Paleomagnetic study of the Ordovician Table Head Group, Port au Port Peninsula, Newfoundland

1988 ◽  
Vol 25 (9) ◽  
pp. 1407-1419 ◽  
Author(s):  
Stuart A. Hall ◽  
Ian Evans
Keyword(s):  
North America ◽  
Geomagnetic Field ◽  
Head Group ◽  
Time Interval ◽  
Reverse Polarity ◽  
Middle Ordovician ◽  
Early Ordovician ◽  
Stable Component ◽  
Paleomagnetic Study ◽  
Two Component

A paleomagnetic study of the Ordovician Table Head Group in the Port au Port Peninsula of western Newfoundland reveals a simple two-component magnetization history comprising a reversely magnetized, stable southeasterly remanence with a, shallow to moderate inclination, and an unstable present-day overprint. Pole positions for the stable component, both with and without tectonic tilt correction, correspond with the Early to middle Ordovician pole positions for North America, suggesting this remanence is early. Although the nature of the geomagnetic field in the Ordovician is not well known, the polarity observed is consistent with that reported from other mid-Ordovician studies and appears to reflect a predominance of reverse polarity for this time interval. Evidence of significant rotation of any of the sites studied is absent, indicating that the continental margin in this region acted in an integral rather than a fragmented fashion during deformation.Previously published and new, but preliminary, results from the Early Ordovician St. George Group indicate the presence of two stable components of remanence. These components have similar south-southeasterly declinations but differ in inclination. The shallow to intermediate positive inclination component has a direction that is broadly compatible with Early Ordovician poles from North America. The shallow negative inclination component observed in rocks of similar age from other parts of western Newfoundland appears to be consistent with a later remagnetization of this unit.

New echinoderms from the Early Ordovician of west Texas

1994 ◽  
Vol 68 (2) ◽  
pp. 324-338 ◽  
Author(s):  
James Sprinkle ◽  
Gregory P. Wahlman
Keyword(s):  
North America ◽  
El Paso ◽  
Middle Ordovician ◽  
Early Ordovician ◽  
West Texas ◽  
Lower Ordovician ◽  
Single Well ◽  
Thecal Plate ◽  
Stem Segments

Four specimens of blastozoan and crinozoan echinoderms are described from the Lower Ordovician El Paso Group in the southern Franklin Mountains just north of El Paso, west Texas.Cuniculocystis flowerin. gen. and sp., based on two partial specimens, appears to be a typical rhombiferan in most of its morphologic features except that it lacks pectinirhombs and instead has covered epispires (otherwise known only from Middle Ordovician eocrinoids) opening on most of the thecal plate sutures. The covered epispires inCuniculocystisindicate that some early rhombiferans had alternate respiratory structures and had not yet standardized on pectinirhombs, a feature previously used as diagnostic for the class Rhombifera.Bockia?elpasoensisn. sp. is a new eocrinoid based on one poorly preserved specimen that has a small ellipsoidal theca and unbranched brachioles attached to a flat-topped spoutlike summit. It is the earliest known questionable representative of this genus and the only one that has been described from North America.Elpasocrinus radiatusn. gen. and sp. is an early cladid inadunate crinoid based on a single well-preserved calyx. It fits into a lineage of early cladids leading to the dendrocrinids and toCarabocrinus.Several additional separate plates, stem segments, and a holdfast of these and other echinoderms are also described.

The brachiopod Ptychopleurella lapworthi (Davidson) from the Ordovician of Girvan, S.W. Scotland

1986 ◽  
Vol 60 (4) ◽  
pp. 845-850 ◽  
Author(s):  
D. A. T. Harper
Keyword(s):  
North America ◽  
Sedimentary Facies ◽  
Upper Ordovician ◽  
Middle Ordovician ◽  
Identity Recognition ◽  
Early Ordovician ◽  
Iapetus Ocean ◽  
Northwestern Margin

The small, distinctive, glyptorthinine brachiopod Ptychopleurella Schuchert and Cooper is widely distributed in rocks of early Ordovician to late Silurian age. Several species are known from the Barr and Ardmillan successions (middle-upper Ordovician) of the Girvan district, S.W. Scotland, one of which, ‘Orthis Lapworthi’ Davidson, has not been described in modern terms as there has been some confusion concerning its true identity. Recognition of this species of Ptychopleurella permits comparison with congeners elsewhere, strengthens the correlation of this part of the Girvan Succession with the middle Ordovician of North America, and provides a more complete record of this genus in the slope sedimentary facies of the northwestern margin of the Iapetus Ocean.

