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

Bosio, Giulia; Gioncada, Anna; Malinverno, Elisa; Di Celma, Claudio; Villa, Igor M.; Cataldi, Giuseppe; Gariboldi, Karen; Collareta, Alberto; Urbina, Mario; Bianucci, Giovanni (2019). Chemical and petrographic fingerprinting of volcanic ashes as a tool for high-resolution stratigraphy of the Upper Miocene Pisco Formation (Peru). Journal of the Geological Society, 176(1), pp. 13-28. Geological Society of London 10.1144/jgs2018-071

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Tephra layers are a unique tool for stratigraphy. Their geologically instantaneous deposition together with their dispersion on wide areas make them a powerful instrument for dating and correlating simultaneous events at different localities. In this paper, we present and discuss the application of tephra fingerprinting in the upper Miocene portion of the Pisco Formation.
The Pisco Formation is an important marine vertebrate Fossil-Lagerstätte that crops out in the Ica Desert, on the Peruvian Coast. The paleontological relevance of this formation has entailed the necessity of reconstructing a chronostratigraphic framework, using 40Ar/39Ar dating and correlations on volcanic ashes. Tephra layers from the volcanic activity of the Peruvian Andes are very frequent in the Pisco Formation and many represent a primary air-fall deposition on the sea floor. Therefore, many samples from tephra layers within measured stratigraphic sections at different sites in the study area were fingerprinted with a combined approach, using petrographic and chemical analyses on phenocrysts and volcanic glasses, as well as glass shard morphology and granulometric analyses, performed on 53 selected samples, collected from different localities. Based on the results obtained, we correlate some tephra layers in two localities at several km distance, where the correlation between the measured sections was previously only hypothetical. Major element chemistry of biotite proved a valuable tool to discriminate different tephra in the upper Miocene portion of the Pisco Formation, due to the presence of peraluminous as well as metaluminous compositions.
This work shows that it is possible to apply tephra fingerprinting and tephrostratigraphy in a complex situation such as that represented by the Pisco Formation, where a high number of apparently similar volcanic ash layers were deposited in a shallow marine environment. The application of this correlation method allows us to increase the chronostratigraphic detail in the studied interval of the Pisco Formation, providing an accurate framework in which fossil marine vertebrates are stratigraphically located. The obtained high-resolution chronostratigraphy will greatly clarify the evolutionary history of Cetaceans.

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