PALEOTOPOGRAPHIC AND VOLCANO-TECTONIC EVOLUTION OF THE UPPER CLARNO FORMATION IN THE POGUE POINT 7.5' QUADRANGLE, NE OREGON: INSIGHTS FROM THE MIDDLE MIOCENE DINNER CREEK TUFF

The distribution and geometry of the middle Miocene Dinner Creek Tuff in the Pogue Point 7.5’ quadrangle, provides insight into the nature of the Eocene to Miocene volcano-tectonic and paleotopographic evolution of NE Oregon. The tuff mantles a > 1 km thick, fluvially incised succession of interstratified, basaltic andesite, andesite, and dacite lavas and volcanogenic sedimentary deposits equivalent to the Eocene Clarno Formation. The volcanogenic sedimentary strata that locally dominate the succession consist of typically massive, thick- to very thick-bedded, matrix- and clast-supported pebble-boulder conglomerate and breccia generated by sediment gravity flow processes. Intercalated within these deposits are medium- to thick-bedded, horizontal- and cross-stratified gravelly sandstone generated by tractional processes. The dominance of basaltic andesite and andesite gravel clasts in the upper two-thirds of the succession and dominance of dacite clasts in the lower third are evidence for derivation from progressively less silicic source rocks. The Dinner Creek Tuff in the Pogue Point area is an erosionally resistant, devitrified, welded, rhyolitic ash flow tuff that locally varies in thickness between 0 and 40 m. This tuff originated ~ 30 km south of the quadrangle (near Castle Rock and Iron Mountain) between ca. 15.9 and 15.4 Ma. The identity of this tuff has been confirmed with the use of XRF major and trace element analyses. Geologic mapping and field relations reveal that an angular unconformity exists between the Dinner Creek Tuff and the older Clarno Formation, implying an episode of faulting and folding that either accompanied or followed Clarno deposition. Field relations also indicate that the Dinner Creek Tuff was faulted following its emplacement. Subsequent erosion of the Dinner Creek and underlying Clarno strata led to the development of a rolling and at least locally incised topography that helped direct and concentrate the later Strawberry Volcanics lava flows. Even though the Dinner Creek Tuff is not preserved everywhere in the Pogue Point 7.5’ quadrangle, the unit nevertheless provides a very useful marker bed that helps separate the Clarno Formation from the overlying Strawberry Volcanics.