Abstract

Aerial photography from the 1940s through 1950s of the Wisconsinan and Holocene portion of the lower Mississippi alluvial valley from 32°N to 34.6°N identifies three previously unrecognized areas that have circular to elliptical tonal anomalies similar in size, shape, and spacing to the aerial photography signature of sand blows previously documented in this region by trenching. Coring, ground conductivity surveying, trenching, sedimentological/soil analysis, and age analysis by 14C and luminescence techniques demonstrate that these three areas are fields of seismically induced sand blows of late Holocene age.

The sand blows have a “popcorn” pattern indicative of hydraulic fracturing of surficial clay and a local seismic source. We found evidence for one sand-blow episode in Richland Parish, Louisiana, between 300 and 5,900 yr B.P., for two episodes in Morehouse Parish, Louisiana, that both occurred between 650 and 3900 yr B.P., and for two in Bradley County, Arkansas circa 1400 to 1760 yr B.P. and circa 1050 to 1400 yr B.P. The older buried blow in Morehouse Parish has a large vent several meters wide, which we attribute to lateral spreading rather than hydraulic fracturing, perhaps triggered by a large earthquake.

Thus at least three (probably more) paleoearthquakes between 700 and 8,000 years ago caused the sand blows in south Arkansas and north Louisiana, but more data is needed to establish a clear chronology. Based on spatial and temporal relationships, we speculate that the paleoearthquakes that triggered these sand venting episodes accompanied reactivation of the Triassic Saline River fault zone or related faults of the rifted Gulf of Mexico margin.

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