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Preface
The problems of Middle and Upper Devonian stratigraphy within the Appalachian basin are briefly discussed with particular reference to biostratigraphic and lithostratigraphic approaches. Six stratigraphic correlation diagrams are presented.
Paleomagnetic data confirm geological evidence for tropical paleolatitudes for deposition of the Catskill Delta and related Old Red deposits in Europe. A tectonic model constrained by paleomagnetic data suggests that the Catskill deposits are the product of the Acadian orogeny of the Northern Appalachians as a complex continent-continent collision between Armorica (Hercynian Europe), Laurentia (cratonic North America) and possibly Gondwana, with the Traveler terrane (central New England and New Brunswick) rotated and compressed in between.
The Middle Devonian Acadian orogeny affected the entire Appalachian orogen from Newfoundland to Alabama with varying intensities of deformation and metamorphism. Part of the erosional debris from this uplifted area of tectonism was shed westward into the adjacent Appalachian basin. The basin subsidence and filling varied along its length, but not in concert with the adjacent tectonism. In the Early Devonian, the basin was stable with very little subsidence. Beginning in the Middle Devonian and continuing throughout the Late Devonian, the eastern part of the basin (now Pennsylvania and central New York) underwent rapid subsidence, where the largest volume and coarsest sediments were deposited as the Catskill Delta. To the southwest, grain size and thickness progressively decrease to the thin sequence of black shales in eastern Tennessee and Kentucky. To the north (northern New York, eastern Ontario and southern Quebec), no real evidence exists that a Devonian basin developed—the sediment from the orogenic zone probably passed over the Laurentian shield to the Michigan basin and perhaps elsewhere. The Acadian orogeny did not impinge upon the basin and its contained sediments, in distinct contrast with the Taconian and Alleghanian orogenies, both of which directly affected the basin, albeit in different ways. Reported Devonian structures within the basin are insignificant (growth folds), questionable (radiometric dating of faults), or probably incorrect (folding and angular unconformity). Acadian structures that occur between the basin and the main tectonic belt include open, upright folds and steep faults; cooling of Taconian metamorphic terrane during the Devonian and Early Carboniferous is also indicated for these rocks.
The Catskill Delta complex and the Acadian Orogeny: A model
The Catskill Delta complex is interpreted to be the aggregate of delta-alluvial wedges and associated facies that developed in the central Appalachians and on adjacent parts of the stable craton from the Early-Middle Devonian transition to the Middle Mississippian during the Acadian orogeny. Recent interpretations of the Acadian orogeny suggest that it probably was related to oblique convergence and transcurrent movement along a major strike-slip fault zone separating the eastern margin of the North American landmass from a linear continental fragment called the Avalon terrane. Distribution of clastic wedges and basinal deposits resulting from this orogeny support a general southwestward progression of orogeny and indicate that the major clastic wedges emanated from areas near promontories on the continental margin during successive phases of Acadian deformation. Three and possibly four such tectophases have been noted. Each tectophase appears to represent increased convergence or possible collision between a specific continental promontory and the Avalon terrane, but some delta development occurred continually along many parts of the orogen in response to each tectophase. The four tectophases are: (1) Collision near the St. Lawrence promontory during the Early-Middle Devonian transition with initiation of the Catskill Delta complex represented by the Needmore and Esopus shales and associated clastics near promontories. (2) Southward migration of deformation and collision near the New York promontory during the Middle Devonian with the development of a large peripheral basin having an east-dipping, western paleoslope. This basin was filled with cyclic delta clastics and carbonates of the Hamilton Group and Tully Limestone. (3) Southward migration of deformation and collision near the Virginia promontory during the Late Devonian to earliest Mississippian accompanied by intense clastic influx of the Genesee-through-Canadaway groups. As a result, the basin was progressively filled from the east so that basinal environments migrated westward out of the peripheral basin and onto adjacent parts of the stable craton. Eventually the basin was filled and a regional west-dipping paleoslope was established. (4) Migration of deformation southward from the Virginia promontory during the Early to Middle Mississippian as basinal environments in cratonic seas were destroyed and Pocono and equivalent clastic wedges essentially filled the epicontinental sea. Middle Mississippian carbonates mark the end of the Acadian orogeny and Catskill Delta complex.
