A new mass death assemblage in Lower Cretaceous strata of east-central Utah contains well-preserved skeletons representing an ontogenetic series of individuals of Utahraptor, and at minimum two iguanodont grade ornithischian skeletons. The dinosaurs were entombed in ovoid-lensoidal, fine-grained sandstone sills linked by sandstone pipes and/or dikes and another basal lensoidal mass with scattered and broken iguanodont and sauropod bones and to an underlying gravelly sandstone bed. Exposed in the excavation high-walls are syndepositional normal-faults bounding graded ripple strata. Multiphased fluid over-pressurization in an artesian setting creating the structures. Trapping, killing, and subsequent burial mechanism was generated by variations of pressure in a localized artesian spring system that breached the surface and is the first such mechanism documented with numerous dinosaur victims.