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A stratigraphic study of the carbonate deposits from the interval between the uppermost Sinemurian and the base of the Upper Pliensbachian is presented herein. For this purpose, four expanded sections of the Asturian Basin, in Northern Spain, were sampled. The sections are located between Rodiles E section (Villaviciosa municipality) and Playa de Vega section (Ribadesella municipality). The ammonites recorded in over 100 successive levels of this sector enabled the substantial improvement of the biochronostratigraphic scale of reference; making it comparable with those defined for other basins of the Iberian Peninsula and other areas of the western Tethys. The lower boundary of the Pliensbachian is indicated by the first appearance of Gemmellaroceras aff. aenigmaticum (Gemmellaro) below the first appearance of the genus Apoderoceras. The upper boundary is defined by the first record of the Protogrammoceras. (Matteiceras) subgenus which in some levels is associated with Amaltheus stokesi (Sowerby). The boundaries between the zones and subzones were able to be established accurately in most cases. The ammonite assemblages are similar to those established for the NW European Province. Some intervals, however, include species typical of the Mediterranean Province. Among these, the Taylori Subchronozone (Gemmellaroceras) and the transition between the Jamesoni Subchronozone and the Valdani Subchronozone (Tropidoceras) can be highlighted. Additionally, taking into consideration the succession between of ammonite genera and species, 27 horizons were identified, 24 of which correspond to the Lower Pliensbachian, and the other 3 to the uppermost Sinemurian and the lowermost part of the Upper Pliensbachian. Moreover, the identification of 3 of the Lower Pliensbachian horizons were based upon the evolution of the Mediterranean Tropidoceras., CRUE-CSIC, Sociedad Pública de Gestión y Promoción Turística y Cultural del Principado de Asturias, Depto. de Geodinámica, Estratigrafía y Paleontología, Fac. de Ciencias Geológicas, TRUE, pub

Anoxic conditions, recorded as black shale deposits, were widely developed on the seafloor during the Toarcian (Early Jurassic). This event, known as the Toarcian Oceanic Anoxic Event (T-OAE), was characterized by massive release of volcanogenic CO2 from the Karoo-Ferrar large igneous provinces, global warming, sea-level rise, and a second-order mass extinction among marine invertebrates. This event also resulted in perturbations to geochemical cycles, being marked by a negative excursion in isotopic carbon, and marked changes in trace metals. In particular, molybdenum (Mo), which is taken up by organic fraction of sediment in quantity only under euxinic conditions (i.e., anoxic, free H2S), should show elevated total organic carbon (TOC)-normalized concentrations in anoxic facies. However, Mo/TOC ratios declined significantly in T-OAE sediments despite the existence of widespread anoxia. Two hypotheses have been proposed to explain this phenomenon: the ¿global model¿, which proposes a generalized Mo drawdown in the Toarcian ocean; and the ¿local model¿, which argues for hydrographic restriction (interruption of water exchange with the open ocean) and, thus, Mo drawdown in local basins. Within this context, our study compares Mo, TOC, and sedimentological data from twenty sections representing Toarcian oceanic basins globally. A case-by-case analysis of the studied settings clarifies Mo behavior during the T-OAE, revealing that low Mo in some SW Tethyan and Panthalassic sections is due to a well-oxygenated setting (pelagic/hemipelagic and ramp carbonates) inhibiting Mo fixation. On the contrary, several shallow basins in the NW and E Tethys regions (epicontinental intrashelves troughs and barrier/lagoon settings) exhibit high Mo and TOC combined with low Mo/TOC, reflecting accumulation in a euxinic and hydrographically restricted setting. Notably, open-marine Panthalassic settings can exhibit high Mo and TOC contents accumulated under unrestricted and intermittent eu

