WIDESPREAD SOFT-SEDIMENT DEFORMATION INTERVALS IN SILURIAN STRATA OF THE APPALACHIAN BASIN: SEISMICALLY INDUCED EVENT HORIZONS
Silurian deformed intervals display a range of sedimentary features that supply information about the environment and timing of deformation. The deformed strata are typically composed of shales, overlain by laminated siltstone to fine-grained sandstones/calcarenites. The shale typically shows little evidence of bioturbation; burrows that are present are sharply defined, indicating formation in firm muds. The overlying silty/sandy layers commonly contain hummocky to swaley cross bedding, suggesting lower shoreface deposition. These beds display sharp basal contacts with sole marks (e.g., scratches, prods, and flutes), suggesting deposition on firm, over-compacted mud substrates, which subsequently became liquefied owing to shaking. Deformation of the surrounding sediments apparently involved gel to sol transitions in thixotropic muds. In the gel state the muds were cohesive enough to record sole marks and to support the load of overlying silts/sands. However, during shaking episodes muds flowed upward as diapers. Truncation of the upper surfaces of deformed intervals indicates that deformation occurred near the sediment-water interface. Taken together, the data suggest component sediment layers were not deformed during initial deposition, but later, during shallow burial. The most parsimonious mechanism for widespread shaking is large-scale earthquakes; i.e., these are seismites.
The occurrence of deformation prone facies within FSSTs of Silurian depositional sequences was regulated by glacio-eustasy. Flexure of the cratonic margin resulted in large-scale earthquakes that triggered widespread shaking and soft-sediment deformation. Thus, these event beds provide a series of time horizons within the Silurian strata of the Appalachian Basin against which other correlation schemes may be calibrated.