ALLUVIAL TERRACING IN A FORELAND-BASIN “SHEET” CONGLOMERATE AS AN INDICATOR FOR SIGNIFICANT ISOSTATIC UPLIFT IN AN OROGENIC BELT

The Early Cretaceous Cadomin Conglomerate of the Deep Basin area, Alberta, Canada, is a relatively thin (5- to 20-m thick) sheet-like, coarse-grained braided-fluvial deposit that overlies a significant angular unconformity, extending at least 250 km from the ancestral Canadian Cordillera into the Western Canada Foreland Basin. This “sheet”, however, accumulated over a period of 12 to 18 Ma, making its rate of deposition excessively slow, approximately 0.1 cm/ka. When examined in detail, the Cadomin is laterally segmented into alluvial terraces that mantle the walls of broad, north-draining valleys, where the oldest terraced-conglomerate interfluve (as indicated by extensive pedogenesis) sits 20-m above the youngest valley fill. Thus, instead of continuous deposition over millions of years, the Cadomin alluvial system cycled between periods of aggradation and incision with an apparent net loss of elevation in the fluvial profile, more consistent with estimated rates of deposition.

Furthermore, accumulation of the Cadomin was synchronous with an observed magmatic hiatus in the Canadian Cordillera. Therefore, it is hypothesized that an orogenic hiatus, possibly through rollback of the subducting slab, allowed for erosional redistribution of mass from the inactive cordillera. Isostatic rebound caused incision of relatively shallow Cadomin-aged valleys in the foreland basin, alluvial terraces in those valleys being deposited when sediment flux in the streams was periodically high. Importantly, uplift was great enough to create a slope upon which streams could transport coarse-grained detritus several hundred kilometers. Overall, this terraced-sheet model, where present in alluvium of a foreland basin, may be an excellent indicator of long-term orogenic quiescence and significant isostatic uplift in a collisional belt.