Biostratigraphy, geochronology and sedimentation rates of the upper Miocene Pisco Formation at two important marine vertebrate fossil-bearing sites of southern Peru

Abstract

The Mio-Pliocene Pisco Formation, Peru, is a world renowned Konservat-Lagerstätte. Beside its potential in preserving important information on the history of the Pacific Ocean during the Miocene, the Pisco Fm. provides palaeontologists with the unique opportunity to study in detail the evolution of marine vertebrate fauna during the Neogene. Because diatomites are one of the main lithologies of the Pisco Fm., previous authors have hypothesised exceptionally high sedimentation rates related to the deposition of diatom frustules as the main cause leading to the formation of the Lagerstätte. However, sedimentation rates were never calculated. With the perspective to build a solid stratigraphic framework for the whole Pisco Fm., we started to investigate two important fossiliferous sites: Cerro Colorado and Cerro Los Quesos. Within these two areas, measurement of multiple stratigraphic sections combined with extensive field mapping of a number of distinct marker beds was carried out, which allowed a high-resolution correlation and creation of a local lithostratigraphic framework. Integrated tephrostratigraphy, 40 Ar/ 39 Ar dating and diatom biostratigraphy permitted an accurate chronostratigraphy to be established. Both biostratigraphic and radiometric ages converge to indicate a late Miocene age for the two sites, amending previous attribution of Cerro Colorado and Cerro Los Quesos to the middle Miocene. Age models for the site of Cerro Los Quesos reveal that sedimentation rates are high but in line with those of highproductivity environments, thus invalidating the hypothesis of exceptional sedimentation rates as the main cause for the formation of the Lagerstätte. These results are extremely important also in the perspective of building a complete stratigraphic framework for the Pisco Fm.

Looking for the key to preservation of fossil marine vertebrates in the Pisco Formation of Peru: new insights from a small dolphin skeleton

“…The excellent agreement between biostratigraphic data and a few well-constrained 40 Ar/ 39 Ar radiometric ages from volcanic ash layers provides useful constraints on the depositional age of the three Pisco sequences. At present, due to the lack of direct biostratigraphic and radiometric data, the age of P0 is constrained between by the age of the upper portion of the Chilcatay Formation (18.02±0.07 Ma from 40 Ar/ 39 Ar radiometric ages on a volcanic ash layer from the top of the Chilcatay strata at Cerro Submarino, Di Celma et al, 2018b) and the onset of deposition of the overlying P1 allomember strata at Cerro Colorado (dated between 9.5 and 9.10±0.04 Ma by means of diatom biostratigraphy and 40 Ar/ 39 Ar dating); the base of P2 allomember is younger than 8.5 Ma (Gariboldi et al, 2017).…”

How whales used to filter: exceptionally preserved baleen in a Miocene cetotheriid

“…The latter corresponds to the P2-8 horizon of Di Celma et al (2017), and is located above a more widely distributed horizon termed P2-6 or the 'Flor' marker bed (Bianucci et al 2016;Di Celma et al 2017). At the highly fossiliferous locality of Cerro Los Quesos, the Flor marker bed defines the lower boundary of 'Member F', which corresponds to the uppermost sediment package of the P2 sequence (Di Celma et al 2017), and includes two volcanic ash layers that have been 40 Ar/ 39 Ar-dated to 6.93 AE 0.09 Ma and ≥ 6.71 AE 0.02 Ma, respectively Gariboldi et al 2017; Fig. 1).…”

Chemical and petrographic fingerprinting of volcanic ashes as a tool for high-resolution stratigraphy of the upper Miocene Pisco Formation (Peru)

“…Recently, the stratigraphy of the Pisco Formation has been re-examined in detail (Di Celma et al, 2016a, 2016b2017). In particular, in order to place its fossil vertebrates in a proper chronostratigraphic framework, new stratigraphically-constrained sediment samples for diatom biostratigraphy and 40 Ar/ 39 Ar isotope ages have been collected and analyzed (Gariboldi et al, 2017). The fossiliferous strata of the Pisco Formation exposed along the western side of the Ica River Valley have been divided into three fining-upward depositional sequences named, from the oldest to the youngest, P0, P1 and P2 and bounded by three regionally extensive erosional surfaces, PE0.0, PE0.1, and PE0.2 (Di Celma et al, 2017).…”

“…The Pisco Formation is a diatomite-rich marine unit with numerous layers of volcanic ash representing primary air-fall events from the Peruvian Central Andes volcanoes and a very important tool to define its stratigraphic framework (Di Celma et al, 2017;Gariboldi et al, 2017). So far, their use has been limited to field correlations within a single locality and to obtain 40 Ar/ 39 Ar ages.…”

“…In the Pisco Formation, however, the high number of ash layers interbedded within a monotonous diatomrich succession, the similarity of their geochemical features and components, and the possibility that sea currents affected both their grain size and composition during deposition, hamper the unequivocal identification of these ashes directly in the field. In this paper, we focus on some of the ash layers from the P2 depositional sequence, which is the youngest unconformity-bounded unit documented so far in the upper Miocene portion of the Pisco Formation (Di Celma et al, 2017) and the most studied one for its high paleontological content. We fingerprint tephra layers of the P2 sequence by using major element chemistry of biotite and glass combined with petrographic and granulometric features, with the aim to correlate stratigraphic sections in different localities and highlight the feasibility of an approach based on major element biotite chemistry.…”