Friday Field Foto #48: Wall of plane- and ripple-laminated sandstone
If you ever find yourself around Guadalupe Mtns National Park in west Texas and southeastern New Mexico, take a day to do the hike up Shumard Canyon on the Western Escarpment. It’s a full day and parts of it quite strenuous, but worth it. To do it, you need to get a key from the park ranger for the Williams Ranch trail. The trailhead is at the end of 4-wheel drive road that can get washed out sometimes … takes about a half-hour to drive it.
When you get up into Shumard Canyon you’ll be within an ancient submarine canyon succession (the siliciclastic and turbiditic Brushy Canyon Formation overlying the carbonate Cutoff and Victorio Peak formations). One particularly spectacular exposure of the Brushy Canyon is a vertical cliff of nearly 100% plane- and ripple-laminated fine sandstone.
The photograph below is a vertical mosaic taking advantage of the vertical layout of a blog post. You’ll have to scroll down to see a person for scale. In addition to the within-bed sedimentary structures, there are a few intervals of inclined stratification that one might interpret as a large bedforms or barforms. For a slightly bigger version, click on photo.
For you turbidite geeks, one thing to consider … if you didn’t have the context of this area being within a submarine canyon-fill, do you think you would interpret it that way?
Happy Friday!
–
check out more photos from a trip to this region last September
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Clastic Detritus 
What’s that thing that looks like an upside down unconformity about 1/3 of the way up?
hmmm … a 1/3 of the way up … well, when you look at a more lateral view of this whole succession a bunch of truncation and ‘downlap’ surfaces are apparent. I would interpret some rather large barform-like features that migrated around and, in some cases, eroded into underlying sets (if you look a little closer, you can see some scour surfaces too). All the traction structures (plane- and ripple-lamination) is consistent with this. But, I’ve never studied this exact location in any more detail than that.
From the photo, the sediments look rather soft, or not stronlgy lithified (?). Otherwise, I think parts of that sequence look a little like some turbidite beds exposed south of Anchorage, AK – although those are somewhat coarser grained, usually show some graded bedding, and overall have thicker individual beds.
The vertically stitched photo is a really good concept – one that for some reason I’ve never even thought about doing.
Silver Fox … these rocks are lithified (they’re Permian), but maybe from a hard rock perspective all unmetamorphosed sandstones look soft :)
Ha! Yes, that’s a good one. And some of the rocks I’m thinking of are called “meta-graywackes” – although I hadn’t really thought about it that much – they aren’t really very “meta” from some perpectives. :)