M.S. Bell, M.K. Reagan, R.R. Anderson, and C.T. Foster Jr.


The Geological Society of America
Special Paper 302, 1996, p. 221-234
The Manson Impact Structure, Iowa: Anatomy of an impact crater
C. Koeberl and R.R. Anderson, eds.


Drilling at the M-1 site on the flank of the central peak intersected the most diverse suite of breccias encountered at a single location within the Manson Impact Structure. Here, we describe and categorize breccias from the lower 108 m of the drill-core, which are noteworthy because of the abundant clasts from the crystalline basement. The M-1 core impact breccias roughly preserve the original stratigraphy of the target lithologies. The uppermost breccias are rich in Phanerozoic and Proterozoic sedimentary clasts, and contain less than a few volume % of crystalline lithologies or shock-melted and shock-metamorphosed materials. The breccias focused on here underlie a sharp but irregular contact. The uppermost of these crystalline clast breccias (unit 1, 106.4-146 meters depth) is unsorted, matrix-rich, and predominately consists of shock-melted and shock-metamorphosed clasts from a limited number of Proterozoic basement lithologies. Proterozoic shale clasts are rare, although the shale content of the matrix is significant. The uppermost portion of this breccia (unit 1a) has a nearly isotropic to finely crystalline matrix, whereas the lower portion (unit 1b) has a more coarsely crystalline matrix and reaction coronas around quartz. The lowermost 53 m of the M-1 core (unit 3, 161-214.3 meter depth) consist of breccias with relatively coarse-grained unrecrystallized matrices, high clast abundances, more diverse clast lithologies, rare solidified melt clasts and emplacement temperatures considerably below the solidus. Between these two breccias is a zone where the two lithologies intermingle, but do not homogeneously mix (unit 2, 146-161 meters depth). Unit 3 is interpreted to be crater lining deposits that formed as the crater was excavated. Unit 1 also may have been emplaced by this mechanism, although the distinct break in lithology between the melt-rich and melt-poor breccias and the poorly-sorted nature of the melt-rich breccias suggest that unit 1 may have been emplaced by a different mechanism than unit 3. This mechanism may have been a density current that originated in the ejecta curtain and flowed back into the crater. In either case, the mingling of the breccias may have been related to uplift of the central peak. The lithologies of the crystalline clast breccias are similar to those encountered at the West Clearwater Impact Structure, which had a similar size, but had less of a sedimentary veneer before impact. However, a clast-poor melt sheet like that of West Clearwater, has not been encountered at the Manson Impact Structure. In addition, the volume of the melt at Manson is significantly less than for West Clearwater. This may be related to the dispersal of the melt material by explosive expansion of volatiles derived from sediments in the target area as suggested by Kieffer and Simonds (1980).


Kieffer, S.W., and Simonds, C.H., 1980, The role of volatiles and lithology in the impact cratering process: Reviews of Geophysics and Space Physics, v. 18, p. 143-181.