Browsing by Author "Read, J. Frederick"

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    Community gradients in the Martinsburg formation (Ordovician), southwestern Virginia
    Springer, Dale A. (Virginia Polytechnic Institute and State University, 1982)
    Studies in modern ecology indicate that most species are distributed independently along environmental gradients according to their individual requirements. Steep gradients often produce species associations separated by discontinuities; gradual gradients produce broadly-overlapping distributions. Approaching the distribution of species populations as a continuum, using gradient analysis, avoids artificial sub-division of totally intergrading distributions, yet permits discontinuities to emerge where present. Faunas of the Martinsburg Formation (Ordovician) in southwestern Virginia offer an excellent opportunity to test the applicability of gradient analysis in a paleoecological setting. A broad spectrum of environments, from nearshore to open-marine, elastic to carbonate-dominated facies, provide both temporal and geographic variation against which to evaluate changes in species distributions. Five classical, Petersen-type communities were recognized in the Martinsburg using cluster analysis: 1) Lingula, 2) bivalve, 3) Rafinesquina, 4) Onniella, and 5) Sowerbyella-dominated associations. Two gradient analysis techniques, ordination and Markov analysis, revealed the same basic associations. However, ordination and Markov analysis permit arrangement of these associations along one or more interpreted environmental gradients. Factors related to water depth and distance from elastic source areas, particularly bottom stability and disturbance frequency, appear to have been the most important of a complex of interrelated physical parameters. The high-stress, nearshore end of the Martinsburg gradient complex was occupied by a Lingula association, followed seaward by an association of bivalves adapted to less stressed environments. Low-stress, open-shelf environments were occupied by Rafinesquina, Onniella, or Sowerbyella-dorninated associations. Broad overlap among these articulate brachiopod communities reflects variations within the open-shelf habitat.
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    Morphological diversification of the ptychopariid trilobites in the Marjumiid biomere (Middle to Upper Cambrian)
    Sundberg, Frederick Allen (Virginia Tech, 1990)
    A morphometric analysis of Cambrian trilobites shows that extinction events sharply reduced the morphological diversity of ptychopariid trilobites living on the shelf at the lower boundaries of the Marjumiid, Pterocephaltid, and Ptychaspid biomeres (Middle to Upper Cambrian). These extinction events not only separate the trilobite assemblages that characterize each biomere, but they also separate similar sequences of morphological diversification. During the initial deposition of each biomere, the shelf was repopulated by a limited number of ptychopariid species that had similar morphologies of very limited range and type. These initial faunas then underwent a morphological diversification evolving similar morphologies during the deposition of the remainder of each biomere. This pattern of repeating episodes that begin with similar morphologies of trilobites which then undergo similar morphological diversifications suggests that the extinction events at the base of each biomere limited the range of morphologies from which new taxa could evolve. Each new biomere assemblage was unable to build upon the adaptations of the specialized taxa of the previous biomere. Only the limited range of similar morphologies of the initial repopulating trilobites was the source from which new taxa could evolve. As a result, new taxa “reevolved the wheel” each time they became adapted to the vacated habitats that were previously occupied in the underlying biomere. In contrast, near the Cambrian-Ordovician boundary, the extinction event at the base of the Lower Ordovician Symphysurinid biomere did not reduce the range of morphologies as drastically as the previous biomere extinctions in the Cambrian. A more diverse initial fauna occurs in the Symphysurinid biomere. This fauna did not undergo a morphological diversification during deposition of the biomere and some regions of morphospace previously occupied in the Cambrian were not reoccupied in this biomere. Taxa of the lower Ordovician were relatively canalized in their morphology. In the Marjumiid biomere (Middle to lower Upper Cambrian), the morphological diversification of ptychoparid trilobites was not significantly influenced by either paleogeography, depositional settings, or migration of taxa from outside North America. The morphological diversification of the ptychopariid assemblage is also seen in a subset of ptychopariids that consists of the Superfamilies Raymondinacea, Asaphiscacea, and Marjumiacea. These superfamilies are the numerically abundant and most diverse taxonomic groups in the Marjumiuid biomere. The patterns of morphological diversification in both the Order Ptychopariida and the combined assemblages of the Superfamilies Raymondinacea, Asaphiscacea, and Marjumiacea were relatively consistent among different paleogeographic regions and were not influenced by general lithofacies. The consistency of the regional patterns and the lack of influence of general lithofacies on these patterns indicate that the overall pattern of morphological diversification is the result of in situ evolution of taxa within the North American Craton and not the result of shifting biofacies or the migration of taxa from outside North America. The morphological diversification of ptychoparids in the Marjumiid Biomere is composed of two distinct phases. The morphological expansion of the Superfamily Ptychopariacea (the ehmaniellids) in the Ehmaniella Biozone is an early phase of diversification. The morphological expansion of the Superfamilies Norwoodiacea, Solenopleuracea (exclusive of the Family Solenopleuracea), Asaphiscacea, Raymondinacea, and Marjumiacea in the Bolaspidella to Crepicephalus biozones represent the later phase of diversification. This second phase is also composed of two morphological expansions, an early expansion in the Superfamilies Norwoodiacea and Solenopleuracea in the Bolaspidella and Cedaria biozones, and a later expansion in the Superfamilies Asaphiscacea, Raymondinacea, and Marjumiacea in the Cedaria and Crepicephalus biozones. The ptychopariid assemblages of the younger Ptychaspid biomere (Upper Cambrian) also display a two-phase morphological diversification.
