The non-marine Lower Cretaceous of El Montsec thrust-sheet (South-Central Pyrenees)

Abstract

The aim of this study is resolving uncertainties on the lithostratigraphy and chronostratigraphy of the non-marine Lower Cretaceous of El Montsec thrust sheet, which contains the most developed record of such facies in the central Pyrenees (Catalonia, Spain). The materials, traditionally known as “El Montsec Charophyte Limestones”, overlie upper Berriasian marine limestones and belong in fact to two different stratigraphic units, separated by an angular unconformity, which is represented by an irregular karstic surface. The lower unit, newly defined as the Cova dels Lladres Intraclastic Limestone Formation, is composed by metric-scale fining-upward parasequences with black intraclast conglomerates at the base overed by charophyte limestones at the top. This first nonmarine unit is attributed to the lower Barremian Eurasian charophyte biozone Atopochara trivolvis triquetra. The upper non-marine unit, (revisited El Montsec Charophyte Limestone), is made of metric-scale fining-upward parasequences of charophyte limestones. Ferruginous marls at the base of the upper unit have yielded charophytes of the Hemiclavator neimongolensis neimongolensis Eurasian charophyte biozone, corresponding to the early–late Barremian boundary. El Montsec Charophyte Limestone Formation transitions laterally to La Pedrera de Rúbies Lithographic Limestones Formation, which includes the two famous fossil Konservat Lagerstätten of La Pedrera de Meià and La Cabroa. In sum, the non-marine Lower Cretaceous succession of El Montsec thrust sheet shows at its base a stratigraphic gap ranging from the upper Berriasian to the early Barremian. Furthermore, the angular unconformity separating the Cova dels Lladres Formation and El Montsec Formation clearly reflects the strong tectonic activity associated with the Barremian Iberian rift, which has been linked with the opening of the Bay of Biscay.