Fossil wonders of anoxic worlds: Linking marine ingressions to early cretaceous Konservat-Lagerstätten from Brazil

Abstract

Konservat-Lagerstätten are sedimentary deposits containing fossils with non-biomineralized structures and soft tissues. They are essential for studying past organisms and communities and constitute a key tool for recognizing ancient environments. Such deposits are rare in the geological record, as their genesis requires highly particular environmental, chemical, and biological conditions. In NE Brazil, two Konservat-Lagerstätten are well known in the Crato and Romualdo formations of the Lower Cretaceous (Aptian-Albian) sedimentary succession of the Araripe Basin. Specimens containing preserved soft tissues are also found in deposits of the Barbalha and Ipubi formations. Exceptionally preserved fossils in these four units are linked to distinct episodes of marine ingression during the final stages of the South America–Africa breakup and the opening of the South Atlantic Ocean. Marine waters progressively flooded marginal and continental settings, leading to the expansion of the oxygen minimum zone. The Batateira fossil beds in the Barbalha Formation and the Santana Konservat-Lagerstätte in the Romualdo Formation are associated with global oceanic anoxic events (OAE1a and OAE1b, respectively). The Ipubi fossil bed is also linked to a significant marine ingression event, which led to the formation of black shales and culminated in evaporite deposition. Conversely, the Crato Konservat-Lagerstätte is the only deposit occurring in a spatially restricted area associated with high alkalinity and salinity local conditions. Our data clearly show that the formation of these deposits with exceptionally preserved fossils is strongly controlled by marine incursions, followed by basin restriction and anoxia, which occurred successively in the early Cretaceous history of the Araripe Basin.