Seismic Evidence Supporting a New Interpretation of the Cody Terrace Near Cody, Wyoming

Abstract

Seismic studies of the Cody terrace complex bordering the Shoshone River near Cody, Wyoming, disclose: (1) no relationship between surficial terrace form and underlying bedrock topography; (2) irregular bedrock topography with relief up to 50 feet; (3) thickness of gravel and silt overlying bedrock averaging about 60 feet and reaching a maximum of more than 100 feet. These data suggest that the Cody terrace is an alluvial terrace, not a rock-cut terrace as previously postulated. Although interrupted by the Shoshone canyon, the gravel in the Cody terrace is believed to be the correlative of the gravel in the lower terrace complex in the South Fork valley which is traceable into valley-train deposits fronting the moraine at Ishawooa. The Cody gravel is therefore in part glaciofluvial. The Cody terrace was probably formed in three stages: (1) cutting of the broad bedrock floor beneath the gravel during a period of lateral cutting and slow downcutting from the Powell surface; (2) deposition of a gravel fill to the height of the highest Cody bench during a period of aggradation associated with advance of glaciers in the upper Shoshone Valley; (3) cutting of the benches of the Cody terrace in the fill by lateral planation and slow downcutting during and after retreat of the ice from the Shoshone drainage area. These results underscore the usefulness of seismic studies in geomorphic work.