Geochemical characteristics of glacial Lake Agassiz sediments and new ages for the Moorhead Phase at Fargo, North Dakota, USA

The cause and age of the Moorhead low-water Phase of glacial Lake Agassiz remains uncertain. New geochemical (X-ray fluorescence (XRF) and elemental analysis) and chronological (optically stimulated luminescence (OSL)) data are used to test for evaporative enrichment within lacustrine sediment from Rabbit Lake, a small basin just above the highest Lake Agassiz strandline, and from two Lake Agassiz sediment cores at Fargo, North Dakota. Increases in quartz and gypsum interpreted to be of aeolian origin suggest increased aridity at Rabbit Lake sometime after 13 540–13 750 cal years BP. From the Fargo cores, lacustrine sediment of the Brenna and Sherack formations did not show convincing evidence for evaporative enrichment. However, this result is complicated by an erosional contact at the top of the Brenna Formation. A thin middle sand unit between the Brenna and Sherack formation clays is stratigraphically equivalent to the Poplar River Formation, West Fargo Member, but its properties differ from the fluvial sand of the West Fargo Member. Four OSL ages from the organic-poor, middle sand unit at Fargo range between 12.8 ± 0.2 and 13.5 ± 0.2 ka (with ±1.6 ka uncertainty) and suggest lake level fell at Fargo at, or before, 13.1 ± 0.2 ka, the average of the OSL ages. With different sedimentological properties, and a difference of ∼1750 years between the new OSL ages and previously published ages on the West Fargo Member sand, additional work is required to determine whether the middle sand unit is a new member of the Poplar River Formation, recording an earlier and different depositional environment than the West Fargo Member. From a plot of available ages for the Moorhead Phase, the regression remains poorly constrained in time.