacent Catskill Sea. Deltaic processes, wave-related processes, and tides shaped the shore. Wave-related processes and bioturbation modified and reworked the shallow marine sediments while turbidity currents and slow deposition from suspension were most effective over the rest of the basin.
Although gas has been produced from Devonian black shales rich in organic matter in the Appalachian basin since 1821, the subsurface stratigraphy and the correlation of the gas-productive rocks were not well known because criteria were not available to subdivide the fine-grained rocks of the Devonian shale sequence. However, as the use of gamma-ray wire-line geophysical logs became widespread in the 1960's, data became available for identifying and tracing individual beds of black shale. Because organic matter in the black shales preferentially absorbed uranium during its transportation and deposition, the black shales are characterized by strong positive deflections on gamma-ray logs. By comparing gamma-ray logs from many closely spaced wells and correlating their conspicuous black shale log signatures with named stratigraphic units that crop out on the periphery of the basin a stratigraphic framework was established for the Devonian shale sequence, the relation of black shales in the New York Devonian section to the Chattanooga Shale of central Tennessee was resolved. Previously suggested correlations based upon conodont zonation were corroborated by the gamma-ray stratigraphy. -from Authors
Organic-carbon-rich shales of the lower Marcellus Formation were deposited at the toe and basinward of a prograding clinothem associated with a Mahantango Formation delta complex centered near Harrisburg, Pennsylvania. Distribution of these organic-carbon-rich shales was influenced by shifts in the delta complex driven by changes in rates of accommodation creation and by a topographically high carbonate bank that formed along the Findlay-Algonquin arch during deposition of the Onondaga Formation. Specifically, we interpret the Union Springs member (Shamokin Member of the Marcellus For-mation) and the Onondaga Formation as comprising a single third-order depositional sequence. The Onondaga Formation was deposited in the lowstand to transgressive systems tract, and the Union Springs member was deposited in the trans-gressive, highstand, and falling-stage systems tract. The regional extent of parasequences, systems tracts, and the interpreted depositional sequence suggest that base-level fluctuations were primarily caused by allogenic forcing-eustasy, climate, or regional thermal uplift or subsidence-instead of basement fault reactivation as argued by previous workers. Paleowater depths in the region of Marcellus Formation black mudrock accumulation were at least 330 ft (100 m) as estimated by differences in strata thickness between the northwestern carbonate bank and basinal facies to the southeast. Geochemical analysis indicates anoxic to euxinic bottom-water conditions. These con-ditions were supported by a deep, stratified basin with a lack of circulation. Copyright © 2014. The American Association of Petroleum Geologists. All rights reserved.
Analysis of more than 900 wireline logs indicates that the Middle Devonian Marcellus Formation encompasses two thirdorder transgressive-regressive (T-R) sequences, MSS1 and MSS2, in ascending order. Compositional elements of the Marcellus Formation crucial to the successful development of this emerging shale gas play, including quartz, clay, carbonate, pyrite, and organic carbon, vary predictably within the proposed sequencestratigraphic framework. Thickness trends of Marcellus T-R sequences and lithostratigraphic units reflect the interplay of Acadian thrust-load-induced subsidence, short-term base-level fluctuations, and recurrent basement structures. Rapid thickening of both T-R sequences, especially MSS2, toward the northeastern region of the basin preserves a record of greater accommodation space and proximity to clastic sources early in the Acadian orogeny. However, local variations in T-R sequence thickness in the western, more distal, area of the basin may reflect the reactivation of inherited Eocambrian basement structures, including the Rome trough and northwest-striking cross-structural discontinuities, induced by Acadian plate convergence. Episodes of block displacement locally warped the basin into northeast-southwest-trending regions of starved sedimentation and/or erosion adjacent to depocenters in which regressive systems tract deposits were ponded. Block movement appears to have initiated in late Early Devonian time, resulting first in thinning and local erosion of the Oriskany sandstone in northwest Pennsylvania. This study, in addition to providing the basis for a predictive sequence-stratigraphic model that can be used to further Marcellus exploration, tells of a foreland basin more tectonically complex than accounted for by simple flexural models. Copyright © 2011. The American Association of Petroleum Geologists. All rights reserved.