Paleomagnetic study of the Late Ordovician–Early Silurian platform sequence of Anticosti Island, Quebec

1986 ◽  
Vol 23 (12) ◽  
pp. 1880-1890 ◽  
Author(s):  
Maurice K. Seguin ◽  
Allen A. Petryk
Keyword(s):  
Late Ordovician ◽  
Residual Magnetization ◽  
Time Interval ◽  
Reverse Polarity ◽  
Common Component ◽  
Magnetization Component ◽  
Paleomagnetic Study ◽  
Middle Carboniferous ◽  
Component C ◽  
Anticosti Island

Eighteen sites (152 samples, 441 specimens) from the Late Ordovician — Early Silurian sequence of Anticosti Island were studied paleomagnetically. Six sites were collected in the Vauréal Formation, seven in the Ellis Bay Formation, three in the Becscie Formation, and one each in the Gun River and Jupiter formations. The lithologies sampled consisted of limestones, sandy limestones, marls, and sandy calcareous shales drawn from a sedimentary platform sequence of predominantly inter-bedded limestones and shales. The specimens were demagnetized in alternating fields (AF) and thermally and were found to be quite stable. Two components of magnetization were isolated. The memory carrier is fine- to medium-grained magnetite; AF and thermal cleanings are about equally efficient. The average directions of residual magnetization are D = 167°, I = 37°, α95 = 18°; D = 315°, I = −24°, α95 = 17 °for components B and C, respectively. The corresponding paleopoles for the B and C components are 129°E, 19°N (dp = 12°, dm = 21°) and 341°E, 16°S (dp = 10°, dm = 18°).Component A is most probably composed of component C and the present Earth's field (PEF); it was chiefly observed in the more altered Vauréal Formation and in the altered top part of the cored samples. The most common component in the Ellis Bay, Becscie, Gun River, and Jupiter formations is component C. Component B is mainly found in the Becscie and Ellis Bay formations. Both components B and C are isolated in the 300–500 °C and 10–40 mT ranges. Component C is either older than component B or synchronous and of reverse polarity to component B. If this last alternative is correct, then the Ordovician and Silurian formations are overprinted by a pre-Kiaman magnetization. The more plausible scenario is the following: component B is secondary, of pre-Middle Carboniferous age, and overprinted on an older secondary magnetization (component C), the acquisition age of which is confined to the Silurian–Carboniferous time interval.

Paleomagnetism of the Harp Lake Complex and associated rocks

1977 ◽  
Vol 14 (6) ◽  
pp. 1187-1201 ◽  
Author(s):  
E. Irving ◽  
R. F. Emslie ◽  
J. K. Park
Keyword(s):  
North America ◽  
Geomagnetic Field ◽  
Great Depth ◽  
Time Interval ◽  
Central Pacific ◽  
Remanent Magnetism ◽  
Low Latitudes ◽  
The World ◽  
The Mean ◽  
Harp Lake

The remanent magnetism of anorthositic rocks from the Harp Lake Complex consists of two antiparallel components, M and H, defined on the basis of their coercivities. M has remanent coercivities generally in the range 100–1000 Oe (7.96 × 103–7.96 × 103 A/m) but sometimes exceeding 2000 Oe (159 × 103 A/m), and is carried by magnetite and hematite, the former being the predominant contributor. H has remanent coercivities in excess of 2000 Oe (159 × 103 A/m), and is carried by hematite probably containing some ilmenite in solid-solution. The mean direction, irrespective of sign, based on 110 samples from 24 collecting sites is 270°, + 01° (α95 = 6°) with a pole 02°N, 154°W (A95 = 4°). This is considered to be the average direction of the geomagnetic field during initial cooling following intrusion of the complex at about 1450 Ma. At this time the complex was at great depth. The Harp dikes, which intrude the complex, have steeper inclinations, (263°, + 44°, α95 = 7°, pole 19°N, 132°W). The country rocks in the thermal aureole have directions roughly similar to those of the complex, but deflected towards the directions in the dikes (272°, + 29°,α95 = 10°, pole 14°N, 144°W). It is suggested that the country rocks were magnetized during final uplift of the area but prior to the intrusion of the Harp dikes. These and other paleomagnetic poles for the interval 1300–1500 Ma fall in the central Pacific indicating that Laurentia was in low latitudes. Previous reviewers have connected these poles by a simple polar loop, but these new observations indicate that the polar path may be more complicated, and a doubly looped path is suggested as a means of reconciling the results. There are about 20 accurately determined pole positions for the interval 1500–1200 Ma from North America, but only 4 from the rest of the world, and it is not yet possible to determine from such an unbalanced body of data the positions of other Precambrian shields relative to North America during this time interval.

