Lower toarcian foraminifera of the San Andrés section (Basque-Cantabrian Basin)

Abstract

A study of 11 samples from the Pliensbachian-Toarcian transition in the San Andres Section yielded a total of 4756 specimens representing 50 taxa. The species are here described and the diversity and abundance trends of the foraminiferal assemblages are shown. Lagenina is the best-represented suborder. Lenticulina toarcense is the most abundant species and is present from the Spinatum Zone to the Bifrons Zone. Miliolina, Robertinina, Spirillinina and Textulariina are also recorded, but in smaller proportions. The succession of foraminiferal assemblages in the Upper Pliensbachian Lower Toarcian of the examined section shows a number of significant biostratigraphic events, including a step-wise extinction of typical long-ranging Lower Jurassic species such as Mesodentalina matutina, Ichthyolaria sulcata, Astacolus speciosus, Icizthyolaria intumescens, Prodentalina terquemi, Planularia pulchra, Planularia inaequistriata, Astacolus matutinus, Saracenella sublaevis and Marginulina spinata in the Tenuicostatum Zone. Stratigraphically short-ranging species, such as Eoguttulina palomerensis and Planularia obonensis are found in the Spinatum and Tenuicostatum zones. Finally, an important renewal and diversification event, including the first record of the genus Citharina and its species C. colliezi and C. clathrata, occurs in the Serpentinum Zone. Comparison of these assemblages with those identified in other European, North African and American basins reveals similar patterns.