Seismogenic
normal faulting during deposition of the late Cretaceous Wahweap
Formation, Kaiparowits Basin, Utah
Jenesky, Timothy A., Tindall, Sarah E., Scalise,
Renee L., Simpson,
Edward
L., and Simpson, Wendy S., 2005, Seismogenic
normal
faulting
during
deposition
of the late Cretaceous Wahweap Formation, Kaiparowits Basin, Utah
[abs]: Rocky Mountain Section - 57th Annual Meeting (May
23–25, 2005), Geological Society of America Abstracts with Programs,
Vol. 37, No. 6, p. 10.
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.