PROXIMAL SEISMITES IN THE UPPER CRETACEOUS UPPER AND CAPPING SANDSTONE MEMBERS OF THE WAHWEAP FORMATION IN GRAND STAIRCASE-ESCALANTE NATIONAL MONUMENT, UTAH

Localized seismogenic normal faulting affected sedimentation near the top of the upper and the base of the overlying capping sandstone members of the Late Cretaceous Wahweap Formation in Grand Staircase-Escalante National Monument. This study recognizes and describes features associated with the localized faulting, including sag pond deposits, megaloads, and “icicle” and “arrowhead” structures. The genesis of these structures was controlled by the initial interbedded lithologies, such that variations in rheological contrast produced different seismogenic structures. Sag pond deposits developed after fault movement generated an internally draining pond. Subsequently, the pond filled with rheologically weak organic-rich mudstones,and these were later intruded by seismically mobilized sand producing dikes and sills within the mudstones. Megaload structures developed as the capping sandstone foundered into the rheologically stronger upper member where the latter is dominantly composed of mudstone-clast intraformational conglomerate. Internally the capping sandstone megaloads lack primary stratification (S1) but are characterized by secondary lamination (S2) that most likely was generated during seismic activity. Where the capping sandstone is in contact with even stronger sandstones of the upper member, “arrowhead” and “icicle” structures are developed and are associated with the strongest rheology. “Icicle” structures resemble Raleigh-Taylor structures with the capping sandstone forming the “icicle” or pointed structure indicating that the upper member was of higher viscosity than the capping sandstone. Right-lateral and left-lateral shear zones are present in the axial regions of “icicles” and “arrowheads” and S2 lamination is bent into parallelism with the shear zones. The systematic orientations of these shear zones parallel the orientations of local faults, and are consistent with Laramide stress fields.