Early Ordovician Research Articles

Bioerosional innovation for living in carbonate hardgrounds in the Early Ordovician of Sweden

Some of the world's oldest macroborings occur in hardgrounds in lower Ordovician (Arenig) limestones exposed on the island of Öland, southern Sweden. The trace fossils, which are described here as Gastrochaenolites oelandicus isp. nov., appear to be dwelling structures excavated in the indurated substrate by invertebrates of unknown taxonomic affinity. They are the oldest examples of this ichnogenus. The appearance of a macroboring life habit at this early time represents a revolutionary new adaptive strategy for inhabiting carbonate hardgrounds. However, this innovative strategy apparently was not successful for the long term, because this particular macroboring taxon seems to have disappeared shortly after its early Ordovician appearance.

The Cadomian active margin (North Armorican Massif, France): a segment of the North Atlantic Panafrican belt

Within the European Variscan belt, the Armorican Massif (northwestern France) contains an isolated fragment of the North Atlantic Panafrican belt — the Cadomian block. Mostly unaffected by Variscan deformation, this block is composed of Neoproterozoic rocks, ranging in age from 620 to 540Ma (Brioverian), locally overlying a composite basement of Icartian (2000Ma) or Pentevrian (750Ma) age.The Cadomian tectonics, the last magmatic manifestations of which are dated at around 540Ma, were sealed by a cover of Early Cambrian rocks in Normandy and Early Ordovician rocks in Brittany.Five units are distinguished in this Cadomian block — Trégor, Saint-Brieuc, Guingamp, Saint-Malo and Fougères. Although the units are bound by major faults, the overall coherence of their different protolithic characteristics suggests that they can be fitted into a simple geodynamic model.The Trégor Unit, represented by the North Trégor Batholith of crustal affinity and its Icartian host rock, probably reflects an early Cadomian evolution.The Saint-Brieuc Unit, essentially made up of magmatic rocks of juvenile affinity, includes two assemblages juxtaposed by the Cadomian tectonics: (1) a volcanosedimentary succession formed in a typical island-arc setting reflecting subduction around 610Ma; and (2) a volcanic–plutonic complex emplaced around 590Ma, in an extensional active continental margin of Pentevrian age.The Guingamp Unit, predominantly migmatitic, includes a tectonic slice of deep origin, the extrusion of which accompanied the exhumation of the migmatites; it may represent the trace of a possible suture.The Saint-Malo Unit is composed of Brioverian detrital rocks, the significance of which is uncertain — passive continental margin or accretionary prism; it is structured around a late migmatitic dome (540Ma).The Fougères Unit corresponds to the outer domain of the Cadomian orogeny; it was only weakly deformed by Cadomian tectonics, and is intruded by the large Mancellian Batholith of crustal origin, dated at 540Ma.Together the structural units are interpreted in the framework of an evolving active margin juxtaposing continental margin, marginal basin and volcanic arc, before being shortened during the Cadomian convergent tectonics.

Geochemical evolution of arc magmatism during arc-continent collision, South Mayo, Ireland

The Lough Nafooey arc, in the western Irish Caledonides, collided with Laurentia during the Early Ordovician. This event is recorded in the stratigraphy of the South Mayo trough, the preserved forearc basin of this system. Lavas at the base of the oldest Lough Nafooey Group show intraoceanic arc composition. eNd(t) decreases and light rare earth elements (REEs) become more enriched upsection in the Lough Nafooey Group (ca. 495‐ 481 Ma), reflecting early collision with Laurentia. The subsequent Tourmakeady Group (ca. 481‐470 Ma) is rhyolitic, light REE enriched, and has strongly negative eNd(t). These rocks were erupted during the Grampian orogeny. The Rosroe and Mweelrea Formations (,470 Ma) show wide scatter of La/Sm and Nb/Zr values, suggesting mixed mantle sources. This correlates with rapid exhumation of the adjacent Connemara metamorphic terrane. The chemical evolution of the arc supports models of collision, orogeny, and tectonic unroofing within ;15 m.y., and shows that genesis of magmas more enriched than continental crust can occur during arc-continent collision, clarifying the potential role of arc volcanism in continental crust formation.

