Sedona, Arizona, situated in the Red Rock country below the Mogollon Rim of north-central Arizona (Fig. 1), is surrounded by hills of red to beige Paleozoic-age sandstones and siltstones. Nearby hills rise from gently undulating topography; at about the break in slope at the base of these hills are at least seven sinkholes. In 1990, the U.S. Forest Service, concerned for the safety of unwary visitors, prompted a geologic study of one sinkhole, Devils Kitchen. This article summarizes study results of the geometry, origin and potential hazards of Devils Kitchen sinkhole. Publication of the complete unabridged study with additional illustration as an Arizona Geological Survey Contributed Report is pending.

Historic Observations: 1880’s to 1989

One of the earliest descriptions of Devils Kitchen, which involved a collapse event there, was retold by long-term resident Albert E. Thompson (1968), "My parents were living in Sedona in the early1880s and heard the crash when the spot caved in. Mother said the dust from the cave-in filled the air all day and the sun looked like it was shining through heavy smoke. Her brother, Jim James, was the first one to see the new hole in the ground".

In late 1989, a second historic collapse event occurred at Devils Kitchen, enlarging the opening by as much as 1/3rd. The 1989 event was largely limited to the north wall, where a gigantic block, detached along three bounding walls from its caprock, rotated outward into the opening without dropping to a lower elevation. Edges of the newly broken rocks are highly angular and the surface soil has not yet begun to slough off.

Collapse of the southernmost wall of the sinkhole pre-dated historic collapse events and probably represents an early, formative event in the history of Devils Kitchen. In contrast to the fresh, angular appearance of historically broken blocks, blocks of the south wall display rounded edges and the surfaces carry a patina of manganese oxide that suggests open exposure over several hundred years.

General Information about Sinkholes

The most common cause of sinkhole formation is from the collapse of caves in limestone bedrock. Atmospheric carbon dioxide dissolved in rainwater yields weakly acidic groundwater that slowly dissolves limestone along fractures. Over long periods of geologic time, slow rates of dissolution are capable of producing gigantic underground caverns. During the formation of a cave the slowly enlarging chambers form below the water table. If a cave is finally unroofed by erosion or if the groundwater table drops below the cave floor, it becomes a “dry” cave capable of forming secondary stalagmites and stalactites. The caves below Sedona, however, still lie below the water table.

The Devils Kitchen Sinkhole

Unlike typical karst topography, outcrops in the immediate vicinity of the Devils Kitchen sinkhole contain no limestone. The upper walls of the collapse area are made up of massive sandstone of the Schnebly Hill Formation, while the lower walls consist of weaker, shaly siltstone of the Hermit Formation. The sinkhole almost certainly formed as the result of deep-seated collapse of caverns in the subjacent Redwall Limestone.

Figure 2 displays a stratigraphic cross section of the rock strata in the vicinity of the Devils Kitchen sinkhole. The stratigraphic subdivisions have been derived from various recent sources and are sufficiently accurate for the purposes of this report.

The schematic cross section shows that the top portion of the Devils Kitchen sinkhole is at an approximate elevation of 4510 feet above sea level. The bedrock lip of the Devils Kitchen sinkhole is situated in Schnebly Hill Sandstone; contact with the underlying Hermit formation lies about 40-60 feet below the sinkhole lip. Below the Hermit lies the massive Supai [Esplanade] Sandstone, which is approximately 430 feet thick and rests conformably on top of the approximately 180 feet thick, Redwall limestone.

Towering above the sinkhole are the imposing cliff-forming sandstones of the Schnebly Hill Formation. And about 520 feet above the sinkhole is a thin rock strata known as the Fort Apache Limestone. This is the same thin limestone bed that forms the "Merry-Go-Round" off Schnebly Hill Road and the "deck" of Steamboat Rock.

The Fort Apache Limestone above and the hidden Redwall Limestone below formed in shallow, tropical seas. The intervening "Red Rock" succession of Schnebly Hill, Hermit, and Supai Formations were deposited close to sea level and sometimes within inter-tidal areas. The rock succession from Redwall Limestone to the Fort Apache Limestone ranges in age from approximately 350 to 285 million years old. All of the present-day landscape of the Sedona area, including the retreating cliff rims, canyons, and buttes formed within the past 10 million years.

Mapping the Devils Kitchen Sinkhole

Figure 3 shows a plan map of the Devils Kitchen sinkhole area on a scale of 1 inch equals 40 feet. Steel pins were placed at points A, B, and Q and were driven flush to the ground. Map surveying was done using a tripod-mounted, Brunton compass and measuring tape with point A as the master control. An arbitrary datum of zero [foot] elevation was assigned to this point; measurements are recorded for either above or below this reference. Extrapolating from the topographic map suggests that point A is about 4510 feet above sea level.

Figure 4, a longitudinal cross section along the N-S axis of the sinkhole (see figure 3 for location), illustrates key geologic features of the upper portion of the sinkhole. Note the arcuate fractures beneath the overhanging south wall (left side). As a column of rock collapses upward its roof is composed of arcuate shells of rock that slab inward into the breccia column (see Fig. 5). What this illustrates on surface is a "holing through" of the upward migrating collapsing column that has been blind until it breaks through to the surface. There may be many more such collapse areas around Sedona but until one breaches the surface it is not yet a "sinkhole"

Geomorphic Features Controlling Sinkhole Development

The Devils Kitchen is situated on pervasive NW-trending joints (Fig. 3). These joints are regional in scope and may have originally formed during the Laramide Orogeny when the Earth's crust was warped and uplifted. The joints, subtle as they may appear, probably proved effective over the last 75 million years in channeling groundwater to depth.

