There are numerous forms of karst processes on the Colorado Plateau in northern AZ including breccia pipes, shallow or surficial karst/cave formation, and salt karst. While breccia pipes have been in the headlines as of late due to the increased demand of Uranium, the sinkholes near Holbrook are interesting in their own way in part because their formation is ongoing. True breccia pipes in northern AZ form through the upward propogation of cavern collapse in the Redwall limestone. Salt karst or sinkhole formation near Holbrook is the result of salt and gypsum dissolution within the Supai Formation. Like breccia pipes, dissolution and collapse in the subsurface propogates upwards resulting in a sinkhole at the surface. Most sinkhole features in the Holbrook Basin are concentrated along the Holbrook Anticline (actually a monocline). Flexure along this feature is related to dissolution of deep salt beds. In addition to sinkholes, numerous tension cracks have formed due to the bending of rigid bedrock overlying zones of dissolution along the monocline. Overall, up to 500 sinkholes and fissures have been recognized between Springerville and Winslow (Neal, 1998).

A large cluster of sinkholes lies ~25 miles SSW of Holbrook (10 miles NNW of Snowflake) is known simply as "The Sinks". Dozens of sinkholes pockmark the landscape, some as large as 500 feet across. There are 2 main fields of sinkholes beginning just north of Hwy 277 west of Snowflake.

The holes in the McCauley Sinks locale below are up to 580 feet across. The pattern of sinkhole formation here approximates two concentric arcs. Perhaps this is a result of a larger scale of subsidence manifested by numerous smaller features. In addition to round sinkholes, numerous tension cracks due to subsidence are found near the sinkholes. One locale is creatively named "The Cracks".

Some area wells have very high salinities indicating dissolution is still ongoing. How many sinkholes or tension cracks lie just below the surface? The Arizona Department of Water Resources has recently documented at least one area which has potential for new sinkhole formation. InSAR interferograms covering a cluster of sinkholes east of the McCauley Sinks shows two distinct areas of ground subsidence. The scale of these features closely matches existing sinkholes in the area. It seems likely that we are witnessing the incipient stages of sinkhole formation in this area.

Information from this post was largely derived from AZGS Open File Report 02-07, "A Review and Bibiography of Karst Features of the Colorado Plateau, Arizona" by Ray Harris. This report references Neal 1998, Variations in evaporite karst, Holbrook Basin, Arizona [abstract]: Geological Society of America, Abstracts with Program, Cordilleran Section, p.33.

Thanks to Brian Conway at ADWR for sharing his InSAR data and figure.