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    Regional dolomitization of Early Ordovician, Upper Knox Group, Appalachians
    Montañez, Isabel Patricia (Virginia Polytechnic Institute and State University, 1989)
    The Early Ordovician, Upper Knox Group consists of meter-scale shallowing-upward cycles that were deposited on a low-sloping ramp. Cycles formed in response to short term (<100 k.y.) eustatic sea-level fluctuations and typically have well developed tidal flat caps. Cycles are bundled into five transgressive-regressive sequences which correspond to third order (1-10 m.y.) sea-level fluctuations defined by Fischer plots. The Upper Knox Group is 90% dolomite of which greater than 75% predates Middle Ordovician, Knox Unconformity development. Early dolomitization occurred penecontemporaneously with tidal flat progradation during fifth-order (up to 100 k.y.) sea-level falls as indicated by: abundant dolomite in cycles with well-developed tidal flat caps and scarce dolomite in cycles with no or thin laminite caps; decrease in dolomite abundance with distance below tidal flat caps; dolomitized cycles decrease basinward; and dolomite clasts veneer cycle tops and the Knox Unconformity surface. Third-order sea-level fluctuations also strongly controlled early dolomitization as indicated by Fischer plots; limestone, subtidal-dominated cycles correspond to third-order sea level rises and completely dolomitized, peritidal-dominated cycles correspond to third-order sea level falls. "Early" dolomite was metastable and its geochemical composition was modified during initial stabilization by marine brines during progradation of each cycle, and by mixed fresh/marine waters of the Knox aquifer associated with unconformity development. Much "early" dolomite however, remained metastable into the deep burial environment where it was replaced and overgrown by burial fluids as suggested by: covariant trends between crystal size, mole % CaCO₃, Sr²⁺, Mn²⁺ and δ¹⁸O; similar regional trends defined by stable isotope values of "early" dolomites and burial dolomites; and water-rock modeling of trace element and stable isotopic trends. Trace element and stable isotope compositions of least-altered "early" dolomite however, record a memory of a precursor evaporative dolomite. Cathodoluminescent dolomite stratigraphy defines five generations of burial dolomite that can be correlated over 100,000 km². Burial dolomites postdate a regional dissolution event attributed to migration of organic acid-rich fluids through the Knox carbonates. Regional dolomitization occurred coeval with Late Paleozoic deformation and was closely associated with MVT mineralization and hydrocarbon migration. The δ¹⁸O values and trace element contents of burial dolomites in conjunction with fluid inclusions, suggest that burial fluids were warm (135 to 200°C), saline (13 to 22 wt. % NaCl equiv.), ¹⁸O-enriched (+2 to +9 % SMOW) fluids with geochemical compositions similar to present day basinal brines. Mn²⁺ and Fe²⁺ contents of the dolomites suggest a redox control over Mn and Fe fluid chemistry, and in conjunction with regional δ¹³C trends, likely record precipitation from organic acid-rich fluids. Regional trace element and δ¹⁸O trends record a basinal fluid source and regional northwestward flow. Stable isotope values of burial dolomites and fluid inclusions from dolomites and associated minerals, define a prograderetrograde sequence that formed during basinwide, gravity-driven fluid flow which developed in response to Late Paleozoic thrusting and uplift.