Despite carbon-cycle perturbations at a global scale during the early Toarcian, the extent of anoxia during the ∼182-Ma Toarcian Oceanic Anoxic Event (T-OAE) remains in debate. A common factor in the development of oceanic anoxia is watermass restriction, which is thought to have been important in the NW European Seaway, but whose influence elsewhere is relatively unstudied. Here, we analyze Mo/TOC (a proxy for watermass restriction) and redox proxies (e.g., C/P) in two sections of the Asturian Basin (northern Iberian Paleomargin), and we integrate these results with data from a suite of global Toarcian sections in order to reassess the relationship of euxinia and local hydrographic restriction during the T-OAE. The Asturian Basin study sections accumulated in oxic to dysoxic waters, punctuated by two episodes of intermittently euxinic bottomwaters or porewaters around the Tenuicostatum/Serpentinum zonal boundary. This area was an unrestricted carbonate ramp during the T-OAE that was not sufficiently reducing for its Mo/TOC ratios (2.3–4.5) to accurately record the degree of watermass restriction. Regionally, Lower Toarcian black shales exhibit elevated Mo and TOC concentrations (21–42 ppm Mo, 12–18% TOC) along with exceptionally low m (e.g., ∼0.3 in the Paris Basin, ∼0.4 in the Cleveland Basin, and ∼ 2.0 in the Qiangtang Basin), providing evidence of highly restricted, euxinic bottomwaters in Northwestern (epicontinental intrashelf troughs) and Eastern (barrier-lagoon) Tethyan settings. In contrast, in other basins across the SW Tethys and Panthalassic oceans, low contents of Mo and TOC (mostly <5 ppm Mo, <3% TOC) indicate unrestricted watermass conditions and at most intermittent bottomwater or porewater euxinia in pelagic to hemipelagic environments, although intermittently euxinic bottomwaters developed also in deep open-marine settings. The results of our analysis support both (1) a major role for hydrographic restriction in modulating the local expression of t

[ES] Las formaciones Tereñes y Lastres, de edad Kimmeridgiense (Jurásico Superior), afloran a lo largo de tres sectores acantilados de la costa centro-oriental de Asturias, denominada “La Costa de los Dinosaurios”. La Formación Tereñes es una sucesión margo-calcárea lumaquélica que representa un ambiente de lagoon de plataforma. La Formación Lastres, superpuesta a la anterior, es dominantemente areniscosa con recurrencias margosas y lumaquélicas y se interpreta como una sucesión de sistemas deltaicos de domino fluvial que desembocan en una cuenca restringida por un umbral tectónico. El presente estudio se centra en las sucesivas facies de abandono del sistema deltaico que constituyen hemiciclos transgresivos; estos a su vez forman parte a menudo de ciclos transgresivos-regresivos de escala métrica. El rasgo más destacable de dichos hemiciclos transgresivos son las acumulaciones lumaquélicas con rasgos tafonómicos muy variados en función de diversos factores, principalmente las diferentes facies del sistema deltaico afectadas por el abandono: canales distributarios principales, canales de desbordamiento, crevasse-splays, barras de Desembocadura, bahías entre distributarios, etc. En aquellos casos con el registro más completo pueden identificarse en dichos hemiciclos de muro a techo las siguientes facies y superficies de discontinuidad: A) Superficie de máxima regresión; B) Facies de abandono incipiente; C) Superficie transgresiva de erosión; D) Facies lumaquélica transgresiva temprana; E) Facies lumaquélica transgresiva tardía y F) Facies lumaquélica de la zona de máxima inundación. Las facies B muestran bioturbación intensa con predominio de hizocorallium irregular y Thalassinoides de relleno areniscoso. La facies D está formada por areniscas y contiene a menudo grandes bivalvos desarticulados (lag transgresivos con Laevitrigonia), algunos cantos blandos y fragmentos vegetales. En la facies E, menos areniscosa y más rica en matriz arcillosa que la anterior, predomin

