Irving Formation
| Irving Formation | |
|---|---|
| Stratigraphic range: | |
| Type | Formation |
| Underlies | Vallecito Conglomerate |
| Overlies | Twilight Gneiss |
| Thickness | Over 10,000 feet (3,000 m) |
| Lithology | |
| Primary | Metavolcanic rock |
| Other | Metasedimentary rock |
| Location | |
| Coordinates | 37°35′42″N 107°30′32″W / 37.595°N 107.509°W |
| Region | San Juan Mountains, Colorado |
| Country |  United States |
| Type section | |
| Named for | Irving Peak |
| Named by | Ernest Howe |
| Year defined | 1904 |
  Irving Formation (the United States)  Irving Formation (Colorado) | |
The Irving Formation is a Precambrian geologic formation found in the San Juan Mountains of southwest Colorado, US. It is thought to be Statherian in age (1800 to 1790 million years old.)[1]
Description[edit]
The formation consists of a variety of lithologies (rock varieties) including amphibolite, various schists and gneisses of intermediate to felsic composition, quartzite, metasiltstone, and banded iron formation. A bed of conglomerate is found near the base of the formation in some locations. The beds show indications of mild retrograde metamorphism and dip steeply to the north.[2] It was intruded by the Twilight Gneiss between 1780 and 1770 million years ago (Mya).[1] The complex is older than the Vallecito Conglomerate.[2]
The formation underlies Irving Peak in the San Juan Mountains of Colorado[3] and is exposed across the western and northern Needle Mountains. It is at least a few thousand meters (several thousand feet) thick.[2]
The unit is interpreted as a portion of an island arc accreted to the southern margin of Laurentia as part of the Yavapai Province between 1.8 and 1.755 Gya.[1] Metasedimentary rocks of the formation are interpreted as turbidites derived from the island arc.[4]
Economic resources[edit]
The formation was surveyed in 1969 for iron ore. Magnetite-rich beds were found in a few locations but were not judged economical to exploit.[5]
History of investigation[edit]
The unit was first named as the Irving Greenstone by Ernest Howe in 1904.[6] The definition was expanded by Fred Barker in 1969, who also renamed the formation as the Irving Formation.[2]
References[edit]
- ^ a b c Whitmeyer, Steven; Karlstrom, Karl E. (2007). "Tectonic model for the Proterozoic growth of North America". Geosphere. 3 (4): 220. doi:10.1130/GES00055.1.
- ^ a b c d Barker, Fred (1969). "Precambrian geology of the Needle mountains, southwestern Colorado". U.S. Geological Survey Professional Paper. 644-A. doi:10.3133/pp644A.
- ^ Cross, Whitman; Howe, Ernest (1905). "Description of Needle Mountains quadrangle [Colorado]". U.S. Geological Survey Geologic Atlas of the United States Folio, Needle Mountains folio. Vol. 131. Retrieved 19 April 2022.
- ^ McLennan, S.M.; Hemming, S.R.; Taylor, S.R.; Eriksson, K.A. (March 1995). "Early Proterozoic crustal evolution: Geochemical and NdPb isotopic evidence from metasedimentary rocks, southwestern North America". Geochimica et Cosmochimica Acta. 59 (6): 1153–1177. doi:10.1016/0016-7037(95)00032-U.
- ^ Steven, T.A.; Schmitt, L.J.; Sheridan, M.J.; Williams, F.E.; Gair, J.E.; Klemic, H. (1969). "Mineral resources of the San Juan primitive area, Colorado, with a section on iron resources in the Irving Formation". U.S. Geological Survey Bulletin. 1261-F. doi:10.3133/b1261F.
- ^ Howe, Ernest (September 1904). "An Occurrence of Greenstone Schists in the San Juan Mountains, Colorado". The Journal of Geology. 12 (6): 501–509. doi:10.1086/621173.