Coquimbo Formation

The Coquimbo Formation was first noted by naturalist and researcher Charles Darwin during his visit to Chile aboard in the 1830s. Darwin observed prominent sedimentary structures (such as cross-stratification on marine terraces) and marine fossils (cetacean bones, shells, shark teeth, among others) of "Tertiary" age at the coast of the Coquimbo Region, approximately more than northward of the Navidad coast. In 1985, Steinmann G. analyzed the marine deposits around the Quiriquina Island of Bay of Concepción with observations to other deposits from Chile. He erected the term "Coquimbo Stufe" for the marine sediments along the coast of Coquimbo. Author Rubén Martínez-Pardo noted that ever since the observations made by Darwin, most of the Neogene northern-central marine deposits of Chile have been wrongly referred to this unit. Ramón Moscoso and team in 1982 published a large geologic map focused on the Atacama and Coquimbo regions of Chile, where they, in view of the confusion regarding the designation of the Coquimbo Formation, proposed to reserve the term Coquimbo Formation for the set of marine sediments located along the coastline between 28° and 30° S of the Coquimbo Region. Culebrón Park Strata of the Coquimbo Formation are largely exposed across creeks and slopes of the Culebrón Park (Parque el Culebrón) of the region. Coquinas, sandstones, and other sediments, as well as abundant fossilized mollusks, can be observed all over the park. Despite the paleontological nature of the site, the Culebrón Park has undergone multiple instances of cleaning-up due to pollution caused by littering, with several areas of the park having become littering spots. During the summer of 1984, reforestation work at the area of the Culebrón Park lead by the National Forest Corporation (CONAF) revealed two relatively complete and articulated cetacean skeletons in nearby creeks with outcrops of the Coquimbo Formation. The individuals were unearthed within the same stratigraphic context from yellowish sandstones and some coquinas at a depth no greater than , and set apart from each other by approximately . Upon the discovery of such remains, the Archaeological Museum of La Serena was contacted in order to manage the fossils. The museum later reached out to the Chilean National Museum of Natural History for further study. Later in 1988, geologist Patricia Z. Salinas published a brief description of the cetaceans and overall discovery, as well as describing the strata of the Coquimbo Formation in which they were found. As stated by Salinas, the cetaceans could not be recovered and studied due to their fragile preservation and were left in field, where consolidation treatments were applied for their protection, pending the establishment of a local museum. Though initially reported as "semifossils" by Salinas despite their age, Lomas del Sauce During the year 2013, geological survey was carried out on a land lot near El Sauce avenue of Coquimbo city, about southeast of Bahía Herradura, in the context of the real estate project Lomas del Sauce. As the lineaments progressed, abundant fossil material was discovered from fossiliferous coquinas and sandy silt assigned to the Coquimbo Formation when trial pits were performed on terrain. Out of the 10 trial pits, one yielded two or three semi-articulated cetacean skeletons including a partial skull. The fossils from this locality, mostly represented by mollusks shells and partial sea birds and fish remains, were recovered and excavated by grids, covering an area of about 32 m2. Reported material was set out to be stored at the Sala de Colecciones Biológicas of the Catholic University of the North of Coquimbo. Soil and sediment characterization of the Lomas del Sauce locality was achieved through the modelling of sub-surfaces and stratigraphic columns. Larger fossils, such as the bones of cetaceans, were extracted and secured in field jackets. Throughout the study of the locality, further fossil taxa was discovered, such as the semi-aquatic ground sloth Thalassocnus, which was previously only known from the similar Pisco Formation and Bahía Inglesa Formation. In 2017, the Thalassocnus material was formally described, contributing to the marine mammal record of the Coquimbo Formation. The specimen is now stored within the collections of the Chilean National Museum of Natural History. ==Description==

The Coquimbo Formation belongs to a discontinuous series of Cretaceous-Neogene sedimentary basins and outcrops of the formation are located across the coasts of the Coquimbo Region, Chile, where multiple marine terraces are exposed. With about in thickness, the Coquimbo Formation is uncomfortably deposited over mesozoic crystalline basement composed of intrusive rocks. It consists predominantly of yellowish, sometimes phosphatized, sands, sandstones, siltstones, highly bioclastic limestones, coquinas, and conglomerates. At the locality of Carrizalillo and nearby creeks, unusual sedimentary structures are present, derived from strong tidal wave processes and elutriation. ==Depositional environment==

