Revisión de la Biozona de Ogygiocaris araiorhachis (Trilobita, Tremadociano tardío) en la región de Pascha-Incamayo, Cordillera Oriental, Argentina: Parte 1: Bioestratigrafía

a revision of the ogygiocaris araiorhachis trilobite zone recorded in the saladillo and parcha formations (pascha-incamayo area, quebrada del toro, salta province) is conducted. stratigraphic distribution of the trilobite fauna, as well as graptolites and palynological associations are presented for the first time for the pascha-incamayo area. the o. araiorhachis zone is late tremadocian in age, and not arenigian as previously supposed, extending from the araneograptus murrayi to the hunegraptus copiosus graptolite zones, and associated with the messaoudensis-trifidum acritarch assemblage. its range is extended to the upper part of the saladillo formation and to the parcha formation. the association of the trilobite kayseraspis asaphelloides harrington with the messaoudensis-trifidum acritarch assemblage in the cerro san bernardo area (salta) suggests that the kayseraspis asaphelloides and the ogygiocaris araiorhachis zones were broadly coeval. hence, these late tremadocian zones do not represent a true temporal succession as proposed by harrington in the trilobite scheme of 1957. in contrast, they portray geographically restricted ecological expressions in response to lateral environmental shifts. differences in oxygenation levels appear to be the most influential control. accordingly, the k. asaphelloides and o. araiorhachis zones are of limited use for biostratigraphical purposes and for regional correlations.