GSA Connects 2022 meeting in Denver, Colorado

Paper No. 102-6
Presentation Time: 9:00 AM-1:00 PM


KHAN, Sabrina, Johns Hopkins University, Department of Earth and Planetary Sciences, 301 Olin Hall, 3400 N. Charles Street, Baltimore, MD 21218, THOMPSON, Alexandria, College of Earth, Ocean, and Atmospheric Sciences, Oregon State University, Corvallis, OR 97331, LUO, Flora, University of Michigan, 500 S. State Street, Ann Arbor, MI 48109, RENTERIA, Olivia, Texas Tech University, 2500 Broadway, Lubbock, TX 79409, GALLANT, Elisabeth, USGS, Hawaiian Volcano Observatory, 1266 Kamehameha Avenue, Suite A-8, Hilo, HI 96720 and MARSHALL, Anita, Department of Geological Sciences, University of Florida, Gainesville, FL 32611

Recent field studies by the GeoSPACE Field Course (NSF Award 2023124) of a northern Arizona cinder pit (V185) located within the San Francisco Volcanic Field uncovered evidence for successive, small-scale explosive events influenced by the presence of water. Analysis of a well-preserved outcrop at the rim of the pit (35.180833, -111.339444) reveals the eruption sequence yielded six primary stratigraphic units, presented here from oldest to youngest: (1) poorly-consolidated volcanic breccia with Toroweap and Kaibab Formation bombs, (2) unconsolidated tephra fallout unit, (3) reverse-graded welded unit with vesicular basalt and Coconino Formation lithics, (4) bedded surge deposits, (5) matrix-supported volcanic breccia with vesicular basalt and Coconino Formation lithics, and (6) grain-supported oxidized scoria with caliche coating.

We hypothesize that the oldest volcanic breccia unit represents the initial eruption event, incorporating large fragments of country rock, with subsequent layers indicating the occurrence of ash fall, pyroclastic surges, and brief lava flows. Bomb sags, geodes, caliche, and welded surge deposits found throughout the outcrop suggest surface and subsurface water was present over the course the eruptive cycle. Alternating beds of oxidized scoria and volcanic breccia near the top of the outcrop signal the progressive drying out of the site.

This stratigraphic analysis was conducted through traditional field mapping techniques as well as detailed photogrammetry survey to produce a 3D model of the outcrop. To learn more about the GeoSPACE Field Course and our work at V185, search for the keyword “GeoSPACE” in the conference program.