Paleogeography, paleoclimate, and sedimentary processes of the Late Devonian Catskill Delta
The Late Devonian Catskill Delta is made up of marine and non-marine facies built up on the flank of the tectonic Appalachian Peninsula during assembly of the Old Red (Laurasian) Continent. Much of the continent was under the influence of tropical climates showing a wide range of rainfall. Over the delta, the climate was either tropical wet and dry or desert, due in part, to a rainshadow effect caused by the mountains to the east. Streams showed great variations in discharge and an extended period of drought was an annual event over the region. Alluvial processes were dominant on land. Braided streams deposited the coarsest sediments on alluvial fans and sinuous, channelized streams deposited sand and mud on the alluvial plains. Interfluves on the alluvial plains were sufficiently long-lived to permit the formation of carbonate soils. Plants were most common near stream courses. Fine sand and mud were carried across the shoreline in distributaries to the floor of the adjacent Catskill Sea. Deltaic processes, wave-related processes, and tides shaped the shore. Wave-related processes and bioturbation modified and reworked the shallow marine sediments while turbidity currents and slow deposition from suspension were most effective over the rest of the basin.
Controls on development of Catskill Delta complex basin-facies
Dark, commonly black, fissile shales represent the basinal facies of the Catskill Delta complex. This dark-shale facies originated in the Appalachian Basin during the Emsian (late Early Devonian) and subsequently spread beyond the Appalachian Basin thus suggesting the importance of regional controls such as paleogeography, paleoclimate, and tectonic regime. The Catskill delta debouched into a subsiding peripheral basin on the eastern margin of a nearly enclosed, equatorial sea. Enclosure prevented deep-water incursions from other water bodies and enhanced the production and concentration of organic matter. Existence in a warm, rainy equatorial belt increased organic productivity and probably formed a nearly permanent thermohaline water stratification that prevented vertical circulation and resulted in anaerobic bottom conditions. Episodes of uplift and deformational loading in the rising Acadian Mountains resulted in closely following or concomitant periods of subsidence through isostatic adjustment in the adjacent peripheral basin. Hence, episodic Acadian uplift not only formed the source of the Catskill clastic wedge, but was also responsible for forming the peripheral basin that received the wedge. Basin subsidence isolated deltaic sediments from other parts of the sea, enhanced water stratification, and augmented net transgression. Rapid aggradation of sediments in nearshore areas due to transgression and the formation of a rainshadow due to uplift in the mountains caused decreased sedimentation on the delta, while concomitant basin subsidence produced abrupt deepening, strong water stratification (bottom anoxia), and migration of clastic-deficient (dark-shale) basinal environments shoreward. During intervening periods of tectonic quiescence, subsidence eventually halted, and erosion outstripped the effects of uplift in the mountains, so that the rainshadow was destroyed and abundant clastic influx resumed. As a result, the delta prograded rapidly across former basinal environments. Decreasing subsidence, shallower water, and abundant clastic influx destroyed basinal, bottom anoxia, so that marginally- to highly-oxygenated bottom conditions resumed throughout all delta environments. Five such major cycles of dark, basinal shales alternating with coarser clastics occur in the Catskill Delta complex of the Appalachian Basin. The progressive westward migration of successive cycles reflects the westward migration of Acadian deformation and accompanying basin infilling.
