San Andres Formation, United States
From Wikipedia, the free encyclopedia
| San Andres Formation | |
|---|---|
| Stratigraphic range: | |
 San Andres Formation near El Malpais National Monument | |
| Type | Formation |
| Sub-units | Bonney Canyon & Fourmile Draw Members |
| Underlies | Grayburg Formation |
| Overlies | Glorieta & Yeso Formations |
| Thickness | 400 m (1,300 ft) |
| Lithology | |
| Primary | Limestone |
| Other | Dolomite, sandstone |
| Location | |
| Coordinates | 33°14′10″N 106°45′36″W / 33.236°N 106.760°W |
| Approximate paleocoordinates | 4°24′N 33°30′W / 4.4°N 33.5°W |
| Region | Texas New Mexico |
| Country | United States |
| Extent | San Juan Basin (subsurface) San Andres, Zuni & Sangre de Cristo Mountains (outcrop) |
| Type section | |
| Named for | San Andres Mountains |
| Named by | W.T. Lee |
| Year defined | 1909 |
  San Andres Formation, United States (New Mexico) | |
The San Andres Formation is a geologic formation found in New Mexico and Texas. It contains fossils characteristic of the late Leonardian (Kungurian) Age) of the Permian Period.[1]
The formation is the most widely exposed Paleozoic formation in the state of New Mexico, cropping out in a zone 80–110 kilometers (50–68 miles) wide that extends from west Texas to central New Mexico and with additional exposures in the Zuni Mountains and Sangre de Cristo Mountains. It is also prominent in the subsurface in the San Juan Basin and in eastern New Mexico.[2]
The formation consists mostly of massive marine limestone. The limestone is typically cherty and poorly fossiliferous, but fossils are locally abundant.[3] The total thickness is up to 400 meters (1,300 ft).[4]
The formation is transitional with the underlying Glorieta Sandstone or Yeso Group. Where the basal sandstone beds are a minor component compared with marine limestone, these beds are assigned to the Rio Bonito Member of the San Andres Formation and no Glorieta Sandstone is mapped. Where the basal sandstones contain little or no marine limestone, the lower beds are mapped as Glorieta Sandstone and no Rio Bonito Member is mapped. This transition from clastic to marine beds represents a late Leonardian marine transgression that was the last to reach northern New Mexico for the next 150 million years.[4] The remaining members are the Bonney Canyon Member, a relatively thin section of thin-bedded limestone with local gypsum beds, and the upper Fourmile Draw Member, which is primarily marine carbonates. The Fourmile Draw grades into the Brushy Canyon and Cherry Canyon Formations in the Delaware Basin and contains considerable gypsum in the Guadalupe Mountains. The entire San Andres Formation tends to become more dolomitic to the north.[4]
The formation is conformably overlain by the Grayburg Formation or, where this is not present, unconformably by Triassic units.[5][1]
Fossil content
The formation contains fossils of the gastropod Euomphalus, the scaphopods Plagioglypia canna and Dentalium mexicanam, and the bivalve Aviculipinna.[3]