Ochre in Sedimentary Rock: Sources in the Central Great Plains

Abstract Red ochre may be found in igneous, metamorphic, or sedimentary rock, but igneous and metamorphic sources formed in localized geological events are easier to define. In sedimentary landscapes, ochre sources can be thought of as the geologic formations from which ochre is collected. This study provides the first description of red or red-firing ochre sources in the sedimentary Central Great Plains, based on 17 geologic ochre samples from five contexts: Cretaceous Pierre Shale; Cretaceous Niobrara Formation, Smoky Hill Chalk member; Cretaceous Carlile Shale; Cretaceous Dakota Formation; and Permian system siltstone and shale. Ochre analysis with powder X-ray diffraction reveals mineralogical differences—particularly differences in iron and sulfate minerals—between two defined ochre sources. Source 1 is the Cretaceous Dakota formation, with exposures on the eastern side of the study area. Source 2 includes younger strata exposed to the west: the Cretaceous Carlile Shale, Niobrara, and Pierre Formations. Source 2 ochre is yellow but becomes red at 250°C–500°C. Samples from a third potential source, Permian siltstones and shales (“red beds”; Tucker 2001:60), lacked identifiable iron oxides or hydroxides in this analysis and may not have been used as ochre.