New Mexico Geological Society Annual Spring Meeting — Abstracts

The Peralta Tuff Member of the Bearhead Rhyolite is superbly exposed in the Peralta CanyonfTent Rocks area in the southeastern Jemez Mountains. These tuffs record episodic, rhyolitic, volcanic activity during the late Miocene, and are composed of primary and reworked pyroclastic-flow (PF), tephra-fall (TF), and pyroclastic-surge (PS) deposits, along with intercalated braided-stream deposits.

The Tuffs of West Mesa (TWM) and the Tuffs of Lower Peralta Canyon (TLPC) are informally named eruptive units within the Peralta Tuff. In most places, the TWM consists of 20 m of PF and minor PS deposits erupted from a vent in the Bearhead Peak dome complex. The TLPC consist of 50 m of PF, PS, and TF and were erupted from a vent located in lower Peralta Canyon, 7 km SE of Bearhead Peak.

Granulometric, vesiculation, petrographic, and stratigraphic data were collected to determine the interaction between the rhyolitic magmas and the groundwater during the TWM and TLPC eruptions. The TWM eruption began with a phreatomagmatic (PM) phase resulting in the formation of a tuff ring at least 70 m thick. Subsequent magmatic PF traveled, 6 km from the vent leaving a ≥ 20 m deposit. The TLPC eruption began with several PM eruptive cycles, each of which began with wet, ash-rich, low-angle cross-bedded PS, progressed through a massive poorly-sorted PF, and ended with a drier, pumiceous, cross-bedded PS. Abundant clasts of underlying basin-fill sediments decrease upwards in the cycles. Approximately 30 m of the vent's tuff ring was deposited in this manner. These cycles gave way to 20 m of pumiceous, cross-bedded PS, containing only rare sedimentary fragments. Explosive eruptive activity ended with the deposition of a 2 m TF. Extrusion of a dome is inferred to have occurred following the explosive activity from each vent.

Near the intersection of Peralta and Colle Canyons the TWM and TLPC are intercalated. The absence of inter-eruptive sediments, erosional surfaces, or paleosols indicates that these two eruptions were simultaneous.

Despite the close proximity of the two vents and simultaneity of the eruptions, the differing substrates are thought to have influenced the amount of groundwater available to each magma. The greater phreatomagmatic component of the TLPC eruption is apparently related to the mixing of magma with the underlying basin-fill sediments at a topographic position ≥ 300 m lower than the TWM vent, which is underlain by relatively dry, older, volcanic rocks that could not supply enough groundwater to sustain a PM eruption.