A window into a middle Eocene mesotrophic mixed siliciclastic-carbonate system from the South American Pacific margin (Paracas Formation, Pisco Basin, Peru).

The Paracas Formation of the Pisco Basin (southern Peru) includes the lower Los Choros Member and the upper Yumaque Member and preserves key paleoenvironmental information on the shallow-marine tropical ecosystems of the eastern Pacific in middle Eocene times. This study focuses on five stratigraphic sections and a few minor outcrops of the Los Choros Member exposed in the Ica Desert. Sedimentological observations, X-ray diffraction data, and micropaleontology reveal a vertical transition from coarse-grained, mixed siliciclastic-carbonate bioclastic deposits resting unconformably on the pre-Cenozoic basement, to finer-grained fossiliferous mudstones of the overlying Yumaque Member. Large benthic foraminifera from the Los Choros Member and nannofossils from the base of the overlying Yumaque Member constrain the deposition of the former to around 43 Ma. Skeletal assemblages include both large and small benthic foraminifera along with mollusks, barnacles, red algae, echinoderms and bryozoans, pointing to a warm-temperate to tropical, shallow-marine setting characterized by moderate hydrodynamic energy and mesotrophic conditions. Despite being taxonomically distinct, the carbonate-producing systems of the paleo-Pacific display functional similarities with the coeval Tethys platforms. These systems evolved independently until the Oligocene, when lepidocyclinids expanded into the Tethys, thereby replacing rapidly the indigenous, nummulitid-dominated faunas. The factors behind the evolutionary success and dispersal of the lepidocyclinids are still largely unknown, but the sedimentological and paleoecological features of the Los Choros strata suggest that thermal adaptability played a major role.