Ste. Genevieve Fault Zone

Location

Union and southwestern Jackson Counties, Illinois (J, K-4, 5), northwestward to Franklin County, Missouri

References

(selected) Flint 1926, S. Weller and St. Clair 1928, J. Weller and Ekblaw 1940, Desborough 1958, 1961a, b, Segar 1965, Heyl et al. 1965, Tikrity 1968, Olsson 1966, McCracken 1971, Heyl 1972, Gibbons 1972, Adair 1975, Nelson and Lumm 1985, Devera 1986, Clendenin et al. 1989, Nelson 1991, Kolata and Nelson 1991a

Description

The Ste. Genevieve Fault Zone crosses the western Sparta Shelf and partly separates the Ozark Dome from the Illinois Basin (fig. 2). Various segments of the fault zone have been named individually. The largest fault of the Ste. Genevieve Fault Zone in Illinois has been called the Rattlesnake Ferry Fault or Rattlesnake Ferry Fault Zone. The Pomona, Atwood, and Delta Faults, and the Harrison Creek Anticline may be related to the Ste. Genevieve Fault Zone.

At least two periods of movement occurred along the fault zone, as documented by S. Weller and St. Clair (1928). The first documented movement took place in late Middle Devonian time and the second, late in the Mississippian to early in the Pennsylvanian Periods. Displacement was opposite for the two episodes. Post-Pennsylvanian displacement may have taken place but cannot be proven because no Pennsylvanian or younger rock is preserved on the southwest side of the fault zone.

Devonian faulting took place mainly in Missouri, but it may have extended eastward into Illinois (fig. 9). This movement raised the Sparta Shelf relative to the Ozark Dome. The uplift began during or immediately after deposition of the Grand Tower Limestone (early Middle Devonian) and ended during deposition of the New Albany Shale (fig. 63). The northern block eroded as it rose. Coarse detritus from the Sparta Shelf was deposited in the Beauvais Sandstone and St. Laurent Formation south of the fault in Missouri, whereas silt and clay-sized sediment was incorporated into the time-equivalent Alto and Lingle Formations in southwestern Illinois (Nelson and Lumm 1985, Devera 1986). Eroded strata as old as upper Ordovician were then overlain unconformably by thin New Albany and younger Mississippian strata on the Sparta Shelf, whereas a complete Silurian and Devonian section is preserved south of the fault zone. As much as 1,000 feet (300 m) of vertical offset is indicated.

The late Mississippian-Pennsylvanian deformation was more extensive than the Devonian movement. All faults exposed at the surface along the Ste. Genevieve Fault Zone in Illinois probably date from the second episode. The Carboniferous uplift reversed the Devonian so that the Ozark Dome rose relative to the Sparta Shelf and Illinois Basin (fig. 10, 63). The time of movement is demonstrated by angular unconformity between Chesterian and Pennsylvanian strata (Ekblaw 1925), eroded clasts of older rocks from the Ozark Dome incorporated into the lower Pennsylvanian Caseyville Formation (Poor 1925), and trends of Pennsylvanian paleochannels (Desborough 1961a, b). Unconformities within Caseyville strata near Pomona are attributed to contemporaneous deformation (Desborough 1961a, b).

The structure resulting from the Carboniferous uplift can be characterized as a steeply dipping reverse fault and an associated monocline. Structural relief exceeds 3,000 feet (900 m) in places. Toward the northwest end of the Ste. Genevieve Fault Zone in Missouri, the fault zone is narrow and nearly vertical, and end folding is minimal. Southeastward, as the sedimentary thickness above basement increases, folding becomes more prominent. Surface faulting dies out entirely in northern Union County, Illinois. Proprietary seismic reflection profiles indicate that faulting continues at depth beneath the surface monocline. The Ste. Genevieve Fault Zone may continue as a basement feature into the northern Mississippi Embayment.

Several authors including Heyl (1972), Viele (1983) Clendenin et al. (1989), and Schultz et al. (1992) have proposed that strike-slip displacement has occurred along the Ste. Genevieve. Among these, only Schultz et al. (1992) offered supporting field evidence. They reported that fold orientations, joint patterns, slickensides, and other kinematic indicators are consistent with left-lateral oblique slip.

Recent mapping by Nelson and Devera (1994) in northern Union County, Illinois, has disclosed offsets of Eocene sediments along faults near the southeast end of the Ste. Genevieve Fault Zone. Post-Eocene faults strike north-north- west, dip nearly vertical, and outline narrow pull-apart grabens. This new finding emphasizes the long, complex tectonic history of the Ste. Genevieve.

Figure(s)

Figure 9   Paleogeography of Illinois during the Middle Devonian Epoch. Figure 10   Structures active from late in the Mississippian Period through early in the Pennsylvanian Period. Figure 63   Cross section of the Ste. Genevieve Fault Zone illustrating two episodes of movement. The northern fault (now concealed by alluvium) moved down to the south in the Middle to Late Devonian Epochs. The southern fault, a high-angle reverse fault, moved down to the north late in the Mississippian and in the Pennsylvanian Periods. Note the preserved Devonian strata between faults (Wittenberg Trough).