SLEUTHING EPICENTER DIRECTION FROM SEISMITES: CRETACEOUS WAHWEAP FORMATION, COCKSCOMB AREA, GRAND STAIRCASE-ESCALANTE NATIONAL MONUMENT, UTAH

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.