Origin of nonbedded pyroclastic rocks in the Cathedral Cliff diatreme, Navajo volcanic field, New Mexico

Abstract

Cathedral Cliff is a well-exposed mid-Tertiary phreatomagmatic diatreme in the Navajo volcanic field, New Mexico. Post-eruptive erosion, estimated between 500 and 1000 m, exposes the deep part of the diatreme. Cathedral Cliff provides an opportunity to examine the transition between the bedded pyroclastic rocks of an upper diatreme and the nonbedded pyroclastic rocks of the subjacent lower diatreme. Bedded pyroclastic rocks occupy 49% of the mapped diatreme area, nonbedded pyroclastic rocks occupy 45%, and the remaining 6% consist of sedimentary megablocks. The bedded pyroclastic rocks range from coarse tuff to medium lapilli tuff and from lithic to juvenile rich. They were emplaced during eruption at the bottom of the crater by pyroclastic density currents, pyroclastic fallout, and as ballistic bombs and blocks. As the eruption continued, they subsided into the diatreme to their current location. Bedding ranges from well-preserved (“continuous beds”) to partially destroyed (“broken beds”). Syn-eruptive subsidence, the passage of debris jets through the broken beds, as well as a concentration of explosions nearby, explains the partial destruction of the bedding. The nonbedded pyroclastic rocks are massive and poorly sorted and consist of coarse tuff, lapilli tuff, and tuff breccia. On average, they are coarser grained and more juvenile rich than the bedded rocks. Nonbedded rocks form zones with steep contacts that cross-cut each other and the bedded rocks. In places, they have gradual transitions into broken beds. We interpret the nonbedded rocks as formed by the passage of debris jets through existing diatreme infill and from the destruction and recycling of the bedded deposits, with juvenile clast enrichment due to dike fragmentation.