R.R. Anderson and B.J. Witzke

The Geological Society of America
27th Annual North-Central Section Meeting
Kalamazoo, Michigan, April 28-29, 1994
1994 Program with Abstracts, v. 26, no. 5, p. 2


Continuing study of 12 research cores recovered from the Manson Impact Structure (MIS) in north-central Iowa has advanced our understanding of the largest intact impact structure in the U.S. and one of the best preserved complex craters on Earth. Of the impact rocks encountered in the cores, one of the most enigmatic is the Phanerozoic Clast Breccia (PCB), a poorly consolidated polymict breccia that is preserved as the upper impact unit in all regions of the MIS. The PCB is a matrix-supported breccia with clasts ranging in size from mm to in excess of 100 m. Clasts are dominated by Cretaceous marine rocks, with subordinate Paleozoic carbonate-dominated sedimentary rock clasts, minor clasts of Proterozoic Red Clastics, and very rare clasts of Proterozoic crystalline rocks and impact melt-rock. The abundance of each clast lithology is inversely related to its pre-impact depth of burial. Parallel deformation features and other indicators of hypervelocity are very rare in these clasts. The matrix is a light gray, calcareous, sandy, silty shale that contains scattered to common Cretaceous foraminifera in some samples. PCB clast composition varies within the MIS, especially near its base, reflecting lithologies in underlying breccia units. The PCB is compositionally similar to the Bunte Breccia described from the Ries Crater (Germany). Bunte Breccia is interpreted as a mixture of proximal impact ejecta and material excavated by secondary cratering processes, mobilized and emplaced by turbulent debris surge. At the Ries Crater, Bunte Breccia is found only in the area immediately outside the crater rim. At Manson all impact materials not structurally preserved within the crater have been removed by post-impact erosion. However, unlike Ries Bunte Breccia, the PCB is found inside the crater, recovered in cores from the Terrace Terrane, the Crater Moat, and even in the Central Peak Pit. Manson PCB appears to be Bunte Breccia that was originally deposited outside the MIS, but was remobilized and transported into the crater as a debris flow with sufficient energy to carry it for at least 17 km, across the Terrace Terrane, to the floor of the Crater Moat (about 2 km below the crater rim), and up at least 2 km to the top of the Central Peak. One possible mechanism for driving this debris flow is water, surging back into the crater following an impact into a shallow marine environment.