Silicified foraminifera from the Lower Carboniferous Genicera Formation (Cantabrian Mountains, Northern Spain) and their palaeoenvironmental interpretation

A section through the Lower Carboniferous (Mississippian) Genicera (= Alba) Formation, subdivided in ascending order into the Gorgera, the Lavandera and the Canalón Member, in the southern Cantabrian Mountains was measured and sampled for stratigraphical and biofacies analyses. 29 limestone samples were processed with formic acid, 11 of which yielded abundant silicified foraminiferal faunas. Additionally, studies on taphonomy, stratigraphy, microfacies and accompanying macrofauna were carried out. The foraminiferal fauna is subdivided into two groups with different abundance patterns. The first group always shows a comparable low abundance (individual taxa rarely exceed 10% of the whole fauna), whereas the second group frequently shows high abundance (individual taxa rarely constitute less than 10% of the whole fauna, but may constitute up to 70%). Based on abundance patterns and taxonomy, two faunal associations can be distinguished, named after their occurrence within the members of the Genicera Formation as "Gorgera Association" and "Canalón Association", respectively. The Gorgera Association consists of a) high abundance patterns of Trochammina mehli and Rectoseptatournayella pyriformis; b) low abundance patterns of Thurammina spp., Tolypammina cyclops, T. irregularis, Hyperammina kahlleitensis, H. stabilis, Reophax northviewensis, Foraminifera aff. Moravammina? constricta. The Canalón Association is characterized by a) high abundance patterns of Hyperammina rockfordensis, H. sp. A, Kunklerina kunklerensis and Ammobaculites gracilis: b) low abundance patterns of Thurammina spp., Ammodiscus siliceus, Tolypammina cyclops, T. irregularis, Tolypammina spp., Hyperammina aperta, H. kahlleitensis, H. stabilis, Reophax northviewensis and Trochammina mehli. With the exception of Rectoseptatournayella pyriformis, Ammobaculites gracilis and perhaps Foraminifera aff. Moravammina mehli, all species cover the entire stratigraphical range of the studied section. Hence, evolutionary influence on the formation of the respective associations is rather small. Taphonomic influence on the formation of the foraminiferal associations, either by post-mortem transport or by diagenetic alteration, also fails to explain the recognized associations. The study of the accompanying fauna, microfacies and regional geology shows palaeobathymetry to be the main or only palaeoenvironmental factor that changed significantly throughout that part of the studied section which yielded the foraminiferal associations. As the succession of the Genicera Formation is interpreted to reflect a continuous sea-level rise, it is assumed that benthic faunas shifted with time through different bathymetrical zones and had to cope with the depth-related change of palaeoenvironmental parameters like oxygen concentration, temperature and availability of food. The proposed palaeobathymetrical hypothesis is compared with a contemporaneous model for the Deseret Basin of the western United States.