Facies architecture and high-resolution sequence stratigraphy of an Upper Cretaceous platform margin succession, southern central Pyrenees, Spain

Excellent exposures of Upper Cretaceous (Santonian) carbonate platforms on the northern flank of Sant Corneli anticline (southern central Pyrenees) provide detailed information of facies architecture in both depositional strike and dip directions. Basic accretional units are differentiated by facies contrast across mappable surfaces. These surfaces do not show clear evidence of subaerial erosion and are correlated basinward with bedding planes across which there are subtle changes in skeletal composition. Two types of basic accretional units have been identified based on bedding patterns, internal facies architecture and skeletal composition: (1) Rudist buildups consist of a rudist and coral belt at the platform margin, passing landward into a slender-hippuritid lithosome, locally overlain by a bioclastic blanket that passes basinward, into bioclastic "apron-like" clinobeds and into fine-grained packstone/ wackestone. (2) Calcarenite wedges consist of yellow-brown, benthic- foraminifer-rich grainstones to grain-dominated packstones, with scattered rudist shells and small coral colonies, passing basinward into mud-dominated packstones to wackestones, with variable siliciclastic content (quartz sand to silt and clay). Rudist buildups and calcarenite wedges alternate, although not rhythmically. These changes in platform skeletal composition reflect changes in the dominant type of carbonate-producing biota independently of the changes in accommodation. Both types of basic accretional units: rudist buildups and calcarenite wedges, form simple sequences and parasequences according to internal lithofacies arrangement and inferred sea-level cyclicity (cycles or paracycles). High-frequency sea-level cyclicity fits in the Milankovitch frequency band. Long-term changes in accommodation governing aggradation, progradation and backstepping of basic sequences and parasequences reflect tectonic influence rather than long-term changes in eustatic sea level. © 2005 Elsevier B.V. All rights reserved.