Devonian Time Research Articles

The Siluro-Devonian evolution of the southern Midland Valley of Scotland

AbstractGeological studies along the southern margin of the Midland Valley of Scotland, together with previously published work, suggest a strike-slip model for its evolution during Silurian and Devonian times. The data emphasize the ‘older Caledonian’ influence and persistent, albeit intermittent, transcurrent activity along the Southern Upland Fault which is the boundary between the Midland Valley and Southern Upland terranes. A comparison of the geological characteristics of the Silurian and Devonian rocks in the southern Midland Valley with characteristics from strike-slip basins elsewhere suggests that the Midland Valley rocks were deposited and locally deformed under sinistral strike-slip regimes with two main episodes of transtension — the first acting in Llandovery—Wenlock times and the second in Lower Old Red Sandstone times. A strike-slip model involving transpression may explain why there is a strong unconformity between the Llandovery—Wenlock and the Lower Old Red Sandstone strata in the Pentland Hills and Girvan inliers but only a marked change in sedimentary facies at the equivalent horizon in the inliers of the central Midland Valley. The Lower Old Red Sandstone was probably deposited in a transtensional regime, but the succession is complicated by the widespread Lower Old Red Sandstone calc-alkaline magmatic event. The mid- Devonian deformation is also variable in its effects and the more easterly trend of theen echelonfold axes relative to the Southern Upland Fault trend is indicative of sinistral transpression. Carboniferous and older rocks were affected by smaller dextral movements on the Southern Upland Fault and related faults which started during Namurian times when a change to an overall dextral regime took place.

A Middle Silurian-Early Devonian Magmatic Arc in the Qinling Mountains of Central China

An early to middle Paleozoic magmatic arc was discovered in the Qinling orogenic belt of central China that marks the boundary between the North China craton and the South China craton. The arc was built on the deformed southern passive continental margin of the North China craton, which has been modified by the accretion of an oceanic island arc in early Paleozoic time. U/Pb analyses of zircons from granites indicate short-lived arc activity from Middle Silurian to Early Devonian time. Trace element analyses as well as REE analyses of I-type granites are consistent with the interpretation that the magmatic arc developed above a north-dipping subduction zone. Termination of magmatic activity was probably caused by the initiation of collision between the North China craton and the South China craton. This interpretation is consistent with faunal data and places new constraints on the Paleozoic tectonic evolution of the Qinling belt.

Archaeopteris (Progymnospermopsida) from the Devonian of southern Africa

Archaeopteris notosaria sp. nov. based on one fertile and numerous sterile leafy branches is described from the Grahamstown By-Pass locality, Witpoort Formation (Witteberg Group) Upper Devonian, and represents the first unequivocal record of the genus in southern Africa. This occurrence is used, in its palaeogeographical context, to support the suggestion that climatic gradients in Late Devonian times were less steep than they are at present.

Archaeopteris(Progymnospermopsida) from the Devonian of southern Africa

Archaeopteris notosariasp. nov.based on one fertile and numerous sterile leafy branches is described from the Grahamstown By-Pass locality, Witpoort Formation (Witteberg Group) Upper Devonian, and represents the first unequivocal record of the genus in southern Africa. This occurrence is used, in its palaeogeographical context, to support the suggestion that climatic gradients in Late Devonian times were less steep than they are at present.

Northwest-verging folds and the northwestward movement of the Caledonian Jotun Nappe, Norway

A family of asymmetric, northwest-verging back folds and crenulations occur on a regional scale in the Caledonian décollement zone in southwestern Norway. They are the result of reversal of the emplacement movement of the Jotun and other nappes in Early Devonian time. The back folds may have formed progressively by two different mechanisms: actively by buckling of the layering, and more passively by transfer of slip across the layering (reverse-slip crenulations) to balance the thinning of the zone caused by foliation-parallel slip and shear band formation. The former mechanism is considered to be most important during the initial stages of fold development, gradually giving way to the latter mechanism. The hinge lines of these folds appear to have remained stationary during deformation. This is explained by the observation that the hinge lines lie within or close to the shear plane, i.e. the plane of no rotation in simple shear deformation. Integration of strain across the deformation zone indicates a magnitude of back movement in the order of ca. 20–36 km, and it is argued that the deformation was close to simple shear.

