BIOGENIC SEDIMENTARY STRUCTURES PRODUCED BY WORMS IN SOUPY, SOFT MUDS: OBSERVATIONS FROM THE CHATTANOOGA SHALE (UPPER DEVONIAN) AND EXPERIMENTS

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 successive layers, internal fills of individual burrows often display an array of complex, convoluted features, which appear to result from mixing of soft to soupy black muds.

Assuming that, as in modern sediments, these burrows were produced by elongate worm-like organisms, experiments were made to study the relationships between the general morphology of burrowers (simple worms, worms with appendages, etc.), sediment viscosity, and textural features within burrows. In a first set of experiments, colored plaster layers were superimposed, and sets of variably shaped, solid rubber bait worms were pulled through these layers. In the second set of experiments, large earth worms were introduced into the plaster substrate. Hardened plaster 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 resemble 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.
5) Structures produced by earthworms are generally the same as those produced in the rubber bait worm experiments, but differ in detail because of a different style of locomotion (peristalsis).

The experiments strongly suggest that the burrows observed in the Chattanooga Shale were indeed produced by worm-like animals that moved through semi-fluid muds.

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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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POSSIBLE INDICATORS OF MICROBIAL MAT DEPOSITS IN SHALES AND SANDSTONES: EXAMPLES FROM THE MID-PROTEROZOIC BELT SUPERGROUP, MONTANA, U.S.A.

 It has been suspected for some time that microbial mats probably colonized sediment surfaces in many terrigenous clastic sedimentary environments during the Proterozoic. However, domination of mat morphology by depositional processes, post-depositional compaction, and poor potential for cellular preservation of mat-building organisms make their positive identification a formidable challenge. Within terrigenous clastics of the Mid-Proterozoic Belt Supergroup, a variety of sedimentary structures and textural features have been observed that can be interpreted as the result of microbial colonization of sediment surfaces. Among these are: (A) domal buildups resembling stromatolites in carbonates; (B) cohesive behaviour of laminae during soft sediment deformation, erosion, and transport; (C) wavy-crinkly character of laminae; (D) bed surfaces with pustular-wrinkled appearance; (E) rippled patches on otherwise smooth surfaces; (F) laminae with mica enrichment and/or randomly oriented micas; (G) irregular, curved-wrinkled impressions on bedding planes; (H) uparched laminae near mud-cracks resembling growth ridges of polygonal stromatolites; and (I) lamina specific distribution of certain early diagenetic minerals (dolomite, ferroan carbonates, pyrite). Although in none of the described examples can it irrefutably be proven that they are microbial mat deposits, the observed features are consistent with such an interpretation and should be considered indicators of possible microbial mat presence in other Proterozoic sequences.

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Sedimentary features indicating erosion, condensation, and hiatuses in the Chattanooga Shale of Central Tennessee: relevance for sedimentary and stratigraphic evolution

 Recent investigations show that the finely laminated and highly carbonaceous Upper Devonian Chattanooga Shale of central Tennessee accumulated in relatively shallow water, prone to influence 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 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, (2) basal sand lags (some mm to cm thick), (3) bone beds, (4) abundant reworked pyrite, (5) conodont or Lingula lags, (6) soft sediment deformation of shale laminae below erosion surface, and (7) localized low-angle truncation of shale laminae.

On erosion surfaces, the concentration of reworked pyrite (framboids, fecal pellets, fills of Tasmanites cysts, pyritic ooids) can in places lead to pyrite 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 minimal 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 (marker horizons).

Erosion or condensation on the cm scale might be due to local conditions, and seems 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 may well be the result of more regional phenomena, such as a 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.

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Pyrite Ooids: A conundrum of texture vs mineralogy.

Although rarely reported, pyrite ooids occur in terrigenous sediments ranging in age from Proterozoic to Jurassic. Size and texture are identical to carbonate or iron ooids. Replacement textures indicate that some occurrences originated through diagenetic pyritization of non-pyritic ooid precursors. Other occurrences, however, where pyrite ooids are associated with carbonaceous shales, show no evidence of replacement. Rounded fragments of broken pyrite ooids that are coated with successive pyrite cortexes, abraded laminae, and alternation of texturally distinct cortexes suggest that these latter pyrite ooids are primary.

Their texture and mineralogy suggest that necessary conditions for their formation are:

(1) vigorous agitation and wave reworking, and (2) reducing conditions at the sea floor. The latter requirement is in conflict with the shallow water and aerated conditions typically associated with agitation and wave reworking, thus our conundrum.

Laser microprobe analysis of 34S/32S ratios in pyrite ooids of suspected primary origin shows:

Textural observations and sulfur isotope data are consistent with a two-stage model for formation of primary pyrite ooids. During stage 1, wave reworking winnows and rounds small diagenetic pyrite concretions. During stage 2, rounded grains undergo shallow burial and accretion of more diagenetic pyrite. Pyrite ooids grow through repeated cycles of reworking, burial, and accretion. The model also implies that black shales associated with pyrite ooids may be of shallow water origin. Further research may reveal alternative pathways for pyrite ooid genesis through comparisons of textural, geochemical, and sedimentological characteristics of diverse occurrences.

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DETECTION OF EROSION SURFACES IN SHALES WITH HIGH RESOLUTION IMAGE SCANS – EXAMPLES FROM THE DOWELLTOWN MEMBER OF THE CHATTANOOGA SHALE, UPPER DEVONIAN OF CENTRAL TENNESSEE.

Evidence of large scale erosional events exists throughout the Chattanooga Shale, Upper Devonian of central Tennessee. Erosional features were examined in the Dowelltown Member of this unit, consisting of alternating black, brown, and greenish-gray shale layers, 0.5 to 15 cm thick. Erosional features can be detected by detailed analysis of contact types between those beds and their relationship to trace fossils. In a number of places, top portions of trace fossils are truncated at contacts. Burrows found in dark colored matrix are often filled with lighter colored sediment that on first glance appears to have been derived from overlying light beds. Enhanced images, however, reveal that in a number of instances these lighter fills are truncated as well. This suggests that another light bed was erosionally removed prior to deposition of the light bed that we now see. The truncated burrow fills are the only remaining evidence for these eroded beds.

Because of the generally small size of these trace fossils, evidence for erosion can only be observed under high magnification. On the other hand, viewing large portions of a sample is desirable for defining erosion surfaces. Because these two requirements are difficult to combine with standard thin section analysis, we chose to make high resolution scans of polished rock slabs or sample photos, and then to view these images with the graphics program Photoshop TM. Digital manipulation of contrast and color allowed enhancement and contrasting of sedimentary features, particularly erosion surfaces, that would otherwise have gone unnoticed. Thus, by using the computer as a "microscope", we were able to extract important new information from rocks that previously were thought mostly devoid of sedimentary features. This information contributed to an improved understanding of the depositional history of deposition of the Chattanooga Shale.

