A DETACHMENT FAULT on THE SOUTHERN FLANK OF MOUNT HILLERS; EVIDENCE FOR THE LACCOLITH MODEL

Past field studies of the Henry Mountains of southeastern Utah have focused upon characterizing intrusive growth and deformation mechanisms (Gilbert, 1877; Hunt, 1953; Jackson and Pollard, 1988). Recent observations on the Cretaceous-aged Dakota sandstone beds south of Mt. Hillers provide evidence regarding the laccolith versus stock controversy. Channels, ripple marks and cross beds in the sandstones indicate that these beds which dip steeply to the south are overturned. We interpret these as evidence of a detachment fault between the Dakota Sandstone and the underlying Brushy Basin Member of the Morrison Formation. Such a fault is consistent with the laccolith model in which the intrusion was emplaced horizontally and inflated vertically.