Why did the fissure cross the road? New and old earth fissure activity in Cochise County, Arizona

Joseph P. Cook, ARIZONA GEOLOGICAL SURVEY, Tucson

Continued...

Fissures in the Three Sisters Buttes Study Area

Earth Fissures Formed in 2010.  The new earth fissures north of the Sulphur Hills most likely opened during rapid erosion coinciding with heavy rains in late July 2010. The period of time over which subsurface erosion took place prior to surface failure is unknown but could span years. Narrow surface cracks are difficult to observe due to heavy grass cover and historical disturbance by cattle and agriculture.

Figure 3. Section of the Three Sisters Buttes earth fissure study area containing the three new fissures cutting Parker Ranch Road. Fissures A, B, and C opened in summer 2010 while fissures to the south were evident in aerial photos from the late 1970s. Continuous fissures are indicated in black and discontinuous fissures are indicated in red.

The cumulative length of the open surface crack portions of the new fissures exceeds 1.3 miles (Fig. 3): it is possible, too, that hairline cracks and voids extend beyond the mapped fissure in the subsurface. Some sections of these fissures are narrow cracks less than six inches across or disconnected lineations of potholes. Other sections have experienced extensive surface collapse resulting in open surface cracks approximately 15 feet across (Fig. 4). The deepest measurable section of the earth fissure complex exceeds nine feet although numerous open cracks and collapse features visible in the base of the open portion of the fissures likely extend to much greater depths. The easternmost newly-opened earth fissure cuts across local drainages, affecting downstream vegetation and drainage patterns. Although the upstream channel is presently wide and unincised, future runoff will pour into the deeper earth fissure enhancing headward erosion along the drainage where there was little incision before. The downstream end of the channel is now abandoned and will no longer receive flow.

Historical Earth Fissures. The newly-opened earth fissure complex is not the first time fissures have been discovered in the area. An earlier generation of earth fissures appears in aerial photos as far back as 1978. Due to capture of precipitation and surface flow, vegetation along older fissures is often twice as tall as that observed in the undisturbed ground away from the fissure. Older fissures can be difficult to access because of the dense tumbleweed and mesquite trees that populate them.

The AZGS recently released the Three Sisters Buttes earth fissure map showing nearly 19 miles of earth fissures concentrated north of the Sulphur Hills and surrounding the Three Sisters Buttes approximately three miles to the northwest. In 2009, the AZGS released the Dragoon Road Earth Fissure study area map detailing nearly six miles of earth fissures southwest of Willcox Playa. Some of these fissures have repeatedly broken the asphalt at the intersection of W Dragoon Road and Cochise Stronghold Road leading to numerous road repairs and the installation of a road sign by Cochise County warning of possible earth fissures. Other fissures in the Dragoon Road study area underlie the coal ash ponds of the Apache Generating Station.

Figure 4. Field photos of newly opened earth fissures. A) A large open section of fissure C from figure 3, note fence stretched across open fissure. B) Deep, steep-sided section of the same fissure. C-D) Narrow surface cracks and aligned potholes representative of portions of earth fissures that have not yet opened completely. A subsurface void likely underlies these features and the overlying soil may one day collapse into it.

Mapping of earth fissures near Bowie and San Simon in Cochise County continues, while work in northern Sulphur Spring Valley north of Willcox begins later this year.

 

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ARTICLE AUTHOR:
Joseph P. Cook, Research Geologist, Arizona Geological Survey
joe.cook@azgs.az.gov















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