Jurassic-Cretaceous paleogeographic evolution of the northern Mixteca terrane, southern Mexico

Abstract

A model of Jurassic-Cretaceous paleogeographic evolution for the northern portion of the Mixteca terrane is presented. The interpretation is based mainly upon stratigraphic analysis of three different areas of southern Puebla and northern Oaxaca, where Mezosoic beds are well exposed. The oldest beds of the Mesozoic sequence (Toarcian(?)-Aalenian) are exposed in the Tezoatlán-El Rosario area where they consist of conglomerate sandstone, siltstone, and shale strata with some coal layers. These beds accumulated after a change from an uplift tectonic regime, active since Early,Triassic, to an extensional tectonics with subsidence. In response to this change a group of tectonic basins with a north-south orientation formed in the Mixteca terrane together with the development of a fluvial system with a general slope toward the south. The continental sequence which accumulated during the Middle Jurassic in this region has a maximum thickness of 2,300 m and shows strong lateral variations, representing transitions from alluvial fans to floodplaius and channel deposits. During Bajocian and Bathonian this region experienced brief marine transgressions of Pacific origin which covered the southern part of the Mixteca terrane up to a line situated between Tezoatlan and Huajuapan de Leon. From Callovian to Hauterivian times, shallow marine conditions dominated and calcareous and detrital deposits accumulated in the southern part of the studied region, which had been occupied by the fluvial system. The Late Jurassic paleogeography featured a shallow bay (Tlaxiaco Bay) connected to the south with the Pacific Ocean. A connection between this bay and the Gulf of Mexico might r.ave initially occurred between Tithonian and Valanginian times. There is no evidence to support the existence of a direct marine connection between the northern part of the Tlaxiaco Bay and the southern portion of the Sierra Madre Oriental area during Middle Jurassic and most Upper Jurassic times. There are no indications of Barremian-aptian marine sedimentation in most of the Mixteca terrane. Only to the north. in the San Juan Raya area, a continuous marine Valanginian-Aptian sequence has been reported. During this time, subaerial monogenetic volcanism locally occurred near Santo Domingo Tonala. From Albian until probably Turonian, a broad calcareous platform evolved over the Mixteca and Oaxaca terranes and may have extended to the Actopan platform, north of the present Mexican Volcanic Belt. In post-Turonian time, general uplift contemporaneous with folding affected the studied region. Only in the southwestern part of the Mixteca terrane the marine sedimentation was apparently continuous until the Maestrichtian.