Paleontological constraints on successive paleogeographic positions of Precordillera terrane during the early Paleozoic

BENEDETTO, LUIS; SANCHEZ, TERESA M.; CARRERA, MARCELO G.; BRUSSA, EDSEL; SALAS, MARIA JOSE

Laurentia-Gondwana connections before Pangea

Geological Society of America

Año: 1999; p. 21 - 42

Faunal data from Cambrian and Ordovician rocks of the Precordillera terrane are examined in order to refine their biogeographic relationships with Laurentia and Gondwana. This study is based principally on benthic organisms (sponges, bryozoans,brachiopods, bivalves and ostracods), but data from graptolites were also considered. The data lead us to recognize four successive stages of Precordilleran biogeqraphic evolution: (1) Laurentian stage (Cambrian-Tremadoc), (2) isolation stage (Arenig-earIy Llanvirn), (3) pre-accretion stage (LIanvirn-Caradoc) and (4) Gondwanan stage (Hirnantian-SiIurian). Each denotes its specific position during the rifting-drifting-collision sequence and each reflects a different pattern of faunal exchange. During the Laurentian stage, the nearly compIete identity with Appalachian faunas supports a close geographic connection between Precordillera and Laurentia. The isolation stage begins when taxa that have not been recorded in Laurentia appear for the first time in the Precordillera basin. Through this stag the Laurentian faunal influence decreases and thec number of endemic Baltic-AvaIonian genera correlatively increases. The pre-accretion stage is characterized by its paucity of Laurentian forms, incoming of Gondwanan taxa, and an unusually high Ievel of endemicity. The Iatter may reflect a degree of geographic isolation, and may in part be one to biologic factors related to dispersal mechanisms. Finally, the Gondwanan stage starts after accretion of the Precordillera terrane at the end of the Ordovician. Faunal data reviewed here support the "far traveled microplate" hypothesis that is consistent, in general, with the geologic evidence.