Dynamics of beach-ridge formation along a migrating strandplain with implications for paleo-environmental assessment, Illinois Beach State Park, southwestern Lake Michigan

This paper provides insights into shoreline morphodynamics and ridge-plain physiographic compart-mentalization along the migrating Zion Beach-ridge Plain, a mainland-attached strand along the high-energy, wave-dominated SW coast of Lake Michigan. Results of UAS-based topographic monitoring dur-ing the recent decadal lake-level high (2018-2020) captured the earliest phase of the ridge-formation process along several beach sites. This process-based study of net-erosional (i.e., littoral updrift) and net-accretionary (i.e., littoral downdrift) parts of the system offers much-needed context for assessing relict strand architectures. Embayed beach-ridge plains of the Laurentian Great Lakes have long served as paleo-environmental archives. They form from water-level changes and can be used to interpret iso-stacy. The preservation of beach ridges here is facilitated by unidirectional infilling. Much less is known about how nearshore littoral processes impact unconfined strand systems and whether useful paleoen-vironmental information absent within embayed system analogs might be elucidated from their complex architecture. We herewith promote a model of punctuated development. Major episodes of shoreline transgression are manifested as continuous ridgelines of topographic prominence that, along the north-ern, erosive strand, truncate older ridges and are onlapped by younger ones. These major discontinuities reflect environmental changes greater in magnitude than decadal water-level oscillations observed to have formed ridgelines in recent decades. The temporal offset across the most lakeward of these major erosional strandlines is on the order of 1 kyrs and it formed between 1.8 ka and 0.9 ka, based on available C-14 ages, coincidental with a regional shift in dominant storm-wind pattern to one promoting higher -energy littoral dynamics.& COPY; 2022 International Association for Great Lakes Research. Published by Elsevier B.V. All rights reserved.