LAKE ERIE'S DETROIT DELTA OR RIVER RAISIN FAN/DELTA OR BOTH?

Data from vibracores and jetted holes from Lake Erie's Western Basin indicate a distinction between the sand bodies identified lakeward of the mouth of the River Raisin and those attributed to the flow associated with the Nipissing Flood. The Nipissing Flood, which peaked about 3900 years BP, emanated from the Detroit River when the flow from the upper lakes was redirected through Lake Erie. This flood may have created a peak on the water level rebound curve for Lake Erie. The three sand deposits are not continuous; two of them are on the glacial plane and the other in a trough cut into the glacial deposits. The Nipissing Flood event may have eroded the trough. The sediment package on the glacial plane at River Raisin consists of a lower sand-based fining upward sequence, commonly ending in an organic unit, overlain by another sand unit that may be topped with modern soft mud. The Detroit River package has a lower sand unit followed by a fining upward sequence capped by the modern soft mud. The upper (more recent) sand unit is absent except in very isolated pods.

Prior to the Nipissing Flood a River Raisin delta sourced by sand reserves to the west may have dominated the area. Assuming this, the flood would have 1) cut a trough in the glacial surface northeast of the River Raisin, 2) reworked the previous deposits into both a sand sheet on the glacial plane in the River Raisin area, and the lower reworked sand unit in the trough. Suspended sediments then settled to create the fining upward sequence found in both areas. As lake level again fell below its present level, the River Raisin delta was reconstructed, as evidenced by the upper sand unit. Isolated areas of organics were deposited on the shallow platform and finer grained material continued to fill the trough. If this upper sand were associated with the Nipissing Flood, one would expect the sediments to have accumulated in the trough and not to the south of the trough. These sands are not likely associated with the "Detroit delta". The Nipissing Flood event is probably the source for the lower sand unit in the trough but this deposit may be better described as a channel fill. Subsequent flooding due to the continuing isostatic rebound at Buffalo has since drowned the River Raisin delta and river mouth, cutting off the sand supply, and depositing the most current fine mud sediments.