R.R. Anderson and B.J. Witzke

Iowa Academy of Science
106th Session
April 22-23, 1994, St. Ambrose University, Davenport, IA
1994 Program Abstracts, p. 17


Continued research on cores from the Manson Impact Structure (MIS) has advanced our understanding of the largest intact impact structure in the U.S. and one of the best preserved complex craters on Earth. One of the most enigmatic rock units encountered 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 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 a turbulent debris surge. At the Ries Crater, Bunte Breccia is found only in the area immediately outside the crater rim. 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. The probable mechanism for driving this debris flow is water, surging back into the crater following an impact into a shallow marine environment. Breccia similar to the PCB has been reported within only one other crater, the Chesapeake Bay Crater, where it is also interpreted as Bunte Breccia swept back into the crater by water following a marine impact.