Terry L. Tolan

Intracanyon flows of the Columbia River Basalt Group in the lower Columbia River Gorge and their relationship to the Troutdale Formation

The Priest Rapids Member (Wanapum Basalt) and the Pomona Member (Saddle Mountains Basalt) of the Columbia River Basalt Group crossed the Miocene Cascade Range into western Oregon and Washington as intracanyon flows about 14 m.y. and 12 m.y. ago, respectively. A thick, allogenic, bedded palagonite complex underlying the Priest Rapids intracanyon flow originated when the first Priest Rapids flow (Rosalia chemical type) interacted with a shallow lake on the Columbia Plateau, displacing water that flushed hyaloclastic debris into an incipient ancestral Columbia River channel. The Priest Rapids flow then overfilled the canyon, forcing the river northward, where it established the Bridal Veil channel. The Bridal Veil channel, part of a river system that extended across the Columbia Plateau, was only partly filled by the Pomona flow, allowing the river to remain in this channel through the Cascade Range. The Troutdale Formation is made up of (1) older alluvial gravels deposited below and above the Pomona intracanyon flow while the ancestral Columbia River occupied the Bridal Veil channel and (2) younger, more varied alluvial deposits characterized by clastic and hyaloclastic debris from Boring and High Cascade volcanism, spread over a much wider area, when aggradation overfilled the Bridal Veil channel less than 6 m.y. ago. The shift of the lower Columbia River to its present course took place during this time of maximum alluviation, aided by scattered eruptions of younger volcanoes of the Boring and High Cascade Lavas. The present Columbia River Gorge was formed in post-Troutdale time by entrenchment of the Columbia River during rapid uplift of the Cascade Range of northern Oregon during the past 1 to 2 m.y.

New aeromagnetic data reveal large strike-slip (?) faults in the northern Willamette Valley, Oregon

High-resolution aeromagnetic data from the northern Willamette Valley, Oregon, reveal large, northwest-striking faults buried beneath Quaternary basin sediments. Several faults known from geologic mapping are well defined by the data and appear to extend far beyond their mapped surface traces. The Mount Angel fault, the likely source of the Richter magnitude (M L ) 5.6 earthquake in 1993, is at least 55 km long and may be connected in the subsurface with the Gales Creek fault 25 km farther northwest. Northeast of the Mount Angel fault, a 60-km-long, northwest-striking anomaly may represent a previously unrecognized dextral-slip fault beneath the towns of Canby and Molalla. Vertical offsets along the Mount Angel fault increase with depth, indicating a long history of movement for the fault. Dominantly northwest-trending, relatively straight faults, consistent stepover geometries, offset magnetic anomalies, and earthquake focal mechanisms suggest that these faults collectively accommodate significant dextral slip. The 1993 earthquake may have occurred on a left-stepping restraining bend along the Mount Angel–Gales Creek fault zone.

Late Cenozoic structure and correlation to seismicity along the Olympic-Wallowa Lineament, northwestern United States: Discussion and reply

The Columbia Plateau-portion of Raiszs (1945) Olympic-Wallowa Lineament (OWL) is a geologically fascinating feature whose origin, structural significance, and age have been the subject of numerous geologic studies over the years because of its proximity to nuclear and hydroelectric facilities. Despite all the work that has been done, some aspects of this complex feature are still not well understood; therefore we recognize the need for the continued study of this feature and welcome new efforts that further our understanding. However, Mann and Meyer have overlooked an extensive body of published (and publicly available) data on the OWL, and this has resulted in numerous mistakes and erroneous interpretations/conclusions in their paper with little new insight into the subject. In the following sections, we discuss some of the more significant problems with Mann and Meyers paper based upon the existing literature and the results of our ongoing research.