Crato Lake Deposits: Rocks to Preserve an Extraordinary Fossil Lagerstätte

The Crato Formation is the main lacustrine unit of the Araripe Basin, known for its limestone layers since the nineteenth century, rich in well-preserved Early Cretaceous fossils. Since then, numerous biota has been studied, including abundant plant remains, thus granting the status of a Konservat Lagerstätte to the formation. Despite the paleontological interest, it is known that the host rocks of fossils are more than just limestone, yet include marls, shales, sandstones, and more. The study of these rocks complements environmental analyses of the Crato Formation, and they are essential in sedimentological and taphonomical research of lake deposits. In this subject, different authors carried out several studies, being the main approaches by means of analyses of fieldwork facies studies, petrography, mineralogy, microfossil studies, and organic and inorganic geochemistry. This effort involved data obtained from outcrops, quarries, and well logs. The current panorama deals with a very dynamic lacustrine system, sometimes fragmented in several lakes but eventually reassembled in a general tendency of enlargement. The main control was probably the rainfall regime under orbital forcing of cycles estimated to last 200–500 ka each. Alternating arid and slightly wet periods, six main carbonate units were recognized, each representing shallow to deep waterbodies fed by water, sediment, and nutrients through ephemeral deltas. Life thrived in four paleoenvironmental zones, from underwater saline lakes to the small hills around them. Once the lakes were very calm, the carcasses suffered little from transport; therefore, taphonomical studies have focused on chemical transformations inside the water column and sediment.