TSUNAMITES AS SEISMITES:A PROBABLE EXAMPLE FROM THE MIDDLE DEVONIAN DUFFIN BED, NEW ALBANY SHALE, SOUTH-CENTRAL KENTUCKY

Locally in south-central Kentucky, the Duffin Bed is the basal unit of the New Albany Shale, except in a few structurally low places where it overlies black shales of the Carpenter Fork Bed. The Duffin consists of poorly sorted, millimeter- to meter-scale, dolomitic clasts from underlying units, floating in a dark, organic-rich, matrix, unconformably overlying Ordovician, Silurian or older Devonian carbonates. The breccias occur in two to three well-defined, crudely graded beds near Devonian synsedimentary fault zones. The breccias were deposited uniformly in fault-bound basins and on the downthrown margins of faults, but persist in a thinned state on intervening uplands or on the upthrown margins of the faults. Included body fossils are reworked from underlying units, but trace fossils from upper parts of some units suggest a shallow-marine setting. The breccias have been interpreted as debris-flow deposits derived from the ancient fault scarps, but this interpretation cannot account for thinned breccias on the uplands, the organic-rich matrix, or the uniformity of deposition in separated fault basins. However, presence of coeval, liquefaction structures typical of seismites in more distal, facies equivalents suggests the likelihood of contemporaneous seismicity and related tsunamis. Activity on the faults may have generated the seismicity, and the resulting tsunamis could have ripped up organic-rich muds of the Carpenter Fork Bed and mixed them with eroded clasts from the unconformity, depositing thin, organic-rich breccia layers on the uplands and thicker layers in the lows. Definite evidence of tsunamis is absent, but the association of features and circumstances rules out more common alternatives in favor of tsunamis. Although unlike the soft-sediment deformation typical of most seismites, because of their likely association with Devonian seismicity, these sedimentary breccias are nonetheless probably seismites.