Pyrite framboids are densely packed, generally spherical aggregates of submicron-sized pyrite crystals. In this study, a survey was made of framboid size distributions in recently deposited sediments from euxinic (Black Sea; Framvaren Fjord, Norway; Pettaquamscutt River Estuary, Rhode Island, USA), dysoxic (Peru Margin), and oxic (Wallops Island, Virginia, USA; Great Salt Marsh, Delaware, USA) environments. Pyrite framboids in sediments of modern euxinic basins are on average smaller and less variable in size than those of sediments underlying dysoxic or oxic water columns. Down-core trends indicate framboid size distribution is a sediment property fixed very early during anoxic diagenesis, generally within the top few centimeters of burial. Size distributions in modern sediments are comparable with those in ancient sedimentary rocks, evidence that framboid size is preserved through advanced stages of diagenesis and lithification. It is proposed that where secondary pyrite growth is limited, as to preserve primary pyrite textures, framboid size distribution may be used to indicate whether fine-grained sedimentary rocks were deposited under oxic or anoxic conditions.The Crystal Size Distribution Theory relates framboid size to growth time and rate. On the basis of this theory, the characteristic smaller sizes of framboids in sediments of modern euxinic basins reflect shorter average growth times relative to oxic or dysoxic environments. In euxinic environments, framboid nucleation and growth occurs within anoxic water columns, and growth times are, on average, shorter because of hydrodynamic effects than when framboid nucleation and growth occurs within anoxic sediment porewaters underlying oxic water columns. A maximum framboid growth time of 0.4 years is indicated for framboids forming in the water columns of euxinic basins.
The study of sedimentary pyrite morphological characteristics provides useful information on depositional environments and early diagenic processes and can be used as an indicator of redox conditions in ancient
lake bottom waters. The results of this study in the Songliao basin in northeastern China show that euhedral
crystals and framboids are the dominant pyrite forms in the mudstones of the first member of the Qingshankou Formation (K2qn1). The framboidal size distribution indicates that during the deposition of K2qn1, redox conditions in the bottom water fluctuated from oxic–dysoxic to euxinic–anoxic to oxic–dysoxic.
The presence of euxinic–anoxic bottom water suggests that an anoxic event developed in the Songliao
ancient-lake and continued for 196 ka. The development of an anoxic bottom environment was the most
important factor in hydrocarbon source rock formation in the Songliao Basin.
A detailed study of size distributions of framboidal pyrite in Holocene Black Sea sediments establishes the timing of a change from deposition under an oxic water column to deposition under an anoxic and sulfidic water column. In the most recent carbonate-rich sediments (Unit I) and in the organic carbon-rich sapropel (Unit II), framboid size distributions are remarkably uniform (mean diameter = 5 μm); over 95% of the framboids in Unit I and Unit II are < 7 μm in diameter. These properties of framboidal pyrite are consistent with framboid nucleation and growth within an anoxic and sulfidic water column, followed by transport to the sediment-water interface, cessation of pyrite growth due to the exhaustion of reactive iron, and subsequent burial. In contrast, the organic carbon-poor sediments of lacustrine Unit III contain pyrite framboids that are generally much larger in size (mean diameter = 10 μm). In Unit III, over 95% of the framboids are < 25 μm in diameter, 40% of framboids are between 7 μm and 25 μm, and framboids up to 50 μm in diameter are present. This distribution of sizes suggests framboid nucleation and growth within anoxic sediment porewaters. These new data on size distributions of framboidal pyrite confirm that the development of water-column anoxia in the Black Sea coincided with the initiation of deposition of laminated Unit II sapropels.
