- UW Department of Geology and Geophysics
A diverse Oligocene insect and plant Lagerstätte has been identified recently from paper shales in a sequence of fine-grained deposits exposed on the west side of Canyon Ferry Reservoir, southeast of Helena, Montana. Limited excavation and preparation thus far has led to the discovery of at least 37 families of insects and more than 40 taxa of plants (leaves, seeds, and stems). Preservation of the insect fossils is outstanding, with color patterns, feeding and reproductive structures commonly present. Quality of the preservation compares favorably with that of other Tertiary insect and plant Lagerstätten, such as Green River, Florissant, and Republic. The geologic affinities of the site are not certain, although the deposit has been regarded broadly as Oligocene, based on the mammalian biostratigraphy and an ash-flow tuff stratigraphically well below the insect horizon that has a U-Pb radiometric date of 32.0 ± 0.1 Ma.