QUATERNARY DEFORMATION OF THE PORTLAND HILLS, OREGON – CONSTRAINTS FROM THE LIGHT RAIL TUNNEL

A geologic cross section of the Portland Hills, Oregon has been digitally compiled by the USGS from observations and mapping done by the late Ken Walsh and his thesis advisor, the late Dr. Marvin Beeson, during 1993-98 construction of a 4.8 km-long light rail tunnel through the Hills. Included in the compilation is additional structural, geochronologic, geochemical, gravity, and video information gathered by the USGS in the tunnel. The cross section is based on the geologic mapping of both eastbound and westbound tunnel bores along its entire length and the integration of that data with analysis of core material and boring logs from 54 deep, vertical borings along the tunnel route (roughly N80°E). These observations provide important constraints on the style and rate of Quaternary folding and faulting in the Portland Hills and associated fault zones.

The cross section shows the Portland Hills to be a broad anticlinorium of Columbia River Basalt Group (CRBG; 15.5 Ma) overlain by fluvial sediments, Pleistocene Boring Basalt, and a thick mantle of Pleistocene loess. Boring basalt flows from Cornell Mt. (1.2 Ma) , Sylvan Hill (1.1 Ma), and Elk Point (0.12 Ma) are exposed in the western third of the tunnel. The Boring lavas interfinger with fine grained fluvial sediments and overlie Sentinel Bluffs and Winter Water members of the Grande Ronde Basalt (CRBG), which dip 25° W towards the Tualatin basin above the W-dipping Sylvan thrust in the CRBG. The Boring flows above the thrust also dip west about 8° and are cut by two vertical strands of the northwest-trending Sylvan Creek-Oatfield fault, the easternmost of which places a thrust sliver of 15.5 Ma Grande Ronde Basalt on top of a 1.2 Ma Boring basalt flow. Minimum post-CRBG slip on the Sylvan thrust is 500 m, and post-1.2 Ma vertical component of slip on the Sylvan Creek-Oatfield fault are 75-100 m and 7-20 m on the eastern and western strands, respectively. Shearing has been mapped in the 120 ka Elk Point flow, but offset has not been determined. We interpret the west-dipping Sylvan fault as a roof thrust and the near vertical faults as splays off an inferred, deeper, east-dipping floor thrust. Horizontal slickensides mapped on the Sylvan Creek-Oatfield fault suggest a component of strike slip deformation which has not been quantified.