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 th e Rio Grande Rift, New Mexico help to establish architectural models with m ore general application to basin analysis. The process of toe-cutting can l ead to complete fan destruction and may be a response to climate change, te ctonic tilting, fault propagation or a combination of these variables. It g ives rise to: near horizontal erosion Surfaces cut in fan sediments steep f an-margin scarps: progressive up-fan incision from the scarp by a network o f 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 abando ned axial river channel and floodplain deposits, gradually onlapping the er oded scarp and its upstream network of incised channels. Toe-cutting has im portant stratigraphic basin analysis and economic consequences: bajada depo sits subject to the process exhibit appreciable extra groundwater and petro leum reservoir potential in the intercalations of more porous and permeable axial fluvial sediments.