The early depositional history of the Pisco Formation (middle to Upper Miocene, Peru).

The East Pisco Basin of Peru is a world-renowned Fossil-Lagerstätte that has yielded an abundant and exceptionally well-preserved record of marine vertebrates within a sedimentary succession ranging in age from the middle Eocene to, at least, the Late Miocene. Owing to its remarkable wealth of fossil discoveries, the Miocene Pisco Formation is the most famous unit in the basin. Its stratigraphic architecture has been recently redefined in the Ica River Valley, with the identification of three depositional sequences (P0, P1 and P2, in ascending order), separated by extensive unconformities that testify to periods of subaerial exposure correlated with major climatic cycles. While P1 and P2 provided abundant diatom markers and several volcanic ash layers dated through the 40Ar/39 Ar method (P1, 9.5–8.6 Ma; P2, 8.4–6.7 Ma), the exposures of P0 that defined this unit were found to be barren of microfossils and lacked ash layers, thus their dating relied solely on strontium isotope stratigraphy (14.7-12.6 Ma). Here, we analyze a new section including P0 and P1 at Cerro Tiza. It represents a deeper portion of the Miocene Pisco Basin in the Ica River Valley, and features abundant siliceous markers, allowing for a good biostratigraphic control of the early Pisco deposits. Notably, the biostratigraphic age assignment of P0 in this work (14.2-12.9 Ma) confirms previous age estimates obtained through the strontium-isotope ratio and allows a chronological correlation with the Laberinto, Pampa and Naranja members of the Pisco Formation in the Laberinto area. Furthermore, stratigraphic markers and one ash layer from the P1 sequence indicate that deposition of this unit started earlier than hitherto recognized (slightly before 10 My), thus shortening the extension of the stratigraphic gap separating the two sequences. The microfossil assemblages also allowed for better constraining the paleoclimatic and paleoceanographic conditions that characterized the early depositional phases of the Pisco Formation.