La cuenca terciaria continental del Diapiro de Murgia: la Formación Izarra (Oligoceno superior-Mioceno inferior, Cuenca Vasco-Cantábrica)

The tertiary continental Izarra Formation (Alava, Basque-Cantabrian Basin) is formally described for the first time. Its materials were deposited on a small basin originated in the central part of the Murgia diapir. Those materials are subdivided in four members which are: a) a basal unit of red marls and conglomerates formed in diapiric-fringe alluvial systems; b) the intermediate member consists of marls and limestones with paedogenic structures deposited in a temporary inundated outer alluvial plain; c) a top unit with calcareous lutites and fossiliferous limestone concretions (Konservat-Lagerstatten) of lacustrine origin; and d) a calcareous tufa unit overlying the basal member and originated next to hard-water springs whose stratigraphic correlation with the intermediate and top members is not clear. The chemical content of the waters was at least locally influenced by sulfates and carbonates derived from the diapir and surrounding cretaceous carbonates and ore deposits.