DES PLAINES STRUCTURE

DES PLAINES STRUCTURE

by: Charles O’Dale

  • Type: Complex
  • Age Ma: <280a – PERMIAN
  • Diameter: 8 km
  • Location: N 42° 03’ W 87° 52’
  • Shock Metamorphism:  Shatter cone fragments in dolomite.

a The age of the “Des Plaines Disturbance” has been estimated on geologic grounds (Koeberl et al 1996).

Shatter cones documented in the Glasford and Des Plaines impact structures.
The 8-km diameter Des Plaines Structure exhibits complex faulting and shock features such as percussion fractures and strain lamellae, as well as a few shatter cones. The center of the crater lies under Big Bend Lake on the Des Plaines River. Seismic reflection data suggest that there are numerous other faults within the bedrock of Cook County. Courtesy of United States Meteorite Impact Craters.

I took this image of the Des Plaines Structure immediately after take-off from the Chicago O’Hare International Airport. We are over the south rim of the structure looking south.

I walked into the swamp to get as close as I could to the point of impact of the Des Plaines structure. The impact point is ~100 m into the bush (and yes there were lots of mosquitoes!).

Anomalous geologic conditions in the northwest Chicago suburb of Des Plaines were noted by water well drillers as early as the 1890s. It was first suggested that it was faulting in the area. In 1944 the tectonic map of the United States identified the area of faulting as the “Des Plaines Disturbance”, which has generally been used since. The disturbance was labeled a “cryptovolcanic structure” in 1945. The Des Plaines Disturbance is roughly circular and about 5 miles (8 km) in diameter. It lies on the east flank of the Wisconsin Arch and is surrounded by Silurian bedrock that dips gently eastward. Within the disturbance, rocks ranging from Champlainian to Pennsylvanian subcrop beneath Pleiscarbonates. The Deland Oil Field was discovered in Silurian glacial deposits. The oldest rock at the bedrock surface is found at the center of the disturbance and is more than 800 feet (240 m) above its expected position. This is the central peak consisting of uplifted brecciated Middle Ordovician St. Peter Sandstone. The central peak is surrounded by a crater moat consisting mainly of shales and by a faulted and displaced megablock zone, and is marked by a distinct negative gravity anomaly. Shatter cone fragments in dolomite were revealed in drill cuttings. Strongly brecciated and fractured St. Peter Sandstone with quartz grains that show strain lamellae and crystallographically oriented PFs. No PDFs have been reported. The age can only be constrained to less than 280Ma. (Koeberl et al 1996).

References

[see – METEORITE]

Brent Dalrymple, Radiometric Dating Does Work! Reports of the National Center for Science Education

Christian Koeberl, Raymond R. Anderson; Manson and company: Impacts structures in the United States. Geological Society of America, Special Paper 302, 1996.

University of New Brunswick, 2012.