Sandwich Fault Zone
Location
Central Ogle to southern Will County (B-5 to C-7)
References
Willman and Payne 1942, Willman and Templeton 1951, Green 1957, McGinnis and Heigold 1961, Beck 1965, McGinnis 1966, Heigold 1972, Kolata et al. 1978, Kolata et al. 1983
Description
The largest fault zone in northern Illinois, the Sandwich Fault Zone, is approximately 85 miles (135 km) long and 0.5 to 2 miles (0.8-3 km) wide. It strikes overall about N60°W and the southwest side is upthrown along most of the zone. The Sandwich Fault Zone is known from surface exposures, primarily in quarries and other artificial cuts, and from water well records. The zone displaces rocks ranging from the Upper Cambrian to Silurian. The oldest surficially exposed rock in Illinois is dolomitic sandstone of the Cambrian Franconia Formation adjacent to the Sandwich Fault Zone in Lee and Ogle Counties.
A structure-contour map of the Franconia Formation (fig. 17) reveals a gentle regional east-southeast dip on the flank of the Wisconsin Arch northeast of the Sandwich Fault Zone. The Franconia is deformed into the highly asymmetrical Ashton Anticline southwest of the fault zone. The Franconia dips at approximately 100 feet per mile (approximately 1°) on the long southwest limb of the Ashton Anticline. The steeper northeast limb of the arch is truncated against the fault zone. Displacement of the fault zone increases from the ends toward the midpoint, where it reaches a maximum of about 800 feet (240 m).
All of the good exposures of the Sandwich Fault Zone are near the ends of the zone where the net displacements are down to the southwest, which is opposite to the major displacement in the poorly exposed central portion of the zone. Faults observed in quarries and other manmade cuts are dominantly vertical or steeply inclined dip-slip normal faults (fig. 64). Most faults display little or no drag; however, some small faulted monoclines suggest faulting increases with depth. Also observed were small thrust faults and reverse kink bands within shale in a horst between two large normal faults at Vicks Pit near Channahon, Will County. In spite of these local compressional faults, the fault pattern is indicative of northeast-southwest horizontal extension near the two ends of the Sandwich Fault Zone. The configuration of the central part of the zone suggests that the basement block southwest of the fault zone was raised and tilted back toward the southwest. Possibly, the Sandwich Fault Zone has been subjected to multiple episodes of deformation under different stress fields.
Regionally the Sandwich Fault Zone is approximately in line with the Kankakee Arch to the southeast and is southeast of the Plum River Fault Zone. The Sandwich and Plum River Fault Zones display similar structural styles but have different trends and apparently do not interconnect. The Sandwich Fault Zone is nearly parallel with, and has overall throw opposite to, the Peru Monocline at the northwest end of the La Salle Anticlinorium. Both structures may have formed simultaneously when the crustal block between them rose (Kolata et al. 1978). The fact that the Sandwich Fault Zone exhibits dominantly brittle failure (faulting), whereas the La Salle shows dominantly ductile failure (folding), is not explained. The degree of faulting along the Peru Monocline is not known because of outcrop scarcity and lack of good well control on the steep flank. The Sandwich Fault Zone is observed at mostly lower structural levels (closer to basement) than the La Salle; potentially, faults originating in basement would die out upward, particularly in Pennsylvanian shale. Another possibility is that the Peru Monocline is underlain by a reverse fault, which would have induced compression, favoring folding of overlying strata. Whether the Sandwich Fault Zone and the Peru Monocline developed at the same time or at different times is not clear.
The youngest rocks demonstrably displaced by the Sandwich Fault Zone are Upper Silurian. Mississippian (?) and lowermost Pennsylvanian sandstone, shale, and coal are preserved in sinkholes in faulted Silurian dolomite at Vicks Pit, Will County.
Neither Kolata et al. (1978) nor I, while examining these Pennsylvanian rocks, could determine whether they were deformed by faults. Pleistocene glacial sediments along the Sandwich Fault Zone are not deformed. The Sandwich Fault Zone may be the same age as the Peru Monocline. The major uplift was during late Mississippian to early Pennsylvanian time; the lesser uplift was during and after Pennsylvanian time.
References
Figure(s)
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