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Sequence Stratigraphy in the Chattanooga Shale: Unraveling the Depositional History of a Black Shale Succession

The Late Devonian Chattanooga Shale of Tennessee and Kentucky constitutes the distal end of the Catskill Delta clastic wedge. Although conventionally thought of as deposited in deep stagnant waters, in the study area it was deposited within the reach of storm waves and contains evidence of benthic life.

Detailed examination of outcrops and drill cores has revealed a series of areally extensive erosion surfaces within the Chattanooga Shale. These erosion surfaces are sequence boundaries and reflect drops of sea level accompanied by erosion of parts of the underlying black shale succession. New black shale blankets were deposited during subsequent sea level rises. Tracing these erosion surfaces and the enclosed black shale sequences across the study area provides the foundation for a 14-fold stratigraphic subdivision. Correlation of sequences and sequence boundaries was accomplished through detailed outcrop and core descriptions, matching of lithologic and sedimentary features, and gamma ray spectrometry.

The currently studied part of the succession was deposited along the comparatively shallow Cincinnati Arch region. There, erosion at sequence boundaries provides for lithologic, sedimentologic, and compositional contrasts (gamma ray spectrometry) that facilitate their recognition. In the deeper water areas east (Appalachian Basin) and west (Illinois Basin) of the Cincinnati Arch, sequence boundaries are most likely not marked by erosive features. Their recognition in these deeper water settings is a remaining challenge before we can arrive at a comprehensive sequence stratigraphic framework for the Late Devonian black shale complex of the eastern US.

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Possible Tubular tempestites in the Chattanooga Shale: Indicators of complex erosive episodes?

Previous investigations of the finely laminated and highly carbonaceous strata of the Upper Devonian Chattanooga Shale have shown a variety of indications of shallow water conditions (erosion surfaces, storm deposits) and of benthic life on the seafloor (trace fossils). In a study of trace fossils, sand filled burrows were encountered in various places. This observation was puzzling for two reasons: (1) "decompaction" of trace fossils indicates that Chattanooga surface muds contained 65-70% water and were of soupy consistency; (2) there were no sand layers overlying these burrows. Subtle erosive truncations of burrow tops indicate that the original sand layers were not preserved, leading to burrows with an "exotic" sediment fill. This situation is analogous to so-called "tubular tempestites", sediment filled burrows that constitute the only preserved record of storm sedimentation in some modern shallow marine environments.

These burrows do record a more complex depositional history than simply the passage of storms. Generating a sand layer to allow their infill required at least the winnowing of several decimeters of mud. This implies removal of soupy surface muds and exposure of underlying stiff compacted muds. Burrowers colonized this newly produced firmground, and their burrows were covered and filled with sand during a later storm event. Finally, this sand layer was removed by yet another erosive event that also partly truncated the burrows.

The sand filled burrows, though inconspicuous, record prolonged episodes of winnowing and erosion (possibly due to lowering of sea level), the benthic colonization of firmgrounds, sand movement over this surface by storm currents, and more erosion that removed all traces of the prior sand layer. Rather than recording a single storm event like their modern counterparts, each horizon of sand filled burrows provides evidence for a succession of storm produced erosive events that continued for an extended time period and may be the only tangible evidence for a substantial hiatus.

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Colonization hierarchy of a Devonian sea floor by burrowing organisms in the context of sediment properties: – Examples from the Chattanooga Shale of central Tennessee.

In the Dowelltown member of the Chattanooga Shale the most common burrow types are (1) unnamed biodeformational structures, (2) Palaeophycus, (3) Chondrites, (4) Planolites, and (5) Teichichnus. Although often found together in a given layer, these burrows were emplaced in muds of differing water content. Reconstruction of various burrow types suggests that borrowing took place in muds of soupground to firmground consistency. Cross-cutting relationships suggest that emplacement of traces within a given layer took place gradually, rather than being the result of a single endobenthic community.

The factors that controlled what type of burrow was emplaced at a given time seem primarily to have been substrate consistency (water content) and the oxygen content of the overlying waters. At most times oxygen levels appear to have fluctuated between oxic and dysoxic, with intermittent episodes of near anoxic conditions. The normal surface substrate seems to have been a soupy, watery mud with as much as 70% water. Burrows that indicate a firm sediment were either emplaced in deeper, more compacted sediment layers (e.g. Planolites, Teichichnus), or were emplaced in firm muds that were exposed because erosion had removed the soft surface layer (e.g. Chondrites).

Trace fossil associations were studied by examination of relative deformation, morphology, and cross cutting relationships. The context of these various observations led to the establishment of a hierarchy of traces within individual layers. Emplacement of burrows most likely took place in the following order, (1) biodeformational structures, (2) Planolites, (3) Palaeophycus, (4) Teichichnus, and (5) Chondrites. Most of the traces appear to have been produced by vermiform organisms, probably polychaete worms. In modern oxygen restricted environments, polychaetes typically dominate the ecosystem. Following what is known from modern polychaete assemblages, we have produced a model of successively changing polychaete communities that could have produced the observed sedimentary fabrics.

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The Flynn Creek Impact Structure of Tennessee: Evidence for shock deformation and infill mechanisms.

The Flynn Creek Impact Structure is located in north central Tennessee, approximately 8 km south of Gainsboro. The meteorite that produced the structure hit a thick succession of Ordovician carbonates in Middle Devonian times and caused a circular crater of approximately 3.6 km diameter and 150 m depth. An impact breccia (basal breccia), up to 40 m thick, was deposited on the crater floor and in areas surrounding the crater.

Recent experimental work indicates that shock metamorphism of carbonate rocks produces substantially lower intensity and broadening of characteristic peaks for dolomite and calcite. Samples from the basal breccia of the Flynn Creek structure that were examined for these effects show a lowering of the hkl (104) peak of dolomite by about 50%. In addition, the half height full width of the hkl (104) peak increases by as much as 0.066 degrees. These observations parallel experimental results and suggest that changes of dolomite XRD parameters in these breccia samples record impact related internal lattice strain. Our results suggest that this methodology could also be applied to other suspected impact structures and impact breccias, in order to verify shock metamorphism in absence of other features, such as shattercones and shocked quartz.

