geological formation
A geological formation, or simply formation, is a body of rock having a consistent set of physical characteristics ( lithology) that distinguishes it from adjacent bodies of rock, and which occupies a particular position in the layers of rock exp ...
of
sedimentary rock
Sedimentary rocks are types of rock that are formed by the accumulation or deposition of mineral or organic particles at Earth's surface, followed by cementation. Sedimentation is the collective name for processes that cause these particles ...
in south-central Chile. The sediments of the formation were deposited during the
Late Oligocene
The Chattian is, in the geologic timescale, the younger of two ages or upper of two stages of the Oligocene Epoch/Series. It spans the time between . The Chattian is preceded by the Rupelian and is followed by the Aquitanian (the lowest stage ...
and
Middle Miocene
The Middle Miocene is a sub-epoch of the Miocene Epoch made up of two stages: the Langhian and Serravallian stages. The Middle Miocene is preceded by the Early Miocene.
The sub-epoch lasted from 15.97 ± 0.05 Ma to 11.608 ± 0.005 Ma (million y ...
epochs. The formation's lower sections are made up of
conglomerate
Conglomerate or conglomeration may refer to:
* Conglomerate (company)
* Conglomerate (geology)
* Conglomerate (mathematics)
In popular culture:
* The Conglomerate (American group), a production crew and musical group founded by Busta Rhymes
** Co ...
,
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
mudstone
Mudstone, a type of mudrock, is a fine-grained sedimentary rock whose original constituents were clays or muds. Mudstone is distinguished from '' shale'' by its lack of fissility (parallel layering).Blatt, H., and R.J. Tracy, 1996, ''Petrology. ...
some of which is rich in organic material. Additionally there are thin beds of
tuff
Tuff is a type of rock made of volcanic ash ejected from a vent during a volcanic eruption. Following ejection and deposition, the ash is lithified into a solid rock. Rock that contains greater than 75% ash is considered tuff, while rock cont ...
and
coal
Coal is a combustible black or brownish-black sedimentary rock, formed as rock strata called coal seams. Coal is mostly carbon with variable amounts of other elements, chiefly hydrogen, sulfur, oxygen, and nitrogen.
Coal is formed when dea ...
. The formation's composition indicates that sedimentation occurred in an
estuarine
An estuary is a partially enclosed coastal body of brackish water with one or more rivers or streams flowing into it, and with a free connection to the open sea. Estuaries form a transition zone between river environments and maritime environment ...
(paralic) and marine environments.
Stratigraphically
Stratigraphy is a branch of geology concerned with the study of rock layers (strata) and layering (stratification). It is primarily used in the study of sedimentary and layered volcanic rocks.
Stratigraphy has three related subfields: lithostrati ...
it overlies the
Bahía Mansa Metamorphic Complex
The Bahía Mansa Metamorphic Complex or BMMC ( es, link=no, Complejo metamórfico Bahía Mansa, CMBM), also known as the Western Series, is a group of metamorphic geologic formations of the Chilean Coast Range in southern Chile. It consists main ...
and is similar in age and type to
Lacui Formation
Lacui Formation ( es, Formación Lacui) is a marine Miocene sedimentary formation located in Chiloé Island with minor outcrops near Carelmapu on the mainland. Gastropod shells are the most common macrofossils of Lacui Formation. According to Serna ...
Santo Domingo Formation
Santo Domingo Formation ( es, Formación Santo Domingo) is a mainly marine Miocene sedimentary formation located in south–central Chile.Encinas et al., 2008 The formation was defined by R. Martínez Pardo and Mario Pino in 1979 and named after ...
to the north. It is overlain across an
angular unconformity
An unconformity is a buried erosion surface, erosional or non-depositional surface separating two Rock (geology), rock masses or Stratum, strata of different ages, indicating that sediment deposition was not continuous. In general, the older layer ...
by
Pliocene
The Pliocene ( ; also Pleiocene) is the epoch in the geologic time scale that extends from 5.333 million to 2.58Quaternary
The Quaternary ( ) is the current and most recent of the three periods of the Cenozoic Era in the geologic time scale of the International Commission on Stratigraphy (ICS). It follows the Neogene Period and spans from 2.58 million years ...
sediments. The formation is
intruded
Intrusive rock is formed when magma penetrates existing rock, crystallizes, and solidifies underground to form '' intrusions'', such as batholiths, dikes, sills, laccoliths, and volcanic necks.Intrusive RocksIntrusive rocks accessdate: March ...
by
porphyritic
Porphyritic is an adjective used in geology to describe igneous rocks with a distinct difference in the size of mineral crystals, with the larger crystals known as phenocrysts. Both extrusive and intrusive rocks can be porphyritic, meaning all ...
trachyte
Trachyte () is an extrusive igneous rock composed mostly of alkali feldspar. It is usually light-colored and aphanitic (fine-grained), with minor amounts of mafic minerals, and is formed by the rapid cooling of lava enriched with silica and al ...
of Oligocene to Miocene age (
Ancud Volcanic Complex
Ancud Volcanic Complex ( es, Complejo Volcánico de Ancud) is a volcanic complex of Oligocene and Miocene age located around Ancud with exposures in Chiloé Island, the Chilean mainland and smaller islets. Three subunits are recognized in the compl ...
). The outcrops of the formation are restricted to a NW-SE strip near Caleta Parga north of the estuary of
Maullín River
Maullín River( es, Río Maullín) is a river of Chile located in the Los Lagos Region. The river originates as the outflow of Llanquihue Lake, and flows generally southwestward, over a number of small waterfalls, emptying into the Gulf of Coronad ...