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The Carboniferous carbonate rocks cropping out in the NE of the Cantabrian Zone formed part of a microbial boundstone-dominated carbonate platform characterized by high-relief mar- gins and steep depositional slopes. Although the most continuous and best-preserved cross-sections of this carbonate platform, showing its seismic-scale depositional profile, have been intensely studied (Sierra del Cuera and Las Llacerias), other are poorly documented from a sedimentological point of view. This paper describes the San Antolín-La Huelga section, where the excellent exposures and the well-preserved depositional fabrics make it a reference section for the characterization of the lithofaties belts forming this particular depositional system. [ABSTRACT FROM AUTHOR]

Abstract: A Pennsylvanian (Bashkirian–Moscovian) high-rising, microbial boundstone-dominated carbonate platform (>12,000 km2) developed in the external margin of a Variscan foreland basin. Intensely studied in the northern domain (Cuera area), the stratigraphy and sedimentology of the southern domain (the Picos de Europa Province) is here revisited. Distribution of the Bashkirian facies (Valdeteja Formation) defines a ramp/low-angle platform with an internal zone (central and eastern sectors of the Picos de Europa Province) consisting of huge amounts of massive, cement-rich microbial boundstones, surrounded by an external zone comprising lower-slope boundstone-derived breccias and basinal lime mudstones. From the Bashkirian–Moscovian transition to the early Kasimovian (Picos de Europa Formation), the carbonate platform moderately aggraded acquiring a high-rising and flat-topped geometry and then prograded rapidly into the external zone, where platform-derived carbonate sediment and clay, sourced from the orogen, were deposited. Seismic-scale exposures of the western margin of the shelf show angular-bedding relationship between basin/toe-of-slope, slope and platform-top stratal domains; with slope inclination up to 32° and a topographic relief of 550 m. The platform evolution above described for the Picos de Europa Province significantly differs with respect to the Cuera area, where platform mostly prograded during the Bashkirian and subsequently aggraded during early Moscovian. The evolutionary differences between these two carbonate domains, which are geographically close, are interpreted as changes in tectonic subsidence distribution, acting as the major factor controlling the large-scale platform geometries in the active foreland basin context. Finally, the tectonic overload due to the emplacement of the Cuera thrust sheets during the Moscovian–Kasimovian transition changed the platform growth style from progradational to aggradational and consequently the stacking pattern of the platform-top cycles in the Picos de Europa Province. [Copyright &y& Elsevier]

The depositional slope of the Sierra del Cuera, a Carboniferous (Bashkirian–Moscovian), high, steep-margined carbonate platform, provides excellent cross-sections of lithofacies zonations and associated stratal geometries. The steep (30–40°) and nearly planar upper slope is dominated by massive sheet-like layers of microbial, cement-rich boundstone, which alternate with intercalations of red-stained bryozoan cementstone with crinoids and brachiopods. The slightly gentler (20–26°) and concave-upward lower slope is characterized by clast-supported resedimented deposits. The upper slope extended from platform break to ca. 300 m water depth, whereas lower slope sediments were deposited in water depths up to 600–700 m, at which level the slope beds flatten to a few degrees (toe-of-slope) and interfinger with spiculitic and argillaceous basinal sediments. Between 250 and 450 m water depth, boundstones and breccias alternated in a transitional zone. The lower slope sediments include clast-supported breccias with radiaxial fibrous cement in interparticle space, mud- to clast-supported breccias with red-stained carbonate mud matrix and packstone to grainstone and rudstone beds. Most of the clasts comprise boundstone reworked from an upper slope setting and smaller grains are platform and slope derived. A pervasive submarine cementation occurs along the upper two-thirds of the flank and this stabilized the slope.Slope deposition is interpreted as follows. During active boundstone accretion, microbial boundstone layers slid off and formed breccia tongues extending from the lower upper slope down to the toe-of-slope. Rock falls and avalanches were generated whenever the shear strength of the substrate of loose (or partly lithified) sediment was exceeded. Upper-slope boundstone accretion and shedding, independent of the depth of light penetration, controlled most of the depositional processes on the slope. Cement-dominated intervals are considered to be related to early highstand (and/or flooding) phases. Relative sea-level fluctuations and/or associated changes in the water conditions are believed to be responsible for intervals of low boundstone production or cement precipitation. Whether the in situ boundstone and breccia are preferentially related to lowstand or highstand periods is, as yet, unclear. [Copyright &y& Elsevier]

A steep-margined carbonate platform is developed in the Carboniferous synorogenic foreland basin of northern Spain. Dips of 60–90° produced during Late Carboniferous thrusting enable cross-sections of a 4-km-wide portion of the marginal area of this platform (Las Llacerias outcrop) to be studied in aerial photographs at a seismic scale. Three stratal domains are observed: (1) a horizontal-bedded platform; (2) a clinoformal-bedded margin with a relief of up to 500 m; and (3) a low-angle toe-of-slope, where slope beds interfinger with basin sediments. The slope shows well-bedded sigmoidal clinoforms with depositional dips ranging from 15° to 32°. Based on lithology and stratal patterns, four facies groups have been recognized: (1) a flat-topped platform, in which thick algal boundstone, skeletal packstone–grainstone and peloidal micrite wackestone with a poorly rhythmic character prevail; (2) the platform margin and upper slope, characterized by microbial boundstone spanning a bathymetric range of ≈150 m measured from the break of slope; (3) a slope, predominantly composed of margin-derived rudstones and breccias; and (4) a toe-of-slope to basin zone, where a cyclic alternation of spiculitic siltstones, packstone to grainstone calciturbidites and rudstone/breccia is visible. Five successive stages of platform development are deduced: (1) Bashkirian: flooding of the pre-existing Serpukhovian platform giving rise to the nucleation of a low-angle ramp to the south-east of the study area with microbial mud-mound accumulations, and breccias and calciturbidites on the margins; (2) Early Moscovian: an influx of siliciclastic sediment buried part of the platform and reduced the area of carbonate sedimentation; (3) Moscovian: aggradation and progradation of the carbonate system produced an extensive steep-margined and flat-topped shallow-water platform (shelf system); (4) Latest Moscovian–earliest Kasimovian: drowning of the platform; and (5) Kasimovian: covering of the platform by marly calcareous ramp sediments. [ABSTRACT FROM AUTHOR]