STRATIGRAPHY OF TOLIMAN REGION, QUERETARO STATE, CENTRAL MEXICO AND ITS ROLE IN THE EVOLUTION OF THE CONTINENTAL MARGIN AND TERRANE ACCRETION

Base on recent fieldwork, we found important differences in composition and association of pre-cretaceous rocks compared to what was described previously. The results presented here are preliminary and its regional implications have not been straighten out yet. Carrillo (2000) grouped the pre-cretaceous rocks of the area in one formation (San Juan de la Rosa). However we found that is actually two units separated by an important deformational event. The oldest we named El Chilar Complex and the youngest we proposed to keep the name of San Juan de la Rosa Formation. The El Chilar Complex is made of a highly deformed succession of quartz-rich sandstone and shale. This succession forms a matrix within which tectonic blocks of different lithologies can be found. The blocks are strongly deformed and in sheared contact with the matrix, and vary in size from centimeters to hundreds of meters. They are made of the same quartz-rich sandstone and shale, black and white chert, diabase to dioriote, and limestone. The age of the El Chilar Complex remains undetermined, but pre-Upper Jurassic, as suggested by the regional stratigraphy. At present fossil determinations are in progress. The characteristics of the El Chilar Complex suggest that it might represents an accretionary prism. We considered this unit might be the possible limit of the western margin of Oaxaquia, which is the Grenvillian basement of eastern Mexico. If we find more evidence that constrain that the complex was formed in a subduction complex, this would indicate that the Proterozoic backbone of Mexico (Oaxaquia) is much narrower than previously thought, and that all rocks to the west are accreted. However, further mapping, geochemical and isotopic analysis are necessary. The El Chilar Complex is covered unconformably by the Jurassic San Juan de la Rosa Formation made up of marine felsic volcanic and volcaniclastic rocks. We postulate that the San Juan de la Rosa Formation probably corresponds to the southward continuity of the Jurassic continental volcanic arc, described in northeast Mexico by Grajales et al. (1992) and Norris et al. (1995) among others.