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.

Core Name Drilled County Location
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.

Core Date
Interval (m)
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
  lake sediments
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



Core location map
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.