March/April Geological Society of America Bulletin media highlights

Avulsion—the abandonment of a river channel for a new course—influences the long-term distribution of water and sediment on many flood plains, deltas, and alluvial fans but more field data are needed to test avulsion concepts and models. This study uses optically stimulated luminescence (OSL) dating to establish an avulsion history for the meandering Klip River, eastern Free State, South Africa. Bedrock outcrop in the channel bed indicates that the long-term sedimentation rate is negligible, but OSL ages show that five avulsions have been completed during the last 30,000 years in association with meander belt development. Field evidence suggests that an avulsion is initiated when floodwater that originates on the upstream muddy flood plain drains back to the channel through low points in levees or bank tops. This forms a new channel that in subsequent floods erodes through the flood plain to connect with the original channel farther upstream. Over time, flow and sediment is diverted along the new channel, and it gradually develops meanders whilst the original channel is abandoned. European settlement in the valley in the late 1800s initiated an ongoing avulsion; rapid erosion of a new channel is proceeding alongside abandonment of a 1–2 km reach, suggesting that avulsions are probably completed in 100–150 years. The findings support previous suggestions that sedimentation rate is a primary control on the frequency and processes of avulsion but the study emphasizes the need for further avulsion studies in settings where sedimentation rates are very low.

Volcano evolution and eruptive flux on the thick crust of the Andean Central Volcanic Zone: 40Ar/39Ar constraints from Volcán Parinacota, Chile
John Hora, University of Wisconsin–Madison, Dept. of Geology & Geophysics, Madison, WI 53706, USA; et al. Pages 343-362.

Keywords: Andes, Central Volcanic Zone, Parinacota, 40Ar/39Ar geochronology, eruptive rates.