GEOPHYSICAL AND PALEOSEISMIC INVESTIGATION OF THE COMMERCE GEOPHYSICAL LINEAMENT, IDALIA, MISSOURI: DEFORMATION ASSOCIATED WITH PRIMARY TECTONIC FAULTING?

The Commerce geophysical lineament (CGL), a 400-km-long, 5-to 10-km-wide, NE-trending magnetic and gravity anomaly, extends from central Arkansas to southern Illinois, and is associated with diffuse contemporary microseismicity. In the Bloomfield Hills of SE Missouri, geomorphic mapping, acquisition of seismic reflection data and excavation of paleoseismic trenches near Idalia, Missouri (south Holly Ridge site) provide evidence of late Pleistocene faulting that is coincident with the surface projection of the CGL. We investigated a prominent, NE-trending linear swale that is bounded on the south by a 2-to 3-m-high north-facing escarpment and an apparent deflected creek, and to the north is bounded by the north Holly Ridge site that was previously investigated by the Missouri Dept. of Nat. Resources. The geomorphology aligns with NE-trending linear valleys, escarpments, drainages, including springs and bedrock notches mapped NE and SW of the site. Interpretation of seismic reflection data acquired across the Bloomfield Hills and the site indicates that near vertical faults extending across the Tertiary/Cretaceous boundary and projected to the surface generally coincide with faulting observed at north Holly Ridge and in trenches at the south Holly Ridge site.

Trenches excavated across the linear swale and escarpment at the south Holly Ridge site exposed Tertiary deposits, Plio-Pliestocene reworked Mounds gravel, and Sangamon Geosol that are either overlain by or in fault contact with Pleistocene Roxana and Peoria loess deposits. In addition, we interpret late Pleistocene to Holocene (?) colluvial wedge deposits and Holocene alluvium. The trenches expose near-vertical NE striking faults that offset loess and colluvium present along the eastern margin of the linear valley. The apparent coincidence of deep-seated faulting imaged from the geophysical profiles with near-surface faulting at the Holly Ridge sites suggests that the deformation is of primary tectonic origin, and may be related to the CGL. We are awaiting C¹⁴ and luminescence results to better estimate the timing of deformation. In addition, we are combining and evaluating data between the north and south Holly Ridge sites to better characterize the origin of faulting.