Juan R. Bahamonde

Origin and Significance of Isotope Shifts in Pennsylvanian Carbonates (Asturias, NW Spain)

ABSTRACT The primary variability of the composition and properties of seawater is much greater in the shallow coastal zones than in the main body of ocean water. An inadequate understanding of this variability, as well as different diagenetic environments, severely limit the interpretation of the stable-isotope record of shoalwater carbonates. In order to investigate this primary and diagenetic variability along a Bashkirian-Moscovian platform-to-basin transect, 13C and 18O analyses have been performed on more than 1000 matrix micrite, carbonate cement, and brachiopod shell samples. In isotope analysis, these different carbonate materials tend to complement each other, inasmuch as they have different advantages and shortcomings. The resulting data reveal spatial trends in 13C and 18O signatures from platform top (lower values) to basin (higher values). In the case of 13C from pristine brachiopods, this trend can be explained by the long residence time (aging) of platform-top water masses. In the case of brachiopod 18O, this variance is interpreted to reflect temperature differences between warm surface and colder bottom water separated by a permanent thermocline at about 150 to 200 m beneath the shelf break. Micrite and marine cement isotopic values from the platform interior were reset (lowered) during pervasive early meteoric diagenesis. In contrast, micrite and marine cement isotopic values from the outer platform, slope, and basin show higher values close to the assumed Pennsylvanian seawater isotopic composition. This implies that isotopic data from shoalwater carbonates (including pristine brachiopod shells) might not necessarily reflect paleoceanographic trends of the open-ocean water masses because of changes in coastal water-mass isotope signature and interaction with early meteoric fluids.

Emplacement of the Cuera and Picos de Europa imbricate system at the core of the Iberian-Armorican arc (Cantabrian zone, north Spain): New precisions concerning the timing of arc closure

The final stages of the Variscan orogeny (mostly Carboniferous) in the Western European Variscan belt involved the development of the Iberian-Armorican arc, which is cored by the Cantabrian zone (NW Iberia). The Cantabrian zone is the foreland of the Western European Variscan belt, and it is interpreted to record the waning stages of the closure of the Rheic Ocean. The distalmost tectonic units within the Cantabrian zone (the Cuera Unit and the Picos de Europa Province) were the last tectonic units emplaced at the core of the Western European Variscan belt orocline. Together, they form an imbricate system and associated wedge-top basins that are key to understanding the development of the orocline. The emplacement of the Cuera Unit and the Picos de Europa Province occurred in the latest Pennsylvanian, between Moscovian and Gzhelian times. New detailed mapping together with stratigraphic, sedimentologic, and biostratigraphic data analysis of syntectonic successions and structural information constrain the timing and evolution of this imbricate system. Our analysis indicates that the thrust sheets were emplaced roughly perpendicular to previous tectonic units of the Cantabrian Variscan foreland fold-and-thrust belt, most probably during the oroclinal bending of the SW European Variscan belt that formed the Iberian-Armorican arc. The N-S–directed imbricate system was characterized by a shallow dip of the topographic surface (α 150 ± 15 km) and complex structure of the orogenic wedge are thought to result from the progressive increase in the dip of the basal decollement during bending of the underlying Gondwana lithosphere and may reflect the closure of the Iberian-Armorican arc.

Growth and demise of Late Carboniferous carbonate platforms in the eastern Cantabrian Zone, Asturias, northwestern Spain

Abstract Geometric relationships of seismic-scale stratal patterns observed on aerial photographs of Late Carboniferous carbonate platforms in the Cuera Region in Asturias, northwestern Spain, allow the identification of two discrete stages of platform development with different styles of deposition. The first phase, represented by the Bashkirian Valdeteja Formation, is characterized by progradation of more than 10 km with steep clinoforms of up to 35°, and slope heights up to 650 m. During the second phase, corresponding to the Moscovian Picos de Europa Formation, the platform aggraded mainly vertically, nearly 850 m. Both phases of platform growth were terminated by major relative sea-level lowstands in conjunction with local filling of the adjacent basins by shales and mudstones that onlap the platform flanks. Calibration with outcrops suggests a subdivision into the following lithofacies, directly related to the depositional profile: (1) platform interior, parallel, well- to thick-bedded skeletal pack- to grainstones; (2) margin, algal- and sponge-dominated bioherms with little internal structure; (3) slope, clinothems of medium- to thick-bedded margin-derived breccia beds; (4) toe-of-slope, alternating grain- and packstones with spiculites; and (5) basin, marly shales, shales, sandstones, and calcareous mudstones. Relative sea-level changes caused by regional tectonic movements are suggested to be responsible for the generation of the two platform phases; sea-level highstands triggered the nucleation and growth of the platforms, whereas major sea-level lowstands in combination with terrigenous infill of the basins terminated the platform growth. In addition, it is proposed that the termination of Late Carboniferous carbonate platforms is caused by the combined effect of a sea-level fall, subaerial exposure and erosion, and, finally, a rapid transgression that drowned and polluted the platform with shales and mudstones. Local variations in the infilling of the basins controlled the variations in the style of progradation. The subsidence rates for the vertically aggrading phase (Picos de Europa Formation) were approximately twice that for the progradational phase (Valdeteja Formation) and may explain the aggradational versus progradational character.

