SURFICIAL GEOLOGY OF PICTURED ROCKS NATIONAL LAKESHORE, MICHIGAN – INDIAN TOWN, MELSTRAND, WOOD ISLAND SE, GRAND PORTAL POINT, TRAPPERS LAKE, AU SABLE POINT SW, AU SABLE POINT SE, AU SABLE POINT, GRAND SABLE LAKE, AND GRAND MARAIS 7.5 MINUTE QUADRANGLES

Pictured Rocks National Lakeshore consists of approximately 69 km of scenic coastline along the southern shore of Lake Superior in the Upper Peninsula of Michigan. The park is about 300 km² and is almost fully incorporated within the ten 7.5 minute quadrangles presented in this surficial geology map (covering about 1,330 km²). Before this project, Pictured Rocks and other significant glacial landforms in the area lacked detailed (1:24,000) surficial geology maps characterizing sediment-landform assemblages in the region. This new series of ten detailed quadrangle maps are essential for a more comprehensive understanding of glacial history and geomorphic evolution of this popular landscape.

This map was produced by integrating extensive field study of surficial materials with several additional datasets, such as digital elevation models (DEMs), aerial imagery, and non-invasive geophysical surveys. Sediment observation and classification in the field was primarily completed in shallow hand-augured borings around 1.5 m deep, except in areas of greater exposure (e.g. sand/gravel pits). 372 sediment samples were classified throughout the mapped area. Additionally, 356 passive seismic surveys were conducted within and just outside the mapped area to provide subsurface information concerning the thickness and extent of surficial geologic units.

The western quadrangles are characterized by thin (<5 m) accumulations of undifferentiated glacial and postglacial sediment above shallow bedrock. Subtle diamicton ridges in these quadrangles are interpreted to trace former ice margin positions that connect to previously established positions at the crest of the Grand Marais and Munising outwash fan complexes in the eastern quadrangles. These fan complexes are among the most prominent glacial landforms in the region, along with a series of meltwater kame terraces adjacent to the north. The eastern quadrangles show thick accumulations of quartz-rich sand that are interpreted as different glacial outwash units. A newly available high-resolution (60 cm) LiDAR DEM and hillshade was especially valuable to differentiate between outwash units, Holocene sand accumulations, and interpreting subtle features, like those representing former ice margin positions in the west, that would otherwise have been unrecognized.