Interpreting ancient marine seismites and apparent epicentral areas for paleo-earthquakes, Middle Ordovician Lexington Limestone, central Kentucky

Three horizons of soft-sediment deformation in the Middle Ordovician Brannon Member of the Lexington Limestone, a platform carbonate sequence in central Kentucky, are in stratigraphic, structural, and temporal circumstances that suggest seismogenic origins. However, because such deformation may not be a unique response to seismicity, application of four concurrent criteria to such horizons can reduce the ambiguity. For the Brannon horizons, concurrence of these 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 the Brannon horizons has effectively generated isoseismal maps that were used to pinpoint 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, such interpretations must remain tentative until better data and modern, marine analogues can be found to illustrate the influence of site effects in similar marine facies. The example of Brannon seismites shows that seismicity was an important process affecting epicontinental sedimentation and that well-constrained, seismically deformed beds are potentially important chronostratigraphic event horizons, which can provide information about the nature and occurrence of ancient Phanerozoic earthquakes.