Mid-Quaternary in North America

Abstract

Middle Pleistocene time is assigned to Marine Isotope Stages (MIS) 19–6 and falls entirely within the Brunhes Normal Chron. Oxygen isotope excursions in marine sediment are interpreted as representing glacial–interglacial cycles because light isotopes are preferentially evaporated from the oceans and during glaciations may be stored long term in ice sheets. The magnitude of the marine isotope excursion is therefore interpreted as an indirect record of the volume of terrestrial ice sheets. Nevertheless, the presence, location, extent, and character of Middle Pleistocene ice sheets require independent documentation and verification. In the midcontinent and the northwestern areas of North America, the sediment record of glaciation spans the Pleistocene and extends to the Pliocene in places, based on strata dated by a number of methods. Glacial deposits are being increasingly assigned to Middle Pleistocene time. Where exposed at the surface, they have been historically recognized as Illinoian in age and are separated from Late Pleistocene deposits, also normally magnetized, on the basis of a variety of relative and quantitative dating methods. In the subsurface, where soils and weathering zones may not be preserved, datable strata include ashes, lavas, calcrete soils, loess, and marine layers. The most compelling evidence that restricts a glacial deposit to the Middle Pleistocene is normal remanent magnetism, which places it after the Brunhes–Matuyama reversal, combined with an independent date (e.g., the radiometric age of volcanic ash or the cosmogenic exposure or burial dating of a surface). In some cases, combinations of methods have been used to constrain the horizon or deposit to a particular marine isotope stage. Middle Pleistocene glaciations in North America, as they have been thus far identified, are not everywhere the most extensive glaciations, as the large δ 18 O isotopic excursions might suggest. In eastern North America, the interpreted limits for glacial advance are very similar to late Wisconsinan limits and much of pre-late Wisconsinan record of glaciation is missing in the subsurface. In the midcontinent, Early Pleistocene, reversely magnetized glacial deposits are more extensive in many places. In the northwest, ice centers interpreted for the Middle Pleistocene were similar to Late Pleistocene centers but were less extensive.