THE PROPERTIES AND FORMATION OF EROSIONAL PIPE-SHAPED STRUCTURES IN IGNIMBRITES AROUND VALLES CALDERA

In three separate ignimbrite deposits of the Bandelier Tuff Formation around Valles Caldera (La Cueva, Otowi, and Tshirege Members, preferential weathering has exposed columnar or pipe-shaped structures. The features result from the compositional alteration of the ignimbrite creating a hardened resistance to erosion in elongated volumes of the non-welded deposits. These features are manifested with different widths and lengths, however, it is thought that their origins can be attributed to the similar alteration processes and conditions. This alteration was previously assumed to be vapor-phase alteration (Cas and Wright, 1988), but further analysis has shown the presence of zeolites suggesting the alteration was the result of saturation of the ignimbrite by meteoric water.

The structures exist as groups (1 to 30 features) and here is a strong positive correlation between the diameter and spacing of pipes that could be representative of the capture area for water draining into the deposit. If the pipes started to form immediately after deposition of the ignimbrite, in a similar way to fumarolic pipes, they would likely be a more widespread phenomenon in the deposit. Rapidly occurring welding and vapor-phase alteration of the top of the ignimbrite may have stopped water reaching the interior of the deposit, except where valleys were rapidly incised. Each area where pipes occur must therefore experience localized conditions, which control formation at the time the valley is being formed. The suggestion is that landslides on oversteepened, friable valley walls exposed fresh, still hot, unaltered ignimbrite in which pipe structures could form if the material had cooled from its depositional temperature to within a suitable, specific range. An abundant supply of water from high rainfall and/or stream activity is also implied.

The presence of the pipes gives an insight into the erosional evolution of ignimbrite-filled valleys over a period of almost 2 million years. One conclusion is that the valley systems in the Bandelier ignimbrites developed quickly, within the time scale of cooling of the ignimbrite sheets, and the walls have remained little modified in places since that time.

[Statements herein are those of the authors and do not necessarily reflect the view or regulatory position of the US NRC]