Mancos Shale
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The Mancos Shale or Mancos Group is a Late Cretaceous (Upper Cretaceous) geologic formation of the Western United States.
The Mancos Shale was first described by Cross and Purington in 1899 and was named for exposures near the town of Mancos, Colorado.
Geology
The unit is dominated by mudrock that accumulated in offshore and marine environments of the Cretaceous North American Inland Sea. The Mancos was deposited during the Cenomanian (locally Albian) through Campanian ages, approximately from 95 million years ago (Ma) to 80 Ma.
Stratigraphically the Mancos Shale fills the interval between the Dakota and the Mesaverde Group.
The lower marine Mancos Shale conformably intertongues with terrestrial sandstones 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), 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 classification of the Great Plains region. Accordingly, various units of the Colorado Group are recognized within the Mancos in those areas where their distinct facies can be recognized.
Occurrences
The Mancos occurs in the Basin and Range Province, 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 basins that often intertongue with other formations. The subunits and intertonguing formations (in italics) in each basin, in stratigraphic order, are:
| Black Mesa Basin Upper shale member Hopi Sandy Member Middle shale member Lower shale member Chama Basin Upper shale unite El Vado Sandstone Member Middle shale unit Cooper Arroyo Sandstone Member Juana Lopez Member Lower shale unit Greenhorn Limestone Graneros Shale Estancia Basin (Mancos Group) Niobrara Formation Carlile Shale Greenhorn Limestone | Paradox Basin West and northwest Masuk Member Emery Sandstone Blue Gate Shale Ferron Sandstone Tununk Shale North Buck Tongue Castlegate Sandstone Juana Lopez Member Northeast and east Juana Lopez Member Greenhorn Limestone Piceance Basin Buck Tongue Anchor Mine Tongue Main body | San Juan Basin Mulatto Tongue Dilco Coal Member of Crevasse Canyon Formation Niobrara Calcareous Shale Carlile Shale Greenhorn Limestone Graneros Shale Paguate Tongue of Dakota Formation Clay Mesa Tongue Socorro Basin D-Cross Tongue Gallup Sandstone Pescado Tongue Tres Hermanos Formation Rio Salado Tongue Twowells Tongue of Dakota Formation Whitewater Arroyo Tongue | Uintah Basin Anchor Mine Tongue Sego Sandstone Upper shale member Frontier Sandstone Middle shale member Aspen Shale Lower shale member Mowry Shale |
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 Mancos, Colorado, and along the Mancos River nearby. The two geologists also traced the unit into the Telluride, Colorado, area. W.T. Lee had traced the unit north into the Grand Mesa 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 layers corresponding closely to formations of the Colorado Group further east. By 1944, Rankin had concluded that most of the formations of the Colorado Group could be identified as lithostratigraphic 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.