CHARACTERIZING AND MAPPING THE POST-SUPERERUPTION SITGREAVES TUFF IN MEADOW CREEK BASIN, SOUTHERN BLACK MOUNTAINS, NW ARIZONA

Sitgreaves Tuff (SGT; Thorson, 1971) is a post-supereruption ignimbrite in the southern Black Mountains near the supereruptive caldera of the Peach Spring Tuff (18.8 Ma; Ferguson et al. 2013). SGT is therefore a prime subject for studying volcanic processes and landforms in post-supereruption terranes. We used existing reconnaissance geologic mapping (Liggett & Childs, 1982), field observations, and multispectral remotely sensed imagery in conjunction with geochemical analyses (Wallrich et al. 2016, GSA abs.) to subdivide SGT into three subunits and create a bedrock geologic map and structural cross-sections of Meadow Creek basin, the type locality of SGT.

SGT1 is ~18 meters thick, consists of fall and pyroclastic flow deposits, and is less chemically evolved than upper SGT units. SGT2 is ~140 meters thick and consists largely of pyroclastic flow deposits. Within the flow sequence of SGT2 are a single ~2m fall deposit and several horizons with large cogenetic lithics up to 2m in diameter. SGT3 is ~30 meters thick and consists of volcaniclastic sediments that show evidence of fluvial reworking. SGT1 likely represents an initial eruptive episode that caused relatively small pyroclastic flows. SGT2 represents more massive and violent eruptions that triggered pyroclastic flows powerful enough to transport large boulders.

Field relations suggest that the contact between SGT and the Meadow Creek trachyte lava dome (MC) is a buttress unconformity and that the MC played a role in constraining the deposition of SGT pyroclastic flows. Thinned SGT deposits at the west edge of Meadow Creek basin suggest Sitgreaves pass was a topographic high during SGT’s emplacement. The history of the basin involves the emplacement of Gold Road lava, Antelope Rhyolite (AN), and MC (in that order); SGT units were then deposited via repeated pyroclastic eruptions that draped over AN and flowed up against MC. SGT2 deposits include co-genetic lithic horizons towards the west of the basin. The westward location of co-genetic lithics, the thinning of SGT2 units to the east, the likely paleo-ridge at the west of the basin, and the MC dome to the north suggest that SGT pyroclastic flows flowed west to east. The massive co-genetic lithic horizons within SGT, which are limited to Meadow Creek basin, suggest a nearby source, likely to the west of the study area.