Biostratigraphical correlationsof Early Devonian ichthyofaunas of the Old Red Continent

The Old Red Continent was a Devonian landmass which included North America; northern Europe and severalterranes. In Early Devonian times; vertebrate-bearing localities were situated all around this landmass. A major problem of biostratigraphical correlation occurs between the Lower Devonian vertebrate zonations and the standard conodont zonation. All boundaries stratotypes are very poor in vertebrate remains; or yielded vertebrate taxa which do not allow to discriminate the stages boundaries. However; preliminary data lead to biostratigraphical correlations between some other vertebrate-bearing formations and conodont zones; particularly in the Emsian.

Palaeozoic sequences and evolution of the Calabrian‐Peloritan Arc (Southern Italy)

ABSTRACTThree Palaeozoic sequences belonging to three different basins crop out in the Calabrian–Peloritan Arc. Their age covers the time span from middle (?) Cambrian to early Carboniferous. The sequences comprise terrigenous, volcanic and carbonate rocks, and show low‐grade metamorphism. The basement is of pre‐middle Cambrian age, crops out in the Calabrian–Peloritan Arc, and was metamorphosed prior to the opening to the Palaeozoic basins.The Palaeozoic basins existed no later than the middle Carboniferous, with inferred maximum crustal extension in the Cambro‐Ordovician. By Devonian time, the tectonic regime was compressional overall with middle Devonian island‐arc type volcanic activity that continued until the closure of the basins. Approximately 330 Myr, the Palaeozoic sequences experienced low‐pressure greenschist facies metamorphism and continuing subduction controlled the Variscan tectonogenesis with 280 Myr island‐arc type intrusive magmas. Subduction ceased and late‐stage 280–270 Myr granitic magmas were emplaced during continental collision.The crustal sector carrying the Palaeozoic basins is interpreted as comprising fragments of an active continental margin, one of the several microplates, located at the southern margin of the Euro‐Asia continent which faced a large (at least 1500 km in width) ocean basin.

A new species of Droharhynchia (Brachiopoda) from the lower Middle Devonian (Eifelian) of west-central Alaska

A new species of the genus Droharhynchia Sartenaer is established from lower Eifelian strata of west-central Alaska and the northwestern Brooks Range of Alaska. Droharhynchia rzhonsnitskayae n. sp. occurs in the Cheeneetnuk Limestone of the McGrath A-5 quadrangle, west-central Alaska, and the Baird Group of the Howard Pass B-5 quadrangle, northwestern Alaska. These occurrences extend the lower biostratigraphic range of both the genus and the subfamily Hadrorhynchiinae into the Eifelian. They also suggest close geographic proximity of the Farewell terrane of southwestern and west-central Alaska and the Arctic Alaska superterrane of northern Alaska during Devonian time.

Paleozoic Plate-Tectonic Evolution of the Tarim and Western Tianshan Regions, Western China

The plate-tectonic evolution of the Tarim basin and nearby western Tianshan region during Paleozoic time is reconstructed in an effort to further constrain the tectonic evolution of Central Asia, providing insights into the formation and distribution of oil and gas resources. The Tarim plate developed from continental rifting that progressed during early Paleozoic time into a passive continental margin. The Yili terrane (central Tianshan) broke away from the present eastern part of Tarim and became a microcontinent located somewhere between the Junggar ocean and the southern Tianshan ocean. The southern Tianshan ocean, between the Tarim craton and the Yili terrane, was subducting beneath the Yili terrane from Silurian to Devonian time. During the Late Devonian-Early Carboniferous, the Tarim plate collided with the Yili terrane by sinistral accretional docking that resulted in a late Paleozoic deformational episode. Intracontinental shortening (A-type subduction) continued through the Permian with the crea...

