THE ORTLEY SEGMENT OF THE COLUMBIA HILLS ANTICLINE, WASHINGTON AND OREGON, REVISITED

Recent mapping (2008-09) in combination with observations made by the author during regional mapping by the USGS (1979-1981) has provided a more complete understanding of the (1) surrounding context and (2) specific details of Ortley Anticline (OA) spectacularly exposed by the Columbia River just east of the town of Lyle, Washington. A northwest- trending right-lateral strike slip fault (Rowena fault [RF]) intersects and offsets the Columbia Hills anticline (CHA) near Rowena, OR and controls the position of the Columbia River at Rowena Gap. The pattern of folding and faulting of the CHA changes across the RF with virtually all north-directed fault slip ending there. The very visible tectonic breccia walls seen on the WA side attenuate abruptly to the southwest in OR at or near the RF. The RF also offsets the NE trending out-of-(Mosier)syncline Memaloose thrust fault within the Columbia River Channel. The fault plane of this south-verging thrust is clearly defined where it rises out of the Mosier syncline and crosses through cliffs above the town of Lyle, WA. The OA is a steeply NW verging uplift with two south-dipping faults bounding its leading edge. The westernmost of the faults is exposed at highway level and bounds a steeply rotated footwall (FW) slice. This block contains shattered and overturned Sentinel Bluffs (SB) member at highway level, overlain by well-exposed Frenchman Springs Member flows (Ginkgo & Sand Hollow). The main frontal fault (MFF) bounds the east side of the rotated footwall block (RFWB). Correlating the Ginkgo/Sand Hollow contact between the FW, RFWB, and into the hanging wall of the MFF permits determination of the amount of fault movement: The vertical component of separation, for example, is approx. 60 m for the RFWB and approx. 150 m for the MFF. These modest offsets highlight the greater importance of folding in accommodating shortening in this area. The tectonic breccia walls, at and above the highway, are formed within SB member flows, which were mechanically weaker than the largely intact underlying Winter Water member flows. The shattering of SB flows began as interlayer slip progressed during the steepening of the anticline limb. The SB breccia walls are offset by the MFF clearly indicating that layer-parallel brecciation occurred prior to faulting in the Ortley structure.