Depositional facies and stratal geometry of an Upper Carboniferous prograding and aggrading high-relief carbonate platform (Cantabrian Mountains, N 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 outer-platform 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 12degrees to 2degrees. Steep clinoforms with dips of 20-28degrees 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 6degrees, consistent with lithofacies deepening seaward. The basinward dip is attributed to the downward shift of upper-slope 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 30-45degrees 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.