A petrographic study of the basal breccia and overlying deposits suggests the following scenario for the history of the early crater fill: (1) Immediately after the impact, brecciated material settled back into the crater and formed a chaotic, massive, "basal breccia". (2) Rainfalls caused materials on the crater rim to be washed back into the crater and led to deposition of internal sediment within breccia cavities. (3) The area was flooded by the Chattanooga Sea in the earliest Frasnian (conodont data). (4) Waves reworked carbonate ejecta in shallow water areas surrounding the crater and caused deposition of a thin bedded carbonate breccia within the crater (increasing proportions of rounded vs angular carbonate grains). (5) After a further rise of sea level black shale deposition commenced in the crater and in surrounding areas.

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A detailed study of gamma ray logs from the Laconia field in southern Indiana: Internal stratigraphy of the New Albany Shale and implications for sequence stratigraphy.

The New Albany Shale of southern Indiana is a Middle to Late Devonian black shale succession that constitutes an important hydrocarbon source rock in the Illinois Basin. Like in other Devonian black shales of the eastern US, there is widespread production of natural gas out of the New Albany Shale.

Recent outcrop studies of the New Albany Shale and its lateral equivalent, the Chattanooga Shale, show that these black shale successions can be subdivided into as many as 14 sequences on the basis of widespread erosion surfaces. Individual sequences can be traced in outcrop from southern Tennessee to northwestern Kentucky and have allowed substantial refinement in understanding the depositional history of these rocks.

Comparing gamma ray logs from the Laconia field of southern Indiana with outcrop gamma ray profiles from northwestern Kentucky shows that sequences recognized in outcrop can also be recognized in the subsurface, and reveals inconsistencies between the outcrop based stratigraphic nomenclature in Kentucky and the subsurface based stratigraphy in Indiana. This issue will be addressed at depth in a continuation of this study.

Although the gamma ray logs vary from well to well, careful examination shows that (1) practically every gamma ray peak can be correlated from well to well; (2) increases of gamma ray counts coincide with transgressive episodes; and (3) differences between logs can be related to erosive truncation at the top of individual shale packages. These observations allow the tracing of sequence boundaries from surface outcrops into the subsurface. In future studies of these Middle to Late Devonian black shales, this will be the basis for making the connection between conformable sequences of the Illinois Basin interior (deep water) and discordant sequences of the Cincinnati Arch region (shallow water).

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A Regional Study of Sequences in the New Albany Shale of the Southeastern Illinois Basin (Indiana) with Gamma Ray Logs and Well Cores.

The New Albany Shale of southern Indiana is a middle to late Devonian Black Shale unit that constitutes an important hydrocarbon source rock in the Illinois Basin. Going from east to west, the New Albany Shale thickens and changes its lithologic characteristics. These changes reflect the gradual deepening from the shallow water regions on the Cincinnati Arch to the deeper water regions of the Illinois Basin.

In outcrop studies from Tennessee and central Kentucky, recognition of widespread erosion surfaces allowed a sequence stratigraphic subdivision of this black shale succession. Gamma Ray logs from southern Indiana show that these subdivisions can be carried into the subsurface west of the New Albany outcrop belt. Systematic tracing of these sequences through the Illinois basin may in the future allow substantial refinement in the understanding of the depositional history of these rocks.

The observed variability between adjacent gamma ray logs is attributed to the erosional truncation at the top of individual shale packages. Additional variability is introduced due to the fact that some shale packages that are present in western Indiana have been completely lost to erosion in eastern Indiana and Kentucky. The transgressive base of individual sequences typically coincides with an increase in gamma ray intensity.

Future study of these shales will be the basis of making a better connection between the conformable sequences of the Illinois Basin interior and the discordant sequences of the Cincinnati arch region.

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

Increasingly, microbial communities are recognized for playing a potentially important role in defining and modifying surface sediment characteristics in various settings, ranging from terrestrial, through 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) irregular wrinkled bed surfaces; (E) ripple patches; (F) laminae with mica enrichment; (G) irregular, curved-wrinkled impressions on bedding planes; and (H) lamina specific distribution of early diagenetic minerals (dolomite, ferroan carbonate, pyrite). Positive identification of a fossil microbial mat requires one 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, the above features (especially when found in combination) are highly suggestive of microbial mats, and can serve as guides to sediments that may have accumulated under the influence of microbial mats.

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. Jet Rock, Monterey Formation, Green River Formation), with wavy to crinkly kerogen-rich laminae being the main indication of possible microbial mat origins. More evidence is clearly desirable. Although 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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Distribution and Deposition of Mudstone Facies in the Upper Devonian Sonyea Group of New York

 Petrographic and small-scale sedimentary features of the mudstone-dominated Upper Devonian Sonyea Group of New York were examined in order to (1) improve our understanding of mudstone facies, (2) examine coeval mudstone facies within a broader depositional context, and (3) promote a simple methodology for the study of mudstones. Six mudstone facies are distinguished, each characterized by several component mudstone types or subfacies. Sedimentary conditions and environments are reconstructed from primary sedimentary structures and bioturbation characteristics. Mudstone facies are characterized by soils and flood deposits in the coastal-plain region, by rapid sediment deposition and frequent reworking in nearshore areas, by storm-dominated offshore transport on a wide, basin-margin platform, by turbidite slope deposits below storm wave base, and by bottom currents and slow settling in the distal, "deep"-basin black mudstones. The latter, although commonly thought of as deposits of a stratified anoxic basin, contain indications of benthic life, such as burrows, disrupted laminae, and clay/silt fecal pellets. These observations are incompatible with a stratified-basin model, and indicate the need to search for alternative models of black shale accumulation.

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Pyrite Ooids and Oncoids from the Winnipeg Formation (Ordovician of Saskatchewan): Textural and geochemical observations in support of a primary Origin

Pyrite ooids are enigmatic sedimentary particles that occur in sediments ranging in age from Archean to Jurassic. Their texture suggests highly energetic and agitated conditions, but their mineralogy seems to require a reducing and stagnant environment. Because of this conflict, they have traditionally been thought to represent diagenetic replacements of precursor iron oxide, chamosite, or carbonate ooids. Pyrite ooids from the Winnipeg Formation, however, provide strong evidence that pyrite ooids can indeed form as primary sediment grains.

