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
Region Bío Bío Region
Country Chile
Type section
Named for Caleta Ranquil
Named by Juan Tavera, 1942

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 Formation (43° S) two nearby Miocene marine formations.[3]

The base of Ranquil Formation is the so-called “main unconformity” which is thought to have 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]

Fossil content

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

See also

References

  1. 1 2 García A., Floreal (1968). Ceccioni, Giovanni, ed. El Terciario de Chile Zona Central (in Spanish). Santiago de Chile: Ediorial Andrés Bello. p. 25–57.
  2. 1 2 3 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. 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: 3–16. 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.
  5. ↑ Le Reoux, 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. Retrieved 29 July 2016.
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