A new geologic map of the Twin Rocks 7.5 minute quadrangle primarily located within Capitol Reef National Park, south-central Utah, provides stratigraphic and structural detail not previously available. This map has also been instrumental in understanding the evolution and development of fluvial terraces associated with Sulfur Creek and the structural geology of the backlimb of the Miners Mountain uplift. Nine bedrock stratigraphic formations and eight types of Quaternary deposits were mapped throughout the quadrangle. Bedrock stratigraphy ranges in age from Permian to Jurassic. New details absent on previous geologic maps include members of the Chinle and Moenkopi Formations and the Jurassic Page Sandstone, a stratigraphic unit herein separated from the Navajo Sandstone. Terraces associated with Sulfur Creek record the central pathway of ancient streams rather than the lateral extent of the floodplain. Volcanic boulder-rich terrace deposits were likely created as stream channels were clogged with volcanic boulders and subsequently abandoned. The boulder-fill effectively armored the underlying softer bedrock. As the stream moved away from the abandoned, boulder-filled channel, it eroded and downcut into the adjacent softer mudstone bedrock, rather than eroding through the more resistant boulder alluvium. Thus, the abandoned boulder-filled channel becomes elevated relative to the stream. This inverted topography is preserved as elevated fluvial terrace deposits. This style of preservation of linear terraces developed over a broad area is in contrast to nearby terraces along the Fremont River which are preserved as "steps" cut into the resistant sandstones of the Glen Canyon Group along the Waterpocket Fold. These terraces have been used to identify changes in the location of Sulfur Creek through time. Kinematic analysis of structures in the backlimb of the uplift show a principle compressive stress orientation nearly perpendicular to the uplift axis and rotated 30° counter clockwise from the stress indicated by deformation bands measured in the forelimb. These data suggest that stress transmitted through the basement is partitioned and rotated in the backlimb, likely due to decoupling and differential slip in strata with low shear strength. Such decoupling would allow the stress to be rotated perpendicular to the resisting fold axis, rather than parallel to the far-field stress transmitted through the basement. Sandbox models produced in this study display boundary perpendicular structures similar to those measured in the backlimb of the Miners Mountain uplift.



College and Department

Physical and Mathematical Sciences; Geological Sciences



Date Submitted


Document Type





Miners Mountain, geology, structural geology, kinematic analysis, kinematics, backlimb, strain partitioning, fluvial architecture, Capitol Reef, Capitol Reef geology, Sulfur Creek, clog and abandon



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Geology Commons