Tuesday, October 18, 2011

Volcanic Ramblings Part 6: Cascades Slop Over Into Basin and Range

We managed to consistently get started at a decent time each day of the trip, aiming to gather for breakfast at 8:00. Since we had driven about an hour south from the turn-off to Crater Lake, that meant we had to drive an hour north- though I recently realized there IS a more direct, probably significantly shorter route that would have taken us to the SW entrance. But wait'll you see the juicy outcrops we would have missed in that case!

Descending into the Klamath Basin, numerous ~N/S normal faults have opened up some terrific exposures I had never bothered to look at carefully- though the next one (after this) in particular is one I've wistfully watched as I sailed past it more times than I care to count over the decades. But here's the thing: the rock is steep, not particularly competent, so mass wasting into the road is commonly a problem, route 97 is sort of shoehorned in there where it fits, often with little space for pull-outs or break-down lanes... and it's a fast, busy and dangerous road. So opportunities to geologize need to be chosen carefully, with an eye toward safety.

All that said, we did find a few pre-park stops which, though they chewed up and spat out enormous amounts of precious time (the commodity which was in shortest supply for the whole trip), were definitely worth it. For most of the stops on the trip, I tried to remember to take a shot or two of helpful landmarks for anyone else who would like to check the spots out for themselves someday, and so I could more confidently and precisely find them in Flash Earth. The lead photo above is the intersection of Algoma Road with Route 97. We were able to get off the road, park in a pull-out on the south side, get behind the Jersey Barrier (which was very nice to have between ourselves and traffic), and walk a hundred yards or so south to an interesting-looking face we had noticed only moments before.

Here's the first shot of the day ( Note that all the photos in this series will get much larger with a click.):So what are we looking at? Well, that there is an itty-bitty normal fault in poorly sorted, extremely immature volcaniclastic sand (mostly) and silt. The grains looked to be mostly lithic, which is why I describe it as "extremely immature." The offset looks to be about an inch. Here's a view from a different angle:Isn't that purty? I'm quite fond of sed structures, both original and soft deformation, so I keep an eye out for them at stops like this. Not much of note at this one, but enough to convince me this stuff was reworked by water, not just air fall ash. (Obligatory admission: I'm no expert, and I'm always open to correction and/or questioning.)
Backing off from the same spot, with the lovely Dana Hunter for scale, you can get a better sense of the over-all look of the outcrop, and see that the fault is roughly parallel to the cliff face, suggesting this is a subsidiary fault to the much larger one that created this escarpment.Looking a bit farther along, there's a lens of very dark sediment that I took to be a small cut-and-fill structure at the time, but it occurs to me it might be a small channel from near the start of this sedimentation cycle. The next unit down is a brick red layer that looked as if it might be a thick ash-fall unit. Looking south along the face, below, it's quite striking. But at the far end, you can see the same kind of stratification as above the brick red stuff.
Walking south, down-section, more of the same under the bricky stuff...
Turning to look north, yep, it's pretty clear that structurally, we're in basin and range. I would guess (and later 65 mph reconnaissance geology seemed to confirm) that the sorts of rocks and sequences we're seeing here would be up there, too.Below, a block of stretched-vesicle mafic rock that fell from a lava flow above the sediments first shown with the fault...
...and a coarse cobbly layer in the lower section of sediment.
So what can we say about this environment (dons semi-irresponsible speculation helmet)? Well, it's very clearly dominated by a nearby source of volcanic rocks... oh, I didn't show you the view across the highway yet, did I?That'd be Mt. McLoughlin, the southernmost major Cascade peak in Oregon. (I doubt that particular volcano contributed much to this outcrop, though.) But I have to admit, I'm not very confident describing the depositional environment. I'm guessing that we're looking at a braided stream drainage, mostly choked with ash, but with larger channels carrying flows competent enough to move larger boulders. The material is too coarse and poorly sorted to be lacustrine (as the modern-day lake across the road) but I'm not seeing enough in the way of cross bedding and channel features to feel comfortable with what I'd consider a more typical meandering fluvial environment. There are quite a few flows in between thick piles of sediments, so we're not so far from the vents that sufficiently fluid lavas couldn't reach this spot, but they're a fairly small proportion of the pile... less than a quarter for sure, maybe closer to 10-15%.

In terms of age, I really don't know, but I'd bet pre-Pleistocene, because the magnitude of the fault offsets (which post-date the sedimentation) would require at least one or two million years.

One last shot, taken as we crossed the causeway in the lead photo, with a question for Callan Bentley... what was the term for a ramp between two vertically offset blocks? You mentioned it in your posts on structure in the Bishop Tuff. Followup: Callan responds swiftly with the response "relay ramp." That there below is one.


Callan Bentley said...

Relay ramp!

Brian Romans said...

some of that interlaminated stuff looks like some delta-front deposits I've seen