Ranquil Formation

Ranquil Formation
Stratigraphic range: Miocene–Pliocene
Type	Geological formation
Underlies	Tubul Formation
Overlies	Lebu Group
Lithology
Primary	Conglomerate with clay and silt matrix, breccia, sandstone, siltstone, mudstone
Other	Conglomerate with sand matrix
Location
Coordinates	37.6°S 73.7°W / -37.6; -73.7
Approximate paleocoordinates	37.9°S 71.4°W / -37.9; -71.4
Region	Bío Bío Region
Country	Chile
Type section
Named for	Caleta Ranquil
Named by	Juan Tavera
Year defined	1942
Ranquil Formation (Chile)

The Ranquil Formation (Spanish: Formación Ranquil) is a Miocene and Pliocene sedimentary formation located in Arauco Province in south–central Chile, including outcrops in Mocha Island. The formation has its greatest thicknesses in the south-west, where its sediments were largely deposited in marine conditions. It overlies unconformably sedimentary formations of the Paleocene-Eocene Lebu Group.[1] The formation is part of the fill of Arauco Basin which is a sedimentary basin that extends south of Concepción.[2]

Macrofossils of the formation are similar to those of Navidad (34° S) and Lacui Formations (43° S), two nearby Miocene marine formations.[3]

The base of the Ranquil Formation is the so-called "main unconformity", which is thought to have been formed by erosion during a period of tectonic inversion.[4]

The formation was first defined in 1942 by Juan Tavera.[1]

Units

The formation has been subdivided into five units, with the lowermost being made up of sandstone 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 and the so-called Huenteguapi sandstone.[2] The sediments of Huenteguapi sandstone evidences that a megatsunami struck the coast of south–central Chile in the Pliocene,[2] which has been linked to the hypothetical Eltanin impact.

Fossil content

The Ranquil Formation contains the following trace fossils: Zoophycos, Chondrites, Phycosiphon, Nereites missouriensis, Lockeiasiliquaria, Parataenidium, Ophiomorpha, Rhizocorallium and possibly also Psammichnites.[5]

See also

• Arauco Peninsula

References

1. García A., Floreal (1968). Ceccioni, Giovanni (ed.). El Terciario de Chile Zona Central (in Spanish). Santiago de Chile: Ediorial Andrés Bello. pp. 25–57.
2. Le Roux, J.P.; Nielsen, Sven N.; Kemnitz, Helga; Henriquez, Álvaro (2008). "A Pliocene mega-tsunami deposit and associated features in the Ranquil Formation, southern Chile" (PDF). Sedimentary Geology. 203 (1): 164–180. Bibcode:2008SedG..203..164L. doi:10.1016/j.sedgeo.2007.12.002. Retrieved 11 April 2016.
3. Finger, Kenneth L.; Nielsen, Sven N.; Devries, Thomas J.; Encinas, Alfonso; Peterson, Dwan E. (2007). "Paleontologic evidence for sedimentary displacement in Neogene forearc basins of Central Chile" (PDF). PALAIOS. 22 (1): 3–16. Bibcode:2007Palai..22....3F. doi:10.2110/palo.2005.p05-081r. S2CID 59383760. Retrieved 28 July 2016.
4. Becerra, Juan; Contreras-Reyes, Eduardo; Arriagada, César (2013). "Seismic structure and tectonics of the southern Arauco Basin, south-central Chile (~ 38°S)". Tectonophysics. 592: 53–66. Bibcode:2013Tectp.592...53B. doi:10.1016/j.tecto.2013.02.012.
5. Le Roux, Jacobus P.; Nielsen, Sven N.; Henríquez, Álvaro (2008). "Depositional environment of Stelloglyphus llicoensis isp. nov.: a new radial trace fossil from the Neogene Ranquil Formation, south-central Chile". Revista Geológica de Chile. 35 (2): 307–319. doi:10.5027/andgeov35n2-a06. Retrieved 29 July 2016.