| Arauco Basin | |
|---|---|
| Cuenca de Arauco | |
 Arauco Basin is located in the westernmost part of central Chile | |
  | |
| Coordinates | 37°05′S73°10′W / 37.083°S 73.167°W /-37.083; -73.167 |
| Etymology | Arauco Peninsula |
| Location | Southwestern South America |
| Country |  Chile |
| State(s) | Bío Bío Region |
| Characteristics | |
| On/Offshore | Both |
| Boundaries | Accretionary prism of theChile trench (W) Accretionary complex intruded by theCoastal Batholith of central Chile (E) |
| Area | ~8,000 km2 (3,100 sq mi) |
| Hydrology | |
| Sea(s) | Eastern Pacific Ocean |
| Geology | |
| Basin type | Forearc basin |
| Orogeny | Andean |
| Age | Maastrichtian-Pliocene |
| Stratigraphy | Stratigraphy |
| Field(s) | Chilean coal |
TheArauco Basin (Spanish:Cuenca de Arauco) is asediment-filled depression –asedimentary basin– in south-centralChile. In the context ofplate tectonics it is classified as aforearc basin. The basin has an approximate area of 8,000 square kilometres (3,100 sq mi) and at its deeper parts the surface of its sedimentary fill reaches 200 metres (660 ft) below sea-level.[1] The basin is interpreted as being part of an uplifted part of thecontinental shelf.[2] To the west it bounds an activeaccretionary prism that lies next to theChile trench and to the east it bounds metamorphic basement representing a fossilPaleozoicaccretionary complex that has been intruded by theCoastal Batholith of central Chile.[3]
Traditionally the centre ofcoal mining in Chile, large-scale coal mining in Arauco Basin ended in the 1990s.[4] Given a high density ofgeological faults that have displaced the coal beds and the thin nature of these (less than one metre) mining activity in Arauco Basin has proven difficult tomechanize.[5]
The sedimentary fill has a maximum thickness of ca. 2,000 metres (6,600 ft). Parts of the basin are on land inArauco Peninsula where Eocene coal-bearing rocks of marine and continental origin and Eocene age are exposed. On top of these rocks and toward the centre of the peninsula Miocene and Pliocene sedimentary rocks exists. Towards the east Cretaceous sedimentary rocks crop out.[2]The sedimentary formations defined in Arauco Basin include:
| Name[3][6] | Environment[6][7] | Age[6][8] | Lithology[6] | |
|---|---|---|---|---|
| Tubul Formation | Marine | Pliocene | Sandstone,siltstone | |
| Ranquil Formation | Marine | Miocene–Pliocene | Conglomerate, sandstone, siltstone,shale | |
| Lebu Group | Millongue Formation[note 1] | Marine and continental | Eocene | Shale, siltstone |
| Trihueco Formation | Marine and continental | Eocene | Sandstone, shale, coal | |
| Boca Lebu Formation | Marine | Eocene | Clay-rich andcalcareous siltstone, sandstone | |
| Curanilahue Formation | Marine and continental | Paleocene | Massive sandstone, coal, clay-rich sandstone | |
| Quiriquina Formation | Marine | Late Maastrichtian | Sandstone, conglomerate | |
Pliocene Tubul Formation is the oldest formation in the basin that has not beenfolded. It lies on anunconformity that cuts across all other formations of the basin. At present it reaches 100 metres (330 ft) above sea level in some locations and is dissected by a number of small valleys.[6] The base of the Ranquil Formation is the so-called "main unconformity" which is thought to have formed byerosion during a period oftectonic inversion.[3]
A three-stage model of evolution has been proposed for the Arauco Basin. First a phase of extension in the Late Cretaceous and Paleogene. Then a basin inversion lasting from the Middle Eocene to the Miocene causing the uplift and erosion that created the "main unconformity" and finally a post-inversion phase ofstrike-slip faulting in the Pliocene andPleistocene.[3] The subduction of theMocha fracture zone under the basin that begun about 3.6 million years ago is believed to have caused the uplift of the basin plus some further tectonic inversion and contraction.[3]
| Sedimentary basins | |
|---|---|
| Faults | |
| Batholiths | |
| Metamorphic complexes | |
| Terranes | |
