The Mancos Shale or Mancos Group is a

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 ...

(Upper Cretaceous)

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 ...

of the

Western United States The Western United States (also called the American West, the Western States, the Far West, the Western territories, and the West) is List of regions of the United States, census regions United States Census Bureau. As American settlement i ...

. The Mancos Shale was first described by Cross and Purington in 1899 and was named for exposures near the town of

Mancos, Colorado Mancos is a statutory town in Montezuma County, Colorado, United States. The population was 1,196 at the 2020 census, down from 1,336 in 2010. The town is in southwestern Colorado, at the base of Mesa Verde National Park, and holds the trade ...

Geology

Uinta Piceance Basin stratigraphic column

The unit is dominated by

mudrock Mudrocks are a class of fine-grained siliciclastic sedimentary rocks. The varying types of mudrocks include siltstone, claystone, mudstone and shale. Most of the particles of which the stone is composed are less than and are too small to ...

that accumulated in offshore and marine environments of the Cretaceous North American Inland Sea. The Mancos was deposited during the

Cenomanian The Cenomanian is, in the International Commission on Stratigraphy's (ICS) geological timescale, the oldest or earliest age (geology), age of the Late Cretaceous epoch (geology), Epoch or the lowest stage (stratigraphy), stage of the Upper Cretace ...

(locally

Albian The Albian is both an age (geology), age of the geologic timescale and a stage (stratigraphy), stage in the stratigraphic column. It is the youngest or uppermost subdivision of the Early Cretaceous, Early/Lower Cretaceous epoch (geology), Epoch/s ...

) through

Campanian The Campanian is the fifth of six ages of the Late Cretaceous epoch on the geologic timescale of the International Commission on Stratigraphy (ICS). In chronostratigraphy, it is the fifth of six stages in the Upper Cretaceous Series. Campa ...

ages, approximately from 95 million years ago ( Ma) to 80 Ma.

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 ...

the Mancos Shale fills the interval between the

Dakota Dakota may refer to: * Dakota people, a sub-tribe of the Sioux ** Dakota language, their language Dakota may also refer to: Places United States * Dakota, Georgia, an unincorporated community * Dakota, Illinois, a town * Dakota, Minnesota ...

and the Mesaverde Group. The lower marine Mancos Shale conformably

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 terrestrial

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 ...

s and mudstones of the Dakota and in its upper part grades into and intertongues with the Mesaverde Group. The shale tongues typically have sharp basal contacts and gradational upper contacts. Whereas in the plains east of the Rocky Mountains certain mappable marine shales are identified as formations (e.g., Skull Creek,

Graneros Graneros is a Chilean commune and city in Cachapoal Province, O'Higgins Region. Demographics According to the 2002 census of the National Statistics Institute, Graneros spans an area of and has 25,961 inhabitants (12,992 men and 12,969 women) ...

), correlated deposits within the distribution of the Mancos are named as tongues of the greater Mancos Formation. Thus, the classification broadly corresponds with the

Colorado Group Colorado is a geologic name applied to certain rocks of Cretaceous age in the North America, particularly in the western Great Plains. This name was originally applied to classify a group of specific marine formations of shale and chalk known ...

classification of the

Great Plains The Great Plains is a broad expanse of plain, flatland in North America. The region stretches east of the Rocky Mountains, much of it covered in prairie, steppe, and grassland. They are the western part of the Interior Plains, which include th ...

region. Accordingly, various units of the Colorado Group are recognized within the Mancos in those areas where their distinct facies can be recognized.

Occurrences

Mancos Shale badlands in Capitol Reef NP

The Mancos occurs in the

Basin and Range Province The Basin and Range Province is a vast United States physiographic region, physiographic region covering much of the inland Western United States and Northern Mexico, northwestern Mexico. It is defined by unique basin and range topography, charac ...

, the Colorado Plateau Province, and the San Juan Mountains Province.

Structural basins

The Mancos is a diverse unit, with dozens of named subunits in different

structural basin A structural basin is a large-scale structural geology, structural formation of rock stratum, strata formed by tectonics, tectonic warping (Fold (geology), folding) of previously flat-lying strata into a syncline fold. They are geological dep ...

s that often intertongue with other formations."Colorado River Basin Stratigraphy: Mancos Shale"
United States Geological Survey The subunits and intertonguing formations (in italics) in each basin, in stratigraphic order, are:

History of investigation

The Mancos Shale was first named by Charles Whitman Cross and C.W. Purington in 1899, for outcrops near the town of

and along the Mancos River nearby. The two geologists also traced the unit into the

Telluride, Colorado Telluride is the county seat and most populous town of San Miguel County, Colorado, San Miguel County in the southwestern portion of the U.S. state of Colorado. The town is a former silver mining camp on the San Miguel River (Colorado), San M ...

area. W.T. Lee had traced the unit north into the

Grand Mesa The Grand Mesa is a large mesa in western Colorado in the United States. It is the largest flat-topped mountain in the world. It has an area of about and stretches for about east of Grand Junction between the Colorado River and the Gunnison ...

area, defining it as all marine shale between the Dakota and the Mesaverde. It was subsequently traced into Utah and New Mexico. During their work in New Mexico in 1924, J.B. Reeside Jr., and F.H. Knowlton found that the Mancos Shale could be divided into

biostratigraphic Biostratigraphy is the branch of stratigraphy which focuses on correlating and assigning relative ages of rock strata by using the fossil assemblages contained within them.Hine, Robert. "Biostratigraphy." ''Oxford Reference: Dictionary of Biology ...

layers corresponding closely to formations of the

further east. By 1944, Rankin had concluded that most of the formations of the Colorado Group could be identified as

lithostratigraphic Lithostratigraphy is a sub-discipline of stratigraphy, the geological science associated with the study of strata or rock layers. Major focuses include geochronology, comparative geology, and petrology. In general, strata are primarily igneous ...

members of the Mancos Shale as well. The unit was raised to group rank by C.E. Jamison in 1911, and is sometimes given group rank in New Mexico and Utah as well.

References

{{Chronostratigraphy of Colorado, Mesozoic state=expanded Shale formations of the United States Upper Cretaceous Series of North America Cretaceous United States Campanian Stage Cenomanian Stage Coniacian Stage Santonian Stage Turonian Stage Cretaceous Arizona Cretaceous Colorado Cretaceous formations of New Mexico Cretaceous geology of Texas Cretaceous geology of Utah Cretaceous geology of Wyoming Geology of the Rocky Mountains

Geology

Occurrences

Structural basins

History of investigation

See also

References