| Popotosa Formation | |
|---|---|
| Stratigraphic range:Neogene 26.4–7 Ma | |
 Popotosa Formation at one of its reference sections, Canoncito de las Cabras, New Mexico | |
| Type | Formation |
| Unit of | Santa Fe Group |
| Underlies | Sierra Ladrones Formation |
| Overlies | South Canyon Tuff |
| Thickness | 1,447 m (4,747 ft) |
| Lithology | |
| Primary | Volcaniclastics |
| Other | Tuff |
| Location | |
| Coordinates | 34°18′00″N107°02′38″W / 34.30000°N 107.04386°W /34.30000; -107.04386 |
| Region | New Mexico |
| Country | United States |
| Type section | |
| Named for | Canada Popotosa |
| Named by | C.S. Denny |
| Year defined | 1940 |
  Popotosa Formation (the United States) Show map of the United States Popotosa Formation (New Mexico) Show map of New Mexico | |
ThePopotosa Formation is ageologic formation inNew Mexico. It preservesfossils dating back to theNeogeneperiod. These include the Socorro flora, notable for its fine preservation of plant reproductive structures.
The Popotosa Formation is a thick (up to 1,447 meters (4,747 feet)) sequence ofvolcaniclastic beds with a few interspersedash beds. It is exposed along theRio Grande rift in theSocorro area.Radiometric dating of interbeddedflows gives it an age of 26.4 to 7 million yeawrs (Ma), corresponding to the lateOligocene toMiocene.[1] It lies ontuffoutflow sheets of theMogollon-Datil volcanic field, primarily theSouth Canyon Tuff, and is overlain by theSierra Ladrones Formation.[2]
The formation is interpreted as deposition offanglomerates (mostly derived from the Mogollon-Datil volcanic field to the southwest) andplaya sediments in aclosed basin in the early stages of rifting along the Rio Grande rift. It is thus typical of the lowerSanta Fe Group.[1] The formation was severely deformed in the late Miocene or early Pliocene and some beds dip as much as 60 degrees.Faults displace the formation hundreds to thousands of meters.[3]
Deformation in the middle Miocene caused the area to subside at a rate that exceeded the sediment supply, forming a topographicallyclosed basin in which the Popotosa Formation was deposited. Increasing tilt rates created a series of regionalunconformities. Whentectonic activity finally slowed in the latest Miocene and early Pliocene, sedimentation exceededaccommodation. The basin began to fill, spilled over, and became an open basin as it wasintegrated into the ancestralRio Grande river system. Sediments deposited after integration became the Sierra Ladrones Formation.[4]
The Popotosa Formation is the original locality for the Socorro flora, estimated to be 20 to 15 million years old. The Socorro flora is notable for its impressions ofjuniper foliage,angiosperm leaflets, and floral parts. It is of particular interest for its fine preservation, including of reproductive structures. The flora is dominated byCalliandra leaflets but also containsJuniperus[2]
The formation has yielded a fossil of the pig-likeoreodontMerychyus major major from anarroyo nearSan Antonito.[5]
The formation containslithium-richsmectiteclay beds with up to 1250 parts per million of lithium. However, the beds discovered as of 1979 are not extensive enough for economic exploitation.[3]
The unit was first described by C.S. Denny in 1940, who named it for exposures near Canada Popotosa.[6] It was assigned to the lower Santa Fe Group by M.N. Machette in 1978.[7]
