UPPER ORDOVICIAN SEISMITES OF KENTUCKY: A SEDIMENTOLOGICAL AND SEQUENCE STRATIGRAPHIC APPROACH

Seismites are abundant in the Late Ordovician strata of Kentucky. These deformed intervals are prominent at several levels within Shermanian and Edenian age strata and have been the focus of recent studies on seismites. These marine strata were deposited during times of active tectonism. Far-field tectonics was the probable mechanism by which seismic waves were concentrated in this area. These seismites display a range of sedimentary features including: saucer structures, shale diapirs, homogenous ellipsoids, foundered blocks, and intraformational conglomerates. These features give information about the environment and timing of deformation. Mobilization of mud caused deformation of the surrounding sediments. Many Late Ordovician shales contain swelling clays, likely from reworking of volcanic ash. We suggest it is the thixotropic properties of these clays that allow them to become unstable, in going from a gel to sol state. In the gel state these muds were cohesive enough to record prod marks, flutes, and grooves, as is witnessed on the undersides of pillow structures. However, during episodes of seismic shaking muds flowed upward as diapirs, evacuating from the lower part of a deformed zone to be redeposited on top. Calcarenite and calcisiltite beds most commonly behaved ductily; however, there are cases of brittle deformation in the form of foundered blocks and small scale thrust faults, indicating early cementation of sediments. Thrusts and overturned folds, in the Frankfort area, have a vergence to the north, parallel to depositional dip. Truncation of the upper surfaces of deformed zones indicates that deformation occurred near the sediment-water interface. Hardgrounds are common within these intervals and formed after deformation, as they occur upon truncation surfaces. The most widely exposed seismite interval in the Ordovician of Kentucky occurs in the late highstand strata of the upper Bromley Shale and can be traced for over 9000 km2. In northeastern Kentucky this zone of seismites occurs within a channel, owing to the erosive base of the late highstand interval. These and other late highstand deposits in the Upper Ordovician strata of Kentucky typically contain an abundance of well-sorted fine-grained calcarenite interbedded with shale, the sediments that most commonly compose seismites.