IMPLICATIONS OF A SAND VOLCANO IN THE LATE CRETACEOUS UPPER MEMBER OF THE WAHWEAP FORMATION

A vertical slice through a well-preserved sand volcano was discovered 1.8 meters above the base of the Late Cretaceous upper member of the Wahweap Formation at the Gut in Grand Staircase – Escalante National Monument, Utah. The internal structure of the sand volcano permits the reconstruction of the vertical fluid flow pathways and interpretation of seismogenesis during local faulting.

The 1.3 meter long conduit for the sand volcano cuts across eight beds composed of lithic sandstone. In its lower reaches the conduit is a narrow dike-like feature cross cutting a low-permeability fine-grained sandstone seal. Above the seal, the pipe widens and the pipe edges are more diffuse in high permeability sandstone, whereas the pipe contracts in diameter through lower permeability sandstones. The change in character of the pipe and the structureless sandstone near the pipe indicates that lateral flow to the conduit was greater in the high permeability zones. Within the pipe a winnowed concentration of pebbles 30 cm below the vent indicates that vertical fluid flow was of suffucient velocity to elutriate fine sand forming the surface sand volcano. The internally massive, cone-shaped surface volcano is slightly asymmetrical measuring ~120 cm in apparent diameter and ~20 cm in height. The volcano onlaps the lee face of a fluvial dune.

Modern sand volcanoes often develop with high magnitude seismic events proximal to faults. This sand volcano is in close proximity to a series of normal faults with preserved syntectonic deposits in the Waheap Formation indicating that the fault slip history included some intense high magnitude seismic activity.