A POSSIBLE SEISMITE FROM AN UPPER ORDOVICIAN (ROCKLANDIAN) CARBONATE STRANDLINE

The Ordovician (Rocklandian) Carters Limestone of Tennessee consists of tropical-type carbonate platform facies deposited late in the Blountian tectophase of the Taconic orogeny. In middle Tennessee, Lower Carters skeletal-pelletal facies typically indicate a wave- and storm-influenced setting on a shallow shelf lithotope. In contrast, Upper Carters facies generally indicate a low-energy, peritidal setting. In most outcrops, the Deicke K-bentonite bed (449.8 +/- 2.3 Ma), resulting from a plinian eruption, separates the Lower and Upper Carters. Two lesser bentonites in the Lower Carters further aid lithocorrelation.

Along the Nashville Dome’s northwestern flanks, roadcuts on or near Tennessee Highway 840 reveal a somewhat different Lower Carters facies character. Sedimentary structures and textures reveal a transient shoreface zone with variable fairweather- and storm-wave energy. One locality hosts three stacked, meter-scale grainstone-dolomudstone couplets representing intertidal berm and backberm deposits formed on a low-energy, dissipative beach. However, supratidal features are absent.

The berm-backberm facies association provided a reverse density gradient (RDG), and soft sediment deformation structures (SSDS) later developed therein. These include ball and pillow, convolute bedding, mud diapirs, and carbonate breccias. While nearby outcrops contain likely upper shoreface deposits, more intense bioturbation (ii 3-4) destroyed any significant RDG, making soft sediment deformation less likely. Both carbonate foreshore deposits and carbonate SSDS rarely appear in the rock record. In this instance, the former was critical to the development of the latter.

Researchers have proposed numerous SSDS triggers, but only three are likely in this setting: storm wave-, tsunami-, and seismic-induced deformation. Sedimentological evidence supports only the latter trigger. Widespread SSDS -- an important criterion for seismic trigger recognition -- do not occur at this stratigraphic level. Nonetheless, a seismic origin cannot be ruled out. Increased facies variation on this carbonate platform may indicate tectonic loading of the peripheral foreland, or reactivation of basement faults within the craton. Either mechanism could also help account for the associated SSDS.