SEISMOGENIC NORMAL FAULTING DURING DEPOSITION OF THE LATE CRETACEOUS WAHWEAP FORMATION, KAIPAROWITS BASIN, UTAH

Often the effect of seismogenic faulting on sedimentation is difficult to isolate. A preliminary examination of the Late Cretaceous Wahweap Formation delineates the possible impacts of seismogenesis on sedimentation. The Wahweap Formation examined is exposed in southern Utah near the northern end of the Laramide East Kaibab monocline. A N50E-striking normal fault, with a trace length of 2 km, displaces Cretaceous strata in the steep limb of the monocline. Patterns of sedimentation and soft-sediment deformation in the vicinity indicate fault activity during Wahweap deposition.

Preliminary analysis of the Capping Sandstone Member of the Wahweap Formation indicates the presence of fluvial and eolian environments. Convoluted beds contain dish and pillar features and form distinct, laterally continuous horizons. These soft-sediment deformed beds signify major disruption or liquefaction and are best interpreted as seismites. Thirty measurements of trough cross-beds and foresets establish a paleocurrent, trending S69E and crossing the fault trace, during deposition of the upper Wahweap Formation. Clast counts in fluvial conglomerates within the uppermost 5 m of the Wahweap Formation show major changes across the fault trace. North of the fault on the footwall, chert composes 62% of clasts, while south of the fault on the hanging wall, chert accounts for only 37%. Mudstones peak at the fault, encompassing 51% of all clasts, but the percentage declines to 33% within 1 km south of the fault. The discovery of breccias adjacent to and onlapping the fault scarp on the down-dropped hanging wall indicates active faulting during deposition of fluvial and eolianite systems in the Capping Sandstone Member. Distorted beds, a terrestrial environment, clast distribution, and landslide breccias support seismically active normal faulting during Wahweap deposition.

The cause of faulting has not yet been determined, but the presence of Cretaceous normal faults on the steep limb of the East Kaibab monocline may signal the initial onset of Laramide tectonism in the southern Utah region.