TECTONIC CHARACTERIZATION OF A 150 KM TRANSECT OF THE COLUMBIA RIVER, WESTERN OREGON AND WASHINGTON, AS CHARACTERIZED BY A SEISMIC REFLECTION, AEROMAGNETIC, AND GEOLOGIC MAP DATA

We integrated 150 km of seismic-reflection data with combined high- and low- resolution aeromagnetic surveys and geologic maps to characterize the tectonic framework along the Columbia River from 30 km offshore to St. Helens, Oregon. We identify several potentially active northwest- to northeast-trending faults that cross and perhaps control the Columbia River channel. Each geologic and geophysical technique offers a differing insight into the fault history. We reprocessed 24-channel industry seismic-reflection data with an emphasis on the upper 2 km to identify and characterize the vertical offsets on these faults. We used the aeromagnetic data to further constrain the regional extent of these faults and also to characterize deeper structures that were not imaged by the seismic data. Geologic map data provided the ground-truth and age control necessary to identify hazards associated with these structures.

Miocene and Eocene volcanic rocks define the largest seismic boundaries along the regional transect. Offshore, the volcanic rocks define the acoustic basement with the seismic data recording the character of the overlying sedimentary basins. The offshore basins contain large, regionally folded reflections with unconformities and laterally terminating reflection packages. The unconformities likely correspond to changing sea-level conditions, while the laterally terminating reflections correspond with regionally significant magnetic anomalies that may signify Neogene or younger faulting. Integration with nearby borehole information may help identify the temporal characteristics of the mapped offshore faults. Onshore, we observe offsets in the seismic reflection data that correspond with mapped oblique-slip faults and large offset magnetic lineaments cored by Eocene volcanic rocks in the vicinity of Astoria, Oregon and also further east. Numerous smaller offset faults also truncate seismic reflections. Geologic maps help place age control on the seismic and magnetic boundaries. Further integration of each data type will allow us to better assess the risks associated with the potentially active fault structures in the region.