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 (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 322 km (200 mi) northward of the Navidad coast.[1] In 1985, Steinmann G. analized 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.[2] 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.[3] 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.[4]
View of outcrops of the Coquimbo Formation at the 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.[5][6]
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 40 cm (16 in), and set apart from each other by approximately 400 m (1,300 ft). 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.[7]
Photographs of the two fossil cetacean individuals from the Culebrón Park
Line diagram of the cetaceans
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.[7] 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.[8] Though initially reported as "semifossils" by Salinas despite of their age,[7] Yañez and Canto stated that individuals eventually dissociated in the field preventing attempts for future studies.[8]
In 2021 the Geological Society of Chile formally recognized the Culebrón Park as a geositio (lit. geosite), that is, an important locality of either geological or paleontological value. Documentation and petitions were conducted by at-the-time Geologythesist 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.[9][10]
Diagram featuring the cetaceans skeletons found at Lomas del Sauce (yellow, red, and purple)
During the year 2013, geological survey was carried out on a land lot near El Sauce avenue of Coquimbo city, about 2.5 km (8,200 ft) 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 sandysilt 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.[11]
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 slothThalassocnus, which was previously only known from the similar Pisco Formation and Bahía Inglesa Formation.[12][13] 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.[14]
Marine terrace at Caleta Los Hornos, which follows the sequence from Quebrada Honda
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.[3] 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 (such as cetaceans, brachiopods, bivalves, and gastropods) 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 (of different composition), 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.[18]
The fossil record of shallow water-based taxa such as Crassostrea, Heterodontus, Incatella or Thalassocnus also suggests a shallow marine environment upon deposition.[19] The foraminiferan fossil assemblage from Bahía de Guanaqueros indicates several changes in sea levels and depositional environments, from neritic to bathyal marine conditions.[20] 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.[21]
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.[22]
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.[23] 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.[14]
Map indicating the location of the cetaceans from Park El Culebrón, in Coquimbo city
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.[15][16][17] In Tongoy, the formation runs across creeks that feature large marine terraces.[17] In Coquimbo, the formation can be traced as far as the Culebrón Park, with a more consistent extent at Quebrada Las Rosas.[24]
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.[17] The marine terraces of the Coquimbo Formation are relatively large sedimentary bodies, and they were carved in these sediments as a result of the Pliocene-Quaternary marine transgressions and regressions that occurred in this area.[3][17] The marine terrace levels at Coquimbo and La Herradura bays preserve unconformities that indicate that they were periodically resedimented during successive high level-sea tides, which is also supported by the aminostratigraphy of mollusks.[25]
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.[15] It is subdivided into 16 lithostratigraphic units of coastal marine origin. In ascending order these range from Unit 1 to Unit 16. The lowermost unit belongs to the lower Miocene, with the third-deepest unit dated at 11.9 ± 1.0 Ma. The uppermost unit of the formation is estimated at 1.2 Ma.[17]
It is proposed that the Coquimbo Formation is equivalent to the Bahía Inglesa Formation. Both formations show similarities in sedimentation (such as correlation of lithostratigraphic units) and fossil content, and they could represent distinct units of the same geological formation, pending further analyses.[26] 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.[27]
Lithostratigraphy of the Coquimbo Formation from Tongoy[17]
Restoration of Otodus megalodon 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.[14][26] Invertebrates such as molluscs are particularly abundant in the facies of the formation, represented by bivalves and gastropods, and to a lesser extent barnacles. Vertebrates are recurrent faunal components but are usually fragmentary in the record of the Coquimbo Formation, best represented by bird and cetacean remains. Like Bahía and Pisco, fish are well documented, including historically notorious taxa such as the megatoothed Otodus, the extant Carcharodon, and extinct Cosmopolitodus.[28][19] The semiaquatic ground sloth Thalassocnus was otherwise only known from the Pisco Formation, but its fossil record is now reported from numerous geological units across Miocene-Pliocene deposits of South America including Argentina and Chile, specifically the Coquimbo Formation.[14][29] 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.[19]
A baleen whale. Formation of origin not stated, but it is likely the Coquimbo Formation.[40] Previously known as "Megaptera", the species is not related to Megaptera,[41] and is now assigned to Plesiobalaenoptera.[42]
Bahía de Tongoy, Bahía de Carrizalillo, Caleta Chañaral, La Cantera Baja, La Herradura, Lomas del Sauce, Quebrada Camarones, Quebrada El Culebrón, Quebrada Las Rosas.
^Darwin, C. R. (1846). "CHAPTER VIII - NORTHERN CHILE: CONCLUSION". Geological Observations on South America: Being the Third Part of the Geology of the Voyage of the Beagle, under the Command of Capt. Fitzroy, R. N. during the Years 1832 to 1836. Cambridge University Press. pp. 128−129. doi:10.1017/CBO9780511910180.010.
^Steinmann, G. (1895). "Das Auftreten und Alter der Quiriquina-Schichten". Neues Jahrbuch für Mineralogie, Geologie und Paläontologie (in German). 10: 64−94.
^Moscoso, R. D.; Nasi, C.; Salinas, P. (1982). "Geología de la hoja Vallenar y parte norte de La Serena, Regiones de Atacama y Coquimbo". Carta Geológica de Chile (55): 1−100.
