NEW RESEARCH CORE DRILLING IN THE MANSON IMPACT STRUCTURE: A DESCRIPTION OF THE ROCKS RECOVERED AND PRELIMINARY INTERPRETATIONS

by
R.R. Anderson and B.J. Witzke

Iowa Academy of Science
105th Session
April 23-24, 1993, Luther College, Decorah, Iowa
1993 Program Abstracts, p. 70

ABSTRACT


In 1991 and 1992 the Iowa Department of Natural Resources - Geological Survey Bureau (GSB) and the U.S. Geological Survey drilled 12 research cores in the Manson Impact Structure (MIS), the largest intact impact structure in the U.S. and a potential K-T boundary impact site. Cores were drilled in each of the 3 major terranes in the structure, and the rocks recovered have been broadly grouped into 5 major lithologies. The most widespread of these was (1) Sedimentary Clast Breccia, encountered in all regions of the MIS and interpreted as a post-impact debris flow. This breccia was up to 191 m thick with entrained blocks as large as 102 m. Other lithologies included: (2) Crystalline Clast Breccia, a polymictic breccia composed of basement clasts in a sandy or melt-rock matrix, which probably formed along the brecciated-to-melted Transient Crater floor and then uplifted on the Central Peak; (3) Central Peak Crystalline Rock, basement gneisses and granites that underlie the Crystalline Clast Breccia and form the core of the Central Peak; (4) Intact Cretaceous Strata, structurally preserved on down-dropped blocks in the MIS Terrace Terrane; and (5) Ejecta Flap, a stratigraphically inverted sequence of Proterozoic and Paleozoic strata also encountered in the Terrace Terrane. Post-impact lake sediments were not encountered, although anticipated by MIS models. Over 20 geoscientists from across the U.S and Europe as well as GSB geologists are currently investigating MIS core material. The results of this research will greatly expand our understanding of the MIS, its age, and relationship to events at the K-T boundary.