Content uploaded by Jenaro L. García-Alcalde
Author content
All content in this area was uploaded by Jenaro L. García-Alcalde on Apr 09, 2016
Content may be subject to copyright.
... The stratigraphical sequences of the Northern Spanish Devonian have been well documented (e.g. Barrois, 1882;Comte, 1959;García-Alcalde et al., 2002;García-López, 2002) and are well age-constrained using macrofossil and conodont evidence (e.g. García-López and Bastida, 2002) (Fig. 2). ...
... The sedimentary sequence represents a near continuous sequence through the Devonian and has been well documented regarding stratigraphical correlation, sedimentology, palaeontology and biostratigraphy (e.g. García-Alcalde et al., 2002;Gutiérrez-Marco et al., 2019). The sediments are entirely marine in origin and exhibit a broad alternation between calcareous and clastic deposition. ...
Dispersed spore assemblages are described from the Early Devonian (late Lochkovian-late Emsian) Rañeces Group (Asturias) and equivalent La Vid Group (Leon) of Northern Spain. The lower part of both groups consists predominantly of limestone and yields only impoverished palynological assemblages. However, the upper parts contain more clastic sediments and yield rich assemblages of well preserved palynomorphs. These are dominated by marine forms but also include abundant spores. Sixty-two spore taxa are reported from the Valporquero Formation (Leon). These all belong to a single assemblage that can be equated with the lindlarensis-sextantii Assemblage Zone (mid-early late Emsian age) erected in northern Gondwana. The sequences are independently age constrained by conodont and invertebrate macrofossil evidence and the ages derived from these studies are concordant with the assigned spore zones. The Spanish spore assemblages are from deposits that accumulated around the Armorican Terrane Assemblage (ATA), a group of islands that lay between the continents of Laurussia and Gondwana. Jacaard Index analysis indicates that the Valporquero Formation spore assemblage is endemic, and not closely related to those previously described from Laurussia or Gondwana, although it has more in common with the latter. This suggests that the Armorican Terrane Assemblage was widely separated from both Laurussia and Gondwana, but was located closer to and had more connectivity with Northern Gondwana.
... Consequently, detailed description and reference to the original studies are here avoided, since it overpasses the subject of this synthesis. It may be found in extended and recent publications (Dégardin 1988;Sanz-López 1995, 2002a, b, 2004Dégardin et al. 1996;Delvolvé et al. 1996;García-López et al. 1996;Majesté-Menjoulas et al. 1996;García-Alcalde et al. 2002). Sanz-López (2002a, 2004 described several sedimentary domains modifying the previous Devonian facies-area defined in the Pyrenean Axial zone (Mirouse 1966;Mey , 1968Boersma 1973;Zwart 1979) and the Basque massifs (Heddebaut 1975) (Fig. 8.16). ...
... A drowning event was recognized at the top of the Villech Fomation (upper Emsian) and followed by the progradation of resedimented carbonate of the Comabella Formation in the ECd (Sanz-López 1995;Montesinos and Sanz López 1999). Late Emsian to basal Eifelian distal siliciclastic wedges arrived to the Western Pyrenees (Socotor and Acherito Beds in the Sd and Fd) and limestone beds yielded brachiopods from the Emsian-Eifelian interval (Requadt 1974;Juch and Schafer 1974;García-Alcalde et al. 2002). Distal siliciclastic deposits were sedimented in the deepest and subsiding areas of the Central Pyrenees (Vilaller Formation in the Bd and SNd; Boneu and Entecada formations in the Cd). ...
Diverse microphytoplankton assemblages, including 72 taxa belonging to 27 genera of acritarchs and prasinophyte phycomata, have been recovered from Middle Devonian rocks in northern Spain, revealing an endemic flora dissimilar to coeval assemblages. These deposits are of early Givetian age and consist of the laterally equivalent Naranco, Huergas and Gustalapiedra formations of Asturias, León and Palencia provinces. At the time, Iberia was part of the Armorican Terrane Assemblage, a comparatively isolated island chain positioned between Laurussia and Gondwana. The studied formations represent a marine transect across a nearshore–offshore gradient and consist of a large clastic unit sandwiched between extensive carbonate deposits. This clastic unit incorporates the Kačák Event, an important global extinction event associated with marine anoxia. Herein, the suite of generally well-preserved microphytoplankton assemblages is described and their stratigraphical and biogeographical importance are considered. The microphytoplankton represents a single assemblage deposited in a short interval and is interpreted as being endemic. The assemblage is only moderately similar to contemporary assemblages from Laurussia and Gondwana and, although certain characteristic Middle Devonian taxa are present, other common species such as Arkonites bilixus and Tyligmasoma alargada are absent. While no unique taxa are found here, the taxa which are present represent a particular combination of species not seen elsewhere. Certain taxa appear which may have discordant temporal ranges, though no major inferences can be made from them as only two taxa both occur in significant numbers and have a confident identification. This assemblage adds to our knowledge of phytoplankton paleobiogeography, representing a significantly endemic assemblage within the generally cosmopolitan microphytoplankton flora of the Middle Devonian.
