Peoria Folds (New)
Location
Peoria, Fulton, McDonough, Schuyler, and Brown Counties
References
Wanless 1957
Description
A series of subtle anticlines and synclines mapped by Wanless (1957) are designated the Peoria Folds. Although many of these folds do not meet the usual standards of named features, the close parallelism of the folds is intriguing and indicates an origin in a regional stress field.
Individual folds named by Wanless are the Astoria Anticline, Bardolph Anticline, Brereton Anticline, Bryant Syncline, Bushnell Syncline, Canton Syncline, Elmwood Syncline, Fairview Syncline, Farmington Anticline, Littleton Anticline, Ripley Syncline, St. David Anticline, Sciota Anticline, Seville Anticline, Table Grove Syncline, and Versailles Anticline. They were mapped from surface and subsurface data on various Pennsylvanian and Mississippian horizons. Nearly all strike slightly north of east. They are linear to slightly arcuate and the convex side is to the north. The folds plunge eastward with regional dip. Most have less than 100 feet (30 m) of structural relief; dips on the flanks are generally measured in a few tens of feet per mile (less than 1°). The southern limbs of anticlines are generally steeper than the northern limbs.
Not mentioned by Wanless is the correspondence these minor folds have with topography. The east-northeast alignment of small streams is apparent not only in the area mapped by Wanless, but also farther west in McDonough and Schuyler Counties (fig. 58). The pattern on high-altitude aerial photographs and satellite images is striking. This is remarkable, considering that the entire region has been glaciated. Nowhere else in Illinois is topography apparently so strongly influenced by bedrock structure through glacial drift.
Wanless did not speculate about the origin of the folds.
The presence of so many apparently parallel flexures of similar length and amplitude tempts one to theorize that horizontal compression was involved, but no likely source of compressive force can be named. Another tantalizing idea is of a possible structural link to the Upper Mississippi Valley Zinc-Lead District. Cobb (1981) reported abnormally high concentrations of sphalerite in Pennsylvanian coal seams of the Peoria district. The zinc sulfide occurs in joints, fractures, elastic dikes, and similar disturbed zones in coal. The sulfide is chemically very similar to sphalerite from the Upper Mississippi Valley District. Major folds of the latter, like the Peoria Folds, dominantly trend east to west. Southward on the Western Shelf, however, the principal trend of structures is northwest. The northwest-trending structures appear to be products of faulting in basement, possibly inherited Precambrian trends. The nature and origin of the Peoria Folds require further study.
Figure(s)
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