Late Ordovician Time Research Articles

Late Ordovician brachiopods Rafinesquina lata Whiteaves, 1896 and Kjaerina hartae n.sp. from southern Manitoba and the Hudson Bay Lowlands

Examination of the Late Ordovician strophomenid brachiopod Rafinesquina lata Whiteaves, 1896 from the Selkirk Member of the Red River Formation of southern Manitoba reveals that the two syntypes actually represent two genera and two species, Oepikina lata and Kjaerina hartae n.sp. Specimens typically described as Rafinesquina lata by Whiteaves are large, thick-shelled, strongly but evenly concavo-convex, with randomly spaced, fine, concentric growth lamellae and a high ventral interarea. Affinity of the species to Oepikina lies in its unequal parvicostellae of the ventral valve, conspicuous dorsal peripheral rim, robust cardinal process, and strong lateral trans-muscle septa. Kjaerina hartae is easily distinguished from O. lata by its strongly geniculate and less massive shell, a much lower ventral interarea, prominent concentric rugae, and, internally, considerably weaker trans-muscle septa. The shells of O. lata and K. hartae are unusually large and suggest an open, shallow- and warm-water, subtidal depositional environment for the original sediments of the Selkirk Member of the Red River Formation. Common occurrences of K. hartae in the upper Portage Chute and Surprise Creek formations (Bad Cache Rapids Group) of the Hudson Bay Lowlands, together with other brachiopods and other groups of fossils, support the concept that the epicontinental seas once covering the Hudson Platform and the Williston Platform were connected during Late Ordovician (Maysvillian) time.

Upper Ordovician palaeogeography of the Bohemian Massif: implications for Armorica

SUMMARY The palaeogeographic position of Armorica in the upper Ordovician is still rather unclear, due mainly to the lack of reliable palaeomagnetic data. To help resolve this, a palaeomagnetic study of Caradocian and Ashgillian sediments and volcanics of the Barrandian Basin in the central Bohemian Massif (Czech Republic) has been carried out. A total of 29 sites were sampled, including four from a diabase sill intruded into Hirnantian-age sediments, thus providing a baked contact test. Three components of magnetization, labelled A, B and C, have been resolved, in addition to a low-temperature component which corresponds to the present-day direction of the local geomagnetic field (D/I= 360°/67°). Component A (southerly and shallow before bedding correction) is only sporadically present, and corresponds to a remagnetization event of Permo-Carboniferous age which is well known throughout Hercynian Europe. Component B, present as stable endpoints in the sill, and as a secondary component in the host-rock sandstones, yields an overall mean direction of 360°/39°, α95= 9.3°, k= 52.3 in situ, and 355°/21°, α95= 9.3°, k= 52.3 after bedding correction. Component C, identified as stable endpoints in sedimentary and volcanic rocks, passes both a fold test and a baked contact test and is interpreted as being the primary direction of magnetization. The overall mean direction for C is 183°/−59°, α95= 9.5°, k= 30.3 (nine sites) after bedding correction which corresponds to a palaeopole position of 80°S; 360°E. This translates into palaeolatitudes of 40°S for the area studied in latest Ordovician times, but indicates large amounts of rotation (up to 170°) of the Bohemian Massif before final consolidation of Hercynian Europe. Although such large amounts of rotation are difficult to accommodate, they are consistent with palaeomagnetic results obtained from lower Ordovician and upper Silurian rocks elsewhere in the Barrandian Basin. If the Bohemian Massif is interpreted as being an integral part of the Armorican microplate, then these results indicate that prior to Caradoc times, Armorica rifted away from the northern margin of Gondwana, and that by Ashgillian times it was located at intermediate palaeolatitudes.

