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.
