Estratigrafía del Cretácico en el sondeo del Sifón de los Yesos (Torrelaguna) y su correlación con afloramientos de superficie

Abstract

Several hydrogeological boreholes for survey purposes have been drilled by the Canal de Isabel 11 near Torrelaguna (Madrid). One of those boreholes, at the Sifón de los Yesos, was drilled out with complete core recovering and cut across detrital and evaporitic rocks of Paleogene age and, partly, the Upper Cretaceous series mainly composed of dolostones, marls and sandstones. Six units have been distinguished in the Cretaceous of the borehole, from base to top: 1) Marls and dolostones with minor sandy intercalations (23.6 m); 2) Thick-bedded yellow dolostones (23.4 m); 3) marly dolostones and marls with minor dolostones (8 m); 4) thick-bedded yellow dolostones (17.4 m); 5) medium to thick-bedded white dolostones (21.3 m); and 6) Calcareous breccias alternating with white dolostones and red mudstones (28.5 m).
These units have been correlated with the nearby Barranco de las Cuevas section, which is a key section in order to understand the stratigraphical relationships for the Cretaceous south of the Central System, because of several lithostratigraphical units are poorly dolomitized here and thus their facies and sedimentary environments of deposition can be studied and understood. From base to top borehole unit l is correlated with the Tranquera Fm.; borehole units 2, 3 and 4 (lower part) are correlated with Hortezuelos Fm.; borehole units 4 (upper part) and 5 are correlated with Hontoria del Pinar Fm.; and finally, borehole unit 6 is correlated with Burgo de Osma Fm.
There are clear differences between these units in surface outcrop and in the borehole relating to colour, which is related to the organic matter content and its oxidation state, and appearance, marly intercalations are weak intervals in surface meanwhile correspond to harder and more compacted intervals in the borehole. The recognition of the borehole facies for the Cretaceous is a very useful stratigraphical tool for future drilling campaigns allowing a more detailed interpretation of the cores.