Integrated biostratigraphy and paleoenvironmental setting of the initial deposits of the Pisco Formation (Peru): Middle Miocene to early Late Miocene

Abstract

The East Pisco Basin is a world known Fossil-Lagerstätte that provided an abundant and exceptionally well-preserved record of marine vertebrate fauna (Bosio et al., 2021) within a middle Eocene to late Miocene sediment succession. In particular, the Miocene Pisco Formation is the most studied: its stratigraphic architecture has been recently redefined in the Ica River Valley (Di Celma et al., 2017, 2022) with the identification of three depositional sequences, namely P0, P1, P2 in ascending order, separated by extended unconformities which testify subaerial exposure and correlate with major climatic cycles (Di Celma et al., 2018). While P2 (8.4-6.7 Ma) and P1 (9.5-8.6 Ma) provided abundant diatom markers and common volcanic ash layers that were dated through the 40Ar/39Ar method (Bosio et al., 2020a), P0 (14.7-12.6 Ma), lacking microfossils and ash layers, was chronologically constrained by Strontium Isotope Stratigraphy (Bosio et al., 2020b). A new section sampled at Cerro Tiza in the Ica River Valley provided a fossiliferous P0 and the basal P1,  allowing to constrain the age and paleoenvironment of these early deposits of the Pisco Formation. The age of P0 is now constrained by the occurrence of Denticulopsis hyalina (14.9-13.1 Ma), D. simonsenii (14.5-8.7 Ma), D. vulgaris (~13.5-8.5 Ma), Koizumia adaroi (14.2-9.0 Ma) and Crucidenticula nicobarica (15.1-12.3 Ma) - similar to the assemblage observed further south in the Laberinto area (DeVries et al., 2021) and well in agreement with isotopic dating - in a high productivity (abundant Thalassionema) coastal setting (common phytoliths, Paralia, Melosira, Actinoptychus and Grammatophora). The early deposits of P1 are constrained by the occurrence of Lithodesmium reynoldsii (10.0-9.0 Ma), D. vulgaris (~12.0-8.6 Ma) and D. praekatayamae (9.6-8.6 Ma) still in a high productivity (common Thalassionema, Chaetoceros) coastal setting (common Delphineis, Actinoptychus, Grammatophora). Few to abundant calcareous nannofossils (Reticulofenestra pseudoumbilicus, R. perplexa, Discoaster variabilis, Coccolithus pelagicus) are also here reported for the first time, testifying to a more open coastal environment, as compared to the previously-analyzed settings located more on-shore.