PETROGRAPHY AND PERMEABILITY OF THE MESA RICA SANDSTONE AT AUTOGRAPH ROCK, OKLAHOMA

A reconnaissance field and laboratory study was conducted to determine the petrography and permeability of the sandstone exposed at Autograph Rock, OK. Autograph Rock is a 30' high cliff along Cold Springs Creek that hosts over 200 inscriptions dating from 1850. The historic inscriptions are carved into the (100-110 Ma) Mesa Rica Sandstone (Dakota Group). Autograph Rock was an important stop along the Santa Fe Trail because of the excellent and dependable water supply at this location. The Mesa Rica Sandstone is a quartz-rich sandstone characterized by numerous small-scale cross-stratification sets. Petrographic analysis indicates that the Mesa Rica Sandstone at Autograph Rock is very porous and poorly-cemented. Modern weathering of the sandstone at Autograph Rock is due to both biogeophysical and biogeochemical weathering. Much of the disintegration of the sandstone can be attributed to the influence of lichens. The lichens readily penetrate the sandstone, locally increase permeability, and contribute to enhanced weathering and degradation of the sandstone and the inscriptions. The presence or absence of lichens seems to be the most important biogeochemical weathering factor affecting the inscriptions. A mini-permeameter was used to characterize and quantify the permeability of the Mesa Rica Sandstone at Autograph Rock. Preliminary results from the permeameter field study indicate that the lichen-free, cross-bedded sandstone exposures have similar permeability values (excluding concretions, subaerial exposure planes, iron oxide diffusion rings, and certain other variant bedding features); data from lichen-covered exposures indicate increased permeability. This increase in permeability is likely due to increased subsurface fungal penetration. The lichen-covered inscriptions and rock surfaces are therefore subject to a higher degree of degradation relative to lichen-free surfaces.