In the study area, the Winnipeg Formation consists primarily of bioturbated sandstones with a high matrix content, and layers enriched with pyrite ooids and oncoids. Initial pyrite formation appears to be concentrated in the mucus linings and fills of burrow tubes. The resulting pyrite concretions are later reworked, fragmented, and rounded, and provide the cores of pyrite ooids and oncoids. Broken pyrite ooids that are coated with pyrite cortexes, abraded laminae, and alternation of texturally distinct cortexes suggest that pyrite cortexes covering these cores are a primary deposit. No remnants of potential precursor materials were found. Microprobe data show very low levels of Al, Si, K, Ca, and P in pyrite cortexes. The abundance of these elements is the same as in associated pyrite concretions and framboids, suggesting that replacement of non-pyritic ooids did not occur. Sulfur isotope data from concretions, framboids, and ooids are within the same range, and are about 20-30 permill below coeval seawater. This indicates sulfate reduction close to the sediment surface in an essentially open system. Alternating cortexes with isotopically "lighter" and "heavier" sulfur, as well as wide fluctuations in 34S/32S ratios from core to margin, also suggest that a simple replacement origin is unlikely.

Pyrite ooids and oncoids probably formed in shallow areas of low sedimentation. Reworking produced rounded pyrite particles, and intermittent burial ("resting" periods) in a reducing sediment allowed for further pyrite accretion. Alternating these conditions produced primary coated pyrite grains (ooids/oncoids) that were concentrated in reworked horizons.

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Experiments on the Self-Compaction of Fresh Muds: Implications for Interpreting the Rock Record

Although many mudstone successions contain laminae of sand and silt that are from less than a mm to several mm thick, rarely is the significance of these laminae reflected upon. Considering that freshly deposited muds are essentially of fluid consistency, a basic question is whether sand/silt deposition on such muds could possibly have resulted in stable laminae, or whether it was necessary to erode the semi-liquid surface layer prior to their deposition.

To illuminate this question some simple experiments were conducted. A tank with suspended mud was subdivided into cells, and the mud allowed to settle under its own weight. At formation of a mud-water interface, the water content of the mud was 96%. Over time, 0.5mm thick sand layers were placed on top of successive cells. Initial sand deposits (at 86% water content) caused fluidization of the mud and immediately sank beneath the surface. At later placements (84% water content or less) the sand layers remained on the surface for a few minutes before disappearing, and at a water content of 82.8% (after 5.5 hours of settling) the sand layer remained stable on the mud surface. Later examination of the sediments showed that the sand that sinks in forms ball-and-pillow structures, and that the pillows occur at successively shallower depth as water content decreases. Experiments were conducted in freshwater as well as in saltwater (3% salinity), with essentially identical results.

The experiments reveal that freshly deposited muds can indeed support a thin sand/silt layer shortly after deposition, and that a preceding erosive interval may not be required to explain analogous layers in the sedimentary record. In natural environments, however, current and wave activity will most likely have a destabilizing effect on the structural integrity of watery muds. Thus, our results probably place an upper limit (in terms of water content) on the stability of freshly deposited muds. Further experiments will be conducted to evaluate the impact of current and wave activity, as well as the influence of the thickness of sand/silt layers, so as to arrive at a more realistic appraisal of silt and sand laminae in mudstones.

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Ichnofacies: Burrow community, or Hierarchy of Emplacement? Examples from the Dowelltown Member of the Chattanooga Shale, Upper Devonian, central Tennessee.

The concept of ichnofacies has long been used to identify supposedly diagnostic bioturbation patterns. Those patterns can in turn be used to determine the environment of deposition and physical properties of the substrate, as well as oxygen levels in the water above it. By convention, those patterns are attributed to certain benthic and endobenthic communities, which resided in a given area for a certain period of time. What is often not taken into consideration is the fact that a burrow assemblage often consists of burrow types that may not necessarily have co-existed as the same community. Soft sediments, such as preserved in the rocks of the Dowelltown Member, Chattanooga Shale, Upper Devonian of central Tennessee, are rich in trace fossils that are readily observed due to mixing of muds of contrasting colors. The most common types are: Palaeophycus, Chondrites, Planolites, and Teichichnus. All of these types can be readily observed in the same layers, but textural observations suggest that they were produced by organisms that burrowed through the same sediment at different times. Variable degrees of compaction and cross-cutting relationships strongly suggest that different burrow types were emplaced successively, thus putting in question the concept of a single community. For example, although found in the same layers, compaction of Planolites suggests a water content of 65% at emplacement, whereas Chondrites suggests water contents of 45%. These observations suggest that the Dowelltown trace fossil assemblage represents the cumulative record of several successive communities in the same layer, and each community may have existed under different environmental conditions. These findings indicate that caution needs to be exercised when using ichnofacies for interpretation of paleoenvironmental conditions.

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"In Situ" Quartz Silt in Devonian Mudstones from the Eastern US:  A Petrographic and Geochemical Investigation by Scanning Cathodoluminescence and Ion Microprobe Mass Spectrometry

Mudstones are the most abundant but least understood sediment type, and they contain the lion's share of recorded earth history. A basic task of sedimentologists trying to decipher the origin of any type of sediment is careful petrographic examination and determination of provenance. In mudstones, the fine grain size and diagenetic alteration of clay minerals has long defied efforts to investigate their history in detail. Silt size quartz grains in mudstones, on the other hand, are an abundant constituent and are chemically and mechanically resistant. Scanning cathodoluminescence (CL), a recent innovation, has created new opportunities for the in depth examination of sedimentary particles. It allows even very small (silt size) grains to be examined for CL textures, and enables distinction of quartz from a variety of source rocks.

Application of this technique to quartz silt grains in Late Devonian mudstones of the eastern US has revealed that a large portion (up to 80%) of the quartz silt in this succession has actually been produced "in situ", very close to the sediment-water interface. The textural identification of "in situ" quartz silt was verified with oxygen isotope analyses of quartz grain by ion microprobe. Detrital quartz silt typically has relatively light (metamorphic) oxygen isotope values (d18O approx. +10), whereas "in situ" quartz grains show substantially heavier values (d18O approx. +28) typical for diagenetic quartz elsewhere. Silica derived from the dissolution of opaline skeletons of radiolaria was reprecipitated in pore spaces and algal cysts while the muds contained 70% water and were essentially uncompacted. Unless examined with scanned CL, this "in situ" quartz silt is readily mistaken for detrital silica derived from the adjacent landmass.

If what has been observed in these Devonian mudstones is a widespread phenomenon, we may have in the past (1) seriously misinterpreted the sedimentary record, (2) missed an important component of the global silica cycle, (3) overlooked a whole aspect of paleoproductivity, and (4) made considerable errors in the paleogeographic reconstruction and interpretation of mudstone-rich basins.