Evolution of the Early Paleozoic mélange at the eastern margin of the Argentine Precordillera

At the eastern margin of the Argentine Precordillera, Early Paleozoic clastic sediments occur which are interpreted to represent sedimentary mélange deposits of (? Ordovician-) Silurian age. Their olistolith content as well as different pebble types from conglomerate olistoliths provide information about the sedimentary history and the source areas of clastic input during preSilurian times. Gravitational sliding of the olistoliths into an eastern basin was followed by a compressional event during Late Devonian to pre-Upper Carboniferous times. The latter is interpreted to be the result of a ± W-E crustal shortening between the Eastern Precordillera and a basement complex to the east. Late Tertiary compression affected the already steeply inclined and deformed deposits. There was no evidence found to postulate large-scale strike-slip faulting between the Eastern Precordillera and the Sierras Pampeanas Basement Complex during Paleozoic and/or Tertiary times. The formation of both the eastern clastic deposits of the Precordillera and the western slope of the Cambro-Ordovician carbonate platform is interpreted to be the result of large-scale crustal extension with block rotations during Middle to Late Ordovician times.

Palaeomagnetism of the Tan y Grisiau granite, North Wales: Evidence for a subvolcanic origin in Late Ordovician times

AbstractProgressive thermal demagnetization of samples from the Tan y Grisiau granite defines a coherent easterly positive characteristic remanence (D/I = 124.9/60.3°;, 42 samples, R = 40–51, a95 = 4.8°;) residing in magnetite. An ancient reversal of magnetization is recovered in the highest blocking temperature spectrum of a few samples and suggests that a cooling‐related dipolar axis is recorded by this pluton. Only facies of the granite which have been reddened, probably by submagmatic streaming, have recorded a stable remanence. Adjustment for tilt yields a very steep remanence (D/I = 193/88°;) incompatible with any known Early Palaeozoic and younger field direction from Britain. The in situ remanence has a similar declination to the primary magnetization in Late Ordovician dolerites from the Welsh Borderlands and yields a comparable palaeolatitude (41.5°;S). It is concluded that the Tan y Grisiau pluton was magnetized in Late Ordovician times after deformation. Folding in this region is therefore interpreted to be substantially of Taconic (Late Ordovician) origin and not Acadian in age. As both in situ and tilt‐adjusted remanence directions are incompatible with Silurian and younger palaeofield directions from Britain, the pluton is interpreted as a subvolcanic component of the North Wales igneous province. Large anticlockwise rotation of Avalonia is identified between Late Ordovician and Late Silurian times.

Late Ordovician – early Silurian transgressive sedimentation in the Jämtland basin, central Swedish Caledonides

Abstract The sedimentary facies of the Kyrkås Quartzite Formation, Ede Quartzite Formation, and Rönnöfors Conglomerate from the meta‐sedimentary sequences of the Lower Allochthon of Jämtland, central Swedish Caledonides are described and interpreted. The biostratigraphical correlation, which includes new graptolite evidence from the Kyrkås Quartzite, indicates that the transgressive sedimentation represented by these coarse clastic units was initiated in late Ordovician times. Debris flow conglomerates of the Rönnöfors Conglomerate, and slumping in the Kyrkås Quartzite, indicate basement uplift and instability in the Jämtland basin. A local metamorphic source contributed much of the matrix and clasts of the Rönnöfors Conglomerate, but both the Kyrkås and Ede formations were derived from sedimentary source areas. Palaeocurrents that flowed from the west, and the probably wide inshore platform areas of carbonate mud flats fringing Baltica towards the east, mean that much of the coarse clastic sediment for the Kyrkås and Ede formations may have been derived from longshore transport, e.g., from late Precambrian or Cambrian sandstones. Cherns, L. & Karis, L., 1995: Late Ordovician – early Silurian transgressive sedimentation in the Jämtland basin, central Swedish Caledonides. GFF, Vol. 117 (Pt. 1, March), pp. 23–30. Stockholm. ISSN 1103–5897.