Nonmarine facies of the Middle and Late Devonian Catskill coastal alluvial plain
Nonmarine Catskill facies were deposited by many rivers flowing northwestward from an eastern source area and comprise a suite of sedimentary rocks that (1) thin and decrease in average grain size from southeast to northwest, (2) increase in average grain size from base to top, (3) are dominantly red in color, (4) are variable both laterally and vertically, and (5) were deposited mainly by either meandering or braided streams. The marine to nonmarine transition is a complex interval composed of various facies that represent repeated alternations of marine and nonmarine depositional environments. It is frequently many tens of meters thick. Meandering-stream deposits comprise sandy to gravelly, crossbedded, channel facies and finer-grained, overbank facies arranged in fining-upward sequences. Delta-plain meandering-stream deposition was by streams of low gradient, high sinuosity, shallow water depth, and low erosional capability. The cycles generated are usually only a few meters thick and are dominated by finer-grained floodbasin sediments. Alluvial-plain meandering-stream deposition was by streams of moderate to high gradient, moderate sinuosity, moderate water depth, and moderate to large erosional capability. The generated cycles are frequently many meters thick, dominated by the coarse-grained, channel facies, and show evidence of considerable channel erosion. The braided-stream facies comprises trough-crossbedded sand deposited in shallow to moderate water depths on an alluvial plain. These deposits are almost exclusively gray in color and are sandwiched between the delta-plain and alluvial-plain meandering-stream sequences. The end of Catskill deposition was accompanied by local erosion of the alluvial plain, basin-wide transgression, and demise of the Acadian Mountains as a source area.
Shallow marine strata of the Devonian delta complex range from lower delta plain with some marine influence to lagoon, barrier island, shallow shelf, shelf margin, and gentle or steep slope, progressing seaward to deep-water deposits of black shales and some pelagic limestones. Shallow marine depositional history and lithofacies at any one place vary through time depending on: (1) tectonic uplift of source area and rate of erosion, (2) subsidence of the major basin, (3) sea-level changes, (4) localized basement tectonics, (5) random shifts of delta distributaries, (6) extent of delta progradation across the basin, (7) tidal amplitude, (8) longshore drift, and (9) intensity of mass wastage of the delta front. Deposition of prodelta muds immediately above the Oriskany Sandstone and Wallbridge Discontinuity began the history of the Catskill Delta complex. The initiation of prodelta siltstones shifted southward from New York in the Cazenovia Age to Tennessee in the Bradford Age. Maximum delta lobe progradation occurred simultaneously in all delta lobes in the mid-Bradford Age. The location with maximum detrital accumulation rate in eastern outcrop belts shifted southward from New York in the Middle Devonian to west-central Virginia in the very late Devonian, to southwestern Virginia at the beginning of the Mississippian. The area uplifted east of New York appears to have been reduced from the dominant sediment-input center as the locus of Acadian maximum uplift progressed southward.
Upper Devonian turbidite sequence, central and southern Appalachian basin: Contrasts with submarine fan deposits
The Upper Devonian turbidite sequence in the central and southern Appalachian basin reaches 1400 m in thickness and consists of siltstone turbidites interbedded with mudstone, claystone, and shale. Facies and paleocurrent analyses, based on approximately 6100 m of detailed measured section at 35 localities and 700 measurements of directional structures, indicate major differences between the Upper Devonian sequence and submarine fan and ancient flysch deposits. Compared to flysch and submarine fan deposits, the Upper Devonian turbidites are finer grained and thinner bedded, reflecting the smaller size and lower maximum velocity of the turbidity currents. The stratigraphic transition from turbidites to overlying deltaic rocks shows a different succession from that expected for proximal deposits of submarine fans associated with fan channels and canyons. This transition in the Upper Devonian deposits is gradual and marked by upward-thickening and -coarsening sequences of evenly-bedded turbidites and lenticular beds of crossbedded sandstone. Higher energy mass flow deposits such as debris flows, massive and pebbly sandstone, and conglomerates are notably absent. Paleocurrent data are remarkably uniform, both areally and vertically through the stratigraphic section. They indicate a uniform dispersal pattern, transverse to the basin axis, for nearly 600 km along depositional strike. This suggests turbidity currents had multiple point sources along the basin margin and that the paleoslope had little topographic relief on it. Dip-oriented, short-lived turbidite lobes were built and abandoned as the source of turbidity currents shifted. Migrating delta distributaries, instead of fixed feeder channels, probably supplied sediment to the slope.