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BIOGENIC SEDIMENTARY STRUCTURES PRODUCED BY WORMS IN SOUPY, SOFT MUDS: OBSERVATIONS FROM THE CHATTANOOGA SHALE (UPPER DEVONIAN) AND EXPERIMENTS

Abundant trace fossils occur within rhythmically interbedded black and gray shales of the Chattanooga Shale (Upper Devonian) in central Tennessee. Burrows cross the boundaries between layers and tend to obscure the contacts between alternate layers. Infills of individual burrows often display an array of complex, convoluted features, apparently due to mixing of soft to soupy black and gray muds.

Assuming that the burrowers were elongate worm-like organisms, experiments were made to study the relationships between the general morphology of burrowers (simple worms, worms with appendages, etc.), sediment viscosity, and textural features. Rubber bait worms were pulled through superimposed layers of plaster in a first set of experiments. Comparable experiments were then conducted with earthworms. The resulting textures were studied by sawing the hardened plaster blocks perpendicular and horizontal to bedding. The explored viscosities ranged from that of heavy motor oil to that of lithium grease (at 25oC).

The study shows that (1) convolute textures observed in plaster experiments closely resemble those seen in Chattanooga Shale burrows; (2) the degree of convolution increases as the vicosity of the substrate decreases; (3) the length of the worm determines the extent of mixing between layers; (4) mixing patterns produced by smooth worms and worms with appendages, although similar, contrast in detail; (5) earth worms and bait worms produce generally similar structures, but show certain differences due to contrasting styles of locomotion (peristalsis vs. unidirectional pull). "Virtual" compaction and decompaction of digital images shows close resemblance between experimentally produced structures and burrow textures from the Chattanooga Shale, suggesting that the latter were indeed produced by worm-like animals that moved through semi-fluid muds.

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Developing a Sequence Stratigraphic Framework for the Late Devonian Chattanooga Shale of the southeastern US: Relevance for the Bakken Shale

The Late Devonian Chattanooga Shale of Tennessee and Kentucky is in most areas a thin black shale deposit of less than 10 meters thickness. It is a distal equivalent to the almost 3000m thick Catskill sequence, and encompasses most of the Frasnian and Fammenian.

Although originally thought of as a continuously, though slowly, deposited deep water shale unit, the Chattanooga Shale shows internal erosion surfaces at various scales as well as storm deposits. These features suggest comparatively shallow water deposition (tens of meters water depth), and a stratigraphic record with numerous interruptions of various duration. A number of erosion surfaces are laterally extensive and can be traced from exposure to exposure through the entire outcrop area (over distances of as much as 300km). Erosion surfaces of this type are considered sequence boundaries sensu Vail, and tracing them has made it possible to subdivide the Chattanooga Shale into as many as 14 sequences.

These sequence boundaries are very subtle. The following features are indicative of their presence in a given outcrop: 1) sandy, silty, or pyritic lag deposits of up to several cm thickness; 2) sharp-based shale beds; 3) low-angle truncation of shale beds; 4) scoured surfaces; and 5) soft-sediment deformation in underlying shales. Tracing erosion surfaces from outcrop to outcrop is based on a combination of: 1) petrographic matching of lag deposits; 2) the petrography and microfabrics of individual shale packages; 3) conodont data; and 4) gamma ray surveys.

Collection of further conodont data and extension of this stratigraphic framework to adjacent areas may eventually lead to a comprehensive stratigraphic framework for the entire Late Devonian black shale complex of the eastern US. Comparison between distal and proximal successions may allow differentiation of truly eustatic events from those due to tectonism and sedimentation.

The lower Bakken is of Late Devonian age, and is probably equivalent to the upper half of the Gassaway Member of the Chattanooga succession. Cursory examination of drill core has shown lag deposits and evidence of erosive gaps. These features could be physical evidence of sequence boundaries. They should be traceable through careful examination of cores in conjunction with geophysical logs.

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Sequence Stratigraphic Correlations in Devonian Black Shales of the eastern US: Relationship to Global Sealevel Variations

Late Devonian black shales of Tennessee, Kentucky, Indiana, and Ohio have long been thought of as deposited in the deepest portions of stagnant basins. Several years of reexamination, however, reveal a substantially more complex picture.

While undoubtedly deep basin deposits in some areas, there is good evidence that in other areas these black shales were deposited in shallow water, within reach of storm waves and with abundant benthic life. "Shallow" water black shales are most common on and along the flanks of the Cincinnati Arch. Outcrop studies in Tennessee and Kentucky revealed extensive erosion surfaces, formed in response to sea level drop, that are the basis of a sequence stratigraphic subdivision. In the subsurface of Indiana, sequence boundaries are identified by a combination of core studies and tracing of gamma ray signatures from the outcrop belt. In addition, truncation of gamma ray motifs provides independent confirmation of sequence boundaries in the subsurface.

At present, the succession of black shales has been subdivided into 14 sequences, ranging in age from Givetian to uppermost Famennian. Sequences may diminish in thickness or disappear completely as we approach the Cincinnati Arch, reflecting onlap as well as erosion during emergence of the arch. Utilizing available biostratigraphic data, lithostratigraphic correlations, and matching of transgressive-regressive cycles, it is possible to link the sequences in the study area to equivalent Devonian strata in Iowa, New York, and to the global Devonian sea-level curve. Because there are more sequences than can be accommodated by global eustatic variations, some sequences may have a tectonic origin. Ongoing biostratigraphic studies are likely to clarify assignment of sequences to global eustatic variations.

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A study of cyclic vs event sedimentation in a shallowing upwards mudstone succession: the upper Placid Shale (Virgilian) of the Fort Worth Basin, Texas

Upper Pennsylvanian strata of north-central Texas are cyclic in nature and exhibit rapid lateral and vertical facies changes. Lowstand deposits include exposure surfaces, paleosols, and fluvial-deltaic complexes. The Placid Shale is part of the Brad Formation of the Canyon Group, and is of lowermost Virgilian age. It was deposited on the eastern shelf of the Midland Basin as part of a minor deltaic cycle, and is probably correlative with the Weston Shale of the South Bend cycle in the midcontinent region. In the study area it is separated by a disconformity from the overlying Ranger Limestone.

The upper Placid Shale was measured on a cm scale to document in detail the variations in sand vs mud content, bioturbation, and diagenetic phases such as siderite. The base of the section is mud dominated and consists of 3-5 cm thick mudstone intervals with thin (mm to sub-mm) lenses and laminae of silt and fine sandstone that alternate quite regularly with fine sand layers of 1-2 cm thickness. Internally the sand layers may show flasers, wavy laminae, and cross-lamination. Upwards in the section the thickness of sandy intervals gradually increases. Contacts between sandstone and mudstone intervals tend to be gradational, and ripple cross lamination is more clearly developed. Towards the top of the succession sandstone beds with sharp erosive base, low angle internal cross lamination, and bioturbated tops occur within this rhythmically bedded succession.

