Sedimentological controls on plant-fossil preservation in an Eocene caldera-lake fill: a high-resolution, age-constrained record from the Tufolitas Laguna del Hunco, Chubut province, Argentina

Abstract

Caldera lake sediments of the early Eocene Tufolitas Laguna del Hunco (Chubut Province, Argentina)host one of the world’s best-preserved and most diverse fossil plant assemblages, but the exceptional quality of preservationremains unexplained. The fossils have singular importance because they include numerous oldest andunique occurrences in South America of genera that today are restricted to the West Pacific region, where many ofthem are now vulnerable to extinction. Lacustrine depositional settings are often considered optimal for preservationas passive receptors of suspended sediment delivered, often seasonally, from lakeshores. However, caldera lakescan be influenced by a broader range of physical and chemical processes that enhance or decrease fossil preservationpotential. Here, we use Laguna del Hunco to provide a new perspective on paleoenvironmental controls onplant fossil preservation in tectonically active settings. We establish a refined geochronological framework for theLaguna del Hunco deposits and present a detailed history of processes active during ~ 200,000 years of lake fillingfrom 52.217 6 0.014 Ma to 51.988 6 0.035 Ma, the time interval that encompasses nearly all fossil deposition.Detailed facies analysis shows that productive fossil localities reside within high-deposition-rate beds associated withhigh-energy density flows and wave-reworked lake-floor sediments, challenging traditional views that low-energyenvironments are required for well-preserved plant fossils. These results demonstrate that even delicate fossil componentslike fruits and flowers can survive high-energy transport, underscoring the importance of rapid burial as aprimary control on fossil preservation. Short, steep sediment-transport networks may facilitate terrestrial fossilpreservation by limiting opportunities for biochemical degradation on land and providing relatively frequent, highenergydepositional events, which quickly transport and bury organic material following events such as landslidesfrom steep, wet, surrounding slopes. Our new model for plant taphonomy opens a path toward finding and understandingother exceptional biotas in environments once considered unlikely for preservation.