CASE OF THE MISSING MARBLES: IRON OXIDE CONCRETIONS OF GRAND STAIRCASE ESCALANTE NATIONAL MONUMENT, UTAH

Numerous spheroidal golf-ball sized iron oxide concretions (cemented mineral masses, commonly known as “moqui marbles”) comprise a unique resource of Grand Staircase Escalante National Monument (GSENM) of Utah. Although concretions occur in many place worldwide, the Jurassic Navajo Sandstone examples are unique in their quantity, size, geometric, shapes, texture, internal structure, and mineralogy. The Navajo Sandstone is a widespread formation representing an ancient desert system (190 mya), and it comprises distinctive landscape exposures in southern Utah and in several of the state’s national parks including Canyonlands, Arches, Zion, and Capitol Reef. Diagenetic iron-rich fluids precipitated iron oxide cements to make the hard, resistant marbles that weathered out of the weakly cement sandstone host, resulting in impressive accumulations of concretions lags on the landscape.

Although the concretions are a protected resource of GSENM, over 40,000 were stolen off of public lands for resale and profit, and recovered by the Bureau of Land Management (BLM) law enforcement in 2012 and 2015. Evidence of the unethical disturbance was sobering as the thieves used heavy equipment to scrape across weathered sandstone surfaces. BLM needed proof that the marbles were from GSENM, which was provided by geologic characterization and comparison with concretions from other sites. The cases were recently settled and four individuals pled guilty to the felony charges. For BLM, the case of the stolen concretions was unusual in that the marbles had a specific and established monetary value. Some of the confiscated concretions have been used for educational purposes, but the majority are still being held. The concretions within their geologic setting should be preserved intact and remain protected for future generations as a rich geoheritage resource that also has important analog applications for planetary science. In order to prevent illegal collection, the geologic community and the public can help by reporting thefts and discouraging the resale market by not purchasing concretions that were likely removed illegally from public lands.