Sugarloaf Mountain Tephra - A pleistocene rhyolitic deposit of base-surge origin in Northern Arizona

Sugarloaf tephra forms an apron surrounding Sugarloaf Mountain, a rhyolitic dome northeast of the San Francisco Peaks, Arizona. Tuffring deposits, exposed for 180° around the north side of the endogenous dome, have a maximum thickness of 75 m. Three types of bedding (or depositional) units are recognized in the tuff ring: plane beds, massive beds, and sand-wave beds. A related 3- to 10-m-thick air-fall deposit extends northeast of the tuff ring. The dome, dated at 212,000 yr B.P., and its associated tephra deposits predate the adjacent Illinoian-age glacial deposits. Interaction of a ground-hugging fluidized density flow with a cohesionless substrate is the model used for base-surge transport of the tephra deposits. Particle-size data show that the tuffring textural types represent discrete subpopulations. The two principal roots extracted by factor analysis of the size data are assigned to viscous and inertial shear at the base of the surge flow. Sandwave beds form under the influence of viscous shear in a dilute-phase fluidized system with a thin traction layer. Plane beds develop through inertial shear in a 1- to 10-cm-thick bed-load layer beneath a dilute-phase fluidized surge cloud several meters thick. Massive beds result from a combination of viscous and inertial forces operating in a deflating, dilute- to dense-phase fluidized cloud. Solid fraction of the flowing cloud and thickness of the bed-load layer are the critical factors in developing the textures of the tuffring deposits.