Stratigraphy and Biochronostratigraphy of the Lower Pliensbachian (Jurassic) from the Asturian basin (Northern Spain)

Abstract

A stratigraphic study of the carbonate deposits from the interval between the uppermost Sinemurian and the base of the Upper Pliensbachian is presented herein. For this purpose, four expanded sections of the Asturian Basin, in Northern Spain, were sampled. The sections are located between Rodiles E section (Villaviciosa municipality) and Playa de Vega section (Ribadesella municipality). The ammonites recorded in over 100 successive levels of this sector enabled the substantial improvement of the biochronostratigraphic scale of reference; making it comparable with those defined for other basins of the Iberian Peninsula and other areas of the western Tethys. The lower boundary of the Pliensbachian is indicated by the first appearance of Gemmellaroceras aff. aenigmaticum (Gemmellaro) below the first appearance of the genus Apoderoceras. The upper boundary is defined by the first record of the Protogrammoceras. (Matteiceras) subgenus which in some levels is associated with Amaltheus stokesi (Sowerby). The boundaries between the zones and subzones were able to be established accurately in most cases. The ammonite assemblages are similar to those established for the NW European Province. Some intervals, however, include species typical of the Mediterranean Province. Among these, the Taylori Subchronozone (Gemmellaroceras) and the transition between the Jamesoni Subchronozone and the Valdani Subchronozone (Tropidoceras) can be highlighted. Additionally, taking into consideration the succession between of ammonite genera and species, 27 horizons were identified, 24 of which correspond to the Lower Pliensbachian, and the other 3 to the uppermost Sinemurian and the lowermost part of the Upper Pliensbachian. Moreover, the identification of 3 of the Lower Pliensbachian horizons were based upon the evolution of the Mediterranean Tropidoceras.