Mechanisms of Sediment Storage and Recycling on the Great Plains: Implications for the Rocky Mountain-Gulf of Mexico S2S System (Late Miocene–Pliocene)

The Cenozoic history of the Rocky Mountain–Gulf of Mexico source-to-sink system is complicated by long term (>106 a.) sediment transfer inefficiency on the Great Plains. Here we present new evidence of sediment storage and recycling mechanisms within late Miocene–Pliocene fluvial systems from northeastern Colorado to east-central Nebraska. We analyzed 544 fluvial ridges from an exhumed, telescopic megafan. Late Miocene deposits have an axial azimuth of 60° and an extant longitudinal slope of 0.33%. Latest Miocene and earliest Pliocene deposits have an axial azimuth of 73° and a longitudinal slope of 0.19°. The inferred apex migrated 70 km, from north-northeastward in the late Miocene to eastward in the early Pliocene. These characteristics show that uplift and differential tilting were the chief drivers of fan telescoping in the late Miocene. From the latest Miocene to Pliocene, an increase in grain size, tenfold increase in median channel width (from 80 to 800 m), and uniform incision of the fan axis show that increased stream power drove landscape denudation. Nevertheless, equivalent Pliocene deposits in the subsurface of central Nebraska show widespread aggradation of multiple gravel bodies. In all these systems, proximal incision was broadly contemporaneous with distal aggradation, which would have dampened, delayed, and distorted tectonic and climatic signals over geological timescales. Such factors should be considered when interpreting the stratigraphic record of distant downstream sinks.