Estratigrafía y paleoambiente asociados a un Gomphoteriidae (Cuvieronius hyodon) en Tzintzuntzan, Michoacán, México

Abstract

The stratigraphic, petrographic and pollinic description of the fluvial volcanic-sedimentary column of the Barranca Rancho Viejo, Tzintzuntzan and north of the Pátzcuaro lake is presented. This column contains vegetation remnants and bone fragments, including a well preserved jaw of a gomphothere, which was recovered from a volcanic lahar deposit intercalated with fluvial deposits and a pyroclastic succession associated to the basaltic monogenetic volcanism of the Cerro Catio. On the basis of the dental morphology, the gomphothere has been classified as Cuvieronius hyodon. The stratigraphic description indicates that the deposit of this lahar was associated to a volcanic succession with a predominant pyroclastic fall, scarce pyroclastic flows and a basal basaltic lava flow with olivine ± ortopyroxene. The complete volcanic sequence was emplaced within a fluvial endorreic basin restricted to the northern portion of the Pátzcuaro lake. Isotopic 14C analysis of a woody fragment found in a horizon underlying the fossil fragments, provided an age of 26 ky ± 190 BP. This late Pleistocene age can be assigned to the gomphothere as well as to the basaltic volcanic event of the Cerro Catio.

Petrographic and pollinic studies of the sediments and basaltic tephra show that the volcanic activity was characterized by variable intensity, with recurrent waning periods, followed by volcanic quiescence. Moist climatic conditions favored the devitrification process of the volcanic ash and the weathering of other minerals, which provided good conditions for recovering of the mesophillic vegetation: Fraxinus, Acer, Corylus, Ulmus, Betula, and Juglans. In agreement with other authors, we conclude that the preponderant paleoenvironment of the Pátzcuaro lake during late Pleistocene was similar, although colder, to present-day climate. In this paleoenvironment, the presence of the terrestrial mammals was favored by the availability of relatively abundant mesophytic vegetation, which was seemingly perturbed by the volcanic exhalations associated to the monogenetic volcanism of the Michoacán-Guanajuato volcanic field.