Proximal
seismites in the Upper Cretaceous Upper and Capping Sandstone
Members of the Wahweap Formation in Grand Staircase-Escalante
National Monument, Utah
Wolf, Hannah
L., Tindall,
Sarah E., Simpson,
Edward
L., Simpson, Wendy S., Bernard, Jonathan, and Jenesky, Timothy,
2007, Proximal
seismites
in
the
Upper
Cretaceous
Upper and Capping Sandstone Members of the Wahweap Formation in
Grand Staircase-Escalante National Monument, Utah [abs]: 2007
GSA Denver Annual Meeting (28–31 October 2007), Geological Society
of America Abstracts with Programs, Vol. 39, No. 6, p. 631.
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.