Glasford Structure
Location
Northeastern T7N, R6E, Peoria County (D-4)
References
Wanless 1957, Buschbach and Ryan 1963, Buschbach and Bond 1974, McHone et al.1986b
Description
Various names have been applied to the unique structure northeast of Glasford. Wanless (1957) showed a dome on his map of the Colchester Coal Member (Pennsylvanian), but he did not name it. Buschbach and Ryan (1963) called the feature the Glasford explosion structure, whereas Buschbach and Bond (1974) referred simply to the Glasford Structure. Treworgy (1981) revised the name to Glasford Disturbance, probably for the sake of consistency with the Des Plaines Disturbance. McHone et al. (1986b) reverted to Glasford Structure, a name that is used here because it is simple, generic, and conforms to the most common usage for similar structures in other areas. The Glasford Structure is difficult to classify because it is partly a dome and partly a chaotic breccia. It is considered a probable meteorite impact feature (astrobleme).
Indications of a dome based on coal test drilling prompted Central Illinois Light Company to evaluate the Glasford Structure as a gas storage site. Between 1959 and 1961 they drilled a number of test holes, some of which were cored. They also conducted a gravity survey, which showed a very strong positive anomaly (Otto 1962). Drilling revealed a roughly circular dome, about 2.5 miles (4 km) in diameter, coinciding with a circular 0.7-milligal gravity high. All units and intervals from Pennsylvanian through Silurian are domed and thin somewhat across the structure. Closure increases downward from about 100 feet (30 m) on the Pennsylvanian Colchester Coal to 250 feet (75 m) on the Fort Atkinson Limestone (Cincinnatian). The Maquoketa Group (Cincinnatian) is 75 to 110 feet (23-34 m) thicker on the dome than it is elsewhere (Buschbach and Ryan 1963). An intensely shattered breccia of older rocks is beneath the Maquoketa. A well drilled near the apex of the dome cored 1,500 feet (450 m) of breccia without reaching undisturbed rock. The breccia consists of large and small blocks of dolomite, sandstone, and other materials in a matrix of finely broken to pulverized sand and rock fragments. The blocks themselves are greatly fractured or internally smashed and recemented (fig. 39). Buschbach and Ryan identified fragments of several Cambrian formations 800 to 1,000 feet (240-300 m) above their normal stratigraphic position. Ordovician rocks, normally distinctive, were not recognized within the breccia. Buschbach and Ryan also observed that the intensity of deformation seemed to be diminishing near the bottom of the cored hole.
Buschbach and Ryan concluded that the Glasford Structure was produced by impact of an extraterrestrial body as Maquoketa deposition began. The Glasford area was covered by a shallow sea at that time. The crater was filled with the anomalously thick lower Maquoketa and then covered by younger sediments. Continuous rebound of the brecciated core or effects of differential compaction resulted in thinning and doming of all strata overlying the impact breccia.
McHone et al. (1986b) identified shatter cones in cores from the Glasford Structure. These are the first identified features indicative of shock metamorphism, and they strengthen the case for impact origin of the deep-seated brecciation.
Central Illinois Light Company is storing natural gas in the vuggy Silurian dolomite arched across the Glasford Structure.
See also DES PLAINES DISTURBANCE, HICKS DOME, OMAHA DOME.
Figure(s)
|
