R.R. Anderson, B.J. Witzke, and D.J. Roddy

The Geological Society of America
Special Paper 302, 1996, p. 45-88
The Manson Impact Structure, Iowa: Anatomy of an impact crater
C. Koeberl and R.R. Anderson, eds.


In 1991 and 1992 a series of twelve research cores were drilled into the Manson Impact Structure (MIS) by the Iowa Department of Natural Resources - Geological Survey Bureau and U.S. Geological Survey, as part of a U.S. Continental Scientific Drilling Program sponsored research project. The original goal of the drilling was to geologically characterize the MIS, recover material to more accurately date the age of crater formation, sample impact melt-rock that may yield an impactor signature, and sample the different materials associated with the MIS for comparison with exotic materials found at the K-T boundary. A total of 1901 m (6178 ft) of drilling was completed in the MIS central peak, crater moat, and terrace terrane. A variety of problems were encountered in the process of drilling the highly brecciated and often poorly-consolidated impact materials, however most were resolved and a total of 1283 m (4170 ft) of core was recovered. Six primary impact rock lithologies were encountered, and described using non-genetic, lithologically descriptive terms, (1) crystalline basement rock, (2) crystalline clast breccia with sandy matrix, (3) crystalline clast breccia with melt matrix, (4) Keweenawan shale breccia, (5) Phanerozoic clast breccia, and (6) overturned strata. Most of these breccias can be classified using "standard impact crater terminology": i.e., crystalline basement rock and overturned strata="impactite," crystalline clast breccia with sandy matrix="fragmental breccia," and crystalline clast breccia with melt matrix and Keweenawan shale breccia="suevite." There are some problems in the definitions of these standard impact crater terms when they are applied to the materials observed in the MIS. One breccia, the Phanerozoic clast breccia, does not apparently fit any of these "standard terms." It is currently interpreted as a mixture of Bunte breccia-like deposits and fallout suevite, that were swept back into the crater very shortly after crater formation. This report describes the drilling of the 12 Manson research cores and the rocks retrieved in the cores (including core logs) and provides preliminary interpretations of the impact materials recovered.