VISCOSITY CONTRAST BETWEEN CHERT AND IRON OXIDE BASED ON FOLD WAVELENGTH MEASUREMENTS

The viscosity of biphase materials is strongly dependent on the arrangements of the phases of the materials. In order to better understand viscosity contrasts in simple biphase materials, we have analyzed fold wavelengths of chert layers in samples of a banded iron formation from the NW Seminoe Mountains, Wyoming. The thickness of chert and hematite layers varies from 6.6 to 1 mm and 15.2 to 0.5 mm, respectively. When adjacent to thick layers of hematite, chert layers form parallel folds. However, when several chert layers are only separated by thin hematite layers, the limbs of the folded chert layers are attenuated, forming similar folds. The interfaces between some chert and hematite layers are altered to prismatic grunerite, which formed after deformation, but indicates deformation temperatures in excess of 550^oC (Forbes, 1977). Based on wavelengths of chert layers of various thicknesses and grain sizes, the strength contrast between chert and iron oxide varies from about 5:1 to about 140:1. Narrow, isolated layers of chert are weaker than thick or multiple, closely separated layers of chert which control larger scale wavelengths of folds.