DEVELOPMENT OF GREAT LAKES LAKE-LEVEL CURVES FROM THE SEDIMENTOLOGIC STUDY OF STRANDPLAINS OF BEACH RIDGES

Beach ridges are roughly shore-parallel ridges of sand that mark the former shoreline position of a lake. They occur along coasts with a positive net supply of sediment. In the Great Lakes, these features are best preserved in embayments where sediment is captured from the littoral system, but beach ridges are also a component of spits and some promontories. Beach ridges have a core of water-laid deposits overlain by dune sand. They are formed by fluctuations in lake level with an incipient ridge forming in the final stages of a lake-level rise. Consequently, the elevation of foreshore (swash zone) sediments within a beach ridge record the elevation of the water as it approaches a high stand of lake level.

The systematic collection of the foreshore sediments across a strandplain of beach ridges can be used to reconstruct paleo lake levels. Vibracorers are useful in collecting foreshore sediment, producing a relatively undisturbed sample of the entire foreshore sequence. The physical characteristics of individual foreshores vary between standplains and within an individual strandplain. Commonly, they are coarser and more poorly sorted than overlying dune and underlying upper shoreface deposits. Foreshore sediments have normally and some inversely graded horizontal parallel-laminations or subhorizontal to high-angle parallel-laminations that dip toward the lake. Basal foreshore deposits may be coarser and more poorly sorted than the rest of the foreshore. These sediments represent the plunge point that is present at the base of the swash zone. It is the elevation of these sediments that is one of the best indicators of past lake level.

Strandplains of beach ridges may have intra-ridge swales containing wetlands. If the wetland forms and begins accumulating organic sediment soon after the ridge lakeward of it forms, a radiocarbon date of basal wetland sediment will approximate the age of the next lakeward ridge. By collecting a representative number of radiocarbon dates, a graph of relative lake levels through time can be produced.