First occurrence of Janospira from the Early Ordovician of Australia

First occurrence of Janospira from the Early Ordovician of Australia

Early Paleozoic (Late Cambrian–Early Ordovician) acritarchs from the metasedimentary Baden-Baden–Gaggenau zone (Schwarzwald, SW Germany)

The metasediments of the low-grade metamorphosed Baden-Baden–Gaggenau zone of the northern Schwarzwald (southwestern Germany) have been analyzed palynologically. From 133 samples representing different metasedimentary units, only three samples of the upper part of the Traischbach Serie provide extremely poorly-preserved palynomorphs. The assemblage consists of the galeate acritarch genera Caldariola, Cymatiogaleaand Stelliferidium, as well as specimens of diacromorph and polygonomorph acritarchs. Although determinations are difficult at the generic level and essentially impossible at the specific level, the assemblage can be attributed to an interval between the Late Cambrian and Early Ordovician. This is the first biostratigraphical age assignment for the metasediments of the Baden-Baden–Gaggenau zone, which can possibly be correlated with the Villé Unit of the northern Vosges Mountains (eastern France).

Epizoan-sponge Interactions in the Early Ordovician of the Argentine Precordillera

Abstract Evidence for biotic interaction is not obtainable easily in the fossil record. Epizoans cemented to the skeletons of host organisms provide an opportunity to study possible host–epizoan interactions and the ecological structure of hard substratum communities. One such possibility is sponges and their associated encrusters, but few such occurrences have been documented from the fossil record. Early Ordovician limestones of the San Juan Formation in the Argentine Precordillera bear an important sponge fauna consisting mainly of demosponges. This Early Llanvirn assemblage has the highest diversity, and most sponges were subjected to epizoan encrustations. A total of nineteen species included in eleven genera of sponges were examined to record epizoans encrustations. Type and relative abundance of epizoans were compiled for each sponge species. Five kinds of epizoans were recorded: bryozoans, stemmed echinoderms, brachiopods, other sponges, and scars and borings of undetermined organisms. Crinoids an...

Epizoan-Sponge Interactions in the Early Ordovician of the Argentine Precordillera

Epizoan-Sponge Interactions in the Early Ordovician of the Argentine Precordillera

Cambro-Ordovician palaeomagnetic and geochronologic data from southern Victoria Land, Antarctica: revision of the Gondwana apparent polar wander path

SUMMARY We present new palaeomagnetic and isotopic data from the southern Victoria Land region of the Transantarctic Mountains in East Antarctica that constrain the palaeogeographic position of this region during the Late Cambrian and Early Ordovician. A new pole has been determined from a dioritic intrusion at Killer Ridge (40Ar/39Ar biotite age of 499±3 Ma) and hornblende diorite dykes at Mt. Loke (21°E, 7°S, A95=8°, N=6 VGPs). The new Killer Ridge/Mt. Loke pole is indistinguishable from Gondwana Late Cambrian and Early Ordovician poles. Previously reported palaeomagnetic poles from southern Victoria Land have new isotopic age constraints that place them in the Late Cambrian rather than the Early Ordovician. Based upon the new palaeomagnetic and isotopic data, new Gondwana Late Cambrian and Early Ordovician mean poles have been calculated.

