FROM SOURCE TO SINK, LATE DEVONIAN ALGAL CYSTS (TASMANITES) DELIVERED TO THE GULF OF MEXICO DURING THE LAST GLACIATION: PART II, THE SOURCE

Radiocarbon ages and careful observations have established occurrences of reworked upper Devonian tasmanitids in distant isolated sedimentary basins: Pleistocene glacigenic sediment of the western Great Lakes region, and continental-slope deposits in the Gulf of Mexico (GOM). The source of the microfossils include the New Albany Shale (Indiana, Illinois), Atrim Formation (Michigan), and Ohio Shale (Ohio). Our studies have been limited chiefly to glacial sediment in Illinois, but tasmanitids also likely occur in Ohio, and perhaps more abundantly, in Indiana. We have noted the highest concentrations of tasmanitids in deposits of ice-walled lakes.

The first and last occurrence ages of tasmanitids from the GOM are about 28.5 to 17.8 cal. kyr B.P. The first occurrence age is consistent with advance ages of the Lake Michigan and Huron-Erie lobes of the south-central Laurentide Ice Sheet during the last glaciation. Tasmanitid abundance in Lake Michigan lobe sediment is sparse in sediment dating from ca. 28.5 to 22.5 cal. kyr B.P., suggesting similar aged material in the GOM was sourced primarily from the Huron-Erie lobe.

The last occurrence age of GOM tasmanitids is 17.8 cal. kyr B.P., but these fossils are common in glacigenic diamicton and Lake Chicago deposits dating as young as about 15.2 cal. kyr B.P. Accounting for the age discrepancy is sequestering of tasmanitids in the sediment of lakes Chicago and Whittlesley that were proglacial to the Lake Michigan and Erie lobes, respectively.

Our present dataset indicates that reworked tasmanitids are found in neither pre-Michigan Subepisode (Wisconsin Episode) GOM deposits nor in their glacial counterparts in the western Great Lakes region. If this relationship holds, or is largely true, we posit that most erosion of the Great Lakes, save Lake Superior, occurred during the last glaciation.