RECENT studies1-8 have documented significant short-term vertical fluctuations in the position of the oxic-anoxic interface (chemocline) in the waters of the Black Sea, the world's largest anoxic basin. Natural5,9 and anthropogenic3,4,8,10 influences have been invoked as possible causes of the observed fluctuations, but it has been difficult to establish the relative importance of these two forcings. One reason is that observations of the magnitude of chemocline displacement have not extended sufficiently far in the past to eliminate the possibility of anthropogenic changes in fresh-water input. Here we present chemical analyses of shelf sediments, collected from the Bosporus region of the Black Sea, which contain a record of past water column chemistry. We find that the chemocline in this region rose by at least 40-50 m >=250-300 years ago, precluding anthropogenic forcing as a viable cause. Although our results do not rule out an anthropogenic cause for the recent variations, they do show that natural perturbations more than twice as large as the more recent changes have occurred in the past.
An integrated lithologic, paleontologic, and multi-proxy geochemical study of the Middle Devonian Oatka Creek Formation, a black shale in the northern Appalachian Basin, indicates that a number of different factors contributed to organic carbon-rich black shale deposition. Conditions leading to this organic-rich sedimentary deposit were ultimately controlled by a relative sealevel rise, dominantly eustatic but with possible contributions from local tectonics, which cut off the supply of carbonate to the basin. Geochemical proxy evidence - such as Mo/Ti, Fe/Ti, C o r g , S p y , and δ 3 4 S p y - suggests that as sealevel continued to rise after the carbonate supply was cut off, a threshold was crossed at which point conditions in the basin shifted from dominantly anoxic to dominantly euxinic (anoxic-sulfidic bottom waters). Concurrent with the shift to dominantly euxinic conditions, the supply of siliciclastic sediments was cut off, resulting in a condensed horizon, as evidenced by the elemental ratios of Si/Al and K/(Fe+Mg) and the relative concentration of eolian silt as determined petrographically and from scanning electron microscopy. Sediment starvation in the basin appears to have facilitated the biogeochemical (re)cycling of C, N, and P. Specifically, the elemental ratios of C, N, and P and the stable carbon isotope composition of organic matter suggest that the preferential regeneration of P under anoxic conditions (and of N during the oxic phase of oxic/anoxic oscillation) led to enhanced primary production in surface waters, thereby maintaining euxinic conditions in the bottom waters through respiration of settling organic matter. Finally, it is observed that, though conditions in the basin seem to have remained consistently anoxic-sulfidic for some time after the initial shift to euxinic conditions, a progressive increase in siliciclastic sedimentation led to a corresponding decrease in the enrichment of various redoxrelated elements, illustrating the overriding control that sedimentation can have on geochemical proxy records.
Most workers attribute the preferred grain orientation that defines the shale fabric to the mechanical collapse of a high-porosity clay microfabric under increasing overburden pressure. Scanning electron microscope analysis of samples of the Upper Devonian (Frasnian) Rhinestreet shale of western New York State collected from near early diagenetic carbonate concretions provides further evidence of this mechanism. Specifically, mudstones recovered from pressure shadows immediately adjacent to lateral edges of concretions are characterized by an open fabric of randomly arranged clay domains-the cardhouse fabric. Laterally equivalent shale samples collected only 0.2 to 0.3 m distant the pressure shadows, however, reveal a low-porosity, strongly oriented shale fabric in which clay particles mold around incompressible quartz silt and euhedral pyrite grains and pyrite framboids. These observations suggest that the organic Rhinestreet shale accumulated as water-rich flocculated clay. The open clay fabric observed in pressure-shadow mudstones was preserved by the incompressible concretions during burial. Flocculated clay beyond the shielding effect of the concretions, however, collapsed during progressive burial to form the strongly oriented shale fabric.
The successive Seboomook-Littleton (northern Appalachians) and Catskill-Pocono (central Appalachians) clastic wedges suggest oblique convergence and southwestward migration of Acadian orogeny beginning in Early Devonian and continuing into Early Mississippian. Wrench-fault movement in Maritime Canada coincided with deposition of all but the earliest components of the Catskill-Pocono clastic wedge and continued into the Pennsylvanian. Contrasts between a thin, Lower to Middle Devonian shallow-shelf facies in the Alabama Appalachian fold-thrust belt and a time-equivalent, thick, shallowing-upward sedimentary to volcanic succession in the adjacent Talladega slate belt are interpreted to reflect a wrench-fault basin. A wrench-fault setting for Devonian rocks in Alabama integrated with manifestations of oblique convergence during the Acadian orogeny in the central and northern Appalachians can be accommodated in dextral transpression along the entire length of the Acadian Appalachian orogen.