Lithofacies character and architecture across a Pennsylvanian inner-platform transect (Sierra de Cuera, Asturias, Spain)

ABSTRACT Seismic-scale outcrops of lower Pennsylvanian steep-margined carbonate platforms in northern Spain (Sierra de Cuera, Cantabrian Mountains) provide the opportunity to link depositional facies to platform-interior architecture. Lithofacies character, vertical stacking patterns, and spatial distribution of depositional facies and shallowing-upward cycles were investigated to provide a semiquantitative depositional model for inner-platform strata. A 70-m-thick succession of eight shallowing-upward cycles (2.5-15 m thick) was studied across a 2-km-wide transect. Cycles consist of algal bioherms and skeletal packstone (lithofacies association B) deposited in an open marine and sub-wave-base environment, which are locally lateral to crinoid-dominated packstone (lithofacies association D). Lithofacies association B developed on a transgressive, one-meter-thick high-energy coated-grain grainstone (lithofacies association A) and is overlain by open marine to restricted lagoonal deposits (lithofacies association C) indicative of decreased paleo-water depth. Cycle boundaries are marine flooding surfaces occurring at the base of lithofacies A and the top of lithofacies C. Petrographic and outcrop evidence of subaerial exposure is conspicuously lacking. Cycle and lithofacies thickness varies laterally, and all the lithofacies can be either continuous for the 2 km width of investigation or discontinuous, terminating within tens to hundreds of meters. In most of the cycles, the lithofacies succession changes laterally because of lateral facies transitions (between lithofacies B and D) and pinching out of lithofacies A and C. Meter-scale variations in depositional topography are related to the stacked lens-shaped bioherms assigned to lithofacies B and appear to control the thickness and lateral continuity of the overlying facies. Sierra de Cuera strata do not resemble other Pennsylvanian cycles driven by high-frequency, high-amplitude glacio-eustasy, which are commonly characterized by subaerial exposure surfaces developed on subtidal deposits, except for the absence of peritidal facies and the presence of low-relief algal bioherms on the platform top. The necessary accommodation to enhance the growth of bioherms on the platform top was probably due to high-frequency (100-240 ky) moderate-amplitude (nearly 40 m) sea-level fluctuations combined with subsidence rates of 70-140 m/My. Tectonic subsidence might have been responsible for the lack of evidence of subaerial exposure surfaces in the examined Moscovian strata. The studied inner-platform lithofacies character and architecture were controlled by the interplay of high-frequency changes in accommodation and the presence of meter-scale depositional topography attributed to lateral variations of carbonate accumulation.

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

SummaryDonezella 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. ThoughDonezella 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 ofDonezella 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°) to the basin floor. This unique feature allows reliable estimates of paleo-water depth and distance from the platform margin.Donezella specimens are interpreted asin 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 containDonezella and a diverse fossil assemblage of calcareous algae, bryozoans, and foraminifers.Donezellas delicate network appears fortified by thein situ precipitation of peloidal micrite. On the upper slopein situ precipitated peloidal micrite, abundant radial fibrous cement, and fenestellid bryozoans are the major components of the boundstone facies, along withDonezella and a eskeletal community similar to the outer platform one. On the slope,in situ Donezella were observed down to paleowater depths up to 200 m.Sedimentologic, petrographic, and microfacies analysis ofDonezella 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 eitherDonezella 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 ofDonezellas skeleton and its sedimentological occurrences are not exclusively indicative of an affinity with Chlorophyta. Therefore, it is suggested thatDonezella should be considered as amicroproblematicum organism.The data presented in this study contribute to the inter-pretation of comparableDonezella accumulations in carbonate depositional systems where limited outcrop exposures do not allow correct evaluation of the geometry and facies distribution.