Based on the record of foraminiferans at Tongoy, it has been suggested that the deposits at this area were deposited in the lower zone of a continental shelf with relatively warm waters. Diego M. Partarrieu in his 2022 doctorate thesis proposed at least four sedimentary episodes across the localities of Coquimbo, with the two oldest episodes situated in a beach depositional environment, and the two youngest episodes occurring within shallow, intermareal waters. The changes in depositional environments were also followed by rapid faunal turnover and large-scale climatic change. Taphonomy The changes in sea level throughout the history of deposition of the Coquimbo Formation have been also important for taphonomic alterations to fossils. At the locality of Quebrada Chañaral the fossils of vertebrates show unusual features such as their nodulization within sediments, iron and phosphate mineralization, and high roundness. In addition, many vertebrate fossils show fragmentation and erosion on their bone surfaces likely originated from long exposure to the environments. These taphonomic features are interpreted as reworking from older fossils that were ultimately deposited in younger sediments due to marine regressions. Similar conditions have reported from other discoveries, such as Thalassocnus from Lomas del Sauce. The specimen was found with most elements in articulation except for the forelimb and pelvis remains, indicating that the skeleton suffered displacement before burial. The in situ unearthing paired with the breaking of some of the bones suggests that these were broken just before or during the burial. Other taphonomic features are reported, such as the development of a phosphatic crust at the bottom of the fossiliferous bed that mineralized most elements, or the attachment of bivalve internal molds and shells onto bones. ==Geographic extent==

The Coquimbo Formation is largely deposited along the coasts and creeks of the Coquimbo Region. It can be found outcropping from Tongoy city to Caleta Chañaral (and Chañaral de Aceituno). The latter does not form part of the Coquimbo Region and is instead located at the southernmost extension of the Atacama Region, making this area the northernmost extension of the Coquimbo Formation. In Tongoy, the formation runs across creeks that feature large marine terraces. In Coquimbo, the formation can be traced as far as the Culebrón Park, with a more consistent extent at Quebrada Las Rosas. ==Tectonic evolution==

In the area of Tongoy, the Coquimbo Formation was deposited in an ancient bay that was formed in a graben or half-graben, with a normal fault dipping east derived from local and regional tectonic movement as recorded in the sedimentary successions and geological structures of this area. The tectonic movement is associated with the subduction of the Juan Fernandez Ridge and a second oceanic plateau which caused the uplifting of the continental crust and changes in the sea level, along with faulting. ==Stratigraphy and age==

The Coquimbo Formation was sedimented during the Neogene discordantly covering the basament scarps of the region, most notably the northern sector of La Serena, Carrizalillo. In 2024, Benjamín A. Araya and team proposed a division of the Coquimbo Formation intro three members, characterized as a Lower Member of Tortonian-Messinian age composed of yellowish-gray muds and silts with iron and magnesium nodules, diatomites, and fine-grained sands with intercalations of coquina and phosphate deposits; a Medium Member of Messinian-Zanclean age comprising silty sands, silts, floatstone, and conglomerates; and lastly an Upper Member of Zanclean-Piacenzian age including stratified, sand-grained coquinas that are overlaid by rudstones, coarse-grained sands, and matrix-supported coquinas. ==Paleobiota of the Coquimbo Formation==

'' along other marine fauna. This shark is an important component of the paleofauna of the Coquimbo Formation and similar units The paleobiota of the Coquimbo Formation is extensively similar to those of the Bahía Inglesa and Pisco formations, also sharing similar depositional environments. During the Pliocene, the coasts of the Coquimbo Region served as nursery areas for Carcharodon carcharias, as evidenced in the high abundancy of juvenile teeth in multiple fossil localities belonging to the Coquimbo Formation. Algae Diatoms Dictyochales Dinoflagellates Ebriids Foraminiferans Invertebrates Barnacles Bivalves Brachiopods Decapods Gastropods Invertebrate traces Ostracods Vertebrates Birds Cetaceans Fish Bony fish Eels Rays Sharks Seals Sloths ==See also==