QUATERNARY DEFORMATION ASSOCIATED WITH THE COMMERCE GEOPHYSICAL LINEAMENT, HOLLY RIDGE, IDALIA, MISSOURI

The Commerce geophysical lineament (CGL) is a 600-km-long, 5-to 10-km-wide, NE-trending magnetic and gravity anomaly, which extends from northeast Arkansas to central Indiana. Geomorphic mapping, seismic reflection and ground penetrating radar data, paleoseismic trenching, and borehole information collected near Idalia (Holly Ridge South site), southeast Missouri, provide evidence of late Pleistocene deformation 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. The geomorphologic features are aligned with regional NE-trending linear valleys, escarpments, drainages, springs and bedrock notches mapped NE and SW of the site. At Holly Ridge South, seismic reflection data acquired across the mapped CGL image near-vertical faults across Tertiary/Cretaceous stratigraphic contacts, coinciding with faults exposed in excavations at the site.

Trenches excavated across a linear escarpment in the Bloomfield Hills, southeast Missouri, expose faulted and warped Tertiary deposits, Plio-Pliestocene reworked Mounds gravel, Sangamon geosol, and Pleistocene Roxana and Peoria loess. We also observed warping and faulting(?) of a younger soil developed in the Peoria loess. A trench and borehole transect trending normal to the SW projection of the near-surface faulting, and across a Holocene fan with subtle tectonic-related geomorphology, exposed possibly warped Tertiary to Pleistocene deposits, liquefaction-related features cross-cutting Peoria loess, and undeformed early Holocene alluvium. Preliminary analyses of trench, borehole and GPR data constrain the most recent event at the site between the late Pleistocene and early Holocene. Lastly, the combined results suggest that deformation observed at the site and coincident with the CGL is of primary tectonic origin based on: (1) an apparent alignment of deep-seated faulting with near-surface deformation, (2) linear continuity of apparent tectonic-related geomorphology, and (3) an absence of landslide-related features in a trench excavated across a hypothesized toe.