Early-Middle Ordovician conodont paleobiogeography with special regard to the geographic origin of the Argentine Precordillera: A multivariate data analysis

Statistical comparisons of conodont faunas from many parts of the world were carried out in an effort to shed light on one of the most discussed, and most controversial, problems in the lower Paleozoic geology of South America, namely, the geographic origin of the exotic terrane in western Argentina known as the Precordillera. The similarity between the conodont faunas from the Precordilleran La Silla, San Juan, Gualcamayo, and Yerba Loca Formations and many coeval faunas from Laurentia, as well as from other parts of the world, was assessed using the Jaccard Index. The analysis of faunas from six biostratigraphic intervals in the Lower and Middle Ordovician shows that the earliest Ordovician (Tremadocian) faunas cluster with those of Laurentia, whereas slightly younger faunas show less obvious provincialism. The conodont faunas of the Middle Ordovician (early Darriwilian) of the Precordillera again show dominantly Laurentian affinities. The hypothesis that the Precordillera rifted from the Ouachita embayment and moved across part of the Iapetus Ocean to dock with western Gondwana in Ordovician time is not clearly supported by the conodonts (and other non-conodont phosphatic microfossils). The similarity with faunas in southern Laurentia (mainly from the El Paso area of Texas and southern New Mexico) is high in the Tremadocian. The expected similarity decrease with presumed increase in distance from Laurentia later in the Early Ordovician is not evident in the conodont faunas. Similarity between the two regions (mainly the Precordillera and the Marathon area of Texas) remains about the same through the early Middle Ordovician. It is concluded that the conodonts, the best known and most widespread fossil group in the study areas, do not provide conclusive evidence of the geographic origin of the Precordillera. © 2010 The Geological Society of America. All rights reserved.