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Paper No. 223-8
Presentation Time: 10:45 AM
MAGMATIC INSIGHTS FROM A SEDIMENTARY SEQUENCE IN A DYNAMIC VOLCANIC CENTER, BLACK MOUNTAINS, AZ
The >640km3 (DRE) Peach Springs Tuff (PST) and its source, the Silver Creek Caldera, have been a recently renewed area of study as a well exposed example of a supereruption. In this study, we sought to understand more precisely the magmatic, environmental, and topographic evolution of this supervolcano by examining a sedimentary section 5km north of the caldera rim in the Black Mountains of NW AZ (Union Pass quadrangle). Our mapping along a 4km long N-S transect followed an erosive unconformity that coincides in time and stratigraphy with the locally absent PST. A welded tuff overlying the unconformity is 17.8Ma, and the youngest date from the pre-unconformity trachytes is 18.9Ma, indicating that the 18.8Ma PST outflow is missing (Lang et al 2008). XRF, SEM, and thin section analyses of sediments and lavas from this sedimentary basin show the unconformity marks a boundary that divides trachytic lavas and their derivative sediments from rhyolitic, and mostly explosive, volcanism, indicating a shift in magmatism near the time of the PST eruption. Pre-unconformity brown sandstones are rich in plagioclase, with sanidine and biotite as minor to accessory minerals, and display a variety of lithic clasts (mostly trachyte clasts). Post-unconformity sediments are pink and white slightly reworked pumiceous (40-60% pumice) tuffs rich in sanidine, followed by quartz, biotite, and plagioclase. Basaltic trachyandesite lavas interbedded with the sandstones were the last material erupted below the unconformity and provide evidence for a dynamic magmatic system leading up to the PST eruption (Lee, et al 2014 GSA). In one location, the post-PST unit fills a channel in the top of the pre-PST sandstones at the contact, and in most locations the pre-PST sandstones become friable. In some locations they grade up into the post-PST strata. These observations indicate an erosional, rather than nondepositional unconformity. Though nondeposition cannot be ruled out merely by the presence of an erosive surface where we expect to see the PST, the unconformity may represent removal of the PST via erosion shortly after eruption. This relatively quick turnaround from a sedimentary basin to an erosive surface could suggest tectonic activity or resurgent uplift at or near the time of the PST eruption.