Abstract The Ruff formation, the informal A-1 carbonate unit of the lower Salina Group, underlies the A-1 evaporite and overlies the A-2 evaporite in the subsurface of the Michigan basin. It basically is brown to dark grayish brown, fetid, unfossiliferous carbonate mudstone that ranges from limestone to dolomitic limestone and dolomite. The Ruff formation can be divided into five lithofacies, based upon associations of subordinate lithologie constituents: microlaminated mudstone, leached mudstone, pelletal wackestone-packstone, thinly laminated mudstone, and nodular anhydrite. Deposition primarily was in shallow subtidal to infratidal, low-energy, hypersaline reducing environments, and secondarily on ephemeral tidal to supratidal flats. In southeastern Michigan, the basal portion of the Ruff formation is intertidal-flat algal-laminated mudstone that was transgressive over the A-1 evaporite, a contiguous supratidal deposit. The remaining Ruff formation primarily is subtidal to infratidal microlaminated mudstone that grades into infratidal to intertidal pelletal wackestone and packstone peripheral to the Niagaran reefs. Deposition of these lithofacies was interrupted circumferential to the reefs by development of ephemeral tidal flats in which nodular anhydrite was deposited. The uppermost Ruff formation is the subtidal facies-equivalent of the supratidal deposit of the lowermost A-2 anhydrite. Within the Ruff formation, differences in lithofacies distribution among various areas of the fore-slope-shelf region, and other stratigraphie variations {Mantek, 1973) are responses to different tectonic regimes within the basin. These variations have resulted in conflicting interpretations of the “reef-evaporite” relation in the Michigan basin, depending on the locations of studies.

ABSTRACT This field guide discusses the dune types and processes, ecology, and geomorphic history of the largest freshwater dune systems on the southeastern shore of Lake Michigan. From north to south, stops include P.J. Hoffmaster State Park, Gilligan Lake/Green Mountain Beach Dune, Saugatuck Harbor Natural Area, and Grand Mere and Warren Dunes State Parks, Michigan. All of the sites are low, perched transgressive dune complexes. Moving from the lake inland, the typical dune complex in this area consists of incipient foredunes, an established foredune ridge, a parabolic dune complex, and a back-dune ridge complex. All stages of ecological succession are typically present in the larger dune complexes. Surface changes in Lake Michigan dunes are driven by spatial gradients in sand flux, which, in turn, are determined by a complex interaction among wind dynamics, vegetation patterns, and preexisting topography. Surface change patterns are modified by seasonal effects, with the majority of sand transport being associated with strong storms in the autumn, winter, and early spring. Sand can be temporarily stored in niveolian deposits during the winter, leading to oversteepened slopes, which collapse during the spring thaw. Current dune complexes largely formed during and after the rise in lake levels to the Nipissing high lake level, ca. 4.5 ka. Broad fields of relatively low dunes developed during the lake-level drop following the Nipissing high. Beginning with the rise to the Algoma high lake level, ca. 3.2 ka, the lakeward edges of these fields were episodically reworked, forming large parabolic dune complexes. A period of widespread dune stability formed the Holland Paleosol, a spodic inceptisol. Dune growth and migration resumed prior to European settlement of the area and continues today. Foredune complexes grow wider and higher during periods of low lake levels, but narrow during periods of high lake level due to scarping at their lakeward edges.

This study investigates the impacts of extratropical cyclones on Lake Michigan dune complexes by integrating field measurements and meteorological data from sites along the southeastern shore. Surface changes and wind velocities were monitored at Hoffmaster State Park, Saugatuck Harbor Natural Area, and Mount Baldy at Indiana Dunes National Lakeshore from October to April in 2010–2011 and 2011–2012. Over 70% of the events with wind speeds at least two standard deviations above the mean were associated with extratropical cyclones. The wind directions depended on the cyclone path, with westerly or southerly components most common. Local conditions moderated the effects of storm winds on surface change. The greatest surface changes measured in a trough blowout at Saugatuck Harbor Natural Area were associated with regional winds with a component blowing up the lee slope that produced bifurcated windflow within the trough. While the orientation of a given dune strongly influences the amount of surface change, it does not always follow a simple pattern deduced from dune geometry. Surface changes at Hoffmaster State Park and Saugatuck Harbor Natural Area during a normal winter (2010–2011) and an unusually warm winter (2011–2012) suggest that colder weather conditions inhibited net transfer of sand from the beach but had less impact away from the shore. Moisture also inhibited sand transport, but strong storm winds moved wet sand, sometimes over long distances at Indiana Dunes National Lakeshore. Overall our results show that winds associated with extratropical cyclones play a vital role in the development of Lake Michigan dune complexes.

ABSTRACT This field guide explores the geomorphology, ecology, contemporary processes, sedimentary structures, and geomorphic history of the large freshwater dune systems on the southeastern shore of Lake Michigan. Recent research studies on varying aspects of the dunes are highlighted at each stop. From north to south, these stops include P.J. Hoffmaster State Park near Muskegon, Michigan; Gilligan Lake and Green Mountain Beach southwest of Holland, Michigan; Saugatuck Dunes State Park and Saugatuck Harbor Natural Area, both near Saugatuck, Michigan; Warren Dunes State Park and Grand Mere State Park between the Indiana–Michigan border and Benton Harbor, Michigan; and Mount Baldy on the eastern edge of the Indiana Dunes National Lakeshore, Indiana. All of the complexes described are low perched transgressive dune complexes that are migrating inland over former lake plains or baymouth bars. Moving from the lake inland, the typical dune complex in this area consists of incipient foredunes, an established foredune ridge, a parabolic dune complex, and a back-dune ridge complex. All stages of ecological succession—beginning with a pioneer community dominated by beach grasses and ending with a mesic forest dominated by oak, maple, and beech—are typically present in the larger dune complexes. Like coastal dunes everywhere, surface changes in Lake Michigan dunes are driven by spatial gradients in sand flux, which, in turn, are determined by a complex interaction among wind, vegetation patterns, and preexisting topography. The patterns of surface change are modified by seasonal effects, with the majority of sand transport being associated with strong storms in the autumn, winter, and early spring. Sand can be temporarily stored in niveolian deposits during the winter, leading to oversteepened slopes, which collapse during the spring thaw. A variety of sedimentary bed forms and structures can be viewed in dunes along the southeastern shore of Lake Michigan, including wind ripples, lag deposits, raindrop impressions, adhesion ripples, adhesion warts, eolian turrets, sand pedestals, surface patches of fine-grained dark sand, pinstripes, paleosols, cross-bedding, climbing ripple lamination, niveolian deposits, and avalanche lobes. Most of these features are best seen immediately after strong storms in the autumn and winter. Remnants of older dune surfaces are exposed in a few places in back-dune ridge complexes; however, the current dune complexes are largely a product of events that occurred during and after the rise in lake levels to the Nipissing peak (ca. 4.5 ka). Broad fields of relatively low dunes developed during the drop in lake levels following the Nipissing peak. Beginning with the rise to the Algoma high lake level (ca. 3.2 ka), the lakeward edges of these fields were episodically reworked, forming the large parabolic dune complexes. A period of widespread dune stability resulted in the development of the Holland Paleosol, a particularly well-developed paleosol with Spodosol characteristics. Widespread dune growth and migration resumed prior to European settlement of the area and continue today.