Late-stage Pyroclastic Flow and Fall Deposits From Volcán Ceboruco, Mexico: Insights From a Small Volume Caldera-Forming Eruption

Volcán Ceboruco, located in western Mexico, erupted 3 to 4 km³ of rhyodacitic to dacitic magma, known as the Jala Pumice, forming a 3.7-km-wide caldera (approx. 1000 A.D.). Late-stage deposits differ significantly from early ones in eruption style, overall lithic content, pumice composition, accidental lithic types, and erupted volume. Detailed stratigraphic correlation between pyroclastic fall, surge, and flow deposits of the Jala Pumice indicate that a progressive shift occurred from an early convective region (Phase I), to a transitional regime feeding both a Plinian convective column and dilute density currents (Phase II), to an almost fully collapsing regime producing mostly dense pyroclastic flow deposits and lithic-fall deposits (Phase III). Phase I deposits contain less than 15 wt.% lithics. In contrast, the lithic content of Phase II deposits range between 20 and 60 wt.%, and between 65 and 90 wt.% in Phase III deposits. A systematic change in pumice composition is observed in the Jala Pumice sequence, where 95% of the juvenile component in Phase I deposits is rhyodacite pumice (white pumice) compared to 85% of the juvenile component in Phase III deposits being dacitic pumice (gray pumice). Banded pumice clasts, consisting of intermingling streaks of the two magma compositions, are found only in Phase II and III fall deposits. Accidental lithic type populations also vary through the Jala Pumice. Early deposits contain an abundance of surficial and granitic lithics, whereas late-stage deposits are entirely composed of surficial lithics and Sierra Madre Ignimbrite fragments. The erupted volume of Phase I deposits is dramatically different than the Phase III layers. Phase I deposits, composed of 2 pyroclastic fall layers, account for approximately 70% of the total erupted volume of the Jala Pumice, whereas late-stage deposits, composed of pyroclastic flow and small volume lithic-fall deposits, account for less than 2% of the total erupted volume. In addition, despite the abnormally high lithic content observed in the late-stage flow deposits, referred to as the Marquesado and North-Flank PFD's, they are clearly pyroclastic in origin due to their long run-out distance (>13 km), and dynamic facies changes with distance and topography. We conclude that foundering of the caldera produced an excessive amount of lithic material, some of which was incorporated into late-stage pyroclastic flows and block and ash falls, with collapse, subsequently choking the conduit and eventually shutting down the eruption.