Stratigraphic control of rare-earth pattern types in Mid-Proterozoic sediments of the Belt Supergroup, Montana, U.S.A.

Within sediments of the Helena embayment, an eastern extension of the Mid-Proterozoic Belt basin, three different types of REE patterns were identified. The lower part of the investigated sequence (Chamberalin Shale and Lower Newland Formation) is a uniform shale interval which accumulated during a period of tectonic quiescence. The REE patterns of the shales (normalized to NASC) are either flat or LREE enriched, lacking Eu deficiencies. The superjacent unit (Newland Transition Zone) contains variable amounts of feldspathic sandstone. It indicates rejuvenation of the hinterland and regression. The uppermost unit (Upper Newland Formation) consists of alternating packages of carbonates and shales. REE patterns in shales of the Newland Transition Zone and the Upper Newland Formation have negative Eu anomalies. This drastic change of REE patterns was observed in all stratigraphic sections.

The source rocks of the Beltian sequence were probably dominated by granitoid gneisses and migmatities, and rocks of this composition show negative Eu anomalies (against NASC) in many other places. The observed negative Eu anomalies in the shales were probably inherited from the source rocks. However, the patterns of the Chamberlain Shale and the Lower Newland Formation are not as easy to explain, because the source rock seems not to have changed during deposition of the Beltian sequence. Perhaps more intense chemical weathering during Chamberlain Shale - Lower Newland time obscured the negative Eu-anomlies in the residual clays. Adsorption of LREE’s on clays during transport may have caused the LREE-enriched patterns.

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The possible role of benthic microbial mats during the formation of carbonaceous shales in shallow Proterozoic basins

A considerable portion of the upper member of the Mid-Proterozoic Newland Formation, Meagher County, Montana, consists of carbonaceous silty shales (striped shale facies). This type of shale facies is common in Mid-Proterozoic basins and is host to several major base metal deposits. The striped shales were deposited in a subtidal setting, basinward of carbonates characterized by crypalgal laminites, mudcracks, and flat-pebble conglomerates. The carbonaceous silty shales are considered remnants of benthic microbial mats. Irregular internal laminae, patterns of particle trapping, mechanical deformation during penecontemporaneous soft-sediment deformation and filamentous microbiota provide evidence for this interpretation. The dolomitic clayey shale contains graded silt/mud couplets, and these are interpreted as storm layers. Modern subtidal microbial mats can only survive under special conditions, but in the Proterozoic, it is suggested that benthic microbial mats colonized the shallow seafloor during periods of low sediment input, leading to the formation of carbonaceous shales.

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Sedimentation in epithermal veins of the Bohemia mining district, Oregon, USA: Interpretations and significance

Open spaces in epithermal veins of the Bohemica mining district, Oregon, USA, filled with sediments during hydrothermal activity. These sediments consist mainly of chalcedony, rock fragments, and vein quartz fragments. In addition, hematite is deposited during stage 3 of the vein development. Observed sedimentary features include draping laminae, erosion surfaces, slumping, and graded bedding. Such sediments can be used for the reconstruction of the original orientation of vein systems, because the sediment laminae are initially deposited horizontally. Vein sediments record variations in fluid flow due to self-sealing, fracturing, and cessation of hydrothermal activity. Investigation of vein sediments therefore provides an additional tool to unravel the geologic history of epithermal systems. The chalcedonic vein sediments record large temperature drops and highly silica supersaturated waters, probably due to fracturing and pressure release. Hematitic vein sediments indicate sulfide deficient hydrothermal fluids.

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Storm-dominated epicontinental clastic sedimentation in the Mid-Proterozoic Newland Formation, Montana, U.S.A.

The Newland Formation, a shale dominated Mid-Proterozoic sequence, occurs in the Big Belt and Little Belt Mountains, USA. Within this sequence two types of storm deposits were recognized: A) medium to coarse grained hummocky cross-stratified sandstones, B) silt/mud couplets in the shales. These storm deposits probably formed in response to storms of variable strength. Evaluation of possible transport processes shows that thye storm sands were probably deposited during unusually strong storms by basinward flowing gradient currents, and the available data suggest that these sandstones were at least partially transported in bedload. The sandstones were probably derived from nearshore sand bars and show systematic lateral changes in frequency of occurrence, mean bed thickness, and cumulative thickness of sandstone, from basin margin to basin centre. Silt/mud couplets were probably the products of relatively weak storms, and from comparison with modern examples one may conclude that they were probably carried into the basin in suspension.

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Redistribution of rare earth elements during diagenesis of carbonate rocks from the Mid-Proterozoic Newland Formation, Montana, USA

REE data for carbonate rocks from the Proterozoic Newland Formation, Belt Supergroup, show variable degrees of relative REE enrichment when normalized to a composite shale of the Newland Formation. Most conspicuous are relative enrichments in LREE and positive Eu-anomalies. Possible coprecipitation of REE's in carbonates from seawater is much too small to account for the REE enrichment, thus only diagenetic REE enrichment has to be considered. By far the strongest LREE enrichment is observed in carbonates with interbedded shales. During diagenesis REE apparently moved from the shale partings to the adjacent limestone beds. Massive limestone units have considerably smaller REE enrichment (except in proximity to shales), suggesting that REE addition from formation waters is less efficient for supplying REE's than is local redistribution between shale and limestone beds. Early precipitation of diagenetic silica in pore spaces reduces REE enrichment in limestone beds by reducing their permeability. Dolostones are cemented by pervasive, early diagenetic silica and lack significant REE enrichment except for Eu, probably because silica cement prohibited access of REE-bearing pore waters. Positive Eu-anomalies indicate reduction of Eu to the divalent state during diagenesis and relatively large mobility of divalent Eu.

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Storm sands with swaley cross-stratification in the Lower Miocene Taliao Formation, Taiwan

A sedimentological investigation of a sandstone interval in the Early Miocene Taliao Formation of northern Taiwan shows that these sediments were deposited in a nearshore to offshore setting. The presence of hummocky cross-stratification, swaley cross-stratification, deep erosional scours, and intraformational conglomerates indicates that storms were an important depositional agent during the accumulation of these sediments. Combining information from trace fossils and primary sedimentary structures allows a threefold subdivision of the sequence into deposits of the upper shoreface, lower shoreface, and inner shelf. Swaley cross-stratification is a relatively recently recognized feature of storm influenced shoreface deposits, and has so far only been reported from a few localities. In the Taliao Formation it is restricted to upper shoreface deposits and was initially, in smaller outcrops, mistaken for low-angle beach cross-stratification. This experience indicates that reexamination of shoreline deposits from the geologic record may well lead to the recognition of many more occurrences of swaley cross-stratification.

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The coincidence between macroscopic paleocurrent indicators and magnetic lineation in shales of the Precambrian Belt basin

Paleocurrent flow direction in shale sequences can be inferred from sedimentary structures of associated lithologies, from orientation of fossils, from alignment of silt and sand-sized particles in shales, from mapping of scalar properties of a shale sequence, and from the orientation of concretions. Not all of these methods relate to shale properties directly, and most of them are very time-consuming and may even require fortuitous circumstances to be applicable. Consequently, even though shales constitute about 60% of all sediments, paleocurrent data from shale sequences are sparse. Thus, in order to improve our understanding of the evolution of sedimentary basins, it would be beneficial to have an accurate and conveniently applicable method of paleocurrent determination for shales.

The anisotrpy of magnetic susceptibility (AMS) of a number of shale samples from the Precambrian Belt Supergroup carries a fabric-related lineation that coincides with paleocurrents indicated by cross-laminated silt beds within the shales. Magnetic foliation decreases as dolomite content increases because of diagenetic dolomite growth, but magnetic lineation is only slightly affected. These data indicate that primary flow direction as represented by AMS measurement of these samples is preserved and therefore the AMS method can provide a rapid and accurate way to determine current-flow systems in shale-dominated sedimentary sequences.

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Facies and origin of shales from the Mid-Proterozoic Newland Formation, Belt basin, Montana, U.S.A.

Shales constitute more than 60 percent of the world's sediments, yet while facies models for sandstones and carbonates are at a high level of sophistication, the study of shales has clearly lagged behind. In the Mid-Proterozoic Newland Formation six major shale facies types, deposited in nearshore to basinal environments, are distinguished on the basis of bedding characteristics, textural features, and the proportions of silt, clay and carbonate. Textural features of these shale types are related to sedimentary environments as deduced from associated lithologies. The shales are undisturbed by bioturbation, and their textural and sedimentary characteristics reflect subaqueous growth of microbial mats, erosion and deposition by storms, deposition of flocculated vs. dispersed clays, continuous slow background sedimentation, winnowing by waves or currents, and subaerial exposure.

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The origin of the Neihart Quartzite: A basal deposit of the Mid-Proterozoic Belt Supergroup, Montana, U.S.A.

The Neihart Quartzite is the basal quartz arenite unit (~270 m thick) of the Mid-Proterozoic Belt Supergroup of western North America. Petrographic studies indicate a source area with plutonic granitic, metamorphic, and felsic volcanic rocks. Extreme textural maturity and bimodality indicate an episode of aeolian transport for the detrital quartz grains. The lower 80% of the Neihart Quartzite were probably deposited by braided streams, whereas the upper 20% were deposited in shoreline environments. Residual material that was "stored up" on the pre-Beltian cratonic surface and underwent aeolian reworking was the likely source material for most of the Neihart Quartzite. Less mature sediments in the top portion of the Neihart Quartzite indicate uplift and erosion of new source material during Neihart deposition. Other known cratonic quartz arenites, such as the St. Peter Sandstone (Ordovician), Lamotte Sandstone (Cambrian), and Flathead Quartzite (Cambrian), are thin (10's of meters thickness) and exhibit sheet-like geometry. In contrast, the Neihart Quartzite and its probable lateral equivalents are considerably thicker and increase in thickness towards the central portions of the basin. It thus appears that Belt sedimentation began with accumulation of a basal quartz arenite unit, and that sand for that unit was transported by braided streams from the surrounding craton to a gradually subsiding Belt basin.

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Pyrite mineralization in microbial mats from the Mid-Proterozoic Newland Formation, Belt Supergroup, Montana, U.S.A

The Mid-Proterozoic Newland Formation, a shale dominated unit of the Belt Supergroup, was deposited in an eastern extension of the Belt basin, the Helena embayment. A variety of different shale types can be distinguished. Of particular interest for this study is a shale facies that has been interpreted to be result of microbial mat growth (resulting in carbonaceous shale beds) interrupted by storm deposits (causing deposition of graded silt/mud couplets). Alternation of carbonaceous beds with silt/mud couplets gives these shales a characteristic striped appearance. Along the basin margins a pyrite-rich subfacies of these striped shales is found locally, consisting of laminated pyrite beds that alternate with non-pyritic silt/mud couplets. Laminated pyrite beds in pyritic striped shales are interpreted as mineralized microbial mats because of wavy-crinkly laminae and because of direct association with unmineralized striped shales that contain microbial mat deposits. Excess iron in pyritic shale horizons was probably supplied by terrestrial runoff in colloidal form. Iron hydroxides, introduced by rivers into basin-marginal lagoons, flocculated, and where then incorporated into microbial mats and reduced to pyrite upon burial.

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A sulfur isotope study of pyrite genesis: The Mid-Proterozoic Newland Formation, Belt Supergroup, Montana

Different generations of sedimentary pyrite from the Mid-Proterozoic Newland Formation, USA, have been analysed for their sulfur isotopic compositions. The results indicate bacterial sufate reduction as the pyrite forming process. The d34S values for early diagenetic pyrite, around -14v, are in contrast to dominantly more positive values for many other Middle Proterozoic units. A progressive reduction of sulfate availability during diagenesis can be recognized by an increase in d34S content (Rayleigh Distillation) as well as through detailed petrographic observations. Contemporary seawater had a sulfur isotopic ratio between +14 and +18o/oo as measured from sedimentary barite within the unit.

