ANATOMY OF A MIGRATORY, LATE HOLOCENE STRAND COMPLEX: A GPR-BASED ARCHITECTURAL STUDY OF THE ZION BEACH-RIDGE PLAIN, SW LAKE MICHIGAN

The Zion Beach-ridge Plain (ZBRP) defines the northernmost portion of the Illinois coast of SW Lake Michigan, where it hosts a variety of unique ecosystems within its ‘washboard-style’ ridge-and-swale topography (e.g., pannes). The sands comprising this coastal lithosome were sourced by southward littoral fluxes and the present strand contains an ~5-kyr paleorecord. An effort is underway to map, in spatiotemporal detail, the depositional architecture of the ZBRP using GPR (~12 trackline km along shore-perpendicular road sections), LiDAR data (2012 NOAA topo-bathymetry), historic shoreline positions (from nautical charts and aerial photographs), lithologic data (cores and auger descriptions), and geochronological information (C-14 and OSL ages). Our goal is to better understand how the shape of the underlying till surface and late Holocene climate changes have influenced shoreline and strand morphodynamics.

Shore-perpendicular GPR transects (250 MHz) resolve lakeward-inclined reflectors of high amplitude within the upper 5 m of the strandplain subsurface. These are interpreted as former foreshore profiles. Radar surfaces truncating landward reflections and/or characterized by onlap of more lakeward ones indicate ravinement. Associated overwash deposits are inferred by the presence of landward-dipping reflections beneath lobate topographic features that occur along the backside of prominent ridgelines. Zones of foreshore-profile amalgamation (where overwash is absent) correlate spatially with high-elevation (relatively), compound ridge complexes shown to obliquely truncate landward (i.e., older) ridgelines in map view.

Unlike embayed strandplains of the Niagara escarpment, often studied for paleohydrographic information, the Zion Beach-ridge Plain is a Great Lakes strand that has undergone near-continuous reworking along its northern extent and accretion along its southern. This general migratory dynamic has been punctuated by extreme events, which are manifested in its highly compartmentalized architecture. Potential culprits of this are changes in lake level and/or storm climate (e.g., prevailing storm-wind directions). Efforts are underway to refine the strand geochronology for an improved evolutionary model and for regional contextualization.