T102 The Virtue of Fieldwork in Volcanology, Sedimentology, Structural Geology, and Tectonics (Posters): A Session to Honor Cathy Busby, MGPV Distinguished Geological Career Award Recipient of 2020

Monday, 10 October 2022
9:00 AM - 1:00 PM
Exhibit Hall F

PRELIMINARY FIELD STUDY OF A SMALL PHREATOMAGMATIC VENT IN THE SAN FRANCISCO VOLCANIC FIELD, AZ

Paper number: T102-7
Program: Technical Sessions
Time: 9:00 AM – 1:00 PM

Authors

Abstract

Preliminary field studies of Vent 185 (V185) of the San Francisco Volcanic Field indicate that the feature was formed by a series of phreatomagmatic eruptions driven by water-magma interactions. V185 is a poorly-studied, ~380 m elongate pit located off I-40 east of Flagstaff, AZ. The site has been heavily modified as a former cinder quarry but retains several outcrops suitable for geologic analysis. We conducted preliminary remote sensing, geologic mapping, and stratigraphic and mineralogical analysis of V185 to determine the formation and eruptive evolution of the site as part of the GeoSPACE Field Course (NSF Award 2023124). Initial spectroscopic analysis indicated that the crater is likely mafic in composition. Subsequent field work supported this finding and determined that the dominant rock types in the crater are poorly-welded, highly-oxidized scoria and layered volcanic breccias with sedimentary and basaltic xenoliths. Strike and dip relationships of three in situ outcrops suggest that they may be the remains of an eroded or quarried tuff ring. These deposits are consistent with an origin involving multiple phreatomagmatic eruptions. This origin is supported by similarities between V185 and other known phreatomagmatic vents in the area, such as V235 and Rattlesnake Crater.

Our field mapping and analysis of V185 identified several facies indicative of a series of phreatomagmatic eruptions with varying depositional processes and environments over a short time. Layered, matrix-supported volcanic breccias likely formed from ash fallout after explosive eruptions. Well-bedded surge deposits contain bomb sags and duneforms consistent with wet, high-energy surges. Younger units of poorly-welded scoria likely formed after a groundwater source had been removed. Water also played a role post-emplacement, as evidenced by secondary mineralization which formed caliche coatings, crack infill, and geodes in cavities throughout the outcrops. This preliminary field study provides a foundation for future work focused on constraining the eruptive sequence of V185 and investigating its relationship with other phreatomagmatic vents in the area.

For more information on the GeoSPACE mission and our findings at V185, search “GeoSPACE” in the conference abstracts.

Geological Society of America Abstracts with Programs. Vol. 54, No. 6

doi: 10.1130/abs/2022AM-379511

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