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Camp Verde landslides

AZGS at Work for Arizona: Landslides along the I-17 corridor in the Verde Valley

This is the third blog post profiling the work of AZGS geoscientists investigating and mapping landslides along U.S. Interstate-17 (I-17) from Anthem, 25 miles north of Phoenix, to Flagstaff in northern Arizona. 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.

To view Part I. Identifying landslide deposits and hazards on the I-17 Corridor
             Part II. Landslides of Black Canyon City and Sunset Point Visitor's Center

Part III. After cresting the Black Hills of central Arizona, northbound I-17 plunges nearly 2,000 feet to the floor of the Verde Valley (3,100 feet AMSL). On its descent it slices through roadcuts of basalt lavas, tephra beds, and sedimentary deposits of the Hickey Formation. Rock falls here are common but are confined to rockfall catchment areas behind Jersey barriers, thus mitigating impact to the roadway. (Figure 1).

Verde Valley is a large late Miocene-Pliocene structural basin that has been dropped down across the Verde fault zone, a major normal fault zone at the base of the Black Hills. Volcanic rocks of the Hickey Formation that cap Mingus Mountain at about 7000 feet above sea level have been dated as young as 11 Ma. Equivalent volcanic rocks encountered in deep wells in the axis of Verde Valley at about 1500 feet above sea level; thus, there has been about 5500 ft of net displacement across the Verde fault zone.

As the basin subsided, it filled with sand and gravel deposits shed from the Black Hills and the Colorado Plateau margin to the north. In the basin center carbonates (limestone), silt and clay accumulated in marshes and shallow lakes. These deposits collectively are the Verde Formation (Nations et al., 1981).  Deposits of the 3,100-feet thick Verde Formation accumulated in Verde Valley from about 8-2.5 Ma (Bressler and Butler, 1978). Once the Verde River began to drain to the southeast about 2.5 Ma, downcutting began. This has resulted in dramatic dissection and erosion of Verde Formation deposits in the valley.

I-17 on east slope of the Black Hills
Figure 1. Hickey Formation volcanics cropping out in an I-17 roadcut on the east slope of the Black Hills above the Verde Valley.

The 5.4-mile stretch of I-17 from Copper Canyon to Montezuma Castle opened to traffic near Camp Verde in August 1978 and was the final section of the I-17 roadway completed (ADOT). After crossing the Verde River north of Camp Verde, I-17 begins its gradual ascent to the Colorado Plateau. For the first 6.5 miles, the highway winds through friable exposures of bleach-white lacustrine rocks of the Verde Formation before encountering equivalent clastic sedimentary deposits derived from the valley margin that grade and intermix with the lacustrine deposits. Thirteen miles north of Camp Verde I-17 encounters Late Miocene ‘ramp basalts’ that subdue this section of the Mogollon Escarpment of the SW Colorado Plateau.

Older Landslides south of Camp Verde. A number of Quaternary-Tertiary landslide deposits of the Verde Valley comprise chaotic blocks of Tertiary basalt and Tertiary sedimentary rocks (DeWitt and others, 2008). Descending the east slope of the Black Hills on I-17, two landslide clusters crop out north and south of the roadway (Figure 2). Four landslide deposits hosted in Miocene Hickey Formation clastic sedimentary rocks are situated roughly 1.0 to 2.5 miles north of the roadway at ~4,000 to 4,800 feet AMSL (Table 1). To the south, four landslide deposits identified by DeWitt and others (2008) crop out roughly 4.0 to 6.0 miles from the roadway at elevations of 5,200 to 5,600 feet AMSL.  The age of landslide deposit is uncertain; Dewitt and others (2008) concluded that landslides of cluster 2 formed sometime from the Holocene to the Pliocene. The type of landslide motion is uncertain, but some combination of rotational and translational is most reasonable.

Overview of Verde Valley. Landslides in yellow
Figure 2. Landslide clusters near Camp Verde, Arizona.
TABLE 1. Attributes of Camp Verde Landslides Clustered Groups 1 (4 landslides), 2 (4 landslides), and 3 (10 landslides): Aggregate perimeter and area; approximate elevation range, and host material.
Group #           Perimeter                          Area                             Elevation Range       Host Rock

Group 1: 3.75 miles (6.04 km);     0.25 sq miles (0.65 sq km);  4,800-4000 ft; Hickey Formation clastics
Group 2: 7.44 miles (11.98 km);   0.61 sq miles (1.58 sq km);  5,600-5,200 ft; Hickey Formation clastics
Group 3: 16.93 miles (27.26 km); 1.11 sq miles (2.87 (sq km); 3,300-3250; Verde Formation lacustrine

Newly identified landslides east of Camp Verde. In the axis of the Verde Valley, 1 to 4miles east-southeast of Camp Verde, at roughly 3,250-3,000 feet AMSL, we identified 10 landslide deposits of cluster 3 (Table 1) using 2017 NAIP (National Agriculture Imagery Program) topographic data. We have since field-checked and validated 9 of the 10 landslide deposits.

For a 90-second 'Landslides in the Verde Formation' video with narrative by Brian Gootee: .

These landslides occur in the Verde Formation and postdate incision by the Verde River and its tributary, West Clear Creek. The hummocky, youthful appearance of the landslide surfaces points to a latest Pleistocene to Holocene age. The single largest landslide (Figure 3), 4.32 miles perimeter (6.96 km), 0.41 square miles (1.06 square km), anchors the southeast end of cluster #3, where it is bounded by AZ 260 and adjacent to a rapidly urbanizing area.
Landslides of the Verde Formation are of the translation – rotation type and mimic the Black Canyon City and Sunset Point slides with mildly rotated blocks near the headscarp and progressively more chaotic blocks toward toe.

Landslides hosted in the Verde Formation along axis of Verde Valley.
Figure 3. The southeastern most landslide of Cluster 3 bounded by Highway 260. Drone imagery by Brian Gootee; annotated by Joe Cook.

Landslide threat to the I-17 Corridor. The extant older landslide deposits in the Black Hills west of Camp Verde (Figure 2) do not constitute a direct threat to the I-17 corridor. They are miles from the highway and there is no evidence that they are currently unstable.
Younger landslides of the Verde Formation east of Camp Verde (Cluster 3, Fig.2) pose no immediate threat to I-17. In several cases, the toe of landslides masses undergird Highway 260 and impinge on nearby neighborhoods. Currently, I-17 traverses the landslide-prone Verde Formation for more than 10 miles as the road climb north towards the Mogollon Escarpment.  Fresh cracks along the limestone mesa cap, and a surplus of  limestone boulders and blocks along upper slopes are indicative of unstable ground.

This portion of the I-17 corridor requires close examination to mitigate potential hazards associated with future localized collapse of the lacustrine deposits of the Verde Formation.

Verde Formation exposed on I-17
Figure 4. Lacustrine rocks of the Verde Formation exposed in road cut along the I-17 corridor. (Photo by M. Conway)

Part IV pending. Landslides on near the intersection of Highway 179 and I-17.

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.

AZGS staff contributors: Joe Cook, Brian Gootee, Phil Pearthree, Mike Conway

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