THE DISTRIBUTION OF SEISMITES: A SEQUENCE STRATIGRAPHIC AND SEDIMENTOLOGICAL APPROACH

Seismites are not ubiquitous throughout the rock record. They are restricted to times of active tectonism, as well as to specific portions of depositional cycles corresponding to deformation prone sedimentary architecture. Eight large-scale (3rd-order) depositional sequences from the Late Ordovician together with examples from the Early to Middle Silurian of the eastern United States are examined for presence of specific deformation features and stratigraphic position of those intervals. Within these strata seismites are developed in two specific depositional facies. First, rhythmite facies include rhythmically interbedded, thin, tabular calcisiltites and shale representing deep shelf deposition in early highstand systems tracts. Second, laminite facies begin with a basal shale overlain by laminated, fine-grained calcarenites interbedded with thin mud drapes, which preferentially form in the late highstand. Seismites display a range of sedimentary features including: convolute bedding and intraformational conglomerates (restricted to rhythmite facies), and saucer structures, mudstone diapirs, ellipsoids, foundered blocks (restricted laminite facies). These features give information about the environment and timing of deformation. Mobilization of mud caused deformation of the surrounding sediments. We suggest it is the thixotropic properties of the clays within these muds that allow them to become unstable, in going from a gel to sol state. In the gel state the muds were cohesive enough to record prod marks, flutes, and grooves, as is witnessed on the undersides of saucer structures. However, during episodes of seismic shaking muds flowed upward as diapirs, evacuating from the lower part of a deformed interval to be redeposited on top. Truncation of the upper surface of deformed intervals indicates that deformation occurred near the sediment-water interface and is occasionally marked by a bored and encrusted hardground. The regular distribution of deformation prone facies within a single depositional sequence seems to be regulated by eustatic fluctuations. Tectonics simply provides the triggering mechanism to disrupt deformation prone strata. The record of deformed intervals provides a meter of intensity and timing of pulses of tectonism during orogeny.