A comparison of fluvial megafans in the Cordilleran (Upper Cretaceous) and modern Himalayan foreland basin systems

The Campanian-Maastrichtian Hams Fork Conglomerate Member of the Evanston Formation in northeastern Utah and southwestern Wyoming consists of a widespread (>10 000 km2) boulder to pebble, quartzitic conglomerate that was deposited by east-southeastward-flowing, gravelly braided rivers on top of the frontal part of the Sevier fold-thrust belt and in the adjacent foredeep of the Cordilleran foreland basin. In northeastern Utah the conglomerate was deposited in a lobate fan-shaped body, up to 122 m thick, that trends southeastward away from its principal source terrane in the southern end of the Willard thrust sheet The Willard sheet contains thick Proterozoic quartzite units that produced highly durable clasts capable of surviving long-distance fluvial transport. Although the main source of sediment for the Hams Fork Conglomerate was the Willard sheet, the active front of the thrust belt lay 40-50 km to the east along the Absaroka thrust system. Displacement along the Absaroka system uplifted and topographically rejuvenated the Willard sheet, and antecedent drainages carried detritus from hinterland source terranes into the proximal foreland basin. Although topographic ridges associated with fault-propagation anticlines along frontal thrusts locally influenced transport directions, they provided relatively little sediment to the Hams Fork Conglomerate. Lithofacies, paleocurrent, and isopach data indicate that the Hams Fork Conglomerate was deposited in fluvial megafans and stream-dominated alluvial fans, similar in scale and processes to megafans and alluvial fans in southern Nepal and northern India that are forming along the proximal side of the Himalayan foreland basin system. The Himalayan fluvial megafans have areas of 103-104 km2, slopes of 0.05°-0.18°, and are deposited by large transverse rivers that are an-tecedent to frontal Himalayan structures and topography. The main fluvial channels on the upper parts of the megafans are anastomosed and braided at bankfull stage but commonly have braided thalwegs at low-flow stage. Downstream, these channels become predominantly braided and meandering and ultimately merge with the axial Ganges trunk river system. Stream-dominated alluvial fans in the Himalayan foreland basin system fringe the topographic front of the fold-thrust belt in the intermegafan areas. These fans have areas of ∼102 km2 and slopes of ∼ 0.5°. The proximal parts of both types of fans are dominated by extremely coarse (boulder-cobble) bedload that is in transit mainly during the monsoon. The prevalence of fluvial megafans in the modern and Miocene Himalayan foreland and in the Upper Cretaceous-lower Tertiary stratigraphic record of the Cordilleran foreland suggests that these types of deposits may be the volumetrically largest gravel accumulations in nonmarine foreland basin systems.