ACCUMULATIONS OF FERGANITES (FUSULINACEA) IN SHALLOW TURBIDITE DEPOSITS FROM THE CARBONIFEROUS OF SPAIN

As occurs commonly in upper Kasimovian and lower Gzhelian strata from several parts of Eurasia, the lower part of the Puentelles Formation (Cantabrian Zone, NW Spain) contains high concentrations of reworked Ferganites tests. No other large fusulinacean taxa are recorded in these beds, which are interpreted as shallow turbidite deposits derived from flood-dominated fan-delta and river-delta systems. These alluvial to shelfal lobes developed in a tectonically active setting and were located in the proximal sectors of a marine carbonate ramp, with hyposaline water conditions. In contrast, the upper member of the Puentelles Formation contains more varied fusulinacean assemblages, including Ferganites and other large fusulinaceans. This upper part is characterized by diverse biota, skeletal wackestone deposits with abundant boundstone intervals (algal bafflestones and mud mounds), and represents the gradual backstepping from detrital lobes and re-establishment of normal marine conditions, lacking clastic influx from the hinterland. The Ferganites species occurring in the lower member seem to have lived in near-shore, high-energy environments in which their durable morphology was advantageous. These forms seemingly were well adapted to relatively low salinity conditions, which otherwise greatly restricted faunal diversity. Four Ferganites forms from the lower member of the Puentelles Formation (including a new species, Ferganites martinezi ) are described.

Facies belt of a Carboniferous carbonate platform (San Antolín- La Huelga section, NE Cantabrian Zone, Nothern Spain)

Entre la importante coleccion de fosiles que guarda el Museo del Departamento de Geologia de la Universidad de Oviedo, procedentes principalmente de la cordillera Cantabrica, se destaca en este trabajo el material tipo (holotipo y paratipos) de 55 especies de braquiopodos carboniferos. Se mencionan el holotipo y paratipos de estas especies (7 de ellas son, a su vez, especies tipo de sus respectivos generos, y uno de estos ultimos es el genero tipo de una subfamilia), se indica su procedencia y edad, asi como la sinonimia correspondiente y se figura el holotipo. Se destaca la importancia de esta vertiente ligada a la investigacion y la obligacion que tiene la institucion de conservar, catalogar adecuadamente y poner este material a disposicion de la comunidad cientifica.

The Sierra del Cuera (Pennsylvanian microbial platform margin) in Asturias, north Spain

ABSTRACT Stratal geometry and lithofacies architecture from carbonate outcrops provide critical information for subsurface prediction of rock types, their spatial distribution, and reservoir quality. Such observations guide and support exploration, appraisal, development, and production strategies. The Pennsylvanian Sierra del Cuera (SDC) carbonate platform outcrop of the Cantabrian Mountains in north Spain offers such information through a seismic-scale cross section of a microbial, unrimmed, and high-relief platform to basin transect, deposited during an icehouse period. The SDC outcrops provide alternative platform margin models to those developed from coral–algal-dominated Neogene carbonate systems and serve as direct analogs for supergiant fields such as Tengiz, Kashagan, and Karachaganak in the Pricaspian Basin of Kazakhstan and potentially for other high-relief platforms with microbial margins. Key features of the SDC platform are (1) a flat-topped platform with a rollover into a steep (30°–40°) and high-relief (600–850 m [1960–2790 ft]) depositional slope and (2) the presence of a microbial boundstone factory from the platform break to nearly 300 m (984 ft) paleowater depth on the slope, stabilized by pervasive marine cementation. The SDC represents a nonactualistic type of carbonate platform margin where the microbial carbonate factory on the upper slope controlled the rate of progradation instead of the platform top–sourced sediment input. These features have significant implications for the interpretation of the evolution and controlling factors of carbonate depositional systems in outcrops and in the subsurface.

Constraining the Timing and Amplitude of Early Serpukhovian Glacioeustasy With a Continuous Carbonate Record in Northern Spain

During the Late Paleozoic Ice Age (LPIA, 345–260 Ma), an expansion of ice house conditions at 330 Ma caused a nearly synchronous, global unconformity. Subaerially exposed paleotropical carbonates were dissolved by meteoric waters, mixed with the light terrestrial carbon, and recrystallized with overprinted, diagenetic dC values. In Northern Spain, development of a rapidly subsiding foreland basin kept local sea level relatively high, allowing continuous carbonate deposition to record dC without meteoric overprint. The Spanish sections show a 2& increase in dC that can be modeled as the ocean’s response to the creation of a significant light carbon sink through widespread meteoric diagenesis of marine carbonates during the near-global hiatus. About 15–35 m of sea level fall would have exposed a large enough volume of carbonate to account for the positive excursion in dC of oceanic DIC. Combining the dC data with high resolution biostratigraphy and new ID-TIMS U-Pb zircon ages from interbedded tuffs, we calculate that the depositional hiatus and glacioeustatic fall caused by the early Serpukhovian phase of ice growth lasted for approximately 3.5 My.