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.
