GRAIN SIZE, SORTING, AND MINERALOGICAL ANALYSIS OF BEACH SEDIMENTS: A CASE STUDY FROM THE SOUTHERN SHORE OF LAKE SUPERIOR, ONTONAGON, MICHIGAN

Analysis of grain-size distribution of modern lacustrine sediment provides insights into sedimentary sources and transport histories, offering analogs for ancient deposits. This study analyzes 38 sand samples from a ~11 m (~30 cm separation) transect spanning nearshore to backshore zones on a private beach near Ontonagon, Michigan, on Lake Superior’s southern shore. Grain size, sorting, roundness, and mineralogy were assessed, with grain-size distribution calculated using Gradistat V8. Five sediment groups were identified: (1) Coarse gravel to medium sand in moderately well to very poorly sorted nearshore deposits, indicating high-energy wave activity; (2) Poorly sorted very coarse to medium sand on the shoreline, reflecting moderate energy; (3) Poorly sorted medium gravel to fine sand on the beach face, showing variability in wave reworking; (4) Well-sorted medium to fine sand in berm/backshore zones, associated with active reworking; (5) Moderately to well-sorted medium to coarse sand in vegetated stable backshore areas, affected during very high energy storms. Grain shape and mineralogical analysis of sediment reveals highly-rounded and well-sorted grains, consistent with Precambrian rock sources (super mature sediments) including sandstones, conglomerates, and basalts. These findings illustrate the interplay of energy, sediment transport, and depositional stability, aiding in interpreting ancient lacustrine environments.