Geological evidence, supported by biogeographical data and in accord with palaeomagnetic constraints, indicates that “one ocean” models for the Variscides should be discarded, and confirms, instead, the existence of three Gondwana-derived microcontinents which were involved in the Variscan collision: Avalonia, North Armorica (Franconia and Thuringia subdivided by a failed Vesser Rift), and South Armorica (Central Iberia/Armorica/Bohemia), all divided by small oceans. In addition, parts of south-eastern Europe, including Adria and Apulia, are combined here under the new name of Palaeo-Adria, which was also Peri-Gondwanan in the Early Palaeozoic. Oceanic separations were formed by the break-up of the northern Gondwana margin from the Late Cambrian onwards. Most of the oceans or seaways remained narrow, but – much like the Alpine oceans – gave birth to orogenic belts with HP-UHP metamorphism and extensive allochthons: the Saxo-Thuringian Ocean between North and South Armorica and the Galicia-Moldanubian Ocean between South Armorica and Palaeo-Adria. Only the Rheic Ocean between Avalonia and peri-Gondwana was wide enough to be unambiguously recorded by biogeography and palaeomagnetism, and its north-western arm closed before or during the Emsian in Europe. Ridge subduction under the northernmost part of Armorica in the Emsian created the narrow and short-lived Rheno-Hercynian Ocean. It is that ocean (and not the Rheic) whose opening and closure controlled the evolution of the Rheno-Hercynian foldbelt in south-west Iberia, south-west England, Germany, and Moravia (Czech Republic). Devonian magmatism and sedimentation set within belts of Early Variscan deformation and metamorphism are probably strike-slip-related. The first arrival of flysch on the forelands and/or the age of deformation of foreland sequences constrains the sequential closure of the Variscan seaways (Galicia-Moldanubian in the Givetian; Saxo-Thuringian in the Early Famennian; Rheno-Hercynian in the Tournaisian). Additional Mid- to Late Devonian and (partly) Early Carboniferous magmatism and extension in the Rheno-Hercynian, Saxo-Thuringian and Galicia-Moldanubian basins overlapped with Variscan geodynamics as strictly defined. The Early Carboniferous episode was the start of episodic anorogenic heating which lasted until the Permian and probably relates to Tethys rifting.
The formation and destruction of supercontinents requires prolonged convergent tectonics between particular plates, followed by intra-continental extension during subsequent breakup stages. A specific feature of the Late Paleozoic supercontinent Pangea is the prolonged and diachronous formation of the collisional belts of the Rheic suture zone coeval with recurrent continental breakup and subsequent formation of the mid-ocean ridge systems of the Paleo- and Neo-Tethys oceans at the Devonian and Permian margins of the Gondwana plate, respectively. To decide whether these processes are causally related or not, it is necessary to accurately reconstruct the plate motion of Gondwana relative to Laurussia. Here we propose that the strain pattern preserved in the continental crust can be used for the reconstruction of ancient plate kinematics. We present Euler pole locations for the three fundamental stages of the Late Paleozoic assembly of Pangea and closure of the Rheic Ocean: (I) Early Devonian (ca. 400 Ma) collisional tectonics affected Gondwana at the Armorican Spur north of western Africa and at the promontory of the South China block/Australia of eastern Gondwana, resulting in the Variscan and the Qinling orogenies, respectively. The Euler pole of the rotational axis between Gondwana and Laurussia is positioned east of Gondwana close to Australia. (II) Continued subduction of the western Rheic Ocean initiates the clockwise rotation of Gondwana that is responsible for the separation of the South China block from Gondwana and the opening of Paleo-Tethys during the Late Devonian. The position of the rotational axis north of Africa reveals a shift of the Euler pole to the west. (III) The terminal closure of the Rheic Ocean resulted in the final tectonics of the Alleghanides, the Mauritanides and the Ouachita–Sonora–Marathon belt, occurred after the cessation of the Variscan orogeny in Central Europe, and is coeval with the formation of the Central European Extensional Province and the opening of Neo-Tethys at ca. 300 Ma. The Euler pole for the final closure of the Rheic Ocean is positioned near Oslo (Laurussia). Thus, the concomitant formation of convergent and divergent plate boundaries during the assembly of Pangea is due to the relocation of the particular rotational axis. From a geodynamic point of view, coupled 45 collisional (western Pangea) and extensional tectonics (eastern Pangea) due to plate tectonic reorganization is fully explained by slab pull and ridge push forces.
