CLASTIC PIPE SOURCES AND STRATIGRAPHIC RELATIONSHIPS: EVIDENCE FOR MULTIPLE LIQUEFACTION EVENTS IN THE MIDDLE JURASSIC CARMEL FORMATION, SOUTHERN UTAH

The Jurassic Carmel Formation of southern Utah provides exceptional exposures of clastic pipes (injectites) that record major events of liquefaction/fluidization induced by strong ground motion. This mapping and characterization study of ~ 900 small-scale vertical pipes in a 36 km² area shows six important traits. 1) All 900 clastic pipes ~ 10 cm to 7m in diameter and up to 9 m tall occur solely within the Paria River Member. 2) The pipes are predominately composed of upper fine- to upper medium-grained (sometimes coarse) quartz sandstone with volcanic granules/pebbles, and rip-up clasts of shale and occasional sandstone. Pipes are encased in sabkha host rock of primarily fine- to medium-grained quartz sandstone with some fluvial volcaniclastic and conglomerate beds. 3) Internally, the pipes display concentric structure with coarser-grained rinds of higher porosity and permeability on the outer edges. 4) In eight percent of the pipes there are vertically oriented, crosscutting pipes that are preferentially located in the outer area of the original pipe. 5) Small, synsedimentary reverse, normal, and antithetical faults in the host rock are common, along with undulating and contorted bedding. Collectively, these features demonstrate evidence for both brittle and ductile sediment behavior. 6) Pipe textures and internal characteristics vary over short vertical distances, sometimes in response to local changes in the host bed.

Pipe characteristics suggest formation during a series of rapid, violent liquefaction events where sediment was expulsed upward, sourced from local sandstone layers. The source bed was likely pressurized during strong ground motion due to an overlying shale confining layer. The brittle and ductile behavior of the host rock as well as pieces of entrained host rock in the pipes suggests moderate sediment cohesion at the time of emplacement. The various styles of synsedimentary faulting reflect volume changes due to upward liquefied injection and initial loss of grain contacts, which swells sediment volume, balanced by downward collapse and dewatering, which reduces sediment volume. Coarser-grained outer rims of initial pipes served as porous conduits for successive generations of water-saturated sediment, potentially from a strong earthquake and related aftershocks.