Experience from other parts of the world show that caves commonly line up along fracture zones and often develop into large vertical chimney-like features at the intersection of two or more fractures. The Devils Kitchen map displays prominent NW-trending joint sets and a subordinate NNE-trending set of joints. It is probable that where these joints project downward to the Redwall Limestone a large cavern had developed at this site. Upward stoping, or caving, took place until the upward advancing collapse [breccia] column finally broke through to the ground surface as shown schematically in Figure 2.

Water wells in the vicinity exploit solution cavities in the Redwall, providing for most of the municipal water for Sedona. There is no way of knowing how many caverns exist below the ground surface but water wells indicate that they are abundant. At the present time rain and snowfall on the Colorado Plateau provides the aquifer recharge in the porous Redwall limestone. This water passes beneath the Sedona region from the Mogollon Rim toward the Verde Valley in a southwesterly direction. The slow water movement can be likened to that flowing through a saturated sponge that is inclined at a very low angle toward the southwest. The former cave that once lay below the Devils Kitchen sinkhole had grown so large by groundwater dissolution of the limestone that it finally collapsed to form the sinkhole we see today.

Figure 5 illustrates how this process of upward-stoping took place. It is highly unlikely that the entire column of rock fell at one time, although it probably did advance by catastrophic pulses. If a collapse of this magnitude took place below the ground surface in a "blind" collapse zone it may be capable of generating an audible "bump" on the surface in much the same manner as that experienced from a large underground mine blast. Such reports should not be taken as frivolous in nature, but be regarded with some concern. While highly unlikely, such events may provide advanced warning to future sinkhole development.

Calculating the Size of the Collapsed Cave System

A cursory examination of the surface map, longitudinal section and stratigraphic section indicate that a subsurface column of breccia ~200 feet in diameter would extend downward for 660 feet to the top of the former Redwall Limestone cave. From sinkhole surface to the top of the Redwall, this represents a cylinder of rock with a volume of 20,735,000 cubic feet. Assuming the entire Redwall Limestone strata were dissolved over its 180 foot height this would have allowed for a 200 foot diameter cave to have been present in the Redwall Limestone prior to the collapse.

Because broken rock occupies a greater volume than solid rock (approximately 1.3 times as much volume) this means that as the collapse progressed – with concomitant volume expansion -- it finally came close to equilibrium at the present ground surface when about 27,000,000 cubic feet of breccia occupied the old cave chamber with the column stretching up to the surface. If correct, this calculated volume of a cavern in the Redwall (180 feet high and 200 feet in diameter) is suggestive of a cave network to rival that of Mammoth Cave National Park of Kentucky.

Hazards at the Devils Kitchen Sinkhole

The Devils Kitchen sinkhole formed catastrophically and will continue to enlarge over geologic time. Man's brief occupancy of the area has been too short to fully appreciate the scope of these highly unpredictable developments. Figure 6 shows the potential hazard areas at the sinkhole.

Certain edges of the Devils Kitchen sinkhole are at greater risk of collapse while others are deemed less likely to collapse. It should be emphasized, however, that no degree of certainty exists at this site. The area requires regular inspection and posted safety precautions should be strictly adhered to. The present policy of driving jeeps on tours along the southern access road and viewing from the southwestern comer of the sinkhole is considered by the writer to be about as safe as any other human activity. The obvious danger point is in the extreme south wall where many people walk unknowingly onto a severe overhang.

But as long as this overhang is pinned-in by the rock buttresses on the west wall (marked in red and orange), the yellow area should remain locked in place. If the areas shown in red and orange collapse at some time in the near future, as one anticipates, the entire situation should be re-evaluated.

Viewed today, new fractures on the north side are very evident when compared to the older south wall which exhibits well-rounded cliff edges and a surface weathering patina. The recent collapse has severely undercut the north wall making it highly unstable (Fig. 4). This area should continue to spall off over the near future. A warning sign should be placed at the edge of the unstable block marked in red (dark cross-hatching) on the hazard map (Fig. 6). Also, warning signs should be posted on the west side and eastern ends of the north wall to warn hikers to stay back from the edge.

The surface map (Fig. 3) shows a large number of arcuate fracture zones along the southwestern portion of the sinkhole. As indicated on the hazard map (Fig. 6), I believe that over a long enough period of time the sinkhole may expand in this direction. There is no way of ascertaining how long this might take. It could progress by incremental steps, it could collapse catastrophically, or it might remain as is for several centuries.

Last, visitors should approach the edge of the sinkhole prudently. Geologists should examine the sinkhole and environs frequently for evidence of any new fractures that may presage imminent collapse.

additional photos

		(Click to Enlarge)

Acknowledgments

Mark Avery, Pink Jeep Tours of Sedona, Arizona, made the initial request, at the prompting of the U.S. Forest Service, for a geologic and geohazard evaluation of Devils Kitchen.

References Cited

Thompson, A.E., 1968, Early Days, 1968, Sedona Westerners, p. 60