mid-Silurian Times Research Articles

New paleomagnetic results for Ordovician and Silurian rocks of the Tarim Block, Northwest China and their paleogeographic implications

The kinematics of the Tarim Block is poorly understood prior to the late Paleozoic and this study aims to clarify motions during Ordovician and Silurian times. We report the paleomagnetic record from 12 sites in Ordovician marine sediments and 21 sites in Silurian fine-grained sandstones and mudstones from the Aksu and Kalpin areas of northwestern Tarim. Together with our previously reported preliminary results, the Ordovician sediments isolate a sporadically-preserved pre-Cenozoic characteristic remanence with dual polarity. Although a positive fold test can only define the remanence as pre-Cenozoic, the corresponding paleomagnetic pole (33.7°S, 185.0°E, dp/dm = 2.7°/4.0°) is removed from post-Silurian paleopoles from Tarim and combined with evidence from magnetic fabrics indicating a quasi-primary origin. A characteristic remanence isolated at 17 sites in Silurian sediments from the Aksu and Kalpin areas includes 7 sites with suspected contamination from regional Permian igneous activity. The 10 remaining sites, all from Kalpin, yield a positive fold test and a paleopole at 19.1°N, 172.9°E (A95 = 5.5°) significantly removed from younger paleopoles from Tarim indicating a Late Llandovery to Wenlock remanence age. The new Mid-Late Ordovician paleopole places the Tarim Block at ~32°S, probably in the vicinity of the Arabian margin of Gondwana. Rapid APW between ~460 and ~433 Ma records large differential dextral motions of Tarim together with other continental blocks along the Paleo-Asian Ocean margin which brought the China continental blocks into proximity with Australia by mid-Silurian times in low (~9°N) northerly paleolatitudes.

Heydrichia (?) poignantii, sp. nov. (Sporolithaceae, Sporolithales, Rhodophyta), a 100 million year old fossil coralline red alga from north-eastern Brazil, and a new Hauterivian record of Sporolithon from Switzerland

Fossil specimens of Heydrichia (?) poignantii, sp. nov. (Sporolithaceae, Sporolithales, Rhodophyta), representing the first confirmation of the genus in the fossil record, were discovered in thin sections of Albian limestones from the Riachuelo Formation, Sergipe Basin, and in thin sections of Albian - Cenomanian limestones from the Ponta do Mel Formation, Potiguar Basin in north-eastern Brazil. A detailed morphological-anatomical account of the species is provided, and its placement in Heydrichia is discussed in relation to current classification proposals. Comparisons with the four other known species of the genus, all non-fossil, show that H. poignantii is the only known species of Heydrichia in which thalli are encrusting to sparsely warty to horizontally layered with overlapping lamellate branches that commonly appear variously curved or arched, and in which thalli have sporangial complexes that become buried in the thallus. The evolutionary history of Heydrichia remains uncertain, but available data suggest that the genus may have diverged from the sporolithacean genus Sporolithon, known as early as Hauterivian times (c. 129.4-132.9 ± 1 Ma) from Spain (and newly reported here from Switzerland), or it may have arisen from a graticulacean alga such as Graticula, dating from mid-Silurian times (c. 427-435 Ma). Current data also suggest that Heydrichia is more likely to have arrived in Brazil from Central Atlantic waters than from higher latitude South Atlantic waters. This implies that currently living species in southern Africa probably arose later from ancestors further equatorward in the South Atlantic, although confirming studies are needed. All non-fossil species of Heydrichia are known only from the southern hemisphere.

