OPPORTUNISTIC STRATIGRAPHY: GLACIAL LAKE SEDIMENTS REVEALED DURING THE MAY 15-16, 2006 SUNCOOK RIVER AVULSION IN EPSOM, NEW HAMPSHIRE

On May 15-16 2006, massive flooding caused the Suncook River in Epsom, NH to instantaneously change its course (Wittkop et al., 2007; NE GSA Guidebook to Field Trips in NH, adjacent ME, and MA), exposing a stratigraphic sequence of dark grayish brown clay with thin (~1 cm) woody debris layers. Based on the composition and location of these sediments, it is likely that they were deposited in Glacial Lake Hooksett during the end of the last ice age. A radiocarbon date from peat collected from Glacial Lake Hookset deposits near Bow, NH suggests the lake existed ~ 13,000 years B.P. (Caldwell et al., 1978; GSA abstract). In June 2008, we cored approximately 2 meters of sediment in the clay rich portion of the recently exposed strata near Epsom, NH. In order to establish the origin and age of this section, we examined the fine and coarse fraction sediment compositions, diatom biostratigraphy, variability and source of organic carbon content downcore using a CHN analyzer, and AMS 14C dating techniques. Our results show that the mineral assemblages are dominated by quartz, micas, feldspars, and clay minerals in the coarse and fine factions. Diatoms assemblages remain fairly constant downcore, with five freshwater genera consistently dominant: Navicula, Stauroneis, Tabellaria, Cyclotella, and Melosira. Terrestrial organic matter is present throughout the section and concentrated in 3 distinct layers near the base of the section. The organic carbon content downcore averages 3.08 wt. %, but gradually decreases from maximum of 4.19 wt. % near the top to a minimum of 1.5 wt. % near the bottom. Two radiocarbon dates (11,950 +/- 60 B.P. RC years and 12,250 +/- 60 B.P. RC years) in stratigraphic succession, obtained from two of the organic layers deep in the core, when calibrated to calendar years B.P (Calib ver. 5.0.2; 2 sigma ranges 13,692-13,964 and 13,924-14,420, respectively) establish a late Pleistocene age for these sediments. Measured C and N on the organic carbon down core yield an average ratio of 13.3, consistent with a soil organic carbon source. Based on these results, we can conclude that this recently exposed clay dominated sequence was deposited in a low energy, freshwater, diatom and organic carbon rich, terrestrial environment within the known proximity of and age of Glacial Lake Hooksett.