Palaeobiogeographic affinities of the reef faunas from the earliest Pragian in the Cantabrian Zone (NW Spain)

Devonian reef faunas in the Cantabrian Zone are well known through several works done by the Research Group on Devonian Reefs from Oviedo University. This Group has established up to seven Devonian reef episodes of different magnitude and some of them were widely studied (Méndez-Bedia et al., 1994, Fernández et al., 1997, among others). The first episode was developed during the earliest Pragian and is recorded in some thin-bedded biostromal limestones with a rich fauna of corals and stromatoporoids. This interval has been studied within the context of the project CGL2005-03715/BTE and the results are currently in press. The aim of this work is to display the palaeobiogeographical affinities shown by the corals and stromatoporoids occurring in this first reef episode

Earliest Pragian (Early Devonian) corals and stromatoporoids from reefal settings in the Cantabrian Zone (N Spain)

The oldest reefal episode in the Cantabrian Zone (earliest Pragian) consists of small biostromal patch reefs, mainly built by corals and stromatoporoids, and developed on a storm-dominated ramp. Four outcrops provide the stratigraphic framework in which these reef facies developed, and these permitted an interpretation of their depositional setting in terms of a relatively distal or protected shelf. We systematically describe three species of rugose corals, five species of tabulate corals, and six species of stromatoporoids. This fauna is allocated to three Pragian fossil associations. Association 1 is mainly composed of massive tabulate corals and stromatoporoids. Association 2 contains dominant branching rugose and tabulate corals. Finally, association 3 is represented by tiny massive tabulate corals. Each association occurs at a specific location within a framework of high-frequency deepening upward cycles, being related to a specific depositional setting. This mode of occurrence suggests that their development was tuned by relative base-level oscillations, forming during rises that took the sea-bottom to relatively deep or sheltered conditions, with rare reworking by storm-related currents. Finally, a comparison of this reefal fauna with examples of similar age from elsewhere is presented in order to explore their affinities

Earliest Pragian (Early Devonian) corals and stromatoporoids from reefal settings in the Cantabrian Zone (N Spain)

p. 301-323The oldest reefal episode in the Cantabrian Zone (earliest Pragian) consists of small biostromal patch reefs, mainly built by corals and stromatoporoids, and developed on a storm-dominated ramp. Four outcrops provide the stratigraphic framework in which these reef facies developed, and these permitted an interpretation of their depositional setting in terms of a relatively distal or protected shelf. We systematically describe three species of rugose corals, five species of tabulate corals, and six species of stromatoporoids. This fauna is allocated to three Pragian fossil associations. Association 1 is mainly composed of massive tabulate corals and stromatoporoids. Association 2 contains dominant branching rugose and tabulate corals. Finally, association 3 is represented by tiny massive tabulate corals. Each association occurs at a specific location within a framework of high-frequency deepening upward cycles, being related to a specific depositional setting. This mode of occurrence suggests that their development was tuned by relative base-level oscillations, forming during rises that took the sea-bottom to relatively deep or sheltered conditions, with rare reworking by storm-related currents. Finally, a comparison of this reefal fauna with examples of similar age from elsewhere is presented in order to explore their affinities.S

First record of chambered hexactinellid sponges from the Palaeozoic

p. 129-130Most chambered sponges (the polyphyletic group of "Sphinctozoa") are hypercalcified types and most of them probably belong to the Demospongia. "Spinctozoa" occur from the Cambrian to the Recent and are the most abundant sponges in Late Palaeozoic and Triassic reefs and shallow water limestones. Among hexactinellid sponges, chambered forms are very rare including taxa only from the Late Jurassic and the Late Triassic of Europe, Russia, Tadjikistan, Iran or China. There are five genera described Casearia Quenstedt, Caucasocoelia Boiko, Dracolychnos Wu & Xiao, Pseudo-verticillites Boiko and Innaecoelia Boiko, the latter of which is synomised with Casearia by most authors.S

Late Palaeozoic lithostratigraphy of the Andean Precordillera revisited (San Juan Province, Argentina)

New data on some neopalaeozoic lithostratigraphic units of the central sector of the Argentinean Precordillera are presented. The western Precordillera displays occasional outcrops of siliciclastic rocks of the Late Palaeozoic. These rocks are separated by two main unconformities related to the development of the Chanic (Upper Devonian-early Carboniferous) and Gondwanan (late Carboniferous-early Permian) orogens. A major unconformity marks the beginning of the Andean cycle in middle Permian times. The Del Ratón Formation (800 m) in the studied area is made up of conglomerates and can be divided into two units. The basal part (Lower Member) was deposited in a fjord environment. The Upper Member was generated by an alluvial system that was mainly dominated by upper flow-regime plane beds, whereas its uppermost part accumulated in a fan delta setting. The clast provenance and the main palaeocurrents suggest the existence of a significant topographic high to the W and NW of the present outcrops. The El Planchón Formation (1400 m) is mainly composed of shales and sandstones as a result of sedimentation in a deep-sea fan environment. These lutite-dominated materials graded laterally northwards into conglomerates in a glacial setting. The Del Ratón and El Planchón formations of the Lower Carboniferous correspond to the synorogenic Chanic sedimentation (Chanic foreland basin). Thereafter, the stratigraphic succession underwent deformation which led to its prolonged subaerial exposure and rubefaction. In Permian times, a marine transgression gave rise to the Del Salto Formation (600 m), which is constituted by beach deposits with some aeolian reworking and which lies unconformably over the Lower Carboniferous rocks. Conglomerates (Quebrada del Alumbre Formation, 100 m) produced by an alluvial system prograded episodically into coastal areas. The Escombrera Formation (350 m) is composed of beach deposits prograding seawards. This formation probably represents the end of the Palaeozoic sedimentary record. Thereafter, a playa-lake and an alluvial fan system were generated (Quebrada de la Arena Formation). This unit (750 m) was the last infill of the Gondwanan retroarc foreland, the total thickness of which was about 4000 m.Centro de Investigaciones Geológica

