The Glasford Structure in Peoria County, Illinois, was recognized as a buried meteorite impact crater in the early 1960s but has gone largely unstudied for the past several decades. Recent, multidisciplinary re-examination of Glasford in light of 21st-century planetary science has revealed a wealth of new information about the crater and the processes that created it. The Glasford Structure was formed by the impact of a ~200-m-diameter meteorite into shallow Ordovician seas. The impact profoundly disrupted the target rock on both a macroscopic and microscopic scale to a depth of several hundred meters. After rock was ejected from the transient crater by the force of the impact, underlying strata rebounded upwards to fill the void, creating a megabrecciated uplift in the center of the crater. The high-pressure shock wave generated shatter cones and shocked quartz grains that serve as proof of a hypervelocity impact. ‘Impact fracking’ shattered the carbonate target rocks and injected cataclasites that propped open the newly formed cracks, facilitating impact-induced hydrothermal circulation. Resurge processes and subsequent normal marine deposition produced unique crater fill deposits (the “Kingston Mines” unit) which contain marine fossils. Graptolite specimens retrieved from the Kingston Mines place the time of deposition at approximately 455 ± 2 Ma (Late Ordovician, Sandbian). This age determination potentially links the Glasford impact to the “Great Ordovician Meteor Shower,” an increase in the rate of terrestrial meteorite impacts attributed to the breakup of a large object in the main asteroid belt.
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About the speaker
Mr. Monson is a member of the Petroleum Geology section of the Illinois State Geological Survey. He has been employed at ISGS since 2010. Monson holds a B.S. in geology and an M.S. in paleontology from the University of Iowa and is a Ph.D. candidate in the Geology Department of the University of Illinois at Urbana-Champaign. His research interests include clastic sedimentology and stratigraphy, Lower Paleozoic paleontology, meteoritic impact processes, and the history of geology.