The Pinkerton Trail 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 is found in the

Four Corners 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. Most of the Four Corners regio ...

region of the

United States The United States of America (USA), also known as the United States (U.S.) or America, is a country primarily located in North America. It is a federal republic of 50 U.S. state, states and a federal capital district, Washington, D.C. The 48 ...

. It contains

fossils 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 preserved ...

characteristic of the Atokan and Desmoinesian Ages of the Pennsylvanian.

Description

The Pinkerton Trail Formation is the lowest member of the Hermosa Group, a

group A group is a number of persons or things that are located, gathered, or classed together. Groups of people * Cultural group, a group whose members share the same cultural identity * Ethnic group, a group whose members share the same ethnic iden ...

of geological formations deposited in the interior and margins of the Paradox basin during the Pennsylvanian. The Pinkerton Trail Formation consists of gray

limestone Limestone is a type of carbonate rock, carbonate sedimentary rock which is the main source of the material Lime (material), lime. It is composed mostly of the minerals calcite and aragonite, which are different Polymorphism (materials science) ...

with occasional beds of black

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 ...

. North of Durango, it contains significant

clastic Clastic rocks are composed of fragments, or clasts, of pre-existing minerals and rock. A clast is a fragment of geological detritus,Essentials of Geology, 3rd Ed, Stephen Marshak, p. G-3 chunks, and smaller grains of rock broken off other rocks by ...

sediments Sediment is a solid material that is transported to a new location where it is deposited. It occurs naturally and, through the processes of weathering and erosion, is broken down and subsequently sediment transport, transported by the action of ...

. It rests on the Molas Formation and is overlain by the Paradox Formation. Its thickness is up to in the subsurface. The formation is exposed in the

Durango, Colorado Durango is the home rule city that is the county seat and the most populous municipality of La Plata County, Colorado, United States. The city population was 19,071 at the 2020 United States census. Durango is the home of Fort Lewis College ...

area and is present in the subsurface in the

San Juan basin 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 ...

and the Paradox basin. It was deposited by the advance of the sea (a '' transgression'') over the region, which deposited limestone atop the continental sediments of the Molas Formation.

Fossils

The formation is fossiliferous, containing

crinoids Crinoids are marine invertebrates that make up the class Crinoidea. Crinoids that remain attached to the sea floor by a stalk in their adult form are commonly called sea lilies, while the unstalked forms, called feather stars or comatulids, are ...

and

fusulinids The Fusulinida is an extinct Order (biology), order within the Foraminifera in which the Foraminifera test, tests are traditionally considered to have been composed of wikt:microgranular, microgranular calcite. Like all forams, they were single- ...

that place its age in the Atokan to Desmoinesian Ages of the Pennsylvanian.

History of investigation

The formation was first designated by Sherman Wengerd and John Strickland in 1954 as part of their work raising the Hermosa Formation to group stratigraphic rank. It was named for exposures at the Pinkerton Trail, about north of

. Baars, Parker, and Chronic proposed a subsurface reference section in 1967 in the Paradox basin.

References

{{Reflist Carboniferous Colorado Carboniferous southern paleotropical deposits Limestone formations of the United States Shale formations of the United States