ANOMALOUS UPPER ORDOVICIAN DEBRIS FLOWS OF OHIO AND KENTUCKY: ARE THEY SEISMITES?

Upper Ordovician seismites and debris flows of Ohio and Kentucky are highly deformed calcisiltite or calcarenite beds containing ball-and-pillow structures, convolute lamination, load casts, flame structures, sedimentary breccias, and synsedimentary folding. Seismites occur in thin, laterally persistent zones traceable for many kilometers; these zones contain one or more deformed beds. These zones are separated by thick sequences of undeformed strata. Debris flows occur in seismite zones and as anomalous, lenticular deformed beds in the otherwise undeformed sequences between seismite zones. These anomalous debris flows are generally restricted to a single outcrop and occur in the upper portions of shallowing sedimentary cycles. Upper and lower contacts are sharp and in some cases truncate adjacent undeformed strata. Elongate pillows are weakly to strongly imbricated, float in a shale or argillaceous wackestone matrix, and are rarely thrust into the adjacent undeformed strata. Some anomalous debris flows are capped by beds or sheets of wackestone matrix and contain rafted packstone and grainstone clasts incorporated from the adjacent undeformed strata.

Clearly, debris flows present within seismite zones were likely triggered by earthquakes. However, the triggering mechanism for anomalous debris flows is problematic. Seismic shock is unlikely because individual flows are restricted to very limited geographic areas, and flows occur in shallowing cycles containing numerous, laterally persistent, undeformed calcisiltites and calcarenites. Storm-wave liquefaction is improbable because anomalous debris flows occur only rarely in shallowing cycles dominated by tempestites. Load-induced liquefaction is doubtful because the underlying sediments are undeformed. We suggest anomalous debris flows represent localized gravity-induced slumping associated with seismic activity and peripheral and anti-peripheral bulge migrations of the Taconic Tectophase.