The Mancos Shale was first described by Cross and Purington in 1899[1] 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
Campanianages, approximately from 95 million years ago (
Ma) to 80 Ma.
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. As such, various units of the Colorado Group are recognized within the Mancos in those areas where their distinct facies can be recognized.[3]
The Mancos is a diverse unit, with dozens of named subunits in different
structural basins that often
intertongue with other formations.[4] The subunits and intertonguing formations (in italics) in each basin, in stratigraphic order, are:
Tununk Member of the Mancos Shale below the capping Ferron Sandstone Member. West side of the San Rafael Swell, Emery County, Utah.
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.[1] 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.[19] It was subsequently traced into Utah[20] and New Mexico.[21]
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.[3] The unit was raised to
group rank by C.E. Jamison in 1911,[22] and is sometimes given group rank in New Mexico[23] and Utah[24] as well.
^
abcRankin, Charles H. (1944).
"Stratigraphy of the Colorado Group, Upper Cretaceous, in Northern New Mexico"(PDF). New Mexico Bureau of Mines and Mineral Resources Bulletins (20). New Mexico School of Mines: 5. Retrieved 2018-08-13. ...that all divisions of the Colorado group (Mancos shale) as described in southern Colorado, except the Fort Hays limestone and the Apishapa shale, can be recognized in northern New Mexico.
^Walton, P.T. (1944). "Geology of the Cretaceous of the Uinta basin, Utah". Bulletin of the Geological Society of America. 55 (1): 91–130.
Bibcode:
1944GSAB...55...91W.
doi:
10.1130/GSAB-55-91.
^Lee, Willis Thomson (1912). "Coal fields of Grand Mesa and the West Elk Mountains, Colorado". United States Geological Survey Bulletin. 510.
doi:
10.3133/b510.
hdl:2346/65145.