Arizona State and Local Government Documents Collection

The purpose of this research project is to determine the origin of the materials used to construct the Black Hills Dam in order to restore the landscape to pre-dam conditions. The Black Hills Dam site is located in northern Scottsdale, Maricopa County, at 33.75° North, 111.80° West. The goals of this project are to characterize the surficial deposits and local geology of the dam site. This report presents our findings, interpretations and conclusions based on background research, a site visit to the dam site, and technical discussions with the City of Scottsdale engineer and planners.

Seven sinkholes surround the city of Sedona in Coconino and Yavapai Counties, Arizona. They occur in surface bedrock of Permian age Esplanade Sandstone, Hermit formation, and Schnebly Hill Sandstone, but the causative source is from the collapse of subsurface water-filled caves in Mississippian Redwall Limestone that underlies those formations. The original Mississippian-age Redwall karst surface has undergone two additional phases of dissolution enlargement in later geologic time. The first occurred after the Laramide uplift of the Mogollon Highlands when northeast flowing streams penetrated the exposed Redwall Limestone erosion surface, and the second took place following the generation of the Verde graben ~10 million years ago when regional drainage reversal took place. Pre-graben Miocene basalt lava flows that overlie eroded Paleozoic strata are present on either side of, and faulted within, the Verde graben closed depression. Post-graben erosion generated the Mogollon Rim escarpment in the northern portion of the Verde Valley and allowed surface streams to erode the broad Dry Creek and Margs Draw valleys. Oak Creek fault, and the erosion of its canyon, is much younger than the faulting that generated the Verde graben.

Over time, water flow through the Sedona area evolved from surface flow to dominantly groundwater flow, mainly due to leakage through abundant northwest-southeast oriented rock joints and permeable fault zones into underlying cavernous Redwall Limestone. USGS oxygen isotope studies show that water recharge entering the northeastern part of the Upper Verde watershed, and passing beneath the greater Sedona area, originated high on the Colorado Plateau above 6900 feet before discharging at a rate of ~15 millions of gallons per day at artesian springs in the Page Springs area to the southwest of Sedona. Dissolution caves in the underlying Redwall Limestone have now enlarged to the point where their sandstone roof rocks have collapsed into limestone caves over the past several thousand years. Devils Kitchen sinkhole has historic records of collapse in the 1880s, 1989 and 1995, and it will continue to collapse in future years.

Six additional sinkholes are in various stages of collapse from modern time and possibly to the end of the last Ice Age. While the danger of future collapse is probably minimal to humans, unregulated septic leakage into hidden sinkhole breccias within the town limits could contaminate groundwater being tapped for municipal use or the contamination of the Page Springs outflow. The report contains geologic maps, cross sections, photographs and individual features of the sinkholes as of the end of 2009.

At the request of the U.S. Forest Service to Red Rock Jeep Tours of Sedona a geologic report on the condition of the Devils Kitchen sinkhole was required for the safe continuation of Jeep visits to the site (Fig. 1). Mark Avery of the jeep company contacted me to study the site and write up my findings. The study of sinkholes in the Sedona area has been of interest to the writer for some time and the present study is hoped to shed some light on these fascinating geologic features. This report is offered as a public contribution at the cost of publication and without fee.