^ abde los Arcos, S.; Pérez, L.; Rincón, I.; Gil, C.; Mourgues, A.; Chávez-Hoffmeister, M.; Severino, S.; Partarrieu, D.; Contreras, K. (2014). "Nuevo Yacimiento Paleontológico del Plioceno de la Formación Coquimbo"(PDF). In Moreno, K.; Nielsen, S.; Abarzúa, A.; Silva, N.; Leiva, F. (eds.). IV Simposio Paleontología en Chile. Valdivia: Universidad Austral de Chile. p. 86.
^ abcdefghijklmnopqrstuvwxAraya, S. C.; Araya, B. A.; Antiquera, B.; Gonzales, M. T.; Oyanadel, P. U.; Rivadeneira, M. M.; Villafaña, J. (2024). "Ensambles de microfósiles para la determinación de paleoambientes en sedimentos Neógenos de la Bahía de Guanaqueros, norte de Chile". In Valenzuela, A.; Tapia, I.; Manfroi, J.; Chávez, M.; Moisan, P.; Robles, X. (eds.). Libro de resúmenes(PDF). Copiapó: III Congeso Chileno de Paleontología. p. 128.
^García, C. A. L. (2019). Análisis de facies de sedimentos neógenos marinos en Coquimbo (30°S), Chile: significado paleoambiental e implicancias tectónicas (Thesis) (in Spanish). Santiago: Universidad de Chile. hdl:2250/170934.
^Rincón, I.; Nielsen, S. N.; Rivadeneira, M.; Chávez, M. (2024). "Análisis tafonómico de fósiles de vertebrados de la Formación Coquimbo en el sector de Quebrada Chañaral, Región de Atacama, Chile". In Valenzuela, A.; Tapia, I.; Manfroi, J.; Chávez, M.; Moisan, P.; Robles, X. (eds.). Libro de resúmenes(PDF). Copiapó: III Congeso Chileno de Paleontología. p. 141.
^Araya, B. A.; Rivadeneira, M. M.; Urbina, P. O.; Tejos, M. G.; Castillo, S. A.; Villafaña, J.; Antiquera, B.; Mora, Y. H.; Contreras, K.; Cisterna, K.; Nielsen, S. N. (2024). "Propuesta estratigráfica y paleontológica para Formación Coquimbo, norte de Chile". In Valenzuela, A.; Tapia, I.; Manfroi, J.; Chávez, M.; Moisan, P.; Robles, X. (eds.). Libro de resúmenes(PDF). Copiapó: III Congeso Chileno de Paleontología. p. 129.
^Quiñones, S. I.; Zurita, A. E.; Miño-Boilini, Á. R.; Candela, A. M.; Luna, C. A. (2022). "Unexpected record of the aquatic sloth Thalassocnus (Mammalia, Xenarthra, Folivora) in the upper Neogene of the Puna (Jujuy, Argentina)". Journal of Vertebrate Paleontology. 42 (1): e2109973. Bibcode:2022JVPal..42E9973Q. doi:10.1080/02724634.2022.2109973. ISSN0272-4634. S2CID252327107.
^ abSantelli, M. B.; Río, C. J. (2019). "New Neogene taxa of the tribe Chlamydini Teppner, 1922 (Pectinidae, Bivalvia) of southern South America". Journal of Paleontology. 93 (6): 1088−1104. doi:10.1017/jpa.2019.50.
^ abAlvarez, M. J. (2019). "Phylogenetic analysis of the genus Retrotapes del Río, 1997 (Bivalvia, Veneridae) and systematic analysis of its taxa from Chile". Journal of Paleontology. 93 (4): 685−701. doi:10.1017/jpa.2018.110.
^ abcdefghijklmnopqHoffmeister, M. C.; Toro, A. V.; Villafaña, J.; Oyanadel, P.; Carrillo, J.; Rincón, I.; Rivadeneira, M. (2024). "Los vertebrados fósiles del Plioceno de Bahía Carrizalillo, Región de Atacama, Chile". In Valenzuela, A.; Tapia, I.; Manfroi, J.; Chávez, M.; Moisan, P.; Robles, X. (eds.). Libro de resúmenes(PDF). Copiapó: III Congeso Chileno de Paleontología. p. 138.
^Gutstein, C.; Horwitz, F.; Valenzuela-Toro, A. M.; Figueroa-Bravo, C. P. (2015). "Cetáceos fósiles de Chile: Contexto evolutivo y paleobiogeográfico". In Rubilar-Rogers, D.; Otero, R.; Vargas, A.; Sallaberry, M. (eds.). Vertebrados Fósiles de Chile. Santiago: Publicación Ocasional del Museo Nacional de Historia Natural. pp. 339−383.
^Dathe, F. (1983). "Megaptera hubachi n. sp., ein fossiler Bartenwal aus marinen Sandsteinschichten des tieferen Pliozäns Chiles" [Megaptera hubachi n. sp., fossil whale of the marine sandstone beds of the lower Pliocene of Chile]. Zeitschrift für Geologische Wissenschaften (in German). 11 (7): 813−848.
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