A new look at early bivalve phylogeny

Abstract In the 30 years since publication of the bivalve Treatise , (Moore, R. C. (ed.) 1969. Treatise on Invertebrate Paleontology. Part N. Mollusca 6, Bivalvia , Geological Society of America and University of Kansas) important new faunas have been described from the early and mid Cambrian and from the early and mid Ordovician. These contain significant new forms, including some long-ranging intermediate groups, that indicate the relationships between the principal bivalve clades, but lack of fossils from the late Cambrian and earliest Ordovician is a major hindrance. The principal phase of bivalve diversification followed on from the evolution of the filibranch gill in the latest Cambrian or earliest Ordovician. The fundamental division of the class is into two subclasses, Protobranchia and Autolamelli branchiata; links between the two can be demonstrated in the early Ordovician. Major divisions of each subclass are recognized as superorders. Within the Protobranchia, the Nuculoida developed specialist food-gathering palps and an enlarged foot. Diverging early from the protobranch stock were other bivalves that lived symbiotically with sulphur-oxidizing chemoautotrophic bacteria; this allowed colonization of anaerobic substrates and produced two distinct stocks: the deeply infaunal anteriorly elongate Solemyoida and the shallower infaunal Nucinelloida. The Autolamellibranchiata, initially identified by strongly asymmetrical hinges, diversified in three directions, each characterized by distinctive hinges. The Trigonioida were characterized by ligamental nymphs and frequently denticulate teeth, and rapidly regained greater symmetry; the Anomalodesmata also developed a strong ligamental insertion within nymphs and largely lost their dentition, whilst the Heteroconchia, principally with a shell including a complex crossed-lamellar structure, had various combinations of cardinal and lateral teeth. Heteroconch diversifications were mainly in the Mesozoic and Cenozoic, but one Ordovician group, the Glyptarcoidea, is a good ancestor for the Pteriomorphia. The following new taxa are proposed: Cardiolarioidea superfam. nov., Eritropidae fam. nov. and Catamarcaidae fam. nov.

A new record ofColpocoryphe (Trilobita) from the Skiddaw Group of the Cross Fell inlier, northern England

The trilobite Colpocoryphe sp. is described for the first time from the Kirkland Formation, Llanvirn, of the Skiddaw Group in the Cross Fell inlier. It is considered to be closely related to a form previously recorded from the upper Arenig of the Lake District. This new record supports the previously suggested Gondwanan aspects of the northern England Avalonian fauna during the early Ordovician. Copyright © 2000 John Wiley & Sons, Ltd.

Simulating particle suspensions by extended point-force distributions in a Navier-Stokes scheme

The Ediacaran–Early Ordovician interval is of great interest to paleogeographer's due to the vast evolutionary changes that occurred during this interval as well as other global changes in the marine, atmospheric and terrestrial systems. It is; however, precisely this time period where there are often wildly contradictory paleomagnetic results from similar-age rocks. These contradictions are often explained with a variety of innovative (and non-uniformitarian) scenarios such as intertial interchange true polar wander, true polar wander and/or non-dipolar magnetic fields. While these novel explanations may be the cause of the seemingly contradictory data, it is important to examine the paleomagnetic database for other potential issues.This review takes a careful and critical look at the paleomagnetic database from Baltica. Based on some new data and a re-evaluation of older data, the relationships between Baltica and Laurentia are examined for ~ 600–500 Ma interval. The new data from the Hedmark Group (Norway) confirms suspicions about possible remagnetization of the Fen Complex pole. For other Baltica results, data from sedimentary units were evaluated for the effects of inclination shallowing. In this review, a small correction was applied to sedimentary paleomagnetic data from Baltica. The filtered dataset does not demand extreme rates of latitudinal drift or apparent polar wander, but it does require complex gyrations of Baltica over the pole. In particular, average rates of APW range from 1.5° to 2.0°/Myr. This range of APW rates is consistent with ‘normal’ plate motion although the total path length (and its oscillatory nature) may indicate a component of true polar wander. In the TPW scenario, the motion of Baltica results in a back and forth path over the south pole between 600 and 550 Ma and again between 550 and 500 Ma. The rapid motion of Baltica over the pole is consistent with the extant database, but other explanations are possible given the relative paucity of high-quality paleomagnetic data during the Ediacaran–Cambrian interval from Baltica and other continental blocks.A sequence of three paleogeographic maps for Laurentia and Baltica is presented. Given the caveats involved in these reconstructions (polarity ambiguity, longitudinal uncertainty and errors), the data are consistent with geological models that posit the opening of the Iapetus Ocean around 600 Ma and subsequent evolution of the Baltica–Laurentia margin in the Late Ediacaran to Early Ordovician, but the complexity of the motion implied by the APWP remains enigmatic.

