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

Joseph P. Cook, ARIZONA GEOLOGICAL SURVEY, Tucson

Introduction

In late July 2010, landowners north of the Sulphur Hills, 10 miles southeast of Willcox Playa in central Cochise County, Arizona, were cut off from their farms and homes (Fig. 1) because E. Parker Ranch Road, east of Kansas Settlement Road, was cut by three freshly ruptured earth fissures rendering the road impassable. Repair by Cochise County in order to make the roads passable was completed on July 28, 2010.

Figure 1. Location map of Three Sisters Buttes Earth Fissure Study area in central Cochise County, Arizona.

The three fissures trend roughly north-south forming curvilear cracks subparallel to one another. Each fissure formed in the flat, relatively unincised mesquite grasslands north of the Sulphur Hills. While no homes were impacted by the new fissures, one abandoned homesite lies approximately 150 feet from the westernmost fissure. At present, the earth fissures crossing Parker Ranch Road have been filled but cracks remain open on either side and possibly at depth. A road sign warning of the possibility of earth fissures has been installed near the west end of Parker Ranch Road.

Causes of Earth Fissure Formation

Earth fissures are cracks, seams, or separations in the ground surface that form in response to tensional forces related to uneven ground subsidence that accompanies overpumping groundwater (Arizona Land Subsidence Group, 2007). Groundwater levels in the vicinity of the Three Sisters Buttes fissures have dropped from 106 feet up to 260 feet since the early 1950s (Arizona Department of Water Resources GWSI database).

As water levels fall, pore spaces between valley filling sediments formerly filled with groundwater are free to compact. This compaction results in ground subsidence at the surface. Earth fissures commonly manifest near the base of bedrock hills or ranges because the sediments overlying the buried bedrock are thinner and thus compact less than the thick sediments underlying the central valley area (Fig. 2). This model of earth fissure formation is observed in the distribution of fissures of the Three Sisters Buttes study area (Arizona Geological Survey, 2011).

Figure 2. Schematic diagram of earth fissure formation by differential compaction, tensional cracking, and subsurface erosion.

The initial appearance of incipient earth fissures is a hairline surface crack, often crossing or perpendicular to local drainage patterns. Rainwater and overland flow are able to infiltrate the crack and percolate into the subsurface leading to piping erosion that eventually forms an open underground void aligned with the still narrow surface crack. When the void becomes large enough the undermined surface soil may abruptly collapse into the open part of the fissure below.

 

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















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