INTERPRETING ANCIENT MARINE SEISMITES: EVIDENCE FROM THE MIDDLE-LATE ORDOVICIAN LEXINGTON LIMESTONE, CENTRAL KENTUCKY

Horizons of soft-sediment deformation abound in the Lexington Limestone, a Middle-Late Ordovician, carbonate-platform sequence in central Kentucky. Although any of these horizons could be seismites, only a few of the deformed horizons like those in the Brannon Member occur in stratigraphic, structural, and temporal circumstances that allow a definite seismite interpretation. Because such deformation is not a unique response to seismicity, application of four concurrent criteria to such horizons can reduce the ambiguity. For the Brannon horizons, concurrence of criteria, which include: deformation consistent with a seismogenic origin, deformation in widespread, temporally and stratigraphically constrained horizons, deformation that shows systematic increases in frequency or intensity toward a likely epicentral area, and the ability to exclude other possible causes, strongly supports a seismogenic origin; other criteria reinforce this interpretation. Because deformation intensity, as indicated by preserved sedimentary structures, can reflect original energy input, mapping distribution of deformation types in Brannon horizons has effectively generated isoseismal maps that can be used to infer apparent epicentral areas. Although use of these maps with other stratigraphic data has suggested interpretations regarding ancient epicenters, earthquake directivity, earthquake clustering, magnitude and recurrence intervals, these interpretations must remain tentative until better data and modern, marine analogues can account for uncertainties like site effects. Nonetheless, the example of Brannon seismites shows that seismicity was at times an important process in epicontinental sedimentation and that well-constrained, deformed beds of seismogenic origin are potentially important chronostratigraphic event horizons, which can provide information about the nature and occurrence of very ancient Phanerozoic earthquakes.