Diversification of bivalvesin the Ordovician

The Ordovician was the most significant Period in bivalve diversification. From a small Cambrianstock of palaeotaxodonts, the most fundamental radiation occurred in the early Ordovician. An intrinsic factor was the most significant, involving the evolution of the feeding gill within the palaeotaxodonts. This fundamental morphological change allowed development of more effective feeding strategies, permitting bivalves to take advantage of the increased Ordovician nutrient supply. This allowed rapid size increase and diversification, and colonization of a variety of habitats from entirely infaunal into semi-infaunal and epifaunal. By the end of the early Ordovician, seventeen families belonging to six subclasses have hitherto been recognised. Early Ordovician bivalves are restricted to peri-Gondwanan shelves and favoured inshore clastic sedimentary environments; it was not until the mid Ordovician that bivalves were able to migrate to other continents, but initially remained largely confined to nearshore clastic facies. The succeeding late Ordovician radiation was largely driven by extrinsic factors, as the development of low-latitude carbonate platforms promoted the evolution of many epifaunal groups, particularly the modiomorphoids and the pteriomorphians — the latter often characterized by a calcitic outer shell layer.

Trans-Iapetus contrasts in the geological development of southern Scotland (Laurentia) and the Lakesman Terrane (Avalonia)

Abstract The Iapetus Ocean was a major feature separating Laurentia and Avalonia in the early Ordovician. The early Palaeozoic, Laurentian margin of the ocean is preserved in the UK in inliers in the Midland Valley of Scotland and in the Southern Uplands Terrane. The oldest rocks, of similar age to the deep-marine facies of the Skiddaw and Manx Groups opposite on the Avalonian margin, form the fragmentary ophiolitic sequences preserved in the Ballantrae and Highland Border complexes. These, together with equivalents displayed more extensively in Newfoundland, show that a succession of volcanic arcs and back arc basins formed and were accreted onto the Laurentian margin as a result of subduction during the late Cambrian and early Ordovician. In contrast, a suprasubduction zone extensional regime may have dominated the Avalonian margin at that time. Evidence for the progressive destruction of the ocean by northward directed subduction is preserved in the Caradoc-Wenlock rocks of the Southern Uplands. The tectonostratigraphic configuration, with fault-bound slices containing thick turbidite sandstone sequences, of individually restricted duration, resting southwards on progressively younger oceanic mudrocks, is suggestive of an accretionary complex, although the precise situation is debated. Interpretation of the structural complexity of the Southern Uplands is reliant upon extensive biostratigraphical data, prior to the availability of which the understanding was at a level comparable to that of the Manx Group at the present time. The late Llandovery-Wenlock Hawick and Riccarton Groups in the Southern Uplands continue the general tectonostratigraphical pattern. These younger turbidites are, however, of distinctive lithological character and show similarities with Wenlock to Ludlow sandstone-dominated sequences in the Windermere Supergroup in the southern Lake District and the Niarbyl Formation on the Isle of Man. This correlation between the Laurentian and Avalonian margins confirms that the Iapetus Ocean was no longer a significant feature by the mid-Silurian. The Southern Uplands accretionary thrust front migrated southwards on to the Avalonian foreland during the late Silurian as the Avalonian plate was subducted beneath Laurentia. Deformation in the Southern Uplands was largely complete prior to emplacement of c. 400 Ma granite plutons, whereas the Acadian deformation of the Skiddaw and Manx Groups was concentrated in the early Devonian at c. 390 Ma. However, similarities in structural style in the two areas seem to arise from the operation of similar mechanisms.

