2 September 2011

Friday fold: duplex structure in the gastropod limestone

Posted by Callan Bentley

The Friday fold is a duplex structure, seen this last summer at Sandy Hollow, a classic field mapping locale west of Block Mountain (north of Dillon and west of Twin Bridges), in Montana. A duplex structure is a bunch of shingled-up folded and faulted strata (here’s an example from Scotland), usually bounded above and below by less-deformed layers. This particular duplex appears on the east limb of the Sandy Hollow Anticline, in the limestone layer known as “the gastropod limestone,” so called because of its rich concentration of snail fossils. You can enlarge this stitched panorama by clicking on it for a 4000-pixel-wide version:

You’ll probably need to enlarge it to see the details of the duplex, since it’s very wide but not especially tall.

Here’s a zoomed-in look at the most intriguing portion of the outcrop, with my annotations following:

You can make the annotated one bigger by clicking on it.

You can see a bunch of asymmetric and overturned antiforms and synforms, often broken on the overturned limb and transformed into small-scale thrust faults with a few meters of displacement. Them’s some seriously messed up rocks! Note that the deformation only goes so deep – and that there is a relatively undeformed stratum below all this chaos – small scale folds between the “duplexified” strata and the undeformed “floor” of the duplex suggest some sort of less competent layer in there that accommodated the strain with a “lubrication” effect.

As a bonus, here’s a single fold in the gastropod limestone, a little further along strike to the south, with my trusty Swiss Army knife for scale:

Happy Friday, and Happy Labor Day weekend for my American readers. The Mrs. and I are off for our traditional pilgrimage to Dolly Sods. Enjoy your holiday!