EPISODIC DEPOSITION AND INCISION OF THE THIRD DAM ALLUVIAL-FAN COMPLEX IN LOGAN CANYON, BEAR RIVER RANGE, NORTH-CENTRAL UTAH

Three levels of dissected surfaces form a north-sloping alluvial-fan complex above Third Dam at Mill and Spring Hollows in Logan Canyon. Details at sampling pits combined with OSL age dates, plus previous research, establish a sequence of deposition and incision related to upstream glacial-interglacial episodes, related changes in stream regimen, incursion of Lake Bonneville upvalley to the fans, and later entrenchment.

The higher and intermediate fans were deposited ~30-40ka (Marine Stage 3), prior to Lake Bonneville's rise into Logan Canyon during the Bonneville highstand ~18 ka. In addition to an erosional interval between formation of the two higher fan surfaces, boulders later deposited ~14 ka across the intermediate surface indicate that deep entrenchment to form the inset lower fan surface was during rapid desiccation after the Provo level of Lake Bonneville.

The Logan River probably was perennial through the time involved, likely in a channel across the north toes of the higher fans, perhaps above its present level. The persistence of a knickpoint at the present west margin of the fans may result from resistant dolomite in the nearly horizontal Devonian Hyrum Formation here, near the axis of the Logan Peak Syncline.

Absence of older Quaternary fan deposits here suggests effective removal by Logan River during high glacial discharges, perhaps aided by ice-burst floods. However, these fans resisted removal during the most recent glacial episode (Marine Stage 2), including impingement by Lake Bonneville. These data improve understanding of local geomorphic responses to climate change during the late Pleistocene and early Holocene and develop a local chronology of fan-forming episodes.