April 29, 2011

Columbia Basin Trip: Day 1 - a Lahar and an Anticline

Only now at the end of April do I have a moment to breath, as two tough terms that went by like a blur have come to their inevitable summer conclusion, and the search for work placement begins. But before the hunt begins, a getaway for a few days was what my figurative spirit called for. I'm the outdoors type (is any geoscience type not?), thus I'm allergic to staying indoors when I have some time off. After perusing through Ellen Morris Bishop's Hiking Oregon's Geology book, I was inspired to try out some of the listed hikes. The trip I took along the Columbia River Gorge was astoundingly beautiful, and the forces of nature were out to make it an interesting few days.

After a few hours driving, the first stop was an aside from I90-East, to visit the roadcut that exposes the Ellensburg Formation lahars (N 47° 06.041 W 120° 41.705). Driving along the windy stretches of central Washington, this slice of roadside geology was quite the treat. The lahar members of the formation stick out quite noticeably from among the semi-arid grass veneer over basaltic lava flows typical of the CRBG region. This lahar exposure, resulting from a muddy mix of water with pyroclastics & ash originating 50km away, is but one example of over a dozen such deposits which have been discovered around Kittitas and Yakima counties.

This particular Miocene lahar deposit has some unique features, including an erratic conglomerate boulder that was carried by the great erosive force,  and a plethora of broken-up crossbeds. This is typical of the stratigraphy of lahar deposition, which generally includes a bottom layer of eroded country material, followed by the bulk lahar layer composed of grains ranging from silt - boulder size, and finally the top layer of mostly sand with interbedded bits of gravel & pebbles arranged in a disarray of swooping crossbeds, indicative of a highly turbulent current during deposition. When scrutinizing the roadcut, I could feel the sandy texture, and noticed various discontinuous crossbed forms. The granules were mostly hornblende, feldspar, and some distinct pink Ca pyroxene minerals.
The generic X-sec of a pumice-laden lahar (left); discontinuous crossbed forms near the base (middle, pen for scale); the roadcut along route 10 showing the poorly sorted Ellensburg Formation lahar mass (right)

Shortly after the stop at the lahar was the first hike on the trip, and it turned out to be one of the best hikes I've had in a long time. This is BBC → Beautiful Basalt Country. A tributary creek of the Yakima River canyon, called the Umtanum, has a never-ending trail that takes hikers through a varied landscape of exotic vegetation, eclectic creatures, and fantastical basalt shapes. Obviously I focused on the latter, but tried my best to frame shots to include the colorful blooming trees and the out-of-sync redwoods I spotted. The basalt exposures became more plentiful and protruding the further I hiked in, to the point where I could see plenty of examples of entablature, pillow, and columnar basalt all within a small radius. The multiple flood basalt lava flows of the CRBG that effused throughout the Neogene gave the Umtanum creek canyon its layered characteristics, with varied thicknesses and formational conditions combining to define the Yakima Basalts you would see there.
Example of eroded/oxidized remnants of columnar Yakima Basalts
Boundaries between lava flows is visible on many faces in the Umtanum creek canyon. The purple line divides the lower columnar basalt from the proceeding pillow basalt above. Some surface expression of flows are hidden behind talus or broken up.
Columnar, Hackly, Pillow, all visible on one face

On my way out, a last pullover harkened to me, maybe because my GPSr kept bleeping at me that an earthcache was nearby. Lo and behold a small monument to plate tectonics + fluvial processes in geomorphology. What did it say? To frame it geologically: The Yakima Basalts and Ellensburg Formation lava flows are quite thick, on the order of a few kilometers, and fluvial downcutting into them still has a ways to go (core drilling down 3 km's still hadn't reached a different basement lithology). On top of that thickness, folding into anticlines & synclines occurred due to north-south trending compression. One particular anticline, the Umtanum ridge anticline, is noticeable if you have a good eye for the big picture. It stretches for over 80km, and with such a large scale and broad sweep it might be hard to notice the bend that defines the anticline.
Evolution of Yakima river course during folding

Due to the slow warping matching the slow downcutting of the Yakima river, the river has cut its course through the basaltic layers and created a water gap. The river retained most of its sinuosity whilst doing so, spending its erosive force cutting through the basalt rather than cutting a straighter channel as the gradient increased. Another great earthcache with lots of information and excellent diagrams provided by local organizations, in this case the Washington State Parks and Recreation Commission.

There were other interesting stops along the routes and highways, where roadcuts exposed features and structures that demanded scrutiny. I saw no less than several examples of tilting, faulting, and various forms of basalt in numerous shapes that would make cumulus clouds envious. The vegetation of the Umatilla Plateau & Yakima Folds ecoregions gave a splash of variety & color in springtime that is quite unique and unparalleled. The next day took me into new and interesting places, more CRBG but in different flavors. Day 2 post to follow...

No comments: