SOFT SEDIMENT DEFORMATION AND INJECTITES OF THE JURASSIC CARMEL FORMATION, SOUTHERN UTAH: SEDIMENTARY RECORDS OF STRONG GROUND MOTION

Numerous, well-exposed sandstone injectites or clastic pipes in fluvial to sabkha facies of the Jurassic Carmel Formation of southern Utah record evidence of extensive liquefaction from strong ground motion. Nearly all the pipes occur within the Paria River Member of the Carmel Formation and rarely within the Crystal Creek and Windsor members. Approximately 900 separate injectities of tens of centimeters to meter scales were mapped in three separate sites covering a 6.5 by 5.5 km area. The pipes crosscut predominately sandstone with some thin mudstone beds, intruding upwards as much as 25 m in vertical extent. They typically show a concentric coarsening outward in grain size and concentration of lithics towards the edges in a rind. The Paria River Member contains several ash beds, fluvial transported volcanic bombs, and volcaniclastic rich beds indicating contemporaneous volcanic activity likely associated with the Cordilleran arc system to the southwest.

The pipes have a complex diagenetic history showing preferential fluid flow via the vertical fluid conduits, particularly where the grain size is coarser around the perimeter. Bleaching rinds record the movement of reducing fluids, where there is common carbonate cement, localized iron oxide cement, and some large sparry calcite crystals.

These clastic pipes form under a unique set of circumstances including a high water table, which saturates a body of loose unconsolidated sand beneath a lower permeability layer. The dense field of pipes with broad vertical and lateral extent suggests large Jurassic earthquakes that affected the liquefaction-susceptible sabkha sediments. Repacking of the grains increases the pressure within the system and liquefies the sand causing it to scour upwards into pipes. The liquefaction likely occurred in one major event sourced from local fluvial sandstones within the Crystal Creek Member. Although seismites have been commonly recognized in the stratigraphic record, these pipes are closely tied to the complex interplay between sabkha and fluvial depositional environments along with a high water table, with pipe alignment approximately parallel to the paleoshoreline.