SLOPE-PARALLEL FRACTURES IN THE NAVAJO SANDSTONE OF THE COLORADO PLATEAU: RELATIONSHIPS TO DENUDATION, WATER TABLES, AND CONCRETIONARY IRONSTONES

S.J. Martel’s theoretical work and field studies of granite landforms in the Sierra Nevada have shown that homogeneous rocks within compressive tectonic regimes can develop curving (convex-up), slope-parallel, opening-mode fractures. These joints open perpendicular to free surfaces and form at depths < 100 m. Shallow, slope-parallel fractures are also abundant in extensive outcrops of the Jurassic Navajo Sandstone of southern Utah. In crossbedded sandstone, broad (100’s of square meters) fractures with low curvature can develop. Outcrops of structureless sandstones (those that have been liquefied by seismic shocks or bioturbated), however, develop fractures with unique polygonal patterns. The outcropping polygons extend about 5m² and are domed (convex-upward). Plumose structures are preserved on the tops of some polygons, demonstrating that polygons formed in the subsurface as opening-mode fractures. Cross-sectional views along road-cuts show interconnected networks of convex-up sandstone lenses. Lenses near the surface are broken by younger, surface-perpendicular joints, resulting in a pattern that is doubly polygonal-- the “elephant skin weathering” of some authors.

 Large (1x1x4 m), ovoid ironstone concretions are locally abundant in the Navajo and formed via oxidation of siderite concretions of the same size and shape. Through-going, vertical joints inside these smooth-surfaced ironstones are lined by iron oxide and pre-date growth and oxidation of the siderite. Cm-scale diffusion of ferrous iron is required for joint linings; this means that the sandstone was water-saturated when oxides accumulated. Slope-parallel fractures that dip >10° cut (and post-date) all iron oxide in the ironstones now exposed along upper canyon slopes and drainage divides. Some ironstones exposed along canyon floors, however, contain horizontal and polygonal fractures that are lined by iron oxide and therefore pre-date siderite oxidation. As the Navajo was uplifted and exhumed, siderite was everywhere oxidized while near the water table. Along upper canyon slopes and drainage divides, precipitation of joint-lining oxides was complete long before ironstones approached steeply sloping land surfaces. Along canyon floors, however, this oxidation was active in the shallow subsurface—the critical zone.