SEISMIC SHAKING SOURCE FOR DEFORMATIONS IN MARINE SEDIMENTS

Limestone, shale, and sandstone strata, typically decimeters in thickness, are ubiquitous throughout north central and northeastern Kentucky and are exposed in virtually every roadcut and ravine. These strata were deposited in a marine shelf setting, and are mainly Ordovician or Devonian in age. Many tens of beds contain deformed zones that encompass tens to hundreds of sq. km in areal extent. Various types of plastic deformations occur, including convolute bedding, intraformational conglomerates, saucers, and shale diapirs. These beds also occasionally host faults and small sand-filled dikes.

Recently, numerous field studies have been conducted in order to assess whether the deformations were seismically induced. Many of these studies are presented in GSA Special Paper 359 (2002), Ancient Seismites. The consensus there is that seismic shaking triggered most (if not nearly all) of the deformations, although definitive evidence such as a pattern of deformations around a fault is typically missing. Thus, the possibility remains that these deformations are related to storm waves, sudden sediment accumulation, local slumping, or debris flows.

We observed sedimentary relations at one site near Tannery, Kentucky that almost certainly demonstrate a seismic origin. Here, convoluted sandstone beds roughly five to ten decimeters thick have severely warped both overlying and underlying shale beds, and in some locations the sandstones exhibit water escape structures. The sediments in this vicinity were deposited in deep seawater, as evidenced by the regional presence of thick carbonaceous (black) shales and pyrite, both of which form in oxygen-deficient environments. This suggests that a rapid accumulation of sediments and storm wave-induced deformation are unlikely. In addition, these beds are flat lying suggesting that local slumping likely did not trigger the deformations. This leaves debris flows and seismicity as potential triggers. However, the large water escape structures suggest a very rapid buildup of porewater pressure occurred in the underlying sediments, and strongly support a seismic trigger. This mechanism also is consistent with the coeval Taconic Orogeny. We suspect that such sedimentary relations can be applied when assessing origin in many subaqueous settings.