| Irving Formation | |
| Type: | Formation |
| Period: | Statherian |
| Prilithology: | Metavolcanic rock |
| Otherlithology: | Metasedimentary rock |
| Namedfor: | Irving Peak |
| Namedby: | Ernest Howe |
| Year Ts: | 1904 |
| Region: | San Juan Mountains, Colorado |
| Underlies: | Vallecito Conglomerate |
| Overlies: | Twilight Gneiss |
| Thickness: | Over 10000feet |
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]
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]
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]
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]