THE VEXING CHRONOLOGY OF LAKE AGASSIZ’S TINTAH SHORELINE

Over the past decade Optically Stimulated Luminescence (OSL) dating has been highly successful in constraining the depositional ages for three of the four major shoreline complexes in the southern basin of Lake Agassiz. The Herman, Norcross and Campbell have all been successfully and reproducibly dated using OSL with ages of 14.1 ± 0.3 ka, 13.6 ± 0.2 ka and 10.5 ± 0.3 ka, respectively. However, deposits of the Tintah shoreline complex have not thus far yielded a robust set of reproducible ages. Near the southern outlet, where much of the past work has been focused, Tintah ridges are very weakly expressed on the landscape and are often composed of course-grained lag deposits unsuitable for OSL dating. To overcome these challenges a sampling strategy targeting better expressed, sand-rich beach ridge segments further north, in Marshall and Roseau counties of northern Minnesota, was undertaken. A total of seven samples were collected from ridge deposits mapped as “lowest” Tintah to help constrain the timing of the Tintah-Campbell gap. Six of the seven ages were tightly clustered with a group mean of 16.4 ± 0.2 ka. This is significantly older than any previously reported Tintah age and, in fact, is significantly older than the now, well-established age of the earliest Lake Agassiz shoreline, the Herman. In addition to reviewing existing OSL ages for the Tintah shoreline, this presentation will focus on derivation of the new ages and substantiation of the results. These findings instigate a complex problem for current interpretations regarding glacial dynamics in the region, but since ice margin age constraints as well as sub-lobe interpretations throughout this region are convoluted, the new age data should provide a much-needed opportunity to re-evaluate the status quo.