Sleuthing epicenter direction from
seismites: Cretaceous Wahweap Formation, Cockscomb Area, Grand
Staircase-Escalante National Monument, Utah
Wolf, H.,
Simpson,
Wendy
S.,
Simpson,
Edward
L., Tindall,
Sarah, Bernard, J.J., Jenesky, T.A., Orsulak,
M., and Tester, E.W., 2006, Sleuthing
epicenter
direction
from
seismites: Cretaceous Wahweap Formation,
Cockscomb Area, Grand Staircase-Escalante National Monument, Utah
[abs]: Rocky Mountain Section–58th Annual Meeting (17–19 May 2006),
Geological Society of America Abstracts with Programs, Vol. 38, No.6,
p. 6.
The upper
and capping sandstone members of the Upper Cretaceous Wahweap
Formation, within the Cockscomb area of Grand Staircase-Escalante
National Monument, Utah, contain discrete zones of soft sediment
deformation. Examination of types and distribution of soft sediment
deformation indicates these features satisfy the criteria for seismic
origin. These criteria include 1) laterally continuous horizons, 2)
deformation horizons separated by non deformed zones, 3) soft sediment
deformation structures having experimental analogs, 4) association with
a seismically active area (sediments were deposited during faulting),
and 5) geographic variation in intensity.
Detailed examination of seismite fold axes shows a systematic change in
orientation along the Cockscomb. From north to south, the mean
orientation of fold axes rotates progressively from east-trending to
south-trending. A qualitative intensity scale of soft sediment
deformation based on field observations and experimental data from
literature was constructed. This qualitative scale varies from 0 (no
deformation) to 5 (intense disruption of stratification). From north to
south, a progressive change from a 5 to 2 was observed along the
contact between the upper and the capping sandstone members.
When the trends of fold axes, coupled with changes in intensity ratings
are plotted on a map of the field area, they indicate a radial pattern
with a point of origin north and west of the study area. Assuming the
fold axes are parallel to the direction of seismic wave propagation,
this pattern is consistent with the geometrical spreading of earthquake
waves originating from epicenters north and west of the study area.
