MINERALOGIC EVIDENCE OF PRESERVATIONAL CONDITIONS IN THE OLIGOCENE CANYON FERRY INSECT LAGERSTATTE, SOUTHWEST, MONTANA

The Oligocene Canyon Ferry insect lagerstatte, exposed in outcrops of clayshale in the Cenozoic Bozeman Group along the shores of Canyon Ferry Reservoir in southwest Montana, has been tentatively inferred from taxonomic associations to record deposition of fossil-bearing horizons in a lacustrine setting characterized by significant volcaniclastic sediment input. In order to begin assessing this interpretation, we utilized x-ray diffraction (XRD) and scanning electron microscope (SEM) methods to determine the bulk mineralogical content as well as the elemental composition of the clayshales and fossils preserved within them. Our XRD results revealed that the clayshales consist primarily of quartz and phyllosilicate (primarily clay) minerals. This suggests that deposition in this environment was dominated by terrigenous siliciclastic input. Our SEM results revealed an abundance of diatoms with occasional volcanic glass shards comprising the fossiliferous laminae. Analysis of the diatoms reveals them to be composed of quartz. The fossil insects possess an elemental composition identical to that of the clayshale sediment matrix and are interpreted as phyllosilicate (clay) mineral films. These results suggest a complete replacement mode for preservation of the insect fossils with a strong controlling influence of diatomaceous biofilms rather than burial in volcanic ash driving insect accumulation, entombment, and subsequent fossilization. Absence of carbonate minerals suggests that the insects accumulated in a ponded water setting dominated by relatively persistent input of suspended terrigenous clastic sediment with biogenic carbonate production lacking. The presence of diatoms also suggests rapid entombment of and/or minimal diagenetic alteration of the lagerstatte fossil assemblage.