Eustatic and tectonic control on the distribution of marine seismites: examples from the Upper Ordovician of Kentucky, USA

摘要

Widespread zones of soft-sediment deformation are not ubiquitous throughout the rock record. Rather, they are associated with orogenic sediments and are concentrated in specific portions of depositional sequences characterized by deformation prone facies. Upper Ordovician soft-sediment deformation features exposed in Kentucky are here interpreted as the result of seismically induced thixotropic fluidization of muds resulting in foundering of denser silts and carbonate sands and corresponding diapiric injection of muds. Severity of soft-sediment deformation is associated with sediment density gradients, bed thicknesses, and varying magnitudes of triggering events. The widespread nature (>5000 km~2) of multiple deformation horizons argues for a seismic trigger resulting from reactivation of multiple basement faults. Eight large-scale (3rd order) depositional sequences were examined for the presence of specific deformation features and stratigraphic position of those intervals. The regular distribution of deformation prone facies throughout the Upper Ordovician seems to have been regulated by eustatic fluctuations. Seismicity generated by far-field tectonics provided the triggering mechanism to disrupt deformation prone strata. The record of seismites and their co-occurrence with K-bentonites provides a meter of intensity and timing of tectophases of the Taconic Orogeny.