* Astronomy

Blobrana · L

RE: Manson Impact Structure

Blobrana · L

The Manson Impact refers to a meteorite that struck what now is Pocahontas County, Iowa. Buried 100 to 300 feet below the town of Manson is the geologic record of the impact, which struck with the force of 10 trillion tons of TNT

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Blobrana · L

Title: Evidence from the Crow Creek Member (Pierre Shale) for an impact-induced resuspension event in the late Cretaceous Western Interior Seaway
Authors: Ryan D. Weber and David K. Watkins

The 13-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.

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Blobrana · L

The impactor is considered to have been a stony meteorite about two kilometres in diameter. The impact disrupted granite, gneiss, and shales of the Precambrian basement as well as sedimentary formations of Palaeozoic age, Devonian through Cretaceous.

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Latitude: 42.5934°N, Longitude: -94.5446°W
Diameter 35 km

Manson Impact Structure.kmz
Google earth file (3kb, kmz)

Blobrana · L

Seventy-four million years ago, near the end of the Cretaceous Period, central Iowa lay near the shoreline of an inland seaway that separated eastern North America from rapidly rising mountains to the west. The low-lying Iowa landscape was home to a rich and varied population of plants and animals, including dinosaurs and small mammals. These organisms lived in a fern-rich, mixed conifer and deciduous forest with a warm, moist climate much like todays Gulf Coast. The environment dramatically changed when a stony meteorite, over one mile in diameter, weighing about 10 billion tons and travelling about 45,000 miles per hour, blasted through the atmosphere and crashed to earth.
In the fraction of a second that it took the meteorite to penetrate about one mile into the ground, the shock wave created by the initial contact with the surface reached the back side of the meteorite and its potential energy was transformed to kinetic energy, the equivalent of about 10 trillion tons of TNT. An electromagnetic pulse moved away from the point of impact at nearly the speed of light, and instantly ignited anything that would burn within approximately 130 miles of the impact (most of Iowa). The shock wave toppled trees up to 300 miles away (Chicago, Minneapolis, and St. Louis), and probably killed most animals within about 650 miles (Detroit and Denver). The blast left a crater over 24 miles in diameter centred in an area of unimaginable death and destruction.
Today there is no land surface expression of the crater that exists 100 to 300 feet below the town of Manson (Calhoun County), which lies near the centre of the crater that bears its name.

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