A late-Holocene surficial sandblow in the western lowlands of southeast Missouri: A paleoseismic milestone

Shallow trenches excavated across an elongate mound of sand showed several characteristics consistent with an extrusive paleoearthquake origin. Numerous clastic dikes, sills, and intruded cavities immediately underlie the sandblow; and discontinuous eruptive vents and occasional small clasts were located within the sandblow. An adjacent ditch exposure indicates that the sandblow formed by the venting of sand up through seven or more meters of fine-grained alluvium--a likely result of seismic liquefaction. Soil-morphological characteristics which suggest a prehistoric age include: (1) dominant yellowish brown Munsell color (10YR 5/4 and 5/6) beneath the A horizon of the sandblow, (2) absence of bedding planes in the sandblow, and (3) absence of a buried A horizon immediately beneath the sandblow. In contrast, sandblows produced by the great 1811--12 New Madrid earthquakes are typically dull colored, often have traces of bedding just under the A horizon, and are intermittently underlain by buried A horizons of the pre-earthquake surface soils. Formation of this prehistoric sandblow may have occurred contemporaneously with paleoliquefaction and tectonic deformation previously identified by others in the vicinity of New Madrid, Mo. A calibrated [sup 14]C date of 770--1020 A.D. on wood obtained just beneath the sandblow overlaps considerably a calibrated paleoearthquake timing of more » 539--991 A.D. for a site 30 km northeast of New Madrid. Assuming that these data represent the same event and that the seismic source was near New Madrid, the 60--70 km epicentral distance to this large sandblow in the Western Lowlands suggests a paleoearthquake magnitude of about 7.0 or greater. « less