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Boletín de la Sociedad Geológica Mexicana

versão impressa ISSN 1405-3322

Resumo

GALVAN-ESCOBEDO, Iris G. et al. Registro paleopalinológico de la formación San Gregorio (Barreno B4), Baja California Sur, México. Bol. Soc. Geol. Mex [online]. 2017, vol.69, n.1, pp.35-58. ISSN 1405-3322.

In La Purísima region, Comondú, Baja California Sur, the borehole B4 was extracted from the San Gregorio formation (upper Oligocene-lower Miocene). It contains continental and marine palynomorphs. The aim of this study was to examine the borehole palinomorphs and analyze their richness and α diversity. We propose the vegetation types in which these elements could develop based on continental microfossils. Also the deposition environment was interpreted based on the marine microfossils. The richness for pollen-spores and dinoflagellate cysts assemblages was n = 161 and n = 28 respectively. The total richness was n = 189. The α-diversity (H’) of the pollen-spores assemblage was 2.61 and the evenness (J’) was 0.51. The dinoflagellate cysts diversity H’ was 2.33 and J’ was 0.70. The total diversity H’ was 3.02, and J’ was 0.57. Using CONISS analysis (constrained incremental sum of squares clustering), three zones in the stratigraphic column of the borehole were defined. Zone A, was characterized by the presence of Operculodinium dinoflagellate cysts and a scarcity of pollen-spores. Zone B, was defined by continental palynomorphs, highlighting the presence of Chenopodipollis, Graminidites, Liliacidites pollen and Lusatisporis dettmannae spores, as well as Achomosphaera, Lingulodinium and Polysphaeridium dinoflagellates cysts. Zone C, was established by the Tubulifloridites pollen record. The chronostratigraphic pointers were: Corsinipollenites, Ephedripites claricristatus, Momipites coryloides and M. tenuipolus. The pollen-spore assemblages suggests the presence of two plant paleocommunities: a cloud forest in the highlands at the regional level, and a tropical deciduous forest at the local level near to the deposit basin. The presence of marine palynomorphs in the sequence suggest that these deposits correspond to a shallow marine environment near the coast.

Palavras-chave : pollen-spores; dinoflagellates; diversity; paleovegetation; San Gregorio formation.

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