Late Cretaceous and Cenozoic Geology of the New Madrid Seismic Zone

Abstract

Structure-contour maps constructed from well, seismic reflection, and outcrop data of the tops of the Paleozoic section, Upper Cretaceous section, Paleocene Midway Group, and Eocene section illustrate the post-Paleozoic structure of the New Madrid seismic zone region. Isopach maps of the Late Cretaceous section, Midway Group, and Eocene section help constrain the timing of the structural events. These maps, which encompass much of the northern Mississippi embayment, reveal reactivation of the underlying late Precambrian/Cambrian Reelfoot rift during Midway Group deposition but no reactivation during Late Cretaceous or Eocene deposition. The structure-contour maps also indicate a subtle, south-plunging depression on the tops of the Paleozoic, Upper Cretaceous, and Midway Group along the axis of the northern Mississippi embayment that we have called a trench. This trench is 50-km wide, has a maximum depth of 100 m, and appears to have formed during the Eocene. The trench's western boundary coincides with the Blytheville arch/Lake County uplift and its southeastern margin underlies Memphis, Tennessee. The Blytheville arch/Lake County uplift is the structure responsible for the New Madrid seismic zone, and thus it is possible that the southeastern margin of the trench is also a fault zone. Northern Mississippi embayment post-Paleozoic stratigraphy consists of sands, silts, and clays that thicken from 477 m at New Madrid, Missouri, to 987 m near Memphis, Tennessee. The uniformity of these sediments indicates their elastic properties and therefore seismic velocities are very similar; however, variations in cementation and unconformities within the section may influence seismic-wave propagation. Manuscript received 17 June 1999.