CONFUSING SEISMITES IN EOLIAN SANDSTONES WITH TRACE FOSSILS

Broad exposures of well-defined wind-ripple laminae and avalanche cross-strata within Paleozoic and Mesozoic eolian sandstones provide exceptional opportunities to study soft-sediment deformation. Even with these high-quality outcrops, however, differentiating physical from biogenic structures can be difficult. The Escalante Member of the Entrada Sandstone (south-central Utah) contains abundant tracks and burrows. One type of structure that Loope (2008) interpreted as a burrow is typically ~2-4 cm in diameter, up to a meter long, and has a cone-shaped terminus that is truncated by wind-ripple laminae. Because these structures terminate at numerous, closely spaced stratigraphic horizons, Loope concluded that brief burrowing intervals were interrupted by similarly brief intervals of dune migration.

Meter-scale folds and faults are common in many eolian sandstones, and likely record dune collapse toward zones of liquefaction beneath interdunes with high water tables (Horowitz, 1982; Bryant & Miall, 2010). Soft-sediment folds and faults are rare in the Navajo Sandstone of Zion National Park, but vertical pipes <2-6 cm in diameter, and up to ~3 m long are locally abundant. Rows containing up to 20 even-spaced pipes record water escape localized by coseismic, lateral spreading. These pipes and associated dikes expand and terminate within cross-strata, indicating that water repeatedly escaped to sloping dune surfaces and built many distinct generations of sand blows. Some sand blows formed at least 1.5 m above the water table. The Zion cross-strata lack signs of life; the pipes and dikes record earthquake sequences in which many events exceeded M_w5 (Loope et al. 2013). The absence of meter-scale folds and faults suggests that a low water table prevented dune collapse.

The long rows of vertical pipes in the Navajo (a new observation) drove the seismic interpretation at Zion. In retrospect, the Entrada structures with conical termini that are present at many close-spaced stratigraphic intervals are probably also seismogenic. The abundance of traces in Entrada outcrops and the absence of obvious evidence for liquefaction led to misinterpretation of the Entrada structures.