SANDSTONE AND GRAVEL PROVENANCE IN THE FERRIS FORMATION (UPPER CRETACEOUS/PALEOGENE, HANNA BASIN, WYOMING)

The Ferris Formation comprises hundreds of sandstone and conglomerate fluvial channel bodies deposited in the Hanna Basin (south-central Wyoming) during the early Laramide orogeny. Channel bodies within the unit show distinct spatial clustering that may reflect depositional self-organization over long time scales. In the northern Hanna Basin the Ferris Formation dips 80° exposing a cross section of Cretaceous/Paleogene basin fill across the present day land surface, providing an opportunity to evaluate spatial and temporal changes in sand and gravel provenance and look for evidence of tectonic controls on stratigraphic architecture.

Gravel composition was measured at thirty-four locations throughout the study area, where composition, size, and texture were recorded for ≥100 clasts. Additionally sandstone petrography was conducted on seven medium-grained sandstone samples from across the study area. Differences between gravel counts from the same sand body were used to establish a background level of variability for comparing gravel composition between sand bodies. Clast size contributes to compositional variation between samples and was considered when comparing sand bodies with different grain-size distributions.

Gravel composition in the Ferris Formation reflects locally available source material and is dominated by chert pebbles reworked from the Cloverly Formation (Upper Jurassic) and Archean granite clasts. Low in the Ferris Formation gravels are dominated by chert clasts until the first appearance of granite ~600 m into the section, at which point granite and chert comprise 80-90% of gravel clasts. This unroofing of crystalline bedrock is also evidenced in sandstone petrography by a steady increase in the percentage of potassium feldspar present in the sand bodies. Sediment composition was not found to vary spatially within the study area. This suggests that tectonic control of sediment supply was not the primary cause of channel-body clustering in the Ferris Formation.