Abstract

GALVAN-ESCOBEDO, Iris G. et al. Paleopalynology and U-Pb zircon dating of the El Cien Formation (Oligocene), Baja California Sur, Mexico. Bol. Soc. Geol. Mex [online]. 2020, vol.72, n.2, 00007.  Epub Dec 28, 2020. ISSN 1405-3322.  https://doi.org/10.18268/bsgm2020v72n2a300819.

Borehole (B187) near San Juan de la Costa, Baja California Sur, was collected. The strata of this borehole consists of the El Cien Formation. Dating by U-Pb method in Zircons indicated that the sediments were deposited 27.5 million years ago (upper Oligocene). The B187 paleopalynological research shows that in these strata, palynomorphs of both continental (pollen and spores) and marine (dinoflagellate cysts) origin are preserved, which were quantified and identified. The palynomorphs’ richness consists of 248 taxa, 230 were continental and 18 dinoflagellates. The α diversity index of total palinomorphs was H’ = 3.3, with uniformity J’ = 0.6. Values of α diversity for continental and dinoflagellates groups were H’ = 3.6, J´= 0.7 and H’ = 1.8, J´= 0.6, respectively. The CONISS cluster analysis defined three palynozones throughout the borehole stratigraphic column: palynozone A, distinguished by a poor richness of both, pollen-spores and dinoflagellates; in contrast, palynozone B concentrated the highest richness and diversity of continental palinomorphs and dinoflagellates; and palynozone C, had the lowest palinomorphs richness. The assemblage of continental microfossils were dominated by Chenopodipollis (Amaranthaceae-Chenopodiaceae), Graminidites (Poaceae), Psilatricolpites (Leguminosae), Liliacidites (Liliaceae, Bromeliaceae y Palmae) and the Stercualiaceae type, all of which are currently associated with the presence of tropical deciduous forest. On the other hand, pollen of Eucommia, Fraxinoipollenites (Fraxinus), Betulaceae, Jungladaceae, are all allochthonous elements belonging to tropical mountain cloud forests, and diverse spores highlighted by Lusatisporis dettmannae (Selaginella oregana). In the dinoflagellates assemblage, both Operculodinium and Spiniferites ramosus abundance.

Keywords : continental and marine palynomorphs; alpha diversity; tropical deciduous forest; tropical mountain cloud forests; palynomorphic comparisons.

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