During the Devonian, continental ecosystems underwent major transformations including increases in primary and secondary production and in trophic differentiation and specialization. These are consequent on adaptations in vascular plants which increased resource acquisition and assimilation and reduced the effects of disturbance. The adaptations were probably initiated in relatively stable, rich, low disturbance habitats; their development involved a positive feedback with habitat characteristics so that improvements in utilization of resources and regulation of disturbance actually added to the availability of resources and moderated disturbance. Progressive development of this feedback system produced significant changes in the physical characteristics of continental habitats through the course of the Devonian. In particular, by late Devonian, the rate of chemical weathering must have been higher, and the rates of runoff, throughflow, and sheet and rill erosion lower with a concomitant reduction in variations in stream discharge. These changes would have had major consequences for alluvio-deltaic sedimentation, especially for determination of channel regimes and of depositional and early diagenetic patterns in overbank environments. Detailed studies of Catskill sediments can help to elucidate the extent of ecosystem changes.
The flora of the Catskill clastic wedge
In the last 25 years, many paleobotanists and palynologists have focused on the paleoflora of the Catskill clastic wedge, to take advantage of the sequence of abundant floral remains. In this interim a considerable body of paleobotanical information has accrued. It is clear now that these fossil plants are not a localized paleoflora. At the generic level many of them are to be found in Europe, Siberia, China, Australia and, less commonly, in Africa. Most of these genera are of the plant lineages that give rise to the dominant floras of the Carboniferous. Unlike most animal megafossils, it is the form and position of the reproductive organs of the plant that have been primarily used in placing the plant in its rightful position in major evolutionary groupings. Yet plant megafossils are mostly found as fragments of the vegetative plant, as leaves, stems and occasionally cellular anatomy. Because of this, it has been only recently that a tentative scheme of sequential megafossil assemblages in the Siluro-Devonian has appeared to augment earlier palynological schemes. Several prolific localities are discussed and one example of exceptional preservation is illustrated. These details show the kind and extent of information obtainable from the Catskill delta deposits, which then give clues to depositional environments and climatic conditions at the time of deposition.
Upper Devonian marginal-marine deposits exposed at Ashcraft Quarry, northern-most Pennsylvania, are unusual in that they contain limestone in addition to the sandstone and shale which is prevalent in the Catskill clastic wedge. A 3 m. thick lower unit is a lateral-accretion deposit, composed mainly of planar and cross-stratified sandstones with subordinate wavy-flaser bedding. Erosion surfaces beneath sandstones are overlain by intraformational breccias containing transported crinoid, brachiopod, bivalve and plant remains. Paleocurrents are unidirectional westward, but current ripples rarely indicate bidirectional paleoflow. The 3.5–5.5 m. limestone unit, comprising skeletal grainstone interbedded with calcareous sandstone, fines up or coarsens up from an extensive erosional base, and shows lateral-accretion bedding. The lime-grainstone contains abraded fragments of crinoids, brachiopods, bivalves, gastropods, and fish bones; also ankerite concretions, shale chips and plant remains. The whole unit is mainly large-scale cross-stratified, with bidirectional paleocurrents; and its top surface is marked by sandwaves, interfering wave and current ripples, and abundant burrows. The upper unit comprises interbedded sandstones, siltstones and shales. Sheet-like and channel-filling sandstones have basal skeletal lags, large- and small-scale cross-stratification, planar stratification, and hummocky cross-stratification. Finer-grained strata have wavy-lenticular bedding (with wave and current ripple marks), concretions, and abundant burrows. Throughout the exposure fauna) diversity is low relative to coeval marine-shelf facies, and trace fossils belong mainly to the Skolithos ichnofacies. The depositional environment of the lower unit, the limestone unit, and immediately adjacent beds, is laterally migrating sand bars adjacent to curved tidal channels with strong tidal-current asymmetry, probably in an estuary with marginal intertidal flats. Overlying deposits were introduced by periodic unidirectional currents and reworked by waves, possibly in a brackish coastal bay.