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Pyritic shales and microbial mats: Significant factors in the genesis of stratiform Pb-Zn deposits of the Proterozoic?

Extensive horizons of pyritic shale occur in Mid-Proterozoic sediments of the eastern Belt basin, Montana, U.S.A. These pyritic shales are of striped appearance. Laminated pyrite beds alternate with non-pyritic shale beds. Laminated pyrite beds have wavy-crinkly internal laminae and are interpreted as mineralized microbial mats. Pyrite is essentially the only sulfide mineral in these shales. Pyritic shale horizons occur along the basin margins, and it is feasible that colloidal iron was introduced by rivers into basin marginal lagoons and then incorporated into microbial mats and reduced to pyrite. The pyritic shales in the Newland Formation show great similarity to those that host the Pb-Zn deposits of Mt. Isa and McArthur River. It is suggested that pyritic shales of this kind are relatively common in Mid-Proterozoic shales, and that the processes that led to the occasional formation of Pb-Zn ore bodies in these shales are not related to those that formed the pyritic shales themselves.

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Distribution of REE in the eastern Belt Supergroup: Implications for stratigraphic correlations and basin evolution

Shales of the Mid-Proterozoic Newland Formation, Belt basin, contain a geochemical marker horizon that is characterized by appearance of negative Eu-anomalies in shale REE patterns. REE pattern changes appear to be related to changes in weathering intensity and tectonic activity in the hinterland. Stratigraphic and sedimentologic considerations suggest that this REE-marker horizon defines an approximate time-line within the sequence. Comparison of REE patterns of the Newland Formation and the coarse clastic LaHood Formation that was deposited to the south, show that the lower portions of the Newland Formation were deposited prior to LaHood sedimentation. Such a correlation implies that early Belt sediments may have covered a much larger area than delineated by the outline of the present day Belt basin and that, contrary to earlier views of basin evolution, the half-graben configuration of the eastern Belt basin was established at some later point of basin history.

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Significance of styles of epicontinental shale sedimentation in the Belt basin, Mid-Proterozoic of Montana, U.S.A.

Within the strongly shale dominated sediment fill of the Mid-Proterozoic Belt basin a large variety of shale types can be distinguished. Sedimentological investigations of several formations have yielded valuable data about lateral associations of shale types as well as about the principal mechanisms of shale deposition. Depositional environments of shales in the Belt basin range from red bed mudstones of ancient flood plains to deep water mudstones in a turbidite setting. Graded silt/mud couplets are ubiquitous in most shales of the Belt basin. However, they differ in detail and can be related to a variety of depositional processes, such as low density turbidites, sheet floods, storms, and wave reworking. In several formations there is evidence that microbial mats colonized the sediment surface and probably protected the sediment surface from erosion.

Shales from comparable settings in Phanerozoic shales lack in many cases the silt/mud couplets that are so commonly observed in shales from the Belt basin and other Proterozoic epicontinental basins. This difference probably reflects the proliferation of bioturbating organisms in the Phanerozoic. The absence of benthic microbial mats in Phanerozoic shales is probably due to the evolution of metazoan grazers towards the end of the Proterozoic.

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The origin and economic potential of disseminated Pb-Zn mineralization in pyritic shale horizons of the Mid-Proterozoic Newland Formation, Montana, U.S.A.

Pyritic shale horizons of the Newland Formation (Belt Series, Mid-Proterozoic) show striking similarities to Proterozoic pyritic shales elsewhere that are host to major lead-zinc deposits, such as Mt. Isa and McArthur River. However, in contrast to these deposits the pyritic shales of the Newland Formation contain only minute quantities of lead and zinc. Elevated concentrations of lead and zinc are only found in pore spaces of intercalated sandstone beds. Petrographic and geochemical data indicate that pyritic shale deposition and elevated lead-zinc concentrations in sandstone beds are unrelated, that base metal mineralization is controlled by initial porosity, and that iron and base metals were derived from different sources.

Petrographic studies show that base metal sulphides are diagenetic and resemble disseminated mineralization described from sandstone hosted lead-zinc and copper deposits in the Belt basin and elsewhere. The absence of orebodies of disseminated lead-zinc mineralization in sandstones of the Newland Formation may be due to a comparatively thin sedimentary sequence below the sandstone occurrences, as well as to unfavourable geometry and small volume of sandstone bodies.

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Facies and deposition of a mixed terrigenous-carbonate suite in a Mid-Proterozoic epicratonic sea: The Newland Formation, Belt Supergroup, Montana, U.S.A.

Carbonate rocks of the Newland Formation are characterized by a relatively large content of terrigenous matter (up to 50%). Simultaneous terrigenous clastic and carbonate sedimentation is indicated by lateral facies associations and small scale relationships between shale and carbonate beds. Main lithofacies types are: (A) "molar tooth carbonates" (fine crystalline dolostone interbedded with shales), deposited in a nearshore mudflat to lagoonal setting; (B) cherty dolostone (minor clay content) with cryptalgal laminations, deposited nearshore in an intermittently exposed setting; (C) bedded limestone, deposited in a probably storm-wave dominated shallow offshore setting; (D) heterolithic limestone (limestone-marl cycles), deposited in a storm-dominated shallow offshore setting; (E) millimetre-laminated limestone, a "starved basin" facies that was deposited far offshore.

Facies distribution within the basin was primarily controlled by water depth (implying energy of depositional environment as well as distance from shoreline) and proximity to the source of terrigenous material. Carbonate-siliciclastic mixing involved simultaneous operation of "Punctuated Mixing", "Facies Mixing", and "In Situ Mixing" processes. Microbial mats were probably important factors in carbonate production. Because of their ability to thrive even under conditions of considerable siliciclastic sedimentation they were most likely the reason why carbonate production in nearshore environments was not negatively affected by influx of large proportions of terrigenous clastics.

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A combined petrographical-geochemical provenance study of the Newland Formation, Mid-Proterozoic of Montana

A provenance study was conducted on the Mid-Proterozoic Newland Formation, in which petrographic features of sandstones and geochemical characteristics of shales were integrated to arrive at an internally consistent interpretation.

Sandstones of the Newland Formation are typically arkosic sands and arkoses with very well rounded quartz and feldspar grains and only minor amounts of extrabasinal rock fragments. The predominant feldspar types are K-spar and microcline, feldspar grains are smaller than quartz grains, and feldspars show little alteration due to weathering. Detrital modes of Newland sandstones (QFL diagrams) indicate that they were derived from a stable cratonic source. These petrographic features imply a source area dominated by granites and granitoid gneisses, semi-arid to arid climate, tectonic quiescence, and overall peneplain conditions.

Shales of the Newland Formation are dominated by illite, quartz silt, and fine crystalline dolomite. They have small La/Th ratios, relatively large Hf contents, and small contents of Cr, Co, and Ni, all indicative of derivation from crust of granitic composition. Small TiO2/Al2O3 ratios also suggest source rocks of granitic composition. The average chemical index of alteration (CIA) for Newland shales is 71.8, which in light of the probable granitoid source indicates modest amounts of chemical weathering. Relatively large SiO2 contents and large K2O/Na2O ratios reflect derivation from stable cratonic areas and tectonic quiescence.

Thus, in general the petrography of sandstones and geochemistry of shales provides the same provenance clues for the Newland Formation. One notable discrepancy between the two approaches is that the sandstones indicate an arid to semi-arid climate with very minor chemical weathering, whereas the CIA of the shales indicates at least modest amounts of chemical weathering. This indicates on one hand the need to better calibrate the CIA with a larger variety of muds from modern climatic settings, and on the other hand the possibility that this discrepancy is due to transport segregation.

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Determination of basinwide paleocurrent patterns in a shale sequence via anisotropy of magnetic susceptibility (AMS): A case study of the Mid-Proterozoic Newland Formation, Montana

It has been demonstrated that in shales of the Mid-Proterozoic Newland Formation the long axis of the AMS ellipsoid A) coincides with the flow azimuth indicated by macroscopic paleoflow indicators and B) is inclined in an upcurrent direction. Following this finding, oriented samples were collected from the Newland Formation and its lateral equivalents over the entire extent of the southeastern Belt basin, the so-called Helena embayment. Two successive stratigraphic units were sampled to determine possible changes in paleocurrent patterns with changing basin configuration. The study resulted in the recognition of coherent paleocurrent patterns that are consistent with earlier stratigraphic and sedimentologic studies. The described investigation is the first of its kind to apply the AMS method to shales on a basinwide scale, and its success suggests that this method could also be successfully applied to paleocurrent investigations of other basins that contain extensive shale sequences.

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Evidence for episodic high energy events and shallow water deposition in the Chattanooga Shale, Devonian, central Tennessee, U.S.A.

The Upper Devonian Chattanooga Shale of central Tennessee, a classical black shale, was deposited in an epicontinental setting, west of the Appalachian foredeep. Its finely laminated and highly carbonaceous nature is commonly interpreted to indicate deposition in comparatively deep and stagnant water. Interbeds of bioturbated greenish-gray shale, indicating oxygenated bottom waters, are commonly ascribed to pycnocline fluctuations. However, laminated fine sand and silt and hummocky cross-stratification (HCS) at the base of some of these beds indicates interaction of storm waves with the seabed, and suggests that greenish-gray shale beds are post-storm mud drapes.

Other interesting features are inclined-undulose erosion surfaces that are conformably overlain by shale beds, sets of inclined shale beds that suggest low-angle cross-bedding, and clearly and uniformly developed alignment of clay particles (magnetic fabric studies). These observations show that the seabed was at times subject to prolonged erosion by bottom currents (erosion surfaces), agitation and reworking by storm waves (HCS and greenish-gray shale beds), and sediment transport by long-lived bottom currents (particle alignment). The epicontinental sea setting and the presence of HCS and other storm produced features suggest a relatively shallow water depth (possibly only a few tens of meters). Together with abundant evidence of variably strong bottom currents and bioturbation of black and gray shale beds this suggests that abundant planktonic organic matter production rather than stagnant bottom waters are the primary cause for black shale formation.

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Anomalous iron distribution in shales as a manifestation of non-clastic fluvial iron supply to sedimentary basins: Relevance for pyritic shales, base metal mineralization, and oolitic ironstone deposits

In previous investigations, nearshore pyritic shale horizons in the Mid-Proterozoic Newland Formation were interpreted to be due to "non-clastic" colloidal iron supply by streams. New data on the chemical composition of shales in the Newland Formation support this interpretation. In these shales, Fe and Al show a positively correlated trend that intercepts the Fe axis above the origin. These relationships suggest control of Fe by clays (via iron oxide coatings on clay minerals), and presence of an additional, "non-clastic iron" component. Shales from stratigraphic intervals during which pyritic shale horizons were deposited plot above the Fe/Al trend typical for the remainder of the Newland Formation.

Pyritic shale horizons in sediments are favourable hosts for base metal deposits of the pyrite replacement type. Fe/Al relationships as found in the Newland Formation may help to identify stratigraphic horizons in other sedimentary basins that contain pyritic shale horizons and potentially base metal mineralization.

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Early diagenetic silica deposition in algal cysts and spores: A source of sand in black shales?

Devonian black shales deposited on the North American craton contain abundant Tasmanites cysts. Although these are typically flattened because of compaction, a small proportion is filled with diagenetic silica. The latter are spherical to ellipsoidal (0.1-0.5 mm), and filled with chalcedony, micro- and megaquartz, and with single quartz crystals. Chalcedonic cyst fillings are preserved best in chert and phosphate nodules, whereas megaquartz and single quartz crystals are most common in shale matrix. Together with colloform textures, this suggests that the various silica types originated from recrystallization of early diagenetic silica deposits.

