HOLOCENE SAND BLOWS IN NORTHEAST LOUISIANA

We examined aerial photography of eight parishes in northeastern Louisiana within the Wisconsinan and Holocene portion of the Lower Mississippi Valley for possible seismically-induced sand blows. Two areas or “fields” in Morehouse, Richland, and Franklin Parishes of northeastern Louisiana have circular to elliptical tonal anomalies that are similar in size, shape and spacing to the aerial photography signature of sand blows to the north in the Mississippi Embayment, but are well outside the known region of liquefaction associated with the New Madrid seismic zone (NMSZ) in the northern embayment (>330 km to the north). Following our previous approach of coring, ground conductivity mapping, trenching, sedimentological/soil analysis, and age analysis by ¹⁴C and luminescence techniques, we conclude that these features are fields of seismically-induced sand blows of late Holocene age. We also recognized another area in Franklin Parish as relict wind blown silt dunes rather than sand blows. We found evidence for two sand blow episodes in Morehouse Parish that occurred between 700 and 3830 yr B.P. and for one in Richland Parish between 330 and 5500 yr B.P. The older blow in the Morehouse Parish field has a large vent several meters wide, and we interpret this older blow as a lateral spread sand blow that may have been triggered by a larger earthquake than the later blows that show a “shotgun” surface pattern indicating hydraulic fracturing rather than lateral spreading. The age constraints of the Louisiana sand blows can accommodate any of three dates of possible regional events in southern Arkansas liquefaction fields (700, 2300, and 3500 yr B.P.). Based on spatial and temporal relationships, we speculate that the paleo-earthquakes that triggered the sand venting episodes in northeastern Louisiana were along the Saline River fault zone in southern Arkansas.