Devonian-Carboniferous Boundary Research Articles

Shallow-Water Sedimentation as illustrated in the Upper Devonian Baggy Beds

Summary The Baggy Beds (440 m thick) form a transgressive sequence of shallow-water marine and delta-like sediments of Upper Famennian age between the continental Pickwell Down Sandstone and the overlying neritic Pilton Beds (which straddle the Devonian-Carboniferous boundary). The Baggy Beds are divided into lower and upper divisions (the lower division containing the bulk of the sandstones) and a number of members. There is a strong diversity of facies, and nine main facies are recognized; they show little evidence of cyclicity but their stratigraphical relationships are important for their interpretation. Penecontemporaneous erosion, principally attributed to wave-action, was often intense, and has contributed more than any other factor to the overall aspect of the marine facies. The bulk of the sediment comprises the shales, siltstones, and very fine sandstones of the Diplocraterion yoyo facies group, which display all gradations from non-life (letal) graded siltstones and shales to intensely bioturbated sandstones, in which the trace-fossil D. yoyo is generally conspicuous. The Reynard facies comprises graded sets of fine to very fine sandstones, the upper portions of which commonly show oscillation ripple or are bioturbated. The sets are intercalated singly in the D. yoyo facies, or form cosets (11 per cent of Baggy Beds). These two facies, which yield a small neritic fauna, are considered to represent deposits of the delta-front platform, or a like off-shore environment, with the cosets of Reynard facies for the most part representing a nearer-shore environment. The other facies (with an interpretation of their depositional environment) are as follows: Arenicolites curvatus facies (6 per cent), which include thin, fine to very fine sandstones with A. curvatus and bioturbated sandstones: sub-beach environment. Rough facies (1 per cent), thin sets of shelly, cross-stratified sandstones with Dolabra , and intraformational conglomerate of local derivation: near-shore, shelly, and possibly intertidal channel fills. Gull facies (1 per cent), cross-stratified fine well-sorted sandstones: near-shore submarine dune depositis. Lingula facies (8 per cent), bioturbated and pene-contemporaneously eroded graded siltstones and shales with patches of disarticulated Lingula and thin bellerophontoid limestones: lagoonal or restricted bay environment, possibly brackish. Timber and Cormorant facies (11⋅5 per cent), channel-fill sediments with parallel and cross-stratified fine to very fine sandstones of northerly provenance with much plant debris: distributary fills. (Cormorant subfacies represents deposition mainly under upper flow regime.) Tag facies (5 per cent), multiple channel-fill sediments deposited under lower flow intensity with fine to very fine sandstones and silt-grade sediments, in which small-scale cross-lamination is important: interpretation doubtful, possibly distributary mouth bar deposits, or crevasse fill or the fill of an off-shore barrier inlet. Hoe facies (1⋅5 per cent), shales and siltstones associated with Timber and Tag facies: deposited under conditions of low flow intensity, or in fresh-water lakes. Plant remains and, rarely, Chondrites? parvus . A model is erected (Fig. 27) suggesting that the sediments were deposited in the vicinity of a delta or similar fluvially influenced environment, subject to a moderate tidal range, though largely on the delta-front platform, or relatively inshore on the continental shelf.

THE HOMOLOGIES OF THE LABYRINTHODONT CENTRUM.

The nature of the vertebrae has long been used to define the major taxa within the Labyrinthodontia and to assign a place to the group within the classification of the Amphibia. The most diagnostic feature is the mode of ossification of the region of the centrum. Early labyrinthodonts had the centrum ossified in two principal and separate regions, an intercentrum (hypocentrum), often an anteroventral crescentic wedge, and a pleurocentrum, lying posterior and often dorsal to the intercentrum. It is now fairly generally agreed that the rhachitomous vertebra represents the primitive labyrinthodont condition (Romer, 1947). In this type a wedge-shaped intercentrum lies in front of laterally paired, dorsal, pleurocentral blocks (e.g. Eryops: Fig. 4B). However, for many years there was little evidence of rhachitomous vertebrae below the Upper Carboniferous and Watson (1912, 1919, 1926, 1929) regarded the embolomerous vertebra, typically associated with the Coal Measure anthracosaurs, as the most primitive type. Description of the vertebrae in some Rhipidistia, the fish group ancestral to the tetrapods, and in the earliest known labyrinthodonts, the Ichthyostegalia (Jarvik, 1952), gives added support to the contention that the rhachitomous vertebra is primitive. Both rhipidistians, such as Eusthenopteron from the Upper Devonian, and the ichthyostegids from the Devonian-Carboniferous boundary had a typical wedgeshaped intercentrum and paired posterodorsal blocks, which may be homologous with the rhachitomous pleurocentrum (Fig. 1, A and B). Romer's views are further confirmed by the report of rhachitomus vertebrae articulated with a skull of the loxommid Megaloce phalus (Baird. 1957). reDresenting a group placed by Watson, in the absence of associated vertebrae, within the Embolomeri. Vertebrae little removed from the rhachitomous type have also recently been described by Romer (1964) in the Lower Carboniferous Pholidogaster, probably to be regarded as a very primitive anthracosaur (Fig. 1C). Romer's (1947) scheme for the evolution of the various types of vertebrae found within the labyrinthodonts is well known and is reproduced in Figure 2. To the types illustrated must be added the unique and aberrant pattern found in the plagiosaurs (Panchen, 1959), whose derivation is quite uncertain (Fig. 6). For many years the theory of the morphology of vertebrae originally proposed by Gadow and Abbott (1895) colored the thinking of all workers in the field and was taken as a point of departure for consideration of vertebral structure in the labyrinthodonts. The theory of Gadow and Abbott, which was extended to apply to the tetrapods by Gadow in 1896, was based on the now untenable assumption that modern elasmobranchs showed the basic scheme for all craniate vertebrae, and specifically that the structure of tetrapod vertebrae could be derived by reductions or loss of elements of the elasmobranch pattern. Like many persistent but erroneous theories, Gadow's proposed a nomenclature which was widely adopted. He assumed that all vertebrae were formed from four pairs of symmetrically arranged cartilaginous elements, termed arcualia, which he named individually. Embryological study of the origin of vertebrae in living Amphibia (see references in Williams, 1959) made it clear that Gadow's theory was of little help here. The ossification of the centrum was perichordal and in

