Dalton Wells: Geology and significance of debris-flow-hosted dinosaur bonebeds in the Cedar Mountain Formation (Lower Cretaceous) of eastern Utah, USA

Abstract

The Lower Cretaceous Dalton Wells dinosaur locality, north of Moab, Utah, consists of a 2 m thick, stacked succession of four fossil-rich bonebeds with an estimated collective lateral extent of more than 4000 m 2. To date, 215 m 2 of the bonebeds have been excavated and more than 4200 vertebrate fossil field specimens have been collected. The site occurs at the base of the Yellow Cat Member of the Cedar Mountain Formation, and lies unconformably on Upper Jurassic strata of the Morrison Formation. The age of the assemblage is tentatively accepted as Barremian. Nine dinosaur taxa are recognized, making Dalton Wells one of the richest (abundance and diversity) Lower Cretaceous dinosaur localities in the world. The bonebeds are rare examples of fossiliferous subaerial debris flow deposits. At least four flows originated an undetermined distance up-slope from an Early Cretaceous site of dinosaur bone concentration and flowed through the site carrying bones short distances toward an adjacent area of lakes, ponds and wetlands. Host sediments most likely accumulated on the eastward-dipping slopes of a backbulge in a foreland basin. The overall depositional setting was alluvial–lacustrine in a warm-to-hot and seasonally wet-and-dry climate. The consistent expression and location of lacustrine facies in both the Brushy Basin Member of the Morrison Formation and the Yellow Cat Member of the Cedar Mountain Formation indicate that high rates of subsidence typified the Dalton Wells area both before and after the J–K transition, and further suggest that there was some form of long-term to episodic and localized influence on subsidence. The depositional slope required to generate debris flows may have been established by forebulge crustal flexure, but could also have been established or amplified by the high rates of subsidence that maintained the lacustrine paleoenvironments in the field area. Concretionary deposits in the Yellow Cat Member at Dalton Wells are diagenetic and apparently resulted from groundwater precipitation, likely enhanced by the presence of organics (bones) and lacustrine carbonates. The regional and cross-facies expression of calcretes in the Yellow Cat Member combined with the rarity of pedogenic indicators confirms that they are not paleosols. Mineral precipitation likely occurred as groundwater flow was established in response to foreland basin development. It remains unknown under what specific circumstances animals died and carcasses accumulated prior to being reworked by debris flows. However, the stratigraphic association of seasonally wet-and-dry facies throughout the Yellow Cat Member suggests that drought-induced mortality events were probably common. In a seasonally wet-and-dry environment, debris flows may have been triggered by intense rainfall or seismic events.