POLYGONAL CRACK SYSTEMS IN THE JURASSIC NAVAJO SANDSTONE: A WEATHERING ANALOG TO MARS

Small-scale (cm- to m-scale) crack systems are widely developed on outcrops of the Jurassic Navajo Sandstone in southern Utah and northern Arizona. The eolian sandstone is relatively porous and varies from isotropic/massive to slightly anisotropic perpendicular to cross bedding. Individual cracks form consistently perpendicular to outcrop surfaces regardless of slope or aspect. Cracks only extend inward several centimeters to decimeters from weathering surfaces and are absent on fresh surfaces exposed by exfoliation and rock falls, indicating that the crack systems are weathering features. Crack systems with well organized 5- to 6-sided polygons form in areas with massive sandstone (from soft-sediment deformation and burrowing) and along some bed-parallel outcrop surfaces, reflecting generation of near-surface, uniform tensile stress. Rectangular crack systems form on steeper outcrop surfaces with bedding anisotropy. Individual high-angle cracks curve to maintain orthogonality to tangential cross beds and locally terminate against bed-parallel cracks. Different polygonal sizes and nested patterns may be controlled by the age and thickness of the weathering “layer”. Many outcrop surfaces with polygonal patterns also display small-scale (cm) domal relief, interpreted to reflect enhanced weathering along and near cracks. Cracks combined with surface-parallel exfoliation results in local spalling of thin polygonal patches. The Navajo Sandstone crack examples are well developed in the desert regime where there are large temperature and moisture fluctuations that influence weathering.

Similar polygonal crack patterns in the Burns formation (e.g., Wopmay rock) imaged by the Mars Exploration Rover (MER) Opportunity suggest a relatively massive, porous host rock subjected to modern or relict weathering on the surface of Mars. Processes that generate near-surface tensile stress include contraction/expansion cycles associated with relatively rapid and large temperature variations, moisture changes (which may also produce partial dehydration of some minerals), and possible near surface dissolution/precipitation of minerals.