TRENDS IN LATE CRETACEOUS SEISMITES: UPPER AND CAPPING SANDSTONE MEMBERS, WAHWEAP FORMATION, KAIPAROWITS BASIN, UTAH

The Late Cretaceous upper and capping sandstone members of the Wahweap Formation accumulated in a subsiding foreland basin. This study demonstrates the utility of mapping seismites in detail to separate the influence of regional versus local tectonic activity influencing foreland basin sedimentation.

Soft-sediment deformation structures have been reported in the capping sandstone member. The vertical and horizontal distribution of these soft-sediment deformation structures was systematically studied and their origin was determined to be seismogenic by comparison to well established criteria. A qualitative scale applied to these seismites uses a scheme from 1 to 4 that is based on the predominance of liquefaction versus fluidization processes, and on scales and types of sedimentary structures. In addition, orientations of soft-sediment fold axes were recorded. Construction of a northwest to southeast cross-section demonstrates: 1) a reduction in stratigraphic thickness and percentage of conglomerates to the southeast, 2) the presence of a lower seismite, a middle aseismic zone and an upper seismite zone within the capping sandstone, 3) merging of the lower and upper seismite zones to the southeast, and 4) the utility of seismite zones as a possible correlation tool in nonfossiliferous strata. Isoseismal maps generated from the qualitative scale indicate a decrease in overall intensity from west-northwest to southeast in both the upper and lower seismic zones and along the contact between the upper and capping sandstone members. Local normal fault effects are superimposed on the regional trends. In addition, cumulative fold orientations indicate west-northwest regional epicenters.