A Late Ordovician high‐energy temperate‐water carbonate ramp, southern Quebec, Canada: implications for Late Ordovician oceanography*

ABSTRACTThe Trenton Group (Late Ordovician), the youngest carbonate unit in the Taconic foreland basin of southern Quebec, is a tripartite unit with a distinctive coarse‐grained middle part, the Deschambault Formation.Lithofacies of the Deschambault Formation are dominated by coarse‐grained bioclastic/intraclastic limestones; finer‐grained lithofacies are ubiquitous but subordinate. The complete spectrum of lithofacies indicates sedimentation ranging from above fairweather‐ to below storm‐wave base. Skeletal components are indicative of the modern temperate‐water bryomol association. Non‐skeletal elements are represented by peloids and intraclasts. Accretion rates from areas of continuous sedimentation were low (<14 cm/103 years). From sedimentological and faunal evidence, it is proposed that the Late Ordovician Deschambault ramp was bathed by temperate waters. The model compares favourably with modern cool‐water shelves rimming the southern edge of the Australian continent.Palaeomagnetic data locate southern Quebec in a low latitudinal setting during the Late Ordovician. Upper Ordovician facies distribution in eastern Canada and progressive disappearance of some faunal provinces through Late Ordovician time are used to conclude that the initiation of the Late Ordovician glaciation that covered most of Gondwana was instrumental in easing northward movement of cold oceanic currents. This resulted in the rapid contraction of the southern hemisphere warm‐water tropical belt from a 30° latitudinal‐wide zone in the early Caradoc to a 15° zone in the late Caradoc.

Urinary endocrine analyses to evaluate menstrual function in female populations exposed to occupational or environmental hazards

The Ural–Mongol belt (UMB), between Siberia, Baltica and Tarim, is widely recognized as the locus of Asia's main growth during the Paleozoic, but its evolution remains highly controversial, as illustrated by the disparate paleogeographic models published in the last decade. One of the largest tectonic units of the UMB is the Kokchetav–North Tien Shan Domain (KNTD) that stretches from Tarim in the south nearly to the West Siberian Basin. The KNTD comprises several Precambrian microcontinents and numerous remnants of Early Paleozoic island arcs, marginal basins and accretionary complexes. In Late Ordovician time, all these structures had amalgamated into a single contiguous domain. Its paleogeographic position is of crucial importance for elucidating the Paleozoic evolution of the UMB in general and of the Urals in particular. The Aral Formation, located in Kyrgyzstan in the southern part of the KNTD, consists of a thick Upper Devonian (Frasnian) basalt–andesite sequence. Paleomagnetic data show a dual-polarity characteristic component (Dec/Inc = 286° / + 56°, α95 = 9°, k = 21, N = 15 sites). The primary origin of this magnetization is confirmed by a positive test on intraformational conglomerates. We combine this result with other Paleozoic data from the KNTD and show its latitudinal motion from the Late Ordovician to the end of the Paleozoic. The observed paleolatitudes are found to agree well with the values extrapolated from Baltica to a common reference point (42.5°N, 73°E) in our sampling area for the entire interval; hence coherent motion of the KNTD and Baltica is strongly indicated for most of the Paleozoic. This finding contradicts most published models of the UMB evolution, where the KNTD is separated from Baltica by a rather wide Ural Ocean containing one or more major plate boundaries. An exception is the model of Şengör and Natal'in [A.M.C. Şengör, B.A. Natal'in, Paleotectonics of Asia: fragments of a synthesis, in: A. Yin and M. Harrison (eds.), The tectonic evolution of Asia, Cambridge University Press, Cambridge (1996) 486–640], in which coherent paleolatitudinal motion of Baltica and the KNTD is hypothesized — the latter as part of the Kipchak Arc. We suggest a parallel hypothesis, which explains coherent motion of the KNTD and Baltica. In particular, we argue that if a basin with oceanic crust ever existed between the KNTD and Baltica, it was a narrow one without (significant) active spreading in Middle to Late Paleozoic time. Notably, the paleogeographic position of Siberia during the Middle Paleozoic and hence, the width of the Khanty–Mansi Ocean between Siberia, on the one hand, and Baltica–KNTD, on the other hand, remains largely unconstrained, because of the paucity of high-quality Silurian, Devonian and Carboniferous paleomagnetic results from Siberia.

