Ash-cloud surge origin and influence of local topography for stratified and cross-stratified deposits beneath Bandelier Tuff ignimbrite (Tshirege Member, Pajarito Plateau, New Mexico, USA)

Ignimbrites, with their relatively massive and poorly-sorted characteristics, are commonly underlain by finer-grained, stratified and cross-stratified deposits (basal deposits). The former are often inferred to reflect emplacement from high particle concentration pyroclastic flows, while the stratified and cross-stratified deposit are interpreted to have been deposited by low-concentration, dilute pyroclastic surges. We document basal deposits from the upper (Tshirege) member of the large-volume Bandelier Tuff, which erupted from the Valles caldera (New Mexico, USA). Two paleotopographic settings are characterized: (1) plateau settings that contained approximately-radial shallow canyons, valleys and interfluves, and (2) settings associated with rugged topography of pre-Bandelier Tuff lava domes. We find that the stratified and cross-stratified basal deposits consist of the same components as the overlying ignimbrite matrix but are selectively finer grained. In plateau settings the basal deposits thicken somewhat with distance from the caldera and on paleo-interfluves relative to locations near paleovalley bottoms. Basal deposits are thickest, and have the best developed cross-stratification, in topographic lows in the lava dome settings. We infer that the basal deposits were deposited by ash-cloud surges that consisted of fine particles entrained from underlying pyroclastic flows. The ash-cloud surges either (1) ran ahead of the more sluggish flows and onto interfluves in the relatively low-relief plateau settings, followed by burial by aggrading ignimbrite as pyroclastic flows reached a location; or (2) overtopped topographic highs that temporarily blocked pyroclastic flows in the lava dome setting, until ignimbrite aggradation allowed the flows to spill over and cover the basal deposits. Larger thicknesses of basal deposits reflect longer times of deposition from ash-cloud surges before ignimbrite inundation. The result is a sort of inverted stratigraphy, where ash-cloud surge deposits, originating atop the moving pyroclastic flows, ran ahead or over the surrounding terrain and deposited first.