R.R. Anderson

The Geological Society of America
27th Annual North-Central Section Meeting
Rolla, MO, March 29-30, 1993
1993 Abstracts with Program, v. 25, no. 3, p. 2-3


In 1991 and 1992 the Iowa Department of Natural Resources - Geological Survey Bureau and U.S. Geological Survey drilled a series of 11 research cores in the Manson Impact Structure, the 35 km diameter K/T boundary-age impact site in north-central Iowa. The cores investigated the three major terranes within the structure: (1) the Terrace Terrane, a ring of structurally down-dropped blocks that lie just within the outer limits of the structure, (2) the Central Peak of uplifted crystalline basement rocks and crater floor material, and (3) the Crater Moat, the depression Terrace Terrane in search of the structurally-preserved, upper-most Cretaceous impact surface. One (M-3) penetrated about 88 m of structurally preserved Turonian strata, erosionally truncated well below the impact surface. The second (M-4) reached a depth of 375 m, penetrating into an overturned flap of Proterozoic and Paleozoic strata. The target impact surface and structurally-preserved latest Maastrichtian strata (eroded outside of the Manson Structure) probably lie within a hundred meters below the base of the M-4 core. Post-impact crater fill sediments and their paleontologic record were the targets of drilling in the Crater Moat. Two of these cores (M-6 and M-9) encountered a Sedimentary Clast Breccia (SCB), interpreted as a debris flow, and one core (M-2) encountered a thin interval of mudstone (possibly lake sediments) above the SCB. Six cores penetrated into the Central Peak of the Manson Structure. One core (M-5) penetrated directly into Proterozoic gneiss, basement rocks uplifted with the Central Peak. One core (M-7) penetrated directly into Crystalline Rock Breccia with a Glassy Matrix (CRBGM), interpreted as impact melt-rock, and continued into basement gneiss. Four cores (M-1, M-8, M-10, and M-11) encountered SCB at the bedrock surface and continued into CRBGM and then into basement gneiss. Numerous impact materials were encountered in these cores including sanidine that crystallized from the melt-rock and is currently being dated by Michael Kunk (USGS-Reston). Shock-deformed minerals were encountered in abundance, and the gneisses of the Central Peak display pseudotachylite glass fracture fills. The debris flow (SCB) was encountered in all three crater terranes, reaches a thickness of at least 244 m, and moved megaclasts as large as 98 m. It apparently mantled the entire crater, and may have been transported by water, suggesting impact into a shallow Maastrichtian epicontinental sea.