LATE PLEISTOCENE DELTAS IN THE LOWER PENINSULA OF MICHIGAN, USA

In Monograph 53, Leverett and Taylor (L & T) identified over 20 late Pleistocene deltas in the Lower Peninsula of Michigan. These deltas, now relict, can be used to interpret paleoenvironmental conditions, e.g., wave energies and prevailing wind and longshore drift directions. Most of the deltas identified by L & T are located at the junction of a major present-day river and a relict shoreline. The deltas show a distinct bulge along the former shoreline and are either elongate- or cuspate-shaped in plan view. In this study I focused on these deltas, but also attempt to identify and describe deltas in Lower Michigan that may have been overlooked by L & T. All were presumably emplaced during the late Wisconsin. The study utilized a variety of data within a GIS environment: a statewide USGS 7.5’ digital raster graph, a seamless 10 m digital elevation model (DEM), county-level NRCS SSURGO soils data, and depth profiles constructed from water well and oil/gas logs. DEMs were particularly useful where they could be ‘flooded’ to various elevations that L &T and more recent works identified as relict shorelines. Maps of soil wetness and soil textural characteristics, as derived from soils data, were also employed in detecting deltas and outlining their boundaries. More than 30 deltas, not previously recognized by L & T, were identified during this analysis. Similar to those identified by L & T, many of the new deltas protrude from an escarpment or shoreline berm, and are fan-shaped. Furthermore, most of the deltas are composed of sandy, well-drained sediments and have graded longitudinal profiles. Many are perched above a relatively low relief, poorly drained landscape. However, unlike several deltas recognized by L & T, I found that many such deltas occur (1) adjacent to a formerly unknown shoreline or do not connect to a modern river, (2) have complex forms, and/or (3) are broad features, deposited by more than one river, leading to consolidation of deltaic deposits. The methodologies used to identify and delineate delta boundaries are a focus of this study; they may be useful for mapping relict deltas in other regions of the Great Lakes. Detailed mapping of late Wisconsin deltas will ultimately aid in a better understanding of the paleocoastal and terrestrial conditions during the late Pleistocene.