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    A sedimentological and structural analysis of the Proterozoic Uncompahgre Group, Needle Mountains, Colorado
    Harris, Charles William (Virginia Polytechnic Institute and State University, 1987)
    Siliciclastic sediments of the Proterozoic Uncompahgre Group can be subdivided into stratigraphic units of quartzite (Q) and pelite (P); these units include a basal, fining- and thinning-upward retrogradational sequence (Q1-P1) that records the transition from an alluvial to a shallow-marine setting. Overlying the basal sequence are three thickening- and coarsening-upward progradational sequences (P2-Q2, P3-Q3 and P4-Q4) that were influenced by tide-, storm- and wave-processes. The progradational units are subdivided into the following facies associations in a vertical sequence. Outer-to inner-shelf mudstones, Bouma sequence beds and storm beds of association A are succeeded by inner-shelf to shoreface cross-stratified sandstones of association B. Conglomerates and cross-bedded sandstones of upper association B represent alluvial braid-delta deposits. Tidal cross-bedded facies of the inner shelf/shoreface (association C) gradationally overlie association B. Interbedded within the tidal facies in upper association C are single pebble layers or <1 m-thick conglomerate beds and trough cross-bedded pebbly sandstones. Single pebble layers could be due to storm winnowing whereas conglomerates and pebbly sandstones may record shoaling to an alluvial/ shoreface setting. A temporally separated storm/alluvial and tidal shelf model best explains the origin and lateral distribution of facies in the progradational sequences. The presence of smaller progradational increments in the mudstone dominated units (P3) and the recurrence of facies associations in the thick quartzite/conglomerate units (Q2, Q3, Q4) suggests that external cyclic factors controlled sedimentation. A composite relative sea level curve integrating glacio-eustatic oscillations and long-term subsidence may account for the evolution of the thick progradational sequences of the Uncompahgre Group. Sedimentary rocks of the Uncompahgre Group have been subjected to polyphase deformation and greenschist facies metamorphism. Phase 1 structures (localized to the West Needle Mountains) include bedding-parallel deformation zones, F₁ folds and an S₁ cleavage. Phase 2 coaxial deformation resulted in the development of upright, macroscopic F₂ folds and an axial-planar crenulation cleavage, S₂. In addition basement-cover contacts were folded. Phase 3 conjugate shearing generated strike-parallel offset in stratigraphic units, a macroscopic F₃ fold, and an S₃ crenulation cleavage. In addition, oblique-slip, reverse faults were activated along basement-cover contacts. The Uncompahgre Group unconformably overlies and is inferred to be parautochthonous upon ca. 1750 Ma gneissic basement that was subjected to polyphase deformation (DB) and amphibolite facies metamorphism. Basement was intruded by ca. 1690 Ma granitoids. Deformation of gneissic and plutonic basement together with cover (DBC) postdates deposition of the Uncompahgre Group. The structural evolution of the Uncompahgre Group records the transition from a ductile, north-directed, fold-thrust belt to the formation of a basement involved “megamullion" structure which was subjected to conjugate strike-slip faulting to accommodate further shortening. DBC deformation may be analogous to the deep foreland suprastructure of an orogenic belt that developed from ca. 1690 to 1600 Ma in the southwestern U.S.A ..
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    Sedimentological constraints on Precambrian crustal evolution in northern New Mexico
    Soegaard, Kristian (Virginia Polytechnic Institute and State University, 1984)
    The Precambrian of northern New Mexico is part of an extensive 1,800 to 1,500 m.y. terrane stretching from Colorado through northern New Mexico into central Arizona. Three lithostratigraphic sequences are present in New Mexico. The oldest consists of 1,760 to 1,720 m.y. metamorphosed bimodal volcanic and volcaniclastic rocks to which no basement has been recognized. This juvenile crust developed as a magmatic arc complex and represents an early period of crustal instability. Between 1,755 and 1,700 m.y., the volcanogenic sequence was intruded by voluminous, coeval granodiorites and tonalites which stabilized the early crust. Unconformably overlying the volcano-plutonic terrane is a thick sequence of metamorphosed quartz arenites and subordinate argillites, the Ortega Group, which accumulated on a stable continental shelf. The inner shelf was dominated by tidal processes with subordinate reworking by storm and fair-weather waves. Storm processes were responsible for deposition on the outer shelf. The Ortega shelf sloped gently to the south and experienced an overall transgression which culminated in drowning of the outer shelf with onlap of black basinal muds from the south. Stable shelf sedimentation resulted from prolonged thermal subsidence following cratonization of the juvenile volcanogenic crust by intrusion of granitoid plutons. The third lithostratigraphic sequence, the Marquenas Quartzite, consists of polymictic meta conglomerates and texturally-immature meta sandstones deposited in a braided-alluvial environment. These terrigenous sediments were supplied from the south and pebble compositions indicate derivation from the underlying volcanogenic and shelf sequences. The Marquenas Quartzite signifies cannibalization of the underlying magmatic arc and shelf. succession in response to deformation of the cratonic margin to the south. The transition from arc volcanism to stable-shelf sedimentation and subsequent deformation of the craton margin in northern New Mexico has been recognized in central Arizona and southern Wisconsin between 1,800 and 1,630 m.y. This common crustal evolution suggests that a proto-North American craton margin opening to the south existed from central Arizona and northern New Mexico into southern Wisconsin at ca. 1,700 m.y. and was destroyed between 1,660 m.y. and 1,630 m.y.
