PETROLOGY OF THE HOPEMAN SANDSTONE (PERMO-TRIASSIC), MORAY FIRTH BASIN, SCOTLAND
To further characterize the petrology of the Hopeman, ten samples were collected from various locations along the coastal outcrop, including flat-bedded (3), cross-bedded (4), and deformed strata (2), and a clastic dike (1). Sorting, rounding, and mineral composition data was obtained by petrographic analyses of thin sections of each sample. To evaluate the sorting, the longest axes of at least 400 grains were measured from 3-5 sections per slide, perpendicular to bedding. The standard deviation of the grain sizes was then used to determine sorting, according to Johnson (1994). The rounding of the quartz and orthoclase grains was estimated using the rho scale by Powers (1953) and Folk (1955). Mineral composition and porosity data was obtained by point counting.
Of the ten sections, one was well-sorted and nine were poorly sorted. The standard deviation of 300 random grains from each slide (excluding the clastic dike) was 0.81, indicating that the sandstone was poorly sorted overall. The average rounding values of the quartz and orthoclase grains were 3.3 and 3.0, respectively. While composition varied for each section, point count data indicated that, overall, the sandstone consisted of 69.14% quartz, 6.39% orthoclase, and trace amounts of musvcovite and other minerals.
These results are surprising for an eolian sandstone. Modern cross-bedded desert dunes are typically well- to moderately sorted and do not contain micas. The poor sorting, sub-rounded to angular orthoclase grains, and presence of micas in the Hopeman therefore suggest that some re-evaluation of the accepted depositional model may be required.