11-8 Rheoignimbrites of the Davis Mountains Volcanic Field, Trans-Pecos Texas

Session: Using Volcanic Deposits to Help Us Understand Volcanic and Magmatic Processes

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Abstract:

Three rheoignimbrites of the Davis Mountains Volcanic Field (DMVF) form a spectrum of increasing rheomorphism from readily discernible pyroclastic origin to crypto-rheoignimbrite difficult to distinguish from silicic lava. Gomez Tuff (DRE >200 km³) is as much as ~ 500 m thick within its source caldera, but over much of its outcrop, preserved thickness is ~ 20 m. It has a non-welded pyroclastic base, and, where the upper surface was protected from erosion, an upper non-welded zone. Breccia zones are absent. A prominent vitrophyre with shard texture occurs as the base of the densely-welded zone. Where thick (>80 m), laminar flow structures are common and pyroclastic textures are typically obliterated through granophyric crystallization, although all exposures of Gomez contain characteristic lithic inclusions. Fort Davis Tuff (Barrel Springs Group) preserves a pyroclastic base in many sections, but granophyric densely-welded zones do not show shard texture. Lithic inclusions are sparse, and thick sections (~80 m) have a prominent upper breccia. One exposure has a 1-m thick basal breccia. The enigmatic Sleeping Lion Rhyolite, typically cited as a silicic lava, displays prominent basal and upper breccias; shard texture is only preserved in vitrophyre, and lithic inclusions are rare (except within its possible intracaldera facies).

Differential rheomorphism was influenced by magma temperature and topography. Gomez Tuff (no breccia and quartz phenocrysts) had an estimated eruption temperature of 750 ^oC; Fort Davis Tuff (breccia and no quartz phenocrysts) had an estimated magmatic temperature of ~900 ^oC, based upon comparison with similar units. Abundantly porphyritic Sleeping Lion Rhyolite (again no quartz) was emplaced within a valley system that funneled flow along a 40-km path that promoted extensive brecciation.

DMVF is well-known for extensive silicic lava. The relative absence of widespread air-fall deposits associated with the rheoignimbrites and magma lumps ejected with ash in some, suggest pyroclastic flow and lava eruption were favored over more energetic pyroclastic fall eruption. Rheoignimbrites may be identified by their greater thickness variation than silicic lava, greater lithic inclusion content, and local preservation of pumice and shard texture.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-5841

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