Crustacean disparity through the Phanerozoic: comparing morphological and stratigraphic data

Abstract Crustaceans have been an important component of marine diversity and biomass since the earliest Phanerozoic. With a relatively well-documented fossil record, they provide an excellent subject for a continuous study of disparity (approxiamtely bodyplan variety) from the Cambrian to the Recent. A data base of 135 morphological characters forms the basis for cladistic and morphospace studies at the ordinal and sub-ordinal level. Gross cladistic topology is: (Eumalacostraca+Hoplocarida vs Maxillopoda) vs Phyllopoda (paraphyletic). Each of these groups is of approximately equal disparity, and occupies a distinct region of the morphospace plot. A few problematical fossils (e.g. Waptia and Odaraia ) fall close to the base of the tree. Comparison of the cladogram with stratigraphic range data indicates the location of probable ghost lineages, and randomization procedures provide a statistical test of the goodness of fit of a given set of stratigraphic ranges to a given tree topology. Disparity indices are calculated at series and stage intervals. Observed range data indicate that Cambrian disparity was approximately one third its present level. The Earliest Ordovician saw a marked decrease, with an increase and subsequent plateau through rest of the period. Increases through the Silurian and Devonian corresponded to the radiation of branchiopods, cephalocarids, and latterly the Eumalacostraca and Hoplocarida. By the end of the Carboniferous, observed disparity had reached over four fifths of Recent levels, and the remaining history of the group saw a gradual but slightly irregular increase up until the end of the Tertiary. Indices of disparity incorporating ghost lineages exhibit less marked peaks and troughs, with fewer perturbations overall. Cladistically-implied disparity in the Lower Cambrian is estimated at three quarters of that in the Recent. Rarefaction is used to compare actual levels of disparity at each time interval with the mean for a similar number of taxa selected randomly from the list of all realized bodyplans. Most intervals preserved a range of forms more disparate than the mean of random samples drawn from the pool of all the taxa considered. From the Triassic to the Recent this difference was intermittently significant. Once occupied, extremes of morphospace tend not to fall vacant again.

Crustacean disparity through the Phanerozoic: comparing morphological and stratigraphic data

Crustaceans have been an important component of marine diversity and biomass since the earliest Phanerozoic. With a relatively well-documented fossil record, they provide an excellent subject for a continuous study of disparity (approxiamtely bodyplan variety) from the Cambrian to the Recent. A data base of 135 morphological characters forms the basis for cladistic and morphospace studies at the ordinal and sub-ordinal level. Gross cladistic topology is: (Eumalacostraca+Hoplocarida vs Maxillopoda) vs Phyllopoda (paraphyletic). Each of these groups is of approximately equal disparity, and occupies a distinct region of the morphospace plot. A few problematical fossils (e.g.WaptiaandOdaraia) fall close to the base of the tree. Comparison of the cladogram with stratigraphic range data indicates the location of probable ghost lineages, and randomization procedures provide a statistical test of the goodness of fit of a given set of stratigraphic ranges to a given tree topology. Disparity indices are calculated at series and stage intervals. Observed range data indicate that Cambrian disparity was approximately one third its present level. The Earliest Ordovician saw a marked decrease, with an increase and subsequent plateau through rest of the period. Increases through the Silurian and Devonian corresponded to the radiation of branchiopods, cephalocarids, and latterly the Eumalacostraca and Hoplocarida. By the end of the Carboniferous, observed disparity had reached over four fifths of Recent levels, and the remaining history of the group saw a gradual but slightly irregular increase up until the end of the Tertiary. Indices of disparity incorporating ghost lineages exhibit less marked peaks and troughs, with fewer perturbations overall. Cladistically-implied disparity in the Lower Cambrian is estimated at three quarters of that in the Recent. Rarefaction is used to compare actual levels of disparity at each time interval with the mean for a similar number of taxa selected randomly from the list of all realized bodyplans. Most intervals preserved a range of forms more disparate than the mean of random samples drawn from the pool of all the taxa considered. From the Triassic to the Recent this difference was intermittently significant. Once occupied, extremes of morphospace tend not to fall vacant again.

