Structural observations and stratigraphic variability in Jurassic strata, Upheaval Dome, Canyonlands National Park, Utah, USA

Abstract

Upheaval Dome is a structurally deformed topographic depression located in Canyonlands National Park, southeast Utah. Multiple hypotheses for its origin have been proposed by various scientists over many years of research. The two remaining viable hypotheses are at opposite ends of the geologic spectrum, one proposing long-term deformation of the structure, while the other proposes a catastrophic meteorite impact. (1) The seminal paper by Jackson et al. (1998) suggests that Upheaval Dome was created due to the growth and subsequent pinch-off of a salt diapir sourced from the Pennsylvanian Paradox Formation. Their conclusions were based on various growth geometries in Jurassic age strata. (2) Perhaps the most influential paper proposing a meteorite impact at Upheaval Dome is by Buchner and Kenkmann (2008), titled "Upheaval Dome, Utah, USA: Impact origin confirmed". In this paper only two grains of shocked quartz are identified, out of 120 standard thin sections. Based on these thin sections comprising medium-coarse sand grains, only ~0.00043% of grains displayed evidence of high-pressure deformation. For shocked quartz to confirm a meteorite impact there must be abundant shocked grains (2-5%), and ~0.00043% cannot be considered abundant (French and Koeberl, 2010). Prior to this study there has been no attempt made to combine an in depth stratigraphic investigation of exposed, accessible formations with structural and lithologic observations in the Upheaval Dome area. Analysis of stratigraphic field data for Triassic to Jurassic-aged strata reveals: (1) stratigraphic thicknesses from measured sections range from 7 meters to 224 meters in the Kayenta Formation, and projected thicknesses in cross sections can exceed 400 meters; (2) distinct changes in facies distributions in relation to mapped structural features; (3) localized angular discordances, such as angular unconformities and onlaps, at the contact between formations or within individual formations. Analysis of structural features at Upheaval Dome reveals: (1) synclinal axes and associated depositional centers shift throughout the Jurassic; (2) stratigraphic thicknesses across normal faults from hanging to footwall blocks are unequal on the scale of meters to tens of meters; (3) thrust faults verge dominantly to the southeast regardless of the side of the dome they are located on; (4) blocks of Triassic Chinle Formation encased in the younger Jurassic Wingate Sandstone adjacent to dog tongues suggests the involvement of a brief period of allochthonous salt break out after the deposition of the Chinle. Petrographic analysis was inconclusive, as there were no shocked grains, nor any clasts of the Paradox Formation present in younger formations. The research presented in this study strongly indicates that long-term deformation occured at Upheaval Dome during the Early Jurassic and possibly in older less well exposed units. Evidence supporting long-term deformation includes growth strata, changes in facies distributions, shifting formation depocenters, angular discordances, and growth faults. Sparse indicators of catastrophic are also present in the form of sparse shocked quartz grains and poorly developed shatter cones. To accommodate these juxtaposing deformational regimes an evolution of Upheaval Dome is presented here that relies on an early meteorite impact to initiate active diapirism leading eventually to a passively growing salt diapir. This explanation would account for the petrographic evidence supporting meteorite impact, as well as the growth geometries in the Triassic-Jurassic aged strata surrounding Upheaval Dome. Upheaval Dome has historically been one of the most controversial geologic features in the United States. It is important for geologists to understand the genesis for this structure as it is an extremely well exposed field example of a meteorite impact, pinched-off salt diapir, or a combination of the two, and can help further understand similar structures found around the world at the Earth's surface, or in the subsurface.