Depositional style and tectonostratigraphic evolution of the El Bierzo Tertiary Subbasin (Pyrenean orogen, NW Spain)

El Bierzo Tertiary sub-basin (Oligocene–Miocene, NW Spain) is a small remnant of the western Duero Basin, a nonmarine broken foreland basin developed in front of the Cantabrian Mountains (Pyrenean orogen). The alluvial infill of El Bierzo Tertiary sub-basin consists of a coarsening-upward succession from fluvial (Toral Formation) to alluvial-fan deposits (Las Médulas Formation) and reflects the uplift of the Cantabrian Mountains, in the north, and then of the related Galaico-Leoneses Mountains, in the south. These alluvial deposits show signs of having been laid down mainly by catastrophic flows (flood-dominated systems) and consist of three main depositional elements, namely, flood-plain fines, and lobe and channel conglomerates and sandstones. The vertical stacking patterns of these deposits and their relationships to the Alpine structures permit to unravel the tectonosedimentary evolution of the basin. The alluvial-plain element is the main constituent of a wide unconfined alluvial plain (Toral Formation) during the early stages of basin evolution, whereas the channel and lobe elements form a set of relatively small, laterally confined alluvial fans (Las Médulas Formation) fed first from the north and then from the south. Las Médulas deposits form two superposed units, the lower unit, cut by the Alpine thrusts, shows a progradational character, and the upper unit, which postdates most of the thrusts but not the youngest ones, displays a composite retrogradational trend. This organisation reflects the interplay between thrust emplacement and alluvial-fan sedimentation and suggests that maximum progradation took place during the climax of Alpine deformation

Depositional facies and stratal geometry of an Upper Carboniferous prograding and aggrading high-relief carbonate platform (Cantabrian Mountains, NW Spain)

Seismic-scale continuous exposures of an Upper Carboniferous (Bashkirian–Moscovian) carbonate platform (N Spain) provide detailed information about the lithofacies and stratal geometries (quantified with differential global positioning system measurements) of microbial boundstone-dominated, steep prograding and aggrading platform margins. Progradational and aggradational platform-to-slope transects are characterized by distinct lithological features and stratal patterns that can be applied to the understanding of geometrically comparable, high-relief depositional systems. The Bashkirian is characterized by rapid progradation at rates of 415–970 m My)1. Characteristic outerplatform facies are high-energy grainstones with coated intraclasts, ooids and pisoids, moderate-energy algal-skeletal grainstones to packstones and lower energy algal packstone and boundstone units. The Moscovian aggradational phase is characterized by aggradation rates of 108 m My)1. Coated-grain shoals are less common, whereas crinoidal bars nucleated in well-circulated settings below wave-base. Boundstones form a belt (30–300 m wide) at the platform break and interfinger inwards with massive algal-skeletal wackestones (mud-rich banks). The progradational phase has divergent outer-platform strata with basinward dips of 12 to 2. Steep clinoforms with dips of 20–28 are 650–750 m in relief and possibly sigmoidal to concave in the lower part. The basinward-dipping outer-platform strata might be depositional for less than 6, consistent with lithofacies deepening seaward. The basinward dip is attributed to the downward shift of upperslope boundstone, forced by late highstand and relative sea-level fall, and to compaction-induced differential subsidence during progradation. The aggradational phase is characterized by horizontally layered platform strata. Clinoforms steepen to 30–45 reaching heights of 850 m and are planar to concave. The evolution from progradation to aggradation, at the Bashkirian–Moscovian boundary, is attributed to increased foreland-basin subsidence and decreased boundstone accumulation rates. Progradation was primarily controlled by boundstone growth rather than by highstand shedding from the platform top. Within the major phases, aggradational–progradational increments are produced by third- to fourth-order relative sea-level fluctuations

Microfacies and paleoenvironments of Donezella accumulations across an Upper Carboniferous high-rising carbonate platform (Asturias, NW Spain)

Donezella is a problematic organism that during the mid-Carboniferous (latest Serpukhovian to Moscovian) characterized carbonate depositional systems in Europe, North Africa, Russia, Kazakhstan, and North America. Though Donezella is generally included in the green calcareous algae, it has been attributed to different systematic groups and its classification and paleoecology still remain controversial. This work focuses on the distribution of Donezella across a carbonate platform (Sierra de Cuera) of Upper Carboniferous (lower Bashkirian-lower Moscovian) age located in the Cantabrian Mountains (Asturias, NW Spain). Sierra de Cuera exhibits a well-exposed cross-section from the horizontal platform through a steep slope (30\ub0) to the basin floor. This unique feature allows reliable estimates of paleo-water depth and distance from the platform margin. Donezella specimens are interpreted as in situ when they form a network supporting cement-filled primary cavities and the ramified skeletons are surrounded by micrite coatings, often with a peloidal fabric, or by early marine cement. In the platform interior, Donezella is associated with phylloid algae and occurs in mud-rich low-relief bioherms. Towards the platform margin, massive units of boundstone are characterized by clotted peloidal micrite and radial fibrous cement-filled primary cavities. They contain Donezella and a diverse fossil assemblage of calcareous algae, bryozoans, and foraminifers. Donezella's delicate network appears fortified by the in situ precipitation of peloidal micrite. On the upper slope in situ precipitated peloidal micrite, abundant radial fibrous cement, and fenestellid bryozoans are the major components of the boundstone facies, along with Donezella and a skeletal community similar to the outer platform one. On the slope, in situ Donezella were observed down to paleo-water depths up to 200 m. Sedimentologic, petrographic, and microfacies analysis of Donezella accumulations in the different facies belts of Sierra de Cuera strongly suggest that this problematic organism was able to thrive over a large depth range, in low-energy but also in moderately agitated environments or in settings with temporary increase in current action, and in organic, physical-chemical and oceanographic conditions that enhanced the precipitation of peloidal micrite. The interval of water depth inferred from the well-exposed slope geometry of Sierra de Cuera suggests that either Donezella might not have belonged to the green calcareous algae or, alternatively, this depositional system was influenced by particular paleo-oceanographic conditions that extended the euphotic zone below the average depth. The morphology of Donezella's skeleton and its sedimentological occurrences are not exclusively indicative of an affinity with Chlorophyta. Therefore, it is suggested that Donezella should be considered as a microproblematicum organism. The data presented in this study contribute to the interpretation of comparable Donezella accumulations in carbonate depositional systems where limited outcrop exposures do not allow correct evaluation of the geometry and facies distribution

