Abstract

Dinoflagellate cysts from the LB-5 core in the San Gregorio Formation (SGF) from the La Purísima area, Baja California Sur, Mexico, limit the age of the studied succession to Oligocene–Miocene (28–20 Ma). Our results allow correlation of this core with the type locality and another well-dated sections of the SGF in the region, namely the nearby La Purísima section, with an unequivocal Oligocene–Miocene boundary (O/M – 23 Ma). Lithology and dinoflagellate cyst data indicate mainly marine sedimentary environments, with a transgression from continental at the base to upper bathyal (200–500 m) in the O/M level (∼243 m), and a regression towards the top of the unit. Dinoflagellate cysts are virtually absent in some intervals, probably due to oxidation during diagenesis. The abundance of heterotrophic taxa during intervals with high cyst concentrations indicates that palynological preservation was adequate for quantitative analyses in the rest of the samples. Samples with dinoflagellate cyst concentrations >2000 cysts/gram of sediment (c/g sed), the abundance of heterotrophic taxa, and the presence of phosphoritic layers and diatomites indicate high biogenic productivity during the deposition of the SGF. Intense upwelling conditions in the area are probably associated with the cold events Mi-1 (∼23 Ma) near the O/M and Mi-1a (∼21.7 Ma) at ca. 170 m depth. Quantitative dinoflagellate cyst data in the LB-5 core suggest lower productivity (mean = 203 c/g sed) in the Oligocene than during the Miocene (mean = 848 c/g sed) interval. The absolute abundances of the Miocene intervals indicate similar dinoflagellate cyst concentrations in the LB-5 core and modern samples from the Pescadero Basin in the southern Gulf of California.

摘要

来自墨西哥南下加利福尼亚州 La Purísima 地区圣格雷戈里奥组 (SGF) 的 LB-5 核心的甲藻包囊将研究的演替年龄限制在渐新世-中新世 (28-20 Ma)。我们的结果允许将该核心与该地区 SGF 的类型位置和另一个年代久远的部分相关联，即附近的 La Purísima 部分，具有明确的渐新世 - 中新世边界（O / M - 23 Ma）。岩性和甲藻包囊数据表明主要是海洋沉积环境，在O / M水平（~243 m）从底部的大陆到上深海（200-500 m）的海侵，并向单元顶部回归。甲藻囊肿在某些时期几乎不存在，可能是由于成岩过程中的氧化。在高孢囊浓度期间异养类群的丰度表明孢粉保存足以对其余样品进行定量分析。甲藻包囊浓度 > 2000 包囊/克沉积物 (c/g sed) 的样品、异养类群的丰度以及磷质层和硅藻土的存在表明 SGF 沉积过程中的生物成因生产力很高。该地区强烈的上升流条件可能与 O/M 附近的寒冷事件 Mi-1（~23 Ma）和 ca. 170 米深度。LB-5 核心中的定量甲藻包囊数据表明，渐新世的生产力（平均 = 203 c/g sed）低于中新世（平均 = 848 c/g sed）间隔。