A SURVEY OF THE GEOLOGIC HISTORIES OF ALLEGAN COUNTY, MICHIGAN PEAT BOGS: A RECORD OF HYDROSERAL SUCCESSION

We studed five Allegan County bogs to investigate regional patterns in their stratigraphies and geologic histories. Each bog, save one, has an emergent center surrounded by a moat. The remaining bog has a partially submerged surface. We collected 3 to 4 sediment cores along a traverse from edge to center at each bog. We used smear slides to determine the core’s composition. Glacial lake plain sand with no siliceous microfossils underlies all five bogs indicating that each bog began as a small, shallow lake formed in depressions left by the retreating glaciers. In Fern Bog, the basal sand is interbedded with gyttja layers. In the two northernmost bogs (Minor Lake and Sundew Bogs), a basal marl layer, indicating a rich fen, developed next. A basal clay layer atop the sand is present in the three southern bogs (Fern, Mosquito, and Allegan Bogs). An algal gyttja with diatoms and sponge spicules occurs next in each bog, indicating a deepening of the water body. Peat with sponge spicules, suggesting a shallowing of the water, formed next. However, a second layer of marl deposited prior to this peat formation in Sundew Bog. Peat without sponge spicules overlies the peat with sponge spicules in each bog, except Fern Bog. This indicates the development of a raised bog mound above the water surface. The emergent peat layer is thickest in the centers of the bogs and becomes thinner towards the edges, disappearing entirely in some cases. Currently, the raised bogs are vegetated with a Sphagnum- and/or heath-rich plant community. In Sundew Bog the peat occurs in a floating mat, while it is anchored to the bottom in the other emergent bogs. Fern Bog has a partially submerged surface dominantly vegetated with Cyperaceae and fern species. Collectively, there is a regional pattern of terrestrialization and acidification in the development of these bogs. However, there are regional differences in their initial stages in that the northern bogs formed marl and the southern bogs did not do so. In addition, Sundew Bog currently has a floating bog mat and Fern Bog has not yet developed a raised mound. It is possible that the bogs represent different stages in hydroseral succession from a submerged bog to an anchored, emergent bog mound with a possible intermediate floating mat stage.