Coquimbo Formation
The Coquimbo Formation is a littoral, sedimentary, and fossiliferous geological formation that chiefly crops out along the coast of the Coquimbo Region, Chile. It is dated back to the Miocene to Middle Pleistocene. The lithology of the formation comprises sands, sandstones, siltstones, limestones, coquinas, and conglomerates. The strata and facies of the Coquimbo Formation reflect a complex marine depositional history, varying from deep to shallow water paleoenvironments controlled by tectonic events and turbulent, successive changes in sea the level that developed large marine terraces within the strata which is recorded in the form of marine transgressions and regressions.
The Coquimbo Formation preserves a rich fossil marine assemblage, including molluscs, barnacles, brachiopods, fish, seabirds, and marine mammals such as cetaceans and aquatic ground sloths. As a whole, the paleoenvironments, and paleofauna of the formation have experienced strong changes and successions through time mainly due to large climatic and tectonic events. In both lithology and fossil content the Coquimbo Formation is very similar to other units of South America, such as the Pisco Formation of Peru or Bahía Inglesa Formation of the Atacama Region, which is also found in Chile.
History
The Coquimbo Formation was first noted by naturalist and researcher Charles Darwin during his visit to Chile aboard the HMS Beagle in the 1830s. Darwin observed prominent sedimentary structures and marine fossils 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
of the Coquimbo Formation are largely exposed across creeks and slopes of the Culebrón Park 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 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. In 1991 researchers Jose Yañez and Jhoann Canto made observations regarding the identification of the cetacean fossils, correcting their taxonomic family. In their brief note, the team also criticized the lack of a more meticulous procedure regarding the report and conservation of these valuable fossils, such as the storage of detailed photographs. Though initially reported as "semifossils" by Salinas despite of their age, Yañez and Canto stated that individuals eventually dissociated in the field preventing attempts for future studies.
In 2021 the Geological Society of Chile formally recognized the Culebrón Park as a geositio, that is, an important locality of either geological or paleontological value. Documentation and petitions were conducted by at-the-time Geology thesist Benjamín A. Araya, who advocated for the recognition and protection of the Culebrón Park. Although the fossiliferous value of the park and implications for the Coquimbo Formation have been in knowledge for a long time, no recognitions had ever been given before.
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. The sedimentological record from Quebrada Honda preserves data that can reflect two major depositional environments during its history of deposition. Towards the bottom of the sequence, sediments and fossil content suggests a low-energy, deep-water marine paleoenvironment with high ecological activity/dynamics. The abundance of phosphorite and marine vertebrates, the contribution of polymictic clasts, and angular unconformities from the successive layers of strata of this section, indicates periods of turbulent rising sea levels with the posterior mark of a marine transgression which can also be reflected in the decreasing abundancy of the fossil content towards the roof of the sequence, especially brachiopods. The presence of phosphorite deposits suggests that during the deposition of sediments the environment was subject to marine upwelling, where microorganisms and vertebrate remains precipitated phosphate deposits. Towards the roof the sequence a turbulent deposition can be inferred from the abundant emergence of unconformities as well as conglomerates and coquinas likely derived from the paleofauna of the older, low-energy deposits. These events are interpreted as products from a high energy tsunami or storm tides. Quebrada El Culebrón preserves a similar sedimentological record.The fossil record of shallow water-based taxa such as Crassostrea, Heterodontus, Incatella or Thalassocnus also suggests a shallow marine environment upon deposition. The foraminiferan fossil assemblage from Bahía de Guanaqueros indicates several changes in sea levels and depositional environments, from neritic to bathyal marine conditions. Constanza A. García in 2019 identified multiple depositional environments during the history of deposition of Quebrada Las Rosas, El Culebrón, and Los Clarines, which were characterized by alternations of marine regressions and transgressions. Many of these depositional environments are reflected in the lithology and taphonomy of fossils, such as coquinas with dissolution of calcareous material or the presence of shallow trace fossils.
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.