Abstract

Tephra layers are a unique tool for stratigraphy. Their geologically instantaneous deposition across wide areas make them a powerful instrument for dating and correlating distant localities. In this paper, we apply tephra fingerprinting for high-resolution stratigraphic purposes in the upper Miocene portion of the Pisco Formation, cropping out along the southern Peruvian Coast. The Pisco Formation diatomaceous strata host an important marine vertebrate Fossil-Lagerstätte, whose palaeontological relevance has entailed the necessity of reconstructing a detailed chronostratigraphic framework using 39Ar–40Ar dating and tephra correlations. Distal ashes from the Central Andes volcanoes are frequent in the Pisco Formation, but their similar glass composition and mineral assemblage, joined to the shallow marine environment limiting tephra preservation, could be unpromising for tephra-based correlations. In this work, tephra layers from measured stratigraphic sections were fingerprinted with a combined petrographic and microanalytical approach, including glass shard morphology and granulometric analyses. Based on the obtained results, we correlated sections at several kilometres of distance, highly increasing the chronostratigraphic detail. Major element chemistry of biotite proved a valuable tool to discriminate the Central Andes metaluminous to peraluminous tephra deposited in the Peruvian forearc basins. This work highlights the applicability of tephra fingerprinting in unfavourable shallow marine environments.

Supplementary material: Compilation of tephra data in Table S1 (field observations, granulometric parameters, petrographic data and EPMA analyses on glass), Table S2 with EPMA single analyses on glass, Table S3 with EPMA analyses on biotite, 39Ar–40Ar step heating results (Fig. S1), measured stratigraphic logs (Fig. S2) and biotite chemical diagrams (Fig. S3, S4, S5, S6) are available at https://doi.org/10.6084/m9.figshare.c.4201484

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