Soot and palynologic analysis of Manson impact-related strata (Upper Cretaceous) of Iowa and South Dakota, USA David J. Varricchio a, * , Russell F. Raven b , Wendy S. Wolbach c , William C. Elsik d , Brian J. Witzke e a Earth Sciences, Montana State University, Bozeman, MT 59717, USA b 6324 Wisteria Way, San Jose, CA 95129, USA c Department of Chemistry, DePaul University, Chicago, IL 60014-3214, USA d The MycoStrat Connection, PO Box 549, Snook, TX 77878, USA e Iowa Geological Survey,109 Trowbridge Hall, Iowa City, IA 52242-1319, USA article info Article history: Received 7 June 2007 Accepted in revised form 8 June 2008 Available online 21 June 2008 Keywords: Manson impact structure Impact structures Impact craters Pierre Shale Palynology Soot abstract The Campanian Manson impact structure of Iowa represents the best-preserved, large-diameter complex crater within the continental U.S. To assess the timing and potential mode of crater infilling and the possible presence of an impact event horizon, we analyzed samples from both within and distal to the impact structure for their elemental carbon, soot and palynomorphs. Within the impact structure, identifiable soot occurred in fragmented impact breccia and suevite but not in lower impact-melt breccia. Although most of this soot likely represents reworking of material from older Cretaceous marine shales, one high soot concentration occurs with melt material in a Keweenawan Shale–Phanerozoic clast breccia mix. This represents the first association of soot and impact-generated materials within an impact structure and the best sample candidate for Manson impact-generated soot. No palynomorphs occurred in the impact melt breccia. Overlying suevite (Keweenawan Shale clast breccia) of the central peak yielded sparse and thermally altered palynomorphs, indicating deposition prior to full cooling of the crater debris. Presence of easily degraded soot also argues for rapid backfilling of the crater. Distal samples from South Dakota represent the Sharon Springs and Crow Creek members of the Pierre Shale 230 km northwest of the Manson impact structure. Although containing shocked grains, the Crow Creek preserves no soot. In contrast, the Sharon Springs, generally considered as predating the Manson impact, has significant soot quantities. Palynomorphs differ markedly across the unconformity separating the two members with the Crow Creek containing more terrestrial forms, normapolles, and older reworked palynomorphs, consistent with a terrestrial impact to the east. Origin of the Sharon Springs soot remains unclear. Given soot occurrence within four of the five Cretaceous marine units sampled, the relatively shallow, anoxic bottom conditions of the Western Interior Cretaceous Seaway may have simply favored soot preservation. Until a better understanding of the broader occurrence and preservation of soot is achieved, some soot-impact associations will remain ambiguous. Ó 2008 Elsevier Ltd. All rights reserved. 1. Introduction Past analyses of impact craters have identified ejecta horizons containing various signature materials, such as coesite, shocked quartz, and concentrations of elemental carbon, soot, and platinum group elements. Recognition of these horizons within a known stratigraphic context can resolve age uncertainties about impact events, offer search locations for extraterrestrial material, and provide insights of potential impact effects on biotic systems (Chao et al., 1960; Alvarez et al., 1980; Koeberl, 1989; Koeberl and Sigurdsson, 1992: Wolbach et al., 1998; Poag, 1999). This study analyzed samples from the Upper Cretaceous (Campanian) Manson impact structure of northwest Iowa (Izett et al., 1998) and con- temporary strata within the Pierre Shale of South Dakota for soot and palynomorphs in order to discover evidence of 1) impact- generated regional fires, 2) other environmental perturbations, and 3) a regionally identifiable impact horizon. The Manson impact structure represents the best-preserved, large diameter (36 km), complex crater yet discovered within the continental United States (Hartung and Anderson, 1996; Koeberl and Anderson, 1996)(Fig. 1). The asymmetrical nature of the crater indicates the impactor entered the Earth’s atmosphere from the southeast at a low elevation angle (<45  ) distributing ejecta * Corresponding author. Earth Sciences, Montana State University, 200 Traphagen Hall, Bozeman, MT 59717, USA. E-mail address: djv@montana.edu (D.J. Varricchio). Contents lists available at ScienceDirect Cretaceous Research journal homepage: www.elsevier.com/locate/CretRes 0195-6671/$ – see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.cretres.2008.06.005 Cretaceous Research 30 (2009) 127–134