Fracturing along the Grand Hogback, Garfield County, Colorado

The several sets of fractures found in steeply dipping Mesaverde strata along the Grand Hogback, northwest Colorado, have previously been interpreted to be either (1) entirely of pre-uplift origin, or (2) entirely caused by stresses created during folding and uplift. However, based on fracture characteristics measured at 32 locations along the Grand Hogback, we suggest that only the major fracture set formed prior to uplift, and corresponds to subsurface fractures in nearby flat-lying reservoirs. These are the oldest fractures, which developed in the Mesaverde Group and lower Wasatch Formation as a set of west-northwest trending, through-going, regional fractures in response to horizontal compression. Secondary sets of northwest through northeast trending, cross-cutting fractures post-date the west-northwest fracture set and can be related to stresses caused by structural deformation and exposure of the strata along the Grand Hogback. These cross-cutting fractures are suggested to have resulted from several different mechanisms: (1) as relaxation fractures that formed subsequent to the early fracture event, (2) as fractures that formed on the north-south trending section of the Grand Hogback in response to cross-sectional folding, and (3) as fractures that formed on the sinuous and east-west trending sections of the Grand Hogback in response to planarmore » folding. The cross-cutting fractures are developed widely along the Grand Hogback but are rare in the subsurface. Conjugate shear fractures in the lower Wasatch Formation formed in response to east-west compression and post-date folding. 17 refs., 12 figs.« less