Origin of modem quartzarenite beach sands in a temperate climate, Florida and Alabama, USA

Quartzarenite coastal sands extending from eastern Louisiana eastward to Apalachee Bay, Florida, are anomalous: their position 7 degrees north of the Tropic of Cancer contrasts with most other known modem quartzarenites, most of which are in a tropical setting. To determine the origin of these quartzarenite beach sands, we compared the mineralogy of samples taken from Alabama and Florida beaches, rivers that supply sand to the coast, and well cuttings representative of sandstone bedrock exposed in the Alabama coastal plain. To help assess the abundance of recycled quartz, and accepting the conventional wisdom that rounded sand-size quartz grains are recycled, we quantified the roundness of quartz grains in thin sections of river, beach, and well samples. We also determined the abundance of recycled grains with authigenic quartz using cathodoluminescence. River sands on Precambrian and Paleozoic bedrock in the study area have subarkose and sublitharenite compositions. However, as far as 200 kin inland from the coast, river sands have attained quartzarenite composition and all rivers are presently delivering sand with at least 97% quartz to the coast. Rivers develop quartzarenite sand composition where they traverse poorly consolidated Tertiary sandstones, all of which we sampled are composed of > 95% quartz. Published experimental work indicates that abrasional rounding of sand-size quartz by rivers is insignificant and rounding in beaches is extremely slow. Hence, the abundance of quartz grains with some degree of rounding (96% for beaches; > 75% for rivers) further attests to the abundance of recycled quartz. (c) 2007 Published by Elsevier B.V.