Pennsylvanian carbonate platforms adjacent to deltaic systems in an active marine foreland basin (Escalada Fm., Cantabrian Zone, NW Spain)



The Pennsylvanian marine foreland basin of the Cantabrian Zone (NW Spain) is characterized by the unique development of kilometre-size and hundred-metre-thick carbonate platforms adjacent to deltaic systems. During Moscovian time, progradational clastic wedges fed by the orogen comprised proximal alluvial conglomerates and coal-bearing deltaic sequences to distal shelfal marine deposits associated with carbonate platforms (Escalada Fm.) and distal clay-rich submarine slopes. A first phase of carbonate platform development (Escalada I, upper Kashirian-lower Podolskian) reached a thickness of 400 m, nearly 50 km in width and developed a distal high-relief margin facing a starved basin, nearly 1000-m deep. Carbonate slope clinoforms dipped up to 30° and consisted of in situ microbial boundstone, pinching out downslope into calciturbidites, argillaceous spiculites and breccias. The second carbonate platform (Escalada II, upper Podolskian-lower Myachkovian) developed beyond the previous platform margin, following the basinward progradation of siliciclastic deposits. Both carbonate platforms include: (1) a lower part composed of siliciclastic-carbonate cyclothems characterized by coated-grain and ooid grainstones; and (2) a carbonate-dominated upper part, composed of tabular and mound-shaped wackestone and algal-microbial boundstone strata alternating at the decametre scale with skeletal and coated-grain grainstone beds. Carbonate platforms initiated in distal sectors of the foreland marine shelf during transgressions, when terrigenous sediments were stored in the proximal part, and developed further during highstands of 3rd-order sequences in a high-subsidence context. During the falling stage and lowstand systems tracts, deltaic systems prograded across the shelf burying the carbonate platforms. Key factors involved in the development of these unique carbonate platforms in an active foreland basin are: (1) the large size of the marine shelf (approaching 200 km in width); (2) the subsidence distribution pattern across the marine shelf, decreasing from proximal shoreline to distal sectors; (3) Pennsylvanian glacio-eustacy affecting carbonate lithofacies architecture; and (4) the environmental conditions optimal for fostering microbial and algal carbonate factories.