SOOT AND POLLEN FROM MANSON IMPACT-RELATED STRATA (UPPER CRETACEOUS) OF IOWA AND SOUTH DAKOTA

The Campanian Manson impact structure of Iowa (USA) represents the best-preserved, large-diameter complex crater yet discovered within the continental U.S. To assess the timing and potential mode of crater infilling and to establish the presence of an impact event horizon, we analyzed samples from both the MIS and more distal locations from South Dakota for their elemental carbon, soot and pollen content. No significant remains occurred in the lowest portion of crater fill, the impact melt breccia. The next horizons of suevite, the Keweenawan Shale clast breccia (KSCB), yielded sparse and thermally altered pollen indicating deposition prior to full cooling of the crater debris. Identifiable soot was present only in the upper portions of the crater-fill suevite, the Phanerozoic clast breccia (PCB) and PCB-KSCB mixtures. The highest soot concentration occurred in a reworked clast of Cretaceous marine shale from the PCB. This and the absence of soot from any portions of the fill lacking Cretaceous clasts, suggests that the soot in part represents reworked material. Although its origin remains ambiguous, presence of easily degraded soot argues for rapid backfilling of the crater. South Dakota samples represent the Crow Creek and Sharon Springs members of the Pierre Shale 230 km northwest of MIS. 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. In comparison to Sharon Springs, the Crow Creek samples contain higher amounts of terrestrial forms, moss and fern spores, Normapolles pollen, and older (Devonian-Cretaceous) reworked palynomoprhs. The abundance, taxonomic make-up, and recycled palynomorphs are consistent with a terrestrial impact to the east. Origin of the Sharon Springs soot remains unclear. The relatively shallow, anoxic bottom conditions of the Cretaceous Interior Seaway may have favored soot preservation. Alternatively, deposition of Sharon Springs in the sample area (Yankton) may be younger and contemporary with the impact. Until a better understanding of the broader occurrence and preservation of soot is achieved, some soot-impact associations will remain ambiguous.