Facies de relleno y origen de la excavación albiense de Aldabide (Macizo de Gorbeo, Bizkaia)

Abstract

The Lower Albian Aldabide unit, located at the NW comer of the Urgonian Gorbea Massif (Bizkaia, northem Spain), consists mainly of big limestone olistoliths filling a scoop-shaped surface on top of autochthonous marls and marly limestones. Detailed mapping and sedimentological analysis of the Aldabide unit have revealed the presence of four main resedimentated intervals, with platform- derived blocks up to 100 m. long, separated by authochthonous marls. A retrograding slumping followed by drag related with debris flows is invoked to explain the origin of the basal truncational unconformity, whose minimun relief is 32m. Mostly drag action by debris flow seems to have caused the truncation at the base of each of those megabreccias. A relevant tectonic pulse, coupled or not with an eustatic change, could well explain the origin of the unconformity at the base of the Aldabide unit, while minor pulses of both, tectonism and eustatism, could explain the intermittent appearence of megabreccias on top of it.