Syndeformational
origin for internal laminations of spectacular seismogenic load
structures in the Late Cretaceous Wahweap Formation, Grand
Staircase-Escalante National Monument, Utah
Wizevich,
Michael
C., Simpson,
Edward
L., Hilbert-Wolf,
Hannah L., and Tindall,
Sarah E., 2009, Syndeformational
origin
for
internal
laminations
of spectacular seismogenic load structures in the Late Cretaceous
Wahweap Formation, Grand Staircase-Escalante National Monument,
Utah [abs]: 2009 Portland GSA Annual Meeting (18-21 October
2009), Geological Society of America Abstracts with Programs, Vol.
41, No. 7, p. 121. .
The
Wahweap Formation, informally subdivided into lower, middle, upper
and capping sandstone members, accumulated in fluvial systems within
the active Cordilleran foreland basin of south central Utah. In
Grand Staircase-Escalante National Monument (GSENM), the East Kaibab
monocline is offset by 3 normal faults with features including
growth strata, sedimentary (colluvial) breccia of cannibalized
sediment, soft-sediment deformation, and sag pond deposits, which
indicate that the faults were active during Wahweap deposition.
In the Bull Flat area of GSENM, adjacent to a fault, large-scale,
pendulous load structures are preserved over an area >104 sq. m.
The loads formed because of an inverted density profile,
earthquake-generated liquefaction, and growth of irregularities (a
Rayleigh-Taylor instability) on the sandstone-sandstone contact of
the tan, lithic upper and white, quartzose capping members. Load
structures consist of 2 types: bulbous megaloads and wedge-shaped
fracture loads (‘icicles’ in cross-section); both forms suggest that
deformation along the interface was dominated by sinking of the
capping member relative to diapiric rise of the upper member. Wedge
loads appear both as a series of ~0.5 m spaced parallel features,
< 1 m tall, and along the extremities of megaloads or other wedge
loads (forming ‘arrowheads’). All wedge loads contain
well-developed, medial shear deformation bands that thin downwards
through the wedge, but extend into the upper member.
In proximity to the loads the upper member sandstone is featureless
except for a bed of mudstone rip-up clasts, 4.5 m below the contact,
which appears to define the lower limit of megaload movement.
Internally throughout all loads are well-defined laminations,
oriented parallel to the outside form of the loads and asymptotic to
deformation bands. Rare crosscutting of subhorizontal laminations
(depositional?) by internal laminations near the edge of loads,
suggests a secondary syndeformational origin for internal
laminations. Thin sections reveal laminations are defined by
elongate grain alignment and alternating packed/porous laminae.
Possible processes for the formation of the laminations include
fluid generation, dilatant or shear granular deformation, or
vibrofluidization; their origin remains enigmatic.