MODE OF ROOT TRACE PRESERVATION AND PALEOSOL HYDROLOGY: EXAMPLES FROM THE BIGHORN BASIN, WYOMING

Alluvial paleosols are well documented in the Paleogene Fort Union and Willwood formations of the Bighorn Basin, Wyoming. In the McDermotts Butte area of the Basin, the paleosols exhibit different modes of root trace preservation that provide information on ancient soil moisture. Many of these root traces are preserved as zones of depletion and accumulation of iron and/or manganese in and around the former root. Analyzing the mineralogy of the root traces and their color patterns enhances interpretations of the ancient drainage conditions of the paleosols.

Common in relatively well drained red paleosols are root traces consisting of grey, iron depletion zones with red rims, whose color indicates hematite accumulation. Powdery calcium carbonate is locally present within the grey depletion zones. These features are typical of surface water gley processes that caused iron and manganese to move from the root channel outward to the soil matrix and carbonate precipitation in the channel as the soil dried. More poorly drained purple paleosols have root traces consisting of iron depletion zones surrounded by yellow-brown (goethite) rims that also indicate surface water gley processes. In many paleosols, the red and yellow-brown accumulation rims were partly removed by continued depletion, producing abundant irregular-shaped mottles.

The somewhat poorly drained paleosols also show root traces preserved as goethite (yellow-brown) accumulations. The goethite accumulations are contained within grey depletion zones, indicating movement of iron from the matrix towards the root channel probably as a result of groundwater gley processes. The goethite accumulations are generally intermingled with lignite, representing preservation of the root organic material. Some root traces are preserved as tube-shaped concentrations of small (0.1-0.2 mm diameter) black spheres whose color indicates a mix of manganese and iron oxides. These are most common in paleosols with low chroma (grey) matrix colors, indicating poor drainage. Paleosols that are even more poorly drained show root traces preserved in jarosite, which is an oxidation product of pyrite. In addition, root traces in the more poorly drained (low chroma) paleosols are sparse and do not penetrate deeply.