FIRST ROUND OF RESEARCH ON THE MANSON IMPACT STRUCTURE ANSWERS MANY QUESTIONS....RAISES MANY MORE

by
R. R. Anderson

Iowa Academy of Science
107th Session
April 21-22, 1995, Wartburg College, Waverly, IA
1995 Program Abstracts, p. 17

ABSTRACT

The first round of research on the recently obtained drill cores from the 37 km diameter Manson Impact Structure in north-central Iowa is now completed. Key among the discoveries is the new age of the structure, 73.8 Ma, about 9 million years older than initially determined and no longer considered related to the Cretaceous/Tertiary boundary. Other important discoveries include the anomalously large size of the crater's central peak (possibly an indication of an oblique impact), the identification of the Phanerozoic-clast breccia (a poorly-understood deposit that filled the crater very shortly after crater formation), the preservation of an overturned ejecta flap (probably overlying the pre-impact surface), and the probable existence of distal impact ejecta in the Crow Creek Member of the Pierre Shale in South Dakota and Nebraska. Studies of secondary mineralization indicate that a post-impact hydrothermal system reached temperatures as high as 360ºC, primarily driven by elevation of the geotherm with the central peak. Detailed examination of the impact rocks has led to the discovery of several rarely observed and unexplained shock metamorphic features including "toasting" of quartz grains, shock-induced polycrystallinity, and altered alternate albite twins. New questions include the environment at the impact site (marine or terrestrial), the origin of several of the impact breccias and the processes that governed their deposition, the dynamics of ejecta emplacement and sorting as observed in the Crow Creek Mbr., and the effect of the impact on flora and fauna in the region. The Manson structure is one of the best-preserved complex impact structures on Earth, and apparently one of the few with preserved distal ejecta. The application of current research on the Manson structure and the results of future studies will undoubtedly help to advance our understanding of impact processes.