Saturday, July 12, 2014

Geo 730: July 10, Day 556: Stumped

From an Ore Bin article by Irene Gregory:
   Scattered deposits of Tertiary fossil (petrified) woods are to be found throughout most of the Western Cascades adjoining the eastern side of Oregon's Willamette Valley, but those deposits making up the area known as the Sweet Home Petrified Forest in Linn County are among the most abundant and well known.

   The abundance of the area's fossil wood is evident even to the casual traveler. It may be seen crushed or as fill in driveways, in fences and retaining walls, and in decorative garden work. Larger pieces -- stumps and logs -- mark driveway entrances and hold up mailboxes; barn floors have been built of it and abandoned wells filled with it. Farm people of an earlier day considered it a great nuisance -- a feeling perhaps carried over, justifiably, by today's landowners at times harried by avid rock hunters.
That article makes for some interesting reading. In particular, the list of identified taxa is pretty amazing, in terms of diversity and the climates they indicate. The photo above is of the narrower but much taller stump at the Sweet Home Community Museum. The two pieces displayed there are outside, so you can stop and see them, even if the museum is closed. As Gregory comments in the article, the quality of preservation is remarkable. In thin section, it's essentially identical to modern, unfossilized, wood slices of the same type.

Photo unmodified. March 9, 2012. FlashEarth location.

Geo 730: July 9, Day 555: Roll Over

Heading back toward Corvallis, we got off I-5 in the North Eugene-Coburg area to get a better look at an outcrop we'd passed at other times. It's quite visible from the freeway, but there's no good or safe spot to stop and get a better look-see. I took a number of photos here, but I'm not really happy with any of them.
Here's an annotated version. I'm not sure if we're actually seeing the right/south margin of the dike here, so that's dotted. Also, now that I have the photo marked up and loaded, it occurs to me the sedimentary rocks are more likely Oligocene age. The dike rolling over into a sill is probably a Western Cascades hypabyssal intrusion.

Photo unmodified. March 9, 2012. FlashEarth location.

Geo 730: July 8, Day 554: Technical Terminology

The technical term for these rocks is "squarshed." (Actually, I think that's more drag folding over a low-angle reverse fault.)

Photo unmodified. March 9, 2012. FlashEarth location.

Friday, July 11, 2014

Geo 730: July 7, Day 553: Slant

A closer view of the right limb of the previous post's anticline.

Photo unmodified. March 9, 2012. FlashEarth location.

Geo 730: July 6, Day 552: Another Anticline

The right limb is more evident than the left, but I trust you can see both sides. In this case, we have a clear enough view of the strike and dip to see the anticline is plunging away from us- generally speaking, to the east.

Photo unmodified. March 9, 2012. FlashEarth location.

Thursday, July 10, 2014

Geo 730: July 5, Day 551: Auntie Kline II

A close-up of the anticline in the previous post. Part of the reason I suspect this was "semi-soft" deformation is that the rock does not seem to be metamorphosed at all... the mud layers are quite friable. If this had been subjected to enough heat and pressure to plastically deform the rock, I'd expect the fine-grained portion, at least, to show some signs of that.

Photo unmodified. March 9, 2012. FlashEarth location.

Geo 730: July 4, Day 550: Auntie Kline

A sweet little anticline along Garden Valley Road and the Umpqua River downstream from Roseburg, Oregon. We almost never get to see nice folding like this in Western Oregon. There is plenty of mild regional deformation, but very little in outcrop-scale. It's a lot easier to find sample-size folding, but that's mostly soft-sediment, penecontemporaneous deformation... though actually, this may be soft or "semi-soft" deformation too. No one seems to be very confident about just what is going on here.

Photo unmodified. March 9, 2012. FlashEarth location.

Geo 730: July 3, Day 549: What a Drag

Looking more closely at the reverse fault in yesterday's photo, drag folding is apparent in the hanging block. This is the clue I'm using to establish that it is, in fact, a reverse fault, not a low-angle normal fault. The beds are too rhythmic and self-similar for me to have any confidence in correlating across the fracture.

Photo unmodified. March 9, 2012. FlashEarth location.

Wednesday, July 9, 2014

Geo 730: July 2, Day 548: Folds Plus Minor Reverse Faults

A bit up the road from yesterday's photo, the disturbed nature of the rocks in this cut are more apparent. There looks to be a minor reverse fault behind the wire and down the slope from the oak in the right middle near the top, and another that looks incipient to the left of that same tree. This is Flournoy Formation, which in this area is normally gently tilted, but not deformed to anywhere near the degree we see here. This is a restricted, and as far as I know, unique, occurrence of highly deformed rocks of this area and age. (Older, Klamath Province rocks can be extremely deformed, but these are of Coast Range affinity.) It doesn't appear to be related to tectonism. A possible explanation is that it was a large, but localized, slump before the rocks had been buried too deeply. As I mentioned yesterday, it seems that the now-accepted term for that sort of thing is a mass transport deposit.

Photo unmodified. March 9, 2012. FlashEarth location.

Tuesday, July 8, 2014

Geo 730: July 1, Day 547: Moving Along

After leaving Bushnell Rock and the conglomerate behind us, we finished our drive into Roseburg on Route 42, got on I-5, traveled north a bit, then got off onto the Garden Valley Road to do a side loop and look at some very odd folds in the Flournoy Formation. I posted a couple of panoramas from this area from my visit to it with Chris Rowan last year in the Geo 365 series, on July 6 and July 13, but this was my first visit to this spot in about 20 years. I was introduced to the concept of mass transport deposits back in December. While this was conjecturally described as a possible olistostrome by one field trip leader, I wonder if this might be another example of a mass transport deposit, one with quite a bit of deformation but not outright disruption.

Photo unmodified. March 9, 2012. FlashEarth location.

Geo 730: June 30, Day 546: Hammer for Scale

The move is complete, the new place is nice (though awfully hot in the afternoon and early evening), and while my body has not yet entirely forgiven me, it's not as achy, stiff and sore as it has been the last couple days. So back to the rocks. Above we see the same conglomerate as in the previous Geo 730 post, this time with a hammer for scale. There are parts of this outcrop where imbrication is clearly evident, indicating a right-to-left current direction. This makes good sense; the deeper basin was to the left (north, going by modern geography), and the land and depositing river system to the right. However, if there is imbrication in this photo, it's subtle; I wouldn't use this picture to illustrate the concept.

Photo unmodified. March 9, 2012. FlashEarth location.