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    Sedimentology and diagenesis of Lower to Middle Cambrian carbonate platform, Shady Dolomite, Virginia
    Barnaby, Roger Joseph (Virginia Polytechnic Institute and State University, 1989)
    Drill cores through the Lower to Middle Cambrian Shady Dolomite carbonate platform (600 to 1200 m thick) in the Austinville, Virginia, region allow the evolution of the carbonate platform from a gently sloping ramp, to a high relief, rimmed shelf to be documented. The Shady Dolomite forms the initial carbonate foundation for the overlying Cambrian-Ordovician carbonate shelf sequence, which persisted for about 30 m.a. until it was destroyed by incipient collision during the Early Ordovician. The Shady Dolomite records several episodes of dolomitization during burial, coeval with late Paleozoic deformation. Rare relict corcs of zone 1 dolomite were replaced and overgrown by zone 2A dolomite, the dominant replacement phase. After replacement dolomitization, the sequence was subjected to dissolution and fracturing, followed by Pb-Zn mineralization, zones 2B, 3 and 4 dolomite cement, sphalerite, quartz, and calcite. Zone 1 dolomite apparently has similar isotopic and trace element composition as zone 2A dolomite. Zone 2A dolomite (δ¹⁸O = -10.2 to -7.0 °/oo PDB; δ¹³C = +1.0 to +1.6 °/oo PDB) is depleted in ¹⁸O and enriched in ¹³C relative to marine cements (δ¹⁸O = -7.5 to -6.1; δ¹³C = +0.2 to +0.8), reflecting precipitation at elevated temperatures from fluids in equilibrium with the host limestone. Zone 2B dolomite cement has identical δ¹⁸O values as zone 2A dolomite, indicating precipitation from similar fluids for the two dolomite generations. The Mn and Fe contents of zones 2A and 2B dolomite likely reflect a pH control over the fluid Mn and Fe chemistry; their similar low total Sr and nonradiogenic ⁸⁷Sr/⁸⁶Sr, imply that Sr was largely derived from the limestone precursor. Zones 3 and 4 dolomite cements (δ¹⁸O = -13.8 to -11.3; δ¹³C = -0.7 to +0.9) are depleted in δ¹⁸O relative to previous dolomites, recording hotter fluids. Zone 3 dolomite is depleted in Fe, due to pyrite precipitation whereas zone 4 dolomite cement has relatively high Mn and Fe contents. Zones 3 and 4 dolomite cements and later calcite are enriched in total Sr and have high ⁸⁷Sr/⁸⁶Sr, indicating late radiogenic Sr-enriched brines. Fluid inclusions indicate that zones 2A and 2B dolomite precipitated from warm (100-175°C), saline (23-26 wt.% NaC1 equiv.) fluids, followed by later hotter (175-225°C) more saline (30-33 wt.% NaCl equiv.) fluids. Pressure solution of the Knox Group dolomites during overthrusting provided much of the Mg²⁺ for dolomitization, this Mg²⁺ was transported by regional gravity-driven fluid flow that developed in response to tectonic uplift.
