PERMIAN STRATA OF THE SAN ANGELO AREA CONTAIN SYSTEMATIC FRACTURE SETS

Early Permian (Leonard Series) San Angelo and Blaine Formation outcrops in the San Angelo, Texas area, in the Eastern Shelf of the Permian basin, display three sets of vertical joints or shear fractures. Two main fracture sets strike 358 and 085. A third set, found to date only near Twin Buttes Reservoir, strikes approximately 300. Low Twin Buttes Reservoir water levels in 2011 exposed San Angelo Formation outcrops normally under water. Six other extensive exposures also provided data. In most exposures, surface trends of all systematic fractures were measured along several traverses at different orientations. Traverses averaged 10 m length. The San Angelo Formation contains resistant, tan, quartz sandstone ledges interstratified within maroon shale and mudstone. Sandstone beds contain well-sorted, sub-angular to sub-rounded, 0.25 - 0.35-mm-diameter grains, abundant hematite concretions, and common laminations and crossbeds. The overlying lower Blaine Formation contains cream-colored sandstone and mudstone. Beds are horizontal in all exposures. The three sets of vertical joints or shear fractures are not mineralized and ornamented joint surfaces have not been discovered. Fracture spacing varies between sets, ranging from 0.3 - 1.5 meters. Systematic fractures are visible on some aerial photos. E-W-striking fractures correlate with joints in the Palo Duro Basin related to late Paleozoic Ouachita orogeny folding (Collins and Luneau, 1986). Shaw (2006) recognized four post-Cretaceous fracture sets (NE, NW, N-S, and E-W) in the Edwards Plateau that control stream drainage patterns. N-S- and NE-striking fractures could be related to two phases of Basin and Range extension (31-10 Ma and 10-0 Ma) or to Laramide contraction (70-50 Ma). Sparse N-S fractures also cross-cut Tertiary or Quaternary caliche-cemented conglomerate that directly overlies Permian exposures.