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.