Thin sandstone beds in the Chattanooga Shale (e.g., Bransford Sandstone) contain abundant quartz sand that is much coarser than the detrital quartz component of underlying black shales. Because of this, their quartz component is thought to have been transported over considerable distances from the basin margin. However, because certain shale horizons contain as much as 10% silicified cysts that upon reworking could have yielded quartz grains of fine to coarse sand size, the quartz component of these sandstone beds may actually have formed in situ.

Indeed, petrographic examination of the sandstone beds shows them to contain quartz grains with morphological and textural features of "cyst" quartz (e.g. rounding, sphericity, chalcedony, pyrite inclusions, lobate grain margins). Thus, silica deposition in algal cysts may provide a significant component of intrabasinal quartz sand in shale sequences. Distinction of this type of quartz from extrabasinal detrital quartz is important to the reconstruction of the depositional history of shale sequences.

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Sedimentologic, geochemical, and mineralogical features of the Belt Supergroup and their bearing on the lacustrine vs marine debate

Comparisons of Belt rocks with sediments of the lacustrine Green River Formation have been used to promote the idea that the Belt Supergroup as well might have accumulated under lacustrine conditions. However, comparison to other sedimentary sequences with similar facies types, but clearly marine association, shows that such an approach yields ambiguous results at best. Because a lacustrine basin is in essence a closed system, the bulk chemistry of its sedimentary fill should compare closely to the average composition of surrounding source rocks. Major element data show large compositional discrepancies between the likely source rocks of the Belt Supergroup and the average composition of major stratigraphic units of the Belt Supergroup. These discrepancies are most pronounced for highly soluble elements (Na, Mg, Ca) and are best explained through continuous exchange (export/import) with the Proterozoic ocean. The observed chemical imbalances are incompatible with a lacustrine setting for the Belt Supergroup. Sulfur content and sulfur isotope data of Belt rocks also suggest connection to the ocean reservoir. Presence of glauconite in various Belt units further confirms the marine character of the Belt basin.

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Small scale sedimentary iron deposits in a Mid-Proterozoic basin: Viability of iron supply by rivers

The Helena embayment, an eastern extension of the Mid-Proterozoic Belt basin, Montana, USA, contains extensive horizons of pyritic shale along the basin margins. These pyritic shale horizons may contain several hundred million tons of iron in pyrite. The source of the iron is problematic, and it is proposed here that it was introduced into the basin by continental runoff. Pyritic shale units were deposited after major regressions (or pulses of coarse clastic sedimentation), thus suggesting a mode of iron introduction similar to Phanerozoic oolitic ironstones. The sedimentary record of the Beltian sequence indicates a semi-arid climate, a hinterland of low relief, and low sedimentation rates. A combination of these parameters with data from recent environments provides physical and chemical constraints on a "fluvial" model of iron introduction. With such a model the average size of the drainage basin and the amount of introduced iron can be estimated. Sufficient iron is introduced within the temporal and physical limits that are imposed by data from the sedimentary record and by data on iron transport in recent terrestrial waters, to form the pyritic shale horizons in the Helena embayment.

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Sedimentary structures, textures, and depositional settings of shales from the Lower Belt Supergroup, Mid-Proterozoic, Montana, U.S.A.

The Belt Supergroup is a thick (20km) shale-dominated sequence that accumulated in an epicontinental basin between 1450 and 850 Ma. Sedimentary features show that shale facies can be interpreted in terms of depositional setting, and that shales of the Newland Formation were deposited in shallow water. The absence of certain features in the Prichard Formation, such as microbial mat deposits and cross-lamination in silt/mud couplets, points towards deposition in a deep basin, and sediment supply by nepheloid flows and reworking by bottom currents are indicated by indistinctly and sharply bounded silt laminae respectively. Possible eolian input is indicated by scattered silt in "deep" water facies of the Newland Formation, and by even silt laminae of large areal extent in the Prichard Formation. Small scale sedimentary features are still recognizable in shales that have undergone low-grade metamorphism. Thus, an approach to shale sedimentology as shown here can also be applied to metamorphosed shale sequences.

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Paleoflow patterns and macroscopic sedimentary features in the Late Devonian Chattanooga Shale of Tennessee: Differences between the Appalachian Basin and the American Craton.

Previously, paleocurrent data from the Chattanooga Shale and its lateral equivalents suggested deposition on a westward dipping paleoslope (westward paleoflow). However, new paleocurrent data (measurements of anisotropy of magnetic susceptibility, AMS) are not in agreement that view. They indicate eastward flowing paleocurrents in eastern Tennessee, and irregular flow in central Tennessee.

Macroscopic sedimentary features also point to significant differences in sedimentary setting between eastern and central Tennessee. In eastern Tennessee, the Chattanooga Shale intertongues with turbidites of the Brallier Formation, lacks shallow water features, and may have been deposited in depths between 100-200m. In central Tennessee, HCS siltstones and sandstones, shale-on-shale erosion surfaces, lag deposits (bone beds), and ripples, suggest relatively shallow water (tens of metres) and reworking of the sea bed by waves and strong currents.

A shallow water platform in central Tennessee, and deeper water conditions in eastern Tennessee require an eastward dipping sea floor (paleoslope) between these two areas. Further east however, where the Chattanooga Shale interfingers with the Brallier Formation, the paleoslope was inclined in the opposite direction (westward flowing paleocurrents). These two opposing slopes formed an elongate trough along the eastern margin of the Appalachian Basin.

The portions of the Chattanooga Shale whose AMS data suggest eastward paleoflow, occur between the shallow water Chattanooga of platform origin and the area where it interfingers with the Brallier Formation. Thus, the eastward dipping paleoslope suggested by sedimentary features is in good agreement with AMS paleocurrent data.

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Reflection of deep vs shallow water deposition by small scale sedimentary features and microfabrics of the Chattanooga Shale in Tennessee

The Chattanooga Shale was deposited on a continental platform in central Tennessee, and in a slope/peripheral trough setting in eastern Tennessee. Careful examination of shale samples shows distinct differences between these two settings. A total of 14 shale types has been distinguished on the basis of sedimentary features, lithologic associations, and small scale sedimentary sequences. Sedimentary features attributable to fine-grained turbidites characterize most of the shales from eastern Tennessee. On the basis of the clay/silt ratio two types of fine-grained turbidites can be distinguished. They may represent deposition in an interlobe slope (large clay/silt ratio) and lobe margin environment (small clay/silt ratio). Silt laminae produced by storm induced wave and current reworking characterize the shales of the platform region. Differences in energy levels, probably reflecting seafloor morphology, are indicated by sedimentary features of the various shale types.

Bioturbation in these shales is subtle and has been underestimated in the past. Burrows of the platform region are more elaborate and complex than those of the trough region. Even units that to lack visible signs of bioturbation contain peloids that could be fecal pellets of polychaete worms or similar surface dwellers. This observation indicates that oxygen levels in the bottom waters were not as low as previously assumed and that truly anoxic conditions were rare in the Appalachian Basin.

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The influence of sea level changes and possible pycnocline shifts on benthic communities in the Finis Shale (Virgilian) near Jacksboro, north-central Texas

The Finis Shale Member (Upper Pennsylvanian, Virgilian, Cisco Group, Graham Formation; Barnes, 1987) in the Jacksboro-Graham area is situated in the lower middle part of a well preserved transgressive-regressive cycle, the Finis Shale Cycle. Below it, the cycle begins with a lenticular sandstone, a mudcracked greenish shale, and a poorly developed transgressive limestone correlative with the Salem School Limestone Member of the Graham Formation. Exceptionally well preserved in situ benthic invertebrate fossil assemblages in this cycle allow for a detailed paleoecological investigation, providing information about possible past environmental conditions relating to the water column and substrate. Marine benthic communities preserved within the Finis Shale Cycle begin with a very shallow water Myalina community, a very thin (10 cm) mudstone containing a Triticites community, followed by a deeper water, relict mature Crurithyris-Paraconularia community, a Crurithyris-Glabrocingulum community, and a relatively mature Hustedia-Phymatopleura community which persists through the transgression-regression shift (and disappears a few decimeters above). A Rhipidomella-Dentalium community is topped by a thick (30 cm) interval of abundant Triticites within the regressive portion of the cycle. The base of the Jacksboro Limestone tops section. The described community changes could be interpreted as primarily due to changes in sea level, sediment supply, and energy input. However, only 20 miles to the southwest, fissile, poorly fossiliferous black shales dominate the lower part of the Finis Shale, suggesting that lateral pycnocline shifts may also have influenced living conditions of benthic communities.

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Tempestites from the Mid-Proterozoic Newland Formation, Montana

Some sandstones in the transition between Lower and Upper Newland Fm., Big Belt and Little Belt Mts., Montana, were previously interpreted as turbidites. However, the primary sedimentary structures do not follow a regular Bouma sequence, and are incompatible with deposition from turbidity currents. Depositing currents fluctuated in strength and only sometimes progressively waned. It is considered, that the sandstones were deposited from ebb currents generated by storm surges, and that each bed records a single storm event.

The sandstones are interbedded with shales, range in thickness from a few centimeters to more than a meter, and are laterally impersistent. Parallel to depositional strike the distribution of sandstone beds varies greatly over short distances. Perpendicular to it they show systematic lateral changes in frequency of occurrence, mean bed thickness, and cumulative thickness of sandstone, indicative of derivation from a northern source. The sandstones show planar to undulatory erosive bases and non-erosive tops. With increasing bed thickness the basal surfaces become more clearly erosive and show locally deep scouring and flute casts. In the northernmost exposures the sandstones are thickest, contain carbonate pebbles at the base, are locally interbedded with flat pebble conglomerates, can occupy erosional channels, and show large scale tabular crossbedding. Towards the south, parallel lamination, low angle cross-stratification, wavy bedding, and ripple cross-lamination become more abundant.

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Cryptalgal laminites and storm layers in a Proterozoic shale: Proceedings of the Oregon Academy of Science

The Upper Newland Fm. in Meagher Co., Montana, consists of interstratified packages of shales and carbonates. The shales consist of carbonaceous silty shale interlaminated with clayey shale (striped shale facies). Layer thickness ranges from a few mm to several cm. Intimately associated carbonates with cryptalgal laminites and flat pebble conglomerates are evidence of a subtidal setting for the striped shales.

The carbonaceous silty shales are considered to be fossil algal mats. Irregular internal laminations, patterns of particle trapping, mechanical deformation during penecontemporaneous soft sediment deformation, oversteepened laminae, and algal filaments furnish evidence in favour of this interpretation. The clayey shales often have silt with grading, graded rhythmites, and ripple cross-lamination in the bottom portion of the layer. In places the silt/mud couplets drape over layers of intraformational conglomerates. A storm origin is evisioned for the silt/mud couplets. by comparison, modern subtidal sediments of the North Sea contain similar storm layers of fine sand and silt, covered by a post-storm mud drape.

At present subtidal algal mats can only survive under special conditions, but in the Precambrian they extensively colonized the shallow portions of sedimentary basins. Before the start of grazing and bioturbation by metazoans, striped shales were very common in Precambrian basins.

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Shale facies in basin analysis: An example from the Proterozoic of Montana

Shales constitute more than 60% of the world’s sediments, yet while facies models for sandstones and carbonates are at a high level of sophistication, the study of shales has clearly lagged behind. Four principal groups of shale facies types can be distinguished in the Mid-Proterozoic Newland Fm., Montana, by bedding characteristics, internal structures, and the proportions of silt, clay, and carbonate. From stratigraphic mapping each group can be placed relative to basin margin and center (group A most marginal, group D most basinal). the sedimentary environment of the shales can be deduced from associated sandstone and carbonate units.

