A Summary of Results


by  D.J. Roddy, E.M. Shoemaker, and R.R. Anderson

The Geological Society of America
1994 Annual Meeting
Seattle, WA, October 24-27, 1994
1994 Program with Abstracts, v. 26, no. 7, p. A-337


The Manson structure, buried beneath glacial drift in western Iowa, is a 74-ma-old impact crater 36.5 km in diameter. In 1991, the Manson Impact Structure (MIS) Research Program was initiated as a multi-year study to examine this feature and produce an integrated view of the cratering event and its subsequent effects on the terrestrial environment. These studies have substantially expanded our understanding of the Manson impact event and its role in the continuum of large impacts that have affected the Earth.

During 1991-1992, 12 holes were drilled along an E-W line through the east half of the structure and its rim, producing 1283 m of core. The Iowa Department of Natural Resources - Geological Survey Bureau, as part of the MIS cooperative program with the USGS, conducted the drilling along an AMOCO seismic reflection line that extends from near the center of the MIS through the east rim. Two holes were drilled in the terraced part of the crater walls, three in the crater floor region, and seven in the Central Peak. Using these cores, research teams have been conducting detailed petrologic, stratigraphic, shock-metamorphic, isotopic dating, paleomagnetic, geochemical, geophysical ,and biostratigraphic studies. Field geophysical studies, such as seismic reflection, down-hole logging, and gravity, also were conducted to provide structural information.

The most significant Manson findings include (a) a new isotopic 40A/39A age of 74 ma; (b) core analyses showing impact time was during normal geomagnetic polarity; (c) amount of displacement of basement rock in central uplift (over 5 km); (d) identification of complex melt unit overlying at least part of the Central Peak; (e) recognition of overturned and stratigraphically inverted near-field ejecta deposits on terraced crater walls; (f) recognition of far-field ejecta, possibly tsunami-deposited, with shock metamorphic features in Upper Cretaceous Pierre Shale 240-500 km distant; (g) documentation of rim collapse by core and geophysical analyses; (h) recognition of partial filling of crater by landslides and/or other possible flow-type deposits by core and geophysical analyses; (i) determination that crater erosion was only moderate, leaving central uplift and terraced walls still intact; and (j) recognition of long-term hydrothermal activity in crater-fill deposits.

These conclusions were drawn directly from the Manson Impact Structure Research Program which has been jointly sponsored by the U.S. Geological Survey (USGS), the Iowa Department of Natural Resources - Geological Survey Bureau (IDNR-GSB), the National Science Foundation (NSF), U.S. Department of Energy (DOE), and the National Aeronautics and Space Administration (NASA); the USGS, DOE, and NSF joint sponsorship is part of the U.S. Continental Scientific Drilling Program.