The Burro Canyon Formation is an Early Cretaceous Period sedimentary geologic formation, found in western Colorado, the Chama Basin and eastern San Juan Basin of northern New Mexico, and in eastern Utah, US.

Description

The

lithology The lithology of a rock unit is a description of its physical characteristics visible at outcrop, in hand or core samples, or with low magnification microscopy. Physical characteristics include colour, texture, grain size, and composition. Lit ...

of the Burro Canyon Formation is dominated by medium- to fine-grained sandstone, much of which has rounded chert and quartz pebbles above scour surfaces. The Burro Canyon Formation was deposited in a braided-stream system. The formation overlies the

Morrison Formation The Morrison Formation is a distinctive sequence of Late Jurassic, Upper Jurassic sedimentary rock found in the western United States which has been the most fertile source of dinosaur fossils in North America. It is composed of mudstone, sandsto ...

, with the boundary placed at the first conglomeratic sandstone bed. It underlies the Dakota Formation, from which it is distinguished by the presence of green

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. ...

and the absence of carbonaceous material. It is similar to the Cedar Mountain Formation on the west side of the Colorado River and the two formations have been interpreted as interfingering alluvial fans from separate source regions. The Jackpile Member of the Morrison Formation may be its lateral equivalent.

Fossils

Palynomorphs near the top of formation indicate a middle to late Early Cretaceous age. The formation also contains the

pelecypod Bivalvia (), in previous centuries referred to as the Lamellibranchiata and Pelecypoda, is a class of marine and freshwater molluscs that have laterally compressed bodies enclosed by a shell consisting of two hinged parts. As a group, biva ...

mollusks ''Protelliptio douglassi'' and '' Unio farri'' and the conifer ''Frenelopsis varians''.

Economic geology

The Burro Canyon Formation in the southern Chama Basin has been evaluated for its potential for uranium deposits. These are not economical to recover at 2020 prices.

Investigative history

The formation was first named by W.L. Stokes and D.A. Phoenix in 1948 for exposures in Burro Canyon east of the Dolores River. Stokes concluded by 1952 that the formation is equivalent to the Cedar Mountain Formation, with which it is continuous near

Dewey, Utah Dewey is a ghost town in along the Colorado River in southeastern Grand County, Utah, United States. Description Originally named ''Kingsferry'', it began in the 1880s when Samuel King built and operated a ferry across the Colorado River (but ...

. Robert Fillmore considers the two formations to be distinct enough to warrant retaining the distinction in name, and puts the boundary along the Colorado River. E.R. Swift proposed the name Deadman's Peak Formation for similar beds in the Chama basin, but this was rejected by A.E. Saucier.

Footnotes

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