Group A consists of wavy interlaminated silt and mud, may contain small shrinkage cracks, and was deposited near shore and intermittently exposed. Group B consists of silty mud with non-parallel stratification planes, randomly oriented clays, and ripped up fragments of microbial mats. It was deposited in a very shallow subaqueous environment with frequent agitation. Group C consists of interstratified silty carbonaceous shale and graded silt-mud couplets, forming in quiet, shallow water by growth of microbial mats and episodic sediment supply by storms. Group D consists of even layers of a mixture of mud, silt and lumpy fragments of microbial mats, which settled in the deepest part of the basin and were not later reworked.

Shales of the Newland Fm., like other Precambrian shales, are undisturbed by bioturbation. Their primary structures are preserved in detail, thus making them uniquely suited for studying the conditions of shale deposition.

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A novel use of rare earth element patterns in stratigraphy and basin analysis

The Proterozoic Newland Fm. (Belt Supergr.) of the Little Belt Mts./Montana can be subdivided into 3 major stratigraphic units. The Lower Newland Fm. is a uniform dolomitic shale sequence which accumulated slowly during a period of tectonic quiescence, and REE patterns of shales (normalized to NASC) are either flat or LREE enriched. the Newland Transition Zone contains variable amounts of feldspathic sandstones in addition to the shales. It indicates rejuvenation of the hinterland and regression. the Upper Newland Fm. consists of alternating packages of shales and carbonates. REE patterns in the last two units have negative Eu anomalies. This drastic change in REE patterns was observed in all of the 4 stratigraphic sections that contained all 3 stratigraphic units. the pre-Beltian source rocks are mainly granitoid gneisses and migmatites, and rocks of this composition show negative Eu anomalies (against NASC) in many other places. The observed negative Eu anomalies in the shales are probably inherited from the source rocks. However, the patterns of the Lower Newland Fm. are not as easy to explain, because the source rocks seem to have been the same throughout Newland deposition. It appears that more intensive chemical weathering prevented the preservation of the negative Eu anomaly in the residual clays, and that adsorption of LREE on clays during transport caused the LREE enriched patterns.

A change in tectonic regime and weathering is recorded by the change in REE patterns, establishing quasi time-lines in a basin where the use of index fossils is not possible.

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Mobility of rare earth elements (REE) during carbonate diagenesis of the Mid-Proterozoic Newland Formation, Montana

Whole rock REE analyses of carbonate rocks from the Newland Formation were normalized to a composite average of associated shales. With this method of normalization an enrichment of light REE (LREE), relative to their terrigenous component, is visible in the carbonates. In addition, a positive Eu-anomaly becomes obvious. Separate REE determinations on soluble fractions and insoluble residues confirm that above normalization method allows to "see through" the terrigenous overprint of whole rock REE patterns of carbonates.

Strongest LREE enrichment is observed in limestones that are intimately interbedded with shales. Comparison of REE contents between limestones and shale partings suggest that REE were transferred from the shale partings into the limestone beds during diagenesis. Early diagenetic silica precipitation in carbonate rocks appears to control diagenetic LREE addition, because the amount of diagenetic silica is inversely proportional to LREE enrichment. LREE enrichment is very low or absent in dolostones, which show pervasive silicification. Positive Eu-anomalies indicate reduction of Eu to the divalent state during diagenesis and relatively high mobility of divalent Eu.

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Mixed terrigenous-carbonate sedimentation in the Mid-Proterozoic Newland Formation, Montana

Carbonate rocks of the Newland Formation are characterized by a relatively large content of terrigenous matter (up to 50%). Simultaneous terrigenous clastic and carbonate sed-imentation is indicated by lateral facies associations and small scale relationships between shale and carbonate beds.

Main lithofacies types are: (A) fine crystalline dolo-stone interbedded with shales, up to 50% terrigenous matter, with mudcracks and edgewise conglomerates, deposition in nearshore mudflat to lagoonal setting; (B) massive cherty dolostone with minor clay content, algal laminations and in-cipient mudcracks, nearshore, intermittently exposed set-ting; (C) massive bedded limestone, only small amounts of terrigenous material, current and wave formed ripples, shal-low offshore setting; (D) heterolithic limestone (alter-nating limestone and shale beds), large proportion of terri-genous material, erosive channels, intraformational conglom-erates, current and wave formed ripples, shallow offshore setting; (E) mm-laminated, massive bedded limestone (purest carbonate facies), no current-produced sedimentary struc-tures, deposited in deepest portions of the basin.

Nearshore dolostones developed as facies A where supply of terrigenous material was relatively high, and as facies B between areas of terrigenous input. Limestones of facies C and D were deposited in essentially the same shallow offshore setting, but contrast in the amount of terrigenous sedimentation; a reflection of relative proximity to terrigenous input. Facies E was deposited in the center of the basin, farthest removed from terrigenous input.

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Sedimentary structures, textures, and depositional settings of shales from the Mid-Proterozoic Belt basin

The Belt Supergroup of Montana, Idaho, and British Columbia is a thick shale-dominated sequence that accumulated in an epicontinental basin between 1450-850 m.y. ago. Shales from the Newland Formation in the eastern part of the basin have been studied in detail, and six major shale facies types are distinguished on the basis of bedding characteristics, textural features, and the proportions of silt, clay and carbonate. Textural features of these shale types are related to sedimentary environments as deduced from associated lithologies. A variety of sedimentary features, such as graded silt/mud couplets, ripple cross-lamination, graded rhythmites, massive bedding, non-parallel uneven bedding planes, random arrangement of clays and micas, deformation of algal mat fragments, shrinkage cracks, intraformational conglomerates, wave ripples, gradual variations of sedimentary components, and the contacts between interbedded lithologies (sharp vs gradual) serve to elucidate the depositional environment of these shales. Their textural and sedimentary characteristics reflect subaqueous growth of microbial mats, erosion and deposition by storms, deposition of flocculated vs. dispersed clays, continuous slow background sedimentation, winnowing by waves or currents, and subaerial exposure. The various shale facies types reflect differences in the conditions of deposition, from nearshore to basinal. Shales from the variably metamorphosed Prichard Formation, a lateral equivalent of the Newland Formation to the west, still show clearly visible small scale sedimentary features (graded silt/mud couplets and tiny silt ripples are predominant). Deposited in a generally deeper setting than shales of the Newland Formation, they contain several distinguishable shale types, though differences between shale types are more subtle.

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Distribution of rare earth elements in stratigraphic units of the Belt Supergroup

The Mid-Proterozoic Belt Supergroup (NW USA, SW Canada), a thick (20 km) shale dominated succession, was deposited in an epicratonic basin over a time span of 600 m.y. Samples (30% sandstones, 70% shales) from the various stratigraphic units of the Belt Supergroup were analysed for REE. REE patterns (NASC normalized) show great variability and reflect the combined influences of source rock, weathering, transport, and diagenesis. Seven different REE pattern types were recognized and interpreted as follows: (A) Flat patterns = derivation from average continental crust of granodioritic composition; (B) LREE-enrichment = increased abundance of granitic rocks in the source area; (C) HREE-enrichment in shales = abundance of mafic source rocks; (D) HREE-enrichment in sandstones = enrichment in detrital zircon (transport fractionation); (E) Eu-enrichment (sandstones only) = due to abundance of detrital feldspars; (F) Negative Eu-anomalies (common in shales throughout most of the sequence) = the Mid-Proterozoic crust as a whole contained a large proportion of granitoid gneisses and migmatites; (G) Positive Ce-anomalies (red beds) = Ce-scavenging by iron hydroxides during diagenesis. Related studies indicate that the survival of source rock related negative Eu-anomalies depends on a decrease in weathering intensity. Some units with uniform and consistent REE pattern types bear promise as geochemical marker horizons.

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Pyrite mineralization in microbial mats from the Mid-Proterozoic Newland Formation, Belt Series, Montana, U.S.A.

Within the Mid-Proterozoic Newland Formation, a shale-dominated unit of the Belt Supergroup that was deposited in the eastern Belt basin, a variety of different shale facies types can be distinguished (Schieber, 1989, Sedimentology, v. 36, p. 203-219). Of particular interest for this study is a shale facies that has been interpreted to be the result of microbial mat growth (resulting in carbonaceous shale beds) interrupted by storm deposition (causing deposition of graded silt/mud couplets). Alternation of carbonaceous beds with silt/mud couplets gives these shales a characteristic striped appearance. Along the basin margins a pyrite-rich sub-facies of these striped shales is found locally, consisting of laminated pyrite beds that alternate with non-pyritic silt/mud couplets. Laminated pyrite beds in pyritic striped shales are interpreted as mineralized microbial mats because of wavy-crinkly internal laminae and because of the direct association with unmineralized striped shales that contain microbial mat deposits. Horizons of pyritic striped shale contain on the order of several 108 tons of iron in pyrite. The iron was probably supplied by terrestrial runoff in colloidal form. Iron hydroxides, introduced by rivers into basin marginal lagoons, flocculated, and were then incorporated into microbial mats and reduced to pyrite upon burial. Pyritic shales of very similar appearance to those in the Newland Formation have been described from several other Proterozoic basins, and are in some instances associated with stratiform lead-zinc deposits.

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Application of anisotropy of magnetic susceptibility (AMS) to paleocurrent studies in shale sequences.

A pilot study on the relationship between AMS and macroscopic paleocurrent indicators in shales of the Newland Formation (Proterozoic of Montana) showed that AMS can be used to predict paleoflow directions in these shales. In an extension of this study oriented cores were taken in outcrops of the lower and upper member of the Newland Formation. Sedimentologic and stratigraphic studies indicate a significant change in basin configuration between these two stratigraphic units. As expected, a change in the AMS-defined regional paleoflow pattern was clearly visible between the two members of the Newland Formation. These results show clearly that the AMS method can be used successfully to determine paleoflow patterns in shale sequences. Preliminary studies of other shale sequences indicate that AMS can be utilized for paleoflow detection in these units as well. Thus, for many shale sequences AMS may be the only widely applicable method of paleoflow investigation.

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Evidence for stabilization of terrigenous mud and sand surfaces by microbial mats in the Mid-Proterozoic Belt Supergroup.

The mention of fossil microbial mats typically evokes the image of three-dimensional domal structures (commonly known as stromatolites) in carbonate rocks. Research on modern terrigenous sediments has shown that there as well sediment surfaces may be covered with microbial mats. However, because physical sedimentation processes dominate the depositional environments of terrigenous clastics, these microbial mats typically exhibit planar morphology.

The Belt Supergroup constitutes a thick (20 km) Mid-Proterozoic sedimentary sequence that is dominated by terrigenous clastics. Formations in which evidence for planar microbial mats on terrigenous sediment surfaces has been found are the Newland, Mt. Shields, Snowslip, McNamara, Shepard, and Revett. In sandy units the following features are considered indicative of microbial mat colonization: 1) rippled erosion patches on otherwise smooth surfaces; 2) intervals of wavy-irregular layering within current laminated beds; 3) coherent behavior of microbial layers during erosion and soft sediment deformation; 4)ferroan carbonate cements. In muddy units microbial mats are indicated by: 1) wavy-crinkly fine lamination; 2) lateral gradation of fine laminae into domal buildups; 3) coherent behavior of microbial laminae during erosion and soft sediment deformation; 4) large contents of organic carbon; 5) dolomite cement.

Above observations strongly suggest that most sediment surfaces of shallow water terrigenous environments in sediments of the Belt Supergroup were covered and stabilized by microbial mats between sedimentation events. In the absence of metazoan grazers this was probably common for Proterozoic shallow water clastic deposits in general.

