GRAIN SIZE DEPENDENT ASPECT RATIO AND CLAST ORIENTATION IN SEDIMENTARY ROCKS: IMPLICATIONS FOR PALEOCURRENT DETERMINATION AND STRAIN ANALYSIS

Previous analyses of clast size, orientation, and aspect ratio in sedimentary and metasedimentary rocks suggest interrelationships between the three factors. Recognition that clast aspect ratios and orientation are grain size dependent has important implications for depositional processes that influence subsequent paleocurrent and/or strain analysis. Using conglomerates from the Late Cretaceous Tuscaloosa Formation of the Gulf Coastal Plain in Georgia, and Attalla Chert Conglomerate of the southern Appalachian foreland basin in Alabama, we show that sedimentary clast populations exhibit grain size dependent anisotropy with respect to orientation (phi) and aspect ratio (Rf). Using software designed for strain analysis (Ellipsefit), clast analysis of the two samples suggests phi and Rf values vary systematically with grain size. By targeting equal numbers of sand, granule and pebble sized grains (n>500 grains), we were able to identify two important relationships. First, average Rf values decrease with decreasing grain size. Second, phi variation decreases with increasing grain size. This observation holds true for both bedding parallel and bedding perpendicular surfaces. These critical observations help explain apparent grain size dependent strain partitioning in deformed metaconglomerates (e.g., Cheaha Quartzite), with larger grain size populations recording higher bulk strain values than populations of smaller grains. Digital and mathematical modeling of Rf and phi anisotropy reveals similar results. The results of this work suggest statistical analysis of grain orientation in sedimentary rocks can provide valuable paleocurrent information in the absence of macro structural features (e.g., crossbeds). In addition, results indicate that grain size plays an important factor in interpreting the results of Rf-phi strain analysis.