Subduction-related magmatism of late Ordovician age in eastern England

AbstractDeep boreholes show that plutonic and volcanic igneous rocks comprise an important component of the Caledonian basement in eastern England. The isotopic compositions of these rocks reveal that many of them are of late Ordovician age (440–460 Ma), and their geochemical compositions suggest calc–alkaline affinities. The intermediate (diorite-tonalite) plutonic rocks are associated with a prominent northwest–southeast trending belt of aeromagnetic anomalies extending from Derby to St Ives, Hunts., which is interpreted to work the plutonic core of a calc-alkaline magmatic arc. It is inferred that this arc was generated by the subduction of oceanic lithosphere, possibly from the Tornquist Sea, in a south or southwest direction beneath the Midlands Microcraton in late Ordovician times. The age and geochemical composition of concealed Ordovician volcanic rocks in eastern England, and hypabyssal intrusions of the Midlands Minor Intrusive Suite in central England, is compatible with such a hypothesis.

Factors Controlling Simpson Group Production in Central Oklahoma

ABSTRACT The area of study includes Townships 7 through 9 North, Ranges 1 through 3 West, Cleveland and McClain Counties, Oklahoma. Sandstones of the five Simpson Group Formations have produced more than 65,000,000 barrels of oil from this 324 square mile area. Production is from structural traps, generally small faulted anticlines ranging in size from 80 to 750 acres. Cumulative oil production per field ranges from 36,000 to 11,760,000 barrels of oil. The distribution of oil production from within the Simpson Group has been considered enigmatic because, although all of the sandstone members are present throughout the study area, some fields have attained production from all of the sandstone members and others produce from only one horizon. The Simpson shales are not capable of generating hydrocarbons and the oil produced from the Simpson Group probably originated from the Devonian/Mississippian Woodford Shale. The McClain County fault juxtaposed the Woodford Shale with the Simpson Group enabling lateral oil migration from the Woodford into the Simpson sandstone. Also, tortuous migration pathways allowed Woodford oil to ultimately accumulate in Simpson reservoirs which involve downthrown younger rocks adjacent to upthrown older rocks. Slight structural movements occurred contemporaneously with the deposition of the Simpson Group creating semi-parallel northeast-southwest oriented thick and thin trends. Local structural movements were active during part or all of Simpson deposition. This created stratigraphic variations in the individual Simpson Group Formations which caused the apex of a field to migrate (or even vanish) with depth. Consequently, lower Simpson structures may not be reflected by upper Simpson structures, and conversely, upper Simpson structures may not continue with depth. Variations in Viola deposition enhanced the vertical discontinuity of structures within the study area so that mapped Viola structures may not reflect underlying Simpson structures. Furthermore, a dramatic change in the orientation of the structural grain began to appear in late Ordovician (Viola) time. Subsequent movements enhanced this later structural orientation producing two acute structural trends which control the entrapment of oil. This study demonstrates that the dual structural imprint is probably the single most important factor in controlling the distribution and accumulation of hydrocarbons within the area. The study also identified stratigraphic conditions within the Simpson Group sandstone which may affect their productivity.

Baltica. A synopsis of vendian-permian palaeomagnetic data and their palaeotectonic implications