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Non-clastic fluvial iron supply to the Mid-Proterozoic Belt basin: Evidence from major element composition of shales and carbonates.

Limestones of the Newland Formation (Belt basin) were deposited in shallow offshore to basinal settings and show a very clear linear relationship between Al2O3 and Fe2O3 with the Al/Fe regression line going through the origin of a Al2O3/Fe2O3 diagram. This indicates that essentially all the iron in the limestones can be related to the terrigenous fraction, and probably was carried in mainly as iron oxide coatings on clay minerals. Dolomites of the Newland Formation were deposited in nearshore lagoons and mudflats and contain clearly elevated iron contents (1-1.5%) when compared to the limestones. A "primary" nature of iron enrichment is suggested by the micritic nature of most of these dolomites. Shales of the Newland Formation also show a linear relationship between Fe and Al, but the regression line intercepts the Fe2O3 axis at approximately 1.5-2.0%. These observations indicate that in addition to the ever present terrigenous iron component (clays, clay coatings etc.), there is a non-lithogenic component as well. The fact that during times of small terrigenous sedimentation this latter component is only recognizable in basin marginal dolomites, whereas it is present throughout the basin when terrigenous sedimentation predominates, suggests that it is introduced along the basin margins by continental runoff. Considering the oxidation state of the Mid-Proterozoic atmosphere it is likely that iron was carried into the basin in colloidal form (iron oxyhydroxides). Comparable relationships between Fe and Al should be found in sequences with oolitic ironstones if as hypothesized rivers supplied colloidal iron to form these deposits.

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Application of anisotropy of magnetic susceptibility (AMS) to paleocurrent studies in several shale basins.

Regional paleoflow investigations in shales of Precambrian (Newland Formation), Ordovician (Utica and Athens Shale), Devonian (Chattanooga Shale), and Jurassic (Posidonia Shale) age show good agreement of AMS azimuths with paleoflow azimuths determined independently from orientation of ripple marks and skeletal remains of various organisms (e.g. graptolites and belemnites). In shales that were deposited under influence of bottom currents in moderately deep epicratonic basins the imbrication of magnetic fabric elements is in downcurrent direction (Newland Formation, Posidonia Shale). In contrast, shales deposited as the distal portions of a turbidite system (Utica and Chattanooga Shale) show imbrication of magnetic fabric elements in upcurrent direction. At least in the case of the Newland Formation magnetic fabric imbrication and orientation of clay minerals seems to coincide, suggesting that magnetic fabric mimics petrofabric. Contrasting fabric imbrication between shales that were deposited by density currents and those deposited under influcence of bottom currents could reflect differences in the physical conditions of sedimentation. AMS azimuths from two deformed shale units (Newland Formation, Chattanooga Shale) indicate that flow related fabrics are not seriously affected by folding. The data further confirm the applicability of the AMS method to paleocurrent studies in shales.

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A comparison of provenance clues from interbedded shales and sandstones of the Mid-Proterozoic Newland Formation, Belt Series, Montana, U.S.A.

In provenance studies of sandstones petrographic observations are the main source of information, whereas geochemical criteria are commonly used in provenance studies of shales. Rarely are these two approaches applied to the same sedimentary sequence and results compared for internal consistency. Sandstones in the Newland Formation indicate a source area dominated by granites and granitoid gneisses (arkosic sands, K-Spar and microcline dominant), semi-arid to arid climate (well rounded quartz and feldspar, feldspar unaltered), peneplain conditions, and derivation from a stable cratonic source area (QFL diagrams). Newland Formation shales also indicate derivation from a granitic source (small La/Th and TiO2/Al2O3 ratios, large Hf contents, small Cr, Co, and Ni contents). Negative Eu-anomalies indicate granitoid gneisses and migmatites in the source area. Modest amounts of chemical weathering are indicated by a CIA of 69, and a stable cratonic setting is indicated by large K2O/Na2O ratios and large SiO2 contents. The main discrepancy between the two provenance approaches appears to be that shales indicate a larger degree of chemical weathering than one would expect from sandstone petrographic data. Transport segregation serves as a possible explanation for this discrepancy. There also seems to be a definite need for a better calibration of the CIA with muds from modern climatic settings.

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Evidence for episodic high energy events and shallow water deposition in the Chattanooga Shale, Devonian, Central Tennessee, USA

The upper Devonian Chattanooga Shale of central Tennessee, a classical black shale, was deposited in an epicontinental setting, west of the Appalachian foredeep. Its predominantly finely laminated and highly carbonaceous nature is commonly interpreted to mean that it was deposited in comparatively deep and stagnant water. Interbeds of bioturbated greenish-gray shale, an indication of more oxygenated bottom waters, have been ascribed to (1) basinward retreat of anaerobic bottom waters; (2) complete disappearance of anoxic conditions for brief intervals; (3) injection of oxygen into bottom waters by distal turbidity currents. Some of these beds have laminated fine sand and silt at the base and may show hummocky cross stratification (HCS), an indication that they resulted from the action of storm waves on the seabed and that greenish gray shale beds are post-storm mud drapes that settled rapidly from suspension. This conclusion is supported by microfabric studies (random clay fabric). Black shale beds may also show bioturbation that can extend into the underlying greenish-gray shales.

Other interesting features are inclined-undulose erosion surfaces that are conformably overlain by shale beds, sets of inclined shale beds that suggest low-angle cross-bedding, and clearly and uniformly developed alignment of clay particles (magnetic fabric studies). These features indicate occasional strong bottom currents and migration of large scale bedforms (possibly ancient analogs of mudwaves on modern shelves).

These observations show that the seabed was at times subject to prolonged erosion by bottom currents (erosion surfaces), agitation and reworking by storm waves (HCS and greenish-gray shale beds), and sediment transport by long-lived bottom currents (mudwaves, particle alignment). The epicontinental sea setting, the muddy sea bottom (wave damping), and the presence of HCS and other storm produced features suggest a relatively shallow water depth (possibly only a few tens of meters). Together with abundant evidence of variably strong bottom currents and bioturbation of black and gray shale beds this suggests that abundant planktonic organic matter production rather than stagnant bottom waters are the primary cause for black shale formation.

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Determining paleoflow patterns in a shale dominated epicontinental basin by using anisotropy of magnetic susceptibility (AMS).

The shale dominated Newland Formation (Mid-Proterozoic of Montana, USA) was deposited in an eastern extension of the Belt basin, the Helena embayment. Because the present shape of the Belt basin is largely an erosional feature, the original shape of the embayment is a matter of debate. Main questions are: (A) was the embayment originally much larger and had no resemblance to its present shape (parallel paleocurrent pattern would strongly indicate this possibility); (B) does the present outline of the embayment mimic its original shape (would be indicated by a circular or concentric paleocurrent pattern).

Compilation of AMS data from the lower member of the Newland Formation show a largely parallel pattern in N-S direction that shows no relationship to the present basin outlines. Data from the upper member of the Newland Formation show a change to a combined radial-concentric pattern. An eastward swinging (concentric) flow component dominates the northern embayment, and a concentric northerly flowing component prevails in the south. The data indicate that during deposition of the lower member of the Newland Formation the basin was much larger than presently preserved and covered large portions of the craton, and that the basin configuration probably changed to an east-west trending trough or half-graben during deposition of the upper member. The latter is in good agreement with stratigraphic and sedimentologic data from the Newland Formation and its lateral equivalents, suggesting a north-west to east-west trending shoreline in the northern embayment and an east-west trending southern shoreline with localized depocenters that supplied sediment northward into the basin.

Except where deposition by density currents can be suspected, the imbrication of magnetic fabric elements appears to be in upcurrent direction and thus allows not only detection of the sense of flow, but also the direction of flow. Reverse imbrication of fabric elements in density current deposits appears to be related to an as yet undefined "antidune effect". Successful application of the AMS method of paleocurrent determination to shales of the Newland Formation shows that this method also has considerable potential in investigation of other shale basins.

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Observations suggestive of persistent bottom currents and episodic high energy events during deposition of the Chattanooga Shale, Devonian of Tennessee: Implications for probable water depth and sedimentation conditions.

The predominantly finely laminated and highly carbonaceous nature of the Cattanooga Shale in central Tennessee is commonly thought to indicate comparatively deep and stagnant water. Interbeds of bioturbated greenish-gray shale have been ascribed to (1) basinward retreat of anaerobic bottom waters; (2) complete disappearance of anoxic condi-tions for brief intervals; (3) injection of oxygen into bottom waters

by distal turbidity currents. These beds may have laminated fine sand and silt with hummocky cross-stratification (HCS) at the base, suggesting that storm wave action reached the sea bed and that greenish gray shale beds are post-storm mud drapes. Random clay fabrics (rapid settling from suspension) are supportive of that conclusion. Inclined-undulose erosion surfaces that are conformably overlain by shale beds, sets of inclined shale beds that suggest low-angle cross-bedding, and clearly and uniformly developed alignment of clay particles (magnetic fabric studies) suggest occasional strong bottom currents and migra-tion of large scale bedforms (ancient analogs of mudwaves?). Thus, the seabed was at times subject to prolonged erosion by bottom currents (erosion surfaces), agitation and reworking by storm waves (HCS and greenish-gray shale beds), and sediment transport by long-lived bottom currents (mudwaves, particle alignment). The epicontinental sea setting, the muddy sea bottom (wave damping), and the presence of HCS and other storm produced features suggest a relatively shallow water depth (possibly only a few tens of meters). Together with abundant evidence of variably strong bottom currents and bioturbation of black and gray shale beds this suggests that abundant planktonic or-ganic matter production rather than stagnant bottom waters are the primary cause for black shale formation.

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A smorgasbord of meso- and mega-Scale sedimentary features in shales and their possible environmental significance.

Shales from a variety of sedimentary settings and geologic ages show an assortment of small scale (meso-scale) sedimentary features that can be related to depositional processes. For example, internal laminae show a large range in thickness and lamination styles (even, discontinuous, lenticular, wrinkled etc.), which can represent quiet settling, sculpting of the sediment surface by bottom currents, and growth of microbial mats respectively. Internal lamina features, e.g. grading (a), random clay orientation (b), preferred clay orientation (c), sharp basal contacts (d), and sharp top contacts (e) may be interpreted as indicative of (a) event-sedimentation (e.g. floods, storms, turbidites), (b) flocculation or sediment trapping by microbial mats, (c) settling from dilute suspension, (d) current flow prior to deposition, and (e) current flow during or after deposition. Other features are mudcracks, load casts, flame structures, bioturbation, graded rhythmites, fossil concentrations, cross-lamination, loop structures, and gradual variation in background sedimentation components, all of which carry information about conditions of sedimentation. No single feature can be used to pinpoint specific depositional conditions and environments, but assemblages of sedimentary features will probably turn out to be environmental indicators once more sedimentological case studies of shale sequences have been undertaken. Shales may also show outcrop scale (mega-scale) sedimentary features such as large scale cross-stratification (mudwave migration?) and "sandstone" and "conglomerate" beds that consist entirely of shale particles (infrequent high energy events? uplift and erosion of shale sequences?). All in all, careful study of shales reveals many sedimentary features useful for interpretation of sedimentary environments.

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Relationships between microfabrics, mesofabrics, and sedimentary environments of shales from the Mid-Proterozoic Newland Formation, Montana.