Preservation of extraterrestrial 3He in 480-Ma-old marine limestones

We have measured the helium abundance and isotopic composition of a suite of Lower Ordovician marine limestones and associated fossil meteorites from Kinnekulle, Sweden. Limestone 3He/ 4He ratios as high as 11.5 times the atmospheric value in fused samples and up to 23 times atmospheric in a single step-heat fraction indicate the presence of extraterrestrial helium, and demonstrate that at least a fraction of the extraterrestrial 3He carried by interplanetary dust particles must be retained against diffusive and diagenetic losses for up to 480 Ma. The carrier phase has not been identified but is not magnetic. Extrapolation of high-temperature 3He diffusivities in these sediments is consistent with strong retention of extraterrestrial 3He under ambient Earth-surface conditions. Combination of the observed helium concentrations with sedimentation rates estimated from conodont biostratigraphy suggest that the flux of extraterrestrial 3He in the Early Ordovician was about 0.5×10 −12 cm 3 STP cm −2 ka −1, ignoring potential post-deposition helium loss. This value is indistinguishable from the average 3He flux estimated for the Cenozoic Era. In contrast, previous studies of fossil meteorites, Ir abundances, and Os isotopic ratios in the limestone suggest that the total accretion rate of extraterrestrial material during the studied interval was at least an order of magnitude higher than the Cenozoic average. This disparity may reflect significant post-depositional loss of 3He from IDPs within these old limestones; if so, the match between the Ordovician flux and the Cenozoic average would be fortuitous. Alternatively, the size distribution of infalling objects during the Early Ordovician may have been enriched only in extraterrestrial material too large to retain 3He during atmospheric entry heating (>∼30 μm). The fossil meteorites themselves also preserve extraterrestrial helium. Meteorite 3He concentrations of 2 to 9×10 −12 cm 3 STP g −1 are several orders of magnitude lower than found in most modern meteorites, suggesting very substantial helium loss (probably >99.9%) from these chemically altered objects. The meteorites carry 3He concentrations only a factor of a few higher than the host limestones. The meteorites themselves cannot be the source of the extraterrestrial 3He observed in the limestones.

Cambrian bivalved arthropods

This monograph is part of a series of publications by the two authors and their collaborators (Williams et al. 1994; Hou et al. 1996; Melnikova et al. 1997; Williams & Siveter, in press) aiming toward a comprehensive inventory of an important component of the Palaeozoic arthropod fauna: the bradoriids and the phosphatocopids. These two groups of small bivalved arthropods occur worldwide from the early Cambrian to the early Ordovician. From the time of their discovery in the late 19th century, these fossils were referred to the ostracod crustaceans until their soft parts were discovered in the Upper Cambrian of Sweden (phosphatocopids of the orsten faunal assemblage; Muller 1979, 1982), the Middle Cambrian of Australia (Walossek et al. 1993), the Lower Cambrian of Great Britain (a phosphatocopid baby, Hinz 1983), and the early Cambrian of China (Chengjiang fauna; Hou et al. 1996 for the braSeilacher, A. 1992: Vendobionta and Psammocorallia: lost constructions of Precambrian evolution. Journal of the Geological Society, London 149,607413. Seilacher, A. 1994: Early multicellular life: Late Proterozoic fossils and the Cambrian explosion. In Bengtson, S. (ed.): Early Life On Earth Nobel Symposium No. 84, 389400. Columbia University Press, New York, N.Y. Valentine, J.W. 1992: Dickinsonia as a polypoid organism. Paleobiology

The strontium isotopic composition of Ordovician and Silurian brachiopods and conodonts: relationships to geological events and implications for coeval seawater

