Syndeformational origin for internal
laminations of spectacular seismogenic load structures in the Late
Cretaceous Wahweap Formation, Grand Staircase-Escalante National
Michael C., Simpson,
Edward L., Hilbert-Wolf,
L., and Tindall,
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. .
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.