Permeability and fracture patterns in extrusive volcanic rocks: Implications from the welded Santana Tuff, Trans-Pecos Texas
The Santana Tuff of Trans-Pecos Texas is a nonwelded to thoroughly welded, fractured, vitric crystal tuff. The unsaturated welded portions of the Santana Tuff were examined to determine its matrix permeability and fracture patterns; more than 1,100 permeability measurements on tuff surfaces were taken with a gas minipermeameter. Independent variables included the degree of surficial weathering, the presence of rock varnish, and the orientation of the surface with respect to direction of emplacement and flattening. The mean permeabilities for fresh, weathered, and varnished surfaces were 55.33 md, 5.03 md, and 3.31 md, respectively. The fresh-surface measurements are significantly different from the other two surface types, both of which were from statistically equivalent populations. Analyses on cut surfaces showed about a 30% decrease in permeability perpendicular to (flow direction and) flattening. This trend was also evident on the varnished and weathered surfaces. Weathering rinds (including the varnish) decrease the permeability. In thin section, the rinds exhibit a thin layer of dark minerals (mostly Fe oxides). SEM photographs of the weathered and varnished surfaces show a lamellate texture indicative of coating by smectitic clay particles. Fracture patterns were examined at six outcrops, spanning 9 km. A pavement was mapped at one outcrop. Rose diagrams and stereonets were prepared from more than 300 observations. Although the composite data show two main orientations of nearly vertical cooling fractures, the fracture trends vary considerably even between out-crops in proximity to each other. Field observations indicate that fractures are continuous across individual flow boundaries. At the mapped outcrop, the fracture density and trace-length are fractal between the 0.5 m and 3 m range (the fractal dimension of fracture density was 1.44, and the length of the fracture was scale-dependent, but a fractal dimension of fracture lengths was not able to be determined). A lower percentage of abutting fractures was observed here than has been noted in similar studies in Nevada. This study demonstrates that weathering along fractures reduces fracture-matrix interactions in such rocks and that fracture patterns on a regional scale may be difficult to characterize.