Coniacian facies and environments of the Iberian basin: Evolution, and the problem of the facies mixing with implications for basin paleogeography and paleoceanography

Abstract

Abstract The Coniacian carbonate sediments of the Iberian basin were deposited in a homoclinal ramp grading upwards to a distally-steepened ramp, with a major coastal siliciclastic fringe. Twenty-four facies were recognized and grouped into four main depositional environments: outer, mid, inner ramp (including shoal and lagoon subenvironments), and coastal (with carbonate tidal-flat and siliciclastic coastal subenvironments). The more outstanding biogenic components show a mixture of sunlight-dependent, phototrophic, organisms (mainly large benthic foraminifera) and nutrient-dependent, heterotrophic, organisms (mainly rudists), being remarkable the scarce presence of corals; nutrients supplied from the emerged mainland were probably a source for the development of heterozoan organisms. Three main stages of the ramp evolution were: (1) narrow homoclinal ramp with a fringe of coastal siliciclastic sedimentation; (2) drowning and outer ramp widening with siliciclastic sedimentation; and (3) distally steepened ramp with facies aggradation and progradation of the mid and inner ramp sediments. Siliciclastic distribution is problematic since the sands sourced to the basin should have been rapidly and widely redistributed along the basin considering the common storm, wave, and tidal processes shown by the sedimentary facies. The presence of a clockwise NW-flowing longshore current is necessarily assumed to originate such distribution, which was probably induced by dominant external currents around Iberia. These clockwise gyres facilitated the larval dispersion to this enclosed basin and the local presence of upwelling influences; that could have been another source of episodic nutrient-rich waters from the deep ramp, which consequently favored heterozoan development even in the more proximal and relatively shallower facies.