RIDGE-PLAIN PROMONTORIES OF THE NORTH AMERICAN GREAT LAKES AS DEPOSITIONAL ARCHIVES OF CHANGING LITTORAL DYNAMICS

While depositional proxy records within embayed ridge plains of the Great Lakes have helped with reconstructing late Holocene glacio-isostatic movements and regional changes in water balance, ridge-plain promontories have been overlooked as repositories of paleo-environmental information. Exposed to the high-energy wave/current conditions of the open lake environment, they undergo extensive reworking over time and tend to migrate in direction of prevailing littoral transport. The utility of two different Great Lakes ridge-plain promontories as repositories of information on littoral hydrodynamic and alongshore sediment-transport processes is herewith evaluated.

We have studied the physiography, stratigraphy, and geochronology of the late Holocene Zion Beach-ridge Plain of southwestern Lake Michigan and the Presque Isle Peninsula of southcentral Lake Erie, both of which have historically experienced erosion along the littoral updrift and accretion along the littoral downdrift. Stratigraphic assessment is based on ground-penetrating radar imagery and sediment-core information, while physiographic assessment made use of available coastal LiDAR datasets. Geochronological information includes archival C-14 age dates and results of optically stimulated luminescence (OSL) dating of targeted sand deposits.

Ridge sets, demarcated by high-relief (comparatively) erosional strandlines, compartmentalize the studied ridge-plain promontories. C-14 and OSL-based geochronological data suggest that (1) major lake-level fluctuations, beyond those of historical changes with decadal, meter-scale lake-level variances; and/or (2) changes in storm pattern (e.g., energy conditions of the littoral zone) are responsible. Shelter from alongshore hydrodynamic processes reduces these geomorphic signatures within coastal embayments. This underscores the importance of expanding reconstructive efforts to promontory systems for a more complete assessment of regional paleoclimate.