One hundred and fifty-six 87Sr/ 86Sr ratios were measured from Ordovician and Silurian brachiopod shells, marine calcite cements, and conodonts in order to establish the secular strontium isotope curve for the coeval seawater. Preservation of the brachiopod shell material has been evaluated by petrographic and geochemical criteria and only the well preserved internal secondary layer of the shells has been utilized for strontium isotope measurements. The results document a gradual decrease in 87Sr/ 86Sr, from 0.7091 to 0.7087, from Tremadoc to Llandeilo, a sharp decline to 0.7078 during the late Llandeilo-early Caradoc; little change during Caradoc and the Ashgil; and a steady rise to 0.7087 through the Silurian. These long-term (10 7 yr) variations, with magnitudes in the range of 10 −3, are interpreted to be controlled primarily by continental collisional tectonics and its associated erosion and weathering. The gradual decrease in 87Sr/ 86Sr ratio during the Early Ordovician may record the reduction in uplift and weathering rates due to waning of the Pan-African orogenies. The rapid decline near the Llandeilo/Caradoc boundary suggests a strong hydrothermal flux likely due to increased sea-floor spreading and a possible superplume event. The latter may have caused the prominent transgressive phase, the largest in the Phanerozoic, which would have muted continental flux input. Although the Caradoc was the main interval for the Taconic Orogeny, its impact on the Sr continental flux may have been delayed until the early Llandovery. This effect, complemented by reworking of glacial deposits near the Ordovician-Silurian boundary and enhanced by phases of the Silurian Salinic Orogeny, may have combined to give the progressive increase in the strontium isotope ratio through the Silurian. The scale and directionality of these changes makes the strontium isotope curve valuable for dating and correlation purposes.

Comparative study of the Lower Ordovician ironstones of the Iberian Massif (Zamora, Spain) and of the Armorican Massif (central Brittany, France)

ABSTRACT The Lower Ordovician ironstones of the Iberian Massif (Zamora) and of the Armorican Massif (central Brittany) occur in units of varying thickness (0.2-2.5 m). They are interstratified in mainly sandy formations, with numerous sedimentary structures and trace fossils showing that they were deposited in a nearshore environment (intertidal or subtidal). Mineralogically, they consist of quartz, magnetite, hematite, apatite, and chamosite, associated with biotite in Zamora and with stilpnomelane in central Brittany. These mineralizations were probably formed together with the other ironstones of the ferriferous belt of southwest Europe, within a broad shelf that extended along the northern margin of the Gondwana continent during the Early Ordovician. In the sedimentary basin, the iron, leached out of emergent continental rocks, gave rise to direct precipitation of iron oxyhydroxides (goethite and hematite) under oxidizing conditions from sea water. Below the sediment-water interface small crystals of magnetite and/or iron phyllosilicates (berthierine?) were formed under reducing conditions. During later diagenesis and metamorphism, the goethite and hematite and the berthierine were probably transformed into magnetite and chamosite, respectively, the second generation of chamosite and/or magnetites were precipitated, and those minerals that already existed were recrystallized.

Taconian orogeny in the New England Appalachians: Collision between Laurentia and the Shelburne Falls arc

Tectonic models of the Ordovician Taconian orogeny in western New England usually invoke a collision between the Laurentian margin and a magmatic arc identified as the Bronson Hill arc. However, in central Massachusetts and southern New Hampshire, rocks in the Bronson Hill arc are 454 to 442 Ma and therefore younger than much of the Taconian deformation and metamorphism in western New England and eastern New York, which began by 470 Ma. U-Pb and single-grain evaporation zircon ages combined with geochemical analyses reveal the presence of an older magmatic arc, the Shelburne Falls arc, that formed west of the Bronson Hill arc at 485 to 470 Ma. The Shelburne Falls arc formed above an east-dipping subduction zone by the Early Ordovician. The Taconian orogeny was the result of the collision between Laurentia and the Shelburne Falls arc beginning ca. 475 to 470 Ma. The younger Bronson Hill arc formed above a west-dipping subduction zone that developed along the eastern edge of the newly accreted terrane during the final stages of and subsequent to the Taconian orogeny. The Taconian orogeny ended when plate convergence between Laurentia and Iapetus was accommodated by the newly developed west-dipping subduction zone instead of by crustal shortening in the Taconian thrust belt. The tectonic history of the New England Appalachians is inconsistent with a Middle Ordovician collision between Laurentia and the proto-Andean margin of Gondwana.