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| Arauco Basin | |
|---|---|
| Name | Arauco Basin |
| Country | Chile |
| State | Biobío Region |
| Region | Arauco Province |
| Period | Miocene–Holocene |
| Type | Forearc and pull-apart basin |
Arauco Basin is a Neogene to Quaternary sedimentary basin on the Chilean margin, located in the Biobío Region near the city of Concepción, Chile and the town of Arauco, Chile. The basin records interactions between the Nazca Plate and the South American Plate and contains important records of volcanic, tectonic and sedimentary processes associated with the Andean orogeny. It is a site of hydrocarbon exploration, coal mining and paleontological study linked to regional events such as the Chile Triple Junction and Pleistocene glacioeustatic changes.
The basin lies within the coastal margin of central-southern Chile adjacent to the Pacific Ocean and northeast of the Mocha Island area. Regional geology links the basin to the larger context of the Andes Mountains, the Coastal Cordillera (Chile), and the nearby accretionary complex of the Chiloé Block. Bedrock around the basin includes metamorphic complexes correlated with the Paleozoic sequences found in the Coastal Batholith of central Chile and plutonic bodies related to the South American Plate magmatic arc. The modern coastline and shelf are influenced by features associated with the Peru–Chile Trench and the distribution of Neogene volcanic centers like Llaima Volcano and Villarrica Volcano.
Stratigraphic successions in the basin span from Miocene through Pliocene to Pleistocene and Holocene deposits. Major lithostratigraphic units include marine turbidites, fluvial conglomerates, deltaic sandstones, and coal-bearing strata that correlate with regional units such as the Ranquil Formation and the Cofralca Formation recognized in nearby outcrops. Volcaniclastic layers intercalated within the succession are tied to eruptive episodes from the Southern Volcanic Zone of the Andes. The basin stratigraphy contains unconformities and sequence boundaries reflecting regional sea-level changes attributed to glacioeustasy during the Pleistocene glaciations.
The basin formed in the trench-parallel margin influenced by oblique convergence between the Nazca Plate and the South American Plate, with local modifications from transform and transpressional motions along faults comparable to the Liquiñe-Ofqui Fault Zone. The proximity to the Chile Triple Junction during Neogene times affected slab geometry and slab window processes, altering mantle flow and magmatism similar to effects documented for the Patagonian Andes. Basin subsidence history reflects flexural and strike-slip components comparable to other forearc basins such as the Valdivia Basin and the Osorno Basin.
Deposits record a range of depositional environments from shallow marine shelves and estuaries to fluvial floodplain systems and coaly swamps. Turbiditic sequences suggest sediment gravity flows sourced from uplifted parts of the Coastal Cordillera (Chile) and transported across submarine fans analogous to systems described at Corral Bay and the Reloncaví Estuary. Deltaic and strandplain units show channels, cross-bedding and plant-bearing horizons comparable to depositional architecture in the Itata Basin. Interbedded volcaniclastics reflect explosive eruptions from volcanic centers like Calbuco and Osorno Volcano impacting sediment supply and diagenesis.
The structural framework is dominated by a network of reverse, normal and strike-slip faults accommodating margin-parallel shortening and local extension. Major fault systems segment the basin into half-grabens and pull-apart depocenters similar in style to structures observed near the Arauco Peninsula and the Bío Bío River valley. Deformation phases correspond to regional compressional episodes of the Andean orogeny and to coeval transtensional events linked to changes in plate convergence vectors documented along the Peru–Chile Trench.
The basin hosts economically significant coal seams historically mined in the Arauco area by companies including regional operations tied to Chilean industrial development around Coronel, Chile and Lota, Chile. Hydrocarbon exploration has targeted potential petroleum systems with source rocks, maturation beneath burial, and structural traps analogous to smaller forearc accumulations documented elsewhere on the Chilean margin. Aggregates, sand and volcaniclastics are quarried for construction, and geothermal potential has been evaluated in relation to nearby volcanic centers such as Villarrica Volcano.
Fossiliferous horizons preserve plant remains, pollen assemblages and marine invertebrates that inform reconstructions of Neogene–Quaternary paleoclimates and paleoenvironments. Palynological records compare with Pleistocene sequences from Magallanes Region and coastal sites near Valdivia to track shifts in vegetation during glacial–interglacial cycles. Marine faunas show affinities to assemblages described from the Sarmiento Formation and other South American Neogene marine units, aiding biostratigraphic correlations with basins along the Pacific coast of South America.