SIKESTON RIDGE: PLEISTOCENE EROSIONAL REMNANT OR QUATERNARY FAULT-BOUNDED BLOCK WITHIN THE NEW MADRID SEISMIC ZONE?

Recent seismic reflection data are interpreted by others to suggest that the margins of Sikeston Ridge (SR), a 3.5- to 10-m-high, north-trending late Pleistocene remnant extending from New Madrid to north of Sikeston, Missouri, are coincident with Quaternary faults that merge at depth with Precambrian rift faults. If true, such faults may represent previously unrecognized seismic sources within the New Madrid seismic zone. To assess the presence or absence of potentially active structures along the margins of SR, we produced a surficial geologic map of SR, followed by drilling and trenching.

Analysis of geomorphic and geologic data favor a non-tectonic origin for SR. Interpretation of aerial photography and LANDSAT images, coupled with geologic mapping, demonstrate that late Pleistocene-Holocene paleochannels produced scalloped patterns along the ridge margins suggestive of past erosional trimming. Topographic profiles across the ridge suggest an absence of eastward and westward tilting from potential displacement along inferred Quaternary faults. Borehole data acquired across the western ridge margin to assess a potentially uplifted late Pleistocene paleochannel show no correlative marker units projecting across the ridge margin.

Paleoseismic trenches excavated at Knox and Davis Cemeteries, near Kewanee, Missouri, exposed faults and grabens coincident with the western escarpment of SR. Exposed stratigraphic relations indicate that late Pleistocene deposits record <0.25 m of total west-side down vertical separation. The net fault displacement is negligible relative to the 3-to 5-m-high western escarpment at these sites. Radiometric data and stratigraphic relations collected from the trenches indicate that the faulting most likely occurred during the 1811-1812 earthquakes, and likely is related to lateral spreading rather than primary tectonic faulting. Trench exposures confirm the presence of buried paleochannels inset into the base of the western escarpment, thus supporting a fluvial origin for the western margin of SR. In summary, geomorphic, surficial and subsurface geologic data collected and analyzed during this study strongly support an erosional, non-tectonic origin for SR.