PALEOECOLOGY AND TAPHONOMY OF THE GRASSY MOUNTAIN FORMATION

The Grassy Mountain Formation (hereafter: GMF) of southeastern Oregon includes middle-Miocene volcaniclastic and tuffaceous sediments, deposited in an intermontane basin circa 15 million years before the present. For over a century, permineralized remains of stumps, logs, and branches have been recovered from several localities but to date no scientific assessment of the taxonomy and paleoecology of this specific flora has been attempted.

During summer 2017, we began a long-term study of the GMF paleoflora. Outcrops were searched and specimens were brought back to the Orma J. Smith Museum, College of Idaho, for examination. Specimens were prepared and examined under a digital microscope. On well-preserved specimens tree-ring data were recorded including quantity and size of both the early and late-wood. Fossil GMF wood specimens on display at the Orma J. Smith Museum were also analyzed in addition to field collections. Microstructure of conductive tissue (xylotomy) permitted assignment of fossils to genus level; albeit with several problematic forms. Over twenty tree taxa were identified in the GMF paleoflora, including common genera such as Quercus and Betula and more exotic genera like Taxus and Magnolia. This flora and its record of growth annuli indicates a wetter and more subtropical ecology than that of today but marked by distinct seasonal changes.