R.R. Anderson, B.J. Witzke, and J.B. Hartung

The Geological Society of America
Special Paper 307, 1996, p.527-540
New Developments Regarding the KT Event and Other Catastrophes in Earth History
G. Ryder, S. Gartner, and D. Fastovsky, eds.


In 1992 the Iowa Department of Natural Resources - Geological Survey Bureau and U.S. Geological Survey Branch of Astrogeology completed the last of 12 research cores drilled in the Manson Impact Structure. In excess of 1,200 m of drill core was recovered; almost all were impact materials (materials emplaced as a direct result of the impact). Six primary impact materials were encountered. They include:

Crystalline Basement Blocks. Large blocks of crystalline basement materials lifted up over 5 km with the crater's central peak.

Crystalline Clast Breccia with Sandy Matrix. Blocks of brecciated crystalline rocks in a matrix of disaggregated crystalline rock and mineral fragments.

Crystalline Clast Breccia with Melt Rock Matrix. Blocks of brecciated crystalline rocks and mineral grains (dominantly quartz) in a melt rock matrix.

Keweenawan Clast Breccia. Clasts of brecciated dark gray to black Keweenawan (Middle Proterozoic) shales, siltstones, and sandstones mixed with light gray-green, devitrified melt rock.

Phanerozoic Clast Breccia. A matrix-dominated breccia composed primarily of clasts of Cretaceous marine shale clasts with lesser numbers of Paleozoic carbonate clasts, rare clasts of Proterozoic Keweenawan clastic rocks, and very rare clasts of crystalline rock and melt rocks in a medium gray calcareous sandy shale matrix.

Impact Ejecta. An overturned flap of latest ejecta on the terrace terrane.

The Crystalline Basement Blocks (CBB), Proterozoic gneisses and granites which form the core of the central peak, were encountered in 4 cores. Pseudotachylite dikes of melt rock and crystalline rock breccia intrude these blocks. The Crystalline Clast Breccia with Sandy Matrix (SM) lies above the CBB on the central peak and was encountered in 5 cores. Rare clasts of melt-rock and siltstone reflect minor mixing with overlying units. The SM is interpreted as impact-brecciated lower transient crater floor, similar to Ries crater suevite (Hörz, 1982). The SM grades upward into the Crystalline Clast Breccia with Melt Rock Matrix (MRM), recovered in 5 cores. The central portion of the MRM was apparently the hottest, displaying the maximum melt/clast ratio, sanidine, and zones of solid state diffusion (Reagan et al., 1993). The MRM also displays rare siltstone clasts and crystalline rock clasts with melt-rock mantles (probable airborne accretion) suggesting minor mixing of overlying materials. The MRM is interpreted as transient crater lining, however a fall-back origin has also been suggested (Bell et al., 1993). A mixture of SM and MRM was encountered in 2 cores, indicating mixture of the units by slumping or sliding. The uppermost of the central peak impact units is the Keweenawan Clast Breccia (KCB), encountered in 4 cores. The KCB is a mixture of unmelted and partially melted dark gray to black shale-dominated Proterozoic clastic rocks mixed with melt rock. This relatively thin unit is interpreted as the innermost portion of the transient crater, with unmelted Keweenawan Clastic rocks displaced downward during crater formation, then lifted up on the crest of the Central Peak.

On the terrace terrane an overturned flap of late-stage Crater Ejecta (CE) was encountered in one core. This unit included an overturned sequence of Keweenawan Upper Red Clastics overlying Cambrian, Ordovician, and Devonian marine carbonate and clastic rocks. While all stratigraphic units in this interval appear to be present, they are represented by only about 17% of their normal thicknesses. This core reached the maximum depth attainable with the rig that was used for the drilling, however the possiblity of encountering the structurally-preserved Upper Cretaceous impact surface less than 100 m lower (~500 m below land surface) presents a tempting target for future drilling.

The most unique impact material encountered in the Manson core drilling was the Phanerozoic Clast Breccia (PCB). The dominance of rock clasts from the shallowest pre-impact stratigraphic units (clast composition by volume is inversely related to pre-impact depth) and the scarceness of shock-melted or shock-deformed minerals is identical to Ries Crater Bunte breccia, described most recently by Newsom et al. (1990). Bunte breccia is, by definition, primary crater ejecta formed by the combined effects of ground surge and secondary cratering, and as such is restricted to regions outside of the transient crater. However, in the Manson Crater, PCB was encountered in 8 cores, in all regions of the crater (Figure 1), implying that if this material had a Bunte breccia-like origin, it must have been transported back into the crater by some extraordinary process. The most likely medium for such transport is water, displaced by the bolide impacting into a shallow marine environment, rushing back into the evacuated crater region. The presence of marine conditions in the Manson area does not preclude a latest Cretaceous impact, since erosion has left an equivocal record of conditions at that time. Recovery of the structurally-preserved impact surface by core drilling in the crater terrace would resolve the questions of impact environment and age.


Schematic illustration of
        the impact materials
Figure 1. Schematic illustration of the impact materials encountered in the Manson Impact Structure and their relative locations.


Bell, M.S., Reagan, M.K., Foster, Jr., C.T., and Anderson, R.R., 1993, Petrography of the crystalline breccias from the Manson M-1 core: Compositional correlation to target lithologies: Geol. Soc. Amer. Abst. with Prog., v. 25, no. 6, p. A-222.

Hörz, F., 1981, Ejecta of the Ries Crater, Germany, in Silver, L.T., and Schultz, P.H. (eds), Geological Implications of Impacts of Large Asteroids and Comets on the Earth: Geol. Soc. Amer. Spec. Paper 190, p. 39-56.

Newsom, H.E., Graup, G., Iseri, D.A., Geissman, J.W., and Keil, K., 1990, The Formation of the Ries Crater, West Germany; evidence of atmospheric interactions during a larger cratering event, in Sharpton, V.L. and Ward, P.D. (eds), Global Catastrophes in Earth History: Geol. Soc. Amer. Spec. Paper 247, p. 195-206.

Reagan, M.K., Foster, Jr., C.T., and Bell, M.S., 1993, Origin of the upper crystalline clast-bearing impact melt breccia in the Manson M-1 core: EOS (Transactions of the American Geophysical Union), v.74, no. 44, p. 386-387.