LOW-RELIEF MOUND-LIKE FORMS ALONG THE GRAND RIVER (CHARITON COUNTY, MO): A PALEOSEISMIC ORIGIN?

An irregular landscape atop part of a fluvial terrace is observed near the confluence of the Missouri and lower Grand Rivers in north-central Missouri, consisting of low-relief mounds highlighted by tone and texture variations visible in aerial photography. This study aims to estimate the age and explore potential explanations for the formation of these mounds. Possible origins for their formation include: (1) burial of dunes or similar depositional landforms, (2) erosion of a previously uniform landscape, or (3) surface disruption from subsurface liquefaction. To test these hypotheses, we collected high resolution LiDAR data (> 150 pts/m²), close-range aerial photogrammetry, conducted soil augering to the saturated zone, and excavated some soil test pits. We also acquired a 256m fixed-spread seismic refraction profile to examine variations in the elevation of the water table and shallow variations in the seismic velocity structure.

Elevation data reveal a poorly drained, “egg-carton” topography, with no throughgoing drainages or channels. Soil auger profiles indicate significant differences between the troughs and mounds, the former with soil rich in clay covering thin red sand, while the latter has clay loam covering thick silty sand with a light buff color, grading to medium grain at depth. Profiles were terminated upon reaching saturation depth, ranging from 3.5m to 4m. A soil test pit in a sandy deposit shows no apparent sedimentary structures and produced a worked lithic fragment that indicates human inhabitation and a Late Pleistocene age. Interpretation of the seismic tomographic profile is ongoing but should enhance field observations of the water table topography. Overall, the absence of sharp gullying on the mounds does not support erosional formation. The soil profiles support either the dune burial or liquefaction hypotheses. The study area’s high-water table and fine loam capping soil unit lend additional support to the liquefaction hypothesis. The results from this study will be integrated with the results from the ongoing paleoseismic investigation to better understand if the origin of the mounds is depositional or seismic in nature.