RECOGNITION OF THE 1811-1812 NEW MADRID EARTHQUAKES IN REELFOOT-LAKE,TENNESSEE SEDIMENTS USING POLLEN DATA

Reelfoot Lake is located within the New Madrid Seismic Zone, a region characterized by ongoing seismic activity and the locus of a series of large earthquakes (m(b)>7) during 1811-1812. Coseismic uplift and sub sidence from the 1811-1812 events formed the lake basin from a partial ly inundated alluvial bottomland forest. Lithologic, chronologic, and palynologic data from a vibracore are used here to characterize the 18 11-1812 earthquake record in lacustrine sediments. The stratigraphic r ecord consists of a poorly consolidated upper silt, an intervening 10- cm sand layer, overlying a compact lower silt. Calibrated radiocarbon age estimates on wood samples from both silt units indicate deposition during historical time (1490-1890 AD). Better age estimates were obta ined by correlating pollen assemblage data from the upper and lower si lt with the historical record of land-use change in the Reelfoot Lake region. Two factors resulted in changing plant distributions (and henc e pollen assemblages) in Reelfoot Lake sediments: 1) altered drainage patterns of Reelfoot Creek and Bayou de Chien resulting from 1811-1812 uplift and subsidence, and 2) deforestation and subsequent cultivatio n beginning approximately 1850 AD. The upper silt is characterized by a oak/cedar arboreal pollen (AP) assemblage, showing a mixture of upla nd and alluvial bottomland AP influx from the region to the open lake basin. Non-arboreal pollen (NAP) in the upper silt shows increasing ab undance of Composites, particularly ragweed pollen indicating cultivat ion. This unit was deposited after the 1811-1812 earthquakes. The inte rvening sand layer was apparently emplaced by earthquake activity, or represents colluvium derived from most recent(1811-1812) coseismic upl ift of Reelfoot scarp, which forms the western margin of the lake. The lower silt is characterized by a baldcypress/cedar AP assemblage with minor percentages of other flood-tolerant AP genera, interpreted as a baldcypress-dominated bottomland forest. Pollen influx in this enviro nment is dominated by gravity-component deposition from local sources. The NAP in the lower silt shows that ragweed is rare or absent, sugge sting pre-settlement conditions and deposition prior to 1811-1812.