ERUPTIVE AND DEPOSITIONAL PROCESSES OF NARBONA PASS MAAR VOLCANO, NAVAJO VOLCANIC FIELD, NAVAJO NATION, NEW MEXICO

We reconstruct the eruptive history of Narbona Pass maar (NPM), a center in the mid-Tertiary Navajo volcanic field (NVF) on the Colorado Plateau, to determine the temporal evolution of the eruption, dominant depositional mechanisms, influence of liquid water on deposit characteristics, geometry and evolution of the vent, efficiency of fragmentation, and the relative importance of magmatic and external volatiles. The basal deposits form a thick (5-20 m), massive lapilli tuff to tuff-breccia deposit. This is overlain by alternating bedded sequences of symmetrical to antidune cross-stratified tuff and lapilli tuff; and diffusely-stratified, clast-supported, reversely-graded lapilli tuffs that pinch and swell laterally. This sequence is interpreted to reflect an initial vent clearing phase that produced concentrated pyroclastic density currents, followed by a pulsating eruption that produced multiple density currents with varying particle concentrations and flow conditions to yield the well-stratified deposits. The lack of features such as accretionary lapilli and soft sediment deformation suggests that little to no liquid water existed in the density currents during deposition. Juvenile material is dominantly present as blocky fine ash and finely-vesiculated fine to coarse lapilli pumice. This indicates that phreatomagmatic fragmentation was predominant, but also that the magma was volatile-rich and vesiculating at the time of eruption. The top of the phreatomagmatic sequence abruptly contacts the overlying minette lava flows, which suggests no significant repose period between the two eruptive styles.

Our results indicate that the NPM maar-diatreme eruption had several characteristics distinct from those of other NVF centers: (1) while the explosive eruption was driven primarily by the interaction with external water, the abundant pumice suggests that magmatic volatiles were also present and important; (2) downward excavation beneath NPM was much shallower than at other NVF diatremes, a consequence of regional variations in aquifer thickness; and (3) the vent migrated laterally or widened during the eruption, controlled by subsurface lithology and availability of external water to the magma. Our findings at NPM offer several new perspectives on the evolution of maar-diatreme volcanoes.