Rapid infilling of a fresh earth fissure in southern Pinal County, Arizona w/ comment by Ken Fergason
In January 2017, J.P. Cook, AZGS’ Earth Fissure program manager, reported on a nearly two-mile long fissure in the Tator Hills earth fissure study area of southern Pinal County (Cook, 2017). The fissure was divided into sub-equal lengths of an older, northern tract, and a younger southern tract.
The northern tract first appeared prominently in Google Earth imagery in December 2014. When examined in 2017, it was largely backfilled with sediments from sidewall collapse and incision along captured drainages. The younger, southern tract did not appear in Google Earth imagery until April 2016. In contrast to the north section, it displayed sharp vertical walls, a deep v-shaped geometry (maximum depth about 30 feet), with piles of angular soil blocks along wider open sections (Figure 1a).
Cook revisited the younger, southern tract in January 2019 (Figure 1b); two years after his first visit. His observation’s, bulleted below, show a rapidly degrading and infilling fissure. Figure 2 from the Houston Ave. fissure in Apache Junction illustrates extensive headward erosion that may accompany some earth fissures.
• When first examined, the fissure was steep-sided and tapered to a deep narrow crack as much as 30 feet deep. In January 2019, maximum depth was approximately 8 to 10 ft deep, indicating 20 to 22 feet of backfill. Fissure sidewalls remain steep-sided but infilling with sand and sediment from captured drainages produced a broad, flat-bottomed channel.
• Fresh headcutting, incision and erosion along drainages entering the fissure from the east is dramatic. Drainages entering the fissure from the west are abandoned. Figure 2 provides a drone image overview of extensive headcutting along an earth fissure.
• Fissure width has increased over the past two years as large blocks of sidewall have collapsed into the fissure. Parallel cracks running along the fissure and leaning pillars of soil along some deeper sections indicate this process is ongoing.
• At the southern-most extent, what were once narrow cracks or discontinuous potholes in January 2017 are now connected to form a continuous crack several feet wide and up to 10 feet deep. The overall fissure length is unchanged since 2017.
• An abundance of tire tracks testifies to a number of fissure visits by 4-wheel track vehicles. Note: anyone driving across a fissure zone should exercise extreme caution, especially along the edge of a fissure. Sidewalls are highly unstable and fissure zones frequently include multiple fissures, some of which may be incipient, but prone to collapse, nonetheless.
• Coyote and javelina tracks are evidence that animals use the fissure channel as a transportation corridor.
Final comment. Rapid backfilling of the southern Pinal County earth fissure does not reflect the history of other fissures. To this day, the long-lived Roger fissure in Maricopa County (Arizona Geological Survey, 2009) maintains steep sidewalls and a deep V-shaped channel. Additionally, it is not uncommon for fissures to reopen and recapture the original deep V-shaped geometry.
AZGS staff will continue to monitor the Tator Hills fissures remotely via Google Earth imagery and with annual or biannual visits.
COMMENT by Ken Fergason
Ken is a Sr. Geologist @ Wood Environment & Infrastructure Solutions, Inc. & former President of the Assn. of Environmental & Engineering Geologists
My experience is that rapid backfilling of earth fissures happens fairly regularly, particularly when earth fissures have captured a drainage and begin to function in part as a drainage feature themselves. In these cases the fissure often rapidly backfills during storm events. If subsidence slows, stops, or ground deformation at the fissure stops, then the fissure will essentially become a drainage and will become nearly indistinguishable from other drainages other than an odd orientation (this has happened to a few older fissures near McMicken Dam in Maricopa County and a few other old fissure locations that I've visited). If deformation continues, then a cycle forms where the fissure fills up in a big storm, then cracks, then a big storm comes along and washes out the backfill. Then it will all repeats. The fissures out near Hawk Rock do this a lot, especially those south of Hawk Rock near the Siphon Draw Wash Basin.
I think this is a sort of thing that we may start seeing a lot more as groundwater gets utilized more as part of the whole drought contingency plan for the CAP water. It's looking like a huge part of the contingency is to pump a whole of groundwater. This will eventually lead to more subsidence in places that have been relatively quite for the past 10, 20 or 30 years. My rough guess is that in about 10 years things will start get really interesting.
Select earth fissure resources
Arizona Geological Survey, 2009, Earth Fissure Map of the Tator Hills Study Area: Pinal County, Arizona: Arizona Geological Survey Digital Map - Earth Fissure Map 11 (DM-EF-11) V2.0, map scale 1:24,000.
Arizona Geological Survey, 2009, Earth Fissure Map of the Harquahala Study Area: Maricopa County, Arizona: Arizona Geological Survey Digital Map - Earth Fissure Map 14 (DM-EF-14), map scale 1:24,000.
Conway, B., 2015, Land subsidence and earth fissures in south-central and southern Arizona, USA. In: Land Subsidence Processes, Journal of Hydrogeology, 7 p., DOI 10.1007/s10040-015-1329-z
Cook, J.P., 2017, Discovery of a large earth fissure in the Southern Picacho Basin, Pinal County, Arizona. Arizona Geological Survey Open-File Report, OFR-17- 01, 7 p., 1 appendix.
Harris, R.C. and Pearthree, P.A., 2002, A Home Buyer's Guide to Geologic Hazards in Arizona. Arizona Geological Survey Down-to-Earth #13, 48 p.
Harris, R.C., 1999, Field Guide to Earth Fissures and other Land Subsidence Features in Picacho Basin. Arizona Geological Survey Open File Report, OFR-99-26, 55 p.
Posted 1/26/2019 - AZGS Staff: JP Cook | M Conway