CORE DRILLING IN THE MANSON IMPACT STRUCTURE PROVIDES ABUNDANT RESEARCH MATERIALS AND NEW INSIGHTS INTO THE GEOLOGY OF A WELL-PRESERVED COMPLEX IMPACT CRATER
R. R. Anderson, B.J. Witzke, D.J. Roddy, and E.M. Shoemaker
The Geological Society of
1994 Annual Meeting
Seattle, WA, October 24-27, 1994
1994 Program with Abstracts, v. 26, no. 7, p. A-337
The recovery of 1283 m of drill core from the Manson Impact Structure (MIS) in 1991-1992 provided significant new material for numerous investigations of the structure. The research drilling program, sponsored by the Iowa Department of Natural Resources - Geological Survey Bureau and the U.S. Geological Survey, produced 12 cores from all terranes of the structure; the Central Peak (7 cores), Crater Moat (3 cores), and Terrace Terrane (2 cores). Six impact lithologies were identified and assigned non-generic classifications: (1) crystalline basement rocks (CR), (2) crystalline clast breccia with sandy matrix (SM), and (3) melt matrix (MM), (4) Keweenawan shale breccia (KSB), (5) Phanerozoic clast breccia (PCB), and (6) overturned strata (OS). The CR includes large blocks of basement gneiss and granite displaying pseudotachylite dikes and multiple microscopic shock metamorphic features. The SM is a fragmental breccia composed of clasts of crystalline basement rocks in a matrix of sand- to silt-size crystalline rock fragments and mineral grains. Virtually all grains show multiple shock metamorphic features. The MM is dominated by a matrix of devitrified melt-rock and finely powdered crystalline rocks with recrystallized quartz and sanidine and minor crystalline rock clasts and represents suevite and/or melt layer. The KSB is a suevite, dominated by clasts of Proterozoic (Keweenawan) shale and devitrified melt-rock. The CR, SM, MM, and KSB were encountered only on the Central Peak. The OS is an inverted and thinned sequence of Proterozoic and Paleozoic clastic strata, encountered only on the Terrace Terrane and interpreted as a late-stage ejecta flap, structurally-preserved on a down-dropped block. The PCB is a matrix-dominated unit with large clasts dominated by the shallowest pre-impact strata (Cretaceous shale) with lesser numbers of Phanerozoic clasts and rare clasts of Proterozoic material and meltrock. The PCB is found everywhere in the MIS (including the Central Peak Pit) and was probably originally emplaced outside of the transient crater as Bunte breccia-like deposit overlain by fall-out suevite, then transported back into the crater very shortly after crater formation.
Numerous research projects are in progress or have recently been completed, and will be presented in this theme session and published in upcoming GSA Special Papers or professional journals. Among the new information is a slightly modified average crater diameter of 36.5 km (max. 37.8 km NW; min. 35 km NE). Also, the presence of PCB, in over 90% of the drill penetrations of the MIS, implies the nearly complete preservation of underlying impact materials, tempting targets for future drilling. Current and future research will make the MIS one of the best-studied complex impact structures on Earth.