New Mexico Geological Society Annual Spring Meeting — Abstracts

Kilbourne Hole, a late Pleistocene maar in the Potrillo volcanic field, lies on the trace of the Rio Grande rift-related Fitzgerald-Robledo fault system in southern New Mexico. It formed as the result of a phreatomagmatic eruption that broke through the pre-existing Santa Fe Group sediments and 0.1 Ma Afton basalt flow, and deposited the exposed ejecta rim. Subsequent calderalike collapse and erosion are responsible for the size of the modern crater.

The preserved ejecta rim displays classic feature associated with base surge deposits. These features include: sandwave, planar and massive bedforms previously described by Wohletz and Sheridan (1979), as well as accretionary lapilli, impact sags with plastic deformation due to ballistically emplaced blocks, and soft sediment structures. Each of these bedforms and depositional structures can be attributed to different water/magma ratios. Hence, the stratigraphy of these deposits mimic relative changes in the water/magma ratio during the eruption. Accretionary lapilli, and massive beds with associated soft sediment structures increase upward in the section, and thus reflect increasing amounts of water in the surge cloud as the eruption progressed.

Further evidence for the progressively increasing amounts of water is preserved in compositional variations within the section. The base surge is predominantly composed of accidental material from the Santa Fe Group and the Afton basalt flow; the fraction of juvenile basalt fragments increases away from the base of the deposit. Thus, both bedform and compositional variations in the stratigraphic sequence of the base surge deposits at Kilbourne Hole illustrate the dynamic interplay of magma and water in the formation of base surge deposits.