SEISMITE INTENSITY STUDIES IN THE WAHWEAP FORMATION, SOUTHERN UTAH: UNRAVELING SYNDEPOSITIONAL FAULT MOVEMENTS AND THE DIFFERENTIAL EFFECTS IN THE HANGING WALL AND FOOTWALL BLOCKS

Soft-sediment deformation structures related to local faulting are abundant in the upper member of the Late Cretaceous Wahweap Formation along the Cockscomb, Grand Staircase-Escalante National Monument, Utah. Two faults crosscut the upper member, dividing it into southern, central, and northern blocks, each a few miles wide. A semi-quantitative scale was used to record seismite intensity in stratigraphic sections measured proximal and distal to the faults in each of the blocks. Higher intensity deformation was recorded on the down-dropped hanging wall than in the adjacent footwall of each of the faults. This analysis demonstrates the utility of seismites to reveal differential soft-sediment deformation in the hanging wall and footwall.

The recognition of sag pond deposits and growth strata in the lowermost portion of the hanging-wall section, adjacent to the southern fault, indicates significant fault movement during initial deposition of the upper member. However, seismite intensity is relatively low in this interval. In contrast, the lower section of the center block contains sand volcanoes and collapse blocks that reveal the most intense soft-sediment deformation in the hanging wall of the northern fault. In both instances fault movement in the lower portion of the upper member was significant, although movement associated with the southern block was apparently less seismogenic. The middle of the upper member, in the central block, displays high seismite intensity that is not found in the sections in the southern block. It is probable that the northern and southern faults were active simultaneously throughout deposition of the upper member, but the northern fault was more seismogenic.

Fault activity appears to have concluded first at the southern fault. Movement along the north fault clearly continued beyond that of the southern fault, as it offsets the younger capping sandstone member of the Wahweap Formation. Seismic intensity increases at the upper member to capping sandstone member contact in the northern block, but is related to motion on a more northerly fault.