SPILLOVER MODELS FOR AXIAL RIVERS IN REGIONS OF CONTINENTAL EXTENSION- THE RIO-MIMBRES AND RIO-GRANDE IN THE SOUTHERN RIO-GRANDE RIFT, USA

The Pliocene-early Pleistocene history of the ancestral Rio Grande and Quaternary history of the Rio Mimbres in the southern Rio Grande rift , New Mexico, illustrate how axial rivers may alternately spill into a nd subsequently abandon extensional basins. Three types of spillover b asins are recognized, based on the angle at which the axial river ente rs the basin and whether it descends the hanging wall dip slope or foo twall scarp to reach the basin floor. In the Mimbres basin type, the a xial river enters and flows through the spillover basin nearly paralle l to the footwall scarp, resulting in a narrow belt of basin-axis-para llel channel sand bodies located near the footwall scarp. In contrast, an axial river may enter a spillover basin at a high angle to its axi s, either descending the hanging wall dip slope (Columbus basin type) or footwall scarp (Tularosa basin type), and construct a fluvial fan, consisting of radiating distributary channels orientated nearly perpen dicular to the basin axis. Faulting exerts significant control on rive r spillover by creating the topographic gaps through which the axial r iver moves and by terminating spillover by subsequently uplifting or t ilting the gap. Spillover may also be autocyclic in origin, as a resul t of aggradation to the level of a pre-existing gap, headward erosion creating and/or intersecting a gap, or simple river avulsion upstream of a gap. Predicting facies architecture in the three types of spillov er basins is critical to successful subsurface exploration for hydroca rbon reservoirs, groundwater aquifers or placer mineral deposits.