Abstract

Three distinctive fault sets offset the rocks of the northeast-trending San Rafael Swell, east-central Utah. Faults in all three sets are much alike; all are normal, high angle, linear to slightly sinuous, and range in length from several hundred feet to as much as 10 mi (15 km). Many are paired and form grabens. (1) The most prominent set, confined essentially to the crest and east flank of the Swell, consists of faults that trend northwest. This set appears to be the northwest extension of similar faults in the Paradox basin of southwestern Colorado and southeastern Utah. (2) The second less abundant set consists of a series of west-striking faults confined chiefly to the north end of the Swell. These faults may be part of the northwest-striking fault set, for northward, each fault within the northwest-striking fault set gradually changes strike to a more westward orientation. Near the north end of the Swell the faults trend almost due west. (3) A third set, consisting of faults that strike north and northeast, is confined to the west flank of the Swell. These faults may be related to similar faults that break the crest of the Wasatch Plateau, west of the Swell. Alternatively, they may reflect extensional forces; if so, they are tectonic features wholly unrelated to salt dissolution.

The origin of the faults sets is uncertain. Collapse, resulting from solution of salt (halite) of different ages, may have played some role in their development. The northwest-striking fault set most likely stems from solution of salt of the Paradox Formation (Middle Pennsylvanian age) of the Hermosa Group. The west-striking faults may also stem from solution of Paradox salt. The north-striking faults, if related to the faults along the crest of the Wasatch Plateau, may reflect solution of evaporites, possibly salt, contained within the the Carmel Formation or its correlative, the Arapien Shale, of Middle Jurassic age.