Insights into the late Cenozoic configuration of the Laurentide Ice Sheet from40Ar/39Ar dating of glacially transported minerals in midcontinent tills

Abstract

Glacial sedimentary sequences in the north central United States record multiple advances of the Laurentide Ice Sheet (LIS) since ∼2 Ma. Although the tills found in these sequences were deposited by southward flowing glacial lobes, little information is available on the geometry of flow lines in the interior of the LIS during any one glaciation, and the provenance of glacial deposits older than the last ice advance is largely unknown. Systematic changes in the composition of midcontinent tills and other paleogeographic considerations, however, raise the possibility of significant shifts in the trajectory of flow lines feeding the lobes of the southwestern LIS margin. Here we constrain till provenance using40Ar/39Ar ages of individual hornblende and feldspar grains retrieved from tills representing several glaciations since ∼2 Ma. Hornblende grains show40Ar/39Ar ages that indicate erosion of Paleoproterozoic (∼1.7–2.0 Ga) and late Archean (>2.5 Ga) rock sources, whereas feldspar grains show a broad range of Paleoproterozoic ages (∼1.4–2.4 Ga). Dating of hornblende and feldspar minerals in single pebbles suggests that this latter distribution of ages is related to the greater sensitivity of feldspars to thermal resetting during minor tectonic events. Accordingly, the range of40Ar/39Ar ages for the predominant population of Paleoproterozoic hornblende and feldspar grains in our samples is consistent with a source from terrains forming the Churchill province of the Canadian Shield, while the small population of Archean‐age grains likely reflects a source from the southwestern tip of the Archean Superior province that crops out near the study area. These results indicate that midcontinent tills were deposited by ice derived from the northwestern (Keewatin) sector of the LIS. The nearly identical distribution of hornblende and feldspar ages in the till samples identifies the Keewatin ice dome and the related ice flow to the midcontinent as long‐standing features of the LIS throughout the late Pliocene‐Pleistocene glaciations.