During the late Frasnian, 11 fourth-order progradational/retrogradational marine sedimentation cycles were deposited in the Appalachian foreland basin. Mapping based primarily on subsurface data demonstrates the continuity of these cycles over a distance of 700 km. Cyclicity in distal facies occurs as alternations of organic-rich and organic-poor shales, two of the organic shales can be correlated with the transgressive “Kellwasser Beds” of Europe. In more proximal facies, recurring lobes of siltstone and sandstone were deposited. Based on lithologic indices, the temporal pattern shows significant variation in the strength of relative facies change during deposition. In particular, two times of particularly pronounced progradation correspond to previously recognized eustatic sea-level falls. The correlation of portions of Appalachian basin depositional cyclicity with global sea-level events suggests that the entire sequence of 11 cycles, with estimated average duration of around 100–150 ka, were the result of high-frequency eustatic sea-level changes. This in turn would be consistent with a brief period of late Frasnian glaciation, as others have previously suggested.
Reports the results obtained through construction of six detailed cross sections connecting western New York with northern, central, and northwestern Ohio; southwestern Ontario; and southeastern Michigan, using the gamma-ray logs of 94 wells. The sections were drawn at a vertical scale of 1 in. to 50 ft. and were extended into measured outcrops or mine shafts to confirm the identification of subsurface units. These detailed sections are too large to reproduce in a professional journal. They consequently have been placed on open file in the office of the New York State Geological Survey. Three severely reduced and generalized sections using widely spaced wells are published here.-from Author
The lower Cambrian Hetang Formation, a black shale sequence, contains a stone coal (a flammable, organic-rich mudstone) unit. Abundant pyrite framboids occur in this unit, with the average mean sizes falling in a narrow range from 4.6 to 5.4 μm and the maximum diameters being around 10 μm. The size distribution pattern of the pyrite framboids indicates a euxinic depositional environment. The redox-sensitive trace metals of the stone coal samples reveal a large enrichment of Mo (10–180 times),V (4–40 times), U (10–60 times) and Ni (2–20 times) compared to the average upper continental crust value, consistent with an anoxic environment for their deposition. The redox-sensitive element ratios (Th/U, V/(V+Ni), V/Cr) indicate that the depositional environment for the lower part of the stone coal unit was the most anoxic and euxinic. In contrast, a sponge biota including eleven species of demosponges and hexactinellids and two undetermined forms indicate an oxic or dysoxic environment. To reconcile these two facts, we propose that although an anoxic/euxinic environment predominated during the deposition of the lower Cambrian Hetang Formation black shales, occasional currents may also have brought free oxygen to the bottom water column to allow the growth of the sponges on the sea floor.
Pyrite synthesis was examined in the laboratory, and the chemical, crystallographic and textural results were compared with
naturally occurring pyrite in marine sediment. The path of pyrite formation begins with reaction of hydrogen sulfide and dissolved
iron. The first product is either amorphous iron sulfide or mackinawite with composition FeS (sub 0.9) . In the presence of
limited oxygen, this can change to hexagonal pyrrhotite, FeS (sub 1.1) . Further reaction of either of the above phases with
elemental sulfur will produce greigite, Fe 3 S 4 and, finally, pyrite, FeS 2 . The greigite assumes the morphological character of spherules. Pyrite framboids were found to form only on spherical nuclei.
Thus, both chemical and textural evidence is provided for the pathway of pyrite formation.
