GLASFORD IMPACT STRUCTURE
by: Charles O’Dale
- Type: Complex
- Age Ma: 443.8 – 485.4a – ORDOVICIAN
- Diameter: ~4 km
- Location: N 40° 36.1’ W 089° 47.1’
- Shock Metamorphism: shatter cones
a Geologic estimated age: Buschbach, T. C., Ryan, R. 1963.
The Glasford structure was recognized as a dome in the 1950s, while exploration for coal was occurring in the region. A gravity map revealed a buried dome at a depth of 350m. The local power company took several cores for the purposes of considering the dome for underground storage of natural gas. Drill cores revealed that this dome uplift contained denser, lower rocks, with large randomly-oriented blocks mixed together in a matrix of finer brecciated material. A complex pattern of faulting suggested that the dome was the central uplift of an impact crater. This was later confirmed with the discovery of shatter cones in blocks of fractured dolomite. (Paul Hodge).
Glasford (Illinois) cryptoexplosion structure
Geological Society of America Special Paper – estimate that the impact occurred during the earliest Cincinnatian (about 451 million years ago), based on interpretation of stratigraphic and structural evidence, and provide a solid stratigraphic constraint at or before the lowermost expression of the Upper Ordovician Maquoketa Shale, and during or after the deposition of the Late Cambrian Franconia and Eau Claire Formations, both of which were observed as displaced and uplifted units within the structure. This means the impact occurred between about 443.8 and 485.4 million years ago (Buschbach and Ryan 1963).
Ordovician Explosion Structure at Glasford, Illinois
Cores from a deep well at the crest of a dome about 2½ miles in diameter near Peoria, Illinois, show a normal sequence of Paleozoic strata down to the Upper Ordovician Maquoketa Shale, but the underlying 1,500 feet of rocks are intensely disordered. Middle and Lower Ordovician strata normally underlying the Maquoketa are not recognized in the disturbed zone. However, the core penetrated several jumbled blocks of Cambrian formations uplifted about 1,000 feet above their normal stratigraphic position. The disturbed section consists chiefly of brecciated dolomite, with minor amounts of compact, mylonitic sandstone, and contorted beds of red and green shale. Dark argillaceous bands and shale partings show many small-scale faults. Dips are randomly oriented and attain up to 90° throughout much of the section, but in the lower 400 feet of the well the apparent bedding planes dip southeast and the angle decreases with depth from 70° to 40°. The chaotic condition of the pre-Maquoketa rocks suggests that this structure was caused by a violent explosion which probably took place in early Cincinnatian time. The writers believe the explosion was the result of meteorite impact. Thinning and arching of strata overlying the breccia indicate that there was gradual and continuous structural development of the dome from Late Ordovician to at least Pennsylvanian time and perhaps to the present. (Robert Ryan T. C. Buschbach 1963)
Brent Dalrymple, Radiometric Dating Does Work! Reports of the National Center for Science Education
Buschbach, T. C., Ryan, R., Ordovician explosion structure at Glasford, Illinois. Bulletin of the American Association of Petroleum Geologists, v. 47, pp. 2015-2022. 1963.
Dietz, R. S., McHone, J. F., Astroblemes recently confirmed with shatter cones (abstract). Meteoritics, v. 26, p. 332. 1991.
Hodge P, Meteorite Craters and Impact Structures of the Earth, Cambridge University Press
McHone, J. F., Sargent, M.L. and Nelson,W.J., Shatter cones in Illinois: Evidence for meteoritic impacts at Glasford and Des Plaines (abstract). Meteoritics, v. 21, p. 446. 1986.
Nelson, W.J., View Profile. In Geology of Illinois, Champaign, IL, United States (USA): University of Illinois at Urbana-Champaign, Institute of Natural Resource Sustainability, 2010.
Snyder, F. G., Gerdemann, P. E., Explosive igneous activity along an Illinois-Missouri-Kansas axis. American Journal of Science, v. 263, pp. 465-493. 1965.