Lower Mississippian (Osagean) brachiopods from the Santiago Formation, Oaxaca, Mexico: stratigraphic and tectonic implications

Abstract

Four species of spiriferid brachiopods freom the Santiago Formation of Oaxaca are described. The species described belong to the orders Athyridida (Actinoconchus lamellosus), Spiriferida (Torynifer pseudolineatus), and Spiriferinida (cf. Syringothyris sp. and Punctospirifer gigas new species). The association between A. lamellosus and T. pseudolineatus indicates an Early Mississippian (Osagean) age for the Santiago Formation. The Carboniferous succession in the area was originally divided into the Santiago Formation (formed by lower and upper members) and the Ixtaltepec Formation. The studied fauna belongs to the lower member of the Santiago Formation. Because Pennsylvanian brachiopods were found in strata of the upper member, it is now considered to have more affinity with the Ixtaltepec Formation. Therefore, we propose a change in the stratigraphy of the area to include the upper member of the Santiago Formation in the Ixtaltepec Formation. This change in the stratigraphy is supported because a major lowangle normal fault places the former upper member of the Santiago Formation on the lower member and indicates a tectonic contact between both members of the original Santiago Formation. Brachiopods freom the Santiago Formation are similar to those in the Midcontinent province located in the eastern and central regions of the United States. The affinity of these Mississippian faunas has important tectonic implications, because Ordovician and Silurian faunas of Oaxaquia have strong biogeographic affinities with GondwananEuropean provinces. Therefore, there may have been a geographic connection between southeastern Mexico and North America during Lower Carboniferous times. This could change previous interpretations, which suggest that the collision of Oaxaquia with North America occurred during Permian time. (C) 2002 Elsevier Science Ltd. All rights reserved.