TELESCOPIC FLUVIAL FANS AS SIGNAL-BUFFERING MECHANISMS OF LATE NEOGENE SEDIMENT STORAGE AND RECYCLING ON THE HIGH PLAINS IN COLORADO AND NEBRASKA, USA

Late Neogene fluvial sediments on the central to northern High Plains are frequently interpreted within the nebulous conceptual framework of an alluvial plain, although a few authors have demonstrated the existence of localized paleovalley fills or generally implied that fluvial megafan deposition occurred. In this paper, we present strong evidence for a late Miocene to Pliocene megafan on the southern Cheyenne Table, in the South Platte Valley, and on the High Plains in extreme northeastern Colorado and the southern Nebraska Panhandle. In this study area, we mapped >500 fluvial ridges that can be separated into five groups of diverging paleochannel segments comprising successive, nested, fan-like patterns. Together, these five groups form a feature ~125 km long and 90 km wide, which is centered around the WSW–ENE axis of the present South Platte Valley. The four oldest and topographically highest groups (Groups 1 through 4) of paleochannel segments lie on the uplands flanking the valley; they also exhibit extension angles that successively decrease downdip from 96° to 47°. The longitudinal slopes of these four groups of paleochannel segments successively decrease from 0.33% to 0.19%. The youngest and topographically lowest group (Group 5) includes paleochannel segments confined to a terrace within the South Platte Valley, and there is no appreciable extension angle for these segments. The longitudinal slope of this youngest group is identical to that of the present South Platte River (0.16%). The downgradient migration of inferred megafan apices and the east-northeastward propagation of intersection points between successive longitudinal slopes are evidence that the fan underwent telescoping during the post-late Miocene tilting of the High Plains. Furthermore, the uniform entrenchment of the megafan apparent in Groups 4 and 5, as well as an increase in median channel width of more than 500 m, suggest that stream power drove later stages of telescoping and incision. We maintain that telescoping megafans such as the one we propose could be buffering agents of tectonic, climatic, and geomorphic signals in other modern and ancient sediment routing systems.