In light of recent additions to the Palaeozoic palaeo-magnetic data-base, particularly for the Ordovician era, a revised apparent polar wander (APW) path for Baltica has been constructed following a rigorous synthesis of all Late Precambrian-Permian data. The APW path is characterized by two prominent loops. Firstly, a Late Precambrian-Cambrian loop probably relating to a rifting event and secondly, a younger loop relating to a Mid-Silurian (Scandian) collision event. These features imply major change in plate-tectonic reconfiguration. Baltica probably represented an individual continental unit in Early Palaeozoic times and was positioned in high southerly latitudes in an “inverted” geographic orientation. In such a reconstruction Baltica was separated from the northern margin of Gondwana by the Tornquist Sea and from Laurentia by the Iapetus Ocean. The Tornquist Zone is thus interpreted as a passive or dextral transform margin during the early Palaeozoic. While undergoing counter-clockwise rotations (up to 1.6°/Ma), Baltica drifted northward through most of the Palaeozoic; except for a short period of southerly movement in Late Silurian-Early Devonian times after collision with Laurentia. Rapid movements in latitude (up to 9 cm/yr) are noted in Late Precambrian/early Palaeozoic times and significant decrease in velocities throughout Palaeozoic time probably reflect the progressive amalgamation of a larger continent by Early-Devonian (Euramerica) and Permian (Pangea) times. The Tornquist Sea had a principal component of palaeo-east-west orientation. Hence it is difficult to be precise in the timing of when micro-continents such as Eastern Avalonia and the European Massifs ultimately collided along the southwestern margin of Baltica. These micro-continents are considered to have been peripheral to Gondwana (in high southerly latitudes) during the Early Ordovician. Eastern Avalonia clearly had rifted off Gondwana by Llanvirn-Llandeilo times and may have collided with Baltica during Late Ordovician times, although the present available Silurian palaeomagnetic data from Eastern Avalonia may suggest collision in Late Silurian times. Across the Iapetus facing margin of Baltica, Laurentia was situated in equatorial to southerly latitudes during most of the Lower Palaeozoic. These continents collided in Mid-Silurian times, i.e. a first collision between southwestern Norway and Greenland/Scotland which gave rise to the early Scandian Orogeny (425 Ma) in southwestern Norway possible followed by a later, but less dramatic, Scandian event in northern Norway at around 410 Ma. Since Baltica was geographically inverted in early Palaeozoic times, the collisional margin could not have been a margin that once rifted off Laurentia as assumed in a number of plate-tectonic models.

The Ordovician volcanics of the Elmtree–Belledune inlier and their relationship to volcanics of the northern Miramichi Highlands, New Brunswick

An investigation of the geology and chemistry of the basic igneous rocks in the Elmtree and Belledune inliers in northern New Brunswick shows that the bulk of the Middle Ordovician rocks of the ophiolitic Fournier Group are best interpreted as the products of volcanism and sedimentation in an extensive ensimatic back-arc basin southeast of a volcanic arc. The oceanic back-arc-basin igneous rocks form the basement to renewed arc-related basaltic volcanism in late Middle to Late Ordovician time. The Fournier Group is separated from the structurally-underlying, shale-dominated Elmtree Formation of the Tetagouche Group by an extensive tectonic melange, which incorporates lenses of serpentinite, mafic volcanic rocks, and sedimentary rocks of both the Tetagouche and Fournier groups. The mafic volcanic rocks in the Elmtree Formation correlate best with those intercalated with the lithologically similar sediments of the Llandeilian–Caradocian Boucher Brook Formation in the northern Miramichi Highlands. The melange and the present structural amalgamation of the Tetagouche and Fournier groups result from closure of the marginal basin by northward-directed subduction at the end of the Ordovician. Most mafic suites in the Elmtree and Belledune inliers can be chemically correlated with similar suites in the northern Miramichi Highlands, showing that the two areas are not separated by a terrane boundary.

Early Paleozoic orogenic belt of the Andes in southwestern South America: Result of Laurentia-Gondwana collision?

The late Precambrian to early Paleozoic age rock units of the Pampean ranges, the Puna, and the North Patagonian massif of southwestern South America constitute the Famatinian orogenic belt. They are interpreted as an Ordovician collisional belt between the Occidentalia terrane and the Gondwana craton. They include mafic and ultramafic belts of Neoproterozoic to early Paleozoic age. An intense tectonothermal event resulted from the collision; syntectonic granitoids represent crustal melting. In that collision syntectonic to late-tectonic foreland basins developed. The recently proposed juxtaposition of Laurentia and East Antarctica-Australia in the Neoproterozoic raises the possibility that Laurentia and western South America were close together in the early Paleozoic, and therefore that the Famatinian belt resulted from Laurentia-Gondwana collision. Occidentalia, which is bordered by a Cambrian carbonate platform similar to that of eastern North America, may be a sliver detached from Laurentia during Late Ordovician time.

