Fossil occurrences in the Upper Cenomanian-Lower Turonian at Ganuza, northern Spain: an approach to Cenomanian/Turonian boundary chronostratigraphy

An Upper Cenomanian to Lower Turonian section has been sampled in the vicinity of Ganuza, northern Spain. Ammonoids, inoceramids and microfossils including foraminifera and calcareous nannofossils have been studied. We recognize three planktonic foraminiferal biozones: theRotalipora cushmani,Whiteinella archaeocretaceaandHelvetoglobotruncana helveticaZones. We also characterize theEiffellithus turriseiffeliiandQuadrum gartneriZones, based on nannofossil biostratigraphy. Macrofaunal occurrences are scarce but allow us to characterize theMetoicoceras geslinianumandMammites nodosoidesZones, and two inoceramid assemblages. In theRotalipora cushmaniZone the following sequence of biohorizons is recorded from bottom to top: (a) last occurrence (LO) ofCorollithion kennedyi; (b) LO ofRotalipora greenhornensis; (c) LO ofAxopodorhabdusalbianus; (d) first occurrence (FO) ofEuomphaloceras septemseriatum, (e) LO ofLithraphidites acutus; (f) LO ofRotalipora cushmani. In theWhiteinella archaeocretaceaZone the sequence of events is as follows: (g) FO ofCalycoceras naviculare; (h) LO ofMicrostauruschiastius; (i) FO ofQuadrum intermedium; (j) FO ofHelvetoglobotruncana praehelvetica; (k) FOs ofKamerunocerassp. andMytiloidessubmytiloides; (l) FO ofMytiloides kossmati kossmati; (m) FO ofKamerunoceras calvertense; (n) FO ofQuadrum gartneri; (o) FO ofMammites nodosoides; (p) FO ofMytiloides mytiloides. The top of this zone is marked by the FO ofHelvetoglobotruncana helvetica.A possible Cenomanian-Turonian (C/T) boundary can be drawn between FOs ofKamerunocerassp. andMytiloidessubmytiloidesand the FO ofMytiloides kossmati kossmati. Successive FOs ofH. praehelveticaandQ. gartneriare also good proxies for the C/T boundary.