GEOLOGIC CROSS SECTIONS THROUGH THE ROSEBURG 30’ X 60’ QUADRANGLE, OREGON: NEW CONSTRAINTS FROM POTENTIAL FIELD MODELING

Geologic and potential-field data were examined to address fold and thrust structures in the Roseburg 30’ x 60’ quadrangle. Suturing of the Paleocene-Eocene oceanic Siletz terrane to the continent during early Eocene produced a fold and thrust belt in which the Jurassic and lower Cretaceous Dothan accretionary complex and Rogue volcanic arc were thrust NW over the Coast Range basalt basement. To elucidate the subsurface structure of the fold and thrust belt and the nature of the terrane boundary at depth, we constructed three geologic cross sections constrained by new geologic mapping, potential-field data, and deep exploratory wells. Restorable geologic cross sections were used as starting points for simultaneous modeling of aeromagnetic and gravity profiles in which structurally and genetically distinguishable units were characterized by their density, remanent magnetization, and magnetic susceptibility.

Restoration of our cross sections indicates that the Siletz terrane and Umpqua Group have been shortened 26 km, or 37 percent. The detachment below the fold and thrust belt deepens SE, from 1.8 km beneath the Umpqua basin to about 3.5 km near the suture. Geophysical profiles derived from the geologic models are in good agreement with observed gravity and magnetic anomalies over the Mesozoic terranes and the broad Umpqua arch of the Coast Range basement. On the Umpqua arch, we infer reversely magnetized subaerial basalt flows overlying an intrusive complex in order to satisfy the gravity high, aeromagnetic low, and depth to the Coast Range basalt basement based on well data. Near the suture, geophysical modeling suggests sediments (Paleocene-Eocene strata?) are thicker in the lower plate than previously recognized. The terrane boundary dips southeast 30° at depth (> 900 m) and steepens to 70° at the surface. The considerable thickness and depth of the Umpqua Group clastic sedimentary rocks and the nature of the complex syn-depositional structures found throughout them may have significant implications for hydrocarbon exploration.