Palaeogeography of the Siberian platform during middle Palaeozoic Times (∼450–400 Ma): new palaeomagnetic evidence from the Lena and Nyuya rivers

New reliable palaeomagnetic data from the Siberian platform help in deciphering its palaeogeography during ∼450–400 Ma. Geochronology of late Devonian mafic sills provides time constraints for tectonic deformation along the southern margin of the Siberian Platform and thus a minimum age for the regional magnetic overprint. From a late Ordovician–Silurian sedimentary section of the Nyuya syncline in the southern part of the Siberian platform with the Devonian sills hosted nearby in early Palaeozoic sediments, pre-folding presumably primary magnetization was isolated from the sediments during stepwise thermal cleaning. High unblocking temperatures imply that haematite is the main carrier of magnetization. The sample-mean direction for 37 Ordovician samples from nine sites in stratigraphic coordinates is Ds = 168.5, Is = −5, ks = 22.3 and α 95 = 5.1 and for 77 Silurian samples from six sites is Ds = 193.9, Is = 20.9, ks = 16 and α 95 = 4.2. Another component recorded in both the Silurian and Ordovician samples is pre-folding, with a sample-mean direction of Ds = 204.4, Is = 37.9, ks = 27.2 and α 95 = 2.7 for 104 samples from eight sites. This component was probably formed during a regional remagnetization event, which took place in post-early Silurian time. Putting this secondary component into a framework with available Palaeozoic data and geochronology further constrains its age to be early Devonian.The isolated components yield new late Ordovician, early Silurian and early-middle Devonian palaeomagnetic poles. The revised middle Palaeozoic segment of the apparent polar wander path (APWP) for the Siberian platform provides new palaeogeographic constraints. Our data suggest that in late Ordovician the platform was situated in equatorial latitudes and was rotated 180° with respect to its present position. During middle-late Ordovician time, the platform did not experience any notable latitudinal drift. It started drifting to the north in the late Ordovician, and by the late Silurian it had travelled ∼1500 km northwards and had rotated ∼30° counter-clockwise (CCW). During late Silurian time, the platform continued northward drift and CCW rotation, and by the early Devonian it had drifted ∼1100 km northwards and rotated 10° CCW relative to its Silurian position. After that, the rotation of the platform changed to clockwise (CW), and by the late Devonian had drifted another 1500 km to the north and had rotated ∼60° CW.We evaluated palaeomagnetically viable positions from 450–400 Ma of the three largest Laurasian cratons, Siberia, Baltica and Laurentia, based on the new data and previously published APWPs. Contrary to several published reconstructions, the Siberian platform could not have been situated to the north of the Caledonian suture in mid-Silurian time, but was probably located either at the eastern or the western side of Laurussia. The new data are compatible with an early Devonian position of Siberia similar to the modern Eurasian configuration. They also support the post- early or middle Devonian relative rotation between the Aldan and Angara blocks of the Siberian platform.

U-Pb Silurian age for a gabbro of the Platinum-bearing belt of the Middle Urals (Russia): evidence for beginning of closure of the Uralian Ocean

Abstract The Platinum-bearing Belt of the Urals consists of a series of zoned ultramafic bodies obducted onto the passive continental margin of the East European Craton during the Palaeozoic. Conventional U-Pb and secondary ionization mass spectrometry U-Th-Pb analyses of single zircon grains from a pegmatitic gabbro of the Kumba massif provide Mid-Silurian ages of 425 ± 3 Ma and 419 ± 10 Ma, respectively, interpreted as dating crystallization of the gabbroic magma. This contrasts with the c. 360 Ma age of the neighbouring Kytlym massif and indicates that the Uralian Platinum-bearing Belt is best interpreted as remnants of an island-arc oceanic lithosphere that started forming in Mid-Silurian times and had a protracted lifetime of about 70 Ma. The Uralian Platinum-bearing Belt is thus coeval with the Sakmara arc of the Southern Urals, and is of similar age to other ultramafic bodies such as the Kempersai and Mindyak massifs. Ophiolitic fragments, now preserved along the Main Uralian Fault, may thus represent the assemblage of contemporaneous oceanic and arc terranes brought together during the final stages of the evolution of the Uralide orogen. The age of the Kumba massif, in addition, indicates that the Uralian Ocean underwent contractional events as early as the Mid-Silurian, which suggests a possible link with the pivotal rotation of Baltica.

