The Ranquil Formation ( es, Formación Ranquil) is a

Miocene The Miocene ( ) is the first epoch (geology), geological epoch of the Neogene Period and extends from about (Ma). The Miocene was named by Scottish geologist Charles Lyell; the name comes from the Greek words (', "less") and (', "new") and mea ...

and

Pliocene The Pliocene ( ; also Pleiocene) is the epoch in the geologic time scale that extends from 5.333 million to 2.58sediment 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, sa ...

ary

formation Formation may refer to: Linguistics * Back-formation, the process of creating a new lexeme by removing or affixes * Word formation, the creation of a new word by adding affixes Mathematics and science * Cave formation or speleothem, a secondar ...

located in

Arauco Province Arauco Province ( es, Provincia de Arauco) is one of four provinces of the Chilean region of Bío Bío. It spans a coastal area of just south of the mouth of the Biobío River, the traditional demarcation between the nation's major natural reg ...

in south–central Chile, including

outcrop An outcrop or rocky outcrop is a visible exposure of bedrock or ancient superficial deposits on the surface of the Earth. Features Outcrops do not cover the majority of the Earth's land surface because in most places the bedrock or superficia ...

s in

Mocha Island Mocha Island ( es, link=no, Isla Mocha ) is a small Chilean island located west of the coast of Arauco Province in the Pacific Ocean. The island is approximately in area, with a small chain of mountains running roughly in north-south direction. ...

. The formation has its greatest thicknesses in the south-west, where its sediments were largely deposited in marine conditions. It overlies

unconformably 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 ...

sedimentary formations of the

Paleocene The Paleocene, ( ) or Palaeocene, is a geological epoch that lasted from about 66 to 56 million years ago (mya). It is the first epoch of the Paleogene Period in the modern Cenozoic Era. The name is a combination of the Ancient Greek ''pal ...

Eocene The Eocene ( ) Epoch is a geological epoch that lasted from about 56 to 33.9 million years ago (mya). It is the second epoch of the Paleogene Period in the modern Cenozoic Era. The name ''Eocene'' comes from the Ancient Greek (''ēṓs'', " ...

Lebu Group. The formation is part of the fill of

Arauco Basin The Arauco Basin ( es, Cuenca de Arauco) is a sediment-filled depression –a sedimentary basin– in south-central Chile. In the context of plate tectonics it is classified as a forearc basin. The basin has an approximate area of and at its deep ...

which is 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 subside ...

that extends south of Concepción.

Macrofossil Macrofossils, also known as megafossils, are preserved organic remains large enough to be visible without a microscope. The term ''macrofossil'' stands in opposition to the term microfossil. Microfossils, by contrast, require substantial magnifica ...

s of the formation are similar to those of Navidad ( 34° S) and Lacui Formations ( 43° S), two nearby Miocene marine formations. The base of the Ranquil Formation is the so-called "main unconformity", which is thought to have been 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 dis ...

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 ...

. The formation was first defined in 1942 by

Juan Tavera Juan Tavera (1917–1991) was a Chilean geologist and paleontologist. His most important work was on the marine invertebrate fossils of the formations of Algarrobo, Arauco and Navidad. Tavera's work contributed to an increased understanding o ...

Units

The formation has been subdivided into five units, with the lowermost being made up of

sandstone Sandstone is a clastic sedimentary rock composed mainly of sand-sized (0.0625 to 2 mm) silicate grains. Sandstones comprise about 20–25% of all sedimentary rocks. Most sandstone is composed of quartz or feldspar (both silicates ...

and shale, and the second lowest one being made up of a conglomerate. The middle unit is made up of mudrock and massive sandstone. At some places the middle unit is overlain by a unit made up of sandstone with thin layers of conglomerate and sandstone that has been bioturbated. The uppermost unit include a

breccia Breccia () is a rock composed of large angular broken fragments of minerals or rocks cemented together by a fine-grained matrix. The word has its origins in the Italian language, in which it means "rubble". A breccia may have a variety of ...

and the so-called ''Huenteguapi sandstone''. The sediments of Huenteguapi sandstone evidences that a

megatsunami A megatsunami is a very large wave created by a large, sudden displacement of material into a body of water. Megatsunamis have quite different features from ordinary tsunamis. Ordinary tsunamis are caused by underwater tectonic activity (movemen ...

struck the coast of south–central Chile in the

Pliocene The Pliocene ( ; also Pleiocene) is the epoch in the geologic time scale that extends from 5.333 million to 2.58Eltanin impact The Eltanin impact is thought to be an asteroid impact in the eastern part of the South Pacific Ocean that occurred around the Pliocene-Pleistocene boundary approximately 2.51 ± 0.07 million years ago. The location was at the edge of th ...

Fossil content

The Ranquil Formation contains the following

trace fossil A trace fossil, also known as an ichnofossil (; from el, ἴχνος ''ikhnos'' "trace, track"), is a fossil record of biological activity but not the preserved remains of the plant or animal itself. Trace fossils contrast with body fossils, ...

s: ''

Zoophycos ''Zoophycos'' is a somewhat cosmopolitan ichnogenus thought to be produced by moving and feeding polychaete worms. Appearance ''Zoophycos'' occurs in two forms, one planar, and one which resembles a corkscrew. In the latter helicoidal form, ...

'', '' Chondrites'', '' Phycosiphon'', '' Nereites missouriensis'', '' Lockeiasiliquaria'', '' Parataenidium'', ''

Ophiomorpha ''Ophiomorpha'' is an ichnotaxon, usually interpreted as a burrow of an organism (specifically a crustacean) living in the near-shore environment. The burrow lining is more or less smooth on the inside, and densely to strongly mammalated or nod ...

'', '' Rhizocorallium'' and possibly also '' Psammichnites''.

References

{{Geology of Chile Geologic formations of Chile Miocene Series of South America Pliocene Series of South America Neogene Chile Conglomerate formations Geology of Biobío Region Coasts of Biobío Region

Units

Fossil content

See also

References