North Horn Formation

The North Horn Formation is a widespread non-marine sedimentary unit with extensive outcrops exposed in central and eastern Utah. The formation locally exceeds 1100 m in thickness and is characterized by fluvial, lacustrine, and floodplain dominated lithologies, representing a terrestrial, high energy, depositional environment. The sediments date from Late Cretaceous (Maastrichtian) to early Eocene in age and include the K-Pg extinction event boundary; however, this boundary is extremely difficult to locate and there is no strong stratigraphic evidence available that indicates a specific marker bed such as an iridium rich clay layer. Thus far, the only visible evidence is represented in the form of faunal turnover from dinosaur to mammal-dominated fossil assemblages. Taxa from the Cretaceous part of the formation include squamates, testudines, choristoderes, crocodyliforms, shark, fishes, amphibians, mammals, dinosaurs, eggshell fragments, trace fossils, molluscs, plant macrofossils, such as wood fragments, and palynomorphs. Characteristic dinosaur taxa include the ceratopsian Torosaurus utahensis, the titanosaurid sauropod Alamosaurus sanjuanensis, and the theropod Tyrannosaurus rex; however, the most frequently occurring taxon in the Cretaceous strata of the North Horn Formation is the teiid squamate Polyglyphanodon. Fauna recorded from Paleocene strata within the formation appear to be far more diverse and over 70 different taxa have been identified, including frogs, numerous multituberculate, protoeutherian, periptychid, and phenacodontid mammals, crocodyliforms, choristoderes, trace fossils, and palynomorphs.