Palaeomagnetism of the Ross of Mull granite complex, western Scotland: lower Palaeozoic apparent polar wander of the Orthotectonic Caledonides

The Ross of Mull granite complex was emplaced near the western margin of the Orthotectonic Caledonides of Scotland in early–mid-Silurian times. Anisotropy of magnetic susceptibility reflects the post-tectonic character of the pluton and contrasts with magnetic fabrics in adjoining country rocks which record the regional D3 deformation. Palaeomagnetic study identifies two dominant magnetizations. An ‘A’ component is magnetite-resident with distributed blocking temperatures (100–500 °C) and normal polarity (D/I=339/−50°, 30 samples, α95=5.6°, palaeomagnetic pole at 12°E, 1°S, dp/dm=5.0/7.5°); it was probably acquired during initial cooling. A ‘B’ magnetization (unblocking temperatures 500–580 °C, D/I=195/+21°, 40 samples, α95=4.3°, palaeomagnetic pole at 339°E, 22°S, dp/dm=2.4/4.5°) is of dual polarity and comparable to magnetizations widely recorded in the Younger Granite Suite of the Scottish Caledonides. It does not correlate with the timing of regional isotopic closure as defined by K/Ar thermochron ages and appears to have been imparted during a regional thermochemical event at low temperatures (≈250–150 °C) and a late stage of cooling of the orogen (≈420–410 Ma). Metamorphic facies in the aureole and adjoining country rocks record examples of both A and B magnetizations, together with a low-blocking-temperature (0–350 °C) component acquired during late Palaeozoic/early Mesozoic rifting in the Hebridean Basin. Palaeomagnetic poles from the Orthotectonic Caledonides define a late to post-tectonic magnetization record equivalent to a general east to west apparent polar wander trend of late Ordovician–Lower Devonian age. A short-term reversal of this trend is identified during the earlier part of Silurian times so that mid–late Silurian poles are located in a similar position to late Ordovician poles. The overall path coincides with the contemporaneous record from the Paratectonic Caledonides of England and Wales from c. 455 Ma. It is therefore concluded that the three-plate model of the Caledonides (incorporating the collision of Laurentia and Baltica in mid-Ordovician times to produce the Orthotectonic Caledonides, and the closure of the Iapetus Ocean between Baltica–Laurentia and Avalonia in late Ordovician times to produce the Paratectonic Caledonides) was unified prior to the oldest magnetizations preserved in the Orthotectonic Caledonides. Certain second-order discrepancies remain and may either reflect the incompleteness of the present database or the effects of subsequent regional rotation and strike-slip movements prior to full welding of the orogen by late Lower Devonian times.

Silurian provenance variation in the Southern Uplands terrane, Scotland, assessed using neodymium isotopes and linked with regional tectonic evolution

ABSTRACTThe progressive changes in the provenance of Silurian greywacke turbidites in the Southern Uplands terrane reflect geotectonic events at the Laurentian continental margin during closure of the Iapetus Ocean. In the northern Gala Group, juvenile andesitic detritus in some beds gives εNd values no lower than −4·2; more commonly, quartzo-feldspathic greywackes have εNd values in the −5·5 to −6·7 range, produced by the mixing of juvenile plutonic and Proterozoic basement detritus during arc unroofing. In the southern (younger) Gala Group, Proterozoic εNd values range down from −7·7 to −11·2 with only sporadic evidence for a juvenile component. An abrupt change is seen between the Gala Group and its tectonostratigraphical successor, the Hawick Group. In the latter, εNd values have a compact range between −4·7 and −6·6, indicating the renewed dominance of a more juvenile, plutonic provenance. Regional variations in the Sr/Rb ratio suggest that this was more evolved than the source of the Gala Group plutonic material. The Wenlock greywackes of the Riccarton Group have εNd values in the range −5·1 to −7·8, overlapping with the Hawick Group and with coeval greywackes from both the Midland Valley and Lakesman terranes. Overall, the data support proposals that the Iapetus Ocean had effectively closed by mid-Silurian times. Conversely, data from greywacke boulders in the basal Old Red Sandstone conglomerate of the Midland Valley terrane militate against its Wenlock juxtaposition with the Southern Uplands.