Se estudia la cronoestratigrafía, biosedimentación y, en particular, la sucesión de los ammonoideos que, con un registro relativamente pobre, se encuentra en varias secciones del Sinemuriense superior (cronozonas Obtusum y Oxynotum) de la costa de Asturias, entre la Playa de Peñarrubia (Gijón) y la Playa de Vega (Ribadesella). Si se exceptúa el límite inferior de la cronozona Obtusum, todas las unidades cronoestratigráficas han podido ser caracterizadas con bastante precisión. En la cronozona Obtusum (subcronozonas Obtusum y Stellare) se suceden en el tiempo siete especies de Asteroceras (A. aff. confusum-A. confusum-A. obtusum-A. cf. margaritoides-A. stellare-A. blakei) y Aegasteroceras (A. sagittarium), que de forma puntual pueden estar asociadas a escasos Epophioceras, Xipheroceras, Promicroceras y Cymbites. La subcronozona Denotatus se caracteriza por la sucesión de tres especies de Eparietites (E. undaries-E. impendens-E. glaber) asociadas a raros Promicroceras. En la cronozona Oxynotum se registran Palaeoechioceras, Gagaticeras (G. cf. exortum-G. gagateum) y Oxynoticeras (O. simpsoni) en la subcronozona Simpsoni y Oxynoticeras (O. oxynotum) asociado a Cheltonia (C. accipitris) en la subcronozona Oxynotum. En la parte media y superior de esta subcronozona, se registran asimismo escasos Paroxynoticeras y escasos Bifericeras y Gleviceras. En el aspecto paleobiogeográfico, la sucesión obtenida es típica de la Provincia del NO Europeo con la inclusión de algunos elementos más propios de la Provincia Mediterránea en la parte media y superior de la cronozona Obtusum (Asteroceras cf. suevicum, E. glaber) y en la parte media de la cronozona Oxynotum (Paroxynoticeras salisburgense)., Ministerio de Economía y Competitividad (MINECO), Universidad Complutense de Madrid, Sociedad Pública de Gestión y Promoción Turística y Cultural del Principado de Asturias, Depto. de Geodinámica, Estratigrafía y Paleontología, Fac. de Ciencias Geológicas, TRUE, pub

Detailed bed-by-bed ichnological and sedimentological analysis of the early Toarcian Oceanic Anoxic Event (T-OAE) in two sections from the Asturian Basin – Rodiles and Lastres – reveals changes in the macrobenthic tracemaker community mainly induced by oxygen fluctuations. Ichnological analysis enabled us to document an abundant and diverse trace fossil assemblage, with ten ichnogenera, probably corresponding to alternations between the Zoophycos ichnofacies and the distal expression of the Glossifungites ichnofacies: Arenicolites, Chondrites, Diplocraterion, Halimedides, Palaeophycus, Planolites, Rhizocorallium, Thalassinoides, Trichichnus and Zoophycos. Six ichnofabrics were differentiated: laminated?, Diplocraterion–Thalassinoides, Chondrites-mottled, Chondrites–Palaeophycus–Planolites, Chondrites–Halimedides–Planolites and Chondrites-laminated. Ichnological features (including ichnodiversity, distribution of traces and cross-cutting relationships) allowed us to characterise a well-developed multi-tiered macrobenthic tracemaker community, as well as distinguish different tiering models associated with variations in bottom- and pore-water oxygenation. Anoxic conditions associated with the T-OAE corresponded mainly, but not exclusively, to black shale facies. Minor-order oxygen fluctuations from suboxic/anoxic to disoxic/oxic conditions developed during the middle part of the T-OAE. Laminated intra-T-OAE dark and light grey marls show lithological features similar to those of bioturbated pre- and post-event marls; thus, the ichnological information provides evidence that the T-OAE had a larger effect on biota than on depositional features.

Recent ichnological analysis conducted in two sections (Rodiles and Lastres) of the Asturian Basin revealed the presence of Halimedides Lorenz von Liburnau 1902, which occurs just above the black shales related to the end of the Toarcian Oceanic Anoxic Event (T-OAE). Halimedides is associated with recovery of the trace-maker community after the re-establishment of favourable, oxic, conditions. The appearance of Hal- imedides after the T-OAE event, previously not registered, supports the close relationship of the trace maker with oxygen conditions, as occurs in other anoxic events including the Cretaceous OAE-1a and OAE-2. Also, a relationship between morphometric and palaeoenvironmental parameters is observed: occurring larger and densely chambered specimens in darker, weakly oxygenated facies, while smaller and sparsely chambered forms are registered in lighter, better oxygenated sediments.