Oligocene–Miocene dinoflagellate cysts from the San Gregorio Formation, La Purísima area, Baja California Sur, Mexico

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.

采自墨西哥南下加利福尼亚州拉普里西马地区圣格雷戈里奥组（San Gregorio Formation, SGF）LB-5岩芯的沟鞭藻囊孢（dinoflagellate cyst），将本次研究的层序年代限定为渐新世-中新世（28~20 Ma）。本研究结果实现了该岩芯与区域内圣格雷戈里奥组标准剖面及另一处定年明确剖面的对比，即邻近的拉普里西马剖面，二者均存在明确的渐新世-中新世分界（O/M——23 Ma）。岩性与沟鞭藻囊孢数据显示，研究层序主体发育于海相沉积环境，经历了从底部陆相环境向渐新世-中新世分界处（约243 m深度）的上半深海（200~500 m水深）环境的海侵过程，随后向层序顶部发生海退。部分层位中几乎无沟鞭藻囊孢产出，这大概率是成岩作用期间的氧化作用所致。在囊孢浓度较高的层位中，异养类群占比可观，表明其余样品的孢粉保存质量足以开展定量分析。当沟鞭藻囊孢浓度＞2000囊孢/克沉积物（c/g sed）、异养类群丰度较高，且地层中存在磷质层与硅藻土时，指示圣格雷戈里奥组沉积时期区域生物生产力处于较高水平。该区域曾发育强烈的上升流环境，这大概率与渐新世-中新世分界附近的Mi-1冷事件（约23 Ma）以及约170 m深度处的Mi-1a冷事件（约21.7 Ma）相关联。LB-5岩芯的定量沟鞭藻囊孢数据显示，渐新世时期的生物生产力（平均为203 c/g sed）低于中新世时期（平均为848 c/g sed）。中新世层位的绝对丰度数据表明，LB-5岩芯与加利福尼亚湾南部佩斯卡德罗盆地的现代沉积物样品中，沟鞭藻囊孢浓度水平相近。