ABSTRACT

Carbonate ramps are special carbonate depositional settings, characterized by very gentle slopes extending from the coast to offshore without a marked break in slope. In the Cantabrian Mountains of Northern Spain a carbonate ramp developed during the Early Devonian (Abelgas Formation). An inner ramp tidal-flat complex bounded by small oolite shoals (Dolomite Member) developed on the southern edge of the Cantabrian Block and to the south and west passed into the mid ramp, where shallow-water storm-influenced limestones and shales (Lumajo Member) and nodular limestones (Wavy Limestone Member) were deposited. Later, skeletal banks formed in nearshore areas (Millaro Limestone Member) whereas deeper ramp facies (Limestone-Marlstone Member) consist of regularly bedded pelagic limestones, ma lstones, and shales.

Evolution of the ramp was mainly controlled by eustatic sea-level fluctuations. An initial sea-level rise near the Silurian/Devonian boundary resulted in a reduction in siliciclastic supply and the initiation of carbonate sedimentation in a ramp setting. The ramp was finally drowned towards the close of the Pragian by a second rapid global sea-level rise. The resulting sequence-stratigraphic pattern shows a stack of two ramp systems, which can be grouped into a type 1 sequence at the base of the formation and a subsequent type 2 sequence including the overlying Esla Formation.