Geochemistry, single zircon ages and Sm–Nd systematics of granitoid rocks from theGóry Sowie (Owl Mts), Polish West Sudetes: evidence for earlyarc-related plutonism

Granitoid gneisses as well as their migmatitic and anatectic derivates were investigated from theGóry Sowie (Owl Mts) Massif of SW Poland in the central West Sudetes. The gneisses and migmatites aretectonically interlayered with paragneisses, and experienced several consecutive tectono-metamorphicevents. Geochemically, the granitoid gneisses are calc-alkaline and similar to orogenic granite suites, whichtherefore lends support to a subduction-related origin. Single zircon Pb–Pb evaporation ages suggest that the gneiss precursors were emplaced between 473 and488 Ma, and most samples analysed contain zircon xenocrysts with minimum ages between 1124 and2620 Ma. An early phase of high-grade metamorphism is documented by an age of c. 440 Ma for ananatectically derived granite. Two late Variscan granites have an age of c. 333 Ma. Nd isotope systematicssuggest variable involvement of crustal material in the generation of the granitoids. We speculate that the Góry Sowie Massif was part of Avalonia in early Palaeozoic times, derived fromthe northwestern margin of Gondwana, probably northern South America. Granitoid emplacement in theOrdovician took place along a Japan-type active continental margin while Avalonia drifted towardsBaltica and the Tornquist Ocean was consumed. Collision with Baltica occurred in mid-Silurian times. Our data are compatible with an active magmatic arc extending from southern England to thesoutheastern Sudetes in Cambrian and Ordovician times.

Paleomagnetic constraints on Siluro-Devonian Laurentian margin tectonics from northern Appalachian volcanics

Paleomagnetic analyses of Silurian mafic volcanics from the overstep sequence of sedimentary and volcanic rocks deposited over the Central Mobile Belt of the northern Appalachians provide insight into the mid-Paleozoic tectonic history of the Laurentian margin. Stepwise thermal demagnetization of the subaerial mafic volcanics of the Ludlovian Fivemile Brook Formation of northwestern Maine reveals two ancient high-temperature, dual-polarity remanences. A tilt-corrected mean direction yields an inclination of 41° (σ I = 3.9°, N = 4 sites) and the magnetization is interpreted as a near-primary magnetization acquired well before the end of Early Devonian deformation. An in situ mean direction ( D = 165°, I = 35°, α 95 = 6°, k = 65, N = 10 entries from 8 sites), is interpreted as a secondary overprint with a pole (22°S, 306°E) near the Early Carboniferous segment of the North American apparent polar wander path (APWP). Conglomerate and fold tests of the high-temperature characteristic remanence preserved in Wenlockian subaerial mafic volcanics of the Bryant Point Formation and a red bed of the South Charlo Formation, both of the Chaleur Group of northeastern New Brunswick, constrain paleolatitude and deformational age. The inclination-only fold test peaks at 50% unfolding with a mean inclination of −35° (σ I = 8.5°, n = 148). Synfolding acquisition of magnetization is Wenlockian, based on a negative conglomerate test at the base of the section and a positive conglomerate test at the top of the section. Clockwise streaking of site mean directions away from a predominantly northerly declination is consistent with post-middle Wenlockian dextral shear in the Chaleur Bay region. Comparison of a locus of paleomagnetic pole positions with the North American APWP supports a Silurian age of the magnetization of the Chaleur Group. The age and synfolding nature of this remanence furthermore requires that deformation associated with the Acadian orogeny in New Brunswick began by mid-Silurian times. Moreover, inclinations from these units indicate that, within paleomagnetic resolution, there has been no significant latitudinal displacement with respect to stable North America since the Silurian.

