VOLCANIC STRATIGRAPHY, STRUCTURE AND GEOCHRONOLOGY OF INTRA-CALDERA FILL IGNIMBRITES OF THE 28 MA BURSUM CALDERA, MOGOLLON DATIL VOLCANIC FIELD, NEW MEXICO

The 28 Ma Bursum Caldera in the western Mogollon Datil Volcanic Field erupted two extensive ignimbrite sequences followed by smaller rhyolitic lava flows, domes and pyroclastic deposits. The succession includes the Bloodgood Canyon Tuff outflow sheet, the Apache Springs intracaldera tuff, and resurgent volcanic rocks of the Fanney Rhyolite and Deadwood Gulch pyroclastic member of the Fanny Rhyolite. The three high-grade and very high grade ignimbrites are intercalated with non-welded ignimbrites, and volcaniclastic sandstone deposits, with paleosols dispersed throughout. These rocks are overlain by intermediate composition lava flows.

Here we re-examine the stratigraphy, structure and geochronology of ring fracture domes and pyroclastic sequences of the intra caldera fill and resurgent volcanic rocks to determine eruptive volumes, vent locations and timing of extension in the Bursum Caldera. We redefine the vent location of the Deadwood Gulch member pyroclastic deposits based on thickness and welding and eastern edge of the Bursum caldera. Deposits are divided into five main phases each characterized by a different eruption style and/or deposit emplacement mechanism. The eruption sequence starts with the eruption of the Bloodgood Canyon and Apache Springs tuffs, which cause caldera collapse. These eruptions are followed by a period of quiescence and volcaniclastic deposits prior to small volume domes and lava flows erupting along the structural boundaries and interior fractures of the caldera. Rhyolite emplaced in volcanic units of both episodes point to volatile-poor magmas during which rhyolite flows and domes formed. The final rhyolite unit erupted is a single cooling unit ignimbrite of the Deadwood Gulch member erupted. Thicknesses increase for this unit to the east with the only observed densely welded section of the ignimbrite occurring along the center of the Bearwallow Mountain quadrangle. The source of the Deadwood Gulch member is inferred to be along the eastern collapse margin of the Bursum Caldera. The final eruptive unit is the andesites of the Last Chance Andesite representing a transition to intermediate composition magmatism in the area.