Quantitative interpretation of ancient river systems in the Oneonta Formation, Catskill Magnafacies
The Oneonta Formation (Catskill Magnafacies) in south-central New York is composed of two lithofacies associations: (1) Medium- to very fine-grained, cross- and planar-stratified sandstone bodies, with bedsets (lithofacies) arranged into one or (usually) more erosively based storeys. Individual storeys generally have upward-fining lithofacies, also lateral-accretion bedding and channel fills: (2) Interbedded mudstones and erosively based sandstones with a diversity of primary sedimentary structures, calcareous concretions, plant remains and trace fossils. Upward-fining bedsets (lithofacies) are sheet-like or channel-filling, and are arranged in meter-scale lithofacies sequences. The sandstone bodies are interpreted as deposits of laterally migrating and aggrading single-channel (perennial) rivers. Vegetated point-bar tops were subjected to both sheet floods and chute-channel formation. Minor low-flow deposition occurred on bar surfaces. Quantitative reconstruction of bankfull channel geometry and hydraulics gives channel widths, mean depths and slopes of approximately 60 m., 2.5 m., and 10 −4 respectively: low reconstructed bend sinuosities (1.1 to 1.2) are supported by paleocurrent data, and help to explain the dominantly coarse-grained channel fills associated with chute cut-off. Sinuosity changes during lateral migration are documented quantitatively. The sandstone-mudstone lithofacies association is interpreted as overbank flood deposits, on levees, crevasse channels and splays, and flood basins. Plant and faunal activity, and soil-forming processes, were abundant. Periodic channel-belt diversions (avulsions) caused the meter-scale lithofacies sequences in this association, also the cyclicity of the two lithofacies associations. If avulsion frequency averaged about once per 10 3 years, floodplain deposition rates are estimated at about 2 × 10 −3 m/year.
The Devonian Catskill Formation has been divided into four magnafacies (Mf) which have been correlated throughout Pennsylvania. Lithologies in 28 sections and wells were assigned to 10 facies on the basis of grain size, color, bed forms, fossils, and other sedimentary features. Repeating patterns of facies were used to identify magnafacies. Mf-A, the basal magnafacies, is composed dominantly of interbedded marine and non-marine shale and fine sandstone and is inferred to have been deposited mainly in a mud-rich tidal-flat environment. Mf-B is dominated by thick red shales accompanied by thin, fine-grained sandstones and is interpreted as a low-energy fluvial deposit on an inactive arid coastal plain. Mf-B is interrupted occasionally by thin, transgressive sandstones of tidal origin. Mf-C is composed of thick sandstones deposited by braided rivers and localized in three major northwest-trending zones of sediment input. Mf-D is composed of thick, fining-upward cycles with sub-equal amounts of sandstone and red shale. It is inferred to have been deposited by meandering rivers and is thickest in the same areas that Mf-C is thick. Mf-A and Mf-B are richer in sand near the sediment-input centers. In most sections, the Mf are present in the order A, B, C, and D upward, with C missing in many areas and more complex relations in extreme northeastern Pennsylvania. All Mf tend to thin northwestward. This pattern of distribution presumably results from northwestward progradation of the fluvial environment. Numerous small non-economic Cu-U occurrences are concentrated in areas of thick, shale-rich Mf-B lying between the major sediment-input areas. Most are closely associated with marine transgressions. A few occurrences are in the upper part of Mf-A and in Mf-D, but none is in Mf-C. Most of the Cu-U occurrences are localized by small accumulations of plant trash in shallow fluvial or tidal channels. In contrast, Wyoming-type, roll-front uranium occurrences are localized near Jim Thorpe in large channel sandstones of thick Mf-D in the sediment-input areas. The Cu and U occurrences appear to have formed during diagenesis by migration of metalliferous pore fluids.
