IDENTIFYING SUBTLE LITHOLOGIC DISCONTINUITIES IN SOILS AND USING THAT INFORMATION TO INTERPRET LANDSCAPE EVOLUTION: EXAMPLES FROM MICHIGAN

Soils provide important information about surficial sediments. Many soils have formed in more than one parent material, each separated by a lithologic discontinuity. Identifying these subtle and often-missed discontinuities is a challenge, but the potential rewards are great. Another step in unraveling soil-sedimentary history involves identifying the sedimentary origins of the soil parent materials. Then, after the soil parent materials have been identified and their origins determined, maps of parent materials or surficial deposits can be created in a GIS, using NRCS soil maps as the original data source. This methodology has been standard fare at Michigan State University for several years.

In this presentation I discuss several examples of how we have used this approach to unravel the evolution of some Michigan landscapes. Of particular note is the case of the sandy soil uplands of northern Lower Michigan, where a thin (<50 cm) silty deposit has been identified. This silt is probably an ablation deposit, let down during stagnant deglaciation. The sandy till below provides another example of how soils can be used to identify paleoprocesses. It was long assumed to be an outwash deposit but based on fabric and texture, has been shown to be a basal till. Taken together, these two examples illustrate how a detailed analysis of soils and their parent materials can facilitate the understanding of landscape evolution, and potentially also aid in future soil mapping endeavors.

Another example, this time from NE Lower Michigan, will illustrate the importance of mapping soils in a GIS as a means to identify the areal extent of past depositional systems. Here, we identified a thin lacustrine clay deposit in some soils and mapped the extent of these particular soil series in a GIS. The maps helped to determine that a high-level paleolake, previously unrecognized, had indeed existed on this landscape. Using this spatial information, we next examined soils on previously-submerged drumlin crests, formed in loamy till. These soils contained fewer coarse clasts and more fine sands in their upper sola, suggesting that the upper 50 cm of till (above this subtle lithologic discontinuity) had somehow been wave-worked. Once this wave-worked sediment had been identified and characterized, we were able to identify it in soils elsewhere, and use it as evidence of past periods of submersion.