STRATIGRAPHIC, SEDIMENTOLOGIC, AND MINERALOGIC CHARACTERIZATION OF THE GOODENOUGH FORMATION (MIOCENE?), GRAND MESA, CO

The informal Goodenough formation (previously called the "unnamed unit" on USGS maps) consists of mudrock, sandstone, conglomerate, and limestone that crops out on Grand Mesa, CO, at elevations ranging from 2,900 to 3,380 m. The total areal extent is estimated at between 775 and 875 km². The age of the Goodenough is poorly constrained. It unconformably overlies the Eocene Uinta and Green River Formations (47-48 Ma) and is overlain by mafic lava flows that range in age from 9.45 to 10.99 Ma. It may contain silicic ash beds of probable Miocene age. Goodenough deposits are broadly similar to strata in the Glenwood Springs-Carbondale region that are mapped as Miocene and Oligocene basin fill. All of these units pre-date integration of the upper Colorado River system. Because Goodenough deposits are the westernmost preserved fill unit, its age constrains when sediment from the Southern Rocky Mountain region began to arrive on the Colorado Plateau via the ancestral Colorado River system.

The sedimentologic and stratigraphic characteristics of the Goodenough formation are poorly understood because it has been disrupted by glaciation and large-scale mass wasting. It ranges in thickness from 15 to 274 m and thins from southeast to northwest. In central and eastern Grand Mesa, it can be subdivided into two intervals. The lower interval, which is dominated by variegated, mottled, bentonitic siltstone and claystone and cherty limestone (wackestone-packstone), was deposited in lacustrine and flood-plain settings. The upper interval consists of fine to very coarse grained, poorly sorted, lithic to arkosic sandstone, granule-pebble-cobble, polylithic conglomerate, and variegated mudrock, and was deposited by fluvial processes. Conglomerate clasts are mainly diorite and andesite, but also include granite, meta-granite, gneiss, schist, quartzite, and marble. Sandstone framework grains are dominated by diorite, andesite, quartz, plagioclase, and K-feldspar. These observations suggest that the source areas for the fluvial facies were mid-Tertiary intrusives in the West Elk and Elk Mountains, plus arkosic strata of the Pennsylvanian-Permian Minturn and Maroon Formations from the western flanks of the Sawatch and Gore Ranges.