Abstract

Syndepositional fractures are common in Upper Permian strata of Dark Canyon, Guadalupe Mountains, New Mexico, USA, and are typically oriented parallel to the platform-margin trend with subvertical traces. Fracture apertures range from millimeter- to meter-scale with sediment, microbialite, breccia, and cement infill. Syndepositional normal faults are less frequent, and commonly are filled with fault-breccia and flanked by damage zones. Syndepositional fault offsets range from tens of centimeters up to 18 meters. In some instances, fault offsets were large enough to alter sedimentation patterns and stratal architecture of the Tansill shelf in the form of growth monoclines or small-scale grabens. Evidence for flow of dolomitizing fluids along syndepositional deformation features consists of narrow (centimeter- to meter-scale) dolomite halos along the fractures and faults, and dolomite bodies that extend laterally for several meters to tens of meters away from faults and fractures. Halos are interpreted to have formed where deformation features crosscut low-permeability strata. Dolomite bodies formed where deformation features intersected more permeable strata and dolomitizing fluids flowed laterally into the surrounding host rock. Isotopic data from individual fracture-controlled dolomite bodies indicate temporal variability in the dolomitizing fluids, suggesting that those fluids moved through different syndepositional fractures systems at different times.