Rhythmic sand/mud interbedding in the lower and middle part of the succession is interpreted as climatic rather than depositionally forced cyclicity on the basis of siderite concretions in the sandy intervals. Siderite suggests very slow sediment accumulation and thus sand concentration via winnowing rather than through increased sediment supply. Upward thickening of sandy intervals suggests increased wave reworking due to shallowing conditions. Sharp based sandstone beds in the uppermost part of the section are interpreted as hummocky cross-stratified storm deposits, and indicate deposition within storm wave base. Erosion at the top of the Placid Shale interval indicates that after the gradual sediment buildup during its deposition, sea level dropped prior to renewed transgression and deposition of the Ranger Limestone.

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A detailed study of depositional history and sequence boundaries in the middle member of the Ranger Limestone (Virgilian) of the Fort Worth Basin, Texas

Upper Pennsylvanian strata of north-central Texas are cyclic in nature and exhibit rapid lateral and vertical facies changes. Lowstand deposits include exposure surfaces, paleosols, and fluvial-deltaic complexes, highstand deposits can be developed as carbonaceous shale units and carbonate horizons. The Ranger Limestone is part of the Brad Formation of the Canyon Group, and is of lower Virgilian age. It was deposited on the eastern shelf of the Midland Basin as part of a transgressive carbonate cycle, and is correlative to the Iatan-Westfalia limestone interval in the midcontinent region.

The middle Ranger unconformably overlies the lower Ranger carbonates and consists at the base of interbedded mudstones and sandstones. The latter show basal erosion, hummocky cross stratification, and in places cross-bedding. Mudstones contain silty/sandy laminae and large sandstone pillows. Upwards the sand/mud ratio decreases, sand layers are characterized by ripple cross-lamination, and mudstone becomes dominant. This is interpreted as a deepening trend. An undulose erosion surface cuts down into these mudstones, and is overlain by a thick "lumpy" limestone unit with a blocky appearance. In detail, this unit consists of irregular limestone clasts that are the same as found in the lower Ranger Limestone, and are mixed with mudstone lumps and stringers of the same appearance as found below the erosion surface. Reworked limestone and mudstone clasts suggest that the "lumpy" unit formed due to subaerial exposure and erosion. The upper surface is hummocky, and hummocks are clay free, have a mottled texture, are well cemented by carbonate, and resemble thrombolite buildups. The latter may have formed after initial transgression. This unit is then overlain by interbedded fossiliferous mudstones and limestones, and grades into the overlying upper Ranger Limestone.

The middle Ranger displays two transgressive-regressive cycles and two sequence boundaries. The basal unconformity (sequence boundary 1) and shallow water siliciclastics mark the first TR cycle, the "lumpy" limestone unit (overlies sequence boundary 2) and overlying fossiliferous deposits mark the second cycle.

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New insights into the deposition of the Chattanooga Shale in Tennessee and possible applications to lateral equivalents in North-Central Arkansas

Previous investigations of the finely laminated and highly carbonaceous strata of the Upper Devonian Chattanooga Shale in Tennessee have shown a variety of indications of shallow water conditions (erosion surfaces, lag deposits, storm deposits), periods of extremely slow sediment accumulation (hiatus deposits), and of benthic life on the seafloor (trace fossils). Chattanooga surface muds contained 65-70% water, were of soupy consistency, and show many indications that intermittent erosion of the seafloor was a widespread phenomenon.

Detailed examination of outcrops and drill cores has revealed a series of laterally extensive erosion surfaces within the Chattanooga Shale that are considered sequence boundaries, reflecting drops of sea level accompanied by erosion of parts of the underlying black shale succession. Tracing these erosion surfaces and the enclosed black shale sequences across the study area provides the foundation for a 14-fold sequence stratigraphic correlation scheme, and was accomplished through detailed outcrop and core descriptions, matching of lithologic and sedimentary features, and gamma ray spectrometry.

Published descriptions of petrographic features in the Chattanooga Shale of north-central Arkansas, such as abundant pyrite, lag deposits of sand grains, mineralized Tasmanites cysts, and conodont lags, closely resemble observations made in Tennessee and suggest comparable conditions of shallow water deposition. These black shales overlie a sandy transgressive lag and thin south to north, an analogous situation to the Chattanooga onlap on the Cincinnati Arch in Tennessee. Conodonts suggest that the shales are correlatives of the Gassaway member of the Chattanooga Shale in Tennessee. Experience from Tennessee suggests that it should be possible to define sequences in the Arkansas Chattanooga Shale based on a combination of petrographic features, conodont data, and erosion surfaces. This should allow us to link this succession to already recognized sequences in Tennessee. Closer study of the Chattanooga Shale in Arkansas may provide a critical link to extend sequence stratigraphic correlations from the Appalachian Basin all the way to the Midland Basin.

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OBSERVATIONS ON SEDIMENT REWORKING AND BIOTURBATION IN THE CAMP RUN MEMBER OF THE NEW ALBANY SHALE GROUP, INDIANA

The Camp Run Member of the New Albany Shale is a highly carbonaceous Late Devonian mudstone unit that is found over large areas of the Illinois Basin. It consists of thick beds of carbonaceous mudstone (up to several meters thick) that are separated by thin intervals of greenish-gray mudstone (a few inches thick at best). In past studies it has been interpreted as the result of largely anoxic depositional conditions, briefly interrupted by oxygenation events that are marked by beds of greenish-gray and bioturbated mudstones.

Close inspection, however, reveals that this interpretation is overly simplistic, and that there are many additional clues to environmental conditions that have not been reported previously. For example, whereas remains of the marina alga Tasmanites are widespread and preserved in a flattened state due to compaction, in some intervals concentrates and lags of mineralized (pyrite, quartz) Tasmanites cysts are found. These probably indicate conditions of extremely slow deposition accompanied by winnowing of the seafloor. Thin silt laminae elsewhere in these carbonaceous mudstones also suggest current or wave reworking of the substrate. Although these black shales appear to be unbioturbated except where in contact with greenish-gray mudstone intervals, they nonetheless contain several features that can be attributed to bioturbation. These features are (1) random disruptions of silt laminae that cause sharp instead of tapered terminations; (2) tube-like structures that have undergone strong compaction and are filled with dark brown mudstone that can only be recognized through substantial contrast enhancement of shale images; (3) stacked horizontal burrow traces that may show internal back-fill structures and homogenization of mud fabrics, and may give a subtle dark-brown to black banding to these mudstones. In a drill core from Harrison County in southern Indiana, these bioturbation related features strongly dominate the Camp Run interval, and indicate that benthic colonization was the rule rather than the exception during Camp Run deposition. In combination, all of these observations suggest that dysoxic conditions were typical for Camp Run deposition, and that surface productivity rather than anoxia caused the accumulation of highly carbonaceous mudstones.

