SEDIMENTARY-CLAST BRECCIAS OF THE MANSON IMPACT STRUCTURE, IOWA

by
B.J. Witzke and R.R. Anderson

The Geological Society of America
Special Paper 302, 1996, p. 115-144
The Manson Impact Structure, Iowa: Anatomy of an impact crater
C. Koeberl and R.R. Anderson, eds.

ABSTRACT


A variety of compositionally distinct sedimentary-clast breccias, all with a dominant clast composition of sedimentary target rocks, are recognized as crater-filling units within the Manson impact structure. These include: 1) Phanerozoic-clast breccia, with the majority of sedimentary clasts derived from the shallower parts of the pre-impact stratigraphic sequence in a silty-sandy argillaceous matrix; 2) an overturned ejecta flap of fractured blocks derived from Keweenawan (Middle Proterozoic) and Phanerozoic sources and preserved in general inverted stratigraphic order; and 3) Keweenanwan shale-clast breccia characterized by shale and mudstone clasts derived from the lower part of the Middle Proterozoic sedimentary sequence in a variably argillaceous or melt-rich matrix. The overturned ejecta flap is restricted to the megablock zone of the Manson structure, and the Keweenawan shale-clast breccia is known only from the central peak area. However, the Phanerozoic-clast breccia is recognized as the last stage of crater-filling material across all structural terranes within the Manson structure.

Emplacement mechanisms for these breccias remain speculative, but their composition, distribution, and relative stratigraphic positions suggest possible relationships to stages of crater excavation and collapse. The overturned ejecta flap is interpreted to represent late-stage collapse of the ejecta curtain as the transient crater approached its maximum extent. The origin of the Keweenawan shale-clast breccia is enigmatic, but its emplacement may be related to early-stage collapse of the transient crater walls and uplift and collapse of the central peak. The Phanerozoic-clast breccia is compositionally similar to the Bunte breccia at the Ries Crater, but, unlike the Bunte breccia, the Phanerozoic-clast breccia occurs inside, not outside, of the crater rim. The Phanerozoic-clast breccia was probably derived from deposits of primary ejecta and secondary surface scour outside the crater, and from strata in the terrace headscarps. Phanerozoic-clast emplacement across the crater resulted from large-scale transport of these materials back into the crater, possibly related to back-surge in a shallow sea and/or to crater collapse.