Paleotopographic influence on basal deposit emplacement in a large caldera-forming eruption: field study of the Upper Bandelier Tuff, Valles caldera, NM

Abstract:

The Upper Bandelier Tuff (UBT) is a 400 km³ sequence produced during the eruption of Valles Caldera, NM (1.25 Ma). This study focuses on the deposit between the Plinian fallout deposit and the ignimbrite sequence, called a “basal deposit” due to its location at the base of the ignimbrite sequence. The UBT basal deposit is described in detail at two key locations between 10-16 km from the caldera. In other case studies, similar basal deposits have been interpreted to result from sedimentation from a dilute flow, such as the ash-cloud surge that precedes and moves on top of a concentrated pyroclastic current. Observations of the UBT basal deposit are consistent with basal deposits at smaller calderas, suggesting that although the size of the eruption may differ, basal deposit emplacement processes are similar.

Paleotopographic setting at the two study areas had major influence on the characteristics of the basal deposit. At Guaje Canyon (10 km from the caldera) the paleotopography was dominated by rhyodacite lava domes. The basal deposit in this location is highly variable in thickness (0.9-2.9 m) with dune cross-stratification and internally erosive layers. This location also includes a lithic tuff breccia layer in the lowermost overlying ignimbrite, with clasts derived from nearby lava domes that constrain the flow path of concentrated pyroclastic currents. Concentrated currents were initially blocked by domes, but ash cloud surges decoupled and continued over the lava domes. This allowed for deposition of a thick, highly cross-stratified basal deposit. Eventually, aggrading ignimbrite filled in topography such that the concentrated pyroclastic current could spill into the next valley and bury the surge deposits.

At Pueblo Canyon (13-16 km from the caldera) the basal deposit has a more consistent thickness (0.6-1 m), with plane parallel to low angle cross stratification. Outcrops interpreted to represent paleo-side drainages have thicker basal deposits and show soft-sediment deformation. Lithic clasts from the basal deposit are used to constrain possible flow paths. Pyroclastic currents flowed down the drainage and concentrated portions were confined to the drainage bottom, gradually filling it with aggrading ignimbrite. Ash-cloud surges were not confined, leading to thicker basal deposits in side drainages.