Origin of the Late Ordovician Lepidocyclus brachiopod fauna in North America and its biogeographic significance

Three rhynchonellid brachiopod genera, Hiscobeccus, Lepidocyclus, and Hypsiptycha, are the most diagnostic elements of the Lepidocyclus fauna of North America in Late Ordovician time. These are characterized by relatively large, strongly biconvex to globular shells with coarse imbricating growth lamellae and, internally, with septiform cardinal processes in brachial valves. Among the three genera, Hiscobeccus appears the earliest, now known from rocks of late Trentonian-Edenian age in the Canadian Rocky Mountains and Mackenzie Mountains. Morphologically, Hiscobeccus is distinguished from the other two genera by its open delthyrium in the pedicle valve. Early forms of Hiscobeccus show close morphological similarity to Rhynchotrema in their non-globular biconvex shells covered by strong growth lamellae only in the anterior portions. It has been suggested that Hiscobeccus evolved from the Rhynchotrema wisconsinense stock through increase in shell size, globosity, and strength of growth lamellae. Earliest species of Rhynchotrema has been documented convincingly from rocks of early Trentonian age, and the derivation of Hiscobeccus most likely took place during the mid-Trentonian. Lepidocyclus and Hypsiptycha evolved from either Rhynchotrema or Hiscobeccus by developing a pair of deltidial plates covering the delthyrium.Rhynchotrema and other rhynchonellids that evolved before mid-Trentonian time are common to the North American (Laurentian) and the Siberia-Kazakhstan paleocontinents. In contrast, Hiscobeccus, Lepidocyclus, and Hypsiptycha that evolved after the mid-Trentonian are virtually restricted to Laurentia. Therefore, Rhynchotrema marked the last successful intercontinental migration of rhynchonellids during their Llandeilian-Caradocian cosmopolitanism. The pronounced provincialism of the North American Lepidocyclus fauna may have been caused by a number of factors. Facies control is not likely the explanation because these rhynchonellids occur in nearly all the inland and marginal platform seas of Laurentia and commonly are found together in the same types of rocks. Plate tectonics and sea-level changes are considered major causes. The Ordovician rhynchonellids lived in shallow marine (intertidal-subtidal) environments and were incapable of crossing vast, deep oceanic barriers because of their sedentary mode of life and short-lived motile larval stages. The widening of the ocean between North America and Siberia, coupled with high sea-level stand, may have created a sufficiently wide oceanic barrier to interrupt faunal mixing between the two paleocontinents by late Trentonian time. Moreover, the rise in sea level would have resulted in the disappearance of island faunas, which could have served as stepping stones for intercontinental migration of shallow-water benthic faunas during low sea-level stand.

Ordovician magnetostratigraphy: a correlation of global data

Palaeomagnetic studies where primary magnetizations are established from well-dated rocks have been compiled to construct a magnetostratigraphic time-scale for the Ordovician period. An excellent correlation of magnetozones is observed between independent studies of the Baltic and South Siberian platform sequences. Supplemental datasets from other continents concur with the Baltic and Siberian polarity data. Early Ordovician times were mainly characterized by a reverse field. Rapid reversals then occurred during Llanvirn and Llandeilo times. Late Ordovician times were dominated by a normal polarity field. Some gaps remain in the polarity record, notably for late Caradoc-early Ashgill and mid-Tremadoc times.

Comment on “Palaeomagnetic results from volcanic rocks of the Shelve Inlier, Wales: evidence for a wide Late Ordovician Iapetus Ocean in Britain” by C. McCabe and J.E.T. Channell

In a recent paper, McCabe and Channell [1] report a Late Ordovician palaeomagnetic pole from the Shelve Inlier, Wales, and argue for a Iapetus Ocean of 30 degrees latitudinal width between Laurentia and Southern Britain in Late Ordovician times. They further suggest that Southern Britain lay in high latitudes adjacent to the northern margin of Gondwana at this time. Unlike McCabe and Channell we contend a primary Llanvirn (Mid-Ordovician) remanence age for the Shelve volcanics. Additionally, several important points regarding the palaeomagnetism of Southern Britain may have been overlooked by McCabe and Channell. In particular, re-examination of a previous palaeomagnetic study of the Shelve Inlier [2] reveals a dual polarity remanence, which passes a fold test at 95% confidence, and indicates temperature southerly palaeolatitudes of 32S _+ 8.

