Deglaciation in the Upper Peninsula of Michigan since the last glacial maximum and its relationship to tunnel valleys found in the Lake Superior basin.

摘要

The Upper Peninsula of Michigan (Upper Peninsula) has undergone several glaciations as evidenced by the presence of ice-advance moraines, extensive channels, and outwash complexes. In addition, networks of erosive valley-type depressions have been identified in the Lake Superior and Michigan basins bordering the Upper Peninsula. These features are 1-2 km wide, and are up to 200 m deep and are identified as tunnel valleys---subglacial channels found at the margin of ice sheets, thought to evacuate meltwater from the base of the glacier (Halls and West, 1971; Patterson et al., 2003; Regis et al., 2003). These valleys are hypothesized to have yielded multiple, episodic meltwater releases over time, potentially impacting deglaciation processes. To assess this, a geophysical, geomorphological/sedimentological, and chronologic examination was performed, including: (1) determining the presence of tunnel valleys across the Upper Peninsula, (2) identifying geomorphic or sedimentologic signatures of episodic meltwater releases, and (3) establishing radiocarbon chronology of ice retreat.;The geophysical survey suggested the discontinuous presence of erosional valley-type features, ranging from 180 to 400 m in relief. The geomorphic investigation supported the existence of tunnel valleys on the Upper Peninsula, identifying landforms associated with the valleys (i.e.-kettled fan outwash complexes, discontinuous eskers and channels). Episodic meltwater releases are likely, based on multiple, large-scale outwash fans, lobate kettled plains, and erosional bars. Lastly, deglaciation is thought to be time transgressive as the LIS retreated northward, based on radiocarbon dating of kettle lake organics. The LIS is suggested to have been in the Lake Superior basin by 8500 14C yr BP. These examinations indicate deglaciation in the Upper Peninsula was shaped by the presence of tunnel valleys found in the Lake Superior basin.