Revisión bioestratigráfica de las pizarras del Ordovícico Medio en el noroeste de España (zonas Cantábrica, Asturoccidental-leonesa y Centroibérica septentrional)

A complete review of more than one hundred Middle Ordovician fossil localities, distributed within the north western part of the Hesperian Massif, shows that the sedimentation of the dark shales (Luarca Fo rmation and equivalents) that overlie Arenig quartzites with Cruziana (the 'Armorican Quartzite' facies) was not as uniform as it has been supposed for the whole NW Spain. These shales were mainly deposited during the Oretanian in the West Asturian-Leonese Zone and in nort h e rn Central-Iberian Zone (Domain of the Ollo de Sapo Antiform). In these zones, the top of the unit is close to the Oretanian/Dobrotivian boundary, without any fossils with proved Dobrotivian age. In the Cantabrian Zone, clay sedimentation started in the latest Oretanian and continued during the Dobrotivian. Several local or regional stratigraphic gaps are proposed and characterized for the whole study area. In addition, the most recent paleog e ographical reconstructions proposed are discussed, in accordance with new paleoecological and paleobiog e ographical data. These data indicate that sedimentation took place in open shelf areas, relative ly deeper than in the southern Central Iberian shelf, and with trough areas where some mesopelagic elements are recorded. We identified a total of 97 different fossil taxa (67 from Oretanian rocks and 45 from Dobrotivian rocks), remarkable among which are the first known appearance of certain trilobites and ostracodes, the latest record of other taxa, and also the presence of some taxa in common with Avalonia and Baltica, that were previously unknown from any area of SW Europe

Las sucesiones estratigráficas del Paleozoico inferior y medio

13 páginas, 20 páginas.-- Editor: García Cortés, Angel.-- Capítulo 2.Peer reviewe

Late Palaeozoic lithostratigraphy of the Andean Precordillera revisited (San Juan Province, Argentina)

New data on some neopalaeozoic lithostratigraphic units of the central sector of the Argentinean Precordillera are presented. The western Precordillera displays occasional outcrops of siliciclastic rocks of the Late Palaeozoic. These rocks are separated by two main unconformities related to the development of the Chanic (Upper Devonian-early Carboniferous) and Gondwanan (late Carboniferous-early Permian) orogens. A major unconformity marks the beginning of the Andean cycle in middle Permian times. The Del Ratón Formation (800 m) in the studied area is made up of conglomerates and can be divided into two units. The basal part (Lower Member) was deposited in a fjord environment. The Upper Member was generated by an alluvial system that was mainly dominated by upper flow-regime plane beds, whereas its uppermost part accumulated in a fan delta setting. The clast provenance and the main palaeocurrents suggest the existence of a significant topographic high to the W and NW of the present outcrops. The El Planchón Formation (1400 m) is mainly composed of shales and sandstones as a result of sedimentation in a deep-sea fan environment. These lutite-dominated materials graded laterally northwards into conglomerates in a glacial setting. The Del Ratón and El Planchón formations of the Lower Carboniferous correspond to the synorogenic Chanic sedimentation (Chanic foreland basin). Thereafter, the stratigraphic succession underwent deformation which led to its prolonged subaerial exposure and rubefaction. In Permian times, a marine transgression gave rise to the Del Salto Formation (600 m), which is constituted by beach deposits with some aeolian reworking and which lies unconformably over the Lower Carboniferous rocks. Conglomerates (Quebrada del Alumbre Formation, 100 m) produced by an alluvial system prograded episodically into coastal areas. The Escombrera Formation (350 m) is composed of beach deposits prograding seawards. This formation probably represents the end of the Palaeozoic sedimentary record. Thereafter, a playa-lake and an alluvial fan system were generated (Quebrada de la Arena Formation). This unit (750 m) was the last infill of the Gondwanan retroarc foreland, the total thickness of which was about 4000 m.Centro de Investigaciones Geológica

Microfábricas pedogénicas y de exposición subaérea en el sistema volcánico y lacustre de la formación san ignacio (Carbonífero Superior-Pérmico Inferior), Cordillera Frontal, Argentina

