Depositional environment of upper Miocene to lower Pliocene diatom-rich deposits of the Bahía Inglesa Formation inferred from benthic foraminiferal assemblages

Neogene diatom-rich deposits of north-central Chile represent fundamental archives for studying the dynamic relationship between sea surface temperatures and paleoclimatic fluctuations in the Atacama Desert. To ensure a reliable correlation between the (often discontinuous) marine and terrestrial archives, a well-calibrated stratigraphic framework is needed. In this sense, this study presents microfossil and sedimentological data from a c. 9m-thick diatomaceous mudstone deposit cropping out at the Quebrada Tiburón (27°42' S, 70°59' W, Bahía Inglesa Formation) for paleoenvironment investigations. Biostratigraphy, chemostratigraphy, and tephrochronometry correlated the diatomaceous mudstones with the upper Messinian and Zanclean (c. 6.1-3.6 Ma). To further improve the paleoceanographic interpretations and stratigraphic framework of the sequence, we also measured stable oxygen isotopes (δ¹⁸O) from two shallow infaunal benthic foraminifera species (Bulimina falconensis and Uvigerina striata/peregrina). However, the diatom-rich succession shows distinctive horizons of bioturbation, often associated with overlaying sandstones of variable thicknesses which are intercalated with the diatomaceous mudstones. These repetitive bioturbation cycles warrant caution when interpreting δ¹⁸O data, as they may imply potential hiatuses in sedimentation. At the same time, the observed cycles may provide the opportunity to establish a sequence stratigraphic framework for the deposition of the diatomaceous mudstones.

Benthic foraminiferal assemblages from the mudstones indicate high export productivity and limited oxygen supply at the seafloor related to a coastal upwelling setting. The low-diversity fauna is mostly dominated by individuals < 150 μm of Bolivina granti/pacifica, Epistominella obesa, and Eubuliminella bassendorfensis. Across the Miocene/Pliocene boundary, the relative abundances of Bolivina granti/pacifica and Epistominella obesa respectively increase and decrease, and Bolivina aenariensis disappears. This faunal shift may be explained by changes in the amount and/or frequency of organic matter input. In the intercalated sandstones, Bolivina granti/pacifica, Eubuliminella bassendorfensis, and Epistominella obesa are still the most abundant species, with variable minor contributions of uvigerininds, Bolivina advena and shallow-water taxa such as Buccella peruviana. Notably, the faunal composition of these sandstones differs from neritic upper Tortonian to lower Messinian and upper Pliocene sandstones below and above the studied section, respectively, in which cibicids and Buccella spp. are dominant. In further steps, increased resolution of the assemblage data and statistical analysis combined with sedimentological data will provide more insights into the depositional processes to explain the observed sedimentary cycles. Understanding these processes will help to interpret the δ¹⁸O record and potentially establish a sequence stratigraphic framework for the section.

This study contributes to CRC 1211 “Earth-Evolution at the dry limit”, funded by the Deutsche Forschungsgemeinschaft (DFG).