Definition
The Abanico Formation (also referred to as the Abanico Group) is a geologic unit in central Chile that comprises a sequence of late Cretaceous to early Paleogene sedimentary and volcaniclastic rocks. It records the transition from the Mesozoic to the Cenozoic in the Andean forearc basin of the region.
Overview
The formation outcrops primarily along the central Andes, extending from approximately 29° S to 33° S latitude, north of Santiago. It attains thicknesses of up to 3 kilometres in some sections and underlies younger volcanic sequences of the Neogene and Quaternary. Stratigraphically, the Abanico Formation rests unconformably on older Paleozoic metamorphic and igneous rocks and is itself overlain by the Miocene–Pliocene volcanic deposits of the Andes. The unit is subdivided in several members that reflect variations in depositional setting, including fluvial, lacustrine, and pyroclastic environments.
Etymology / Origin
The name “Abanico” is derived from the Spanish word abanico (“fan”), a reference to the fan‑shaped alluvial deposits that characterize parts of the formation. The term was first applied by Chilean geologists in the early 20th century to rocks exposed near the Abanico River basin, which drains the central Andes westward toward the Pacific.
Characteristics
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Lithology: The formation consists of interbedded sandstones, mudstones, conglomerates, siltstones, and a substantial component of volcaniclastics such as tuffs, ash‑flow deposits, and basaltic lapilli tuffs. Locally, ignimbrite layers up to several metres thick are present, indicating episodic explosive volcanism.
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Depositional Environment: Sedimentary facies suggest deposition in a forearc basin influenced by both fluvial systems (braided rivers and alluvial fans) and lacustrine settings. The presence of widespread volcaniclastic material points to contemporaneous arc volcanism in the western Andes, with ash and pyroclastic flows reworked into the basin.
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Age: Radiometric (U‑Pb, Ar‑Ar) and palynological data constrain the formation to the latest Cretaceous (Campanian–Maastrichtian, ~ 83–66 Ma) through the early Paleogene (Paleocene–Eocene, ~ 66–34 Ma). The precise timing varies among members, reflecting a protracted period of sedimentation.
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Fossil Content: Palynological investigations have recovered diverse pollen and spores indicative of temperate to subtropical vegetation. Sparse macrofossils include leaf impressions of angiosperms and conifers. No definitive dinosaur body fossils have been reported from the core succession, although fragmentary tracks have been noted in isolated outcrops.
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Structural Context: The Abanico Formation is deformed by thrust faulting and folding associated with the Andean orogeny. It forms part of the “Abanico Accretionary Complex” that records crustal shortening and uplift during the early stages of mountain building.
Related Topics
- Andean Orogeny – the tectonic processes responsible for the uplift of the Andes and the formation of forearc basins.
- Chilean Geology – the broader geological framework of the Chilean Central Andes, including the overlying “Quillay” and “Mendoza” volcanic units.
- Forearc Basin Sedimentation – the study of sedimentary fill in tectonic basins situated between a subduction trench and volcanic arc.
- Cretaceous–Paleogene Transition in South America – regional comparisons of contemporaneous formations such as the “Mendoza Group” (Argentina) and “Mendoza Formation” (Chile).
This entry reflects current scientific consensus as of 2026. No substantial disputes regarding the existence, age, or primary characteristics of the Abanico Formation are documented in peer‑reviewed literature.