Detrital Mineral Dating of the Cretaceous/Paleogene(?) Ohio Creek Conglomerate, Western Piceance Basin, Colorado

Abstract

The origin and age of the Ohio Creek Conglomerate of western Colorado is controversial. Prior studies using paleontological data (pollen, vertebrate fossils) have produced contradictory results. Vertebrate fossil data from near Rifle, Colorado support an Early Paleocene age for this unit whereas palynological data from the western Piceance Basin support a Paleocene or a Late Cretaceous age. Regarding the origin of the Ohio Creek Conglomerate, there is debate as to whether the Ohio Creek represents a discrete stratigraphic unit or a zone of weathering formed within Upper Cretaceous strata.

Detrital mineral dating using both U-Pb dating of zircon and ⁴⁰Ar/³⁹Ar geochronology of sanidine were used to provide age estimates for sandstones and pebbly conglomerates of the uppermost Williams Fork Formation, the Ohio Creek Conglomerate, and the basal Wasatch Formation at Mesa, Colorado, and nearby Atwell Gulch. The Williams Fork Formation consists of laterally extensive intervals of fine- to medium-grained cross-bedded sandstone with an uppermost light-gray kaolinite-rich zone. The Ohio Creek is heterolithic and includes tan- to purple-colored, fine- to medium-grained, cross-bedded sandstones with local lenses of granule- and pebble-sized multi-colored chert and quartzite. The base of the Ohio Creek is represented by shale and bedded fine-grained sandstone that sharply overlies the uppermost kaolinitic Williams Fork sandstones. The basal Wasatch Formation is characterized by medium- to coarse-grained, cross-bedded sandstone and pebbly sandstone with a sharp erosional base. This distinctive erosional contact persists laterally throughout the outcrop belt. Clasts are dominated by multi-colored chert and quartzite, although a few rare volcanic clasts are also present.

At Mesa, Colorado, results of the detrital zircon analysis of the Ohio Creek sample produced a youngest single grain age of 63.1 ± 0.7 Ma and the youngest nine grains yielded a weighted mean age of 64.7 +0.15/-0.73 Ma. Zircon data for the overlying Wasatch Formation sample produced a youngest single grain age of 64.2 ± 0.7 Ma and the five youngest grains yielded a weighted mean age of 64.3 ± 0.6 Ma. At nearby Atwell Gulch, results of the detrital zircon analysis of the Ohio Creek sample produced a youngest single grain age of 72.9 ± 8.6 Ma; there were not enough young grains to produce a mean age. Zircon data for two samples of the overlying Wasatch Formation sample produced a youngest single grain age of 62.0 ± 5.0 Ma and 63.3 ± 8.0 Ma. These same samples produced weighted mean ages of 65.3 ± 0.5 Ma and 65.5 ± 1.4 Ma. Results of the detrital sanidine dating are pending.

Despite the imprecision of individual U-Pb zircon dates, the data suggest that the Ohio Creek Conglomerate and overlying Wasatch Formation are both similar in age and straddle the Cretaceous-Paleogene boundary. There is no obvious evidence of a major unconformity separating these two units. Further assessment of this interpretation will require results of the sanidine data, which should yield age constraints with better precision than the zircon analyses.

AAPG Datapages/Search and Discovery Article #90374 © 2020 AAPG Rocky Mountain Section Meeting, 2020 Vision: Turn Hindsight to Foresight, Grand Junction, Colorado, September 13-15, 2020 (CANCELLED)