The history and architecture of lacustrine depositional systems in the northern Lake Michigan basin

The post-glacial history of the Great Lakes has involved changes in lake le vels that are equivalent in vertical extent to the Pleistocene changes in g lobal sea level and changes in sediment accumulation by at least two orders of magnitude. In the sediments of the northern Lake Michigan basin, these radical changes in base level and sediment supply are preserved in detailed records of changing depositional environment and the impact of these chang es on depositional architecture. The seismic sequences of the sediment fill previously described in Lake Huron have been carried into northern Lake Mi chigan and used to map the history and architecture of basinal deposition. As the Laurentide Ice Sheet retreated northward in the early Holocene, it o pened progressively deeper channels to the east that allowed the larger lak es to drain through the North Channel, Huron, and Georgian Bay basins. At t he end of the Main Algonquin highstand, about 10,200 (radiocarbon) yrs ago, the eastern drainage passage deepened in a series of steps that defined fo ur seismic sequences and lowered lake levels by over 100 m. Near the same t ime a new source of sediment and meltwaters poured across the Upper Peninsu la of Michigan and into the northern Lake Michigan basin from the Superior basin ice lobe. A marked increase in deposition is seen first in the northe rn part of the study area, and slightly later in the Whitefish Fan area at the southern end of the study area. Accumulation rates in the area graduall y decreased even as lake levels continued to fall. Drainage directly from t he Superior basin ended before the beginning of the main Mattawa phase abou t 9,200 (radiocarbon) yrs ago. Although individual lowstand systems tracts are at the most a few hundred y rs in duration, their geometries and seismic character are comparable to ma rine systems tracts associated with sea level falls of similar magnitudes. In some of the thicker lowstand deposits a second order cyclicity in sedime ntation can be detected in the high resolution seismic records.