ABSTRACT

We have used sets of oriented samples taken from modern eolian, fluvial and beach environments to study the effectiveness of three common methods of determining sedimentary grain orientation. The first of these methods, point counting of individual grain orientation directions in thin section, was used to verify the results from two alternative methods, namely measurement of quartz optic c-axis orientation (the photometric method) and measurement of magnetic susceptibility anisotropy.

For the photometric method, we show how the use of an analyser wavelength of 0.733µ and a thin section thickness of 20µ considerably improves the method's accuracy. We also conclude that misalignment of optic c-axes to individual grain elongation directions has little effect on the validity of the method.

For the magnetic susceptibility anisotropy method, we have related magnetic parameters such as lineation and foliation to the amount of dispersion in theoretical assemblages of ellipsoidal grains. In the samples we studied, the maximum to intermediate susceptibility differences are very small. Nevertheless, the maximum susceptibility directions of samples from a single site are usually well grouped and have mean directions that agree well with those obtained from both the photometric method and from point counting individual grains in thin section.

Finally, we show that results from all three methods are consistently correlated with cross-bed dip directions in the environments from which the samples we used were taken.