SYNDEFORMATIONAL ORIGIN FOR INTERNAL LAMINATIONS OF SPECTACULAR SEISMOGENIC LOAD STRUCTURES IN THE LATE CRETACEOUS WAHWEAP FORMATION, GRAND STAIRCASE-ESCALANTE NATIONAL MONUMENT, UTAH

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.