ABSTRACT U.S. Geological Survey (USGS) Monograph 53 by Frank Leverett and Frank Taylor identified 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 identified 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, first identified 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 profiles interpreted from descriptive water well and oil/gas logs. DEMs were particularly useful, because they can be “flooded” 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 profiles. 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.

A northeast-trending graben was hypothesized to extend southwest of Saginaw Bay to the Mid-Michigan Gravity High, based on interpretation of Landsat 1 imagery, stream drainage maps, and sparse well-log data. The edges of the graben were thought t o extend along and southwest of the Pinconning oil field on the northwest side, and the Quanicassee River on the southeast side. Subsequent analysis of digital terrain, magnetic, gravity, seismic, and well-log data showed that no unequivocal evidence for a discrete, simple graben within the originally defined limits could be found. However, the new data indicated that the proposed edges of the “graben” correspond to structural lineaments (monoclines and anticlines) expressed within the Paleozoic section and on the bedrock surface. These structural features correlate with basement contacts and/or fault zones inferred from interpretation of magnetic and gravity images. These possibly basement-controlled structural lineaments influenced depositional patterns intermittently during the Paleozoic, as evidenced by the presence of northeast-trending highs within the limits of the “graben” on isopach maps of Pennsylvanian, Mississippian, and Devonian stratigraphic units. Rapid thinning and facies changes in Middle and Lower Ordovician units across the southeastern edge of the “graben,” coupled with its correlation with northeast-trending positive gravity and magnetic anomalies, suggest that this is a significant structural feature, possibly controlled or influenced by the Grenville Front.