Devils Tower National Monument: Paleontological resources inventory (non-sensitive version)

Tweet, Justin; Santucci, Vincent

Devils Tower National Monument (DETO), the first United States national monument, is world-famous for the monument’s namesake feature, Devils Tower, an unusual igneous feature rising over the plains and bluffs of northeastern Wyoming within the northwestern Black Hills. This feature, of long-debated origin, formed in surroundings composed of much older sedimentary rocks deposited in and around shallow seas from the Permian to Cretaceous periods. The youngest sedimentary rocks long ago eroded to nothing but residuum, in the process exposing the igneous body that erosion has sculpted into the Tower. However, several formations of Permian to Jurassic age are exposed around the Tower within DETO. Fossils are found at various locations throughout the monument, primarily from the Middle–Upper Jurassic Sundance Formation. The presence of fossils near Devils Tower has been known since at least the beginning of the 20th century, but geologic work at DETO has almost always been focused on the Tower itself, with the overall character and depositional settings of the sedimentary rocks as a secondary focus and fossils considered only in passing until recently. Exploration of the monument over the past few years has located several new fossil localities, expanding and illuminating the scope of the monument’s paleontological resources. These fossils and the information they provide about the rocks help to provide a more complete picture of the story of DETO. In ascending order (oldest to youngest), the geologic formations at DETO include the Spearfish Formation (Permian–Triassic), Gypsum Spring Formation (Middle Jurassic), and four distinct members of the Sundance Formation (Middle–Upper Jurassic), with an unconfirmed report of a Morrison Formation (Upper Jurassic) outlier. These formations are frequently overlain by unconsolidated deposits, such as talus from the Tower, residuum possibly from the Lakota Formation, and alluvium in the floodplain of the Belle Fourche River. The Spearfish and Gypsum Spring formations appear to be unfossiliferous. On the other hand, the Sundance Formation preserves a variety of marine fossils, primarily fragments of belemnite cephalopods and beds of bivalve shells in DETO. Rare evidence of vertebrates has also been found. This unit was deposited approximately 167 to 160 million years ago during multiple marine transgressions (advances) and regressions (retreats), which are responsible for the different members of the formation. Exposures of the most fossiliferous parts of the Sundance Formation are limited in DETO, but they show thriving shallow marine ecosystems. Small numbers of fossils have also been reported from the Upper Jurassic Morrison Formation, Lower Cretaceous ?Lakota Formation, and an unusual “agglomerate” unit of igneous origin, although all but possibly the Morrison Formation occurrence are displaced from their original contexts. Many of the fossils of DETO are accessible from trails or roads, which increases their risk of loss through unauthorized collection or damage (intentional or unintentional), and there is anecdotal evidence of occasional collecting at the monument. In addition, the deep cultural significance of the Tower and associated resources leads to some complications for paleontological resource management. DETO has had site monitoring programs in the past, and it is recommended that they resume.

Devils Tower National Monument: Paleontological resources inventory (non-sensitive version)

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