Lateral erosion ('toe-cutting') of alluvial fans by axial rivers: Implications for basin analysis and architecture

We document the neglected phenomenon of lateral erosion ('toe-cutting') of alluvial fans by non-incising axial river channels. Field examples from the Holocene of the Big Lost River basin, Idaho and the Plio-Pleistocene of the Rio Grande Rift, New Mexico help to establish architectural models with more general application to basin analysis. The process of toe-cutting can lead to complete fan destruction and may be a response to climate change, tectonic tilting, fault propagation or a combination of these variables. It gives rise to: Near horizontal erosion surfaces cut in fan sediment; steep fan-margin scarps; progressive up-fan incision from the scarp by a network of channels; soil formation up-fan away from the incised channel network: A deposit of axial alluvium that overlies the erosion surface and onlaps the scarp. Once avulsion occurs to take the axial channel away from the bajada margin, distinctive 'healing-wedges' of fan alluvium prograde across abandoned axial river channel and floodplain deposits, gradually onlapping the eroded scarp and its upstream network of incised channels. Toe-cutting has important stratigraphic basin analysis and economic consequences: Bajada deposits subject to the process exhibit appreciable extra groundwater and petroleum reservoir potential in the intercalations of more porous and permeable axial fluvial sediments.