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Geochronology of Laramide synorogenic strata in the Denver Basin, Colorado

U.S. Geological Survey, MS 963, Box 25046, Denver Federal Center, Denver, CO 80225, U.S.A.

Only a few radiometric ages are available to help constrain the depositional history of Laramide synorogenic strata of the Denver Basin. This paper provides an evaluation of previous radiometric work in the basin and presents analyses of six recently sampled tuffs and tonsteins. Although tuffs and tonsteins are known to exist in Upper Cretaceous parts of the strata, ages for these are either yet to be determined or are unreliable. The few reliable ages, based on ⁴⁰Ar–³⁹Ar, conventional K-Ar, and fission-track dating, fall into two groups: early Paleocene and early Eocene. A pronounced hiatus, related either to erosion or nondeposition, can be shown to exist between 63–64 Ma and 54 Ma. Some anomalous results will require additional fieldwork and analysis to resolve.

Key Words: Denver Basin stratigraphy • Paleocene • Eocene • ⁴⁰Ar–³⁹Ar dating • geochronology • tuffs • tonsteins

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