STRATIGRAPHIC RECONSTRUCTION OF THE LATE HOLOCENE ZION BEACH-RIDGE PLAIN, SW LAKE MICHIGAN: TARGETING OVERWASH SANDS FOR OSL DATING

The Zion Beach-ridge Plain (ZBRP), located along the SW Lake Michigan coast, is a migrating strand whose depositional architecture offers valuable insights into changing littoral dynamics. While embayed Great Lakes beach-ridge plains have served as paleo-environmental archives of late Holocene water budgets and glacio-isostatic adjustments, few studies have addressed the complex evolution of migrating ridge-plain promontories that evolve by erosion along the littoral up-drift and accretion along the littoral down-drift side. This presentation documents results from ongoing work addressing how changing storm-wind/wave patterns may have found proxy manifestation within associated strand deposits.

Data from recent topo-bathymetric monitoring activities along the ZBRP shoreline provide a process-based blueprint for stratigraphic reconstructions. Overwash deposits, emplaced during lake-level highstands (followed to varying degrees by eolian modification), mark the creation of new strand terrains. These are preferentially preserved along the down-drift, net-accretionary part of the system and are recognized in GPR imagery as landward-dipping reflections that onlap more continuous paleo-topographic surfaces. Overwash deposits are captured in core as interbedded sand and gravel beds. Medium to fine-grained sand units were targeted for OSL dating.

ZBRP terrain physiography is compartmentalized across today’s net-erosive (up-drift) part of the system, where unconformable ridge-set boundaries are resolved in GPR, LiDAR, and the OSL dataset. This contrasts the continuous ridge chronosequence mapped along the net-accretionary portion of the system, where ridgelines are parallel, and no significant hiatus is resolved. The most recent ridge-set boundary along the net-erosive strand truncates conformable ridgelines dating to between 1.5 ka and 2.2 ka. This suture zone is onlapped by lakeward ridge terrains of <1.2 ka. Ongoing work will attempt to frame new insights within a regional paleoclimate context, addressing the possibility of shifting storm-wind patterns and/or major lake-level changes (of greater magnitude than recently observed) as potential culprits of inferred punctuations in millennial-scale geomorphic strand development in the alongshore direction.