Friday, October 31, 2014

Geo 730: October 31, Day 669: Faulty Loading?

The faults from yesterday's panorama are visible in the lower right, but here we can also see the overlying Columbia River Basalt, which I'm nearly convinced now invaded in an igneous form of ball-and-pillow structure. That form of deformation is normally the purview of soft sediments but in the case of Elephant Rock, the denser, sinking, mass was basaltic lava. The vertical columnar joints strongly imply that the feature is not a dike, as it appears to be, and it took a couple days of puzzled frustration for me to come up with a plausible alternative.

Yesterday, I was looking over the nature of the offset on these small faults, supposing they were associated with subsequent tectonism following the lithification of these two units. Looking at this photo, I have another guess to toss out: they look as if they may be fairly similar in strike to the outcrop's face. If that's the case, the fault just to the left of the lens cap is farther under the basalt cylinder than the one roughly a foot to the right. This in turn suggests that the faults might be loading structures. The inner sediments, more heavily loaded, sheared downwards with respect to the less-heavily loaded sediments outward. Keep in mind, that's just a guess, but it would imply the faults were contemporaneous with the basalt emplacement.

Photo unmodified. July 15, 2014. FlashEarth Location.

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