JOINTING OF CATACLASTIC DEFORMATION BANDS IN A THREE-DIMENSIONAL MECHANICAL NETWORK: OBSERVATIONS FROM THE WATERPOCKET FOLD, SOUTHERN UTAH

Subseismic structures like joints and deformation bands are significant considerations in hydrocarbon reservoirs because spatial variations in their characteristics can affect the tortuosity of flow. Cataclastic deformation bands at Oak Creek Canyon on the Waterpocket Fold represent conjugate strike-slip shear zones with internal Riedel shear geometry, creating a network of stiff, tabular zones in a wide range of orientations. The exposure presents an opportunity to examine relationships among (1) deformation bands acting as stiff mechanical units in variable orientations, (2) far-field tension, and (3) joints within the deformation bands.

At Oak Creek Canyon a persistent joint set crosscuts both deformation bands and surrounding Navajo Sandstone indicating the direction of far-field tension responsible for jointing. Short, discontinuous joints crosscut deformation bands and terminate at boundaries between bands and weaker host sandstone. Orientations of the discontinuous joints cluster parallel to the persistent joint set so all are attributed to the same far-field stress. Band-confined joints parallel the persistent joints, and are perpendicular to the bands they cross, where deformation band strike parallels the tension direction inferred from persistent joints.

However, at Oak Creek Canyon most deformation bands are oblique to the far-field tension associated with jointing because of the complex conjugate Riedel geometry of the bands. The orientations of discontinuous, band-confined joints vary systematically as a function of deformation band strike. Bands that strike clockwise from the inferred tension direction contain joints striking slightly clockwise relative to the persistent joint set, and bands counter-clockwise from the tension direction contain joints oriented slightly counter-clockwise from the persistent joint set. Where bands are oblique to the far-field tension direction the band-confined joints are oblique, rather than perpendicular, to deformation bands.

Observations of the jointed deformation band network at Oak Creek Canyon suggest a more general relationship in which stiff layers oriented oblique to a subsequent stress state may develop joints that are oblique both to mechanical layers and to far-field stresses.