Friday Fragment: For BrianR

I've been terribly remiss on my rock-blogging, and I apologize. These photos have been on my drive for nearly 3 months now; time to post them.

The rock is a sample of the King's Valley Siltstone (Tkv), a volcaniclastic sedimentary member of the Siletz River Volcanics (Tsr). The latter forms the "basement" of Siletzia, as described in the 3rd to last paragraph of this post. (The lowest and generally oldest rock in a regional sequence, "the basement" also sort of implies that it extends down into the earth far enough that for practical purposes, it doesn't really matter what's below it).

Tsr is mid to late Eocene in age, and I'll post on it some other time. It's thought to be, at its base, ocean ridge basalt. On top of that was formed a ridge of hot-spot, Hawaiian-style volcanoes. The importance of Tkv is that it shows these volcanoes emerged from the ocean and formed what must have been fairly substantial islands off the coast. (At the time, the coast would have been somewhere near the current axis of the Cascade volcanic arc, and these islands a little outboard of the current coastline) Had Tsr not emerged from the ocean, there couldn't have been enough erosion to produce these turbidites.

Tkv is intimately related to Tsr in a number of ways. First, near the contact, Tsr flows and pillows are interfingered with strata of Tkv. Second, as mentioned, the main source material of Tkv is weathered and eroded Tsr basalt. (At the site I collected this, there are also some interbeds of ashey rhyolitic tuff, which may be derived from the Challis volcanics of central Washington- but I can't vouch for that) As a result of the second point, the compositions of these two units are quite similar, and in weathered outcrops or drive-by geology, they can be difficult to distinguish.

The Kings Valley Siltstone member is fairly limited in areal extent, showing up in a band paralleling the Corvallis fault, another local feature I haven't discussed yet. It's also limited in terms of outcrops; I've only found a few, to the west on Marys Peak and to the north in Dunn State Forest (a unit of OSU's research forest land). Some of these outcrops, though, can knock your socks off. My favorite is a gravel quarry in Dunn SF, where one can clearly see the above mentioned interfingering, and I once found a most excellent feeding structure with radiating green phosphorite(?) worm poop. Carried the slab (~ 16x10x4 inches) home on my back too, about 10 miles. A friend of mine also found a magnificent turtle fossil in an outcrop near the town of Kings Valley- I tried to convince her that it might be an important fossil, and that she should show it to a researcher, but I couldn't get her to. It was about the size of a box turtle shell, but one corner had broken off, and you could see bone in the matrix. More mundane fossils (mostly mollusks, in my experience) can be found occasionally, but I've never seen a concentration of fossils anywhere.

From what I can remember, Tkv is a maximum of about 500 feet thick; that fits with my own trompings about. The top of the unit is pretty chaotic and unsorted, as is the bottom. The middle seems to be dominantly turbidites. Glassy basalt (sideromelane) and palagonite are abundant in the bottom of the unit, but absent from much less abundant in the middle, and then more near the top, though not as well preserved as at the bottom. I interpret these features to indicate less transport near the top and bottom, and from a more weathered source at the top.

So despite the lack of much research on this unit, I personally suspect it has a lot to offer, including a sedimentary record of the emergence, then erosion and submergence of a Hawaiian Island analogue, an admittedly spotty record of late Eocene life, but unique with respect to its location and time. It's also a beautiful area with overall good access- forestry (and the roads that go with it) rules a few miles to the west of my current position.

The pictures show three faces of the piece I have in my possesion at the moment, and show the graded bedding constituting the base of a single turbidity flow; the overlying finer sediments weather and crumble so fast they're very dificult to collect. And geo people should be able to see immediately that they're upside-down from the original deposition orientation- the "top" made a flatter, more stable position for photographing. Non-geo people should note that as you start from the top, the grains are larger, and as you move down, they get smaller. Turbidity currents (another thing I need to discuss in more detail at another time) start suddenly, then gradually slow over a period of hours to (maybe) a day or so. So the larger grains will drop out and settle first, then progressively smaller and smaller grains. The first deposited material is at the bottom of a sequence, later materials will be above, on top of, earlier materials, so the largest grains are at the bottom. Which happens to be on top in this picture, because the rock is "upside down."

Now here's the treat: I've learned how to upload big pictures, so the link below each will take you into a fantasy land of explicit geological detail.

BTW, this post is dedicated to BrianR, because he's always going on about turbidites, and I think he'll get a kick out of these.

I wanna see!
Big as a House! (just realized the first two are actually the same face from slightly different angles. Oh, well)
Penny the size of a grapefruit! (This is a slightly weathered face, but still fresh enough to see the texture)

I also wanted to mention that the glittery bits you can see are mostly the zeolite cement and some patches of calcite. Location information for this sample can be gleaned from the GoogleEarth screen caps below, or leave a comment for further info.

(Outcrop is immediately west of where the power line (~straight n/s swath) crosses the lower road (winding, ~e/w swath at the bottom).