New Mexico Geological Society Annual Spring Meeting — Abstracts

Joints caused by relief of in situ stresses associated with the present-day topography are the dominant fracture type in moderately indurated sandstones of the Cliff House Sandstone (Upper Cretaceous) at Chaco Culture National Historical Park and in the San Jose Formation (Eocene) in the Navajo Reservoir area.

Rim joints (Kelley and Clinton, 1960, p. 16) found along cliffed valley walls are vertical to nearly vertical fractures which break through entire sandstone beds and strike parallel to topographic contours. Rim joints cut sandstone beds ranging in thickness from a foot to about 50 feet, the maximum thickness observed. Away from the valley walls into the hillsides, these same sandstones appear to be virtually unjointed. At anyone outcrop, rim joints have a consistent (systematic) orientation but adjust closely (over several hundred feet horizontally) to the strike of the local topographic slope. This topographyconforming alignment suggests that the joints result from rock relaxation towards an unsupported surface, with primarily gravitational stresses causing failure. Polygonal or network pattern joints (Howard and Kochel, 1988) are nonsystematic fractures generally with five to six sides.

Polygon size varies from about one foot across with a few inches of fracture penetration to about ten feet across with many feet of vertical fracture development. Polygonal joints occur away from cliff edges mostly on the upper surfaces of massive sands; however, polygonal joints may grade into rim joints. Polygonal joints are probably caused by mechanical or thermal stresses involving expansion of massive sandstone upward toward a free surface, as opposed to rim joints where primarily downslope-directed gravitational stresses operate to cause rock failure.

Rim joints that break through thick sandstone beds produce extensive, planar, joint surfaces locally several hundred feet long and as much as 50 feet in height. On standard-scale aerial photographs (about 1:20,000), cliff segments and associated strips of vegetation growing in fissures where mass wasting has caused joint separation are mappable as photolinears.

Field evidence suggests outcrops of moderately indurated sandstones of Upper Cretaceous and younger rocks in the study area have virtually no regional joints. The well-jointed aspect of these rocks along canyon walls is the result of relief of in situ stresses associated with topography and exposure. Joint orientation, joint density, and photolinears from these rim joints do not indicate subsurface fracture patterns. A "complete", surface-based, fracture analysis of the San Juan Basin and other analogous Rocky Mountain foreland basins must also include the search for: regional, layer-confined, joint sets developed in well-indurated (usually carbonate-cemented) rock units; deep-penetrating, fracture zones (lineaments); and recognition of the linear expression of wind-derived landscapes.