New Mexico Geological Society Annual Spring Meeting — Abstracts

Mt. Withington cauldron was originally described as a nearly circular late Oligocene structure encompassing the entire north half of the San Mateo Mountains. Our recent detailed mapping locates northern and southern cauldron-margin segments within this proposed structure. A previously mapped west-facing structure in the western Magdelena Mountains is reinterpreted as another cauldron margirt segment. Together, these segments suggest a northeast-elongated elliptical shape for the redefined Mt. Withington cauldron, with the western margin not yet defined. The cauldron is filled with at least 800 m of 27.4 Ma South Canyon Tuff, which contains lithic breccias formed by collapse of oversteepened cauldron walls. Outside the cauldron, pre-cauldron units are overlain by less than 200 m of South Canyon Tuff.

The northern cauldron margin segment, near Monica Saddle, is a near vertical fault that bounds bottomless South Canyon Tuff to the southeast. The southern segment, near Wildcat Peak, is a northwest-tilted (15 degrees) flexure unconformably buried by the South Canyon: Tuff. Clast-supported lithic breccias within South Canyon Tuff overlying this flexure are interpreted as proximal lag deposits derived from a nearby vent. A moat sequence, exposed along the southern margin, consists of rhyolite domes, alluvial fart deposits, and eolian sandstone. The tuff of Turkey Spring (petrographically similar to the South canyon) overlies the moat deposits, and the South Canyon outside the cauldron.

Intra-cauldron South Canyon Tuff is invaded by northwest trending epithermal gold veins in the Rosedale Mining District. Voluminous pre-cauldron high-fluorine, high-silica rhyolite lavas occur northwest of the cauldron.