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.

DSCN0881 fold in Grinnell clastics

It proudly displays an asymmetric / overturned anticline.

DSCN0881 fold in Grinnell clasticsanno

The outer arc of the hinge of this anticline shows extensional fractures in the red argillite “coating”:

DSCN0882 fold in Grinnell clastics with extension cracks

DSCN0882 fold in Grinnell clastics with extension cracksanno

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).

Happy Friday!