SEDIMENTATION, STRUCTURE AND TECTONICS OF THE UMPQUA GROUP (PALEOCENE TO EARLY EOCENE), SOUTHWESTERN OREGON

Abstract

A major change in sedimentary and structural style occurs in Eocene strata exposed along the southern margin of the Oregon Coast Range. Lithofacies of the early Tertiary Umpqua Group have been described, mapped and assigned to likely depositional environments. Submarine fan and slope facies (upper Roseburg Formation) overlie Paleocene basaltic basement rocks to the north, whereas fluvial, deltaic and shallow marine facies (Lookingglass Formation) overlie Franciscan-equivalent strata to the south along the flank of the Klamath Mountains. These two depositional systems are gradational into one another, and were prograding northwestward until about 52 Ma. Means of clast compositions from sandstones and conglomerates from both the Roseburg and Lookingglass Formations suggest derivation from identical recycled orogen or arc-continent collision sources in the Klamath Mountains. Change from Klamath-parallel to more north-south structural trends is well displayed within early Eocene strata of the Umpqua Group. Five major fault systems involve lower Umpqua (Roseburg and Lookingglass) strata, and were active while deposition was taking place. All these faults ceased to be active at about 52-50 Ma, and are overlapped by the middle Eocene Tyee Formation. Regional strain analysis indicates more than 20 percent shortening by right-lateral convergence during early Eocene time. The structural style and syn-tectonic deformation of marine slope facies suggest deposition in an active subduction complex until about 52 Ma. Structural trends in the southern Oregon Coast Range parallel those in the adjacent Klamath Mountains until the end of the early Eocene. At 52-50 Ma, subduction apparently ceased as incoming seamounts clogged the trench, and may have jumped to an outboard position near the present day coastline. In middle Eocene time, the newly developed forearc region rapidly filled with sediments from a much sandier depositional system. Paleomagnetic studies of relatively undeformed Tyee forearc strata indicate as much clockwise rotation as the much more deformed, underlying volcanic basement of the Oregon Coast Range. Rotation of the Oregon Coast Range as a single crustal block must have occurred after, rather than during seamount accretion to the continental margin, which was essentially complete by 52 Ma.