The Niquivil Section, Precordillera of San Juan, Argentina, proposed gssp for the Lower/Middle Ordovician boundary

• Guillermo L. Albanesi ^[1] ; Marcelo G. Carrera ^[1] ; Fernando L. Cañas ^[2] ; Matthew Saltzman ^[3]
  1. [1] Universidad Nacional de Córdoba

     Universidad Nacional de Córdoba

     Argentina

     
  2. [2] Universidad Nacional de Río Cuarto

     Universidad Nacional de Río Cuarto

     Argentina

     
  3. [3] Ohio State University

     Ohio State University

     City of Columbus, Estados Unidos

     

  Mostrar afiliaciones +
• Localización: Serie correlación geológica, ISSN-e 1666-9479, ISSN 1514-4186, Vol. 17, Nº. 1, 2003
• Idioma: inglés
• Enlaces
  • Texto completo (htm)
• Resumen

  • Since the International Subcommission on Ordovician Stratigraphy (SOS) adopted the first appearance datum (FAD) of the conodont Tripodus laevis to mark the base of the Middle Ordovician Series (Webby, 1998), diverse problems arose upon the selection of this biostratigraphic marker. The Whiterock Narrows Section at Monitor Range, Nevada, proposed by Finney and Ethington (2000) as GSSP has been found to be of questionable usefulness in several respects. Detailed analyses of problems concerning both the selected conodont key species and the proposed section were posted on the "Ordovician Stratigraphy Discussion Forum" internet site (http://seis.natsci.csulb.edu/ordstrat1/default.htm).

    The position of the Lower/Middle Ordovician Series boundary in the Argentine Precordillera was recently determined by means of conodont biostratigraphy to be the FAD of Tripodus laevis s.l. in the middle part of the San Juan Formation, Yanso Section, Potrerillo Mountain (Albanesi et al., 1998). In search of this horizon in graptolite facies, Ortega and Albanesi (1999) investigated new sections in northern Precordillera, which resulted in the recognition of the lower (but not lowest) Middle Ordovician biozones based on the Isograptus complex (beginning with I. victoriae maximus Zone), which begins at the T. laevis Zone (Albanesi et al., 1999). Graptolite biohorizons that lie within the critical interval we are considering were still not found in the Precordillera. After examination of diverse alternatives, Albanesi and Carrera (2001, and referred internet address) proposed the Niquivil Section of Central Precordillera as GSSP for the base of the Middle Ordovician by the FAD of the conodonts Texania heligma Pohler and Protoprioniodus aranda Cooper. This particular level was selected for being located just above the widely recognized Oepikodus evae Zone (referred taxa are considered key substitutes after the absence of T. laevis sensu stricto in the Precordillera).