Implications of new luminescence (OSL) dates from sediments beneath a giant erratic boulder on the Iowan Surface, northeast Iowa

The Iowan Surface (IS) in northeast Iowa is strewn with spectacular erratic boulders. 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 on a foundation of Paleozoic marine rocks dipping gently to the southwest. Emplacement of erratic boulder trains are most often attributed to transport and deposition by glacial ice, but have also been shown to form during catastrophic megaflood outwash events. The IS 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 IS is blanketed with sandy and gravelly surficial deposits, some of which show anastomosing stream patterns. The lack of continuous loess on the IS, and the distribution and thickness of loess surrounding it may also suggest one or more outwash events has occurred. Newly obtained OSL dates from directly beneath one of the giant erratic boulders are integrated with previously reported OSL and radiocarbon dates from the study area, contributing to a more comprehensive understanding of the development of the IS.