Tuesday, March 17, 2015

Geo 1095: March 16, Day 805: Reverse Grading

On the approach to Tombstone Pass, on Route 20 east of Corvallis, in the Western Cascades, there are a number of appealing roadcuts, some of which have convenient pullouts. Despite having sailed merrily past them many times, I had never had time or opportunity to stop and look more carefully at them prior to a trip to the area with Dana on  October 9, 2012.

Though this was not the droid outcrop I was looking for, it did have some interesting features, and the one I was looking for was just around the corner. And when I say "just," I walked to it in about the same time it took for Dana to move the car there.

Looking at the cut, though, there are a couple of things that pop out immediately. There are two distinct sets of deposits here. From this perspective, the lower one looks fairly homogenous, with a kind of "badlands" morphology (It's even more distinct in the next cut). The upper one consists of angular, poorly sorted clasts ranging from quite fine to very large. However, despite being poorly sorted, there is a distinct trend for the clasts to get larger from the base of that deposit up to maybe 15-20 feet above that line. This is backwards from most examples of graded bedding, where the individual fragments start coarse and become more fine upwards, and thus is called "reverse grading." It indicates that even though deposition was occurring, the energy of the flow was increasing. It's an excellent indicator of a debris flow, and can often be seen in cuts in modern alluvial fans. This one, being composed of volcanic rock, would properly be called a lahar. Here's a crop to show the reverse graded portion more closely:
We didn't cross the road here, so I didn't realize until we went around the corner how puzzled I would be by the lower unit. And in passing, it looks as if there might be a third, finer-grained, and better-bedded third unit, cut into the second, as a "V" near the center of the photo. Note the gap in the larger clasts in that area.

Photo unmodified. October 9, 2012. FlashEarth Location.

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