Gila Group

The group is primarily conglomerate composed of clasts (rock fragments) weathered from nearby outcrops. There are also occasional thin sandstone and tuff beds and basalt flows. Total thickness is 1,000–1,500 feet (300–460 m).[1] Radiometric ages of basalt flows within the group range from 12.1 million years (Ma)[2] to 1 Ma.[3]

The group is similar in age and character to the Santa Fe Group. Both are sedimentary fill of extensional basins. The two groups are separated by the Continental Divide, and the Gila Group is separated from formations of similar age to the north by the Rim Divide, which separates the San Francisco and Salt River drainage basins to the south from the Little Colorado River drainage basin to the north.[4]

The Gila Group has yielded a diversity of fossils at Pearson Mesa (32.643°N 108.968°W / 32.643; -108.968). These include four species of turtle, a heron, and fifteen mammal species. The latter includes the armadillo-like Glyptotherium arizonae, which is found in the southwestern United States only in beds of early Irvingtonian age.[5]

The unit was first named as the Gila Conglomerate by G.K. Gilbert in 1875 from his participation in the Wheeler Survey. Only a type area was defined.[1] By 1903 F.L. Ransome had mapped the unit into the Globe area.[6] P.A. Wood divided the unit into the Bonita beds, Solomonsville beds, and Frye Mesa beds in 1960.[7] In 1963, L.A. Heindl raised the unit to group rank and divided it into the San Manuel, Quiburis, and Sacaton Formations,[8] but M.H. Krieger and coinvestigators removed the San Manuel and Quiburis Formations from the Gila and abandoned the Sacaton Formation, restoring the Gila to formation rank.[9] J.C. Ratte and coinvestigators redesignated the unit as the Gila Formation based on its diverse lithology.[2]

Cather and his coinvestigators redefined the extent of the Gila Group in 1994 as part of a sweeping revision of stratigraphy in western New Mexico and eastern Arizona. They retain it at group rank.[4]

• Cather, Steven M.; Chamberlin, R.M.; Ratte, J.C. (1994). "Tertiary stratigraphy and nomenclature for western New Mexico and eastern Arizona" (PDF). New Mexico Geological Society Field Conference Series. 45: 259–266. Retrieved 25 August 2020.
• Gilbert, G.K. (1875). "Report upon the geology of portions of Nevada, Utah, California, and Arizona, examined in the years 1871 and 1872". Publication of the Wheeler Survey. 3 (1): 17–187.
• Heindl, L.A. (1963). "Cenozoic geology in the Mammoth area, Pinal County, Arizona". U.S. Geological Survey Bulletin. 1141-E: E1–E41. doi:10.3133/b1141E.
• Krieger, M.H.; Cornwall, H.R.; Banks, N.G. (1974). "Big Dome Formation and revised Tertiary stratigraphy in the Ray-San Manuel area, Arizona". U.S. Geological Survey Bulletin. 1394-A: A54–A62. doi:10.3133/b1394A.
• Morgan, Gary S.; Lucas, Spencer G. (2000). "Biostratigraphic significance of late Neogene vertebrate fossils from the Gila Group, Pearson Mesa, southwestern New Mexico and southeastern Arizona" (PDF). New Mexico Geological Society Field Conference Series. 51: 211–200. Retrieved 30 June 2021.
• Ransome, F.L. (1903). "Geology of the Globe copper district, Arizona". U.S. Geological Survey Professional Paper. Professional Paper. 12. doi:10.3133/pp12. hdl:2346/61710. Retrieved 30 August 2020.
• Ratte, J.C. (1989). "Geologic map of the Bull Basin quadrangle, Catron County, New Mexico". U.S. Geological Survey Geologic Quadrangle Map. GQ-1651. Retrieved 31 July 2020.
• Ratte, J.C.; Bove, D.J.; McIntosh, W.C. (1994). "Geologic map of the Milligan Mountain quadrangle, Catron County, New Mexico". U.S. Geological Survey Geologic Quadrangle Map. GQ-1736. Retrieved 31 July 2020.
• Wood, P.A. (1960). "[unknown title]". Arizona Geological Society Guidebook (2).