A Pennsylvanian microbial boundstone-dominated carbonate shelf in a distal foreland margin (Picos de Europa Province, NW Spain)

Abstract

A Pennsylvanian (Bashkirian–Moscovian) high-rising, microbial boundstone-dominated carbonate platform (> 12,000 km 2) 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.