The Campanian Manson impact structure of Iowa represents the best-preserved, large-diameter complex crater within the continental United States. The related bolide struck from the southeast at a low angle, potentially distributing ejecta downrange to the northwest across the Western Interior Cretaceous Seaway. Here, we (1) examine possible correlation of Manson impact horizons across the Cretaceous seaway to terrestrial formations of Montana, and (2) test a large hadrosaur bone bed from the Two Medicine Formation for evidence indicative of the Manson impact. The study includes geochronology; palynomorph, soot, and geochemical analyses; and physical searches for impact ejecta. The impact ejecta–bearing Crow Creek Member of the marine Pierre Shale can be correlated to the SB2 discontinuity in the Judith River and Two Medicine Formations of Montana based on radiometric dates, ammonite zonation, and an association with the onset of the Bearpaw transgression. A 40 Ar/ 39 Ar analysis of an associated bentonite bed dates the hadrosaur bone bed (TM-003) to 75.92 ± 0.32 Ma referenced to MMhb-1 at 523.1 Ma. This bentonite and associated lacustrine units suggest a potential correlation with the SB2 and the Crow Creek Member. However, our examination of the bone bed produced no definitive impact evidence. The combined analyses did reveal three unusual aspects: (1) an abundance of Ulmoideipites sp., (2) a high soot content, and (3) elemental and mineralogical changes suggestive of distinct geochemical units. A major wildfire followed by a postcatastrophe bloom dominated by Ulmoideipites sp. likely preceded the eventual debris flow that generated the bone bed. The SB2 discontinuity and the 33n.3r magnetic subzone represent traceable stratigraphic markers that could serve as guides in future exploration for Manson impact evidence in terrestrial formations west of the seaway.

Abstract This field trip covers sedimentological and paleontological research being conducted on the Jurassic Aztec Sandstone and Lower Cretaceous Willow Tank Formation in Valley of Fire State Park. Valley of Fire State Park is located in southern Nevada, just outside of the town of Overton. The Jurassic Aztec Sandstone is equivalent to the Navajo and Nugget Sandstones; these formations together record an aerially large erg complex along the western margin of North America during the time of deposition. Invertebrate and vertebrate ichnofossils are not uncommon in portions of these Jurassic formations. The Willow Tank Formation is composed of the deposits of both a braided and anastomosed fluvial system. This system drained off the paleohigh of the Sevier fold and thrust front to the west, during Early Cretaceous time. Recently a diverse vertebrate assemblage has been discovered from this formation. The fauna of the Willow Tank Formation are similar to other Early Cretaceous faunas from western North America. The vertebrate remains recovered include three taxa of fish, three to four taxa of turtle, crocodilian, iguanodontian, thyreophoran, dromaeosaur, tyrannosauroid, two theropod ootaxa, and a titanosauriform. In addition to the vertebrate elements, two fern morphotypes have been found. Through the course of this field trip participants will see extensive exposures of Aztec Sandstone, including vertebrate ichnofossils. Participants will also hike to vertebrate bearing-beds of the Willow Tank Formation.