Influence of bedrock geology on sedimentation in Pre-Late Wisconsinan alluvial fans in the Canadian Rocky Mountains

Abstract

Abstract Late Pleistocene alluvial fan deposits in Jasper National Park, Canadian Rocky Mountains, unconformably overlie Precambrian and Paleozoic clastic and carbonate rocks and stratigraphically underlie Late Wisconsinan glacial deposits. The paleofan sequences were deposited in an inferred, cool temperate, alpine to subalpine paleo-environment. They mainly contain clast-supported diamicton interpreted as noncohesive debris flow deposits. Fluvial gravels are less common and cohesive debris flow deposits (matrix-supported diamictons) are rare. Although noncohesive debris flows dominate the fan sequences, the bedrock geology of the fan source areas exerts a strong control on sedimentation patterns by influencing susceptibility to debris slide initiation, availability of large clasts and matrix fines and drainage basin shape (and hence discharge flashiness). Paleofans in the Main Ranges with mainly gently dipping, clastic source rocks are dominated by thick, bouldery debris flow sediments whereas fans with steeply thrusted, interbedded carbonate and shale source rocks in the Front Ranges show thin, pebbly to cobbly debris flow beds with interbedded flood flow and normal stream flow gravels. These observations indicate that debris flow events were less frequent and of higher energy in the Main Ranges than in the Front Ranges. Front Range basins are long and narrow with complexly interbedded and deformed rocks, resulting in the relatively small grain size of transported materials, a greater susceptibility to small, frequent, debris slides, and locally, more uniform discharges during flood events. The prevalence of high-magnitude, low-frequency, debris flow events in the Main Range paleofans, and lower magnitude, higher frequency events in the Front Range fans may be of relevance to paleoenvironmental interpretations of ancient alluvial fan deposits, as well as to modern hazard studies and mitigation efforts mountainous areas.