The stratigraphic record of the Eocene–Oligocene transition in the East Pisco Basin (Peru): a phytoplankton perspective

The East Pisco Basin of southern Peru is renowned worldwide for its exceptionally well-preserved marine vertebrate fossils, recovered from sedimentary successions spanning the middle Eocene to the early Pliocene. Although the lower Eocene–Oligocene deposits of the Otuma Formation have been extensively investigated, the uppermost Eocene to lower Oligocene strata remain comparatively understudied. Notably, these strata encompass the Eocene–Oligocene transition (~ 34 Ma), a critical interval characterized by profound global climatic and ecological changes. This study presents an integrated biostratigraphic analysis of the Otuma Formation based on calcareous nannofossils, diatoms, and silicoflagellates. The investigated deposits were laid down in two distinct sub-basins: an eastern, land-proximal sub-basin at Cerro Tiza and a western, more distal sub-basin at Quebrada Perdida (Media Luna). In addition, 39Ar–40Ar dating of a volcanic ash layer provides an independent chronological tie point for the age model at Cerro Tiza. At both localities, the Otuma Formation comprises calcareous siltstones and diatomite layers deposited from the Priabonian (37.71–33.90 Ma) to the Rupelian (33.90–27.82 Ma). Sedimentation rates derived from the age model average 75.5 m/My at Cerro Tiza and 8.1 m/My at Media Luna. A progressive shift from calcareous nannofossil-dominated assemblages to diatom-dominated assemblages throughout the sections reflects a general trend toward cooler conditions and increased nutrient availability, consistent with global patterns during this interval. However, diatom assemblages do not indicate the establishment of permanently eutrophic conditions. Differences in microfossil assemblages, together with an order-of-magnitude contrast in sedimentation rates between Cerro Tiza and Media Luna, point to markedly distinct palaeoenvironmental conditions in the two sub-basins. These differences are interpreted as the result of tectonic controls and contrasting positions within the Peruvian forearc system.