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| Chañarcillo Formation | |
|---|---|
| Name | Chañarcillo Formation |
| Period | Devonian |
| Age | Early Devonian to Middle Devonian |
| Type | Formation |
| Primary lithology | sedimentary carbonates, shale |
| Other lithology | sandstone, conglomerate |
| Region | Atacama Region, Chañaral |
| Country | Chile |
| Named for | Chañarcillo |
| Named by | Ignacio Domeyko |
| Year ts | 19th century |
Chañarcillo Formation is a Devonian sedimentary succession widely studied for its fossil assemblages, carbonate platforms, and associated mineralization in northern Chile. The unit records reefal and platform-margin facies with rich invertebrate faunas that have informed regional correlations with coeval units in Argentina, Bolivia, and Peru. It has economic importance due to silver-bearing mineral deposits exploited since the 19th century.
The formation lies within the coastal cordillera of the Atacama Desert and records deposition on a Paleozoic continental margin adjacent to the Paleo-Pacific Ocean. Regional tectonics related to the evolution of the Andean orogeny and interactions between the Nazca Plate and the South American Plate influenced basin subsidence, sediment supply, and accommodation. The succession includes platform carbonates, bioclastic limestones, and interbedded siliciclastic horizons that demonstrate shifts between shallow-marine carbonate production and deeper siliciclastic influx tied to tectonic pulses and eustatic changes documented in contemporaneous units such as the Belén Formation and the Mitu Group.
Stratigraphically, the formation overlies older Cambro-Ordovician sequences and is overlain by younger Devonian–Carboniferous strata; local unconformities link it to regional sequences like the Hualfín Formation and the Los Pinos Formation. Lithologies include massive limestone, bedded bioclastic carbonates, micritic mudstones, nodular chert horizons, and sporadic sandstone and conglomerate beds that reflect storm, reef-front, and slope environments. Facies analysis identifies reef core buildups, fore-reef talus, and lagoonal deposits comparable to facies models from the Antofagasta Region and the Precordillera.
The fossil assemblage is diverse, with abundant brachiopods, trilobites, crinoids, corals (both rugose and tabulate), bryozoans, and mollusks including gastropods and bivalves, permitting biostratigraphic correlation with classic Devonian faunas from Euramerica-affinity provinces and Gondwanan localities such as Falkland Islands and Tasmania. Microfossils and foraminifera help refine age models alongside macrofossils used in regional biogeographic studies that involve institutions like the Natural History Museum, London, the Smithsonian Institution, and the Museo Nacional de Historia Natural (Chile). Notable taxa recorded in the succession have been cited in monographs and faunal lists produced by researchers associated with the University of Chile, the University of Buenos Aires, and the Comisión Chilena del Cobre (CODELCO) research programs.
Biostratigraphic indicators place deposition in the Early to Middle Devonian, correlating with the Lochkovian, Pragian, and Emsian stages in different sections; conodont data and coral zonations support these assignments. Depositional environments range from peritidal and lagoonal settings to reefal buildups and outer-platform basins, influenced by relative sea-level fluctuations synchronous with global Devonian transgressive-regressive cycles recognized in sections from the Old Red Sandstone continent and the Holy Cross Mountains research. Oxygenation events and periodic anoxia in restricted basins are inferred from black shale intervals and carbonate geochemistry studied by geochemists at the Universidad de Concepción.
The formation is associated with significant silver and base-metal mineralization historically mined at districts including Chañarcillo (mining district), Combarbalá, and nearby lodes exploited during the Chilean silver rush of the 19th century. Mineralization styles include stratabound sulfide and supergene enrichment, with ores hosted in carbonate breccias, veins, and replacement bodies. Exploration and economic geology studies by firms such as Codelco and international consultancies reference the formation when targeting carbonate-hosted vein and replacement deposit models analogous to deposits in the Central Chile metallogenic belt and comparisons to classic carbonate-hosted silver districts like those in Mexico and Peru.
Exposures are concentrated in the coastal ranges of northern Chile, notably near the town of Chañaral and surrounding localities in the Atacama Region where the type sections were described. Correlative outcrops extend along strike into the Coquimbo Region and subsurface equivalents are recognized in regional seismic and borehole data that tie the unit to similar Devonian successions in northwestern Argentina and the Altiplano basin.
Initial description and naming in the 19th century are attributed to geologists working in Chilean mining districts, with later systematic stratigraphic, paleontological, and geochemical investigations carried out by researchers affiliated with the Universidad de Chile, the Instituto de Investigaciones Geológicas (INGEOMINAS), and international teams from institutions like the Geological Society of America and the International Union of Geological Sciences. Subsequent studies integrated biostratigraphy, sedimentology, and economic geology, producing regional correlation schemes cited in syntheses published by national geological surveys such as the Servicio Nacional de Geología y Minería (SERNAGEOMIN).
Category:Geologic formations of Chile Category:Devonian System of South America