Frank R. Ettensohn

Controls on the origin of the Devonian-Mississippian oil and gas shales, east-central United States☆

Abstract An abundance of organic matter, development of bottom anoxia in a stratified water column, and absence of major clastic dilution are the principal factors controlling the accumulation of organic-rich black shales. During the Devonian and Early Mississippian, such shales became abundant across much of east-central United States, apparently controlled by a unique coincidence of paleoclimatic, paleogeographic and tectonic factors. Like most major black-shale depositional episodes, Devonian-Mississippian deposits were formed during a time of global transgressive and greenhouse states. During this time, the east-central United States was located in the subtropical trade-wind belt where marine and terrestrial organic productivity would have been high and where surface waters would rarely have become cool enough to overturn and disrupt the water stratification necessary to generate bottom anoxia. Location in the trade-wind belt may also have enhanced upwelling which would have increased organic productivity and the likelihood of anoxia. Perhaps most importantly, however, black-shale deposition coincided with early phases of tectonism, when, because of rapid deformational loading in the orogen, subsidence in the adjacent Appalachian foreland basin far outstripped sedimentation. In the resulting deep-water conditions, water stratification and accompanying anoxia were soon established. Because most of the deformational load at this stage was subaqueous, there was no major source of clastic dilution, so that clastic starvation prevailed and largely organic-rich sediments accumulated. Thus hydrocarbon source beds of broad extent like the Devonian-Mississippian black shales, are rarely attributable to a single cause. They typically result from a complex interplay of factors unique to a given time and place.

Defining the nature and location of a Late Devonian–Early Mississippian pycnocline in eastern Kentucky

The pycnocline, the intermediate layer in a tripartite, density-stratified water column, is an important element in recent explanations of the origin of anaerobic sediments. Dysaerobic sediments deposited where the pycnocline intersects the bottom are distinct from overlying aerobic and underlying anaerobic sediments and were used to define the position of the paleopycnocline on a Late Devonian-Early Mississippian sea bottom in eastern Kentucky. Surface and subsurface mapping of the Bedford-Berea sequence and equivalent parts of the New Albany and Chattanooga Shales reveals approximately coeval aerobic, dysaerobic, and anaerobic facies that permitted mapping of the paleopycnocline, apparently the first time the actual sea-bottom position of a paleopycnocline has been defined. The Bedford-Berea (Devonian-Mississippian) sequence is situated between the black Ohio Shale (Devonian) below and the black Sunbury Shale (Mississippian) above. The black shales reflect deposition in a deepening, transgressive sea where anaerobic conditions were easily established and maintained. The intervening Bedford-Berea sequence, however, represents deposition in aerobic and dysaerobic environments during a shallowing, regressive episode as deltas prograded to the south and west in eastern Kentucky. The gray sandstones, siltstones, and shales of the Bedford-Berea sequence have been interpreted as pinching out between the black shales above and below in east-central Kentucky. Each of the above units, however, has a distinctive signature on gamma-ray logs. Comparison of gamma-ray logs and radioactivity profiles with corresponding cores, drillers9 logs, and exposures indicates that black-shale equivalents are present beyond the point where the gray Bedford Shale pinches out; this also is supported by limited biostratigraphic data. This transition from gray Bedford shales to black-shale equivalents marks the approximate position of the ancient pycnocline and has been mapped in the subsurface and on the outcrop in eastern Kentucky. The ancient pycnocline closely parallels the Bedford-Berea delta front, suggesting major control by the delta. The continued influx of oxygen-rich, sediment-laden waters from the delta apparently prevented the northward and eastward migration of the pycnocline up the delta front. The introduced oxygen and sediment, however, were effective only in breaking up the pycnocline over a distance of ∼25 to 40 km from the active delta fronts. Stratigraphic, paleontologic, and geographic constraints indicate that the paleopycnocline was not horizontal but sloped northeastward from shallower waters near the Cincinnati Arch to deeper waters off the delta fronts.

