THE ROLE OF GRAIN SIZE DEPENDENT CLAST ORIENTATION AND ASPECT RATIO IN RF-PHI STRAIN ANALYSIS: COMPARISON OF MATHEMATICAL AND DIGITAL MODELS WITH NATURALLY DEFORMED ROCKS.

Rf-phi strain analysis of deformed sedimentary rocks from the southern Appalachians and Picuris Mountains (New Mexico) reveal a pattern of increasing calculated bulk strain (Rs) with increasing grain size (sand to pebbles). Digital deformation of an image of an undeformed conglomerate yielded the same results, with populations of larger clasts (e.g., pebbles) producing higher Rs values than populations of smaller clasts (e.g., sand). In order to test the underlying causes for this apparent grain size dependent strain partitioning, we analyzed undeformed conglomerates from the Gulf Coastal Plain of Georgia (Tuscaloosa Formation) and southern Appalachian foreland of Alabama (Attalla Chert Conglomerate). That work revealed grain size influences on clast orientation and aspect ratio, with larger grains having a higher average aspect ratio (Rf), as well as a more restricted range of clast orientations (phi) than smaller grains. The collective results suggest grain size dependent clast orientation and aspect ratio acquired during sediment deposition can have a significant influence on Rf-phi calculations of Rs. To further test the influence of clast orientation and aspect ratio anisotropies on calculated Rs via Rf-phi, we created a mathematical model using the formula for an ellipse that allows for clast population analysis during progressive deformation. In this model, Rf and phi values for clasts of varying size (n>1000) were catalogued in an Excel spreadsheet and mathematically deformed from an initial Rs value of 1 (undeformed) up to an Rs value of 4 for three different populations of grains: isotropic with respect to Rf and phi, anisotropic with respect to Rf, and anisotropic with respect to phi. Results from this mathematical model were consistent with the results from digitally and naturally deformed rock. Implications from this work suggest that grain size should be considered when using Rf-phi strain analysis to estimate Rs in deformed sedimentary rocks.