Constructional dynamics for a lower Cretaceous carbonate ramp (Gorbea Massif, north Iberia)

The lower Albian distally steepened Egalezaburu carbonate ramp is composed of four stacked cycles with homogeneous characteristics throughout the section. Relative sea-level changes accounted for cycle formation and changes in facies distribution. During transgression and highstands, fossiliferous boundstones were deposited on the inner ramp, and argillaceous mudstones formed on the mid- and outer ramp. Shallow marine benthic communities flourished in open, moderate-energy environments, controlled by the depth of the photic zone. During lowstands, shallow-water sedimentation shifted basinward, and skeletal grainstones dominated shallow-water environments on the mid-ramp, controlled by the depth of the wave-base. Meanwhile megabreccias accumulated locally in oversteepened sites, at the mid- to outer ramp transition. The resulting cycles differ from the inner to the mid-ramp: shallowing-upward muddy cycles formed in the inner ramp during transgressions and highstands, as deepening-upward cycles with a basal grainy unit formed on the mid-ramp during lowstand to early highstand. Photic zone and wave-base levels, controlled by relative sea-level fluctuations, were the main controls on facies types and distribution and cycle development for the Egalezaburu ramp.