SUBSURFACE ARCHITECTURES MAPPED ALONG HARBOR-HEADLAND BEACHES OF THE LAURENTIAN GREAT LAKES USING GPR: A UNIFYING STRUCTURAL FRAMEWORK EXPLAINED BY RADAR FACIES PATTERNS AND SEQUENCE STRATIGRAPHIC PRINCIPLES

The shallow stratigraphy of depositional harbor-headland beaches along Michigan's Lake Superior and Ohio's Lake Erie coastlines was imaged with high-resolution GPR in an effort to develop an architectural blueprint for net-progradational lacustrine strands, important sedimentary archives for assessing margin-wide littoral dynamics. Subsurface mapping was undertaken across four beach systems along shore-perpendicular transects, extending from lakeward limits of foredune development to the upper swash zone. GPR data underwent basic post-processing to eliminate noise and amplify weakened signals at depth. Hyperbola-velocity calibrations established mean radar velocities on the order of 0.10 m/ns. No topographic corrections were required as imaging through flat and horizontal backshore regions ensured avoidance of non-vertical beam orientations. Subsurface interpretations are based on core stratigraphic principles.

Studied systems conform to a unifying architectural blueprint. Three primary GPR facies are identified throughout imaged subsurface regions, distinguished by radar pattern and stratigraphic association. Parallel, horizontal radar units occupying the uppermost depth intervals represent modern aeolian backshore deposits. They are unconformably underlain by parallel, seaward-inclined radar units, which are imaged to depths of up to ~4 m and represent advancing foreshore environments. Lastly, hummocky to chaotic radar units comprise the sectional basal deposits and are interpreted as former nearshore bedforms upon which foreshore environments prograded. While the stratigraphic superposition of these three distinct facies remains fixed, lateral variances in imaged clinoform shape, elevation, and preservation are suggestive of lake-level forcing. In particular, evidence of erosional truncation and onlap resolved in the subsurface generally coincides with known paleoshoreline positions affiliated with low and rising lake-level conditions, respectively. Conformable sedimentary packages, defined by little change in prograding clinoform geometry, document near-continuous lateral accretion of the headland beaches while unconformities, recognized by highly distinct stratal termination patterns, attest to punctuation of this growth by episodic erosion.