SEDIMENTARY STRUCTURES IN A BROAD INTERDUNE DEPRESSION (DUNE SLACK) IN A LAKE MICHIGAN COASTAL DUNE COMPLEX: A GPR STUDY

A GPR study was undertaken of the broad (0.5 km long) interdune depression (dune slack) between massive parabolic coastal dunes and lower inland back dunes along the southeastern Lake Michigan coast southwest of Holland, Michigan. Two different sedimentary packages are evident in the GPR transects across the depression. The eastern (inland) package is made up of many short (2-5 m) dome shaped (doubly dipping) reflectors stacked onto each other. To the west this package down laps onto an eastward dipping, 20 m long continuous reflector. The package west of this reflector has numerous continuous reflectors (80 m or longer), with subdued topographic relief (~1 m), truncated by the layers above them. Between these long reflectors are numerous parallel short reflectors (1-10 m) which usually dip to the east. Five holes extending to the water table at ~3m in depth were augered along a traverse through the depression. A sesquioxide layer (Bs horizon), representing a truncated soil, was found at or near (within 1 m) of the surface throughout the traverse. This indicates a relatively long period of landscape stability, allowing for this soil horizon to form. OSL analyses of sand from the surrounding back dunes indicate burial of the sediment beneath this soil at ~3000-4000 cal. YBP. An A horizon was discovered at 2.2 m depth in one auger hole, where it intersects a long continuous GPR reflector. However, paleosols were not found where other holes intersected similar continuous reflectors. The sediment within all of the holes was medium grained sand, skewed toward finer grain sizes, and homogeneous, suggesting probable eolian deposition. This work provides insight into eolian sedimentation in interdune depressions. The presence of short doubly dipping reflectors in the eastern package suggest deposition around isolated patches of vegetation along a sandy plane, forming small shadow dunes. The eastward dipping short reflectors and buried topography in the western package suggests deposition in low (1-2 m) migrating dune forms.