STRUCTURAL EVIDENCE FOR SYNTECTONIC SEDIMENTATION WITHIN THE CRETACEOUS WAHWEAP FORMATION, KAIBAB UPLIFT, UTAH

The East Kaibab monocline on the Colorado Plateau trends NNE and extends from the Grand Canyon in northern Arizona to near Bryce Canyon in southern Utah. Near the northern terminus of the monocline two curved faults, each more than 2 km in strike length, display approximately 0.5 km of apparent right-lateral offset of east-dipping Jurassic and Cretaceous strata exposed in the steep limb of the monocline. The faults strike ~N50E and dip 60W-80E; previous workers interpreted them as reverse, right-lateral faults that formed late during Laramide development of the East Kaibab monocline.

The Cretaceous Wahweap Formation appears to be approximately three times thicker on the southeastern side of each fault than on the northwestern side. Apparent thickness of the Wahweap is greatest immediately adjacent to each fault, and becomes thinner with distance south of the faults. In order to determine whether the apparent thickness change is an illusion caused by a shallowing of bedding dip in the steep limb of the monocline, or represents thickening possibly resulting from deposition during fault movement, we measured strike and dip of bedding along three transects at distances between 0.5 km and 3 km south of one of the curved faults. The upper and lower stratigraphic boundaries of each transect correspond with distinctive marker beds in the Cretaceous stratigraphy. Cross sections constructed using transect data show that thickness of bedding adjacent to the fault is between 400-520 ft, whereas 3 km south of the fault the thickness is180-190 ft. Different subsurface interpretations along each transect result in slightly different calculations of sediment thickness, producing the thickness ranges noted above, but constraints provided by surface data indicate that a change in bedding orientation is not sufficient to account for the apparent thickening of the Wahweap Formation adjacent to the fault. Based on these new data, we hypothesize that stratigraphic thickening on the southeastern side of each fault represents syntectonic sedimentation during the Cretaceous, and that the steeply dipping, curved faults formed as listric normal faults before being rotated to their current orientations in the steep limb of the East Kaibab monocline.