Subsidence analysis of Eastern Avalonian sequences: implications for Iapetus closure

Basin subsidence analysis, employing the backstripping method, indicates that fundamentally different basin-generating mechanisms controlled Silurian basin development in northern England and in Wales. Accelerating subsidence with time suggests that the dominant control on basin formation in northern England was flexure of the Eastern Avalonian lithosphere. Mid-Wales, however, displays dominantly decelerating subsidence, suggesting an extensional control. The errors involved with the backstripping method, particularly concerning Lower Palaeozoic application, have been addressed. In conjunction with sedimentological and tectonic data, the subsidence patterns for the Windermere Supergroup indicate deposition in a flexurally-loaded foreland basin which migrated southwards with time. Similar data for the Powys Supergroup of Mid-Wales imply an extensional control on Silurian deposition, possibly in a strike-slip setting. However, later Pøídolí and early Devonian Welsh sedimentation may represent the onset of foreland basin deposition. The new data are consistent with a diachronous closure of the Iapetus ocean, related to rotational collision of Eastern Avalonia with Laurentia during the early Silurian. Early Silurian oblique collision produced transtension in Wales. Subsequent rotation of Eastern Avalonia produced more orthogonal collision. Thus, continental flexure was felt initially in northern England in late Wenlock times, followed by migration of the resultant foreland basin to the southeast into Wales in early Devonian times. The versatility of the backstripping method is shown by this application to Lower Palaeozoic sequences.

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.

Oroclinal bending in the Caledonides of western Ireland

New palaeomagnetic data from the Caledonides of western Ireland indicate that the Silurian rocks of South Mayo underwent oroclinal bending, following folding, in Siluro-Devonian time. Bending was accommodated on faults cutting the Silurian sequence, and was driven by strike slip motion across the Antrim-Galway Line, a recently recognized major curvilinear lineament. We propose that the Silurian rocks in the east of the region suffered up to 30° clockwise rotation, increasing towards the west to approximately 70°. This newly inferred differential rotation of the Silurian rocks, about a sub-vertical axis, implies that the underlying Dalradian rocks of the Connemara Massif to the south must also have undergone clockwise rotation in late Silurian/early Devonian time. This is consistent with published palaeomagnetic data from the Connemara Gabbro and accounts for the swing in strike of the Caledonian orogen in this part of western Ireland. From this we deduce that the swing in strike of the Caledonian rocks elsewhere on the west coast of Ireland might have a similar explanation.

Silurian paleogeography of Armorica: New paleomagnetic data from central Bohemia

Due to the lack of reliable paleomagnetic data from Armorica for mid‐Ordovician to Late Devonian times, the drift history for this tectonic element is poorly understood. In order to help rectify this problem, a paleomagnetic study of Upper Silurian volcanic, intrusive and sedimentary rocks from the Barrandian Basin of the Central Bohemian Massif (Czech Republic) has been carried out. This basin comprises a sequence of unmetamorphosed lower Paleozoic rocks, the folding of which has an uppermost age limit of Early Carboniferous. Stepwise thermal demagnetization of the samples collected (182 samples from 21 sites), and principal component analysis of both great circle and stable endpoint data reveal a total of three directions of magnetization. The first (labelled A) is in the direction of the present‐day local Earth's magnetic field (D/I = 360°/67°) and is thus interpreted as being of recent origin. The other two directions, termed B and C, comprise the intermediate and high blocking temperature directions respectively. The overall in situ mean direction obtained for B, identified between 150° and 320°C, is 195°/−8° (D/I), k = 18.5, α95 = 13.2° (eight sites). It fails the fold test and yields an in situ paleo‐south pole position of 42°S, 354°E. This coincides with the Late Carboniferous segment of the apparent polar wander (APW) path for stable Europe and is considered to be a secondary overprint of this age. The third magnetization (termed C) is identified between 320° and 600°C, gives an overall mean direction of 208°/−40°, k = 96.8, α95 = 5.7° (eight sites), after bedding correction. It passes fold and reversal tests on both local and regional levels and is interpreted as representative of the Late Silurian paleofield direction in the Bohemian Massif; it yields a paleo‐pole position of 55°S, 325°E. Assuming a normal polarity, the inclination value obtained translates into paleolatitudes of 23°S for this part of Armorica and implies a position adjacent to the southern margin of Baltica. This therefore indicates closure of the intervening ocean by Late Silurian times. However, adopting this polarity also implies large‐scale (up to 140°) anticlockwise rotations of the Bohemian Massif prior to deformation in the Early Carboniferous.

