Original paper

Early Aptian sedimentary record and OAE1a in Cuchía (northern Spain): new data on facies and ammonite dating

García-Mondéjar, Joaquín; Owen, Hugh G.; Fernández-Mendiola, Pedro Angel

Abstract

Knowledge of the Early Aptian sedimentary history of the western Basque-Cantabrian Basin (BCB) is advanced by new data from the Cuchía section (Cantabria province). In ascending order, the sedimentary units comprise the Umbrera Formation (shallow-water calcarenites), Patrocinio Formation (intraplatform basin marls and sandstones), and San Esteban Formation (shallow-water rudist platform). These formations are analysed litho-, bio- and sedimentologically. The succession terminates upward with the rudist platform limestones of the Reocín and Barcenaciones formations (Albian), formerly included in the San Esteban Formation. The calcarenitic Umbrera and marly Patrocinio Formations display a general deepening-to-shallowing succession punctuated by small-scale shallowing upward cycles. The Patrocinio Formation, already reported to contain the OAE1a, has provided new information of relative age based on ammonites collected bed by bed throughout the whole succession and underpins a newd δ13Corg curve that confirms and clarifies the geochemical signature of the OAE1a in this section. A correlation of the ammonite succession in Cuchía with that of the Igaratza section at the other end of the BCB permits the establishment of the relative time range of the OAE1a in the BCB. The OAE1a in the BCB encompasses the D. forbesi and D. deshayesi zones as well an interval in which there is a transition from the D. deshayesi Zone to the Dufrenoyia furcata Zone containing a mixed ammonite fauna. These new datings disprove previous attributions of the OAE1a to the D. forbesi Zone only and provide key crucial information that helps to refine the bio-chemostratigraphic history of the BCB and enables worldwide correlations.

Keywords

cuchíaoae1aammonite zonationcarbon stable isotopesearly aptiansedimentology