A POSSIBLE SAND BLOW FIELD IN THE CENTRAL ARKANSAS RIVER VALLEY OF THE SOUTHWESTERN MISSISSIPPI EMBAYMENT

Analysis of 1:20,000-scale aerial photographes (ca. 1976 to 1982) reveals circular to elliptical light tonal anomalies along ancestral Holocene courses of the Arkansas River on the Lincoln and Jefferson County line in eastern Arkansas. The tonal quality, size (10 to 30 m diameter), and geomorphic position (Holocene natural levees and crevasse splays) of these anomalies are identical to tonal anomalies seen on aerial photos (ca. 1951 to 1981) in two nearby regions primarily in Desha County and in Ashley County, Arkansas. These two regions (50 and 80 km to the southeast) were confirmed by trenching studies to be fields of paleoseismic sand blows. Thus, we tentatively interpret the Lincoln-Jefferson County tonal anomalies as sand blows.

Radiocarbon and luminescence dates from our previous work show the Ashley County and Desha County liquefaction fields each experienced at least three earthquakes large enough to vent some sand in the last 6000 years. In the Lincoln-Jefferson County field, sand blows are present on natural levees and crevasse splays of early Stage 2 (~3000 to 4500 yBP) and older courses of the Arkansas River, but not on late Stage 2 or Stage 1 courses, suggesting a mid-Holocene earthquake caused the sand blows on the aerial photos.

The seismic source of these liquefaction fields may be the New Madrid seismic zone 200 km to the northeast. However, the same ancestral Arkansas River courses pass through the Lincoln-Jefferson County field and the Desha and Ashley County sand-blow fields, but tonal anomalies characteristic of sand blows are absent along these courses between the fields. This discontinuous sand-blow distribution within continuous alluvial deposits suggests that rather than a New Madrid source there are at least three local seismic sources capable of causing liquefaction over areas of ~500 sq km (Lincoln-Jefferson County and Ashley County fields) and ~1500 sq km (Desha County field). Witnessed liquefaction events of this size in other regions have been produced by M6 to 6.5 earthquakes.