ASYMMETRY OF THE MANSON IMPACT STRUCTURE: EVIDENCE FOR IMPACT ANGLE AND DIRECTION

by
P.H. Schultz and R.R. Anderson

The Geological Society of America
Special Paper 302, 1996, p. 397-418
The Manson Impact Structure, Iowa: Anatomy of an impact crater
C. Koeberl and R.R. Anderson, eds.

ABSTRACT


The Manson Impact Structure (MIS) is proposed to be the result of an oblique impact (20-30 from the horizontal) from the southeast. Such an interpretation is based on similarities between asymmetry in the MIS and patterns produced in laboratory experiments observed on planetary surfaces. Diagnostic signatures include the following features: (1) maximum central uplift (corresponding to deepest penetration) offset uprange from the geometric center; (2) breached central peak complex parallel to the trajectory; (3) large central uplift diameter relative to diameter; (4) larger diameter and greater disruption transverse to (rather than along) the trajectory; (5) maximum structural rim uplift transverse to the trajectory with minimum uplift downrange (northwest); and (6) shallower than expected excavation. The proposed origin for the MIS helps to account for the preservation of sheared but in-sequence strata from the depths of only 2 km on the central peaks. It is also consistent with the nature, distribution, and sequence of ejecta deposits including high-speed downrange ejecta represented by materials in the Crow Creek Member of the Pierre Shale and the matrix-supported ejecta debris draping the clastic overturned rim sequences and central peaks.