Marine and continental assemblages from San Gregorio Formation (Late Oligocene-Early Miocene), La Bocana Region, Baja California Sur, Mexico.

RAMIREZ-ARRIAGA, E.; PRAMPARO, M. B.; GUERSTEIN, G.R.; MARTINEZ -HERNANDEZ, E.; GALVAN-ESCOBEDO, I.G; CORONA-ESQUIVEL, R.; MARMOLEJO-RODRIGUEZ A.J.

Querétaro

Simposio; 108th Annual Meeting GSA Cordillera Section; 2012

Geological Society of America

San Gregorio Formation was deposited when Baja California was part of a stable shelf and it was attached to the Mexican mainland. This formation lies unconformably on the Tepetate Formation, assigned to the late Cretaceous to Eocene. The stratigraphy of San Gregorio Formation is characterized by a lower member composed of shales, sandy shales with interbedded sandstones, siltstones and finegrained compact, of sandstones presenting alteration by silicification. The upper member is composed of silicified black shales, sandstones with concretions and thin bands of chert. According to radiometric dating, micro and macrofossils information, the age of the San Gregorio Formation has been considered Late Oligocene – Early Miocene (Gastil et al., 1979; Hausback, 1984; Applegate, 1986; Helenes and Carreño, 1999).et al., 1979; Hausback, 1984; Applegate, 1986; Helenes and Carreño, 1999). The present research (DGAPA-PAPIIT IN105411) focuses on the well preserved palynologicalassemblages recovered from San Gregorio Formation, including both marine and continental microfossils. Based on quantitative analysis is possible to propose palenvironmental models related to the distribution patterns of marine palynomorphs. On the other hand, pollen and spores assemblages reflect the composition of plant communities evolved near the depositional area. Both marine and continental palynomorphs provide information about paleoclimatic changes as well as biostratigraphic data that will be important for future correlations. Preliminary results from the La Bocana "LBA” well indicate the dominace of dinoflagellate cysts, including a morphological complex of Spiniferites spp. , Operculodinium centrocarpum, Tuberculodinium vancampoae and Lingulodinium sp., among others, suggesting the analyzed part of San Gregorio Formation was deposited in an estuarine environment condition or in marine marginal waters. Furthermore, Tuberculodinium vancampoae, Chiropteridium lobospinosum and Lentinia serrata have chronostratigraphic value. Pollen assemblages give evidence of regional temperate vegetation integrated by Pinus and cloud forest, as well as local semiarid conditions that allowed the development of grassland with Amaranthaceae -Chenopodiaceae, Poaceae, and Ephedra. Additionally, neighboring communities of low tropical deciduous forest and thorn forest were recorded.