MORE LOGS ON THE BEACH FIRE: ADDITIONAL AGES FOR THE TINTAH SHORELINE OF LAKE AGASSIZ

Lake Agassiz was a massive glacial lake covering 1,500,000 sq km in central Canada as well as portions of western Minnesota (MN), eastern North Dakota (ND) and far northeastern South Dakota (SD). The lake existed from 14.1 to 8.2 ka. Lake Agassiz formed during the recession of the Des Moines and Red River Valley lobes of the Laurentide Ice Sheet (LIS) that had extended as far south as Des Moines, Iowa. There are four widely recognized strandline complexes associated with the southern arm (MN/ND/SD) of the glacial lake: the Herman, Norcross, Tintah and Campbell. The spatial extent and ages of these strandlines are critical to understanding the deglacial and post-glacial history of the midcontinent and the lake’s relationship to global climate change. The ages of the Herman and Norcross strandlines are well constrained by optically stimulated luminescence (OSL) dating to 14.1 ± 0.3 ka and 13.6 ± 0.3 ka, respectively. The Herman strandline complex represents the highest level that Lake Agassiz attained. The age of the youngest of these shorelines, the Campbell, is also well constrained by both OSL (10.5 ± 0.3 ka) and radiocarbon dating. The age of the Tintah shoreline has yet to be confirmed, but three prior OSL ages ranging from 13.4 to 13.6 ka have been obtained. Chronoquest is an on-going undergraduate student research effort that has been evaluating the age of Tintah strandlines in western Minnesota using OSL techniques. Previous Chronoquest work in far northeastern MN counties of Roseau and Marshall yielded a highly consistent age data set averaging 15.5 ± 0.2 ka. This data gave rise to the interpretation that an older, pre-Agassiz glacial lake, temporarily referred to as Lake Venetz, may have existed in the area. This lake would have been bounded on most sides, other than the south, by glacial ice. The current study followed the “Tintah” shoreline complex south into Polk, Marshall, and Pennington counties of MN. Our team will present OSL ages from Tintah shorelines in these counties and evaluate if they are consistent with geomorphic inferences placing the age of the Tintah at ~12.2 ka, or with past OSL ages of ~13.5 ka, or ages of ~15.5 ka consistent with proposed “Lake Venetz.”