Diagenesis, thermal maturation and burial history of the Upper Cambrian Bonneterre dolomite, southeastern Missouri; an interpretation of thermal history from petrographic and fluid inclusion evidence

In this report, fluid inclusion, thermal alteration index (TAI) and petrographic data are used to interpret thermal maturation and burial history of the Bonneterre Formation of southeastern Missouri. TAI visual estimates, based on indigenous Upper Cambrian acritarchs, indicate maximum thermal exposure into the mid-upper peak gas window. This interpretation is in agreement with maximum fluid inclusion homogenization temperatures which average 235°C (455°F or an approximate vitrinite equivalent of 1.7% R 0). Thin-section petrography and fluid inclusion evidence further indicate that three separate episodes of burial have occurred. Initial burial, of unknown depth or thermal exposure, culminated in a period of uplift and karstic subaerial exposure possibly during Late Ordovician time. Maximum burial and thermal exposure to the mid-upper peak gas window followed, and is represented in the rock by a high-temperature (235°C), randomly oriented two-phase fluid inclusion population in matrix dolomite. Subsequent uplift, possibly coincident with Ouachita overthrusting, resulted in a variety of diagenetic fabrics, including late burial cements and fractures. Secondary two-phase fluid inclusions within these late calcite cements and those occurring in linear trains that cut across matrix dolomite record lower average homogenization temperatures than their unoriented precursors (average T h = 170°C, or an approximate equivalent of 1.3% R 0), further indicating that a significant amount uplift has occurred since maximum burial. Present-day bottom hole temperatures of only 74°C (an approximate equivalent of 0.7% R 0) indicate that uplift has occurred after this second burial episode. This event probably coincides with the development of the New Madrid Fault system during the Tertiary, which still active today. These results demonstrate the utility of fluid inclusion microthermometry complex burial/thermal histories.

Age and setting of the Bronson Hill magmatic arc: A re-evaluation based on U-Pb zircon ages in southern New England

New U-Pb zircon ages of selected rocks from the Bronson Hill anticlinorium in southern New England are as follows: Microcline gneiss of the Dry Hill Gneiss in the Pelham dome, believed to be metamorphosed alkali rhyolite, is confirmed as Late Proterozoic, yielding an age of 613 ± 3 Ma. Another sample shows strong evidence of zircon inheritance with a minimum mean age of 1402 Ma. Plagioclase-rich gneisses with associated amphibolites (Swanzey, Pauchaug, Monson, and Fourmile gneisses), now believed to represent metamorphosed intrusive igneous rocks, yield ages from 454 to 442 +3/-2 Ma, thus ranging from early Late through latest Ordovician, consistent with other absolute ages of the Oliverian plutonic series in the region. Quartz-phyric rhyolite from the upper member of the Ammonoosuc Volcanics and rhyolite tuff from the Partridge Formation are dated at 453 ± 2 Ma and 449 +3/-2 Ma, respectively, indicating that eruption of felsic lavas in the overlying volcanic section spanned an interval of approximately 4 m.y. Additional zircon ages are reported from a crosscutting but deformed tonalite dike in the Swanzey Gneiss (381 ± 3 Ma) and a coarse-grained gabbro from the Prescott Intrusive Complex (407 +3/-2 Ma) intrusive into the Partridge Formation. Metamorphic overprinting of the region during both the Acadian and Alleghanian orogenies is indicated by U-Pb titanite ages of 357 ± 5 Ma and 292 ± 5 Ma from a tonalite gneiss on Prescott Peninsula and the Dry Hill Gneiss, respectively. The above ages and other zircon ages, as well as petrography and geochemistry, of the Fourmile, Monson, Swanzey, Pauchaug, and other plagioclase-rich gneisses of the Bronson Hill anticlinorium suggest that they represent the plutonic root of a calc-alkaline magmatic arc at least 250 km long that was produced partly or entirely on a continental margin in Late Ordovician time (ca. 454-442 Ma). The physically overlying cover sequence of chemically different tholeiitic arc-volcanic and sedimentary rocks of the Ammonoosuc Volcanics and Partridge Formation, previously thought to rest unconformably on the plagioclase-rich plutonic gneisses, is now shown to have had a magmatic history overlapping with them. The zircon ages in the plutonic gneisses and the cover sequence indicate that both are the same age or slightly younger than the Late Ordovician (Caradocian) emplacement of the Giddings Brook slice of the Taconian allochthons, and too young to have formed above an early and medial Ordovician subduction zone.

