RAPID LATE PLEISTOCENE INCISION ALONG S. ANNA RIVER IN THE VIRGINIA PIEDMONT- EFFECTS OF A CHANGING CLIMATE ON MID-ATLANTIC DRAINAGES AND RELEVANCE TO PALEOSEISMIC INVESTIGATIONS IN THE CENTRAL VIRGINIA SEISMIC ZONE

Late Pleistocene optically stimulated luminescence (OSL) ages for upland deposits along the S. Anna River Valley show that rapid incision occurred there, similar in timing and magnitude to incision along the Potomac and Susquehanna Rivers as determined by cosmogenic exposure dating (Reusser et al., 2004, Science, v. 305, p. 499-502). Reusser et al. measured ~15-20 m of incision dating from ~35 ka to ~13 ka and concluded that this was a regional effect of changing climate on glaciated and non-glaciated Mid-Atlantic rivers. Our work focused on 2 sites in the vicinity of the 2011 M5.8 Louisa County earthquake; Site A is near the earthquake’s epicenter and Site B is in the vicinity of the up-dip projection of the earthquake, ~5 km upstream from Site A. At Site A, ~1m-thick, clast-supported, imbricated-gravel/sand deposit that is ~15 m above the floor of modern S. Anna River (AMSAR) yielded an OSL age of 42.0 ± 4.2 ka from the middle of the gravel and 48.6 ± 2.2 ka from the base of the gravel. This gravel is overlain by ~1m-thick, matrix-supported colluvium with an OSL age of 22.3 ± 1.2 ka and an eolian silt dated at 29.1 ± 2.9 ka. At Site B, multiple terrace deposits at differing elevations were dated: 1) at ~20 m AMSAR, the OSL age of a clast-supported gravel/sand deposit was 42.8 ± 3.2 ka; 2) a 1m-thick, clast-supported deposit at ~18 m AMSAR yielded OSL ages of 34.3 ± 2.6 ka (base) and 25.1 ± 2.6 ka (top); 3) at ~11m AMSAR, 2 samples from a clast-supported gravel/sand deposit that is cut by fractures thought to be of tectonic origin yielded ages of 23.8 ± 1.0 ka and 26.9 ± 1.1 ka; 4) at ~10 m AMSAR, a sand/silt overbank deposit yielded ages of 22.0 ± 0.6 ka, and an underlying cross-bedded gravel/sand deposit yielded ages of 27.5 ± 2.5 and 28.3 ± 2.2 ka. This basal gravel either drapes a ~1m paleo-scarp of possible tectonic origin or is offset by a SE-dipping fault; 5) at ~8.5m AMSAR, an overbank deposit of fine sand/silt was dated at 17.2 ± 0.7 ka. These data demonstrate that there is a Late Pleistocene fluvial history preserved on the landscape in the epicentral area of the 2011 earthquake. This history is critical for determining possible regional neotectonic deformation related to prehistoric earthquakes, and may include effects of surface-rupturing deformation other than that mentioned above.