Named for Las Animas Co, CO in Southern Rocky Mountain region. Occurs in subsurface in a belt 130 km (80 mi) long and as much as 50 km (30 mi) wide. Type well for main part is the R. W. Lange No. 1 Government test in SW1/4 NW1/4 sec 10, T29S, R62W. Reference well for upper part is Phillips Petroleum Co. No. 1 E. W. Haskins NE1/4 NE1/4 sec 23, T29S, R56W. Consists principally of dark-gray to black slate, phyllite, fine-grained, dark-gray graywacke, and chert. Carbonate and volcanic rocks present in upper part. Exceeds 1,700 m (5,575 ft) in thickness. Exact thickness not known. Thought to overlie Precambrian granite of 1,300-1,400 m.y. age. Is overlain by Permian rocks assigned to Fountain or Sangre de Cristo Formation at type. Author prefers Sangre de Cristo. Is overlain by Upper Cambrian rocks at reference well variously assigned to Sawatch Quartzite, Peerless Formation, Lamotte Sandstone, Bonneterre Formation, lower part Arbuckle Group, or Reagan Sandstone. Selection of term Reagan preferred by author. Classified as Proterozoic Y age. Closely resembles Tillman Metasedimentary Group of Wichita Mountains province, southern OK. Was deposited in a deep, west-northwest trending trough of rift origin.

Source: GNU records (USGS DDS-6; Denver GNULEX).

Lies on granite of Berthoud Plutonic Suite and on older gneisses in the subsurface. Is therefore younger than 1,400 m.y. Thought to be a filling in a trough of rift origin in a terrane of 1,400 m.y. mesozonal granites. Has a similar trend to Tillman Metasedimentary Group with which it is correlated. Thought to be at least 1,200 m.y. and less than 1,400 m.y. Age assignment changed from late Precambrian to Middle Proterozoic. Is in Southern Rocky Mountain region.

Source: GNU records (USGS DDS-6; Denver GNULEX).