Paleomagnetic results from volcanic rocks of the Shelve Inlier, Wales: evidence for a wide Late Ordovician Iapetus Ocean in Britain

In order to shed light on the early Paleozoic paleogeography of Iapetus-bordering continents, samples for paleomagnetic study were collected from middle Ordovician (Llanvirn) volcanic rocks of the Shelve Inlier, Welsh Borderland. The folding within the inlier is constrained to late Ordovician (Ashgill) time, thus offering a fold test with tight age brackets. After stepwise thermal demagnetization, a pre-folding characteristic magnetization component was isolated from 95 specimens. The mean of 11 site mean directions for the characteristic magnetization gives D/I=116.4/+ 67.9(k = 89.4; α 95 = 4.9° ), and the paleomagnetic pole lies at 26.6°N, 36.1°E ( δp = 6.9°, δm = 8.2°). The fold test is positive at the 99% confidence level, indicating that the magnetization was acquired during the middle or late Ordovician. We infer from geological considerations that this magnetization is not a primary thermoremanence, but was acquired during late Ordovician time as a result of a thermochemical disturbance of that age. Our results indicate that southern Britain was near 51° south latitude during the later Ordovician. Since middle to late Ordovician paleolatitudes for Gondwana and Laurentia are well-determined, our result can be used to test various scenarios for the closure of Iapetus. We conclude that: (1) the Iapetus Ocean in Britain still had a latitudinal width of some 30° at this time, thus rendering a postulated Ordovician closure unlikely; (2) southern Britain, Avalon, and Hercynian Europe all occupied high latitudes during the later Ordovician, consistent with a paleoposition near the northern margin of Gondwana; and (3) the Acadian orogeny in Britain and North America could have been the result of the collision of Gondwana and Laurentia during early Devonian time.

Middle to Late Ordovician rocky bottoms and rocky shores from the Manitoulin Island area, Ontario: Discussion

The Precambrian Lorrain Quartzite is exposed on Manitoulin, Great Cloche, and Birch islands in Lake Huron off the mainland of Ontario. Metamorphism created a massive unit, which later formed a resistant angular unconformity overlain by a succession of strata deposited from Middle to Late Ordovician times. At the start of Ordovician sedimentation, topographic relief was at least 125 m when the quartzite hills formed an archipelago of small, rocky islands. The Ordovician–Precambrian unconformity provides insight on a changing series of depositional environments over rocky bottoms and rocky shores. Four Ordovician units are well represented. The Swift Current Formation includes some red beds, while the younger Cloche Island and Cobourg formations are carbonates. The Collingwood Formation is a black shale. The first three include eroded quartzite clasts typical of a rocky-shore setting. The unconformity marked by the Collingwood Formation indicates a rapid transgression with little time for the accumulation o...

Middle to Late Ordovician rocky bottoms and rocky shores from the Manitoulin Island area, Ontario: Reply

The Precambrian Lorrain Quartzite is exposed on Manitoulin, Great Cloche, and Birch islands in Lake Huron off the mainland of Ontario. Metamorphism created a massive unit, which later formed a resistant angular unconformity overlain by a succession of strata deposited from Middle to Late Ordovician times. At the start of Ordovician sedimentation, topographic relief was at least 125 m when the quartzite hills formed an archipelago of small, rocky islands. The Ordovician–Precambrian unconformity provides insight on a changing series of depositional environments over rocky bottoms and rocky shores. Four Ordovician units are well represented. The Swift Current Formation includes some red beds, while the younger Cloche Island and Cobourg formations are carbonates. The Collingwood Formation is a black shale. The first three include eroded quartzite clasts typical of a rocky-shore setting. The unconformity marked by the Collingwood Formation indicates a rapid transgression with little time for the accumulation o...