Saturday, September 10, 2011

Volcanic Ramblings Part 3: Salt Creek Falls

There's not too much of note to see on the drive down I-5 from the Corvallis exit to Route 58 just south of Eugene... at least little I'm not jaded to. So with an exception I'll tack on at the end, our first stop was at Salt Creek Falls. As the second tallest waterfall in Oregon, it's an enjoyable, lovely stop for anyone, even non-geotypes. I must say, Dana and Cujo359 got an earful of my griping about the $5 day-use fee for what used to be a nice, free roadside rest.

Dana has focused mostly on the glacial aspect of this spot in her post- and that is indeed an important part of the story- so I'll look more carefully at the bedrock geology. The Cascade Volcanic arc has an interesting evolutionary history. I have no idea if the pattern is unusual, but it youngs and narrows toward its eastern margin. In other words, when volcanism initiated about 35 mybp, following the docking of Siletzia (the Coast Range block), and subduction started outboard of that newly accreted terrane, it spread from the eastern side of the Willamette Valley to central Oregon. As time passed, though, the axis of active volcanism narrowed to what is now a fairly confined band along the eastern margin of what was originally a wide swath of activity. So the arc- at least in Oregon- can be divided into two portions: the Western (older) Cascades, and the High (younger) Cascades. The Western Cascades were more explosive overall, and produced a huge pile of pyroclastic rocks, volcanoclastic sediments and debris flows/lahars. They were also the ash source for the John Day formation in central to northeastern Oregon, famous for its preservation of Cenozoic mammals and other organisms.

According to a conceptual model developed by Ed Taylor, and apparently largely accepted on the basis of subsequent work by him, his students, and others, the origin of the modern Cascades began with the foundering of the ancestral "Plio-Cascades" 4-5 mybp into what is referred to as the "High Cascade Graben," as shown schematically above. The image that understandably comes to mind when one pictures these mountains are the big stratovolcanoes, but they are the exceptions, not the rule. Most of the High Cascade arc actually consists of a low ridge of basalt, andestic basalt, and basaltic andesite.

So what does all this have to do with Salt Creek Falls? When we stopped here with Sharkey, it was not for the pretty water feature, but because it was the boundary between the Western and High Cascades. The lower layer of rock, of Western Cascade affinity, is one I have not had an opportunity to look at closely, but judging from its fragmentary nature at a distance, I feel safe saying it's probably either lahar deposits or pyroclastic rocks of some kind- dark in color, so probably fairly mafic in composition. Overlying the fragmented rock is a thick intracanyon High Cascade mafic flow- Dana has found a source saying it's andesite, and I have no reason to say differently. I never could distinguish the mafic lavas without a petrological scope. So in a nutshell, we have a thick, competent, vertically-jointed rock overlying a soft, easily weathered and eroded rock.

This is a classic set-up for a waterfall.

Water erodes the softer rock and undercuts the overlying hard rock, which, because of its vertical jointing, slabs off to form a vertical face. While in most cases, erosion would incise and smooth the streambed, in this case, the physical arrangement of components acts to maintain a shear face. There are certainly other ways waterfalls can form, but I think of the "hard-over-soft" scenario as being the most common.

Well, isn't that sweet. Blooger has decided it controls the vertical and horizontal again. Sorry, I'm tired of fighting with this for now... maybe I can figure out how to fix it later.
Looking downstream, ~west, from the top of the falls.
This is perhaps not the best possible view to show it, but the landscape is pretty clearly the product of two distinct phases of erosion: a broad, glacially carved valley, followed by Holocene incision by Salt Creek.
The falls from the north rim...
...and from halfway down the trail.
Lovely columns.

No comments: