Updated Stratigraphic Framework for Late Pleistocene Glacial Deposits of the Saginaw Lobe of the Laurentide Ice Sheet, Indiana Paper

Abstract

Northern Indiana is covered with up to several hundred feet of Quaternary-age sediment which accumulated during several glacial stages of the Pleistocene ice age. The stratigraphic framework of Wisconsin Episode units that compose the upper part of this sediment package has been described by different authors using non-standardized terms or leaving orphaned or unnamed units. Therefore, developing a regional glacial stratigraphy that can aid in water and aggregate resource identification and management has become a difficult task. The interlobate region of north-central Indiana is particularly challenging, located between well-defined deposits of the Lake Michigan Lobe on the west and the Huron-Erie Lobe on the east. While the region has long been regarded as primarily a product of the Saginaw Lobe, studies conducted over the last three decades have demonstrated a more complex, multi-lobe origin for the interlobate area, significantly reducing the known surface extent of Saginaw Lobe deposits.

In this publication, we assessed 16 lithostratigraphic units that have been formally and informally proposed in publications and maps, along with a host of informal morphostratigraphic units introduced in recent generations of geologic maps. Following guidelines of the North American Stratigraphic Code, we establish formal lithostratigraphic units for diamicton-dominated sediments and associated glaciofluvial units deposited by the Saginaw Lobe of the Laurentide Ice Sheet in northern Indiana during the Wisconsin Episode. We recommend one major new unit to be elevated to the rank of formation: the Elkhart Formation. This new unit includes three well-defined members: the Baugo Creek Member, Newbury Member, and Lagrange Member. Criteria used to differentiate these units, both among themselves and from deposits of adjacent lobes, in order of relevance, are: gravel lithology, diamicton matrix standardized color, grain size, and calcite/dolomite ratios. Heavy mineral assemblages of fine sand and diamicton matrix chemical composition were not sufficiently distinctive to differentiate stratigraphic units.

Former lithostratigraphic units used informally in earlier reports are either aggregated and named in this report following guidelines or suggested to be abandoned. Morphostratigraphic units such as moraine ridges and outwash fans are retained only for context in the description of lithostratigraphic units. Units of formal status that are no longer considered necessary in the area as lithostratigraphic units are proposed to be abandoned. The geographic extent and stratigraphic position of the Saginaw Lobe deposits—between the Late Wisconsin Lake Michigan Lobe glacial deposits to the west and north, and the Late Wisconsin Huron-Erie Lobe glacial deposits to the east, and bound by pre-Wisconsinan sediments at the base—makes the exercise presented here relevant for a future update in defining units of the adjoining lobes.