Dana is near the bench, looking at the monument in the middle left. Most of the lowlands in this view are covered with a veneer of Pleistocene sediments, including Missoula Floods deposits, and modified by Holocene river processes. Underlying that is the Eocene Spencer Formation, which is the shallow water facies of the older Tyee Turbidites. However, the rock we're standing on is Siletz River Volcanics, an early Eocene oceanic plateau. This unit extends south to Roseburg, where it's known as the Roseburg Volcanics, and north to the Olympic Penninsula and southern Vancouver Island, where it goes by the name of the Crescent Volcanics. Interestingly, the rocks are older at the northern and southern ends, and youngest in the middle. This is thought to represent a hot spot on a spreading ridge; picture an amalgam of the Hawaiian Islands and Iceland.
There is a major fault along the base of this hill that runs NE to SW, the Corvallis fault, which I personally think represents an oblique-slip, but primarily strike-slip, fault. Getting a general sense of vertical offset is fairly easy, but difficult to estimate precisely due to poor exposures and folding near the fault. Getting a sense of horizontal offset is next to impossible, as far as I've been able to tell. My belief that it's primarily strike-slip comes from two lines of evidence: first, it's more consistent with the stress-strain patterns that would have been present during the mid-Neogene, and second, nearly every slickenside I've seen in and around this fault zone is close to horizontal. Another item of importance is that this fault has a wide (1000-2000 foot) crush zone of cataclastic material. Neither a normal or reverse fault with roughly 2000 feet of vertical offset would have a 1000 foot crush zone; strike-slip faults tend to have wider crush zones. I consider this supporting evidence, though, not persuasive in and of itself.
The fault is sutured in at least a few places by Oligocene intrusives, which are not broken. This suggests the fault has not moved since then, that is, for about 30 million years, and it's not considered to be a significant seismic risk.
If you visit this park, especially on a damp day (the best views are clear days in the winter, which are rare but spectacular, and in the early summer before the haze has a chance to set in and muck up the sights) watch the ditches as you drive in along the ridge line. The soils will be kind of orangey-yellow. This is a cue you're on Tyee or Spencer Formation bedrock. Just before you get to the parking lot, there's a dip in the ridge; this is where you're crossing the Corvallis Fault. In the park itself, not only are there abundant chunks of basalt float that have survived weathering and pedogenesis, but the soils are a dark brick red. Weathering goes fast in our mild, moist climate, and soil colors become a thing one pays attention to, even if, like me, your color acuity is poor.
Photo run through Paint.net's "auto-level" processor to try to cut through the haze and bring out some details. October 8, 2012. FlashEarth Location- not certain this is exactly the spot, but close enough, and I think the bench and monument are just below (south of) the cross-hairs.
Followup: Just realized you can see a color difference in the soils on opposite side of the fault in FlashEarth: Compare this (on basalt bedrock) to this (on sedimentary bedrock). The Corvallis Fault runs between these two spots in this area, and that view is backed out enough to include the other two. The color difference is even more distinct when it's damp/wet.
This Week's Geo-Quiz: Other Planets
6 months ago