Abstract

The dextral strike-slip New Madrid North Fault (NMNF) is interpreted by others to have ruptured during the 23 January 1812, M 7.8 earthquake. Although widespread liquefaction was associated with this earthquake and is evident today, the NMNF's surface trace remains enigmatic. Our surficial geologic mapping in southeast Missouri identifies two prominent northeast-trending, 3- to 3.5-km-long lineaments (North and South Farrenburg lineaments) traversing Sikeston Ridge. The lineaments are expressed by apparent right-laterally deflected paleodrainages, scarplets, swales, linear troughs, and tonal contrasts in late Pleistocene deposits. Northeast-trending contemporary microseismicity and previously inferred NMNF locations align partly with the lineaments. Borehole data show a linear change in thickness (5 m) of post-Tertiary alluvium trending N30°–35°E for 35 km across both Sikeston Ridge and a Holocene fluvial surface east of the ridge. This thickness change coincides with microseismicity, apparent right-laterally separated isopachs, aeromagnetic anomalies, and the North and South Farrenburg lineaments. Our interpretation of S-wave seismic-reflection profiles acquired across the North Farrenburg lineament also suggests warped and faulted Quaternary deposits.

Four trenches excavated across the North Farrenburg lineament exposed late Pleistocene interbedded clay, silt, and sand. The trenches also exposed linear liquefaction vents that trend N20°E to N55°E, near-vertical faults striking N30°E and N42°E, and a possibly warped Pleistocene paleochannel. Vented sand extends upward nearly to the ground surface, suggesting that the vents were produced during the 1811–1812 earthquakes. Oriented sediment samples collected from the fault zones and vents and analyzed with an SEM identified fractured sand grains across the faults, suggesting that the displacement exposed in the trenches may be of primary tectonic origin. Similar sediment samples collected from undeformed sand and from the sand vents show no evidence of grain fracturing and reduction. We hypothesize that the geomorphic, geologic, seismic reflection, trench, and microtextural data strongly suggest that the North Farrenburg lineament, as well as the South Farrenburg lineament, may be the surface expression of an underlying tectonic fault.

You do not currently have access to this article.