Stratigraphic evidence from the Appalachian Basin for continuation of the Taconian orogeny into Early Silurian time

Abstract Traditional interpretations of the Appalachian Basin during Silurian time suggest a period of tectonic stability between Taconian and Acadian orogenies. However, recent interpretations of evidence from deformation and igneous sources in the northern Appalachians indicate Silurian tectonism centered on and near the St. Lawrence promontory and that this tectonism probably effected sedimentation in parts of the Appalachian Basin during much of Silurian time. Of special interest is the tectonism that extended from latest Ordovician into Early Silurian time and the nature of its relationships with known orogenic events. Although evidence and interpretations from deformation and igneous sources have become increasingly well established, there has been little support from the stratigraphic record. Now, however, criteria based on the implications of flexural models, namely the nature and distribution of unconformities, the presence of flexural stratigraphic sequences, and the distribution in time and space of dark-shale-filled foreland basins, provide stratigraphic evidence from the Appalachian Basin that supports Early Silurian (Medinan; early Llandoverian) tectonism related to Taconian orogeny. In particular, the distribution and local angularity of the Ordovician–Silurian or Cherokee unconformity suggest major tectonic influence and a latest Ordovician to Early Silurian inception for that tectonism. An overlying flexural stratigraphic sequence represented by the Lower Silurian Medina Group and the presence of a dark-shale-filled foreland basin reflected by the Power Glen–lower Cabot Head shales support interpretations of flexural subsidence related to deformational loading. Moreover, the distribution in space and time of the foreland basin containing these shales indicates that the basin is more likely a continuation of the northwestwardly shifting trend of earlier Taconian basins than that of later Salinic basins. Although the kinematic regime may be different from that of earlier Taconian tectophases, the stratigraphic evidence supports a northeastward extension of the Taconian orogeny into present-day eastern Canada during Early Silurian time and illustrates the usefulness of flexure-based stratigraphic interpretations in understanding the timing and extent of some orogenies.

Assembly and dispersal of Pangea: Large-scale tectonic effects on coeval deposition of North American, marine, epicontinental, black shales

Abstract Both before and after inclusion of Laurussia in Pangea, the continent was a site of extensive epicontinental, marine, black-shale deposition, but from Pennsylvanian to Jurassic time when North America was an integral part of Pangea, the pattern of black-shale deposition was one of long-term decline. In North America the decline seems to have been greatest in Late Permian and Triassic times. Although this decline could have reflected a period of global cooling and a related decrease in organic productivity brought on by conditions associated with the supercontinent state, mapping the distribution of black shales in time and space on North American parts of Pangea suggests that the restricted availability of suitable repositories for organic-rich sediments may have been an equally important cause. In fact, mapping shows that the distribution of North American, Pangean, marine, black shales was greatest during Mississippian assembly of Laurussia when foreland-basin-type repositories were abundant and again during Late Jurassic fragmentation when rift-basin-type repositories were abundant. In both cases, tectonically conditioned basins formed the major repositories and promoted certain conditions that enhanced early basin anoxia. During Late Permian and Triassic time, when Pangea had been assembled, neither compressive orogenies nor crustal extension were major influences on North America. Consequently, suitable repositories were minimal and so was the extent of black-shale deposition. However, the continued presence of even a few major black-shale deposits during this time of minimum suggests that even low organic productivity was not a primary cause of decline and points to the possible significance of active continent assembly and breakup in generating tectonic-basin repositories conducive to accumulation and preservation of the organic matter that is nearly always present in quantities great enough to form major black-shale deposits.

Palaeoecology and sedimentology of the dysaerobic Bedford fauna (late Devonian), Ohio and Kentucky (USA)

Abstract Oxygen-deficient biofacies models rely on lithologic and paleontologic attributes to identify distinctive biofacies interpreted to reflect levels of oxygenation in anaerobic, dysaerobic, and aerobic parts of a stratified water column. This study of the Bedford fauna from the Bedford Shale of Ohio and Kentucky and from adjacent black-shale units reports faunal distributions different from those predicted by the accepted models. This study suggests that, although oxygenation was an important factor that determined the taxonomic makeup of the fauna, bacterially mediated nutrient recycling and substrate characteristics were more important than oxygenation in determining faunal distribution in the dysaerobic zone.

Protosalvinia Dawson and Associated Conodonts of the Upper Trachytera Zone, Famennian, Upper Devonian, in the Eastern United States

Abstract Protosalvinia first occur in association with conodonts of the Upper trachytera Zone and below the Three Lick Bed in the Ohio Shale and the Ellicott Shale of the central and northern Appalachian Basin, as well as in the Clegg Creek Member of the New Albany Shale of the Illinois Basin. In the Chattanooga Shale of the southern Appalachian Basin, Protosalvinia are found no lower than the Upper marginifera Zone or associated with obviously reworked conodonts in the Middle expansa Zone. Regionally Protosalvinia are associated with a disconformity and may be found with conodonts of the Lower expansa Zone.

