9 January 2015
Friday fold: a surging rippled whale of Grinnell Formation
Posted by Callan Bentley
Along Going-to-the-Sun Road in Glacier National Park, you can see this outcrop of Mesoproterozoic Grinnell Formation quartzite (former quartz sandstone) and red argillite (oxidized mud rock). These strata are part of the Belt Supergroup.
It proudly displays an asymmetric / overturned anticline.
The outer arc of the hinge of this anticline shows extensional fractures in the red argillite “coating”:
Those photos were taken by Tom Biggs (UVA) in 2013. Here’s a shot I took in 2010 of the same fold, showing its cross-sectional profile:
Note the primary sedimentary structures: the ripples (bedform) and their guts (cross-bedding), as well as evidence of fluctuating current strength (mudchip rip-up clasts “suspended” in quartzite):
I look forward to revisiting this fold in July 2015 as part of my Rockies field course (for which, by the way, applications are now open).