GEOHAZARD ASSESSMENT OF A POTENTIAL RIM COLLAPSE AT DEVILS KITCHEN: RISK TO VISITORS AT AN ACTIVE SINKHOLE NEAR SEDONA, NORTHERN ARIZONA

Much of Northern Arizona stratigraphy is capped by shales and sandstones and underlain by karst. This

stratigraphic sequence allows for sinkhole formation via solution collapse breccia pipes (SCBP).

Locally observed sinkholes of this nature are both relatively young and highly active. The

potential of a rim collapse will be assessed for Arizona’s most active SCBP: Devils Kitchen of

Sedona, AZ, a major tourist attraction. Easy accessibility is offered via jeep tours or a short hike,

making this destination a favorite. Devils Kitchen is estimated to have been formed in the early

1880’s, while the most recent major collapse events occurred in late 1989 and again in 1995. If a

collapse event occurred while visitors were along the rim, the result could be fatal. Devils

Kitchen’s surface opening is measured at 45 by 23 m with a depth ranging from 12 to 18 m. The

surface opening and chamber walls are exposed in the Permian-aged Schnebly Hill formation: a

massive cliff-forming sandstone with interbedded siltstone. We identified seven fracture-

delineated slabs for study. Along those slabs, we measured the surface fracture orientation and

width over time to identify fracture growth. Five trips were made to the field site over six

months to measure fracture widths. A photogrammetric technique, Structure from Motion, was

then utilized to construct a three-dimensional model of Devils Kitchen to better analyze spatial

relationships and geometries of fracture sets. Preliminary data suggests that three of seven

slabs around the rim of Devils Kitchen may be showing signs of movement. Slab 1(A) was

measured at 113.0 +/- 0.7 mm and showed an increase to 115.5 +/- 0.8 mm. Slab 2 was

measured at 19.4 +/- 0.3 mm and showed an increase to 20.4 +/- 0.5 mm. Slab 3 was measured

at 59.6 +/- 0.8 mm and showed an increase to 64.9 +/- 1.0 mm. Additionally, a more distal

control fracture was initially measured to be 24.2 +/- 0.3 mm and the greatest deviation from

the initial measurement was 24.8 +/- 0.4 mm; evidence for fracture stability. Monitoring of this

site will continue through April 2020 to further examine rates of fracture growth and to

interpret the rim collapse geohazard associated with Devils Kitchen. We suggest that further

monitoring is continued and that proper signage is established at Devils Kitchen to ensure

public safety as the top priority.