The Basal Choteč or jugleri Event, close above the Lower–Middle Devonian boundary, has been regarded as a minor but important eustatic transgressive event, which is characterized by significant environmental changes, faunal extinction, appearance of new forms, and maximum radiation, particularly among the goniatites. This study contributes to a more precise stratigraphic allocation of the event, and provides a reconstruction of paleoenvironmental settings in the type area of the event, the Prague Basin (Czech Republic). The beginning of a transgression is recorded already in the Třebotov Limestone (partitus Zone, Eifelian, early Middle Devonian). The basin-wide change in the sedimentation pattern (onset of peloidal and crinoidal grainstones (calciturbidites) of the Choteč Formation) corresponding to the uppermost partitus and costatus conodont zones, base of Nowakia (Dmitriella) sulcata
sulcata dacryoconarid Zone, and Pinacites
jugleri goniatite Zone is interpreted here to be linked to a maximum flooding of the basin. A hypothesis of enhanced nutrient load during sedimentation of the Choteč Formation is suggested here as a triggering mechanism for intense micritization and peloid formation and prasinophyte blooms, which could be, along with a greater depositional depth, responsible for oxygen deficiency and consequent reduction of diversity and habitat tracking among benthic invertebrates.
The ability to correlate with high precision among geological successions is critical in evaluating the global coincidence and therefore magnitude and duration of geological events. This is a difficult problem given resolution uncertainties and ambiguities in biostratigraphic data sets. The magnetostratigraphy susceptibility (MSS) method, based on low-field bulk magnetic susceptibility (MS) measurements of closely spaced samples, provides a well-established, abiotic, high-resolution correlation tool in stratigraphy, that when combined with bio-, chemostratigraphic, and other geophysical techniques, offers ways to resolve correlation problems. In addition, such data sets lend themselves to time-series analysis, where developing high-resolution timing of events becomes possible. This paper examines the final episode (equivalent to the Geneseo Bioevent in the Appalachian Basin) of the important Taghanic Biocrisis known from globally distributed Middle Devonian geological sections. The Geneseo Bioevent is important because the extinction of some major fossil groups occurred during this time, while other groups declined significantly, including many benthonic groups. However, the maxima of extinctions does not appear to have occurred everywhere simultaneously. To help resolve timing of this bioevent, results are reported for two Middle Devonian successions from Northern Spain, the S. Huergas de Gordón and Punta Boletos sections, both showing the final Taghanic Biocrisis, Geneseo interval, and correlate these successions to the same interval within an independent, global database. Time-series analyses of the MSS data sets from the Spanish sections was performed using Multi-Taper (MTM) and Fourier Transform (FT) methods, and have extracted well-defined Milankovitch bands from the primary MS data from each section. Using FT and MTM values for eccentricity at ~ 400 kyr and ~ 100 kyr, a floating-point time scale was developed for the two Spanish sections that provides age resolution to ~ 50 kyr for these successions. The results show that the Geneseo Bioevent in Northern Spain occurred within a narrow time window estimated to have lasted for ~ 70 kyr. This time window falls within the Polygnathus ansatus (uppermost part)–Ozarkodina semialternans conodont zones, just at the Middle–Upper Givetian boundary of the Middle Devonian in what appears to be the upper, regressive phase of T–R cycle IIa.
In the Cantabrian Mountains (NW Spain) a strong difference exists between Emsian articulate brachiopod species diversity compared to that of other remaining Devonian stages (169 Emsian species versus 210 species for the rest of the Devonian). Historical, paleogeographic and eustatic reasons are presented that could in part explain this numerical asymmetry. The Emsian stratigraphy of the Asturo-Leonian domain Emsian is briefly discussed and related to both global bio-events and eustatic and magnetosusceptibility events. Spiriferids are the most diversified Emsian brachiopod group (44 species). Despite this fact only nine of these spiriferid species have adequately been described and/or figured in recent times. This paper focuses on the Asturo-Leonian domain upper Emsian Spinelloidea and Cyrtospiriferoidea. Nine species (five new) are described and/or figured belonging to six genera (four new). The meaning of the term “crural plates” and its diagnostic value in the taxonomic characterization of the Family Spinellidae is discussed.
This paper presents a review and critical analysis of the literature on Devonian floras of the Iberian Peninsula. Although the known outcrops of Devonian strata in the Iberian Peninsula are marine, in some cases, a few fragmentary remains of vascular plants are associated with faunal remains. Records include largely specimens from the Lower Devonian of Barrancos (Alentejo, Portugal) and the Upper Devonian of Sierra de Hornachos in Badajoz province, southwest Spain; the remainder consists of drifted plant fragments from scattered sites in the Iberian Peninsula ranging in age from Lochkovian to Upper Devonian–Earliest Carboniferous. The vegetation inferred for the Lower Devonian of the Iberian Peninsula is mainly based on palynological data and corresponds to herbaceous types characterised by bryophytes, rhyniophytes (Horneophyton, Cooksonia, Rhynia), trimerophytes (Psilophyton, Pertica and Hostinella), primitive lycophytes (Drepanophycus) and incertae sedis such as Nothia and Chaleuria, all flora that developed near the coast in low-lying and, at least periodically, wet areas. In the Middle Devonian, two vegetation strata can be recognised: herbaceous (Psilophyton) and semi-arboreal (Cladoxylales). Although three levels of vegetation in the Upper Devonian, have been described from outcrops worldwide, the scarce available data from the Iberian Peninsula only indicate an arborescent lycopsid vegetation and species with uncertain botanical affinity such as Sphenopteridium keilhauii Nathorst.
ResearchGate has not been able to resolve any references for this publication.