Palaeozoic palaeogeography of Laurentia and its margins: a reassessment of palaeomagnetic data

SUMMARY The recognition of wide-spread remagnetization events during the Palaeozoic has greatly added to the problems of reconstructing the Palaeozoic drift history of Laurentia and in recent years increasing emphasis has been placed on evaluating the relative reliability of palaeomagnetic data, in an attempt to overcome some of these difficulties. In the light of these developments a revised apparent polar wander (APW) path has been constructed following a rigorous evaluation of all published Palaeozoic palaeomagnetic data from Laurentia and its Palaeozoic margins (i.e. North America, northern Britain, Northern Ireland and Greenland). The use of data from the Laurentian margins, when corrected for the opening of the North Atlantic, has resulted in a revision of previously published Palaeozoic palaeogeographic reconstructions and these new palaeogeographic reconstructions for Laurentia are in good agreement with faunal and lithological data. A comparison of this new APW path with other APW paths from Avalonia and Baltica reveals a very close agreement between all three paths from mid-Silurian times onwards, indicating that the closure of the Iapetus ocean and the Tornquist sea was essentially complete by this time. Laurentia occupied equatorial latitudes from Cambrian to early Silurian times and its drift history was dominated by anti-clockwise rotation (up to 0.7°Ma−1) and low latitudinal drift rates (up to 3 cm yr−1). Late Silurian and early Devonian times were characterized by increased rates of anti-clockwise rotation (up to 1.3°Ma−1) and southerly directed latitudinal drift (rates up to 13 cm yr−1) of the continent, which place Laurentia in moderate southerly latitudes (30–40°S) by the early Devonian. These palaeolatitudes are comparable with those from Avalonia and Baltica for this time indicating that the amalgamation of Eurmerica had already taken place. By late Devonian time Laurentia had begun to drift north (rates up to 5 cm yr−1) towards equatorial latitudes by mid-Carboniferous time.

The closure of the Iapetus Ocean and Tornquist Sea: new palaeomagnetic constraints

Early Wenlock volcanic rocks from the East Mendips inlier, SW England, yield primary palaeomagnetic directions which indicate that southern Britain (Eastern Avalonia) occupied a palaeolatitude of 13 ± 5°S in Mid-Silurian (430 Ma) times. When combined with coeval data from Laurentia, the southern Laurentian margin (Scottish terranes), and Baltica, the collective data indicate that: (1) the British and Scandinavian sectors of the Iapetus Ocean were closed, to within the limits of the palaeomagnetic resolution, by the Early Wenlock; (2) Acadian deformation across Britain, which culminated in Early Devonian time, post-dated initial docking of Eastern Avalonia and Laurentia; (3) a previous palaeomagnetic requirement for the Tornquist Sea to remain open into Mid-Silurian times is removed, thereby reconciling palaeomagnetic and biogeographical constraints upon the convergence of southern Britain and Baltica. Recently-acquired palaeomagnetic data from southern Britain and Baltica have facilitated more reliable reconstructions of the early Palaeozoic palaeogeography of Northern Europe (Fig. 1). Palaeomagnetic data now record the progressive drift of southern Britain across the Iapetus Ocean during Ordovician times (Torsvik & Trench 1991; Channell et al. 1992 ) and a predominant northward drift and counterclockwise rotation of Baltica from Cambrian to Devonian times ( Torsvik et al. 1991 a Torsvik et al. 1992 a; Perroud et al. 1992 ). Nevertheless, details on the closure history of the Iapetus Ocean and Tornquist Sea (Fig. 1) have remained sketchy due to a sparseness of coeval Late Ordovician–Mid-Silurian data from the bordering continents. These palaeomagnetic uncertainties permit competing geological models. In this contribution, we address two outstanding problems relating to the closure

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.

