27 Structural Geology and Tectonics I

Friday, 21 March 2008
8:00 AM - 12:00 PM

PREFERENTIAL DEVELOPMENT OF DEFORMATION BANDS WITHIN GRAINFLOW TONGUES OF THE NAVAJO SANDSTONE ALONG THE EAST KAIBAB MONOCLINE, UT-AZ

Paper number: 27-2
Program: Technical Sessions
Time: 8:20 AM – 8:40 AM

Authors

Abstract

Deformation bands are thin (~ 1mm), semi-brittle shear zones that accommodate fault offset. They are especially common in porous, quartzose eolian sandstones of the Colorado Plateau. At The Dive, a steep outcrop of Jurassic Navajo Sandstone along the East Kaibab Monocline near the Arizona-Utah border, subtle differences in sandstone characteristics have exerted a strong influence on deformation band development. Cross-strata within the Jurassic Navajo Sandstone are composed of interbedded grainflow (avalanche) tongues and wind-ripple deposits. Grainflows are highly porous, well-sorted, and composed of well-rounded, medium sand; wind-ripple deposits contain mm-scale laminae composed of well-rounded medium grains and very-fine sand to coarse silt. Within the synclinal hinge of the monocline at The Dive, deformation bands are present only within the uppermost 50 m of the formation. Grainflow tongues contain very abundant, closely spaced (1-20 cm) deformation bands, but in directly adjacent wind-ripple deposits, bands are much less common and more widely spaced (30-300 cm). Offset along these deformation bands is typically imperceptible; some bands show mm-scale offset. Bands are oriented N-S, parallel to the trend of the monocline, and dip 75-90°. In thin section, abundant shattered quartz grains delineate deformation bands. In outcrops consisting exclusively of wind-ripple deposits, deformation bands are absent, and instead systematic jointing is prevalent.

A compaction (burial) origin for the bands is unlikely because: a) they are restricted to the uppermost Navajo; b) they strike parallel to the monoclonal hinge; and c) there is clear evidence of cataclasis. The bands formed after lithification, during Laramide deformation, and were localized in the monoclonal hinge by small compressional and extensional stresses near the mechanical boundary at the top of the Navajo. The greater abundance of deformation bands within grainflow tongues relative to wind-ripple deposits reflects preferential formation in better-sorted sandstone with larger pore spaces.