sediment
Sediment is a naturally occurring material that is broken down by processes of weathering and erosion, and is subsequently transported by the action of wind, water, or ice or by the force of gravity acting on the particles. For example, sand ...
-filled depression –a
sedimentary basin
Sedimentary basins are region-scale depressions of the Earth's crust where subsidence has occurred and a thick sequence of sediments have accumulated to form a large three-dimensional body of sedimentary rock. They form when long-term subsiden ...
– in south-central
Chile
Chile, officially the Republic of Chile, is a country in the western part of South America. It is the southernmost country in the world, and the closest to Antarctica, occupying a long and narrow strip of land between the Andes to the eas ...
. In the context of
plate tectonic
Plate tectonics (from the la, label=Late Latin, tectonicus, from the grc, τεκτονικός, lit=pertaining to building) is the generally accepted scientific theory that considers the Earth's lithosphere to comprise a number of large te ...
s it is classified as a forearc basin. The basin has an approximate area of and at its deeper parts the surface of its sedimentary fill reaches below sea-level. The basin is interpreted as being part of an uplifted part of the
continental shelf
A continental shelf is a portion of a continent that is submerged under an area of relatively shallow water, known as a shelf sea. Much of these shelves were exposed by drops in sea level during glacial periods. The shelf surrounding an island ...
. To the west it bounds an active
accretionary prism
An accretionary wedge or accretionary prism forms from sediments accreted onto the non-subducting tectonic plate at a convergent plate boundary. Most of the material in the accretionary wedge consists of marine sediments scraped off from the do ...
that lies next to the Chile trench and to the east it bounds metamorphic basement representing a fossil
Paleozoic
The Paleozoic (or Palaeozoic) Era is the earliest of three geologic eras of the Phanerozoic Eon.
The name ''Paleozoic'' ( ;) was coined by the British geologist Adam Sedgwick in 1838
by combining the Greek words ''palaiós'' (, "old") and ...
accretionary complex
An accretionary wedge or accretionary prism forms from sediments accreted onto the non-subducting tectonic plate at a convergent plate boundary. Most of the material in the accretionary wedge consists of marine sediments scraped off from the d ...
coal mining in Chile
In Chile, coal mining is restricted to a few places located in its southern half. Energy originating from coal stands for 11,6% of Chile's electricity consumption.Explotacion Reservas CNE. Accessed on September 10. 2012. Given a high density of geological 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 to
mechanize
''Mechanize'' is the seventh studio album by American heavy metal band Fear Factory. It is the only album to feature Gene Hoglan on drums and the first since 2001's '' Digimortal'' to include original guitarist and founding member Dino Cazares, ...
The sedimentary fill has a maximum thickness of ca. . Parts of the basin are on land in
Arauco Peninsula The Arauco Peninsula (Spanish: península de Arauco), is a peninsula in Southern Chile located in the homonymous Arauco Province. It projects northwest into the Pacific Ocean. The peninsula is located west of Cordillera de Nahuelbuta. Geologicall ...
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.
The sedimentary formations defined in Arauco Basin include:
Pliocene Tubul Formation is the oldest formation in the basin that has not been folded. It lies on an
unconformity
An unconformity is a buried erosional or non-depositional surface separating two rock masses or strata of different ages, indicating that sediment deposition was not continuous. In general, the older layer was exposed to erosion for an interval ...
that cuts across all other formations of the basin. At present it reaches above sea level in some locations and is dissected by a number of small valleys. The base of the Ranquil Formation is the so-called “main unconformity” which is thought to have formed by
erosion
Erosion is the action of surface processes (such as water flow or wind) that removes soil, rock, or dissolved material from one location on the Earth's crust, and then transports it to another location where it is deposited. Erosion is d ...
during a period of
tectonic inversion
In structural geology inversion or basin inversion relates to the relative uplift of a sedimentary basin or similar structure as a result of crustal shortening. This normally excludes uplift developed in the footwalls of later extensional fau ...
.
Tectonic evolution
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 of
strike-slip fault
In geology, a fault is a planar fracture or discontinuity in a volume of rock across which there has been significant displacement as a result of rock-mass movements. Large faults within Earth's crust result from the action of plate tectoni ...
ing in the Pliocene and
Pleistocene
The Pleistocene ( , often referred to as the ''Ice age'') is the geological Epoch (geology), epoch that lasted from about 2,580,000 to 11,700 years ago, spanning the Earth's most recent period of repeated glaciations. Before a change was fina ...
. The subduction of the Mocha 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.
See also
*
Caldera Basin
Caldera Basin ( es, Cuenca de Caldera) is a sedimentary basin located in the coast of northern Chile west of Copiapó. The basin has a fill of marine sediments of Late Cenozoic age. With a north-south extension of and an east-west width of the ...