A biostratigraphic method based on a quantification of the characters of Devonian tracheophytes

Biostratigraphic applications of Devonian plant macrofossils are unusual because of the rarity of these fossil plants. Moreover, the taxonomic determination of these plants is too often either arguable or impossible. Here is proposed a biostratigraphic method based on a quantification of individual plant characters and not on determination of whole fossils. The method is the following: (1) each biocharacter found at a given locality receives a score according to whether it is primitive or derived, so that the biocharacter score increases with increasing derivation; (2) the biostratigraphic coefficients of well-dated localities are calculated, the biostratigraphic coefficient of a given plant locality being the mean of all the biocharacter scores of this locality; (3) the scores of these well-dated localities are used to build a reference scale to which localities under investigation will be compared. A detailed example is developed for Early Devonian times. Various plant biocharacters of seven fossiliferous outcrops have been quantified and the subsequent scores of these localities are presented. This example shows that biostratigraphic results obtained by using the quantification method are comparable in precision with dating based on palynological assemblage zones. Evolutionary aspects of the method are also discussed.

The southern border of Baltica—a review of the present state of knowledge

Abstract The southern border of Baltica was for many years drawn along the NW part of the Tornquist Zone. This important boundary between segments of the pre-Cadomian East European and the Palaeozoic West European Platforms is now redefined on the basis of geophysical and well data. Most probably it runs south of the Ringkobing-Fyn High in Denmark. The transects Bornholm-Rugen and Scania-Holstein are used to discuss its position and tectonic character. Two main interpretations are possible: In model (1), the boundary coincides with a steep fracture zone (the Jylland-Mon Fault) that forms the northern front of the Caledonian folded Lower Palaeozoic in the southern forefield of Baltica. Because of significant differences in the tectonical, lithological and thickness development on both sides of this fault, it is interpreted as the northern boundary of a Caledonian terrane (Rugen-Pomorze Terrane) accreted to and locally overthrusted onto Baltica at Early Devonian times. The south adjoining area between this terrane and the northern front of the mid-European Variscides is thought to represent a mosaic of predominantly Cadomian terranes, probably with intervening Caledonian folded complexes. This mosaic of terranes is assumed to have been amalgamated to a North German Superterrane during late Caledonian diastrophism at the latest. In model (2), the boundary is ∼50 km south of the Caledonian front. It coincides with the buried Trans-European Fault that has been documented by deep seismic sounding. In this model, the Caledonian-deformed complexes south of the Ringkobing-Fyn High are interpreted as nappe structures of a Lower Palaeozoic marginal belt thrusted onto the southern edge of Baltica. Hence it follows that the area to the north of the mid-European Variscides is most probably built up of the internal parts of a North German-Polish Caledonide orogen.

