Boletín de la Sociedad Geológica Mexicana
Volumen 69, núm. 1, 2011, p. 35 ‒ 58
Registro paleopalinológico de la formación San Gregorio (Barreno B4), Baja California Sur, México
Iris G. Galván-Escobedo1,*, Elia Ramírez-Arriaga2, Alfonso Valiente-Banuet3, Ebandro Uscanga-Mortera1, Edmundo García-Moya1, Josué Kohashi-Shibata1
1 Posgrado en Botánica, Colegio de Postgraduados, Campus Montecillo, Carretera México-Texcoco Km. 36.5, Montecillo, Texcoco, 56230, Estado de México.
2 Laboratorio de Palinología, Dpto. de Paleontología, Instituto de Geología, Universidad Nacional Autónoma de México, Ciudad Universitaria, 04510, Coyoacán, Ciudad de México.
3 Departamento de Ecología de la Biodiversidad, Instituto de Ecología, Universidad Nacional Autónoma de México, Ciudad Universitaria, 04510, Coyoacán, Ciudad de México.
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.
Keywords: pollen-spores, dinoflagellates, diversity, paleovegetation, San Gregorio formation.