Geomorphic and Tectonic Implications of the Post-Laramide, Late Eocene Erosion Surface in the Southern Rocky Mountains

Abstract

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.

An Exhumed Late Paleozoic Canyon in the Rocky Mountains

“…Young 1983). The Eocene erosion surface of the modern Rockies is an example of an ancient form in a modern orogenic belt (Epis and Chapin 1975) but one that is undergoing active dissection and thus may not persist. Ancestral Unaweep Canyon is an exhumed form and hence was protected from destruction until only recently; however, its survival is odd because unlike other examples of ancient landforms, it occurs in a region characterized today-but more notably during the late Paleozoic-by profound orogenesis and significant relief far from a cratonal setting.…”

Geologic history and hydrocarbon potential of Late Cretaceous-age, low-permeability reservoirs, Piceance Basin, western Colorado

“…Mountains (Epis and Chapin, 1975). Remnants of this surface are still preserved beneath 9.7-million-year-old basalts in the basin at a present-day elevation of about 10,000 ft (Marvin and others, 1966).…”

Geophysical Constraints on the Flexural Subsidence of the Denver Basin

“…As a result, an extensive erosion surface was formed across the regions currently occupied by mountains and plains alike (Scott, 1975;Epis and Chapin, 1975;Bradley, 1987). This erosion surface is preserved below volcaniclastic sedimentary rocks of Oligocene and early Miocene age (Morse, 1985).…”