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A REGIONAL STUDY OF INTERNAL SEQUENCE STRATIGRAPHY OF THE NEW ALBANY SHALE OF THE ILLINOIS BASIN WITH THE HELP OF GAMMA RAY LOGS

The New Albany Shale of southern Indiana is a middle to late Devonian Black Shale unit that constitutes an important hydrocarbon source rock in the Illinois Basin. Going from east to west, the New Albany Shale thickens and changes its lithologic characteristics. These changes reflect the gradual deepening from the shallow water regions on the Cincinnati Arch to the deeper water regions of the Illinois Basin.
        In outcrop studies from Tennessee and central  Kentucky, recognition of widespread erosion surfaces allowed a sequence stratigraphic subdivision of this black shale succession. Gamma Ray logs from  southern Indiana show that these subdivisions can be carried into the subsurface west of the New Albany outcrop belt. Systematic tracing of these sequences through the Illinois basin may in the future allow substantial refinement in the understanding of the depositional history of these rocks.
        The observed variability between adjacent gamma  ray logs is attributed to the erosional truncation at the top of individual shale packages. Additional variability  is introduced due to the fact that some shale packages  that are present in western Indiana have been  completely lost to erosion in eastern Indiana and  Kentucky. The transgressive base of individual  sequences typically coincides with an increase in  gamma ray intensity.
        Future study of these shales will be the basis of  making a better connection between the conformable  sequences of the Illinois Basin interior and the discordant sequences of the Cincinnati arch region.

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Diagenetic origin of quartz silt in mudstones and implications for silica cycling

Mudstone—the most abundant sedimentary rock type, composed primarily of clay- or silt-sized particles—contains most of the quartz found in sedimentary rocks. These quartz grains, which are chemically and mechanically resistant and therefore preserve their characteristics well, have long been considered to be derived from the continental crust. Here we analyse quartz silt from black shales in the eastern USA, dating back to the Late Devonian period (about 370 million years ago), using backscattered electron and cathodoluminescence imaging and measure oxygen isotopes with an ion probe. Our results indicate that up to 100% of the quartz silt in our samples does not originate from the continental crust. Instead, it appears to have precipitated early in diagenesis in algal cysts and other pore spaces, with silica derived from the dissolution of opaline skeletons of planktonic organisms, such as radiolaria and diatoms. Transformation of early diatoms into in situ quartz silt might explain the time gap between the earliest fossil occurrences of diatoms about 120 Myr ago and molecular evidence for a much earlier appearance between 266 or even 500 Myr ago. Moreover, if many other mudstone successions show similarly high proportions of in situ precipitated—rather than detrital—quartz silt, the sedimentary record in mudstones may have been misinterpreted in the past, with consequences for our estimates of palaeoproductivity as well as our perceptions of the dynamics and magnitude of global biogeochemical cycling of silica.

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Deposition of the Chattanooga Shale in Central Tennessee: A complex history of Late Devonian transgressions and regressions

The Chattanooga Shale in central Tennessee, long considered an essentially continuous succession of Frasnian and Fammenian black shales, reveals a complex depositional history on close inspection. Two regional unconformities, caused by subaerial exposure of the Cincinnati Arch, allow subdivision of the succession into three packages.

Deposition of package one commenced in the Late Givetian, generally under shallow water conditions. Shallow water sands and sandy lags pass upwards into interbedded sandstones and black shales. Approximately during the MN5 conodont zone, the sea retreated from the area, and erosion removed the previously deposited sediments over most of the area.

During conodont zones MN6-7, the area was flooded again and black shales that are commonly referred to as the Dowelltown member (Conant and Swanson, 1961), were deposited. During Dowelltown deposition, smaller scale sea level variations led to formation of submarine erosion surfaces during sea level drops, and deposition of black shale blankets during transgressions. In the uppermost Frasnian the sea again retreated from the area and caused subaerial erosion of previously deposited black shales. Dowelltown erosion is most severe near the crest of the Cincinnati Arch, and diminishes further east and west of the Arch.

In the lowermost Fammenian, the sea again flooded the region and deposition of black shales, commonly known as the Gassaway Member (Conant and Swanson, 1961) commenced. As before, smaller scale sea level variations led to formation of submarine erosion surfaces during sea level drops, and deposition of black shale blankets during transgressions. In the uppermost Fammenian (preasulcata zone) the sea again retreated, and subaerial erosion over the Cincinnati Arch led to variable removal of previously deposited black shales.

Recognition of large and small-scale transgressive regressive cycles allows for a sequence stratigraphic re-appraisal of the Chattanooga Shale, and a better understanding of the geologic history of the region. Jeff Over (SUNY Geneseo) kindly helped with identification of conodont faunas and advise on conodont literature.

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The Flynn Creek Crater in Tennessee: Learning from Crater Sedimentation about Timing and Impact Conditions

The Flynn Creek crater in north-central Tennessee was produced when a cometary body struck a flat lying succession of Ordovician carbonates. Conodont data from Devonian (Upper Givetian to Lower Frasnian) lag deposits that overly the Ordovician succession in the region, indicate flooding by the Devonian sea during the upper Givetian. A lack of shale accumulation suggests that these initial Devonian sediments were deposited in frequently winnowed shallow water. In the context of storm deposits found in the overlying Chattanooga Shale, the water depth during lag accumulation was probably on the order of 10 meters or less.

Within the crater, fractured and deformed bedrock is overlain by very coarse and chaotic breccia. The top portion of the breccia systematically fines upwards. Depending on location, one or more additional units of fine breccia and/or sand to silt size carbonate debris with weakly developed bedding are encountered above the graded breccia unit. Layers of bedded breccia are separated from the latter, as well as from each other, by thin, soft partings that consist of a mixture of clays, quartz sand and silt, carbonate debris, phosphatic debris, and conodonts. The conodonts are lower Frasnian in age and overlap with those found in lag deposits outside of the crater. These beds are in turn overlain by Devonian black shales.

Detailed study of crater stratigraphy and sedimentary features suggest the following sequence of events: (1) impact in shallow water during the lower Frasnian (381-382m.y.); (2) formation of the basal chaotic breccia as a fall-back deposit; (3) deposition of graded breccia as displaced water rushed back into the crater; (4) while the sea was still shallow, ejected material was washed back into the crater by storm-induced waves and currents; (5) with rising sea level, black shales were able to accumulate, first in the crater, and later also outside the crater.

