The carbonate signal and calcareous nannofossil distribution in an Upper Jurassic section (Balingen-Tieringen, Late Oxfordian, southern Germany)

Abstract

Total abundance, assemblage composition and the stratigraphic distribution of the six most abundant species of calcareous nannofossil were analysed in the Upper Oxfordian succession of Balingen-Tieringen (western Swabian Alb, south Germany). This locality represents a deep-shelf (less than 200 m) environment that is strongly influenced by its proximity to the Jura shallow platform. Nannofossil abundance negatively correlates with carbonate content as well as with accumulation rate. Calcareous nannofossils thus did not produce the bulk of carbonates, and their abundance was essentially controlled by the import of allochthonous carbonate mud. Gradual increase in carbonate content in the Balingen-Tieringen succession can be related to an intensified carbonate mud export from the Jura shallow platform to the Swabian Alb deep shelf during the Late Oxfordian. Changes in the nannofossil assemblage composition along the profile are interpreted as primary although a partial control by diagenesis on the assemblage can not be completely excluded. The coccoliths Lotharingius hauffii, Watznaueria britannica, Cyclagelosphaera margereli, Watznaueria barnesae, Watznaueria manivitae and the incertae sedis Schizosphaerella spp. display their highest relative abundance one after the other, in a stratigraphic order. This nannofossil succession parallels the gradual increase in allochthonous carbonate mud accumulation on the deep shelf as well as the gradual decrease in the intensity of siliciclastic input recorded on the Jura shallow platform. This suggests a climatic control by humidity on shallow-platform carbonate production and export basinwards, and on nutrient availability in deep-shelf surface waters that itself controlled the nannofossil assemblage composition. Therefore, the succession of highest relative abundance of L. hauffii, W. britannica, C. margereli, W. barnesae, W. manivitae and Schizosphaerella spp. can represent a trophic preference continuum from more eutrophic to oligotrophic conditions.