Abstract

A geologic investigation of the city of Memphis and southern Shelby County, Tennessee reveals Quaternary faulting. Structure contour maps and cross-sections of the top of the Pliocene-Pleistocene Upland Complex (Lafayette gravel), Eocene Upper Claiborne Group, and Eocene Lower Claiborne Group reveal two 20-m down-to-the-northwest faults that strike approximately N30°E The western fault is herein called the Memphis Fault and the eastern, the Ellendale Fault.

The Ellendale Fault is projected to pass beneath the Wolf River flood plain in east Memphis. At this location, a cut bank exposes flood-plain sands that are folded into an anticline with a wavelength of ∼ 100 m, an amplitude of ∼4 m, and a hinge oriented 10°N7°W. The folded sands are truncated by overlying, flat-lying, clayey silt overbank sediments. A 1-m-wide earthquake liquefaction sand dike was found above the crest of the anticline, in the overbank sediments. 14C dates reveal that the folding occurred between A.D. 390 and 450, and the liquefaction occurred post A.D. 450. Seismic-reflection lines indicate that the fold extends to more than 60 m depth into the Lower Claiborne Group, has a length of more than 1 km, and is thus tectonic in origin. The reflection lines also reveal a N25°E down-to-the-west fault with 5 m of displacement in the Upper Claiborne Group that we believe is within the Ellendale Fault zone. We believe the anticline formed during ∼5 m of Quaternary right-lateral strike-slip movement on the N25°E-trending fault. Based on these observations, we also believe that the Ellendale Fault, and perhaps the Memphis Fault, may pose seismic threats to the city of Memphis and Shelby County, Tennessee.

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