DEFINING GRADIENTS IN A SAMPLE OF SEDIMENTARY ROCK

Abstract

SUMMARYThree orthogonal planes (two vertical and one horizontal with respect to the apparent bedding structure) of a core sample of seemingly graded graywacke sandstone have been petrographically analyzed in order to determine the form and characteristics of the lithologic gradients in the specimen.Compositional and textural properties were sampled in thin section according to an orthogonal grid pattern on each of the three faces.Variability among rows and columns (plus interaction) with respect to each petrographic variable was tested by analysis of variance using a two way classification. A larger number of properties show significant variation among the rows of the sampling grid (thus in a vertical direction) than in the horizontal plane of the apparent bedding.Quality control models with confidence limits, which display graphically trends in the individual constituents, indicate that feldspar proportion, mica proportion, and quartz grain size exhibit significant gradients (above the level of analytical error) in the direction perpendicular to the bedding. Quartz proportion oscillates in a non‐systematic fashion, and quartz grain shape shows no trend in the vertical planes of the specimen. The petrographic variability observed in the horizontal plane of the bedding is generally trendless.Partial trend surfaces show that the trends in quartz grain size are dominant, but are not simple in form according to the comparative reductions in total variation achieved by fitted surfaces of successively higher degree. Again, the major portions of variability are explained by the surfaces fitted to planes perpendicular to the bedding. The textural homogeneity of the rock in the horizontal (bedding) plane is reflected in the relatively subdued trends exposed by the surfaces in this direction.Taking grain size as an example, a 1: 1 correspondence is established between this property and the theoretical effects of settling in a liquid medium by a functional relationship; it is shown that only 25 to 40% of the total grain size variation in the specimen is attributable to this process.It is concluded that the interpretation of natural sedimentation processes from analysis of petrographic properties is complex due to the interaction of the processes and the interdependencies among the measurable properties which result.