Tectonic analysis of the Forester Creek anticline, Larimer County, Colorado

The Forester Creek Anticline is a flexural slip fold in a layered sandstone and shale sequence in the Upper Big Laramie River Valley, Colorado. The anticline is upright and symmetric with an axis trending approximately north-south and plunging 25° to the south. The object of this paper is to present information concerning the mechanics of folding from field observation, laboratory experiments, and theoretical considerations.;The sandstones of the Muddy and Cloverly formations behaved in a brittle fashion, while the shales behaved in a ductile manner. This is consistent with experimental work done on sedimentary rocks at the same temperature and pressure that existed at the time of deformation.;The fracture pattern is a result of the total movement picture. The joints may not have opened until later, but the strain energy was present from the time of bending.;From observations on the Cloverly and Muddy sandstones, four principal fracture subpatterns were recognized in the fold. The principal stress axes for two of the subpatterns coincided, whereas the other two were superimposed on the pattern at a later time of movement. The layering played a significant role in the orientation of the stress axes during the folding process.