THE LANDSLIDE THAT CAUSED A FOREST FIRE; A STUDY OF AN UNUSUAL THERMAL ANOMALY

On August 21, 2004, a landslide started a three-acre fire in a remote area of the Dick Smith Wilderness of the Los Padres National Forest, California. The fire was quickly extinguished but subsurface temperatures in excess of 550oF and a sulfur odor remained. Research by National Forest, USGS and UC Santa Barbara geologists and hydrologists suggests a possible cause for the thermal anomaly is the exothermic oxidation of iron sulfide minerals (pyrite and marcasite), which then burns disseminated organic material found in the local shale.

The thermal anomaly is centered in a three-acre, highly disrupted slide deposit in a recently active, seventeen-acre rotational/translational landslide. The landslide is within the Eocene shales and siltstones of the Juncal Formation, a distal submarine fan deposit that contains traces of iron sulfides, organic material, and carbonate (Dibblee, 1966; Onderdonk, 2003).

A “Drive Probe” was used to install three monitoring wells, where the highest temperature recorded was 584oF at 11.5' depth in December 2004. Subsequent sampling trips to the slide suggest an average cooling of approximately 30oF degrees since December 2004. Our data indicates the heat sources are extremely localized in areas of <1 m2 and <10 meters deep, which often experience high canyon winds. Springs in proximity to the landslide are cold, commonly precipitate gypsum, and exhibit dissimilar chemistry to the closest hot springs. Soil, rock, gas, and water samples are currently being analyzed at the USGS, Menlo Park using a wide variety of geochemical and electron microscopy techniques in order to test hypotheses regarding the cause of the thermal anomaly.