Stratigraphy and palaeoenvironment of Upper Miocene diatomaceous mudstones (Coquimbo Formation) at Tongoy, Chile

Diatom-rich sediments exposed along the coastline of northern and central Chile are fundamental archives of coastal upwelling properties associated with the Humboldt Current System in the Neogene. One of the southernmost occurrences of Neogene diatom-rich sediments is known from the Tongoy Peninsula (30.3°S). This study presents new bio- and magnetostratigraphic data from diatomaceous mudstones in the Quebrada Las Salinas, together with an assessment of the depositional environment. The results constrain the outcrop to the upper Tortonian to Messinian (c. 7.6–6.0 Ma). If the absence of Fragilariopsis miocenica is considered a true biostratigraphic signal, the range may be constrained to the uppermost Tortonian (c. 7.6–7.3 Ma). The comparison with previous studies suggests that Neogene diatom-rich deposits in the study area are highly diachronous within and between quebradas, extending from the Tortonian to the Pleistocene. Assemblages of siliceous microfossils indicate coastal upwelling in the Tongoy Palaeobay, increasing in intensity over the studied time interval. The intensification of coastal upwelling may have been the result of the coeval strengthening of the Antarctic Circumpolar Current and the Humboldt Current System.