LATE MIOCENE PALEOENVIRONMENTAL CONDITIONS LEADING TO MARINE VERTEBRATE MASS DEATH ASSEMBLAGE IN THE NORTHERN CHILEAN COAST

A dense accumulation of over 40 well-preserved, frequently articulated marine vertebrate fossils excavated from the Mio-Pliocene Bahía Inglesa Formation in the Chilean Atacama Region (27°S) has been attributed to repeated mass mortality events during the late Miocene. The timing of accumulation and paleoenvironmental conditions allowing for the preservation of these catastrophic mass death assemblages are not well-understood. We present a revised chronostratigraphic framework for the Mina Fosforita (MF) and Cerro Ballena (CB) localities based on detailed litho-ichnofacies analyses, sequence stratigraphy, and U-Pb detrital zircon ages of ashes and sandstones, which suggest that both deposited in shoreface environments with distinct coastal bay configurations before and after 6.06 ± 0.32 Ma, respectively. The MF locality features a 26.4-m-thick, fining-upward succession of coquina beds and fossil-rich, trough cross-stratified sandstones separated from bioturbated ash and diatomite beds by a nodular phosphate hardground interval. The 9-m-thick section exposed at CB is composed of coarsening-upward, fine- to-medium-grained sandstones with abundant marine vertebrate and trace fossils, along with wave-ripple lamination and hummocky cross-stratification, characteristic of a barrier-protected, lower-to-middle shoreface depositional system with low-intensity, storm-related sedimentation. These localities reflect a transition between an unsheltered middle-to-upper shoreface to shelf environment at MF driven by the onset of a transgression before 6 Ma and subsequent deposition of CB strata in a barrier-protected shoreface farther north. Results suggest that rapid burial due to increased sedimentation rates associated with local sea-level rise and coastal upwelling allowed for the high preservation quality of marine vertebrates at both sites, which aligns with published taphonomy from CB that notes the absence of significant scavenging and disarticulation of cetacean fossils.