Micromorphology, mineralogy, and genesis of soils and fracture fills on the Pajarito Plateau, New Mexico

Soils are natural bodies which function as open systems within a landscape. A wide variety of processes contribute to, and are reflected in, the properties of individual soil profiles (pedons). Material and energy fluxes occur into, within, and through soils as a function of parent material, climate, topography, organisms, time, and a variety of possible other factors (Jenny, 1941). Ideally, the existing properties of an individual soil can provide information about the environment(s) in which it formed, and the processes that have acted upon it.

The town of Los Alamos, and the Los Alamos National Laboratory (LANL), are located on the Pajarito Plateau in north-central New Mexico (Figure 1). Environmental concerns have recently focused attention on the numerous fractures in the Bandelier Tuff, the series of volcanic rock units that make up most of the plateau. These fractures have come into question as possible conduits for transport of contaminants downward through the tuff. This study arose out of a need to evaluate the potential for contaminant transport in the fractures.

Because the fractures are typically filled, or partially filled, with soil-like material, and appear to be physically continuous with the soils on the surface of the Pajarito Plateau, it was decided to approach the question of the fractures from a soil genesis and morphology standpoint. Specifically, it was believed that soil characterization techniques, including soil micromorphological and mineralogical analyses, could provide information about the dominant processes (past and present) acting in the soils and fractures.

The specific objectives of this research were to investigate: (1) the physical, mineralogical and chemical nature of fracture-filling materials in the Bandelier Tuff, as well as associated surface soils; (2) the relationships among fracture-fills, tuff bedrock, and surface soils of the Pajarito Plateau; (3) the processes responsible for the development of the fracture-fills; and (4) the likely sequence of events leading to the present morphology of the soils and fracture-fills.