A facies model for an Early Aptian carbonate platform (Zamaia, Spain)

The Cretaceous (Early Aptian, uppermost Bedoulian, Dufrenoyia furcata Zone) Zamaia Formation is a carbonate unit, up to 224 m thick and 1.5 km wide, which formed on a regional coastal sea bordering the continental Iberian craton. A high-resolution, facies-based, stratigraphic framework is presented using facies mapping and vertical-log characterization. The depositional succession consists of a shallow estuarine facies of the Ereza Fm overlain by shallow-water rudist limestones (Zamaia Fm) building relief over positive tectonic blocks and separated by an intraplatform depression. The margins of these shallow-water rudist buildups record low-angle transitional slopes toward the adjacent surrounding basins. Syn-depositional faulting is responsible for differential subsidence and creation of highs and lows, and local emplacement of limestone olistoliths and slope breccias. Two main carbonate phases are separated by an intervening siliciclastic-carbonate estuarine episode. The platform carbonates are composed of repetitive swallowing-upward cycles, commonly ending with a paleokarstic surface. Depositional systems tracts within sequences are recognized on the basis of facies patterns and are interpreted in terms of variations of relative sea level. Both Zamaia carbonate platform phases were terminated by a relative sea-level fall and karstification, immediately followed by a relative sea-level rise. This study refines our understanding of the paleogeography and sea-level history in the Early Cretaceous Aptian of the Basque-Cantabrian Basin. The detailed information on biostratigraphy and lithostratigraphy provides a foundation for regional to global correlations.