THE EFFECTS OF GRAIN-SIZE DEPENDENT CLAST ORIENTATION AND ASPECT RATIO FOR STRAIN ANALYSIS IN DEFORMED SEDIMENTARY ROCKS: A CASE STUDY FROM THE TUSCALOOSA FORMATION OF GEORGIA (USA)

Strain analysis using the Rf-phi technique is commonly conducted on deformed sedimentary rocks since clasts are generally assumed to be randomly oriented (-90 to +90 phi) with a range of aspect ratios (Rf) that are independent of clast size. Some workers, however, have found that Rf-phi bulk strain calculations (Rs) can vary within an individual sample depending on the size of the grains selected for analysis. As an example, clasts of deformed metaconglomerate from the lower greenschist facies Cheaha Quartzite (Talladega belt, Alabama) and amphibolite facies Marquenas Formation (Picuris Mountains, New Mexico) showed a systematic increase in calculated Rs from smaller to larger grains (sand to cobbles). Differences in Rs from smaller to larger grain size populations varied by as much as 127%. Analysis of deformed (i.e., folded but not metamorphosed) conglomerates from the southern Appalachian foreland basin yielded similar results, with smaller grain size populations (sand) yielding lower calculated Rs values than larger grain size populations (pebbles) during Rf-phi analysis. Analysis of clast orientation and aspect ratio from a conglomerate in the barely deformed Attala Chert (Alabama foreland) revealed aspect ratio and clast orientation values that varied by grain size populations (sand to pebbles), which could explain the apparent grain-size dependent “strain partitioning” observed during Rf-phi analysis of highly deformed conglomerates. In this project, we use a strain analysis program (Ellipsefit) to analyze a conglomerate from the Late Cretaceous Tuscaloosa Formation of Georgia that has experienced no deformation since deposition. Bedding parallel views from multiple samples provide both clast orientation and aspect ratio data for hundreds of sand to pebble-sized grains that should reveal anisotropies that would affect both Rf and phi values in a deformed rock.