scholarly journals Late Ordovician Palynomorphs

1985 ◽  
Vol 4 (1) ◽  
pp. 11-26 ◽  
Author(s):  
S. G. Molyneux ◽  
F. Paris
Keyword(s):  
New Species ◽  
North America ◽  
Late Ordovician ◽  
Middle Ordovician ◽  
Early Ordovician ◽  
Core Samples ◽  
Type Section ◽  
A New Species

Abstract. ACRITARCHSOrdovician acritarchs have been recorded in five core samples collected between 2520 ft. and 3000 ft. in Well E1-81, and ten cutting samples taken between 12150 ft. and 13240 ft. in Well J1-81A. All the assemblages recovered are of Late Ordovician age; no Early Ordovician or Middle Ordovician assemblages have been identified.Investigations have so far concentrated on the acritarch assemblages from Well El-81. The highest three Ordovician samples from depths of 2520 to 2550 ft., 2552 to 2557 ft., and 2562 to 2567 ft., yielded similar assemblages which include Veryhachium irroratum, V. cf. lairdii, V. oklahomense?, V. subglobosum, V. trispinosum, Villosacapsula setosapellicula and a new species, Striatotheca sp. A. Navifusa similis? is represented by one specimen in the sample from 2552 to 2557 ft. Another specimen from the same sample is tentatively referred to Aremoricanium syringosagis. Specimens of Baltisphaeridium, Peteinosphaeridium, Leiofusa and Eupoikilofusa occur throughout the interval 2520 to 2567 ft. but are rare. Commonly occurring species include V. irroratum and V. setosapellicula. V. irroratum has been recorded from the Middle Ordovician of North America (Loeblich & Tappan, 1969) and the Caradoc of England (Turner, 1984) but Cramer & Diez (1979) maintain that it has its acme in the Ashgill. V. setosapellicula is common in the Sylvan Shale of Oklahoma (Loeblich, 1970) which is generally understood to be of Ashgill age, but is rare in the Eden Shale (Caradoc) of Indiana (Colbath, 1979) and in the type section of the Caradoc Series in Shropshire, England (Turner, 1984). . . .

scholarly journals Phylogenetics of the Early Paleozoic Archaeogastropoda

1992 ◽  
Vol 6 ◽  
pp. 300-300
Author(s):  
Peter J. Wagner
Keyword(s):  
North America ◽  
Common Ancestor ◽  
Cladistic Analysis ◽  
Early Paleozoic ◽  
Single Family ◽  
Morphometric Traits ◽  
Systematic Analysis ◽  
Middle Ordovician ◽  
Early Ordovician ◽  
Character Complexes

The Archaeogastropoda are one of the most abundant orders of the Paleozoic, but they have received surprizingly little systematic analysis. I have conducted a cladistic analysis of archaeogastropod species appearing during the initial radiation of the clade during the latest Cambrian through the Ordovician. The analysis included over 100 species. Most specimens were from North America, but some European and Asian material was included. The hypothesized phylogeny also includes an additional 40 Ordovician species from a previous cladistic work on a single family (the Lophospiridae), and only the stem members of this clade were included here.The analysis used 54 meristic and 12 morphometric traits encompassing 154 character states. This seemingly large number of character states for a group considered character-poor can be attributed to: 1) the breadth of morphologic diversity produced during the initial phases of the clade's radiation, resulting in nearly all conceivable morphologies being included here; and 2) the high number of character states for “types” of selenizones and sinuses, morphologies absent on most extant gastropods.The results of the analysis suggest the following:1. There were two large clades present by the Early Ordovician that most closely correspond to present definitions of the Murchisonoidea and Euomphaloidea. The clades shared an anisostrophically coiled common ancestor and thus were not derived separately from bellerophonts. However, the Macluritidae appear to have been derived separately from bellerophonts, corroborating a widely held hypothesis.2. Early Paleozoic species presently considered “pleurotomaroids” comprise a polyphyletic assemblage, with the major families independently derived from either the murchisonoid clade (the Eotomaridae, Lophospiridae and Phanerotrematidae) or from the euomphaloid clade (the Liospiridae, Luciellidae and post-Middle Ordovician members of the Raphistomatidae). It had been assumed previously that pleurotomaroids represent the least-derived anisostrophically coiled gastropods, as well-developed selenizones and sinuses were considered primitive traits. However, those features actually represent derived conditions of the Ordovician that appear to have been secondarily lost in later lineages; thus seemingly “modern” appearing early Paleozoic species often simply retained non-derived conditions. As the Mesozoic Pleurotomaria can not be linked with any Early Paleozoic clades, there is no basis for classifying early Paleozoic gastropods in the Pleurotomaroidea.3. The Subulitoidea and Loxonematoidea (which may be ancestral to the Caenogastropoda) evolved separately from murchisonoids. The origins of the Trochoidea are not clear. If the Holopeidae represent the stem members of the Trochoidea, then trochoids evolved from euomphaloids. However, the Platyceratidae also have been linked with trochoids. The earliest known members of that group possess many unique homologies and no obvious synapomorphies with any other archaeogastropods. The taxon can not be linked reliably with holopeids or any other taxon included in this study, and I do not discount the possibility that platyceratids were evolved separately from bellerophonts.4. Gross shell convergences abound, as turritelliform and planispiral-to-hyperstrophic shells both evolved at least four times. However, the combinations of character states used to achieve these gross morphologies differ among clades, allowing “homoplasies” of character complexes to be recognized.

