The Elbert Formation is a

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 ...

that crops out in the

San Juan Mountains The San Juan Mountains is a high and rugged mountain range in the Rocky Mountains in southwestern Colorado and northwestern New Mexico. The area is highly mineralized (the Colorado Mineral Belt) and figured in the gold and silver mining industry ...

of southwestern

Colorado Colorado is a U.S. state, state in the Western United States. It is one of the Mountain states, sharing the Four Corners region with Arizona, New Mexico, and Utah. It is also bordered by Wyoming to the north, Nebraska to the northeast, Kansas ...

. The formation contains

fossil A fossil (from Classical Latin , ) is any preserved remains, impression, or trace of any once-living thing from a past geological age. Examples include bones, shells, exoskeletons, stone imprints of animals or microbes, objects preserve ...

s indicating it is upper Devonian in age.

Description

The Elbert Formation is divided into the lower McCracken Sandstone

Member Member may refer to: * Military jury, referred to as "Members" in military jargon * Element (mathematics), an object that belongs to a mathematical set * In object-oriented programming, a member of a class ** Field (computer science), entries in ...

and upper unnamed member. The McCracken Sandstone Member consists mostly of gray to brown very fine to coarse grained

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 ...

, with lesser amounts of dolomite and gray to green

shale Shale is a fine-grained, clastic sedimentary rock formed from mud that is a mix of flakes of Clay mineral, clay minerals (hydrous aluminium phyllosilicates, e.g., Kaolinite, kaolin, aluminium, Al2Silicon, Si2Oxygen, O5(hydroxide, OH)4) and tiny f ...

. The sandstones are very highly cemented, with almost no pore space. The member is distinguished from the underlying Ignacio Formation by its relative lack of

feldspar Feldspar ( ; sometimes spelled felspar) is a group of rock-forming aluminium tectosilicate minerals, also containing other cations such as sodium, calcium, potassium, or barium. The most common members of the feldspar group are the ''plagiocl ...

. The McCracken Sandstone Member was first identified in the subsurface, where the

Aneth Formation The San Juan Basin is a geologic structural basin located near the Four Corners region of the Southwestern United States. The basin covers 7,500 square miles and resides in northwestern New Mexico, southwestern Colorado, and parts of Utah a ...

is sometimes found between the McCracken Member and the Ignacio Formation. It crops out at the surface near the

Needle Mountains The Needle Mountains are a subrange of the San Juan Mountains of the Rocky Mountains located in the southwestern part of the U.S. State of Colorado. Much of the range is protected in the Weminuche Wilderness of the San Juan National Forest. The ...

. Total thickness is up to . The remainder of the Elbert Formation consists of thin-bedded brownish-gray dolomite and sandstone, green to red shale, and small amounts of

anhydrite Anhydrite, or anhydrous calcium sulfate, is a mineral with the chemical formula CaSO4. It is in the orthorhombic crystal system, with three directions of perfect cleavage parallel to the three planes of symmetry. It is not isomorphous with the ...

. It is well exposed at the surface in the Piedra River canyon. The formation is overlain by the Ouray Formation. Maximum thickness is up to . The McCracken Sandstone Member is interpreted as shallow marine sand deposited during a rise in sea level (a '' transgression''). The upper member contains casts of salt crystals,

stromatolites Stromatolites ( ) or stromatoliths () are layered sedimentary formations ( microbialite) that are created mainly by photosynthetic microorganisms such as cyanobacteria, sulfate-reducing bacteria, and Pseudomonadota (formerly proteobacteria) ...

, and fish fossils suggesting it was deposited in a

tidal flat Mudflats or mud flats, also known as tidal flats or, in Ireland, slob or slobs, are coastal wetlands that form in intertidal areas where sediments have been deposited by tides or rivers. A global analysis published in 2019 suggested that tidal ...

environment during a time of increasing sea depth and

tectonic Tectonics ( via Latin ) are the processes that result in the structure and properties of the Earth's crust and its evolution through time. The field of ''planetary tectonics'' extends the concept to other planets and moons. These processes ...

stability. The area was at the western edge of the

Transcontinental Arch The Transcontinental Arch refers to the islands of North America which extended from New Mexico to Minnesota and the Great Lakes region. These islands were present during the Sauk sequence, the earliest cratonic sequence. As a result of the Sauk s ...

and the formation may record deposition along a rocky shoreline, a depositional environment rarely preserved in the geological record.

Fossils

The Elbert Formation contains remains of fossilized fish characteristic of the upper Devonian.

Economic resources

The Elbert Formation extends in the subsurface into

Utah Utah is a landlocked state in the Mountain states, Mountain West subregion of the Western United States. It is one of the Four Corners states, sharing a border with Arizona, Colorado, and New Mexico. It also borders Wyoming to the northea ...

, where the McCracken Sandstone Member is an important

petroleum reservoir A petroleum reservoir or oil and gas reservoir is a subsurface accumulation of hydrocarbons contained in porous or fractured rock formations. Such reservoirs form when kerogen (ancient plant matter) is created in surrounding rock by the prese ...

at Lisbon Valley.

History of investigation

The formation was first designated by Charles Whitman Cross in 1904 for exposures at Elbert Creek. The McCracken Sandstone Member was designated in 1955 by R.L. Knight and J.C. Cooper. Evans, Maurer, and Holm-Denoma recommended reassigning the McCracken Sandstone to the underlying Ignacio Formation in 2019.

References

{{Reflist Devonian Colorado Sandstone formations of the United States Geologic formations of Colorado