The Molas Formation is a geologic formation that is found in the

Four Corners The Four Corners is a region of the Southwestern United States consisting of the southwestern corner of Colorado, southeastern corner of Utah, northeastern corner of Arizona, and northwestern corner of New Mexico. The Four Corners area ...

region of the United States. Its age is poorly constrained but is thought to be Namurian (late

Mississippian Mississippian may refer to: * Mississippian (geology), a subperiod of the Carboniferous period in the geologic timescale, roughly 360 to 325 million years ago *Mississippian culture, a culture of Native American mound-builders from 900 to 1500 AD ...

to middle

Pennsylvanian Pennsylvanian may refer to: * A person or thing from Pennsylvania * Pennsylvanian (geology) The Pennsylvanian ( , also known as Upper Carboniferous or Late Carboniferous) is, in the International Commission on Stratigraphy, ICS geologic timesca ...

Description

The Molas Formation is a sequence of continental

red beds Red beds (or redbeds) are sedimentary rocks, typically consisting of sandstone, siltstone, and shale, that are predominantly red in color due to the presence of ferric oxides. Frequently, these red-colored sedimentary strata locally contain ...

. It is divided into three members. The lowermost Coalbank Hill Member is up to of purplish-red to reddish-brown

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.Blatt ''et al.'' 1980, p ...

and

conglomerate Conglomerate or conglomeration may refer to: * Conglomerate (company) * Conglomerate (geology) * Conglomerate (mathematics) In popular culture: * The Conglomerate (American group), a production crew and musical group founded by Busta Rhymes ** Co ...

. This is overlain by an informal middle member, consisting of

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

, sandstone, and conglomerate, that is typically around . The upper member is typically around that includes maroon to light gray sandstone. The upper member grades into the overlying Pinkerton Trail Formation. The Coalbank Hill Member may be a

terra rossa Terra rossa (Italian for "red soil") is a well-drained, reddish, clayey to silty soil with neutral pH conditions and is typical of the Mediterranean region. The reddish color of terra rossa is the result of the preferential formation of hematite ...

(a residual soil formed by weathering of the underlying

Leadville Limestone The Leadville Limestone is a Mississippian geologic formation in the western United States.correlate with the

Log Springs Formation The Log Springs Formation is a geologic formation in the Jemez, Nacimiento, and Sandia Mountains of New Mexico. Its age is poorly constrained but is thought to be Namurian (late Mississippian to early Pennsylvanian). Description The Log Spri ...

in the Jemez Mountains of New Mexico. It has also been interpreted as a

loess Loess (, ; from german: Löss ) is a clastic, predominantly silt-sized sediment that is formed by the accumulation of wind-blown dust. Ten percent of Earth's land area is covered by loess or similar deposits. Loess is a periglacial or aeolian ...

trapped on the

karst Karst is a topography formed from the dissolution of soluble rocks such as limestone, dolomite, and gypsum. It is characterized by underground drainage systems with sinkholes and caves. It has also been documented for more weathering-resistant ro ...

surface of the underlying limestone. The middle member and much of the upper member were deposited in a river system that reworked the Coalbank Hill Member sediments, while the upper member includes some marine beds. The boundary between the middle and upper members is set where

kaolinite Kaolinite ( ) is a clay mineral, with the chemical composition Al2 Si2 O5( OH)4. It is an important industrial mineral. It is a layered silicate mineral, with one tetrahedral sheet of silica () linked through oxygen atoms to one octahedral ...

clay becomes more abundant than illite clay. The formation is exposed in the San Juan Mountains, the

Sangre de Cristo Mountains ) , country= United States , subdivision1_type= States , subdivision1= , parent= Rocky Mountains , geology= , orogeny= , area_mi2= 17193 , range_coordinates= , length_mi= 242 , length_orientation= north-south , width_mi= 120 , w ...

, the Mosquito Range, the Sawatch Range, and at Glenwood Springs, Colorado. It exists in the subsurface throughout the San Juan basin.

Fossils

The Coalbank Hill Member is devoid of fossils, but the upper member contains some marine limestone beds with brachiopods, bryozoans,

echinoderms An echinoderm () is any member of the phylum Echinodermata (). The adults are recognisable by their (usually five-point) radial symmetry, and include starfish, brittle stars, sea urchins, sand dollars, and sea cucumbers, as well as the sea li ...

, and forams indicating a middle Pennsylvanian age.

History of investigation

The formation was first designated by Charles Whitman Cross and Ernest Howe in 1905 for deposits near Molas Lake. Sherman Wengerd and John Strickland first divided the formation into three informal members in 1954, and William Merrill and Richard Winar formally designated the Coalbank Hill Member in 1958.

References

{{Reflist Carboniferous Colorado Carboniferous southern paleotropical deposits