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 region of the

United States The United States of America (U.S.A. or USA), commonly known as the United States (U.S. or US) or America, is a country primarily located in North America. It consists of 50 states, a federal district, five major unincorporated territori ...

. It contains fossils 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 ide ...

of geological formations deposited in the interior and margins of the

Paradox basin The Paradox Basin is an asymmetric foreland basin located mostly in southeast Utah and southwest Colorado, but extending into northeast Arizona and northwest New Mexico. The basin is a large elongate northwest to southeast oriented depression for ...

during the Pennsylvanian. The Pinkerton Trail Formation consists of gray

limestone Limestone ( calcium carbonate ) is a type of carbonate sedimentary rock which is the main source of the material lime. It is composed mostly of the minerals calcite and aragonite, which are different crystal forms of . Limestone forms whe ...

with occasional beds of black shale. North of Durango, it contains significant clastic

sediments Sediment is a naturally occurring material that is broken down by processes of weathering and erosion, and is subsequently transported by the action of wind, water, or ice or by the force of gravity In physics, gravity () is a fundame ...

. It rests on the Molas Formation and is overlain by the

Paradox Formation In geology, the Paradox Formation Is a Pennsylvanian age formation which consists of abundant evaporites with lesser interbedded shale, sandstone, and limestone. The evaporites are largely composed of gypsum, anhydrite, and halite. The formatio ...

. Its thickness is up to in the subsurface. The formation is exposed in the Durango, Colorado 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 ...

and the

. 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 animals that make up the class Crinoidea. Crinoids that are attached to the sea bottom by a stalk in their adult form are commonly called sea lilies, while the unstalked forms are called feather stars or comatulids, which are ...

and

fusulinids The Fusulinida is an extinct order within the Foraminifera in which the tests are traditionally considered to have been composed of microgranular calcite. Like all forams, they were single-celled organisms. In advanced forms the test wall was dif ...

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

References

{{Reflist Carboniferous Colorado Carboniferous southern paleotropical deposits