Tectonic Tilting and Reorganization of an Aluvial Fan to a Dendritic, Erosional River Network: the Example of the Ogallala Gravels
Abstract
Deposition of the Mio-Pliocene Ogallala gravels in the foreland of the Rocky Mountains represents a great natural experiment in landscape evolution. Starting about 20 million years ago the flux of sediment shed off the Rocky Mountains increased, likely in response to dynamic uplift of the Rockies and tilting of the High Plains. This event shifted the high plains from a state of erosion to deposition. The flux of sediment formed huge alluvial megafans, burying the pre-existing river network and effectively "repaving" the western High Plains. Today we are witnessing the re-establishment of a new river network that is dissecting, capturing and eroding these sediment fans. By mapping the modern drainage basins and noting the channel gradient with respect to the normalized length parameter, χ, we identify two types of basins in the high plains. The remnants of the alluvial megafans are drained by long narrow basins with low normalized steepness and nearly no concavity, reflecting little incision since formation. In contrast, the fan remnants are surrounded by basins with a dendritic structure and efficient water and sediment routing, resulting in low values of chi and correspondingly low elevation. The boundary between these two basin types is commonly an erosional escarpment, demonstrating that the trellis basins are consuming the fan deposits by lateral divide migration and successive river capture. We present scaling arguments that show that lateral escarpment advance is nearly an order of magnitude faster than the upstream (knickpoint) propagation of channel entrenchment. This process of landscape evolution has important implications for water in the high plains. Deprived of an efficient channel network, fan surfaces remain uneroded, preserving the Ogallala sediments, and the poorly-drained, poorly integrated surface retains ephemeral water for wetland habitat and aquifer recharge, illustrating how the surface hydrology reflects landscape evolution on million year timescales.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2016
- Bibcode:
- 2016AGUFM.T31A2879W
- Keywords:
-
- 3322 Land/atmosphere interactions;
- ATMOSPHERIC PROCESSESDE: 1815 Erosion;
- HYDROLOGYDE: 8175 Tectonics and landscape evolution;
- TECTONOPHYSICSDE: 8177 Tectonics and climatic interactions;
- TECTONOPHYSICS