A detailed study of the size distribution of framboidal pyrites in the black shales of the Upper Permian Ravnefjeld Formation was performed to evaluate the redox state of the Late Permian ocean. In contrast to framboidal pyrites in bioturbated sediments, the smaller and less variable size distribution of pyrite framboids in the laminated shales of the Ravnefjeld Formation provides persuasive evidence for sulfidic (H2S-rich) bottom-water conditions in the East Greenland Basin. However, the S isotope compositions of both pyrite populations show a similar distribution. The widespread δ34S values of pyrites (−41.2‰ to −28.2‰) in the black shales of the Ravnefjeld Formation indicate a large fractionation (up to 52.7‰) relative to seawater sulfate, and may record different pathways of sulfur cycling in sulfidic water columns as well as within sediments. The new data from the East Greenland Basin indicate that environmental stress such as widespread sulfidic conditions could have caused the biotic crisis in the Late Permian.
http://geology.gsapubs.org/content/32/12/1037.abstract
Fabric and pyrite framboid size analysis of Permian to Jurassic samples from the Mino-Tamba terrane of Japan provide an 80 myr redox history from the Panthalassa Ocean. Fully oxygenated conditions dominated but were punctuated by three phases of anoxia (euxinia) during the Permo-Triassic boundary, the late Early Triassic (Spathian Stage) and the late Early Jurassic (Toarcian Stage). A Permo-Triassic superanoxic event of 10 myr duration is thus resolved into a complex redox history of dysoxic-anoxic background conditions punctuated by short-lived euxinic pulses. Both the end-Permian and Toarcian anoxic episodes began abruptly with the development of siliceous claystones, characterized by a framboid-dominated pyrite fraction and no organic C enrichment, interpreted to record euxinic conditions. These facies are overlain by organic-rich shales with slightly larger framboids, more crystalline pyrite and bioturbation indicating accumulation beneath a better-ventilated, higher productivity water column. Thus, an initial phase of ocean-wide stagnation was followed by improved circulation attributed to the onset of warm saline bottom water circulation. Five radiolarian crises during our study interval show a complex relationship with redox conditions. The latest Permian mass extinction coincided with the onset of ocean euxinia but two other crises (Middle Permian and end Triassic), occurred during fully oxygenated intervals. Radiolarian radiation events are also not consistently linked with redox conditions: diversification in the early Late Permian coincides with the onset of weakly dysoxic conditions, late Early Triassic and late Early Jurassic radiations coincide with euxinic events, and the basal Jurassic radiation occurred during a fully oxic phase.
The Salton Sea is a large, saline, closed-basin lake in southern California. The Sea receives agricultural runoff and, to
a lesser extent, municipal wastewater that is high in nutrients, salt, and suspended solids. High sulfate concentrations (4xhigher
than that of the ocean), coupled with warm temperatures and low-redox potentials present during much of the year, result in
extensive sulfate reduction and hydrogen sulfide production. Hydrogen sulfide formation may have a dramatic effect on the
iron (Fe) geochemistry in the Sea. We hypothesized that the Fe(II)-sulfide minerals should dominate the iron mineralogy of
the sediments, and plans to increase hypolimnetic aeration would increase the amount of Fe(III)-oxides, which are strong adsorbers
of phosphate. Sequential chemical extractions were used to differentiate iron mineralogy in the lake sediments and suspended
solids from the tributary rivers. Iron in the river-borne suspended solids was mainly associated with structural iron within
silicate clays (70%) and ferric oxides (30%). The iron in the bottom sediments of the lake was associated with silicate minerals
(71% of the total iron in the sediments), framboidal pyrite (10%), greigite (11%), and amorphous FeS (5%). The ferric oxide
fraction was <4% of the total iron in these anaerobic sediments. The morphological characteristics of the framboidal pyrite
as determined using SEM suggest that it formed within the water column and experiences some changes in local redox conditions,
probably associated with alternating summer anoxia and the well-mixed and generally well-aerated conditions found during the
winter. The prevalence of Fe(II)-sulfide minerals in the sediments and the lack of Fe(III)-oxide minerals, suggest that the
classic model of P-retention by Fe(III)-oxides would not be operating in this lake, at least during anoxic summer conditions.
Aeration of the hypolimnion could affect the internal loading of P by changing the relative amounts of Fe(II)-sulfides and
Fe(III)-oxides at the sediment/water interface.