The Pennsylvanian-age Tensleep Formation in south-central Wyoming is comprised of repeated limestones, sandy limestones, and sandstones. Strata of these varied lithologic units are folded over Beer Mug Anticline and cut by numerous intersecting fractures. The anticline, with a near-vertical forelimb and backlimb dip up to 50^o, provides an ideal analog for fracture systems in tightly folded Paleozoic hydrocarbon reservoirs. Fracture type and degree of development vary systematically with lithology, structural position, and degree of folding. Fracturing is most intense towards the core of the anticline, which locally consists of brecciated, oil-stained rock with large-scale vuggy porosity. Most of these strata exhibit inherited (F₀) fracture patterns that predate folding, as well as fold-related extension fractures that trend approximately normal (F₁) and parallel (F₂) to the axis of folding.

Within the Tensleep sandstones, bedding-parallel slip is common between beds—especially between limestone and sandstone units, and along the large dune cross-bed foresets. The meter-scale limestone beds interbedded within the thicker Tensleep sandstone units accommodated much of the larger-scale, bed-parallel shear through brecciation, although brecciation is not always obvious due to thorough recementation and weathering. In fact, at a gross scale, many of the limestone beds give the appearance of being massive and unfractured. Granulation, brecciation, pressure solution, and cementation likely occurred nearly simultaneously with deformation.

Natural hydraulic or injection fractures with irregular strikes occur within both sandstone and limestone strata. Some injection fractures are parallel to the ESE-WNW-striking, early strain extension fractures, suggesting an age and/or mechanical relationship.

Shales overlying the Tensleep Formation accommodated strain by ductile deformation and bedding-parallel shear. Numerous small but systematic thrust faults in the Alcova Limestone, which acted as a stiff strut within the ductile shaley sequence, provide strong evidence that the maximum compressive stress was in the horizontal plane and approximately ESE-WNW prior to tilting of the beds. This stress and its local variations plausibly accounts for most of the fractures, faults, and folds observed in the Paleozoic strata at Beer Mug Anticline.