Conodonts as biostratigraphic tools for redefinition and correlation of the Cambrian–Ordovician Boundary

1988 ◽  
Vol 125 (4) ◽  
pp. 349-362 ◽  
Author(s):  
J. F. Miller
Keyword(s):  
North America ◽  
Boundary Point ◽  
Outer Part ◽  
Head Group ◽  
Eastern Canada ◽  
Green Point ◽  
Hierarchical Arrangement ◽  
Stratotype Section ◽  
Evolutionary Lineages ◽  
Zones And Subzones

AbstractThe Cambrian–Ordovician Boundary is recognized at different horizons on various continents by utilizing several fossil groups. Conodonts are abundant, diverse, and less provincial and facies-controlled than other fossils in this interval; many species are widespread and some are cosmopolitan. Strata representing the Cambrian–Ordovician Boundary interval from Asia, Australia, and North America can be correlated easily when those strata are from cratonal or shelf environments. Strata from slope facies have fewer conodonts, fewer taxa, and taxa may have different ranges compared with cratonal and shelf facies. It may be easier to correlate intercontinentally within cratonal/shelf facies than to correlate intracontinentally from cratonal/shelf facies to slope facies.A new hierarchical arrangement of conodont interval zones and subzones is proposed for western North America. These include the Proconodontus tenuiserratus Zone, the P. posterocostatus Zone (new), the P. muelleri Zone (new), the Eoconodontus Zone (new, with Eoconodontus notchpeakensis and Cambrooistodus minutus Subzones), the Cordylodus proavus Zone (emended, with Hirsutodontus hirsutus, Fryxellodontus inornatus, and Clavohamulus elongatus Subzones), the Cordylodus intermedius Zone (new, with Hirsutodontus simplex and Clavohamulus hintzei Subzones), the Cordylodus lindstromi Zone (new), and Cordylodus angulatus Zone (new).The boundary point for the base of the Ordovician System will be chosen so as to coincide with a correlatable conodont zonal boundary; other fossil groups will support correlation of this boundary point. Three alternative horizons are being considered. The base of the Cordylodus proavus Zone is the most distinctive but is regarded by some as older than is appropriate for the base of the Ordovician. The base of the Cordylodus intermedius Zone is recognizable by faunal changes in several evolutionary lineages at a level that is slightly younger than the presently recognized boundary in Australia, North America, and parts of Asia but slightly older than the base of the Tremadoc Series of Europe. The base of the C. lindstromi Zone can be recognized in most areas by the lowest occurrence of the nominate species, which is of questioned taxonomic validity and uncertain biostratigraphic utility; this horizon is closest to the base of the Tremadoc Series of Europe.Choice of a stratotype section has been narrowed to the Cow Head Group in Newfoundland, eastern Canada; and to the Fengshan and Yehli formations in Jilin Province, northeastern China. The Cow Head Group was deposited on and near the base of the continental slope. Erosion by debrisslide breccias resulted in a significant hiatus near the boundary interval in the Broom Point sections, a condition contrary to guidelines for choice of boundary sections. This may be less of a problem in other sections, such as Green Point. The Dayangcha section in China was deposited on the outer part of a continental shelf and has acritarchs, conodonts, graptolites and trilobites. Additional study of the China section is needed to document fully the ranges of critical conodont taxa.

