The Cenomanian–Turonian boundary event and dinocyst record at Ganuza (northern Spain)

Abstract

An excellent record of the Cenomanian–Turonian transition zone occurs in an expanded section near the village of Ganuza, northern Spain. The sequence at Ganuza shows no evidence of stratigraphic discontinuities, and consists of an alternation of marl, marly limestone and limestone, which were deposited in a middle to outer shelf environment. Our results from palynological residues of 34 samples from the section indicate that number of cysts per gram of dry bulk sediment varies between a total of 7 and 236, and includes more than 130 taxa. Species diversity varies between 25 and 49 species and subspecies per sample. The Spiniferites ramosus group, together with two other important taxa, Trichodinium castanea and Exochosphaeridium phragmites dominate cyst assemblages. Other common taxa include Palaeohystrichophora infusorioides, Canningia reticulata, Pterodinium cingulatum, Coronifera oceanica, Florentinia mantellii, Cleistosphaeridium ? aciculare, Dapsilidinium laminaspinosum, and Xenascus ceratioides. Successive last occurrences of Florentinia cooksoniae, Litosphaeridium siphoniphorum and Epelidosphaeridia spinosa (uppermost Cenomanian), and the first occurrence of Senoniasphaera rotundata (lower Turonian) are used to characterize the transition from Cenomanian to the Turonian. Our results show that dinocysts occur continuously throughout the Ganuza section across the C/T boundary event marked by only a slight decrease in species diversity. The most significant variation in the Ganuzan section in relation to the C/T boundary event is a decrease in cyst abundance from Late Cenomanian to earliest Turonian. The decline occurred just above a level marked by a benthic foraminiferal turnover, as well as the extinction of Rotalipora cushmani. Dinocyst minimum abundances occurred slightly later than minimum abundances of nannofossils. Our dinocyst results are thus consistent with a decrease in productivity during the latest Cenomanian. A new cycle is marked by a strong recovery of dinocysts immediately above the boundary event, although a subsequent decrease recurred higher in the section. Dinoflagellates have a more general opportunistic behaviour than foraminifera and calcareous nannoflora, and such behaviour may offer a viable explanation to account for the different patterns of response observed between the dinocysts and the other microfossil groups.