La Biozona Bifurcatus en el Sector Nororiental (Rama Aragonesa) de la Cordillera Ibérica: Asociaciones de ammoniles y bioestratigrafía.

La Bioestratigrafia de la Biozona Bifurcatus (Oxfordiense Medio), en la Cordille1-a Iberica nororiental (Rama Aragonesa), esta basada en la sucesion de asociaclones registradas de ammonoideos del genero Perisphinctcs WAAGEN, y mas concretamente del subgenero Perisphincws (Dichoiomoceras~ Bu KMAN. El limite inferior de esta biozona. se localiza en la base de la subbiozona inferior, Stenocycloides, y viene marcado por la sustitucion de los Dichotomosphinctes BUCKMAN del grupo P.(l)ichtes.) warrae Bt'sowsy, por los primeros Dichotornoceras de gran talla, de los grupos bi/i?rcatoides (Esv)--.stenocycloides (SIFM[RisozK[). El limite superior se corresponde con la base de la Biozona Bimamrnatum, y se caracteriza por la aparicion (lei ~~flCF() Jsp~pelt oteros, coincidiendo aproximadamente con la desaparicion de los ultimos representantes cie Perisphiiu.tes. El estudio de las asociaciones de ammonoideos permite reconocer las subdivisiones bioestratigraficas establecidas para esta biozona por otros autores en las distintas localidades estudiadas. Buaspidoceras hypselurn Oi'pvi.,, especie indice de la subbiozona inferior dc ia Biozona Bimammatum, se encuentra ya presente en la partc superior de la Biozona Bifurcatus. La presencia de varios ejemplares de Perisphinctes (Ampthiha) arinienss oleom o/ng/a, ni 48: 11-23. Servicio dc Publicaciones U.C.M. Madrid, 1996 12 Julia Bel/o Villalba ABST RA CT The Biostratigraphy of the Middle Oxfordian Bifurcatus Zone in [he Iberian Chain, is based on the stratigraphic succession of the ammonite genus Perisphinctes WAAGLN, subgenus Per. U)ichotomoceras) BLJCKMAN. The lower boundary is placed aL the base of the lower, Stenocycloides Subzone, and is marked by the replacement of the late Dichotomosphinctes of the Per. <`Dicbtes.) wartae group BuKowsKi by the carliest representatives of Per. <`Dichotomoceceras,) of the bifurcatoides ENAY-Stenocyc/oides SIEMIRADZKI groups. The upper boundary which also corresponds to the MiddleUpper Oxfordian boundary, is placed at the base of the Himammatum Zone. which is marked by the l7irst appearance of the genus Epipelto ceras. A detailed biostratigraphic study has been carried out in the northeastern part of the Iberian Chain (E Iberia). The recorded ammonite sucessions in [he different studied localities, have allowed characterising (he biostratigraphic subdivisions currently established for the Bifurcatus Zone. Ihe species Euaspidoceras hypse/um (OPILtL) is already found at the upper part of the Bifurcatus Zone. On the other hand, the record of the species Perisphinctes (Atnpthilia) ariniensis (MELENDEZ) in the lastest levels of this Biozone, support the reeent proposal (MELENDEZ & FONTANA, 1993) of [he new biohorizon (Arieniensis Horizon) at the top of the Bifurcatus Zone, Grossouvrei Subzone. Finally, a progressive southeastwards wedging out of the Stenocycloides Subzone is observed, between the localities of Ricla and Arino, a stratigraphic gap spanning the whole subzone having been recorded in this section.