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Differentiating Black Shale Sequences with Visible Light Reflectance

Rather than representing an essentially continuous succession, Late Devonian black shales in the eastern US were found to consist of stacked sequences, reflecting multiple transgressive-regressive cycles. These sequences are bounded by erosion surfaces, and shales of successive sequences display subtle but distinct petrographic differences. In an attempt to aid differentiation of black shale packages, we used visible light reflectance to characterize bulk composition and subtle differences between successively deposited shales.

In a preliminary study, reflectance measurements were taken at closely spaced intervals on samples from the Devonian New Albany Shale. Comparison of reflectance data shows that visible light reflectance measurements indeed allow distinction between shale packages. Although the reflectance curves themselves show little visible difference, the first derivative curves of the reflectance signatures show systematic differences between shale packages. Smoothing the data with a narrow window, moving average highlights these differences. Maxima and minima in the first derivative curves, as well as the overall slope of the curve change systematically as we move from shale package to shale package. These differences probably reflect primarily changes in the content of clay minerals, silt, organic matter, and pyrite.

Studies are in progress to (1) verify that the observed vertical changes in reflectance characteristics persist laterally within the New Albany Shale, and (2) to correlate reflectance changes with petrographic characteristics and geochemical data. Initial results of this study are very encouraging and refinement of this approach may in the future provide a reproducible and objective method for the characterization of black shale successions.

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Organic Petrography of Late Devonian Black Shales: What’s in it for Sedimentologists?

Devonian black shales of the eastern US have been studied in detail along various lines of inquiry, including outcrop study, macro- and micropaleontology, ichnofossil, gamma ray spectroscopy, microscopic examination of thin sections in transmitted and reflected light, electron microscopy (SEM, BSE), electron microprobe, carbon and sulfur isotopes, and organic geochemistry. Just like the seven blind men examining the elephant, each one of these approaches only delivers a partial answer concerning the origin and history of these rocks.

Recent studies of sedimentary features in the Chattanooga and New Albany Shale have for example shown that erosive features are much more common than previously assumed, and that these shales also contain evidence of storm wave reworking of the seabed. A number of erosion surfaces within this black shale succession have now been traced over large distances and provide the foundation for a sequence stratigraphic re-interpretation of these rocks and detailed inter-basinal correlations.

Study of trace fossils has revealed that bioturbation, although subtle in many instances, is nonetheless much more widespread than previously believed. Together with sedimentological observations and stratigraphic considerations, this suggests that high surface productivity, rather than anoxic conditions were the likely cause for preservation of the large quantities of organic matter in these shales. Understanding the nature of this organic material is of crucial importance for developing realistic scenarios of black shale genesis.

The thin development and comparatively small accumulation rates for silt and clay attest to the distal setting for these shales, and allowed accumulation of comparatively large proportions of organic matter. A possible proxy for the nature of the bottom sediment is the organic muck that accumulates in swamps or at the bottom of ponds. There, the great majority of the organic material, regardless if of floral or faunal origin, is broken down by bacteria within a matter of months into a mass of largely unidentifiable organic particles and extracellular bacterial slime. Although one can still find identifiable material (cuticles, ribs and veins of leaves, algal cysts, animal hard parts, etc.) within this mass, slime and amorphous material strongly dominate.

With this in mind, what can we hope to gain from petrographic examination of organic material in Devonian black shales? The dominant macerals in these shales are alginite and bituminite, with minor amounts of fusinite and vitrinite. Most variability with respect to sedimentary conditions is observed with respect to alginite and bituminite. For example, black shales deposited during conditions of maximum flooding (high sea level) contain abundant alginite and are comparatively low in bituminite (finely mixed with mineral matrix), whereas those deposited during transgressions (relatively shallow water) are strongly dominated by streaks of bituminite. In the latter case, most of the original alginite shows various stages of alteration to bituminite. In both instances, shales were deposited at very small rates of accumulation, but the preponderance of bituminite in the latter (shallow water) case may reflect overall better oxygenation and more complete bacterial degradation of organic material. Black shales deposited as distal equivalents of regressive episodes appear to fall between these two extremes.

Returning to our bottom muck of organic particles and extracellular bacterial slime, this sediment also has interesting physical properties. When suspended, the material flocculates quickly and the resulting sediment takes on the a jelly-like consistency within a few hours of settling. Although consisting of little more than water (90% or more), this jelly layer is able to support sand grains and other large particles on its surface, and resists deformation. Thin streaks of "coal" (1-5mm thick) that have been observed in our Devonian black shales, are usually interpreted as coalified branches and logs. An alternative interpretation would be that they actually have originated as layers of organic jelly that was later buried by more "normal" mucks consisting of a mixture of minerals and organic matter (interpretation based on sedimentological and petrographic observations).

In light of possible climatic forcing of the abundance of certain macerals (e.g. alginite), it is tempting to also interpret variations of maceral ratios (e.g. vitrinite/bituminite) within depositional cycles of black shales in terms of climatic factors. Rapid degradation of organic matter during deposition and early diagenesis, however, indicates many potential complications. For example, observations made on low-stand and high-stand black shales suggest alternatively that such variations may instead reflect sea level oscillations that caused variations in bottom sediment reworking and the degree of dysoxia.

In this context, geochemical approaches to understanding the origin of the Chattanooga/New Albany Shales may also produce ambiguous conclusions. Geochemical data concur with petrographic and sedimentologic observations that suggest a varied origin for successive black shale intervals, and HI/OI characteristics have been used to propose simultaneous supply of terrigenous and marine organic matter. Differentiation of anoxic from dysoxic conditions was made on the basis C/S relationships and degree of pyritization. Yet, comparison of these data with distribution of bioturbation features shows significant inconsistencies.

In the standard practice of sedimentary geology, organic petrology is infrequently used to address geologic questions. Yet, black shales consist to a significant proportion of organic matter, and thus their organic particles hold a substantial share of the contained information. Because the history of a sedimentary deposit is far better understood when we base it on the summation of its individual particles instead of on bulk properties (such as TOC), organic petrology can provide another "window" for the study of these rocks and of sedimentary sequences in general. Although probably inadequate for elucidating the source of the organic matter in black shales in detail, organic petrology can provide new clues and insights for the sedimentologist. A tight integration of sedimentology, organic petrology, and geochemistry should help us to arrive at an understanding of the origin of black shales that is fully consistent with the available data, and addresses the underlying mechanisms.

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© Jürgen Schieber, UTA Department of Geology
Last updated: September 07, 2000.