Sponges from the Wordian (middle Permian) of the Las Delicias Formation, Coahuila, northwestern Mexico

Reconstructing the geological evolution of central and western Mexico during the end of the Paleozoic and the beginning of the Mesozoic is very diffi cult because of a lack of exposures. The few outcrops available, and indirect information obtained from geophysical and geochemical data suggests that Central and Western Mexico are made up of a mosaic of pre-Jurassic terranes, and that previously defi ned terranes are mostly composites of basements of different origins. Most of those terranes are allochthonous with respect to North America, but some developed not far from their present position. It has been suggested that the Coahuila and Sierra Madre terranes (Oaxaquia block), part of Gondwana during Early Paleozoic, collided with North America by Late Paleozoic time. However, their Mississippian faunas of North American affi nity suggest that the collision might have occurred earlier. The nature of the basement of the Central terrane is unknown, but it is inferred to be allochthonous because there is an accretionary prism at its NE boundary. The basement of the Parral and Tahue terranes is formed by a deformed volcano-sedimentary complex of Early Paleozoic age, whose origin and paleogeographic evolution remains unknown. The Caborca and Cortes terranes are formed by Proterozoic metamorphic complexes and an accreted eugeoclinal Paleozoic sedimentary wedge. The basement of the Zihuatanejo terrane is made up of Triassic ocean-fl oor continental-rise assemblages accreted in Early Jurassic time. An overview of new stratigraphic and geochronologic data indicates that a number of tectonic events occurred during Late Paleozoic to Early Mesozoic time. A continental arc with a paleo-Pacifi c, east-dipping subduction zone evolved from Carboniferous to Early Permian time in eastern Mexico (Oaxaquia), and it was in part contemporaneous to deformation in the Ouachita belt. This was followed by a period of volcanic quiescence during middle Permian. A more felsic arc, with a different distribution of the volcanic axis, developed along all the paleo-Pacifi c margin in the Permo-Triassic. Terranes in northwestern Mexico show a completely different geological evolution during the Carboniferous and Permian time. They were characterized by passive margin sedimentation and by folding and thrusting of eugeoclinal rocks in the Mississippian and Late Permian. By Late Triassic, a passive or rifting Centeno-Garcia, E., 2005, Review of Upper Paleozoic and Lower Mesozoic stratigraphy and depositional environments of central and west Mexico: Constraints on terrane analysis and paleogeography, in Anderson, T.H., Nourse, J.A., McKee, J.W., and Steiner, M.B., eds., The Mojave-Sonora megashear hypothesis: Development, assessment, and alternatives: Geological Society of America Special Paper 393, p. 233–258. doi: 10.1130/2005.2393(08). For permission to copy, contact editing@geosociety.org. ©2005 Geological Society of America. 234 E. Centeno-Garcia spe393-08 3rd pages