Ranquil Formation

{{Short description|Miocene and Pliocene sedimentary formation in south-central Chile}}

{{Infobox rockunit

| name = Ranquil Formation

| image =

| caption =

| type = Geological formation

| period = Pliocene

| age = MiocenePliocene

| prilithology = Conglomerate with clay and silt matrix, breccia, sandstone, siltstone, mudstone

| otherlithology = Conglomerate with sand matrix

| namedfor = Caleta Ranquil

| namedby = Juan Tavera

| year_ts = 1942

| region = Bío Bío Region

| country = Chile

| coordinates = {{coord|37.6|S|73.7|W|display=inline,title}}

| paleocoordinates = {{coord|37.9|S|71.4|W|display=inline}}

| unitof =

| subunits =

| underlies = Tubul Formation

| overlies = Lebu Group

| thickness =

| extent =

| area =

| map = {{Location map+ | Chile

| relief = 1

| width = 150

| float = center

| places =

{{Location map~ | Chile

| lat_deg = -37.6

| lon_deg = -73.7

| mark = Yellow ffff00 pog.svg

| marksize = 12

}}

}}

| map_caption =

}}

The Ranquil Formation ({{langx|es|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.{{cite book |last=García A. |first=Floreal |date=1968 |title=El Terciario de Chile Zona Central |editor-last=Ceccioni |editor-first=Giovanni |location=Santiago de Chile |publisher=Editorial Andrés Bello |pages=25–57 |language=es}} The formation is part of the fill of Arauco Basin which is a sedimentary basin that extends south of Concepción.

Macrofossils of the formation are similar to those of Navidad (34° S) and Lacui Formations (43° S), two nearby Miocene marine formations.{{cite journal |last1=Finger |first1=Kenneth L. |last2=Nielsen |first2=Sven N. |last3=Devries |first3=Thomas J. |last4=Encinas |first4=Alfonso |last5=Peterson |first5=Dwan E. |date=2007 |title=Paleontologic evidence for sedimentary displacement in Neogene forearc basins of Central Chile |url=http://www.ucmp.berkeley.edu/people/klf/KLF_files/Palaios.pdf |journal=PALAIOS |volume=22 |issue=1 |pages=3–16 |doi= 10.2110/palo.2005.p05-081r|bibcode=2007Palai..22....3F |s2cid=59383760 |access-date=28 July 2016}}

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.{{cite journal|last1=Becerra|first1=Juan|last2=Contreras-Reyes |first2=Eduardo |last3=Arriagada |first3=César |title=Seismic structure and tectonics of the southern Arauco Basin, south-central Chile (~ 38°S) |journal=Tectonophysics|date=2013|volume=592|pages=53–66 |doi=10.1016/j.tecto.2013.02.012|bibcode=2013Tectp.592...53B}}

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 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.{{cite journal |last1=Le Roux |first1=J.P. |last2=Nielsen |first2= Sven N. |last3=Kemnitz |first3=Helga |last4=Henriquez |first4=Álvaro |date=2008 |title=A Pliocene mega-tsunami deposit and associated features in the Ranquil Formation, southern Chile |url=http://repositorio.uchile.cl/bitstream/handle/2250/125260/Le%20Roux_J_P.pdf?sequence=1 |journal=Sedimentary Geology |volume=203 |issue=1 |pages=164–180 |doi= 10.1016/j.sedgeo.2007.12.002|bibcode=2008SedG..203..164L |access-date=11 April 2016}} The sediments of Huenteguapi sandstone evidences that a megatsunami struck the coast of south–central Chile in the Pliocene, 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.{{cite journal |last1=Le Roux |first1=Jacobus P. |last2=Nielsen |first2=Sven N. |last3=Henríquez |first3=Álvaro |date=2008 |title=Depositional environment of Stelloglyphus llicoensis isp. nov.: a new radial trace fossil from the Neogene Ranquil Formation, south-central Chile |url=http://www.scielo.cl/scielo.php?script=sci_arttext&pid=S0716-02082008000200006 |journal=Revista Geológica de Chile |volume=35 |issue=2 |pages=307–319 |doi= 10.5027/andgeov35n2-a06|access-date=29 July 2016|doi-access=free }}

See also

  • {{annotated link|Arauco Peninsula}}

References