Cotter (1982) demonstrated a logical sequence of depositional facies in the Tuscarora Sandstone, both vertically and laterally, from braided river fluvial deposits to the southeast, progressing through local “estuarine” and regional beach facies, to marine(?) shelf sand waves to the northwest. Several examples of each facies were sampled and subjected to quantitative grain-shape analysis. Although the outcropping Tuscarora has suffered intense diagenesis (pressure solution and quartz overgrowth), these effects are less obvious in thin sections cut parallel to bedding, where the gross two-dimensional grain shapes are relatively unmodified. Preliminary results comparing the Fourier Analysis (FA) methods of Ehrlich and colleagues (1970 to 1982) and Boon (1982) with the Rotated Radials-Factor Score (RR-FS) method of Parks (1981) are presented.
In the RR-FS method, the two-dimensional silhouettes of several hundred grains from each sample are electronically digitized and the raw data (100+ points per grain) are stored on floppy disks. From this point on, the analysis is performed under operator control by a series of FORTRAN programs implemented on a microcomputer. For each grain, the digitized outline is rotated about a calculated “center of gravity” to a least squares best fit with an empirical “reference shape”; and 36 equal-angle spaced radials are calculated. The data set for each sample (400+ grains) is reduced by R-mode Factor Analysis. Computed factor scores are used to classify the gross grain shapes into a limited number of distinguishable categories. Associations of shape types are compared to specific facies.
