SYNTECTONIC SEDIMENTATION IN THE EAST KAIBAB MONOCLINE: EVIDENCE FOR THE TIMING OF THE ONSET OF LARAMIDE DEFORMATION IN SOUTH-CENTRAL UTAH

The East Kaibab monocline on the Colorado Plateau formed by faulting and folding during the Late Cretaceous and early Tertiary Laramide orogeny. Two northwest-dipping faults, each with ~0.5 km of right-lateral separation, offset Cretaceous strata in the steep limb of the monocline, where it trends northward along the western margin of the Kaiparowits basin in southern Utah. Aerial photographs and maps of the East Kaibab monocline show apparent differences in thickness of the upper member of the Cretaceous Wahweap Formation across each fault trace. Changes in true thickness across the faults would indicate that the area experienced syntectonic sedimentation during Cretaceous faulting, and that the faults were originally southeast-dipping normal faults associated with the onset of monoclinal uplift. If this hypothesis is correct, then sediments should increase in thickness from north to south across each fault trace. Alternatively, it is possible that faulting occurred after the deposition of sediment, and differences in outcrop width represent dip variation and structural position within the monocline. To evaluate the importance of true thickness variation versus structural position, we collected GPS locations and bedding orientations across the upper member along three transects: 1) north of the faults; 2) between the faults; and 3) south of the two faults. Data were used to construct cross sections showing dip variation and structural position within the steep monoclinal limb. Precise measurement of the upper member thickness revealed that the thinnest sediment package (92m) lies north of the faults. Between the faults, the upper member is 132m thick, and south of the southern fault it is 262m thick. Our data confirm that faulting was active during the deposition of the upper member of the Wahweap Formation, causing thickness variations across normal faults immediately before monoclinal uplift. This newly-recognized syntectonic sedimentation associated with the East Kaibab monocline pinpoints the timing of onset of Laramide deformation on the western margin of the Colorado Plateau.