Wednesday, January 14, 2015

Geo 1095: January 14, Day 744: Sedimentary Interbeds

This is a marginally useful view of the sedimentary rocks between the two large Grand Ronde Columbia River Basalt flows. The lens cap is 52 mm in diameter (2 inches, for practical purposes), but that's a little misleading, because the darker rocks just above it are back a good deal farther than the rocks it's resting on, and the uppermost light layer, as well as the rock below it, are closer to the camera. So the sense of scale is likely at least a little misleading. Next, the jumbled-looking rock the cap is resting on could easily be taken for some kind of breccia. As best as I can recall, though, it's just more horizontally bedded sediment. But it's weathered, and quite a bit of talus has spalled off onto that slope, basically obscuring and confusing what it is we're looking at.

On the other hand (and the reason I decided to post this photo), it does appear to show distinct compositional zoning between layers. If I was being forced to guess as to the reason, I'd conjecture that the lighter layers represent sporadic felsic and ashy Cascade eruptions, and the darker layers represent times when the major sediment source was erosion of exposed CRB surfaces upstream from here. But I'm not being forced to guess, so I won't.

Photo unmodified. August 30, 2012. FlashEarth Location (approximate).

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