Sinistral transpression and the Silurian closure of Iapetus

In recent years conflicting models have been proposed for the late Caledonian closure of the Iapetus ocean between Laurentia, Baltica and the Avalonian terranes. Recently published structural and stratigraphic evidence from Britain, Scandinavia, East Greenland and Newfoundland is reviewed and shows that Western Avalonia, Eastern Avalonia and Baltica all docked sinistrally against Laurentia in the Silurian. Western Avalonia collided sinistrally against the previously accreted Gander and Dunnage arc terranes on the Appalachian margin of Laurentia in mid-Silurian time and then shifted dextrally during the Acadian orogeny in the Devonian. Oblique collision of Baltica with the Greenland margin induced southeasterly crustal stacking in the Scandian orogen and sinistral transpression in the NE Greenland Caledonides, and was followed by more nearly orthogonal convergence. Eastern Avalonia underwent anticlockwise with the Scottish ‘corner’ of Laurentia, rotating into a re-entrant between Laurentia and Baltica. Some implications of this Silurian closure model are that convergence in the Tornquist zone was modest at that time, and that the Acadian (Devonian) deformation in the northern Appalahians and Britain had a subsequent external cause, most likely the impingement of Armorica and Iberia due to the northward drift of Gondwana.

Palaeomagnetism of a Caledonian ophiolite complex, Karmøy, southwest Norway

Abstract The palaeomagnetism of the Karmoy ophiolite suite emplaced at the southwest extremity of the Scandinavian Caledonides in Middle-Upper Ordovician times is described. Low intensities of magnetisation and low Q n values in the bulk of the constituent facies are linked to the effects of low grade metamorphism, but ultramafic pods that have experienced serpentinisation yield well-defined single component magnetisations. A predominant (A) component of magnetisation isolated in 35 samples yields a mean of D I = 51° 58° (α 95 = 5°) . This population is discrete from a subsidiary (B) component recognised in 17 samples with a mean direction of 145° 63° (α 95 = 8) ; a minor (C) component ( D I = 330° 60° , α 95 = 8° ) is also identified and small numbers of anti-parallel components are identified in each case. An Upper Ordovician ignimbrite from Kattnakken, northeast of Karmoy, yields an in situ component of D I = 270° 38° ; together with similar components recognised elsewhere, this mean ( ‘D’ ) direction is most closely compatible with directions recognised in Upper Silurian-Lower Devonian rocks from the Scottish sector of the Caledonides; it is interpreted as an overprint of this age, but may include a component of local rotation. Two interpretations are considered for the remanence record in the Karmoy complex. The ‘A’ component could record the influence of a thermal event linked to uplift of the continental margin and development of the adjoining North Sea Basin in Mesozoic times if this margin has been tilted seawards by ∼ 20° since magnetisation. No plausible tectonic model can bring the ‘B’ component into conformity with the post-300 Ma apparent polar wander (APW) path and it is alternatively proposed that both the ‘A’ and ‘B’ components record Upper Ordovician post-emplacement remanences. With this interpretation the data complement a range of palaeomagnetic results from Middle-Upper Ordovician igneous and metamorphic rocks from North Atlantic bordering terranes yielding intermediate to high palaeolatitudes and contrasting with older and younger data. Collectively they imply that the Iapetus Ocean had effectively closed by these times, and that large APW movements took place between mid-Ordovician and mid-Silurian times.

Comparison of Late Ordovician Epicontinental Seas and Their Relative Bathymetry in North America and China

Six widely separated areas with Upper Ordovician strata in Canada and the United States are compared with six localities in southern China. Cyclic sedimentation, including evaporites, carbonates, and phosphatic black shales, occurred in relatively shallow epicontinental seas of North America. As many as three Ashgill cycles of regionally different styles may have been coeval throughout North America in response to modest changes in sea level. Environments in South China primarily included carbonates and black shales with a far more uniform distribution. Absence of comparable sedimentary cycles confirms that the platform bathymetry of South China was consistently deeper than in North America. The almost complete exposure of North China (Sino-Korean Plate) by Late Ordovician time underscores the fact that independent cratons have different bathymetric histories. By mid-Silurian time, South China also was fully exposed. Such hypsographic variation is critical to the intercontinental correlation of features related to eustasy, water chemistry, and even the patterns of extinctions.

