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Abstract: Recognition of Seismogenic Influence During Paleozoic Epicontinental Sedimentation: Examples from the Middle Ordovician Lexington Limestone, Central Kentucky

ETTENSOHN, FRANK R.; M. A. KULP, and N. RAST

Stratigraphic recognition of seismogenic horizons has been largely limited to Quaternary sediments in active-margin settings. Elsewhere in the geologic record and in other geographic settings, especially in epicontinental situations, the possibility of seismites has been largely ignored due to the erroneous notion that earthquakes are not common on "stable" cratons and partly due to the inability to distinguish seismogenic features from similar structures with different causes. In the Lexington Limestone several horizons of penecontemporaneous, soft-sediment deformation are commonly ascribed to downslope movement, but the facts that these horizons developed in an epicontinental setting where slopes were not major factors and exhibit features that are geometrically and lithologically identical to Quaternary counterparts, suggest that seismogenic origins may be just as likely. Other evidence for seismogenic origin includes co-occurrence of deformed horizons with known fault segments, suggesting reactivation of basement precursors and attendant earthquakes, presence of deformed layers enclosed within and locally crosscutting undeformed horizons, association of deformed horizons with likely tsunamites, and presence of micrite veins. Moreover, on a regional scale, the coincidence of many facies boundaries in the Lexington Limestone with structural features known to have basement precursors during a time of likely Taconian far-field tectonism suggests the likelihood of Ordovician synsedimentary tectonism and concomitant earthquakes in the central Kentucky area. Clearly, paleoseismicity is also present in the older Phanerozoic sedimentary record, and it should be recognized as another important aspect of marine epicontinental sedimentation.

AAPG Search and Discovery Article #90939©1997 AAPG Eastern Section and TSOP, Lexington, Kentucky