Sunday, November 30, 2014

Riprap, Slickensides, and a Thanksgiving Walkabout

I went for a walkabout yesterday, two days after eating my way into a holiday food coma. That spiral cut ham–scalloped potatoes–pecan and cranberry dressing–vegetable medley–cranberry sauce–two pieces of pie–champagne and wine Thanksgiving dinner needed to be dealt with. So after nibbling on some leftover ham and forking my way across a hefty slice of breakfast pumpkin pie, I headed over to the Santa Clara Reserve to spend a few hours with friends, hiking and socializing in the great southern Utah outdoors.

164DaveRJJeff
I have enjoyed hikes with these guys for several decades

The hike was advertised to be around five miles (but turned out to be more like six and a half) and a couple hundred feet up and down over rolling desert terrain. The area is a magnet for local mountains bikers, folks so intent on staying on their bikes that they seem to miss seeing anything beyond a three–foot radius of the front of their handlebars. We spent a good part of the morning hollering “bike coming up in the rear!” and edging off the trail so as not to be bowled over by those very handlebars barreling up behind us.

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Working off that turkey dinner!
The Santa Clara River, usually a trickling stream, morphed into a raging torrent in the 2005 floods here in Washington County, and $61 million of federal disaster aid has gone a long way in providing funding for stabilizing the banks with riprap. More flooding in 2010 could have been as destructive as that in 2005 but the riprap apparently helped preserve the banks. How long before another 100–year flood comes along to wipe out all that hydrological engineering, though, is anyone’s guess.
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Riprap stabilizes the banks of the Santa Clara River
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A small dam spans the Santa Clara River, most likely for irrigation purposes
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Black basalt riprap stabilizes the banks of the Santa Clara River - for now
As we traipsed across the rock–strewn expanse, fragments of shiny slickensides on sandstone jumped out at me like a sharp reflection of sun on a windshield. A Slickenside is a polished area of rock along a fault surface, indicating movement or sliding of one rock against another. Southwest Utah is riddled with faults both large and small, with recent major crustal movement being down–to–the–west normal faulting related to Basin and Range extension, around 20 million years ago. However, older crustal compression and mountain building events during Sevier Orogeny times occurred even earlier, some 100 million years ago. Either or both could have been the culprit and caused the slickensides.

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Slickenside on sandstone
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Slickenside fragment
 
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Slickenside polishing

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