SURFICIAL GEOLOGY AND GEOLOGIC HAZARDS OF BONITA AND RHYOLITE CANYONS, CHIRICAHUA NATIONAL MONUMENT, ARIZONA

In cooperation with the National Park Service, GEOCORP program, and the AZ Geological Survey, we conducted surficial geologic mapping in the inhabited canyons of Chiricahua National Monument (CHIR) to better define geologic hazards. CHIR is located in the northern Chiricahua Mtns in the Basin and Range Province in southeast Arizona. Steep mountainous terrain drains to valley floors where nearly all park facilities are located, so flooding, debris flows and rockfalls are all potential hazards. The Chiricahua Mtns have been subject to large wildfires in the past few decades, including the 2011 Horseshoe 2 Fire that burned 90% of the mountain range. The fire burned mixed conifer, oak brush, pinyon, juniper, ponderosa pine, and grasses, leaving hillslopes bare and decreasing soil permeability. The amount of runoff in the downstream canyons increased, generating frequent flooding and endangering infrastructure, including the park entrance, main road, headquarters, visitor’s center, and employee housing. While there were no significant debris flows within the monument, there were many in the mountains surrounding it.

Mapping was done in the field and in ARCMAP using .25-meter LiDAR generated in January 2015 and recent aerial imagery. Axial valley units are associated with Bonita Creek where flooding is the dominant hazard. Pebbles and cobbles dominate the active stream channel. The lowest terrace above the active channel shows signs of recent flooding, whereas higher terraces do not. Bouldery debris flow deposits dominate steep tributary fans. The most active parts of these systems are fairly narrow small valleys incised into older Holocene, fan surfaces. Locally, young fine-grained fans inset against the coarser older fans, indicating that some modern systems do not move larger clasts. Coarse, very poorly sorted, somewhat imbricated boulder deposits cover most of the valley floor along Rhyolite Creek; the modern channel is incised several meters below them. The coarse bouldery deposits suggest that Holocene debris flows dominated erosion and deposition in much of CHIR. Climatic conditions or fire patterns earlier in the Holocene may have generated more, larger debris flows. Studying these paleo-debris flows and their relationship to fire can help inform possible future patterns impacted by climate change.