GEOMORPHIC, MINERALOGIC, AND CHEMICAL DIFFERENCES BETWEEN THE TUNUNK AND BLUE GATE MEMBERS OF THE CRETACEOUS MANCOS SHALE

In the area between Hanksville and Cainville, UT there are noticeable differences in the processes, rates, and types of erosion between the Tununk and Blue Gate Members of the Cretaceous Mancos Shale. These two shale members are separated by the Ferron Sandstone Member, a fluvial-deltaic sequence that acts as a cap-rock to the Tununk. The Blue Gate is capped by the Cretaceous Muley Canyon Sandstone. Both shale members form classic badland landscapes. Erosional differences include (a) development of long, sharp-crested ridges on the Blue Gate as opposed to short ridges with broad rounded crests on the Tununk, (b) the presence of remnant paleo-pediments on the Blue Gate, but their absence on the Tununk, (c) hillslope erosion of the Tununk at rates twice the erosion rates of the Blue Gate, (d) deeper and more closely spaced rills on the Tununk slopes, and (e) 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 ridge morphology, texture of the pediment soil surfaces, and erosion rates) may be the result of differences in the physical and chemical properties of the shales. Based on a limited number of soil and rock samples there appear to be subtle, possibly significant, differences in mineralogical and chemical composition of the Tununk (11 samples) and Blue Gate (4 samples). Blue Gate has about four times as much organic carbon as the Tununk which, due to the stabilizing effect of organic carbon on soil, may contribute to the morphologic differences between the members. A similar result may be caused by greater dolomite content of the Blue Gate (about 6% versus approximately 3% in the Tununk). The clay mineralogy and content of both members is similar. Both members are sources for S which forms the sodium sulfate salts that commonly form on the badland surfaces. The mean Se content of the Blue Gate (3.1 ppm) is significantly greater than that of the Tununk (0.3 ppm). Further work is required to confirm the suggested mineralogical and chemical differences between the Tununk and Blue and the links between mineralogy, chemistry, and geomorphology.