PRELIMINARY RESULTS OF U.S. GEOLOGICAL SURVEY - IOWA DNR GEOLOGICAL SURVEY BUREAU MANSON CORE DRILLING PROJECT
R.R. Anderson, J.B. Hartung, B.J. Witzke, E.M. Shoemaker and D.J. Roddy
The Geological Society of
Special Paper 293, 1994, p. 237-248
Large Meteorite Impacts and Planetary Evolution
B. Dresser, R.A.F. Grieve, and V.I. Sharpton, eds.
The U.S. Geological Survey and Iowa Department of Natural Resources - Geological Survey Bureau completed, in 1992, a two-year core drilling program in the Manson Impact Structure, a 35-km diameter Cretaceous-Tertiary boundary-age feature located in north-central Iowa. A total of 12 cores sampled in excess of 1,200 m (4,000 feet) of crater rocks, supplementing the two previously drilled shallow cores from the structure. The cores penetrated the three major terranes in the Manson Impact Structure: the Terrace Terrane, Crater Moat, and Central Peak. Preliminary interpretations identified several important impact-related lithologies in these cores. The most widespread is sedimentary clast breccia, a post-impact polymictic breccia or mixtite that mantles at least part of all crater terranes. Two types of crystalline clast breccias were cored on the Central Peak, one with a melt-matrix, the other with a matrix dominated by silt- to sand-sized grains. Large blocks of basement gneiss that formed the interior of the Central Peak were encountered in several cores. Within the Terrace Terrane, structurally-preserved Cretaceous strata and an overturned ejecta flap of Proterozoic and Paleozoic sedimentary rocks were encountered. Only sedimentary clast breccia was encountered in the Crater Moat. The preliminary investigation of these cores has provided significant information on the geometry and history of the Manson Impact Structure, but it has also prompted many more questions.