Origin of a Late Middle Ordovician Seismite-like Deposit: Did It Jump, or Was It Pushed?
Clay Harris, Holly Baker, and Crystal Lewis
Middle Tennessee State University, Department of Geosciences, Murfreesboro, TN 37132, [email protected]
Comparison with Upper Ordovician seismites in Kentucky and Virginia suggests a late Middle Ordovician (middle Mohawkian) seismite occurs on the Nashville Dome's eastern flank. We observed several soft-sediment deformation features in a Lower Carters Limestone (Turinian) outcrop, including ball-and-pillow structures and convolute bedding. We attribute these features primarily to liquefaction due to shear stress. Similar deformation features are not present in other area outcrops, even those within 10 km. Nor is there evidence for an alternate source of strong shear; for example, intense tidal or wave action. However, normal faults with less than a meter of offset occur at this location, and larger-scale normal faults occur less than two km away.
Outcrop evidence indicates the offsets are younger than the soft-sediment deformation, but this does not preclude earlier, related seismic shocks. We therefore interpret this to be a seismite. Consequently, a literature search suggests this deposit — the only known seismite of the Lower Carters in Middle Tennessee — may be the earliest documented seismite from southern Laurentia coincident with the Taconic orogeny.
The host sediments are interbeds of bioturbated (Phytopsis) dolomicrite-siltite and well-laminated packstonegrainstone. Interplay between fair weather, storm, and tidal processes on a carbonate shoal resulted in cm- to dm-scale, rhythmically bedded sand- and mud-dominated layers, which are uncharacteristic of the Lower Carters. Wave action(?) discouraged burrowing fauna (ii 2-3), preserving sharp boundaries between the mud and sand layers. Had less of a textural gradient existed, the degree of sediment disturbance would probably have been minimal, leaving no seismic record.
AAPG Search and Discovery Article #90095©2009 AAPG Eastern Section Meeting, Evansville, Indiana, September 20-22, 2009