NEW RESULTS FOR THE DURATION OF THE MOORHEAD PHASE OF GLACIAL LAKE AGASSIZ: THE REDWOOD LOOP SITE, NORTH DAKOTA

Because we suspect that some ages for the Moorhead Phase of glacial Lake Agassiz record involve “reworking,” we revisited the site with the oldest Moorhead age. The Redwood Loop site is a now-filled basement excavation on the Grand Forks Air Force Base. The stratigraphy observed is the same as that described in the drainage ditch where the oldest wood of Moorhead age was collected decades earlier (W-723: 10,960±300 14C yr BP, redated TAM1: 10,820±190 14C yr BP). The lithostratigraphy consists of a grey stony diamicton overlain by ~2 m of littoral sediments consisting of units of laminated sand and pebbles, discontinuous 10-cm thick sandy peat, cross-bedded sand, ripple cross-laminated sand, a pebble lag, plane-bedded sand, and massive sand, all capped with 0.6 m of silty clay containing the topsoil. We obtained 6 ages from wood at the sand/diamicton interface, which range from 10,340±75–10,000±70 14C yr BP. We also dated a stump disturbed by the excavator (two samples, 10,160±70 and 10,010±70), four samples from an in situ root thought to be from the same stump (10,370±75–10,710±75), and plant macrofossils from the sandy peat. Plant and fossil beetle analyses of the peat indicates a boreal forest/sedge wetland, representative of a cooler climate than those recovered from similar aged deposits 130 km south, indicating the existence of a north-south thermal gradient similar to that which exists today. Radiocarbon dates from spruce needles, sedge seeds, and a poplar wood fragment from the base of the peat are: 10,080±75, 10,150±75, and 10,520±70, respectively. Ages for the same materials from the top of the peat are: 10,440±80, 10,260±80, and 10,330±75, respectively. The delicate nature of the sedge seeds encased by intact perigynia (papery sheaths), and the intact black spruce needles precludes any significant reworking of this deposit. The 14C ages fall within the “radiocarbon plateau” and where there are complicated structures (wiggles). After calibrating and summing the probabilities of the 14C ages for each ‘group' of samples, the oldest is the root at 12,840–12,060 cal yr BP, and the youngest is the stump at 12,040–11,290 cal yr BP (2 sigma range). This exercise suggests a maximum duration of 1530 cal years between deposition of the diamicton and subsequent lake transgression. Preliminary OSL age analysis of two samples within the profile are consistent with the radiocarbon ages.