Relaciones tectónicas y sedimentarias entre la serie del Cadí y la zona del Pedraforca (Pirineo catalán)

Abstract

A Tectonic and sedimentary relations between the Cadí Unit and Pedraforca Unit (Catalonian Pyrenees, Spain)
The Pedraforca Unit forms, to the east of the Segre Fault, the eastern part of the allochthonous series of the southern Pyrenees. It was generally accepted that the material of the Pedraforca Unit, mainly Mesozoic, is made up of a group of allochthonous units (“Pedraforca nappe” in the Seguret sense) coming from the North and displacing over the Eocene of the Cadí Unit; this last unit (relative autochthonous) is constituted of sedimentary strata superposed on the Paleozoic rocks of the Pyrenees Axial Zone to the SE of Andorra. This interpretation is rejected by the authors. We remember the essential stratigraphic characteristics of the Cadí Unit and of those of the Pedraforca Unit (eastern prolongation of the allochthonous series of the southern Pyrenees situated to the W of the Segre). We insist on the common stratigraphical series of both units (marine Upper Senonian, Maestrichtian-Paleocene of “garumnian” facies, marine Lower-Middle Eocene). In both units we observe the suppression of stratigraphical series from the W to the E. We demonstrate that the lithological succession of the western CadI (sector of Fornols) and the succession of the most eastern part of the Pedraforca Unit (sector to the SW of La Pobla de Lillet) have some very peculiar characteristics, exclusively of these two sectors: heavy mineral assemblages in Maestrichtian sandstones, typical detrital beds (“La Nou conglomerates”, “Fornols breccias”), onlap disposition of the Ilerdian…. But, at present, these two sectors are separated by 40 km. from W to E. Then we propose to displace the eastern point of the Pedraforca Unit to the neighbouring western part of the Cadí Unit replacing towards the west the “Central Unit of the southern Pyrenees”.
This movement would be carried out principally along the “Cadí Fault”, a big fault, with a strike slip and a vertical slip of kilometric order, between the Cadí Unit and Pedraforca Unit. After remembering the several explanations proposed for the tectonic emplacement of the Pedraforca Unit, the authors describe in detail some important sectors situated along the Cadí Fault or in its proximity, between the Paleozoic klippe of the Montsec de Tost (Nogueras Unit) and the S of La Pobla de Lillet. This fault, with strong dip towards the S, intersects the different formations of the Eocene series of the CadI Unit. Various tectonic arguments justify the strike slip of this fault, whose age is essentially Middle Upper Eocene, but which has reacted in an important way during the Neogene.
Therefore we cannot consider the “Cadí Fault” as the basal thrust plane of a “Pedraforca nappe”, of Eocene age, which would have been made in almost vertical position in a posterior time. We consider it to be a profound fault affecting the basement with a prolongation towards the W, in the interior of Nogueras Unit. At the same time the Cadí Fault connects, to the SW, with the profound Segre Fault. The Cadí and Segre Faults are partially strike-slip faults, both delimit a crustal block which, during and after the emplacement (in the Eocene) of the allochthonous units of the southern Pyrenees, would have displaced towards the E with a certain clockwise rotation, with respect to Axial Zone and to the basement of the “Central Unit of the southern Pyrenees” to the W of the Segre.