Downs Anticline

Part of the La Salle Anticlinorium

Location

McLean and eastern De Witt Counties (D, E-6)

References

Heigold et al. 1964, Howard 1964, Clegg 1972

Description

The Downs Anticline is a major element of the La Salle Anticlinorium and forms the west edge of the belt at this latitude. It is a strongly asymmetrical structure with a steep west flank and a gently sloped east one. A series of domes occurs along the crest of the Downs Anticline. Named domes include, from north to south, the Lake Bloomington, Hudson, and Wapella East. Clegg (1972) also included the Parnell and De Land domes as part of the Downs Anticline, but those domes are offset southeast of the axis of the Downs Anticline. Structural relief is expressed at all mapped horizons and increases with depth. Maximum relief on the Danville Coal Member (Pennsylvanian) is about 325 feet (100 m) (Clegg 1972); Stevenson et al. (1981) showed relief of more than 700 feet (200 m) on the New Albany Group (Devonian-Mississippian) at the same point. Borehole and seismic data from gas storage projects (Buschbach and Bond 1974) show that the Hudson and Lake Bloomington Domes affect the basal Cambrian Mt. Simon Sandstone. The Downs Anticline, in all probability, is a basement structure.

The Downs Anticline, which plunges abruptly southward, is separated from the Osman Monocline by the Colfax Syncline. Northward, the Downs flattens out, loses definition, and ends in offset relation to the Peru Monocline.