TRACE FOSSILS IN THIN-BEDDED VOLCANICLASTIC SILTSTONES AND SANDSTONES OF THE OLIGOCENE TO MIOCENE ROXY FORMATION, JACKSON COUNTY, OREGON

The Oligocene to Miocene Roxy Formation is approximately 1100 m thick composed of basalt and andesite lava flows, volcanic and mudflow breccias, volcaniclastic mudstones, siltstones, sandstones, and conglomerates, and subordinate amounts of vitric and crystal tuff. These sediments are consistent with deposition in a continental volcanic setting associated with the Western Cascades. Thirty samples containing trace fossils were collected from a 1.5 m interval of thin- to medium-bedded, planar stratified, volcaniclastic siltstones and very fine-grained sandstones. This newly discovered trace fossil locality yielded specimens of Spirodesmos, Plangtichnus, and Treptichnus ichnogenera and are most likely associated with the Mermia ichnofacies. Spirodesmos are 1 mm-diameter traces that exhibit a spiral-shaped form with a radius of 2.5 cm on the bedding surface of laminated siltstones. Spirodesmos at this locality are found in association with Treptichnus and Plangtichnus in thin-bedded siltstones. Plangtichnus consist of 3 to 9 cm length, 1 mm-diameter traces, with convex and concave epirelief on the bedding surfaces of laminated siltstones and very fine-grained sandstones. Treptichnus are 1 mm-diameter traces with a zigzag pattern on the bedding surfaces of laminated siltstones similar to Plangtichnus. Treptichnus are differentiated from Plangtichnus by the presence of short, punctuated marks on the bedding surfaces. These trace fossils have been previously reported from non-marine siltstones of the Pennsylvanian Mansfield Formation in southern Indiana. This is the first reported occurrence of these ichnogenera from a volcaniclastic, continental setting.