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

Abstract

This paper provides insights into shoreline morphodynamics and ridge-plain physiographic compartmentalization 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 during 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 isostacy. 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 paleoenvironmental 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 northern, 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.