Late Pleistocene Deltas in the Lower Peninsula of Michigan, USA. In: Quaternary Glaciation of the Great Lakes Region: Process, Landforms, Sediments, and Chronology

Michael Luehmann and Randall J. Schaetzl

U.S. Geological Survey (USGS) Monograph 53 by Frank Leverett and Frank Taylor identifi ed more than 20 deltas of late Pleistocene age in the Lower Peninsula of Michigan. To that list, we add many additional deltas discovered during the course of our research. These “relict” deltas are important proxies for paleoenvironmental conditions, particularly wave energies, as well as prevailing wind and longshore drift directions. If dated, they can help to constrain the chronologies of ice retreat and proglacial lake stages. In plan view, relict delta morphologies usually protrude from a paleolake shoreline and are often elongate or cuspate shaped. Most of the deltas identifi ed by Leverett and Taylor have this morphology and are located at the junction of a major present-day river and a relict paleolake shoreline. In this chapter, we map and discuss these deltas, fi rst identifi ed by Leverett and Taylor, while also identifying and describing the other, newly found deltas. All of these deltas formed during the marine isotope stage 2 ice retreat, roughly 28–13 ka. To identify and characterize them, we utilized a variety of data within a geographic information system, mainly a statewide USGS 7.5′ digital raster graphic, a 10 m digital elevation model (DEM), county-level Natural Resources Conservation Service soil data, and schematic lithologic depth profi les interpreted from descriptive water well and oil/gas logs. DEMs were particularly useful, because they can be “fl ooded” to various elevations of paleolakes. Maps of soil wetness and textural characteristics were also useful in detecting and delineating deltas. In sum, we mapped 61 deltas; 27 had been known from previous works, whereas 34 are newly reported in this study. Most are composed of sandy, well-drained sediments and have smooth, graded longitudinal profi les. Of these, most are perched above a relatively low-relief, poorly drained lake plain. However, unlike several deltas recognized by Leverett and Taylor, we found that many of the newly reported deltas are (1) adjacent to one or more formerly unknown shorelines, (2) not associated with a modern river, (3) complex, and/or (4) broad, coalesced features, deposited by more than one river, with fan-like morphologies. The methods that we used to identify and delineate these deltas can be applied to other regions. Mapping like the kind reported here will aid in a better understanding of the paleocoastal and terrestrial conditions during the late Pleistocene.