The Poison Canyon Formation is a

geologic 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 ...

in the

Raton Basin The Raton Basin is a geologic structural basin in southern Colorado and northern New Mexico. It takes its name from Raton Pass and the town of Raton, New Mexico. In extent, the basin is approximately east-west, and north-south, in Huerfano an ...

Colorado Colorado is a U.S. state, state in the Western United States. It is one of the Mountain states, sharing the Four Corners region with Arizona, New Mexico, and Utah. It is also bordered by Wyoming to the north, Nebraska to the northeast, Kansas ...

and

New Mexico New Mexico is a state in the Southwestern United States, Southwestern region of the United States. It is one of the Mountain States of the southern Rocky Mountains, sharing the Four Corners region with Utah, Colorado, and Arizona. It also ...

. The formation was deposited from the

late Cretaceous The Late Cretaceous (100.5–66 Ma) is the more recent of two epochs into which the Cretaceous Period is divided in the geologic time scale. Rock strata from this epoch form the Upper Cretaceous Series. The Cretaceous is named after ''cre ...

through 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), ...

Description

The Poison Canyon Formation consists of thick

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 ...

beds separated by beds of

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.Blatt, H., and R.J. Tracy, 1996, ''Petrology.'' New York, New York, ...

and

siltstone Siltstone, also known as aleurolite, is a clastic sedimentary rock that is composed mostly of silt. It is a form of mudrock with a low clay mineral content, which can be distinguished from shale by its lack of fissility. Although its permeabil ...

. It is found throughout most of the

. The sandstone is

arkosic Arkose () or arkosic sandstone is a detrital sedimentary rock, specifically a type of sandstone containing at least 25% feldspar. Arkosic sand is sand that is similarly rich in feldspar, and thus the potential precursor of arkose. Components ...

and coarse-grained to conglomeratic. The mudstone and siltstone beds

weather Weather is the state of the atmosphere, describing for example the degree to which it is hot or cold, wet or dry, calm or stormy, clear or cloud cover, cloudy. On Earth, most weather phenomena occur in the lowest layer of the planet's atmo ...

to yellow, are rich in

mica Micas ( ) are a group of silicate minerals whose outstanding physical characteristic is that individual mica crystals can easily be split into fragile elastic plates. This characteristic is described as ''perfect basal cleavage''. Mica is co ...

, and are not resistant to

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 ...

. The total thickness of the formation is up to . The formation grades below into the Raton Formation, with the transition often very gradual, up to . In the western part of the Raton Basin, the formation

intertongues A geological contact is a boundary which separates one rock body from another. A contact can be formed during deposition, by the intrusion of magma, or through faulting or other deformation of rock beds that brings distinct rock bodies into contac ...

with and partially replaces the Raton Formation. The two are distinguished by color (the Raton Formation is gray and the Poison Canyon Formation is yellow to orange), by the absence of arkosic sandstone in the Raton Formation, and by the absence of

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 Chemical element, elements, chiefly hydrogen, sulfur, oxygen, and nitrogen. Coal i ...

from the Poison Canyon Formation. In addition, river channel deposits in the Raton Formation are up to six times wider and five times deeper than river channels in the Poison Canyon Formation, and the channels in the Poison River Formation tend to be isolated and lack any sheet-like amalgamation. The Poison Canyon Formation underlies the

Cuchara Formation The Cuchara Formation is a geologic formation in Colorado. It preserves fossils dating back to the Ypresian stage of the Eocene period, or Wasatchian in the NALMA classification.derived its

sediments 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 ...

from the San Luis uplift to the southwest and the

Wet Mountains The Wet Mountains are a small mountain range in southern Colorado, named for the amount of snow they receive in the winter as compared to the dry Great Plains to the east. They are a sub-range of the Sangre de Cristo Mountains, in the southern Roc ...

uplift to the north and northwest. Its age ranges from

(where the formation partially replaces the Raton Formation) to

Fossils

The Poison Canyon Formation contains sparse fossilized plant remains characteristic of the Paleocene.

Economic resources

The formation is a significant freshwater aquifer in the

Canon City, Colorado Canon or Canons may refer to: Arts and entertainment * Canon (fiction), the material accepted as officially written by an author or an ascribed author * Literary canon, an accepted body of works considered as high culture ** Western canon, th ...

area.

History of investigation

The unit was first named as the "Poison Canyon conglomerates" by R.C. Hills in 1888. Hills later clarified the definition, separating the uppermost beds of his measured section to the Huerfano Formation.

References

{{Chronostratigraphy of Colorado, Cenozoic state=expanded Paleogene Colorado Paleogene formations of New Mexico Sandstone formations of the United States Mudstone formations of the United States Siltstone formations of the United States

Description

Fossils

Economic resources

History of investigation

See also

References