EARLY HOLOCENE EOLIAN ACTIVITY IN EASTERN UPPER MICHIGAN DROVE AGGRADATION DURING TRANSGRESSIONS OF GLACIAL LAKE MINONG

Perpetual early Holocene drainage of glacial Lake Minong eastward through the Saint Mary’s River to Lake Huron has long been assumed. Nonetheless, new evidence from the foundation plans of six highway bridges spread across eastern Upper Michigan suggests a southerly flow path through the Tahquamenon Valley, south across the present watershed divide at Danaher, thence to the Manistique Valley to Lake Chippewa. Minong’s outlet lay at ~220 m along the present Tahquamenon River near Joy’s Island. Interbasin flow was probably intermittent during the dry early Holocene. The interbasin channel was active as the Laurentide Ice Sheet retreated from the Superior Basin and again during episodic receipt of meltwater from Lake Agassiz. These contentions must be reconciled with the recent finding that Minong transgressed ~ 12 meters over four decades just prior to the breach of the Nadoway Drift Barrier (NDB) at the Lake’s southeastern corner ~ 9.5-9.0 ka. A primary question comes immediately to mind: channel aggradation of 12 meters would require a sediment supply far in excess of that available from immediate channel banks. How could this sediment have been delivered to the channel?

Reflections from four km of ground penetrating radar (GPR) at the present inter-basin divide at Danaher suggest that large amounts of sediment were ultimately delivered to the connecting channel by eolian processes. Radar reflections in medium sand > 10m beneath Danaher suggest truncated and buried dunes that formed prior to ~10.6 ka. These dunes likely had an origin similar to the thousands of dunes that spread across eastern Upper Michigan as the Great Lakes became closed basins ~8.9 yr ka. As capacity of the bedrock notch at Minong’s outlet was exceeded, overland flow spread south and west across the Tahquamenon Valley, entraining sand from thousands of dunes and driving aggradation of the broad flow path. Since prior episodes of eolian activity probably occurred during the early Holocene, dunes could have filled dry connecting channels in the lee of prevailing NW winds on several occasions. Such infilling and attendant dune building served to build up Danaher, the present watershed divide. Danaher was abandoned and became the head of the modern Tahquamenon River as the NDB, 50 km to the east, was breached near 9.1 ka.