La Formación Boñar (Cretácico Superior, norte de León): estratigrafía, geoquímica y potencial productor de roca ornamental

Zusammenfassung

The Boñar Formation (Coniacian-Campanian, possibly reaching the Maastrichtian) is a mainly carbonate succession, some 300 m thick, which includes some shaly, marlstone and sandstone intervals. It crops out between the southern edge of Cantabrian Zone and the northern border of the Cenozoic Duero Basin (northern León province). New outcrops located near the town of Boñar have allowed to make a more complete stratigraphic description of the Formation, in which four lithological units (members) have been distinguished. From base to top, these are: the Calcarenitic Member (M-1), the Argillaceous and Sandy Member (M-2), the Heterolithic Member (M-4) and the Dolomitic Member (M-4). The Boñar Formation deposition has been linked to the global Upper Cretaceous sea level rise, which flooded the underlying fluvial deposits of the Voznuevo Formation. The well indurated calcarenites of the members M-1 and M-3 are used in the building industry as rough stone, whereas some dolostones of the Member M-4 constitute the so-called “Piedra de Boñar”, which has been traditionally used as ornamental stone. The “Piedra de Boñar” is a light beige to yellowish-beige dolomitized micrite, occasionally lime mudstone, which locally displays irregular grey patches, representing an inconvenient as to its use as ornamental rock. The beige hue is due to recent oxidation processes, usually due to the flow of meteoric waters along bedding surfaces, fractures or joints; whereas the grey spots are those zones preserved from oxidation. Fields studies carried out in this work show that the potential resource of the “Piedra de Boñar” are still significant (at leas 15 Mm3). The areas where the “Piedra de Boñar” displays the best conditions for commercial exploitation are the northern limb of the Las Bodas syncline and the southern limb of the La Losilla anticline, between Llamera and Barrillos de las Arrimadas villages.