Palinoestratigrafía de la región de tepexi de Rodríguez, Puebla, México - Implicaciones cronoestratigrá ficas
Por:
Martínez-Hernández E., Ramírez-Arriaga E.
Publicada:
1 ene 1999
Resumen:
The undifferentiated Cenozoic beds in this region have yielded valuable palynological assemblages, ranging in age from Paleocene to Oligocene, although initially the Tertiary sequence was considered as Pliocene-Pleistocene, based on the mammal tracks at the Pie de Vaca locality. This confusion increased due to the presence of Quaternary sediments with horses, glyptodonts and mammoths, which overlie unconformably the Pie de Vaca Formation. Even though at first glance me palynological assemblages indicated abundance of taxa (Pinus sp., grasses, etc.) that are present in the Quaternary of central Mexico, plus the overweighted evidence of Pliocene-Pleistocene tracks in the region given by the vertebrate specialists, the initial palynological results were biased to support a Pliocene-Pleistocene age, although the Neogene age for the Cenozoic sequence was never discarded. The paleobotanical researches arose again the question of age assignment of the Cenozoic rocks in this region, due to the paleobotanical discovery of Cedrelospermum and Eucommia, taxa that occurred during the early Tertiary of North America. This issue triggered a new palynological study of the volcaniclastic sequence, based upon the total pollen content and different kinds of biozones, which finally allow the palynostratigraphical assignment of those beds to the Eocene. The palynological assemblages indicated that during the Paleogene several plant communities were present locally and regionally, forming different mosaics of vegetation, that were distributed along an altitudinal gradient. Regionally, the forest of conifers (Picea and Pinus) occupied the mountain ranges, together with abundant grasses and Ephedra spp., where the climate was cold-temperate. In contrast, the slopes of hills were more humid with temperate to subtropical climate with a mesophyll community characterized by the abundance of Engelhardtia (Momipites spp.), Cedrelospermum, Eucommia, Betula, Ulmus, Juglans and Platanus. Near the deposit basin, the climate regime became subtropical drier, with abundance of legum fruits and abundant pollen of Caesalpinaceae, Mimosaceac, Bombacaceae, Burseraceae, grasses, Ulmaceae, which could indicate savanna to scrubland vegetation. The above mentioned vegetation types should be considered as a model, since it is clear that the type is distinguished because of its physiognomic peculiarities and not because of its taxonomic composition. Supporting this mosaic of vegetation, it is the fact that in the macrofossils, conifer remains are rare and the leaves mat predominate in this florula are rather small, indicating that the vegetation growing near the site of deposition was subject to a dry and subtropical regime. Based upon the chronostratigraphic relevance of certain taxa, such as Pterocarya, Erdtmanipollis, Aglaoreidia, Pandanus and several Momipites spp., the age of the assemblages can be considered as Eocene. Furthermore, the presence of freshwater algae confirms the lacustrine environment of these beds.
Filiaciones:
Martínez-Hernández E.:
Instituto de Geología, Universidad Nacional Autónoma de México, Delegación Coyoacán, 04510 D.F., Mexico
Ramírez-Arriaga E.:
Instituto de Geología, Universidad Nacional Autónoma de México, Delegación Coyoacán, 04510 D.F., Mexico
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