Microfabrics of shales are a very important feature of these rocks and represent the cumulative record of the history of transport, deposition, and burial of these fine-grained rocks. Shales used for this study cover a wide range of sedimentary environments and from thin section study six shale facies types have been identified. Preliminary SEM studies have shown overall correlation between meso- and microfabrics. For example: (1) shales that have random mesofabric because of flocculation show edge to face relationships of clays; (2) well aligned clays are found in shales that show preferred extinction under the petrographic microscope; (3) wavy stringers of well aligned clays that alternate with laminae of more randomly oriented silt and clay are found in layers that resulted from microbial mat growth. The improved resolution of the SEM also allowed observation of small scale textural features, such as changes of lamina styles that are due to fluctuations and/or changes in transport conditions, and thus adds a new dimension to our understanding of the depositional history of shales. It appears that with some effort fabric features of shales or combinations thereof can be related to specific facies types and sedimentary environments, as well as to basic processes of sedimentation, such as flocculation, settling of dispersed clays, agglutination on microbial mats etc.

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Sedimentologic, geochemical, and mineralogical features of the Belt Series and their bearing on the lacustrine vs marine debate.

The question whether the Belt Supergroup was deposited in an epeiric sea or an intracratonic lacustrine basin has occupied geologists for almost a century. Paleogeographic reconstructions that suggest an epicontinental or even an episutural setting for the Belt basin and that place Siberia along the western margin of Laurentia, have helped to revive a lacustrine interpretation.

A major supporting argument was Winston's (1986) comparison of Missoula Group strata to the lacustrine Eocene Green River Formation. Lacustrine deposition of the Green River Formation is indicated by epicratonic location, centripetal facies distribution, freshwater biota, and unusual evaporite minerals that could not have formed from evaporation of marine waters. The latter three features are not documented in the Belt Supergroup. Remaining similarities are a possible epicratonic setting and a general resemblance in sedimentary structures and facies from basin margin to basin center. It should also be noted that upon direct inspection the resemblance is not as close as one might perceive from comparing published descriptions. One could with equal justification cite the Triassic of central Europe and the western US, as well as the Late Devonian of the Appalachian basin, as containing intervals that are sedimentary analogs of the Missoula Group. Yet in the former case an epicontinental marginal marine basin, and in the latter two cases alluvial aprons and coastal plains bordering a marine foredeep basin are indicated. Obviously, sedimentary structures and facies alone do not necessarily furnish sufficient proof for either a marine or a lacustrine setting. Inland seas and lake basins both have comparatively small energy input via wave and/or tidal action, and in absence of biological indicators sedimentary structures and facies primarily reflect the interplay between energy level and sediment supply. It is therefore not surprising that it is very difficult to distinguish lacustrine from epeiric sea sediments with physical features alone.

I propose here a geochemical mass-balance approach to test a lacustrine vs marine interpretation. If the Belt basin was indeed an analog to the "closed" Green River basin, particles and solutes produced by weathering in the surrounding drainage would ultimately end up in the basin and the average chemical composition of the basin fill should closely match the average composition of its source rocks. Conversely, with an oceanic connection a net export or import of soluble species (in particular Na+, Mg2+, Ca2+, SO42-) may show up as a significant compositional imbalance between the average source rock and the average basin sediment. The underlying considerations for such a calculation are outlined below.

Although distinctions can be made on the basis of certain trace elements and isotopes, with respect to major elements Precambrian shields are of fairly uniform composition. Thus, with regard to major elements, the (unknown) craton that is placed to the west of the Belt basin in various reconstructions should have been of very similar average composition as the Canadian Shield to the east. Because the Canadian Shield underlies and borders the known portions of the Belt basin, and because its average composition is close to the average shield composition of Taylor and McLennan (1985), it is thought to closely reflect the average source rock of Belt strata.

Newly acquired data and those of Ross (1963) were used to calculate average compositions for main lithologies (shale, sandstone, carbonate) in each formation. For each major subdivision of Belt strata (Lower Belt, Ravalli Group, Middle Belt Carbonate [MBC], Missoula Group), as well as for the total Belt Series, weighted average compositions were calculated. Weighting took into account proportions of lithologies, thickness and extent of formations, and eroded margins of the present Belt basin. To correct for eroded margins the sandstone proportion was enhanced so that the assumed sandstone/shale ratio reflects the overall ratio of these lithologies in the sedimentary record. The fact that the averaged units were deposited over time intervals on the order of 100 million years, the domination of sedimentary mass balance by shales, and the observation that (on a carbonate-free basis) amounts and proportions of immobile elements (Al, Si, Ti, Fe) came very close to the Canadian/Precambrian Shield average, suggests that this is a viable approach. In principle the sandstone/shale ratio of a sedimentary sequence will be affected by changes in the intensity of chemical weathering. However, the chemical index of alteration (CIA) varies little for the successive formations of the Belt Series and indicates moderate chemical weathering (CIA 65-70). Even changing the assumed proportions of sandstones and shales by %10 percent does not significantly change the results of mass-balance calculations.

Because the Belt Supergroup represents a 600 m.y. time interval, it is conceivable that both export and import of soluble species occurred repeatedly, possibly cancelling each other in a grand mass balance. Although a weighted average for the Belt Supergroup nonetheless shows that the Belt basin was not a closed system, averages have been computed for major stratigraphic subdivisions in order to detect possible changes in export/import patterns through time. Figure 1 shows that for most of Belt deposition there was a net export of large quantities of Na, Ca, P, and Mn. Elements shown in Figure 1 are either highly soluble (Na, Mg, Ca, K) and/or may show considerable mobility during sediment diagenesis (P, Mn). As far as removal of elements is concerned, two scenarios come to mind, (1) communication with the Proterozoic ocean, and (2) storage of elements in evaporite minerals and later dissolution by meteoric waters. For example, with regard to Na in scenario (2) one would need to accommodate (on average) approximately 0.3 meters of NaCl or trona for each 10 m of sediment. Furthermore, in lacustrine systems like the Green River Formation evaporites are not evenly distributed but instead concentrated in the central portion. Thus, by comparison with the Green River Formation, centrally deposited strata of the Belt Series should in places contain very abundant sodic evaporites (in excess of 20%). It seems highly unlikely that quantities as large as that could have been dissolved from Belt strata without leaving a trace. The only indication of sodic evaporites in the Belt Series are halite cast horizons, but these are neither numerous nor extensive enough to have contained significant quantities of Na. One is thus left with scenario (1) and the implication of a connection to the Proterozoic ocean. That there was not only outflow but also influx of solutes through such a seaway during certain times in Belt history is shown by data from the MBC. During its deposition a 240-400% excess of Mg and Ca respectively was supplied to the basin, most likely from an ocean reservoir. For the Ravalli and Missoula Groups Figure 1 indicates influx of Mg but nonetheless an outflow of Ca, reflective of the fact that dolomite is the predominant carbonate phase in these rocks. On a more detailed level, substantial Mg and Ca influx is seen in the Empire and Shepard Formation (Ravalli and Missoula Group respectively). Figure 1 also shows consistent enrichment of K in Belt rocks, in amounts ranging from 19% to 40%. In the Belt Series illite is the dominant clay mineral and probably dominates the K mass-balance. Considering that this illite probably was derived from precursor smectite, it appears possible that the present K excess came originally from marine formation waters and was bound to Belt sediments during diagenetic illite formation. Thus, general K-enrichment of Belt sediments suggests marine influence throughout the depositional history. Approximately 40-50% of the potentially available P and Mn were removed throughout Belt deposition and must have entered the ocean reservoir. Considering the likely oxygen content of the Proterozoic atmosphere, Mn should have entered the basin in oxidized form, probably together with iron oxide as coatings on sediment grains. Because of the generally reducing character of the majority of Belt sediments large quantities of this "detrital" Mn could have been remobilized during early diagenesis and entered the overlying basin waters. For analogy, approximately 45% of the Mn accumulating in todays Mississippi delta is lost to overlying seawater.

Early diagenetic sulfides in the Belt Series have so strongly negative sulfur isotope values as to require open system conditions with respect to sulfur. This observation indicates that the Belt sea was connected to the Proterozoic ocean rather than being an isolated inland basin. Likewise, from analyses of rocks from the eastern Belt basin and the Prichard Formation it is safe to say that the lower Belt Supergroup contains on average 0.5-1% sulfur in sedimentary pyrite. Such sulfur concentrations are an order of magnitude above the average for the Canadian Shield (0.06%), requiring influx of sulfur from the ocean reservoir and confirming my conclusion above. Sulfur isotope data from mineralized zones in lower Belt strata indicate that the sulfur source was seawater sulfate, suggesting that hydrothermal fluids contributed only negligible amounts of sulfur to the Belt basin.

Boron concentrations in illite of the MBC are comparable to those found in Phanerozoic marine sediments. Schopf (1980), after an extensive review of information on ancient sea water, concluded that about 2 b.y. ago the oceans had reached modern salinity and composition, and that since then ocean chemistry has changed very little. Considering that Belt illite was not recycled sedimentary material, its boron content may indeed be indicative of marine basin waters. Sr contents of many limestone samples from the Newland Formation lie between the values established for calcite and aragonite in contact with seawater. Because later diagenetic overprint typically lowers Sr concentrations, the Sr data indicate that Newland limestones formed in marine waters.

Eby (1977) found abundant indications of vanished evaporites in the MBC. From his detailed description of sedimentary features it appears that the general mineral sequence was dolomite - calcium sulfate - halite. This puts considerable limits on possible water compositions and strongly suggests that the MBC evaporites precipitated from marine waters. The same reasoning applies to observations by White (1977) concerning the Altyn Formation, similarly suggesting marine waters for the Lower Belt.

Herringbone cross-stratification and reactivation surfaces have been described from the Belt Series at Glacier Park and suggest influence of tidal currents at least in the northern portion of the basin. Further south possible tidal features are ambiguous or absent. If, as envisioned by Cressman (1989), the Belt basin was a narrow gulf that opened to the Proterozoic ocean, it could well be that tidal features are more prominently developed closer to the opening of that gulf.

Chamosite was found in the Newland Formation and glauconite has been reported from the Ravalli and Missoula Groups. Both minerals form in shelf areas of modern seas and are common in Phanerozoic marine deposits. Glauconite in particular is considered a reliable indicator of marine conditions.

In conclusion, sedimentological criteria are not useful to differentiate between a lacustrine or marine origin of the Belt Series, whereas chemical mass balance considerations, sulfur isotopes, and concentrations of B and Sr all suggest that the Belt basin was marine. Pseudomorphs of evaporite minerals show the same minerals and mineral sequence to be present as typically found in marine evaporites, requiring precipitation from marine waters. Glauconite formation in Belt sediments also necessitates marine conditions. Its presence has been known for over 20 years and it is still incumbent upon those favoring a lacustrine Belt basin to explain how it got there. The data available for the Belt basin indicate connection to and exchange with the Proterozoic ocean. Variable salinity of the Belt sea (ranging from brackish through normal to hypersaline) is indicated by the geologic record and was probably caused by changing relationships between continental runoff, basin volume, and water depth.

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Facies and origin of shales in Upper Devonian Sonyea Group, New York.

