Tephra Blanket Record of a Violent Strombolian Eruption, Sunset Crater, Arizona

Abstract:

New fieldwork provides a detailed description of the widespread tephra of the ~1085 CE Sunset Crater eruption in the San Francisco Volcanic Field, Arizona, and refines interpretation of the eruptive sequence. The basal fine-lapilli tephra-fall-units I–IV are considered in detail. Units I and II are massive, with Unit I composed of angular to spiny clasts and II composed of more equant, oxidized clasts. Units III and IV have inversely graded bases and massive tops and are composed of angular to spiny iridescent and mixed iridescent and oxidized angular clasts, respectively. Xenoliths are rare in all units (<0.1%): sedimentary xenoliths are consistent with the known shallow country rock (Moenkopi and Kaibab Fms); magmatic xenoliths are pumiceous rhyolite mingled with basalt. Unit II is less sideromelane rich (20%) than Units I, III, and IV (60–80%). Above these units are at least two more coarse tephra-fall units. Variably preserved ash and fine-lapilli laminae cap the tephra blanket. This deposit is highly susceptible to reworking, and likely experienced both syn- and post-eruptive aeolian redistribution. It appears as either well sorted, alternating planar-parallel beds of ash and fine lapilli with rare wavy beds, or as cross- or planar-bedded ash. The tephra blanket as a whole is stratigraphically underlain by a fissure-fed lava flow and lapilli-fall units are intercalated with two larger flows.

Mean grain size is coarsest in Unit I but coarsens in Units II–IV. Units I, III, and IV are moderately to poorly sorted with no skew. Unit II is better sorted and more coarse-skewed. Units I and III are slightly more platykurtic than II and IV. Without considering possible spatial effects introduced by dispersion patterns, bootstrap ANOVA confidence intervals suggest at least Unit II sorting and skewness are from distinct populations. Isopachs indicate Units I and II were associated with a 10-km-long fissure source. After or during Unit II’s deposition, activity localized to Sunset Crater. Units III and IV were emplaced with waxing to sustained activity, and followed by at least two more sustained episodes. Two lava flows began effusing from the cone during this period and remained active after explosive activity ceased. Primary tephra deposition ended with a period of small discrete explosions.