La Formación Boñar (Cretácico Superior, norte de León): estratigrafía, geoquímica y potencial productor de roca ornamental

[ES] La Formación Boñar (Coniaciense-Campaniense, posiblemente llegando al Maastrichtiense) en una sucesión eminentemente carbonatada con intercalaciones arcillosas y arenosas que supera en algunas zonas los 300 m de espesor. Aflora entre el borde meridional de la Zona Cantábrica y el margen septentrional de los depósitos cenozoicos de la Cuenca del Duero (norte de la provincia de León). Nuevos afloramientos, descubiertos en las inmediaciones de Boñar, han permitido hacer una descripción completa de la misma, en la que se han diferenciado cuatro miembros, que de muro a techo son: Miembro Calcarenítico (M-1), Miembro Arcilloso-Arenoso (M-2), Miembro de Alternancias (M-3) y Miembro Dolomítico (M-4). Su depósito ha sido relacionado con el ascenso generalizado del nivel del mar durante el Cretácico Superior, que inundó los ambientes deposicionales de la Formación Voznuevo, sobre la que descansa. Las calcarenitas de los miembros M-1 y M-3 son rocas usadas en mampostería, mientras que las dolomías del miembro superior (M-4) constituyen la denominada “Piedra de Boñar”. Se trata de micritas dolomíticas (ocasionalmente calizas mudstone) de tonos beige-ocres que localmente pueden presentar unas zonas o manchas irregulares grises, lo que supone una merma en su calidad como roca ornamental. La coloración beige-ocre está relacionada con procesos de oxidación recientes, en relación a circulación de fluidos a través de superficies de estratificación, pequeñas fracturas y diaclasas; mientras que las manchas grises corresponden a áreas que preservan el color de la roca con un bajo grado de oxidación. El estudio de campo realizado, pone de manifiesto recursos de “Piedra de Boñar” de al menos 15 Mm3 y reconoce como áreas más favorables para la ubicación de nuevas explotaciones el flanco norte del sinclinal de Las Bodas y el flanco sur del anticlinal de La Losilla, desde la proximidades de Llamera a Barrillos de Las Arrimadas.[EN] The Boñar Formation (Coniacian-Campanian, possibly reaching the Maastrichtian) is a mainly carbonate succession, some 300 m thick, which includes some shaly, marlstone and sandstone intervals. It crops out between the southern edge of Cantabrian Zone and the northern border of the Cenozoic Duero Basin (northern León province). New outcrops located near the town of Boñar have allowed to make a more complete stratigraphic description of the Formation, in which four lithological units (members) have been distinguished. From base to top, these are: the Calcarenitic Member (M-1), the Argillaceous and Sandy Member (M-2), the Heterolithic Member (M-4) and the Dolomitic Member (M-4). The Boñar Formation deposition has been linked to the global Upper Cretaceous sea level rise, which flooded the underlying fluvial deposits of the Voznuevo Formation. The well indurated calcarenites of the members M-1 and M-3 are used in the building industry as rough stone, whereas some dolostones of the Member M-4 constitute the so-called “Piedra de Boñar”, which has been traditionally used as ornamental stone. The “Piedra de Boñar” is a light beige to yellowish-beige dolomitized micrite, occasionally lime mudstone, which locally displays irregular grey patches, representing an inconvenient as to its use as ornamental rock. The beige hue is due to recent oxidation processes, usually due to the flow of meteoric waters along bedding surfaces, fractures or joints; whereas the grey spots are those zones preserved from oxidation. Fields studies carried out in this work show that the potential resource of the “Piedra de Boñar” are still significant (at leas 15 Mm3). The areas where the “Piedra de Boñar” displays the best conditions for commercial exploitation are the northern limb of the Las Bodas syncline and the southern limb of the La Losilla anticline, between Llamera and Barrillos de las Arrimadas villages.SIEl presente estudio ha sido financiado por la Excma. Diputación Provincial de León, dentro del marco de Ayudas a la Investigación de 1999