Upper Devonian sediments of the Sonyea Group were deposited by the Catskill Delta complex in a foreland basin. Good stratigraphic, paleoecologic, and sedimentological control allows shale fabrics to be related to environmental parameters. Shale types are distinguished by composition, assemblage of lamina types, lamina thickness and continuity, gradual vs sharp lamina contacts, co-genetic lamina sequences, type and degree of bioturbation, sedimentary structures, and color. Onshore-offshore changes of shale facies are mirrored by changes in sedimentary textures and interpretable in terms of depositional conditions. Flood plain shales reflect overbank deposition, settling in oxbow lakes, and pedogenic processes. Basin marginal shales reflect rapid mud deposition in front of river mouths (shale with silt ripples), and reworking by waves and organisms in areas in between (biolaminated fabric, vertical burrows, HCS). Offshore/outer shelf shales are characterized by silty and muddy tempestites, a thriving invertebrate fauna, and abundant bioturbation. Absence of wave reworking, possible deposits from river plumes, fine-grained turbidites, and moderate to minor bioturbation are typical for shales deposited on the slope (or clinoform). In basin floor shales (black, laminated) indistinct uniform lamination vs rhythmic lamination (silty shale, silt, clay) suggests undisturbed background deposition vs episodic sediment reworking by currents respectively. Although inconspicuous, bioturbation (burrows, disrupted laminae) and fecal pellets suggest colonization of the sediment surface even during supposedly anoxic conditions. Examination of shales from other parts of the Catskill clastic wedge shows repetition of the same facies types in the same overall context, suggesting that the shale facies scheme worked out for the Sonyea Group is applicable to the Catskill Delta in general.

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Paleoflow patterns and macroscopic sedimentary features in the Late Devonian Chattanooga Shale of Tennessee: Differences between the Appalachian Basin and the American Craton.

Previously, paleocurrent data from the Chattanooga Shale and its lateral equivalents suggested deposition on a westward dipping paleoslope (westward paleoflow). However, new paleocurrent data (measurements of anisotropy of magnetic susceptibility, AMS) are not in agreement that view. They indicate eastward flowing paleocurrents in eastern Tennessee, and irregular flow in central Tennessee.

Macroscopic sedimentary features also point to significant differences in sedimentary setting between eastern and central Tennessee. In eastern Tennessee, the Chattanooga Shale intertongues with turbidites of the Brallier Formation, lacks shallow water features, and may have been deposited in depths between 100-200m. In central Tennessee, HCS siltstones and sandstones, shale-on-shale erosion surfaces, lag deposits (bone beds), and ripples, suggest relatively shallow water (tens of metres) and reworking of the sea bed by waves and strong currents.

A shallow water platform in central Tennessee, and deeper water conditions in eastern Tennessee require an eastward dipping sea floor (paleoslope) between these two areas. Further east however, where the Chattanooga Shale interfingers with the Brallier Formation, the paleoslope was inclined in the opposite direction (westward flowing paleocurrents). These two opposing slopes formed an elongate trough along the eastern margin of the Appalachian Basin.

The portions of the Chattanooga Shale whose AMS data suggest eastward paleoflow, occur between the shallow water Chattanooga of platform origin and the area where it interfingers with the Brallier Formation. Thus, the eastward dipping paleoslope suggested by sedimentary features is in good agreement with AMS paleocurrent data.

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Reflection of deep vs shallow water deposition by small-scale sedimentary features and microfabrics of the Chattanooga Shale in Tennessee.

The Chattanooga Shale was deposited on a continental platform in central Tennessee, and in a slope/peripheral trough setting in eastern Tennessee. Careful examination of shale samples shows distinct differences between these two settings. A total of 14 shale types has been distinguished on the basis of sedimentary features, lithologic associations, and small scale sedimentary sequences. Sedimentary features attributable to fine-grained turbidites characterize most of the shales from eastern Tennessee. On the basis of the clay/silt ratio two types of fine-grained turbidites can be distinguished. They may represent deposition in an interlobe slope (large clay/silt ratio) and lobe margin environment (small clay/silt ratio). Silt laminae produced by storm induced wave and current reworking characterize the shales of the platform region. Differences in energy levels, probably reflecting seafloor morphology, are indicated by sedimentary features of the various shale types.

Bioturbation in these shales is subtle and has been underestimated in the past. Burrows of the platform region are more elaborate and complex than those of the trough region. Even units that to lack visible signs of bioturbation contain peloids that could be fecal pellets of polychaete worms or similar surface dwellers. This observation indicates that oxygen levels in the bottom waters were not as low as previously assumed and that truly anoxic conditions were rare in the Appalachian Basin.

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Relationship of depositional environment to microfabric features of a shale unit in the Mid-Proterozoic Newland Formation, Montana

Scanning electron microscope (SEM) examination of a shale unit from the Newland Formation reveals textural features that are beyond the resolution of the petrographic microscope. For example, seemingly homogenous appearing layers of shale may actually be composed of interbedded laminae of aligned vs randomly oriented clay minerals, indicating a more complex depositional history. Another interesting feature are shale intraclasts (0.02-0.05 mm in size) within shale beds. Differential compaction around these intraclasts and unabraded corners suggest that these were hardened at time of deposition and were not transported very far. These observations suggest erosion of dried out mud crusts and redeposition of fragments thereof. In general the microfabrics show a moderate to low degree of clay mineral alignment. In part this can be ascribed to differential compaction around silt grains, but there is also evidence for an original lack of preferred orientation. For example, silt sized mica flakes, rather than being subparallel or parallel to lamination, show in many instances random orientation. At higher magnification one can see domains of randomly oriented clay minerals with edge to face and edge to edge contacts. These textures are typical for flocculated clays. Above observations suggest deposition of these shales in fairly shallow water, nearshore mixing of terrestrial waters with saline basinal waters, and fluctuating energy levels (episodic wave reworking, tides?). This compares well with previous assessments of the depositional environment of this shale unit and suggests that despite diagenetic modifications, microfabric features of shales can be useful environmental indicators.

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Relationships between benthic communities and sedimentary environments in the Pennsylvanian Finis Shale near Jacksboro, Texas.

In a new and complete exposure near Jacksboro, unweathered and highly fossiliferous Finis Shale overlies greenish laminated shales with mudcracks, and has a transgressive sandstone at the base. Five or more superimposed fossil assemblages can be distinguished. The basal sandstone is overlain by a thin red mudstone with calcareous concretions and abundant Myalina shells (very shallow water), followed by a thin sandy mudstone bed that contains abundant shell fragments and fusulinids. Then follows a black to dark-gray shale unit with abundant conularids, brachiopods, fossil seed pods and wood fragments (pyritic preservation). Modern analogs suggest that the latter might have been reworked from mangrove soils in the course of transgression. The next higher unit is a thin, highly fossiliferous horizon of light gray shale with phosphatic nodules and abundant evidence of erosion and reworking (disarticulated and broken brachiopod shells, epibionts on nodules). The uppermost unit is a dark gray shale with a brachiopod and bivalve dwarf fauna at the bottom, changing upwards into an assemblage where in contrast very large specimens are conspicuous in addition to rugose corals and soft bottom sponges. Faunal and sedimentological characteristics such as degree of bioturbation, population density, dwarf faunas, shell morphology (e.g. thin vs massive shells and spine development in brachiopods), and faunal assemblages reflect a complex interplay of oxygen levels, food supply, strength of bottom currents, turbidity, and substrate properties (firm vs soft). Careful examination of these characteristics allows detailed reconstruction of living conditions and depositional environments of a Pennsylvanian core shale.

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The influence of sea level changes and possible pycnocline shifts on benthic communities in the Finis Shale (Virgilian) near Jacksboro, north-central Texas.

The Finis Shale Member (Upper Pennsylvanian, Virgilian, Cisco Group, Graham Formation; Barnes, 1987) in the Jacksboro-Graham area is situated in the lower middle part of a well preserved transgressive-regressive cycle, the Finis Shale Cycle. Below it, the cycle begins with a lenticular sandstone, a mudcracked greenish shale, and a poorly developed transgressive limestone correlative with the Salem School Limestone Member of the Graham Formation. Exceptionally well preserved in situ benthic invertebrate fossil assemblages in this cycle allow for a detailed paleoecological investigation, providing information about possible past environmental conditions relating to the water column and substrate. Marine benthic communities preserved within the Finis Shale Cycle begin with a very shallow water Myalina community, a very thin (10 cm) mudstone containing a Triticites community, followed by a deeper water, relict mature Crurithyris-Paraconularia community, a Crurithyris-Glabrocingulum community, and a relatively mature Hustedia-Phymatopleura community which persists through the transgression-regression shift (and disappears a few decimeters above). A Rhipidomella-Dentalium community is topped by a thick (30 cm) interval of abundant Triticites within the regressive portion of the cycle. The base of the Jacksboro Limestone tops section. The described community changes could be interpreted as primarily due to changes in sea level, sediment supply, and energy input. However, only 20 miles to the southwest, fissile, poorly fossiliferous black shales dominate the lower part of the Finis Shale, suggesting that lateral pycnocline shifts may also have influenced living conditions of benthic communities.

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The utility of small scale sedimentary structures in shales.

Although quite inconspicuous or even invisible in outcrop, small scale sedimentary structures in shales contain information about depositional conditions and history of a shale. When polished slabs and petrographic thin sections are examined, many seemingly drab shales show a large range and variability of sedimentary features. Examples ranging from Proterozoic to Eocene demonstrate how small scale sedimentary structures can be used to detail sedimentary conditions. Among the most common structures are thin silt layers. These may for example indicate deposition by density currents (grading, fading ripples), storm reworking and transport (graded rhythmites), wave winnowing (fine even laminae with scoured bases), and bottom currents (silt layers with sharp bottom and top). Gradual compositional changes between e.g. clay and silt dominated laminae, are another commonly observed feature, and are suggestive of continuous (although slow) deposition, possibly from deltaic sediment plumes and shifting nepheloid flows. Upon close inspection shales may also reveal clay-filled mud cracks, brecciation due to desiccation, and sands or conglomerates that consist entirely of shale particles. The latter can for example form as a result of soil erosion (pedogenic particles), erosion of cracked mud crusts, and submarine scouring of mud substrates by strong currents. Biologic agents may produce microbial laminae and protection of mud surfaces from erosion, or may manifest themselves as bioturbation and destruction of primary fabrics. However, in many instances sufficient proportions of primary features survive bioturbation, and in many instances observation of bioturbation features indicates substrate firmness, event deposits (escape traces), and rates of deposition.

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Authigenic silica deposition in spores: A mechanism to produce "in situ" quartz sand in black shales?

Devonian black shales that were deposited on the North American craton contain abundant Tasmanites spores. Although these are typically flattened because of compaction, a small proportion of spores is filled with diagenetic silica. They are spherical to ellipsoidal (0.1-0.4mm), and filled with chalcedony, chert, poly-crystalline quartz, and mono-crystalline quartz. Chalcedonic spores are best preserved in nodules (chert, phosphate), whereas other forms of silica and particularly monocrystalline quartz are most common in shale matrix. Together with colloform relict textures this suggests that the various silica types originated from recrystallization of early diagenetic amorphous silica.

Thin sandstone beds that are found in the Chattanooga Shale (e.g. Bransford Sandstone) contain abundant fine to medium quartz, much coarser than the detrital quartz component of underlying black shales. Because of this, their quartz component is thought to have been transported over considerable distances (Conant and Swanson, 1963). However, because certain shale horizons contain as much as 5% silicified spores that upon reworking could have yielded fine to medium quartz grains, the quartz component of these sandstone beds may actually have formed in situ.

Petrographic examination of sandstone beds shows them to contain indeed a considerable proportion of quartz grains with morphological and textural features of "spore" quartz (e.g. rounding, sphericity, chalcedony, pyrite inclusions, lobate grain margin). Thus, silica deposition in spores may provide a significant component of intrabasinal fine to medium quartz in shale sequences. Distinction of this type of quartz from "truly" detrital quartz (extrabasinal) is important to the reconstruction of the depositional history of shale sequences.

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Depositional cycles in the Chattanooga Shale of Tennessee: Evidence of orbital forcing?

Recent field work shows the presence of cyclic deposition in the Chattanooga Shale of central Tennessee. The observed cycles can be grouped into three types:

1) alternating greenish-gray and black shale, 5-10cm/cycle
2) alternating black and dark gray shale, 5-10cm/cycle
3) "banded" black shale, 1-2cm/cycle

Cycles of type 1 characterize the upper Dowelltown member, those of type 2 occur in the lower Dowelltown and the lower and middle Gassaway member, and cycles of type 3 occur in the upper Gassaway member.

