AN 8-EVENT, JURASSIC EARTHQUAKE SEQUENCE OF ~1YR DURATION RECORDED BY CLASTIC PIPES AND DIKES WITHIN DUNE CROSS-STRATA, ZION NATIONAL PARK, UTAH

Earthquakes are not isolated events; they occur in sequences. Sequences on or near plate boundaries can include more than 10 events >M_w5, and typically are complete within 1yr. The Lower Jurassic Navajo Sandstone accumulated as a westward-thickening wedge, and reaches ~700 m thick in SW Utah. We mapped more than 1,000 vertical pipes and dikes in uncontorted eolian cross-strata of the Navajo at two small study sites in Zion National Park. Pipes are 2-5 cm in diameter and up to 3 m long; dikes are ~6 cm wide. At each mapped site, a 5 m-wide swath of clustered water-escape structures lies directly above and below a near-horizontal bounding surface. Dikes are restricted to wind-ripple strata that lie above bounding surfaces; pipes are common both above and below bounding surfaces. In map view, most pipes are arranged in lines. Near bounding surfaces, pipes merge upward with dikes that trend parallel to the lines of pipes. Pipes formed in grainflows—homogeneous, well-sorted sand lacking cohesion. Dikes formed in more-cohesive, poorly sorted, wind-ripple strata. As liquefaction began, expanding subsurface grainflows caused spreading within capping, unsaturated wind-ripple strata near the land surface. Pipes rose from the grainflows to the surface, following trends of cracks in the wind-ripple strata. Because wind-ripple strata had low cohesion, a depression formed around each rupture, and ejected sand built upward to a flat-topped surface rather than forming a classic sand volcano. In one 3m² portion of the map area, a cluster of about 20 pipes and dikes, many with truncated tops, record eight stratigraphically distinct seismic events. The large dunes that deposited the Navajo cross-strata likely moved ~1m/yr. When, in response to seismic shaking, a few liters of fluidized sand erupted onto the lowermost portion of the dune lee slope through a pipe, the erupted sand dried and was buried by climbing wind-ripple strata as the large dune continued to advance downwind. We conclude that the pipes and dikes of our study sites are products of a distinct earthquake sequence that lasted about 1 yr and included at least eight >M_w5 earthquakes. Sand intrusions and volcanoes within and between laminae produced during steady dune migration (modern or ancient) can function as faithful recorders of earthquake sequences.