PROGRESS TOWARD QUATERNARY DISPLACEMENT RATES ON THE MEEMAN-SHELBY FAULT AND JOINER RIDGE HORST, EASTERN ARKANSAS

Two structures in the Reelfoot Rift in eastern Arkansas, the Joiner Ridge horst and the Meeman-Shelby fault, are of particular concern because of their proximity to Memphis, Tennessee. Empirical scaling estimates based primarily on fault length suggest earthquakes along the Meeman-Shelby fault could be up to M_w 6.9. Understanding the recent displacement history of these structures is crucial to determining the seismic hazard at Memphis, which is <10 km from the Meeman-Shelby fault and 50 km from Joiner Ridge. Seismic reflection profiles of the Meeman-Shelby fault and Joiner Ridge show vertical displacement of subsurface alluvium at the Eocene-Quaternary disconformity of 28 m and 20 m, respectively, but the displacement is not apparent at the surface at either site as a scarp. The Holocene alluvium overlying the Meeman-Shelby fault is ~10 ka and Joiner Ridge is overlain by the ~ 12 ka Morehouse terrace, but the age of the displaced alluvium at depth is unknown. Our primary goal is to determine the age of the basal alluvium that is vertically displaced to calculate a slip rate for the Meeman-Shelby fault and Joiner Ridge.

We drilled two continuous cores through the Mississippi River alluvium and into Eocene strata adjacent to the Meeman-Shelby fault (44 m) and on Joiner Ridge (40 m). Cores were split in a mobile dark lab, where one half was wrapped in light-tight containers for optically stimulated luminescence (OSL) dating, and the other half was used for analyses such as physical core descriptions, grain size measurements, and detailed magnetic susceptibility measurements. Downhole geophysical logs were also acquired at each core site. The stratigraphy of the Meeman-Shelby core is consistent with a meandering river point bar sequence, although the lower 4 m of the alluvial section may be Pleistocene glacial outwash. The Joiner Ridge core exhibits a thick overbank clay unit underlain by at least two coarse-grained glacial outwash units with an intervening silty paleosol that are likely Pleistocene deposits. These core descriptions and the forthcoming OSL ages will constrain fault slip rates for these structures and provide “type sections” for the Holocene and Morehouse alluvial sections that encompass large areas of the Mississippi River Valley.