OSL DATING AND SEDIMENTOLOGICAL ANALYSIS OF PRE-HERMAN GLACIAL LAKE STRANDLINES NEAR MILNOR, NORTH DAKOTA

Discontinuous ridges of sand and gravel near Milnor, North Dakota occur at higher elevations than the Herman strandlines of glacial Lake Agassiz. These were mapped as the Milnor beaches by Warren Upham (USGS Monograph 25, 1895). Later studies suggested non-lacustrine origins for these strandlines; instead attributing ridges in the area to a former watercourse of the Sheyenne River referred to as the Milnor Channel by Baker (USGS Pro. paper 550-B, 1966). This study focuses on discontinuous ridge segments located in northeastern Sargent and southeastern Ransom Counties, North Dakota. Three topographically prominent strandlines that were mapped as Milnor beaches (Upham, 1895; Plate XXVII), as well as one location associated with the Sheyenne River paleo-channel, were sampled for optically stimulated luminescence (OSL) dating. In addition to OSL dating, grain-size analyses were conducted for each sample. Our results indicate that only one of the three sampled ridge segments has sedimentology and stratigraphy consistent with beach deposits. Similarly, age results do not indicate a consistent depositional period for the individual segments that have been considered contemporaneous beach deposits in the past. A highlight of this presentation is a single, but intriguing age for the Milnor beach.