ABSTRACT

The fabrics of non-opaque grains of oriented siltstones from the Red Peak Member of the Chugwater Formation (Triassic) are anisotropic in planes parallel to bedding. Six of seven samples show a preferred orientation averaging 360° (all orientations are for nothern hemisphere). A sample with disturbed bedding is nearly isotropic in the bedding plane. A mean paleocurrent direction of 21° was obtained from asymmetric, parallel ripple marks in the interval that contained the oriented samples of the fabric study.

Siltstone fabrics in thin sections perpendicular to bedding show little imbrication but less dispersion than in thin sections parallel to bedding. Two samples, sectioned parallel to the preferred grain orientation, deviate from the bedding plane by about +8° and -1°. Similar values occur, however, in four samples that were sectioned perpendicular to the preferred orientation.

Measurement of 1174 opaque grains in thin sections parallel to bedding yielded a mean elongation (length/width) ratio of 1.62, a standard deviation of 0.43 and a median interval of 1.50-1.59. Similar measurement of 639 grains in thin sections perpendicular to bedding yielded a mean elongation ratio of 1.79, a standard deviation of 0.50 and a median interval of 1.70-1.79.

Opaque grains are anisotropic in planes parallel to bedding with a mean direction of 324°, differing by 36° from the mean of the non-opaque grain fabrics. In planes perpendicular to bedding, the "preferred inclination" of elongate opaque grains corresponds closely with the bedding plane. There is little difference in imbrication between the opaque and non-opaque grains in planes perpendicular to bedding.

The mean (324°) for the preferred orientations of the opaque grains nearly coincides with the declination of the predominant (normal and reverse) magnetic directions, suggesting that the two are related.