Abstract

The Tununk and Blue Gate Members of the Cretaceous Mancos Shale are shallow marine deposits that are separated by the Ferron Sandstone Member, a fluvial-deltaic sequence. Despite the somewhat similar depositional environments, there are subtle differences in mineralogical and chemical composition that may contribute to noticeable differences in the processes, rates, and types of erosion between the two shale members. A possibly important chemical difference is that the Blue Gate has about four times as much organic carbon as the Tununk. Observed erosional differences include (1) development of badland topography and presence of remnant paleo-pediments on the Blue Gate, but their absence on the Tununk, (2) wind erosion of the Tununk, (3) fluvial erosion of the Tununk at twice the erosion rates of the Blue Gate, (4) deeper and more closely spaced rills on the Tununk slopes, and (5) hard and flat soil surfaces on modern Blue Gate pediments compared to “fluffy” and hummocky surfaces on Tununk pediments. Some of these observed differences (such as the presence of remnant paleo-pediments) could be the result of the greater thickness of the Blue Gate, whereas others (such as response to wind erosion, texture of the pediment soil surfaces, and rates of fluvial erosion) may be the result of differences in the physical and chemical properties of the shales.