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Previous Research
Pryor and Ross (1962) mapped the geology of an area including the Olmsted 7.5-minute Quadrangle. They showed no faults near Olmsted, the nearest being close to Thebes, Illinois. Ross was the first geologist to map faults in the Olmsted area. Based on borehole data, Ross’s map depicts the “America Graben” running northeast from Cairo through Olmsted and Grand Chain. This narrow downfaulted block corresponds to the area of thickest Cretaceous strata. Other faults mapped by Ross have diverse orientations, outlining triangular and polygonal patterns on the map. A structure map (Pryor and Ross 1962, Ross 1964) on the base of the Clayton Formation (Paleocene) depicts no faulting. Ross concluded that the America Graben likely was active during latest Cretaceous time, but prior to Clayton deposition. Ross suggested that some faults in southernmost Illinois (but not near Olmsted) displace units as young as the Mounds Gravel.
Kolata et al. (1981) again mapped the structure of southernmost Illinois, relying chiefly on well data. Their map shows fewer faults than that of Ross (1964) and omits the America Graben. Kolata et al. (1981) recognized the area of thick Cretaceous sediments centered near Olmsted but did not associate it with fault activity. Like Ross, Kolata et al. (1981) mapped the Clayton Formation as dipping uniformly and not faulted. Kolata et al. (1981) also investigated several sites where Cretaceous and younger sediments are deformed. They attributed all of this deformation to non-tectonic processes, such as landsliding and collapse of sinkholes. In conclusion, they found no convincing evidence for Cretaceous or younger tectonic faulting anywhere in southern Illinois.
New Findings
Seismic reflection profiles, borehole data (especially cores), and exposures in clay pits reveal an intricate system of large faults in the Olmsted Quadrangle. The dominant trend is north-south to north-northeast; some small faults strike east-west to east-northeast. Principal movements took place near the end of the Cretaceous Period during deposition of the McNairy Formation. Small displacements continued on some faults during Tertiary and early Quaternary time. The youngest unit that is visibly offset at the surface is the Mounds Gravel. However, earthquake activity within the map area indicates that faults at depth are still active.
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50 MilesOlmstedQuadrangleInferred Cretaceous igneous intrusion, based on geophysical data from Hildenbrand et al. (1982) and Berg and Masters (1994). FluorsparDistrictFluorsparDistrictOZARKDOMEILLINOISBASINMISSOURIILLINOISINDIANAKENTUCKYTENNESSEEARKANSASMemphisedge of Reelfoot RiftNew Madrid Seismic Zoneedge of Mississippi Embaymentedge of Mississippi Embaymentedge of Reelfoot RiftSte. Genevieve Rough Creek Fault SystemCottage Grove Fault SystemWabashValleyFaultFaultSystemSystem92°91°90°89°88°87°38°37°36°35°
Figure 1 Map of central Mississippi Valley, showing the study area and selected geologic features.
