Tephrochronology and biostratigraphy of two exceptional fossil localities in the Pisco Formation (Peru)

An exceptional fossil record of marine vertebrates is hosted in the Pisco Formation, Peru (Bianucci et al., 2015 and references therein), which is attributed to middle Miocene to Pliocene (Brand et. al., 2011). Fossil vertebrate remains are numerous, found in many different levels of the Pisco Formation (spanning more than 10 Ma), and often extremely well preserved. Although crucial to reconstruct evolutionary trends, the chronostratigraphic setting is still poorly constrained. High sedimentation rates favoured by the high productivity of this upwelling region could have played a role in the formation of this exceptional fossil record, but they have not been quantified so far. The Pisco Formation is a mainly diatomaceous sedimentary succession with interbedded volcanic ash layers from the Central Andean volcanoes; therefore tephro- and diatom biostratigraphy are suitable chronostratigraphic tools. Based on a detailed stratigraphic reconstruction (Di Celma et al., 2015), over 70 volcanic ash beds were identified in two non-overlapping sections of about 200 and 300 m respectively. Most ashes have a dacitic to rhyolitic glass composition and a minor crystal fraction of juvenile feldspars and biotite. Crystal-bearing ashes with field characteristics suggestive of distal deposits of single eruptive events were selected for granulometric, SEM-EDS and EPMA analyses, avoiding obvious alteration. Biotite-feldspar pairs from five tuff levels were selected for 40Ar/39Ar dating by step-heating. Feldspars were mostly subtly altered, as shown by very high Cl/K ratios. Some but not all biotites were substoichiometric, i.e. also partly altered. Data mining returns age ranges concordant with stratigraphy: 7.4±0.2 (upper allomember) – 9.0±0.1 (lower allomember) Ma for the older Cerro Colorado section and less well constrained ages around 7 Ma in the Cerros Los Quesos section. Those results agree with biostratigraphic evidence of late Miocene ages for both sections