LATE EOCENE TO MIOCENE LANDSCAPE EVOLUTION OF WESTERN COLORADO: NEW AGE CONSTRAINTS USING DETRITAL SANIDINE 40AR/39AR GEOCHRONOLOGY

Detrital sanidine (DS) ⁴⁰Ar/³⁹Ar geochronology provides a preliminary temporal framework for interpreting the post-Laramide sedimentary history of western Colorado. DS dates constrain the age of the Telluride Conglomerate and point to episodic Oligocene-Miocene aggradation, perhaps associated with internally drained basins.

Telluride Conglomerate outcrops along the northwestern edge of the San Juan Mountains consist of ~15 m of alternating fluvial sandstone and sandy conglomerate with clasts dominated by Precambrian granite and Cretaceous dacite. Paleocurrents show westward flow directions that are interpreted as an expression of incipient uplift associated with Sawatch-Trend calderas. A DS maximum depositional age (MDA) of 34.58 ±0.02 Ma based on the two youngest sanidine grains from an uppermost Telluride Conglomerate sample suggests that this ancient river system is latest Eocene in age. Unconsolidated Neogene(?) sediments that directly overlie the Telluride Conglomerate DS sample have numerous DS grains that range in age from 34-32 Ma, which are absent from the underlying Telluride Conglomerate. This observation supports a latest Eocene age for the Telluride.

East of Grand Mesa, ~300 m of interbedded mudstone and sandstone with minor carbonate, referred to as the Goodenough Formation, are overlain by Late Miocene basalt and overlie the Eocene Green River Formation. The youngest single sanidine date from a reworked ash near the top of the Goodenough is 33.0 ±0.04 Ma, and the next youngest age population (n=8) ranges from 34.6-34.9 Ma. The dates suggest that the Goodenough depocenter was contemporaneous with the Telluride river system, but may have been separated from the rivers by highlands associated with the Gunnison Uplift.

At Grand Mesa proper, DS dating of uppermost Goodenough deposits yielded a MDA of 13.29 ±0.06 Ma based on the four youngest sanidine grains, and the youngest sanidine grain from a second sample from the middle of the Goodenough sequence is 25.09 ±0.50 Ma. The difference in Goodenough DS dates between the two areas suggests that there are complicated stratigraphic relationships that have yet to be resolved. This unit could reflect episodic aggradation and incision, perhaps related to the erosion of the Oligocene West Elk-San Juan volcaniclastic blanket.