Cycles of type 1) show abundant bioturbation in green-gray as well as in black intervals, and also contain storm deposits (mud-tempestites); cycles of type 2) mainly show bioturbation associated with dark-gray shale intervals; and cycles of type 3) are characterized by rhythmic variations in organic matter and/or silt content. These differences in sedimentary features of cycles may suggest that different cycle types reflect differences in depositional setting, such as e.g. water depth, surface productivity, terrigenous sediment input. From available conodont data and recent chronometric calibration of Late Devonian conodont zones, it appears that the approximate duration of cycles of type 1) and 2) was between 100-150Ka, and that cycles of type 3) represent approximately 28Ka. Thus, cycle frequencies fall within the Milankovitch band, possibly representing eccentricity (cycles of type 1 and 2) and precession cycles (cycles of type 3). Because identification of orbital forcing for these cycles would allow considerable refinements in the study of the Late Devonian black-shale sequence, additional studies are in progress to obtain better estimates of cycle length (conodont biostratigraphy), and cycle regularity (spectral analysis).

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Sedimentary expression and stratigraphic significance of erosion surfaces, condensation horizons, and hiatuses in the Chattanooga Shale of central Tennessee.

Recent investigations show that the finely laminated and highly carbonaceous Upper Devonian Chattanooga Shale was deposited in relatively shallow water, and that deposition was influenced by storm waves and episodic erosive events. Recurring erosive events of variable strength and/or duration are indicated by truncation surfaces beneath which from a few centimeters to more than a meter of section is missing. That the seabed was close to or within reach of storm waves through large portions of Chattanooga history is suggested by hummocky cross-stratified sand and silt beds and mud tempestites.

In many places, however, truncation surfaces cut underlying beds at a very shallow angle, or even become conformable to under- and overlying beds. In hand specimen or thin section, these essentially "invisible" erosion surfaces may be associated with one or more of the following features: (1) sharp-based shale beds, with (2) basal sandy layers (some mm to cm thick), and/or (3) abundant reworked pyrite, and/or (4) conodont concentrations, 4) soft sediment deformation of shale laminae below erosion surface, 5) localized low-angle truncation of shale laminae.

On erosion surfaces, the concentration of reworked framboidal pyrite can in places lead to enrichment by a factor of 20-30 relative to "normal" black shale, and may give rise to sharp-based beds of pyritic shale. Diffuse-based pyritic shale beds, lacking an association with erosional features, but otherwise texturally identical, have also been observed. In these beds, the very strong enrichment of diagenetic (pyrite) and biogenic (conodonts) components suggests extremely slow or absent terrigenous sedimentation, probably an indication of condensation and/or hiatuses. Pyritic shale beds as thick as 10 cm have been found, possibly representing as much as 2-3 m of eroded and reworked (sharp-based) or condensed section (diffuse-based). This makes them potentially significant stratigraphic elements.

Erosion or condensation on the cm scale might be due to local conditions, and seem to define macroscopic bedding in outcrops of the Chattanooga Shale. Meter scale erosion and thick pyritic beds on the other hand define substantial discontinuities that they may well be the result of basin-wide lowering of sea-level. In order to see whether these latter features are expressions of sequence boundaries in slowly accumulating black shale sequences, work is underway to test whether they correlate with transgressive-regressive cycles found in the Upper Devonian of the Catskill Delta (Johnson et al., 1985).

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An Atrypid/Leiorhynchid brachiopod fauna in the Chattanooga Shale of southwestern Tennessee: Indication of dysaerobic conditions?

Atrypid and Leiorhynchid brachiopods are locally abundant in the Dowelltown member of the Chattanooga Shale (Late Devonian) of southwestern Tennessee. These brachiopods occur within a sequence of rhythmically interbedded dark-gray and greenish-gray shales (5-15cm thick) that also contains storm-wave reworked sandstone beds.

Greenish-gray shale beds were probably homogenized by infaunal activity (absence of primary laminae) and may contain large numbers of brachiopod shells. Five brachiopod genera were recognized, all of them characterized by thin, broad, round, and relatively flat shells (most shells within 2-2.5cm size range). No other invertebrate fossils were observed. Intervening dark-gray shale beds show well preserved primary laminae and contain essentially no shell material. Dark-gray shales display sporadic to weak bioturbation, mainly vertical to sub-horizontal tubular burrows (3-5mm diameter). Brachiopod shells rest horizontally within the shale, lack preferred orientation, and do not show breakage or abrasion. Within brachiopod bearing intervals, shells may vary in abundance between successive bedding planes, and consecutive shell layers typically are separated by a few millimeters of shale.

With respect to morphological features, the low diversity/high abundance brachiopod assemblage found in these greenish-gray shale beds could be a Devonian analog to the "flat clam" assemblages that seem to characterize several Mesozoic black shale sequences. The latter are thought to indicate adaption to low-oxygen bottom water conditions, possibly involving chemosymbiosis. By analogy, the brachiopods in the Chattanooga Shale seem to indicate low-oxygen, possibly dysoxic bottom waters and rhythmic changes in oxygenation. Potential causes for these rhythmic changes are presently under investigation.

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Lag deposits in the Chattanooga Shale: The importance of diagenetic and biogenic grains and potential for clues to paleoceanography.

The Chattanooga Shale of central Tennessee and southern Kentucky contains an abundance of erosive features. Some erosion surfaces show removal of only a few mm (very limited lateral extent), others as much as 1.5 m of erosion of underlying shale (may extend for more than 100 miles).

A variety of lag deposits is associated with these erosion surfaces. Silty lags (up to 13 cm thick) are common and dominated by quartz silt. Sandy lags (up to 2 cm thick) contain abundant quartz sand, in addition to pyritic, phosphatic, and glauconite grains. Bone beds (up to 2 cm thick) contain abundant fish bone fragments (up to 25 mm) in a sandy matrix. Pyritic lags (up to 70 mm thick) can be subdivided into peloidal lags, spheroidal/framboidal lags, and pyritic ooid beds. Conodont lags (typically less than 1 mm thick) consist of a concentrate of conodonts in a silt matrix. Lingula lags consist of shale bedding planes strewn with Lingula shells.

Cysts of planktonic green algae (0.05-0.8 mm), Tasmanites, filled with pyrite and quartz during early diagenesis and provided in situ quartz sand and pyrite grains (spheroids & framboids). Breakup of early cyst quartz during reworking provided in situ silt size quartz grains (Schieber, JSR, v. 66, p. 175-183). Diagenetic pyritization of fecal pellets produced the dominant grain type in peloidal pyrite lags. Pyrite ooids closely resemble (texture) carbonate ooids, and although this might indicate a primary origin, a very early diagenetic origin is considered more likely. Conodonts, Lingula shells, and fish bones are common biogenic particles in the Chattanooga Shale, and their concentration into lags is to be expected. The majority of sand size grains, all of the larger than sand size particles, and a substantial proportion of the silt size grains are of early diagenetic or biogenic origin. Field observations and relative abundance of grain types in average shale allows ranking of lag deposits by likely depth of erosion. Sandy lags & bone beds: X*102 cm; pyritic lags: X*101 cm; silty lags & conodont lags: X*100 cm; Lingula lags: X*10-1 cm.

Lag types also show stratigraphic control. Whereas silty, conodont, and Lingula lags are found throughout the Chattanooga Shale, sandy lags are dominant in the lower, Dowelltown member, and pyritic lags dominate the upper, Gassaway member. Because lag particles are largely of early diagenetic and biogenic derivation, elucidating their origin and the reasons for stratigraphic distribution of lag types may uncover clues to underlying paleoceanographic controls, such as bottom current regime, terrigenous sediment influx, plankton blooms & nutrient supply, sedimentation rates, and bottom water oxygenation.

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Are sedimentary features associated with worm burrows in the Chattanooga Shale a result of fluid mixing? An experimental approach.

Trace fossils are abundant in the Dowelltown member of the Chattanooga Shale (Late Devonian) in central Tennessee. They occur within a sequence of rhythmically interbedded dark-gray and greenish-gray shales (5-15 cm thick). Burrows cross the boundaries between layers and tend to obscure the contacts between alternate layers. Owing to contrasting colors of alternate layers, internal fills of individual burrows often display an array of complex, convoluted features, which appear to result from mixing of semi-fluid gray and black muds.

Because it is not known what particular invertebrate groups produced these burrows, experiments were made to study the relationships between the general morphology of burrowers, sediment viscosity, and textural features within burrows. Colored plaster layers of differing viscosities were superimposed, and sets of variably shaped artificial bait worms were pulled through these layers. The hardened blocks were sawed perpendicular and horizontal to bedding, in order to study the resulting textures. The experiments were conducted at room temperature, and the explored viscosities ranged from that of butter milk to that of lithium grease.

The results were as follows:

  • 1) The produced convolute textures very closely resembled those observed in burrows from the Chattanooga Shale.

    2) The degree of convolution increases with decreasing viscosity of the substrate.

    3) The length of the worm determines the extent of downward mixing as the worm passes between layers.

    4) There is a distinct difference between mixing patterns produced by smooth worms as compared with those that have appendages.

  • The experiments strongly suggest that the burrows observed in the Chattanooga Shale were indeed produced by worms that moved through semi-fluid muds. It is likely that comparable textures will be recognized in other mudstone successions in the future. Also, experiments such as this may provide a basis for quantifying the initial consistency of surface sediments in ancient mudstone successions.

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    Microbial Mats in Terrigenous Clastics: The Challenge of Identification in the Rock Record

    Increasingly, microbial communities are recognized to play a potentially important role in defining and modifying surface sediment characteristics in various settings, ranging from terrestrial, over marginal marine, to continental margins. Whereas microbial mat presence can be established with comparative ease in modern terrigenous clastics, in sedimentary rocks this poses a big challenge.

    Terrigenous clastics of the Belt Supergroup (Mid-Proterozoic), show a number of features that can serve as microbial mat indicators: (A) domal buildups; (B) cohesive behavior; (C) wavy-crinkly character of laminae; (D) pustular-wrinkled bed surfaces; (E) ripple patches; (F) laminae with mica enrichment; (G) irregular, curved-wrinkled impressions; (H) features resembling growth ridges; and (I) lamina specific distribution of early diagenetic minerals (dolomite, ferroan carbonate, pyrite). Positive identification of a fossil microbial mat requires to find microbial filaments in life position. This is a difficult task even in the case of very favorable preservation of organic matter, and impossible to accomplish in many instances where fossil microbial mats are suspected. Nonetheless, above features (especially when found in combination) are highly suggestive of microbial mats, and can serve as guides to likely microbial mat deposits.

    Whereas the Belt Basin examples are all from shallow water environments, microbial mats may also have played an important role in deeper water oxygen deprived settings, the realm of black shale formation. In areas of modern oxygen minimum zones, microbial mats have been found to thrive at the seafloor, profoundly influencing the chemistry of the sediment/water interface and sediment stability. Possible ancient analogs occur in a variety of black shale deposits (e.g. Bell Island Group, Chattanooga Shale, Jet Rock, Posidonia Shale, Monterey Formation, Green River Formation), with wavy to crinkly kerogen-rich laminae and spongy microfabrics being the main indication of possible microbial mat origins. More evidence is clearly desirable. While microbial mats clearly have the ability to thrive in black shale environments, it will require more research to firmly establish whether, and how extensively, they occupied this niche in the geologic past.

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    © Jürgen Schieber, UTA Department of Geology
    Last updated: August 23, 2000.