The base of the Selandian Stage is defined in the sea-cliff along the main beach of the coastal town Zumaia, Basque Country, northern Spain. The GSSP is at the base of the Itzurun Formation about 49m above the Cretaceous/Paleogene boundary. The best event for global marine correlation is the second radiation of the fasciculith group of calcareous nannofossils, occurring from slightly below to 1.1 m above the GSSP. The marker species for the base of nannofossil Zone NP5 (Fasciculithus tympaniformis) occurs 1.1m above the GSSP. The GSSP level is interpreted to be coincident with a widespread rapid sea-level fall, and to be 30 precession cycles (~630 kyr) above the top of magnetic polarity Chron 27n.
The GSSP of the Selandian Stage is located along the main beach, of the coastal town Zumaia, Basque Country, northern Spain at a latitude of 43° 17' 57.1"N, and a longitude of 02° 15' 39.6"W. The GSSP for the Selandian is about 49m above the Cretaceous/ Paleogene boundary, equivalent to the base of the marls overlying the uppermost limestone bed of the ca. 10 meters of limestone - marl couplets in the upper part of the Aitzgorri Limestone Formation.
The upper 10m of the Aitzgorri Limestone Formation are dominated by limestones with marl intercalations. The overlying Itzurun Formation consists of greyish marlstones, however the lower 2.85 m of marls and marlstones have a reddish color. The GSSP is defined at the abrupt lithological change between the two formations.
The base of the Selandian corresponds to the second radiation of the calcareous nannofossil group, the fasciculiths. Species, such as Fasciculithus ulii, F. billii, F. janii, F. involutus, F. pileatus and F. tympaniformis have their FAD in the interval from a few centimeters below to 1.1 meter above the GSSP. The marker species for the base of nannofossil Zone NP5 (Fasciculithus tympaniformis) occurs 1.1m above the GSSP.
The GSSP occurs 30 precession cycles (630kyr) above the top of magnetochron C27n.