Supraglacial Diamictons at Continental Ice Margins: Pleistocene Alluvial Fan-Flow Till Deposition in Southeastern Michigan: ABSTRACT

Glacial sediments more than 50 m (165 ft) thick were laid down over southeastern Michigan during retreat of the late Wisconsinan continental ice sheet. As a result, the topography of the region is now dominated by recessional moraines composed of thick sequences of outwash sand and gravel, which are in turn capped by up to 7 m (23 ft) of glacial diamicton. Sedimentary structures exposed in many gravel pits in the Fort Wayne moraine suggest that outwash sequences were deposited as proglacial alluvial fan systems which were partially overridden during short periods of ice readvance. Several features, including abruptly truncated trough cross-bedding in gravels and truncated large clasts of previously ice-cemented sand at outwash-till contacts, require shearing at outwash su faces either prior to or during till emplacement. Such outwash till contacts suggest that some of the diamicton was deposited in subglacial settings as lodgement till. Other exposures, however, exhibit gradational outwash till contacts, fluidized mixtures of thin outwash and till layers, and till draped over large sand clasts. Such features require that much of the diamicton was emplaced without truncation at the outwash surface, and suggest that deposition occurred in supraglacial settings as flow till. The distribution of lodgement till and flow till in this region indicates that lodgement tills predominate on proximal (iceward) portions of moraine slopes and that flow tills predominate on distal slopes. This distribution suggests that during outwash till deposition, the front of the continental ice sheet had readvanced only to the moraine crest, and that a single depositional episode gave rise to the entire outwash till sequence. Similar features are typical of other moraines in the region and suggest that, in general, the crests of gravel-cored Pleistocene moraines coincide with the maximum limits of ice readvance and delineate areas of lodgement till deposition on proximal slopes and areas of flow till deposition on distal slopes. End_of_Article - Last_Page 563------------