GRAIN SIZE EFFECTS ON RF-PHI STRAIN ANALYSIS OF DEFORMED ROCKS: A CASE STUDY FROM THE ATTALA CHERT CONGLOMERATE, ALABAMA FORELAND, SOUTHERN APPALACHIANS (USA)

Rf-phi strain analysis conducted on deformed sedimentary rocks from the southern Appalachians and Picuris Mountains of northern New Mexico reveal a surprising, but consistent pattern of increasing calculated bulk strain (Rs) values with increasing grain size. Rf-phi strain analysis using quartz clasts (n>400) in conglomerate and metaconglomerate samples from the Colvin Mountain Sandstone (Georgia), Cheaha Quartzite (Alabama), and Marquenas Formation (New Mexico), with clasts ranging in size from sand to pebbles, yielded Rs values that systematically increased as much as 127% from the smallest to largest grain size populations. We consider a number of potential underlying factors that might explain this variation, including potential differences in rheological behavior between smaller and larger grains, initial grain-size dependent orientation anisotropy, and initial grain-size dependent aspect ratio anisotropy. Digital deformation of undeformed conglomerate images produced the same grain-size dependent Rs variation as analysis of a naturally deformed rock, making it unlikely that Rs values vary due to rheological differences between smaller and larger grains during deformation. In order to assess the potential for grain-size dependent orientation and/or aspect ratio anisotropies, we analyzed >1200 clasts on bedding planes from the relatively undeformed, Ordovician Attala Chert (Alabama). Similar populations of sand (n=412), granules (n=455), and pebbles (n=347) were analyzed as part of this project. Results indicate grain-size dependent variation for both clast orientation and aspect ratio. For clast orientation (phi), larger clasts were more likely to have a preferred long axis orientation than smaller clasts. Aspect ratios for clasts (long axis/short axis) also varied with grain size, with average aspect ratios >1.75 for larger clasts and average aspect ratios of <1.4 for smaller clasts. Collectively, grain-size dependent orientation and aspect ratio anisotropies identified in different clast size populations in the Attala Chert conglomerate could explain apparent grain-size dependent “strain partitioning” in deformed conglomerates. Coupled with previous research, this suggests grain size must be considered during Rf-phi strain studies.