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    Sedimentology and tectonic implications of the Late Proterozoic to Early Cambrian Chilhowee Group in southern and central Virginia
    Simpson, Edward L. (Virginia Polytechnic Institute and State University, 1987)
    Few detailed facies analyses of rift to passive-margin transitions have been undertaken in exhumed orogenic belts. In the central Appalachians, the Chilhowee Group records such an evolution. The Unicoi and basal Hampton Formations record the transition from rifting to opening of the Iapetus Ocean. The majority of the Hampton Formation and the overlying Erwin Formation represent an overall regressive sequence punctuated by five progradational packages that accumulated along a passive margin. The rift to passive·margin phases of sedimentation in the central Appalachians reflect a continuum from fault·influenced to thermotectonic subsidence. Alluvial sediments and intercalated basalts of the lower Unicoi Formation developed in a rift setting. Paleontological data indicate that rifting continued into lower Cambrian time. The upper Unicoi Formation represents the incipient phase of passive-margin sedimentation related to a first-order, sea level rise. Differences in degree of crustal attenuation controlled the distribution of sedimentary environments during transgression. On the most attenuated crust to the east, initial transgressive facies consist of tidal sandwave and sandridge deposits intercalated with proximal and medial braid-pIain deposits. As transgression progressed cratonwards onto less attenuated crust, tidal sedimentation was supplanted by tide- and wave-influenced sedimentation characterized by sandwave complexes, tidal inlets and longshore bedforms. Drowning at the top of the Unicoi Formation is indicated by outer-shelf black mudstones. Deepening may have been enhanced by continued movement along listric faults throughout the incipient phase of passive-margin development. Examination of outcrops of the Hampton and Erwin Formations on different thrust sheets has permitted an across-strike reconstruction of the Early Cambrian Chilhowee shelf in space and time. Progradational packages developed under storm- and fair·weather wave conditions. Coarsening· and thickening-upward sequences on westerly thrust sheets were generated during progradation of shoreface, inner-shelf and outer-shelf environments. Outer-shelf facies predominate on easterly thrust sheets. Intertidal-flat deposits on the most westerly thrust sheet erosively overlie progradational shoreface sediments and developed during transgression in an embayment in which the tidal wave was amplified. More distal transgressive deposits consist of fining- and thinning·upward sequences with glauconitic horizons, and condensed sections in mudstones.
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    Stratigraphy, sedimentology, and diagenetic history of the Siluro- Devonian Helderberg Group, central Appalachians
    Dorobek, Steven L. (Virginia Polytechnic Institute and State University, 1984)
    The Late Silurian-Early Devonian Helderberg Group, Central Appalachians, is a sequence of mixed siliciclastic-carbonate sediments that was deposited during relative tectonic quiescence on a ramp that built out from low-relief tectonic highlands bordering the eastern side of the Appalachian Basin. Three transgressive-regressive sequences are recognized. Each sequence was deposited over 2-3 m.y.; subsidence rates during deposition were 1 to 2 cm/1000 years. Skeletal grainstone/rudstone formed fringing skeletal banks that formed during regression and prograded away from the eastern side of the basin. Thick Middle Devonian siliciclastic sediments buried the Helderberg Group and updip subaerial exposures accompanying the onset of the Acadian Orogeny. Cementation of the Helderberg Group began on the seafloor, but most cements formed under shallow (<300 m depth) to deep burial (300 m to 4 km) conditions. Regional cathodoluminescent zonation patterns in early, clear calcite cements indicate meteoric groundwaters, that become progressively more reducing away from recharge areas, were involved in shallow burial cementation. Progressive downdip reduction of meteoric groundwaters resulted in updip nonluminescent calcite cements that pass downdip into timecorrelative "subzoned" dull cement and finally, nonzoned dull cements. Calculated stable isotopic compositions of Helderberg shallow burial pore fluids are similar to values in modern coastal meteoric groundwaters. Extensive meteoric groundwater systems developed over a 3-4 m. y. period when the Helderberg Group was subaerially exposed along the eastern basin margin and when Helderberg aquifers were confined by fine-grained sediments at <300 m burial depth. Meteoric groundwaters had recharge areas in eastern tectonic highlands which supplied sufficient hydraulic heads to expel connate marine pore fluids and discharge at least 150 km offshore onto the floor of the Appalachian Basin. Void-filling dull calcite cement formed from deep burial (300 m to 4 km) pore fluids with calculated chemical compositions similar to modern oil field brines. Migration of hydrocarbons and high-temperature, high-pressure brines occurred during Late Paleozoic deformation after Helderberg sediments were totally cemented. Brines probably came from eastern overthrusted terranes and migrated through fractures without altering conodont CAI values. Late hydrocarbons probably had several source rocks.