Lava-dome growth and explosive volcanism in the Jemez Mountains, New Mexico: Evidence from the plio-pleistocene puye alluvial fan

Abstract

Abstract The Plio-Pleistocene Puye Formation, north-central New Mexico, is a 200-km2 volcanogenic alluvial fan shed eastward from the Tschicoma volcanic center, part of the Jemez Mountains volcanic field. The fan contains > 15km3 of volcaniclastic material derived from closely spaced lava domes in the northeastern portion of the Tschicoma center. Interbedded in the fan sediments are at least 25 primary pyroclastic units from explosive eruptions of dacitic and rhyolitic lava domes. Tephra occur mainly as pumice falls but include several pumiceous ignimbrites and two thick proximal block-and-ash pyroclastic flows. The upper part of the fan also contains rhyolitic plinian deposits erupted from sources in the central portion of the Jemez field and basaltic ash derived from the central Rio Grande rift. Fanglomeratic (pyroclastic and epiclastic) facies exhibit considerable lateral variation. Primary tephra deposits, however, provide a stratigraphic framework for reconstruction of the growth of individual lava domes. While only volcanic domes and lava flows are exposed in the Tschicoma center, Puye tephra layers show that vulcanian, subplinian and plinian activity, and block-and-ash pyroclastic flows accompanied many dome-forming events. Degradation of these lava domes produced coarse-grained debris flows dominated by lava and dome carapace clasts. From these epiclasts it is possible to identify lithologies of lavas that are either currently buried or have been obliterated by erosion or by collapse of the Valles calderas. The preservation of Puye deposits reflects very high rates of aggradation and gradual down-faulting in the Espanola basin of the central Rio Grande rift. This record of pyroclastic and epiclastic deposition allows detailed interpretation of the evolution of a volcanic center from its pyroclastic and erosion products, and is the first example of a volcanogenic alluvial fan in an intracontinental rift setting to be described in detail. This study emphasizes that interpretation of any ancient volcanic terrain based solely upon data derived from mapping, radiometric dating, and chemical analysis of lavas or existing denuded edifices is likely to greatly underestimate both the role of explosive volcanism in the build-up of volcanic centers and subsequent estimates of the volumes of material produced.