Abstract

Faults, folds, and sand dikes have been identified in late Holocene sediments exposed in an exploratory trench excavated across Reelfoot scarp in northwestern Tennessee. In excess of 3 m of vertical displacement believed to be of deep-seated origin occurs across a 0.5-m-wide zone of east-dipping normal faults near the scarp base. The zone includes the only faults of probable tectonic origin known to cut Holocene sediments in the upper Mississippi embayment. Stratigraphic and geomorphic relationships suggest, however, that little (<0.5 m) or no near-surface fault movement occurred across the zone during the high-magnitude New Madrid earthquakes of 1811–1812. Numerous faults having only minor displacements were mapped elsewhere in the trench. Geologic relationships between the faults and sand dikes indicate that the faults formed contemporaneously with the dikes during high-magnitude earthquakes. Crosscutting geologic features and local geomorphic history suggest that at least two periods of faulting predate sediments deposited before A.D. 1800. Thus historical data and the sediments in the trench record a history of three earthquakes near the trench site strong enough to liquefy sediments and generate faulting. Carbon-14 dates obtained on fresh-water shells indicate that the trench sediments have a maximum age of about 2,000 radiocarbon years. A recurrence interval of ∼ 600 yr or less is suggested for large earthquakes in the New Madrid area.

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