FIRE TEMPERATURE RECONSTRUCTION: ANALYSIS OF KAIBAB FORMATION HEAT EXPOSURE DUE TO THE MANGUM FIRE ARIZONA

The June 2020 high-severity Mangum fire ignited 71,000 acres of Ponderosa Pine forest on the Kaibab Plateau, near the North Rim of the Grand Canyon. The rock formation dominantly affected by the fire was the Permian Kaibab Formation, specifically the Harrisburg Member which is composed of varying units of marine evaporites, chert and siliciclastics. This research is designed to experimentally determine the degree of thermal rock-colour alterations in response to controlled exposures of heat and duration to stimulate various intensities of wildfire conditions. Results will be used on rock samples collected within the Mangum Fire burn areas to testify that thermally altered rock colouration can be used to determine wildfire intensity as a metric of interest to assess recent amplified fire regimes across the western United States.

Rock samples from the Harrisburg Member of the Kaibab Formation consists of surface rocks exposed to heat during the Mangum Fire and rocks of the same unit with minimal exposure located in nearby road cuts. Forest-type and burn severity were noted for the surficial rocks collected from the burn areas while an outcrop description and quick facies analysis was documented for the unburned samples.

Experimentally determined colour change of the unaltered rock colour was first recorded using the Munsell colour scheme prior to being baked in a muffle furnace. Heating increments were done in 50 °C intervals up to 700°C. At first the rock samples were held at set temperatures for 1 hour but then additional assessments tested for varying durations of heating to stimulate a diverse set of wildfire conditions. Other observations such as a decrease in HCl effervescences and rock spalling have been noted as potentially related to low grade metamorphism from heat exposure. Laboratory-induced thermal colour alteration will be compared to the rocks collected from high and medium-low severity burn areas of the Mangum Fire.

The broader goals and outcomes of this project are to assess the potential of using thermally altered rock colour alterations as a metric for wildfire intensity. Application to other fire regimes and rock types will need to be experimentally tested to determine the suitability of this technique elsewhere.