New Mexico Geological Society Annual Spring Meeting — Abstracts

The Organ Cauldron is rotated to the west by late Tertiary extensional faults within the southern Rio Grande rift, exposing a complete 3 km thick cross-section of intracauldron rocks. The oldest of the four major volcanic units filling the cauldron is the Cueva Tuff. It is a high-silica rhyolite compound cooling ash-flow unit that varies from approximately 200 to 550 m in thickness. The Cueva Tuff was deposited on the precaldera Orejon Andesite, and it is overlain by discontinuous deposits of sandstone and conglomerate up to 3 m thick. The Cueva Tuff is characterized by weak welding, high lithic content, low phenocryst content, and by the occurance of numerous tuff beds in comparison to the overlying massive ash-flow tuff units. Individual ash-flow units vary from 7 m to 75 m thick and are interlayered with base surge deposits and accretionary lapilli-bearing fallout deposits up to a few meters thick. Welding and phenocryst content increase up section, while lihtic content and size decrease up section. Angular fragments of andesite resembling percauldron rocks are the most abundant lithic fragment followed bysparsly porphyritic rhyolite and gneiss. Lithics make up a manximum of 20% of the deposits, and are np to 5 cm across. Zones of strong zeolite alteration occur near the base. A strong reversal to move phenocryst-poor tuff occurs about half way up through the deposit. The phenocryst-poor tuff overlies a thin layer of fossilferous tuff, indication of a brief repose in eruptive activity.

The complex stratigraphy of the Cueva Tuff suggests that initial cauldron forming-eruptions were eposodic in nature, and that the eruption columns contained significant amounts of H₂O. Asence of megabreccia suggests that large scarps had not formed in exposed areas.