THE CENOMANIAN/TURONIAN BOUNDARY EVENT: ITS RECORD IN A CARBONATE PLATFORM IN NORTHERN SPAIN

Comparison of several Cenomanian-Turonian boundary sections in northern Spain, Menoyo and Ganuza especially, shows that they are all characterized by a consistent rhythmicity throughout. All these deposits are also rich in macrofossils, microfossils as well as nannofossils. Based on the sequence stratigraphic scheme proposed by Gräfe and Wiedmann (1993), the CTBE occurs during the LSW and TST phases of the third-order cycle UC5/UC6, starting close and beneath its lower boundary. However, the CTBE is not always recorded, because some proximal ramp (inner platform) and deep basin (middle and lower slope) localities of the area have been affected by erosion. Stable isotope curves show a typical pattern: a pre-excursion background, a build-up in two phases of rapid increase in d¹³C values separated by a trough, a plateau, a recovery, and subsequently a new post-excursion background. Key macrofossil bioevents can be recognized and correlated with details in the lithostratigraphy, including northern Spain upper Cenomanian and lower Turonian ammonite zones, incoming of inoceramid bivalves of the genus Mytiloides, and several pulses of faunal events. Patterns of occurrence of benthic foraminifera in the coarse fraction (>250 mm) show two peaks of extinction corresponding to the two build-up phases of the carbon excursion. Key biomarker events for the CTB include the first occurrences of the following taxa: Kamerunoceras sp., Kamerunoceras calvertense and Mammites nodosoides (ammonites), Mytiloides (inoceramids), Helvetoglobotruncana praehelvetica and H. helvetica (planktonic foraminifera), Quadrum intermedium, Q. gartneri and Eprolithus octopetalus (calcareous nannofossils), Senoniasphaera rotundata (dinocysts). They also include the last occurrences of the planktonic foraminifera Rotalipora greenhornensis and R. cushmani, the calcareous nannofossils Corolithion kennedyi, Axopodorhabdus albianus, Lithraphidites acutus and Microstaurus chiastius, the dinocysts Florentinia cooksoniae and Epelidosphaeridia spinosa. The d ¹³ C excursion is the result of increased rates of burial of organic carbon, which sequestered nutrients and caused a decline in productivity. Cyclostratigraphy allows very precise correlation of isotopic and bio-events with other sections in Europe and North America.