A late Eocene erosion surface of low relief, which extended throughout south-central Colorado, provides a post-Laramide, pre-Oligocene, regional structural datum. The age and geomorphic character of the surface are documented for an area of more than 10,400 km 2 in the southern Front Range, Rampart Range, South Park, Thirtynine Mile volcanic field, southern Mosquito Range, upper Arkansas River valley, southern Sawatch Range, and adjacent Great Plains to the east. The surface truncated middle Eocene and older rocks, which were deformed during the Laramide orogeny, and deeply beveled crystalline Precambrian rocks across wide areas. Size, shape, and distribution of the overlying Wall Mountain Tuff and associated gravel units show that the surface sloped gently southward and eastward and merged with the western Great Plains.Correlation of deposits on the surface indicates that it was uplifted 1,500 to 3,000 m and disrupted by block faulting of basin-and-range style in Miocene and later time. Many erosion surfaces now at various levels in the mountainous terrain of the area are faulted segments of this late Eocene surface. 45 on June 29, 2015 memoirs.gsapubs.org Downloaded from
EPIS AND CHAPÍN
It is suggested that erosion surfaces of similar geomorphic character and age developed over a much larger region of the Southern Rocky Mountains province and of the adjoining Basin and Range province to the south and southwest.
INTRODUCTIONEarlier studies of the Thirtynine Mile volcanic field of central Colorado showed that the volcanic pile was underlain by an erosion surface of relatively low relief (Cross, 1894; Chapin and Epis, 1964; Epis and Chapin, 1968; Steven and Epis, 1968). Additional field, petrographic, and radiometrie-age data have provided a better understanding of the stratigraphic sequence of units in the volcanic field and of their regional correlation in outlying areas (Epis and Chapin, 1974). It is now possible to document the age and geomorphic character of the prevolcanic surface across an area of more than 10,000 km 2 and to show that the surface was extensively uplifted and block faulted in Miocene and later time.