Taconian seismogenic deformation in the Appalachian Orogen and the North American Craton

Abstract Originally the paradigm of plate tectonics was largely based on seismological interpretation of earthquakes. It therefore behoves that geological literature should pay more attention to the detection of seismic activity in ancient rocks. The effects of earthquakes on relatively young Tertiary and Holocene deposits have been recognized, commonly in the context of engineering works. In this paper, observational evidence for seismogenic structures in the Lower Palaeozoic carbonate belt of E. North America is presented. The structures range from minor faults and liquefaction effects to breccias and melanges. It is sometimes possible to suggest the dynamics of faults on which earthquakes occurred. Although most of the deformation happened at the margins of the craton and the growing orogenic belt, intracratonic faulting, earthquakes and synsedimentary deformation also took place.

Echinoderms from the lower Silurian Brassfield Formation of east-central Kentucky

Abstract. A new echinoderm fauna is reported from the Brassfield Formation (Rhuddanian, Silurian) of Bath County, Kentucky. The Brassfield Formation was the first extensive marine unit to be deposited following the end-Ordovician glaciation and extinctions and represents several shallow, open-marine facies. These facies supported a diverse pelmatozoan fauna. This report not only extends the geographic distribution of this fauna, but also the temporal range of the fauna back to Rhuddanian time. Six pelmatozoans are reported, including the crinoids Browerocrinus arthrikos n. gen. n. sp., Temnocrinus americanus n. sp., Stereoaster sp., and Dendrocrinus sp.; and the glyptocystitids Brockocystis nodosarius Foerste, 1919, and Anartiocystis whitei Sumrall, 2002. In addition, the asteroid Gordonaster brassfieldensis Blake and Ettensohn, 2009, was reported previously from this locality. Browerocrinus increases the diverse calceocrinid fauna from the Brassfield Formation; Temnocrinus was previously only known from the Homerian (Silurian) of England; and this is the first known occurrence of Stereoaster beyond the greater Dayton, Ohio, region. Furthermore, this is the first Brassfield locality known with two glyptocystitid taxa.

The Complex Morphology of a New Lower Silurian Asteroid (Echinodermata)

Abstract Gordonaster brassfieldensis is a new genus and species of Asteroidea (Echinodermata) described from the Lower Silurian Brassfield Formation of east-central Kentucky. Tentatively assigned to the poorly understood Palaeasteridae, Gordonaster shares much with Ordovician asteroids, yet it also exhibits apparent homoplasies that presage the post-Paleozoic crown group. Available specimens also indicate that the ontogenetic pattern of ossicular addition seen in the crown group was established during the Paleozoic.

Large-scale Tectonic Controls on the Origin of Paleozoic Dark-shale Source-rock Basins: Examples from the Appalachian Foreland Basin, Eastern United States

Recent plays like the Middle Devonian Marcellus Shale and possible prospects like the Upper Ordovician Utica Shale point out the significance of dark-shale source rocks in the Appalachian Basin. Mapping the distribution of such shales in space and time throughout the basin shows that periods of dark-shale deposition coincided with orogenies and the related formation of foreland basins. The fact that foreland basins form and become repositories for organic-rich dark-shale source rocks is mostly the result of deformational loading in the adjacent orogen. Tectonism mostly exerts its control through the flexural effects of deformational loading and subsequent relaxation in the orogen. These flexural processes generate sedimentary responses in the foreland basin that are reflected in a seven-part unconformity-bound cycle, of which dark shales are a major component. Because orogenies comprise a series of smaller deformational events, or tectophases, and each tectophase generates a similar cycle, many foreland basins typically exhibit a cyclic array of dark-shale and intervening clastic units, called tectophase cycles. Thirteen such third-order tectophase cycles, formed during four orogenies, are present in the Appalachian Basin. Using examples of foreland-basin dark-shale units formed during the Ordovician–Silurian Taconian and Devonian–Mississippian Acadian/Neoacadian orogenies, the timing of cycles and migration of successive dark-shale units within them relative to the progress of orogeny are presented as evidence of causal relationships between tectonism and dark-shale sedimentation. However, tectonic influence may extend well beyond the confines of the foreland basin in the form of far-field tensional and compressional forces. This may impel the yoking of foreland and intracratonic basins as well as the reactivation of foreland basement structures—the former allowing dark-shale depositional conditions to move from one basin to the other, and the latter, inaugurating new basins for dark-shale accumulation.