193 Advances and Applications in Geochronology for Interpreting Stratigraphic and Basin Records (Posters)

This session welcomes submissions related to applications of geochronology (e.g., U-Pb, 40Ar/39Ar) in stratigraphic research, including topics such as eruption and maximum depositional age estimation, provenance analysis, numerical modeling of geochronological data, and multi-disciplinary chronostratigraphic characterization.

Tuesday, 21 October 2025
8:00 AM - 5:30 PM
HBGCC, Hall 1

Cenozoic Landscape Evolution of Western Colorado: insights from the Late Eocene to Late Miocene(?) Goodenough unit

Paper number: 193-11
Program: Technical Sessions
Time: 8:00 AM – 8:00 AM

Authors

Abstract

Recently examined fluvial-lacustrine deposits of the Goodenough unit underlie Late Miocene basalt flows in western Colorado, overlie the Eocene Green River Formation, and pre-date the development of the ancestral Colorado River system.  These poorly studied deposits are up to 300 m thick but are often associated with large landslide blocks, which complicates their study. 

The Goodenough unit is differentiated from the underlying Eocene Green River Formation by its generally high proportion of sandstone, the presence of red mudstone, and locally, by the presence of volcanic clasts in the sandstones.  Typical lithofacies include: 1) volcanic pebbly sandstone, 2) arkosic sandstone, 3) smectitic red and gray mudrock, and 4) micritic carbonate beds.  Rare stromatolites are also present.  Volcanic clasts indicate that Goodenough sediments record erosion of nearby volcanic uplands of the West Elk and Elk Mountains and perhaps the Sawatch Range.  Arkosic sandstones suggest that Precambrian rocks or Paleozoic arkosic deposits associated with nearby Laramide uplifts such as the Gunnison uplift and the Sawatch Range were at least partially exhumed at the time of Goodenough sedimentation.  Mudstones and carbonate beds suggest that Goodenough sediments accumulated within a closed basin setting within the Piceance Basin following the Laramide orogeny.

Detrital-mineral dating including both 40Ar/39Ar detrital-sanidine and U-Pb detrital-zircon analyses indicates that the Goodenough unit is <42 Ma and, at least a portion of the strata is 34-35 Ma. The youngest Goodenough deposits have a maximum depositional age of ca. 13 Ma, and they may represent a separate younger stratigraphic unit that overlies the main body of the Goodenough unit. 

In summary, the Goodenough unit is a fluvial-lacustrine deposit that shares broad similarities with older Laramide basin-fill units in the Piceance Basin, but it post-dates the Laramide orogeny.  Instead, the Goodenough sediments probably infilled pre-existing post-Laramide paleotopography perhaps associated with the Rocky Mountain Erosion Surface.  The Goodenough unit marks the final episode of sedimentary aggradation in the Piceance Basin prior to the development of the ancestral Colorado River ca. 11 Ma. 

Geological Society of America Abstracts with Programs. Vol. 57, No. 6

doi: 10.1130/abs/2025AM-412905

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