Hymenozonotriletes lepidophytus kedo, its distribution and significance in relation to the Devonian-Carboniferous boundary

Miospore assemblages of Upper Devonian (Upper Famennian and Strunian) age from Western Australia, North Africa, Belgium, Germany, Canada and the U.S.S.R. are examined. Populations of Hymenozonotriletes lepidophytus are described and the synonymous relationships between this and morphologically similar species are established. The stratigraphic distribution and the possibilities of utilising zones based on biometrically defined differences within the populations of H. lepidophytus for local correlation purposes are discussed. The general composition of Upper Devonian and Lower Tournaisian microfloras is briefly reviewed.

Isotopic Age Determinations on Upper Devonian Rocks from Victoria, Australia: A Revised Estimate for the Age of the Devonian-Carboniferous Boundary

A thick sequence of felsic volcanic rocks in the Cerberean Cauldron of Victoria, Australia, is Upper Devonian as evidenced from a fossil fish fauna found in intercalated sedimentary rocks (Hills, 1958). Radiometric dating of the lavas was undertaken to provide further control as to the age of the Devonian-Carboniferous boundary. Potassium-argon dates on biotites from a nevadite and a biotite rhyodacite range from 340 to 365 million years; from this data, we estimate a minimum value of 362 ± 6 m.y. for their true age. Rubidium-strontium dates on several of these biotites are systematically lower at 352–357 m.y. (λRb 87 = 1.47 × 10 −11 yr −1 ), and this difference seems best resolved by decreasing the value selected for λRb 87 to 1.43 × 10 −11 yr −1 . The Rb-Sr isochron for the basal rhyolite in the Cerberean Cauldron, based on whole-rock and feldspar samples, is significantly older at 367 ± 22 m.y. than the Rb–Sr biotite dates and older than an (apparent) isochron for the nevadite. Its relationship to the K–Ar date of 362 m.y. is obscured by the uncertainty in the Rb 87 decay constant. We conclude that either the extrusion of the lavas occurred over an unexpectedly long period or that loss of radiogenic strontium has occurred from the biotites and that the nevadite Rb–Sr data do not correctly define an isochron. In either case, these results show that the radiometric age for the top of the Devonian should be increased by 5 per cent from 345 ± 10 m.y. (Kulp, 1961) to 362 ± 6 m.y. and possibly by more than 10 per cent, depending on the correct value for the decay constant for Rb 87 . Rb–Sr data for the Barjarg Granite and the Ryans Creek Rhyolite from the nearby Mansfield-Tatong area yield an age of 369 ± 11 m.y., using λRb 87 = 1.47 × 10 −11 yr −1 . Although these rocks are thought to be the same age as the lavas of the Cerberean Cauldron, their stratigraphic position within the Devonian is not conclusively established.

Devonian–Carboniferous Boundary

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Seance a Perpignan

Abstract Report of general meetings held during the eastern Pyrenees field trip of the Societe Geologique de France in September 1958; includes discussion of some of the problematic geologic features observed during the field trip, particularly the color of Gotlandian (Silurian) schists, the evidence of granitization of the Mont-Louis massif, the Albere paleovolcanic series, the Devonian-Carboniferous boundary, and the metamorphism of the Canigou massif.

Devonian-Carboniferous Boundary in Japan

Devonian-Carboniferous Boundary in Japan

The Relationship of the Pilton Beds of North Devon to their Equivalents on the Continent

Having made a general study of the Devonian-Carboniferous junction in North-West Europe and a more detailed one of special areas, the author discusses here the correlation of the Pilton Beds of North Devonshire and the “K beds” of the Bristol and Forest of Dean areas with the Etroeungtian and other horizons near the Devonian-Carboniferous boundary on the Continent.