Middle Ordovician Table Head Group of western Newfoundland: a revised stratigraphy

1980 ◽  
Vol 17 (8) ◽  
pp. 1007-1019 ◽  
Author(s):  
Colin F. Klappa ◽  
Paul R. Opalinski ◽  
Noel P. James
Keyword(s):  
Carbonate Platform ◽  
Head Group ◽  
Group Status ◽  
Middle Ordovician ◽  
Iapetus Ocean ◽  
Lower Ordovician ◽  
Head Formation ◽  
New Formation

Lithostratigraphic nomenclature of early Middle Ordovician strata from western Newfound land is formally revised. The present Table Head Formation is raised to group status and extended to include overlying interbedded terrigenoclastic-rich calcarenites and shales with lime megabreccias. Four new formation names are proposed: Table Point Formation (previously lower Table Head); Table Cove Formation (previously middle Table Head); Black Cove Formation (previously upper Table Head); and Cape Cormorant Formation (previously Caribou Brook formation). The Table Point Formation comprises bioturbated, fossiliferous grey, hackly limestones and minor dolostones; the Table Cove Formation comprises interbedded lime mudstones and grey–black calcareous shales; the Black Cove Formation comprises black graptolitic shales; and the Cape Cormorant Formation comprises interbedded terrigenoclastic and calcareous sandstones, siltstones, and shales, punctuated by massive or thick-bedded lime megabreccias. The newly defined Table Head Group rests conformably or disconformably on dolostones of the Lower Ordovician St. George Group (an upward-migrating diagenetic dolomitization front commonly obscures the contact) and is overlain concordantly by easterly-derived flysch deposits. Upward-varying lithologic characteristics within the Table Head Group result from fragmentation and subsidence of the Cambro-Ordovician carbonate platform and margin during closure of a proto-Atlantic (Iapetus) Ocean.

Paleomagnetism of the Triassic Nikolai Greenstone, McCarthy Quadrangle, Alaska

1977 ◽  
Vol 14 (11) ◽  
pp. 2578-2592 ◽  
Author(s):  
J. W. Hillhouse
Keyword(s):  
British Columbia ◽  
High Temperature ◽  
North America ◽  
Vertical Axis ◽  
Upper Triassic ◽  
Present Position ◽  
Stable Component ◽  
South Central ◽  
Early Mesozoic ◽  
Lamellar Texture

Paleomagnetic evidence indicates that the extensive early Mesozoic basalt field near McCarthy, south-central Alaska, originated far south of its present position relative to North America. Results obtained from the Middle and (or) Upper Triassic Nikolai Greenstone suggest that those basalts originated within 15° of the paleoequator. This position is at least 27° (3000 km) south of the Upper Triassic latitude predicted for McCarthy on the basis of paleomagnetic data from continental North America. The Nikolai pole, as determined from 50 flows sampled at 5 sites, is at 2.2° N, 146.1° E (α95 = 4.8°). The polarity of the pole is ambiguous, because the corresponding magnetic direction has a low inclination and a westerly declination. Therefore, the Nikolai may have originated near 15° N latitude or, alternatively, as far south as 15° S latitude. In addition to being displaced northward, the Nikolai block has been rotated roughly 90° about the vertical axis. A measure of the reliability of this pole is provided by favorable results from the following tests: (1) Within one stratigraphic section, normal and reversed directions from consecutive flows are antipolar. (2) Consistent directions were obtained from sites 30 km apart. (3) Application of the fold test indicated the magnetization was acquired before the rocks were folded. (4) The magnetizations of several pilot specimens are thermally stable up to 550 °C. The stable component is probably carried by magnetite with lamellar texture, a primary feature commonly acquired by a basalt at high temperature during initial cooling of the magma. Geologic and paleomagnetic evidence indicates that the Nikolai is allochthonous to Alaska and that, together with associated formations in southern Alaska and British Columbia, it is part of a now disrupted equatorial terrane.

The halloporid trepostome bryozoans from the Ordovician Simpson Group of Oklahoma

1991 ◽  
Vol 65 (2) ◽  
pp. 200-212 ◽  
Author(s):  
Marcus M. Key
Keyword(s):  
Discriminant Analysis ◽  
North America ◽  
Cladistic Analysis ◽  
Middle Ordovician ◽  
Bryozoan Species ◽  
The Family ◽  
Distinct Lineage

The Bromide Formation of the Middle Ordovician Simpson Group of Oklahoma contains one of the oldest diverse bryozoan faunas in North America. The early divergence of many trepostome clades is revealed in these rocks. Three trepostome bryozoan species belonging to family Halloporidae are described from this fauna. Discriminant analysis is used to define the following halloporid species: Diplotrypa schindeli n. sp., Tarphophragma karklinsi n. sp., and Tarphophragma macrostoma (Loeblich). Preliminary cladistic analysis indicates that the family Halloporidae was already a distinct lineage by the Middle Ordovician. This suggests that by this time, many of the major trepostome clades were already established.

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