AZGS at work for Arizona: Identifying landslide deposits and hazards along the I-17 corridor from Anthem to Flagstaff, AZ

PART I. Landslides of the Agua Fria Canyon/Black Canyon City area

This is the first of several blog posts profiling the work of AZGS geoscientists as they map and investigate landslides along Interstate-17 (I-17) from Anthem (25 miles north of Phoenix) to Flagstaff. This is a preliminary report and has not been reviewed for AZGS editorial standards. AZGS will release an Open-File Report with data, digital maps, and final results later this year.

Introduction. I-17 is the chief route between Phoenix in central Arizona and Flagstaff in northern Arizona. Each day more than 50,000 cars (> 18,250,000 cars annually) traverse the Anthem Way - Sunset Point segment. The I-17 corridor winds through hills, mountains, river valleys, and plateaus, slicing through fractured and weathered rocks. Landslides and rock falls potentially threaten I-17 from the Agua Fria River north to the Mogollon Escarpment, 25 miles south of Flagstaff. Over the next several years the Arizona Dept. of Transportation (ADOT) plans to spend ~$300 million to expand and improve I-17. AZGS’s I-17 landslide assessment project will provide basic data that ADOT can use in designing and constructing highway improvements.

With funds provided by the Arizona Dept. of Emergency and Military Affairs (DEMA), AZGS Research Scientists J.P. Cook and B.F. Gootee are mapping a 15-mile-wide strip centered on the current route of I-17 from Anthem to Flagstaff – a distance of ~100 miles. They are mapping the locations and boundaries of landslides using moderately detailed topographic data derived from 2017 aerial orthophotography in a GIS platform, and reconnaissance field investigations aided by drone surveys.

Results of this study will be integrated with the more than 6,300 landslides in the Arizona Statewide Landslide Database (AzSLID) compiled by AZGS geoscientists (Youberg and others, 2015). The Natural Hazards in Arizona Viewer includes a landslide theme portraying the AzSLID data.

The road from Phoenix to Flagstaff.  Interstate-17 serves as the principal travel corridor connecting Phoenix in central Arizona with Flagstaff to the north, a distance of 140 miles (see profile along I-17 below, taken from Google Earth). From Phoenix, at ~ 1,450 feet above mean sea level (MSL), the interstate climbs steadily before descending through Miocene volcanic and sedimentary rocks along the Agua Fria River near Black Canyon City, 23 miles north of Phoenix.  I-17 climbs out of the Agua Fria Canyon area before plateauing on ~11 million year old basalt lava flows near the Sunset Point rest stop. The road then skirts Black Mesa to the east before encountering small, rolling granitic hills around Cordes Junction (41 miles north of Phoenix). Continuing north, I-17 winds across subdued topography, crossing the Agua Fria River again, and gradually climbing into the Black Hills before descending precipitously,  5,000 feet to 3,100 feet MSL over the course of four miles into the Verde Valley. I-17 then bridges the Verde River and its Holocene deposits near Camp Verde, traverses extensive limestone outcrops of the late Miocene to Pliocene Verde Formation, and then climbs out of Verde Valley on ‘ramp basalts’ of late Miocene age that have buried the Mogollon Escarpment. On the final 20 miles to Flagstaff, I-17 traverses Pliocene and Quaternary basalt lava flows and cinders of the active San Francisco Volcanic Field, and then sparse outcrops of the Permian Kaibab Limestone on the southern outskirt of Flagstaff.  

Landslide features of the Agua Fria River near I-17

Traveling north from Anthem, I-17 crosses Agua Fria Canyon near Black Canyon City. The Agua Fria River drains a large watershed with headwaters east of Prescott; it courses south 120 miles to its confluence with the Gila River. Numerous landslide features have previously been identified on the flanks of Black Mesa just north of Black Canyon City. The Agua Fria River has incised a deep canyon into Black Mesa (on the west) and Perry Mesa (on the east); there is clear evidence of many landslides along parts of the Agua Fria. The landslides along Agua Fria Canyon have occurred where basalt flows of the Hickey Formation overlie fine grained sediments (limestone, shale, silts, and clays) of the Chalk Canyon Formation. Farther north where basalt flows cap granitic rocks there are no known landslides. Fresh or reactivated landslides in the canyon could dam the Agua Fria River, impound water, and threaten downstream communities with catastrophic flooding (e.g., Voight, 1978).

The annotated UAV (drone) image below illustrates the prevalence of landslide deposits in the Agua Fria Canyon just north of Black Canyon City where basalt lava flows cap unconsolidated and easily eroded sediments. The Agua Fria River eroded the basal sediments, which allowed them to fail into the canyon, undermining the more resistant basalts and causing them to slide down. The grassy mesas midway down the slope represent the top of basalt lava flows that slid downslope as much as 150 meters (500 feet). Much more resistant granite is exposed in the side of the canyon in the lower left part of the photo. This is the approximate northern limit of landslides in the canyon; as was noted earlier, no landslides have been detected in areas where basalt flows rest directly on granite.

Our next entry, Part II: Landslides near the Sunset Point overview., will follow the path of I-17 from the Agua Fria River to the top of Black Mesa is on the east flank of Black Canyon – the large valley at the western edge of Black Mesa

Acknowledgments.  We thank James Lemmon (ADOT) for his cogent comments regarding landslides and rock falls along the I-17 corridor. Thanks, too, to the staff at DEMA for assisting with grant development and implementation.

RESOURCES

• Arizona Geological Survey, 2020, Natural Hazard in Arizona Viewer, landslide theme.
• Arizona Public Media, 2019, Landslide Database to Shape Arizona I-17 Expansion Project. Associated Press (16 Aug. 2019), https://news.azpm.org/p/news-topical-sci/2019/8/16/156687-landslide-database-to-shape-arizona-i-17-expansion-project/ 
• Cook, Joseph P., Pearthree, Philip A., Gootee, Brian F., Conway, Brian D. And Youberg, Ann M., Landslides are Surprisingly Large and Widespread in Arizona. Geological Society of America 2016 Annual Meeting in Denver, Colorado, USA.
• Conference 2016, Voight, B., 1978, Chapter 3 – Lower Gros Ventre Slide, Wyoming, USA. Developments in Geological Engineering, v. 14, Part A, pg. 113-162.
• Youberg, A. and five others, 2015, Building a statewide inventory of landslides in Arizona. Geological Society of America 2015 Annual Meeting in Baltimore, Maryland, USA

AZGS Staff who contributed to this post: J. Cook, B. Gootee, P. Pearthree, M. Conway