Saxothuringian zone in Germany and Poland: differences and common features

The classical correlation by kossmat (1927) of the Saxothuringian belt of Germany with the West Sudetes of Poland is re-evaluated with special reference to the crystalline massifs of Munchberg and the Gory Sowie and the surrounding areas. Important common features are: Cambro-Ordovician magmatism and sedimentation; Silurian to Devonian radiolarian cherts in the low grade to unmetamorphosed units; Early Devonian (and older?) pressure-dominated metamorphism followed by rapid uplift; the incorporation of high pressure and mantle rocks; and Early Carboniferous mass flow deposits with ‘Saxothuringian’ clasts. Differing features in the West Sudetes are: tectonic contacts in and around the Gory Sowie are mostly subvertical and mainly (semi-)brittle (in contrast with subhorizontal and ductile contracts in the Munchberg); the Gory Sowie has undergone late re-equilibration of the earlier tectonic fabrics; an ophiolite juxtaposed with the Gory Sowie crystallized in (?) Devonian time and became only statically metamorphosed, as opposed to the regionally metamorphosed Cambro-Ordovician mafic rocks of the Saxothuringian belt; localized depocenters with Upper Devonian to Lower Permian elastic rocks relating to north-west trending strike-slip faults instead of the north-east trending cylindroidal foreland basin in the Saxothuringian. It is possible that the West Sudetes originated as an easterly continuation of the Saxothuringian belt, but then became involved, from the Late Devonian onwards, in a strike-slip system parallel with the margin of the East European Platform.

Relationship between metamorphism and structure in the Skiddaw Group, English Lake District

AbstractRegional variation of white mica (illite) crystallinity in the Skiddaw Group is set against the structural interpretation of Hughes, Cooper & Stone (1993, this issue) in which early- or pre-Ludlow deformation and slaty cleavage development (S1) were succeeded by southward thrusting and an associated development of S1 and S1 crenulation cleavages, possibly during early Devonian times. Kubier index (KI) values are plotted in relation to geological structure for a major part of the Skiddaw Group, and cross-sections constructed. The pattern is interpreted in terms of three processes: (1) diagenetic to low anchizonal burial metamorphism under a relatively high geothermal gradient during the late-arc stage (early to mid-Silurian) which preceded the orogenic phase and formation of S1; (2) upper anchizonal to epizonal metamorphism due to tectonic thickening and slaty cleavage development during the early to pre-Ludlow orogenic phase following closure of Iapetus; (3) late-tectonic uplift of already metamorphosed rocks by southward-directed movement on the Loweswater, Gasgale Gill and Causey Pike Thrusts possibly during early Devonian time. An analogous interpretation is made for the Skiddaw area, though with the addition of major post-S1 contact metamorphism.

The Caledonides of the Anglo-Brabant Massif reviewed

The Anglo–Brabant Massif has formed a positive structural element since the Acadian orogenic phase in mid–Devonian times. As a consequence, the Caledonian basement lies at relatively shallow depth across the massif, encompassing a large area of southern Britain, the southern North Sea, southern Netherlands, Belgium and northern France (Fig.1).

Lasneria globosa gen. et sp. nov., Corallinaceae ancestrale du Dévonien inférieur du Massif armoricain, France

Monospecific rhodoliths constructed by Lasneria globosa gen. et sp. nov. have been recovered at the base of the l'Asnerie section (Early Devonian, Eastern Armorican Massif). L. globosa represents the oldest coralline alga known up to day. The internal structure is anatomically close to that of Tertiary forms and reveals that Corallinaceae were already very advanced in Early Devonian times.

Lorophyton goense gen. et sp. nov. from the Lower Givetian of Belgium and a discussion of the Middle Devonian Cladoxylopsida

Lorophyton goense gen. et sp. nov. is described from Lower Givetian strata at Goé (Vesdre Synclinorium, Belgium). This moderately sized plant resembles a small tree with robust roots and a crown of branches. The vegetative ultimate units are three-dimensional and dichotomize profusely. The fertile ultimate units also are three-dimensional and consist of two parts of unequal size: in each part a pseudo-main axis terminates with a dichotomous division and bears three dichotomously divided laterals. Either all the segments of the fertile units terminate in a pair of pendulous elongated sporangia, or several of them remain vegetative. The cladoxylalean affinities of the plant are discussed and the phylogeny of the Cladoxylopsida during Devonian time is considered.