The Eocene sediment successions of the East Pisco Basin (southern Peru) host an exceptionally rich and well-preserved assemblage of vertebrate fossils. However, due to the dearth of geochronological and biostratigraphic controls as well as of stratigraphic correlations, our understanding of these rocks and their fossil content remains elusive.

This paper provides a comprehensive calcareous nannofossil, diatom, and silicoflagellate biostratigraphic framework for the Eocene strata exposed at four localities along the Ica River Valley, permitting a robust chronological calibration of the marine vertebrate fauna entombed therein and a better definition of important appearance/ extinction events. The Paracas Formation, deposited directly on top of the Proterozoic and Paleozoic rocks of the crystalline basement, is formed by a siliciclastic-bioclastic sand-sized deposit (Los Choros member) and calcareous-terrigenous siltstone (Yumaque member) that was deposited from the Lutetian (47.8–41.2 Ma) through the Bartonian (41.2–37.7 Ma) to the early Priabonian (37.7–33.9 Ma). The unconformably overlying Otuma Formation consists of a basal sandstone, followed by calcareous siltstone intercalated by diatomite layers towards the top. In the study area, the Otuma Formation is Priabonian in age and is truncated at the top by an unconformity at the base of the overlying Miocene Chilcatay Formation. Due to the angular nature of the unconformity, the upper Otuma strata reach the Oligocene elsewhere.

Average sedimentation rates range from 17 to 24 m/My in the Yumaque member of the Paracas Formation and increase to 147–170 m/My in the Otuma Formation. The microfossil assemblages witness a coastal setting with warm-temperate conditions for the Paracas Formation that become slightly cooler (though still temperate) in the upper Otuma Formation. Diatomaceous layers in the upper Otuma Formation indicate an overall increase in nutrient availability, which could reflect the global reorganization of ocean currents at the Eocene-Oligocene transition. However, the taxonomic composition of the diatom assemblage suggests seasonal rather than persistent upwelling conditions.

东皮斯科盆地（秘鲁南部）的始新世沉积序列拥有异常丰富且保存完好的脊椎动物化石组合。然而，由于缺乏地质年代学和生物地层控制以及地层相关性，我们对这些岩石及其化石含量的理解仍然难以捉摸。

本文为伊卡河流域四个地点出露的始新世地层提供了一个全面的钙质纳米化石、硅藻和硅藻生物地层框架，允许对埋在其中的海洋脊椎动物群进行可靠的年代校准，并更好地定义重要的出现/灭绝事件. 帕拉卡斯组直接沉积在结晶基底的元古界和古生界岩石之上，由硅质碎屑-生物碎屑砂粒大小的沉积物（Los Choros 段）和石灰质陆源粉砂岩（Yumaque 段）形成(47.8-41.2 Ma) 到巴托阶 (41.2-37.7 Ma) 到早期的普里亚波阶 (37.7-33.9 Ma)。上覆不整合的 Otuma 组由基底砂岩组成，其次是钙质粉砂岩，顶部夹有硅藻土层。在研究区，Otuma 组为普里亚博尼阶，顶部被上覆中新世 Chilcatay 组底部的不整合面截断。由于不整合面的角度特性，Otuma 上层地层在其他地方到达渐新世。

Paracas 组 Yumaque 段的平均沉积速率范围为 17 至 24 m/My，Otuma 组增加到 147-170 m/My。微化石组合见证了帕拉卡斯组的沿海环境和暖温带条件，在奥图马组的上部变得稍微凉爽（尽管仍然是温带）。Otuma 组上部的硅藻层表明养分可用性总体增加，这可能反映了始新世-渐新世过渡时期洋流的全球重组。然而，硅藻组合的分类组成表明季节性而不是持续的上升流条件。