A POSSIBLE MIDDLE DEVONIAN TSUNAMITE: DUFFIN BED, NEW ALBANY SHALE OF SOUTH-CENTRAL KENTUCKY

The basal unit of the New Albany Shale in south-central Kentucky is the Middle Devonian Duffin Bed, a dolomitic breccia in an organic-rich matrix, ranging from a few centimeters to 7-m thick and unconformably overlying Ordovician to Middle Devonian units. The breccias are composed of angular chert and dolostone clasts from underlying, subaerially exposed Middle Devonian carbonates; clasts range from a few millimeters to more than 2 m in diameter. The breccias infill paleokarst in underlying carbonates, but are more commonly associated with ancient, downdropped fault blocks and grabens. Two to three, well-defined, crudely graded breccia units occur rather uniformly in these structural lows, but thin dramatically on adjacent highs and upthrown blocks. In one well-preserved graben, breccias overlie an in-place black shale that was apparently removed elsewhere along the underlying unconformity. Trace fossils from upper, reworked parts of breccia units suggest shallow-marine deposition. Although previously interpreted as debris-flow deposits from adjacent fault scarps, this interpretation cannot explain thinned breccias on adjacent highs, the organic-rich matrix, and the uniformity of deposition throughout fault basins. However, presence of liquefaction structures typical of seismites in distal facies equivalents suggests the probability of coeval seismicity and related tsunamis. Movement on adjacent faults may have generated the seismicity, and resulting tsunamis could have ripped up underlying, organic-rich muds and mixed them with eroded clasts from the unconformity, depositing thicker layers of organic-rich breccia in the structural lows and thinner layers on adjacent uplands. Indisputable evidence for tsunamis is absent and will probably always be difficult to identify in shallow-water, epicontinental settings, but an association of features and circumstances, especially evidence of coeval seismicity, can rule out more common alternatives and lend support for a tsunami model of origin.