Environment of the Early Paleozoic vertebrates

A comprehensive review of the known pre-Devonian vertebrate localities suggests that the non-marine environment was not invaded until about Wenlock, early mid-Silurian time. Known, well-dated Late Cambrian, Ordovician, and Early Silurian vertebrate localities are all in the marine environment. Considerations of paleogeography for the pre-Wenlock indicate that most of the vertebrate localities are too distant from shorelines to provide any basis for transport from the non-marine environment to a marine depositional site as a favored explanation—the articulated nature of the vertebrates from some of these pre-Wenlock occurrences also supports this interpretation. Reconsideration of both the physiological and embryological evidence relative to living vertebrates, and some related chordates, is also consistent with the view that the vertebrates originated in the marine environment.

A new Silurian microvertebrate assemblage from the Tortworth inlier, Avon, England

A new agnathan assemblage is described from the Wenlock Series of Tortworth, southern England. It includes Logania cruciformis Gross 1967, L. cuneata (Gross 1947), L. ludlowiensis Gross 1967, L. scotica (Traquair 1898) and Thelodus cf. traquairi Gross 1947. This may be a facies fauna, but it has affinities with Baltic and Siberian assemblages and may suggest a more complex palaeogeography for Europe in mid-Silurian times, differing from those already known from England.

Early Paleozoic Evolution of Northern Parts of Canadian Arctic Islands: ABSTRACT

End_Page 2509------------------------------A geosyncline occupied northern parts of the Arctic Islands in Late Proterozoic time. It received sediments from the continent and deepened in a northerly direction. A northwestern belt, which included northernmost Ellesmere Island and the present shelf off Ellesmere and Axel Heiberg Islands, underwent an orogeny in latest Proterozoic or Cambrian time. The orogen behaved as an intermittently rising geanticline, and remained a site of volcanism, plutonism, and metamorphism, from Cambrian to Devonian time. Sediments derived from the geanticline accumulated in a clastic basin on its southeast side. The basin was flanked on the southeast by a subsiding carbonate shelf, in turn grading southward to stable carbonate platforms. Three phases of sedimentation are recognized in the clastic basin in northeastern Ellesmere Island: (1) Middle to Upper Cambrian (?) post-tectonic deltaic deposition; (2) Early to Middle Ordovician deep-water deposition of starved-basin type (radiolarian chert, graptolitic shale, etc.); (3) late Middle Ordovician to Middle Silurian deep-water deposition of flysch-type (graywacke, shale, etc.). The trough must have formed by subsidence of the continental crust rather than by sea-floor spreading, because the deep-water sediments lie on shallow-water sediments and not on volcanics. The trough, which was separated from subaerial parts of the geanticline by a shelf on which carbonates, clastics, and volcanics were deposited, expanded until about mid-Silurian time, then migrated southeast, ahead of the southeast-migrating geanticline. The southeast flank of the trough, characterized by graptolitic shales and limestones, has been traced from northwestern Greenland to northwestern Melville Island. There, starved-basin conditions persisted from Early Ordovician to Early Devonian time. A north-trending belt in the central islands, extending from the stable platform to the geanticline, was elevated in the Early Devonian. The uplift was basement controlled and reflects Precambrian basement trends unrelated to the early Paleozoic basin configuration. An orogeny of the entire northern regions, locally accompanied by intrusion of quartz diorite, occurred in Middle Devonian to Mississippian time. Deformation and uplift proceeded from northwest to southeast. End_of_Article - Last_Page 2510------------