THE STEAMBOAT ROCK SILT: INDIRECT EVIDENCE FOR A MORAINE-DAMMED LAKE IN UPPER GRAND COULEE DURING THE LAST GLACIAL MAXIMUM

Exposures of sediment in the vicinity of Steamboat Rock, upper Grand Coulee, Washington, have been mapped as Quaternary sand and gravel associated with the late Wisconsinan Missoula Floods. Recent mapping shows that surface deposits are underlain by a 15 m thick sequence of silt and sand we designate as the Steamboat Rock Silt. The age is constrained to between 17.24 and 13.5 14Cyr B.P., based on the timing of the Fraser advance and retreat, position of the terminus at maximum advance, and the Missoula Floods. Absolute age criteria in the deposit have not yet been identified. The unit is preserved in the vicinity of Steamboat Rock, with exposures on a peninsula to the south, on an isthmus at the northeast edge, and on a side channel bar about 8 km south of Steamboat Rock.

Bedding in the Steamboat Rock Silt varies from a few cm to nearly 1 m in thickness, and is remarkably continuous along strike. Lower bedding contacts are erosional, and most beds display normal grading. Cross-stratification is present in coarser beds, with amplitudes ranging from about 5 to about 20 cm. Some beds exhibit soft-sediment deformation. Dropstones are common; most are relatively small (3-10 cm), but near the base of the exposed section, dropstones up to 1.5 m in diameter are present. Dropstone lithologies includes basaltic, granitic, and metamorphic rocks. Trace fossils are evident on some bedding surfaces; trace makers are unknown.

The Steamboat Rock Silt is interpreted to represent cyclic, perhaps seasonal deposition into a temporary impoundment behind the terminal moraine deposited during the last advance of the Okanogan Lobe of the Cordilleran ice sheet. The most likely location for the terminal moraine was at the present location of Dry Falls Dam; here, a bedrock sill is exposed. The moraine was breached and the lake drained prior to or during the late Wisconsinan Missoula Floods. The channel geometry of upper Grand Coulee and Steamboat Rock protected parts of the Steamboat Rock Silt from erosion during the later stages of the Missoula Floods.