MANSON IMPACT CRATER
- Type: Central peak
- Age: 74.1 Ma a
- Diameter: ~35Km
- Location: N 42°35’ W 94° 33’
a Dating Method:40Ar/39Ar method at 74.1 Ma (million years; Izett et al 1998)
In the Cretaceous Period, a large meteorite struck the earth at a location near the present town of Manson, Iowa. The heat of the impact melted some of the feldspar crystals in the granitic rocks of the impact zone, thereby resetting their internal radiometric clocks. These melted crystals, and therefore the impact, have been dated by the 40Ar/39Ar method at 74.1 Ma (million years; Izett et al 1998).
No surface evidence exists due to comparatively recent coverage by glacial till. The crater is buried 20 to 90 m below the surface. The area was known from unusual water well drill cuttings in 1912 of deformed rock, “crystalline clast breccia with a melt matrix” as a later report described it. A research investigation was started in 1955, and it was labeled a “cryptovolcanic structure” (a hypothetical volcanic steam explosion). Further investigation was undertaken that proposed an impact origin in 1959 and by in 1966 when evidence of shocked quartz grains which confirmed the impact origin of the structure was produced.
The impact also created shocked quartz crystals that were blasted into the air and subsequently fell to the west into the inland sea that occupied much of central North America at that time. Today this shocked quartz is found in South Dakota, Colorado, and Nebraska in a thin layer (the Crow Creek Member) within a thick rock formation known as the Pierre Shale. The Pierre Shale, which is divided into identifiable sedimentary beds called members, also contains abundant fossils of numerous species of ammonites, ancestors of the chambered nautilus. The fossils, when combined with geologic mapping, allow the various exposed sections of the Pierre Shale to be pieced together in their proper relative positions to form a complete composite section (Figure 1). The Pierre Shale also contains volcanic ash that was erupted from volcanoes and then fell into the sea, where it was preserved as thin beds. These ash beds, called bentonites, contain sanidine feldspar and biotite that has been dated using the 40Ar/39Ar technique.
Evidence from the Crow Creek Member (Pierre Shale) for an impact-induced resuspension event in the late Cretaceous Western Interior Seaway Ryan D. Weber and David K. Watkins
The 1–3-m-thick Crow Creek Member is a unique marlstone with rip-up clasts and a basal coarse layer in the Upper Cretaceous Pierre Shale in South Dakota and Nebraska. Although the Member has been thought to represent a marine transgression along the eastern margin of the Western Interior Seaway, the presence of impact ejecta from the Manson Impact Structure suggests an impact-induced genesis.
An upper Campanian in situ nannofossil assemblage with a lower Campanian reworked assemblage (from older Niobrara Chalk) occurs in the Crow Creek at most localities. The reworked assemblage decreases in abundance upward through the marlstone, a pattern consistent with an origin involving gravitational settling rather than marine transgression. Gray marlstone clasts in the basal coarse layer have nannofossils derived from the underlying Gregory Member and Niobrara Chalk. The reworked assemblage decreases in abundance with increased distances from the Manson Impact Structure and the Sioux Ridge (a paleotopographic high). The nonuniform geographic distribution of reworking suggests that Crow Creek deposition was linked to the Manson Impact. These observations, and a fining upward trend, the presence of impact ejecta, and coeval deposition with the Manson Impact Structure, support a resuspension-event origin for the Crow Creek Member.
G. Brent Dalrymple, Radiometric Dating Does Work! Reports of the National Center for Science Education
Crossey, L. J.,Mccarville, P. Post-impact alteration of the Manson impact structure Lunar and Planetary Inst., Twenty-fourth Lunar and Planetary Science Conference. Part 1: A-F; p 351-352
Samuels, S.H., Manson Ground Exploration
Short, N. M., Gold, D. P., Petrography of shocked rocks from the central peak at the Manson impact structure. Geological Society of America Special Paper 302, pp. 245-265. 1996.