Abstract The geometry, orientation and distribution of deformation bands and fractures in eolian sandstones, siltstones and shales of the San Rafael Desert and Moab Fault area have been investigated. The results show that deformation bands, which are cataclastic in eolian sandstones and disaggregation structures in siltstones, are unevenly distributed throughout the damage zone in the form of individual bands, deformation band zones and deformation band clusters. The density of bands increases with increasing grain size. In thin (<3 m) eolian sandstones deformation band frequency is significantly lower than in thicker eolian sandstones, whereas above this thickness the frequency seems not to be related to layer thickness. Furthermore, large faults do not develop higher concentrations of deformation bands. Somewhat simplified, this suggests that damage zone growth occurs by expansion into its hanging wall and footwall. Still, the highest concentrations of deformation bands occur close to the main fault, which is of importance when considering their effect on fluid flow. Their general fault-parallel conjugate arrangements favour intra-damage zone flow parallel to rather than perpendicular to the fault.

ABSTRACT The Colorado Plateau in the southwestern United States is within the Paleozoic transcontinental arch, an area of thin, cratonic strata. The plateau was broken by latest Mississippian to early Permian Ancestral Rocky Mountain orogenesis, which produced bedrock uplifts that influenced lower Mesozoic sedimentation before Jurassic burial. Clastic sediments shed from uplifts interfinger with eolian Permian strata ultimately derived from eastern Laurentia. Triassic and Jurassic strata of the Colorado Plateau are here divided into five depositional systems, each representing a different sedimentary and tectonic setting and forming stratal associations referred to as “deposystems.” The five deposystems, which largely but not entirely correspond to formation or group names, were deposited during northward continental drift from tropical latitudes (fluvial, tidal, and nearshore marine Moenkopi and fluvial Chinle) through desert latitudes (the erg-dominated Glen Canyon and San Rafael) to temperate latitudes (fluvial Morrison). Paleomagnetically determined paleolatitudes, corrected for inclination shallowing due to postdepositional sediment compaction, place the Glen Canyon and San Rafael eolianites firmly within expected latitudes for desert environmental conditions. Lower Triassic strata of the Moenkopi deposystem form a westward-thickening wedge of fluvial and shallow marine strata and are overlain by entirely fluvial strata of the Chinle deposystem. Both contain 240–280 Ma detrital zircon populations derived from the east Mexico magmatic arc, but more northern Chinle fluvial deposits contain a higher fraction of zircons derived from Paleozoic, Neoproterozoic, and Grenville provinces in eastern Laurentia. Westward thickening of Moenkopi strata is attributed to subsidence in the proforeland basin of the east-vergent Sonoma orogeny in central Nevada, whereas accommodation space for Chinle sedimentation was provided by dynamic subsidence above the upper Triassic subduction zone behind the newly established Cordilleran magmatic arc to the southwest. Overlying, largely Jurassic Glen Canyon and San Rafael deposystems are dominantly eolian. Detrital-zircon geochronologic analysis indicates that eolian sands were derived largely from eastern Laurentia. Interbedded marginal marine, ­lacustrine-sabkha, and fluvial strata have been associated with regional unconformities, but evidence for such unconformities is here regarded as indicating facies transgressions without development of plateau-wide unconformities or disconformities. Upper Jurassic northward continental drift carried the plateau out of the desert belt and into the zone of prevailing westerly winds. This coincided with a flare up of magmatism in the Cordilleran magmatic arc, leading to transgression of Morrison fluvial sediments over erg deposits of the San Rafael deposystem. Eastward dispersal of Morrison sediments marked the initiation of the Cordilleran orogen as the dominant topographic feature of the plateau region.

Abstract Location . The San Rafael Swell is a major domal uplift in the northwestern Colorado Plateau and dominates the structure of east-central Utah. The San Rafael Reef site is on its eastern edge, approximately 13 mi (21 km) west of Green River, in Emery County, Utah (Fig. 1). 1–70 cuts more or less directly across the structure (Rigby and others, 1974). Accessibility . Roadcuts are on right-of-way controlled by the Utah State Highway Commission. Land along side the route is generally controlled by the U.S. Bureau of Land Management.Alluvial flats along the San Rafael River are on private land. Significance of Site . The San Rafael Swell is characteristic of many asymmetric large uplifts of the Colorado Plateau; and itseastern margin, the San Rafael Reef, is representative of monocline of the province. A complete stratigraphic sequence from the Permian White Rim-Cedar Mesa Sandstone up through Triassicand Jurassic rocks to the lower part of the Cretaceus Mancos Shale Fig. 2 is exposed along the highway. These rocks accumulated in a wide variety of depositional enviroments, ranging from marine carbonate and elastic environments to deserts(Rigby, 1978).