In the Argentinian Andes (Frontal Cordillera) the upper part of the late Carboniferous-early Permian San Ignacio Formation is made up of lacustrine-palustrine microbial carbonates and interbedded volcanic deposits. In this lacustrine-palustrine environment a natural monospecific forest was developed. The deposits of this sedimentary-volcanic succession were repeatedly subjected to subaerial exposure and modified by pedogenesis to varying degrees giving rise to paleosoils development. Diagenetic microfabrics were well preserved in the carbonates and volcanic rocks. The carbonate microfabrics comprise a wide spectrum of features consisting of root marks and stumps-related structures (rhizoliths, alveolar texture, tunnel-like structures and coprolites of arthropods), pisoids, coated grains and pseudomicrokarst, cracking, brecciated and nodular fabrics, and grainification also occur corresponding to different stages in the pedogenic evolution. Meteoric dissolution and cementation processes are observed; examples are well identified by scanning electron microscope showing silica-filled voids in partially dissolved carbonates and growths of inorganic carbonate microcrystals or of microbial origin in voids. Other different types of cements can be seen such as discontinuous carbonate crusts, ribbon spar, cavities with silt infillings and pendant cements. The whole set of these microfabrics are indicative of wetting, desiccation and meteoric conditions (vadose and phreatic). The abundance of plant roots and associated micro-organisms mainly of bacterial origin (micro-rods, short rod-shapes, nano-fibres, filaments and nano-spheres) played an important role in the pedogenic and subaerial diagenetic processes affecting these deposits. The immature character of the paleosoils and absence of calcretes point out to short intervals of subaerial exposure due to oscillating fluctuations in water level, intermittent volcanic supply, tectonic subsidence and oscillating climatic conditions. The whole of the macro and microfabrics reveals that the prevailing weather could correspond to an intermediate between semi-arid to sub-humid, however the alternating wetting and drying conditions in which the fossil forest developed and the abundance and diversity of micro-organisms, suggest a transition to sub-humid climate conditions.En los Andes argentinos (Cordillera Frontal), la parte superior de la Formación San Ignacio (Carbonífero superior-Pérmico inferior) está constituida por carbonatos microbiales de origen lacustre-palustre interestratificados con depósitos volcánicos. En ese ambiente sedimentario lacustre-palustre se desarrolló un bosque natural monoespecífico. Los depósitos carbonáticos y volcánicos de esa sucesión fueron afectados por repetidas exposiciones a condiciones subaéreas y modificados por pedogénesis en diferentes grados. Las microfábricas diagenéticas quedaron bien preservadas en los carbonatos y en las rocas volcánicas. Las microfábricas carbonáticas muestran un amplio espectro de características correspondientes a estructuras de raíces y troncos (rizolitos, texturas alveolares, estructuras tuneliformes y coprolitos de artrópodos), pisolitos, granos con recubrimientos, pseudomicrokarst, fracturación, fábricas brechoides y nodulares así como granificación correspondiente a diversos estadios de evolución pedogénica. Además, se observan diferentes procesos de disolución y cementación meteórica de los depósitos de carbonato. Diversos ejemplos, estudiados mediante microscopio electrónico, muestran rellenos silíceos de cavidades existentes en carbonatos parcialmente disueltos y crecimientos de cristales de calcita de origen inorgánico y microbial. Asimismo, se han identificado otros tipos de cementos como costras carbonáticas, esparita cordoniforme, cavidades con rellenos limosos y cementos colgantes. El conjunto de esas microfábricas indica humectación, desecación y condiciones meteóricas (vadosas y freáticas). La abundancia de raíces de plantas y de microorganismos asociados, principalmente de origen bacteriano (microvarillas, microfibras, filamentos y nanoesferas), han jugado un papel importante en los procesos pedogénicos y diagenéticos que afectaron a estos depósitos. El carácter inmaduro de los paleosuelos y la ausencia de calcreta indica que los periodos de exposición fueron cortos, debido a fluctuaciones del nivel del agua, actividad volcánica intermitente, subsidencia tectónica y unas condiciones climáticas variables. El conjunto de macro y microestructuras indica la prevalencia de un clima intermedio entre semiárido y semihúmedo, aunque las condiciones alternantes de sequedad y humedad en las que se desarrolló el bosque fósil y la abundancia y diversidad de microorganismos sugieren una transición a condiciones climáticas semihúmedas.Fil: Méndez Bedia, Isabel. Universidad de Oviedo; EspañaFil: Gallastegui, Gloria. Instituto Geológico y Minero de España; EspañaFil: Busquets, Pedro. Universidad de Barcelona; EspañaFil: Cesari, Silvia Nelida. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Parque Centenario. Museo Argentino de Ciencias Naturales "Bernardino Rivadavia"; ArgentinaFil: Limarino, Carlos Oscar. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Departamento de Ciencias Geológicas; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; ArgentinaFil: Prats, Eva. Universidad de Barcelona; EspañaFil: Cardó, Raúl. Secretaría de Industria y Minería. Servicio Geológico Minero Argentino; ArgentinaFil: Colombo Piñol, Ferran. Universidad de Barcelona; Españ