Controls on late Miocene marine vertebrate bonebed genesis in northern Chile

Abstract

The Bahía Inglesa Formation in the Chilean Atacama Region (27°S) hosts one the world's best-preserved Cenozoic marine vertebrate death assemblages attributed to harmful algal bloom (HAB)-mediated mass mortalities. However, the lack of a well-dated depositional model prevents understanding of the timing of fossil accumulation and associated paleoenvironmental conditions. We present a revised chronostratigraphic framework for the Bahía Inglesa Formation at the Cerro Ballena and Mina Fosforita paleontological sites based on detailed field sedimentology, paleontology, and UPb zircon and phosphate geochronology. We integrate maximum depositional ages (MDAs) of intercalated volcanic ash and sandstone beds with published basinal chronostratigraphy to provide the first correlation between both localities. Detrital zircon UPb ages suggest that the highly articulated cetacean carcasses preserved at Cerro Ballena were rapidly deposited in a barrier-protected shoreface environment at ca. 6.1 Ma during periods of voluminous volcanic ash and diatom accumulation on the shelf. Evidence of disarticulated and taxonomically diverse vertebrate fossils within the Mina Fosforita bonebed suggest deposition in an unsheltered offshore transition to inner shelf environment with strong wave action and rapid flooding conditions between ca. 7.4 and 6.8 Ma. We propose that the fossil accumulations at Cerro Ballena and Mina Fosforita were deposited during distinct sedimentation events controlled by local bay paleogeography and rapidly fluctuating relative sea-level conditions. We compare our chronostratigraphic results with 2969 published magmatic ages from the Central Andes and propose a correlation between the mass deaths of marine mammals in northern Chile and intensifications in volcanism, ocean fertilization, phosphogenesis, and HAB blooms during the late Miocene.