R.R. Anderson, M.K. Reagan, M.S. Bell, and O. Plocher

Preliminary Observations and Interpretations from the Manson M-1 Core: A Look into a K-T Impact Structure
Geological Society of America, Abstracts with Program, 1992, v. 24, no. 4, p. 2


The Manson M-1 core was drilled in 1991 by the Iowa Department of Natural Resources - Geological Survey Bureau and the U.S. Geological Survey. The core, drilled on the eastern edge of the Central Peak of the K-T boundary-aged Manson Impact Structure (MIS), reached a depth of 703 feet and penetrated three primary lithologies, a shale-dominated sedimentary clast breccia (SCB), a crystalline rock breccia with a glassy matrix (CBG), and a crystalline rock breccia with a sandy matrix (CBS). The SCB (204-311 ft) is dominated by centimeter- to meters-sized clasts of Cretaceous shales, siltstones and sandstones, with minor Paleozoic carbonate, sandstone, and shale, and some clasts of Proterozoic Red Clastics shales, siltstones and sandstones. Very rare clasts of crystalline rocks are observed. The clasts are surrounded by a gray matrix composed of sand-, silt-, and clay-sized particles with approximately the same composition as the clasts. Only extremely rare clasts display impact shock effects.

The CBG (311-~485 ft) is composed of centimeter- to meter-sized clasts of felsic igneous rocks with very rare amphibolite clasts and glass fragments. The matrix is glassy, displaying varying degrees of devitrification and including quartz, feldspar, and flow-banded glass fragments. Many of the quartz and feldspar grains are shock-deformed, displaying shock lamellae, kink-bands, and disrupted extinction patterns.

The CBS (~485-703 ft) includes more clasts of amphibolite, less felsic igneous clasts, with generally larger and more abundant clasts than CBG. The CBS matrix is composed of sand- to silt-sized grains of crystalline rocks or minerals derived by the brecciation of these rocks, and shock deformation is pervasive in most grains.

The contact between CBG and CBS is gradational, with glassy matrix being steadily replaced by matrix grains. The size and abundance of grains in the matrix increase downward through the two units as does the mean size of clasts and frequency of shock-deformed grains.

The contact between SCB and CBG is sharp but irregular. Clasts of glassy material from CBG are included up to 17 m in the SCB above the contact, and shale clasts up to 9 m in the CBG below it. Angular fragments of flow-banded glass and rounded glassy fragments with irregular boundaries are included in the shale, suggesting that both solid and liquid phases of glass were included. Shale fragments below the contact are completely surrounded by glassy material, suggesting that it was a liquid when the shale was incorporated.

The preliminary interpretation is that CBG and CBS represent shock-brecciated crystalline rock crater floor that was partially melted from the top down by impact energy. Some movement and mixing is apparent, probably occurring during crater formation or during uplift of the Central Peak. The SCB probably represents a debris flow of ejecta and other crater materials from the crater rim that occurred shortly after crater formation. Evidence indicates that the CBG had probably partially cooled, with a thin, glassy crust that was ripped up and incorporated, along with some still molten material, into the shaly debris. At the same time shale fragments were thrust down into the molten CBG. Subsequent alteration modified some grains and matrix.