2009 Portland GSA Annual Meeting (18-21 October 2009)

Paper No. 10
Presentation Time: 9:00 AM-6:00 PM


RIGGINS, Annelise M.1, SMITH, April2, MEYER, Lauren3, DENNISON, Shannon4, MERKEL, Stephen4, RAMOS, Frank C.2 and MCMILLAN, N.J.2, (1)Earth & Environmental Sciences, New Mexico Institute of Mining and Technology, 801 Leroy Place, Socorro, NM 87801, (2)Geological Sciences, New Mexico State University, Box 30001, MSC 3AB, Las Cruces, NM 88003, (3)Vanishing Treasures Program, Bandelier National Monument, 15 Entrance Road, Los Alamos, NM 87544, (4)Bandelier National Monument, 15 Entrance Road, Los Alamos, NM 87544, anneliseriggins@gmail.com

During the Ancestral Pueblo period (1150-1500 CE), primitive cave dwellings, also known as “cavates,” were hand-excavated into the 1.2 Ma, highly friable, lower Tshirege member of the Bandelier tuff, in Bandelier National Monument, northern New Mexico. Currently the cavates are experiencing extensive deterioration ranging from exterior degradation to cave-ins. Our project intends to characterize compositional and textural variations with depth to determine the mechanisms causing cavate disintegration. The effects of vapor phase alteration, soot deposits from occupational fires and lichen growth were investigated by X-ray Fluorescence (XRF), Scanning Electron Microscopy (SEM), petrographic analysis and accelerated weathering experiments.

Surfaces affected by vapor phase alteration and subsequent lichen growth are harder and more coherent than unaltered areas. SEM and petrography reveal a thin layer of fine-grained material at the surface that may be responsible for the hardening. Major element traverses from the interior to the surface show significant increases in Fe, Mg, Ca, P and Ba. LOI (loss on ignition) is similarly highest at the surface.

After excavation, soot deposits on the ceiling increased the coherence of the freshly exposed tuff. Ceiling samples have a blackened surface with a horizon of orange staining beneath. SEM and light microscopy show that the surface is finer-grained and less crystal rich than the interior. Major element concentrations are constant with depth, though LOI is significantly higher at the surface.

Accelerated weathering experiments include freeze-thaw and exposure to heat. In freeze-thaw experiments, samples were immersed in water before being frozen. A significant loss of cohesion resulted in the disintegration of all samples after six cycles. Heating experiments involved heating separate pieces of unaltered tuff to 300 ºC, 400 ºC, 500 ºC, and 600 ºC. All samples experienced increased surficial cohesion. High temperatures also caused color change in the tuff, from a pale pink to a more intense orange.