RECOGNITION AND SIGNIFICANCE OF MIDDLE ORDOVICIAN PALEOSEISMITES FROM SOUTHWESTERN VIRGINIA

Paleoseismites are pre-Recent sedimentary rocks with soft sediment deformation features, breccias, or graded bedding induced by seismic activity. Middle Ordovician rocks at several localities in the Valley and Ridge Belt of SW Virginia contain sedimentary structures that suggest the rocks may be paleoseismites. On the deeper, southwestern side of a Middle Ordovician epicratonal sea, after paleoturbiditic sections of the Knobs and Paperville formations were deposited, soft-sediment deformation features, notably folds, formed in rocks now exposed on the Pulaski fault block in the area between Lodi and South Holston Reservoir. As soft-sediment deformation commonly occurs independently of tectonism in steep slope to basin plain settings, Knobs and Paperville features are not unequivocal evidence of seismic activity. However, detailed geologic mapping in the Horton Valley area of Russell County to the north reveals Middle Ordovician Bowen Formation tidal flat to shallow-water, subtidal shelf, mixed carbonate-siliciclastic rocks on the Copper Creek-Narrows block that also contain soft-sediment folds and faults that cannot reasonably be attributed to gravitational slope failure. A seismic cause is likely for these bedding-confined features. Stereographic analysis of Bowen slump fold geometries indicates slump movement downslope, generally in the N20°W direction. This slope direction suggests the possible existence in the Middle Ordovician of an upthrown fault block southeast of the depositional site on the shelf. Contemporaneity of the Knobs-Paperville rocks and the Bowen Formation separated by more than 35 km distance cannot currently be verified biostratigraphically, but the general similarity in age suggests that the Knobs-Paperville soft-sediment features are possible paleoseismites. The Bowen paleoseismite occurrence points to an important role for earthquake-induced modification of normal sedimentation patterns during the Taconic Orogeny in southeastern North America.