THE MANSON IMPACT STRUCTURE RESEARCH PROGRAM: NEW RESULTS
D.J. Roddy, E.M. Shoemaker, and R.R. Anderson
VII International Symposium on
the Observation of the Continental Crust Through Drilling
Santa Fe, New Mexico, 1994
The Manson Impact Structure (MIS), located in north-central Iowa, lies at the bedrock surface, but is completely buried by 30 to 70 m of glacial drift. The feature was identified as an impact structure with the discovery of multiple sets of planar deformation features (PDFs) in quartz grains recovered from the center of the structure (Short, 1966). A series of 40Ar/39Ar age determinations from central peak feldspars (Kunk et al., 1987; Kunk et al., 1989; and Hartung et al., 1990) concluded that the MIS formed about 65 Ma, "an age indistinguishable from recent estimates of the age of the K/T boundary" (Kunk et al., 1989) and coincident with one of the Earth's greatest mass extinctions of fauna and flora, possibly related to a large bolide impact (Alvarez et al., 1980).
With the establishment of a probable relationship between the MIS and the K/T boundary, a consortium of the National Science Foundation, the U.S. Geological Survey (USGS) and Department of Energy, and NASA was formed under the auspices of the DOSECC Continental Science Program to investigate the structure by research core drilling. In 1991 and 1992 the USGS, representing the MIS Consortium, assisted the Iowa Department of Natural Resources - Geological Survey Bureau (GSB) in drilling 12 research core holes into the MIS, recovering over 1,200 m of core. Most of the cores were drilled along a line of reflection seismic data (collected by AMOCO Production Co.) that approximates a radius of the MIS. Two cores were drilled in the MIS Terrace Terrane, four cores in the Crater Moat region, and six cores drilled on the Central Peak (Fig. 1). Preliminary logs of the cores were produced by the GSB, several cores were split, with half-splits reposited in the U.S.Geological Survey Core Repository, and selected samples were distributed to MIS Consortium-supported researchers and others.
Table 1. Locations of Manson cores.
|M-1 Pierson||1991||Pocahontas||NE,SE,NE sec. 18 T90N R31W|
|M-2 & M-2A Hoefing||1991||Pocahontas||SE,NE,NE sec. 23 T90N R31W|
|M-3 Allen||1992||Webster||SW,SE,SW sec. 14 T90N R30W|
|M-4 Welch||1992||Webster||NW,NW,NW sec. 23 T90N R30W|
|M-5||1992||Pocahontas||SE,SE,SE sec. 17 T90N R31W|
|M-6||1992||Pocahontas||NE,SE,NE sec. 16 T90N R31W|
|M-7 Boyd||1992||Pocahontas||SE,SW,SW sec. 16 T90N R31W|
|M-8 Van Hoff||1992||Pocahontas||SW,SE,SW sec. 16 T90N R31W|
|M-9 Seifried||1992||Pocahontas||SE,SE,SE sec. 16 T90N R31W|
|M-10 Widlund||1992||Pocahontas||SE,SE,SE sec. 13 T90N R32W|
|M-11 Johnson||1992||Pocahontas||SW,SW,SW sec. 06 T89N R31W|
Table 2. Materials encountered in the Manson cores.
|M-1||1991||PCB||53 - 104||glassy clasts near base|
|CCB-M||104 - 159|
|CCB-S||159 - 211||few glassy clasts|
|M-2||1991||PCB||56 - 247||included 105 m-thick block of Cretaceous Carlile Fm|
|M-2A||1991||Mudstone||44 - 60||possible post-impact|
|M-3||1992||Cretaceous Strata||39 - 88||Carlile, Greenhorn, and Graneros formations|
|M-4||1992||PCB||39 - 176||many Red Clastics clasts|
|Layered Strata||176 - 375||inverted stratigraphic sequence, Red Clastics & Cambrian through Devonian|
|M-5||1992||Gneiss||40 - 42||shock-deformed quartz|
|M-6||1992||PCB||51 - 60|
|M-7||1992||CCB-M||52 - 132||abundant black shale clasts|
|Gneiss||132 - 221||short intervals of CCB-S|
|M-8||1992||PCB||56 - 83|
|CCB-M||83 - 124||abundant black shale clasts|
|Gneiss||124 - 168||short intervals of CCB-S|
|M-9||1992||PCB||59 - 64|
|M-10||1992||PCB||54 - 131||glassy clasts 90 - 131 m|
|CCB-M||131 - 149||calcite druse in fractures|
|Gneiss||149 - 188||short intervals of CCB-S and veins of black pseudotachylite|
|M-11||1992||PCB||51 - 71||small glassy clasts throughout|
|CCB-S||71 - 136||abundant glassy clasts|
|Figure 1. Location of twelve research core holes into the Manson Impact Structure.|
Alvarez, L.W., Alvarez, W., Asaro, F., and Mitchel, H.V., 1980, Extraterrestrial cause for the Cretaceous-Tertiary extinctions: Science, v. 208, p. 1095-1108.
Hartung, J.B., Kunk, M.J., and Anderson, R.R., 1990, Geology, geophysics, and geochronology of the Manson Impact Structure, in Global Catastrophies in Earth History, V.L. Sharpton and P.D. Ward (eds.): G.S.A. Special Paper 247, p. 207-222.
Kunk, M.J., Izett, G.A., and Sutter, J.F., 1987, 40Ar/39Ar age spectra of shocked K-feldspar suggests K-T boundary age for Manson, Iowa, impact structure (abs.): Eos Trans. AGU, v. 68, p. 1514.
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.
Short, N.M., 1966, Shock processes in geology, J. Geol. Edu., v.14, p. 149-166.