DESCRIPTIONS AND PRELIMINARY INTERPRETATIONS OF CORES RECOVERED FROM THE MANSON IMPACT STRUCTURE (IOWA)
R.R. Anderson, B.J. Witzke, J.B. Hartung, E.M. Shoemaker and D.J. Roddy
Lunar and Planetary Science
XXIV, Lunar and Planetary Institute, Houston
Abstracts of Papers, 1993, p. 35-36
A core drilling program initiated by the Iowa Department of Natural Resources - Geological Survey Bureau and U.S. Geological Survey in 1991 and 1992 collected 12 cores totaling over 1,200 m from the Manson Impact Structure, a probable K-T boundary structure (Anderson and Hartung, 1992; Kunk et al., 1989) located in north-central Iowa. Cores were recovered from each of the major structural terranes, with two cores (M-3 and M-4) from the Terrace Terrane, four cores (M-2, M-2A, M-6, and M-9) from the Crater Moat, and six cores (M-1, M-5, M-7, M-8, M-10, and M-11) from the Central Peak (Figure 1). These supplemented two Central Peak cores (1-A and 2-A) drilled in 1953. The cores penetrated five major impact lithologies. Sedimentary Clast Breccia (SCB) is a polymictic breccia, interpreted as a post-impact debris flow that is dominated by clasts of Cretaceous marine shale, lesser numbers of Paleozoic carbonate, sandstone, and shale clasts, minor Proterozoic Red Clastics clasts, and rare clasts of igneous and metamorphic rocks and impact melt-rock. These clasts are in a medium gray, calcareous, sandy, silty, shale matrix. Impact Ejecta is an inverted sequence of Proterozoic Red Clastics and Paleozoic carbonates, apparently emplaced as an overturned ejecta flap. Central Peak Crystalline Rocks (CPC) are Proterozoic gneisses and granites uplifted from the crystalline basement to form the Central Peak. Crystalline Clast Breccia with a Sandy Matrix (CCB-S) is a polymictic breccia composed of clasts of Proterozoic gneisses and granites displaying abundant planar deformation features (PDFs) and other impact deformation features in a matrix of sand- to silt-sized crystalline rock fragments and mineral grains from disaggregated crystalline rocks. Crystalline Clast Breccia with a Melt-Matrix (CCB-M) is a rock composed of clasts similar to the sandy matrix breccia in a dominantly isotropic matrix that displays regions of flow-banding, interpreted as originally melt in some areas, subsequently crystallized to varying degrees. This unit is the impact melt-layer.
Sedimentary Clast Breccia was encountered mantling portions of all 3 major structural terranes in cores 2-A, M-1, M-2, M-4, M-6, M-8, M-9, M-10, and M-11 (Figure 1). Where it overlies the CCB-M on the Central Peak (especially in M-1) the SCB incorporates both angular melt-rock clasts (solid when fractured and entrained in the SCB) and irregular "blobs" of melt-rock (apparently molten or plastic when entrained). This indicates emplacement of the SCB debris flow very shortly after crater formation, when only a thin crust had formed on melt-rock that capped the Central Peak. The composition of the SCB, dominated by the shallowest pre-impact lithologies, is similar to the anticipated composition of the latest, proximal ejecta, which is assumed to be a major source of this debris flow. Maximum SCB in thicknesses penetrated in the Manson Impact Structure include 137 m (M-4) in the Terrace Terrane, 191 m (M-2) in the Crater Moat, and 77 m (M-10) on the Central Peak. The magnitude of these debris flows and their presence in all terranes of the structure, including at the center of the Central Peak (1-A), poses many questions about mechanisms of their transportation, mixing, and emplacement. SCB flows would presumably have traveled across the Terrace Terrane, down to the crater floor in the Moat area, and up and over the Central Peak. Although most previous interpretations of latest Cretaceous paleogeography concluded that epicontinental seas had completely regressed from the North American continental interior (including the Manson site), alternative stratigraphic interpretations suggest that the seaway may in fact still have been present in the area at the close of the Cretaceous. Water displaced by the impact and flooding back into the crater could have been a factor in transport of the massive SCB debris flow.
Crystalline Clast Breccias were encountered on the Central Peak. CCB-S was penetrated in cores 2-A, M-1, and M-11, CCB-M in cores M-1, M-7, M-8, and M-10. While clast in these breccias are almost exclusively crystalline, rare shale and siltstone clasts and rare melt rock clasts are present throughout these units. Additionally, some crystalline clasts are mantled with glassy layers, reminiscent of accretionary lapilli. These clasts and the polymictic character of the breccias suggest some amount of clast movement and mixing. These breccias probably represent impact-brecciated transient crater floor, the CCB-M from a position nearer inside of the transient crater where temperatures were highest. The thickest sequence of CCB-S, 65 m, was penetrated in the M-11 core, and the thickest CCB-M, 80 m, in the M-7 core.
The Central Peak Crystalline Rock was encountered in the 2-A, M-5, M-7, M-8, and M-10 cores. The CPC is dominated by biotite oligoclase quartz gneiss, but includes other gneisses, granitic intrusives, and minor mafic lithologies. In the M-7, M-8, and M-10 cores the CPC is present as large blocks separated by thin intervals of CCB-S, apparently intruded between the blocks. Also present in CPC blocks are thin pseudotachylite-like bands of melt-rock which frequently cut across gneiss foliation.
The overturned flap of Impact Ejecta material penetrated in the M-4 core includes Proterozoic Red Clastic strata overlying Paleozoic Cambrian, Ordovician, and Devonian strata. Although all Paleozoic units present at the time of impact are preserved (in inverted order) in the M-4 core, they are only about 18% of their pre-impact thicknesses.
Samples from these cores are currently being investigated by over a dozen scientists. As their research advances, so will our understanding of the Manson Impact Structure and its relationship to events at the K-T boundary. Samples from these cores are available as research materials from both the U.S. Geological Survey and the Iowa Department of Natural Resources - Geological Survey Bureau. Interested scientists are encouraged to contact the authors.
|Figure 1. Location of the Manson Impact Structure cores and principal crater terranes.|
Anderson, R.R. and Hartung, J.B., 1992, The Manson impact structure: its contribution to impact materials observed at the Cretaceous/Tertiary boundary: Proc. Lunar Planet. Sci., v. 22, 101-110.
Kunk, M.J., Izett, G.A., Haugerud, R.A., and Sutter, J.F., 1989, 40Ar/39Ar dating of the Manson impact structure; a Cretraceous-Tertiary boundary candidate: Science, v. 244, p. 1565-1568.