LEAVE NO STONE UNTURNED: PRELIMINARY OSL DATES FROM BENEATH A GIANT ERRATIC BOULDER, NORTHEAST IOWA

From the “foundlings” of Goethe and Agassiz to Darwin’s boulders in Tierra del Fuego, erratic boulders have fascinated and intrigued many geoscientists. The Iowan Erosion Surface (IES) in northeast Iowa is strewn with spectacular erratic boulders, the largest of which are granitic igneous and metamorphic rock. The stratigraphic context of the IES includes a bedrock foundation of gently dipping Paleozoic sedimentary rocks. The area has been glaciated several times during the early to mid-Pleistocene, leaving a palimpsest of tills and associated sedimentary materials of different ages. Radiocarbon dating in the study area indicates ages that exceed the effective range of the technique.

Emplacement of erratic boulder trains or fans are most often attributed to glacial transport and deposition, but have also been shown to form during catastrophic megaflood outwash events. The IES displays a number of geomorphic features associated with outwash floods including deeply incised bedrock gorges and amphitheater-headed canyons, streamlined upstanding remnant hills, gravel bars, and coarse-grained terraces and paleovalley fill sediments. The IES is covered with sandy and gravelly surficial deposits, some of which show anastomosing stream patterns. The lack of loess on the IES, and the distribution and thickness of loess surrounding it may also suggest the occurrence of one or more outwash events. Features associated with permafrost have also been described from the IES.

Here, we report new OSL dates from beneath one of the giant erratic boulders, integrated with previously reported OSL and radiocarbon dates from the study area in an effort to clarify and investigate the time and method of emplacement of the erratic boulders, contributing to a more comprehensive understanding of the development of the IES.