The Ranquil Formation () 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 (geology), epoch in the geologic time scale that extends from 5.33 to 2.58sediment Sediment is a solid material that is transported to a new location where it is deposited. It occurs naturally and, through the processes of weathering and erosion, is broken down and subsequently sediment transport, transported by the action of ...

ary formation located in

Arauco Province Arauco Province () is one of three 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 regions, Zona Central and ...

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 and other terrestrial planets. Features Outcrops do not cover the majority of the Earth's land surface because in most p ...

s in

Mocha Island Mocha Island ( ) is a Chilean island located west of the coast of Arauco Province in the Pacific Ocean. The island is the location of numerous historic shipwrecks. In Mapuche mythology, the souls of dead people travel west to visit this island. ...

. 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 o ...

sedimentary formations of the

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

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

Lebu Group. The formation is part of the fill of

Arauco Basin The Arauco Basin () 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 deeper parts the surface ...

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 subsidence ...

that extends south of Concepción.

Macrofossil Macrofossils, also known as megafossils, are the preserved remnants of organic beings and their activities that are large enough to be visible without a microscope. The term ''macrofossil'' stands in opposition to the term microfossil. Microfoss ...

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 Surface runoff, water flow or wind) that removes soil, Rock (geology), rock, or dissolved material from one location on the Earth's crust#Crust, Earth's crust and then sediment transport, tran ...

during a period of

tectonic inversion In structural geology, inversion or basin inversion relates to the relative uplift of a sedimentary basin or as a result of crust (geology), crustal shortening. This normally excludes uplift developed in the fault (geology), footwalls of later ...

. The formation was first defined in 1942 by Juan Tavera.

Units

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

sandstone Sandstone is a Clastic rock#Sedimentary clastic rocks, clastic sedimentary rock composed mainly of grain size, sand-sized (0.0625 to 2 mm) silicate mineral, silicate grains, Cementation (geology), cemented together by another mineral. Sand ...

and

shale Shale is a fine-grained, clastic sedimentary rock formed from mud that is a mix of flakes of Clay mineral, clay minerals (hydrous aluminium phyllosilicates, e.g., Kaolinite, kaolin, aluminium, Al2Silicon, Si2Oxygen, O5(hydroxide, OH)4) and tiny f ...

, and the second lowest one being made up of a conglomerate. The middle unit is made up of

mudrock Mudrocks are a class of fine-grained siliciclastic sedimentary rocks. The varying types of mudrocks include siltstone, claystone, mudstone and shale. Most of the particles of which the stone is composed are less than and are too small to ...

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 Rock (geology), rocks cementation (geology), cemented together by a fine-grained matrix (geology), matrix. The word has its origins in the Italian language ...

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

megatsunami A megatsunami is an incredibly large wave created by a substantial and sudden displacement of material into a body of water. Megatsunamis have different features from ordinary tsunamis. Ordinary tsunamis are caused by underwater tectonic activi ...

struck the coast of south–central Chile in the

Pliocene The Pliocene ( ; also Pleiocene) is the epoch (geology), epoch in the geologic time scale that extends from 5.33 to 2.58Eltanin impact The Eltanin impact is thought to be an impact event, 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 impact occurred ...

Fossil content

The Ranquil Formation contains the following

trace fossil A trace fossil, also called an ichnofossil (; ), is a fossil record of biological activity by lifeforms, but not the preserved remains of the organism itself. Trace fossils contrast with body fossils, which are the fossilized remains of part ...

s: '' Zoophycos'', ''

Chondrites A chondrite is a stony (non-metallic) meteorite that has not been modified by either melting or planetary differentiation, differentiation of the parent body. They are formed when various types of dust and small grains in the early Solar Syste ...

'', '' 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 ''Rhizocorallium'' is an ichnogenus of burrow, the inclination of which is typically within 10° of the bedding planes of the sediment. These burrows can be very large, over a meter long in sediments that show good preservation, e.g. Jurassic r ...

'' 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 of Chile Geology of Biobío Region Coasts of Biobío Region

Units

Fossil content

See also

References