The shallow-water strata of the lower West Falls Group (Upper Devonian) in New York can be subdivided into three units on the basis of extensive black shale tongues from the deep-water Rhinestreet Shale. These units are the “Millport equivalent,” “Beers Hill equivalent,” and “Meads Creek equivalent” in ascending stratigraphic order. Two distinctive community-types developed during lower West Falls deposition: a shallower water spiriferacean-rhynchonellide-bivalve species ensemble, and a deeper water assemblage of unattached epibenthic and sessile semi-infaunal brachiopods. During deposition of the “Beers Hill equivalent,” an additional diverse assemblage—the Nervostrophia-Devonochonetes Community—developed in a unique array of muddy, delta platform environments flanked by seaward, delta front sand bars. The evolution of lower West Falls marine ecosystems represents a complex interaction of biological and physical patterns, of which regional differences in substratum type, and the evolution of those differences through time, played a major role.
The evolution of Frasnian marine “community-types” in south-central New York represents a complex interaction of biological and physical processes. The two most important physical factors were shelf profile (bathymetric configuration) and regional differences in substratum type, and temporal changes in both. Offshore marine communities were more sensitive to changes in bathymetric configuration than were the near-shore communities; the latter retained an essentially unchanging ecological structure throughout Frasnian time. Three major biome phases can be discerned in the Frasnian strata of south-central New York. These reflect the progressive shallowing on the shelf, the increase in overall sand content with time, and the geographic expansion of sand-grade substrata in shallow marine habitats.
Ammonoid horizons in the Upper Devonian Genesee Formation of New York: Legacy of the Genesee, Portage, and Chemung
The names Genesee, Portage and Chemung are indelibly linked to the early Upper Devonian stratigraphy of the Catskill delta in western New York. For over a century generations of stratigraphers struggled in the attempt to correlate these groups eastward from the Genesee Valley into sections around the delta-front at Cayuga Lake. Rapid thickening and intertonguing of facies were the chief difficulties, but the misinterpretation of ammonoids also played a major role in obscuring the facies relationships. Manticoceras sinuosum (Hall, 1843), the index fossil of the Portage Group, was mistaken by James Hall and J. M. Clarke and later workers for species of Ponticeras and Koenenites from horizons in the older Genesee Formation east of Canandaigua Lake. Some of these Genesee ammonoid horizons have been traced from the pelagic facies in the basin into the benthic facies of the delta-front. One of these horizons in the Upper Penn Yan and Middle Ithaca Members may record an interval of transgression or standstill in the progradation of the delta. The ammonoid horizons in the Genesee Formation at Cayuga Lake are stratigraphic markers which should aid in the taxonomic and biostratigraphic revision of the benthic groups and in the refinement of the paleoecological associations in the delta-front facies.
Destruction of a Late Devonian ophiuroid assemblage: A victim of changing ecology at the Catskill delta front
The fossilized remains of an isolated but once-thriving assemblage of the brittle starfish Furcaster cf. F. leptosoma (?) (Salter, 1857) record a singular event in the succession of late Devonian (Famennian?) marine life. In similar cases of fossil starfish mortality reported elsewhere, the cause of death was cited as suffocation due to rapid burial. In order to construct a paradigm which reflects the circumstances before and after the destruction of the assemblage in question, trace elements in the rock matrix were analyzed and correlated with neontologic data relative to important elements of the community and its environment. The circumstantial evidence points to burial as the ultimate factor in the death of the starfish. They were probably doomed by a series of subtle changes, of which they were not aware, taking place at the front of the delta they inhabited. The changes, hydrologic in nature, were instrumental in dulling the instincts of the starfish and preventing their escape from burial.