R.R. Anderson, B.J. Witzke, E.M. Shoemaker, D.J. Roddy, and J.B. Hartung

The Geological Society of America
1993 Annual Meeting
Boston, MA, October 25-28, 1993
1993 Abstracts with Program, v. 25, no. 6, p. A-222


The U.S. Geological Survey and Iowa Department of Natural Resources recently drilled 12 cores totaling over 1,200 m in the Late Cretaceous Manson Impact Structure in Iowa. Four principal lithologic units were encountered in these cores: (1) Sedimentary Clast Breccia (SCB), (2) Crystalline Clast Breccia with Melt Rock Matrix (CCB-M) and Sandy Matrix (CCB-S), (3) Central Peak Igneous and Metamorphic Rocks (CP), and (4) an overturned flap of Impact Ejecta (IE). The SCB, recovered from all major structural terranes, is dominated by clasts of Cretaceous marine shale and mudstone, with subordinate Cretaceous sandstone, Paleozoic carbonate, minor Proterozoic red clastics, and rare crystalline rock and impact melt-rock clasts in a medium gray, calcareous, sandy argillaceous matrix. Evidence of impact metamorphism is rare. The SCB reaches maximum thicknesses in excess of 200 m and is interpreted as post-impact landslides that originated at the crater margins. The CCB-M, contains crystalline rock and mineral clasts displaying planar deformation features, shock isotropism, and/or partial melting in a melt-rock matrix that displays flow-banding and devitrification textures. The CCB-M is interpreted as an impact melt-layer, derived primarily from crystalline basement rocks. The CCB-S is dominated by clasts of basement gneiss and granite in a matrix of sand- to silt-size grains of disaggregated crystalline rocks. Most clasts display abundant impact deformation features. The CCB-S is interpreted as the impact-brecciated floor of the transient crater, uplifted on the Central Peak. The CP consists of Proterozoic crystalline basement rocks that form the core of the Central Peak. They are dominated by gneisses and granites, but also include minor mafic and ultramafic rocks; all lithologies display abundant planar deformation features and other shock deformation features. The IE, encountered in one core in the Terrace Terrane, totaled over 200 m of Proterozoic red clastics and Paleozoic carbonates and clastics in overturned stratigraphic succession. The IE is interpreted as an ejecta flap, only partially penetrated by the drill core. Deeper core drilling in this region would penetrate to the structurally-preserved impact surface, a tempting target for future drilling.