Miocene Intra-Continental, Shallow, Subaqueous Volcanism in Verde Valley, Arizona, USA – A Permanent Record of a Temporary Lake

Abstract:

The Hackberry Mountain Volcanic Center (HMVC), Verde Valley, Arizona, USA, is composed of a chemically diverse suite of effusive and pyroclastic rocks that erupted into the southeast end of a modified half-graben basin from ~14-7 Ma. The half-graben, the Verde Basin, was filled with intercalated fluvial, evaporite, marsh, & lacustrine carbonate deposits of the Verde Formation (~7.5-2.5 Ma), the stratigraphy of which suggests a closed continental basin by ~5.5 Ma. Notably, little is known about if or how the extent of those sedimentary depositional systems was controlled by contemporaneous and voluminous volcanism. New geologic mapping of the oldest exposed record of subaqueous volcanism (~9.2-7.4 Ma) reveals facies transitions from shallow subaqueous to subaerial basaltic volcanism that have important implications for the spatial extent, timing, & causes of basin closure and flooding in the Verde Basin. Volcanism in the study area began with widespread phreatomagmatic eruptions of clinopyroxene-rich basaltic tephra from at least four different vents (1A; blt1). This tephra is capped by basaltic lava flows that cooled in pillows and pseudo-pillows (1C, D, E, F). Locally, basalt lavas are cut by clastic dikes, have expanded, quench-fragmented basal breccias in a sandy matrix, peperitic margins (1B), and/or are enveloped in matrix- and clast-supported hyaloclastite (1G). Finally, at least one vent shows a transition to subaerial Strombolian-style eruptions of scoria, bombs, spatter, & rootless lava flows – although we postulate that the entire sequence could be subaqueous based on a capping hyaloclastite (1A; blt2). The mapped extent of textures representative of water-lava & water-magma interactions suggest that the HMVC initially dammed the ancestral Verde River and created a closed lacustrine basin much earlier than previously proposed, perhaps as early as ~7.5 Ma. Despite an exposed record of younger sedimentation indicating repeated spatial and temporal transitions from fluvial to lacustrine deposition in the Verde Fm, a sparse record of early sedimentation during HMVC volcanism makes it difficult to evaluate paleogeography and environmental factors like water depth. It is unclear what methods, if any, can be used to extract water depth from the variety of volcanic textures mapped in the study area.