Stagnation of the Harvard sublobe (Lake Michigan lobe) in
northeastern Illinois, USA, from 24,000 to 17,600 BP and subsequent tundra-like
ice-marginal paleoenvironments from 17,600 to 15,700 BP.
Curry, Brandon B. and Catherine H. Yansa
Glacial deposits of the last glaciation associated with the
Harvard sublobe (Lake Michigan lobe) in northeastern Illinois, USA, occur
between sediment with dateable organics. The lower organics include fragments of
Picea sp. as young as 24 000 270 BP. The supraglacial organics occur sparsely
in laminated silt and fine sand in landforms that are positioned relatively high
on the landscape, such as deposits from ice-walled lakes. These terrestrial
organics yield ages that are 2500 to 1300 14C years older than organics at the
base of sediment successions in nearby kettle basins. Basal 14C ages from four
upland sites range from 17 610 270 to 16 120 80 BP. Our revised
time-distance diagram of the Harvard sublobe now reflects a period of stagnation
from 24 000 to about 17 600 BP. The supraglacial lacustrine silt yielded plant
macrofossil assemblages of primarily tundra plants, including Salix herbacea and
Dryas integrifolia. These plants likely grew in supraglacial and ice-marginal
environments. The ostracode fauna include Cytherissa lacustris and Limnocythere
friabilis. Geomorphic relations and ostracode ecology indicate that more than 17
m of ice buttressed some of the supraglacial lakes.