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The Somerset Coal Field in the southeastern corner of the Piceance Basin contains three distinct fracture types in the Mesaverde Group: open-mode fractures (including sandstone fractures and coal cleats), shear-mode (faults and shear) fractures, and surface joints. The main coal cleat measurements consistently range between N58E and N75E across the study area, with near vertical dips perpendicular to bedding and well-developed butt cleats. Open-mode fractures measured in Mesaverde sandstones have two primary systematic orientations (called Set 1 and Set 2) averaging N76E, 82 SE dip and N18W, 84 SW dip throughout the coal field. Set 1 and Set 2 orientations are similar to major joint sets measured by previous authors in the Piceance Basin. These fractures are generally non-mineralized, orthogonal to bedding, and regularly spaced with very small apertures. The northeast-trending fractures are more common and usually crosscut the northwest fractures. Both joint systems have close spacing on the order of 3-in. average, apertures of less than 0.1 in., and lack facial expression. As observed locally in three coal mines, joints are less common underground than at the surface due to overburden density. However coal cleats, both face and butt measurements, occur with regular frequency and spacing. Mesaverde coalbeds in Somerset are fractured with one dominant face cleat orientation. Mine plans are generally oriented to extract coal within 15 degrees of the main anisotropic coal cleat direction for maximum breakage. Subsurface cleat spacing is usually about 0.5 in. with apertures of less than 0.1 in. Dip-slip shear fractures were also observed underground striking N45W to N78W and dipping between 60 and 76 degrees SW with some regularity. The shear sets are usually associated with tectonic faulting in the mines. Shear faults (both dip-slip and strikes-slip types) observed in Somerset coal mines cut across coal beds and sandstone roof rock. The distribution of these shear faults was observed between adjacent cross-cut walls at West Elk Mine but extend less than 0.5 miles in length. Shear fractures in the surface, however, generally strike northeast (shear fractures trend N47 E, and dip 66 SE at Paonia Reservoir) but are less frequent than the two primary joint systems. Faults in the Somerset Coal Field generally trend NW-SE on the west side near Bowie Mine No.2 and NE-SW on the east side near West Elk Mine. The regional fracture systems measured in Mesaverde rocks in Somerset are considered post-Laramide in age. The youngest joint set measured in the north fork of the Gunnison River valley is related to Tertiary intrusion. Mesaverde sandstone joints measured in Hubbard Creek adjacent to Iron Point stock show a pattern unlike the rest of the valley, striking N75W in one primary direction, with regular, close spacing. These Mesaverde joints are inclined parallel to joints measured in the 10-28 Ma stock itself.
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